Ab initio RHF and MP2 calculations with the standard 6-31G* and
6-311G** basis sets were carried out on
cyanomethyl formate (HCOOCH2CN). The calculations
indicated two stable conformers (Z,ap and
Z,sc) for
this molecule. However, the energy difference between the
conformers varied at different levels of theory.
The calculations with the 6-311G** basis set and correlated wave
functions indicated equal energy for these
conformers, whereas all the other calculations slightly preferred the
Z,ap conformer (by 0.2−2.1 kJ
mol-1).
The existence of the Z,ap and Z,sc
conformers of cyanomethyl formate was confirmed by matrix
isolation
infrared studies in argon. Immediately after deposition the
spectrum showed two peaks both in carbonyl and
asymmetric C−O−C stretching regions. The behavior of these
pairs of infrared peaks upon UV irradiation
and the comparison with the results obtained for related esters
strongly indicate that these doublets are caused
by two different conformers. It was observed that the
Z,ap and Z,sc conformers can be
surmounted thermally
in an argon matrix. This is consistent with the calculated values
of 2.6 kJ mol-1 (MP2/6-31G*) or 3.7
kJ
mol-1 (RHF/6-31G*) for the potential energy
barrier of this process. A conformer interconversion
process,
proceeding from both the Z,ap and Z,sc
to the E,sc conformer, was induced by UV irradiation at
248 nm.
The E,sc conformer is the main product of
isomerization. This was established by ab initio calculations
and
by analysis of the spectral changes in the (CO) and (C−O−C)
stretching regions. Upon prolonged irradiation
at the same wavelength, the E,sc conformer decomposed
to stable products including a hydrogen-bonded
(1:1) complex of CO and cyanomethanol. (CO2 production
was also observed. However, its counterpart
could not be identified, presumably due to further
decomposition.)
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