Showing papers on "Potential energy surface published in 1993"

Global geometry optimization of clusters using genetic algorithms

Abstract: A genetic algorithm is used to find the global minimum energy structure for Si 4 on an empirical potential energy surface. Given a suitable encoding of the cluster geometry, and an exponential scaling of the potential energy values to obtain a fitness function, the genetic algorithm can successfully optimize all degrees of freedom. With the number of potential energy function evaluations as a measure, the genetic algorithm is more economical than either a set of traditional, local minimizations or a molecular dynamics simulated annealing approach

The transition state of the f + h2 reaction.

Abstract: The transition state region of the F + H(2) reaction has been studied by photoelectron spectroscopy of FH(2)(-). New para and normal FH(2)(-)photoelectron spectra have been measured in refined experiments and are compared here with exact three-dimensional quantum reactive scattering simulations that use an accurate new ab initio potential energy surface for F + H(2). The detailed agreement that is obtained between this fully ab initio theory and experiment is unprecedented for the F + H(2) reaction and suggests that the transition state region of the F + H(2) potential energy surface has finally been understood quantitatively.

Femtosecond laser studies of the cis‐stilbene photoisomerization reactions

Abstract: Femtosecond laser studies have been performed on the photoisomerization reactions of cis‐stilbene to obtain the most detailed understanding to date of a polyatomic isomerization reaction in a condensed phase environment. These experiments demonstrate that vibrationally hot product molecules are formed within a few hundred femtoseconds of the escape of the molecule from the cis* region of the potential energy surface. Although the cis to trans reaction may proceed via a twisted intermediate structure, this intermediate is not intercepted on the ∼150 fs time scale. The frictional effects on the cis to trans reaction coordinate are found to be important and account for the anisotropy of the trans product molecules. Specific experiments presented in detail are the absorption spectrum of electronically excited cis molecules (cis*); the anisotropy decays for cis* showing motion along the reaction coordinate; the detection of the trans‐stilbene product using transient fluorescence and transient absorption, confi...

Transition-state spectroscopy via negative ion photodetachment

Abstract: Experiments which directly probe this transition states can provide detailed insight into the most important part of the reaction potential energy surface, enabling us to learn about the microscopic, interatomic forces that control chemical reactivity. One can improve on the resolution obtained in photoelectron spectroscopy by using a different technique, threshold photodetachment spectroscopy. Here, the anions are photodetached with a tunable laesr, and only those electrons produced with nearly zero kinetic energy are collected as a function of laser frequency. this, in principle, yields the same information as photoelectron spectroscopy, namely, the energies of vibrational and electronic levels of the neutral formed by photodetachment, but at considerably higher resolution (0.3-0.4 meV). A comparison of results from the two methods is presented in the following section. 40 refs., 9 figs.

Intermolecular bonding and vibrations of phenol⋅H2O (D2O)

Abstract: Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6‐31G(d,p) and 6‐311++G(d,p) basis sets. Fully energy‐minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest‐energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6‐311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra‐ and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H‐bond axis, and the β2 mode are strongly anharmonically coupled, and a two‐dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h‐phenol⋅H2O and d‐phenol⋅D2O employing two‐color resonance‐two‐photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes.

Determination of an improved intermolecular global potential energy surface for Ar–H2O from vibration–rotation–tunneling spectroscopy

Abstract: A new highly accurate and detailed intermolecular potential surface for Ar–H2O is derived by a direct nonlinear least squares fit to 37 far infrared, infrared, and microwave spectroscopic measurements. The new potential (denoted AW2) gives a much better description of the strong radial dependence of the anisotropic forces and of the binding energy than its predecessor, the AW1 surface [Cohen and Saykally, J. Phys. Chem. 94, 7991 (1990)]. The global minimum on the AW2 potential (De=142.98 cm−1) occurs at the position R=3.636 A, θ=74.3°, and φ=0°. At these coordinates the argon is located in the monomer plane between the perpendicular to the C2 axis (θ=90°) and the hydrogen bonded geometry (θ=55°). This orientation of the minimum is opposite of that found in recent ab initio calculations of Bulski et al. [J. Chem. Phys. 94, 8097 (1991)] and Chalisinski et al. [J. Chem. Phys. 94, 2807 (1991)]. Both sets of authors find a minimum at an antihydrogen bonded geometry corresponding to an orientation Ar–OH (θ=125°).

