Marc C. van Hemert

Variation-perturbation treatment of the hydrogen bond between water molecules

The hydrogen-bond energy of two water molecules has been calculated as a sum of the electrostatic, exchange, induction and dispersion contributions, neglecting the electron correlation within the free monomers. The last two contributions have been evaluated by applying a variation-perturbation procedure and making use of an extensive basis set of contracted gaussian functions. It has been shown that the sum of the electrostatic, exchange and induction energies is very close to the binding energy obtained within the SCF scheme. The dispersion contribution to the hydrogen-bond energy amounts to about 2 kcal/mole and causes substantial reduction of the equilibrium distance of the oxygen atoms. The minimum of the total energy is attained at 2·86 A and its depth is equal to 5·8 kcal/mole. These values are consistent with the experimental results.

Resonances in the photodissociation of OH by absorption into coupled 2Π states: Adiabatic and diabatic formulations

The bound 3 2Π and repulsive 2 2Π states of OH are strongly coupled by the action of the nuclear kinetic energy operator. The process of photodissociation by absorption into the coupled  2Π states is studied theoretically. The adiabatic electronic eigenfunctions and potential energy curves of the 2 2Π and 3 2Π states are calculated using large configuration‐interaction (CI) representations and the nuclear radial coupling matrix elements are obtained by numerical differentiation. The coupled equations for the nuclear wave functions of the two states are set up in an adiabatic and in a diabatic formulation and are solved by numerical integration. The electric dipole transition moments connecting the ground X 2Π state to the 2 2Π and 3 2Π states are computed from the CI wave functions and the resulting photodissociation cross sections of OH arising from absorption into the coupled 2 2Π and 3 2Π states are obtained. Two alternative sets of potential curves, coupling matrix elements, and transition moments are...

ROTATIONALLY INELASTIC AND HYPERFINE RESOLVED CROSS SECTIONS FOR OH-H2 COLLISIONS. CALCULATIONS USING A NEW AB INITIO POTENTIAL SURFACE

Cross sections and rate constants are presented for the rotational excitation of OH in collision with ortho and para‐H2, using a new ab initio interaction potential [Offer and Van Hemert, J. Chem. Phys. 99, 3836 (1993)]. The cross sections are given at a number of energies and are compared with those calculated using an earlier potential energy surface, and with the available experimental results. A strong oscillatory behavior is found in the cross sections for collisions with ground state para‐H2 which was not apparent in earlier calculations. The oscillatory behavior is very much reduced in collisions with ortho‐H2. Rate constants obtained by averaging the cross sections over a Maxwell–Boltzmann velocity distribution are given at a temperature of 300 K. Expressions for calculating the hyperfine resolved cross sections by transforming the S‐matrices are discussed for the case where H2 is no longer constrained to its rotational ground state, and cross sections for transitions between the hyperfine resolved levels are given for collisions with both para and ortho‐H2.

Ab Initio Molecular Dynamics Calculations versus Quantum-State- Resolved Experiments on CHD3 + Pt(111): New Insights into a Prototypical Gas−Surface Reaction

The dissociative chemisorption of methane on metal surfaces is of fundamental and practical interest, being a rate-limiting step in the steam reforming process. The reaction is best modeled with quantum dynamics calculations, but these are currently not guaranteed to produce accurate results because they rely on potential energy surfaces based on untested density functionals and on untested dynamical approximations. To help overcome these limitations, here we present for the first time statistically accurate reaction probabilities obtained with ab initio molecular dynamics (AIMD) for a polyatomic gas-phase molecule reacting with a metal surface. Using a general purpose density functional, the AIMD reaction probabilities are in semiquantitative agreement with new quantum-state-resolved experiments on CHD3 + Pt(111). The comparison suggests the use of the sudden approximation for treating the rotations even though CHD3 has large rotational constants and yields an estimated reaction barrier of 0.9 eV for CH4 + Pt(111).

