Richard Knochenmuss

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‐...

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 25u2009000 cm−1, characteristic of the emission of the excit...

Proton transfer reactions in neutral gas-phase clusters: 1-Naphthol with H2O, D2O, CH3OH, NH3 and piperidine

Abstract The optical spectroscopic properties of 1-naphthol·B n clusters (B=H 2 O, D 2 O, CH 3 OH, NH 3 and piperidine) have been studied in supersonic molecular beams. Proton transfer to the solvent cluster occurs at n ⩾2 for B=piperidine and n ⩾4 for B=NH 3 , while no transfer is observed for B=CH 3 OH up to n ≈26, nor for H 2 O or D 2 O clusters up to n ≈40. The occurrence and cluster size dependence of proton transfer are mainly determined by the proton affinity of B n and by the electrostatic stabilization of the microsolvated ion pairs.

Excited‐state proton transfer in gas‐phase clusters: 2‐Naphthol⋅(NH3)n

Excited‐state proton transfer was studied in supersonically cooled neutral acid‐base clusters of 2‐naphthol⋅(NH3)n with n=1‐10, using size‐selective two‐color resonant two‐photon ionization (R2PI) and fluorescence emission techniques. The smaller clusters (n≤3) show vibrationally resolved spectra and do not exhibit excited‐state proton transfer. Two rotamers (cis and trans) were observed for each cluster size; these exhibit very different electronic‐vibrational couplings to the intermolecular modes, reflecting the orientation of the (NH3)n cluster relative to the electronic transition dipole moment of the aromatic chromophore. Excited‐state proton transfer occurs for n≥4, as evidenced by (a) a large spectral red shift (≊8000 cm−1) of the fluorescence emission band; (b) large increase in the emission bandwidth; (c) similarity of the fluorescence band position and width to that of 2‐naphtholate anion in neutral or basic aqueous solution. In absorption, the S1←S0 bands are substantially broadened for cluster...

Selective spectroscopy of rigid and fluxional carbazole–argon clusters

Two size‐selective spectroscopic techniques were used to experimentally differentiate between nearly rigid (solid‐like) and highly fluxional (liquid‐like) carbazole⋅Arn (n=4–6) clusters produced and cooled in supersonic molecular beams: (1) ionization potential selective resonant two‐photon ionization (IP selective R2PI) spectroscopy; and (2) spectral hole‐burning with R2PI detection. For each cluster size, separate and qualitatively very different electronic spectra were obtained by IP selective R2PI, depending on total ionization energy. At low ionization energies, broad bands of halfwidth ≊50 cm−1 (FWHM) were obtained, which are interpreted as due to fluxional clusters of high internal energy. When ionizing slightly above an abrupt step in the ionization efficiency curve, additional narrow (Δν≊5 cm−1) features appear superimposed on the semicontinuous spectra; these are interpreted as due to (near) rigid clusters with low internal energy. The spectral hole‐burning experiments support this interpretatio...

Broadband near‐infrared luminescence of Cr+3 in the elpasolite lattices Cs2NaInCl6, Cs2NaYCl6, and Cs2NaYBr6

The broadband luminescence of isolated Cr+3 ions in the title lattices is investigated. The 4T2g lowest excited state is found to undergo Jahn–Teller distortion in the eg coordinate, in addition to expanding along the a1g. Quantitative determination of these displacements is made by analysis of extremely rich fine structure, the Ham quenched spin‐orbit splitting of the electronic origins, and the Stokes shifts. The Jahn–Teller effect is found to influence the temperature dependence of lifetimes at less than 50 K. In the yttrium containing hosts, nonradiative deactivation of the 4T2g is observed above 200 K. Analysis of lifetime data with four models for nonradiative decay indicates that the active vibrations are again the a1g and eg. The trends observed among the host materials are rationalized on the basis of the freedom of the 4T2g to relax along the a1g and eg directions.

Isomer-specific spectra and ionization potentials of van der Waals clusters

Abstract Phenanthrene ·Ar n van der Waals clusters exhibit spectroscopically distinct isomers for n =2 and 4. The ionization potentials (IP) of these isomers differ by 33 ( n =2) and 56 cm −1 ( n =4), or about 25% of the total IP shift induced by the cluster. Based on this IP difference, isomer-selective ionization and electronic spectroscopy have been performed.

