POLARIZATION SPECTROSCOPY AND AB-INITIO STUDY OF PHOTOORIENTED MATRIX-ISOLATED ISOTOPOMERS OF PHTHALIC-ANHYDRIDE - ASSIGNMENT OF THE INFRARED-ABSORPTION SPECTRUM

In this paper we provide an assignment of symmetries for all transitio ns observed in the infrared spectrum of phthalic anhydride (1a), based on data determined from polarization studies of photooriented 1a. Pho toselection, using two electronic transitions in phthalic anhydride 1a , at 34083 and 40650 cm(-1) and of A(1) and B-2 symmetry, respectively , produced two differently oriented uniaxial samples of the anhydride immobilized in neon and other noble-gas matrices. Subsequent polarizat ion measurements in the infrared region, combined with polarized Raman data, allowed a complete determination of transition moment direction s for all observed fundamental vibrations. To facilitate definitive sp ectral assignments, we studied three isotopomers of 1a: 1,2-C-13-label ed phthalic anhydride (1b), the perdeuterated phthalic anhydride (1c), and the alpha-C-13-labeled compound (1d). Absolute infrared absorptio n intensities were determined for all observed transitions utilizing a bsolute infrared absorption intensities of CO and CO2, formed in equim olar amounts with o-benzyne (2) in phototransformations of 1, Interpre tation of the experimental results and the final assignments were aide d by quantum mechanical modeling at the SCF/6-31G* level.