Calculation of alcohol conformations by molecular mechanics

The geometries and energies of simple alcohols were calculated with a molecular mechanics force field. The force field requires the application of the charge interaction model with charges calculated by the CNDO/2 method, the importance of electrostatic interactions for the equilibrium of rotamers about the C-O bond exceeds that of van der Waals interactions. The calculated rotamer populations are discussed with regard to the value of ¹H NMR coupling constants ³JHCOH and other experimental data.     

Efficiency of post-Hartree-Fock force field for the interpretation of vibrational spectra ofN,N-dimethylnitramine

Harmonic force fields for the molecule ofN,N-dimethylnitramine were calculated in the RHF/6-31G^* and MP2/6-31G^** approximations. Scaling of the force fields obtained made it possible to reliably interpret the vibrational spectra of light and perdeuterated compounds reported in the literature. The assignment is confirmed by good reproducibility of experimental isotope shifts upon¹⁵N-amino- and¹⁵N-nitrosubstitution. The frequencies of intramolecular vibrations in far IR and Raman spectra as well as in neutron inelastic scattering spectra for the light and perdeuterated samples of solidN,N-dimethylnitramine were identified using the force field calculated with the inclusion of electron correlation (MP2). Although general structures of the force fields calculated in the RHF and MP2 approximations are similar, considerable differences in the force constants of the NO and NN stretching vibrations and especially in the constants of the NO_str/NO_str and NO_str/NN_str interactions remain even after scaling the force fields.     

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.     

A theoretical study of the vibrational spectra,geometry and force field of thiourea

A theoretical force field for the molecular vibrations of thiourea has been determined from ab initio calculations at the Hartree-Fock level using the 3-21G* basis set. The reliability of the force field is analyzed by calculating the vibrational frequencies for the deuterated and ¹⁵N isotopomers. Frequencies calculated from the force field are utilized to critically examine the experimental assignments for thiourea and deuterated thiourea. Theoretical geometry, the calculated IR and Raman band intensifies are analyzed.     

An analytical approach to obtaining a valid solution to the secular equations for cis-disubstituted octahedral metal carbonyls

Using an analytical approach we have been able to obtain a valid solution to the secular equations for cis-ML₂(CO)₄ molecules. The solution has led to the relations by which carbonyl stretching force constants and interaction constants can be calculated directly from ¹²C¹⁶O stretching frequencies. It has been found that calculated force constants are in excellent agreement with those obtained from isotopic enrichment studies and exactly equal to those calculated through the iterative procedure employed by Jernigan et al.     

Bond stretching force constants and compressibilities of nitride,oxide, and sulfide coordination polyhedra in molecules and crystals

Molecular quadratic stretching force constants are calculated for a variety of MX bonds (X = N, O, S) in coordinated polyhedra containing row 1 and 2 metal atoms, M, using SCF molecular orbital methods and 6-31G^* basis sets. The resulting data scatter along three distinct trends, depending on whether the bonds involve row 1 atoms, row 1 and row 2 atoms, or row 2 atoms. When compared with spectroscopically determined force constants, the calculated force constants are found to be 20% larger. A single trend seems to obtain when the calculated force constants are plotted as a function of the effective nuclear charges of the bonded atoms and their interatomic separations. Scaled force constants calculated for the SiO bond are in rough agreement with values provided by spectroscopic measurements for silicic acid molecules and silicate crystals. Polyhedral compressibilities for nitride-, oxide-, and sulfide-coordinated polyhedra are inversely related with force constants calculated for their MX bonds. The close similarity of these compressibilities and those recorded for chemically similar crystals suggests that force constant-compressibility relationships in chemically similar molecular and crystalline systems are not significantly different.     

Ab initio study of force constants and intensities of 1,3,5-trioxane

The structure, vibrational force constants and infrared and Raman intensities have been calculated for 1,3,5-trioxane using a 3-21G basis set. These results have been used to identify some possible inaccuracies in experimental diffraction based structures and in vibrational assignments. It is demonstrated that there is a marked contrast between the trends in the vibrational force constants and in the King's effective atomic charges of the axial and equatorial CH bonds in the series cyclohexane, 1,4-dioxane and 1,3,5-trioxane. The axial CH stretching force constant decreases by 0.04 mdyne Å⁻¹ for each adjacent oxygen atom, whereas that of the equatorial CH bond increases by 0.15 mdyne Å⁻¹ per oxygen. In trioxane the effective atomic charge of the axial hydrogen is twice that of the equatorial. Atomic polar tensors are calculated in a bond oriented frame, and the effect of the oxygens on CH stretching and bending mode intensities discussed. Some properties are also calculated using the 4-31G basis.     