Photodissociation of water. I. Electronic structure calculations for the excited states

Results of ab initio calculations for the four lowest excited states of both A′ and A″ have been discussed. In the multireference configuration interaction calculations, a large Rydberg basis set has been used. Three-dimensional potential energy surfaces, and matrix elements of the transition dipole moment between the excited states and the ground X state, and the electronic angular momentum operator between the A state and the B and X states have been presented. The calculations show that above about 124 nm the photodissociation can be well described by the three lowest electronic states, X, A, and B. The ab initio results of matrix elements of the electronic angular momentum operator allow a realistic nonadiabatic treatment of the photodissociation in the B band. At wavelengths smaller than about 124 nm, the dynamics will be more complicated because of the coupling between various electronic states.

Photodissociation of water. II. Wave packet calculations for the photofragmentation of H2O and D2O in the B̃ band

A complete three-dimensional quantum mechanical description of the photodissociation of water in the B band, starting from its rotational ground state, is presented. In order to include B-X vibronic coupling and the B-A Renner–Teller coupling, diabatic electronic states have been constructed from adiabatic electronic states and matrix elements of the electronic angular momentum operators, following the procedure developed by A. J. Dobbyn and P. J. Knowles [Mol. Phys. 91, 1107 (1997)], using the ab initio results discussed in the preceding paper. The dynamics is studied using wave packet methods, and the evolution of the time-dependent wave function is discussed in detail. Results for the H2O and D2O absorption spectra, OH(A)/OH(X) and OD(A)/OD(X) branching ratios, and rovibrational distributions of the OH and OD fragments are presented and compared with available experimental data. The present theoretical results agree at least qualitatively with the experiments. The calculations show that the absorpt...

Photodissociation of water in the à band revisited with new potential energy surfaces

Theoretical calculations on the photodissociation of water in the first absorption band have been used to test the accuracy of three available potential energy surfaces for the first excited state of water: the well-known coupled electron pair approximation potential of Staemmler and Palma [Chem. Phys. 93, 63 (1985)], and two new multireference double excitation configuration interaction surfaces: the Dobbyn–Knowles surface (unpublished), and the Leiden surface [R. van Harrevelt and M. C. van Hemert, J. Chem. Phys. 112, 5777 (2000)]. Exact quantum mechanical calculations, using the wave packet approach, have been performed for J″>0, where J″ is the initial rotational state of the water molecule. The cross section was found not to depend strongly on the rotational state, so that it is reasonable to compare calculated cross sections for J″=0 with experimental room temperature cross sections. Small and simple corrections were applied to the potential energy surface to improve the agreement between theory and...

FINE-STRUCTURE EXCITATION OF C + AND Si + BY ATOMIC HYDROGEN

We present calculations of cross sections for fine-structure excitation in collisions of carbon and silicon ions in the 2 P state with atomic hydrogen in the ground state. The results are based on accurate calculations of CH + and SiH + molecular potentials, including electronic core correlation and relativistic effects. We find that the energy dependence of the excitation cross sections is largely determined by shape resonances. Our work improves on the results of previous calculations with less accurate potentials. Analytical expressions for the cooling efficiency of C + ( 2 P1=2 )a nd Si + ( 2 P1=2) are given for the temperature interval 15‐2000 K. Subject headingg ISM: atoms — ISM: molecules — scattering

Photodissociation of CH2. I. Potential energy surfaces of the dissociation into CH and H

The photodissociation processes of CH2 into CH and H have been studied using ab initio multireference configuration‐interaction methods. Two‐dimensional potential energy surfaces of the ten lowest triplet states correlating with the seven lowest states of CH have been calculated as functions of bond angle and one C–H bond distance, keeping the other C–H distance fixed at the equilibrium CH2 value. Transition dipole moments connecting the excited states with the ground state have been obtained as well. It is shown that efficient photodissociation of CH2 into CH (X 2Π)+H can occur by absorption from the ground X 3B1 (1 3A‘) state into the 1 3A1 (1 3A’) state at about 6.3 eV. Photodissociation into excited CH (a 4Σ−)+H can take place through the 1 3A2 (2 3A‘) and 2 3B1 (3 3A‘) states, although in a more complex manner since several avoided crossings occur along the reaction path. The 1 3A2 state is a so‐called low‐angle state, which has an equilibrium bond angle of less than 60° and correlates directly with...

Photodissociation of CH2. II. Three‐dimensional wave packet calculations on dissociation through the first excited triplet state

Quantitative results on photodissociation of CH2(X 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three‐dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two‐dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three‐dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment.