Hypervalent ammonium radicals. Effects of alkyl groups and aromatic substituents

Article abstract of DOI:10.1021/jo960320u

Neutralization by collisional electron transfer of gaseous benzylalkylammonium ions produces transient hypervalent radicals whose dissociations depend on the substituents in the aromatic ring and at the amine nitrogen atom. Benzylammonium radical, C₆

Full text of DOI:10.1021/jo960320u

5234
J . Org. Chem. 1996, 61, 5234-5245
Hyp er va len t Am m on iu m Ra d ica ls. Effects of Alk yl Gr ou p s a n d
Ar om a tic Su bstitu en ts
Scott A. Shaffer, Martin Sad´ılek, and Frantisˇek Turecˇek*
Department of Chemistry, Bagley Hall, Box 351700, University of Washington,
Seattle, Washington 98195-1700
Received February 15, 1996X
Neutralization by collisional electron transfer of gaseous benzylalkylammonium ions produces
transient hypervalent radicals whose dissociations depend on the substituents in the aromatic ring
•
and at the amine nitrogen atom. Benzylammonium radical, C6H5CH2NH3 , dissociates mainly by
N-H bond cleavage to give benzylamine. Dissociation of the CH2-N bond to benzyl radical and
•
ammonia is less abundant. Benzylmethylammonium, C6H5CH2NH2CH3 , dissociates by CH2-N,
N-CH3, and N-H bond cleavages to give methylamine, benzyl radical, benzylamine, and
•
N-methylbenzylamine. Benzyldimethylammonium, C6H5CH2NH(CH3)2 , undergoes loss of dimeth-
ylamine and hydrogen, while the loss of methyl is less important. (2,3,4,5,6-Pentafluorobenzyl)-
•
dimethylammonium radical, C6F5CH2NH(CH3)2 , dissociates mainly by fission of the pentafluo-
rophenyl ring to give CnFm fragments with CF• as the dominating product, while bond dissociations
at the hypervalent nitrogen atom are less important. The relative stabilities of pentafluorobenzyl
and tropyl cations and radicals are assessed by ab initio calculations. (3,5-Dinitrobenzyl)-
•
dimethylammonium radical, (NO2)2C6H3CH2NH(CH3)2 , undergoes competitive losses of hydrogen
and NO and intramolecular proton transfer onto the dinitrophenyl ring. Mechanisms for these
reactions are suggested involving dissociative electron attachment at the aromatic ring and
formation of hypervalent ammonium radicals and zwitterionic intermediates.
Sch em e 1
In tr od u ction
Hypervalent ammonium radicals are transient inter-
mediates in one-electron reduction of ammonium ions in
solution or the gas phase. Ammonium radicals, which
are denoted as (9-N-4) according to Perkins et al.,1
formally have nine valence electrons at the nitrogen atom
and exhibit unusual electronic properties and reactivi-
ties.2,3 While undetectable in solution because of their
instability,4 several hypervalent organic ammonium radi-
cals have been generated in the gas phase by collisional
reduction of their ammonium ion counterparts, and their
dissociations have been studied by neutralization-reion-
ization (+NR+) mass spectrometry, as reviewed.2b,5 Briefly,
ammonium cations are prepared by protonation in the
gas phase and accelerated to a kiloelectronvolt kinetic
energy. The fast cations are allowed to collide with a
thermal atomic or molecular target gas, which serves as
an electron donor in a glancing collision. The cation-
donor interaction time is typically <10-14 s, which results
in an essentially vertical electron transfer, such that the
nascent fast neutral species is formed with the geometry
of the precursor ion. The residual ions are removed
electrostatically, and the fast neutrals are analyzed by
mass spectrometry following reionization to cations or
anions.5 Collisional activation,6 unimolecular kinetics,7
photodissociation, and photoionization8 are some of the
auxiliary methods that have been used to elucidate the
energetics and electronic structure of the neutral inter-
mediates.
Simple organic ammonium radicals, CH3ND3•,2c,d (CH3)2-
• 9
ND2 , and (CD2H)(CH3)2NH•,10 show metastability in the
gas phase, such that intact species of microsecond
lifetimes can be detected by mass spectrometry. Other
organic ammonium radicals, e.g., (CH3)4N•, C6H5CH2N-
• 11
•
(CH3)3 ,
n-C7H5NH(CH3)2 , and (CH3)2N(CH2)nNH-
(CH3)2• 12 have been found to dissociate completely within
a few microseconds following their formation in the gas
phase. Aliphatic hypervalent ammonium radicals have
been found to decompose by both N-C and N-H bond
cleavages (Scheme 1), although the relative propensities
for these dissociations depended on the system under
study.9-12
X Abstract published in Advance ACS Abstracts, J uly 15, 1996.
(1) Perkins, C. W.; Martin, J . C.; Arduengo, A. J .; Lau, W.; Alegria,
A.; Kochi, J . K. J . Am. Chem. Soc. 1980, 102, 7753.
(2) (a) Gellene, G. I.; Cleary, D. A.; Porter, R. F. J . Chem. Phys. 1982,
77, 3471-3477. (b) Gellene, G. I.; Porter, R. F. Acc. Chem. Res. 1983,
16, 200-207. (c) Gellene, G. I.; Porter, R. F. J . Phys. Chem. 1984, 88,
6680-6684. (d) J eon, S.-J .; Raksit, A. B.; Gellene, G. I.; Porter, R. F.
J . Am. Chem. Soc. 1985, 107, 4129-4133.
(3) (a) McMaster, B. N.; Mrozek, J .; Smith, V. H. Chem. Phys. 1982,
73, 131. (b) Cardy, H.; Liotard, D.; Dargelos, A.; Poquet, E. Chem. Phys.
1983, 77, 287. (c) Kaspar, J .; Smith, V. H.; McMaster, B. N. Chem.
Phys. 1985, 96, 81. (d) Kassab, E.; Evleth, E. M. J . Am. Chem. Soc.
1987, 109, 1653. (e) Boldyrev, A. I.; Simons, J . J . Chem. Phys. 1992,
97, 6621.
(6) Feng, R.; Wesdemiotis, C.; Baldwin, M. A.; McLafferty, F. W.
Int. J . Mass Spectrom. Ion Processes 1988, 86, 95.
(7) (a) Kuhns, D. W.; Shaffer, S. A.; Tran, T. B.; Turecek, F. J . Phys.
Chem. 1994, 98, 4845. (b) Kuhns, D. W.; Turecek, F. Org. Mass
Spectrom. 1994, 29, 463.
(8) Sadilek, M.; Turecek, F. J . Phys. Chem. 1996, 100, 9610.
(9) Sadilek, M.; Nguyen, V. Q.; Turecek, F. Chem. Phys. Lett.
submitted for publication.
(10) Shaffer, S. A.; Turecek, F. J . Am. Chem. Soc. 1994, 116, 8647.
(11) Beranova, S.; Wesdemiotis, C. Int. J . Mass Spectrom. Ion
Processes 1994, 134, 83-102.
(12) Shaffer, S. A.; Turecek, F. J . Am. Soc. Mass Spectrom. 1995, 6,
1004.
(4) Gedye, R. N.; Sadana, Y. N.; Eng, R. J . Org. Chem. 1980, 45,
3721.
(5) (a) Danis, P. O.; Wesdemiotis, C.; McLafferty, F. W. J . Am. Chem.
Soc. 1983, 105, 7454. For recent reviews, see: (b) Holmes, J . L. Mass
Spectrom. Rev. 1989, 8, 513. (c) McLafferty, F. W. Science (Washington)
1990, 247, 925. (d) McLafferty, F. W. Int. J . Mass Spectrom. Ion
Processes 1992, 118/ 119, 221. (e) Turecek F. Org. Mass Spectrom.
1992, 27, 1087. (f) Goldberg, N.; Schwarz, H. Acc. Chem. Res. 1994,
27, 347.
S0022-3263(96)00320-9 CCC: $12.00 © 1996 American Chemical Society

