Molecular orbital method nitrogen

Molecular orbital methods have been applied with increasing success to the calculation of the UV spectra of the quinolizinium ion (71MI21000) and have been extended to the three benzoquinolizinium ions as well as to some tetracyclic systems having an azonia nitrogen (70G421). 

A mathematical analysis of all four isomeric thiadiazoles by the simple molecular orbital method has provided molecular diagrams of the free base and conjugate acid of each thiadiazole, with electron densities, bond orders, and free valencies. On this basis, predictions have been made concerning the reactivities of the six non-equivalent carbon atoms, the basicities of the nitrogen atoms, and the delocalization energies in these molecules. The 5-position in free 1,2,4-thiadiazole should possess maximum reactivity in nucleophilic substitution reactions. The treatment also accounts for the order of the polarographic half-wave potentials and the position of the absorption maxima in the ultraviolet region of the spectra of 1,2,4- and 1,3,4-thiadiazoles.4... 

Tehan, B.G., Lloyd, E.J., Wong, M.G., Pitt, W.R., Gancia, E. and Manallack, D.T (2002a) Estimation of pKa using semiempirical molecular orbital methods. Part 2. Application to amines, anilines and various nitrogen containing heterocyclic compounds, Quant. Struct. -Act. Relat., 21, 473-485. 

Abbreviations PAH, polycyclic aromatic hydrocarbon DE, diol epoxide PAHDE, polycyclic aromatic hydrocarbon diol epoxide PAHTC, polycyclic aromatic hydrocarbon triol carbocation TC, triol carbocation BaP, benzo[a]pyrene BeP, benzo[e]pyrene BA, benz[a]anthracene DBA, dibenz[a,h]anthracene BcPh, benzo[c)phenanthrene Ch, chrysene MCh, methylchrysene MBA, 7-methyl benz[a]anthracene DMBA, 7,12-dimethyl benz[a]anthracene EBA, 7-ethyl benz[a]anthracene DB(a,l)P, dibenzo[a,l]pyrene MSCR, mechanism-based structure-carcinogenicity relationship PMO, Perturbational molecular orbital method dA, deoxyadenosine dC, deoxycytosine dG, deoxyguanosine MOS, monoxygenase enzyme system EH, epoxide hydrolase enzyme system N2(G), exocyclic nitrogen of guanine C, electrophilic centre of PAHTC K, intercalation constant CD, circular dichroism LD, linear dichroism. 

Application of different molecular orbital methods to the calculation of electron densities of quinolizinium ion and its benzo derivatives led to the results summarized in Table 2. It can be appreciated that in these compounds, nitrogen atoms receive electron density from carbon atoms of the opposite parity, as suggested by perturbation theory <92AHC(55)26i>. Similar Htlckel molecular orbital calculations on berberine (15) and related alkaloids gave an uncommonly high positive... 

Coubeils and Pullman calculated, by means of a molecular orbital method, that the conformation of the side chain attached to the ring nitrogen depended on the folding of the phenothiazine ring systems along the S-N (ring) axis. These data were in agreement with available x-ray... 

An upsurge of interest in the N-methylborazines in the early 1970 s was coupled with a convenient method of synthesis and purification for these compounds The photoelectron spectrum of N-trimethylborazine has been reported. Table 6 summarizes the theoretical and experimental data comparing the location of the molecular orbitals of N-trimethylborazine with those of borazine. The HOMO is predicted and observed to be an e" (w) orbital as in borazine The methyl substitution on nitrogen destabilizes the e" and the a2 jr-orbitals, but does not signiBcantly effect the e (a) orbital. The result is a lowering of the ionization potential for electrons in the two TT-orbitals. This effect, predicted in the dieoretical calculations, was also verified experimentally. 

The molecular orbital description for the nitrite ion just presented was developed without the aid of symmetry considerations and as a starting point, it assumed that a bonds were formed from sp2 hybrid orbitals on the nitrogen and oxygen atoms. Lei us now see how we could have obtained a similar end result by using a method that involves a more formal application of symmetry and does not invoke hybridization. (For a review of symmetry in bonding, see Chapter 3.)... 

The valence bond method has not been used as widely as the molecular orbital approach. With the inclusion of polar structures, however, the valence bond method gives correct orientation for electrophilic substitution and a calculated dipole moment close to the experimental value.100 An application of the one-center method of the 7r-electron system of pyrrole gives electron densities of 1.612, 1.167, and 1.028 on the nitrogen atom and the a- and /3-carbon atoms, respectively.101 Transition energies and the dipole moment by this method are in accord with the observed values. 

There is little to report on the application of molecular orbital calculations to this class of heterocycles. Yamaguchi et al. <91AX(C)590> have observed that the semiempirical AMI method underestimates the double bond character of the bicyclic C—C bond in 2-phenyl-1,2,3-triazolo[4,5-e][l,2,3,4]tetrazine (24) (Table 1). The AMI method has been reported to overestimate bond lengths in other nitrogen heterocycles <92JCS(P1)2779>, and this is possibly attributable to an overestimation of core-repulsion energies for multinitrogen systems. 

We will not pursue the analysis in detail any further. It does illustrate the power of spherical tensor methods, and one can only shudder at the possibility of developing the theory in a cartesian coordinate system, with direction cosines. We list the final values of the molecular parameters for 14N35C1 in table 10.12. The values of the hyperfine constants may be interpreted semi-empirically in the following way. The outmost pair of electrons occupy a 3 /rT molecular orbital and the Fermi contact constants, given in table 10.12, may be compared with the atomic values [144] of 1811 and 5723 MHz for the nitrogen and chlorine atoms respectively one concludes that the s electron character... 

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