The adiabatic correction to the ring puckering potential of oxetane

The adiabatic correction to the Hartree-Fock ring puckering potential of oxetane is presented along with the adiabatic correction to potential functions of H₂, CO, and CO₂. The oxetane calculation allows the identification of the source of the disagreement between theory and experiment for the barrier to ring planarity.     

The cyclopentanone ring puckering

The complete analysis of the H-1 NMR spectrum of cyclopentanone (CPO) and the full ralaxed optimization of its structure at the 4-3IG level have been carried out. The endocyclic torsions angles φ (0) and φ (1) estimated by the R-value method from the vicinal HH couplings are 9 and 12° larger than the respective theoretical 4-3IG values which closely correspond to the empirical electron diffraction (ED) ones. When the theoretical tilts of the CH₂ groups are taken into account the values estimated by the R-value method decrease by ≈4.5°.     

The ring puckering potential function of trimethylene sulfide and trimethylene selenide

The rotational constants A,B and C are functions of the vibrational quantum numbers for the out-of-plane bending modes of trimethylene sulfide and trimethylene selenide. Infrared and microwave spectroscopy have been used to determine the dependence between these parameters. The height and shape of the barrier arising from combination of the ring bending modes is also determined. CNDO/2 calculations are used to predict the position of the hydrogen atoms of the methylene groups in trimethylene sulfide and trimethylene selenide. The four-membered rings are non-planar for several vibrational states of the ring bending modes.     

Some remarks about the ring puckering potential function of 3-methyleneoxetane

The rotational constants A, B, C are functions from the vibrational quantum numbers for the out-of-plane ring bending vibration of 3-methyleneoxetane. Gibson and Harris [1] using microwave spectroscopy had given the dependence between quantum vibrational numbers for the out-of-plane bending mode and rotational constants.In this paper, the agreement between experimental results from the microwave measurements and theoretical ones from the CNDO/2 is analysed. The planarity for the strained four-member ring is confirmed from the information available. The position corresponding to the methylene groups is also predicted from CNDO/2 calculations.     

Semiempirical calculations of internal barriers to rotation and ring puckering: II. Amindo/3 study

The MINDO/3 technique is examined for its applicability to rotational barriers and ring puckering of strained-ring compounds. Comparisons are made with MINDO/2' and INDO on the compounds ethane, acetone, isobutylene, 3-oxetanone, cyclobutane, cyclobutanone, methylenecyclobutane, and 1,1-difluorocyclobutane. The MINDO/3 method improperly predicts all ring compounds to energetically favor the planar conformation as does its predecessor. Some improvement in bond lengths and bond angles is observed by MINDO/3 but the rotational barriers are still underestimated.     

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.     

Density functional calculations of vibrational wavenumbers, ring puckering, and asymmetric CHO potential functions for cyclobutanecarboxaldehyde: comparative study between theoretical and experimental spectra

The structural parameters and energies of the trans-equatorial and gauche-equatorial conformers of cyclobutanecarboxaldehyde c-C₄H₇–CHO were investigated by quantum mechanical DFT-B3LYP (Density Functional Theory-Becke 3 exchange and Lee-Yang–Parr correlation functional) calculations using 6-311G^** basis set. The potential functions for the CHO asymmetric torsion in the equatorial molecule and for the ring puckering inversion were derived. The vibrational wavenumbers were calculated and the potential energy distributions PED among the symmetry coordinates of the normal modes were computed for the two stable conformers of the molecule. The vibrational assignments on the basis of the calculated PED values were compared to the reported ones from experimental data. The vibrational infrared and Raman spectra of the mixture of the trans-equatorial and gauche-equatorial were plotted and the line intensities were compared to the corresponding experimental ones.     

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.

     

Quantum delocalization of benzene in the ring puckering coordinates

The effect of quantum mechanical delocalization of atomic nuclei on the conformation of the six‐membered ring structure in two hydrocarbons, cyclohexane and benzene, is investigated using ab initio path integral approach. A striking feature of benzene species is revealed using ring puckering coordinate representation, which demonstrates that the zero point motion of the heavy atom skeleton dominates over the out‐of‐plane thermal motions of the ring. Even more unexpected is the fact, that this is true not only at low temperature of 150 K, at which such behavior would not be surprising, but also at room temperature, where the nuclear quantum effects are usually of lesser importance, especially in the case of such heavy nuclei as carbon. In view of this finding the planar conformation of benzene, whose equilibrium (T = 0 K) geometry results from the well‐known properties of the electronic structure, can be elucidated also at nonzero temperature. According to our simulations, it appears as a consequence of quantum delocalization of the carbon nuclei rather than a trivial time average over the classical configurations of the puckered ring. This interesting behavior is contrasted with the clearly nonplanar structure of cyclohexane, whose ring puckering states can be unequivocally assigned even if the nuclear delocalization is taken into account. © 2014 Wiley Periodicals, Inc.     

