David Luckhaus

6D vibrational quantum dynamics: Generalized coordinate discrete variable representation and (a)diabatic contraction

A new discrete variable representation (DVR) in generalized vibrational coordinates is proposed together with a new mixed diabatic/adiabatic contraction technique for the treatment of multidimensional vibrational problems up to high vibrational excitations. Formally based on the equidistant Chebyshev DVR in the grid index the new formulation is particularly suitable for multidimensional minimum energy paths. The new Z-matrix DVR proposed in this paper encompasses usual valence coordinates as well as nonlinear maps of coordinates on optimal nonequidistant grids. The pointwise numerical calculation of all kinetic energy terms avoids the algebraic derivation of specialized analytical forms of the kinetic energy adding to the flexibility of the method. With efficient truncation schemes the generalized DVR allows for a compact representation of the time-dependent wave-packet dynamics in up to six dimensions. Vibrationally adiabatic approaches to the detailed modelling of multidimensional quantum-dynamics usual...

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...

The far infrared pure rotational spectrum and the Coriolis coupling between v 3 and v 8 in CH35ClF2

The FIR rotational spectrum of gaseous CH35ClF2 (chlorodifluoromethane, CFC22) is analysed improving vibrational ground state parameters for J up to 80. A new set of rotational and centrifugal distortion constants is given for the C-Cl stretching fundamental v 4. The Coriolis coupling between the two C-F stretching fundamentals v 3 and v 8 is investigated. An effective coupling constant ξ a = 0·38 ± 0·03 cm-1(ξ a 3,8 = 0·56 ± 0·04) was obtained from a rotational analysis of the high resolution FTIR room temperature spectrum (apodized experimental bandwidth: 0·005 cm-1). A ratio of 2 : 1 was estimated for the squares of the vibrational transition moments for the fundamentals v 3 and v 8. A fortran program for simulations and least squares refinements of asymmetric top spectra is described, which allows for the inclusion of any kind of rovibrational coupling between several vibrational states.

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.

Mode selective stereomutation tunnelling in hydrogen peroxide isotopomers

Abstract The six-dimensional tunnelling dynamics of H 2 O 2 , HOOD, and D 2 O 2 are studied on a recently developed analytical global potential energy hypersurface for the electronic ground state of hydrogen peroxide. A new formulation of the harmonic reaction path Hamiltonian approach gives good agreement with the results of a new fully coupled six-dimensional adiabatic channel approach. The experimentally observed pure torsional spectrum and mode specific tunnelling are well reproduced, as are the few known isotope shifts. Predictions are made for the torsional tunnelling spectrum of HOOD and D 2 O 2 .

COMBINED HIGH RESOLUTION INFRARED AND MICROWAVE STUDY OF BROMOCHLOROFLUOROMETHANE

We report a detailed spectroscopic investigation of the chiral molecule bromochlorofluoromethane (CHBrClF) with rotational resolution using a pulsed nozzle beam Fourier transform microwave (FTMW) and a waveguide FTMW spectrometer as well as a supersonic jet interferometric Fourier transform infrared (FTIR) and infrared diode laser spectrometer. The rotational spectrum of CHBrClF has been measured between 8 and 18 GHz. The quadrupole hyperfine components have been fully resolved for the assigned rotational transitions with J⩽18. Three ground state rotational constants, five centrifugal distortion constants, and all five independent elements of the bromine and chlorine quadrupole coupling tensors have been determined for each of the four isotopomers CH79Br35CIF, CH81Br35CIF, CH79Br37CIF, and CH81Br37CIF from about 500 measured transition frequencies of the hyperfine components. The quadrupole coupling tensor has been transformed to its principal axes. The determinable sign combinations of the off-diagonal e...

