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We present a model potential for studying M(q+)(H(2)O)(n=1,9) clusters where M stands for either Na(+), Cs(+), Ca(2+), Ba(2+), or La(3+). The potential energy surfaces (PES) are explored by the Monte Carlo growth method. The results for the most significant equilibrium structures of the PES as well as for energetics are favorably compared to the best ab initio calculations found in the literature and to experimental results. Most of these complexes have a different coordination number in cluster compared to experimental results in solution or solid phase. An interpretation of the coordination number in clusters is given. In order to well describe the transition between the first hydration sphere and the second one we show that an autocoherent treatment of the electric field is necessary to correctly deal with polarization effects. We also explore the influence of the cation properties (charge, size, and polarizability) on both structures and coordination number in clusters, as well as the meaning of the second hydration sphere. Such an approach shows that the leading term in the interaction energy for a molecule in the second hydration sphere is an electrostatic attraction to the cation and not a hydrogen bond with the water molecules in the first hydration sphere.

International audience; Isomer formation in dimeric complexes of a chiral naphthalene derivative (2-naphthyl-1-ethanol) with nonchiral or chiral primary and secondary alcohols (n-propanol, 2-methyl-1-butanol, 2-butanol, 2-pentanol) has been studied by hole-burning spectroscopy. Besides the spectroscopic discrimination between the homochiral and heterochiral complexes, previously observed in the fluorescence excitation spectra, ground-state depletion experiments have shown that each diastereoisomer is cooled in the jet in several isomeric forms. To get information on the structures of the complexes and on the influence of the solvent conformations of these structures, semiempirical calculations that rely on the exchange perturbation method have been performed. It has been shown that the most stable complexes involve a H-bond between the chromophore acting as the donor and the solvent and that they involve anti and gauche conformations of the solvent. The binding energy of the complexes results from a subtle balance between electrostatic and dispersive forces: the complexes involving the gauche and anti conformers of the solvent differ from each other by the amount of dispersion energy relative to the total interaction energy. The increase in the dispersive forces calculated for the complexes with the anti conformers has been related to a larger red shift of the absorption spectrum and is suggested to play a role in the observed chiral discrimination.

A study employing picosecond and subpicosecond excitation in a mass and photoelectron spectrometer is reported for the 7-azaindole (7-AI) dimers, reactive and unreactive. The 7-AI photoelectron spectrum is structured and has a sharp ionisation threshold at 8.17 eV above the neutral ground electronic state. The reactive dimer ionisation threshold was measured as 7.19 eV. The excited-state lifetime of the reactive dimer was measured by a technique that monitors the ionisation signal as a function of pulse duration. 1 + 1 resonance ionisation photoelectron spectra were recorded using 0.8 ps and 5 ps pulses. Our results indicate a lifetime substantially less than a picosecond, however, consistent with recent real time studies. Advantages of the method are discussed.

Results of molecular dynamics simulations of small van der Waals clusters composed of one argon cluster of size 147 or 125 (incomplete outer-layer cluster) and from one to four N2O molecules deposited at thermal relative collision energy on the argon cluster are presented. The potential energy is calculated through the semiempirical Claverie method. We discuss here the necessity and the practical application of fitting some of the potential parameters in order to reproduce the N2O experimental dipole moment value as well as the experimentally observed N2O⋯N2O and N2O⋯Ar equilibrium geometries. We first show that, as in the case of atomic projectiles, a very efficient capture by collision of the N2O molecules by the argon clusters is observed, independently of the initial molecular orientation. Studying trajectories over tens of nanoseconds then gives evidence that the N2O molecules move independently on the surface of the argon clusters, and that the molecules migrate randomly through jump displacements o...

Information about photoinduced electron transfer (ET) in isolated (collision free) molecular systems may be deduced from fluorescence and fluorescence excitation spectra of jet-cooled donor–acceptor (D–A) complexes and bridged donor–acceptor bichromophores (D–B–A). The structures of their different isomeric forms may be obtained by semi-empirical calculations. The relation between structures and essential parameters of ET processes (rates, energy thresholds, etc.) is treated in terms of a model based on the theory of radiationless transitions. Two distinct cases of a rapid (direct) ET corresponding to broad-band excitation spectra and of sequential processes in the systems with narrow-band spectra are discussed. The necessity of further studies and especially of time-resolved experiments at the picosecond timescale is emphasized.