Ab initio calculation of a global potential, vibrational energies, and wave functions for HCN/HNC, and a simulation of the (A-tilde)-(X-tilde) emission spectrum

Abstract: We present a potential energy surface for the HCN/HNC system which is a fit to extensive, high quality ab initio, coupled‐cluster calculations. The new surface is an improved version of one that was reported previously by us [J. A. Bentley, J. M. Bowman, B. Gazdy, T. J. Lee, and C. E. Dateo, Chem. Phys. Lett. 198, 563 (1992)]. Exact vibrational calculations of energies and wave functions of HCN, HNC, and delocalized states are done with the new potential using a new method, which combines a truncation/recoupling method in a finite basis representation procedure with a moveable basis to describe the significant bend–CH stretch correlation. All HCN and HNC states with energies below the energy of the first delocalized state are reported and characterized. All delocalized states up to 18 347 cm−1 above the HCN zero‐point energy and higher energy localized HCN states are also reported and characterized. Vibrational transition energies are compared with all available experimental data on HCN and HNC, including high CH‐overtone states up to 23 063 cm−1. We also report a simulation of the A–X stimulated emission pumping (SEP) spectrum, and compare the results to experiment. The simulation is performed within the Franck–Condon approximation, and makes use of 400 even‐bend wave functions for the ground electronic state, and a realistic vibrational wave function for the first excited bend state in the excited A state. The potential for the A state is slightly modified, relative to one implied by a previously reported force field, to improve agreement with the experimental fundamentals for the A state. In addition, the A‐state wave function is adjusted slightly to improve agreement with the SEP spectrum. We also report Franck–Condon factors for odd bending states of HCN, with one quantum of vibrational angular momentum, in order to compare with the recent assignment by Jonas, Yang, and Wodtke [J. Chem. Phys. 97, 2284 (1992)], based on axis‐switching arguments of a number of previously unassigned states in the SEP spectrum.

Accurate quantum calculation for the benchmark reaction H2+OH→H2O +H in five‐dimensional space: Reaction probabilities for J=0

Abstract: A time‐dependent wave packet method has been employed to compute initial state‐specific total reaction probabilities for the benchmark reaction H2+OH→H2O+H on the modified Schatz–Elgersman potential energy surface which is derived from ab initio data In our quantum treatment, the OH bond length is fixed but the remaining five degrees of freedom are treated exactly in the wave packet calculation Initial state‐specific total reaction probabilities for the title reaction are presented for total angular momentum J=0 and the effects of reagents rotation and H2 vibration on reaction are examined

Full‐dimensional quantum mechanical calculation of the rate constant for the H2+OH→H2O+H reaction

Abstract: The cumulative reaction probability (CRP) (the Boltzmann average of which is the thermal rate constant) has been calculated for the reaction H2+OH↔H2O+H in its full (six) dimensionality for total angular momentum J=0. The calculation, which should be the (numerically) exact result for the assumed potential energy surface, was carried out by a direct procedure that avoids having to solve the complete state‐to‐state reactive scattering problem. Higher angular momenta (J≳0) were taken into account approximately to obtain the thermal rate constant k(T) over the range 300

Symmetry-adapted perturbation theory calculation of the He-HF intermolecular potential energy surface

Abstract: The many‐body symmetry adapted perturbation theory has been applied to compute the Ar–H2 potential energy surface. Large basis sets containing spdfgh‐symmetry orbitals optimized for intermolecular interactions have been used to achieve converged results. For a broad range of the configuration space the theoretical potential energy surface agrees to almost two significant digits with the empirical potential extracted from scattering and infrared spectroscopy data by Le Roy and Hutson. The minimum of our theoretical potential is em=−164.7 cal/mol and is reached at the linear geometry for the Ar–H2 distance Rm=6.79 bohr. These values agree very well with corresponding empirical results em=−161.9 cal/mol and Rm=6.82 bohr. For the first time such a quantitative agreement has been reached between theory and experiment for a van der Waals system that large. Despite such excellent agreement in the overall potential, the exponential and the inverse R components of it agree to only about 20%.

