The Electric Field Gradients at the Nitrogen and Chlorine Sites in 1,2,4,5‐Tetrachloro‐3,6‐Dinitrobenzene and 1,3,5‐Trichloro‐2,4‐Dinitrobenzene. A 14N NMR and 35Cl NQR Single Crystal Study

Abstract 35Cl and 14N nuclear quadrupole coupling tensors were determined at 295 K for 1,2,4,5‐tetrachloro‐3,6‐dinitrobenzene and 1,3,5‐trichloro‐2,4‐dinitrobenzene. The Zeeman split single crystal 35Cl NQR gives the quadrupole coupling constant e2qQh−1(35Cl) = 76.937(4) MHz and the asymmetry parameter η(35Cl) = 0.1207(5) for 1,2,4,5‐tetrachloro‐3,6‐dinitrobenzene, and e2qQh−1(35Cl(1)) = 74.672(6) MHz, η(35Cl(1)) = 0.1097 (10), e2qQh−1(35Cl(3)) = 77.167(6) MHz, η(35Cl(3)) = 0.0969(20), e2qQh−1(35Cl(5)) = 74.422(6) MHz, η(35Cl(5)) = 0.1143(10) for 1,3,5‐trichloro‐2,4‐dinitrobenzene. By high field NMR e2qQh−1(14N) and η(14N) were determined at 295 K. For 1,2,4,5‐tetrachloro‐3,6‐dinitrobenzene it is found e2qQh−1(14N) = 0.757(2) MHz, η(14N) = 0.025(3). Φzz(14N) deviates from the bond direction C  N by about 3.3°, Φxx is parallel to the benzene ring plane. For 1,3,5‐trichloro‐2,4‐dinitrobenzene it is found: e2qQh−1(14N(2)) = 0.722(2) MHz. Φ(14N(2)) = 0.069(3), e2qQh−1(14N(4)) = 0.738(2) MHz, Φ(14N(4)) = 0.064(3). Φzz(14N(2)) and Φzz(14N(4)) nearly coincide with the corresponding C  N bond directions. Φxx(14N(4)) is nearly parallel to the benzene ring plane, whereas Φxx(14N(2)) is twisted about 18° against the ring plane. The relation between the twist angle α and the asymmetry parameter of the 14N electric field gradient is discussed.