Interaction Potential Energy Surface Research Articles

Hydrogen-bond swapping in the benzene-water complex: a model study of the interaction potential

Model calculations have been carried out to characterize the detailed nature of the benzene-water intermolecular interaction potential energy surface. At equilibrium, the calculations show a binding energy of 1303 cm -1 . The optimum water molecule's position is above the plane of the benzene ring and shifted from benzene's symmetry axis by 0.82 A. Of course, the equilibrium position alone does not satisfactorily characterize the benzene-water interactions because of the extreme floppiness of the complex

Accurate a b i n i t i o potential energy computations for the H4 system: Tests of some analytic potential energy surfaces

The interaction potential energy surface (PES) of H4 is of great importance for quantum chemistry, as a test case for molecule–molecule interactions. It is also required for a detailed understanding of certain astrophysical processes, namely, collisional excitation and dissociation of H2 in molecular clouds, at densities too low to be accessible experimentally. Accurate ab initio energies were computed for 6046 conformations of H4, using a multiple reference (single and) double excitation configuration interaction (MRD-CI) program. Both systematic and ‘‘random’’ errors were estimated to have an rms size of 0.6 mhartree, for a total rms error of about 0.9 mhartree (or 0.55 kcal/mol) in the final ab initio energy values. It proved possible to include in a self-consistent way ab initio energies calculated by Schwenke, bringing the number of H4 conformations to 6101. Ab initio energies were also computed for 404 conformations of H3; adding ab initio energies calculated by other authors yielded a total of 772 conformations of H3. (The H3 results, and an improved analytic PES for H3, are reported elsewhere.) Ab initio energies are tabulated in this paper only for a sample of H4 conformations; a full list of all 6101 conformations of H4 (and 772 conformations of H3 ) is available from Physics Auxiliary Publication Service (PAPS), or from the authors. The best existing analytic PESs for H4 are shown to be accurate only for pairs of H2 molecules with intermolecular separations greater than about 3 bohr (1.6 Å). High energy collisions (such as might lead to direct collisional dissociation) cannot be well represented by such surfaces. A more general analytic PES for H4 is required, which will be accurate for compact (high-energy) conformations and for conformations that cannot be subdivided into a pair of H2 molecules. Work in progress on devising such a surface (fitted to the 6101 conformations of this work) will be reported in a forthcoming paper.

Quantum scattering studies of electronically inelastic collisions of CN (X 2Σ+, A 2Π) with He

Using recent ab initio interaction potential energy surfaces for the CN (X 2Σ+, A 2Π)+He system [H.-J. Werner, B. Follmeg, and M. H. Alexander, J. Chem. Phys. 89, 3139 (1988)], we have calculated fully quantum cross sections for inelastic transitions between individual rovibrational levels of the A 2Π and the X 2Σ+ states of CN. We have concentrated on the transitions studied experimentally by Dagdigian and co-workers for CN+Ar, namely transitions between the rotational levels of the A, v=8 and X, v′=12, the A, v=7 and X, v′=11, and the A, v=3 and X, v′=7 vibrational manifolds. In the case of the 8→12 and 7→11 transitions the cross sections are large (0.1–1 Å2), and the dependence on initial Λ doublet level and on final rotational quantum number displays the same subtle alternations as seen experimentally. In the case of the 3→7 transitions, for which the vibrational levels are energetically much more separated, the calculated cross sections for CN+He are extremely small (10−5 Å2), far smaller than observed experimentally for CN+Ar. In order to resolve this discrepancy, we have carried out some additional ab initio calculations for the CN+Ar system, but the change in the interelectronic coupling potential appears not to be large enough to explain the magnitude of the experimental cross sections.

