Van der Waals systems

When we consider a van der Waals system, we can start with the pair interaction as shown in Figure 2.2. The equation giving the pair potential is the 6-12 or Lennard-Jones-Devonshire equation ... 

The first data on gas-phase complexes formed between strong acids and the methane molecnle were reported by Barnes in 1983 [15]. The author observed a CH4 HCI complex in a rare gas matrix that was assigned to a van der Waals system. Its C 3v structure has also been confirmed by gas-phase microwave spectroscopy [16]. Other complexes, snch as CH4 HCN, CH4 HF, and CHj HBr, were reported later [17]. Since these complexes represent C-H- H interactions, then-nature is discussed in Chapter 6. 

For comparison, the authors have probed a complex formed by the same proton-donor molecule and molecular hydrogen. In this very weak complex, HCCH- - (H2), the H- - (H2) distance has been calculated as 2.606 A (i.e., significantly larger than the sum of the van der Waals radii of H). It is extremely interesting that a topological analysis of the electron density also leads to the appearance of the bond critical point in the H- - (H2) direction. However, the Pc and V pc values are very small (0.0033 and 0.0115 au, respectively) compared with those in the HCCH- - -HLi complex (0.0112 and 0.0254 au, respectively). The most important conclusion of this comparison is There is no evident borderline between the dihydrogen-bonded complexes and the van der Waals systems. 

Binary Interaction-Induced Dipoles. Ab initio quantum chemical calculations of interaction-induced dipole surfaces are known for some time (Section 4.4, pp. 159 ff.) Such calculations were recently extended for the H2-He [17] and H2-H2 [18] systems, to account more closely for the dependencies of such data on the rotovibrational states of the H2 molecules [19]. New calculations of the kind and quality are now available also for the H-He system [20], the H2-H system [21], and the HD-He system [22]. New computational methods, called symmetry adapted perturbation theory (SAPT), were shown to be also successful for calculating interaction-induced dipole surfaces of such simple, binary van der Waals systems [23-26]. 

The performance of the SCF-MI procedure was tested on large van der Waals systems, such as clusters of biological interest. In particular, DNA base pairing also in the presence of different interacting charged and neutral species was analysed. 

A SCVB wavefunction incorporating just 25 structures has been shown to produce highly accurate results for the He- -LiH van der Waals system, with marked improvements over a SCVB treatment making use of a much larger number of structures. 

Van der Waals Systems Orbitals, Pitq)alie8 and Reactivity 4.1 Rate of Formatioii, Transformation and Decomposition of vdW Molecules... 

Main entry under title Van der Waals systems. (Topics in current chemistry 93) Bibliography p. Includes index. 1. Van der Waals forces — Addresses, essays, lectures. 2. Molecular orbitals — Addresses, essays, lectures. 3. Infra-red spectrometry — Addresses, essays, lectures. I. Series. QDI.F58 vol. 93 [QD461] 540s [541.276] 80-19620... 

The second study analyzes the behavior of the spiecific heat in the critical region of a spin van der Waals system containing a large but finite number N of spins. The sjjecific heat curve displays a finite jjeak, steep but smooth, which shrinks and turns into a singularity as AT oo. The location TJr of the maximum (the shift) for a finite system is defined by [7Jr — ... 

Opposite to empirical functionals, the PW91 functional was derived without fitting to any data concerning van der Waals systems but it is based on first-principles. 

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