3080 J . Org. Chem., Vol. 67, No. 9, 2002
Geise et al.
chemistry often arises from the more reactive singlet
state, regardless of which is the lowest energy electronic
state. The two most commonly invoked mechanisms
accounting for this reactivity are those by Bethell and
co-workers10 and Griller, Nazran, and Scaiano.11
Thus, before one can begin to understand the reactivity
of a given carbene, one must know the relative energy
difference between the lowest singlet and triplet elec-
tronic states. A recent series of papers has examined this
issue in the case of 2-naphthyl(carbomethoxy)carbene
(2-NCC).12-14
Com p u ta tion a l Meth od s
All geometries were optimized using the three-param-
eter hybrid functional B3LYP19,20 method with the 6-31G*
basis set.21 Analytical second-derivative calculations
confirmed each stationary point to be a minimum by
yielding zero imaginary vibrational frequencies or a
transition state by yielding one imaginary vibrational
frequency. These vibrational frequency calculations also
provided the zero-point vibrational energy corrections,
which were scaled by 0.9806,22 as well as the thermody-
namic corrections necessary to calculate the enthalpies
and free energies at 298 K. Furthermore, single-point
energy calculations were carried out at the B3LYP level
using the 6-311+G** basis set23 on the B3LYP/6-31G*
geometry for all species. All basis sets used six Cartesian
d functions. Spin contamination in the triplet states was
minimal as S2 values were typically between 2.0 and
2.1. In contrast, unrestricted Hartree-Fock based meth-
ods suffer from significant spin contamination for similar
systems.15,16 All energies discussed in this paper will be
at the B3LYP/6-311+G**//B3LYP/6-31G* level of theory
and noted as either an enthalpy or free energy. We should
note that we also attempted CBS-QB324 calculations on
the parent species, and the singlet P CC energy calcula-
tion could not be completed in 200 CPU hours on an
Origin 2000. All calculations were carried out with the
Gaussian 98 suite of programs.25
2-NCC has a characteristic electronic spectrum, al-
lowing for UV-vis study,12,14 and distinct IR bands (due
to the substantially different geometry of the singlet and
the triplet states),12 thereby making it amenable to both
matrix12 and time-resolved infrared (TRIR)13 character-
ization. These studies show that, in nonpolar solvents and
in an argon matrix, 2-NCC has a triplet ground state
and thus can be observed by ESR spectroscopy.12 How-
ever, 2-NCC has a singlet-triplet energy gap that is close
to zero and allows for the experimental observation of
both the singlet and the triplet states simultaneously in
solution-phase TRIR studies. Indeed, three of us have
recently reported that solvent perturbs the singlet-
triplet energy gap of 2-NCC.15
There are a number of factors that can affect the
energetic ordering of the first singlet and triplet electronic
states of a carbene. In the case of an aryl carbene, such
as phenylcarbene, ring substituents can have a substan-
tial effect on the singlet-triplet energy gap.16,17 In this
study, we investigate the energetics of the first singlet
and triplet electronic states in the analogous phenyl-
(carbomethoxy)carbene (P CC) system using density func-
tional theory methods18 in order to gauge the effect of
phenyl ring substitution on the singlet-triplet energy
gap. TRIR methods will be used to provide experimental
validation of the computational results.
Exp er im en ta l Meth od s
Unless otherwise noted, materials were obtained from
Aldrich Chemical Co. and used without further purification.
Acetonitrile and dichloromethane were distilled from CaH2
before use. Hexane and Freon-113 were dried by passage
through a neutral alumina column and stored under argon.
All diazo compounds employed in this study were prepared
by the diazo-transfer method26 from their corresponding
acetates except 4-(N,N)-dimethylaminophenyldiazoacetate,
which was prepared from its hydrazone as detailed in the
Supporting Information. We conducted TRIR experiments
following the method of Hamaguchi and co-workers,27 as
described previously.13
(18) (a) Labanowski, J . W.; Andzelm, J . Density Functional Methods
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(17) A similar effect has also been calculated for phenylnitrenium
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Dulles, F. J .; Falvey, D. E. J . Am. Chem. Soc. 1994, 116, 9787. (b)
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