Semiclassical calculation of the vibrational structure of the B̃ 1B u Rydberg state of t r a n s-di-imide from a b i n i t i o configuration interaction potential energy surfaces

The applicability of the SDCI and CEPA methods to the excited state of the title molecule is investigated. Two basis sets are used, one of triple zeta quality extended with diffuse functions and another which also contained polarization functions. For the subsequent CI calculations closed shell as well as open shell, molecular orbitals are used. We investigate the Pople correction as a way to obtain size consistent results from the SDCI calculation. For each method, a two-dimensional potential energy surface of the 1(4ag,5bu) Rydberg state of trans-di-imide (HNNH) is calculated. The vibrational fine structure in the corresponding B̃←X̃ UV absorption spectrum is derived from these surfaces and the result is compared to the spectrum measured by Neudorfl et al. [P. S. Neudorfl, R. A. Back, and A. E. Douglas, Can. J. Chem. 59, 506 (1981)]. A semiclassical method [K. S. Sorbie, Mol. Phys. 32, 1577 (1976)] is used to obtain the vibrational frequencies. A slightly modified version of the Heller frozen Gaussian approximation [E. J. Heller, J. Chem. Phys. 75, 2923 (1981)] is proposed and used to obtain the intensities of the vibrational bands. We conclude that it is important to use the open shell molecular orbital basis and the SDCI plus Pople correction, or even better, the CEPA. Both methods give good results for the vertical transition energy and excited state geometry. The error in the vibrational frequencies is in the order of 10%, but the NN-stretch mode is best described by the CEPA method.

Trajectory calculations of high temperature and kinetic energy dependent ion–polar molecule collision rate constants

The trajectory method is used to calculate high temperature and kinetic energy dependent ion–polar molecule collision rate constants. A new ion–polar molecule interaction potential energy surface is derived which takes into account the average hard sphere diameter of the colliding partners and the dimensions of the permanent dipole and the induced dipole vectors. A model system, H+3+NH3 is chosen for the calculation. The calculated results show that at low temperatures and kinetic energies, the effects of the size of the ion–molecule partners and the dimensions of the dipole vectors on the collision rate constant are small. At high temperature and high relative kinetic energies, however, these effects are significant. The calculated results are in good agreement with existing experimental kinetic energy dependent reaction rate constants.

Adiabatic and diabatic potential energy surfaces for collisions of CN(X 2Σ+, A 2Π) with He

The interaction potential energy surfaces for CN(X 2∑+, A 2∏)+He have been computed from ab initio MCSCF and MCSCF-CI wave functions using an extensive basis set. In the presence of the He atom the two degenerate components of the CN 2∏ state split into wave functions of A′ and A″ symmetry, and the symmetry of the 2∑+ state reduces to A′. The two adiabatic potentials for the A′ states are transformed to a diabatic basis, which yields a fourth potential energy surface V1, describing the collision-induced electrostatic coupling between the two A′ states. The degree of mixing of the two diabatic A′ states has been determined by integration of the relevant nonadiabtic coupling matrix elements and, in a simpler method, from the coefficients of the MCSCF configurations. Both procedures yield virtually identical results. The nonadiabatic coupling matrix elements are strongly peaked near the CN bond distance at which the X 2∑+ and A 2∏ states cross in the isolated molecule. The diabatic coupling potential V1, however, is only weakly dependent on the CN bond distance, and decreases exponentially with the CN–He separation. Near the classical turning points for room temperature collisions the magnitude of V1 is approximately 50 cm−1. The V1 potential shows a bimodal character as a function of the collision angle θ. These results are discussed in connection with recent experiments of Dagdigian and co-workers.

MINDO/3 potential energy surface for hydrogen-graphite system: Active sites and migration

The potential energy surface for interaction of hydrogen with a cluster model (C 10H 8) simulating the basal plane of graphite has been calculated by using an open shell RHF version of the MINDO/3 method. Results suggest that there exists only an active center, which is coincident with a carbon atom of the model. The migration of chemisorbed hydrogen from an active center to another would take place through a potential barrier of about 2 kcal/mol, with the corresponding saddle point located above the center of a CC bond. According to this result, migration takes place almost freely, following the CC bonds, in agreement with the mechanism suggested by experience.

