The structures and vibrational spectra of small methanol clusters from dimer to decamer have been calculated using a newly developed intermolecular potential which is essentially based on monomer wave functions. Special care has been taken for the description of the electrostatic interaction using a distributed multipole representation and including a penetration term. In addition, the potential model consists of repulsion, dispersion, and induction terms. Based on this potential model cluster structures have been calculated. The lowest energy dimer configuration is linear, while from trimer to decamer for the most stable structures ring configurations were found. Tetramer, hexamer, and octamer have S4-, S6-, and S8-symmetry, respectively. Vibrational spectra of the CO stretch and the OH stretch mode have been determined in the harmonic and in the anharmonic approximation using perturbation theory and variational calculations. Up to the tetramer the experimental spectra of the CO stretch mode are well reproduced, for larger clusters an increasing blueshift with respect to the experimental evidence is found. The experimental data for the OH stretch mode of the dimer are fairly well reproduced in all approximations, however, the spectrum of the trimer can only be reproduced using the variational calculation which includes Darling–Dennison resonance terms.

Structure and hydration of the C4H4 •+ ion formed by electron impact ionization of acetylene clustersInfrared molecular beam depletion spectroscopy of small methanol and acetonitrile clusters embedded in large helium clusters has been studied in the spectral region of the CO stretch and the CH 3 rock mode from 1023 to 1059 cm Ϫ1 . The results are compared with the experimental spectra of the corresponding free clusters generated in adiabatic expansions and calculations based on density functional theory or empirical potential models. For methanol clusters, the two types of experimental results are the same for the dimer and trimer structure. Different isomers are found in cold helium for the tetramer and pentamer, namely a monomer and dimer attached to a cyclic trimer. For acetonitrile clusters in helium, aside from the dimer, different structures are observed. The spectra from the trimer to the hexamer are dominated by structures which contain the antiparallel dimer as building block with D 2d symmetry for the tetramer. They do not correspond to the minimum configurations observed for the free clusters. The fragmentation of the two cluster groups in helium droplets by electron impact ionization is discussed.

The structures of small hydrazine clusters from the dimer to the hexamer have been calculated using a standard site-site intermolecular potential and a newly developed systematic approach which is essentially based on monomer properties. Aside from the repulsive and the attractive dispersion and induction interaction special care is taken for the determination of the electrostatic interaction which is represented by a distributed multipole expansion and a penetration correction. Based on these potentials the vibrational spectra of the N-N stretching and the asymmetric NH 2 wagging mode are calculated using degenerate perturbation theory. While the small shifts of the N-N stretching mode are fairly well reproduced by both potential models, large differences are predicted for the asymmetric NH 2 wagging mode. Here, redshifts of-30 cm Ϫ1 are calculated for the standard and blueshifts of 100 cm Ϫ1 are obtained for the systematic potential in agreement with experiment. The analysis shows that the reason for this behavior is the careful treatment of the electrostatic term in this model.

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Vibrational predissociation spectra of hydrazine ͑N 2 H 4) n clusters have been measured from the dimer to the tetramer using a linetunable, isotopically substituted CO 2-laser in order to fill the frequency gap between 990 and 1010 cm Ϫ1. The clusters are size selected in a scattering experiment with helium atoms. The large blue shifts of the asymmetric NH 2 wag mode at 937 cm Ϫ1 are completely interpreted by calculations based on a recently determined systematic model potential. The gross shifts of 60 cm Ϫ1 for the dimer, 80 cm Ϫ1 for the trimer, and 110 cm Ϫ1 for the larger clusters are explained by the different structures: Cyclic arrangements with two hydrogen bonds per molecule for the dimer, rings with one hydrogen bond per molecule for the trimer, and three-dimensional structures for the larger ones. The peaks in the spectra are caused by characteristic vibrations to which more than one isomer contributes.