The use of the CNDO method in spectroscopy

The Pariser approximation for the two center Coulomb repulsion integrals Γ _rshas been replaced by the Nishimoto-Mataga approximation in the original CNDO/S method. This modification has significantly improved the calculated position of the benzene ¹ B _1u ←¹ A _1g(¹ L _a ) electronic transition in benzenoid compounds. The calculation of transition moments of n — π ^* transitions is also considered. These moments vanish formally in any theory employing the ZDO approximation since integrals of the form 〈2s¦er2p〉 vanish even when the 2s and 2p atomic orbitals are on the same center. In this work the ZDO approximation is abandoned in the evaluation of the electronic transition moment resulting in calculated intensities for n — π ^*, ¹W←¹A, transitions which are in good agreement with experiment.     

Use of the CNDO method for the study of the electronic structure and properties of complexes of the transition metals

Conclusions In our view, the material examined in the present review indicates that, in spite of the appearance of more accurate methods of calculating the electronic structure and the increasing possibilities provided for their realization for fairly complex systems, the applied value of simple semiempirical methods, in particular the CNDO method, is far from exhausted. In the near future, semiempirical calculations will probably provide the chief means of studying the electronic structures of complex molecules, by becoming accessible to an increasing circle of chemists.Leningrad State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 17, No. 3, pp. 549–577, May–June, 1976.     

Energy partitioning with the CNDO method

The partitioning of the total energy from CNDO computations into mono and bicentric terms and into physical components is examined. It is noted that the two-center terms are a measure of the strength of the chemical bond. This relationship is illustrated in the case of a nonclassical bond in a carbonium ion. The consideration of the interaction between vicinal H atoms leads to the discovery of a trans effect which is important in the interpretation of torsional barriers in hydrocarbons. The investigation of the structure of C-C bonds and the analysis of the derivatives of the total energy with respect to a bond length gives hints which are important for the parametrization of a semiempirical theory.

---

Zusammenfassung Die Aufteilung der nach einem CNDO-Verfahren berechneten Gesamtenergie eines Moleküls auf mono- und bizentrische Anteile und deren Zerlegung in physikalische Komponenten wird untersucht. Es wird festgestellt, da\ ein bizentrischer Anteil der Gesamtenergie ein Ma\ für die StÄrke einer chemischen Bindung ist. Dies wird am Beispiel einer nichtklassischen Bindung in einem Carboniumion erlÄutert. Die Betrachtung der Wechselwirkungen zwischen nichtgebundenen H-Atomen führt zur Aufdeckung eines Trans-Effektes, der eine Rolle bei der Interpretation von Torsionsbarrieren in Kohlenwasserstoffen spielt. Die Untersuchung der Struktur von C-C-Bindungen und die Analyse der Ableitungen der Gesamtenergie nach einer BindungslÄnge gibt wichtige Hinweise zur Parametri-sierung einer semiempirischen Theorie.

---

Résumé Etude de la partition de l'énergie totale dans un calclul CNDO en termes mono et bicentriques et en composantes physiques. On remarque que les termes bicentriques donnent une mesure de la force de la liason chimique. Cette relation est illustrée dans le cas d'une liason non classique dans un ion carbonium. Introduisant l'interaction entre atomes H voisins on met en évidence un effet trans important pour l'interprétation des barrières de torsion dans les hydrocarbures. L'étude de la structure des liaisons C-C et l'analyse des dérivées de l'énergie totale par rapport aux longueurs de liason fournit des suggestions importantes pour la paramétrisation d'une théorie semi-empirique.

     

The variation of carbon-carbon bond lengths with environment as determined by spectroscopic studies of simple polyatomic molecules

A brief and critical review of the spectroscopic method of determining molecular structures is given. The difficulties and present experimental accuracies of the method are discussed and comparisons with available diffraction values are made. Spectroscopic data on carbon-carbon bond lengths (accurate to 0·005 Å) which have been accumulated in recent years are summarized. These show a simple dependence on bond environment, namely, that bond lengths increase linearly with an increase in the number of adjacent bonds.     

Infrared emission spectra and equilibrium bond lengths of gaseous ZnH2 and ZnD2

A detailed analysis of the high resolution infrared emission spectra of gaseous ZnH2 and ZnD2 in the 800-2200 cm(-1) spectral range is presented. The nu3 antisymmetric stretching fundamental bands of 64ZnH2, 66ZnH2, 67ZnH2, 68ZnH2, 64ZnD2, 66ZnD2 and 68ZnD2, as well as several hot bands involving nu1, nu2 and nu3 were rotationally analyzed, and spectroscopic constants were obtained. Rotational l-type doubling and l-type resonance, local perturbations, and Fermi resonances were observed in the vibration-rotation bands of both ZnH2 and ZnD2, and equilibrium vibrational frequencies (omega1, omega2 and omega3) were estimated. Using the rotational constants of the 000, 100, 01(1)0 and 001 vibrational levels, the equilibrium rotational constants (B(e)) of 64ZnH2 and 64ZnD2 were determined to be 3.600 269(31) cm(-1) and 1.801 985(25) cm(-1), respectively, and the associated equilibrium bond lengths (r(e)) are 1.524 13(1) angstroms and 1.523 94(1) angstroms, respectively. The difference between the r(e) values of 64ZnH2 and 64ZnD2 is about 0.01%, and is mainly due to the breakdown of the Born-Oppenheimer approximation.     

