A detailed study of the photochemical and discharge-driven pathways taken by gas-phase 1,3-butadiene has been carried out. Photolysis or discharge excitation was initiated inside a short reaction tube attached to the outlet of a pulsed valve. Bath gas temperatures near 100 K were achieved in the reaction tube by the constrained expansion of the gas mixture into the tube, simulating temperatures of relevance in Titan's atmosphere. Photolysis of 1,3-butadiene was initiated at 218 nm with a laser pulse that counter-propagated the reaction tube. Discharge excitation was carried out using discharge electrodes imbedded in the reaction tube walls, enabling the study of the photochemical and discharge products under similar conditions. Products were detected using either single-photon VUV photoionization (118 nm = 10.5 eV) or resonant two-photon ionization (R(2)PI) spectroscopy in a time-of-flight mass spectrometer. Emphasis was placed on characterization of the aromatic products formed, since these may be of particular relevance to Titan's atmosphere, where benzene has been positively identified and 1,3-butadiene is projected as the principle pathway to its formation. Consistent with previous studies of the photodissociation of 1,3-butadiene, C(3)H(3) + CH(3) is the dominant primary product formed. Under the temperature-pressure conditions present in the reaction tube (T approximately 75-100 K, P = 50 mbar), C(6)H(6) is the dominant secondary photochemical product formed. A 1:1 C(4)H(6):C(4)D(6) mixture was used to prove that the C(6)H(6) product was formed by recombination of two C(3)H(3) radicals; however, a careful search for benzene revealed none, indicating that less than 1% of the C(6)H(6) formed in the reaction tube is benzene. This is consistent with expectations for these temperatures and pressures based on previous modeling of propargyl recombination. Two aromatic products were observed from the photochemistry: ethylbenzene and 3-phenylpropyne. Plausible pathways leading to these products are proposed. In the discharge, C(3)H(3) + CH(3) are also identified as significant primary neutral products and C(6)H(6) as a dominant higher-mass product. In this case, the C(6)H(6) was identified as benzene via its R2PI spectrum, appearing with intensity about 10 times larger than any other aromatic formed in the discharge. R2PI spectra of a total of about 15 aromatic products were recorded from the 1,3-butadiene discharge, among them toluene; styrene; phenylacetylene; o-, m-, and p-xylene; ethylbenzene; indane; indene; beta-methylstyrene; and naphthalene. Previously unidentified spectra in the m/z 142 and 144 mass channels were positively identified as the 1,3- and 1,4-isomers of phenylcyclopentadiene and the analogous 1-phenylcyclopentene.

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An analysis of the 1:1 complex of furan and water is presented. In this study, computation and matrix isolation FTIR were used to determine stable complexes of furan:water. Density functional theory and Møller-Plesset second-order, perturbation theory calculations found four, unique geometries for the complex. Two complexes were characterized by C-H···O interactions, one complex was characterized by O-H···O, and the fourth complex was characterized by O-H···π. Optimizations completed using B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 showed the most stable species to be bound by O-H···O interactions. Matrix isolation experiments of mixtures of furan and water held in nitrogen at 15 K showed evidence of stable complexes when probed by FTIR. These signatures grew in intensity when matrices were annealed at 30 K. These vibrational features were predominately associated with perturbation of the water monomer. Additionally, the spectra of complexes containing water isotopologues were recorded. Analysis of spectral features pointed to the presence of a single geometry formed in the matrix, which is best described as a 1:1 complex stabilized by a O-H···O interaction.

Near-pure samples of (E)-phenylvinylacetylene ((E)-PVA) and (Z)-phenylvinylacetylene ((Z)-PVA) were synthesized, and their ultraviolet spectroscopy was studied under jet-cooled conditions. The fluorescence excitation and UV-UV holeburning (UVHB) spectra of both isomers were recorded. The S0-S1 origin of (E)-PVA occurs at 33,578 cm(-1), whereas that for (Z)-PVA occurs at 33,838 cm(-1), 260 cm(-1) above that for (E)-PVA. The present study focuses primary attention on the vibronic spectroscopy of (E)-PVA. Single vibronic level fluorescence spectra of many prominent bands in the first 1200 cm(-1) of the S0-S1 excitation spectrum of (E)-PVA were recorded, including several hot bands involving low-frequency out-of-plane vibrations. Much of the ground-state vibronic structure observed in these spectra was assigned by comparison with styrene and trans-beta-methylstyrene, assisted by calculations at the DFT B3LYP/6-311++G(d,p) level of theory. Both S0 and S1 states of (E)-PVA are shown to be planar, with intensity appearing only in even overtones of out-of-plane vibrations. Due to its longer conjugated side chain compared with that of its parent styrene, (E)-PVA supports extensive Duschinsky mixing among the four lowest-frequency out-of-plane modes (nu45-nu48), increasing the complexity of this mixing relative to that of styrene. Identification of the v'' = 0-3 levels of nu48, the lowest frequency torsion, provided a means of determining the 1D torsional potential for hindered rotation about the C(ph)-C(vinyl) bond. Vibronic transitions due to (Z)-PVA were first identified as small vibronic bands that did not appear in the UVHB spectrum recorded with the hole-burn laser fixed on the S0-S1 origin of (E)-PVA. The LIF and UVHB spectra of a synthesized sample of (Z)-PVA confirmed this assignment.

(E)-Phenylvinylacetylene was shown previously (C.-P. Liu, J. J. Newby, C. W. Müller, H. D. Lee, T. S. Zwier, J. Phys. Chem. A, 2008, 112, 9454.) to support extensive Duschinsky mixing among its four lowest-frequency out-of-plane normal coordinates Q(45)-Q(48) in the S(1)<-- S(0), i.e. (L(a)) A (1)A'<-- X (1)A', electronic transition. The complexity of this mixing is considerably increased relative to that of its parent styrene due to the longer conjugated side chain. Here we quantitatively analyze this change of the motional character of the four non-totally symmetric vibrations upon electronic excitation. The peak intensities of 182 overtone and combination transitions spread over seven SVLF spectra were fit simultaneously with seven parameters in an automated least-squares fitting procedure in which an unweighted least-squares sum was minimized using a pattern search algorithm. The seven parameters consisted of the six Duschinsky rotation angles and the S(1) frequency of normal mode nu(48). The required four-dimensional Franck-Condon overlap integrals were calculated using previously reported recursion relations between harmonic oscillator wavefunctions. As a consistency check, the intensities of all possible 434 electric dipole allowed overtone and combination bands of normal modes nu(45)-nu(48) up to individual vibrational quantum numbers of v = 4 were simulated. The comparison with the experimental intensities revealed with few exceptions very good agreement. The results of the Duschinsky analysis are discussed in light of the pi-pi* electronic excitation as represented by different ab initio (HF, CIS, CASSCF), density functional (B3LYP and BP86) and time-dependent density functional (TD-B3LYP and TD-BP86) methods. Our Duschinsky mixing analysis reveals a challenging complexity that is not quantitatively reproduced by widely used excited state quantum chemical methods. The sensitivity of Duschinsky mixing coefficients to both excited state equilibrium geometries and force fields thus provides a valuable benchmark for the improvement of excited state quantum chemical methods.