Reactivity and structure of the 5-dehydro-m-xylylene anion

Article abstract of DOI:10.1021/jo049555t

The electronic structure of dehydro-m-xylylene anion (DMX^-) has been investigated by using chemical reactivity studies and electronic structure calculations. DMX^- has been generated in the gas phase via the sequential reaction of tri

Full text of DOI:10.1021/jo049555t

Rea ctivity a n d Str u ctu r e of th e 5-Deh yd r o-m -xylylen e An ion
Tamara E. Munsch,† Lyudmila V. Slipchenko,‡ Anna I. Krylov,‡ and Paul G. Wenthold*,†
Department of Chemistry, Purdue University, 560 Oval Drive, West Lafayette, Indiana 47907-2084,
and Department of Chemistry, University of Southern California, 3620 McClintock Avenue,
Los Angeles, California 90089-1062
pgw@purdue.edu
Received March 18, 2004
The electronic structure of dehydro-m-xylylene anion (DMX-) has been investigated by using
chemical reactivity studies and electronic structure calculations. DMX- has been generated in the
gas phase via the sequential reaction of trimethyl-3,5-bis(trimethylsilylmethyl)phenylsilane with
F
- and two molecules of F2. Reactivity and thermochemical properties of the ion indicate a phenyl-
like anion (1a ), consistent with theoretical predictions. Density functional calculations predict a
nonplanar triplet anion, with an allenic singlet anion slightly higher in energy. The driving force
for the out-of-plane distortion is more efficient charge delocalization that is achieved at lower
symmetry.
Distonic ions provide a very useful means for investi-
gating the properties of organic radicals in the gas phase.
Originally defined as the ions that result from ionization
of a biradical or zwitterion1 leading to ion radicals,2,3
distonic ions are now generally considered to include ions
with separated charge and radical centers, and examples
of gaseous distonic ions that contain carbenes,4-8 ni-
trenes,6,9 and even biradicals6,10-12 have been reported.
Thus, distonic ions have been used to investigate the
chemical reactivity of these types of moieties. However,
because of their relationship to the neutral reactive
intermediates, distonic ions also serve as convenient
precursors for mass spectrometric13-15 and spectroscopic16
studies of the investigation of reactive intermediate ther-
mochemistry. In addition to their utility in the investiga-
tion of neutral reactive intermediates, distonic ions are
interesting species in their own right17,18 and with unique
reactivity and unusual electronic structures that result
from ionization of complicated, open-shell wave functions.
We recently reported the measurement of the bond
dissociation energy at the 5-position of m-xylylene,19 and
its use for determining the heat of formation of the
5-dehydro-m-xylylene triradical (DMX). The DMX
triradical has an unusual electronic structure with an
unprecedented open-shell doublet ground state,19 with
three low-spin electrons in three, singly occupied molec-
ular orbitals (σ1π1π1). Ultimately, the measured thermo-
chemical properties for DMX indicate little interaction
between the unpaired σ- and π-electrons in the triradical.
* Corresponding author. Phone: (765) 494-0475. Fax: (765) 494-
0239.
† Purdue University.
‡ University of Southern California.
(1) Yates, B. F.; Bouma, W. J .; Radom, L. J . Am. Chem. Soc. 1984,
106, 5805-5808.
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1992, 92, 1649-1665.
(3) Smith, R. L.; Chou, P. K.; Kentta¨maa, H. I. In The Structure,
Energetics and Dynamics of Organic Ions; Baer, T., Ed.; J ohn Wiley &
Sons Ltd.: Chichester, U.K., 1997; pp 197-261.
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E. L.; Ellison, G. B.; Bierbaum, V. M. J . Org. Chem. 1989, 54, 1846-
1850.
The C-H bond dissociation energy in m-xylylene was
measured by using thermochemical studies involving the
triradical negative ion, DMX-. Whereas the original
study focused on the thermochemical properties of neu-
tral triradical, DMX, the structure of DMX- is also an
interesting question. Addition of an electron to the singly
occupied phenyl σ-orbital (Figure 1) leads to the forma-
tion of an ion consisting of a phenyl anion with a
(5) Hu, J .; Hill, B. T.; Squires, R. R. J . Am. Chem. Soc. 1997, 119,
11699-11700.
(6) Wenthold, P. G.; Hu, J .; Hill, B. T.; Squires, R. R. Int. J . Mass
Spectrom. 1998, 179/180, 173-183.
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Trans. 2 1999, 2249-2256.
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Am. Soc. Mass Spectrom. 1999, 10, 896-906.
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Am. Chem. Soc. 2001, 123, 7923-7924.
1
1
m-xylylene (MX) biradical moiety (σ2π1 π2 ) as repre-
(10) Thoen, K.; Kentta¨maa, H. I. J . Am. Chem. Soc. 1997, 119,
3832-3833.
sented by 1a . However, it is also possible to add an
(11) Price, J . M.; Nizzi, K. E.; Campbell, J . L.; Kentta¨maa, H. I.;
Seierstad, M.; Cramer, C. J . J . Am. Chem. Soc. 2003, 125, 131-140.
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10689-10696.
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597-604.
