The Polarization Propagator Approach as a Tool to Study Electronic Molecular Structures from High-Resolution NMR Parameters

Abstract

Polarization propagators (PP) are powerful theoretical tools that allow carrying out a deep analysis of the electronic mechanisms underlying any molecular response property. The inner projections of the PP and contributions from localized orbitals within the PP approaches described in Section 2 were developed to fully take advantage of this power of analysis for the study of NMR spectroscopic parameters. They are based on the use of localized molecular orbitals (LMOs) related to chemically intuitive concepts to decompose the mathematical expression of these parameters into coupling pathways or shielding pathways. Each of them may be furthermore decomposed into two new objects: (i) perturbators, which give information on the efficiency of a given magnetic perturbation to produce local excitations and (ii) the principal propagator matrix elements which provide deep understanding on the way perturbations are transmitted within the electronic framework of the molecule under study. Applications are presented in Section 3, both within semiempirical and ab initio approaches: the Karplus rule, a general analysis of the signs of J couplings, σ–π decomposition, hyperconjugative effects in transmission of J couplings, general features of 1J couplings, and intermolecular couplings in hydrogen-bonded systems. All applications were especially selected to cover examples in which qualitative physical insight can be gained.