VIBRATIONAL PREDISSOCIATION DYNAMICS OF P-DIFLUOROBENZENE-N-2 COMPLEXES - COMPARISON WITH P-DIFLUOROBENZENE-AR

Abstract

The S-1 vibrational predissociation (VP) dynamics and physical characteristics of the p-difluorobenzene-N-2 (pDFB-N-2) van der Waals complex are reported. The geometry of the complex is proposed to be similar to the geometries of pDFB-Ar, benzene-Ar, and benzene-N-2, with the N-2 about 3.5 Angstrom above the center of the pDFB ring, and, by analogy to the benzene-N-2 complex, with the N-2 nearly freely rotating parallel to the aromatic molecular plane. Upper limits to the S-1 and S-0 van der Waals binding energies of D-0' less than or equal to 240 cm(-1) and D-0'' less than or equal to 213 cm(-1), respectively, were obtained. Only two of the nine observed S-0 pDFB ring modes (nu(6)'' the symmetric ring stretch and nu(8)'' the out-of-plane ring puckering mode) appear to be perturbed by complexation, and these only slightly. In S-1, none of the observed ring levels appears significantly perturbed, but surprisingly, spectroscopic evidence concerning nu(8)' could not be obtained. Tn general, nu(8) (or nu(16a) in Wilson notation) is the most perturbed vibration in aromatic-rare gas van der Waals complexes. Vibrational predissociation from four initial Si ring levels lying within the first 800 cm(-1) of the pBFB-N-2 vibrational manifold was characterized using single vibronic level fluorescence spectroscopy. State-to-state dissociation from each level produces the pDFB product molecule in only a few of many accessible S-1 vibrational levels. Evidence of intramolecular vibrational redistribution (IVR) within the pDFB-N-2 complex is observed for one level. The dissociation is treated by preliminary modeling based on a serial mechanism (involving IVR within the complex followed by VP) that is related to that developed by Kelley and Bernstein [J. Phys. Chem. 1986, 90, 5164] for s-tetrazine-Ar VP. The modeling accounts for the final state selectivity and most but not all of the observed VP channels. The experimental and modeling results are compared with those of the pDFB-Ar complex whose vibrational level structure differs only modestly and in a known way from that of pDFB-N-2.