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(18) Rajca, A. Chem. Rev. 1994, 94, 871-893.
(19) Slipchenko, L. V.; Munsch, T. E.; Wenthold, P. G.; Krylov, A. I.
Angew. Chem., Int. Ed. Eng. 2004, 43, 742-745.
10.1021/jo049555t CCC: $27.50 © 2004 American Chemical Society
Published on Web 07/20/2004
J . Org. Chem. 2004, 69, 5735-5741
5735

Munsch et al.
a phenyl-like anion (1a ), consistent with theoretical
predictions. Calculations indicate a triplet anion, with
an allenic singlet anion slightly higher in energy.
Exp er im en ta l P r oced u r es
All experiments were carried out at room temperature in a
flowing afterglow triple-quadrupole apparatus described else-
where.26 Fluoride ion was generated by electron ionization of
F2 (5% in helium). Collision-induced dissociation (CID) experi-
ments are carried out by selecting the ions with the desired
mass-to-charge ratio using the first quadrupole (Q1) and then
injecting them into the second quadrupole (Q2, radio frequency
only), where they undergo collision with argon gas. The
reactant and product ions are analyzed with the third quad-
rupole (Q3) and are detected with an electron multiplier.
Ma ter ia ls. Gas purities were as follows: He (99.995%) and
F2 (5% in He). Authentic 3,5-bis-formylphenoxide ion was
generated from 1-tert-butyldimethylsilyloxy-3,5-bis-formylben-
zene, prepared by using the procedures described by Albrecht
et al.27 All other reagents were obtained from commercial
sources and were used as supplied.
Trimethyl-3,5-bis(trimethylsilylmethyl)phenylsilane was pre-
pared starting from the tribromide. Treatment of 1-bromo-3,5-
bis(bromomethyl)benzene28 with excess LiCl (10 equiv) in DMF
at room temperature for 12 h under nitrogen leads to chlorina-
tion at the R- and R′-positions.29,30 Diethyl ether was added,
and the mixture was washed with H2O to remove DMF. The
organic layer was dried over MgSO4, and the solvent was
removed in vacuo. The crude 1-bromo-3,5-bis(chloromethyl)-
benzene29 was dissolved in anhydrous THF and added drop-
wise to a refluxing mixture of 12 equiv of trimethylsilyl
chloride and 2.6 equiv of Mg turnings in anhydrous THF.31
The mixture was refluxed for 5 h under nitrogen. Hexane was
added, the mixture was filtered and dried over MgSO4, and
the solvent was removed in vacuo. The yellow residue was
purified by column chromatography with hexane to afford
trimethyl-3,5-bis(trimethylsilylmethyl)phenylsilane as a color-
less oil. 1H NMR (300 MHz, CDCl3): δ 6.85 (s, 2H), 6.64 (s,
1H), 2.03 (s, 4H), 0.23 (s, 9H), 0.02 (s, 18H).
F IGURE 1. Nonbonding molecular orbitals 5-dehydro-m-
xylylene (DMX). C2v symmetry labels are shown in parenthe-
ses.
electron to one of the π-orbitals to create a m-xylylenyl
1
2
2
1
anion with a phenyl radical, 1b (σ1π1 π2 or σ1π1 π2 ),20
where the biradical electronic structure resembles that
of R,3-dehydrotoluene (DHT).21 Advantages of either
structure can be envisioned, as 1a has localized charge
but a delocalized biradical, whereas 1b has a delocalized
(benzylic) charge but a localized phenyl radical. To a first
approximation, structure 1a would be expected to be
lower in energy because the electron affinity (EA) of the
phenyl radical (1.096 eV)22 is greater than that for a
benzyl radical (0.912 eV)22 or m-xylylene (0.907 eV).20
Because ions 1a and 1b are biradical in nature, there
should be several low-lying excited states of different
multiplicities. Given the similarities between the sys-
tems, it may be expected that the properties and energy
separations between states of 1a would be similar to
those of the MX biradical, whereas the states of 1b would
be similar to the states of the DHT biradical. For
example, because the ground state of m-xylylene biradical
is a triplet, with a singlet-triplet energy difference (∆EST
)
of 10.5 kcal/mol,20 ion 1a should have a triplet ground
state. Similarly, R,3-dehydrotoluene is a ground-state
singlet (∆EST ) 1-3 kcal/mol),21 such that 1b is also
expected to have a singlet ground state. Other biradical
states (i.e., open-shell singlet of MX and closed-shell
singlet of DHT) are considerably higher in energy (28
and 62 kcal/mol,23 respectively). The state orderings in
the MX and DHT biradicals (triplet, closed-shell singlet,
open-shell singlet for MX and open-shell singlet, triplet
for DHT) are typical for biradicals with overlapping (MX)
and nonoverlapping (DHT) electronic densities of biradi-
cal orbitals.24,25 However, these predictions ignore any
potential interactions between the σ- and π-systems or
any effects that the charge site has on the electronic
structure of the biradical.
