1 3C high resolution nuclear magnetic resonance studies in polycrystalline tetracyanoquinodimethane

Abstract

1 3C high resolution NMR (nuclear magnetic resonance) studies of solid neutral tetracyanoquinodimethane are performed using different polycrystalline samples: (i) natural isotopic abundance; (ii) 15N isotope enriched; (iii) 13C isotope selectively enriched on the CN groups; and (iv) 13C isotope selectively enriched on the first neighboring carbons of the CN groups. 14N quadrupole effects are observed on 13C spectra of the CN groups, and the quadrupole coupling constant for 14N is obtained: −3.84±0.12 MHz. The 13C isotropic chemical shifts are found: 112.6 ppm for CN groups; 90.8 ppm for the carbons directly bonded to these groups; 132.0 and 133.2 ppm for protonated carbons; and 151.7 ppm for the other carbons on the quinone ring. The principal components of the chemical shielding tensor, the shielding anisotropy, and the shielding asymmetry factor are also obtained for the 13C nuclei in the fragments C–(CN)2, by graphical analysis of CP/MAS spectra. The following values are obtained for 13C nuclei on CN groups: 211±11 ppm, −85±22 ppm (σ⊥ and σ∥); −296±33 ppm (Δσ), and 0 (η). For the other carbon nuclei the values obtained for σ11, σ22, σ33, Δσ, and η are: 157±2 ppm, 75±2 ppm, 39±3 ppm, −77±4 ppm and 1.6±0.1, respectively. The reliability of these results is confirmed by a computer fit of the line intensities.