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In this study, the structure, spectroscopy, and photochemistry of isoniazid (C6H7N3O, INH) were studied by low-temperature infrared spectroscopy and quantum chemistry calculations. According to DFT(B3LYP)/6-311++G(d,p) calculations, 12 minima were found on the potential energy surface of the molecule, corresponding to two cis conformers about the OC−N−N axis (C1, C2) and one form trans about this axis (T), all being 4-fold degenerate by symmetry. The C1 conformer was predicted to be more stable than T and C2, by 20.4 and 22.6 kJ mol−1, respectively. In consonance with these results, only C1 could be observed in low-temperature argon and xenon matrixes as well as in the neat glassy state prepared from the vapor of the compound at 70 °C. The C1 conformer was also found to be the constituting monomeric unit of the crystalline phase of INH produced from warming of the low-temperature neat amorphous state. The infrared spectra of INH in the different phases studied were fully assigned. After UV (λ > 235 nm) irradiation of the matrix-isolated isoniazid, the compound was found to undergo photolysis through two different pathways: a Norris type I α-cleavage leading to production of isonicotinaldehyde and N2H2and a concerted sigmatropic reaction with production of pyridine, CO and N2H2. The latter reaction was found to be nearly two times faster than the former in both argon and xenon matrixes. In addition, both reactions were found to be disfavored in a xenon matrix, which is in consonance with the involvement of (n, π*) excited states in both photochemical processes. [ABSTRACT FROM AUTHOR]

In this work, the conformational space of the pseudosaccharyl ether 3-(allyloxy)-1,2-benzisothiazole 1,1-dioxide (ABID) has been studied by means of infrared spectroscopy and density functional theory (DFT) calculations. Five different low energy conformers (TSk, TC, GSk, GSk‘ and GC,with relative energies of 0.00, 1.97, 2.00, 3.82 and 6.02 kJ mol-1, respectively) were found on the DFT(B3LYP)/6-311G(3df,3pd) potential energy surface of the molecule, all of them differing in the conformation of the allyl substituent. According to the calculations, in the gaseous phase all conformers are significantly populated (TSk:TC:GSk:GSk‘:GC 47%:16%:18%:12%:7%, at 350 K). In the cryogenic matrices, however, only the TSkand TCconformers exist due to isomerization from the higher energy gaucheforms to the most stable transisomers during deposition of the matrix (conformational cooling). The observed conformational cooling is in consonance with the low calculated energy barriers for the GSk→ TSk, GSk‘ → TSkand GC→ TCisomerization processes. Results from annealing experiments in krypton matrix doubtlessly show that in this matrix the order of stability of the TSkand TCconformers is reversed, with the more planar TCform becoming the most stable conformer. [ABSTRACT FROM AUTHOR]

Picolinamide (PA) and isonicotinamide (INA), two structural isomers of pyridinecarboxamide, have been investigated by matrix isolation and low-temperature solid-state infrared spectroscopy, combined with UV ( > 235 nm) photoexcitation and density functional theory and ab initio (MP2) theoretical studies. In consonance with the theoretical data, both PA and INA were found to exist in a single conformation in cryogenic rare gas matrixes. Comparison between the experimental spectra of the matrix-isolated compounds with those theoretically predicted allowed for full assignment of the experimental spectra. In situ UV ( > 235 nm) irradiation of the matrixes showed that only PA reacts, with production of isocyanic acid and pyridine, the first photoproduct further reacting to yield CO NH and cyanic acid. The different photochemical behavior of the two compounds was explained taking into consideration their different structures. The infrared spectra of (i) the low-temperature glassy state resulting from fast deposition of vapors of the compounds onto a substrate cooled to 10 K, (ii) the crystal resulting from the annealed amorphous film of the compound, and (iii) the room-temperature crystals (-phase) of the studied compounds were also obtained, fully assigned and correlated with intermolecular interactions present in the condensed phases, in particular H-bond interactions, showing that these latter are stronger in INA than in PA. [ABSTRACT FROM AUTHOR]

The aliphatic 2H-azirine, methyl 3-methyl-2H-azirine-2-carboxylate (MMAC), has been synthesized and its monomeric form investigated by IR spectroscopy in an argon matrix, at 10 K, as well as theoretically (DFT/B3LYP/6-311G(d,p)). Two low-energy conformers of MMAC (Ct and Cc) were found in the matrix, both exhibiting the cis conformation around the C−O bond but differing in the arrangement around the C−Cbond. The two conformers were photoreactive upon in situ broadband UV excitation ( > 235 nm), yielding nitrile ylide (P1) and ketene imine (P2) type products, which resulted from cleavage of the C−C or C−N bond, respectively. The kinetics of the reactions leading to the formation of P1 and P2 are of first order, with the processes being favored when the reactant is in the Cc conformation. Very interestingly, the C−N bond photocleavage, which is unusual for aliphatic 2H-azirines, was found to be preferred over the generally favored in 2H-azirines C−C bond breakage. This behavior is attributed to the presence in the molecule of the electron-withdrawing methoxycarbonyl substituent, which accelerates the intersystem crossing toward the T1triplet state and, in this way, favors the C−N bond cleavage. In addition to the primary photoprocesses leading to formation of P1 and P2, secondary photoprocesses leading to the decarboxylation and decarbonylation of P2 have been also observed. [ABSTRACT FROM AUTHOR]