Exp er im en ta l Resu lts
The gas-phase synthesis of DMX- involves sequential
reaction of trimethyl-3,5-bis(trimethylsilylmethyl)phen-
ylsilane with F- and two molecules of F2 resulting in
formation of an ion with m/z 103, C8H7-, which is deduced
on the basis of reactivity studies to be DMX- (eq 1). As
has been discussed previously,6 the mechanism of
F2-induced desilylation is believed to occur via electron
transfer from silylated carbanions within a collision
complex, followed by desilylation of the radical by either
F2-‚ or F-. Other products observed in the flow reactor
include the intermediate ions shown in eq 1, m/z 255 and
m/z 179, and their fluoride adducts. In addition, we also
observe smaller amounts of ions at m/z 180, m/z 104, and
m/z 105, which result from reaction of the silylated anions
In this paper, we investigate the electronic structure
of DMX- by using chemical reactivity studies and
electronic structure calculations. We show that the reac-
tivity and thermochemical properties of the ion indicate
(26) Marinelli, P. J .; Paulino, J . A.; Sunderlin, L. S.; Wenthold, P.
G.; Poutsma, J . C.; Squires, R. R. Int. J . Mass Spectrom. Ion Processes
1994, 130, 89-105.
(20) Wenthold, P. G.; Kim, J . B.; Lineberger, W. C. J . Am. Chem.
Soc. 1997, 119, 1354-1359.
(21) Wenthold, P. G.; Wiershke, S. G.; Nash, J . J .; Squires, R. R. J .
Am. Chem. Soc. 1994, 116, 7378-7392.
(27) Albrecht, M.; Kocks, B. M.; Spek, A. L.; Van Koten, G. J .
Organmet. Chem. 2001, 624, 271-286.
(28) Ho¨gberg, H.-E.; Wennerstro¨m, O. Acta Chem. Scand., Ser. B
1982, 36, 661-667.
(22) Gunion, R. F.; Gilles, M. K.; Polak, M. L.; Linenberger, W. C.
Int. J . Mass Spectrom. Ion Processes 1992, 117, 601-620.
(23) Levchenko, S. V.; Krylov, A. I. unpublished results.
(24) Borden, W. T.; Davidson, E. R. J . Am. Chem. Soc. 1977, 99,
4587-4594.
(29) Tashiro, M.; Yamato, T.; Kobayashi, K.; Arimura, T. J . Org.
Chem. 1987, 52, 3196-3199.
(30) Harvey, S.; J unk, P. C.; Raston, C. L.; Salem, G. J . Org. Chem.
1998, 53, 3134-3140.
(25) Slipchenko, L. V.; Krylov, A. I. J . Chem. Phys. 2002, 117, 4694-
4708.
(31) Wrobel, D.; Wannagat, U. J . Organmet. Chem. 1982, 225, 203-
210.
5736 J . Org. Chem., Vol. 69, No. 17, 2004

Reactivity and Structure of 5-Dehydro-m-xylylene Anion
with HF impurity in the fluorine mixture.6 Although the
reaction scheme shown in eq 1 implies initial formation
of a silylated m-xylylenyl anion, which then reacts to give
DMX-, the silylated dehydrotoluene anion is also a
possible intermediate. The exact sequence of reactions
is not known but is not important.
Absolute confirmation of the structure of DMX- was
achieved by using the reaction with O2, which leads to a
product that has the same mass-to-charge ratio as 3,5-
bis-formylphenoxide ion (2). The formation of the phe-
noxide ion can be attributed to oxygen atom abstraction
from O2 by DMX- and sequential oxidation of the
benzylic radicals as shown in eq 2. Oxidation of benzylic
radicals in distonic biradical anions with O2 has been
observed previously by Hu and Squires.12 Similarly, the
phenyl anion reacts with O2 to form phenoxide and other
products.32 However, the phenyl radical can also react
with O2 by oxygen atom abstraction. Although it is
inefficient for neutral phenyl radical because it has a
barrier of 6-7 kcal/mol,33 the barrier in the gas-phase
reaction can be overcome by the ion/neutral complexation
energy.15 Therefore, formation of 2 would be expected
regardless of the electronic structure of the ion. The CID
spectra of 2, prepared by derivatization of DMX-, and of
authentic bis(formyl)phenoxide ion, prepared from 1-tert-
butyldimethylsilyloxy-3,5-bis-formylbenzene,27 are shown
in Figure 2a-b, respectively. The major ionic fragments
are m/z 121 and 92 with measured yields of 5/1 for both
reactant ions. Because of impurities arising from mass
overlap in the spectrum of the derivative, it is not possible
to compare the absolute cross-sections for the two ions.
The excellent agreement between the CID spectra of the
authentic phenoxide and derivative 2 confirms the 5-de-
hydro-m-xylylene structure of the anion but does not
provide insight into the electronic structure.
F IGURE 2. Collision-induced dissociation spectra for m/z 149,
generated by the reaction of m/z 103 with oxygen (a) and from
fluoride induced desilylation of 1-tert-butyldimethylsilyloxy-
3,5-bis-formylbenzene (b). The starred peaks in the spectrum
(b) indicate products attributable to m/z 149 arising from
reaction of O2 with m/z 104.
with CO2 molecules by condensation to form a carboxylate
ion, and the reactivity of the carboxylates reflects the ion
structure. Carboxylates formed from closed-shell anions
are typically not found to undergo addition with NO or
NO2,12 but radical addition has been observed for open-
shell carboxylates, including those derived from benzyne
ions,34 1,3,5-trimethylenebenzene anion12, and m-xylylene
anion.35 The CO2 adduct of DMX- undergoes sequential
addition of two NO or NO2 molecules, indicating biradical
character in the carboxylate. The DMX- ion itself is also
found to undergo addition of up to three NO molecules.
