We have measured the infrared absorption spectrum of C(6)H(5), /X (2)A(1), in an Ar matrix at 10 K. The experimental frequencies (cm(-)(1)) and polarizations follow. a(1) modes: 3086, 3072, 3037, 1581, 1441, 1154, 1027, 997, 976, 605; b(1) modes: 972, 874, 706, 657, 416; b(2) modes: 3071, 3060, 1624, 1432, 1321, 1283, 1159, 1063, and 587. Three different methods have been used for the production of the phenyl radicals. Infrared absorption spectra of five deuterated isotopomers, C(6)D(5), p-C(6)H(4)D, p-C(6)HD(4), o-C(6)H(4)D, and m-C(6)H(4)D, were recorded to compare experimental frequency shifts with calculated (UB3LYP/cc-pVDZ) harmonic frequency shifts. The use of CO(2) or NO as internal standards enabled the experimental determination of absolute infrared intensities. The linear dichroism was measured with photooriented samples to establish experimental polarizations of each vibrational band. True gas-phase vibrational frequencies were estimated by considering the gas-to-matrix shifts and matrix inhomogeneous line broadening. The phenyl radical matrix frequencies listed above are within +/-1% of the gas-phase vibrational frequencies. The C(6)H(5) frequencies from this paper supersede our earlier values reported in J. Am. Chem. Soc. 1996, 118, 7400-7401. See also: http://ellison.colorado.edu/phenyl.
The conformational isomerization dynamics of N-acetyl tryptophan methyl amide (NATMA) and N-acetyl tryptophan amide (NATA) have been studied using the methods of IR-UV hole-filling spectroscopy (HFS) and IR-induced population transfer spectroscopy (IR-PTS), which were developed for this purpose. Single conformations of these molecules were selectively excited in well-defined NH stretch fundamentals. This excess energy was used to drive conformational isomerization. By carrying out the infrared excitation early in a supersonic expansion, the excited molecules were recooled into their zero-point levels, partially refilling the hole created in the ground state population of one of the conformers, and creating gains in population in other conformers. These changes in population were detected using laser-induced fluorescence downstream in the expansion. In HFS, the IR wavelength is fixed and the UV laser tuned in order to determine where the population went following selective infrared excitation. In IR-PTS, the UV is fixed to monitor the population of a given conformation, and the IR is tuned to record the IR-induced changes in the population of the monitored conformer. Besides demonstrating the capability of the experiment to change the downstream conformational population distribution, the IR-PTS scans were used to extract two quantitative results: (i) The fractional populations of the conformers in the absence of the infrared, and (ii) the isomerization quantum yields for each of the six unique amide NH stretch fundamentals (three conformers each with two amide groups). The method for obtaining quantum yields is described in detail. In both NATMA and NATA, the quantum yields show modest conformational specificity, but only a hint of vibrational mode specificity. The prospects for the hole-filling technique for providing insight into energy flow in large molecules are discussed, leaving a more detailed theoretical modeling to the adjoining paper [Evans et al. J. Chem. Phys. 120, 148 (2004)].
The phenyl radical (C 6 H 5 ) is an important species in organic chemistry and combustion processes. [1][2][3] Despite numerous attempts, spectroscopic characterization of C 6 H 5 is far from complete, due to its high reactivity. The energy of this radical has recently been 4 reported [∆ f H 0 (C 6 H 5 ) ) 84.3 ( 0.6 kcal mol -1 ] and is derived from the bond energy of benzene [D 0 (C 6 H 5 -H) ) 112.0 ( 0.6 kcal mol -1 ]. In this paper we report the infrared absorption spectrum of the phenyl radical in an argon matrix at 12 K and propose assignments for the frequencies and intensities
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