VIBRATIONS OF S1 ((1)B(2U)) P-DIFLUOROBENZENE-D(4) - S1-S0 FLUORESCENCE SPECTROSCOPY AND ABINITIO CALCULATIONS

The S1(B-1(2u))-S0(1Ag)) spectroscopy of p-difluorobenzene-d4 (C6D4F2,pDFB-d4) cooled in a supersonic free jet expansion has been characterized by fluorescence excitation (FE) and dispersed fluorescence. The 0(0)0 band lies at 36 987 +/- 1.0 cm-1 (vacuum). The FE band assignments have provided the values of 11 S1 fundamentals. The pattern of Franck-Condon allowed vibrational activity and of vibronically induced transitions is similar to that of pDFB-h4. An exception concerns the a(g) mode v4 that is a prominent source of progressions in d4 on account of harmonic mode scrambling but inactive in h4. Bands involving an out-of-phase mode (v8) that undergoes large frequency change in many aromatics upon formation of rare gas van der Waals complexes have anomalously large intensity when S1 overtones are excited but normal Franck-Condon intensity for S0 overtones. This singular behavior occurs in both h4 and d4. Four Fermi resonances have been detected. A compilation of S1 and S0 fundamentals for both molecules identifies those cases where correlation between S1 and S0 modes is provided by the Franck-Condon mapping of dispersed fluorescence. The v(i)'/v(i)'' ratios for such modes match closely in h4 and d4. This empirical correlation is also observed to hold in the frequencies calculated with ab initio theory. The correlation may be used to estimate (+/-5%) the values of all but six S1 d4 fundamentals that remain unobserved in the spectroscopy. Ab initio as well as semiempirical MOPAC calculations have predicted the structure, the vibrational frequencies, and the normal-mode coordinates in both states of both molecules. All predict pDFB to be planar with D2h symmetry in both states. The usual ab initio scaling to replicate S0 frequencies holds well (+/-5%) also for the S1 frequencies. Several cases of mode mixing appear in comparisons of S1 with S0 motions and of h4 with d4 motions. Other modes are predicted to interchange identities upon electronic excitation. The discovery of this mode switching also reveals that one in-plane C-C stretching mode undergoes a 50% increase in frequency with electronic excitation.