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5235
transfer complex15 whose dissociations may be charge-
induced as observed recently for photoexcited aromatic
molecules in the gas phase.16 Another mechanism for de-
excitation of charge-transfer complexes is photon emis-
sion, as observed for the interaction of anthracene anion
radical with triphenylamine cation radical, which re-
sulted in chemiluminescence.17
Sch em e 2
In this work we investigate the formation and dissocia-
tions of molecules and hypervalent radicals formed by
fast collisional reduction of substituted benzylamine
cation radicals 1+•-3+•, 7+•, and 8+• and ammonium ions
4+-6+, 9+, and 10+. The latter cations allow one to find
Ammonium radicals derived from simple monoamines
have also been studied by theory.3,10 The odd electron is
thought to be deposited into one of the diffuse orbitals
centered at the amine group, where it typically causes a
substantial weakening of one of the N-H bonds.3e,10 In
contrast, N-CH3 bond cleavages were predicted by theory
to require substantial activation energies on the ground
state potential energy surface which should prevent them
from competing with N-H bond dissociations. To explain
the occurrence of N-C bond cleavages, which were
observed experimentally,9-12 formation of higher excited
states in hypervalent onium radicals has been considered
theoretically10 and recently confirmed experimentally.8
An interesting and qualitatively new possibility arises
in fast reduction of larger bi- or polyfunctional am-
monium ions that provide two or more distinct acceptor
sites for electron capture (Scheme 2). Neutralization at
the ammonium center can form a hypervalent radical in
its ground or excited electronic states, in which fragmen-
tations are likely to proceed by cleavages of bonds
adjacent to the nitrogen atom.2,9-12 In contrast, electron
capture by a manifold of unoccupied orbitals located at
a distant atom or functional group will result in the
formation of a zwitterion whose dissociations may be
induced by the charge at either the cationic or anionic
centers and thus lead to distinct products. Previous
ionization conditions under which the initial protonation
site is localized at the amine nitrogen. The electron
capture process can thus be investigated by analyzing
the dissociations taking place at the ammonium center,
in the aromatic ring, or in its substituents. The electronic
properties of the aromatic systems are modified by
introducing electron-withdrawing substituents (F and
NO2 in 7, 9+, and 8, 10+, respectively) that increase the
electron affinity of the molecule through σ and π effects
and thus possibly provide bound states at the aromatic
ring for the incoming electron.18 Deuterium-labeled ions
2a +•, 3a +•, 4a +, 6a +, and 6b+ are used to elucidate the
dissociation mechanisms and detect proton-transfer rear-
rangements.
studies of simple zwitterions of the -CH2XHn type
+
indicated stability or metastability of some of these
species on the microsecond time scale,13 while contradict-
ing theoretical predictions.5b,14 However, dissociations of
those simple systems involved mostly C-X bond cleav-
ages.13
A similar mechanism for electron capture can be
invoked for fast neutralization of bi- or polyfunctional
amine cation radicals. In this case, the electron entering
the semioccupied n-orbital at the nitrogen atom recreates
the stable molecule in its ground electronic state, whereas
electron capture by a distant unoccupied orbital results
in the formation of an excited electronic state. The latter
corresponds formally to an intramolecular charge-
Exp er im en ta l Section
Meth od s. Neutralization-reionization (+NR+, signifying
neutralization of precursor cations and reionization of inter-
mediate neutrals to form cations) spectra were obtained on a
(13) (a) Wesdemiotis, C.; Feng, R.; Danis, P. O.; Williams, E. R.;
McLafferty, F. W. J . Am. Chem. Soc. 1986, 108, 5847. For the hitherto
unsettled controversy about the stability of halogenated ylides, see:
(b) Terlouw, J . K.; Kieskamp, W. M.; Holmes, J . L.; Mommers, A. A.;
Burgers, P. C. Int. J . Mass Spectrom. Ion Processes 1985, 64, 245. (c)
Hop, C. E. C. A.; Bordas-Nagy, J .; Holmes, J . L.; Terlouw, J . K. Org.
Mass Spectrom. 1988, 23, 155.
(14) (a) Pople, J . A.; Raghavachari, K.; Binkley, J . S.; Schleyer, P.
v. R. J . Am. Chem. Soc. 1983, 105, 6389. (b) Yates, B. F.; Bouma, W.
J .; Radom, L. J . Am. Chem. Soc. 1984, 106, 5805.
(15) Bender, C. J . Chem. Soc. Rev. 1986, 15, 475.
(16) (a) Chatorajj, M.; Laursen, S.; Paulson, B.; Chung, D. D.; Closs,
G. L.; Levy, D. H. J . Phys. Chem. 1992, 96, 8778. (b) Chatorajj, M.;
Chung, D. D.; Paulson, B.; Closs, G. L.; Levy, D. H. J . Phys. Chem.
1994, 98, 3361.
(17) Weller, A.; Zachariasse, K. Chem. Phys. Lett. 1971, 10, 590.
(18) (a) Dillow, G. W.; Kebarle, P. J . Am. Chem. Soc. 1989, 111, 5592.
For a review see: (b) Kebarle, P.; Chowdhury, S. Chem. Rev. 1987,
87, 513.