Metastability of isoformyl ions in collisions with helium and hydrogen

The stability of HOC(+) ions under conditions in interstellar molecular clouds is considered. In particular, the possibility that collisions with helium or hydrogen will induce isomerization to the stable HCO(+) form is examined theoretically. Portions of the electronic potential energy surfaces for interaction with He and H atoms are obtained from standard quantum mechanical calculations. Collisions with He atoms are found to be totally ineffective for inducing isomerization. Collisions with H atoms are found to be ineffective at low interstellar temperatures owing to a small (about 500 K) barrier in the entrance channel; at higher temperatures where this barrier can be overcome, however, collisions with hydrogen atoms do result in conversion to the stable HCO(+) form. Although detailed calculations are not presented, it is argued that low-energy collisions with H2 molecules are also ineffective in destroying the metastable ion.

Dissociation of vibronic states of C–1A′ DCN: Quantum treatment

The rates of dissociation of several vibrational states of nπ* excited (C–1A′) DCN have been determined via quantum dynamical means in which only the CD stretching and DCN bending motions are treated. The ab initio configuration interaction potential energy surface used in our earlier classical trajectory study of these same dissociation rates was employed in the present study. The results of this quantal study tend to support our earlier prediction that v2→v1 (bending-to-stretching) energy transfer plays an important role in determining the dissociation rates of these vibronic states. Surprisingly, the absolute rates obtained via the quantum method are in quite close agreement with a certain component of the classically determined rates.

The interaction potential energy surface of O 2Ar

A potential energy surface for the O 2Ar system has been obtained by a multiproperty analysis. The absolute integral cross sections at thermal energy, the high energy scattering cross sections, the second virial coefficients, and the P and R branches in the absorption infrared spectrum of the O 2Ar van der Waals molecule, have been analyzed in order to obtain the isotropic component of the interaction. This average potential curve shows a minimum of 11.5 meV at 3.72 Å. The quenching of glory structure and some features of the infrared spectrum have been used to extract information on the anisotropic component.

Coupled-channel study of rotational excitation of a rigid asymmetric top by atom impact: (H2CO,He) at interstellar temperatures

A quantum mechanical scattering study is carried out to test a collisional pumping model for cooling the 6 and 2 cm doublets of interstellar formaldehyde. The Arthurs and Dalgarno formalism is extended to the collision of an s-state atom with a rigid asymmetric top molecule and applied to rotational excitation of ortho formaldehyde by helium impact. Using a previously determined configuration interaction potential energy surface, the coupled-channel (CC) equations are integrated at 12 scattering energies between 20 and 95°K. Up to 16 ortho formaldehyde states, yielding a maximum of 62 CC equations, are retained to test convergence of computed cross sections. Resonance structure is obtained at ∼20.2, 32.7, and 47.7°K. The computed inelastic cross sections are averaged over a Maxwell–Boltzmann distribution and the resultant rates used to solve the equations of statistical equilibrium for the relative populations. The 6 and 2 cm doublets are found to be cooled only upon inclusion of the j=3 doublet.

Classical S-matrix calculations of vibrational transition probabilities in the Li+-H2system

Describes the first application of the classical S-matrix method to a system for which both experimental transition probabilities and suitable potential energy surfaces exist. Special emphasis is given to the effects on the calculated transition probabilities of vibrational anharmonicity of the hydrogen molecule, and the particular form of the potential energy surface on which the collision takes place. The calculated results are in good agreement with the major features of the experimental data, suggesting that classical S-matrix theory will allow the computation of macroscopic properties such as vibrational relaxation times for gas mixtures with an accuracy limited only by our knowledge of suitable interaction potentials. Although there are discrepancies between the present calculations and the details of the experimental behaviour, these are attributed to uncertainties in the form of the interaction potential energy surfaces at small ion-diatom separations, and the neglect of simultaneous rotational excitation.

Hydrogen bonding between neon and hydrogen fluoride

Hartree-Fock calculations have been carried out to characterize the potential energy surface for interaction of a neon atom with a molecule of hydrogen fluoride. The results exhibit formation of a linear hydrogen bond. Although this bond is weak (0.234 kcal/mole), its strength exceeds that found earlier for the neon-water hydrogen bond.