Sequential addition of NO has been observed with other
open-shell systems.34 This reactivity is consistent with
that expected for either ion 1a or 1b, as they are both
open-shell biradicals.
The question of the phenyl versus the benzyl anion has
been addressed by using reactions with N2O and CS2.
Nitrous oxide reacts with phenyl anion by oxygen atom
transfer (eq 3a) but reacts with benzyl anions by the
addition and loss of H2O (eq 3b).36 As expected, the
The electronic structure of DMX- was characterized
by using chemical reactivity. For example, DMX- reacts
(32) Wenthold, P. G.; Hu, J .; Squires, R. R. J . Mass Spectrom. 1998,
(34) Wenthold, P. G.; Hu, J .; Squires, R. R. J . Am. Chem. Soc. 1996,
118, 11865-11871.
(35) Hammad, L. A.; Wenthold, P. G. J . Am. Chem. Soc. 2000, 122,
11203-11211.
33, 796-802.
(33) Lin, C. Y.; Lin, M. C. Chem. Phys. Processes Comb. 1987, 7/1-
7/4.
J . Org. Chem, Vol. 69, No. 17, 2004 5737

Munsch et al.
reaction shown in eq 3b also occurs with m-xylylene anion
(eq 3c), formed by the reaction of m-xylylene with O-‚.20,35
Given the similarity between the electronic structure of
1b and the m-xylylene anion, it is reasonably expected
that the reaction shown in eqs 3b and 3c should occur
with ion 1b as well. However, the reaction of DMX- with
N2O proceeds to give oxygen atom transfer (eq 4a),
whereas the addition and loss of H2O product (eq 4b) is
not observed at all. On the basis of this observation, we
conclude that the DMX- signal consists of a single ion
structure and that it is a phenyl anion with a triplet
m-xylylene biradical moiety. The presence of 1b, either
in whole or as part of a mixture, would be expected to be
indicated by the reaction with N2O. Benzylic radicals in
the gas phase are essentially unreactive with N2O.12
m-xylylene biradical electronic structure. For example,
the electron binding energy in DMX- (the EA of DMX)
was measured to be 24.9 ( 2.0 kcal/mol.19 This value is
similar to the EA of phenyl radical, 25.3 ( 0.1 kcal/mol,22
but is much higher than the EA of m-xylylene, 21.19 (
0.18 kcal/mol.39 Similarly, the proton affinity of the ion
is measured to be 401 ( 3 kcal/mol,19 nearly the same as
that for phenyl anion, but much higher than that for the
benzyl anion (∼380 kcal/mol).38 Thus, the reactivity and
thermochemical properties of DMX- are all consistent
with an electronic structure with a phenyl anion and
triplet m-xylylene biradical (1a ), as predicted by simple
EA considerations.
Th eor etica l Resu lts
From the electronic structure point of view, adding an
extra electron into the three-electrons-on-three-orbitals
system of the DMX triradical leads to the four-electrons-
in-three-orbitals pattern in DMX-. This leads to many
near-degenerate arrangements resulting in multiconfigu-
rational wave functions. However, if the interaction
between a doubly occupied orbital and two others is weak
due to the symmetry/nodal considerations or spatial
separation, then the system can be more simply described
as an anion and a biradical, and the wave functions can
be described within the traditional two-electrons-in-two-
orbitals biradical model. Because of the extensive elec-
tronic near-degeneracies, the choice of an electronic
structure method is extremely important. An accurate
model needs to provide a qualitatively correct description
of an electronic wave function and also needs to include
dynamical correlation, crucial for quantitative accuracy
in anions. Moreover, to maintain a balanced description
of several states of interest, multistate methods are
preferred to state-to-state approaches.
The reaction with CS2 is also consistent with a phenyl
anion electronic structure. The reaction proceeds by
sulfur atom transfer (25%), and CS2 addition (75%) is
formed. It is well-recognized that sulfur atom transfer
occurs in the reaction of CS2 with highly basic ions. For
example, the reaction of CS2 with phenyl anion has been
measured in this work to proceed by 94% sulfur atom
transfer and 6% CS2 addition (eq 5), whereas the reaction
of CS2 with benzylic anions, including m-xylylene anion,
proceeds only by CS2 addition.35 Thus, the observation
of sulfur atom abstraction with CS2 suggests a phenyl
anion in DMX-. Generally, sulfur abstraction from CS2
by a phenyl radical to form a phenylthiolate radical is
endothermic by ca. 10 kcal/mol37,38 and therefore would
not be expected for ion 1b. However, in this case, the
product would likely be the thiolate anion, which would
be more favorable, and therefore, while the reaction with
CS2 is consistent with the phenyl anion assignment, it
does not rule out structure 1b.
