Martin A. Suhm

Potential energy surfaces, quasiadiabatic channels, rovibrational spectra, and intramolecular dynamics of (HF)2 and its isotopomers from quantum Monte Carlo calculations

We report analytical representations of the six‐dimensional potential energy hypersurface for (HF)2, the parameters of which are closely adjusted to low energy experimental properties such as hydrogen bond dissociation energy (D0=1062 cm−1 ) and vibrational–rotational spectra in the far and mid infrared. We present a detailed analysis of properties of the hypersurface in terms of its stationary points, harmonic normal mode amplitudes, and frequencies for the Cs minimum and C2h saddle point and effective Morse parameters and anharmonic overtone vibrational structure for the hydrogen bond and the HF stretching vibrations. The comparison between experimental data and the potential energy surface is carried out by means of accurate solutions of the rotational–vibrational Schrodinger equation with quantum Monte Carlo techniques, which include anharmonic interactions between all modes for the highly flexible dimer. Two extensions of the quantum Monte Carlo technique are presented, which are based on the clamped...

A new six-dimensional analytical potential up to chemically significant energies for the electronic ground state of hydrogen peroxide

We report calculations of the electronic ground state potential energy surface (PES) of hydrogen peroxide covering, in an almost global fashion, all six internal degrees of freedom by two different ab initio techniques. Density functional theory (DFT) calculations using the Becke 3 parameter Lee–Yang–Parr (B3LYP) hybrid functional and multiconfigurational second order perturbation theory (CASPT2) calculations, both using large basis sets, are performed for a wide range of geometries (8145 DFT and 5310 CASPT2 single-point energies). We use a combined data set of mostly DFT with additional CASPT2 ab initio points and the complete CASPT2 surface to fit a total of four different 6D analytical representations. The resulting potentials contain 70–76 freely adjusted parameters and represent the ground state PES up to 40000 cm−1 above the equilibrium energy with a standard deviation of 100–107 cm−1 without any important artifacts. One of the model surfaces is further empirically refined to match the bond dissocia...

Chirality Recognition between Neutral Molecules in the Gas Phase

Noncovalent interactions are particularly intriguing when they involve chiral molecules, because the interactions change in a subtle way upon replacing one of the partners by its mirror image. The resulting phenomena involving chirality recognition are relevant in the biosphere, in organic synthesis, and in polymer design. They may be classified according to the permanent or transient chirality of the interacting partners, leading to chirality discrimination, chirality induction, and chirality synchronization processes. For small molecules, high-level quantum chemical calculations for such processes are feasible. To provide reliable connections between theory and experiment, such phenomena are best studied in vacuum isolation at low temperature, using rotational, vibrational, electronic, and photoionization spectroscopy. We review these techniques and the results which have become available in recent years, with special emphasis on dimers of permanently chiral molecules and on the influence of conformational flexibility. Analogies between the microscopic mechanisms and macroscopic phenomena and between intra- and intermolecular cases are drawn.

HF DIMER : EMPIRICALLY REFINED ANALYTICAL POTENTIAL ENERGY AND DIPOLE HYPERSURFACES FROM AB INITIO CALCULATIONS

The interaction between two HF molecules as a function of all six internal coordinates is reported as calculated for 3284 selected points in configuration space at counterpoise-corrected explicitly correlated levels and fitted to an analytical expression, which is described in detail. The unweighted rms deviation for all 3284 points is 21 cm−1. Empirical refinements are applied through mixing and scaling of the ab initio data, guided by the comparison of multidimensional nuclear quantum energy levels with experimental data. The resulting semiempirical pair potentials (labeled SC-2.9 and SO-3) contain 67 and 61 freely adjusted parameters and are combined with a four parameter monomer potential of generalized Poschl–Teller type. Various minimum energy paths and cuts are investigated. Major improvements over earlier HF dimer potentials are demonstrated via multidimensional solutions of the nuclear Schrodinger equation. Comparison with other high level ab initio calculations and with various experimental data...

Hydrogen-bonded OH stretching modes of methanol clusters: a combined IR and Raman isotopomer study.

A comprehensive study of the OH and OD stretching fundamentals in clusters of methanol and its isotopomers CH(3)OD, CD(3)OH, and CD(3)OD provides detailed insights into the hydrogen-bond mediated coupling as a function of cluster size. The combination of infrared and Raman supersonic jet spectroscopy enables the observation and assignment of all hydrogen-bonded OH stretching modes of isolated methanol trimer and methanol tetramer. A consistent explanation for the spectral complexity observed more than a decade ago in methanol trimer in terms of low-frequency methyl umbrella motions is provided. Previous explanations based on cluster isomerism or anharmonic resonances are ruled out by dedicated jet experiments. The first experimental lower bound for concerted quadruple proton transfer in S(4) symmetric methanol tetramer is derived and compared with theoretical predictions. The observed isotope effects offer insights into the anharmonicity of the localized OH bond. The performance of harmonic B3LYP and MP2 calculations in predicting hydrogen-bond-induced spectral shifts and couplings is investigated.