5236 J . Org. Chem., Vol. 61, No. 16, 1996
Shaffer et al.
tandem quadrupole acceleration-deceleration mass spectrom-
eter described previously.19 Volatile samples were introduced
into the ion source from a small glass reservoir maintained at
25 °C, and the sample intake was regulated by a Teflon needle
valve to achieve pressures in the range of 8 × 10-6 to 2 × 10-5
Torr. Less volatile samples were introduced from a heated
glass direct probe extending to the ion source. Precursor cation
radicals were generated by electron impact ionization of the
corresponding amines at 70 eV, emission current 500 µA, ion
source temperature 150 °C. Cations 4+-6+, 9+, and 10+ were
prepared by isobutane or ammonia chemical ionization (pro-
tonation with C4H9+ or NH4+, respectively) of the amines in a
tight ion source of our design. The chemical ionization
conditions were as follows: emission current, 1 mA; electron
energy, 100 eV; isobutane pressure, 2 × 10-4 Torr as read on
the ionization gauge located outside the ion source. The
reagent gas pressure and ion source potentials were adjusted
to optimize protonation and minimize electron ionization of
the amines such as to obtain ion abundance ratios of [M +
H]+/[M+•] > 10 for most ions. The deuterated ions 4a + and
detected selectively.19,20 The reported spectra were averaged
over 25-30 repetitive scans obtained at scan rates of 1 s (75
data points) per mass unit.
Collisionally activated dissociation (CAD) spectra of ions at
4 keV laboratory kinetic energy were taken on a Kratos Profile
HV-4 double-focusing mass spectrometer equipped with a
collision cell of our design that was mounted in the first field-
free region and grounded. Oxygen was used as collision gas
at 70% transmittance of the precursor ion beam. The spectra
were obtained by scanning the magnet (B) and electrostatic
(E) sectors while maintaining the B/E ratio constant (B/E-
linked scan). The reported spectra are averages of at least
six repetitive scans obtained at dynamic mass resolution >500.
Ma ter ia ls. Benzylamine, N-methylbenzylamine, and N,N-
dimethylbenzylamine were purchased from Aldrich and used
as received. N,N-Dimethyl(benzyl-d2)amine (3a ) was prepared
by LiAlD4 (Aldrich, 99% D) reduction of N,N-dimethylbenza-
mide in tetrahydrofuran and characterized by its 70 eV EI
mass spectrum: m/ z (rel intensity) 138 (5), 137(M+•, 46), 136
(6.5), 135 (18), 121 (2), 120 (3), 120 (1), 118 (1.5), 94 (9), 93
(47), 92 (4), 91 (5), 79 (1), 78 (1), 77 (2), 69 (2), 67 (7), 66 (5),
65 (3), 64 (2), 63 (2), 61 (4), 60 (100), 59 (4), 58 (3), 53 (1), 52
(1.5), 51 (3), 50 (1), 45 (1.5), 44 (17), 43 (10), 42 (7), 41 (3), 40
(6), 39 (3).
+
+
6b+ were prepared by protonation with ND4 and C4D9
generated by chemical ionization (CI) of ND3 (Cambridge
Isotopes, 99% D) and (CD3)3CD (MSD Isotopes, 99% D),
respectively. The ion source and gas inlet walls were condi-
tioned with D2O at 2 × 10-5 Torr for 1 h prior to the
deuteronation experiments. The precursor ions were passed
N-Meth ylben zyla m in e-N-d 1 (2a ). N-Methylbenzylamine
1.65 g, 13.6 mmol was added to a solution of D3PO4 prepared
from 20 mL of D2O and 1.6 g of P2O5. The homogeneous
solution was stirred at 50˚C, and the deuterium oxide was
distilled off at 50 Torr. The exchange was repeated with a
fresh 20 mL portion of D3PO4. The solution was neutralized
with 20 mL of 4 M NaOD in D2O, the product was extracted
in CH2Cl2 and dried, and the solution was taken down on a
rotary evaporator. The 70 eV mass spectrum of the product,
measured after having conditioned the ion source with D2O
vapor, showed 98% d1 content. Mass spectrum: m/ z (rel
intensity) 123 (2.7), 122 (M-d1+•,39), 121 (M - H, 100), 120
(5.2), 119 (2.3), 118 (1.3). Some back-exchange takes place in
an untreated ion source.
through
a quadrupole mass filter operated at low mass
resolution, accelerated to 8200 eV, and neutralized by collisions
with gaseous CH3SSCH3, (CH3)3N, or Xe at pressures such as
to achieve 70% transmittance of the precursor ion beam. For
the electrostatic potentials used, the precursor ion lifetimes
are estimated at 40-58 µs for m/ z 106-226. The neutral
products were separated by reflecting the remaining ions
electrostatically and then reionized by collisions with oxygen
at pressures such as to achieve 70% transmittance of the
precursor ion beam. The intermediate neutral lifetimes were
in the 4.9-7.2 µs range for precursor ions of m/ z 106-226.
The reionized cations were decelerated by an electrostatic lens,
filtered by kinetic energy, and mass-analyzed by a quadrupole
mass filter, which was scanned in link with the deceleration
voltage.19,20 Such linked scans achieve both product ion
resolution depending on the tunable m/ z bandwidth of mass
analyzer, and precursor ion resolution depending on the fixed
bandwidth of the kinetic energy filter and product/precursor
mass ratio.20 For example, linked-scan detection of the reion-
ized ion at m/ z 31 from 7+• (m/ z 225) achieves precursor mass
resolution of 45, i.e., precursor ions within (2.5 mass units of
m/ z 225 can contribute to the peak intensity at m/ z 31. Some
contamination from the precursor-ion isotope satellites thus
occurs, while product ions from precursors of m/ z <222 are
filtered out.
The use of wide apertures in the collision cells (5 mm) and
ion refocusing by the lens system20 allow efficient collection
of fragments scattered within 0.46° full angle. The reionized
and mass-analyzed products are detected with essentially
identical kinetic energies given by the voltage applied to the
electron multiplier, thus minimizing discrimination effects.21
Collisionally activated dissociation (CAD) of the intermediate
neutrals was carried out by admitting helium in the differen-
tially pumped neutral drift region at a pressure such as to
achieve 50% transmittance of the precursor ion beam. The
drift region was floated at +250 V, so that any cations formed
there had kiloelectronvolt kinetic energies upon exit, T ) (mf/
mp)Tp + 250 eV, where mp and mf are the precursor and
product masses, respectively, and Tp is the precursor kinetic
energy. These ions retain their kiloelectronvolt kinetic ener-
gies and are rejected by the energy filter. Hence, products of
neutral CAD that are reionized in the reionization cell are
N,N-Dim eth yl-2,3,4,5,6-p en ta flu or oben zyla m in e (7). A
solution of 2,3,4,5,6-pentafluorobenzyl bromide (Aldrich, 5 g,
19 mmol) in 45 mL of CH2Cl2 was added drop-wise to 75 mL
of CH2Cl2 solution saturated with anhydrous dimethylamine
(2.38 g, 52 mmol) at 0 °C. After 4 h of stirring, TLC analysis
(silica gel, elution with ethyl acetate/methanol 10:1) showed
that the reaction was complete. The mixture was diluted with
50 mL of CH2Cl2, washed with 4 × 30 mL of 10% aqueous
potassium carbonate, and dried with anhydrous potassium
carbonate. The solvent was distilled off, and the product was
distilled at 76 °C/30 Torr to give 2.37 g (55%) of 7. 1H NMR
(CDCl3): 2.30 (s, 6H), 3.67 (m, 2H). Mass spectrum: m/ z (rel
intensity) 226 (1), 225 (M+•,13), 224 (8), 208 (1), 181 (33), 163
(1.5), 161 (3.5), 135 (1), 117 (2), 93 (4), 91 (4), 81 (2), 69 (2), 59
(3), 58 (100), 51 (2), 44 (4), 43 (28.5), 42 (33), 41 (3), 31 (3), 30
(2).
N,N-Dim eth yl-3,5-d in itr oben zyla m in e (8.) A solution
of 3,5-dinitrobenzyl chloride (Aldrich, 4 g, 18.5 mmol) in 75
mL of CH2Cl2 was added drop-wise to liquid anhydrous
dimethylamine (190 mmol) at 0 °C. The mixture was stirred
for 1 h, diluted with 25 mL of CH2Cl2, washed with water and
5% NaHCO3, and dried with Na2SO4, and the solvent was
distilled off in vacuo. The yellow-brown residue was dissolved
in 50 mL of ether and triturated with hexane at 0 °C. The
solid (mostly hydrochloride salts) was filtered off, and the
solution was taken down on a rotary evaporator to give a
bright-yellow oil that solidified on standing at 0 °C. Yield:
1
2.68 g (64%), single spot by TLC (Silica gel, ethyl acetate). H
NMR (CDCl3): 8.94 (s, 1H), 8.55 (s, 2H), 3.64 (s, 2H), 2.31 (s,
(19) Turecek, F.; Gu, M.; Shaffer, S. A. J . Am. Soc. Mass Spectrom.
1992, 3, 493.
6H). Mass spectrum: m/ z (rel intensity) 226 (8), 225 (M•+
,
(20) Shaffer, S. A.; Turecek, F.; Cerny, R. L. J . Am. Chem. Soc. 1993,
115, 12117.
(21) (a) Potter, W. E.; Mauersberger, K. Rev. Sci. Instr. 1972, 43,
1327. (b) Kurz, E. A. Am. Lab. 1979, 11, 67-82. (c) la Lau, C. In Topics
in Organic Mass Spectrometry, Adv. Anal. Instrum. Meth. Vol. 8;
Burlingame, A. L., Ed.; Wiley: New York, 1970.
66), 224 (40), 208 (3), 181 (22), 178 (15), 135 (10), 133 (2), 132
(34), 90 (9), 89 (46), 77 (12), 76 (6), 75 (16), 74 (10), 63 (66), 62
(22), 59 (44), 58 (42), 57 (15), 51 (18), 50 (14), 46 (11), 45 (5),
44 (43), 43 (31), 42 (100), 41 (16), 39 (41), 30 (82), 29 (13), 27
(7), 15 (21). Amine 8 decomposed to non-volatile products

