Ground states of molecules. 67.MNDO Calculations for compounds containing iodine

MNDO has been parametrized for iodine. Since d atomic orbitals (AOS ) are not included, the calculations are restricted to I^I. Heats of formation, molecular geometries, ionization energies, and dipole moments are reproduced with useful accuracy.     

Ground states of molecules. 56. MNDO calculations for molecules containing sulfur

MNDO has been extended to sulfur, but without inclusion of 3d AO s. Calculations are reported for heats of formation, geometries, dipole moments, and ionization energies of a variety of sulfur-containing molecules. The average discrepancy between calculated and observed heats of formation is larger than for compounds of other elements, a difference probably due, at least partly, to the lower accuracy of the thermochemical data for sulfur compounds. The calculated dipole moments agree well with experiment as do the calculated ionization energies, except for those corresponding to ionization from sulfur “lone-pair” orbitals which are too high by ca. 1 eV, probably as a result of the neglect in NDDO of interactions between inner and valence shell orbitals. As in the case of other third-period elements, the calculated heats of formation of compounds of sulfur in its higher valence states (S^IV, S^VI) were too positive by large amounts, due presumably to the neglect of 3d AO s.     

Ground states of molecules. 53.MNDO calculations for molecules containing chlorine

MNDO calculations of heats of formation, dipole moments, ionization potentials, and structures are reported for a wide range of compounds containing chlorine in its characteristic valence state (Cl^I) and one or more of the elements H, B, Be, C, N, O, and F. The calculated errors in the heats of formation and the dipole moments are not significantly greater than those previously reported for compounds containing no chlorine. First vertical ionization potentials were on average 0.95 eV too high. The ordering of higher cationic states was found to be correct, even for species such as Cl₂O, Cl₂, and HOCl, where ab initio–Koopmans' theorem calculations predict the incorrect ordering. The calculated energies and geometries of compounds such as CIF₃ are qualitatively incorrect, probably because of the lack of 3d atomic orbitals in the orbital basis set.     

MNDO Calculations of silicon-containing molecules

A comparison is made of MNDO and MINDO /3 calculations for saturated silicon-containing molecules, and with experimental values, for heats of formation, molecular geometries, charge distributions, and ionization potentials. Except for bond angles, it is found that with the published parameter values the MINDO /3 program gives more reliable results than MNDO . For unsaturated molecules, a comparison of bond lengths and stabilities of Si multiple bonds as given by the two programs and ab initio methods is made, and large discrepancies between predicted structures are pointed out. Some reasons for the dicrepancies are discussed.     

MNDO calculations for compounds containing aluminum and boron

Parametrization of MNDO for aluminum and comparisons of calculated molecular properties with experimental values indicate the general usefulness of MNDO to study aluminum-containing compounds. Although results are not as good as for molecules containing only C, H, N, and O, they are nevertheless accurate enough to be useful, especially when predictive biases noted in this article are considered. Inclusion of the d orbital in the MNDO scheme may improve the aluminum results. Results for boron-containing molecules not previously reported are also reported in this article.     

Ground states of molecules

Detailed MINDO/2 calculations of naphthalene using the SIMPLEX minimization algorithm are reported. Comparison of results with experiment and various approximations is made.The results are in good agreement with experiment.Part XXII: Bingham,R.C., Dewar, M.J.S.: J. Amer. chem. Soc. (in press).This work was supported by the Air Force Office of Scientific Research through Contract F44620-70-C-0121, and the Robert A. Welch Foundation through Grant F-126.     

Ground states of σ-bonded molecules

Calculations of the bond orders, net charge densities and energy levels for the -electrons in a number of conjugated hydrocarbons by the valence-electron SCF MO method and by the -electron SCF MO method are compared. The distinctions between alternant and non-alternant hydrocarbons, and in bond orders among single, aromatic, and double bonds, established by -electron methods is maintained to a high degree of approximation in the full valence-electron calculations. An excellent correlation exists between the -electron energy levels calculated by the two methods. The valence-electron calculations indicate that the hyperconjugative release of electrons in neutral molecules such as propene and toluene is negligible, but that this release can be substantial in carbonium ions.

