Rotationally resolved UV spectroscopy of the rotamers of indole-4-carboxylic acid: Evidence for charge transfer quenching.

Rotationally resolved electronic spectra of two conformational isomers of jet-cooled indole-4-carboxylic acid (I4CA) and the deuterated forms of the acid (-COOD) and amide (-ND) groups have been obtained using a UV laser/molecular beam spectrometer. The in-plane orientation of the acid group defines the two lowest energy rotamers of I4CA. The S ← S origin bands of the two rotamers and four isotopologues have been fit to asymmetric rotor Hamiltonians in both electronic states.

Rotationally resolved electronic spectroscopy of 3-cyanoindole and the 3-cyanoindole-water complex.

The rotationally resolved electronic spectra of the origin bands of 3-cyanoindole, 3-cyanoindole(d1), and the 3-cyanoindole-(H2O)1 cluster have been measured and analyzed using evolutionary algorithms. For the monomer, permanent dipole moments of 5.90 D for the ground state, and of 5.

Rotationally resolved electronic spectroscopy of the rotamers of 1,3-dimethoxybenzene.

Conformational assignments in molecular beam experiments are often based on relative energies, although there are many other relevant parameters, such as conformer-dependent oscillator strengths, Franck-Condon factors, quantum yields and vibronic couplings. In the present contribution, we investigate the conformational landscape of 1,3-dimethoxybenzene using a combination of rotationally resolved electronic spectroscopy and high level ab initio calculations. The electronic origin of one of the three possible planar rotamers (rotamer (0,180) with both substituents pointing at each other) has not been found.

Electronic spectra of 2- and 3-tolunitrile in the gas phase. II. Geometry changes from Franck-Condon fits of fluorescence emission spectra.

We determined the changes of the geometries of 2- and 3-tolunitrile upon excitation to the lowest excited singlet states from Franck-Condon fits of the vibronic intensities in several fluorescence emission spectra and of the rotational constant changes upon excitation. These structural changes can be connected to the altered electron distribution in the molecules and are compared to the results of ab initio calculations. We show how the torsional barriers of the methyl groups in both components are used as probe of the molecular changes upon electronic excitation.

Electronic spectra of 2- and 3-tolunitrile in the gas phase. I. A study of methyl group internal rotation via rovibronically resolved spectroscopy.

Rotationally resolved fluorescence excitation spectra of the origin bands in the S1 ← S0 transition of 2-tolunitrile (2TN) and 3-tolunitrile (3TN) have been recorded in the collision-free environment of a molecular beam. Analyses of these data provide the rotational constants of each molecule and the potential energy curves governing the internal rotation of the attached methyl groups in both electronic states. 2TN exhibits much larger barriers along this coordinate than 3TN.

Intramolecular structure and dynamics of mequinol and guaiacol in the gas phase: Rotationally resolved electronic spectra of their S1 states.

The molecular structures of guaiacol (2-methoxyphenol) and mequinol (4-methoxyphenol) have been studied using high resolution electronic spectroscopy in a molecular beam and contrasted with ab initio computations. Mequinol exhibits two low frequency bands that have been assigned to electronic origins of two possible conformers of the molecule, trans and cis. Guaiacol also shows low frequency bands, but in this case, the bands have been assigned to the electronic origin and vibrational modes of a single conformer of the isolated molecule.

On the excited state dynamics of vibronic transitions. High-resolution electronic spectra of acenaphthene and its argon van der Waals complex in the gas phase.

Rotationally resolved fluorescence excitation spectroscopy has been used to study the dynamics, electronic distribution, and the relative orientation of the transition moment vector in several vibronic transitions of acenaphthene (ACN) and in its Ar van der Waals (vdW) complex. The 0(0)(0) band of the S(1) ← S(0) transition of ACN exhibits a transition moment orientation parallel to its a-inertial axis. However, some of the vibronic bands exhibit a transition moment orientation parallel to the b-inertial axis, suggesting a Herzberg-Teller coupling with the S(2) state.

