Centro de Investigação em Química, Departamento de Química da Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 687, 4169-007 Porto (Portugal)

Chemical species containing the N-H bond form an important class of compounds with a large variety of applications, from pharmaceutical agents to toxic substances. Thus, these compounds may be found in the building blocks of biomolecules as well as in the majority of chemical industry products. In fact, not only they are relevant in life processes but also can have very different roles in industry acting as antioxidants, complexing agents or in the manufacture of herbicides, surfactants, dyes, pigments, rubber, polymers and biological materials. The N-H bonds play a crucial role in many biological mechanisms, for example, as in the proton-transfer enzymatic reaction catalyzed by acetylcholinesterase, where N-H bonds are cleaved and formed at the imidazole ring present in the Glu327-His440-Ser200 catalytic triad [1]. Also, they are important in the antioxidant activity of phenothiazine and related compounds to prevent premature polymerization or oxidation of plastics, lubricating oils, foods or cosmetics and being equally relevant in free radical reactions [2].

Despite the great potential and wide application of chemicals containing the N-H bond, the information about the reactivity and strength of the N-H bond is still scarce and, in some cases, is contradictory. In fact, even for the most studied simple molecules such as NH3, CH3NH2, (CH3)2NH, PhNH2 and Ph2NH, the experimental homolytic N-H bond dissociation energy (BDE) available in the literature may differ by more than 20 kJ/mol. The main causes for such differences in the gas-phase BDEs are thought to be due to the application of several different experimental techniques and consequent improper introduction of enthalpy corrections.

In the present communication, it will be presented a compilation of calculated gas-phase N-H BDEs for a large series of molecules containing as least one N-H bond. The molecules studied belong to different classes of compounds, namely, substituted amines, amides and anilines, aminoacids, phenoxazines, N,N-alkyl-N'-acyl(thio)ureas, pharmaceutical agents, etc. These values were computed by means of accurate calculations performed at the (RO)B3LYP/6-311+G(2d,2p)//(U)B3LYP/6-31G* level of theory. The computed gas-phase N-H BDEs, at T = 298.15 K, are in range 203.9 - 491.1 kJ/mol, for HNO and HNCO, respectively. Generally, the calculated BDEs are in excellent agreement with available experimental data. Solvent effects in computed N-H BDE will be discussed also.

References:

[1]- Wlodek, S. T.; Antosiewicz, J.; Briggs, J. M. J. Am. Chem. Soc. 1997, 119, 8159.

[2]- Halliwell, B.; Gutteridge, J. M. C. Free Radicals in Biology and Medicine; Clarendon Press: Oxford, U. K., 1985.