CNDO CI calculations on second-row molecules

The CNDO/S method is extended to second-row molecules and parametrized for phosphorus and sulphur. Both sp and spd basis sets are considered. The method is applied to the aromatic molecules phosphorin and thiophen. The uv transitions, ionization potentials and dipole moments of these molecules are satisfactorily explained.     

CNDO calculations of diamagnetic shielding in molecules

An approximate procedure for the calculation of diamagnetic shielding in molecules is presented. The method proposed is based on the ‘complete neglect of differential overlap’ (CNDO) molecular wave functions and is formulated according to the zero differential overlap (ZDO) approximation. The results obtained with several CNDO-type wave functions for diatomic and polyatomic molecules are in very good agreement with non-empirical SCF calculations. The ¹⁴N diamagnetic shielding constants in several molecules were computed and some approximations usually adopted in the interpretation of ¹⁴N chemical shifts are critically discussed. It was shown that in some cases the observed ¹⁴N chemical shifts cannot be interpreted solely in terms of the paramagnetic contribution to the shielding constant.     

Quantum chemical calculations of fluoroethylene molecules by the CNDO and INDO methods

Conclusions A quantum chemical calculation of seven fluoroethylene molecules and two fluorochloroethylene molecules and two fluorochloroethylene molecules by the CNDO and INDO methods has made it possible to explain the experimentally observed change in reactivity and direction of addition reactions at the double bond. The data of a calculation of the relative stability of various isomers also agree with the experimental results.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2199–2202, October, 1973.     

CNDO calculations of N-methyl substituted acrylamides

CNDO calculation is made for N-methyl acrylamide (both incis andtrans configurations) and N,N-dimethyl acrylamide. The charges, bond orders and dipole moments are discussed and compared with those of acrylamide. Thetrans form of N-methyl acrylamide is found to be more stable thancis isomer by 4.5 kcal/mole.     

Ab Initio versus CNDO barrier calculations

Energy barrier curves to internal rotation in P₂H₄ and P₂F₄ have been studied by the ab initio SCF-LCAO-MO method in the gaussian approximation and by the CNDO method. For P₂H₄, two stable rotamers at 75 and 180 of equal energy and separated by a barrier of only 500 cal/mole are predicted, and the cis barrier computes to be 4.1 kcal/mole. A trans-only form of P₂F₄ is the theoretical structure with only an arrest in the barrier curve at the gauche position. The CNDO method does not produce reliable information concerning the number of stable rotamers, their precise configurations, or the heights of the barriers.

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Zusammenfassung Die Kurven der Energiebarriere bei der inneren Rotation von P₂H₄ und P₂F₄ wurden nach der ab initio SCF-LCAO-MO-Methode mit einer Basis von Gaussfunktionen sowie nach der CNDO-Methode untersucht. Beim P₂H₄ ergeben sich zwei stabile Rotamere von gleicher Energie bei 75 und 180 und getrennt durch eine Barriere von nur 500 cal/mol; die cis-Barriere berechnet sich zu 4.1 kcal/mol. Eine reine trans-Form von P₂F₄ ist die theoretische Struktur mit nur einem Sattelpunkt in der Energiekurve bei der gauche-Position. Die CNDO-Methode liefert keine genügend genauen Anhaltspunkte zur Zahl der stabilen Rotameren, ihren genauen Konfigurationen oder zu den Höhen der Barrieren.

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Résumé Les courbes d'énergie donnant les barrières de rotation interne ont été étudiées pour P₂H₄ et P₂F₄ par la méthode ab-initio SCF LCAO MO en base gaussienne et par la méthode CNDO. Pour P₂H₄ on prévoit deux rotamères stables de mÊme énergie à 75 et 180 séparés par une barrière de 500 cal/mole seulement, la barrière cis calculée étant de 4,1 kcal/mole. La structure théorique de P₂F₄ est la forme «tout-trans» avec seulement un arrÊt sur la courbe dans la position gauche. La méthode CNDO ne produit pas d'informations sûres en ce qui concerne les rotamères stables tant du point de vue de leurs configurations que de celui des barrières.

