Assignment of the 13C NMR resonances in trialkylphosphines from spin-lattice relaxation time measurements

Analysis of the ¹³C NMR chemical shift and coupling constant data for a number of straight-chain aliphatic trialkylphosphines and their transition metal carbonyl complexes suggests that complexation leads to: (1) a deshielding of C(1) and an increase in ¹J(¹³C³¹P), (2) a slight shielding of C(2) and a decrease in ²J(¹³C³¹P), and (3) little or no change in the chemical shift for C(3) and a slight increase in ³J(¹³C³¹P). Application of these rules to the assignment of the ¹³C NMR spectrum of P(butyl)₃ led to conflict with prior work. A study of segmental motion in these derivatives via spin-lattine (T₁) relaxation time measurements was therefore performed, and these data are in complete agreement with the proposed assignments. These generalizations must be applied with care, however, since the presence of either unsaturation or branching near the phosphorus can interfere with this pattern.