Raman study of intramolecular frequency noncoincidence effect in dialkyl benzenedicarboxylates

Raman noncoincidence effects (NCE) of the C=O stretching band of dialkyl esters of benzenedicarboxylic (phthalic, isophthalic, and terephthalic) acids were measured as a function of concentration in acetonitrile and dioxane solutions. By quenching the intermolecular vibrational interactions between the C=O groups of neighboring molecules by dilution, the NCE arising from the coupling between two C=O stretching vibrations in a single molecule of the diesters was observed for the first time. The intramolecular NCE values for these molecules were negative and dependent on the relative orientation and distance between two C=O groups in a molecule. Similarly as for most intermolecular NCE, the intramolecular NCE is explained by the transition dipole coupling mechanism. The presence of a single dipolar coupling pair allows one to express the intramolecular NCE values by a simple analytical equation with a few molecular parameters including those for the arrangement of two C=O groups in a molecule. The observed intramolecular NCE values are well predicted, with a limitation of large dipolar distance, by the equation with the geometric parameters obtained from the ab initio optimized structures. In fact, the NCEs provided information about detailed conformational structure of the benzenedicarboxylates in dilute solutions. Since the intramolecular NCE value indeed corresponds to a frequency difference between the in-phase and out-of-phase C=O stretching normal modes of a molecule, the present result suggests that the vibrational coupling between the two C=O internal coordinates in these molecules is mainly due to the transition dipolar interaction. (C) 1999 American Institute of Physics. [S0021-9606(99)00445-6].