Fourth-order diagrammatic MB-RSPT calculations of the interaction energy between two helium atoms

The efficiency of the MB-RSPT in the calculations of the correlation contributions to the interaction energies was investigated, using He₂ as a model Van der Waals system. The attention has been focused on the convergency of the perturbation expansion in the calculations of the interaction energy and on the analysis of the fourth-order terms of MB-RSPT. The rôle of the renormalization term in the correct long-range behaviour of the interaction potential has been emphasized.     

Accurate electrical and spectroscopic properties ofX 1Σ+ BeO from coupled-cluster methods

Summary This paper reports a series of coupled-cluster (CC) calculations through CCSDT on the theoretically challenging ground state of the BeO molecule. Along with CC methods, quadratic configuration interaction (QCI) approximations to CC theory have been used (QCISD and QCISD(T)), which show several dramatic failings. Equilibrium electrical properties (, _xx , and _zz ) and basic spectroscopic properties (r _e, _e,D _e, and infrared intensity (I)) have been computed. Basis set and electron correlation effects are analyzed in order to arrive at accurate values of the dipole moment and polarizability, which are not known experimentally. For the dipole moment, we obtain a value of 6.25 D, with an uncertainty of about 0.1 D. For _xx and _zz , we suggest respective values of 32 and 36 atomic units (a.u.) and error bars of about 1 and 2 a.u. With extended basis sets, the spectroscopic propertiesr _e, _e, andD _e are reproduced to high accuracy, which is the first time this has been achieved for this species byab initio methods. At the highest calculation levels,I is predicted to be very small. AlthoughI has not been measured, some support for this prediction comes from a recent infrared study of BeO-rare gas complexes. The QCI methods are shown to be much more sensitive to basis set, and even with large basis sets yield values of _zz andI which differ from CC results by an order of magnitude and three orders of magnitude, respectively. These differences doubtless arise from the importance of single excitations (T ₁) for this molecule, as several terms involvingT ₁ are neglected in the QCISD approximation compared with CCSD. We also report CC calculations with Brueckner orbitals, which yield results similar to those obtained with restricted Hartree-Fock orbitals.     

IR laser induced UV fluorescence in nitric oxide

We observe UV fluorescence from a gas cell containing NO and Ar, when irradiated by a CO laser line, coincident with an NO fundamental. The power density generated cw by the CO laser does not exceed 1 $\text{kW}\text{cm}{}^2$ in the focal area. It is suggested that V-V pumping is responsible for exciting the NO X ²Π up to υ ≈ 30 from where collisional transfer into A ²Σ and B ²Π is possible.     

Synthese und Konfigurationszuordnung von potentiell antimikrobiellen 5,6-Dihydroxyisobenzofuranonen

Summary Oxidation of isobenzofuranone1 yielded in a diastereoselective reaction epoxide2. Acidolysis of2 resulted in a mixture oftrans-glycols6 a (88%) and7 a (4%), which were separated by crystallization. The relative configuration of6 a and7 a at the chiral centers 3 a, 5, 6, and 7 a was determined by¹H-NMR-spectroscopy and X-ray analysis of O-acetylated and 7 a-methylated derivatives.

     

Effect of vibrational excitation of HBr on the H+HBr abstraction and exchange reactions

The effect of vibrational excitation of HBr on the H+HBrH₂+Br and H+HBrH+HBr reactions has been investigated on the extended LEPS surface (ELEPS) constructed on the basis of quantum chemically calculated points of PES. Together with this surface the LEPS surface of Sudhakaran and Raff [1] was used for comparison at two relative translational energies. A quasiclassical trajectory method was used to study the abstraction and exchange reaction dynamics. The reactive cross section was calculated as a function of the relative collision energy and the vibrational state of HBr. The following conclusions can be drawn from the results of the study: (i) vibrational excitation v=0 v=2 more than doubles the reaction cross section, (ii) the increase in the collision energy is most effectively channelled into the product translational energy.Dedicated to Professor J. Koutecký on the occasion of his 65th birthday     