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5237
when heated in the ion source,22 so that only about 10 scans
could be taken before the vapor pressure dropped below a few
µTorr, causing poorer signal-to-noise ratios in the +NR+ spectra
of 8+• and 10+ when compared with those of ions derived from
chemically stable amine precursors.
tions of 1+•-3+• also occurs abundantly on CAD.28 In
addition to simple-cleavage dissociations, CAD of 2+•
(Table 1) gave rise to C2H4N+ fragments (m/ z 42), which
did not retain the NH hydrogen atom, as evidenced by
the CAD spectrum of 2a +•, which showed no mass shift
due to the N-bound deuterium atom. The C2H4N+ ion is
likely to be formed by loss of the benzylic hydrogen atom
followed by elimination of benzene from the (M - H)+
ion. CAD of benzylamine cation radical 1+• showed
predominant loss of hydrogen, while the benzylic C-N
bond cleavage was a very minor process that gave rise
Ca lcu la t ion s. Standard ab initio calculations of the
2,3,4,5,6-pentafluorobenzyl and 1,2,3,4,5-pentafluorotropy-
lium cations and radicals were carried out using the Gaussian
92 program.23 Equilibrium geometries were optimized with
the 3-21G basis set that was also used to obtain harmonic
vibrational frequencies, which were scaled by 0.9 to calculate
zero-point vibrational energies. The optimized geometries and
vibrational frequencies are available as supporting informa-
tion. The optimized 3-21G geometries were used for single-
point energy calculations with the 6-31G(d) basis set. The size
of these systems (344 primitive Gaussians for the 6-31G(d)
basis set) precluded higher-level post-Hartree-Fock calcula-
tions that would presumably provide more accurate estimates
for the relative stabilities of the cations and radicals under
study.
+
to a weak C7H7 at m/ z 91 (Table 1). Double hydrogen
+
transfer to the aromatic ring gave rise to the C6H7 ion
(m/ z 79), which was abundant in the CAD spectra of both
1
+• (Table 1) and of the (M - H)+ ion formed from 1+• in
the ion source. The (1 - H)+ ion is a plausible interme-
diate in the dissociation of 1+•, as the reaction (1 - H)+
f C6H7 + HCN forms a stable neutral molecule. The
+
major fragments observed in the CAD spectra were
further used as signatures for ion dissociations in the
+NR+ mass spectra of the cation radicals occurring
following collisional reionization.
Resu lts
Hypervalent ammonium radicals formed by neutral-
ization of ammonium ions can dissociate by C-N and
N-H bond cleavages to form stable molecules and the
complementary radicals, which are identified by mass
spectrometry.2,9-12 In +NR+ experiments, products of
neutral dissociations must be reionized to be detected,
which introduces some overlap with the products of ion
dissociations induced by the reionizing collisions. Hence,
the products of ion dissociations must be known to
distinguish both types of processes. Cation radicals
corresponding to reionized molecules and cations corre-
sponding to reionized radicals were therefore investigated
through their collisionally activated dissociation (CAD)
spectra to identify the major products of ion dissociations.
+NR+ spectra of cation radicals were also investigated
for signature ions that could be used to identify the
molecules formed by dissociations of hypervalent am-
monium radicals.
Collisional neutralization of 3+• was studied for CH3-
SSCH3, (CH3)3N, and Xe, as exemplified with the +NR+
spectrum in Figure 1. The spectrum showed a weak peak
of reionized (survivor) 3+• and abundant fragment ions
at m/ z 58 and 91, which were analogous to those formed
by CAD (Table 1). Neutralization with (CH3)3N gave a
+NR+ spectrum which was very similar to that in Figure
1, and which agreed closely to the +NR+ spectrum of 3+•
previously published by Beranova and Wesdemiotis.11
+
+
The CH2dN(CH3)2 and C7H7 ions showed clean mass
shifts upon deuteration (3a +•), e.g., m/ z 58 f m/ z 60 and
m/ z 91 f m/ z 93, which revealed negligible migration
of the benzylic hydrogen atoms in dissociating 3+• and 3.
The +NR+ and CAD spectra of 3+• differ most in the
relative abundance of C2H6N+ (m/ z 44), whose peak is
negligible in Figure 1. The immonium ion at m/ z 58 and
the (M - H)+ ion at m/ z 134 represent specific signatures
for the formation of 3 under +NR+ conditions.
Collision a lly Activa ted Dissocia tion a n d Neu tr a l-
iza tion of Ca tion Ra d ica ls. Electron ionization of the
benzylamines under study forms stable cation radicals
that give rise to abundant peaks in the corresponding
mass spectra.24 Ion dissociations of mass-selected mo-
lecular ions of 1+•-3+• showed mostly simple bond cleav-
ages giving rise to stable immonium CH2dNR1R2+ 25 and
Neutralization-reionization of the N-methylbenzyl-
amine cation radical 2+• gave a negligible survivor ion
at m/ z 121 (Figure 2a). Benzylic cleavages gave rise to
(M - H)+ and C7H7+ ions, which were analogous to those
in the CAD spectrum (Table 1). The C2H4N+ ion is also
prominent in the +NR+ spectrum and represents a
signature fragment. In addition, C6H6+• is an important
fragment ion in +NR+, but less so in CAD. The +NR+
spectrum of the N-D derivative 2a +• (Figure 2b) shows
a clean mass shift m/ z 78 f m/ z 79, which indicates
transfer of the amine hydrogen atom onto the aromatic
ring in the formation of C6H6. The fragments at m/ z 120
and 78 were selected as signatures for the formation of
2 from hypervalent radicals under +NR+ conditions.
The +NR+ spectrum of 1+• shows a negligible survivor
ion (Figure 2c). Benzylic cleavage of one of the C-H
bonds gives rise to the (M - H)+ fragment at m/ z 106,
whereas C-N bond cleavage to give C7H7 is very inef-
ficient. The spectrum is dominated by peaks of aromatic
ring fragments, which are common for the +NR+ dis-
C7H7 ions, as exemplified with the CAD spectra of 3+•
+
and 3a •+ (Table 1). Loss of benzyl radical from 2+• and
3+• is probably accompanied or preceded by hydrogen
migration within the amine moiety to give rise to the
+
stable immonium ions, CH2dNH2 (m/ z 30) and
CH2dNHCH3+ (m/ z 44), respectively.26 The correspond-
ing nitrenium ions, CH3NH+ and (CH3)2N+, due to direct
benzyl loss have been calculated to be unstable in their
singlet states.27 The loss of a benzylic hydrogen atom,
which is prominent in the electron-ionization dissocia-
(22) Patey, A. L.; Waldran, N. M. Tetrahedron Lett. 1970, 3375.
(23) Gaussian 92, Revision C. Frisch, M. J .; Trucks, G. W.; Head-
Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J . B.; J ohnson, B.
G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres,
J . L.; Raghavachari, K.; Binkley, J . S.; Gonzalez, C.; Martin, R. L.;
Fox, D. J .; DeFrees, D. J .; Baker, J .; Stewart, J . J . P.; Pople, J . A.
Gaussian, Inc., Pittsburgh, PA, 1992.
(24) McLafferty, F. W.; Stauffer, D. B. The Wiley/ NBS Registry of
Mass Spectral Data, Vol. 1; Wiley: New York, 1989; p 66, 124, 204.
(25) Bowen, R. D. Mass Spectrom. Rev. 1991, 10, 225.
(28) The (M - H)+ and (M - D)+ ion intensities may be affected by
artifacts in the B/E-linked scans used to obtain the CAD spectra, see:
(a) Morgan, R. P., Porter, C. J .; Beynon, J . H. Org. Mass Spectrom.
1977, 12, 735. (b) Busch, K. L.; Glish, G. L.; McLuckey, S. A. Mass
Spectrometry/ Mass Spectrometry: Techniques and Applications of
Tandem Mass Spectrometry; VCH Publishers: New York, 1988; p 25.
(26) Levsen, K.; McLafferty, F. W. J . Am. Chem. Soc. 1974, 96, 139.
(27) (a) Ford, G. P.; Herman, P. S. J . Am. Chem. Soc. 1989, 111,
3987. (b) Falvey, D. E.; Cramer, C. J . Tetrahedron Lett. 1992, 33, 1705.

5238 J . Org. Chem., Vol. 61, No. 16, 1996
Ta ble 1. Collision a lly Activa ted Dissocia tion Sp ectr a of 1+•, 2+•, 2a +•, 3+•, 3a +•, 4+, 4a +, a n d 6+
Shaffer et al.
relative intensitya
m/z
1+•
2+•
2a +•
3+•
3a +•
(>100)
6.3
4+
4a +
6+
135
134
133
132
131
121
120
119
118
117
116
110
109
108
107
106
105
104
103
94
(>100)
(>100)
21
(17)
3
1.5
7.5
1.3
25
3.8
(>100)
(>100)
30
1.3
3.8
5
(>100)
(56)
1.9
10
28
6.7
6.8
1.7
(42)
6.5
12
2
6.3
1.9
(22)
6.5
4
1
(18)
19
(>100)
10
1.5
3.5
17
2.5
14
5.1
1
1
0.9
3.1
1.9
3
1.9
3
17
100
6.5
10
0.6
0.6
0.6
0.4
16
66
13
21
3
93
92
91
90
89
80
2
16
39
6
12
1.4
3
22
39
7
11
2
15
100
17
16
0.6
7
100
10
9
0.4
12
100
11
2.2
1.2
3
9.6
3
8
10
79
78
77
76
100
10
20
2
3
8
18
4
10
14
1.3
3.3
8
1.3
1.3
3
3.6
3.6
8.4
1.4
0.8
1
2
1.3
4
2.7
2.3
1.6
0.6
67
66
65
64
63
62
60
3.1
3.8
1.6
1.3
1.3
1
1.5
7
2.3
5
1.8
1
2
1
2.6
7.3
2.1
5
1.6
1
9.2
1.9
4.4
1.4
7.3
2.8
7
4.3
1.7
3.8
1.4
6.8
1.2
3
1.3
2.5
100
59
58
1
1
1.3
81
6.5
1.5
19
57
3.4
56
55
2.4
1.6
53
52
51
50
1
3.5
10
5
1.5
3.4
10
3.6
6.6
3.4
1.3
5
3.1
1.3
2.5
1.3
2.5
7.3
4.7
1.2
3.8
2.6
1.2
4.3
2.1
5
49
45
44
43
42
11
9
6.4
100
2
2.1
65
9
5
5.5
27
1.7
6.5
14
4.5
100
2
52
2.5
20
41
39
38
32
31
30
29
28
2
4
4
3
1.3
4.6
2.4
3
1.1
1.3
2.9
40
27
2
47
3
1.0
7.4
1.5
a
Integrated peak areas relative to that of the most abundant peak, excluding (M-nH)+.
sociations of 1-3. The (M - H)+ ion at m/ z 106 is the
only unique signature for 1.
yield a series of fluorinated carbon clusters, e.g., C5F3
(m/ z 117), C5F2+• (m/ z 98), C3F3+ (m/ z 93), C4F2+• (m/ z
86), C5F+ (m/ z 79), C3F2+• (m/ z 74), C2F2+• (m/ z 62), and
CF+ (m/ z 31), with the peak of the latter fragment
dominating the spectrum (Figure 3a). The cleavage of
the C6F5-CH2 bond, which is abundant in ion dissocia-
tions, is negligible on +NR+ as evidenced by the very
small peak of CH2dN(CH3)2+ at m/ z 58 and the absence
of the complementary C6F5+ (m/ z 167) fragment (Figure
3a). Cleavage of the benzylic C-N bond gives rise to
C6F5CH2+ (m/ z 181) and C2H4N+ (m/ z 42). It should be
+
The +NR+ mass spectra of cation radicals 7+• (Figure
3a) and 8+• Figure 4a) show deep dissociations of the
aromatic rings that occur upon collisional electron trans-
fer. Ion dissociations of 7+• in CAD are straightforward
and comprise loss of H• (m/ z 224), benzylic C-N bond
cleavage (m/ z 181), and loss of the pentafluorophenyl
group (m/ z 58, Table 2). These ion dissociations are
analogous to those observed for the unsubstituted ion 3+•.
By contrast, neutralization-reionization of 7+• results in
a pronounced fission of the pentafluorobenzene ring to
noted that C6F5CH2 is a stable radical under +NR+
•