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Zusammenfassung Ladungsdichten, Bindungsordnungen und Orbitalenergien von -Elektronen in einer Reihe von konjugierten Kohlenwasserstoffen werden nach -SCF MO-Methoden sowie nach einer Valenzelektronen-SCF MO-Methode berechnet. Der für die -SCF MO-Methoden typische Unterschied von alternierenden und nichtalternierenden Kohlenwasserstoffen und die Möglichkeit der Klassifizierung der - Bindungsordnungen nach einfachen, aromatischen oder Doppel-Bindungen bleiben bei der vollständigen Behandlung aller Valenzelektronen weitestgehend erhalten. Auch die Orbitalenergien der beiden Methoden korrelieren ausgezeichnet. Die Valenzelektronen-Rechnung zeigt, daß der Einfluß der Hyperkonjugation auf die Ladungsdichte in Molekülen wie Propen und Toluol vernachlässigbar ist, nicht dagegen bei Carboniumionen.

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Résumé Comparaison des indices de liaison, des charges nettes et des niveaux d'énergie obtenus pour un certain nombre d'hydrocarbures conjugués par la méthode SCF MO à électrons de valence et la méthode SCF MO à électrons . Les distinctions entre hydrocarbures alternants et non alternants, et, d'après les indices de liaison, entre des liaisons simples, aromatiques ou doubles, établies par les méthodes à électrons , se maintiennent très bien dans les calculs à électrons de valence. Une excellente corrélation existe entre les niveaux d'énergie des électrons calculés selon les deux méthodes. Les calculs d'électrons de valence montrent que le transfert d'électrons par hyperconjuguaison dans des molécules neutres comme le propène ou le toluène est négligeable, alors qu'il peut s'avérer important dans ions carbonium.

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Part II: Baird, N. C., and M. J. S. Dewar: J. Amer. chem. Soc. 89, 3966 (1967).

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This research was supported by the Air Force Office of Scientific Research through Grant Number AF-AFOSR-1050-67.

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Robert A. Welch Postdoctoral Fellow. 1 Theoret. chim. Acta (Berl.), Vol. 9     

An improved CNDO/2 standard parametrization for bromine containing molecules

Bromine parameters are proposed, matching well the standard values of first and second row atoms, as introduced by Pople et al. This is shown by comparisons of geometries, configurations, conformations, and dipole moments, obtained with different approaches. The new parameter set was found in a very simple and efficient way that may also be useful for other tasks, e.g. in non-empirical calculations.     

Ground states of molecules: Part 57. Vibration frequencies of boron-containing molecules [1]

Vibration frequencies have been calculated for a number of boron-containing molecules using MNDO. Correlations with observed spectra led to estimates of group corrections. Corrected frequencies are used in assigning infrared spectra of aminoborane and B₂N₂H₈ and in predictions of spectra for vinyl borane and B₂N₂H₄.     

Calculation of energies of excited states using MNDO

The energies of the lowest singlet (S₁) and triplet (T₁) states of 28 molecules have been calculated by the “half-electron” (MNDO -HE ) and spin-unrestricted (UMNDO ) versions of MNDO . While most of the calculated values are too negative, because of overestimation of the correlation energy in MNDO -HE and UMNDO , the errors are systematic and depend in an understandable way on the nature of the molecular orbitals (MO S) involved. When appropriate corrections are applied, the calculated energies agree with experiment almost as well as they do for ground states. This justifies the use of MNDO -HE or UMNDO for studies of excited state processes.     

Studies on organophosphorus compounds 52.Structure-reactivity studies of organophosphorus compounds by MNDO calculations

MNDO and MM2(85)methods were used to study the conformation and the structure-reactivity relationship of neutral and acidic phosphorus esters.The calculation results indicate thatfor the most stable conformation,the charge density of phosphoryl oxygen(q_O)is determined notonly by the electronegativity of the substituents,but also by the conformation of the alkoxyl groupson the phosphorus atom.Meanwhile,the conformation of the alkoxyl group provides,as a rule,more important influence on the charge density of the phosphoryl oxygen.However,the energy ofthe highest occupied molecular orbital(EHOMO)is basically dependent on the eletronegativity of thesubstituents,while the donating ability or the withdrawing ability of the neutral phosphorus com-pounds is mainly governed by the EHOMO but not the q_O.This is also true for other kinds of theneutral oxygen-containing compounds.     

