An improved algorithm for satellite-derived UV radiations

The improved algorithm surface irradiance derived from a range of satellite-based sensors (SIDES) is presented in this article. It calculates various types of surface UV intensities, such as biologically weighted or unweighted UV spectra, integrated doses or irradiance at specific wavelengths, using data from satellite instruments. These surface UV data are mainly useful for environmental impact or process studies where high accuracy or a high temporal resolution is required. In contrast to several previous studies, SIDES has been validated with spectral measurements. By this method an averaging of positive or negative deviations over the complete wavelength range is avoided. This is especially important for UV wavelengths around 300 nm where biological effectiveness is highest. The results of SIDES deviate less than 7% from ground-based observations for wavelengths between 295 and 400 nm. In contrast, the corresponding deviations of the joint research center algorithm escalate for shorter wavelengths, reaching 35% at 295 nm. This large deviation is due to an inaccurate interpolation procedure that has been detected by spectral analysis. Thus, spectral validation is demonstrated to be an appropriate tool to detect weaknesses in such an algorithm and provides information essential for improvement.     

MCSCF reaction-path energetics and thermal rate-constants for the reaction of3NH with H2

Summary The intrinsic reaction-path, reactants, transition state and products for the reaction of NH (^3–)+H₂ (¹ _g ⁺ ) NH₂ (²B₁)+H (²S) involving the lowest triplet electronic state of NH₃ were calculated using multi-configuration (MC) SCF methods. The calculated change of internal energy for the reaction of 11.0 kcal mol^–1 agrees with the experimental value within 2 kcal mol^–1. The barrier to reaction is 23.4 kcal mol^–1 high. The harmonic MCSCF reaction-path potential was calculated and canonical variational transition state theory calculations of the rate constants performed over a temperature range from 400 to 2500 K. The computed rate constants are generally two orders of magnitude smaller than those of the comparable reaction of OH with H₂, whereas those of the reverse reaction are by a factor of 20 larger than those of OH₂ with H.     

Mixed valence—Rydberg states

A survey of recent calculations involving the mixing of Rydberg and valence states in the spectra of molecular systems is undertaken. It is pointed out that when such states undergo curve crossings with one another, minimal energy splittings of 1.0–2.0 eV can occur at the respective 50-50 composition points. The significance of such large interactions between valence and low-lying Rydberg states is considered in terms of the properties of the resulting mixed CI states, particularly with reference to the oscillator strengths for the pertinent ground state transitions and also the spatial extension of the corresponding upper orbitals. It is thereupon argued that such mixings have important consequences in the spectra of a wide variety of systems, including those of O₂, ethylene and ethane.     

Photofragmentation of dichloromethanol Cl2CHOH along C-O and C-Cl cleavages: a theoretical study

Multireference configuration interaction calculations are carried out for ground and excited states of dichloromethanol, Cl2CHOH, to investigate two important photofragmentation processes relevant to atmospheric chemistry. Five low-lying excited states (1(1)A", 2(1)A', 1(3)A", 2(3)A" and 1(3)A') in the energy range between 6.4 and 7.5 eV are found to be highly repulsive for C-Cl elongation, leading to ClCHOH (X2A) and Cl (X2P). Photodissociation along the C-O bond resulting in CHCl2 (X2A') and OH (X2II) has to overcome a barrier of about 0.5 eV because the low-lying excited states 1(1)A", 1(3)A' and 1(3)A" become repulsive only after the C-O bond is elongated by about 0.3 A.     

Substrate-reagent complexation ability in enantioselective 1,4 addition reactions

Hydrogen-aarbonyl chelation and magnesium-arene π coordination are proposed to aooount for enhanced enantioselectivïty in 1,4-addition reactions of chiral cuprates.     

Non-empirical calculations on the rydberg states of ethylene

Different methods for representing the upper orbitals in Rydberg transitions are tested by means of a series of ab initio SCF and CI calculations for ethylene and various properties for such diffuse united-atom species are reported. Calculated transition energies indicates the maximum separation between individual components of the (π,3d) Rydberg species to be in the 0.1-0.2 range, with similarly small energy separations being obtained for the corresponding (π, 3p) states     

Theoretical study of hydrogen-bonded formaldehyde complexes

The hydrogen-bonded complexes involving formaldehyde and a series of proton donors of varying strengths, have been investigated at different levels of ab initio MO theory. The structures of the studied complexes were SCF optimized at the 6-31G basis set level. The binding energy was estimated employing basis set superposition correction, zero-point vibrations and MP2 correlation contribution at the different basis set: STO-3G; 6-31G; MP2/6-31G; 6-31G**; MP2/6-31G**; 6-311G(2d, 2p) and MP2/6-311G(2d, 2p). Linear relationships were found of the calculated binding energy with: the calculated shift in the carbonyl stretching frequency, the changes in carbonyl bond length and the optimum value of hydrogen-bond distance; furthermore the calculations confirm a parallel trend between the proton-donor ability and the strength of the hydrogen bond.     

CI method for the study of general molecular potentials

A method of configuration interaction designed for general molecular potentials is outlined. The technique employed to arrive at a symmetrized multideterminantal basis for such calculations relies heavily on certain properties of Abelian groups; in particular the M _L quantum number commonly employed in atomic structure calculations is replaced in the general molecular case by the index p which labels irreducible representations of some appropriate Abelian group. Formation of the desired symmetrized linear combinations of determinants is thereupon accomplished solely by means of a series of simple diagonalizations, a procedure which insures both the linear independence and the orthonormality of the resultant basis set. CI treatments involving the C₄H₄ isomer tetrahedrane (T _d point group) and the linear nitrous oxide NNO molecule are considered in some detail with a view toward illustrating the use of the techniques described herein. Finally, a survey of a series of recent calculations utilizing the CI method is made and it is concluded from these results that the effects of such treatment vary strongly from one system to another, depending in a very specific manner upon the individual characteristics of a given molecule.     

Funktionalisierte Alkinkomplexe von Wolfram(VI). Synthese und Kristallstrukturen von [WCl4(Et?Se?CC?Se?Et)(THF)] und [WCl4(Et?Te?CC?Te?Et)(THF)]

Functionalized Alkyne Complexes of Tungsten(VI). Syntheses and Crystal Structures of [WCl₄(Et? Se? C?C? Se? Et)(THF)] and [WCl₄(Et? Te? C?C? Te? Et)(THF)] The title compounds have been prepared by reactions of [WCl₄(SEt₂)₂] with the alkynes Et? X? C?C? X? Et (X = Se, Te) in CCl₄ solution and subsequent addition of tetrahydrofurane. Both complexes were characterized by crystal structure determinations. [WCl₄(Et? Se? C?C? Se? Et)(THF)]: Space group P2₁/n, Z = 4, structure determination with 2942 unique reflections, R = 0.038. Lattice dimensions at 20°C: a = 934.2, b = 1639.5, c = 1189.5 pm, β = 96.497°. [WCl₄(Et? Te? C?C? Te? Et)(THF)]: Space group P2₁/n, Z = 4, structure determination with 4097 unique reflections, R = 0.067, Lattice dimensions at ?70°C: a = 899.2, b = 1691.9, c = 1213.3 pm, β = 96.82°. The complexes have monomeric molecular structures with the oxygen atom of the THF molecules in trans-position to the side-on bound alkyne ligands.