Semiempirical calculation of harmonic force constants: CNDO/2 and MINDO/2 study of C2H6, C2H4 and C2H2

The complete quadratic force fields of ethane, ethylene and acetylene have been calculated from CNDO/2 and MINDO/2 wavefunctions by the force method. Agreement with experiment is satisfactory for both methods. In the CNDO method the stretching force constants must be scaled empirically to obtain realistic values. It is particularly significant that stretching-deformation and deformation-deformation coupling force constants are correctly reproduced by the CNDO method and with the exception of the rocking-rocking couplings, also by the MINDO method. It is concluded that such calculations may usefully contribute to the determination of force fields.     

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.     

The force field of carbon suboxide

The complete force field for carbon suboxide has been evaluated. The optimum geometry, stretching and interaction force constants are obtained from a combination of the CNDO/force molecular orbital method and the compliance constant formalism.     

Block copolymers : Edited by D.C. Allport and W.H. Janes,applied science,Barking, Essex, 1973, pp. xxiii+ 620, price £12.00

A CNDO/2 parametric adaptation including Fischer-Kollmar and Sadlej modifications is made. This method leads to much better agreement between theory and experiment for bonding force constants of diatomic molecules and ethylenic and saturated hydrocarbons. However, these force constants are very sensitive to first and second derivatives of molecular integrals, so it is not possible to obtain a unique parametric formulation for various types of bond.     

Conformational Effect on CNDO Force Field of Furan-2-Aldehyde

The complete sets of force constants for both O, O-cis and O,O-trans furand-2-aldehyde conformers were calculated using the CNDO force method. The force constants were corrected by ten empirical scaling factors. The calculated and observed frequencies agree well in most cases. Comparing the force constants of the two conformers, the largest deviations can be found in the interaction terms belonging to the aldehyde group.     

Prediction of vtbrational spectra by the CNDO/2 force method: I. Out-of-plane vibrations of benzene and fluorobenzenes

The out-of-plane force constants of benzene and 12 fluorobenzenes have been calculated by the CNDO/2 force method. Two empirical scaling factors had to be applied to obtain good agreement. The mean deviation between the 88 calculated and experimental frequencies is 16 cm^?1.     

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.     

Calculation of optimum geometries and force fields by the CNDO/force method

CNDO/Force calculations have been performed on a series of molecules, H₂CO, F₂CO, CF₄, CHF₃, CH₂F₂ and CH₃F. The optimum geometries and force fields are reported. It is found that the method can successfully predict the geometries of polyatomic molecules. The bending force constants and interaction force constants are, in general, comparable with experimental values both with respect to sign and magnitude. The stretching force constants have higher values than the experimental force constants. However, the trend in stretching force constants of a series of molecules is comparable with that of the corresponding experimental values.     

Determination of force fields for formaldehyde,acetaldehyde and acetone by the CNDO/force method

CNDO/Force calculations have been done for formaldehyde, acetaldehyde and acetone, and the theoretical force fields evaluated. Experimental force fields are obtained from vibrational frequencies using the least-squares refinement method. The initial force fields considered are based on the bending and interaction force constants obtained from the CNDO/Force calculations and the stretching force constants transferred from chemically related molecules. Vibrational frequencies of H₂CO, D₂CO, HDCO, H₂¹³CO and D₂¹³CO for formaldehyde, CH₃CHO, CH₃CDO, CD₃CHO, CD₃CDO and CH₂DCHO for acetaldehyde, and CH₃COCH₃ CD₃COCH₃ and CD₃COCD₃ for acetone are employed in the force field refinements. The final force fields obtained are found to be reasonable with respect to the diagonal and interaction force constants.     

SemiempirischeMO-Berechnungen an Silicium—Fluor-Verbindungen und ihren Hydrolyseprodukten

The series of monomeric silicon compounds containing F and/or OH have been calculated using theCNDO/2 method. The energy values obtained prove useful for qualitative discussion of reactions involving the compounds investigated.CNDO/2-valence force constants have been calculated by two different methods. The absolute values are too high compared with experimental values for these compounds, but their relative order is reflected correctly. Some predictions have been made concerning the spectroscopic data to be expected for the hydrolysis products, which have not yet been studied experimentally.     

A reparametrization of the CNDO method I. Hydrocarbons

The empirical evaluation of the core matrix elements in the CNDO method is modified and the parameters are adjusted to give optimal values for heats of atomization, bond lengths, bond angles and force constants.

