Quantum chemical calculations semiempirical methods

Bond orders, charges on the atoms in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one and its protonated form were calculated by quantum chemical calculations by the semiempirical AMI method. According to the results, the equilibrium conformation of the ring in 1 l//-pyrido[2,l-Z)]quinazolin-l 1-one is planar, while l//-pyrimido[l,2-u]quinolin-1-one adopts a conformation close to a half-chair due to the unfavorable interactions between the oxygen atom of the carbonyl group and the ring C-10 atom in the pen-position (97MI22). 

Quantum-chemical calculations have been carried out for 123 [R = Me, = (CH2) , = 3, 4] using two semiempirical methods AMI and PM3 [94JCS(P2)1561], The heats of formation (AH ) and the population of each tautomer were calculated, the latter being in good agreement with experimental data. 

The simplest discrete approach is the solvaton method 65) which calculates above all the electrostatic interaction between the molecule and the solvent. The solvent is represented by a Active molecule built up from so-called solvatones. The most sophisticated discrete model is the supermolecule approach 661 in which the solvent molecules are included in the quantum chemical calculation as individual molecules. Here, information about the structure of the solvent cage and about the specific interactions between solvent and solute can be obtained. But this approach is connected with a great effort, because a lot of optimizations of geometry with ab initio calculations should be completed 67). A very simple supermolecule (CH3+ + 2 solvent molecules) was calculated with a semiempirical method in Ref.15). 

Since the quantum chemical calculations used to parameterize equations 6 and 7 are relatively crude semiempirical methods, these equations should not be used to prove or disprove differences in mechanisms of decomposition within a family of initiators. The assumption made in the present study has been that the mechanism of decomposition of initiators does not change within a particular family of initiators (reactions 1-4). It is generally accepted that trow5-symmetric bisalkyl diazenes (1) decompose entirely by a concerted, synchronous mechanism and that trans-phenyl, alkyl diazenes (2) decompose by a stepwise mechanism, with an intermediate phenyldiazenyl radical (37). For R groups with equal or larger pi-... 

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). 

Table III compares the bond orders calculated both by the HMO and by the semiempirical methods. The geometry of a molecule may be crucial to quantum chemical calculations it is not known for 4. Whereas Dewar et used an iterative procedure (correlation between bond lengths and bond orders), other workers preferred standard geometry or a combination of naphthalene and furan values. ...

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 energetic preference of the endo form was demonstrated by the first quantum chemical calculations of silatranes performed on 1-hydro- and 1-fluorosilatranes12,181. These calculations employed the CNDO/2 semiempirical method with spd basis set and geometries of the endo and exo forms corresponding to those obtained for 1-methylsilatrane by modified molecular mechanics method. The energy of the Si-e- N coordinative bonding was found to be about 20-25 kcalmol-1 and to be the main contributor to stabilization of the endo structure of silatranes. 

Recent progress in computational hardware and the development of efficient algorithms have assisted the routine development of molecular quantum-mechanical calculations. New semiempirical methods calculate realistic quantum-chemical molecular quantities in a relatively short computational time frame. Quantum-chemical calculations are thus an attractive source for molecular descriptors that can express all of the electronic and geometric properties of molecules and their interactions. Quantum-chemical methods can be applied to QSARs by direct derivation of electronic descriptors from the molecular wave function. 

All quantum chemical calculations are based on the self-consistent field (SCF) method of Hatree and Fock (1928-1930) and the MO theory of Hund, Lennard-Jones, and Mulliken (1927-1929). A method of obtaining SCF orbitals for closed shell systems was developed independently by Roothaan and Hall in 1951. In solving the so-called Roothan equations, ab initio calculations, in contrast to semiempirical treatments, do not use experimental data other than the values of the fundamental physical constants. 

To conclude this section, it should be noted that the calculations of the potential energy surfaces for heterogeneous catalytic reactions, even by semiempirical methods, still remain a matter for the future. Insufficient accuracy of the semiempirical methods, the approximate nature of cluster modeling, the large volume of a configurational space, a variety of possible reaction paths, etc., considerably restrict the utility of quantum chemistry as applied to this field. There is, however, no doubt that these difficulties will be successfully overcome. The value of conclusive quantum-chemical calculations can hardly be overestimated. They are able to answer questions which the most sophisticated and refined experiments would fail to answer. 

In [17], two approaches were taken to assess the role of each reaction pathway quantum-chemical calculations and parabolic simulation of the reaction of addition (semiempirical method of intercrossing parabolas, MIP) [34-36]. Using these approaches in combination, the authors could evaluate independently the reaction rate constants for each pathway and compare their contributions to the total ozonation of olefins of different structures. The comparison results are listed in Table 8. We note a good agreement between the calculated and experimental constants values. 

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... 

In general, quantum chemical calculations may be divided into three broadly defined subdisciplines based on the approaches taken to solve the Schrodinger equation a) semiempirical-based methods b) ab initio-based methods and c) density functional theory. John Pople, ab initio methods, and Walter Kohn, density functional theory (DFT), received the 1998 Nobel Prize in chemistry for their pioneering work in computational quantum chemistry. 

Semiempirical quantum chemical calculations using the complete neglect of differential overlap (CNDO/2) method were made for 1,2-dia-minoimidazole (84KGS1396), 1,2-diaminobenzimidazole (85KGS1402), 1-acylamino-1,2,3-triazoles (87JHC1461), and N,N -diazolyls [80JCR(M)-514]. 

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