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5239
+
intensity of the reionized C7H2F5 intermediate. The
sensitivity of C7H2F5+ to collisional activation was tested
by recording the +NR+ spectra at 70% and 90% transmit-
tances in the neutralization and reionization steps, which
corresponded to 95% and 85% single-collision conditions,
respectively. The spectra showed practically no change
between 90%/90% and 70%/90% +NR+ conditions, which
+
gave 3.75% and 3.80% of surviving C7H2F5 relative to
the total NR intensities (∑INR). At the 70%/70% trans-
+
mittance the relative intensity of C7H2F5 decreased to
3.25% ∑INR. This indicates that C7H2F5+ ions are some-
what more sensitive to collisional activation with O2 than
•
are C7H2F5 radicals to collisional activation with CH3-
SSCH3, although the overall effect is rather weak.
The CAD and +NR+ spectra of C7H2F5 ions are very
+
different. The former shows mostly sequential elimina-
tion of HF molecules (m/ z 161 and 131) and acetylene
(m/ z 155), whereas neutralization results in more pro-
nounced ring dissociations combined with losses of
fluorine atoms. Note that elimination on CAD of CF•
radical is a very minor process in the ion dissociations of
7+• and C7H2F5 (Table 2). The predominating CF+
+
F igu r e 1. Neutralization-reionization (CH3SSCH3/O2) spec-
fragment in the +NR+ spectra thus cannot be accounted
for by reionization of neutral CF• formed by concurrent
CAD of 7+• ion in the neutralization collision cell. By
contrast, reionization of CF• from neutral dissociations
may contribute to the efficient detection of CF+ as the
ionization cross section of the CF• radical is greater than
those of some more highly fluorinated species.29
trum of 3+•
.
It is worth noting that the differences in the CAD and
+NR+ spectra of the C7H2F5+ ions may possibly come from
sampling different mixtures of isomeric structures for ion
and neutral dissociations as is known for the benzyl and
tropylium isomers in the C7H7+ and C7H6F+ systems.30,31
Ab initio calculations (Table 3) indicate that the 1,2,3,4,5-
pentafluorotropylium ion is ca. 24 kJ mol-1 more stable
than the 2,3,4,5,6-pentafluorobenzyl ion at 298 K.
A
benzyl-tropylium isomerization of the ions is therefore
possible, although the energy difference in the pen-
tafluoro system is much smaller than that in unsubsti-
tuted C7H7 (48 kJ mol-1).30 Isomerization in the pen-
+
tafluorobenzyl cation may face a larger energy barrier
than that in the benzyl cation, as discussed for the
monofluorobenzyl and tropylium ions.31 By contrast, the
pentafluorotropyl radical is calculated to be 87 kJ mol-1
less stable than its pentafluorobenzyl isomer at 298 K
(Table 3), and so the latter should not isomerize when
formed by dissociations of neutral intermediates. Both
the pentafluorobenzyl and pentafluorotropyl radicals
show only small energy differences between the corre-
sponding optimized neutral structures and those formed
by vertical electron attachment (3.5 and 29 kJ mol-1
,
respectively, Table 3), indicating small vibrational excita-
tion through Franck-Condon effects.
A still different picture emerges from the CAD and
+NR+ dissociations of cation radical 8+• (Figure 4a). Ion
dissociations are dominated by the aryl-CH2 bond cleav-
F igu r e 2. Neutralization-reionization (CH3SSCH3/O2) spec-
tra of top (a) 2+•, middle (b) 2a +•, and bottom (c) 1+•
.
(29) (a) Tarnovsky, V.; Becker, K. J . Chem. Phys. 1993, 98, 7868.
(b) The analogous CCl+ shows a moderate +NR+ efficiency, 0.0018: Gu,
M.; Turecek, F. Org. Mass Spectrom. 1993, 28, 1135.
(30) From ∆Hf(benzyl+) ) 919 kJ mol-1, ∆Hf(tropylium+) ) 871 kJ
mol-1, see: (a) Baer, T.; Morrow, J . C.; Shao, J . D.; Olesik, S. J . Am.
Chem. Soc. 1988, 110, 5633. (b) Lifshitz, C.; Gotkis, Y.; Ioffe, A.; Laskin,
J .; Shaik, S. Int. J . Mass Spectrom. Ion Processes 1993, 125, R7. For
a recent review of C7H7+ isomers, see: (c) Lifshitz, C. Acc. Chem. Res.
1994, 27, 138.
conditions as the +NR+ spectra of the C7H2F5 cations
+
generated from 7 and C6F5CH2F showed significant
survivor ions (Figure 3b). Furthermore, dissociations of
C7H2F5 on +NR+ give fragments that are very similar
+
to those found in the +NR+ spectrum of amine 7+•. This
indicates that the cleavage of the benzylic C-N bond in
7 could be an important process in spite of the low
(31) McLafferty, F. W.; Amster, I. J . J . Fluorine Chem. 1981, 18,
375.

5240 J . Org. Chem., Vol. 61, No. 16, 1996
Shaffer et al.
F igu r e 3. Neutralization-reionization (CH3SSCH3/O2) spectra of (a) 7+•, (b) C7F5H2+, and (c) 9+.
+
age, giving the CH2dN(CH3)2 ion at m/ z 58 (Table 2).
Benzylic C-N bond cleavage with loss of neutral •N(CH3)2
gives the ion at m/ z 181, which further eliminates NO2
(m/ z 135 and 89). A competing elimination of HNO2 and
NO2 from 8+• gives rise to ions at m/ z 178 and 132,
respectively. CAD of the intermediate 3,5-dinitrobenzyl
ion (or its tropylium isomers, m/ z 181) is consistent with
this interpretation, showing sequential losses of NO2 to
form fragments at m/ z 135 and 89 (Table 2).
In contrast to CAD, +NR+ of 8+• results in the dominant
formation of NO, which gives rise to the peak of NO+ at
m/ z 30 (Figure 4a). Note that neutral NO is not
eliminated by CAD of 8+• (Table 2) nor from its nitro-
group-containing fragments. Hence the formation of NO
on +NR+ cannot be accounted for by reionization of
neutral fragments from ion dissociations concurrent with
neutralization. By comparison, NO2 which is eliminated
by ion CAD (Table 2) gives rise to a much smaller peak

