Nuclear quantum effects in calculated NMR shieldings of benzene; a Feynman path integral study

The Feynman path integral Monte Carlo approach has been coupled to the gauge including atomic orbital formalism in order to analyse the absolute magnetic shieldings of the benzene nuclei under the conditions of thermal equilibrium. The Hamiltonian employed in the derivation of ensemble averaged NMR quantities is of the Hartree-Fock type. The basis set used is of 6–31G quality. The spatial delocalization of the atoms leads to a deshielding of both types of benzene nuclei relative to the shieldings experienced at the minimum of the potential energy surface. This deshielding has to be traced back to bond length elongations in thermal equilibrium. The influence of the nuclear fluctuations on the NMR parameters of benzene is quantum driven up to temperatures of 400 K; classical fluctuations are of minor importance in this low-temperature window.     

The structure of the dihydrohexahelicene formed by the irradiation of styryl benzo [c] phenanthrene under anaerobic conditions

According to Wismontski and Fischer irradiation under anaerobic conditions of 2-styryl benzo [c]phenanthrene (1) gives rise to an isolable dihydrohexahelicene which is easily oxidized into hexahelicene. Its structure obviously different from 5,6-dihydrohexahelicene 4 waselucidated by the analysis of the 500 MHz-NMR spectrum. The structure and configuration: trans-6a, 16d-dihydrohexahelicene with the 16d hydrogen oriented inside the inner helix must be formed by a suprafacial1.5-H shift of only one hydrogen from the primary formed photocyclization product 2.     

On updating triangular products of Householder reflections

Summary In this paper it is shown how a triangular product of Householder reflections can be updated with an arbitrary elementary orthogonal transformation. The amount of work required is 5/2n ² multiplications for absorbing an arbitrary reflection and 3n ² multiplications for absorbing a plane rotation on subsequent indices.     

Very fast MR imaging by field echoes and small angle excitation

An MR imaging technique has been developed producing head and body images of diagnostic quality in only a few seconds acquisition time. The Fourier type imaging technique uses excitation with relatively small excitation angels, echoes produced by gradient inversion, and extremely fast profile repetition. A typical result at 0.5 T is an artifact-free head image of 128 x 128 resolution, 10 mm slice thickness in an acquisition time of 2 seconds.     

Thickness-dependent ordering of water layers at the NaCl(100) surface

We have determined the ordering properties of water adsorbed at room temperature on the rock salt (100) surface under four different conditions: ultrahigh vacuum, dry nitrogen atmosphere, 45% and 75% relative humidity. Details of the atomic structure are determined for both sides of the solid-liquid interface. The top most layer of NaCl shows a small relaxation that changes from an expansion to a contraction with increasing humidity. Under all measured conditions water monolayers with different ordering properties are present at the interface. Surprisingly, we find that the amount of ordering in the first layer is increasing with increasing thickness of the water film. At a solid-liquid interface, the ordering appears to be correlated with the solubility.