14 N NQR study of the effect of protonation on the charge distribution of some N ‐methylated ureas

The 14 N NQR frequencies measured at 77 K for the amine group of the 1:1 salts of nitric acid with methylurea and 1,3‐ and 1,1‐dimethylurea and the phosphoric acid salt of 1,3‐dimethylurea are reported. The analysis of the NQR data showed that the N loses population from its π orbital. The decrease in π population is partially compensated by an increase in the population of the NC σ bonds. The loss of π population is enhanced when the H atoms of the amine group are substituted by electron‐donating groups such as CH 3 .