Ab initio and DFT Computational Methods

Studies of kinetics and dynamics in proton transfer systems frequently require the knowledge of short-Hved intermediates and transition states. These parameters are often difficult to derive experimentally and this is why ab initio and DFT methods have emerged as a powerfiil supplementary tool for obtaining details of a tautomerization process that are not available from experiments and for providing feedback on models used to interpret experimental data [80]. 

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The photo-oxidation of n-butane has been modelled by ab initio and DFT computational methods, in which the key role of 1- and 2-butoxyl radicals was confirmed.52 These radicals, formed from the reaction of the corresponding butyl radicals with molecular oxygen, account for the formation of the major oxidation products including hydrocarbons, peroxides, aldehydes, and peroxyaldehydes. The differing behaviour of n-pentane and cyclopentane towards autoignition at 873 K has been found to depend on the relative concentrations of resonance-stabilized radicals in the reaction medium.53 The manganese-mediated oxidation of dihydroanthracene to anthracene has been reported via hydrogen atom abstraction.54 The oxidation reactions of hydrocarbon radicals and their OH adducts are reported.55... 

It is important first to recall that the distinctions drawn here between localized VB(NBO)-type and delocalized MO-type descriptions are essentially independent of the actual ab initio or DFT computational method on which the analysis... 

The kinetics of the reaction of heptene with H2SO4 at various concentrations has been elucidated by UV spectrometry. The efficiency of proton transfer to carbon on the intramolecular ring closing of enol ethers in Kirby s enzyme model for aldolase and isomerase has been investigated computationally by using ab initio and DFT calculation methods... 

Ab initio and DFT calculations are now routinely applied to study molecules of increasing complexity. Sometimes the results of these calculations are valuable in their own right. But more often than not these results serve as a guide or as a complement to experimental work. Clearly computational chemistry is revolutionizing how chemistry is done. In the remaining part of this chapter, the discussion will be devoted to ab initio and DFT methods. 

Theoretical calculations have played an important and/or crucial role in providing valuable information for establishing general concepts. Recent progress in theoretical methods, efficient computer programs, and powerful computers has greatly advanced the scope of applicability of ab initio and DFT... 

As an alternative to ab initio methods, the semi-empirical quantum-chemical methods are fast and applicable for the calculation of molecular descriptors of long series of structurally complex and large molecules. Most of these methods have been developed within the mathematical framework of the molecular orbital theory (SCF MO), but use a number of simplifications and approximations in the computational procedure that reduce dramatically the computer time [6]. The most popular semi-empirical methods are Austin Model 1 (AMI) [7] and Parametric Model 3 (PM3) [8]. The results produced by different semi-empirical methods are generally not comparable, but they often do reproduce similar trends. For example, the electronic net charges calculated by the AMI, MNDO (modified neglect of diatomic overlap), and INDO (intermediate neglect of diatomic overlap) methods were found to be quite different in their absolute values, but were consistent in their trends. Intermediate between the ab initio and semi-empirical methods in terms of the demand in computational resources are algorithms based on density functional theory (DFT) [9]. 

There have been several studies aimed at using computations to examine hydrocarbon acidity. The proton affinity values for a number of hydrocarbons were calculated by both ab initio and DFT methods. Some of the results are shown in Table 3.39. 

Table 5. Structural parameters for transition state structures for cyclopropene addition to furan computed with ab initio and DFT methods by using 6-31+G(d) basis set.

In recent years there have been significant advances in ab initio and DFT methods. Many groups (including our own) employ these techniques to compute molecular properties of chemical interest as accurately as presently possible. What are the perspeetives of semiempirical molecular orbital methods under these circumstances Which role can they play in the near future ... 

Enthalpies of transition state structures are determined with the above composite ab initio and DFT methods. Enthalpies of transition states structures are calculated as the difference between the calculated value of the TS structure and the value of the stable radical adduct(s) (adjacent product and reactant where both are a single species) at the corresponding levels of calculation. The computational methods for enthalpies are G3MP2B3 and G3 (whenever possible), and DFT. Zero-point energies (ZPVEs) and thermal corrections to 298.15 K are from DFT. Table... 

For comparison, the relative total energy (BE) values computed for DPM with the semieinpirical (AMI), ab initio, and DFT methods are listed in Table 1, as well as geometrical data (torsional angles <1)a and b and angle 0, Scheme Ij. 

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