Computational Chemistry

levels explored in [ 4 ] oxirene was a transition state with an imaginary ring-opening
vibrational mode, functionals have been found which declare it a minimum with
some basis sets: Mawhinney and Goddard found that although very many of their
tested functional/basis combinations gave an imaginary frequency, a few found
oxirene to be a minimum [ 5 ]. In fact, the PBE0 functional found it to be a minimum
with 11 of the 12 bases tried, and with big bases predicted a C 2 H 2 O potential energy
surface similar to that from high-level (CCSD(T)) ab initio calculations. The
speed advantage over CCSD(T) tends to make this DFT method attractive for
such studies, but although the PBE0 functional is not parameterized ([ 5 ], p. 269)
experienced workers might still tend to confer more trust on high-level ab initio
calculations (“...we try to validate the results by comparison with those from ab
initio calculations” [ 6 ]).
A detailed computational study of the ozonolysis of ethyne skirted the question
of the involvement of oxirene by saying that it “will easily revert to [the carbene];
therefore, the oxirene route was not further investigated in this work” [ 7 ].
In recent work (2008) ultrafast photolysis of a potential diazo ketone precursor
ofp-biphenylmethyloxirene failed to detect the oxirene, the UV absorption of
which could, however, have been hidden by another band [ 8 ], and in a combined
experimental/computational (ab initio and molecular dynamics with DFT) study,
flash thermolysis of a formal Diels-Alder adduct was interpreted as affording
acetylmethyloxirene and benzene [ 9 ].

oxirene intermediate

carbene carbene

?

carbene carbene

oxirene transition state

?

a b

oxo carbene a oxirene oxo carbene b

a b

R R R R R R

O O

O

Fig. 9.2 A species with the symmetry of an oxirene scrambles the label in an oxo carbene. But this
does not tell us whether the oxirene is an intermediate or merely a transition state

9.1 From the Literature 563