     

Relationship between EHT and CNDO/2 calculations

Multiplying EHT atomic charges and dipole moments by 0.3 gives approximate CNDO/2 values.Financial support from a CPGA is gratefully acknowledged.     

Thomas-Fermi-Dirac calculations for molecules

The electrostatic potential derived from a solution to the molecular Thomas-Fermi-Dirac equation for F₂ is combined with the exchange potential and modified to give the correct behavior far from the nuclei. One-electron energy levels in this potential are calculated and are in qualitative agreement with SCF orbital energies. Similar computations are carried through for F and Ar, which correspond to the separated and united atoms for F₂. To compensate for errors in the potential, we subtract from molecular orbital energies the difference of TFD and SCF orbital energies for the separated atoms. Now all the orbital energies are correct to a few electron volts.

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Zusammenfassung Das elektrostatische Potential, das sich für F₂ aus der Thomas-Fermi-Dirac Theorie ergibt, wird mit dem Austauschpotential kombiniert und so modifiziert, daß sich das richtige Verhalten in Kernnähe ergibt. Die berechneten Einelektronenenergien sind in qualitativer Übereinstimmung mit SCF-Werten. Analoge Rechnungen für F und Ar werden ausgeführt und als Grenzfälle für Korrekturen verwendet. Dann ergeben sich alle Orbitalenergien bis auf wenige eV richtig.

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Supported by the National Science Foundation under Grant GP-20718.     

CNDO/2 calculations and configuration analyses for some hydrogen-bonded systems

The potential energy curves of hydrogen-bonded systems were calculated for the water, methanol, and formic acid dimers and for the hydrogen maleate ion by a modified CNDO/2 method, the core resonance integrals between-electrons being distinguished from those between-electrons, the different bonding parameters being used for -O- and =O, and the core potential integralsV _AB ^c for O-H, C=O, and O...O being determined semi-empirically. Consequently, the following results were obtained: 1) a potential energy curve with a single minimum atr(O-H)=0.95 Å and with a concavity near 1.70 Å for the linear chain dimers of water and methanol; 2) a symmetrical potential energy curve with two minima atr(O-H)=0.95 and 1.78 Å for the cyclic dimer of formic acid; 3) a flat-bottomed symmetrical potential energy curve for the hydrogen maleate ion. The configuration analysis method was applied to the hydrogen-bonded systems; the contributions of the covalent ((O-H-O)^–1) and ionic (O^–H⁺O^–) structures being 54% and 39%, respectively, for the symmetrical hydrogen bonding of the hydrogen maleate ion.

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Zusammenfassung Die Kurven potentieller Energie von Systemen mit H-Brücken wurden im Fall von H₂O-, CH₃OH- und HCOOH-Dimeren und für das Hydrogen-Maleatanion mittels eines modifizierten CNDO-Verfahrens berechnet, wo für Rumpf-- und --Elektronen jeweils verschiedene Resonanzintegrale und verschiedene Bindungsparameter für =O und -O- verwendet und wo die Rumpfpotential-Integrale für O-H, C=O und O...O semiempirisch bestimmt werden.Die Resultate sind 1) eine Potentialkurve mit einem einfachen Minimum für lineare Ketten von H₂O und CH₃OH, 2) eine symmetrische Potentialkurve mit zwei Minima für das cyclische Diniere von HCOOH und 3) eine Potentialkurve mit einem flachen Minimum für das Maleatanion. Konfigurationsanalyse ergab einen Beitrag von 39% für die ionischen Strukturen.

     

Possibilities of the calculation of UV spectra of molecules with conjugated bonds by the CNDO/S and PPP methods

Theoretical and Experimental Chemistry -     

CNDO calculations of ring puckering potential energy

The ring puckering potential energy functions are calculated for 2,3-dihydrofuran, 2,5-dihydrofuran, cyclopentene and cyclopent-3-enone using the standard CNDO/2 method. The equilibrium conformations are discussed and the ring puckering force constants as well as the dipole moments are evaluated. A more detailed analysis of the influence of angular deformations on the potential function of the cyclopentene molecule is performed.     

