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Understanding the molecular vibrations underlying each of the unknowninfrared 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 isa vital link to uncover the molecular identity of their carriers. This isusually done by customary classifications of normal mode frequencies such asstretching, deformation, rocking, wagging, skeletal mode, etc. A largeliterature on this subject exists and since 1952 ambiguities in classificationsof normal modes via this empirical approach were pointed out by Morino andKuchitsu [1]. New ways of interpretation and analyzing vibrational spectra weresought within the theoretical framework of quantum chemistry [2,3]. Many ofthese methods cannot easily be applied [3] to the large, complex molecularsystems which are one of the key research interests of astrochemistry. Inconsidering this demand, a simple and new method of analyzing and classifyingthe normal mode vibrational motions of molecular systems was introduced [4].This approach is a fully quantitative method of analysis of normal modedisplacement vector matrices and classification of the characteristicfrequencies (fundamentals) underlying the observed IR bands. Outcomes ofapplying such an approach show some overlap with customary empiricalclassifications, usually at short wavelengths. It provides a quantitativebreakdown of a complex vibration (at longer wavelengths) into the contributedfragments like their aromatic or aliphatic components. In addition, inmolecular systems outside the classical models of chemical bonds and structureswhere the empirical approach cannot be applied, this quantitative methodenables an interpretation of vibrational motion(s) underlying the IR bands.

The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis