Potential energy surfaces, calculation empirical

Domanski et al. [179] developed an ab initio force field for liquid carbon dioxide by fitting the LJ parameters and the coulombic terms to the potential energy surface calculated with QM at the MP2 level of theory and the 6-31G basis set Unfortunately, their model does not reproduce the thermodynamic behavior of the liquid state so that an empirical scaUng factor had to be adjusted to experimental data. 

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. 

To calculate N (E-Eq), the non-torsional transitional modes have been treated as vibrations as well as rotations [26]. The fomier approach is invalid when the transitional mode s barrier for rotation is low, while the latter is inappropriate when the transitional mode is a vibration. Hamionic frequencies for the transitional modes may be obtained from a semi-empirical model [23] or by perfomiing an appropriate nomial mode analysis as a fiinction of the reaction path for the reaction s potential energy surface [26]. Semiclassical quantization may be used to detemiine anliamionic energy levels for die transitional modes [27]. 

Example Jensen and Gorden calculated the potential energy surface of glycine using ab initio and semi-empirical methods.This study is of special interest to developers of molecular mechanics force fields. They frequently check their molecular mechanics methods by comparing their results with ab initio and semi-empir-ical calculations for small amino acids. 

HyperChem can calculate transition structures with either semi-empirical quantum mechanics methods or the ab initio quantum mechanics method. A transition state search finds the maximum energy along a reaction coordinate on a potential energy surface. It locates the first-order saddle point that is, the structure with only one imaginary frequency, having one negative eigenvalue. 

Before discussing the kinds of kinetic information provided by potential energy surfaces we will briefly consider methods for calculating these surfaces, without going into detail, for theoretical calculations are outside the scope of this treatment. Detailed procedures are given by Eyring et ah There are three approaches to the problem. The most basic one is purely theoretical, in the sense that it uses only fundamental physical quantities, such as electronic charge. The next level is the semiempirical approach, which introduces experimental data into the calculations in a limited way. The third approach, the empirical one, makes extensive use of experimental results. 

Empirical methods are of two types those that permit potential energy surfaces to be calculated and those that only allow activation energies to be estimated. Laidler has reviewed these. A typical approach is to establish a relationship between experimental activation energies and some other quantity, such as heats of reaction, and then to use this correlation to predict additional activation energies. In Section 5.3 we will encounter a different type of empirical potential energy surface. 

Traditionally, trajectory calculations were only performed on previously calculated (or empirically estimated) potential energy surfaces. With the increased computational speed of modern computers, it has also become possible to employ direct dynamics trajectory calculations [34, 35]. In this method, a global potential energy surface is not needed. Instead, from some... 

A58, 727 (2002). Quantum Calculation of Highly Excited Vibrational Energy Levels of CS2(X) on a New Empirical Potential Energy Surface and Semiclassical Analysis of 1 2 Fermi Resonances. 

Fig. 6.4 Plot of reaction probability vs. initial translational energy for the H + HH = HH + H reaction for a certain empirical potential energy surface (the Porter-Karplus surface). Curves (reading down) are shown for the path shown as PP in Fig. 6.3a. (marked Marcus-Coltrin), the exact quantum mechanical result for the Porter-Karplus surface (marked Exact QM), the usual TST result calculated for the MEP, QQ (Fig. 6.3a) (The data are from Marcus, R. A. and Coltrin, M. E., J. Chem. Phys. 67, 2609 (1977))...

The empirical valence bond (EVB) approach introduced by Warshel and co-workers is an effective way to incorporate environmental effects on breaking and making of chemical bonds in solution. It is based on parame-terizations of empirical interactions between reactant states, product states, and, where appropriate, a number of intermediate states. The interaction parameters, corresponding to off-diagonal matrix elements of the classical Hamiltonian, are calibrated by ab initio potential energy surfaces in solu-fion and relevant experimental data. This procedure significantly reduces the computational expenses of molecular level calculations in comparison to direct ab initio calculations. The EVB approach thus provides a powerful avenue for studying chemical reactions and proton transfer events in complex media, with a multitude of applications in catalysis, biochemistry, and PEMs. 

Molecular dynamic studies used in the interpretation of experiments, such as collision processes, require reliable potential energy surfaces (PES) of polyatomic molecules. Ab initio calculations are often not able to provide such PES, at least not for the whole range of nuclear configurations. On the other hand, these surfaces can be constructed to sufficiently good accuracy with semi-empirical models built from carefully chosen diatomic quantities. The electric dipole polarizability tensor is one of the crucial parameters for the construction of such potential energy curves (PEC) or surfaces [23-25]. The dependence of static dipole properties on the internuclear distance in diatomic molecules can be predicted from semi-empirical models [25,26]. However, the results of ab initio calculations for selected values of the internuclear distance are still needed in order to test and justify the reliability of the models. Actually, this work was initiated by F. Pirani, who pointed out the need for ab initio curves of the static dipole polarizability of diatomic molecules for a wide range of internuclear distances. 

Semi-empirical PM3 calculations" reveal that ylide 23 is a minimum on the potential energy surface and that both steps are exothermic. The enthalpy of the reaction of ylide formation in CH3CN was estimated to be —43 kcal/ mol and the enthalpy of reaction of the second step, 1,2-hydrogen shift, was calculated to be —12.5kcal/mol. 

There is no change in the chemical composition of the reacting species in reactions (a) and (b) nevertheless, it is possible to measure the rate of these reactions by studying the process of the change in the spin state of the nuclei (ortho-para conversion). Theoretically, reactions (a)-(d) are of special interest because for them rather accurate non-empirical calculations of the potential energy surface, as well as detailed, up to quantum mechanical, calculations of the nuclear dynamics during an elementary reaction act can be carried out. 

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