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5241
Ta ble 2. Collision a lly Activa ted Dissocia tion Sp ectr a of
7+•, 8+•, 9+, 10+, C7H2F 5+, a n d C7H5N2O4
+
relative intensity
relative intensity
7+•
9+
C7H2F5 m/ z 8+•
10+ C7H5N2O4
+
+
m/ z
225
224 (>100)
223
(>100)
225
224 (75)
211
(10)
5
4
25
4.6
211
210
209
1.3
8.5
3.7
1.6
5.2
1.1
210
209
15
13
10
3
11
70
10
51
208
194
184
183
182
181 31
180
179
7
208
207
204
193
192
190
182
181
180
179
167
164
163
162
161
155
150
143
137
132
131
130
117
112
111
99
5.1
3.5
1.3
5.9
1.5
11
100
1.2
5
1.6
9.4
100
1
6
39
(>100)
6.6 (>100)
17
178 28
166
165
14
3
9
(30)
3
1
1
13
6
7.5
7.5
28
35
6
20
4
5.4
6.6
11
3.5
3.7
5
1.5
1.4
5.1
1.7
3.8
1.1
2.7
2.7 164
136
1
7
7.2
4.5
4.5
6.5 100
4
12
135
134
133
100
11
20
57
5
2
1.9
12
132 16
7.5 131
2
1
1.5
3.1
1
6
4
5
28
11
10
9
120
119
118
117
105
104
91
90
89
88
78
1.4
F igu r e 4. Neutralization-reionization (CH3SSCH3/O2) spec-
tra of top (a) 8+• and bottom (b) 10+.
2
1
1.1
1.1
2.4
1.1
2
2.8
1.6
1.4
2
5
36
9
1
8
1.6
4.1
2.2
at m/ z 46 after reionization. In contrast to the case of
3+• and 7+•, +NR+ of 8+• shows a very small but detectable
survivor ion, suggesting stabilization in the intermediate
1
6
5
1.7 14
1.2
2
1
4
2
7.5
2.5
16
8
1.5
43
7
6
neutral 8 or reionized 8+•
.
98
93
81
Hyp er va len t Ra d ica ls. Neutralization of ammonium
ions by collisions with CH3SSCH3, (CH3)3N, and Xe was
studied for 6+ prepared by gas-phase protonation of 3
2
1
9
77
2.2 11
80
75
74
69
58
57
51
45
44
43
42
31
4
8.7
4.4
76
75
65
63
62
59
58 100
51
50
45
44
43
42
30
1
6
4.3
5
3
13
6
+
+
1.2
1.1
50
2.7
1.8
2.3
17
1.8
1
4.8
18
2.4
5.4
1.6
2.3 13
with NH4 and C4H9
. Both these gas-phase acids
5
18
7
6.5
100
5.5
4
17
77
6
protonate 3 selectively at the nitrogen atom, as the gas-
phase basicity of the aromatic system is too low to permit
12
1.2
5
+
+ 32
ring protonation with NH4 or C4H9
.
However, pro-
3.2
5.5
tonation with C4H9 (∆PA ) 152 kJ mol-1) is more
exothermic than that with NH4+ (∆PA ) 103 kJ mol-1),33
resulting in somewhat higher vibrational excitation in
the 6+ formed.
+
2
1.7
1.9
7.5
0.9
1
The dissociations of hypervalent 6• proceed by benzylic
CH2-N bond homolysis, as evidenced by the reionized
C7H7+ and (CH3)2NH+•, and by cleavage of the N-H bond,
as evidenced by the presence of the signature ions at m/ z
58 from reionization of the intermediate 3 (Figure 5a).
The branching ratios for the N-H and CH2-N bond
cleavages in hypervalent 6• depend on the precursor ion
internal energy. Neutralization of the more energetic
ions from protonation by C4H9+ led to less hydrogen loss
as judged by the small peak of the signature ion at m/ z
58. Exact quantification of the branching ratios is
hampered by the unknown reionization efficiencies and
their dependence on neutral internal and kinetic en-
ergy.34 However, it appears from the relative abundances
of the signature ions that the benzylic CH2-N bond
cleavage is the prevalent dissociation of hypervalent 6•.
Surprisingly, dissociations by N-CH3 bond cleavage are
much less significant in 6•, as evidenced by the very low
16
3.5 13
1
3
5
Ta ble 3. Tota l En er gies of 2,3,4,5,6-P en ta flu or oben zyl
a n d 1,2,3,4,5-P en ta flu or otr op yl Ca tion s a n d Ra d ica ls
energya
H298
-
6-31G(d) ZPVEb H0
IEa REv
b,c
d
e
species
3-21G
+
C6F5CH2 -758.90926 -763.06379 199
28
29
7.84
7.98
•
C6H6CH2 -759.21722 -763.35311 188
•
C6F5CH2
-763.35179
(VN)f
F5-tropyl+ -758.91142 -763.07343 201
F5-tropyl• -759.17506 -763.31967 187
27
29
6.40
6.82
F5-tropyl•
(VN)f
-763.30857
a
b
Hartree: 1 hartree ) 2625.5 kJ mol-1
.
From 3-21G har-
monic frequencies scaled by 0.9, units of kJ mol-1
enthalpy corrections calculated with the rigid-rotor-harmonic
oscillator approximation. Adiabatic ionization energies from HF/
6-31G(d) calculations and ZPVE corrections, units of eV. Vertical
recombination energies from HF/6-31G(d) calculations, units of eV.
Single-point calculations on optimized ion geometries.
.
298 K
c
d
e
(32) Lias, S. G.; Bartmess, J . E.; Liebman, J . F.; Holmes, J . L.; Levin,
R. D.; Mallard, G. W. J . Phys. Chem. Ref. Data 1988, 17, Supplement
1.
f
(33) From PA(3) ) 954 kJ mol-1, PA(NH3) ) 851.4 kJ mol-1, and
PA(i-C4H8) ) 802 kJ mol-1; see: Szulejko, J . E.; McMahon, T. B. J .
Am. Chem. Soc. 1993, 115, 7839.
relative abundances of the signature ions at m/ z 120 and
78 from reionization of the intermediate 2 (Figure 5a).
Note that the total yields for neutralization and reion-
(34) Harnish, D.; Holmes, J . L. Org. Mass Spectrom. 1994, 29, 213.

5242 J . Org. Chem., Vol. 61, No. 16, 1996
Shaffer et al.
F igu r e 6. Neutralization-reionization (CH3SSCH3/O2) spec-
tra of top (a) NH4+-protonated 5+, middle (b) C4H9+-protonated
5+, and bottom (c) C4D9+-deuterated 5a +.
F igu r e 5. Neutralization-reionization (CH3SSCH3/O2) spec-
tra of top (a) NH4+-protonated 6+, middle (b) ND4+-deuterated
6b+, and bottom (c) C4D9+-deuterated 6b+.
isotope effects of hypervalent 6b• is observed that would
lead to the detection of a survivor ion at m/ z 136.
Deuterium labeling in the benzylic group has no major
affect on the dissociations of 6a •. The label remains
localized in the benzylic group as shown by the clean
mass shifts of m/ z 91 f m/ z 93 and m/ z 58 f m/ z 60.
Neutralization with CH3SSCH3 of ammonium ion 5+
provides +NR+ spectra (Figure 6a,b) that slightly depend
on the precursor ion internal energy. Neutralization of
5+ prepared by the more exothermic protonation with
ization of 2+• are greater than those for 3+• 35
indicating
,
that the reionization efficiency of 2 should not be
significantly smaller than that of 3. Hence the low yield
of reionized 2+• is not due to a discrimination upon
reionization, but rather can be attributed to the inef-
ficient formation of neutral 2 in dissociations of the
hypervalent radical 6•. By comparison, N-CH3 bond
cleavages do occur in hypervalent aliphatic ammonium
+
C4H9 (Figure 6b) results in more extensive CH2-N
• 2
• 9
radicals CH3NH3 , (CH3)2NH2 , and (CH3)3NH•.10
benzylic cleavage compared to that in 5+ prepared by
protonation with NH4+ (Figure 6a). This is documented
by the relative abundances of the signature ions for H
loss (m/ z 44) and benzylic cleavage (m/ z 91) in the
corresponding spectra (Figures 6a,b). The +NR+ spec-
trum of NH4+-protonated 5+ shows fragments at m/ z 120,
104, and 44, typical for the C6H5CH2NHCH3 intermedi-
ate, signifying loss of the ammonium hydrogen from the
hypervalent radical 5• (Figure 6a). Loss of methyl is
indicated by the presence of the C6H5CHdNH2+ fragment
at m/ z 106, which is a signature for the intermediate 1.
This shows that all the bonds adjacent to the hypervalent
nitrogen center in 5• dissociate to some extent, contrast-
ing thus the behavior of 6• (vide supra). Deuterium
labeling in 5a + results in a negligible shift of m/ z 44 to
45 and the absence of deuterium incorporation in most
fragments except for CH3NH2 (Figure 6c). This shows
+
+
N-Deuteration with ND4 or C4D9 appears to have
opposite effects on the branching ratios for the N-D and
CH2-N bond cleavages in 6b• (Figure 5b). N-D bond
cleavage in ND4+-deuterated 6b• is relatively suppressed,
whereas the same cleavage in the C4D9+-deuterated 6b•
is relatively enhanced (Figure 5c). These effects show
that the branching ratios for N-H and CH2-N bond
dissociations depend on the precursor ion internal energy;
however, the mechanism of these energy effects is not
clear. Loss of CH3 remains a minor dissociation pathway
in 6b• as judged from the weak signature ions at m/ z
121 and 79. However, no overall stabilization through
(35) The CH3SSCH3(70% T)/O2(70% T) +NR+ efficiencies, expressed
as the sum of +NR+ ion intensities relative to that of the unattenuated
incident precursor ion beam) were as follows: 2+•, 0.0028; 3+•, 0.00046;
(CH3)2NH, 0.0012.

