THE TRANS-EFFECT OF LONE PAIRS ON INDIVIDUAL X-H BONDS (X=C OR N) - AN AB-INITIO STUDY

Ab initio molecular orbital calculations successfully describe the lon e pair trans effect on XH bonds (X C, N). The diagnosis of the presenc e and a quantitative estimate of the extent of this effect in various molecules may be made by a comparative analysis of XH bond lengths, is olated XH stretching frequencies and isolated XH stretching intensitie s. 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 in dicate that computed isolated XH stretching intensities are a very sen sitive probe of the lone pair trans effect: XH bonds trans to a lone p air 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 an d 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.