Semi-Empirical Quantum Mechanical Calculations

In calculating ionization potentials or electron affinities, we are comparing two 

In spite of the development of new methods of measurements, fewer than 300 molecular electron affinities have been measured in the gas phase [5-7]. With the limited number of experimental Ea it is important to have a simple rigorous quantum mechanical procedure for the calculation of electron affinities of large molecules. Electron correlation is the crucial problem in the calculation of Ea. The correlation energies are significant because Ea represent a small difference between two large quantities. Since an electron is added to the system, the effects of geometry changes and correlation reinforce each other rather than cancel out, as in ionization potentials. 

The quantum mechanical calculations are carried out on a Pentium desktop computer with commercial software. The HYPERCHEM input files (H1N) for the various species contain charge densities and a complete description of the geometric and energy properties of the neutral molecule and anion. These compact files are an efficient way to store and communicate this information [1], 

The linear acenes, benzene to pentacene, are used as examples of the CURES-EC procedure. The results obtained utilizing MINDO/3 and AMI are compared. In addition to calculating the Ea by subtracting the energies of the optimized form, the LUMO of the neutral is compared with the experimental Ea. The electron affinity of hexacene has been estimated from the electronegativity and experimental ionization potential. As a further example of the use of CURES-EC, both the ionization potential and electron affinity of heptacene are estimated. The Ea of octacene and nova-cene are calculated for comparison to values obtained by using Koopman s theorem and a semi-empirical method based on a variable-parameter modification of the Pariser Parr Pople (PPP) approximation to the Hartree Fock equation [10]. 

The valence-state electron affinity of benzene is negative from reduction potentials, but the adiabatic electron affinity is positive. The electron affinities of the other linear acenes up to pentacene have been measured in the gas phase. Multiple values are reported for anthracene and naphthalene. The naphthalene Ea are —0.2 eV by electron transmission and others, and 0.16 0.03 eV and 0.13 0.05 eV by ECD. For anthracene the three values are 0.53 0.01 eV by PES, 0.60 0.10 eV by TCT, and 0.54, 0.60, and 0.68 0.02 eV by ECD. In the PES of anthracene there are also peaks at 0.42, 0.54, 0.60, and 0.68 eV 

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Programs for Semi-empirical Quantum Mechanical Calculations... 

Apractical introduction to molecular mechanics and semi-empirical quantum mechanics calculations, with extensive examples from the MMP2 (not in HyperChem), MINDO/3, and MNDO methods. One of the more accessible books for new computational chemists. 

If you request a semi-empirical quantum mechanical calculation now, HyperChem carries on as well as possible (as described below) in choosing how the atoms for quantum mechanical calculation is capped. If, however, an aromatic benzene ring (with delo-... 

Henry Eyring arrived at Princeton in the fall of 1931 shortly after the appearance of the important paper by Eyring and Polanyi which described semi-empirical quantum mechanical calculations of the potential energy surface for the H -E Ha reaction. Eyring s enthusiasm immediately infected a group of students, some of whom extended the procedure to other reactions while others were interested in more fundamental calculations such as would now be referred to as ab initio. 

The generalised Born equation has been incorporated into both molecular mechanics calculations (by Still and co-workers [Still et al. 1990 Qiu et al. 1997]) and semi-empirical quantum mechanics calculations (by Cramer and Truhlar, in an ongoing series of models called SMI, SM2, SM3, etc. [Cramer and Truhlar 1992 Chambers et al. 1996]). In these treatments, the two terms in Equation (11.61) are combined into a single expression of the following form ... 

Korzunov, V. A., Chuvylkin, I. Y, Zhidomirov, G. M., and V. B. Kazanskiy. 1981. Semi-empirical quantum-mechanical calculations of intermediate complexes in catalytic reaction. The stages of chemisorption of ethanol at the aluminum y-oxide. Kinetika i Kataliz. 22 930-936. 

The e.s.r. spectra of the nitroxide (37) and its deuterio-analogue (38), formed by spontaneous oxidation of their hydroxylamine precursors, were recorded in 1,2-dimethoxyethane or diglyme solution at -50° to -120°C. Two conformers were frozen out at low temperatures, and their geometry was assigned from the observed spin coupling constants with the help of semi-empirical quantum mechanical calculations. ... 

A more recent but closely related explanation of the shift of oxidation potentials by electron-withdrawing substituents is based on results of semi-empirical quantum-mechanical calculations [219, 266-268]. Methods such as MNDO, AMI, PM3, HF/3-21G, or B3LYP/6-31G are used to yield the energy of the highest occupied molecular orbital homo of the anion. Gorrelations of Eqx with Phomo are linear for a given set of compounds of similar structure. 

The partial charges were derived from semi-empirical quantum - mechanical calculations using MOPAC 6.0 with the AMI Hamiltonian [12]. Due to computational limitations, the partial charges of the macrocyclic core and of the ester substituents were calculated separately. The final charge set is shown in fig. 1. The benzene of crystallisation was included using CHARMM 22 parameters [13]. A full list of the parameters will be published elsewhere. 

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