Reorientational Motion of Trihalogenomethyl Groups in Organic Compounds as Studied by 35Cl NQR and 19F NMR Spectroscopy

Abstract

AbstractTrihalogenomethyl derivatives of 4-chloroacetanilide, 4-ClC6H4NHCOCCl3,4-ClC6H4NHCOCF2Cl, and 4 - ClC6H4NHCOCF3 were prepared. The 35Cl NQR and 79FNMR spin-lattice relaxation times T1Q and T1F, respectively, of these compounds were measured at various temperatures. On heating, T1Q of the CCl3 and CF2Cl groups decreased rapidly above ca. 110 and 150 K, respectively, showing the onset of reorientation of the trihalogenomethyl group about the pseudo-triad axis. The activation energies were determined to be 27 and 13 k J mol-1 for the CCl3 and CF2Cl reorientation, respectively. The temperature dependences of the 19F NMR T1F of the CF2Cl and CF3 groups lead to activation energies of 13 and 4.3 kJ mol-1 , respectively, for the same reorientations. T1F of the CF3 group observed above ca. 200 K was attributed to the spin-rotational interaction. The T1Q-1 of ring-chlorine in 4 - ClC 6H4NHCOCCl3 and 4 - ClC6H4NHCOCF3 can be interpreted by the Tn law with n = 2.2 and 1.1, respectively, whereas that of 4-ClC6H4NHCOCF2Cl by the sum of two contributions: Tn law (n = 2.0) and the modulation mechanism due to the CF2Cl reorientation