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5243
∑INR), attesting to the stability of the vertically neutral-
ized benzyl radical (these spectra are given as supporting
information). The effects of collisional reionization were
also studied. Benzyl radical was prepared as a neutral
fragment by collision-induced dissociation with He of
stable 1,2-diphenylethane cation radical and subse-
quently reionized to C7H7+ by collisions with oxygen. The
resulting spectrum again showed a strong peak of surviv-
+
ing C7H7 (19% ∑INR, supporting information). This
indicates that the small peak of C7H7 in the +NR+
+
spectrum of 4• originates from inefficient loss of ammonia
to form the benzyl radical and not from an instability of
the latter.
Neutralization of the fluorinated ammonium ion 9+
results in the dissociation of the benzylic bond as
indicated by the reionized C2H6N+, C2H7N+•, and C6F5-
+
CH2 fragments at m/ z 44, 45, and 181, respectively
(Figure 3c). Loss of the ammonium hydrogen atom is
negligible, as shown by the very small peaks of reionized
+
+
CH2dN(CH3)2 and C6F5 at m/ z 58 and 167, respec-
tively. Loss of methyl is difficult to distinguish due to
an overlap of the presumed CH2dNHCH3+ signature ion
with the isobaric fragment from reionized dimethylamine.
+
Nevertheless, the very small relative abundance of CH3
suggests that loss of methyl from 9• is not important.
Neutralization of ion 10+ results in a spectrum (Figure
4b) which is qualitatively different from those of the other
ammonium ions. Radical 10• shows little dissociation of
the benzylic CH2-N bond, as indicated by the modest
relative abundances of the dimethylamine ions at m/ z
45 and 44. The loss of H from 10• is indicated by the
weak peak of reionized 8+• (m/ z 225) and the CH2dN-
F igu r e 7. Neutralization-reionization (CH3SSCH3/O2) spec-
tra of top (a) 4+ and bottom (b) 4a +.
that D and not H is lost preferentially from hypervalent
5a •, indicating an inverse deuterium isotope effect.
Inverse isotope effects on dissociations of hypervalent
radicals have been noted previously.12,36 In the absence
of detailed potential energy surfaces for the dissociations
of 5• and 6•, the origin of these inverse isotope effects
remains unclear.
+
(CH3)2 fragment at m/ z 58. When compared with the
+NR+ dissociations of cation radical 8+•, those of 10• show
mass shifts of the aromatic fragments, e.g., m/ z 37 to m/ z
38, m/ z 49 to m/ z 50, m/ z 62 to m/ z 63, and (in part)
m/ z 75 to m/ z 76 (Figure 4b). These are indicative of
dissociations involving hydrogen transfer from the am-
monium group to the aromatic ring that takes place
either in the intermediate neutral or following reioniza-
tion. Note, however, that the ring-cleavage dissociations
are much less abundant in CAD of 10+ (Table 2), so their
occurrence on +NR+ is likely to be due to fragmentations
of the neutral intermediate 10•.
Neutralization of the simplest benzylammonium ions
4+ and 4a + induces mostly N-H bond cleavage, whereas
the formation of C7H7 and NH3 by CH2-N bond cleavage
is much less abundant. The N-H bond dissociation is
indicated by the benzylamine signature ions at m/ z 106,
78, and 30 (Figure 7a) and their deutero analogs at m/ z
108, 79-81, and 32 (Figure 7b). The CH2-N benzylic
cleavage is indicated by the complementary C7H7 and
+
NH3 fragments. While the formations from 4a of C7H7
by neutral or ion dissociations are not accompanied by
Discu ssion
H/D exchange, as corroborated by the unshifted m/ z 91
+
in both the NR+ and CAD spectra, the ND3 peak at m/ z
The dissociations of hypervalent radicals 4• - 6•, 9•,
and 10• show effects of methyl substituents at the amine
nitrogen atom and fluorine and nitro substituents in the
aromatic ring. Substitution by methyls at the amino
group relatively enhances cleavage of the CH2-N bond,
which occurs predominantly in 6• but less so in 5• and 4•.
It is noteworthy in this respect that CH2-N bond
heterolysis upon CAD of ammonium cations is much less
sensitive to methyl substitution at the leaving amine
moiety as shown for 4+ and 6+ (Table 1). Cleavage of
the N-CH3 bond increases from 6• to 5•, in spite of the
less favorable statistical factor in the latter, and competes
with the N-H bond cleavage. The latter dominates in
the dissociations of 4•, where it outcompetes the CH2-N
bond cleavage. It should be noted that the product
relative abundances provide relative propensities to bond
cleavage but do not allow direct determination of bond
dissociation energies. In relative terms, methyl substit-
uents at the amino group do affect bond dissociations but
do not increase the lifetimes of the hypervalent am-
20 shows an accompanying peak at NHD2 at m/ z 19. The
origin of the latter fragment is unclear, as H/D exchange
between C6H5CH2 and ND3 is excluded by the clean
formation of the complementary C7H7+. We tentatively
explain the formation of NHD2 as being due to an artifact
from the fragmentation of the d2 isotopomer of 4a + (C6H5-
CH2NHD2+, m/ z 110, Figure 7b, inset), which is coformed
and cotransmitted with 4a +.
The unusually small peak of C7H7+ from 4• raised the
question of the stability of the intermediate benzyl radical
under +NR+ conditions.37 We carried out a series of
measurements in which C7H7+ ions from benzyl bromide
were neutralized with CH3SSCH3 and reionized with O2
under single and multiple collision conditions. All these
spectra showed substantial peaks of reionized C7H7+ (16%
(36) Wesdemiotis, C.; Fura, A.; McLafferty, F. W. J . Am. Soc. Mass
Spectrom. 1991, 2, 459.
(37) Buschek, J . M.; Holmes, J . L. Org. Mass Spectrom. 1988, 23,
765.

