Semiempirical quantum chemical

Conformations of 4-oxo-l,6,7,8,9,9n-hexahydro-4//-pyrido[l,2-n]pyrimi-dine-3-carboxylates and -3-carboxamides were studied by semiempirical quantum chemical calculations at the AMI level (97H(45)2175). While 1-methyl-9n-unsubstituted derivatives adopt a cA-fused conformation, 9a-ethoxy-1-methyl derivatives adopt a trans-fu ed one to avoid a serious non-bonding interaction between 9n-ethoxy and 1-methyl groups, which would be present in an alternative cA-fused conformation. 

One example of the use of semiempirical methodology is provided in an article detailing a molecular-dynamics simulation of the beta domain of metallothionein with a semiempirical treatment of the metal core.73 The beta domain of rat liver metallothionein-2 contains three-metal centers. In this study, three molecular variants with different metal contents—(1) three cadmium ions, (2) three zinc ions, and (3) one cadmium ion and two zinc ions—were investigated using a conventional molecular dynamics simulation, as well as a simulation with a semiempirical quantum chemical description (MNDO and MNDO/d) of the metal core embedded in a classical environment. For the purely classical simulations, the standard GROMOS96 force-field parameters were used, and parameters were estimated for cadmium. The results of both kinds of simulations were compared to each other... 

The heteropolar transition state was also predicted by semiempirical quantum-chemical calculations (78T2315). The ground-state properties of... 

In Section II,B,8 we discussed the question of determining site densities using high-conversion data. We developed a method applicable in the inter-conversion of three isomers when there is a common surface complex for the three possible reactions. We have tested this method using the conversion of 1-butene to cis- and rrans-2-butene over silica-alumina, a system that, according to Hightower and Hall, proceeds through a common surface complex (111). Their conclusion has been confirmed experimentally (112) and by semiempirical quantum-chemical calculations (113). 

For chemical purposes, substitution of total energy hypersurfaces by those based on the heat of formation seems more reasonable, with the difference given by the zero point energy corrections. However, their calculations from first principles can be rather cumbersome (12) and, moreover, for a given variation of some nuclear coordinates it usually can be assumed that the change in zero point energy is small compared to that of the total energy. On the other hand, se eral semiempirical quantum chemical procedures which are appropriately parametrized often yield satisfactory approximations for molecular heats of formation (10) and, therefore, AH hypersurfaces have become rather common. 

In summary, computational quantum mechanics has reached such a state that its use in chemical kinetics is possible. However, since these methods still are at various stages of development, their routine and direct use without carefully evaluating the reasonableness of predictions must be avoided. Since ab initio methods presently are far too expensive from the computational point of view, and still require the application of empirical corrections, semiempirical quantum chemical methods represent the most accessible option in chemical reaction engineering today. One productive approach is to use semiempirical methods to build systematically the necessary thermochemical and kinetic-parameter data bases for mechanism development. Following this, the mechanism would be subjected to sensitivity and reaction path analyses for the determination of the rank-order of importance of reactions. Important reactions and species can then be studied with greatest scrutiny using rigorous ab initio calculations, as well as by experiments. 

In the case of cyclooctatetraene, an electron prefers the isotopicaUy heavier material. At 173 K, the equilibrium constant for -F C Dg = CgHg + CgDg was found to be 1.16 (Stevenson 2007). However, when this anion-radical reacts with cyclooctatetraene dianion (not with the anion-radical), the transferred electrons prefer the isotopicaUy lighter material (Stevenson et al. 1990,1992) as follows CgHg -F = CgHg -F CgDg. The semiempirical quantum chemical consideration led to the... 

The electronic coupling of donor and acceptor sites, connected via a t-stack, can either be treated by carrying out a calculation on the complete system or by employing a divide-and-conquer (DC) strategy. With the Hartree-Fock (HF) method or a method based on density functional theory (DFT), full treatment of a d-a system is feasible for relatively small systems. Whereas such calculations can be performed for models consisting of up to about ten WCPs, they are essentially inaccessible even for dimers when one attempts to combine them with MD simulations. Semiempirical quantum chemical methods require considerably less effort than HF or DFT methods also, one can afford application to larger models. However, standard semiempirical methods, e.g., AMI or PM3, considerably underestimate the electronic couplings between r-stacked donor and acceptor sites and, therefore, a special parameterization has to be invoked (see below). 

