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Vibrations of some aromatic molecules Kydd, Ronald Andrew

Abstract

The infrared spectra of single crystals of naphthalene-d₈, anthracene-h₁₀ and anthracene-d₁₀ are reported with the plane of the incident radiation parallel to all three principal optical directions of the crystals. The polarized measurements extend down to 50 cm⁻¹and all low-energy infrared-active molecular vibrations and many lattice vibrations have been assigned. The laser-excited Raman spectra of single crystals of naphthalene-d₈ and anthracene-d₁₀ also are reported and this data was supplemented by depolarization ratio measurements from solution and from the melt. With the new information available from these studies a re-evaluation of the assignments of the molecular fundamentals of these three molecules has been made. When the list of fundamentals was as complete as possible, attention was directed to the force fields. The out-of-plane field of benzene was reconsidered, and the assumption that interaction constants should be as small as possible was completely supported. Transfer of these force constants to naphthalene was successful; however, it proved to be impossible to fit all the observed non-planar frequencies of anthracene with the force constants developed for benzene. An in-plane modified valence force field designed for benzene was extended to naphthalene and anthracene and refined to fit simultaneously the observed frequencies of all three molecules and their three perdeuterated analogues. The results were compared with the results of a similar calculation carried out by Neto, Scrocco and Califano and presented elsewhere, and certain differences were noted, particularly in the anthracene-h₁₀ and -d₁₀ B₂[subscript u] ring modes. In order to find out how well force fields developed for these molecules would transfer to related but less similar molecules, the vibrations of pyrene and acenaphthene were considered. The infrared spectra of single crystals of pyrene-h₁₀, pyrene-d₁₀ and acenaphthene were measured, with emphasis on the low-frequency regions not previously studied. The data obtained were supplemented by Raman measurements carried out by others in this laboratory and fairly complete assignments of the normal vibrations were possible. The fundamental frequencies of these molecules were calculated with force fields synthesized from the two planar fields mentioned earlier and from the out-of-plane fields of benzene and (for acenaphthene) cyclopentane. Comparison of the observed and calculated frequencies showed that although some fairly large discrepancies did arise, they were few in number and located only in the region of the ring stretching vibrations (above about 1200 cm⁻¹) . The fit to the fundamentals below that energy was most encouraging, and indicated that the transfer of force constants from one molecule to another in order to calculate approximate frequencies was certainly possible.

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