J.W.I. van Bladel

Computational exploration of the six-dimensional vibration-rotation-tunneling dynamics of (NH3)2

In order to address the well‐known problem that the nearly cyclic structure of (NH3)2 deduced from microwave spectra differs greatly from the hydrogen‐bonded equilibrium structure obtained from ab initio calculations, we have calculated the vibration–rotation–tunneling (VRT) states of this complex, and explicitly studied the effects of vibrational averaging. The potential used is a spherical expansion of a site–site potential which was extracted from ab initio data. The six‐dimensional VRT wave functions for all the lowest states with J=0 and J=1 were expanded in products of radial (van der Waals stretch) functions and free‐rotor states for the internal and overall rotations, which were first adapted to the complete nuclear permutation inversion group G36. Although the (expanded) potential is too approximate to expect quantitative agreement with the observed microwave and far‐infrared spectra, we do find several interesting features: The 14N quadrupole splittings and the dipole moment of the complex, whic...

Quantum calculation of vibrational states in the aniline-argon Van der Waals cluster

Theoretical calculations of vibrational intermolecular states of the aniline–argon van der Waals complex for J=0 are reported. A fully‐quantum method (LCHOP) was used in order to describe the van der Waals cluster. Results in the first two electronic states S0 (X 1A1) and S1 (A 1B2) are presented; in the S1 state a comparison with available experimental data is made. We introduce an additive repulsive interaction between N and Ar in the S1 state in order to account for the spectral features observed in larger clusters. Several parametrizations of this term in the potential are discussed with a view to applications to semiclassical simulation of the spectra of the larger An–Arn clusters.

The van der Waals rovibrational states of the Ar–NH3 dimer

Starting from an ab initio anisotropic potential surface, we have calculated the bound rovibrational states of the Ar–NH3 dimer for J=0, 1, 2, and 3. There is good agreement with data available from molecular beam spectroscopy in the microwave and far‐infrared regions. The nature of the states is discussed and they are correlated with free internal rotor states. In spite of the substantial barriers to internal rotation, Ar–NH3 resembles much more a free internal rotor complex than a nearly rigid dimer. Still, the van der Waals vibrations show characteristic differences from free internal rotations. In particular, there is strong mixing between the fundamental stretch vibration of the dimer and the lowest bending mode. We assign the band observed in the far‐infrared region to the lowest, primarily bending, of these modes. Further transitions in the yet unexplored far‐infrared regions are predicted.

The infrared photodissociation spectra and the internal mobility of SF6, SiF4, and SiH4 dimers

We present an analysis of the couplings originating from different intermolecular interactions (electrostatic, exchange, dispersion, induction) which split and shift the frequencies of the vibrational transitions in Van der Waals dimers, and determine their intensities. Model potential calculations illustrate the importance of the various contributions in (SF6)2, (SiF4)2 and (SiH4)2 and their dependence on the monomer orientations. The results, in conjunction with calculated equilibrium structures, barriers to internal rotation and (harmonic) Van der Waals vibrational frequencies, lead to several observations which are relevant for the interpretation of the infrared photodissociation spectra of these complexes. We confirm that in (SF6)2 and (SiF4)2 (orientation-independent) resonant dipole-dipole coupling dominates the appearance of the spectra. For (SiH4)2 we conclude, however, that other than electrostatic terms are not negligible and, moreover, that the electrostatic coupling leads to orientation-dependent vibrational frequencies and intensities. This orientational dependence is related to the large displacements of the hydrogen atoms in the v4 mode of SiH4. We also find that the internal rotations in (SF6)2 and (SiF4)2 are more strongly locked than those in (SiH4)2. Especially the geared internal rotations in the latter dimer could easily occur at the experimental molecular beam temperatures.

The NH3 umbrella motion in the Ar-NH3 dimer

Abstract In the calculation of the van der Waals states of ArNH 3 we have explicitly included the umbrella coordinate which corresponds with the ν 2 vibration and the inversion tunneling of the NH 3 monomer in the complex. We have calculated the rovibrational states of the complex derived from the monomer vibration inversion (υ ± ) states with υ=0 and 1. As expected, we find very little interaction between the υ=0 and the υ=1 states and good agreement with our earlier approximate model for the 0 + and 0 − states, which was compared with experimental far-infrared spectra. The 1 + and 1 −1 states are coupled more strongly with the intermolecular motions, however. Comparison with a recently observed υ=0 → 1 infrared band of ArNH 3 shows that the umbrella coordinate dependence of the ab initio intermolecular potential might still be improved.

Internal motion of two-top molecules: propane and dimethylamine

Abstract Two molecules with two internal rotors each (CH 3 group) were investigated by spontaneous Raman scattering. The torsional (overtone-) spectra yielded the relevant potential parameters for the internal motions. The potentials show characteristic differences due to the C 2v symmetry of propane and C s symmetry of dimethylamine.

Torsional motion of the CH3 groups of propane studied by Raman overtone spectroscopy

Abstract The Raman spectrum of propane is recorded between 300 and 900 cm −1 ; first and third overtones are observed with an Ar + laser intracavity setup. This spectrum is analyzed by means of a model with two hindered and coupled methyl rotors. Since the levels probed extend to the region above the torsional barrier ( V 3 =1353 cm −1 ), all the relevant molecular parameters, in particular the coupling between the methyl rotors, could be accurately determined. By means of a classical approach we have obtained further insight into the qualitative structure of the torsional spectrum.

Analysis of the torsional Raman spectrum of a pyramidal molecule, dimethylamine

Abstract Overtone Raman spectra of dimethylamine (DMA) are analyzed yielding accurate torsional potential parameters. Neglecting the inversion motion of the amino-hydrogen in DMA the group G 18 is the proper molecular symmetry group to analyze the measured torsional Raman spectrum. The character table of G 18 for this basically pyramidal molecule has been derived and is reported here explicitly. The torsional potential parameters are fitted with the introduction of additional terms in the Hamiltonian odd in αi, αi being the torsional angle of top i. The mutual top-top interaction terms yield a significant distortion of the torsional potential surface.