AB-INITIO IPPP-CLOPPA APPROACH TO PERFORM BOND CONTRIBUTION ANALYSIS OF NMR COUPLING-CONSTANTS - (1)J(NH) IN NH3 AS A FUNCTION OF PYRAMIDALITY

A method designed to analyse transmission mechanisms of NMR indirect s pin-spin coupling constants based on the use of localized molecular or bitals and inner-projections of the polarization propagator at the RPA level has been implemented within an ab initio approach, Former semi- empirical versions of this approach proved to be a valuable tool to ob tain insight on transmission mechanisms of NMR parameters. The new fea tures and difficulties found in the implementation of this method at a n ab initio level are discussed. In particular, a localization procedu re is proposed to deal with vacant molecular orbitals. In order to con vey an idea about the potentialities of this approach, in this work a bond contribution analysis of the (1)J(NH) coupling in NH, as a functi on of the pyramidality at the N atom is carried out. The influence of the N lone-pair, the different N-H bonds and the N inner-shell electro ns in the coupling transmission are discussed, for pyramidal and plana r configurations. Trends that are expected to be of general validity a re found. Results obtained are in agreement with previous theoretical studies on (1)J couplings as well as with empirical trends deduced fro m experimentally measured values of (1)J(NH) couplings in different co mpounds. (C) 1998 Elsevier Science B.V. All rights reserved.