M.O. calculations

There is renewed interest in theoretical calculations on cyclophosphazenes. The first non-empirical m.o. calculations (ST0-3G ... 

M.O. Calculations. The serai-empirical molecular orbital calculations were made using the UHF INDO model developed by Pople and co-workers (13), which incorporates the one-center exchange integral. Additionally, instead of assuming standard values for bond distances and angles, full geometry optimization at the INDO level was employed (14). Thus the results do not depend upon an arbitrary choice for the molecular geometry. 

We have chosen to perform our initial M.O. calculations also with MEHT, but have made two modifications to the [PM] parameter choices (i) The set of coulomb integrals (diagonal elements),... 

C5 Hall, M. B Fenske, R. F. Inorg. Chem. 11, 1619 (1972) M. O. calculations of the occupancy of the relevant CO o and w orbitals in a variety of complexes correlated with force constant data... 

This serious limitation of the utility of C-nmr has been recognized (Olah et al., 1970), but dismissed by recourse to the results of extended Hiickel M.O. calculations by Hoffmann (1964). These calculations indicated that the charge on the cationic centre of t-butyl is, in fact, more positive than in the isopropyl ion. Hoffmann noted that his results are at odds with the widely held assumption that a methyl group is a better electron donor than hydrogen but showed that the correct order of carbonium ion stability could be predicted even if a methyl group is electron withdrawing with respect to hydrogen. 

Benzo-annelation across a single bond (e.g. b or h) such as in 43 (Xmax = 603 rnn) or in 45 (Xmax = 572 nm) produces bathochromic effects, as a result of the extension of the conjugated system over two additional double bonds. These effects are reproduced quite well by the rr-electron M.O. calculation. ... 

B. Overlap Integrals.—The two atom overlap integrals (Sy s) between the various atomic orbitals were found in the tabulations available in the literature.39-48 The group overlap integrals (G j s) of interest in the M.O. calculation are related to the S s as... 

M.O. calculations have predicted that all of the heterobenzenes have the same direction of polarization with negative end of the dipole towards the heteroatom67-70). This has been confirmed by substituent effects from pyridine, phos-phabenzene and arsabenzene 27). In each case the 4-methyl derivatives have dipole moments which exceed the parent compound since the electron donating methyl group reinforces the ring dipole. 

It is unlikely that Equation (1) will ever serve as a practical method of calculating log Poct. In fact, its greatest utility comes in the reverse direction calculating H-bond strengths from a series of log P measurements (Taft, 1996). In principle, one should be able to develop quantum chemical parameters which account for II and (1 in Equation (1) and the a term needed for the other solvent pairs. It seems certain that both a and P are extremely sensitive to both electronic and steric effects, (Leahy, 1992 Taft, 1996), and it may be some time before M.O. calculations surpass the simple empirical approaches in evaluating them. 

Hiickel m.o. calculations fail badly with benzimidazole. Localisation energies for the free base and the cation indicated C(4) to be the most reactive position towards electrophilic attack,156157 and led to the false conclusion that substitution involved the free base, the orientation being controlled by charge57 densities. 

Nevertheless, as we have reviewed, there are enigmatic aspects of the electronic states of both UO22 and of UF6. As pointed out by Dr. Ruth McDiarmid142,1521 there is an enormous shift from the weak band of UF6 at 25 800 cm"1 to the first band of WF6 at about 58000 cm-1 whereas the chemical reduction to UFg is not much more eager than to WF5. As seen from Tables 1 and 3, the recent relativistic M.O. calculations have not clarified these problems completely. As soon two or more calculations have been published they show a large dispersion in the predictions of the various details of interest to us. 

Two important quantities q and pcan be deduced from the results of the M.O. calculation. The electron distribution, expressed in units of the elementary charge, is found by calculating the quantity qk cmJc2 for each atom, that is... 

Bosnich has recently reported (47) M. O. calculations leading to the assignment of the L configuration to (—)-[Ru(phen)2X2] and states that the same calculations apply to the cis-bis-bidentate complexes of bipyridyl and acetylacetone, and that the phenanthroline and bipyridyl complexes show very similar circular dichroism spectra in the region of the ligand p-band transitions. He comes to a similar conclusion as a result of studies (48) of mixed ruthenium(II) complexes of phenanthroline and bipyridyl. 

Pople s method 35 made by Fukui and his co-workers this permitted progress in the understanding of the push-pull synthesis 37.38.39.40) various other attempts were based on S,C.F.-M.O. calculations by Nakajima... 

Theoretical Study of the Acid-Base Behavior of Substituted Pyridines in the Gas Phase by Means of Semi-empirical M.O. Calculations (MINDO/3 and MNDO). ... 

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