Hydrogen atom adducts to nitrobenzene: Formation of the phenylnitronic radical in the gas phase and energetics of wheland intermediates

Article abstract of DOI:10.1021/ja001229h

The phenylnitronic radical (1) was prepared in the gas phase by collisional electron transfer to stable C₆H₅NO₂H⁺ cation (1⁺) and found to be stable on the microsecond time scale. The major unimolecular dissociation of 1 was loss of OH radical to form nitrosobenzene as determined by variable-time neutralization-reionization mass spectrometry. Ab initio calculations at the effective QCISD(T)/6-311+G(3df,2p) level and combined Moller-Plesset and density functional theory calculations identified loss of OH as the lowest-energy dissociation of 1 that proceeded at the thermochemical threshold with no reverse activation barrier. Dissociations of 1 by loss of syn- and anti-HONO and a hydrogen atom were more endothermic than loss of OH and had activation barriers above the thermochemical thresholds. The internal energy of 1 formed by electron transfer in the ground electronic state (X) was insufficient to cause the observed dissociations. The dissociations are postulated to take place from the metastable excited electronic B state formed by vertical electron transfer. Wheland intermediates for hydrogen atom additions to the ortho (2), meta (3), para (4), and ipso (5) positions in nitrobenzene were calculated to be 75, 98, 78, and 101 kJ mol^-1 less stable than 1. Radicals 2-4 existed in substantially deep potential energy wells to allow their generation as transient intermediates. Radical 5 resided in a shallow potential energy minimum and was predicted to dissociate exothermically to benzene and NO₂. Relative thermal rate constants for hydrogen atom additions to nitrobenzene were calculated and found to correlate with the regioselectivities for additions of other radicals.

Full text of DOI:10.1021/ja001229h

J. Am. Chem. Soc. 2000, 122, 9511-9524
9511
Hydrogen Atom Adducts to Nitrobenzene: Formation of the
Phenylnitronic Radical in the Gas Phase and Energetics of Wheland
Intermediates
Miroslav Pola´sˇek and Frantisˇek Turecˇek*
Contribution from the Department of Chemistry, Bagley Hall, Box 351700, UniVersity of Washington,
Seattle, Washington 98195-1700
ReceiVed April 7, 2000
Abstract: The phenylnitronic radical (1) was prepared in the gas phase by collisional electron transfer to
stable C6H5NO2H+ cation (1+) and found to be stable on the microsecond time scale. The major unimolecular
dissociation of 1 was loss of OH radical to form nitrosobenzene as determined by variable-time neutralization-
reionization mass spectrometry. Ab initio calculations at the effective QCISD(T)/6-311+G(3df,2p) level and
combined Møller-Plesset and density functional theory calculations identified loss of OH as the lowest-
energy dissociation of 1 that proceeded at the thermochemical threshold with no reverse activation barrier.
Dissociations of 1 by loss of syn- and anti-HONO and a hydrogen atom were more endothermic than loss of
OH and had activation barriers above the thermochemical thresholds. The internal energy of 1 formed by
electron transfer in the ground electronic state (X) was insufficient to cause the observed dissociations. The
dissociations are postulated to take place from the metastable excited electronic B state formed by vertical
electron transfer. Wheland intermediates for hydrogen atom additions to the ortho (2), meta (3), para (4), and
ipso (5) positions in nitrobenzene were calculated to be 75, 98, 78, and 101 kJ mol-1 less stable than 1.
Radicals 2-4 existed in substantially deep potential energy wells to allow their generation as transient
intermediates. Radical 5 resided in a shallow potential energy minimum and was predicted to dissociate
exothermically to benzene and NO2. Relative thermal rate constants for hydrogen atom additions to nitrobenzene
were calculated and found to correlate with the regioselectivities for additions of other radicals.
Introduction
Scheme 1
Additions of small radicals to aromatic compounds have been
of much interest because of their role in processes as diverse
as tropospheric oxidations,1 oxidative damage at tyrosine and
tryptophan residues in proteins,2 and decomposition of energetic
materials.3 In contrast, less is known about radical adducts to
aromatic compounds carrying electronegative substitutents, such
as the cyano and nitro groups.4 This is despite the fact that
nitroaromatic compounds are important biologically active
pollutants and their metabolic reduction5 and tropospheric
degradation have been of interest.6 A kinetic study of the
nitrobenzene-hydroxyl radical reaction showed that nitrobenzene
was less reactive than benzene by a factor of 2.7
Following Wheland’s prediction8 that the para and ortho
positions should be more reactive than the meta position, the
* Author for correspondence. Telephone: (206) 685-2041. Fax: (206)
685-3478. E-mail: turecek@chem.washington.edu.
(1) Atkinson, R. Gas-Phase Tropospheric Chemistry, J. Phys. Chem. Ref.
Data, Monograph No. 2; American Chemical Society: Woodbury, NY,
1994.
(2) Richter, C. In Free Radical Toxicology; Wallace, K. B., Ed.; Taylor
and Francis: Washington, DC, 1997.
(3) Brill, T. B.; James, K. J. Chem. ReV. 1993, 93, 2667.
(4) Urbanski, T. In The Chemistry of the Nitro and Nitroso Groups; Feuer,
H., Ed.; Wiley-Interscience: New York, 1970; pp 64-73.
(5) (a) Kalhorn, T.; Becker, A. R.; Sternson, L. A. Bioorg. Chem. 1981,
10, 144. (b) Becker, A. R.; Sternoson, L. A. Proc. Natl. Acad. Sci. U.S.A.
1981, 78, 2003.
reactivity of nitrobenzene toward radical additions and the
properties of radical intermediates have been studied at various
levels of theory.9 Wheland8 and later Weiss and co-workers10
postulated the existence of transient intermediates (Wheland
intermediates11) formed by radical addition to the benzene ring,
as illustrated for hydrogen atom adducts 1-5 in Scheme 1.
(9) (a) Shashin, S. S.; Rykova, E. A. J. Mol. Struct. (THEOCHEM) 1994,
311, 175. (b) Fabian, J.; Bleisch, S.; Domschke, G.; Mayer, R. Z. Chem.
1981, 21, 265. (c) Ladik, J.; Biczo, G.; Kende, I.; Sumegi, L. Acta Chim.
Sci. Hung. 1969, 61, 381.
(6) Knispel, R.; Koch, R.; Siese, M.; Zetsch, C. Ber. Bunsen-Ges. Phys.
Chem. 1990, 94, 1375.
(7) Arnts, R. R.; Seila, R. L.; Bufalini, J. J. J. Air Pollut. Control Assoc.
1989, 39, 453.
(8) Wheland, G. W. J. Am. Chem. Soc. 1942, 64, 900.
(10) Loebl, H.; Stein, G.; Weiss, J. Chem. Soc. 1949, 2074.
(11) March, J. AdVanced Organic Chemistry, 3rd ed.; Wiley: New York,
1985; p 448.
10.1021/ja001229h CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/16/2000