Ridge method for finding saddle points on potential energy surfaces

Abstract: A new method is proposed for locating saddle points on potential energy surfaces. The method involves walking on the ridge separating reactants’ and products’ valleys toward its minimum, which is a saddle point in coordinate space. Of particular advantage for ab initio calculations, the ridge method does not require evaluation of second derivatives of the potential energy. Another important feature of the method is that no assumptions about the transition state geometry are needed, and it is easy to impose linear constraints on the molecular structure. The ridge method is supplemented by a heuristic detour algorithm, which enables one to deal with unfortunate choices of reactants’ and products’ coordinates. Both algorithms are illustrated by several examples where the complexity of the potential energy surface ranges from a simple analytical formula to a numerical many‐body ab initio potential.

Structures for d0 ML6 and ML5 complexes

Abstract: A second-order Jahn-Teller argument is given to explain why certain d 0 ML 6 and ML 5 molecules will have geometries different from the octahedron and the trigonal bipyramid, respectively, as given by the VSEPR rules. Ab initio molecular orbital calculations were used to explore the potential energy surface for a variety of molecules. In the CrH 6 system, 20 stationary points were located and six were selected for study using a number of basis sets and electron correlation methods. The global minimum appears to be a C 3v (η 2 -H 2 ) 3 Cr isomer which QCISD(T) calculations put at being 165 kcal/mol more stable than the octahedral (On) one

Global energy minimum searches using an approximate solution of the imaginary time Schroedinger equation

Abstract: We present a method for finding the global energy minimum of a multidimensional potential energy surface through an approximate solution of the Schrodinger equation in imaginary time. The wave function of each particle is represented as a single Gaussian wave packet, while that for the n-body system is expressed as a Hartree product of single particle wave functions. Equations of motion are derived for each Gaussian wave packet's center and width. While evolving in time the wave packet tunnels through barriers seeking out the global minimum of the potential energy surface. The classical minimum is then found by setting Planck's constant equal to zero

Excitation function for H+O2 reaction: A study of zero‐point energy effects and rotational distributions in trajectory calculations

Abstract: The excitation function of the H+O2 (v=0)→OH+O reaction has been determined from trajectory calculations using the HO2 DMBE IV potential energy surface. Reactive cross sections for thirteen translational energies, corresponding to a total of a quarter of a million trajectories, have been computed covering the range 65≤Etr/kJ mol−1≤550. Various schemes for analyzing the trajectories, three of which aim to correct approximately for the zero‐point energy problem of classical dynamics, have been investigated. One of these schemes aims to correct also for known requirements on rotational distributions, e.g., for the fact that by Hund’s rules for the coupling of angular momentum the product OH (2Π) molecule always rotates. It has been found that zero‐point energy effects and lowest‐J constraints on rotational distributions may have a crucial role, especially close to the threshold energy of reaction. Agreement with recent measurements of absolute reactive cross sections is generally satisfactory but, unlike exp...

The tert-butyl cation (C4H9+) potential energy surface

Abstract: The CH 49 * potential energy surface (PES) is investigated using level ab initio molecular orbital theory. All structures were fully optimized at Hartree-Fock (HF) and correlated levels (HF/6-31G * , MP2(full)/6-31G * , and MP2-(full)/6-31G ** ) followed by single point energy calculations at MP4sdtq/6-31G ** //MP2(full)/6-31G ** . The C s 1, C 3h 2, and C 3v 3 forms of the tert-butyl cation were investigated

Quantum and quasiclassical calculations on the OH+CO→CO2+H reaction

Abstract: Scattering calculations on the OH+CO→CO2+H reaction are reported using both quantum and quasiclassical methods. The rotating bond approximation is used in the quantum calculations. This method explicitly treats the OH vibration and CO rotation in the reactants and the bending vibration and a local CO stretch in the CO2 product. Analogous quasiclassical trajectory computations are also reported. A potential energy surface obtained as a fit to ab initio data is used. The quantum reaction probabilities are dominated by sharp resonances corresponding to vibrationally excited states of the HOCO complex formed in the reaction. The quantum and quasiclassical lifetimes of these resonances compare quite well with measurements made by Wittig et al. Calculations of differential cross sections, rate coefficients, and CO2 vibrational product distributions are also compared with experimental data. The comparisons of quantum and quasiclassical calculations for models that treat explicitly different numbers of degrees of freedom provide detailed insight into the dynamics of the OH+CO reaction.