On the charge distribution in ethanes and disilanes and correlations with equilibrium bond lengths; an ab initio study

The equilibrium electronic wave-functions for a series of fluoro- and chloro-ethanes and disilanes of general formula M₂H_6−nX_n, (M=C, Si; X=F, Cl), were analysed by the most commonly used methods for electron distribution, using the Mulliken and Löwdin populations, natural atomic orbital (NAO) populations and atoms in molecules (AIM) electron densities. Although the numerical values for local atomic charges vary greatly, all the methods correlate, but in markedly differing ways. The Mulliken charges seem the most selective in relation to systematic change of substituents in the current type of molecular structure. A number of examples occur where the AIM charges at C, Si centres are effectively identical in different molecules, where some differences might have been anticipated. These are often distinguished by Mulliken populations. The fluoroethanes exemplify this, since a plot of the AIM charges (for example on either the F or H centres) against the Mulliken charges for all members of the series, shows three nearly parallel lines, corresponding to those centres with 0, 1 or 2 fluorine atoms on the centre under study. The bond critical points at which the AIM charges are determined seem to be counter to intuition in some cases. This is a density rather than atomic orbital size issue however. The Mulliken and NAO charges seem more reasonable than those from the AIM method. There is an unexpected correlation of the local bond dipoles from the Mulliken analyses, with the calculated equilibrium bond lengths. These correlations lead to bond length values for the non-polarised bonds MX, which agree with data based on covalent radii for some bonds.     

Synthesis of chelate derivatives of o-carboranes with a σ bond between carbon and transition metals

Conclusions From 1-lithium-2-N,N-dialkylaminomethyl-o-carboranes and CuCl, (C₆H₅CN)₂PdCl₂, [(C₂H₅)₂S]₂-PtCl₂, and (C₆H₅CN)₂PtCl₂ were obtained the chelate derivatives of N,N-dialkylaminomethyl-o-carboranes, which contain the carbon-copper, carbon-palladium, and carbon-platinum bond.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 6, pp. 1381–1384, June, 1979.     

Modern spectroscopic techniques in the characterization of Schiff base macrocyclic ligand and its complexes with transition metals

Mn(II), Co(II), Ni(II), and Cu(II) complexes with a new azamacrocyclic tetradentate [N(4)] ligand i.e. 2,3,9,10-tetraphenyl;l,4,8,11-tetraazacyclotetradeca;1,3,8,10-tetraene (L) have been synthesized and characterized by elemental analysis, molar conductance measurements, magnetic susceptibility measurements, mass, (1)HNMR, IR, electronic and EPR spectral studies. On the basis of their non-electrolytic nature, the probable formula of the complexes is proposed to be [M(L)X(2)], where M=Mn(II), Co(II), Ni(II), and Cu(II), X=Cl(-) and NO(3)(-), in dimethylformamide (DMF). All the complexes are of high-spin type and found to have six coordinated, octahedral geometry for Mn(II), Co(II), and Ni(II) complexes, and tetragonal for Cu(II) complexes. Macrocyclic ligand and its complexes have also been screened against pathogenic bacteria and fungi in vitro as growth inhibiting agent.     

Effects of exchange on equilibrium bond lengths of heavy,almost spherical,tetrahedral molecules XH4

The simplest nonrelativistic density functional theory, namely, that of Thomas, Fermi, and Dirac, is used to study the effect of exchange on the equilibrium bond lengths of heavy tetrahedral molecules XH₄. In particular, the limiting bond length as the central atom X becomes infinitely heavy is shown to be reduced by exchange by some 0.34 Å. Comparison with experiment shows that the main features of the bond-length variation through the series CH₄? PbH₄ are reflected by the Thomas–Fermi–Dirac predictions, though the bond lengths remain too large for finite atomic number Z of the central atom. Therefore, a semiempirical correction is proposed, which, however, tends to zero as Z tends to infinity.     

Nuclear magnetic resonance spectroscopic method for the estimation of competitive equilibrium constants

A nuclear magnetic resonance method is described which yields precise estimates of competitive equilibrium constants for systems in which two or more complexing agents compete for common species. Chemical-shift data under rapid exchange conditions are required, but neither analytical not equilibrium concentrations are needed. The method is demonstrated by estimating equilibria of proton exchange between aniline and pyridinium ion, proton exchange between triethylamine and diisopropylammonium ion, and cyclohexaamylose exchange between m- and p-hydroxybenzoic acids. The precisions of the equilibrium constants are similar to or better than those obtained from classical methods and are relatively free from interferences.     

CNDO calculation of polarizabilities including polarization functions in the basis set

Electric polarizabilities are calculated by solving the first- and second-order perturbation equations through a variational procedure. Satisfactory numerical results are obtained for a number of molecules using the CNDO approximation with an extended basis set.