9512 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Pola´sˇek and Turecˇek
Scheme 2
Why does nitrobenzene exhibit different regioselectivities in
radical and electrophilic additions? To answer this question we
now study the energetics of nitrobenzene radicals and the
activation energies of radical additions. Hydrogen atom additions
represent the simplest models for radical additions to nitro-
aromates that can be tackled by experiment and theory. In this
work we prepare radical 1 in the gas phase and study its
unimolecular dissociations using the methods of neutralization-
reionization mass spectrometry (NRMS),20 including temporal
resolution of radical reactions by variable-time NRMS.21 Our
approach starts with protonation of nitrobenzene in the gas-
phase using an acid that targets selectively the nitro group to
produce stable ions 1+ (BH1+, Scheme 2) or using stronger acids
that are capable of protonating the less basic ring positions
(BH2+). Stable ions 1+ are accelerated to a high velocity
(113 750 m s-1) and discharged by fast electron transfer (<10-14
s) from dimethyl disulfide during a glancing collision. This
approach is formally the reverse of the electrochemical reduction
in which electron addition precedes protonation. To obtain a
comprehensive view of hydrogen atom adducts to nitrobenzene,
including relative isomer energies, dissociation energies and
isomerization barriers, we carried out ab initio and density
functional theory calculations at a high level of theory. The
energy data are used to assess the kinetics of competitive
unimolecular dissociations and the branching ratios in hydrogen
atom additions to nitrobenzene. Dissociation mechanisms were
studied using a series of deuterium-labeled radicals (1a-1e).
Hydroxyl radical addition to nitrobenzene was found to produce
o-, m-, and p-nitrophenol, presumably via transient intermediates
of the type shown in Scheme 1. Addition to nitrobenzene of
methyl radicals produced by thermolysis of tert-butylperoxide
formed mostly o- and p-nitrotoluene,12 bearing out Wheland’s
predictions. The reactivity toward radical additions of the ipso-
position and the nitro group has not been studied and their role
in the radical chemistry of nitrobenzene is unclear. Recently,
alkylation of p-dinitrobenzene with trialkylboranes has been
interpreted as involving an alkyl radical addition step,13 and
nucleophilic substitutions in nitroaromates were considered to
include radical intermediates.14
The phenylnitronic radical (1) corresponding to a hydrogen
atom adduct to the nitro group in nitrobenzene has been
postulated to arise by protonation of an anion-radical produced
by electrochemical reduction15a-f or pulse radiolysis of nitroben-
zene in protic solution16a-c (Scheme 2). The evidence for the
transient formation of 1 was inferred from the changes in the
absorption spectrum that showed two maxima at λ ) 418 and
460 nm, which were blue-shifted by 10 nm from those of the
nitrobenzene anion-radical.16,17 In contrast, Cercek concluded
from his pulse radiolysis kinetic measurements that hydrogen
atom addition in nitrobenzene occurred in the ring to yield a
nitrocyclohexadienyl radical.18 Photolysis of nitrobenzene in
tetrahydrofuran as a hydrogen atom donor resulted in the
formation of transient radicals that were tentatively assigned
structure 1 on the basis of electron paramagnetic resonance
(EPR) spectra.19 However, tetrahydrofuran adducts to nitroben-
zene were also detected that complicated the interpretation of
the EPR spectra.19b
Experimental Section
Methods. Measurements were carried out on a tandem quadrupole
acceleration-deceleration mass spectrometer described previously.22
Precursor ions were generated in a tight chemical ionization (CI) source.
Typical ionization conditions were as follows: electron energy 100
eV, emission current 1-2 mA, temperature 280-300 °C, ion source
potential 80 V. CH4, CD4, CH3OH, CD3OD, CH3CN, CD3CN, and CH3-
ONO2 were used as reagent gases at pressures 1.0-1.5 × 10-4 Torr as
read on an ionization gauge located at the diffusion pump intake. CH3O-
ions were generated by negative-ion chemical ionization (NCI) from a
90/10 mixture of CH3OH and N2O, H- ions were prepared by
dissociative electron capture in H2. Typical NCI conditions were as
follows: electron energy 200 eV, emission current 1 mA, temperature
280-300 °C, source potential -80 V. Electron ionization (EI) was
used to generate cation-radicals C6H5NO2+•, C6H5NO+•, and the phenyl
(12) Williams, G. H. In Homolytic Aromatic Substitution; Pergamon:
Oxford, 1960; loc. cit. 90. See also ref 4.
(13) Shifman, A.; Palani, N.; Hoz, S. Angew. Chem., Int. Ed. 2000, 39,
944.
(14) Paradisi, C.; Scorrano, G. Acc. Chem. Res. 1999, 32, 958.
(15) (a) Karakus, C.; Zuman, P. J. Electroanal. Chem. 1995, 396, 499.
(b) Zuman, P.; Fijalek, Z. J. Org. Chem. 1991, 56, 5486. (c) Rao, P. S.;
Hayon, E. Anal. Chem. 1976, 48, 564. (d) Farnia, G.; Mengoli, G.; Vianello.
E. Sci. Chim. 1967, 37, 668. (e) Geske, A. H.; Maki, D. H. J. Am. Chem.
Soc. 1960, 82, 2671. (f) Geske, A. H.; Maki, D. H. J. Chem. Phys. 1960,
33, 825.
(16) (a) Asmus, K.-D.; Wigger, A.; Henglein, A. Ber. Bunsen-Ges. Phys.
Chem. 1966, 70, 862. (b) Asmus, K.-D.; Cercek, B.; Ebert, M.; Henglein,
A.; Wigger, A. Trans. Faraday Soc. 1967, 63, 2435. (c) Lilie, J.; Henglein,
A. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 170.
(20) For most recent reviews see: (a) Zagorevskii, D.; Holmes, J. L.
Mass Spectrom. ReV. 1999, 18, 87. (b) Schalley, C. A.; Hornung, G.;
Schroder, D.; Schwarz, H. Chem. Soc. ReV. 1998, 27, 91. (c) Turecek, F.
J. Mass Spectrom. 1998, 33, 779.
(21) (a) Kuhns, D. W.; Tran, T. B.; Shaffer, S. A.; Turecek, F. J. Phys.
Chem. 1994, 98, 4845. (b) Kuhns D. W.; Turecek, F. Org. Mass Spectrom.
1994, 29, 463.
(17) Iyer, S.; Capellos, C. In AdVances in Chemical Reaction Dynamics;
Rentzepis, P. M.; Capellos, C., Eds; Reidel: Amsterdam 1986; pp 405-
414.
(18) Cercek, B. Nature 1971, 234, 159.
(19) (a) Ward, R. L. J. Chem. Phys. 1963, 38, 2588. (b) Chachaty, C.;
Forchioni, A. J. Chim. Phys., Phys. Chim. Biol. 1968, 65, 1649.
(22) Turecek, F.; Gu, M.; Shaffer, S. A. J. Am. Soc. Mass Spectrom.
1992, 3, 493.

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9513
+
cation C6H5 by dissociative ionization of nitrobenzene. Typical EI
calculation was small as judged from the S2 operator expectation
values that were 0.75-0.77. The optimized structures were characterized
by harmonic frequency analysis as local minima (all frequencies real)
or first-order saddle points (one imaginary frequency). Complete
optimized structures in the Cartesian coordinate format are available
as Supporting Information. The B3LYP/6-31+G(d,p) frequencies were
scaled by 0.963 (ref 28, for other scaling factors see ref 29) and used
to calculate zero-point vibrational energies (ZPVE), enthalpy correc-
tions, and partition functions. Single-point energies were calculated at
several levels of theory. In one set of calculations, MP2(frozen core)30
and B3LYP energies were calculated with the larger 6-311+G(2df,p)
basis set. Spin contamination in the UMP2 calculations was substantial
for the nitrobenzene radicals and transition states, as evidenced by the
spin expectation values S2 that ranged between 1.1 and 1.6. Spin
annihilation using Schlegel’s projection method31 (PMP2)26 reduced
the S2 values to 0.77-0.88 for local minima and 1.1-1.5 for transition
states. In addition, restricted open-shell (ROMP2) calculations32 were
carried out for the entire set of structures to deal with the spin
contamination problem.33 The PMP2 and ROMP2 energies were
averaged with the B3LYP energies according to the empirical procedure
that was introduced previously34 and tested for several systems since.35,36
This resulted in error cancellation and provided excellent relative
energies, denoted as B3-PMP2 or B3-ROMP2,36 as discussed below.
Calculations on closed-shell systems are marked by B3-MP2. In
addition, a composite procedure was adopted that consisted of a single-
point quadratic configuration interaction calculation,37 QCISD(T)/
6-31G(d,p), and basis set expansion up to 6-311+G(3df,2p) through
PMP2 or ROMP2 single-point calculations according to eq 1:
conditions were as follows: electron energy 70 eV, source temperature
280-300 °C, and sample pressure 5-8 × 10-6 Torr. Stable precursor
cations were passed through a quadrupole mass filter operated in the
radio frequency-only mode, accelerated to the total kinetic energy of
8250 eV and neutralized in the collision cell floated at -8170 V. The
precursor ion lifetimes were 30-40 µs. Dimethyl disulfide (DMDS)
was admitted to the differentially pumped collision cell at a pressure
such as to achieve 70% transmittance of the precursor ion beam. The
ions and neutrals were allowed to drift to a four segment conduit,23
where the ions were reflected by the first segment floated at +250 V.
The neutral flight times in standard NRMS measurements were 5.3
µs. The fast neutral species were reionized in the second collision cell
by collision with oxygen at a pressure adjusted such as to achieve 70%
transmittance of the precursor ion beam. The ions formed in the second
collision cell were decelerated, energy filtered, and analyzed by a
quadrupole mass filter operated at unit mass resolution. The instrument
was tuned daily to maximize the ion current of reionized CS2
+•
.
Typically, 40 repetitive scans were accumulated per spectrum, and each
spectrum was reproduced at least three times over a period of several
weeks. Variable-time measurements were carried out as described
previously.21 The neutral flight times that were used to evaluate the
unimolecular dissociation kinetics were 0.51, 1.50, and 2.50 µs.
Collisionally activated dissociation (CAD) spectra were measured
on a JEOL HX-110 double-focusing mass spectrometer of forward
geometry (the electrostatic sector E precedes the magnet B). Collisions
with air were monitored in the first field-free region at pressure to
achieve 70% transmittance of the ion beam at 10 keV. The spectra
were obtained by scanning E and B simultaneously while maintaining
a constant B/E ratio (B/E linked scan).
Materials. Methane (Matheson, 99.97%), CD4 (Cambridge Isotope
Laboratories, 99% D), methanol (Aldrich), CD3OD (Cambridge Isotope
Laboratories, 99% D), CD3CN (Cambridge Isotope Laboratories, 99%
D), dimethyl disulfide (DMDS, Aldrich), nitrobenzene (Aldrich, 99%)
and nitrobenzene-d5 (Aldrich, 99% D), and nitrosobenzene (Aldrich)
were used as received.
ortho-, meta-, and para-D-nitrobenzenes were synthesized from the
corresponding nitroanilines (Aldrich) by diazotation with 1.5 equiv of
NaNO2 in D2O/DCl at -5 °C followed by diazonium salt reduction
with a 5-fold molar excess of D3PO2 (Aldrich, 99% D) at 0 °C
overnight.24 The products were extracted in ether, the solutions were
decolorized with active charcoal, the solvent was distilled off, and the
products were purified by vacuum distillation. The overall yields of
distilled products were 50-70%. The product purity was verified by
gas-chromatography mass spectrometry that showed 99% D1 content
in the labeled nitrobenzenes. Minor impurities were nitrosobenzene
(<1%) and chloronitrobenzenes (<5%) that did not interfere in the
mass spectrometric measurements. Methyl nitrate was prepared ac-
cording to a literature procedure.25
QCISD(T)/6-311+G(3df,2p) ≈ QCISD(T)/6-31G(d,p) +
MP2/6-311+G(3df,2p) - MP2/6-31G(d,p) (1)
This level of theory is intermediate between those of the Gaussian
2 (MP2) method38 which uses the 6-311G(d,p) basis set in the large
QCISD(T) calculation and the G2(MP2, SVP) method39 which uses
the 6-31G(d) basis set instead. We also utilized the previous finding
that restricted open-shell calculations (ROMP2) provided good stabi-
lization energies for small organic radicals.33 A basis set expansion to
effective QCISD(T)/6-311+G(2df,p) was also tested for selected
systems. The calculated total energies are available as Supporting
Information. The relative energies are presented in Tables 1-7.
Franck-Condon energies in vertical neutralization and reionization
were taken as absolute differences between the total B3LYP/6-311+G-
(2df,p) energies of fully optimized ion or neutral structures and those
in which an electron has been added to an optimized cation structure
(27) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 1372 and 5648. (b)
Stephens, P. J.; Devlin, F. J.; Chablowski, C. F.; Frisch, M. J. J. Phys.
Chem. 1994, 98, 11623.
(28) (a) Rauhut, G.; Pulay, P. J. Phys. Chem. 1995, 99, 3093. (b) Finley,
J. W.; Stephens, P. J. J. Mol. Struct. (THEOCHEM) 1995, 227, 357. (c)
Wong, M. W. Chem. Phys. Lett. 1996, 256, 391.
(29) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502.
(30) Møller, C.; Plesset, M. S. Phys. ReV. 1934, 46, 618.
(31) (a) Schlegel, H. B. J. Chem. Phys. 1986, 84, 4530. (b) Mayer, I.
AdV. Quantum Chem. 1980, 12, 189.
(32) McWeeny, R.; Diercksen, G. J. Chem. Phys. 1968, 49, 4852.
(33) Parkinson, C. J.; Mayer, P. M.; Radom, L. J. Chem. Soc., Perkin
Trans. 2, 1999, 2305.
(34) Turecek, F. J. Phys. Chem. A, 1998, 102, 4703.
(35) (a) Turecek, F.; Wolken, J. K. J. Phys. Chem. A, 1999, 103, 1905.
(b) Wolken, J. K.; Turecek, F. J. Phys. Chem. A, 1999, 103, 6268. (c)
Wolken, J. K.; Turecek, F. J. Am. Chem. Soc. 1999, 121, 6010. (d) Turecek,
F.; Carpenter, F. H. J. Chem. Soc., Perkin Trans. 2 1999, 2315. (e) Turecek,
F.; Polasek, M.; Frank, A. J.; Sadilek, M. J. Am. Chem. Soc. 2000, 122,
2361.
(36) Rablen, P. R. J. Am. Chem. Soc. 2000, 122, 357.
(37) Pople, J. A.; Head-Gordon, M.; Raghavachari, K. J. Chem. Phys.
1987, 87, 5968.
(38) (a) Curtiss, L. A.; Raghavachari, K.; Pople, J. A. J. Chem. Phys.
1993, 98, 1293.
(39) (a) Curtiss, L. A.; Redfern, P. C.; Smith, B. J.; Radom, L. J. Chem.
Phys. 1996, 104, 5148. (b) Smith, B. J.; Radom, L. J. Phys. Chem. 1995,
99, 6468.
Calculations. Standard ab initio and density functional theory
calculations were performed using the Gaussian 98 suite of programs.26
Geometries were optimized using Becke’s hybrid functional (B3LYP)27
and the 6-31+G(d,p) basis set. Spin-unrestricted calculations (UB3LYP)
were used for open-shell systems. Spin contamination in the UB3LYP
(23) Turecek, F. Org. Mass Spectrom. 1992, 27, 1087.
(24) (a) Høg, J. H. J. Label. Comput. 1971, 7, 179. (b) Alexander, E.
R.; Burge, R. E. J. Am. Chem. Soc. 1950, 72, 3100. (c) Murray, A.; Williams,
D. L. Organic Syntheses with Isotopes; Wiley-Interscience: New York,
1958; pp 1375 and 1592.
(25) Black, A. P.; Babers, F. H. Org. Synth. 1939, 19, 64.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.;
Peng, C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M.
W.; Johnson, B. G.; Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.