5244 J . Org. Chem., Vol. 61, No. 16, 1996
Shaffer et al.
Sch em e 3
molyses. However, the relative propensities for bond
cleavages in 4•-6• do not correlate with the dissociation
exothermicities. For example, dissociation of the CH2-N
bond in 4• is substantially more exothermic than the
N-H bond cleavage, yet the latter dissociation prevails
in vertically neutralized 4•. Likewise, loss of methyl from
6• is more exothermic than that from 5•; yet this dissocia-
tion is relatively more abundant in the latter radical.
These results show that the cleavages of N-H and
N-C bonds do not follow the usual pattern of competitive
dissociations occurring on the same potential energy
surface, for which a correlation between the activation
energy and reaction thermochemistry could be expected.
It appears that two or more electronic states are accessed
by vertical reduction of ammonium cations and that these
dissociative states may show specific propensities to N-H
and N-C bond cleavages. The observed substituent
effects are then likely to be determined by the popula-
tions of the electronic states of the given reactivity. This
interpretation is consistent with previous studies of
aliphatic hypervalent onium radicals that indicated that
X-H bond homolysis (X ) N,O) should dominate in the
ground electronic state, whereas X-C bond cleavages
become competitive in some excited states.8,10 The nature
of the electronic states that are involved in the formation
of radicals 4•-6• and lead to N-C bond dissociations is
difficult to investigate by experiment or theory, and thus
no specific conclusions regarding their reactivity can be
drawn at this stage.
Sch em e 4
Electron-withdrawing substituents in the aromatic
ring apparently hamper dissociations of the N-CH3 and
N-H bonds in 9• and even the cleavage of the CH2-N
bond in 10•. Instead, the hypervalent radicals undergo
deep fissions of the aromatic rings combined with losses
of substituents. The electronic properties of the aromatic
rings thus clearly have an effect on the dissociations of
bonds adjacent to the ammonium nitrogen atom.
Aromatic ring fissions are common in the dissociations
of 4•-6•, 9•, and 10•, where they give rise to abundant
hydrogenated or fluorinated C4, C3, and C2 fragments. It
is noteworthy that ring fissions in aromatic hydrocarbons
are highly endothermic decompositions that are known
to occur in high-energy processes such as in shock-tube
experiments38 or following multiphoton excitation.39 For
example, the dissociation C6H5CH2• f C4H2 + C3H3 + H2,
which yields the observed fragments at m/ z 50 and 39,
is 579 kJ mol-1 endothermic.32 These endothermic ring
fissions sharply contrast the substantially exothermic
dissociations of bonds in the vicinity of the hypervalent
nitrogen atom (Schemes 3-5). Reactions of such widely
different thermochemistries clearly cannot occur com-
petitively on the same potential energy surface. We
consider two types of effects that can contribute to the
ring fissions observed for neutralized hypervalent radi-
cals.
Sch em e 5
monium intermediates to make them metastable on the
microsecond time scale.
The energetics of bond dissociations in 4•-6• can be
roughly estimated from the known heats of formation of
the cations and neutral products and the estimated
ionization energies of analogous primary, secondary, and
tertiary ammonium ions, e.g., 4.0, 3.6, and 2.9 eV for 4•,
5•, and 6•, respectively.3e The energy data in kJ mol-1
are given in Schemes 3-5 as heats of formation of the
reactants and products.32 Scheme 3 shows that both the
N-CH2 and N-H bond homolyses are exothermic, such
that 4• is metastable with respect to these dissociations.
Thermochemical data also show that 5• and 6• are
metastable with respect to N-H, N-CH3, and CH2-N
bond homolyses of increasing exothermicity in the same
order (Schemes 4 and 5). The observed absence of 4•-6•
of microsecond lifetimes is consistent with these exother-
mic dissociation channels and indicates that there are
only small activation energy barriers for the bond ho-
First, ring fissions probably take place in a fraction of
highly energetic primary neutral fragments, e.g., benzyl
radicals, substituted benzyls, or benzylamines, or in their
corresponding cations following collisional reionization.
The energy needed for the ring fission can be supplied
in part by the exothermicity of the benzyl radical forma-
tion (Schemes 3-5) but can also come from electronic
(38) Lifshitz, A. Private communication.
(39) (a) Grotemeyer, J .; Schlag, E. W. Angew. Chem., Int. Ed. Engl.
1988, 27, 447. (b) Boesl, U.; Neusser, H. J .; Schlag, E. W. J . Chem.
Phys. 1980, 72, 4327. (c) Dietz, W.; Neusser, H. J .; Boesl, U.; Schlag,
E. W. Chem. Phys. 1982, 66, 105.

Hypervalent Ammonium Radicals
J . Org. Chem., Vol. 61, No. 16, 1996 5245
excitation upon collisional electron transfer.7b By com-
parison, endothermic neutralizations of C7H7+ ions with
nitrogen, oxygen, or xenon result in substantial ring
fissions in the radicals formed or in their reionized
counterparts.5e
capture by the aromatic ring in a fraction of neutralized
10•. As discussed by Radom and co-workers,43 nitro
groups, which are both σ- and π-acceptors, lower the
energy of the benzene SOMO by π-delocalization, allow-
ing thus a large portion of the negative charge to flow
into the NO2 group. This negative charge increases the
reactivity of the nitro group; for example, m-dinitroben-
zene anion radicals44 show a dominant loss of NO that
parallels that observed for 10•. The hydrogen transfer
onto the aromatic ring, also observed for 10•, can be
formulated as intramolecular protonation of the aromatic
anion with the ammonium cation. However, the infor-
mation on the initial proton receptor site is lost in the
subsequent fragmentation.
Second, electron capture in the cation may occur in one
of the unoccupied orbitals at the aromatic ring and induce
dissociations remote from the ammonium functionality.
Such zwitterionic states resemble electron-transfer com-
plexes15 in which the anionic and cationic groups are
separated and which are destabilized by an excitation
energy approximated by ∆E ≈ RE(cation) - IE(anion).
The recombination energies (RE, taken as positive val-
ues) of the ammonium groups are close in magnitude to
the ionization energies of the corresponding hypervalent
ammonium radicals (2.9-4.0 eV).3e,10 The ionization
energies (IE) of aromatic anions, which can be ap-
proximated by the corresponding electron affinities, are
typically <2.0 eV. Hence the zwitterionic states produced
by electron attachment must be inherently unstable
toward intramolecular electron or proton transfer that
would quench the zwitterion or to unimolecular dissocia-
tion. Unfortunately, the electronic states in the neutral-
ized intermediates are not probed directly by the present
experiments. However, the electron capture mechanism
is qualitatively consistent with the increased ring fission
in 9• and 10•. Both fluorine and nitro substituents
increase the electron affinity (EA) of the benzene ring,
e.g., EA ) 0.52 and 1.65 eV in hexafluorobenzene and
m-dinitrobenzene, respectively.18 For 9•, electron capture
by the pentafluorophenyl ring should form a loosely
bound electronic state, which can relax by exothermic
electron transfer to the ammonium group to trigger its
dissociations, e.g., cleavage of the benzylic CH2-N group,
as observed. However, most dissociations in 9• occur in
the aromatic ring, suggesting another mechanism of
excitation. We postulate that electron capture may occur
in the dense manifold of Rydberg ns, np, and nd states,
which are interspersed with valence shell excited states
in fluorobenzenes.40 Internal conversion in such an
excited state will deposit sufficient vibrational energy to
drive the highly endothermic ring cleavages in 9•. The
formation of CF by ring fission in 9• finds an analogy in
the laser photolysis of hexachlorobenzene41 or core-
electron excitation of 1,1,1-trifluoroethane and 1,1-di-
fluoroethene.42
Con clu sion s
Reduction of substituted benzylammonium cations by
collisional electron transfer from organic molecules in the
gas phase results in extensive dissociation. Substituents
at the ammonium nitrogen and in the aromatic ring affect
the dissociations of CH2-N, N-H, and N-CH3 bonds in
the transient hypervalent ammonium radicals. Fluorine-
and nitro-substituted benzylammonium radicals undergo
extensive ring fissions which are attributed to electron
capture by the aromatic ring.
Ack n ow led gm en t. Support by the National Science
Foundation (Grants CHE-9102442 and CHE-9412774),
the donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society, and the
University of Washington Royalty Research Fund is
gratefully acknowledged. Computational resources were
provided by the Cornell Theory Center, which received
major funding from the National Science Foundation
and New York state with additional support from the
Advanced Research Projects Agency, the National Cen-
ter for Research Resources at the National Institutes
of Health, IBM Corporation, and members of the
Corporate Research Institute. We also thank Professor
J urgen Grotemeyer and Dr. Viet Q. Nguyen for helpful
discussions.
Su p p or tin g In for m a tion Ava ila ble: Tables of neutral-
ization-reionization spectra and HF/3-21G-optimized geom-
etries in a Z-matrix format (6 pages). This material is
contained in libraries on microfiche, immediately follows this
article in the microfilm version of the journal, and can be
ordered from the ACS; see any current masthead page for
ordering information.
Dinitro derivative 10• shows abundant loss of NO and
hydrogen transfer from the side chain onto the aromatic
ring. We attribute these distinct properties to electron
J O960320U
(43) Birch, A. J .; Hinde, A. L.; Radom, L. J . Am. Chem. Soc. 1980,
102, 3370.
(44) (a) Trenerry, V. C.; Bowie, J . H. Org. Mass Spectrom. 1980,
15, 367. (b) Budzikiewicz, H. Angew. Chem., Int. Ed. Engl. 1981, 20,
624. (c) Stemmler, E. A.; Hites, R. A. Electron Capture Negative Ion
Mass Spectra of Environmental Contaminants and Related Com-
pounds; VCH Publishers: New York, 1988; p 111.
(40) Smith, D. R.; Raymonda, J . W. Chem. Phys. Lett. 1971, 12, 269.
(41) Whetten, R. L.; Fu, K.-J .; Tapper, R. S.; Grant, E. R. J . Phys.
Chem. 1983, 87, 1484.
(42) Boesl, U.; Grotemeyer, J .; Muller-Dethlefs, K.; Neusser, H. J .;
Selzle, H. L.; Schlag, E. W. Int. J . Mass Spectrom. Ion Processes 1992,
118/ 119, 191.