Isomers and isomerization reactions of four nitro derivatives of methane.

The nitro, nitrite, and aci-form isomers and the isomerization reactions of mono-, di-, tri-, and tetra-nitromethanes (NMs) were computationally investigated. The results show that the isomerization displacement of NO(2) by ONO groups is surprisingly thermodynamically favored for the di-, tri-, and tetra-NMs. The molecular stability decreases and the isomerization becomes easier by increasing nitro groups. The largest attraction among substitutes takes place through the central carbon atom in C(ONO)(4) and leads to its higher stability than the C(NO(2))(4) isomer. There is a concerted change of the CO-NO, C-ONO, and CON-O bonds in the nitrite isomers, that is, the weakened CO-NO bond is accompanied with the strengthened C-ONO and CON-O bonds, and vice versa. We only succeeded in finding two tight transition states of isomerization reactions from NO(2) to ONO in the mono- and di-NMs, whereas isomerization reactions to the aci-forms through an intramolecular hydrogen transfer can always be found.