14 N and 35 Cl NQR studies of organophosphorus compounds

14 N and 35 Cl NQR spectra have been investigated for 24 organophosphorus compounds using a pulse technique. The electron populations of the nitrogen lone pair orbital and the NP bond are calculated according to the Townes and Dailey method. The experimental data are interpreted assuming a partial double bond character of the NP bond due to the p π d π interaction and p π σ conjugation of the lone pair electrons of the nitrogen atoms. The effect of the different nature of substituents X on the NP bond populations is observed in X PR n (R 2 N) 3‐n molecules (where X is O, S, Se, or lone pair electrons and n = 0, 1, 2). It can be seen from this dependence that the effective electronegativity of the phosphorus atom is largest in selenophosphoramidates and falls in the sequence PSe > PS > PO > P.