Figure 2. Experimental and simulated fluorescence Stokes shift function 5(f) for coumarin 343 in water. The curve marked Aq is a classical molecular dynamics simulation result using a charge distribution difference, calculated by semiempirical quantum chemical methods, between ground and excited states. Also shown is a simulation for a neutral atomic solute with the Lennard-Jones parameters of the water oxygen atom (S°). (From Ref. 4.)...

For explanation of experimental results and for correlation of charge densities with NMR data, semiempirical quantum-chemical calculations of benzo[c]pyrylium cation have been employed. Interestingly, the first calculation of 1,3-dimethyl-benzo[r]pyrylium cation by the simple linear combination of atomic orbitals/molecular orbital (LCAO/MO) method (70KGS1308) revealed a preference for the resonance from a in which the value of the charge density at C was three times as much as at C3. 

The semiempirical quantum chemical consideration led to the conclusion that the discrepancy can be a consequence of ion-pair formation (Zuilhof Lodder 1995). In the case of cyclo-octatetraene, the ion-pairing phenomenon deserves more detailed explanation. While the dianion of cyclo-octatetraene is completely planar and meets all the requirements of aromaticity, the anion radical of cyclo-octatetraene is a nonaromatic species and is not completely planar. In the equilibria just considered, both the anion radical and the dianion had alkali cations as counterparts. The dialkali salt of the dianion has two cations symmetrically located over and beneath the octagonal plane. Distortions from ion pairing between the dianion plane and these two alkali cations are reciprocally compensated. With the... 

Pankratov, A.N. and Uchaeva, I.M. (2000) A semiempirical quantum chemical testing of thermodynamic and molecular properties of arsenic compounds. Journal of Molecular Structure (Theochem), 498, 247-54. 

Tasi G, Palinko I, Halasz J Naray-Szabo G (1992) Semiempirical quantum chemical calculations on microcomputers. CheMicro Ltd, Budapest... 

In a series of papers various semiempirical quantum chemical procedures were examined for their usefulness to describe ion-ligand interactions (for a review see 7)). 

Weiner B, Carmer CS, Frenklach M (1991) Acetylene Reaction With the Si(lll) Surface A Semiempirical Quantum Chemical Study, Phys Rev B 43 1678-1684... 

Nevertheless, ab initio or semiempirical quantum chemical calculations, with clusters to represent the catalytic solid material, can help build up a model of the catalytic system and study effects of modifications [52], Even simulation of the molecular dynamics of the interaction between reactants or intermediates, together with replication of the unit cell of the catalytic solid in a simple force field, may describe observed effects in a catalytic system and may be able to predict the effect of catalyst modifications [53]. 

Several reviews have been written dealing with ab initio and semiempirical quantum chemical calculations including relativistic effects 1-3,17-20). 

Semiempirical quantum-chemical methods can be subdivided into two groups. The first one covers so-called simple MO LCAO methods, of which extended Hiickel treatment (EHT) (7) and its modification by Anderson (ASED) (5), additionally taking into account core repulsion, are most widely used in chemisorption computations. Anderson s improvement of EHT was aimed at obtaining more reliable values of the total energy and at gaining an opportunity to optimize the geometry of chemisorption structures. The method was mainly used in calculations of chemisorption and catalysis on metals. Its validity for oxide systems, with their rather highly ionic bonds, is formally less justified. 

The AM 1 method143 was applied to the calculations of the two paths a and b of the reaction between the carbonyl and nitrile components according to equation 77. Semiempirical quantum chemical calculations were also carried out for the systems [HCHO + HCN + H+]141 and [H2 -0-CH0 + HCN and H2C = O + HCNCHO]142 which simulate the proton and formyl catalysis, respectively. 

These equations are used in semiempirical quantum chemical calculations of non-linear optical polarizabilities by applying perturbation theoretical expressions [the so-called sum-over-states (SOS) method]. Here we use them to derive some qualitative and very general trends in a few simple model systems. To this end we concentrate on the electronic structure, i.e. on the LCAO coefficients. We do not explicitly calculate the transition frequencies. This is justified for the qualitative discussion below since typical transition energies... 

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