In this paper, we have used the equation-of-motion
approach, which describes excited states as single and
double excitations from the reference state CCSD (coupled-
cluster with singles and doubles model) wave function.40
The traditional method, EOM-EE-CCSD,41 was used for
states that can be described as single excitations from a
(37) Afeefy, H. Y.; Liebman, J . F.; Stein, S. E. In NIST Chemistry
WebBook, NIST Standard Reference Database Number 69; Linstrom,
P. J ., Mallard, W. G., Eds.; National Institute of Standards and
Technology: Gaithersburg, MD, March 2003; pp 20899.
(38) Bartmess, J . E. In NIST Chemistry WebBook, NIST Standard
Reference Database Number 69; Mallard, W. G., Linstrom, P. J ., Eds.;
National Institute of Standards and Technology: Gaithersburg, MD,
March 2003; pp 20899.
(39) Wenthold, P. G.; Squires, R. R. J . Am. Chem. Soc. 1994, 116,
6401.
(40) Purvis, G. D.; Bartlett, R. J . J . Chem. Phys. 1982, 76, 1910-
1918.
The thermochemical properties measured for DMX-
are also consistent with the assignment of a phenyl anion/
(36) Kass, S. R.; Filley, J .; Van Doren, J . M.; DePuy, C. H. J . Am.
Chem. Soc. 1986, 108, 2849-2852.
(41) Stanton, J . F.; Bartlett, R. J . J . Chem. Phys. 1993, 98, 7029-
7039.
5738 J . Org. Chem., Vol. 69, No. 17, 2004

Reactivity and Structure of 5-Dehydro-m-xylylene Anion
closed-shell reference. Its spin-flip counterpart, EOM-
SF-CCSD,25,42-44 was used for biradical states. Some
states (e.g., triplets and closed-shell singlets) were ac-
cessible by traditional single-reference techniques, such
as CCSD(T)45 or density functional theory (DFT).46
1
1
Equilibrium geometries of all the triplet (σ2π1 π2 ,
σ1π1 π2 , σ1π1 π2 ) and closed-shell singlet (σ2π1 π2 )
2
1
1
2
2
0
states of DMX- were optimized at the B3LYP/6-
1
311+G*47,48,49 level of theory. The structures of the B1
(1A’’) and 1A2 open-shell singlet states (σ1π1 π2 and
1
2
σ1π1 π2 ) were optimized at the EOM-CCSD/6-31+G*49,50
2
1
0
2
2
2
level from the σ2π1 π2 and σ0π1 π2 references, respec-
tively. Adiabatic excitation energies between the states
that are well-described by single-reference methods51
were calculated by the B3LYP, CCSD, and CCSD(T)
methods. The energy differences between the two-
1
configurational B1/1A2 open-shell singlets and the cor-
responding 3B1/3A2 triplet states were calculated by EOM-
EE-CCSDand EOM-SF-CCSD.Calculations wereperformed
by using the Q-CHEM52 and ACES II53 electronic struc-
ture packages. Molecular orbitals were visualized by
using Spartan. Natural atomic charges are calculated by
using the Natural Bond Orbital (NBO 4.0) program,54
which is interfaced to Q-CHEM.
F IGURE 3. Relative energies of planar DMX- for phenyl
anion/m-xylylene biradical states (left) and phenyl radical/m-
xylylenyl anion states (right). Adiabatic energies (no ZPE
included) were calculated by B3LYP/6-311+G*, EOM-CCSD/
6-31+G* (1A2), and SF-EOM-CCSD/6-31+G* (1B1) (see text).
The electronic configurations and relative adiabatic
energies of the low-lying states of planar DMX- are
summarized in Figure 3. At the planar C2v geometries,
the correlation between the electronic states of DMX-
and those of MX and DHT is obvious. Electronic states
indicated to the left correspond to an electronic structure
such as that in 1a , with a phenyl anion and m-xylylene
biradical moiety. Electronic states with labels to the right
correspond to an electronic structure like that in 1b. At
the planar geometry, the ground state of the system is
calculated to be 4.9 kcal/mol above the ground state, such
that 1b has a singlet-triplet energy splitting of 0.8 kcal/
3
mol. A second triplet state in 1b, the A2 state, lies 8.2
kcal/mol higher than the ground state. The corresponding
singlet-triplet energy splitting is 1.2 kcal/mol. The
calculated singlet-triplet splitting of planar 1a is 13.1
kcal/mol in favor of the triplet. These values are similar
to the corresponding singlet-triplet splittings in R,3-
dehydrotoluene21 and m-xylylene,20 respectively. Ulti-
mately, the calculated energy ordering for the planar ion
is essentially what would be predicted based on the
electron affinities of phenyl and benzyl anions and the
singlet-triplet splittings in the model biradicals.
3
predicted to be the B2 state, the triplet state of 1a . The
1
lowest energy state of 1b is the B1 singlet state and is
4.1 kcal/mol higher in energy than the ground state. The
3
lowest energy triplet state of planar 1b is the B1 state,
Surprisingly, only one of the planar states described
previously, the 1A2 state, has been found to be a true
minimum with the B3LYP approach, as each of the
(42) Krylov, A. I. Chem. Phys. Lett. 2001, 338, 375-384.
