Low-Temperature 13C Magnetic Resonance in Solids. 3. Linear and Pseudolinear Molecules

The solid-state low-temperature ¹³C NMR spectra of five linear (CO, CO2, OCS, C3O2, acetylene) and four pseudolinear (propyne, 2-butyne, allene, ketene) molecules have been obtained. The assignments of the shielding tensor axes are made from symmetry in all but allene and ketene. In these pseudolinear molecules shift assignments are based on ab initio calculations by the IGLO version of the coupled Hartree-Fock method. Except for carbon monoxide, the parallel component of the chemical shift tensor (σ∥) for C∞ is remarkably constant at −90 ppm from Me4Si, establishing a reference point for discussing the paramagnetic shielding terms. Variations in both the diamagnetic term and off-center shielding terms of both varieties were found to be relatively small. Destruction of the C∞ symmetry in molecules having hydrogen lying off the C-C axis of pseudolinear molecules resulted in major shifts in the tensorial shieldings. Theoretical correlation of the relative experimental shielding values is very good although the agreement of magnitudes in the low-field range worsens as one moves away from the methane shift which is used as a fiducial value to establish the comparison. It is concluded that the theory faithfully represents the shielding phenomena and may be used to assign shielding components to specific spatial orientations when symmetry features or other unequivocal information is lacking. © 1984, American Chemical Society. All rights reserved.

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Author Julio Cesar Facelli Biostatistics & Bioinformatics, Division of Translational Bi ...