The electronic spectroscopy of bichromophoric molecules linked by a OCH 2 O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxymethane (3), has been studied in a supersonic free jet by laser induced fluorescence. The experimental results have been compared to calculations resting on the perturbative CIPSI method, in which the di- and tri-excited configurations involving the π as well the v orbitals have been taken into account. The bis-phenoxymethane (1) molecule shows a single 0—0 transition which is blue-shifted relative to anisole by more than 400 cm −1 . This blue-shift has been theoretically related to the conformation of the bichromophore which displays an out-of-plane distorsion of the ether chain relative to anisole. The calculations clearly show that the observed blue-shift of the transition is related to this distorsion, and not to any electronic coupling between both cycles which is very weak. The single transition experimentally observed corresponds to the most stable structure of (1) which is in a gauche-gauche conformation relative to the CO bonds of the chain. In this structure the cycles are equivalent. The study of van der Waals complexes of (1) with usual solvents confirms this interpretation and shows that the equivalence of the cycles is removed by complexation. This contrasts with the bichromophore (3) whose 0—0 transition is blue-shifted by only 25 cm −1 relative to the 2,6-dimethylanisole subunit. Calculations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steric hindrance introduced by the methyl groups. Moreover the excited state of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of comparison, the fluorescence excitation spectrum of the van der Waals dimer of anisole has also been studied and exhibits a red-shift with respect to bare anisole. The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by means of the exchange perturbation theory.

Potential curves of the ${\mathrm{K}}_{2}$ and NaK molecules have been computed in the framework of pseudopotential methods, over a wide range of interatomic distances. At short internuclear distances, the agreement with experimental data is excellent, the mean deviation between theoretical and experimental spectroscopic constants being 1% for the two alkali dimers. At large interatomic separation, wells and avoided crossings are observed in adiabatic potential curves of highly excited states correlated to the asymptotes close to K[4p]+K[4p] and Na[3p]+K[4p]. For example, the well depths of the $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ states reach 2650 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{K}}_{2}$ and 4330 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the case of NaK. We demonstrate that they correspond to pseudocrossings between covalent states (dissociating into K[nl]+K[n\ensuremath{'}l\ensuremath{'}] or Na[nl]+K[n\ensuremath{'}l\ensuremath{'}]) and ionic states (correlated to ${\mathrm{K}}^{+}$+${\mathrm{K}}^{\mathrm{\ensuremath{-}}}$ for ${\mathrm{K}}_{2}$, ${\mathrm{Na}}^{+}$+${\mathrm{K}}^{\mathrm{\ensuremath{-}}}$, or ${\mathrm{Na}}^{\mathrm{\ensuremath{-}}}$+${\mathrm{K}}^{+}$ for NaK, the negative ion being in the ground state or an autoionizing state). As for ${\mathrm{Na}}_{2}$, these structures may play a crucial role in the interpretation of low-energy collisions. \textcopyright{} 1996 The American Physical Society.

A detailed study of the intrinsic consistency of the semiempirical method of P. Claverie namely, the effects of the basis set and intramolecular correlation on the multipole distributions of molecular subunits and the influence of the electronic population of each atom in the molecular subunit on its van der Waals radius, is performed on some van der Waals dimers. The validity, limits of this model and the appropriate way to use it is established. In particular, the dependence of the geometry and the interaction energy on the basis set chosen and the intramolecular correlation shows that the multipole distribution involved in the calculation of the electrostatic and polarization terms must be derived from a correlated wave function within an extended basis set. Associated to non local methods for finding stationary points, the method of P. Claverie reproduce reliably the intermolecular geometrical parameters observed for the equilibrium structures and the transition states of the dimer and trimer of acetylene. In addition, a study of the equilibrium structures of the ethylene dimer is presented, the aim of being to clarify the considerable uncertainty in their number and their geometry.