On the possibility of measuring the thickness of very thin films using electron probe microanalyzer

The measurement of the thickness of thin aluminum and silver films using an electron probe microanalyzer is described. The method of measurement was studied in order to investigate very thin discontinuous aluminum layers, originated from inorganic residues of photoresists removed from the surface of silicon and other materials by plasma oxidation. The calibration curves, i.e. the dependences of the intensity of the characteristic X-ray emission on the thickness of thin standard films evaporated on the same substrate, were determined for the evaluation of the measurements. These dependences were measured till the smallest thickness 15 nm of the standard film. Then the curves were extrapolated to the origin of coordinates and the film thickness was calculated using the value of the slope of tangent to the calibration curve in the origin. In order to estimate the accuracy of this procedure, the calibration curves were expressed analytically using the mathematical representation of the electron excitation function. It follows from the results that the shape of the calibration curves may be influenced mainly due to electrons scattered from the substrate back into the film and due to fluorescence excitation of X-ray emission. The last effect was markedly observed in Al/Si system.     

Conformational dependence of the electrostatic potential-derived charges of dopamine: Ramifications in molecular mechanics force field calculations in the gas phase and in aqueous solution

The electrostatic potential-derived charges for the catecholamine neurotransmitter dopamine were calculated at the STO-3G and 6-31G* basis sets for six different molecular conformations. The degree of variance of the charges with changing conformations was examined. The 6-31G* basis set produced charges that were more sensitive to changes in conformation than those derived from the STO-3G electrostatic potentials. The implication of the charge variations in molecular mechanics calculations was also investigated. The molecular mechanics results in the gas phase exhibited a variance depending upon the charge set used. The force field calculations varied much less when aqueous solvation was included in the calculations through a continuum model. © 1993 John Wiley & Sons, Inc.     

A Variable-Pressure 2D 1H-NMR Study of the Mechanism of Trimethyl Phosphate Intermolecular Exchange and cis/trains-Isomerisation of Tetrachlorobis(trimethyl phosphate)zirconium(IV)

In chloroform, [ZrCl₄·2(MeO)₃PO] exists in both cis- and trans-isomeric forms. Three reactions can be envisaged in the presence of excess (MeO)₃PO = L: (1) cis-[ZrCl₄·2L] + *L?cis-[ZrCl₄·L*L]+ L; (2) trans-[ZrCl₄·2L] + *L ? trans-[ZrCl₄·L*L] + L; (3) cis-[ZrCl₄·2L]? trans-[ZrCl₄·2L]. To distinguish between these possible reaction pathways, we have used 2D ₁H-NMR spectroscopy. For the first time, variable-pressure 2D exchange spectra were used for mechanistic assignments. cis/trans-Isomerisation was found to be the fastest reaction (in CHCl₃/CDCl₃), with a small acceleration at higher pressure: it is concluded to be an intramolecular process with a slightly contracted six-coordinate transition state. The intermolecular (MeO)₃PO exchange on the cis- and trans-isomer are second-order processes and are strongly accelerated by increased pressure: I_a mechanisms are suggested without ruling out limiting A mechanisms.     

CCSD(T) expectation value calculations of first-order properties

An expectation value approach to calculations of first-order properties using the non-iterative, triple-excitation amplitudes in the coupled cluster wave function is exploited. Three methods are suggested and analysed using the many body perturbation theory (MBPT) expansion arguments. The first method, in which non-iterative triple-excitation amplitudes are used in the expression for the expectation values, makes the wave function accurate through the second order of MBPT. In the second method, which is an extension of the first, effects of triple-excitation amplitudes are coupled with single- and double-excitation amplitudes. The correlated density matrix equivalent through the fourth order to that obtained when CCSDT-la amplitudes are used is employed in the third method. The suggested methods are tested on dipole moment and polarizability calculations for several diatomic closed-shell molecules and are compared to other related approaches. Received: 15 May 1997 / Accepted: 5 June 1997