Intermolecular Potentials, Internal Motions and the Spectra of Van Der Waals Molecules

Abstract

ABSTRACT. The infrared spectra of N2Ar, 02~Ar, N2-N2 and 02-0 2 seem to indicate that these Van der Waals molecules possess orientationally localized (librational) states, as well as (nearly) free internal rotor states. Using the full anisotropic potential, we have calculated the rovibrational bound states of N2-N2 and N2-Ar, and also the ro tatio n al predissociation states of the la tte r complex, and we have evaluated the contributions of these states to the infrared spectra. Thus the infrared spectrum of N2-Ar, and in principle also the spectra of the other dimers, can be understood. The onset of the regular free internal rotor stru c ture cannot be explained from the bound sta te s; i t is due to rather narrow (0.2 to 3 cm~^) rotational resonances lying in the co llisio n continuum. The structure in the lower frequency part of the spectra is caused by transitions between localized lib ra tio n al states; especially this part will be sensitive to the detailed intermolecular p o te n tial. In order to exploit this sensitivity one has to measure the infrared spectra at very low temperature. INTRODUCTIONThe mobility of the constituents in d iffe re n t Van der Waals mole cules varies considerably. Considering, for instance, the series of molecule-rare gas atom dimers, we observe, as one extreme, the nearly free internal rotations of H2 in the H2~X dimers. A theoretical descrip tion of such dimers is best given by using a basis of free rotor func tions, both for expanding the potential and for the rovibrational wave functions [ 1 ]. At the other end of the series are the dimers in which the (rigid) molecule is strongly aspherical, very long or f la t, with one or two of its dimensions larger than the Van der Waals distance between the molecule and the rare gas atom. Internal rotations are completely prevented in that case; at specific angles (and distances) the potential becomes even infinitely repulsive. When the molecular size is comparable with the Van der Waals distance, as in benzene-argon or tetrazine-argon [2], dynamical calculations using a free rotor basis are s t i ll possible, with some special precautions, but they converge 337 A. Weber (ed.), Structure and Dynamics o f Weakly Bound M olecular C om plexes,[337][338][339][340][341][342][343][344][345][346][347][348][349][350][351][352][353][354][355]

Rotational–translational propagation effects in orientational molecular correlation functions

“…Consequently, the interesting properties of the absorbing system will be determined by the parameters characteristic of N 2 -Ar pairs. The rotational constant of N 2 given in the literature is B = 2.013 cm −1 [46] and the parameters of the Lennard-Jones potential (equations ( 16)-( 18)) are found by fitting them to the numerical results in [47]. Of all the spherical components of the potential given in this work only the two with l = 0 and 2 are considered, as the V 4 component is small and purely repulsive.…”