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Three dimensional potential energy surfaces for the collision systems OH(X 2Π)+He and OH(X 2Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate 2Πx and 2Πy states split into two states of 2A′ and 2A″ symmetry, respectively, when the C∞v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close-coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet resolution show, within the same spin–orbit manifold and at low collision energies, a propensity to populate preferentially the e final levels in the F1(2Π3/2) state and an e/f conserving propensity in the F2(2Π1/2) state, while transitions between the two spin–orbit manifolds show a parity conserving propensity. For the v=2 vibrational level kinetic rate coefficients were calculated for a large range of temperatures. The calculated cross sections are in excellent agreement with recent measurements of Schreel, Schleipen, Epping, and ter Meulen.

The accuracy of two density functional derived modelsBLYP/6-31G* and B3LYP/6-31G*is tested to calculate the molecular response to slow neutrons and infrared photons in a series of oligomers of thiophene. In the first type of experiment, the response is a function of the vibrational frequencies and the shapes of the normal modes; in the second, knowledge of the dipole moment surface is also necessary. The combination of the two simulations allows one to conclude that both models give fairly accurate vibrational frequencies and normal modes but may overestimate the infrared response in large systems. For this spectroscopy, BLYP/6-31G* and B3LYP/6-31G* find all the modes present in the experiment to be active. A few modes with modest activity are also calculated to appear strongly in the spectrum. Scaling of the force fields shows the complementary roles of the two methods. BLYP/6-31G* is very accuratescaling factor of 1.00in the calculation of the Cα−Cβ, Cα−Cα, and HCC force constants, and B3LYP/6-31G* does not require scaling of CS, SCC, CCC, and CSC force constants. On the basis of the combined use of the two models, a simple procedure is proposed that should give good agreement with experimental results of conjugated systems.

Inelastic neutron scattering experiments are combined with infrared and Raman data to obtain a uniquely defined description of the intramolecular vibrations of three oligomers of polythiophene. Through refinement of ab initio force fields, the three vibrational spectra of each oligomer are simulated with remarkable accuracy. Two different basis sets of atomic orbitals are used: the first, is 6-31G* and is used to optimize the geometries and calculate the relevant force fields of α-2T and α-4T, the second is 3-21G* and is used for the same purpose for α-4T and α-6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameters is generated for the 6-31G* basis and another for the 3-21G* basis. The parameters are common to both molecules calculated with either basis sets and are believed to be transferable to higher isomers. The fitting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimental infrared and Raman activity, then a fitting of the Inelastic Neutron Scattering profile is performed, finally, the infrared and Raman spectra are calculated with the new normal modes and the ab initio derivatives of the dipole moment and the polarizability. The procedure is iterated until the three spectra of each oligomer are satisfactorily reproduced. For α-4T, two scaled force fields are obtained—one for each basis set—and are shown to yield very similar normal modes. It is important to emphasize that not only the vibrational frequencies but also the spectral intensities are well reproduced by the simulations. Implicitly, this means that the dipole moment and the polarization tensor surfaces calculated ab initio at the potential energy surface minimum are of good quality. The procedure is absolutely general and can be applied to any molecular system. In the present case, it leads to well defined force fields that give us a stringent picture of the vibrations of these molecules.

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 Å, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities for the X and A states of the OH radical and for the Ar atom. These data are used to estimate the contributions of induction and dispersion forces to the long-range OH–Ar potential. The calculated potential energy surfaces have been fitted to an analytical function and used in quantum scattering calculations for collision induced rotational energy transfer in the A state of OH. From the integral cross sections rate constants have been evaluated as a function of the temperature. The theoretical rate constants are considerably larger than the corresponding experimental values of Lengel and Crosley [J. Chem. Phys. 67, 2085 (1977)], but in good agreement with recent measurements of Jörg, Meier, and Kohse-Höinghaus [J. Chem. Phys. (submitted)]. Our potential energy surface has also been used to calculate the bound rovibrational levels of the OH–Ar complex.