9514 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Table 1. Dissociation Energies of Ions
Pola´sˇek and Turecˇek
reaction enthalpya-c
method
1
2
3
4
5
6
7
8
9
rmsd
B3LYP/6-31+G(d,p)
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
ROMP2/6-311+G(3df,2p)
B3-PMP2/6-311+G(2df,p)
B3-ROMP2/6-311+G(2df,p)
exp.
814
814
442
440
439
433
d
436
440
436
443
227
229
289
298
285
300
259
263
223
264
261
262
259
293
269
-154
-128
140
131
139
146
d
149
135
138
144
234
229
224
212
224
213
226
221
242
22
16
41
41
784
786
300
300
281
298
299
203
207
-46
-50
-87
41
19
21
799
800
279
259
236
243
a Including B3LYP/6-31+G(d,p) zero-point corrections and 298 K enthalpy corrections. b In kJ mol-1
.
c Reaction 1: C6H5NO2H+ f C6H5NO2
+•
+
+ H+. Reaction 2: C6H5NO2H+ f C6H5NO2+ + H•. Reaction 3: C6H5NO2H+ f C6H5NO+• + OH•. Reaction 4: C6H5NO2H+ f C6H5
+
syn-HONO. Reaction 5: C6H5NO2H+ f C6H5 + anti-HONO. Reaction 6: C6H5NO2H+ f C6H6 + NO2+. Reaction 7: C6H6 + NO2 f ipso-
+
C6H6NO2 (5+). Reaction 8: C6H5NO2 f C6H5 + NO2. Reaction 9: C6H5NO+• f C6H5 + NO. d The UHF calculations did not converge.
+
+•
+
+
positions are very similar46 and so the former datum has not
or subtracted from an optimized neutral structure. No zero-point
been reexamined here. Gas-phase protonation with CH3CdNH+
(PA(CH3CN) ) 779 kJ mol-1, ref 47) was therefore energetically
possible to occur only in the nitro group to form ion 1+.
Likewise, gas-phase deuteronation with CD3CdND+ introduced
the deuteron in the nitro group to form ion 1a+. Ions 1+ and
1a+ were characterized by collisionally activated dissociation
spectra;48 that of 1+ was in qualitative agreement with the
previous measurements of Crombie and Harrison.49
corrections were applied to the calculated Franck-Condon energies.
Excited-state energies were obtained from single-point calculations
using the complete active space (CASSCF)40 method with the 6-31G-
(d,p) basis set. Oscillator strengths were calculated with the configu-
ration-interaction singles (CIS) method41 and the 6-311+G(2df,p) basis
set.
RRKM calculations used Hase’s program42 that was recompiled for
MS-DOS and run under Windows NT.43 Vibrational state densities were
obtained by direct count of quantum states in 2 kJ mol-1 steps for
internal energies up to 80-120 kJ mol-1 above the threshold. The
rotational states were treated adiabatically44 and the microscopic rate
constants, k(E,J,K), were Boltzmann-averaged over the thermal distribu-
tion of rotational J and K states pertaining to the ion source temperature.
Thermal rate constants were calculated using the standard transition
state theory formulas.45 The activation energies were taken from single-
point calculations, the partition functions were calculated from the
B3LYP/6-31+G(d,p) moments of inertia and scaled harmonic frequen-
cies using the rigid rotor-harmonic oscillator approximation.
Collisional neutralization of stable 1+ and its isotopomers
gave rise to radicals 1, 1a and 1e that were characterized by
NR mass spectra (Figure 1a-c). NRMS of 1 showed a
prominent survivor ion at m/z 124 as evidence that a substantial
fraction of 1 did not dissociate on the 5.3 µs time scale. The
NR dissociations that were observed resulted in the formation
of C6H5NO2 (m/z 123, loss of H), C6H5NO (m/z 107, loss of
OH), C6H6O (m/z 94, loss of NO), C6H5 (m/z 77, loss of
HONO), NO (m/z 30), and aromatic hydrocarbon fragments at
m/z 73-76, 62-65, 49-51, 36-39, and 24-26. These dis-
sociations could occur from the intermediate radical 1 and/or
following collisional reionization to 1+.50 Note that the CAD
spectrum of 1+ (ref 48) showed qualitatively the same fragments
as did the NR spectrum. Hence, the neutral and post-reionization
dissociations needed to be resolved. The stability under NR
conditions of the presumed neutral intermediates was investi-
gated for nitrobenzene, nitrosobenzene and the phenyl radical
(Figure 2a-c). Phenol, another possible intermediate, is known
to be stable in NRMS.23,51 Nitrobenzene showed a moderately
abundant survivor ion and fragments formed by loss of NO2
and subsequent ring cleavages of neutral or ionic C6H5 (Figure
2a). The NR mass spectrum of C6H5NO2 was remarkably similar
to the recent photofragmentation spectrum obtained with 90 fs
laser pulses in which both neutral and ion dissociations were
considered.52 NRMS of nitrosobenzene resulted in substantial
dissociation to C6H5 and NO so that only a weak survivor ion
Results and Discussion
Phenylnitronic Radical (1). Radical 1 was generated by
selective protonation of nitrobenzene followed by collisional
electron transfer (Scheme 2). The topical proton affinities (PA)
in nitrobenzene were calculated previously at an intermediate
level of theory and found to greatly prefer protonation at an
oxygen atom of the nitro group for which PA ) 813 kJ mol-1
was calculated46 in fair agreement with the experimental PA of
nitrobenzene (800 kJ mol-1).47 The B3-MP2 proton affinity for
protonation at the nitro group calculated in the present work,
PA ) 799 kJ mol-1, was in excellent agreement with the
tabulated value (Table 1, reaction 1). The topical PA of the
ipso, ortho, and meta positions in nitrobenzene were calculated
to be substantially lower,for example, 650, 676, and 676 kJ
mol-1, respectively, according to B3-MP2 calculations which
agreed within 10 kJ mol-1 with the previous computational
data.46 The topical proton affinities of the para and ortho
(48) The CAD spectra of 1+ and 1a+ were as follows: 1+ (m/z, rel intens)
26(0.3), 27(0.5), 30(0.7), 36(0.4), 37(0.9), 38(1.2), 39(1.4), 48(0.2), 49-
(0.9), 50(7.2), 51(9.7), 52(0.5), 60(0.2), 61(0.5), 61.5(0.2), 62(3.2), 63(1.1),
64(0.7), 65(2.4), 66(0.7), 73(0.4), 74(2.2), 75(2.2), 76(8.2), 77(34.0), 78-
(4.0), 91(0.2), 93(1.2), 94(6.5), 106(0.2), 107(4.8), 123(2.6). 1a+ (m/z, rel
intens) 24(0.2), 26(0.4), 27(0.4), 30(0.7), 36(0.4), 37(1.2), 38(1.1), 39(0.9),
40(0.4), 41(0.3), 42(0.3), 44(0.2), 48(0.2), 49(0.8), 50(6.6), 51(8.7), 52-
(0.70), 54(0.4), 60(0.5), 61(0.5), 62(0.9), 62.5(2.9), 63(1.0), 64(0.9), 65-
(0.6), 66(1.0), 67(0.6), 73(0.5), 74(1.9), 75(1.7), 76(7.9), 77(31.1), 78(2.6),
79(2.0), 91(0.3), 94(0.5), 95(6.9), 106(0.4), 107(5.4), 123(0.3), 124(2.9).
(49) Crombie, R. A.; Harrison, A. G. Org. Mass Spectrom. 1988, 23,
327.
(40) (a) Hegarty, D.; Robb, M. A. Mol. Phys. 1979, 38, 1795. (b) Eade,
R. H. E.; Robb, M. A. Chem. Phys. Lett. 1981, 83, 362.
(41) Foresman, J. B.; Head-Gordon, M.; Pople, J. A.; Frisch, M. J. J.
Phys. Chem. 1992, 96, 135.
(42) Zhu, L.; Hase, W. L. Quantum Chemistry Program Exchange;
Indiana University: Bloomington, IN, 1994; Program No. QCPE 644.
(43) Frank, A. J.; Sadilek, M.; Ferrier, J. G.; Turecek, F. J. Am. Chem.
Soc. 1997, 119, 12343.
(44) Zhu, L.; Hase, W. L. Chem. Phys. Lett. 1990, 175, 117.
(45) Levine, I. N. Physical Chemistry, 3rd ed.; McGraw-Hill: New York,
1988; p 839.
(46) Eckert-Maksic, M.; Hodoscek, M.; Kovacek, D.; Maksic, Z. B.;
Primorac, M. J. Mol. Struct. (THEOCHEM) 1997, 417, 131.
(47) NIST Standard Reference Database No. 69, February 2000 Release.
http://webbook.nist.gov/chemistry.
(50) Abboud, J. L. M.; Notario, R.; Yanez, M.; Mo, O.; Flammang, R.;
Jagerovic, N.; Alkorta, I.; Elguero, J. J. Phys. Org. Chem. 1999, 12, 787.
(51) Turecek, F.; Drinkwater, D. E.; Maquestiau, A.; McLafferty, F. W.;
Org. Mass Spectrom. 1989, 24, 669.