---

Zusammenfassung Die empirischen Ansätze der CNDO-Methode für die Core-Matrixelemente werden modifiziert und die Parameter abgeändert, um möglichst gute Werte für Bildungswärmen aus den Atomen, Bindungslängen, Valenzwinkel und Kraftkonstanten zu erhalten.

---

Resumé L'évaluation empirique des éléments de matrice de coeur dams la méthode CNDO et les paramèters sont modifiés en vue de dormer des valeurs optimaux pour les énergies d'atomisation, les distances interatomiques, les angles de valence et les constants de force.

     

Predicted vibrational frequencies and assignment of the spectra of uracil and its n,n-dideutero derivative by quantum chemical methods

A recent theoretical study on the vibrational spectrum and force field of maleimide (ref.1) lead to scale factors transferable to molecules of similar structure (ref.2). The theoretical vibrational spectrum of uracil (refs.2–3) calculated from scaled CNDO/2 force constants using the scale factors of ref.1 agrees better with the experimental results (ref.4) than former calculations by $\text{ab initio}$ STO-3G (ref.5) or MINDO/3 (ref.6) methods. The scaled CNDO/2 force field of uracil has also been used to predict the normal frequencies of a set of its deuterated derivatives.     

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.     

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.     

A modified PCILO method

The perturbative configuration interaction using strictly localized molecular orbitals, called the modified PCILO method, for which the use of the Rayleigh-Schrödinger many-body perturbation theory with the Moller-Plesset Hamiltonian partitioning is characteristic, has been proposed in this communication. On the CNDO/2 and INDO levels of Hamiltonian approximations strictly localized molecular orbitals have been constructed by solving modified Roothaan equations. From the zero and second order energy interatomic distances and harmonic force constants for some diatomic molecules have been calculated. The linear dependence of the correlation energy on the number of valence electrons in the series of the molecules CH₄, CH₃F, CH₂F₂, CHF₃ and CF₄ is perfect.     

“Universal” CNDO/2 method (CNDO/2-U)

The Pople-Santry-Segal CNDO/2 method and the modified CNDO/2 version worked out recently by Clack for transition-metal complexes have been extended to the full periodic table.     

Méthode cndo et détermination des constantes de force

Binding force constants calculated by the CNDO II method are very sensitive to the second order derivative of H_μ,ν integrals between orbitals of bonded atoms. This sensitivity is used to obtain a parametric form of these integrals, from which satisfactory force constants for carbon—carbon and carbon—hydrogen bonds are derived. All other characteristics of the method are preserved. The reasons for the observed correlation between bond indices and force-constants are analyzed.     

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.     

A theoretical study of vibrational spectra of tetranitrate-methyl-β-D-glucopyranoside and 2,3-di-O-nitro-methyl-β-D-glucopyranoside

Coupled calculations were carried out of normal vibration frequencies from the point of view of the valence-force field scheme and of absolute IR band intensities by the CNDO/2 method for the tetranitrate-methyl-β-D-glucopyranoside molecule. A good agreement was achieved with the experiments. Normal coordinate analysis was made for 2,3-di-O-nitro-methyl-β-D-glucopyranoside molecule with force constants obtained for the tetra-nitrate-methyl-β-D-glucopyranoside. Before proceeding to the calculation of the spectra of such complex molecules of nitrates of monosaccharides, a complete experimental and theoretical investigation was performed of the vibrational spectra of methylnitrate, which made it possible to determine the deficit of force constants for the calculations of the spectra of nitrosubstituted glucopyranosides. Detailed interpretations of the observable IR spectra of both the nitro-glucopyranosides compounds considered are given. The absorption spectra sensitivity to the spot of the nitrate group localization was discovered. Special attention was focused on analyzing the spectra of nitrates of saccharides for the characteristic split of the band due to the asymmetric stretching vibrations of the ONO₂ groups in the region of 1600–1700 cm⁻¹.     

Approximate SCF CI calculation of excited state geometries of phenol,aniline, para-fluorophenol and para-fluoroaniline

The CNDO/s method has been used to study the electronic structure, spectra, geometry and rotational constants in the ground and the first excited singlet states of phenol, aniline, para-fluorophenol and para-fluoroaniline. The ground state geometry has also been studied using CNDO/2 and INDO methods. Calculated bond length changes following the electronic excitation have been compared with experimental results and ambiguities present in the latter have been resolved.