THEO CHEM Journal of Molecular Structure (Theochem) 330 (1995) 21 I-216 RPA MNDO analysis of the dihedral angle dependence of vicinal J(SnSn) and J(SnC) NMR coupling constants* N.M. Peruchena”, L. Sosaa, G.A. Aucara, R.H. ContrerasbT* “Facultud de Ciencias Exactas ,v Nuturales J Agrimensuru, L’niversidud Nucionul de Nordeste, Corrientes, Argentina hDepartamento de Fihica, Facultad de Ciencias Exuctus y ,Vaturules. b’niversidad de Buenos Aires, Ciudad C’niversitaria, Pahell& 1, (14%) Bueno.~ Aires, Argentina Received 24 September 1993; revised 5 October 1993; accepted 22 December 1993 Abstract Vicinal J(SnCCSn)and J(SnCCC) spin-spin coupling constants are calculated within the RPA MNDO method for different dihedral angles, 8, determined by the intervening bonds.in modelcompounds. For both types of couplings. calculated values closelyfollow Karplus-like dependences. Results arediscussed in terms of experimental values. and the effectsof substituents attached to the coupledatomsare briefly analyzedfor the 6’ = 0” and 0 = 180”conformations. These last values were alsocalculatedusing the RPA AM 1 method. Results indicate that the RPA MNDO and RPA AM I approaches show interesting potential for studyingthe structuraldependences of ?I(SnSn) and3J(SnC) couplings in tin-containing compounds. 1. Introduction Nuclear spin-spin coupling constants are known to be interesting probes for the study of many different molecular structural problems [I]. In particular, three-bond J(N-X-Y -M) couplings are known to depend strongly on the dihedral angle, 0. defined by the N-X-Y-M molecular fragment. Such dependence is commonly known as Karplus-like because of the pioneer work by Karplus [2] on vicinal J(HH) couplings. and it became one of the most frequently used NMR parameters for studying conformations. Several modifications to the original equations were “, Part of a Ph.D. thesis (N.M.P.) to be presented to the University of La Plats. * Corresponding author. Member of the Argentine National Research Council. CONICET. suggested [3-61 and equations for different types of coupled nuclei were presented. A few of them are reviewed in Ref. [7]. In general, they can be considered to represent truncated Fourier series in the dihedral angle, the respective coefficients depending on several parameters such as substi- tuent electronegativity, substituent interactions, the nature of the X and Y atoms belonging to the coupling pathway, the hybridization at atoms N and M, etc. In general, Karplus-like curves are represented by Eq. (1): 3J(NM) = A + BcosCl+ CsinQ +Dcos20+ Esin28 11) Coefficients in Eq. (1) are obtained either empn-i- tally, theoretically or with a combination of both approaches. The theoretical approach presents the 0166-1280/95/$09.50 c 1995 Elsevier Science B.V. All rights reserved SSDI 0166-1280(94)03841-S