Nuclear magnetic shielding tensors for the carbon, nitrogen, and selenium nuclei of selenocyanates - a combined experimental and theoretical approach

The principal components of the carbon, nitrogen, and selenium chemical shi ft (CS) tensors for several solid selenocyanate salts have been determined by NMR measurements on stationary or slow magic-angle-spinning powder sampl es. Within experimental error, all three CS tensors are axially symmetric, consistent with the expected linear geometry of these anions. The spans (om ega) of the carbon and selenium CS tensors for the selenocyanate anion (SeC N-) are approximately 300 and 800 ppm, respectively, much less than the cor responding values for carbon diselenide (CSe2). This difference is a conseq uence of the difference in the CS tensor components perpendicular to the C infiniti symmetry axes in these systems. Ab initio calculations show that t he orbital symmetries of these compounds are a significant factor in the sh ielding. For CSe2, efficient mixing of the sigma and pi orbitals results in a large paramagnetic contribution to the total shielding of the chemical s hielding tensor components perpendicular to the molecular axis. Such mixing is less efficient for the SeCN-, resulting in a smaller paramagnetic contr ibution and hence in greater shielding in directions perpendicular to the m olecular axis.