High-level ab initio calculations at the G3(MP2)//B3-LYP level have been us
ed to study carbomethoxychlorocarbene and related halogenocarbenes and carb
onyl carbenes. Initial calculations at the more accurate W1' level on the s
ubset CH2, HCCl, HCF, CCl2, and CF2 provide support for the reliability of
G3(MP2)//B3-LYP for this type of problem. The W1' calculations also suggest
that the experimental S-T splitting is slightly underestimated for HCCl an
d CF2 and substantially underestimated for CCl2, in keeping with other rece
nt high-level studies. Whereas the parent carbonyl carbenes, namely formylc
arbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to
have triplet ground states, their chloro and fluoro derivatives are predict
ed to have singlet ground states. In particular, carbomethoxychlorocarbene
is predicted to have a singlet ground state, with the singlet-tripler split
ting estimated as -16.0 kJ mol(-1). The barriers to Wolff rearrangement of
the singlet carbonyl carbenes generally (but not always) correlate with the
exothermicity accompanying the production of ketenes. In the case of the p
arent carbonyl carbenes, for which the rearrangement reaction is most exoth
ermic, the barriers lie between about 10 and 30 kJ mol(-1), whereas for the
less exothermic rearrangements of the chloro- and fluoro-substituted carbo
nyl carbenes, the Wolff rearrangement barriers increase significantly to be
tween 58 and 75 kJ mol(-1). The calculated barrier for carbomethoxychloroca
rbene is 58.2 kJ mol(-1).