Matrix isolation IR spectroscopy and high-level ab
initio calculations were applied to investigate the
structure
and vibrational spectra of quinone−pyrimidine heterodimers formed in
low-temperature Ar matrices. A
specially developed experimental technique was used to separate bands
of quinone−pyrimidine dimer from
bands of quinone and pyrimidine monomers and homodimers in the IR
spectra. As a result, nine bands
assigned to the quinone−pyrimidine heterodimer were identified.
Ab initio calculations at the
MP2/6-31+G*,
MP2/6-31++G** and SCF/6-31++G** levels of theory have been
carried out to determine the relative
energies and vibrational spectra of three stable configurations of the
quinone−pyrimidine dimer found
theoretically. These configurations are two planar complexes with
two weak C−H···O and C−H···N
hydrogen
bonds and one stacked complex stabilized by dispersion forces. The
effect of basis set superposition error
(BSSE) on the relative stabilities and the vibrational spectra of the
dimers was also investigated. The non-BSSE-corrected calculations at the MP2/6-31+G* and MP2/6-31++G**
levels of theory predict the stacked
dimer to be the most stable conformer, but accounting for BSSE resulted
in a reverse stability ordering of the
stacked and the planar dimers. The comparison of the observed
frequency shifts with the theoretically predicted
shifts has shown that the planar configuration is responsible for the
experimentally observed bands. This is
in agreement with the stability ordering derived from the
BSSE-corrected relative energies. To account for
the matrix effects on the stability of the planar and stacked dimers,
additional calculations were carried out
using the Onsager's reaction field model and the MP2/6-31++G**
level of theory. These calculations confirm
that the planar H-bonded dimer is the most stable
configuration.
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