The electronic structure of polyfluorinated carbonyl compounds and correlation with ir spectra and reactivity

Calculations of electronic structure, force constants and anharmonicity constants of the C-O valence vibration for a number of fluorinated ketones and ethers have been performed by the CNDO/SW and CNDO/2 methods. The change of association constants for these molecules with phenol is qualitatively explained. Anomalies in the IR spectra are studied, the role of direct fluorine - oxygen interactions is considered. The inductive constants for H(CF₂)₂, and C1(CF₂)₂, are calculated.     

Excited state dipole moments and geometries of the bases of nucleic acids

CNDO/s-CI and VE-PPP methods have been employed to calculate the dipole moments of the bases of nucleic acids in the ground and excited states. A component analysis in terms of μ_hyb^(σ), μ_ch and μ_π has been done using the CNDO/s-CI method and these results have been compared with those obtained by the CNDO/2 and IEHT methods. It is observed that while the CNDO/2 and CNDO/s-CI methods give almost the same total dipole moments, component-wise their predictions are very different.Dipole moments of the molecules have also been studied for the lowest excited singlet and triplet π^* ← π states. It is observed that the conventional method of calculating dipole moments using changes of only the net charges in the excited state does not give correct results for uracil and thymine, for which experimental results are available. Considering deformed non-planar excited state geometries for these molecules, the observed excited state dipole moments have been explained. A method has been suggested to include the effects of non-planarity while calculating the properties of a complex molecule in a π^* ← π excited state. For adenine, guanine and cytosine, the excited state dipole moments are found to be smaller than the ground state values.     

A CNDO/2 study of the OH torsion in phenol and its hydrogen bonded complex with pyridine

The CNDO/2 molecular orbital method has been applied to the study of the OH torsion, in phenol and phenol—pyridine hydrogen bonded complex. The calculated torsional barrier (13.58 kJ mol^?1) and force constant (5.4 × 10^?20 J rad^?2) of phenol agree well with the experimental quantities. The calculated force constant of the corresponding vibration in phenol—pyridine is increased sixfold, reproducing closely the rise in the torsional frequency observed when phenol is complexed to strong acceptors. It is shown that according to CNDO theory, most of the increase can be attributed to the influence of the intermolecular force field and not to a major change in the torsional force constant.     

Valence shell calculations on polyatomic molecules

Dipole moments and charge distributions for twenty molecules of widely different types have been calculated using (a) the CNDO/2 method and (b) a CNDO/2D method in which the orbitals from the CNDO/2 method are deorthogonalized by a Löwdin transformation and are then used to calculate the dipole moments in a rigorous manner. A statistical analysis of the results for the dipole moments calculated by the CNDO/2D method shows that they are in very slightly better agreement with experiment than those from the CNDO/2 method. The net charge distributions from the CNDO/2D method follow more closely the trends of ab initio calculations than do the CNDO/2 net charges.

---

Zusammenfassung Dipolmomente und Ladungsdichten von Molekülen unterschiedlichen Typs wurden mittels des CNDO/2- und CNDO/2D-Verfahrens (d. i. mit delokalisierten Löwdin-Orbitalen als AO's) berechnet. Eine statistische Analyse zeigt, daß die Resultate der zweiten Methode etwas besser als die der ersten den experimentellen Ergebnissen folgen. Das Analoge gilt für die Nettoladungsverteilungen in bezug auf die Trends bei ab initio-Rechnungen.

---

Résumé Les moments dipolaires et les distributions de charge pour vingt molécules de types divers ont été calculés par: a) la méthode CNDO/2; b) une méthode CNDO/2D où les orbitales de CNDO/2 sont déorthogonalisées par une transformation de Löwdin. Une analyse statistique montre que les moments dipolaires calculés par CNDO/2D sont légèrement en meilleur accord avec l'expérience que ceux calculés par CNDO/2. Les distributions de charge de CNDO/2D sont plus ressemblantes à celles de calculs ab-initio que ne le sont les distributions de CNDO/2.

---

---

This work represents part of the Ph.D. Dissertation submitted to the University of Virginia by D. D. S. and was supported by Grants No. 1-F01-GM41986-01 from the National Institutes of Health, Bethesda, Maryland, U.S.A., and No. AF-AFOSR-1184-67 from the Air Force Directorate of Scientific Research.

---

NASA Research Trainee.     

