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In Titan's atmosphere, photochemical pathways that lead to nitrogen heteroaromatics may incorporate photoisomerization of their structural isomers as a final step. (E)- and (Z)-phenylvinylnitrile ((E)- and (Z)-PVN, C6H5--CH==CHCN) are structural isomers of quinoline that themselves possess extensive absorptions in the ultraviolet, and thus may engage in such photoisomerization pathways. The present study explores the vibronic spectroscopy and photo-induced isomerization of gas-phase (E)- and (Z)-PVN in the 33,600-35,850 cm-1 region under jet-cooled conditions. The S0-S1 origins for (E)- and (Z)-PVN have been identified at 33 827 cm-1 and 33 707 cm-1, respectively. Isomer-specific UV-UV hole-burning and UV depletion spectra reveal sharp vibronic structure that extends over almost 2000 cm-1, with thresholds for fast non-radiative decay identified by a comparison between hole-burning and UV depletion spectra. Dispersed fluorescence spectra of the two isomers enable the assignment of many low frequency transitions in both molecules, aided by harmonic frequency calculations (B3LYP/6-311++G(d,p)) and a comparison with the established spectroscopy of phenylvinylacetylene, the ethynyl counterpart to PVN. Both isomers are proven to be planar in both the S0 ground and S1 electronic excited states. (E)-PVN exhibits extensive Duschinsky mixing involving out-of-plane modes whose frequencies and character change significantly in the ππ* transition, which modulates the degree of single- and double-bond character along the vinylnitrile substituent. This same mixing is much less evident in (Z)-PVN. The spectroscopic characterization of (E)- and (Z)-PVN served as the basis for photoisomerization experiments using ultraviolet hole-filling spectroscopy carried out in a reaction tube affixed to the pulsed valve. Successful interconversion between (E) and (Z)-PVN was demonstrated via ultraviolet hole-filling experiments. Photoexcitation of (E)- and (Z)-PVN at their respective S0-S1 origins failed to produce quinoline, a simple polycyclic aromatic nitrogen heterocylcle, within the detection sensitivity of our experiments. Stationary points along the potential energy surface associated with (Z)-PVN quinoline isomerization showed a barrier of 93 kcal/mol associated with the first step in the isomerization process, slowing the interconversion process at the excitation energies used (96 kcal/mol) to timescales beyond those probed in the present experiment. [ABSTRACT FROM AUTHOR]

The alkyl and aromatic CH stretch infrared spectra of inden-2-ylmethyl (I2M, C10H9) and trihydronaphthyl (THN, C10H11) radicals have been recorded under jet-cooled conditions in the ground (D0) and first electronically excited (D1) states using resonant ion-dip infrared (RIDIR) spectroscopy. Previously, the vibronic spectroscopy of a series of C10H9 and C10H11 hydronaphthyl radicals were investigated and their thermochemical properties were evaluated with isomer specificity [J. A. Sebree et al., J. Phys. Chem. A 11, 6255-6262 (2010)]. We show here that one of the m/z 129 spectral carriers characterized in that work was misidentified as 2-hydronaphthyl (2-HN) radical, appearing in a discharge of 1,2-dihydronaphthalene in close proximity to 1-hydronaphthyl radical. The D0-RIDIR spectrum in the alkyl CH stretch region positively identifies the m/z 129 isomer as I2M, whose two-color resonant two-photon ionization (2C-R2PI) spectrum was recently reported by Schmidt and co-workers [T. P. Troy et al., Chem. Sci. 2, 1755-1765 (2011)]. Here, we further characterize the I2M and THN radicals by recording their gas phase IR spectra in the alkyl and aromatic CH stretch regions, and explore the spectroscopic consequences of electronic excitation on the CH stretch absorptions. A local-mode CH stretch Hamiltonian incorporating cubic stretch-bend coupling between anharmonic CH stretches and CH2 scissor modes is utilized to describe their Fermi resonance interactions. Excellent agreement between the experimental and theoretical results facilitates the interpretation of the D0- and D1-state RIDIR spectra of I2M, revealing that upon excitation the alkyl CH stretches decrease in frequency by 70 cm-1, while the allyl-like CH stretches experience a modest blueshift. In comparison, the photophysics of THN are strikingly different in that the IR transitions that possess vibrational motion along the CβH and CδH bonds are absent in the D1-RIDIR spectrum yet are predicted to be present from the theoretical model. Several hypotheses are considered to account for the perturbations to these vibrations. [ABSTRACT FROM AUTHOR]

Titan, a moon of Saturn, has a nitrogen- and methane-rich atmosphere that is similar to prebiotic earth, and is replete with organic nitriles. Pentenenitriles have not yet been detected in Titan's atmosphere or in molecular clouds, but are potential precursors to hetero-aromatic compounds such as pyridine. We performed broadband microwave studies in the 8–18 GHz range on the trans isomer of 3-pentenenitrile (3-PN) and 4-pentenenitrile (4-PN) under jet-cooled conditions. Strong-field coherence breaking (SFCB) was used to selectively modulate the intensities of microwave transitions in a conformer-specific manner for 3-PN, aiding analysis. Two conformers of 3-PN and five conformers of 4-PN were identified and the rotational transitions were assigned. Evidence for methyl internal rotation splitting was observed for both the conformers of 3-PN, and the barrier heights of both conformers was determined experimentally. Comparison is made of the conformational preferences, stability and isomerization barriers through the acquired rotational spectra and potential energy surface (PES) calculations of the structural isomers 3-PN and 4-PN. [ABSTRACT FROM AUTHOR]

The ultraviolet spectroscopy of isoelectronicpair para-diisocyanobenzene (pDIB)and para-isocyanobenzonitrile (pIBN) has been studied undergas-phase, jet-cooled conditions. These molecules complete a sequenceof mono and disubstituted nitrile/isonitrile benzene derivatives,enabling a comparison of the electronic effects of such substitution.Utilizing laser-induced fluorescence (LIF) and resonant two-photonionization (R2PI) spectroscopy, the S0–S1electronic origins of pDIB and pIBN have been identified at 35 566 and 35 443 cm–1, respectively. In pDIB, the S0–S1origin is very weak, with b3gfundamentals induced by vibronic coupling to the S2statedominating the spectrum at 501 cm–1(ν17, isocyano bend) and 650 cm–1(ν16, ring distortion). The spectrum extends over 5000 cm–1, remaining sharp and relatively uncongested overmuch of this range. Dispersed fluorescence (DFL) spectra confirm thedominating role played by vibronic coupling and identify Franck–Condonactive ring modes built off the vibronically-induced bands. In pDIB, the S2state has been tentatively observedat about 6100 cm–1above the S0–S1origin. In pIBN, the S0–S1origin is considerably stronger, but vibronic coupling stillplays an important role, involving fundamentals of b2symmetry.The bending mode of the nitrile group dominates the vibronically-inducedactivity. Calculations carried out at the TD-DFT B3LYP/6-31+G(d) levelof theory account for the extremely weak S0–S1oscillator strength of pDIB and the largerintensity of the S0–S1origins of pIBN and pDCB (para-dicyanobenzene)as nitrile groups are substituted for isonitrile groups. In pDIB, a nearly perfect cancellation of transition dipolesoccurs due to two one-electron transitions that contribute nearlyequally to the S0–S1transition. Thespectra of both molecules show no clear evidence of charge-transferinteractions that play such an important role in some cyanobenzenederivatives. [ABSTRACT FROM AUTHOR]