Nuclear Magnetic Shielding Tensors for the Carbon, Nitrogen, and Selenium Nuclei of Selenocyanates — a Combined Experimental and Theoretical Approach
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Color profile: Disabled Composite Default screen 614 Nuclear magnetic shielding tensors for the carbon, nitrogen, and selenium nuclei of selenocyanates — a combined experimental and theoretical approach Guy M. Bernard, Klaus Eichele, Gang Wu, Christopher W. Kirby, and Roderick E. Wasylishen Abstract: The principal components of the carbon, nitrogen, and selenium chemical shift (CS) tensors for several solid selenocyanate salts have been determined by NMR measurements on stationary or slow magic-angle-spinning powder samples. Within experimental error, all three CS tensors are axially symmetric, consistent with the expected linear geometry of these anions. The spans (Ω) of the carbon and selenium CS tensors for the selenocyanate anion (SeCN–) are approximately 300 and 800 ppm, respectively, much less than the corresponding values for carbon diselenide (CSe2). This difference is a consequence of the difference in the CS tensor components perpendicular to the C∞ sym- metry axes in these systems. Ab initio calculations show that the orbital symmetries of these compounds are a signifi- σ π cant factor in the shielding. For CSe2, efficient mixing of the and orbitals results in a large paramagnetic contribution to the total shielding of the chemical shielding tensor components perpendicular to the molecular axis. Such mixing is less efficient for the SeCN–, resulting in a smaller paramagnetic contribution and hence in greater shielding in directions perpendicular to the molecular axis. Key words: selenocyanates, solid-state NMR, carbon shielding tensors, nitrogen shielding tensors, selenium shielding tensors, ab initio calculations. Résumé : À partir de mesures RMN sur des échantillons de poudre stationnaires ou à rotation lente à l’angle magique de plusieurs sélénocyanates solides, on en a déterminé les principaux composant des tenseurs des déplacements chimiques (DC) du carbone, de l’azote et du sélénium. Dans les limites des erreurs expérimentales, les trois tenseurs des DC sont axialement symétriques, ce qui est en accord avec la géométrie linéaire attendue pour ces anions. Les plages, Ω, des tenseurs du carbone et du sélénium pour l’anion sélénocyanate (SeCN–), sont approximativement de 300 et de 800 ppm respectivement, des valeurs beaucoup plus faible que les valeurs correspondantes observées dans le disélénure de carbone (CSe2). Cette différence est une conséquence de la différence dans les composants du tenseur DC qui sont perpendiculaire aux axes de symétrie C2 dans ces systèmes. Des calculs ab initio montrent que les symétries des orbitales de ces composés sont un facteur important dans le blindage. Pour le CSe2, une combinaison efficace des orbitales σ et π conduit à une contribution paramagnétique importante au blindage total des composantes du tenseur de blindage chimique perpendiculaires à l’axe moléculaire. Une telle combinaison est moins efficace pour le SeCN–;ilen résulte que la contribution paramagnétique est plus faible et, en conséquence, que le blindage est plus grand dans les directions perpendiculaires à l’axe moléculaire. Mots clés : sélénocyanates, RMN à l’état solide, tenseurs de blindage du carbone, tenseurs de blindage de l’azote, tenseurs de blindage du sélénium, calculs ab initio. Bernard et al. 625 Introduction may coordinate to a metal through the nitrogen, the sele- – nium, or by various bridging modes. In the absence of dif- The selenocyanate ligand (SeCN ) has been of consider- fraction data, NMR spectroscopy may provide the most able interest because of its ambidentate nature (1). That is, it reliable information concerning the coordination and struc- ture of these ligands. A review by Duddeck provides an Received January 27, 2000. Published on the NRC Research overview of the 77Se NMR literature including the results of Press website on May 12, 2000. several solution NMR studies of organic selenocyanate com- G.M. Bernard, K. Eichele, G. Wu, C.W. Kirby, and R.E. pounds (2). Carbon-13 and nitrogen-14 or -15 NMR results Wasylishen.1 Department of Chemistry, Dalhousie University, for selenocyanates have been discussed in several reviews Halifax, NS B3H 4J3, Canada. (3, 4). The coordination of metal atoms with several 1Author to whom correspondence may be addressed. selenocyanate anions has been investigated by solution Telephone: (902) 494-2564. Fax: (902) 494-1310. NMR (5, 6). An analogous study of the thiocyanate anion in e-mail: [email protected] copper(I) complexes was conducted using solid-state 13C Can. J. Chem. 78: 614–625 (2000) © 2000 NRC Canada I:\cjc\cjc78\cjc-05\V00-046.vp Friday, May 12, 2000 3:19:26 PM Color profile: Disabled Composite Default screen Bernard et al. 615 NMR (7). In addition to providing insight into the coordina- −µ e2 1 l [3] σ p = 0 ∑∑[ 00ki nnl