LCAO , MO method

A determination of the dissociation constants of the compounds reveals that 5-azauracil (pi a = 6.73) is practically of the same acidity as 6-azauracil and considerably more acidic than uracil, A fundamental difference between 5-azauracil, on the one hand, and 6-azauracil and uracil, on the other, lies in the low stability of 5-azauracil toward acid and especially to alkaline hydrolysis. This fact appears to be in agreement with the differences in electron densities of these substances computed by the simple MO-LCAO method. ... 

If the basic set is chosen to consist of atomic orbitals, this relation forms the fundament for the MO-LCAO method in molecular and crystal theory. In its SCF form this approach was first used by Coulson (1938), and later it has been systematized by Roothaan (1951). More details about the SCF results within molecular theory will be given later in a special section. 

Spin-Spin Coupling.—The relative roles of different coupling mechanisms in organophosphorus compounds have been studied theoretically.66 The spin-spin interaction constants for methylphosphine have been calculated by the MO-LCAO method.67 Unresolved isotropic coupling to boron caused broadening of the vinyl... 

MO LCAO methods, 34 136 Molecular-beam surface scattering, 26 26, 27 Molecular Cage, 34 226 Molecular design in cyclodextrin, 32 427 Molecular dynamics diffusion in zeolites, 42 2, 4-6 argon, 42 20... 

These properties of the d-shell chromophore (group) prove the necessity of the localized description of d-electrons of transition metal atom in TMCs with explicit account for effects of electron correlations in it. Incidentally, during the time of QC development (more than three quarters of century) there was a period when two directions based on two different approximate descriptions of electronic structure of molecular systems coexisted. This reproduced division of chemistry itself to organic and inorganic and took into account specificity of the molecules related to these classical fields. The organic QC was then limited by the Hiickel method, the elementary version of the HFR MO LCAO method. The description of inorganic compounds — mainly TMCs,— within the QC of that time was based on the crystal field... 

Several valence-bond (VB) treatments of heterocyclic compounds were reported in the thirties and forties.1, 2 The known difficulty in applying the VB method to complicated molecules has made an overwhelming majority of authors use the molecular orbital (MO) method. In most cases its simplest version, the naive MO LCAO method, has been used. This approximation differs from the well-known Hiickel... 

Of the more exact methods, the limited configuration interaction (Cl MO LCAO) method and the self-consistent field (SCF MO LCAO) method will be mentioned. In contrast to the HMO method, both of these explicitly take electron repulsion into account. The Cl method is particularly valuable for the calculation of various physical properties, especially electronic spectra. A more detailed description is beyond the scope of the present review the reader is referred to original papers [Cl,17-20 SCF,21-23 and VESCF24 (variable electronegativity)] and to various reviews and monographs.5 25,26... 

Finally, satisfactory parallelism has been found7 between the indices q, p, and F calculated by Amos and Hall97 using the SCF MO LCAO method for five models of pyridine-like compounds and the same indices derived from HMO calculations (SN = 0.5). 

An extensive study has been performed concerning the basicity of aminoazines using the SCF MO LCAO method.165 Moreover, attention has been paid to the influence of various factors influencing the basicity of azines196 and to the relationship between the protonation and solvation entropy.167... 

Simple estimates for the cellular potentials can be extracted from the MO LCAO method in the two-level model eq. (2.96). If one considers the interaction of the type A (A = a, 7r, J) between the (/-orbital of the metal 4>d and the orbital o, representing a mixture of the orbitals of the ligand and corresponding by local symmetry s- and p-orbitals of the metal and taking into account that according to the definition of the... 

Denis, A., and A. Pullman Theoretical study of the electronic properties of biological purines and pyrimidines. III. Theoret. Chim. Acta (Berlin) 7, 110 (1967). Del Re, G. A simple MO-LCAO method for the calculation of charge distributions in saturated organic molecules. J. Chem. Soc. (London) 4031 (1958). 

A detailed theoretical study of the n — ir and v -> ir transitions of (I7a-c) has been made using the SCF MO-LCAO method and the Pariser, Parr and Pople (PPP) modification. The calculated and experimental values are in excellent agreement (79MI43500). The calculated and experimental values of (17c), (18a) and (18c) have also been determined in polyethylene (77JA2855). 

The 7r-electron structures and energies of the singlet tt-tt transitions for a number of l//-pyrrolo[l,2-a jimidazoles (39), l/f-pyrrolo[l,2-f>]-s-triazoles (40) and l//-pyrrolo[2,l-c]-s-triazoles (41) were calculated by the MO LCAO method within the semiempirical self-consistent field (SCF) approximation. A comparison of the data shows that the maximum... 

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 second group of semiempirical MO LCAO methods is constituted by zero-differential overlap methods (9). Two subgroups can be specified here which somewhat conventionally may be called physical and chemical. The former involves CNDO/2, INDO, and some of their modifications, for example, CNDO/S. These methods are directed to the calculations of the electron characteristics charge distributions, dipole moments, polarizabili-... 

FIG. 11. Energy level diagram for the cluster modelling the Cu(lOO) surface, the bare clusters Cut, Cus and Cug evaluated with the standard MO-LCAO method to the left and the calculations for jellium to the right were done using the spherical jeUium model [72,74]. The MO-LCAO results include contribution from the 3d levels while the jellium model only include the free 4s electrons. The one-electron orbitals are characterized by the symmetry notation corresponding to C v symmetry. Since the calculation for jellium were performed within the LSD scheme the one electron levels are split. 

Electronic spectral shifts induced in the model fragment 75 by the introduction of a dimethylamino or cyano group into different positions were computed by MO-LCAO methods. ... 

Del, Re.G. (1958) A simple MO-LCAO method for the calculation of charge distribution in saturated organic molecules./. Chem. Soc., 40, 4031-4040. 

G. Del Re, /. Chem. Sue. l.ondon, 1958, 403 I (1958). A Simple MO-LCAO Method for the Calculation of Charge Distributions in Satur.ited Organic Molecules. 

Calculations by SCF-MO LCAO methods have also been carried out on the oxazinones (1) and... 

There is some preliminary data <74CHE230> based on the MO LCAO method within the semi-empirical self-consistent field (SCF) approximation, on l/f-pyrrolo[l,2-a]imidazoles the maximum TT-electron densities are localized on C-5 and C-7 <84CHEC-I(6)979>. No other theoretical methods have been used. 

Simple HUckel molecular orbital (HMO) calculations on the pyrrolo[2,l-c][l,2,4]triazole (28) suggest that electrophilic attack would occur most readily at C-10, and this prediction was borne out by observations that acid-catalyzed deuteration and bromination by NBS in the dark both occur at this position <85JCR(S)363>. Various reactivity indices have been calculated for a number of pyrrolo[ 1,2-b][, 2,4]triazoles (29) and pyrrolo[2,1 -c][ 1,2,4]triazoles (30) using the MO LCAO method within the semiempirical SCF approximation. These indicate that the 5-position is most susceptible to electrophilic attack, followed by the 7-position <74CHE230>. 

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