Quantum mechanical computations hydrocarbons

Rabinowitz, J. R., and S. B. Little. 1994. Comparison of Quantum-Mechanical Methods to Compute the Biologically Relevant Reactivities of Cyclopenta-Polycyclic Aromatic-Hydrocarbons. Inti. J. Quant. Chem. 52, 681. 

Selim Senkan is noted for his work in environmental engineering, and particularly for his work in the reaction rates of chlorinated hydrocarbons. He writes in Detailed Chemical Kinetic Mechanisms on the impact of efficient numerical algorithms and computational quantum mechanics on the prediction of reaction mechanisms and rates. 

Returning to naphthalene, there seems to be no calculational way to arrive at the correct structure without doing some kind of a quantum mechanical treatment of the 7r-system. The bond orders of the 7T-system determine what the structure will be, and it seems unavoidable. While a quantum mechanical calculation on naphthalene is a problem of sizable dimensions (48 valence orbitals or 58 total orbitals as a minimum basis set), the 7T-system of naphthalene contains but 10 orbitals. Before quantum chemists had computers, they studied in great detail the question of -a separation, and a great body of lore is available to us from those studies (see, for example, Flurry, 1968). It is well known just how to treat planar delocalized hydrocarbons by existing methods, to take advantage of the - separation, and yet obtain accurate results. There are restrictions, however, planarity being a particularly important one. 

The HOMA, TOPAZ, TIR, REPE, and A indices and their aromatic scales for a series of representative benzenoid hydrocarbons are presented in Table 4.10. In order to compare them with the actual electronegativity and chemical hardness-based absolute aromaticities the AIM electronegativity and chemical hardness values are first computed and reported in Table 4.10 based on Eqs. (3.252) and (3.248), respectively then, they were combined with the CFD counterparts for all schemes from Table 3.8 applied on Eqs. (3.375) and (3.376) through employing the semi-empirical AMI quantum mechanically calculation of the involved frontier orbitals and energies the resulted absolute aromaticities are presented in Tables 4.11 and 4.12, respectively. 

Most of the chemical reactions as well as experimental structure determinations are performed in solutions. On the contrary, usual quantum chemical computations usually deal with isolated chemical species. This may lead to erroneous conclusions. For instance, the addition of bromine to an ethylenic hydrocarbon is known for having a different mechanism in the gas phase and in solution. In this example, the velocity constant vary by a factor of 10 when going from carbon tetrachloride to water as a solvent (Reichardt 1979), although the mechanism is the same These features are confirmed by appropriate quantum chemical computations which show that the transition state of ethylene-bromine would be dissymmetric and 55kcal/mol above the van der Waals complex in the case of the isolated species (Yamabe et al. 1988), while with a simple simulation of the solvent effect one finds a symmetric transition state lying 30.79 kcal/mol above the van der Waals complex in a non dipolar solvent and 0.02 kcal/mol in water (Assfeld 1994). 

Computer experiments particularly use quantum chemical approaches that provide accurate result with intense computational cost. Classical or semiempirical methods on the other hand are able to simulate thousands or up to millions of atoms of a system with pairwise Lennard-Jones (LJ)-type potentials [104-107]. Thus, LJ-type potentials are very accurate for inert gas systems [108], whereas they are unable to describe reactions or they do so by predetermined reactive sites within the molecules of the reactive system [109]. van Duin and coworkers [109-115] developed bond-order-dependent reactive force field technique is called ReaxFF as a solution to the aforementioned problems. Therefore, ReaxFF force field is intended to simulate reactions. They are successfully implemented to study hydrocarbon combustion [112,115,116] that is based on C-H-0 combustion parameters, fuel cell [110,111], metal oxides [117-122], proteins [123,124], phosphates [125,126], and catalyst surface reactions and nanotubes [110-113] based on ReaxFF water parameters [127]. Bond order is the number of chemical bonds between a pair of atoms that depends only on the number and relative positions of other atoms that they interact with [127]. Parameterization of ReaxFFs is achieved using experimental and quantum mechanical data. Therefore, ReaxFF calculations are fairly accurate and robust. The total energy of the molecule is calculated as the combination of bonded and nonbonded interaction energies. 

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