Density functional theory thermochemical properties

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]. 

The reactions for which thermochemical properties of transition states are calculated by ab initio or Density Functional Theory methods, oo s are fit by three parameters n, and over the temperature range of 298-2000 K, kao = A T)T exp( a/ 7). Entropy differences between the reactants and transition state structures are used to determine the pre-exponential factor. A, via canonical Transition State Theory [197]. 

Raspopov SA, McMahon TB. A high-pressure mass speetrometrie and density functional theory investigation of the thermochemical properties and strueture of protonated dimers and trimers of glycine. J Mass Spectrom. 2005 40 1536-45. 

Developments in ab initio MO quantum chemistry have resulted in theoretical models [41]. These are able to predict the properties of neutral molecules and ions within so-called chemical accuracy (about 2 kcal/mol). This makes these procedures extremely useful in ion chemistry, especially in thermochemical studies. Another approach is DFT that has attracted intense interest. DFT offers the promise of less drastic scaling with the size of the system than traditional ab initio methods. In general, hybrid density functions frequently provide the best overall agreement and surpass MP2 result of ab initio theory. There have been a number of theoretical studies of the structures and binding energies of alkali ions to many bases, including from free radical, biradical, amino acid, and crown ether to DNA bases (see details in Chap. 3). 

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