High resolution electronic spectroscopy of 4-methylanisole in the gas phase. Barrier height determinations for the methyl group torsional motion.

Rotationally resolved fluorescence excitation spectra of the S(1)<-- S(0) origin band transition of 4-methylanisole have been recorded in the gas phase. The origin band spectrum is split into two subbands owing to tunneling motions along the methyl group torsional coordinate. An analysis of this data provides information about the preferred configuration of the methyl group and the barrier opposing its motion in both the ground and excited electronic states.

High resolution electronic spectroscopy of o- and m-toluidine in the gas phase. barrier height determinations for the methyl group torsional motions.

High resolution electronic spectra of o- and m-toluidine have each been recorded for the S(1)<--S(0) origin band transitions of the isolated molecules. Each spectrum is split into two sub-bands owing to tunneling motions along the methyl group torsional coordinate. Analyses of these data provide information about the preferred configurations of the methyl groups and the barriers opposing their motions in both the ground and excited electronic states.

Lifetime broadening in the rotationally resolved electronic spectra of dibenzothiophene, 2,5-diphenylfuran, and 2,5-diphenyl-1,3,4-oxadiazole in the gas phase. Intersystem crossing dynamics in the statistical limit.

The fluorescence lifetime of the zero point vibrational level of the first excited electronic state of dibenzothiophene (DBT) has been determined to be 1.0 ns by analysis of its rotationally resolved S1 <-- S0 fluorescence excitation spectrum. The S1 lifetime of DBT is substantially shorter than those observed for fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF), analogs of DBT in which the heavy sulfur atom is replaced by lighter ones.

On the role of methyl torsional modes in the intersystem crossing dynamics of isolated molecules.

Rotationally resolved fluorescence excitation spectra of the 0(0)(0) bands of the S1<--S0 electronic transitions of 2- and 5-methylpyrimidine (2MP and 5MP, respectively) have been observed and assigned. Both spectra were found to contain two sets of rotational lines, one associated with the sigma=0 torsional level and the other associated with the sigma=+/-1 torsional level of the attached methyl group. Analyses of their structure using the appropriate torsion-rotation Hamiltonian yields the methyl group torsional barriers of V6''=1.

Rotationally resolved electronic spectra of 9,10-dihydrophenanthrene. A "floppy" molecule in the gas phase.

Rotationally resolved fluorescence excitation spectra of several bands in the S1<--S0 electronic spectrum of 9,10-dihydrophenanthrene (DHPH) have been observed and assigned. Each band was fit using rigid rotor Hamiltonians in both electronic states. Analyses of these data reveal that DHPH has a nonplanar configuration in its S0 state with a dihedral angle between the aromatic rings (phi) of approximately 21.

Rotationally resolved electronic spectra of 2- and 3-methylanisole in the gas phase: a study of methyl group internal rotation.

Rotationally resolved fluorescence excitation spectra of several torsional bands in the S1 <-- S0 electronic spectra of 2-methylanisole (2MA) and 3-methylanisole (3MA) have been recorded in the collision-free environment of a molecular beam. Some of the bands can be fit with rigid rotor Hamiltonians; others exhibit perturbations produced by the coupling between the internal rotation of the methyl group and the overall rotation of the entire molecule. Analyses of these data show that 2MA and 3MA both have planar heavy-atom structures; 2MA has trans-disposed methyl and methoxy groups, whereas 3MA has both cis- and trans-disposed substituents.

Rotationally resolved S1<-- S0 electronic spectra of fluorene, carbazole, and dibenzofuran: evidence for Herzberg-Teller coupling with the S2 state.

Rotationally resolved fluorescence excitation spectra of the S1 <-- S0 origin bands and higher vibronic bands of fluorene (FLU), carbazole (CAR), and dibenzofuran (DBF) have been observed and assigned. Analyses of these data show that replacement of the CH2 group in FLU with a NH group in CAR and an O atom in DBF produces only localized changes in structure, in the ground state. But the three molecules exhibit different changes in geometry when they are excited by light.