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The electronic and infrared spectra of the 5-methyl-2-furanylmethyl (MFM) radical have been characterized under jet-cooled conditions in the gas phase. This resonance-stabilized radical is formed by H atom loss from one of the methyl groups of 2,5-dimethylfuran (DMF), a promising second-generation biofuel. As a resonance-stabilized radical, it plays an important role in the flame chemistry of DMF. The D0-D1 transition was studied using two-color resonant two-photon ionization (2C-R2PI) spectroscopy. The electronic origin is in the middle of the visible spectrum (21934 cm-1 = 455.9 nm) and is accompanied by Franck-Condon activity involving the hindered methyl rotor. The frequencies and intensities are fit to a one-dimensional methyl rotor potential, using the calculated form of the ground state potential. The methyl rotor reports sensitively on the local electronic environment and how it changes with electronic excitation, shifting from a preferred ground state orientation with one CH in-plane and anti to the furan oxygen, to an orientation in the excited state in which one CH group is axial to the plane of the furan ring. Ground and excited state alkyl CH stretch infrared spectra are recorded using resonant ion-dip infrared (RIDIR) spectroscopy, offering a complementary view of the methyl group and its response to electronic excitation. Dramatic changes in the CH stretch transitions with electronic state reflect the changing preference for the methyl group orientation. [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]

The state-dependent spectroscopy of α-methylbenzyl radical (α-MeBz) has been studied under jet-cooled conditions. Two-color resonant two-photon ionization (2C-R2PI), laser-induced fluorescence, and dispersed fluorescence spectra were obtained for the D0-D1 electronic transition of this prototypical resonance-stabilized radical in which the methyl group is immediately adjacent to the primary radical site. Extensive Franck-Condon activity in hindered rotor levels was observed in the excitation spectrum, reflecting a reorientation of the methyl group upon electronic excitation. Dispersed fluorescence spectra from the set of internal rotor levels are combined with the excitation spectrum to obtain a global fit of the barrier heights and angular change of the methyl group in both D0 and D1 states. The best-fit methyl rotor potential in the ground electronic state (D0) is a flat-topped 3-fold potential (V3" = 151 cm(-1), V6" = 34 cm(-1)) while the D1 state has a lower barrier (V3' = 72 cm(-1), V6' = 15 cm(-1)) with Δφ = ± π/3, π, consistent with a reorientation of the methyl group upon electronic excitation. The ground state results are compared with calculations carried out at the DFT B3LYP level of theory using the 6-311+G(d,p) basis set, and a variety of excited state calculations are carried out to compare against experiment. The preferred geometry of the methyl rotor in the ground state is anti, which switches to syn in the D1 state and in the cation. The calculations uncover a subtle combination of effects that contribute to the shift in orientation and change in barrier in the excited state relative to ground state. Steric interaction favors the anti conformation, while hyperconjugation is greater in the syn orientation. The presence of a second excited state close by D1 is postulated to influence the methyl rotor properties. A resonant ion-dip infrared (RIDIR) spectrum in the alkyl and aromatic CH stretch regions was also recorded, probing in a complementary way the state-dependent conformation of α-MeBz. Using a scheme in which infrared depletion occurs between excitation and ionization steps of the 2C-R2PI process, analogous infrared spectra in D1 were also obtained, probing the response of the CH stretch fundamentals to electronic excitation. A reduced-dimension Wilson G-matrix model was implemented to simulate and interpret the observed infrared results. Finally, photoionization efficiency scans were carried out to determine the adiabatic ionization threshold of α-MeBz (IP = 6.835 ± 0.002 eV) and provide thresholds for ionization out of specific internal rotor levels, which report on the methyl rotor barrier in the cation state.