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9515
Figure 1. Neutralization (CH3SSCH3, 70%T)-reionization (O2, 70%T) spectra of (a) 1+, (b) 1a+, and (c) 1e+.
+
was found (Figure 2b). NRMS of C6H5 showed an abundant
survivor ion and dissociations by hydrogen atom losses and ring
fragmentations (Figure 2c).
[1b - H], [1b - D], etc., were fitted into mass-balance
homogeneous linear equations (eq 2-4)
kD/kH[ortho]
[ortho] + 2[meta] + [para] [1b - H]
kD/kH[meta]
2[ortho] + 2[meta] + [para] [1c - H]
kD/kH[para]
2[ortho] + 2[meta] [1d - H]
[1b - D]
Deuterium labeling in the nitro group revealed clean losses
of H (but not D!), OD, and DONO from 1a upon NR (Figure
1b). These were confirmed by the NR mass spectrum of the
complementarily labeled D5-ion (1e+) which showed clean
losses of D, OH, and HONO (Figure 1c). Deuterium labeling
thus proved that the hydroxyl hydrogen atom did not exchange
with the ring hydrogens in stable radicals 1 nor in the course
of radical and/or post-reionization dissociations. The intriguing
loss of hydrogen atom was further studied with radicals 1b-d
produced from specifically labeled ions 1b-d+. The relative
intensities of NR ions formed by loss of H and D, denoted as
)
) 0
(2)
[1c - D]
)
) 0.079 (3)
) 0.079 (4)
[1d - D]
)
to obtain the fractional losses from the ortho, meta, and para
positions marked as [ortho], [meta], and [para]. The best fit was
obtained when considering an average kinetic isotope effect for
the cleavage of the C-H bond as kH/kD ) 3.05. The fractions
favored loss of H from the para position (61%) and both meta
positions (39% total) in 1+, whereas the hydrogen atoms in the
ortho positions and the NO2H group were unreactive. Similar
fractions for the losses from 1+ of the ortho, meta, and para
(52) (a) Kosmidis, C.; Ledingham, K. W. D.; Kilic, H. S.; McCanny,
T.; Singhal, R. P.; Langley, A. J.; Shaikh, W. J. Phys. Chem. 1997, 101,
2264. For other photodissociation studies, see: (b) Galloway, D. B.; Bartz,
J. A.; Huey, L. G.; Crim, F. F. J. Chem. Phys. 1993, 98, 2107. (c) Kosmidis,
C.; Ledingham, K. W. D.; Clark, A.; Marshall, A.; Jennings, R.; Sander,
J.; Singhal, R. P. Int. J. Mass Spectrom. Ion Processes 1994, 135, 229.

9516 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Pola´sˇek and Turecˇek
Figure 2. Neutralization (CH3SSCH3, 70%T)-reionization (O2, 70%T) spectra of (a) nitrobenzene+•, (b) nitrosobenzene+•, and (c) C6H5
.
+
hydrogen atoms were obtained from analysis of CAD spectra.
The absence of loss of H from the NO2H group showed that
nitrobenzene was not formed from 1, nor was its cation-radical
formed from 1+.53 Radicals 1, 1a, and 1e were also generated
from the corresponding ions prepared by highly exothermic
equivalent geminal H and D atoms that should be transferred
to the nitro group with similar probabilities when allowing for
isotope effects. Wheland radical 5 was expected to competitively
eliminate the ipso hydrogen atom and the NO2 group (vide
infra). However, neither of these dissociations was enhanced
in the NR spectrum of [nitrobenzene + D]+ from deuteronation
with CD5+. Hence it follows that Wheland ion structures 2+-
5+ were not present in the ion population formed by the highly
exothermic protonation of nitrobenzene.
+
protonation of nitrobenzene with CH5 or CD5+. Comparison
of the proton affinity of CH4 (543 kJ mol-1 47
) with the topical
proton affinities of the ring positions in nitrobenzene (vide supra)
indicated that ring protonation with CH5+ was >100 kJ mol-1
exothermic and therefore energetically possible for each position.
However, the NR mass spectra of [nitrobenzene + H]+ and
The reason for this observation can be 2-fold. First, the ion-
molecule reaction of nitrobenzene with CH5+ requires formation
[nitrobenzene + D]+ ions from protonation with CH5+ or CD5
of an ion-dipole complex which should slightly favor CH5
+
+
(spectra not shown) were very similar to those of 1+, 1a+, and
1e+, respectively. In particular, losses of OH or OD, and HONO
or DONO cleanly incorporated the proton or deuteron previously
introduced by the gas-phase acid. This ruled out participation
of Wheland structures 2-4 which would contain chemically
interaction with the negatively charged oxygen atoms of the
nitrobenzene dipole. However, an upper-limit estimate from the
locked-dipole theory54 indicated <1% increase of reaction rate
due to locked ion-dipole interaction favoring protonation at
the nitro group.55 Second, even if Wheland structures were
(53) Polasek, M.; Turecek, F.; Flammang, R.; Gerbaux, P. Unpublished
results.
(54) (a) Moran, T. F.; Hamill, W. H. J. Chem. Phys. 1963, 39, 1413. (b)
Theard, L. P.; Hamill, W. H. J. Am. Chem. Soc. 1962, 84, 1134.

