Substituent Effects on Enthalpies of Formation:  Benzene Derivatives

Abstract

High level density functional theory calculations have been carried out for a benchmark set of benzene derivatives, including methyl, ethyl, n-propyl, i-propyl, tert-butyl, phenyl, and benzyl groups as substituents. Geometries were obtained using the B3LYP method and three basis set expansions, namely 6-31G(d), 6-311G(d,p), and 6-311++G(d,p). Final energies were calculated in B3LYP/6-311+G(3df,2p) single-point calculations. Based on these calculations the performance of different theoretical schemes aiming at reproducing substituent effects on enthalpies of formation has been assessed. The poorest performance is obtained when atomization energies or isodesmic reactions are used. No significant improvement is found when using homodesmotic processes. A significant improvement is achieved when the isodesmic processes used involve the unsubstituted parent compound. That means that this procedure can be a good alternative to explore substituent effects on the enthalpies of formation, although the absolute values of this thermodynamical magnitude have still a significant error. The best performance is obtained when different atom equivalent schemes are used, the correlation coefficient of the linear relationship between calculated and experimental values being greater than 0.999.

Thermochemical properties of two benzimidazole derivatives: 2-Phenyl- and 2-benzylbenzimidazole

“…The results of the Knudsen-effusion experiments for 2-BzBIM calculated by means of equation (1) are summarised in table 5, where p represents the vapour pressure, Dm the mass loss during the time t, at temperature T, and dp/p the fractional deviation of the experimental vapour pressures from those computed using equation (8).…”

“…Our research groups have been involved in a systematic experimental and theoretical study about the substituent and ring effects on the energetics of diazole derivatives, derived from imidazole and pyrazole [1][2][3][4][5][6][7][8] and, recently, we extended our research work on bicyclic molecules derived from imidazoles fused to the aromatic ring C [9,10]. The aim of the extension of this research is not only to compare the effect of the substituents in the heterocycle of the bicyclic molecules with the corresponding monocyclic heteromolecules, but also to enlarge the knowledge on the energetics of benzimidazoles, some of which are known to have antiviral activity [11][12][13][14][15][16] used in fungicides [17], in therapy of intoxications [18], as protective agents for metal surfaces of printed-circuit boards [18], as inhibitors on steel corrosion [19], in sunscreen cosmetics [20], etc.…”

“…c Vapour pressure. d dp denotes deviation of the experimental vapour pressures from the values computed using equation(8). and using the literature values for the standard molar enthalpies of formation in the gaseous phase, registered in table 8, and the values measured in this work for 2-phenylbenzimidazole (258.0 ± 2.5) kJ AE mol À1 and 2-benzylbenzimidazole (239.2 ± 4.4) kJ AE mol À1 , the molar enthalpy of reaction…”

Evaluation of the Enthalpy of Formation, Proton Affinity, and Gas-Phase Basicity of γ-Butyrolactone and 2-Pyrrolidinone by Isodesmic Reactions

“…14 However, the results obtained by using these two approaches are not very exact when compared to isodesmic/homodesmic reactions. 15,16 Thus, some methods aimed at the obtention of accurate thermochemical parameters have been developed in recent years. Among these methods are basis set extrapolation, multicoefficient methods, and composite chemical models.…”

“…13,14,21,22 The DFT methods are less demanding in terms of computational costs and provide satisfactory results. 7,[14][15][16] Several works have thoroughly investigated the real ability of different models to predict thermochemical parameters. To this end, Smith and Random tested whether a series of Gaussian models, MP4, and DFT methods were able to reproduce experimental values concerning proton affinity, and they came to the conclusion that the DFT models were the least accurate.…”