The Effect of Substituents on Indirect Nuclear Spin-Spin

PATRICIO F. PROVASI, GUSTAVO A. AUCAR AND STEPHAN P. A. SAUER

INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES

Año: 2003 vol. 4 p. 231 - 248

1422-0067

Ab initio calculations of the spin-spin coupling constants have been carried outfor methan- and ethanimine, methanal- and ethanaloxime at the level of the second orderpolarization propagator approximation with coupled cluster singles and doubles amplitudes(SOPPA(CCSD)) using the aug-cc-pVTZ-J basis sets. Previously we have shown that thismethod can reproduce quantitatively the coupling constants for methanimine; our newresults for methanal- and ethanaloxime agree also very well with the measured couplings. Astudy of both purely geometrical and substituent effects on all coupling constants in the titlecompounds is presented. Analyzing the four contributions to the coupling constants we findthat the stereoelectronic effect of the nitrogen lone pair on the one-bond C-H and C-Ccouplings as well as the corresponding effect for the geminal N-H and N-C couplings isaffected strongly by the -OH substituent. For the one-bond C-N couplings we observe thatthe orbital paramagnetic (OP) contribution is comparable to the Fermi Contact (FC)contribution but opposite in sign and that the spin-dipolar (SD) term amounts to between40% and 85% of the total coupling constants. Changes in the total one-bond C-N couplingscaused by the -OH substituent are also almost entirely due to SD contribution.