Ground states of molecules: Part XLII. Vibrational frequencies of isotopically-substituted molecules calculated using MINDO/3 force constants

The MINDO/3 method is used to calculate vibrational frequencies for ethylene, water and methylamine and their isotopic derivatives. The calculated ²H, ¹³C, ¹⁵N, and ¹⁸O vibrational shifts are in good agreement with the observed values, which suggests the method can be applied with confidence to the calculation of equilibrium and kinetic isotope effects in chemical reactions.     

MNDO calculations of systems containing hydrogen bonds

A modified MNDO method, which can be used in the studies on structures with hydrogen bonds X-H-X, X, X = N, O, is described. Results for a wide range of molecular complexes are reported. Energies of hydrogen bonds are reproduced with useful accuracy. The modified MNDO seems to give more reliable values of hydrogen bond energies and barrier heights of proton transfers than 4-31G ab initio model.     

Perturbation theory for excited states of molecules. I

The problem of perturbations of excited states is discussed and three methods are developed. The first of these uses a zero order wave-function made up of a linear sum of singly excited SCF configurations, whereas the second uses just one of these configurations. The third method is restricted to small -systems, the zero order wave-function being a linear sum of all possible determinants formed from the basis set used. The perturbations considered here are one-electron operators. Example calculations are performed on the butadiene molecule within the context of the -electron approximation.

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Zusammenfassung Für das Problem einer Störung von Einelektronen-Operatoren für angeregte Zustände werden drei Verfahren vorgeschlagen: Erstens die Verwendung einer Zustandsfunktion nullter Ordnung, die eine Linearkombination einfach angeregter SCF-Konfigurationen ist, zweitens die entsprechende Verwendung nur einer ausgewählten Konfiguration. Drittens läßt sich, wenn auch nur bei kleinen -Elektronensystemen, als nullte Näherung eine Linearkombination von allen möglichen angeregten Konfigurationen verwenden. Als Beispiel wird das -Elektronensystem des Butadiens gewählt.

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Résumé Discussion du problème de perturbation pour les états excités et développement de trois méthodes. La première utilise une fonction d'ordre zéro combinaison linéaire de configurations SCF monoexcitées, alors que la seconde utilise seulement une de ces configurations. La troisième méthode est restreinte à de petits systèmes , la fonction d'onde d'ordre zéro étant une combinaison linéaire de tous les déterminants construits dans la base utilisée. Les perturbations envisagées ici sont constituées par des opérateurs monoélectroniques. La molécule de butadiène sert d'exemple dans le cadre de l'approximation à électrons .

     

The fluorination of polyfluorobiphenyls containing bromine

2-Bromononafluorobiphenyl, 2,2′-, 3,3′- and 4,4′-dibromo-octafluorobiphenyl have been fluorinated using CoF₃ at 100–140°. The first two of these compounds lost bromine and gave eicosafluoro-1-cyclohexylcyclohexene and perfluororobicyclohexyl in addition to 2-bromoheptadecafluorobicyclohex-1,1′-enyl. From the latter two biphenyls the corresponding dibromo-eicosafluorobicyclohexyl derivatives were obtained in good yield. 3,3′-Dibromo-octafluorobiphenyl was obtained by the bromination of 2,3,4,6,2′,3′,4′,6′-octafluorobiphenyl which was itself prepared by a coupling reation of 1-bromo-2,3,4,6-tetrafluorobenzene using copper bronze.     

Perturbation theory for excited states of molecules. III

The configuration interaction perturbation theory and the single configuration perturbation theory developed in Paper I is applied to the problem of chemical reactivity for conjugated hydrocarbon molecules. Various related hetromolecules are also considered. It is found that the pattern of reactivity for excited states is much the same as for the ground state.I wish to express my appreciation to Dr. A. T. Amos for constant encouragement during the preparation of this paper and to the S.R.C. for the provision of a maintenance grant.