Ab initio study of the potential energy surface of CH4‐H2O

Abstract: The potential energy surface of CH4‐H2O is calculated through the fourth‐order Mo/ller–Plesset perturbation theory. In an attempt to obtain basis‐set saturated values of interaction energies the extended basis sets are augmented by bond functions which simulate the effects of high‐symmetry polarization functions. The absolute minimum occurs for the configuration involving the C–H‐O hydrogen‐bond in which O‐H points toward one of the faces of the CH4 tetrahedron. The equilibrium C–O separation is equal to 6.8 a0 which corresponds to the bond energy of 0.83 kcal/mol. Due to basis set unsaturation of the dispersion energy the bond energy may still be underestimated by about 0.05 kcal/mol. The secondary minimum involving the C‐H–O hydrogen‐bond is some 0.2 kcal/mol less stable, and the corresponding C–O distance is longer by 0.6 a0. The anisotropy of the potential energy surface is analyzed via the perturbation theory of intermolecular forces. The binding in CH4‐H2O is chiefly due to the dispersion energy whi...

New structure for the most stable isomer of the benzene dimer: a quantum chemical study

Abstract: New and surprising ab initio calculations suggest that the potential curve between two benzene molecules is more complicated than hitherto assumed. In fact, the calculations propose two minima on the potential energy surface of the benzene dimer. The most stable one is found to be the parallel-displaced structure so that the T-shaped structure is now found to be at slightly higher energy. The intermolecular distance found for the T-shaped structure agrees nicely with that predicted from new experiments

On the thermodynamic stability of clathrate hydrate. I

Abstract: The thermodynamic stability of a clathrate hydrate has been investigated by examining the free energy of formation of clathrate hydrate II encaging propane. The total free energy has been divided into several contributions—the interaction between water and guest propane molecules, the entropic contribution arising from the combinations of cage occupancy, and also the free energy due to intermolecular vibrations. The present method avoids some of the fundamental assumptions in the van der Waals and Platteeuw theory. This enables us to assess separately the factors which have a bearing on the thermodynamic stability of the hydrate. Kinetic stability has also been investigated by calculating molecular dynamics trajectories having initially excited several characteristic vibrational modes. We show, for propane in large cages, that the potential energy surface of the guest molecule in a cage has a single minimum and molecular motions can be approximated accurately to a collection of harmonic oscillators. It is...

Ab initio study of hydrogen bonding in the phenol-water system

Abstract: Three hydrogen-bonded minima on the phenol-water, C6H5OH—H2O, potential energy surface were located with 3–21G and 6–31G** basis sets at both Hartree–Fock and MP2 levels of theory. MP2 binding energies were computed using large “correlation consistent” basis sets that included extra diffuse functions on all atoms. An estimate of the effect of expanding the basis set to the triple-zeta level (multiple f functions on carbon and oxygen and multiple d functions on hydrogen) was derived from calculations on a related prototype system. The best estimates of the electronic binding energies for the three minima are –7.8, –5.0, and –2.0 kcal/mol. The consequences of uncertainties in the geometries and limitations in the level of correlation recovery are analyzed. It is suggested that our best estimates will likely underestimate the complete basis set, full CI values by 0.1–0.3 kcal/mol. Vibrational normal modes were determined for all three minima, including an MP2/6–31G** analysis for the most strongly bound complex. Computational strategies for larger phenol–water complexes are discussed. © John Wiley & Sons, Inc.

Analytic Second Derivatives of the Potential Energy Surface

Abstract: One of J. A. Pople's leading contributions in ab initio quantum chemistry was his 1979 paper in which he and his colleagues presented calculations for the analytic second derivatives of the Self Consistent Field Potential Energy surface. This led to the automatic characterization of stationary points as minima or transition states. Recently there has been an upsurge of interest in computational density functional theory (DFT). Here we describe our implementation of analytic gradients and second derivatives for the Kohn-Sham potential energy surface. This parallels similar developments by Pople and his coworkers. We examine Pople's idea of differentiating the weights in the quadrature scheme to ensure that the energy gradient is exactly zero at the energy minimum. We present some calculations on small molecules which are as near ‘exact’ as possible for the LDA and BLYP functional which we use. In other words we use large basis sets and a large number of quadrature points to evaluate the extra (nonanalytical) integrals. We do not use any fitting procedures and no other approximations are introduced.

