Nuclear magnetic double resonance studies of the effect of deuterium substitution upon1J(31P-H) in dimethyl phosphite

Abstract

[Part of first page in lieu of Abstract] Although the effect of isotopic substitution upon the magnetic shielding of a neighbouring nucleus (the secondary isotope effect) is well known [1], there have been few observations of the behaviour of spin-spin coupling constants in these circumstances. Recently two independent groups of workers determined 1 J ( 31 P–H) and 1 J ( 31 P–D) in dimethyI phosphite [(CH 3 O) 2 P(O)H] and its P-deuteriated analogue, but emphasized different aspects of the problem. In the absence of any H–D isotope effect the ratio of these two couplings should be γ (H) / γ (D) , that is 1 J ( 31 P–H) and 1 J *( 31 P–H) [where J * = 1 J ( 31 P–D) γ (H) / γ (D) ] should be equal. One group finds that both J and J * are solvent and concentration dependent, and do not discuss any residual difference which could be ascribed to a true primary isotope effect [2]. The other workers find that J * is circa 3 Hz smaller than J and suggest on intuitive grounds that solvent effects are unlikely to be important [3]. Quadrupole relaxation of the deuterium nucleus ( I = 1) could lead to broadening in the 31 P spectrum and also to a small inward displacement of the lines so that the apparent 1 J ( 31 P–D) would be too small [4]. However, it can be shown that even when the phosphorus lines are broadened to 2 Hz at half-height (and our experiments show that the actual broadening is much less than this) the frequency displacement is less than 0.05 Hz which is insignificant in the present context. The preceding work was done using direct observation of the 31 P or 2 D resonances with attendant problems of low sensitivity, and consequently the solutions examined were fairly concentrated. We now report our results (see table) for these systems which were studied by 1 H-{ 31 P} double resonance experiments so that dilute solutions could be examined [5].