Density functional theory enthalpy

However, even the best experimental technique typically does not provide a detailed mechanistic picture of a chemical reaction. Computational quantum chemical methods such as the ab initio molecular orbital and density functional theory (DFT) " methods allow chemists to obtain a detailed picture of reaction potential energy surfaces and to elucidate important reaction-driving forces. Moreover, these methods can provide valuable kinetic and thermodynamic information (i.e., heats of formation, enthalpies, and free energies) for reactions and species for which reactivity and conditions make experiments difficult, thereby providing a powerful means to complement experimental data. 

Deeth et al. have used density functional theory (DFT) to model water exchange on square-planer [Pd(H20)4]2+ and [Pt(H20)4]2+ (212). Their calculations strongly support that H20 exchange on these complexes proceeds through an a-activation mechanism, in full agreement with experimental assignments. The agreement between the experimental and calculated activation enthalpy is better than lOkJmol-1 for an Ia mechanism, whereas it differs by more than 100 kJ mol-1 for a calculated Id mechanism. 

Deeth et al. used density functional theory to model water exchange on square-planar [Pd(H20)4] and [Pt(H20)4] (90). The calculations strongly support that water exchange proceeds via an a-activation mode on these complexes and the trigonal bipyramidal structures calculated for [Pd(H20)5] and [Pt(H20)5] " were very similar. There is a good agreement between experimental and calculated activation enthalpies for an la mechanism, whereas for an I mechanism, a difference of more than 100 kJ mol is observed. 

The methods available for computing enthalpies of formation fall into two general groups those based on purely empirical schemes and those founded on quantum chemistry. The quantum chemical methods can be further divided into three types ab initio molecular orbital theory, density functional theory, and semiempirical molecular orbital theory. A summary of the types of method used to calculate enthalpies of formation is given in Table 2, along with some specific examples. This is not meant to be a comprehensive tabulation, but rather a list of some of the more popular approaches in use today. Table 3 names some of the commercially available computer programs having capabilities to calculate thermochemical data. 

A great number of studies related to thermochemical properties of QDO and PDO derivatives have been recently described by Ribeiro da Silva et al. [98-103]. These studies, which have involved experimental and theoretical determinations, have reported standard molar enthalpies of formation in the gaseous state, enthalpies of combustion of the crystalline solids, enthalpies of sublimation, and molar (N - O) bond dissociation enthalpies. Table 5 shows the most relevant determined parameters. These researchers have employed, with excellent results, calculations based in density functional theory in order to estimate gas-phase enthalpies of formation and first and second N - O dissociation enthalpies [103]. 

Curtiss LA, Raghavachari K, Redfern PC, Pople JA (1997) Assessment of Gaussian-2 and density functional theories for the computation of enthalpies of formation, J Chem Phys, 106 1063-1079... 

For the first decomposition step the formation of [Bi2Hi2] anion as a possible intermediate phase is discussed [35]. Different enthalpy values in the range —40 to 5 7 kj mol H2 for the first decomposition step at about 277 ° C have been determined from experimental data [35, 36[. From density functional theory (DFT) calculations a reaction enthalpy of 38 kj mol H2 at 27 ° C was confirmed [37]. Cycling experiments performed at 350 ° C under 10 MPa hydrogen lead to re-absorption of more than 3 wt.% hydrogen, showing lhat the second decomposition step is reversible [33, 36]. 

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