Wolfgang Roth

Structure and vibrations of phenol(H2O)7,8 studied by infrared-ultraviolet and ultraviolet-ultraviolet double-resonance spectroscopy and ab initio theory

The vibronic spectra of jet cooled phenol(H2O)7,8 clusters were analyzed with mass selective resonance enhanced two photon ionization (R2PI) and ultraviolet-ultraviolet spectral hole burning (UV-UV SHB). A double resonance technique with an infrared (IR) laser as burn laser (IR-UV SHB) was used to measure the intramolecular OH stretching vibrations of the mass- and isomer-selected clusters. Two isomers of phenol(H2O)7 and three isomers of phenol(H2O)8 could be distinguished via SHB and their IR spectra recorded. The red- or blueshift of the electronic origin relative to the phenol monomer gives valuable hints on the hydrogen bonding between phenol and the water moiety. All IR spectra contain four characteristic groups of OH stretching vibrations which give insight into the structure of the H bonded network. The ab initio calculations show that the minimum energy structures for phenol(H2O)7,8 are very similar to the corresponding water clusters which are based on regular (H2O)8 cubes. Comparison between ex...

Reassignment of ground and first excited state vibrations in phenol

Abstract The phenol molecule has been chosen to demonstrate how dispersed fluorescence spectroscopy can be used to assign hot bands, i.e. vibronic transitions starting from the vibrationally excited electronic ground state. The procedure outlined in this paper provides important information on fundamental out-of-plane vibrations in the electronic excited S1 state that are not observed in jet-cooled spectra. The results obtained lead to a new interpretation of the most intense hot bands in the UV absorption spectrum of phenol. We derive vibrational frequencies for the modes 10b1, 10b1 and 41 (Varsanyis nomenclature) which are different from those given in the literature.

A CASSCF study of the S0 and S1 states of phenol

Abstract Different basis sets and active spaces have been employed to determine the geometries and vibrational frequencies of phenol in its electronic ground and first excited state at the CASSCF level of theory. Good agreement with experimentally determined rotational constants was achieved using Dunnings correlation consistent cc-pVDZ basis set. The electronic origin of the S 1 ← S 0 excitation has been calculated with an accuracy of 0.1 eV considering zero-point energy corrections for both the S 0 and S 1 states. Additionally, S 1 state frequencies were calculated at the CIS/cc-pVDZ level of theory.

Paracyclophanes as model compounds for strongly interacting π-systems. Part 2: mono-hydroxy[2.2]paracyclophane

The structure of the electronic ground- and first excited state of mono-hydroxy [2.2]paracyclophane (MHPC) and the S(1)← S(0) electronic transition have been investigated by resonance-enhanced multiphoton ionisation (REMPI) and by quantum chemical spin-component-scaled-approximate coupled cluster second order (SCS-CC2) computations. The origin of the S(1)← S(0) transition was located at 30,772 cm(-1) (3.815 eV) in the REMPI spectrum. The value has to be compared with a computed excitation energy of 3.79 eV. The vibrational structure of the spectrum confirms a significant geometry change upon excitation along the coordinates corresponding to twist- and shift-motions in the molecule. It gives rise to an experimentally observed progression with a fundamental of +30 cm(-1) and an inverse anharmonicity. From the experimental data a shallow potential along the twist coordinate was derived for the S(1) state. For the shift vibration a wavenumber of +91 cm(-1) was observed, while +85 cm(-1) was computed. The ionisation energy of MHPC was determined to be 7.63 ± 0.05 eV using synchrotron radiation. When compared to earlier results on the parent compound [2.2]paracyclophane and pseudo-ortho-dihydroxy[2.2]paracyclophane it can be seen that already small variations in the substitution pattern have a significant impact on the shapes of the involved potential energy surfaces leading to strong variations in ground and excited state geometries and opto-electronic properties governing the exciton transfer processes.

Laser induced, dispersed fluorescence spectroscopy, and ab initio calculations of p-cyanophenol

The electronic origin of p-cyanophenol has been assigned to the strongest transition in the jet cooled vibronic spectrum at 35547.5 cm−1. Vibrations of the electronic ground state have been measured by means of dispersed fluorescence and those of the first electronically excited state by laser induced fluorescence. An assignment of the S0 state vibrations is presented based upon ab initio calculations at the MP2 and DFT level of theory. Additional CASSCF calculations are presented for both electronic states. The vibronic activity of several low frequency modes points towards a substantial change in geometry of the cyano group upon excitation.

Paracyclophanes as Model Compounds for Strongly Interacting π-Systems, Part 3: Influence of the Substitution Pattern on Photoabsorption Properties

The structures and energetics of the ground and first excited states of [2.2]paracyclophane (PC) and its pseudo-para- (p-DHPC) and pseudo-ortho-dihydroxy (o-DHPC) as well as monohydroxy derivates (MHPC) are investigated by quantum chemical calculations, X-ray crystallography, and resonance-enhanced multiphoton ionization spectroscopy (REMPI) in a free jet. We show that substitution of the aromatic hydrogens in PC causes significant changes of the structure and in particular its change between the ground and the excited state. The structural changes include a breathing mode as well as shift and rotation of the benzene moieties and are rationalized by the electronic structure changes upon excitation. Spin-component-scaled second-order Møller-Plesset perturbation method (SCS-MP2) reproduces the experimental X-ray structure correctly and performs significantly better than ordinary MP2 and the B3LYP methods. The parent propagation method, SCS-approximate coupled cluster second order (SCS-CC2), yields adiabatic excitation energies within 0.1 eV of the experimental values for PC and the investigated hydroxyl derivates as well as the related aromatic molecules benzene and phenol. It is shown that zero-point vibration energy corrections at the time dependent density functional (B3LYP) level are no more accurate enough for that level of theory and have to be substituted by SCS-CC2 values. While the structures of PC and o-DHPC are only slightly modified upon excitation, p-DHPC changes its structural parameters substantially. This is in line with [1 + 1] REMPI-spectra of these substances, which are interpreted with the help of Franck-Condon simulations.

Structures of 1,2,3-trihydroxybenzene in the S0 and S1 states

Abstract In this paper we report on the structure and vibrations of gaseous pyrogallol (1,2,3-trihydroxybenzene) in the electronic ground state (S 0 ) and its first electronically excited state (S 1 ). Both ab initio CASSCF/CASMP2 calculations as well as R2PI spectroscopy have been performed. From the ab initio calculations three minimum energy structures are obtained and the vibrations of two structures are observed in the R2PI spectra. The minimum energy structures differ by their OH torsional angles. The full three-dimensional potential energy surface of the coupled torsional motions is investigated and the three-dimensional eigenvalues are calculated. The most stable structure of pyrogallol contains two intramolecular hydrogen bonds and turns out to be planar in the S 0 state. In the S 1 state the free OH group is rotated out of the plane of the aromatic ring by about 40°. The strong change in geometry of this structure is predicted by the CASSCF calculations and confirmed by the R2PI spectra of pyrogallol and its deuterated species. The low frequency region of the R2PI spectra can be explained by a torsional motion and the out of plane vibration 17b.