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9517
Table 2. Anion Energies
energya
reaction
B3LYP/6-31+G(d,p)
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
B3-PMP2
C6H5NO2 + H- f C6H5NO2H- (1-)
C6H5NO2 + H- f ortho-H-C6H5NO2- (2-)
C6H5NO2 + H- f meta-H-C6H5NO2- (3-)
C6H5NO2 + H- f para-H-C6H5NO2- (4-)
1- + C6H5NO2 f C6H5NO2-• + 1
2- + C6H5NO2 f C6H5NO2-• + 2
3- + C6H5NO2 f C6H5NO2-• + 3
4- + C6H5NO2 f C6H5NO2-• + 4
C6H5NO2-• f C6H5NO2
-423
-291
-301
-296
-433
-303
-314
-309
-332
-205
-242
-224
-434
-305
-314
-310
-5
-6
38
16
55
6
51
-21
51
29
-56
30
-96
-92
7
9
124 (1.29)b
119 (1.24)
204 (2.12)
127 (1.32)
208 (2.16)
121 (1.25)b
115 (1.19)
204 (2.11)
128 (1.33)
208 (2.16)
69.5 (0.72)b
108 (1.11)
193 (2.00)
146 (1.51)
199 (2.06)
95 (0.99)b
111 (1.15)
199 (2.06)
137 (1.42)
203 (2.11)
C6H5NO2H- (1-) f 1
ortho-H-C6H5NO2- (2-) f 2
meta-H-C6H5NO2- (3-) f 3
para-H-C6H5NO2- (4-) f 4
b
a In units of kJ mol-1 at 298 K. Electron affinities in eV.
formed by exothermic protonation, they could be depleted by
exothermic proton transfer to neutral nitrobenzene producing
1+. Under standard chemical ionization conditions, neutral
nitrobenzene is the second most abundant species in the ion
source (after the CI reagent gas), and secondary reactions of it
with high-energy [nitrobenzene + H]+ ions cannot be avoided.
Note that intramolecular isomerization of 2+-5+ to the much
more stable 1+ would result in hydrogen scrambling which was
ruled out by the labeling experiments.
to hydrogen atom elimination to form C6H5NO2-•, as judged
from the calculated reaction enthalpies (Table 2). However,
hydrogen transfer reactions to nitrobenzene were calculated to
be energetically possible and can provide a mechanism for
depletion of ions 1-- 4- (Table 2). Potential energy barriers
can be another factor hampering hydride additions to nitroben-
zene, but these were not studied in this work.
The dissociations of neutral 1 and post-reionization dissocia-
tions of 1+ were distinguished by variable-time measurements.
These yielded phenomenological rate parameters that were used
to quantify the contributions of neutral (kN) and ion (ki) processes
to dissociations observed upon NR. Two dissociations were
investigated: Loss of OH was monitored in the C6H5NO+•
Other attempts at generating ion precursors of 2-5 were
unsuccessful. Gas-phase nitration of benzene with the CH3OH-
NO2 complex was reported to produce the ipso ion 5+ as a
+
reaction intermediate.56 We performed ion-molecule reactions
+
of CH3OH-NO2 with benzene and C6D6 that afforded
+
channel, and loss of HONO was monitored in the C6H5
abundant nitration products at m/z 124 and m/z 130, respectively.
However, the NR mass spectra of the m/z 124 and m/z 130 ions
were identical to those of 1+ and 1e+, respectively. In particular,
the NR spectra of the m/z 124 and m/z 130 ions from gas-phase
nitration showed negligible C6H6 and C6D6 product intensities
that would have been indicative of the presence of 5 or its
deuterium-labeled derivative (vide infra). This strongly indicated
that ion 5+ rearranged rapidly to the more stable isomer 1+, as
postulated previously.56 Rearrangement to 1e+ of the ionic
Wheland intermediate C6D6-NO2+ was also indicated by gas-
phase H/D exchange with methanol that produced C6D5NO2H+
at m/z 129.
channel. Unfortunately, ions from the complementary neutral
fragments, OH and HONO, respectively, were too weak to be
measured reliably.59 The rate parameters indicated participation
of both neutral and ion dissociations, for example, kN(C6H5-
NO) ) (2.8 ( 0.1) × 105 s-1, ki(C6H5NO+•) ) (4.0 ( 0.1) ×
105 s-1, and kN(C6H5) ) (4.8 ( 0.1) × 105 s-1, ki(C6H5+) )
(9.0 ( 0.1) × 105 s-1. However, variable-time measurements
also showed that C6H5NO, when prepared by collisional
neutralization, underwent both neutral and ion dissociations by
•
loss of NO to form C6H5 and C6H5+, respectively. The rate
parameters for unimolecular NR dissociations of C6H5NO
Addition of H- to nitrobenzene was predicted by B3-MP2
calculations to be exothermic when forming anions 1--4-
(Table 2). By comparison, the B3-MP2 calculated electron
affinity of nitrobenzene (0.99 eV, Table 2) was close to the
experimental value (1.00-1.01 eV).47 Despite the favorable
thermochemistry of hydride additions to nitrobenzene, ion-
molecule reactions of H- with C6H5NO2 and C6D5NO2 yielded
only the corresponding nitrobenzene anion-radicals at m/z 123
and 128, respectively. This result was independent of the mode
of H- generation by either dissociative electron capture in H2
(ref 57) or hydride transfer from CH3O- (ref 58). Interestingly,
anions 1-- 4- were found to be substantially stable with respect
were substantial, that is, kN(C6H5) ) (7.4 ( 0.5) × 105 s-1
,
kN(NO) ) (8.2 ( 0.5) × 105 s-1, ki(C6H5+) ) (7.3 ( 0.5) ×
105 s-1, and ki(NO+) ) (1.2 ( 0.1) × 106 s-1. Hence, the
formation of C6H5 (radical or ion) can be due to consecutiVe
dissociations of nitrosobenzene produced as a primary product
from 1. To distinguish the direct and consecutive dissociations
of 1 we resorted to ab initio and density functional calculations
that furnished activation energies and rate constants.
Radical and Ion Energetics. Geometry optimizations with
B3LYP/6-31+G(d,p) found local energy minima for 1-5
(Figure 3) as confirmed by harmonic frequency analysis.
Interestingly, only the planar syn rotamer of 1 of Cs symmetry
was a stable structure, while the quasiplanar anti isomer was a
transition state for internal rotation of the OH group (TS11,
Figure 3). Wheland adducts 2-4 showed unexceptional bond
lengths and angles. The ipso isomer 5 had a long Cipso-N bond
(1.610 Å) that indicated facile dissociation (vide infra). At all
(55) Calculated by using the tabulated values for the dipole moment,
µD ) 4.22 D, and polarizability, a ) 12.9-14.7 × 10-24 cm3, of
nitrobenzene. See: Lide, D. H. CRC Handbook of Chemistry and Physics,
71st ed.; CRC Press: Boca Raton, FL, 1990.
(56) (a) Attina, M.; Cacace, F.; Speranza, M. Int. J. Mass Spectrom. Ion
Processes 1992, 117, 37. (b) Aschi, M.; Attina, M.; Cacace, F.; Ricci, A.
J. Am. Chem. Soc. 1994, 116, 9535. (c) Attina, M.; Cacace, F.; Ricci, A. J.
Phys. Chem. 1996, 100, 4424.
(57) Budzikiewicz, H. Mass Spectrom. ReV. 1986, 5, 345.
(58) (a) Ingemann, S.; Kleingeld, J. C.; Nibbering, N. M. M. J. Chem.
Soc., Chem. Commun. 1982, 1009. (b) Nibbering, N. M. M. AdV. Phys.
Org. Chem. 1988, 24, 1-55.
(59) Vertical ionization of HONO is accompanied by large Franck-
Condon effects that result in facile ion dissociation, see: Polasek, M.;
Turecek, F. J. Phys. Chem. A, 1999, 103, 9241. OH has a substantially
smaller ionization cross section than C6H5NO.

9518 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Pola´sˇek and Turecˇek
Figure 3. B3LYP/6-31+G(d,p)-optimized structures of 1-5 and transition states. Bond lengths in Å, bond and dihedral angles in degrees.
Table 3. Relative Energies of Nitrobenzene Radicals
relative enthalpya,b
Table 4. Dissociation and Adiabatic Ionization Energies of
Nitrobenzene and Nitrosobenzene
reaction enthalpya-c
method
1
2
3
4
5
method
1
2
3
4
5
6
7 rmsd
B3LYP/6-31+G(d,p)
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
ROMP2/6-311+G(3df,2p)
QCISD(T)/6-311+G(2df,p)
UMP2
PMP2
ROMP2
QCISD(T)/6-311+G(3df,2p)
UMP2
0
75
77
67
73
66
72
99
99
82
97
83
98
78
79
71
78
71
78
104
104
87
98
86
B3LYP/6-31+G(d,p)
286 223 182 244 63 939 785 10
281 215 182 247 65 942 780 11
329 253 145 221 76 965 808 26
310 235 151 227 76 959 813 19
331 255 146 222 76 d 810 29
0
0
0
0
0
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
97
ROMP2/6-311+G(3df,2p) 312 236 152 229 76 959 815 19
QCISD(T)/6-311+G(2df,p)
0
0
0
67
65
66
81
79
81
67
66
69
81
81
83
UMP2
PMP2
320 239 152 233 81
327 245 153 234 82
319 237 152 234 82
18
21
17
ROMP2
QCISD(T)/6-311+G(3df,2p)
0
0
0
0
0
0c
66
65
65
72
75
76c
81
80
81
91
98
99c
67
66
69
75
78
80c
80
80
82
UMP2
PMP2
323 241 153 235 82
329 247 154 236 82
323 238 154 236 82
19
22
18
PMP2
ROMP2
B3-PMP2/6-311+G(2df,p)
B3-ROMP2/6-311+G(2df,p)
ROMP2
95
B3-PMP2/6-311+G(2df,p) 305 234 163 234 71 953 794 10
B3-ROMP2/6-311+G(2df,p) 296 225 166 237 71 950 797
exp. 295 228 171 246 76 954 781
(10 (10 (4
101
102c
9
a Including B3LYP/6-31+G(d,p) zero-point and 298 K enthalpy
c
corrections. b In kJ mol-1. 0 K values.
a Including B3LYP/6-31+G(d,p) zero-point corrections and 298 K
•
enthalpy corrections. b In kJ mol-1
.
c Reaction 1: C6H5NO2 f C6H5
levels of theory, 1 was the most stable isomer, followed by the
ortho, para, meta, and ipso Wheland adducts (Table 3).
To judge the quality of the relative energies calculated at
various levels of theory, we obtained 298 K enthalpies for
dissociations and ionization of nitrobenzene and nitrosobenzene,
for which there are experimental thermochemical data.47 Like-
wise, ion dissociation energies were calculated and compared
with values that were derived from experimental enthalpies of
formation. The results are summarized in Tables 1 and 4. The
neutral dissociation energies were approximated by B3LYP and
ROMP2 calculations reasonably well and showed (13 and 16
kJ mol-1 root-mean-square deviations (rmsd), respectively
(Table 4). Since the deviations for B3LYP and ROMP2 energies
were of similar magnitude but showed opposite signs, an
+ NO2 . Reaction 2: C6H5NO f C6H5 + NO•. Reaction 3: C6H6 +
•
•
NO2 f C6H5NO2 + H•. Reaction 4: C6H6 + NO• f C6H5NO + H•.
•
Reaction 5: C6H5NO2 + NO f C6H5NO + NO2. Reaction 6: C6H5-
NO2 f C6H5NO2+•. Reaction 7: C6H5NO f C6H5NO+• d The UHF
.
calculations did not converge.
improved agreement due to error cancellation was achieved for
B3-PMP2 and B3-ROMP2 energies which agreed with experi-
mental data within (10 and (9 kJ mol-1 rmsd, respectively.
Interestingly, the performance of B3-ROMP2 with the present
test set was substantially better than that of the “best” composite
QCISD(T)/6-311+G(3df,2p) procedure that showed rmsd of (18
kJ mol-1 (Table 4). The B3-ROMP2 data were therefore used
to evaluate the energetics of radicals 1-5. The agreement for

