J.P. Flament

Compact and accurate valence bond functions with different orbitals for different configurations: application to the two-configuration description of F2

Abstract A way of computing reliable and very compact ab initio classical valence bond wavefunctions is presented. The method deals with a minimal number of configuration-state functions (CSFs), with only one per Lewis structure necessary to describe the active part of a chemical system since each CSF is allowed to have its specific set of orbitals, different from one CSF to the other. The coefficients of the CSFs and their orbitals, which remain purely local without any delocalization tails, are simultaneously optimized through a non-orthogonal MCSCF technique. The method is applied to the electronic structure of F 2 . The wavefunction involves only two configurations, and yields a dissociation energy of 28.6 kcal/mol, which compares very well with the estimated full-CI result of 29–30 kcal/mol in an analogous basis set.

Ab initio investigation of the ground state potential surfaces of HeNO+ and ArNO+

Abstract New experiments based on resonant multiphoton ionization spectroscopy have allowed the observation of ArNO and ArNO + ground states. We present a new theoretical method, using a CIPSI approach, to calculate the ground state potential surfaces of XNO + complexes (X = He, Ar), which gives results in good agreement with experiment.

MCSCF-Cl calculations of the CO+ ion

Abstract Multiconfiguration self-consistent field + configuration interaction wavefunctions were used to compute potential energy curves of the lowest doublet and quadruplet states of CO + . Spin—orbit splitting the three lowest 2 Π states and dipole moment for the X 2 Σ + , A 2 Π and D 2 Π states have been determined. Transition moments for the A 2 Π-X 2 Σ + ,D 2 Π-X 2 Σ, (3) 2 Π-X 2 Σ + , D 2 Π-A 2 Π and B 2 Σ + -A 2 Π transitions have been calculated. Theoretical oscillator strenghts for the D 2 Π-X 2 Σ + and D 2 Π-A 2 Π transitions were calculated and the results show that these transitions could be observed.

Isomerisation des ions [C2H4O]+·. Une approche SCF AB initio de la tautomerie ceto-enolique en phase gazeuse

Abstract Ab initio molecular orbital calculations are used to investigate isomerisation and fragmentations of ions [CH 2 CHOH] +· ( a ) and [CH 3 CHO] +· ( b ). Calculations carried out at the 3-21G level with complete geometry optimisation and inclusion of electron correlation confirm the stability of a third structure [CH 3 COH] +· ( c ). (The calculated relative energies of a, b and c are −12.0, 0.0 and 2.6 kcal mol −1 , respectively.) Exploration of part of the [C 2 H 4 O] +· potential energy surface (3-21G*/MNDO level) reveals that the isomerisation b → a via both direct 1,3-hydrogen migration or two successive 1,2-hydrogen shifts ( b → c → a ) needs ca. 60 kcal mol −1 . This energy requirement is ⋍ 30 kcal mol −1 greater than that needed by the dissociation into [CH 3 CO] + + H · and thus explains the different behaviour of the stable and metastable ions a and b . The minimum energy reaction path connecting a to [CH 3 CO] + + H · passes through ion c ; the rate-determining step ( a → c ) is responsible for the large kinetic energy released during this decomposition.

Force field for platinum binding to adenine

The antitumor drug cis‐diamminedichloroplatinum(II) (cisplatin) binds preferentially to GpG and ApG sequences of DNA, forming N7,N7 intrastrand chelates. Molecular modeling of the intrastrand adducts have been handicapped, so far, by the lack of force‐field data describing the Pt–guanine and Pt–adenine binding. We used ab initio calculations with relativistic pseudopotentials to evaluate three important parameters for the platinum–adenine model complex [Pt(NH3)3(Ade)]2+: (1) the force constant for the PtN7 bond bending out of the adenine plane; (2) the energy profile for the torsion about PtN7; (3) a set of fractional atomic charges that reproduce the ab initio potential for a number of space points placed around the adduct. A population analysis and comparative study on the tetrammine complex [Pt(NH3)4]2+ have shown that for platinum adenine is a better σ‐donor than NH3, but its capacity as a π‐acceptor is weak.

Ab initio CI study and vibronic analysis of the photoelectron spectra of formaldoxime

Abstract The photoelectron spectrum of formaldoximie, CH 2 NOH, has been re-investigated with higher resolution and interpreted by, means of ab initio SCF Cl calculations. Calculations have confirmed that the states increase in energy as π 1 1 2 2 and have shown the existence of a shake-up peak at ≈15 eV. The calculation of Franck-Condon factors allowed the interpretation of the observed vibrational structure.

MCSCF calculations on the vibrational properties of CN− adsorbed on platinum

Abstract The electronic and vibrational properties of CN − adsorbed on Pt electrodes have been studied through MCSCF calculations on the molecular ions PtCN − and PtNC − . The results are in good agreement with experimental results obtained in situ by the non-linear optical technique of sum frequency generation.

Vibrational modelling in large polycyclic aromatic hydrocarbons

Abstract The scaled quantum mechanical force field (SQM) of a series of polycyclic aromatic hydrocarbons (PAH) of astrophysical interest was calculated at the MNDO level. The set of scale factors for each type of coordinate involved were determined for the smallest molecules of the series, benzene and naphthalene and their deuterated analogues. It the largest molecules, the transfer of the internal coordinates produces a redundant set. The scaling process was modified in order to be able to use a non-redundant set, not destroying symmetry. The resulting mean error in the frequencies is reduced from 30 to 25 cm −1 , if the set of scale factors extracted from benzene is replaced by that calculated from naphthalene.

An SCF CI study of the electronic states of the sulfur dioxide dication (SO2+2)

Abstract The energies of electronic state of SO 2+ 2 at three different geometries have been calculated using a correlated wavefunction. These calculations are discussed in connection with recent results obtained by double-photoionization, double-charge-transfer and double-mass-spectrometry experiments. Our results predict a linear geometry for the dication in its ground state and also suggest the possibility of forming a metastable state of SO 2+ 2 by charge stripping from the singly charged ion.

Determination of contracted atomic basis sets for the study of CN−; application to the MCSCF calculations of its spectroscopic constants

Abstract Contracted atomic basis sets have been constructed from ANOs in order to describe the ground and excited states of C and N and their anions. Application is given with the MCSCF calculation of the vibration frequency of CN − and of other spectroscopic constants. A procedure is proposed to do MCSCF calculations when some problems of convergence arise.