The three lowest triplet potential energy surfaces of HCN have been investigated with the complete active space multiconfigurational self consistent field (CAS/MCSCF) method. Two surfaces are issued from the ground state reactants, while the third one is issued from the first excited state of the reactants, and is coupled to the others by a conical intersection. Of the two surfaces issued from the ground state reactants, one possesses a very small potential barrier in the entrance channel, while the other surface has none, thus making the reaction possible at very low temperature. For each surface, two potential wells have been found, corresponding to bent structures HCN and HNC. These potential wells are separated by high isomerisation saddle points. The approach of H toward CN shows a potential barrier for each of the three surfaces. Some parts of the conical intersection have been calculated and found energetically lower than the ground state reactants. Thus, transitions between surfaces should play an important role in the reaction dynamics.

The core–valence correlation is introduced into ab initio relativistic pseudopotential calculations by modifying the existing core polarization potential. The salient feature of the method presented here is the use of an l‐dependent cutoff parameter (which is related to spherical harmonic functions) for solving the multicenter integrals over the 1/r4 ‐ and r/r3 ‐type operators. The method is tested on the Rb2 and Cs2 molecules considered as two valence‐electron problems. Reliable results for the molecular spectroscopic constants (Re, Te, De, and ωe ) are obtained for the ground state and the lowest excited states. Deviation from the experimental values ranges from 0.05 to 0.1 A for Re, seldom exceeds 2 cm−1 for ωe, and is of the order of 100 cm−1 for De for most of the excited states.

Ab initio MCSCF calculations of the potential energy curves of the first low-lying states of the CO 2+ dication have been performed. A localization procedure of MCSCF orbitals allows us to discuss the bond formation in the framework of valence-bond theory. The non-adiabatic curves corresponding to C + + O + , C 2+ + O and C + O 2+ are described using both orthogonal and non-orthogonal quasi-atomic orbitals, allowing us to re-examine the currently proposed models of bond formation between positively charged species.

We present here a model potential study of the microsolvation of alkali cations M+ (M = Na, K, Rb, Cs) in various solvents (water, methanol, dimethyl ether (DME)). The potential energy surfaces (PES) are explored with the Monte Carlo growth method (MCGM) to find the most significant equilibrium structures of M+−(solvent)n clusters (n = 2, 4). The structures as well as the binding energies are favorably compared to the best ab initio calculations found in the literature and to experimental results. This good agreement is only obtained if we take into account the anisotropy of the polarizability tensor for the solvent molecule. Under these conditions, the atomic parameters included in our model potential framework are found to be transferable from water to methanol and DME. An analysis of the different physical components of the interaction energy shows that the only important n-body term for the description of these systems is the polarization one.

International audience; The o-cyanophenol molecule and its hydrogen-bonded complexes with one and two molecules of water and methanol have been investigated by laser-induced fluorescence excitation, dispersed emission, and IR/UV double-resonance spectroscopy combined with DFT calculations. The sole conformer observed in the jet has a cis geometry due to the stabilizing interaction between the OH and CN substituents. The shifts of the electronic transition and the modification of the OH ground-state frequencies together with the calculated geometry point to a cyclic structure of the complexes. In the 1:1 complexes, the solvent OH binds as a proton acceptor to the phenol OH and as a proton donor to the CN group. The 1:2 complexes involve in a similar way the insertion of the solvent dimer (water)2 or (methanol)2 between the OH and CN substituents of the molecule.

International audience; The trapping of a naphthalene molecule in an argon matrix is simulated using an original method based on classical molecular dynamics calculations. A numerical simulation of the gas mixture deposition on a cold argon surface reproduces the matrix growing process. Three main trapping sites are obtained. The naphthalene replaces four or five argon atoms in the (1 1 1) crystallographic plane, or four argon atoms in the (0 0 1) crystallographic plane of the fcc argon lattice structure. The simulated structures are correlated to experimental site effects: the spectroscopic and dynamic molecular properties depend only on the lattice plane occupied by the naphthalene.

Owing to its exoergicity, the reaction N + CH → CN + H might produce CN in 3 electronic states: X2Σ+, A2Πi and B2Σ+. However, the correlation diagram rules out the production of B2Σ+ state for any symmetry of the reaction complex, either linear or planar. It has been checked experimentally. A vibrational population inversion in A2Πi state has been observed. The excitation limit at v’=16 agrees with the total energy available to the products. Theoretical calculations found no potential energy barrier as expected for an exoergic atom + radical reaction.