A monomers-in-dimers model for carboxylic acid dimers

The OH stretching fundamental band system of carboxylic acid dimers is studied using acetic acid and its isotopomers as a model system. Comparing experimental jet spectra with multidimensional quantum mechanical calculations the origin of the extremely broad vibrational band structure (Δν≈800 cm−1) is found in strong anharmonic resonances involving the OH stretching vibration. Within an adiabatic picture of hydrogen bonding a new monomers-in-dimers model allows to analyze the observed vibrational band structure in terms of the anharmonic quantum dynamics of the CO2H functional group. The results are discussed in terms of the time-dependent population dynamics and its implications for the mode-specificity of the vibrational predissociation of the hydrogen bonds. On a subpicosecond time scale the intramolecular vibrational energy redistribution of the dimer remains effectively localized within the six-dimensional manifold of the internal vibrations of the carboxyl group, conserving its local CS symmetry.

Dipole moment function and equilibrium structure of methane in an analytical, anharmonic nine‐dimensional potential surface related to experimental rotational constants and transition moments by quantum Monte Carlo calculations

The pure rotational spectrum in the far‐infrared and its absolute intensity in the vibrational ground state of CHD3 and CH3D, and the integrated band strength of the N=5 CH‐stretching overtone of CHD3 in the near infrared to visible were measured by high‐resolution interferometric Fourier transform techniques. The far‐infrared data result in permanent electric dipole moments (‖μz0‖=(5.69±0.14)×10−3 D for CHD3, ‖μz0‖=(5.57±0.10)×10−3 D for CH3D), consistent with previous experimental data. The integrated N=5 overtone cross section is found to be (0.828±0.068) fm2. The overtone data are used, together with previous data, to derive a new, nine‐dimensional, isotopically invariant dipole moment function for CH4 within the chromophore model for the CH chromophore in CHD3. With this function, the experimental data can be reproduced to an averaged factor of 1.2, in the best case. In the vibrational ground state, a nine‐dimensional calculation of expectation values on a new, fully anharmonic potential surface was ...

FTIR-spectroscopy of molecular clusters in pulsed supersonic slit-jet expansions

A new approach to the Fourier transform infrared (FTIR) absorption spectroscopy of molecular clusters in pulsed supersonic jets is developed to the point where it is competitive with high-sensitivity laser absorption techniques for intermediate and large molecular systems. A combination of rapid spectral acquisition and of a buffered jet chamber enables the use of intense gas pulses which cover complete interferometer scans. Applications to (N2O)n, (CH3OH)n and (HCl)n demonstrate the capabilities of this technique. Investigations of the association of bulky alcohols and of clusters within clusters illustrate some ongoing research.

A critical analysis of electronic density functionals for structural, energetic, dynamic, and magnetic properties of hydrogen fluoride clusters

We present extensive computational results on density functional calculations for hydrogen fluoride species (HF)n (with 1≤n≤6) and compare them to results from other approaches and experiments, where available. Among the calculated properties we discuss equilibrium structural parameters, vibrational frequencies, electric dipole moments, IR intensities, dissociation energies, barriers for rearrangement by proton tunneling, NMR chemical shifts and spin couplings for 1H and 19F, and magnetic susceptibilities. It is found that density functional (particularly BLYP) and even more so hybrid approaches (particularly B3LYP) provide useful results. However, we show that due to some characteristic deficiencies, these are in general not competitive with more quantitative results from large basis set MP2 calculations. The calculated magnetic properties do not indicate any “aromaticity” connected to a hypothetical electronic ring current. © 1997 John Wiley & Sons, Inc. J Comput Chem 18: 1695–1719, 1997

A NEW AB INITIO BASED SIX-DIMENSIONAL SEMI-EMPIRICAL PAIR INTERACTION POTENTIAL FOR HF

Abstract We present an analytical pair potential for hydrogen fluoride based on 3284 ab initio points at counterpoise-corrected MP2-R12 and lower levels using a large basis set. Empirical refinements are applied through comparison of experimental data with solutions of the multidimensional nuclear Schrodinger equation and appropriate mixing and scaling of the ab initio data. The new potentials are compared to previous dimer potential hypersurfaces and spectroscopic data, yielding best estimates of 19.1(2) kJ/mol for the dimer dissociation energy De and 4.2(2) kJ/mol for the electronic hydrogen bond tunneling barrier.