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

We have investigated the photodecomposition of propionyl chloride, CH3CH2COCl, in an argon matrix at 10 K using FTIR absorption spectroscopy. The decomposition products formed following irradiation at 266, 254, or 248 nm are methyl ketene, CH3CHCO, and HCl; no other photoproducts are observed. We have carried out FTIR polarization studies to determine the relative orientation of the photoproducts and found that the HCl molecule is situated perpendicular to the carbonyl group in the methyl ketene. The orientation in the photoproducts and kinetic studies of the acid chloride dissociation point to a direct elimination mechanism for propionyl chloride decomposition. This is consistent with the direct mechanism proposed for acetyl chloride, CH3COCl, photodissociation in both an argon matrix and the solid crystalline form. We have calculated detailed thermodynamics for CH3COCl and CH3CH2COCl decomposition and found them consistent with the proposed elimination mechanism. We also assign the fundamental vibrational frequencies for the methyl ketene−HCl complex on the basis of ab initio calculations and polarization studies.

Transitions of two different stereoisomers of the He...ICl(X,v" = 0) weakly bound complex, one with a T-shaped orientation and another that is most likely linear, have been observed in laser-induced fluorescence experiments performed in the ICl B-X region. Here we present experimental and theoretical results aimed at confirming the previous assignments and at gaining additional insights into the He+ICl interactions. High resolution action spectra were recorded in the same region to identify those features that could be attributed to transitions of the He...I35Cl(X,v" = 0) isomers and not to higher-order complexes, Hen...I35Cl, where n > or = 2, or I37Cl containing species. Calculations of the rovibronic spectra of the He...I35Cl complexes in the ICl B-X, 2-0 and 3-0 regions were performed using an ab initio potential energy surface for the He+ICl(X,v" = 0) ground state and two different pairwise additive potentials for the He+ICl(B,v' = 2,3) excited states. The rotation-vibration energies and wave functions for the He cdots, three dots, centered I35Cl complexes were obtained for all bound states with total angular momentum J < 10 using both of these potentials. Electronic spectra were generated using these results, assuming that the transition moment lies along the ICl bond and is not perturbed by the presence of the helium atom. The calculations qualitatively reproduce the He cdots, three dots, centered I35Cl action spectrum and strongly support the previous assignments. The calculations also indicate that some of the spectral congestion observed near the linear band may be attributed to transitions of the linear isomer to multiple intermolecular levels in the excited state. Coriolis coupling strongly mixes He cdots, three dots, centered ICl(B,v') states with rotational excitation, making simulations and assignments of the linear band observed in the experimental spectrum difficult.

The ultraviolet and infrared spectra of the C6H6−(C4H2) n complexes with n = 1 and 2 have been formed and studied in a supersonic expansion using resonant two-photon ionization (R2PI), resonant ion-dip infrared spectroscopy (RIDIRS), and IR−UV hole-burning spectroscopy. A T-shaped structure is deduced for the complex, with the C4H2 centered, end on, over the benzene ring. This C−H···π H-bond shifts the frequency of the vibronic transitions of C6H6 by 161 cm-1 to the blue. The acetylenic CH stretch fundamental of C4H2 is localized and split by formation of the π H-bond. The H-bonded CH stretch fundamental is lowered in frequency by about 40 cm-1, increased in intensity by more than a factor of 2, and split by a Fermi resonance that is turned on by the complexation. The C6H6−(C4H2)2 complex has a structure that makes the S1−S0 origin transition weakly allowed and possesses an infrared spectrum that has acetylenic CH stretch absorptions due to free CH, aromatic π-bound CH, and a more weakly π-bound CH. Its structure is tentatively assigned as a cyclic, “pinwheel” structure.

The reactions of metastable diacetylene with benzene and toluene are explored using a molecular beam pumpprobe time-of-flight mass spectrometer. Diacetylene is laser-excited to the 2 1 0 6 1 0 band of the 1 ∆ u r X 1 Σ + g transition, whereupon rapid intersystem crossing occurs to the lowest triplet states. The triplet state diacetylene then reacts with either benzene or toluene as the gas mixture traverses a short reaction tube (∼20 µs). The reactions are quenched as the gas mixture expands into the ion source region of a time-of-flight mass spectrometer where the primary photoproducts are detected using vacuum ultraviolet (VUV) photoionization or resonant two-photon ionization (R2PI). The major products from the reaction of diacetylene and benzene have molecular formulas C 8 H 6 and C 10 H 6 , and are identified as phenylacetylene and phenyldiacetylene using R2PI spectroscopy. The major products from metastable diacetylene's reaction with toluene are C 9 H 8 and C 11 H 8 . The C 9 H 8 product is confirmed as a mixture of o-, m-, and p-ethynyltoluene, with the ortho product dominating. Mechanisms for the formation of the above products are proposed based on deuterium substitution studies of the reactions. The potential importance of these reactions is discussed as they relate to hydrocarbon growth in sooting flames. † Part of the special issue "C. Bradley Moore Festschrift".

Photoelectron spectra are reported for one-photon resonant, two-photon ionization of jet-cooled diacetylene via a number of vibronic levels of the 1 1 ⌬ u state. An improved value for the adiabatic ionization threshold is found to be 82 064Ϯ30 cm Ϫ1 ͑10.175Ϯ0.004 eV͒, in good agreement with the earlier result. The photoelectron spectra of different vibronic bands of the 1 1 ⌬ u state nearly all show long progressions in what appear to be low frequency bending vibrations. At energies just above the ionization threshold, the observed progressions can be understood in terms of excitation of a single Renner-Teller active mode in the ion, with Renner-Teller parameters similar to those of the 4 ϩ trans-bending mode in the ground state acetylene cation.