CNDO/2 calculations of some polyazaindenes

The theoretically estimated dipole moments of indolizine, imidazo[1,2-a]pyridine, imidazo-[1,5-a]pyridine and of pyrazolo[ 1,5-a]pyridine obtained by the CNDO/2 approximation have been compared with the experimental values. The bond angles and bond distances for these polyazaindenes have been estimated.     

Vibrational spectroscopy of protonated imidazole and its complexes with water molecules: ab initio anharmonic calculations and experiments

The results of anharmonic frequency calculations on neutral imidazole (C3N2H4, Im), protonated imidazole (ImH+), and its complexes with water (ImH+)(H2O)n, are presented and compared to gas phase infrared photodissociation spectroscopy (IRPD) data. Anharmonic frequencies are obtained via ab initio vibrational self-consistent field (VSCF) calculations taking into account pairwise interactions between the normal modes. The key results are: (1) Prediction of anharmonic vibrational frequencies on an MP2 ab initio potential energy surface show excellent agreement with experiment and outstanding improvement over the harmonic frequencies. For example, the ab initio calculated anharmonic frequency for (ImH+)(H2O)N2 exhibits an overall average percentage error of 0.6% from experiment. (2) Anharmonic vibrational frequencies calculated on a semiempirical potential energy surface fitted to ab initio harmonic data represents spectroscopy well, particularly for water complexes. As an example, anharmonic frequencies for (ImH+)H2O and (ImH+)(H2O)2 show an overall average deviation of 1.02% and 1.05% from experiment, respectively. This agreement between theory and experiment also supports the validity and use of the pairwise approximation used in the calculations. (3) Anharmonic coupling due to hydration effects is found to significantly reduce the vibrational frequencies for the NH stretch modes. The frequency of the NH stretch is observed to increase with the removal of a water molecule or replacement of water with N2. This result also indicates the ability of the VSCF method to predict accurate frequencies in a matrix environment. The calculation provides insights into the nature of anharmonic effects in the potential surface. Analysis of percentage anharmoncity in neutral Im and ImH+ shows a higher percentage anharmonicity in the NH and CH stretch modes of neutral Im. Also, we observe that anharmonicity in the NH stretch modes of ImH+ have some contribution from coupling effects, while that of neutral Im has no contribution whatsoever from mode-mode coupling. It is concluded that the incorporation of anharmonic effects in the calculation brings theory and experiment into much closer agreement for these systems.     

EHT and CNDO/2 calculations on amino terminal peptides

Quantum Mechanical investigations have been performed by means of EHT and CNDO/2 methods on the three amino terminal peptides: NH ₃ ⁺ -CH₂-CONHCH₃, NH ₃ ⁺ -CH(CH₃)-CONHCH₃ and NH ₃ ⁺ -CH₂-CON(CH₂)₄. The resulting energy maps compare satisfactorily with the distribution of the experimental X-rays data.     

Ab initio calculations of methionines and their protonated forms

Ab initio calculations of molecular and electronic structures of neutral molecules and protonated forms of methionine and its derivatives in the gaseous phase were carried out by the Hartree-Fock method using the 6–31G^* basis set with full geometry optimization. Proton affinities of methionine (1), methionine sulfoxide (2), and methionine sulfone (3) were calculated for different modes of coordination of the proton. The results of calculations demonstrated that in protonated forms of 1 and 3, bonding between the proton and the N atom is most favorable, while in protonated form of 2, bonding between the proton and the O atom of the SO group is most favorable. The proton affinities of the amino acids are as follows: 223.2 (1), 241.2 (2), and 221.5 (3) kcal mol⁻¹,i.e., methionine sulfoxide 2 exhibits the highest proton affinity in the series of the amino acids under consideration. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1487–1490, August, 1998.     

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.