Studies on nitramide and its methyl derivatives with ab initio calculations. IV. The harmonic force field and vibrational spectra of dimethylnitramine and its isotopic derivatives

The complete harmonic vibrational force field of dimethylnitramine has been calculated at the Hartree-Fock level using 4–21G basis set. The harmonic force field was then scaled with scale factors previously derived from N-methylnitramine, and the vibrational spectrum of dimethylnitramine was computed. This a priori prediction, made with no reference to observations on dimethylnitramine, agrees with the experimental IR spectrum in gas phase with a mean deviation of 8.4 cm^?1. Some of the scale factors were reoptimized by fitting of the computed force field to experimental data. The new set of scale factors reduced the mean deviation to 4.5 cm^?1, and was used to predict the vibrational spectrum of deuterated form of dimethylnitramine(-6D). Dipole moment derivatives were also calculated and used to predict infrared intensities which are comparable with experimental values.     

Force fields of allylamine conformers as studied by an ab initio calculation and infrared spectroscopy

The quadratic force constants of the five possible conformers of allylamine were calculated by an ab initio MC method. The infrared spectra of this molecule in the gas phase were remeasured and analyzed on the basis of the calculated vibrational frequencies. A potential energy surface with respect to the skeletal torsions was also calculated. The relative abundances of the S⁺G⁺, ST, CT, CG, and S⁺G^? conformers, predicted to be 44:32:16:7:1 at room temperature, are consistent with the experimental data.     

Equilibrium structure and spectroscopic constants of maleic anhydride

The quadratic, cubic, and semi-diagonal quartic force fields of maleic anhydride have been calculated at the MP2 level of theory employing the cc-pVTZ basis set. The spectroscopic constants derived from the force field are in excellent agreement with the corresponding experimental values. The semi-experimental equilibrium structure has been derived from experimental ground state rotational constants and rovibrational corrections calculated from the cubic force field. This semi-experimental equilibrium structure is in excellent agreement with the ab initio structures computed at the CCSD(T) level of theory and it is closer to the ab initio structure than the purely experimental (or empirical) structures r ₀, r _m⁽¹⁾, and r _m⁽²⁾ obtained by microwave spectroscopy as well as the equilibrium structure derived from gas-phase electron diffraction data.     

Vibrational spectra and force field of terephtalonitrile

The IR and Raman spectra of the two molecules terephtalonitrile and terephtalonitrile-¹⁵N were recorded to permit the general assignment of the vibrational bands observed, in agreement with a D_2h symmetry for these molecules. The general quadratic force field was calculated by the semi-empirical MINDO/3 method from an optimized geometry obtained by the same method. The resulting force field was refined by employing the experimental vibrational frequency data of the two molecules and those of terephtalonitrile-d₄. The final differences between the calculated end experimentally observed frequencies for B_2g and B_3u terephtalonitrile species were within the range ± 0.1 cm⁻¹.     

A general quadratic force field for the diazirine molecule

A general quadratic force field has been calculated for the vibrations of the diazirine molecule by the refinement of a series of force constants obtained recently ab initio by Wiberg et al. for cyclopropene. The calculated force constants have been refined to fit the frequencies for the H₂CN₂ and D₂CN₂ species and the isotopic shifts of the H¹³CN₂ and H₂C¹⁵N₂ species.     

Infrared and Raman spectra of terephthalonitrile and terephthalonitrile-15N2. Force field for out-of-plane vibrations

A general assignment of the vibrational spectra of terephthalonitrile and terephthalonitrile-¹⁵N₂ is proposed on the basis of their infrared and Raman spectra. The relevant symmetry is found to be D_2h. The force field for the out-of-plane vibrations of these molecules was calculated by refining the general quadratic force field obtained by the semi-empirical MINDO/3 method, starting with a geometry optimized by this method. The refined force field reproduces the observed frequencies of the out-of-plane vibrations to better than ±0.5 cm⁻¹.     

Force field for in-plane vibrations of isophthalonitrile

Summary The general quadratic force field for the in-plane vibrations of isophthalonitrile was calculated by the semi-empirical MINDO/3 method. This force field was refined to the frequencies observed experimentally for isophthalonitrile and isotopic shifts of isophthalonitrile-¹⁵N₂. The normal coordinates and the force field in internal coordinates were also calculated from the refined field.

