Hydrogen bonds and van der Waals molecules

The data are obtained from large basis set calculations (see Table 7.6) which include electron correlation (second-order Moeller-Plesset perturbation theory) and which are corrected for basis set superposition error (BSSE), (Boys and Bernardi 1970). While the results for the stronger hydrogen bonds are qualitatively the same with or without the inclusion of correlation, the weaker interactions require the better description of the wave function. 

value for NeHF in Table 7.6 compares well with the value of 0.78 kJ/mol calculated by ONeil et al. (1989) using a large ba.sis set on a size 

The changes in the atomic populations encountered in the formation of the van der Waals complexes are extremely small. There is a predicted transfer of 0.008 e from the base to the acid in ArHF, while the corresponding transfer of 0.001 e in NeHF is equal to the error in an integrated atomic population. There are significant changes in the first and second moments of the atoms in these complexes, however, and these are discussed next. 

The atomic densities of the isolated reactants, as determined by the first moment M(fl) (eqn (6.45)), are polarized in a direction counter to the direction of charge transfer. Thus in HF, F is polarized towards H which is itself polarized away from F and, in HCN, the atoms are polarized in the direction N to C to H. In N2, the atoms are polarized into their non-bonded regions. The atoms in these linear molecules have positive values for the 

Unlike the polarization of the base atom in a regular hydrogen bond interaction, the dipolar polarization of a noble gas atom is towards the hydrogen. In the relatively weak complex of HF with N2, the change in the polarization of the base N away from H is very small. This interaction is transitional between the two patterns of atomic polarizations that result from the mutual penetration of closed-shell systems with little or no accompanying charge transfer, the features common to van der Waals and hydrogen-bonded interactions. 

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