H/D SECONDARY ISOTOPE EFFECTS IN HYDROGEN- AHD HYDRIDE- BRIDGE BONDED COMPOUNDS: 2-AMINOETHANQL AND METHYLDIBORANE

Abstract

From the microwave spectra of multiple isotopic modifications of 2-aminoethanol, for which there is an internal hydrogen bond of the type O-H...N, it has been found that the 0...N distance in the $d_{3} (OD, ND_{2})$ species is shorter than in the normal species by 0.006 {\AA}. Similarly, it has been found that in $d_{5}$ methyldiborane $(CH_{3}B_{2}D_{5})$ the B...B distance is from 0.006 to 0.010 {\AA} shorter than in the normal species. The occurrence of such structural isotope effects precludes simple interpretation of ""substitution structure"" parameters involving the bridge hydrogen. For example, in 2-aminoethanol the ""substitution structure"" parameters $r_{O-H}$ and $\angle COH$, corrected for the small O...N shrinkage which presumably accompanies hydroxy deuteration, are 1.00 {\AA} and $107.4^\circ$, respetively--quite significantly different from the 1.14 {\AA} and $103.7^\circ$ values of the simple substitution structure.