A comparative study of the structure and bonding of HOO,HOS, HSO,and HSS radicals by CNDO/2 and INDO methods

Equilibrium geometries, force constants, barriers to linearity, charge distributions, dipole moments, and electron spin density of HOO, HOS, HSO, and HSS radicals are calculated by CNDO/2 and INDO methods using respectively the original and some recently introduced scheme of parametrization. Three sets of calculations, namely, CNDO/2(sp), CNDO/2(spd), and INDO, are performed, and the results are compared with the ab initio and experimental values, wherever available. A good agreement is obtained for geometry in the case of CNDO/2 (sp) and INDO calculations. The performance of CNDO/2 (spd) calculations in this regard is quite unreliable. The stretching force constants are considerably overestimated by all the methods, while the bending force constants are in reasonable agreement with the ab initio values. With respect to dipole moments, the CNDO/2 values are in better agreement with the ab initio results than the INDO values. In all the cases, the dipole moment vector directions are in complete disagreement with the ab initio predictions.     

Deorthogonalization of atomic originals in the CNDO approach

The procedure for deorthogonalization (D) of atomic orbitals in the semiempirical CNDO approach is reviewed. For comparative studies, CNDO/2, CNDO/2D, and STO -3G calculations of molecular dipole moments and Mulliken populations are carried out on 35 prototype molecules containing H, C, N, O, and F atoms. The calculated values are assessed on the basis of how well they agree with experimental trends, chemical bonding theories, and ab initio molecular orbital (MO) values. Results of analyses indicate that the CNDO/2D values for dipole moments are in reasonable agreement with experimental values, and those for net atomic charges and electron populations bear greater resemblance to the ab initio (STO -3G and 6-31G**) values than the original CNDO/2 values. These findings, together with those of previous investigators, demonstrate unequivocally the advantages of incorporating deorthogonalization into routine CNDO/2 or INDO calculations as a means to obtain reasonable estimates of charge distributions.     

Calculation of excited-state properties of some small molecules: Comparative study of the CNDO/S and CNDO/2-vn−1 potential methods

The transition energy and geometry of the lowest excited (nπ*) singlet and triplet states of CO, CS, HNO, H₂CO, HFCO, and F₂CO molecules are calculated by CNDO /S and CNDO /2-V_N?1 potential methods, and the results are compared with those of experimental and ab initio theoretical studies, wherever available. In the calculation of the vertical transition energy, the performance of the CNDO /S method is seen to be generally more satisfactory than that of the CNDO /2-V_N?1 potential method, while the reverse is true for the excited-state geometry. The CNDO /S method as such fails to describe the geometry of the excited state, but a combined version (CNDO /S-2) of CNDO /S and CNDO /2, as well as the CNDO /2-V_N?1 potential method is fairly successful in this regard.     

Eine Variante des CNDO-Verfahrens unter Einbeziehung vond-Funktionen

A modified handling of thed-functions within the CNDO framework is presented. No change of the (CNDO/2) atomic parameters is necessary from sodium to iodine. One-centre kinetic energy contributions to the core-matrix elements are introduced explicitly. The calculation of dipole moments and equilibrium geometries (estimated with the aid of analytically calculated gradients of the molecular energy) of 45 molecules gives good results,sp- andspd-CNDO/2 have been applied for comparison. Some examples are presented, where the shortcomings of the conventional CNDO/2 are no longer present. A qualitatively accurate picture of the dipole moments of X-Y-molecules (X: Halogen; Y: Hydrogen, Halogen, Alkyl-) is achieved.

     

All-valence-electron scf-mo calculations on the electronic structure and reactivities of some halogenated benzene and azabenzene derivatives

All-valence-electron CNDO/2 SCF-MO calculations have been performed on some fluoro, chloro and methoxy derivatives of benzene, pyridine and the diazines. The computed charge distributions and the dipole moments are discussed and compared with available experimental data. The relative basicities of the derivatives of pyridine and the diazines have been investigated and an attempt made to correlate the results with known pK_a values. Localisation energies for the nucleophilic substitution of halogen by methoxide ion have also been calculated and the predicted relative reactivities compared with experimental observations, where these were available.     

Étude de la tautomérie des halogéno-triazoles-1,2,4 par determination expérimentale et théorique de leurs moments dipolaires

The dipole moments of sixteen 3(5)-halogeno-triazoles have been measured in dioxane at 25 ° C. Experimental dipole moments have been compared with those calculated by two semi-empirical methods CNDO/2 and PCILO, and with those computed vectorially. An energetic approach is also given.It is shown, in _N-unsubstituted 1,2,4-triazoles (for which three tautomeric forms, A, B and C are possible) that the predominating tautomer has a proton on the N1 atom and the electron-accepting substituent on the C3 atom.The effect of the introduction of an electron-accepting or donating substituent in position 3 and 5 of the triazole ring is discussed.     

