CL-35 NQR RELAXATION AND HYDROGEN-BOND IN SOME CHLORAL HEMIACETALS

The crystal structure of chloral ethylhemiacetal has been determined a t 291 K: monoclinic, space group C2-(2)P2(1), Z = 2, a = 854.5(1), b = 594.0(3), c = 853.3(1) pm, beta = 108.30(2)-degrees, R = 0.0962. A sh arp decrease of the spin lattice relaxation time T1 found for the Cl-3 5 NQR of the CCl3 groups in chloral methyl-, ethyl- and n-heptylhemiac etals is attributed to the onset of reorientation of the group over a potential barrier of ca. 39, 37, and 32 kJ/mol, respectively. An unusu al T1 vs. I/T curve with a T1 minimum superimposed on the sharp decrea se of T1 due to the reorientation was observed for each of the three C l-35 NQR lines of nHp-CH. This phenomenon is tentatively ascribed to a fluctuation of EFG caused by jumping motion of the hydrogen atoms in the OH groups.