Christian Ratzer

The structure of phenol in the S1-state determined by high resolution UV-spectroscopy

Abstract The structure of phenol in the electronically excited S1-state has been examined by rotationally resolved UV-spectroscopy of different isotopomers of phenol. The geometry has been fit to the inertial parameters of 12 isotopomers, using different pseudo-Kraitchman fitting strategies. The resulting r0, rs, rm(1), and rm(2) structures, which differ in the amount of consideration of vibrational effects, will be compared among one another as well as to the results of published ab initio studies. The geometry of the -COH substructure has been determined separately for both electronic states by applying Kraitchman’s equations. Independent of the fitting strategy we found a shortening of the CO bond, an increase of the OH bond length and an expansion of the aromatic ring upon electronic excitation. The internal rotation of the hydroxy group causes line splittings that could be observed in the case of the OH species, but remained unresolved for all OD isotopomers. The S1-state lifetimes of the different isotopomers are shown to depend mainly on the presence of the OH function and depend less on the exchange of CH by CD. Thus, the OH stretching mode is most likely the dominant accepting mode, responsible for the rapid internal conversion in phenol.

The structure of 4-methylphenol and its water cluster revealed by rotationally resolved UV spectroscopy using a genetic algorithm approach

The structure of 4-methylphenol (p-cresol) and its binary water cluster have been elucidated by rotationally resolved laser-induced fluorescence spectroscopy. The electronic origins of the monomer and the cluster are split into four sub-bands by the internal rotation of the methyl group and of the hydroxy group in case of the monomer, and the water moiety in case of the cluster. From the rotational constants of the monomer the structure in the S1 state could be determined to be distorted quinoidally. The structure of the p-cresol-water cluster is determined to be trans linear, with a O-O hydrogen bond length of 290 pm in the electronic ground state and of 285 pm in the electronically excited state. The S1-state lifetime of p-cresol, p-cresol-d1, and the binary water cluster have been determined to be 1.6, 9.7, and 3.8 ns, respectively.

Determination of the structure of 7-azaindole in the electronic ground and excited state using high-resolution ultraviolet spectroscopy and an automated assignment based on a genetic algorithm

Abstract The rotationally resolved electronic spectra of four different isotopomers of 7-azaindole (IH-pyrrolo(2,3-b)pyridine) have been measured in order to obtain the geometric structure in the electronic ground and excited state. The electronic origins of the rotationally resolved UV spectra overlap strongly and an assigned fit to single rovibronic lines is hardly possible. We performed an automatized fit based on the genetic algorithm to assign all four spectral components simultaneously and extract the molecular constants. The resulting inertial parameters were used to determine the structure of 7-azaindole in the ground and electronically excited state. It was found that the pyridine moiety expands on electronic excitation, while the pyrrole ring showed only minor geometric changes. From the hybrid-type spectra of three isotopomers, the direction of the Ã∣A′(ππ*)– ∣A′ transition dipole moment for the transition was found to be −21°. Evaluation of the individual line shapes yielded an excited state lifetime of 2.55 ns for 7-azaindole.

A genetic algorithm based determination of the ground and excited (1Lb) state structure and the orientation of the transition dipole moment of benzimidazole

The structure of benzimidazole has been determined in the electronic ground and excited states using rotationally resolved electronic spectroscopy. The rovibronic spectra of four isotopomers and subsequently the structure of benzimidazole have been automatically assigned and fitted using a genetic algorithm based fitting strategy. The lifetimes of the deuterated isotopomers have been shown to depend on the position of deuteration. The angle of the transition dipole moment with the inertial a-axis could be determined to be -30 degrees. Structures and transition dipole moment orientation have been calculated at various levels of theory and were compared to the experimental results.

Structure of 4-fluorophenol and barrier to internal -OH rotation in the S1-state

The structure and barrier to internal rotation of 4-fluorophenol in the ground state and the electronically excited S1-state has been examined by resonantly enhanced two photon ionization spectroscopy and by rotationally resolved laser induced fluorescence spectroscopy of 4-fluorophenol and 4-fluorophenol-d1. The rotationally resolved spectrum of the electronic origin of 4-fluorophenol is comprised of two subbands, which are split by 174.1 ± 0.5 MHz. From the splitting, determined from the HRLIF and several torsional bands observed in the R2PI spectrum an excited state barrier for the internal rotation of the hydroxy group of 1819.0 ± 5 cm−1 was calculated. The subtorsional splitting of 4-fluorophenol-d1 could not be resolved. The experimentally determined structural parameters from a fit to the rotational constants and the barrier to internal rotation in both electronic states are compared to the results of ab initio calculations. The molecule shows quinoidal distortion upon electronic excitation, with a shortening of both the C–O and the C–F bonds.

Rotational isomers of hydroxy deuterated o- and m-cresols studied by ultraviolet high resolution experiments

The laser induced fluorescence spectra of several torsional transitions of the S1 <-- S0 electronic transition were recorded for the hydroxy deuterated o- and m-cresols. Both cis and trans rotamers were observed in a high resolution molecular beam experiment. The spectra were analyzed using a genetic algorithm assisted automatic assignment. The Hamiltonian used included rotational, torsional and rotation-torsion components. Both, high resolution rotationally resolved spectra and low resolution torsional frequencies, were combined to obtain the rotational constants, the direction of the methyl group axis, and the V3 and V6 barriers to internal rotation of the methyl top in the ground (S0) and excited (S1) states. The lifetime of the S1 state is also reported. Quantum interference effects due to the interaction of the internal and overall rotation allowed for determination of the absolute sign of the angle between transition moment and the a principal axis.