Reactivities and structures of compounds of the thiophene and furan series bearing electron-acceptor substituents

The experimental data on orientation in electrophilic substitution reactions of compounds of the furan and thiophene series that bear electron-acceptor substituents were examined. Within the framework of the CNDO/2 method, the electronic structures, dipole moments, and spin-spin coupling constants in the PMR spectra were calculated for model systems — 2-formylthiophene, furfural, and their oxygen-protonated forms. The results of the calculations are compared with the available experimental data on the dipole moments and spin-spin coupling constants, which made it possible to discuss the geometrical structures of the compounds. An analysis of data on the chemical behavior of these compounds demonstrated that, in contrast to the charges found using the -electron approximation, the total charges on the atoms calculated by the CNDO/2 method are rather effective indexes of the reactivity.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 155–164, February, 1972.     

A quatum-mechanical study of metheyl fluoroformate

It is found theoretically that the preferred conformation of methyl fluorofonnate is cis instead of trans as reported previously Semi-empirical(CNDO/2) and ab initio (STO-3G) methods were used and the geometry was partially optimized. The rotational barrier for the methyl group is calculated and its value, 1.07 kcal/mol (CNDO/2) and 1.08 kcal/mol (STO-3G), is in agreement with experiment. Dipole moments are also calculated but are smaller than the experimental values. Finally it is shown how the isomers can be identified using electronic spectra.     

Studies of polarized ethylenes.: PART VII. experimental and theoretical dipole moments and electronic structures by CNDO/2 calculations

The dipole moments of a number of representative “push-pull” ethylenes have been calculated by the CNDO/2 method, and a comparison is made with experimental dipole moments, which are in general surprisingly well reproduced. An analysis of the calculated charge distributions shows that the π-electron displacements are in agreement with qualitative expectations, but no back-polarization of the σ-electrons can be observed. Instead, the distribution of the σ-electrons seems to be governed by atom electronegativities.The experimental dipole moments of some more or less flexible analogues are discussed in relation to conformations and expected charge distributions.     

Valence-shell calculations on polyatomic molecules

Calculations using the CNDO/2, the Extended Hückel (EH) method, and an iterative Extended Hückel (IEH) method are reported for HF, H₂O, NH₃, CO, H₂CO, HCONH₂, HCOOH, HCOF and sydnone. For the IEH method, it is shown that if the dipole moment is calculated by including the atomic dipole moment and the overlap moment (homopolar dipole) as well as the term from the Mulliken populations, then, except for carbon monoxide, the IEH method gives results in good agreement with experiment. The non-iterative EH method predicts dipole moments that are much too high. For molecules with dipole moments smaller than 3 Debyes, the IEH and CNDO/2 methods give similar results, but for molecules with higher dipole moments (formamide and sydnone), the CNDO/2 method gives better agreement with experiment. Comparison of the calculations on sydnone with those on other carbonyl compounds suggests that sydnone is best represented as a resonance stabilized azo-methine imine rather than as a meso-ionic or betaine type compound.

---

Zusammenfassung Rechnungen mittels des CNDO/2- und des erweiterten Hückelverfahrens (iterativ und nichtiterativ) werden für HF, H₂O, NH₃, CO, H₂CO, HCONH₂, HCOOH, HCOF und Sydnon vorgelegt. Im Fall des iterativen Hückelverfahrens zeigt sich, daß die Dipolmomente (außer für CO) gut mit dem Experiment übereinstimmen, wenn man die atomaren Dipol- und die Überlappungsmomente sowie die Terme der Mulliken-Population berücksichtigt. Dagegen sind die entsprechenden Werte des nichtiterativen Verfahrens viel zu groß. Für Moleküle mit Dipolmomenten kleiner als 3 Debye liefert das CNDO/2-Verfahren ähnliche Werte wie die iterative Hückelmethode, für Moleküle mit größeren Dipolmomenten dagegen bessere Resultate. Vergleicht man die Rechnungen für Sydnon mit denen für andere Carbonylverbindungen, so scheint es, daß man es besser als resonanzstabilisiertes Azomethinimin und nicht als Betain auffassen sollte.

