Search

The torsional potential energy surface (TPES) for internal rotation of the CHO group in glyoxal CHOCHO has been derived experimentally by fitting observed energies for the torsional vibration ν7 in both trans- and cis-glyoxal to those calculated with the hindered rotation formalism of Lewis, Malloy, Chao, and Laane. The experimental energies were obtained from S1 → S0 single vibronic level fluorescence (SVLF) spectra of jet-cooled glyoxal. SVLF from the trans levels 73 and 74 plus the cis levels 00 and 51 yield the torsional vibrational energies of all 7n trans levels with n≤14 (except for 713) and that of the cis level 72. The energies of odd trans 7n levels with n≥7 as well as spectroscopic values of any cis 7n level were previously unknown. The best derived TPES fits the observed trans and cis levels to within 0.4 cm-1 except for two cases where the mismatch is 1.0 cm-1. The TPES is defined by the potential energy 2V=∑6n=1Vn (1-cos n[lowercase_phi_synonym]) where [lowercase_phi_synonym] is the torsional angle. For the best TPES, coefficients are (in cm-1) V1=1719.4, V2=1063.5, V3=-53.2, V4=-81.9, V5=21.3 and V6=2.9. For this TPES, the energy separation between the trans and cis potential wells is 1688 cm-1, the barrier to trans → cis internal rotation is 2077 cm-1 and the barrier to cis → trans rotation is 389 cm-1. As one moves from the trans well at [lowercase_phi_synonym]=0°, the torsional barrier occurs at 110°. [ABSTRACT FROM AUTHOR]

The method of chemical timing for obtaining picosecond time-resolved fluorescence spectra has been applied to the measurement of intramolecular vibrational redistribution (IVR) in S1 p-difluorobenzene. In this report, a more detailed examination of the spectroscopy of some vibrationally mixed levels is used in conjunction with the previously measured IVR parameters to construct a generalized picture of the rovibronic level structure giving rise to IVR. Strong, off-resonance interactions are observed to mix levels separated by several cm-1, contributing additional discrete structure to the spectrum but not directly affecting the IVR dynamics. The dynamics, instead, reflect on-resonance interactions between levels separated only by a few tenths of a wave number that give rise to congested background emission. Evidence is seen in comparisons between bulb and jet spectra for the involvement of vibration-rotation coupling in both the off-resonance and the on-resonance interactions. A particularly important and subtle rotational effect is the change of the coupled vibrational field with rotational excitation due to the variation of rotational constants with vibrational excitation, leading to a smoother congested background. [ABSTRACT FROM AUTHOR]