Intramolecular hydrogen bond: Can it be part of the basis set of valence internal coordinates in normal mode analysis?

Abstract

It has been shown earlier[1] that the relaxed force constants (RFCs) could be used as a measure of bond strength only when the bonds form a part of the complete valence internal coordinates (VIC) basis. However, if the bond is not a part of the complete VIC basis, its RFC is not necessarily a measure of bond strength. Sometimes, it is possible to have a complete VIC basis that does not contain the intramolecular hydrogen bond (IMHB) as part of the basis. This means the RFC of IMHB is not necessarily a measure of bond strength. However, we know that IMHB is a weak bond and hence its RFC has to be a measure of bond strength. We resolve this problem of IMHB not being part of the complete basis by postulating ‘equivalent’ basis sets where IMHB is part of the basis at least in one of the equivalent sets of VIC. As long as a given IMHB appears in one of the equivalent complete VIC basis sets, its RFC could be used as a measure of bond strength parameter. The relaxed force constants (RFCs) could be used as a measure of bond strength when the bonds are part of the basis. We define more than one ‘equivalent’ complete VIC basis sets where the intramolecular hydrogen bond is part of atleast one of the basis sets.