Enthalpies of combustion of 2,2′,4,4′,6,6′-hexamethylazobenzene-N,N-dioxide, 2,2′,6,6′-tetramethylazobenzene-N,N-dioxide, 2,4,6-trimethylnitrobenzene, and 2,6-dimethylnitrobenzene: the dissociation enthalpies of the N=N and N-O bonds