We have determined the vibrational band intensities of the 1 A 2←1 A 1 transition of formaldehyde by means of an ab initio calculation. The values obtained fit correctly the experimental evaluations. It is also found that confining the Herzberg-Teller type expansion to the first non-zero term is a poor approximation to the exact value.

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.

An ab initio calculation of dichroic vibrational band intensities has been performed for the n-π ∗ transition of the carbonyl group in acetaldehyde (as a model compound) in a rigid asymmetric conformation. Results show a 10% contribution of vibrational effects adding to the electronic contributions. Different signs are effectively obtained according to the vibrational bands, which is promising for further applications. Concerning the method itself, results are analogous to those obtained for oscillator strengths.

An SCF+CI calculation of the potential energy surface of H2O+ in Cinfinity v symmetry (linear approach) is reported. No energy barrier has been found for the chemical reaction O++H2 to OH++H. Instead, the energy of the intermediate species (O...H...H)+ is found to be 12 kcal/mole below the energy of the input products. This leads to the conclusions that the reaction can actually occur at low temperatures (interstellar clouds) and that the complete potential energy surface is worth computing.

Nous avons developpe une methode de calcul realisant le passage des surfaces de potentiel adiabatiques a des surfaces non-adiabatiques. Cette methode consiste a projeter les voies ouvertes du calcul adiabatique sur un sous-espace [math] de reference et a y definir un hamiltonien effectif qui redonne les projections sur [math] des fonctions propres et les valeurs propres du calcul adiabatique. Nous avons etudie l’influence du choix de la base de reference sur l’exemple du systeme (ArH)+ . La methode donne de bons resultats pour des distances superieures a 4 ua environ.

The nuclear magnetic shielding of 29Si, 31P, 33S and 35Cl in SiH4, PH3, SH2 and ClH has been computed. The inner shells of the heavy atom are not introduced in the calculation for the molecule. Their effect on the valence shell is represented by a pseudo-potential. The agreement with available theoretical and experimental values is satisfactory.

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.

The vertical ionization potentials of the K-shell of carbon have been calculated in CH4, CH3F, CH2F2, CHF3 and CF4. The ab initio SCF CI approach has been used for the first three compounds with an attempt to rationalize the choice of the atomic orbital basis sets. The ionization potentials of CHF3 and CF4 have been extrapolated from the proceeding results. The effect of the CI cannot be neglected in either of these compounds, but it can in CH4.

The jet-cooled fluorescence spectra of perylene excited to the S 1 state with E vib = 0–1600 cm −1 are recorded and analyzed. For E vib −1 only the resonant fluorescence was detected. Ground- and excited-state frequencies of 14 low-frequency normal modes are determined. A drastic change in frequency of the “butterfly” modes upon electronic excitation shows that perylene slightly deviates from planarity in its ground state and is more rigid in the excited singlet state. For a number of levels in the E vib = 800–1600 cm −1 range, the fluorescence is composed of the resonant emission and of non-resonant (“‘relaxed’”) bands. It is shown that apparently single bands in the fluorescence-excitation spectrum correspond to ovelapping bands pumping different molecular eigenstates resulting from the intrastate coupling. The relative role of the anharmonicity and of the Coriolis interaction are discussed. The data are treated in terms of a selective coupling between doorway and hallway states with the coupling constant rapidly decreasing with the difference in the overall vibrational quantum number between initial and final state.

Laser-induced fluorescence spectra for the first allowed electronic transition (22125 cm −1 ) of methylglyoxal (CH 3 COCHO) and its perdeutero analog (CD 3 COCDO) in a supersonic nozzle beam are quantitatively represented assuming that the potential function governing the CH 3 (CD 3 ) rotation is changed during the transition. In the excited state the potential function is ternary ( V 1 = 95 (1 + cos 3θ)cm −1 ) as in the fundamental state ( V 0 = 134.5 (1 - cos 3θ)cm −1 ), but the minima are shifted by an angle of π/3. The spectrum of biacetyl (CH 3 COCH 3 CO) can be reproduced assuming two uncoupled methyl groups undergoing similar conformational changes during the electronic transition (the estimated potential function is V 1 = 117.5 (1 + cos 3θ) cm −1 for each methyl group), in perfect agreement with the most recent assignment of the 0-0 transition. These results are consistent with ab initio calculations for the fundamental and first excited singlet states.

