13C nuclear magnetic resonance of organophosphorus compounds. VI. 13C spin-lattice relaxation times in phosphorus heterocycles

Abstract

Carbon-13 spin-lattice relaxation times have been determined for a series of substituted 2,2,3,4,4-pentamethylphosphetanes (four-membered phosphorus heterocycles), including phosphetanes, phosphetanium salts and phosphetane oxides. Nonprotonated carbons have T 1 values of the order of 8–18 sec while protonated carbons have values ranging from 0.6 to 4.0 sec. The rigidity and, in most cases, isotropic nature of the phosphetanes has allowed comparison of T 1's and subsequent determination of relaxation mechanisms and probing of the ease of internal rotation for each of the several methyl groups on the basic ring backbone. Situations are observed where pseudoequatorial methyl carbons on C-2 and C-4 have relaxation times shorter than those of the presumably more hindered pseudoaxial methyls attached to the same C-2 and C-4 carbons. This points out the danger of directly equating short T 1 values with steric crowding.