The mechanisms of N-nitrosodimethylamine (NDMA) formation from dimethylamine (DMA) and nitrous acid have been investigated at the CCSD/6-311＋G(d,p)//B3LYP/6-311＋G(d,p) level of the theory. Two different pathways have been taken into account. One was that one molecule of nitrous acid directly reacted with DMA, and the other was that two molecules of nitrous acid reacted first to yield nitrogen trioxide, which then reacted with DMA. It was found that the energy barrier of the indirect reaction of DMA with nitrous acid was lower by around 55 kJ/mol than that of the direct pathway. Therefore, the indirect pathway, involving an active intermediate nitrogen trioxide, is the main pathway of NDMA formation. This conclusion is in good agreement with the experimental results in which the rate of NDMA formation is proportional to the square of the concentration of nitrous acid.

Key words: N-nitrosodimethylamine, dimethylamine, nitrous acid, density functional theory, ab initio