The IPPP-INDO method is applied to study different interactions as well as transmission mechanisms of 77Se -77Se coupling constants in cis-and trans-1,2-bis(methy1seleno)ethene. Preferential conformations were determined using a modified version of the PCILO-CNDO method. The preferential conformation of the cis compound is such that nonbonding electron pairs of both Se atoms overlap to some extent. In this conformation a large and positive through-space transmission of the Fermi contact term of the 77Se--77Se coupling takes place, which strongly depends on the Se -Se distances. The similarity between this coupling and others involving different nuclei is discussed. The total calculated coupling is in fair agreement with experimental values for similar compounds. The 77Se -77Se coupling in the transcompound is largely dominated by the orbital interaction, which is mainly transmitted through the vinyl T-electron system. . ' INTRODUCTION -' .
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In a"recent article1 vicinal 77Se -Se coupling con\Stants were reported to have a conspicuous difference for cis and trans arrangements, being much larger in the former than in the latter. A suitable example is given in compounds I and I1 with X = Ph, where Johannsen and Eggertl obtained the following 77Se -77Se couplings: 96.5 Hz and 12.0 Hz, respectively (signs not given).
This conspicuous stereochemical behavior indicates that such couplings constitute a powerful tool for elucidating conformations. However, this power can be notably enhanced if a deeper insight on the interactions as well as on the transmission mechanisms that define the cisltrans difference can be obtained. The resemblance of this large vicinal cis coupling to the corresponding 31P -31P coupling in cis-diphosphinoethenes2 suggests that similar transmission mechanisms are operating in both cases. In a previous article3" that vicinal 31P -31P coupling was studied from a theoretical viewpoint and it was concluded that the overlap of nonbonding electron pairs of both P atoms constitutes an efficient pathway for transmitting through space the Fermi contact term of the spin-spin coupling constant. This result is in good agreement with experimental data3b-3d and with another theoretical analy~is.~" Therefore, should a similar behavior for the lone pairs of two selenium atoms be assessed, it may then be expected that the conformation shown in Scheme 1 is the preferential one for the cis isomer.
*Part of a Ph.D. thesis (H. 0. G.) to be presented to the University of Buenos Aires.
'Member of Carrera del Investigador (CONICET) and author to whom correspondence should be addressed.
fFellowship from CONICET.