Intermolecular Potential Surface Research Articles

Intermolecular potential surfaces from electron gas methods. II. Angle and distance dependence of theA′ andA″ Ar–NO(X 2Π) interactions

Angle dependent intermolecular potential energy surfaces for the two states (2A′ and 2A″) that arise from the interaction of ground (X 2Π) state NO with Ar are calculated using the electron gas model to obtain the short range interactions. The average and difference of the two interaction energies are fit to analytic forms convenient for use in scattering calculations and joined smoothly onto the long range van der Waals potential previously determined. The results, which appear to be of useful accuracy, and the applicability of the electron gas model to such open shell–closed shell interactions are discussed.

Intermolecular potential surfaces from electron gas methods. I. Angle and distance dependence of the He–CO2 and Ar–CO2 interactions

Angle dependent intermolecular potential energy surfaces suitable for use in studies of rotationally inelastic collisions of rigid linear CO2 with Ar and He are calculated using the electron gas model to obtain the short range interactions and joining them smoothly to the long range van der Waals tails of the preceding paper. Smooth analytic fits to the potentials, convenient for use in scattering calculations, are given. The surfaces are very strongly nonspherical. Virial coefficients calculated using the a priori He–CO2 potential are in excellent agreement with experiment, and a simple adjustment of the Ar–CO2 potential gives excellent agreement with both virial coefficients and a potential inferred from high energy scattering data.

Molecular orbital theory of the hydrogen bond. PI electrons as proton acceptors

Ab initio SCF calculations with minimal STO-3G and slightly extended 4-31G basis sets have been performed on dimers having HF and H 2O as proton donors, and the π electrons of C 2H 2, C 2H 4, HCN, and H 2CO as proton acceptors. The equilibrium dimers with C 2H 2 and C 2H 4 have relatively weak hydrogen bonds which form preferentially at or near the midpoint of the C–C π bond. However, distortions of these dimers which move the hydrogen bonded proton away from the midpoint are associated with only a small amount of destabilization. With HCN and H 2CO as proton acceptors, pi hydrogen bonding is most favorable at the negative end of the π bonds. However, these pi dimers are not equilibrium structures on the intermolecular potential surfaces, but relax to structures in which hydrogen bond formation occurs through a lone pair of electrons.

The infrared spectrum of bound state oxygen dimers in the gas phase

Gaseous oxygen was examined from 1480 to 1800 cm −1 at temperatures between 77 and 300°K at path lengths to 200 meters. At all temperatures a broad structureless band is observed which is attributed to induced absorption by colliding pairs of oxygen molecules, however near 90°K a doublet feature appears on top of the broad absorption. The doublet exhibits a quadratic pressure dependence and is assigned to bound state oxygen dimer (O 2) 2. The energy of formation of (O 2) 2, as determined from the temperature dependence of the doublet, is Δ E f = −530 cal/mole. Since the doublet resembles an unresolved P and R vibration-rotation band, models of the dimer are considered which could give the observed doublet separation. A rectangular geometry of (O 2) 2 indicates an intermolecular distance of 2.6 Å. Other geometries give similar distances which are considerably shorter than the Lennard-Jones value of 3.9 Å. A more realistic interpretation of the spectrum is that the doublet represents overlapping transitions from many bound levels of the intermolecular potential surface. There is then no simple relationship between the doublet separation and dimer geometry. However, a dimer in which each O 2 unit is freely rotating would give a very broad absorption unlike the observed doublet feature. So while the system is too complicated to specify the dimer geometry, its structure must be somewhat rigid.