(43) Levchenko, S. V.; Krylov, A. I. J . Chem. Phys. 2004, 120, 175-
185.
(44) Krylov, A. I.; Sherrill, C. D. J . Chem. Phys. 2002, 116, 3194-
3203.
3
others has a single imaginary frequency. For the B2, 3B1,
1
and A1 states, the normal coordinate for the imaginary
frequency distorts the molecule to C2 symmetry. In the
3A2 and 1B1 states, the distortion leads to a planar Cs
structure. Thus, the DFT calculations predict that many
of the stable states of DMX- have non-C2v structures, and
as described next, are nonplanar. This is particularly
striking since out-of-plane distortions perturb conjugation
in the π-system, thus destabilizing it.
(45) Piecuch, P.; Kucharski, S. A.; Bartlett, R. J . J . Chem. Phys.
1999, 110, 6103-6122.
(46) Parr, R. G.; Yang, W. Density-functional theory of atoms and
molecules; Oxford University Press: New York, 1989.
(47) Becke, A. D. J . Chem. Phys. 1993, 98, 5648-5652.
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Phys. 1980, 72, 650-654.
(49) Clark, T.; Chandrasekhar, J .; Spitznagel, G. W.; Schleyer, P.
V. J . Comput. Chem. 1983, 4, 294-301.
The driving force for the nonplanarity of DMX- has
been found to be more efficient charge delocalization that
is achieved at twisted geometries due to the lifting of
symmetry-imposed constraints. When the symmetry low-
ers from C2v to C2, the a1 and a2 orbitals are able to mix
as a-type orbitals, whereas the b1 and b2 orbitals can mix
as b-type orbitals. Similarly, lowering the symmetry from
C2v to planar Cs allows mixing of the a1 and b2 orbitals
(as a′) and of the a2 and b1 orbitals (as a′′). The pair of a′′
orbitals that results is essentially the GVB orbitals in
the m-xylylene biradicals.20 The orbitals that result from
the mixing upon distorting to C2 and Cs are shown in
Figure 4.
(50) Hariharan, P. C.; Pople, J . A. Theor. Chim. Acta 1973, 28, 213.
(51) In the C2v symmetry group, these are the 3B2, 3B1, and 3A2
triplets and the 1A1 singlet. Their electronic configurations are σ2π11π2
,
1
σ1π11π22, σ1π12π21, and σ2π12π2°, respectively, and their wave functions
are single-configurational. In the C2 group, these electronic states mix
with each other, and only 3B and 1A can be described by single-
reference wave functions.
(52) Kong, J .; White, C. A.; Krylov, A. I.; Sherrill, D.; Adamson, R.
D.; Furlani, T. R.; Lee, M. S.; Lee, A. M.; Gwaltney, S. R.; Adams, T.
R.; Ochsenfeld, C.; Gilbert, A. T. B.; Kedziora, G. S.; Rassolov, V. A.;
Maurice, D. R.; Nair, N.; Shao, Y. H.; Besley, N. A.; Maslen, P. E.;
Dombroski, J . P.; Daschel, H.; Zhang, W. M.; Korambath, P. P.; Baker,
J .; Byrd, E. F. C.; Van Voorhis, T.; Oumi, M.; Hirata, S.; Hsu, C. P.;
Ishikawa, N.; Florian, J .; Warshel, A.; J ohnson, B. G.; Gill, P. M. W.;
Head-Gordon, M.; Pople, J . A. J . Comput. Chem. 2000, 21, 1532-1548.
(53) Stanton, J . F.; Gauss, J .; Watts, J . D.; Lauderdale, W. J .;
Bartlett, R. J . Int. J . Quantum Chem. 1992, 879-894.
(54) Glendening, E. D.; Badenhoop, J . K.; Reed, A. E.; Carpenter,
J . E.; Weinhold, F. Theoretical Chemistry Institute; University of
Wisconsin, Madison, WI, 1996.
In the 3B2 triplet state, charge delocalization can be
enhanced by mixing the a1 and a2 orbitals, which requires
J . Org. Chem, Vol. 69, No. 17, 2004 5739

Munsch et al.
6-positions. However, in the C2 geometry, an additional
charge is delocalized from the 5-position to the methylene
groups, thereby reducing the energy penalty due to
charge localization. It should be noted that the overall
energy of the triplet is lowered by only 0.3 kcal/mol as a
result of symmetry lowering, and that the preference for
the non-planar geometry is not found using MP2 or
coupled-cluster theory.
Reduction of symmetry can have dramatic effects of
spectroscopic observables and electronic structure of the
excited states. For example, in substituted benzenes,
small conformational changes in the substituent produce
very large changes the direction of electronic transition
dipole moment (e.g., 50° and more) due to the mixing of
excited nearly degenerate states that are not interacting
in the symmetric nonsubstituted benzene.56-58 Symmetry
can also have a pronounced effect on the bonding and
equilibrium structures of the J ahn-Teller and pseudo-
J ahn-Teller molecules.59-61 The DMX- anion, however,
is an example where low-symmetry equilibrium struc-
tures are preferred because of more efficient charge
delocalization that is not possible at higher symmetry.
