Near-Infrared Radiation
Induced Conformational Change
and Hydrogen Atom Tunneling of 2-Chloropropionic Acid in Low-Temperature
Ar Matrix
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Abstract
Former assignments of the matrix-isolation infrared (MI-IR)
spectrum
of 2-chloropropionic acid are revised with the help of near-infrared
(NIR) laser irradiation induced change in conformer ratios. This method
allows not only the unambiguous assignment of each band in the MI-IR
spectrum to the two <i>trans</i> (<i>Z</i>) and
the <i>cis</i> (<i>E</i>) conformers but also
the assignment of the spectral bands to different matrix sites. The
tunneling decay of the higher-energy <i>cis</i> conformer
prepared from both <i>trans</i> conformers in different
sites is also investigated. It is shown that the tunneling decay time
is very sensitive to the matrix site, especially if the in situ prepared
high-energy conformer has a strained geometry in the matrix cage.
The analysis shows that the kinetics of some <i>cis</i> → <i>trans</i> back conversion processes cannot be fitted by a single
exponential decay. The possible reasons of this observation are examined
and discussed. The present and former results clearly show that, in
addition to tunneling processes, the decay rates strongly depend on
solid-state effects. Therefore, simple theoretical predictions of
decay rates, which do not take into account the solid-state effects,
can only be compared to experimental observations only if experimentally
proven that these effects do not significantly affect the experimentally
measured tunneling rates