High resolution spectroscopy of jet cooled phenyl radical: The nu(1) and nu(2) a(1) symmetry C-H stretching modes

摘要

A series of CH stretch modes in phenyl radical (C6H5) has been investigated via high resolution infrared spectroscopy at sub-Doppler resolution (similar to 60 MHz) in a supersonic discharge slit jet expansion. Two fundamental vibrations of a(1) symmetry, nu(1) and nu(2), are observed and rotationally analyzed for the first time, corresponding to in-phase and out-of-phase symmetric CH stretch excitation at the ortho/meta/para and ortho/para C atoms with respect to the radical center. The nu(1) and nu(2) band origins are determined to be 3073.968 50(8) cm(-1) and 3062.264 80(7) cm(-1), respectively, which both agree within 5 cm(-1) with theoretical anharmonic scaling predictions based on density functional B3LYP/6-311g++(3df,3dp) calculations. Integrated band strengths for each of the CH stretch bands are analyzed, with the relative intensities agreeing remarkably well with theoretical predictions. Frequency comparison with previous low resolution Ar-matrix spectroscopy [A. V. Friderichsen et al., J. Am. Chem. Soc. 123, 1977 (2001)] reveals a nearly uniform Delta nu approximate to + 10-12 cm(-1) blue shift between gas phase and Ar matrix values for nu(1) and nu(2). This differs substantially from the much smaller red shift (Delta nu approximate to -1 cm(-1)) reported for the nu(19) mode, and suggests a simple physical model in terms of vibrational mode symmetry and crowding due to the matrix environment. Finally, the infrared phenyl spectra are well described by a simple asymmetric rigid rotor Hamiltonian and show no evidence for spectral congestion due to intramolecular vibrational coupling, which bodes well for high resolution studies of other ring radicals and polycyclic aromatic hydrocarbons. In summary, the combination of slit jet discharge methods with high resolution infrared lasers enables spectroscopic investigation of even highly reactive combustion and interstellar radical intermediates under gas phase, jet-cooled (T-rot approximate to 11 K) conditions.