Excited-state proton transfer in 1-naphthol·(NH3)n complexes: the threshold size is n=4

Abstract Resonant two-photon ionization, fluorescence and fluorescence-detected excitation spectra of 1-naphthol·(NH 3 ) n clusters are reported and interpreted with emphasis on determining the smallest clusters for which excited-state proton transfer (ESPT) can occur. Consistent with the earliest studies but in contrast to more recent work, the ESPT threshold is found at n =4, not n =3. The confusion is shown to have arisen from weak, broad fluorescence of n =1–3 clusters extending to ∼400 nm. Excitation spectra of genuine ESPT emission at longer wavelengths gives no indication of contributions from n

Accurate dissociation energies of two isomers of the 1-naphthol⋅cyclopropane complex

The 1-naphthol⋅cyclopropane intermolecular complex is formed in a supersonic jet and investigated by resonant two-photon ionization (R2PI) spectroscopy, UV holeburning, and stimulated emission pumping (SEP)-R2PI spectroscopy. Two very different structure types are inferred from the vibronic spectra and calculations. In the edge isomer, the OH group of 1-naphthol is directed towards a C-C bond of cyclopropane, the two ring planes are perpendicular. In the face isomer, the cyclopropane is adsorbed on one of the π-aromatic faces of the 1-naphthol moiety, the ring planes are nearly parallel. Accurate ground-state intermolecular dissociation energies D0 were measured with the SEP-R2PI technique. The D0(S0) of the edge isomer is bracketed as 15.35 ± 0.03 kJ/mol, while that of the face isomer is 16.96 ± 0.12 kJ/mol. The corresponding excited-state dissociation energies D0(S1) were evaluated using the respective electronic spectral shifts. Despite the D0(S0) difference of 1.6 kJ/mol, both isomers are observed in the jet in similar concentrations, so they must be separated by substantial potential energy barriers. Intermolecular binding energies, De, and dissociation energies, D0, calculated with correlated wave function methods and two dispersion-corrected density-functional methods are evaluated in the context of these results. The density functional calculations suggest that the face isomer is bound solely by dispersion interactions. Binding of the edge isomer is also dominated by dispersion, which makes up two thirds of the total binding energy.

Intermolecular dissociation energies of dispersively bound 1-naphthol⋅cycloalkane complexes

Intermolecular dissociation energies D0(S0) of the supersonic jet-cooled complexes of 1-naphthol (1NpOH) with cyclopentane, cyclohexane, and cycloheptane were determined to within <0.5% using the stimulated-emission pumping resonant two-photon ionization method. The ground state D0(S0) values are bracketed as 20.23±0.07 kJ/mol for 1NpOH⋅cyclopentane, 20.34±0.04 kJ/mol for 1NpOH⋅cyclohexane, and 22.07±0.10 kJ/mol for two isomers of 1NpOH⋅cycloheptane. Upon S0→S1 excitation of the 1-naphthol chromophore, the dissociation energies of the 1NpOH⋅cycloalkane complexes increase from 0.1% to 3%. Three dispersion-corrected density functional theory (DFT) methods predict that the cycloalkane moieties are dispersively bound to the naphthol face via London-type interactions, similar to the face isomer of the 1-naphthol⋅cyclopropane complex [S. Maity et al., J. Chem. Phys. 145, 164304 (2016)]. The experimental and calculated D0(S0) values of the cyclohexane and cyclopentane complexes are practically identical, although the polarizability of cyclohexane is ∼20% larger than that of cyclopentane. Investigation of the calculated pairwise atomic contributions to the D2 dispersion energy reveals that this is due to subtle details of the binding geometries of the cycloalkanes relative to the 1-naphthol ring. The B97-D3 DFT method predicts dissociation energies within about ±1% of experiment, including the cyclopropane face complex. The B3LYP-D3 and ωB97X-D calculated dissociation energies are 7-9 and 13-20% higher than the experimental D0(S0) values. Without dispersion correction, all the complexes are calculated to be unbound.