Showing papers by "Robert Berger published in 2006"

Electroweak quantum chemistry for nuclear-magnetic-resonance-shielding constants: Impact of electron correlation

Abstract: One promising route towards the first experimental verification of parity violation (PV) in chiral molecular systems is the detection of line splittings between nuclear magnetic resonance (NMR) spectra of enantiomers. Those numerical methods which can be systematically refined and allow for an accurate and reliable prediction of molecular PV effects will play a crucial role for the preparation and interpretation of such experiments. In this work the ab initio calculation of isotropic parity-violating NMR-shielding constants $({\ensuremath{\sigma}}^{\mathrm{PV}})$ within coupled cluster and multiconfiguration linear response approaches to electroweak quantum chemistry is reported and the results are compared to data obtained at the uncoupled density functional theory level. The ${\ensuremath{\sigma}}^{\mathrm{PV}}$ of the heavy nuclei in hydrogen peroxide, disulfane and diselane (${\mathrm{H}}_{2}{X}_{2}\phantom{\rule{0.3em}{0ex}}\mathrm{with}\phantom{\rule{0.3em}{0ex}}X=^{17}\mathrm{O}$, $^{33}\mathrm{S}$, $^{77}\mathrm{Se}$) computed at the coupled cluster singles and doubles level are found to typically deviate from their electron-uncorrelated counterparts by approximately 20%, while in 2-fluorooxirane, electron correlation alters ${\ensuremath{\sigma}}^{\mathrm{PV}}$ of individual nuclei by almost a factor of 2. It is therefore imperative in the accurate prediction of parity-nonconserving phenomena in NMR experiments that systematically improvable electron-correlating electroweak quantum chemical approaches, such as those presented in this study, are employed.

Pseudotetrahedral Polyhalocubanes: Synthesis, Structures, and Parity Violating Energy Differences

Abstract: All possible pseudotetrahedral, stable polyhalocubanes were prepared, and their structures were proven by NMR spectroscopy and X-ray crystallography. Parity violating energy differences (DeltaEpv) and vibrational frequency shifts were computed. The DeltaEpv values are predicted to be one to two orders of magnitude smaller than those for the corresponding polyhalomethanes. However, the DeltaEpv energy ordering is the same as that for the methane analogues. For both substance classes, the (S) isomers are, with the exception of the bromochlorofluoroiodo derivatives, more stable than the (R) forms.

Doubly protonated benzene in the gas phase.

Abstract: Structural aspects and the unimolecular fragmentations of doubly protonated benzene are studied by means of tandem-mass spectrometry. The corresponding dications are generated by electron ionization (EI) of 1,3- and 1,4-cyclohexadienes, respectively. It is suggested that EI of 1,3-cyclohexadiene leads to the singlet state of doubly protonated benzene, whereas EI of 1,4-cyclohexadiene yields a mixture of singlet and triplet states. Unimolecular fragmentation of doubly protonated benzene exclusively proceeds via dehydrogenation leading to the benzene dication. The proton affinities (PAs) of protonated benzene amount to PA(C(6)H(7)(+))(meta) = 1.9 +/- 0.3 eV for protonation taking place at the meta-position, PA(C(6)H(7)(+))(ortho) = 1.5 +/- 0.2 eV, and PA(C(6)H(7)(+))(para) = 0.9 +/- 0.2 eV, respectively. Various facets of the experiments are compared with density functional theory calculations and generally good agreement is found.

theory study of vibrational frequency shifts in polyhalomethanes

Abstract: In this paper, a two-component density functional theory study of parity violation-induced vibrational frequency shifts in chiral polyhalomethanes is reported and the prospects to detect in these compounds, for the first time, signals of parity violation in molecular systems are discussed. The recent synthesis of enantiomerically enriched CHClFI has renewed interest in examining electroweak corrections for this class of compounds. Utilizing a (quasi-relativistic) two-component zeroth-order regular approximation approach to molecular parity violation, together with density functional theory, the parity violation-induced relative vibrational frequency splittings Δ νpv/ν between the C–F stretching fundamental of polyhalomethane enantiomers are computed. The relative splitting in CHClFI is raised compared with CHBrClF, for which upper bounds were determined experimentally. To facilitate measurement, molecules possessing more pronounced relative splittings are desirable. Therefore, the chiral methane derivative CHAtFI is considered, which exhibits a significantly larger electroweak contribution that induces a vibrational frequency splitting on the order of the experimental resolution previously reported for CHBrClF. Employing compounds containing heavy nuclei such as astatine may thus be necessary with present detection methods.