tion mode of the cation in these complexes, this study ii π 2 − 3 ki 42mEE> n 0 rk showed that conclusions about coordination to the metal n 0 k l drawn from solution NMR studies cannot always be ex- + 00∑∑l nn ki ] ki 3 tended to solids. k r r k With four NMR-active nuclei, 13C, 14N, 15N, and 77Se, µ much information may be gained from solid-state NMR In the above equations, 0 is the permeability of free space, studies of SeCN–. For example, internuclear distances may e and m are the electron charge and rest mass, respectively, be determined from dipolar interactions, and information about while +0| and +n| represent ground and excited electronic – the bonding modes of SeCN is available from isotropic states of the molecule with electronic energy E0 and En,re- chemical shifts and spin–spin coupling constants. Also, ex- spectively. The position vector for electron k is defined by rk amination of stationary powder samples provides the princi- and lki is the angular momentum operator. Equations 2 and 3 pal components of the chemical shift (CS) tensors, which describe the shielding in the directions of the principal com- potentially contain a wealth of structural information. Before ponents of the chemical shielding tensor and are valid only embarking on a solid-state NMR study of selenocyanate–metal if the nucleus of interest is chosen as the gauge origin. Ex- complexes, it is important to understand the shielding ten- pressions for the off-diagonal elements of the chemical sors for the uncoordinated ligand. Towards this end, we pres- shielding tensor and for different gauge origins have been ent the results of solid-state NMR studies of the carbon, derived (14, 15). nitrogen, and selenium CS tensors for selenocyanate salts The contribution to shielding from σ d (eq. [2]) is always + – M [SeCN] ,M=K,NH4,orNMe4. Also presented are the positive, leading to increased shielding relative to the bare preliminary results of a solid-state 13C NMR study of a re- nucleus. Because σ d depends only on the ground electronic lated compound, potassium cyanate. The interpretation of state of the molecule, it can be calculated accurately at rela- the observed magnetic shielding tensors is aided by high- tively low levels of theory and hence is expected to be essen- level ab initio calculations. tially invariant to basis set size or to the effects of electron Selenocyanate anions are thought to be linear; the correlation. Gierke and Flygare (16) have shown that σ d selenocyanate ion of a copper complex which had been re- may be approximated if the molecular structure is known. ported as bent (8) has recently been shown to be linear (9). For example, the shielding for nucleus A in the x-direction Linear systems are ideal for a study of nuclear magnetic may be approximated by: shielding since, in the absence of intermolecular effects, the µ e2 Z orientation of the axially symmetric CS tensor is known ex- [4] σσd ≈+d (free atom) + 0B∑ ()yz22 xxav π 3 B B actly and the unique component of the CS tensor, along the 42mrBA≠ AB C∞ symmetry axis, is dependent only on the diamagnetic σ d term of the shielding (vide infra). The relative simplicity of where av(free atom) is the shielding for the “free” atom. these systems allows one to identify the specific molecular The summation is carried out over all other atoms in the orbitals (MOs) responsible for the paramagnetic shielding. molecule, ZB is the atomic number of the atom being Although carbon and nitrogen shielding tensors are avail- summed, yB and zB are the Cartesian coordinates of the able for most functional groups, corresponding data for sele- atoms which are a distance rAB from the origin (the site of nium are not as widespread (10). The first comprehensive the observed nucleus). The remaining tensor components may experimental study of selenium CS tensors was that of Col- be approximated by cyclic permutations of eq. [4]. In linear lins et al. (11). A surprising feature of the selenocyanate lig- molecules, the component of the chemical shielding tensor σ ands is that the spans of the carbon and selenium CS tensors along the C∞ axis is labelled 2 while the two components σ σ p are much smaller than those of CSe2 (12). With high-level perpendicular to this axis are labelled z. Since || = 0 for – ab initio calculations on SeCN and CSe2, the MOs respon- the nuclei of linear molecules, it follows from eq. [4] that: sible for the different magnetic shielding in these closely re- [5] σσ=≈d σd (free atom) lated compounds were identified. || || av The paramagnetic term (eq. [3]) is a second-order elec- Background theory tronic property which depends on the mixing of ground and excited electronic states of the molecule. These states corre- It is convenient to discuss the nuclear magnetic shielding spond approximately to the occupied and unoccupied MOs σ ( ) of nuclei in linear molecules or ions using Ramsey’s the- (17). The paramagnetic term is usually negative and is in- ory (13).