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Kräftefeld für die nichtebenen Schwingungen von Isophthalsäuredinitril

Zusammenfassung Das allgemeine quadratische Kräftefeld für die nichtebenen Schwingungen von Isophthalsäuredinitril wurde mit der halbempirischen MINDO/3-Methode berechnet. Das Kräftefeld wurde aufgrund der experimentalen Frequenzen von Isophtalsäuredinitril und der Isotopen-Verschiebungen von Isophtalsäuredinitril-¹⁵N₂ verfeinert. Die Normalkoordinaten und das Kräftefeld in inneren Koordinaten wurden auch vom verfeinerten Kräftefeld berechnet.

     

Normal coordinate and infrared band intensities of trichloronitromethane and trichloroacetate ions

Laser Raman and IR spectra in the region 50–3000 cm⁻¹ for trichloronitromethane and trichloroacetate ions were recorded. All observed vibrational bands have been assigned to normal modes. Normal coordinate analyses of these molecules have been carried out in the valence force-field approximation. A set of force constants was obtained leading to good agreement between observed and calculated frequencies. The relative displacements of the atoms resulting from normal coordinate calculations were used to compute the IR band intensity of each mode by the CNDO/2-MO procedure. The intensity calculations confirmed the assignments and supported the calculated force constants.     

Calculation and Measurement of Terahertz Active Normal Modes in Crystalline PETN

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover 1 with the COMPASS 2 force field, CASTEP^[3] and PWscf. 4 The calculations are compared to each other and to terahertz spectra (0.3–3 THz) of crystalline PETN recorded at 4 K. A number of analysis methods are used to characterise the calculated normal modes.     

FT-Raman and infrared spectra,r0 structural parameters,ab initio calculations and vibrational assignment for nitryl chloride

The FT-Raman spectra (2000-30 cm⁻¹) of liquid and solid nitryl chloride, ClNO₂, along with the infrared spectra (2000-80 cm⁻¹) of the gas and solid have been recorded. All six fundamentals are confidently identified and the potential energy distributions determined from the force fields obtained from ab initio calculations. Several different basis sets have been utilized to determine the harmonic frequencies and force constants which are compared to the previously reported valence force constants. Structural parameters have been calculated with these basis sets including electron correlation with MP2, MP3 and MP4 perturbation. The calculated equilibrium structural parameters are compared to the experimental r₀ structural parameters. The spectra of the solid indicate that there are at least two molecules per primitive cell. All of these results are compared to the corresponding quantities for some similar molecules.     

Scaled quantum mechanical (SQM) force field and theoretical vibrational spectrum for benzonitrile

The complete harmonic force field of benzonitrile has been determined by ab initio Hartree—Fock calculations using a 4–21 Gaussian basis set. As force constants are systematically over-estimated at this level, the directly calculated force field was scaled by empirical factors previously optimized for benzene and HCN. Frequencies calculated from this scaled quantum mechanical (SQM) force field confirm the published experimental assignments for benzonitrile, benzonitrile-p-d and benzonitrile-d₅. Aside from the CH (and CD) stretching frequencies, which are strongly affected by anharmonicity, the mean deviation between the observed and calculated frequencies is below 9 cm⁻¹ for each isotopomer. Theoretical i.r. intensities reproduce the main features of the spectra semiquantitatively.     

Vibrational frequencies and assignments for some isotopomers of urcail using a scaled ab initio force field

Vibrational frequencies and IR band intensities for 18 isotopomers of uracil, including deuterated ¹⁵N and ¹⁸O species, have been calculated using the scaled ab initio force field of Ref. 1. The results obtained are compared with available experimental data, and a number of refinements in former assignments are proposed. The good agreement between the calculated and experimental frequencies confirms the reliability of the scaled quantum mechanical-force field.     

The harmonic force fields of dimethyl zinc,cadmium and mercury: A joint theoretical and experimental study

The harmonic force fields of the title compounds have been calculated at the ab initio SCF level using effective core potentials and polarized double-zeta basis sets. After scaling, the theoretical wavenumbers are in good agreement with experiment both for the parent compounds and the perdeuterated isotopomers, and the ¹²C→¹³C shifts are well reproduced. Constraining all but three off-diagonal force constants to their ab initio values in a least squares refinement, semi-empirical harmonic force fields have been derived which account well for all available experimental data. The CH stretching motions are analyzed in terms of an harmonic local mode approach.