The Astrochemistry Implications of Quantum Chemical Normal Modes
  Vibrational Analysis
  Understanding the molecular vibrations underlying each of the unknown
infrared emission (UIE) bands (such as those found at 3.3, 3.4, 3.5, 6.2, 6.9,
7.7, 11.3, 15.8, 16.4, 18.9 mm) observed in or towards astronomical objects is
a vital link to uncover the molecular identity of their carriers. This is
usually done by customary classifications of normal mode frequencies such as
stretching, deformation, rocking, wagging, skeletal mode, etc. A large
literature on this subject exists and since 1952 ambiguities in classifications
of normal modes via this empirical approach were pointed out by Morino and
Kuchitsu [1]. New ways of interpretation and analyzing vibrational spectra were
sought within the theoretical framework of quantum chemistry [2,3]. Many of
these methods cannot easily be applied [3] to the large, complex molecular
systems which are one of the key research interests of astrochemistry. In
considering this demand, a simple and new method of analyzing and classifying
the normal mode vibrational motions of molecular systems was introduced [4].
This approach is a fully quantitative method of analysis of normal mode
displacement vector matrices and classification of the characteristic
frequencies (fundamentals) underlying the observed IR bands. Outcomes of
applying such an approach show some overlap with customary empirical
classifications, usually at short wavelengths. It provides a quantitative
breakdown of a complex vibration (at longer wavelengths) into the contributed
fragments like their aromatic or aliphatic components. In addition, in
molecular systems outside the classical models of chemical bonds and structures
where the empirical approach cannot be applied, this quantitative method
enables an interpretation of vibrational motion(s) underlying the IR bands.