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9519
Figure 4. B3-ROMP2 potential energy diagram for 1. 0 K energies in kJ mol-1
.
neutralization of 1+ were small and resulted in excitations of
<20 kJ mol-1, as documented by the calculated IEa - REv
values (Table 5).
Table 5. Ionization and Recombination Energies of Nitrobenzene
Radicals
ionization and recombination
energiesa,b
Transition states were sought for several dissociations and
isomerizations of 1, as summarized in a potential-energy diagram
(Figure 4). The lowest-energy dissociation of 1 was cleavage
of the N-OH bond that required 145-152 kJ mol-1 at the B3-
PMP2 and B3-ROMP2 thresholds, respectively (Table 6). The
B3-PMP2 potential energy profile along the N-OH coordinate
was studied by stepwise calculations and showed a continuously
endothermic dissociation with no barrier for the reverse addition
of OH• to nitrosobenzene. Dissociation of the C-1-NO2H bond
in 1 showed two isomeric transition states leading to the
formation of the syn and anti rotamers of HONO. Since 1 exists
as a syn isomer only, branching into the isomeric transition states
occurred along the dissociation coordinate at a C-1-NO2H bond
length of about 1.6 Å, beyond which distance the intermediate
syn and anti structures each showed positive force constants
for OH rotation that indicated separate dissociation paths. The
activation energies for the addition of phenyl radical to syn-
and anti-HONO were small, 6 and 7 kJ mol-1, respectively, by
B3-ROMP2 calculations. For energies obtained at the other
levels of theory see Table 6.
method
1
2
3
5
B3LYP/6-31+G(d,p)
7.30
7.72 7.42 7.65
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
ROMP2/6-311+G(3df,2p)
B3-PMP2/6-311+G(2df,p)
7.28 (7.07)c 7.71 7.43 7.65
6.95 (6.74)c 7.53 7.44 7.67
7.05 (6.82)c 7.57 7.39 7.65
6.97
7.07
7.55 7.46 7.69
7.59 7.40 7.67
7.12 (6.90)c 7.62 7.43 7.66
B3-ROMP2/6-311+G(2df,p) 7.17 (6.95)c 7.64 7.41 7.65
a The adiabatic ionization energies include B3LYP/6-31+G(d,p)
zero-point corrections. b In units of electronvolts. c Vertical recombina-
tion energies in parentheses.
the ion energetics was reasonably good for both B3-PMP2 and
B3-ROMP2 where the rmsd were (19 and (21 kJ mol-1
,
respectively. The major sources of error were from the reaction
enthalpies involving the nitrosobenzene cation-radical that
differed from the experiment-derived values by 20-40 kJ mol-1
(Table 1) We note that the experimental adiabatic ionization
energy of nitrosobenzene which is included in the enthalpy of
formation of C6H5NO+• was extrapolated from a poorly resolved
band in the photoelectron spectrum60 and may be underesti-
mated. A comparable fit for the ion energies was obtained from
the B3LYP data alone which showed rmsd of 16-22 kJ mol-1
(Table 1).
The ion and radical energetics are connected through adiabatic
ionization energies (IEa) and vertical recombination energies
(REv). The differences between the IEa and REv values provide
estimates of vibrational excitation in radicals formed by vertical
electron transfer. The calculated IEa and REv are summarized
in Table 5 for selected species. The data showed that 1 had a
lower ionization energy than the nitrocyclohexadienyl radicals
2, 3, and 5. This was mostly due to the substantial stabilization
of ion 1+ relative to the ring protonated nitrobenzene ions 2+-
5+ (cf. Table 1). The Franck-Condon effects in vertical
Dissociation of the O-H bond in 1 required 165 and 170 kJ
mol-1 at the B3-PMP2 and B3-ROMP2 thresholds, respectively
(Table 6). The dissociation path was studied in detail and is
used here as a difficult case in point to illustrate the approach
we also adopted for C-H bond cleavages in 2-5 that showed
better behavior. Full optimizations with B3LYP/6-31+G(d,p)
provided a continuous potential energy profile with a first-order
saddle point at d(O-H) ) 1.84 Å (Figure 5). Single-point
energy calculations along the reaction path showed maxima at
d(O-H) ) 1.80, 1.6, and 1.5 Å, for B3LYP, PMP2, and
ROMP2, respectively, which were displaced toward shorter
O-H distances compared with the fully optimized saddle point
geometry. The averaged B3-PMP2 and B3-ROMP2 energies
peaked out at 1.7 Å, and following ZPVE corrections were used
to estimate transition state energies. The potential energy profile
from the B3-PMP2 and B3-ROMP2 calculations showed a rather
sharp curvature around the saddle point region (Figure 5), which
(60) (a) Rabalais, J. W.; Colton, R. J. J. Electron Spectrosc. Relat.
Phenom. 1973, 1, 83. (b) Egdell, R.; Green, J. C.; Rao, C. N. R.; Gowenlock,
B. G.; Pfab, J. J. Chem. Soc., Faraday Trans. 2 1975, 988.

9520 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Table 6. Dissociation and Activation Energies of 1
Pola´sˇek and Turecˇek
reaction and activation enthalpya-c
method
1
2
3
4
5
6
7
8
B3LYP/6-31+G(d,p)
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
ROMP2/6-311+G(3df,2p)
QCISD(T)/6-311+G(2df,p)
UMP2
168
166
189
186
189
186
173
172
194
198
167
164
187
185
188
185
173
171
194
191
199
193
137
146
141
151
205
200
209
181
221
174
141
145
146
159
147
161
20
14
-5
-2
-2
1
188
159
164
165
167
169
176
157
161
162
153
149
159
190
170
198
118
113
124
4
-1
10
PMP2
ROMP2
QCISD(T)/6-311+G(3df,2p)
UMP2
159
164
165
178
176
178d
167
168
168
183
182
157
161
162
176
174
176d
157
154
164
165
170
173d
186
181
191
205
191
119
114
125
145
152
156d
6
2
13
4
PMP2
ROMP2
B3-PMP2/6-311+G(2df,p)
B3-ROMP2/6-311+G(2df,p)
183
181
6
6.5d
a Including B3LYP/6-31+G(d,p) zero-point corrections. b In units of kJ mol-1 at 0 K. c Reaction 1: C6H5NO2H• f C6H5• + syn-HONO. Reaction
2: C6H5NO2H• f TS(C6H5 ...syn-HONO). Reaction 3: C6H5NO2H• f C6H5• + anti-HONO. Reaction 4: C6H5NO2H• f TS(C6H5 ...anti-HONO).
Reaction 5: C6H5NO2H• f C6H5NO2 + H•. Reaction 6: C6H5NO2H• f TS(C6H5NO2...H•). Reaction 7: C6H5NO2H• f C6H5NO + OH•. Reaction
•
•
8: C6H5NO2H• f C6H6 + NO2 . d 298 K energies.
•
Figure 6. RRKM rate constants (log k versus internal energy) for
dissociations of 1. Reaction 1: 1 f C6H5NO + OH•. Reaction 2: 1 f
C6H5• + syn-HONO. Reaction 3: 1 f C6H5• + anti-HONO. Reaction
4: 1 f C6H5NO2 + H•. Reaction 5: 1 f 2. The dashed line at log
k ) 5.12 corresponds to rate constants for 50% dissociation on the
Figure 5. Potential energy profiles along the O-H coordinate in 1.
experimental time scale.
was caused by the large energy differences between the
transition state and products calculated by PMP2 and ROMP2.
Single point energies were also calculated with the effective
QCISD(T)/6-311+G(3df,2p) at the saddle point geometry, but
the considerable time and memory requirements of these
calculations made reaction path mapping impractical. The
transition state energy from the effective QCISD(T) single-point
calculations (cf. eq 1) were slightly lower than that from B3-
MP2 calculations (Table 6).
The unimolecular isomerization of 1 to 2 was also ac-
companied by a sizable energy barrier. The B3-PMP2 and B3-
ROMP2 barriers for the 1 f 2 isomerization (190 and 192 kJ
mol-1, respectively) agreed well with those from effective
QCISD(T)/6-311+G(3df,2p) (194 and 187 kJ mol-1, respec-
tively, TS12, Table 7). According to the calculated dissociation
and isomerization energetics, the formation of C6H5NO + OH•
should be the preferred unimolecular reaction of 1 at all levels
of theory.
reactions using the RRKM theory. The kuni values that were
based on the B3-ROMP2 transition state energies are shown in
Figure 6. The kuni that were based on B3-PMP2 energies showed
very similar trends and will not be discussed separately. The
calculated kuni strongly indicated that loss of OH should be the
dominant dissociation of 1 from the dissociation threshold up
to the highest internal energy studied (330 kJ mol-1). Losses
of syn- and anti-HONO became competitive with the loss of
OH only at the highest internal energies and could account for
up to 10% of the total dissociation of 1 having >300 kJ mol-1
internal energies. Interestingly, dissociation through the syn-
transition state that had a slightly higher energy in the transition
state was calculated to be faster than that for the anti-isomer.
In contrast, loss of the hydrogen atom from the NO2H group
and isomerization to 2 were calculated to be 3-5 orders of
magnitude slower than loss of OH and therefore should not be
competitive in dissociations of 1 in its ground electronic state.
The unimolecular dissociations of 1 showed substantial kinetic
shifts caused by the finite time scale of the experimental
The calculated transition state energies were used to calculate
unimolecular rate constants (kuni) for the above-discussed