The positions of the two satellites exhibited by the Lyman (Y line of hydrogen in the presence of an argon plasma have been determined in the frame of the quasi-static theory by means of an nb inirio potential. One of the satellites appears to be due to a minimum in the potential of the levels arising in the {Ar '(3~') +H(n = 2)} configuration arid the other satellite to an avoided crossing between these levels and the levels of the {Ar(3p54s) + H'} configuration.

Fluorescence excitation spectra (FES) and fluorescence spectra (FS) under single vibronic level excitation of perylene in a supersonic jet are reported. The fluorescence from the lowest vibronic levels of the S1 state is purely resonant while for δEvib > 62; 1600 cm−1 broad-band emission spectra weakly dependent on νexe are explained by vibrational redistribution with a characteristic time 5

The matrix elements of the transition dipolar momentum IR have been computed between the low-lying states associated to the Lyman–α transition. The orthogonalization of the involved electronic states (obtained by the CIPSI method) has been performed and led to a better value for the asymptotic behavior of the {Ar(3p5, 4s) + H+} configuration. Position of the satellites in the red and blue wing of the line are deduced from the potential-energy curves in the frame of the quasistatic theory.

Les barrieres d'inversion de l'azote dans l'aziridine, l'oxaziridine et la methylene imine et la barriere d'inversion du carbanion dans l'anion vinyle ont ete etudiees a l'aide d'orbitales moleculaires S.C.F. localisees. La variation des interactions entre orbitales localisees (liaisons, couches internes, doublets libres) lors de l'inversion permet de distinguer un effet « primaire » limite au site NH (ou CH⊖), et un effet « secondaire » provenant du reste de la molecule. Les deux facteurs interviennent dans la determination de la barriere d'inversion; toutes les interactions deux a deux sont significatives. L'origine energetique des barrieres ne peut etre localisee en un site donne (par ex. NH) mais est delocalisee sur toute la molecule.

Une etude theorique des barrieres d'inversion de l'azote dans l'aziridine et dans l'oxaziridine a ete effectuee dans le cadre de la methode ab initio SCF-LCAO-MO. — Les barrieres d'inversion calculees sont de 18,3 kcal/mole pour l'aziridine et de 32,4 kcal/mole pour l'oxaziridine. Ces barrieres, ainsi que d'autres proprietes moleculaires, sont en bon accord avec les valeurs experimentales. — L'analyse des resultats fournit une image physique de l'origine des barrieres d'inversion et des effets des heteroatomes.

An ab initio calculation is used to give the form of the ionization potential in the alkane series and to explain the origin of the various contributions to this potential. The question of whether the ionization relates to a σ-type orbital, composed mainly of carbon atomic orbitals, or to a π-type, composed of CH bonds, is considered. The results show a gradual evolution from a CH ionization (13·26 eV) in CH4 to a CC ionization (10·0 eV) at the farthest limit of the series.

Une etude theorique ab initio SCF-LCAO-MO de la methylene imine et de l'anion vinyle a ete effectuee. Les barrieres d'energie calculees pour l'inversion de l'azote et pour la rotation autour de la liaison C=N sont respectivement de 26–28 kcal/mole et de 57 kcal/mole. La barriere d'inversion du carbanion calculee pour l'anion vinyle est de 39 kcal/mole et l'isomerisation des imines se fait donc par inversion de l'azote. Les anions vinyliques ont une configuration stable. L'origine physique de ces barrieres d'inversion est etudiee et caracterisee par les variations des termes energetiques, de la composition de l'OM du doublet libre et des populations electroniques. La comparaison des processus d'inversion du carbanion, de l'azote et de l'oxonium dans la serie CH2=CH⊖, CH2=NH et CH2=OH⊕ a permis de mettre en evidence l'evolution des termes energetiques, de la hauteur de la barriere et des transferts de population.

The charge - transfer reaction between protons and oxygen atoms is critical to the chemistry of the oxygen family; the corresponding rate was evaluated by Field and Steigman (1971) on the assumption of orbiting collisions and statistical probability distribution among the levels. We re-examine this reaction, including the fine-structure excitation process, basing our analysis on a careful description of the different potential curves arising from the O-H+ and 0+-H systems and on the evaluation of the coupling responsible for the transitions.