Samuel Leutwyler

Intramolecular vibrations of small water clusters

The structures, binding energies, and vibrational frequencies of fully optimized water clusters (H2O)n, n=1– 4, were computed with ab initio molecular orbital theory at the SCF level using different basis sets. The SCF procedure allows satisfactory predictions for these properties compared with experimental results. For quantitative predictions of binding energies and geometrical parameters, a basis set including polarization functions is needed. With respect to intramolecular vibrational frequencies and frequency shifts, however, the split valence basis set 4‐31G leads in all cases to the best rationalization of the available experimental data. Analysis of intramolecular force constants, frequencies, and eigenvectors for n=2 to 4 shows that (i) a transition from highly localized (n=2) to highly delocalized (n=4) vibrational modes takes place; (ii) the delocalized O–H vibrations of cyclic (H2O)n clusters (n≥3) can be described as longitudinal/transverse optical phonons; (iii) internal force constants for ...

Intermolecular bonding and vibrations of phenol⋅H2O (D2O)

Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest‐energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H‐bond axis, and the β2 mode...

Proton transfer in neutral gas‐phase clusters: α‐Naphthol⋅(NH3)n

Efficient excited‐state proton transfer in neutral acid–base clusters α‐naphthol⋅Bn has been detected and studied by a combination of laser spectroscopic techniques (resonant two‐photon ionization, fluorescence excitation, and emission spectroscopy). S1 state proton transfer was observed for B=NH3 and n≥4, as evidenced by several criteria: (a) large red shift and substantial broadening of the R2PI spectra of the n≥4 clusters relative to those of the bare α‐naphthol and smaller clusters; (b) very large Stokes shift (∼8000 cm−1) of the emission spectra of the n≥4 clusters; (c) complete broadening of the fluorescence emission band for the n≥4 clusters; and (d) a striking similarity of the emission band position and width of the latter spectra to the emission spectrum of the α‐naphtholate anion in basic aqueous solution. No proton‐transfer reaction was observed for small solvent clusters with B=NH3 and n≤3, nor for any of the pair complexes studied, which involve a single base partner [B=triethylamine, 3‐dime...

Structures and vibrational spectra of water clusters in the self‐consistent‐field approximation

Fully optimized structures were calculated for (H2O)n, n=5 and 8, at the SCF (self‐consistent field) level using the 4–31G and, for n=5, also 6–31G* basis sets. The n=5 cluster was found to have a cyclic structure with five H bonded and five free hydrogens. The n=8 minimum energy structure has almost D2d symmetry, with an approximately cubical oxygen framework and four tetrahedrally arranged free hydrogens; four of the water molecules are single‐ and four are double‐hydrogen donors. Harmonic vibrational frequencies, IR and Raman intensities were calculated for n=5 and 8, as well as for the previously optimized n=2–4 clusters. The band positions and intensities in the 3000–3800 cm−1 region correlate well with IR predissociation spectra of (H2O)n clusters. The O–H stretching frequencies of single‐ and double‐hydrogen donor water molecules are relatively well separated from each other, and both from the frequency region of the free O–H stretches, suggesting a new interpretation for some of the data. The low‐...

Optical spectroscopy of Na3 by two-photon ionization in a supersonic molecular beam

Abstract We report the first measurements of the electronic absorption spectrum of a metal cluster under collision-free gas-phase conditions. Na 3 molecules were produced in a supersonic molecular beam and mass-selective absorption spectroscopy was performed in the visible wavelength region (405–690 nm) by two-photon ionization together with mass-spectrometric detection. Two band systems were found at 625 and 670 nm. The experimental results are compared with recent theoretical calculations.

Proton transfer from 1‐naphthol to water: Small clusters to the bulk

Excited‐state proton transfer from 1‐naphthol to water was studied as a function of solvent system size, from supersonically cooled neutral clusters, 1‐naphthol⋅(H2O)n, n=1–50, to bulk ice and water. Occurrence or nonoccurrence of proton transfer was detected and studied using cluster‐size‐specific laser‐spectroscopic techniques: resonant two‐photon ionization (R2PI) and laser‐induced fluorescence emission. Depending on cluster size or solution phase, three qualitatively different types of excited‐state behavior were observed: (1) For small clusters, n≤7, both the R2PI and fluorescence spectra of the clusters were similar in nature to the spectra of bare 1‐naphthol; (2) the medium‐size clusters (n=8–20) show incremental spectral shifts which indicate successive stages of molecular solvation, and the spectra approach that of 1‐naphthol in bulk ice at n≊20; (3) the fluorescence spectra for large clusters, n≥20, show increasing emission intensity below 25 000 cm−1, characteristic of the emission of the excit...

AN AB INITIO DERIVED TORSIONAL POTENTIAL ENERGY SURFACE FOR (H2O)3. II: BENCHMARK STUDIES AND INTERACTION ENERGIES

A torsional potential energy surface for the cyclic water trimer was calculated at the level of second‐order Mo/ller–Plesset perturbation theory. For the construction of this ab initio surface, the first‐order wave function was expanded in a many‐electron basis which linearly depends on the interelectronic coordinates r12. The one‐electron basis of Gaussian orbitals was calibrated on the water monomer and dimer to ensure that the ab initio surface computed represents the (near‐ ) basis set limit for the level of theory applied. The positions of the free O—H bonds are described by three torsional angles. The respective three‐dimensional torsional space was investigated by 70 counterpoise corrected single‐point calculations for various values of these angles, providing a grid to fit an analytical representation of the potential energy surface. The four symmetry unique stationary points previously found at the Hartree–Fock and conventional Mo/ller–Plesset levels [Schutz et al., J. Chem. Phys. 99, 5228 (1993)...

Fluxionality and low‐lying transition structures of the water trimer

The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies <1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second‐order Mo/ller–Plesset levels, using the 6‐311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen‐bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational‐tunneling path; neighboring...

Multiphoton ionization: mass selective laser-spectroscopy of Na2 and K2 in molecular beams

Abstract A cw dye laser is tuned over the B, A ← X transitions of Na 2 /K 2 while a second laser or a high pressure lamp ionizes the excited states. The ions are detected by mass-spectrometry in the mixture of metal-atom clusters formed in a molecular nozzle beam. The vibronic and gyrovibronic spectra obtained show the correct intensities as compared to laser-induced fluorescence. Perturbations of certain Na 2 A ← X by the a 3 Π manifold and of K 2 B ← X transitions by autoionizing levels are detected. The laser power dependence of the spectral intensities has been clarified.

Electronic spectroscopy of perylene–rare‐gas van der Waals complexes

van der Waals (vdW) complexes of perylene (P) with rare‐gas atoms R (R=Ne, Ar, Kr, and Xe) were synthesized in seeded supersonic free jet expansions and studied by laser‐induced fluorescence spectroscopy. P ⋅ Rn complexes with n=1–4 were identified for all rare gas solvents. The P ⋅ R2 complexes with R=Ar, Kr, and Xe were found to form two spectroscopically distinct species, which are assigned as vdW isomers. The P ⋅ R and P ⋅ R2 intermolecular interactions were calculated using a model calculation based on atom–atom pair potentials, yielding potential energy surfaces, equilibrium structures, and vdW binding energies. The results for P ⋅ R2 support the existence of two different equilibrium structures with almost equal binding energies. The observed microscopic red shifts of P ⋅ R complexes are in excellent agreement with the predictions of microscopic solvent‐shift theory based on a second‐order perturbative treatment. The microscopic red shifts of the larger P ⋅ Rn complexes can be systematized by addit...