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9521
Table 7. Dissociation and Activation Energies of Wheland Intermediates
reaction and activation enthalpiesa-c
method
1
2
3
4
5
6
7
8
9
10
11
12
B3LYP/6-31+G(d,p)
B3LYP/6-311+G(2df,p)
PMP2/6-311+G(2df,p)
ROMP2/6-311+G(2df,p)
PMP2/6-311+G(3df,2p)
ROMP2/6-311+G(3df,2p)
QCISD(T)/6-311+G(3df,2p)
UMP2
PMP2
ROMP2
B3-PMP2/6-311+G(2df,p)
B3-ROMP2/6-311+G(2df,p)
123
115
69
72
73
134
130
110
112
113
105
100
93
54
48
58
52
115
112
99
94
101
87
120
113
64
59
69
133
128
107
111
115
106
95
89
50
48
55
53
122
119
103
102
106
97
-84
-91
-93
-101
-89
-97
-0.2
-0.8
8
179
180
199
203
227
205
154
152
162
156
162
157
7.5
77
64
8
89
88
100
92
126
128
131
120
120
75
73
82
73
70
75d
119
120
121
105
104
88
86
94
89
86
91d
128
128
129
118
117
76
73
82
69
68
72d
127
128
128
111
107
-74
-78
-69
-92
-95
-94d
193
194
187
190
192
3.6
3.4
157
155
93
98d
a Including B3LYP/6-31+G(d,p) zero-point corrections. b In units of kJ mol-1 at 0 K. c Reaction 1: 2 f C6H5NO2 + H•. Reaction 2: 2 f
TS(ortho-H‚‚‚C6H5NO2). Reaction 3: 3 f C6H5NO2 + H•. Reaction 4: 3 f TS(meta-H‚‚‚C6H5NO2). Reaction 5: 4 f C6H5NO2 + H•. Reaction
6: 4 f TS(para-H‚‚‚C6H5NO2 + H•). Reaction 7: 5 f C6H5NO2 + H•. Reaction 8: 5 f TS(ipso-H‚‚‚C6H5NO2). Reaction 8: 5 f C6H6 + NO2.
Reaction 10: 5 f TS(ipso-C6H6...NO2). Reaction 11: TS(12). Reaction 12: TS(23). d 298 K enthalpies.
measurements. These are shown in Figure 6 at log kuni ) 5.12
corresponding to 50% dissociation during the 5.3 µs flight time
of 1. The lowest-energy dissociation of 1 was estimated to have
a 48 kJ mol-1 kinetic shift, which should enhance the kinetic
stability of 1.
to provide a rough estimate of excitation energies and radiative
lifetimes. The active space for the CASSCF calculations
included the top three occupied π-molecular orbitals and two
unoccupied π-orbitals from the virtual orbital space. Energies
were calculated for 1 produced by vertical electron capture in
1+ and for the relaxed structure of the ground-state radical. The
energies for the relaxed and vertically formed 1 were very
similar and need not be discussed separately. The CASSCF
excitation energy for the X f A transition (3.42 eV) was
compatible with a band in the UV absorption spectrum of the
putative [nitrobenzene + H] adduct (3 eV).16-18 The A, B, and
C excited states were due to excitation of a valence electron to
the virtual orbital space. These outer excited states from the
CIS calculations were relevant to the formation of 1 by
femtosecond electron transfer. The A, and C states had
nanosecond lifetimes for radiative transitions to the X state.
Hence, these states were too short-lived to undergo dissociations
on the microsecond time scale. In contrast, the B f X transition
was dipole forbidden (|Rmn| ) 4 × 10-4 au2) and the ensuing
>10 µs radiative lifetime should allow it to participate in
unimolecular dissociations. The B state had a sufficient energy
(calculated as ∼520 kJ mol-1 above the X state) to drive
competitive dissociations by N-O, C-N, and O-H bond
cleavages following internal conversion to a vibrationally excited
X state. Furthermore, direct dissociation from the B state to
excited electronic states of the dissociation products appears to
Are the RRKM rate constants compatible with the experi-
mental rate parameters? The variable-time measurements pointed
•
to neutral dissociations of 1 forming both C6H5NO and C6H5 .
In contrast, the calculated threshold energies suggested that
C6H5• could be formed competitively with C6H5NO only from
highly excited 1. However, C6H5NO formed by loss of OH was
found to dissociate rapidly following reionization to produce
C6H5+, as evidenced by a large rate parameter for the ion dis-
sociation. Hence, the apparent rate parameter for 1 f C6H5• f
+
C6H5 could in part be due to a rate-determining neutral
dissociation 1 f C6H5NO followed by fast dissociation of
reionized C6H5NO+. In this way the temporal profile for the
formation of C6H5+ can show dependence on the time scale for
the slower neutral dissociation.
What drives the unimolecular dissociations of 1? Figure 6
shows that radicals 1 dissociating on the 5.3 ms time scale of
the experiment must have substantial internal energies, estimated
at 180-200 kJ mol-1 depending on the level of theory used, to
access the lowest energy channel. On average, the internal
energy of 1 is composed of the neutral precursor thermal energy
(46 kJ mol-1 for vibrational and rotational enthalpy of nitroben-
zene at 523 K), a 80% fraction of the protonation exother-
micity35b,61 (17 kJ mol-1) and the Franck-Condon energy (EFC)
resulting from vertical electron transfer. Note, however, that
excitation through Franck-Condon effects was remarkably
small in vertically formed 1, for example 14 kJ mol-1 by B3-
ROMP2 (Table 5). Clearly, the sum of the foregoing energy
terms, that is 46 + 17 + 14 ) 77 kJ mol-1, falls short to provide
the vibrational excitation needed for the lowest-energy dissocia-
tion of 1. This leads to the logical conclusion that 1 formed in
the ground electronic state could not dissociate. Hence, the
neutral dissociations must have been driven by formation upon
collisional electron transfer of excited electronic states that
decayed non-radiatively to a vibrationally excited ground state,
or underwent state-specific dissociations to yield the observed
products.
•
be energetically possible for the formation of (C6H5 )* +
HONO. The excited states of C6H5• are known to be 2.47, 4.51,
and 5.00 eV above the ground state,62-64 so that formation from
•
1 of (C6H5 )* + HONO required ∼413 kJ mol-1 and was
accessible from the B state of 1. In contrast, the formation of
(C6H5NO)* + OH• is estimated to require 590 kJ mol-1 (ref
62) which could not be supplied by the excitation energy of
the B state alone. Loss of H• to form an excited state of (C6H5-
NO2)* was estimated62 to require ∼600 kJ mol-1 which was
also above the B state energy.
(62) Calculated on the basis of excitation energies from the corresponding
UV absorption maxima. The λmax values used were: nitrobenzene (240 and
280 nm),62 nitrosobenzene (270 and 290 nm),63a and phenyl radical (248,
275, and 502 nm).63a,b
(63) (a) Nagakura, S.; Kojima, M.; Maruyama, Y. J. Mol. Spectrosc.
1964, 13, 174. (b) Glenewinkel-Meyer, T.; Crim, F. F. J. Mol. Struct.
(THEOCHEM) 1995, 337, 209.
(64) (a) Ikeda, N.; Nakashima, N.; Yoshihara, K. J. Am. Chem. Soc. 1985,
107, 3381. (b) Porter, G.; Ward, B. Proc. R. Soc. Ser. A, Math. Phys. Sci.
1965, 287, 457.
Excited States of 1. Excited states in 1 were addressed by
complete active space (CASSCF(5,5)/6-31G(d,p)) and config-
uration-interaction singles (CIS/6-311+G(2df,p)) calculations
(61) Uggerud, E. AdV. Mass Spectrom. 1995, 13, 53.

