Trans effect of lone-pairs on individual XH bonds (X = C or N): an ab-initio study.

Ab initio molecular orbital calculations successfully describe the lone pair trans effect on XH bonds (X C, N). The diagnosis of the presence and a quantitative estimate of the extent of this effect in various molecules may be made by a comparative analysis of XH bond lengths, isolated XH stretching frequencies and isolated XH stretching intensities. These quantities can be computed by molecular orbital calculations on partially deuterated species in which all but one hydrogen atom are substituted by deuterium atoms. The results reported in this paper indicate that computed isolated XH stretching intensities are a very sensitive probe of the lone pair trans effect: XH bonds trans to a lone pair show a marked increase in IR intensity with respect to CH bonds in conformations other than trans. In this paper the following compounds have been analyzed: H2CO, C2H4, N2H2, CH2NH, CH3Y (Y=OH, OCH3, NH2 and SH) and dioxane. The results show good agreement with the available experimental data obtained from very sophisticated gas phase intensity measurements on partially deuterated compounds.