Dipole Moment of van der Waals Complexes

Dipole Moment of van der Waals Complexes Table 3.1 Dipole moment of N2-N2 dimer (in D)... 

Collision-induced dipoles manifest themselves mainly in collision-induced spectra, in the spectra and the properties of van der Waals molecules, and in certain virial dielectric properties. Dipole moments of a number of van der Waals complexes have been measured directly by molecular beam deflection and other techniques. Empirical models of induced dipole moments have been obtained from such measurements that are consistent with spectral moments, spectral line shapes, virial coefficients, etc. We will briefly review the methods and results obtained. 

For ab initio calculations of all properties of van der Waals complexes like interaction energies, dipole moments, polarizabilities, etc. one should use the basis set augmented with diffuse function, because they allow to describe better the interactions of molecules that are far from each other (distances are larger than those of covalent bonds within molecules). Moreover, the use of a method accounting for the electron correlation (post HF) should be employed. 

Electric properties of van der Waals complex when using ab initio calculations should be, if possible, corrected for the BSSE and BSIE similar to the interaction energy. The BSSE could reach up to 10 % for the dipole moments in the range of the van der Waals wells for the case of a medium sized basis set, like aug-cc-pVTZ (see Fig. 3.1, for example). However, for the systems with multireference character the BSSE correction is not used when a size-inconsistent method is employed. 

A few practical points remain to be discussed. Acceptance of the counterpoise principle implies that all energy terms contained in the final AE must be interpreted in terms of monomer wavefunctions and properties calculated in the full dimer basis set. As mentioned before, the final representation of may in fact be better than would be expected from an estimate of E in terms of the (/ a(Xa) monomer wavefunctions. On the other hand, the final representation of coui and Ei may contain undesirable artefacts such as an unphysical dipole-dipole contribution to Ecoui of He2. These effects have been termed higher-order BSSE , and they will not be removed by applying the 3 correction. In certain applications it may be desirable to remove these artefacts for example by adding a correction coul(l A(J(A)) polar complexes the multipole energy is corrected by inserting better values of the multipole moments , then the dimer-basis multipole energy is the proper reference. The same applies to the calculation of differential properties of van der Waals molecules. Counterpoise corrections have been applied to electron densities , multipole moments and polarizabilities . 

In addition to the total energy of the system, it is desirable to carry out FCI calibration studies of properties such as dipole moments, polarizabilities, and electrostatic forces. For example, in the O + OH- O + H reaction, the preferred approach of the O atom is determined by the dipole-quadrupole interaction. At long distances, this favors a collinear approach to the H atom, whereas for reaction to occur the O atom must migrate to the O end of OH. An accurate description of weakly interacting sy stems such as van der Waals complexes requires a quantitative description of dipole-induced-dipole or induced-dipole-induced-dipole interactions. Further, the dipole moment and polarizability functions of a molecule determine its infrared and Raman spectral intensities. 

The Rabi technique of radio frequency or microwave spectroscopy in atomic or molecular beams [10.14-10.17] has made outstanding contributions to the accurate determination of ground state parameters, such as the hfs splittings in atoms and molecules, small Coriolis splitting in rotating and vibrating molecules, or the narrow rotational structures of weakly bound van der Waals complexes [10.18]. Its basic principle is illustrated in Fig. 10.9. A collimated beam of molecules with a permanent dipole moment is deflected in a static... 

The contributions of the various intermolecular interactions to the vibrational coupling in Van der Waals complexes have been calculated explicitly for (SFe)25 (SiF4)2 and (81114)2. To try and simulate the dimer vibrational spectra (see Section 4) in the frequency range from 880 to 1100 cm , we have also calculated the infrared intensities of the dipole allowed transitions. We concentrate, in particular, on the dependence of the calculated spectra on the monomer orientations. In line with the atom-atom model used for the intermolecular potential, we write the following expression for the vibrational dipole moment operator of a dimer... 

The induced dipole moments of the van der Waals complex ArBFg have been calculated by using ab initio distributed multipole analysis (DMA) and distributed polarizability analysis (DPA) [27], see Boron Compounds 4th Suppl. Vol. 3b, 1992, p. 107. 

Dipole moments of atom-atomic complexes are now studied very well. These complexes are the simplest van der Waals complexes and, as a result, they have been studied first of aU and fully enough (see, for instance [14—21]). Particular analytical forms of dipole moments for atom-atomic complexes can be easily obtained from the general expressions of Sect. 3.1 with a given accuracy using the symmetry properties (Appendix B) of interacting atoms. In particular, Eq. (3.1.9) gives for two interacted atoms (when at least one of them has nonzero quadruple moment) the asymptotic behavior as / ". And when these atoms are in the state s (spherical symmetry) the leading term is caused only by the second... 

The dipole moments of the atom-diatomic complexes X2-Y, in contrast to the atom-atomic ones, depend also on the angle 0 between the axis of diatomic molecule and the axis passing through the atom Y and the molecule X2 (Fig. 3.2a). Note that if the nonrigidity of the diatomic molecule X2 is taken into account, then the additional dependence of dipole moment of the complex on r appears. So, in general we have a surface of the dipole moment for such complex. Nevertheless, these van der Waals complexes are relatively simple yet and they are studied intensively up to now because of their importance (see, for instance, [24—30]). 

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