9522 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Pola´sˇek and Turecˇek
Figure 7. B3-ROMP2 potential energy diagram for Wheland intermediates 2-5. 0 K energies in kJ mol-1
.
Summarizing the discussion of excited states, the experimen-
tally observed loss of OH from 1 can be driven by the formation
of a metastable B state that undergoes internal conversion to a
vibrationally excited, dissociative X state. Since the loss of OH
is predicted by RRKM calculations to be very fast at excitation
energies >250 kJ mol-1 (Figure 6), the measured µs rate
parameter must correspond to B f X internal conversion as
the rate-determining step. The loss of syn- or anti-HONO from
atom additions to the nitro group and the ring positions. The
B3-PMP2 and B3-ROMP2 calculations preferred addition to
the ortho position that had the smallest activation energy Ea )
25-28 kJ mol-1 (Table 8). In contrast, effective QCISD(T)/6-
311+G(3df,2p) predicted addition to the nitro group to be the
lowest activation energy reaction of Ea ) 27-28 kJ mol-1
(Table 8). This discrepancy was caused by the poor performance
of MP2 calculations in treating the transition state energy for
the H atom addition to the nitro group. The deficiencies in the
MP2 energies largely canceled out in the basis set expansion
included in the effective QCISD(T)/6-311+G(3df,2p) energies
which were within 5 kJ mol-1 of the QCISD(T)/6-31G(d,p)
values. Error cancellation was less efficient in the B3-PMP2
and B3-ROMP2 energies where the high activation barriers from
the MP2 calculations were not compensated by the underesti-
mated values from the B3LYP calculations. In contrast, the
activation energies for additions to the ring positions by QCISD-
(T)/6-311+G(3df,2p) were consistently 12-15 kJ mol-1 and
7.7-8.8 kJ mol-1 greater than those from B3-PMP2 or B3-
ROMP2, respectively. (Table 8). However, relative to each other,
the activation energies and rate constants showed very similar
trends for the four series of calculations. For example, the
branching ratios for additions to the ortho, meta, para, and ipso
positions were 76, 7, 17, and 0.1% from QCISD(T)-ROMP2
calculations in essential agreement with the B3-ROMP2 data
(72, 11, 18, and 0.1%, respectively, Table 8).
The calculated absolute rate constants for H atom addition
to the nitro group at 273 K ranged from 2.5 × 107 to 7.4 × 107
s-1 mol-1 cm3, not including the statistical factor due to the
presence of two equivalent oxygen atoms in nitrobenzene. These
values were in reasonable agreement with the single experi-
mental rate constant that was reported as k ) 1.53 × 107 s-1
mol-1 cm3 at an unspecified temperature.18 The addition was
postulated to produce a nitrocyclohexadienyl radical adduct of
an unspecified structure.18 In contrast, the present calculations
suggest that formation of the phenylnitronic radical should be
preferred.
1, which could not be distinguished experimentally from a
stepwise dissociation, 1 f C6H5NO f C6H5NO+• f C6H5
+
+
NO (vide supra), can occur from high vibrational states of the
X state of 1, where the C-N bond cleavages become competitive
with the N-O bond cleavage (Figure 6). In addition, direct
dissociation from the B state of 1 to form an excited state of
•
C6H5 appears to be energetically possible and can contribute
to the observed µs kinetics.
Wheland Intermediates 2-5. The energetics of radicals 2-5
were addressed by B3-PMP2 and B3-ROMP2 calculations
(Table 3). The calculated order of stabilities, ortho > para >
meta > ipso was in qualitative accord with Wheland’s predic-
tions for nitrobenzene reactivity in radical addition.8 The ortho,
meta, and para isomers 2-4 were substantially stable with
respect to isomerizations and dissociations (Table 7). For
example, the isomerizations 2 f 1, 2 f 3, and loss of an ortho-
hydrogen atom required activation energies of 117, 155, and
120 kJ mol-1, respectively (Figure 7). In contrast, the ipso
isomer 5 was only marginally bound with respect to loss of
NO2 which was exothermic by 95 kJ mol-1 at 298 K (Table 7).
The energy barrier from 5 to the transition state for loss of NO2
was only 3 kJ mol-1 suggesting very facile dissociation. The
marginal stability of 5 was indicated by the optimized structure
which showed a long C-1-N bond (1.610 Å, Figure 3). In
summary, the ortho, meta, and para Wheland intermediates 2-4
were predicted to be sufficiently stable to be prepared as separate
structures in the gas phase. In contrast, the ipso isomer 5 existed
in a very shallow potential energy minimum and was predicted
to dissociate rapidly to benzene and nitrogen dioxide.
Hydrogen Atom Addition to Nitrobenzene. Of particular
interest with respect to radical additions to nitrobenzene were
the activation energies for loss of hydrogen atom from 1 and
2-5 and, by microscopic reversibility, the barriers for hydrogen
The calculated relative branching ratios for H atom addition
to the ortho, meta, and para positions in nitrobenzene correlated
with the product yields in other radical additions, for example
those of methyl and phenyl radicals.4,10-12 It should be noted

Hydrogen Atom Adducts to Nitrobenzene
J. Am. Chem. Soc., Vol. 122, No. 39, 2000 9523
Table 8. Activation Energies and Rate Constants for Hydrogen Atom Addition to Nitrobenzene
log Ab
log k250
log k298
log k350
krel
a
a,b
c
d
position
method
ETS
EArrh
ortho
B3PMP2
B3ROMP2
QCISD(T)
UMP2
27.9
25.3
28.5
25.8
12.90
6.95
7.50
7.91
8.37
8.67
9.06
0.64
0.72
36.7
40.0
33.2
31.8
30.2
37.3
40.6
33.7
32.3
30.7
5.11
4.41
5.85
6.16
6.50
6.37
5.78
6.99
7.25
7.54
7.35
6.85
7.88
8.11
8.35
0.80
0.77
0.76
0.14
0.11
PMP2
ROMP2
B3PMP2
B3ROMP2
QCISD(T)
UMP2
meta
para
ipso
NO2
12.91
12.95
12.68
13.28
43.9
46.8
39.0
29.1
27.2
44.4
47.3
39.5
29.7
27.8
3.63
3.02
4.65
6.75
7.13
5.13
4.62
5.98
7.75
8.07
6.30
5.86
7.03
8.53
8.81
0.05
0.05
0.07
0.22
0.18
PMP2
ROMP2
B3PMP2
B3ROMP2
QCISD(T)
UMP2
39.3
42.3
35.4
41.5
38.9
39.8
42.9
36.0
41.4
38.9
4.62
3.99
5.42
4.02
4.55
5.96
5.43
6.63
5.41
5.86
7.01
6.56
7.59
6.50
6.88
0.16
0.17
0.17
0.002
0.001
PMP2
ROMP2
B3PMP2
B3ROMP2
QCISD(T)
UMP2
51.5
55.1
46.6
37.4
32.4
51.5
55.0
46.6
38.9
33.7
1.92
1.17
2.94
5.14
6.24
3.65
3.03
4.51
6.45
7.38
5.00
4.47
5.73
7.49
8.27
0.0008
0.0007
0.001
PMP2
ROMP2
B3PMP2
B3ROMP2
QCISD(T)
UMP2
PMP2
ROMP2
29.4
27.6
27.0
30.7
28.9
28.3
6.86
7.24
7.37
7.90
8.21
.33
8.72
8.98
9.08
a Activation energies in kJ mol-1
.
b From Arrhenius plots of log k versus 1/T. Rate constants in mol-1 cm3 s-1 at indicated absolute temperatures.
d Fractions for H atom additions to the ortho, meta, para, and ipso positions in nitrobenzene.
Figure 8. Orbital diagram for H atom addition to the nitro group in nitrobenzene.
that a methyl radical addition to the nitro group in nitrobenzene
would not yield a stable adduct but would likely produce
nitrosobenzene by a mildly endothermic elimination of a
methoxyl radical (eq 5).
Why are there activation barriers to hydrogen atom additions
to nitrobenzene? The answer to this question was sought by
examining the interactions between the unpaired electron in the
1s orbital of the reacting hydrogen atom and the orbital system
of nitrobenzene, as shown for the addition to the nitro group
(Figure 8) and ipso position (Figure 9). The 1s electron of the
hydrogen atom can interact with the unoccupied 4b1 orbital in
nitrobenzene, which has substantial expansion coefficients at
the oxygen atoms, so that the unpaired electron enters the
developing singly occupied orbital (SOMO, 33a) in the transition
state of the addition. However, the nodal properties within the
•
•
C6H5NO2 + CH3 f C6H5NO + OCH3
∆Hrxn,298 ) 11 kJ mol-1 (5)
Likewise, alkyl or phenyl radical addition to the ipso position
in nitrobenzene is predicted by the present calculations to result
in expulsion of NO2 to yield an alkyl benzene or diphenyl.

9524 J. Am. Chem. Soc., Vol. 122, No. 39, 2000
Pola´sˇek and Turecˇek
Figure 9. Orbital diagram for H atom addition to the ipso position in nitrobenzene.
nitro group of the SOMO in the transition state differ from those
in the nitrobenzene 4b1 orbital (Figure 8). The SOMO develops
by mixing the unoccupied 4b1 orbital with the 2a2 orbital
(HOMO) of appropriate nodal properties. However, the HOMO
is doubly occupied and mixing it with the 4b1 orbital increases
electron energy, which results in an activation barrier.
lowest-energy dissociation of 1 was loss of OH to form
nitrosobenzene which occurred at the reaction thermochemical
threshold without a reverse barrier. However, 1 prepared by
collisional electron transfer did not have sufficient internal
energy to dissociate. The observed dissociations were driven
by the formation of excited electronic states. A metastable B
state was identified by calculations that had both a long radiative
lifetime and sufficient energy to participate in unimolecular
dissociations. The calculated branching ratios for hydrogen atom
additions to the ortho, meta, and para positions in nitrobenzene
correlated well with the experimental branching ratios for
additions of small radicals. Radicals derived from nitrobenzene
represent computationally difficult systems.
Addition to the ipso position shows a different correlation
(Figure 9). Interaction of the hydrogen atom 1s electron with
the 4b1 orbital in nitrobenzene preserves the orbital symmetry,
for example B1 f A′, in the transition state, and does not involve
the HOMO (2a2) which has an A2 symmetry. However, the nodal
properties of the SOMO in the transition state require mixing
of the 4b1 and 3b1 orbitals, indicating valence electron excitation
(Figure 9). Hydrogen atom addition to the para position in
nitrobenzene showed orbital correlations that were analogous
by symmetry to those for the ipso addition. The reasons for
activation barriers in additions to the ortho and meta positions
in nitrobenzene are somewhat subtler. In both the ortho and
meta additions, the 3b1 and 2a2 doubly occupied nitrobenzene
orbitals must mix to provide the 31Râ and 32Râ occupied
orbitals in the transition states that further correlate with
occupied orbitals in the adducts 2 and 3, respectively. In
addition, mixing of the occupied 2a2 orbital with the unoccupied
4b1 and/or 5b1 orbitals is also necessary to achieve appropriate
nodalities of the SOMO’s in the transition states for the ortho
and meta addition. The ensuing electronic excitation contributes
to the activation energy but it can be moderated by the decrease
or increase of electronic energy due to mixing of the 3b1 and
2a2 orbitals.
Despite the high-level of theory employed in this work,
uncertainties remain as to the relative rates of hydrogen atom
additions to the nitro group and the ring positions.
Acknowledgment. Support of this work by the National
Science Foundation (Grant CHE-9712570) is gratefully ac-
knowledged. The computations were performed at the Depart-
ment of Chemistry Computer Facility that received generous
funding from NSF (Grant CHE-9808182). We thank Dr. Martin
Sad´ılek for technical assistance with CAD spectra measurements
and a reviewer for suggesting experiments with ion-molecule
reactions.
Supporting Information Available: Tables of total energies,
optimized geometries, and harmonic frequencies (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Conclusions
Phenylnitronic radical 1 was generated in the gas phase and
shown to be kinetically and thermodynamically stable. The
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