The Astrochemistry Implications of Quantum Chemical Normal Modes Vibrational Analysis

Abstract

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 µm) 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. As a result, further modifications in the structures (modeling) and the generation of the IR spectra (simulating) of the UIE carriers, initiated by proposing a PAH model [5,6], can be implemented in an efficient way. Here fresh results on the vibrational origin of the spectacular UIE bands based on astrochemistry molecular models, explored through the lens of the quantitative method applied to thousands of different vibrational motion matrices are discussed. These results are important in the context of proto-planetary nebulae and planetary nebulae where various molecular species have been uncovered despite their harsh environments.

Morphological study of the nested planetary nebula Hubble 12

“…4, three UIE features at 6.2, 8.3, and 8.6 µm are visible in the wavelength coverage (6.0 -9.0 µm). Normal vibrational analysis demonstrated quantitatively that the appearance of 6.2 µm band is the result of the couplings between the stretching vibration of aromatic sp 2 C-C and in-plane bending of aromatic sp 2 C-H bonds (Sadjadi & Parker 2018). This coupling has a profound effect on the IR signature of organic carbon-based molecules.…”

“…The feature around 11.2 µm is usually attributed to out-of-plane bending mode vibrations of sp 2 C-H bonds, which can be either part of aromatic or olefinic fragments. In the case of honeycomb PAH molecules, this type of vibration can cover a wide wavelength range, starting at 10.5 to 14.0 µm (Sadjadi & Parker 2018).…”

Morphological study of the nested planetary nebula Hubble 12

“…4, three UIE features at 6.2, 8.3, and 8.6 µm are visible in the wavelength coverage (6.0-9.0 µm). Normal vibrational analysis demonstrated quantitatively that the appearance of 6.2 µm band is the result of the couplings between the stretching vibration of aromatic sp 2 C-C and in-plane bending of aromatic sp 2 C-H bonds (Sadjadi & Parker 2018). This coupling has a profound effect on the IR signature of organic carbonbased molecules.…”

“…The basic bond types responsible for 8.3 and 8.6 µm are similar to that for 6.2 µm. The difference is the degree of coupling between two types of vibrations (Sadjadi & Parker 2018). Both bands are essentially assigned to the pure in-plane bending mode vibration of aromatic sp 2 C-H bonds.…”

“…The theoretical study of a large number of vibrational normal modes in complex hydrocarbon species (without any heteroatoms) has provided a grand picture of the vibrational origin of UIE bands (Sadjadi & Parker 2018). These are bonding (6-14.4 µm) and skeleton (beyond 14.4 µm) origin vibrations (Sadjadi & Parker 2018).…”