This type of behavior has been observed previously for
deprotonated benzoquinone, which, like DMX-, adopts
a nonplanar structure to delocalize the charge into the
π-system.62
F IGURE 4. Effects of symmetry reduction on the nonbonding
molecular oribtals in DMX-.
The out-of-plane distortion of the carbon framework
in the triplet ground state is not large. For example, the
C1-C2-C3-C4 dihedral angle is only 5.5° (Figure 5). A
larger out-of-plane distortion is found for the hydrogen
atoms bonded to C4 and C6, which are approximately
10° out of the plane of the aromatic ring. Moreover, the
out-of-plane distortion mainly occurs in the C4-C5-C6
portion of the ring, as the methylene-C1-C2-C3-
methylene portion is essentially planar. As shown in
Figure 5, there is little difference between bond angles
and bond lengths in the C2v and C2 geometries of the
triplet. Much larger differences are observed between the
1
C2v and C2 geometries and charge distributions of the A
1
(1A1) states. The C-C bonds in the planar A1 state are
all essentially 1.40-1.43 Å, with little bond length
alternation, and the bond angles resemble those for
phenyl anion,63 with a C4-C5-C6 angle of 110°. In the
C2 geometry, however, the C4-C5-C6 angle is 126°. The
carbon-carbon bonds to C5 are only 1.34 Å, 0.06 Å
shorter than those in the planar ion, whereas the C3-
C4 and C1-C6 bonds are 0.06 Å longer in the nonplanar
structure. Correspondingly, there is less charge at C5 and
more at the methylene positions and C2 (Figure 5) in the
C2 distorted ion. The geometry and charge distributions
F IGURE 5. Geometries of the C2v (planar) and C2 (nonplanar)
triplet and singlet states of DMX- calculated at the B3LYP/
6-311+G* level of theory. Out-of-plane distortions of the C2
structures are described in the text. The atomic charges
calculated by using the NBO approach are shown in italics.
lowering the symmetry from C2v to C2. Because the
energy gain due to the more extensive delocalization is
more significant than the energy penalty for the breaking
of the conjugation in the π-system, the nonplanar geom-
etry is preferred at the B3LYP/6-311+G* level of theory.
From the electronic structure point of view, this results
in a mixing of the 3B configurations that lowers the
energy due to a configuration interaction. The increased
charge delocalization is reflected in the calculated changes
in natural charges (Figure 5).55 At the C2v geometry, the
charge distribution for the most part reflects the a1
orbital, with almost half the charge at the 5-position, and
the remainder is distributed through the 1-, 3-, 4-, and
1
of the A ion suggest a valence structure like that shown
(56) J oireman, P. W.; Kroemer, R. T.; Pratt, D. W.; Simons, J . P. J .
Chem. Phys. 1996, 105, 6075-6077.
(57) Borst, D. R.; J oireman, P. W.; Pratt, D. W.; Robertson, E. G.;
Simons, J . P. J . Chem. Phys. 2002, 116, 7057-7064.
(58) Panja, S. S.; Chakraborty, T. J . Chem. Phys. 2003, 119, 9486-
9490.
(59) Davidson, E. R.; Borden, W. T. J . Phys. Chem. 1983, 87, 4783-
4790.
(60) Slipchenko, L. V.; Krylov, A. I. J . Chem. Phys. 2003, 118, 9614-
9622.
(61) Slipchenko, L. V.; Krylov, A. I. J . Chem. Phys. 2003, 118, 6874-
(55) The C2v structures of the 3B2 and 1A1 states and the C2 structure
of 1A were also optimized at the MP2/6-311+G* level. The resulting
geometries were very similar to those obtained at the B3LYP level of
theory, differences not exceeding 0.01 Å and 1-2° for bond lengths
and bond angles, respectively.
6883.
(62) Davico, G. E.; Schwartz, R. L.; Ramond, T. M.; Lineberger, W.
C. J . Am. Chem. Soc. 1999, 121, 6047-6054.
(63) Nicolaides, A.; Smith, D. M.; J ensen, F.; Radom, L. J . Am.
Chem. Soc. 1997, 119, 8083-8088.
5740 J . Org. Chem., Vol. 69, No. 17, 2004

Reactivity and Structure of 5-Dehydro-m-xylylene Anion
TABLE 1. Rela tive En er gies of th e Sta ble Sta tes of
DMX- Ca lcu la ted by Differ en t DF T a n d CC Meth od sa ,b
error bar of the employed computational scheme exceeds
0.6 kcal/mol, we believe that the calculated energy gap
3
1
between the B and the A states is reliable due to the
error cancellation, which is supported by the amplitude
analysis of the CC wave function. Thus, the theoretical
calculations predict the ground state of the system to be
the triplet but find that the singlet is slightly higher in
energy.