Differential and integral cross sections for the inelastic scattering of NO (X 2Π) by Ar based on a new ab initio potential energy surface

Abstract: New ab initio potential energy surfaces for the Ar–NO (X 2Π) system are reported based on correlated electron pair approximation (CEPA) calculations. The fitted, rigid‐rotor surface was then used in full close‐coupling calculations of differential and integral cross sections for excitation of NO at a center‐of‐mass energy of 442 cm−1 (0.0548 eV), as well as differential cross sections at lower energies of 119 and 149 cm−1 (0.0145 and 0.0185 eV). The calculated cross sections are compared with those determined using earlier electron‐gas potential energy surfaces and with the results of available experimental measurements. In general, the new CEPA potential energy surfaces yield very good agreement with available experimental integral and differential cross sections. Both theory and experiment reveal a significant tendency for population of final rotational states of Π(A‘) reflection symmetry.

Nonadiabatic effects in the photodissociation of H2S in the first absorption band: An ab initio study

Abstract: The photodissociation of H2S through excitation in the first absorption band (λ≊195 nm) is investigated by means of extensive ab initio calculations. Employing the MRD‐CI method we calculate the potential energy surfaces for the lowest two electronic states of 1A‘ symmetry varying both HS bond distances as well as the HSH bending angle. (In the C2v point group these states have electronic symmetry 1B1 and 1A2, respectively.) The lower adiabatic potential energy surface is dissociative when one H atom is pulled away whereas the upper one is binding. For the equilibrium angle of 92° in the electronic ground state they have two conical intersections, one occurring near the Franck–Condon point. Because of the very small energy separation between these two states nonadiabatic coupling induced by the kinetic energy operator in the nuclear degrees of freedom are substantial and must be incorporated in order to describe the absorption and subsequent dissociation process in a realistic way. In the present work we ...

An ab initio structural and spectroscopic study of acetone—An analysis of the far infrared torsional spectra of acetone‐h6 and ‐d6

Abstract: The far infrared torsional spectra of acetone (CH3)2CO and (CD3)2CO have been determined from ab initio calculations, and the main features of the experimental data assigned. For this purpose, the potential energy surface for the double methyl rotation was determined with fully relaxed geometry into the RHF and RHF+MP2 approximations using a 6–31G(p,d) basis set. The energy values, as well as the kinetic parameters obtained from the optimized geometry, were fitted to double Fourier expansions as functions of the rotational angles in seven terms. The torsional solutions were developed on the basis of the symmetry eigenvectors of the G36 nonrigid group, which factorize the Hamiltonian matrix into 16 boxes. The energy levels and torsional wave functions for each symmetry specie were then obtained diagonalizing each blocks separately. Intensities were obtained from the calculated electric dipole moment variations and the nuclear statistical weights, and were combined with the torsional frequencies to predict the spectra. The calculated band patterns show a multi‐ plet structure and reproduce the main features of the experimental data. The torsional bands of the infrared active ν17 mode were found to be clustered into quartets, (A1→A2, G→G, E1→E1, E3→E4), for the v=0→v=1 fundamental, and (A2→A1, G→G, E1→E1, E4→E3) for the v=1→v=2 first sequence transitions. The G→G transitions were found to be the more intense. The correlation between the calculated and observed spectra allows for an assignment of the major bands.

State-Resolved Differential Cross Sections for Crossed-Beam Ar-NO Inelastic Scattering by Direct Ion Imaging

Abstract: State-testate differential cross sections for inelastic collisions of NO with Ar have been measured in a crossedbeam experiment using time-of-flight ion imaging. Rotational rainbow peaks are observed in the angular distributions, and these move to backward scattering angles with increasing final rotational level. The images are analyzed using a Monte Carlo forward convolution program that accounts for the transformation from the center-of-mass differential cross sections to the experimental image. The results are interpreted using a simple two-dimensional hard ellipse model to provide quantitative insight into the anisotropy of the potential energy surface. For NO (j’ = 18.5), two rainbow peaks are observed. These double rainbows have been predicted for scattering of atoms from heteronuclear molecules, but they have not previously been directly observed in the angular distributions. The analysis is also used to obtain the eccentricity of the hard ellipse potential from the positions of the two rainbow peaks. The angular distributions for the spin-orbit conserving collisions and spin-orbit changing collisions are remarkably similar, even though they were thought to involve two different potential energy surfaces. An alternative mechanism is proposed to account for the spin-orbit changing collisions through non-Born-Oppenheimer coupling of nuclear and electronic motion.

