The formation and decay rate constants of the pairs (M-II, e(s)(-)) involving solvated electron and alkaline earth metal cations (M-II = Sr-II, Ca-II) are determined by pulse radiolysis measurements in tetrahydrofuran (THF). The pairs present a strong reducing character and can reduce biphenyl to biphenylide radical anion. The observed absorption spectra of the pairs are broad and intense, similar to that of (Mg-II e(s)(-)). Compared to the absorption spectrum of solvated electron which is a single band located around 2250 nm, these absorption spectra are shifted to the blue and present two bands. The structure of the pairs is investigated by ab initio calculations, and their absorption bands are studied with an asymptotical method. The observation of two absorption bands for the pairs (M-II, e(s)(-)) is rationalized as a perturbation of the solvated electron by neighboring solutes which stabilize differently its s and p states and split the p states. The absorption spectra are compared with those obtained for the pair of solvated electron and alkali metal cations.

The conformation of the methylene blue (MB)-guanine (GUA) complex in water was investigated by molecular dynamics simulation. It was found that the T-shaped vacuo stable conformation becomes unstable in water and turns into a well-defined stacked conformation. The free energy of the complex formation was determined by the thermodynamic integration method and found equal to -7.2 +/- 0.2 kcal/mol. The variation of the hydration energy upon complexation was found to be positive (about 14 kcal/mol), being compensated by the solute-solute interaction energy (about -21 kcal/mol). The main contribution to this solute-solute interaction energy arises from the van der WaaIs forces (about -14 kcal/mol).

Atomic charges obtained with the fit of the ab initio electrostatic potential suffers of several defects, for instance, chemical meaning is not insured. We have employed a method recently put forward for deriving atomic charges which addresses the issue of chemical meaning and conformational transferability to N,N-dibutylacetamide and ethylenediaminetetraacetate. The charges have been used in molecular dynamics calculations where the interaction with a metallic cation is considered. We found structural parameters for the complexes in good agreement with the available experimental results.

A new set of effective atomic charges of different conformers of alanine dipeptide is presented. These charges are obtained by fitting the electrostatic potential resulting from the ab initio SCF wave function of the system obtained in a 6-31G basis set. A specific fit procedure is used providing charges weakly dependent on the fit points as well as on the geometry of the molecule. It is shown that these charges retain a reasonable chemical meaning. (C) 1999 John Wiley & Sons, Inc.

We propose a new method to derive atomic charges from ab initio molecular orbital calculations. These charges fit accurately the ab initio electrostatic potential and, simultaneously, they retain some chemical meaning; particularly, they are close to the Mulliken charges. The atomic charges were then used to study the methylene blue-guanine complex in vacuo by means of a molecular dynamics simulation. The sensitivity of the results of that simulation with respect to the charges was tested by comparing the results of that simulation with those obtained using a set of MOPAC charges.

The aim of the present article is to present a qualitative description of the ’optimised’ orbitals of molecular systems i.e. of the orbitals resulting from SCF calculations or from MCSCF calculations involving a valence CI : we do not present here a new formal development (although some formalism is necessary), nor a new computational method, nor an actual calculation of an observable quantity ... but merely the description of the orbitals.

The possibility of extracting the initial location of probe molecules in a micelle from the dynamics of their diffusion towards the interface has been studied. Advantages and drawbacks of the simulation of this dynamics with a Smoluchovski-type equation are discussed on the example of bianthryl in a cetyl trimethyl ammonium chloride micelle. It appears that the size and shape of the domain where the extinction of the probe molecules occurs, has a critical role in this approach. Its accurate knowledge is a prerequisite to any precise determination of the initial location of the probes.

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Ab initio calculations of the potential energy surface of the X3Σ− state of OCS2+ have been performed. A high and wide potential barrier is found for the dissociation into CO+ (X2Σ+) +S+ (4S). It appears that it is very unlikely that the system gets through this barrier when it is obtained from the ground state of OCS. In addition, a semi-quantitative correlation diagram between the lowest electronic states of OCS2+ and of CO+ +S+ is proposed. It appears on this diagram that the A 3Σ− state might lead to the ground state fragments via two successive non-adiabatic transitions.

AbstractA close coupled treatment in a vibrational adiabatic representation is applied to the study of molecular photodissociation dynamics. The procedure which is developed here involves three steps: transformation from a diabatic to an adiabatic basis set, truncation of the adiabatic basis set, back transformation to a reduced–diabatic basis set. In the two model cases which are studied, dissociation spectra show complicated peaks and dips, patterns interpreted in terms of shape and Feshbach resonances associated to vibrational predissociation with a relatively high potential barrier in the excited state. An important reduction in the number of channels required for a given final accuracy can be reached by using the reduced–diabatic basis set instead of the usual diabatic one. This is very promising for studying energy partitioning in molecular systems with several internal degrees of freedom taking part in the dynamics.

The rate coefficient for the charge transfer reaction C+2po + H → C 3p+ H+ is calculated with the introduction of the radial coupling between the two 3π states arising from both asymptotic atomic states. the derived rate coefficient at a temperature of 104K is 2 10−15 cm3s−1 which is two orders of magnitude larger than the value previously estimated by Butler and Dalgarno (1980) from a weak spin orbit coupling between the 3Σ− and 3Σ+ molecular states of CH+.

Ab initio SCF Cl calculations of the potential-energy curves of the a 3 II, b 3 σ − and d 3 II states of CH + have been performed. The Cl space has been built using a minimum shell of optimized orbitals (PAOs) plus a unique shell of oscillatory and polarization functions. The results reproduce the energy variations obtained by much larger calculations with a good precision.

A theoretical analysis of the quenching of Ar* in its two metastable states (3P2 and 3P0) by H is presented. It is shown that a simplified treatment of the electronic structure permits a discussion of the dynamics of that type of excitation transfer in terms of the relative position of a crossing (RC) of potential curves and of the region (RD) of decoupling of atomic states by the molecular field. In the present case we explain why the 3P2 state is more reactive than the 3P0 state.

A new method for computing polarized atomic orbitals (PAOs) is described: this method leads to very easy calculations. The space of the resulting PAOs is close to that of MC SCF MOs. Using these PAOs in the frame of a VB calculation has led to the same level of accuracy as the comparable MC SCF calculation for the dissociation energies and the lowest electronic transition energies of H2, H3 and N2.

Ab initio MCSCF-CI calculations have been performed to provide potential energie curves and coupling elements for the colinear (HeN2)+ system. These data are used to study the dynamics of the quasi-resonant He++N2 → He+N2+ (C,v′) reaction in the framework of the Bauer-Fisher-Gilmore multicurve-crossing model. It is shown that the most efficient non-adiabatic transitions take place at helium-N2 distances as short as 4–6 a.u.. For a large rotational excitation of the v′=3 level, as observed experimentally at thermal energy, the v′=3/v′=4 population ratio is correctly reproduced in both the 14N2 and 15N2 isotopes, also as a function of the initial N2(X) vibrational temperature.