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.     

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.     

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.     

PPP and CNDO calculations for protonated molecules

CNDO calculations have been used to obtain the one-centre core integrals for protonated azines required in calculating the ^* absorption spectra of such molecules using the PPP method. Calculated spectra for both the parent and the protonated molecules are obtained in satisfactory agreement with experiment. The changes in the -framework of the molecules on protonation are also discussed in terms of the CNDO results.

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Zusammenfassung CNDO-Rechnungen wurden benutzt, um die Einzentren-Rumpf-Integrale für protonierte Azine zu erhalten, die bei der Berechnung der ^* -Absorptionsspektren mit Hilfe der PPP-Methode benötigt werden.Die berechneten Spektren für die Ausgangsmoleküle und die protonierten Moleküle sind in zufriedenstellender Übereinstimmung mit dem Experiment. Die Veränderungen im -Rumpf der Moleküle bei der Protonierung werden ebenfalls mit Hilfe der CNDO-Resultate diskutiert.

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Résumé Des CNDO ont tét utilisés pour obtenir les intégrales de coeur monocentriques des azines protonées nécessaires au calcul de leur spectre d'absorption ^* par la méthode PPP. Les spectres calculés pour les molécules protonées ou non sont en accord satisfaisant avec l'expérience. Les modifications subies lors de la protonation par le squelette sont discutées en fonction des résultats des calculs CNDO.

     

Ab initio versus CNDO potential surface calculations for Li2O and Al2O

Potential surfaces for Li₂O and Al₂O have been calculated by an ab initio SCF-LCAO-MO method and by the semiempirical CNDO method. For both molecules the semiempirical methods incorrectly imply unreasonable structures with very acute apex angles and very long bond distances — rather more like diatomic Li₂ or Al₂ molecules with O-atoms attached to their bonds. Our ab initio treatment does correctly predict a symmetrical linear configuration for Li₂O with bond distances in excellent agreement with experiment. This method also predicts a linear symmetrical structure for Al₂O, in agreement with experimental gas phase measurements but in disagreement with matrix-isolation studies.     

Multiresolution potential energy surfaces for vibrational state calculations

A compact and robust many-mode expansion of potential energy surfaces (PES) is presented for anharmonic vibrations of polyatomic molecules, where the individual many-mode terms are approximated with various different resolutions, i.e., electronic structure methods, basis sets, and functional forms. As functional forms, the following three representations have been explored: numerical values on a grid, cubic spline interpolation, and a Taylor expansion. A useful index is proposed which rapidly identifies important many-mode terms that warrant a high resolution. Applications to water and formaldehyde demonstrate that the present scheme can increase the efficiency of the PES computation by a factor of up to 11 with the errors in anharmonic vibrational frequencies being no worse than ~ 10cm⁻¹.     

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.     

Studies of crystalline native celluloses using potential energy calculations

Energies for various trial packing arrangements of unit cells for the I and I phases of native cellulose discovered by Sugiyamaet al. were evaluated. Both a rigid-ring method, PLMR, and the full-optimization, molecular mechanics program, MM3(90), were used. For both phases the models that had the lowest PLMR energy also had the lowest MM3 energy. Both calculated models have the chains packed up, O6s intg positions, and the same sheets of hydrogen-bonded chains. The I structure model is essentially identical to that proposed previously for ramie cellulose by Woodcock and Sarko. It is also the same as the best parallel model previously proposed that was based on the X-ray data of Mann, Gonzalez and Wellard, once the various unit cell conventions are considered. Also, the energies from both methods for all three celluloses, I, I and II, are in the order that rationalizes their relative stabilites.     

CEPA calculations of potential energy surfaces for open-shell systems

Potential energy surfaces for the collision of He atoms with NH radicals in the electronically excitedA ³ Π state have been calculated using quantum chemical ab initio methods. The NH distance was kept fixed to its equilibrium value, the range of the HN-He distancesR and HNHe angles γ was chosen to be 4.0≦R≦8.0a ₀, 0≦γ≦180°. The doubly degenerate NH(A ³ Π) state is split upon approach of He into two components, 1³ A′ and 2³ A″, which remain degenerate only for collinear geometries. The resulting two potential surfacesV _A′ andV _A″ are essentially repulsive with shallow van der Waals minima at large distances. An expansion of the sum and the difference potential,V _A′ +V _A″ andV _A′ ?V _A″, respectively, in terms of Legendre polynomials shows that the anisotropic componentsV ₁₀(R) andV ₂₂(R) which mainly govern rotational transitions and Λ-doublet mixing are of the same size. It is therefore expected that these processes as well as fine-structure transitions are similarly probable in NH(A ³ Π)-He collisions. This is in accord with recent experiments of Kaes and Stuhl.