Ab Initio Anharmonic Vibrational Analyses of Non-Rigid Molecules

Abstract: A collection of recent state-of-the-art ab initio results is presefted pertaining to anharmonic vibrational analyses of non-rigid molecules. To demonstrate the performance of electronic structure theory in predicting features of potential energy surfaces to quantitative accuracy, investigations of energy barriers for large-amplitude vibrations in ammonia, isocyanic acid, ethane, and cyclopentene are reported. Anharmonic vibrational analyses based on self-consistent-field and configuration interaction quartic force fields are critically evaluated by focusing on representative results for hydrogen sulfide and nitrous oxide. The use of nonstationary reference structures to improve predictions of anharmonic force fields is reviewed and documented. A summary of an investigation using the complete state-of-the-art armamentarium to elucidate vibrational anharmonicity, vibration-rotation interaction, and the precise r e structure of the quasilinear HNCO molecule is given. Finally, difficulties arising in the application of nonstationary reference geometries in vibrational analyses of large-amplitude motions are highlighted to avert potential pitfalls in future ab initio studies.

Stability of MX2−3 ions in the gas phase and when do ionic molecules have large ionization potentials

Abstract: The present work aims at theoretical consideration of the geometrical and electronic structures of a homologous series of free MXmq−‐type alkali halides (M=Li, Na, K; X=F, Cl; m=1,2,3; q=0,1,2) in order to get insight into their kinetic and electronic stability. At the ab initio Hartree–Fock self‐consistent field (HF‐SCF) level of theory, the lowest energy fragmentation channel leading to the decomposition of the dianions MX32− into MX2− and X− has been investigated. The potential energy surface was found to exhibit a broad, but flat energy barrier to fragmentation. These findings have been confirmed using results from configuration interaction calculations and the molecular dianions are predicted to be long‐lived species formally existing in a metastable state. The stability of the gas‐phase MX32− dianions and of the MX2− fragmentation products with respect to autodetachment of an extra electron has been investigated using ab initio HF‐SCF and Green’s function methods. The inclusion of many‐body effects ...

Multiproperty determination of a new N2-Ar intermolecular interaction potential energy surface

Abstract: A new multiproperty potential energy surface for the N2–Ar intermolecular interaction is reported. The present determination is based upon molecular beam total differential and integral scattering data, taken together with the temperature dependence of the interaction second virial coefficient, transport properties, transport property field effects, and relaxation phenomena, such as pressure broadening of the depolarized Rayleigh line and longitudinal nuclear spin relaxation. The primary fit has been made to the beam scattering and virial data, and refinements to the potential parameters thus determined have been made by employing the data available for the gas phase transport and relaxation phenomena. The potential energy surface employed is an empirical Morse–Morse–spline–van der Waals form, in which the potential parameters depend upon the angle between the N2 figure axis and the line joining the centers of mass of N2 and Ar. No N2 stretching dependence has been included in the present determination. Comparison is made between the present potential energy surface and two other previously published N2–Ar potential energy surfaces. The present potential energy surface provides the best overall agreement for all available gas phase data for N2–Ar mixtures, and can thus be recommended for calculations of all properties of such mixtures that depend upon the intermolecular interaction.

Equilibrium geometry of HC3N

Abstract: A near-equilibrium potential energy surface has been calculated for cyanoacetylene, HC3N, by means of the coupled electron pair approximation (CEPA) using a basis set of 118 contracted Gaussian-type orbitals, and from it vibration-rotation coupling constants αv and 1-type doubling constants qt and qJ t have been calculated for various isotopomers by standard perturbation theory. By the combination of experimental B 0 and theoretical αv values for six different isotopomers we were able to determine an accurate equilibrium geometry: r e(HC(1)) = 1·0624 A, R 1e(C(1)C(2)) = 1·2058 A, R2e(C(2)C(3)) = 1·3764 A and R 3e(C(3)N) = 1·1605 A. A conservative estimate of the error in the equilibrium bond lengths is 0·0005 A.