---

Résumé HF, H₂O, NH₃, CO, H₂CO, HCONH₂, HCOOH, HCOF et la Sydnone on été calculées en utilisant les méthodes CNDO/2, Hückel étendu (EH) et Hückel étendu itératif (IEH). On montre que, pour la méthode IEH, si l'on calcule le moment dipolaire en incluant le moment dipolaire atomique et le moment de recouvrement (dipôle homopolaire) ainsi que le terme provenant des populations de Mulliken, les résultats obtenus sont en bon accord avec l'expérience sauf pour l'oxyde de carbone. La méthode EH non itérative donne des moments dipolaires trop élevés. Pour les molécules de moment inférieur à 3 Debyes, IEH et CNDO/2 donnent des résultats similaires, mais pour les molécules à moments plus élevés (formamide et sydnone) la méthode CNDO/2 donne un meilleur accord avec l'expérience. La comparaison des calculs sur la sydnone avec ceux sur les autres composés carbonylés suggère que la sydnone est mieux représentée comme une azo-méthine imine stabilisée par résonance que comme un composé de type «méso-ionique» ou bétaïnique.

---

---

This work was supported by Grant No. MH-12951-02 from the National Institutes of Health, Bethesda, Maryland, USA.

---

NASA Research Trainee.     

Excited states dipole moments and polarizabilities of uracil and cytosine 5-halo derivatives

Dipole moments and polarizabilities of different excited states of uracil and cytosine 5-halo derivatives have been calculated using solvent shift methods and CNDO/S calculations. The results are discussed in relation to different solute–solvent interactions and the nature of the electronic transition.     

Dipole moments of hydroxy- and methoxy-azobenzene derivatives

The dipole moments of some 4-hydroxy and 4-methoxyazobenzene derivatives were measured in benzene solutions. Results show that (a) there are no appreciable mesomeric interactions between the polar groups in the molecule; (b) evidence is obtained for the hindered rotation of the OH and OCH₃, groups; and (c) the difference between the dipole moments of the title compounds and those of the corresponding benzene derivatives can be referred both to the extension of the conjugated system and to the value of the interaction moment. Experimental results are discussed with the aid of CNDO/2 calculations.     

Density of electronic states of γ-Al2O3 clusters in chemisorption of phenol and butene molecules

In the cluster approximation, by the CNDO/2 method with optimization of the geometry of chemisorbed molecules of phenol and butene, the integral and local densities of electronic states (IDS and LDS) have been calculated for adsorption complexes on -Al₂O₃. When phenol is chemisorbed in molecular form on a Lewis acid center, the LDS of the aluminum ion changes only very slightly, whereas in the case of dissociative adsorption of PhOH, the acidity of the center is considerably reduced, and the intensity of the LDS in the forbidden band drops off. Upon chemisorption of 2-butene on centers differing in accessibility of the aluminum ion, a restructuring of the LDS takes place, with the appearance of new states at the top of the valence band and practically complete suppression of the intensity of the acceptor state in the forbidden band; this corresponds to poisoning of the Lewis centers of the -Al₂O₃ olefins.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 21, No. 3, pp. 310–315, May–June, 1985.     

Studies on pyrazinyl-pyrazylidene tautomerism of pyrazine-acetonitrile derivatives

The equilibria between the pyrazinyl and 2(1H)-pyrazylidene forms of pyrazineacetonitrile derivatives were studied by IR,¹H-NMR and UV spectroscopy. The excess charges and dipole moments for the two tautomers were calculated by the CNDO/2-MO method.

---

Untersuchungen über die Pyrazinyl-Pyraziliden-Tautomerie von Pyrazinacetonitril-Derivaten

Zusammenfassung Die Gleichgewichte zwischen Pyrazinyl- und 2(1H)-Pyrazyliden-Formen von Pyrazinacetonitril-Derivaten wurden mittels IR-, PMR- und UV-Spektroskopie untersucht. Die Überschußladungen und Dipolmomente der zwei Tautomeren wurden nach dem CNDO/2-MO-Verfahren berechnet.

     

Calculation of molecular quadrupole moments and a demonstration of the importance of overlap densities in the theory of polyatomic molecules

A computational method for calculating quadrupole moments from molecular wave functions in a Slater orbital basis set is described. Using both IEHT and CNDO wave functions quadrupole moments for a series of polyatomic molecules are calculated. They are compared with experimental results and the IEHT wave functions are found to give agreement with experiment while CNDO wave functions do not. The importance of bicentric densities (overlap densities) in the calculation of multipole moments is shown. This is followed by a discussion of the usefulness of these wave functions for a quantitative characterization of the electronic structure of large molecules.