The geometry change of 5-cyanoindole upon electronic excitation from the ground to the lowest excited singlet state has been determined from a combined fit of the rotational constant changes upon excitation and the vibronic intensities in various fluorescence emission spectra using the Franck-Condon principle. The so determined geometry change is compared to the results of ab initio calculations and points to an excited state geometry, which is La-like in the nomenclature of Platt. A mode selective coupling of vibronic bands to higher-lying excited states is discussed on the basis of Herzberg-Teller contributions to the Frank-Condon intensities.

Isomer- and mass-selective UV and IR-UV double resonance spectra of the BA3, B2A, and B2A2 clusters of benzene (B) and acetylene (A) are presented. Cluster structures are assigned by comparison with the UV and IR spectra of benzene, the benzene dimer, as well as the BA, BA2, and B2A clusters. The intermolecular vibrations of BA are identified by dispersed fluorescence spectroscopy. Assignment of the cluster structures is supported by quantum chemical calculations of IR spectra with spin-component scaled second-order Møller-Plesset (SCS-MP2) theory. Initial propositions for various structures of the BA3 and B2A2 aggregates are generated with model potentials based on density functional theory combined with the symmetry-adapted perturbation theory (DFT-SAPT) approach. Shape and relative cluster stabilities are then confirmed with SCS-MP2. T-shaped geometries are the dominant structural motifs. Higher-energy isomers are also observed. The detected cluster structures are correlated with possible cluster formation pathways and their role as crystallization seeds is discussed.

The structures of the phenol (Ph)-acetylene (A) clusters PhA1,2,3 and Ph2A1 are assigned on the basis of isomer and mass specific IR-UV double resonance spectroscopy and compared to the structure of the PhA cocrystal. The structures of the PhA1,2,3 clusters are dominated by phenol-acetylene π-hydrogen bonds whereas Ph2A1 binds via OH···OH···C≡C interaction with dominating Ph-Ph hydrogen bond like in the phenol dimer and acetylene attached to the free OH group of the proton acceptor phenol. The macroscopic crystal is a clathrate of phenol with acetylene with hydrogen bridges only between the phenol molecules and not between phenol and acetylene. (1) A possible aggregation pathway is proposed in which larger phenol clusters like Ph6 are cyclic with no free OH available anymore to which acetylene could attach as proton acceptor.

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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.