Spectrum and dynamics of the CH chromophore in CD2HF. I. Vibrational Hamiltonian and analysis of rovibrational spectra

We have measured the complete rovibrational spectrum of dideutero-fluoro-methane (CD2HF) from the far-infrared (20 cm−1) to the visible (14000 cm−1) largely at Doppler-limited resolution. Rovibrational parameters are reported for CH-stretching and bending fundamentals and overtones. The spectra of the coupled stretching and bending modes of the CH chromophore are analyzed quantitatively both in terms of effective Hamiltonians and a three-dimensional model Hamiltonian. A new numerical method for solving the corresponding eigenvalue problem is described briefly. The strong Fermi and Darling—Dennison resonances lead to fast vibrational redistribution on the femtosecond time scale.

Ab initio calculations of mode selective tunneling dynamics in 12CH3OH and 13CH3OH

A simplified formulation of the harmonic reaction path Hamiltonian (RPH) approach is used to calculate mode specific tunneling splittings and stereomutation times in (CH3OH)-C-12 and (CH3OH)-C-13. The experimental torsional spectrum is very well reproduced, as well as the few known isotope shifts. The mode specific changes in tunneling splitting are investigated for the excitation of fundamentals and OH stretching overtones. Good agreement between experiment and the RPH model is obtained, except for excitations of modes which are perturbed by anharmonic resonances. The inverted tunneling splittings (E level below A) experimentally observed for the fundamental transitions of the CH-stretching modes nu(2) and nu(9) and of the CH-rocking mode nu(11) are shown to result from a pure symmetry effect and not from a breakdown of vibrational adiabaticity. Introducing a proper geometrical phase factor but retaining the adiabatic separation of the torsional dynamics yields calculated values of Delta(ν) over tilde (2)=-3.6 cm(-1), Delta(ν) over tilde (9)=-3.2 cm(-1), and Delta(ν) over tilde (11)=-8.2 cm(-1) that are in satisfactory agreement with experimental data. Negative tunneling splittings are also predicted for the asymmetric CH-bending modes nu(4) and nu(10) and the CH3-rocking mode nu(7). A smooth decrease of the tunneling splitting is calculated for increasing OH stretching excitation [Delta(ν) over tilde(nu(1))=6.2 cm(-1),...,Delta(ν) over tilde (6nu(1))=1.5 cm(-1)] in quantitative agreement with experiment [Delta(ν) over tilde(nu(1))=6.3 cm(-1),...,Delta(ν) over tilde (6nu(1))=1.6 cm(-1)]. The effect is shown to result in about equal parts from the increase of the effective torsional barrier and the effective lengthening of the OH bond

The vibrational spectrum of HONO: Fully coupled 6D direct dynamics

The vibrational spectrum of nitrous acid (HONO) in the electronic ground state is derived from fully coupled quantum-mechanical calculations. A global six-dimensional potential hypersurface is directly interpolated from density-functional calculations. A potential-optimized generalized coordinate discrete variable representation is combined with multidimensional successive truncation to yield the complete vibrational spectrum up to the first OH stretching overtone of both stable isomers. Higher OH stretching overtones up to 7νOH are calculated selectively applying the adiabatic contraction technique. The ab initio prediction of the vibrational spectrum is in remarkably good agreement with available experimental data. The results indicate the onset of the intramolecular (1,3)-H-transfer between the first and second OH stretching overtone of cis-HONO.

High-resolution FTIR spectroscopy of CHClF2 in a supersonic free jet expansion

Abstract The infrared spectrum of CHClF 2 (freon 22, chlorodifluoromethane) in the range of the CH stretching fundamental ν 1 and the two CF stretching fundamentals ν 3 and ν 8 has been measured in a supersonic free jet expansion with rotational temperatures of about 50 K and resolutions up to 0.004 cm −1 . The first successful rotational analyses for CH 35 ClF 2 provided band centers and other constants for ν 3 (1108.728 cm −1 ), for ν 8 (1127.284 cm −1 ), and for the ground state by combination differences. The peculiar double-peak structure of ν 1 (3021.6 cm −1 with a side maximum near 3024.6 cm −1 ) is discussed. The results are also important for a better understanding of the IR laser chemistry of CHClF 2 and simulations of its IR absorption as an atmospheric trace gas.