electronic
state
DFT
CCSD
CCSD(T)
schemee
schemec
schemed
3Bf
1Af
1Ag
0.0
4.8
5.1
7.6
0.0
5.9
7.6
0.0
0.6
7.7
h
1A2
11.1
10.0
a
b
Values in kcal/mol; ZPE corrections are not included. The
adiabatic energy differences between the triplet 3B and the closed-
shell singlet 1A were calculated as differences between the total
DFT or CC energies. The adiabatic energy differences between the
3B and the open-shell 1A′′ (1B1)/1A2 states were computed in a two-
step procedure: (i) the energy difference between 3B and 3B1/3A2
was calculated at the DFT or CC levels, respectively and (ii) the
3B1-1B1/3A2-1A2 energy separation was calculated by EOM-CCSD/
6-31+G*. c B3LYP/6-311+G* was used for 3B and 1A and B3LYP/
6-311+G* and EOM-CCSD/6-31+G* for 1A′′ and 1A2 (see footnote
This state ordering is in agreement with the reactivity
studies of DMX-. As noted in the preceding section, the
reactivity of ion 1 with NO and N2O suggests open-shell
character but argues against a benzylic structure. This
would then seem to be evidence against the possibility
of the 1A state being the reactive state of the ion.
Although the singlet ion, 3, is more of a pentadienyl anion
than benzylic, the reaction of N2O with pentadienyl anion
is also found to proceed by addition and loss of water,66
and this reaction is not observed for 1.
d
1
b). CCSD/6-311+G* was used for 3B and A and CCSD/6-311+G*
and EOM-CCSD/6-31+G* for 1A′′ and 1A2 (see footnote b). e CCS-
D(T)/6-311+G* was used for 3B and 1A and CCSD(T)/6-311+G*
and EOM-CCSD/6-31+G* for 1A′′ and 1A2 (see footnote b). f C2
equilibrium geometries optimized at the B3LYP/6-311+G* level
Con clu sion
g
Chemical and theoretical studies indicate that the
ground state of the dehydro-m-xylylene anion is a ground-
state triplet, 3B, consisting of a phenyl anion and a
m-xylylene biradical. The benzylic ion, electronically
similar to the dehydrotoluene biradical, is calculated to
lie 4-5 kcal/mol higher in energy than the ground state.
Triplet carbanions are rare but have been found for other
reduced triradicals12,17 or for carbyne anions.67 This ion
is unique among the known hydrocarbon distonic biradi-
cal anions in that the charge is nominally separated from
the biradical moiety. The lowest energy singlet is not the
biradical but is an allenic structure with a pentadienyl
anion and is calculated to be slightly higher in energy
than the triplet state. Atomic charges calculated for the
singlet ion indicate significant charge density at the
2-position, suggesting that it may be possible to modify
the ground-state electronic structure by using substitu-
ents at that position. Indeed, preliminary calculations
suggest that a cyano substituent at the 2-position of the
ion preferentially stabilizes the singlet state with respect
to the triplet, resulting in a singlet ground state. Experi-
mental studies to test this possibility are currently
underway.
were used. C2v geometry optimized at the EOM-CCSD/6-31+G*
level was used. The equilibrium geometry of this open-shell singlet
is of Cs symmetry. However, we were not able to obtain fully
optimized geometry due to the instabilities caused by the strong
mixing between this state and the closely lying 1A2 (1A′′ in Cs)
state. Calculated at the C2v equilibrium geometry optimized at
the EOM-CCSD/6-31+G* level.
h
as 3, consisting of a pentadienyl (π) anion with an allene
moiety. The bonding shown in 3, confirmed by NBO,64 is
only possible in the lower symmetry geometry, and is
surprisingly reminiscent of that found for deprotonated
cyclooctatetraene,65 which also forms an allene and a
delocalized anion, 4.
The relative energies of the stable states of DMX-,
calculated at various levels of theory, are shown Table
1. At all the levels of theory employed in this work, the
3B state is the ground state of the system. However, the
energies of the singlet states are found to be slightly
higher in energy and depend strongly on the level of
theory. Because of the extensive difference in charge
delocalization, the relative energy of the 1A state at
nonplanar geometries is significantly lower than that of
the planar ion, and is, at the CCSD(T)/6-311+G* level
Ack n ow led gm en t . This work was supported by
the National Science Foundation (P.G.W.: CHE-0137627
and A.I.K.: CHE-0094116), by the Alfred P. Sloan
Foundation, and the donors of the Petroleum Research
Fund administered by the American Chemical Society
(PRF-AC) to A.I.K.
Su p p or tin g In for m a tion Ava ila ble: Cartesian coordi-
nates for optimized geometries and absolute energies used in
this work. This material is available free of charge via the
Internet at http://pubs.acs.org.
1
of theory, lower in energy than the B1 and 1A2 states,
3
only 0.6 kcal/mol higher in energy than the B ground
state. Although the conservative estimate of an energy
J O049555T
(64) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88,
899-926.
(65) Kato, S.; Lee, H. S.; Gareyev, R.; Wenthold, P. G.; Lineberger,
W. C.; DePuy, C. H.; Bierbaum, V. M. J . Am. Chem. Soc. 1997, 119,
7863-7864.
(66) Munsch, T. E.; Wenthold, P. G., unpublished results.
(67) Seburg, R. A.; Hill, B. T.; J esinger, R. A.; Squires, R. R. J . Am.
Chem. Soc. 1999, 121, 6310-6311.
J . Org. Chem, Vol. 69, No. 17, 2004 5741