Potential constants and Coriolis coupling constants of perhalogenated ethylenes: Part 3. trans-ClFC=CFCl, trans-ClBrC=CBrCl and trans-FBrC=CBrF

Based on vibrational frequencies a calculation of a general quadratic force field has been carried out for trans-ClFC=CFCl, trans-ClBrC=CBrCl and trans-FBrC=CBrF via an iterative method. The resulting transformation L matrices were used to evaluate the Coriolis coupling constants and these were used to calculate inertial defects. The calculated values of the inertial defects are lower than those of the respective cis-ethylene-type molecules.     

Potential constants and Coriolis coupling constants of perhalogenated ethylenes Part 1. 1,1-Cl2C=CF2, 1,1-Cl2C=CBr2 and 1,1-F2C=CBr2

Coriolis coupling constants have been calculated from force field computations and used to evaluate the inertial defect of 1,1-dichlorodifluoroethylene, 1,1-dichlorodibromoethylene and 1,1-difluorodibromoethylene. The inertial defect values for the ground vibrational state of 1,1-C1₂C=CF₂ = 0.2450, 1,1-Cl₂C=CBr₂ = 0.3740 and 1,1-F₂C=CBr₂ = 0.4190 amu Ų show corrrespondence with the observed values of similar ethylene-type molecules.     

Multidimensional anharmonic calculation of the vibrational frequencies and intensities for the trans and cis isomers of HONO with the use of normal coordinates

The equilibrium geometry and the potential energy and dipole moment surfaces have been determined for the cis and trans isomers of the HONO molecule by an ab initio Moller–Plesset (MP2) calculation with a wide set of atomic orbitals. The multidimensional anharmonic vibrational Schrodinger equations are solved using the variational method with the Hamiltonian and wave functions written in the normal coordinates of cis and trans isomers. All one- and two-dimensional and a number of three-dimensional vibrational problems are solved to obtain the energy levels and vibrational eigenfunctions. The frequencies and intensities for the fundamental, overtone and some combination bands are determined in good agreement with the available experimental results. The calculation shows the strength of coupling between different vibrational modes and reveals the presence of strong resonances between the (v₁, v₃, v₆) and (v₁, v₃−1, v₆+2) states of cis-HONO. This fact may be important for understanding the energy redistribution between the intermolecular degrees of freedom. The magnitude and direction of vibrationally averaged ground-state dipole moment of both isomers, as well as the direction of transition dipole moments, are in good agreement with the experimental findings. The changes in the values of dipole moment and some geometrical parameters of cis- and trans-HONO on vibrational excitation are also computed.     

An ab initio SCF study of conformational isomerism in HNO2

The structures and complete force fields of cis and trans nitrous acid have been calculated with a (7, 3) basis set. The differences between the two stable isomers are reproduced well. The dipole moments, centrifugal distortion constants, vibrational frequencies and isotope shifts also agree satisfactorily with observed values. The fully optimized structures of rotamers corresponding to intermediate values of internal rotation around the N-O bond have been calculated to investigate structural changes during internal rotation. The considerable changes show the strong influence of π electron delocalization in the planar forms. The barrier is calculated to be 8.7 kcal mol⁻¹ in reasonable agreement with experimental values. Both the cis and trans barriers are calculated to be attractive dominant in contradiction to earlier work.     

Effects of hydration on scale factors for ab initio force constants

Experimentally measured vibrational frequencies from the polar groups of peptides in aqueous solutions do not agree with frequencies calculated from scaled quantum mechanical force fields (SQMFF) using differential scale factors developed for molecules in the vapor phase. Measured stretching frequencies for carbonyl groups are more than 50 wavenumbers lower than the calculated values. On the other hand, frequencies for non-polar groups calculated using these scale factors are relatively accurate. Our goal is to develop a SQMFF that yields accurate calculated frequencies for peptides in aqueous solutions. To this end, we have calculated scale factors for ab initio force constants for formic acid, acetic acid, and acetone using a least squares fit of calculated and experimental frequencies. We compare these scale factors with changes observed in the ab initio force constants calculated for these molecules at various states of hydration. These force constants are calculated using fully optimized geometries for these hydrated molecules using the 4-31G basis. We present a comparison of the experimental and calculated frequencies, along with their potential energy distributions, for both vapor and aqueous phases. The results indicate that scale factors can simulate the effects of solvation on molecular force constants to yield accurate scaled ab initio force fields.     

An ab initio potential energy surface and spectroscopic constants for the XΣg state of NO2

A three-dimensional potential energy function has been calculated for the X¹Σ⁺_g state of NO⁺₂ from ab initio MRD-CI data. With this PE function, converged vibrational calculations have also been performed for ten vibrational states, with the aid of a computer program developed in the present work for this purpose. The calculated harmonic frequencies, vibrational term values and rotational constants are in good agreement with experimental data.     

Prediction of vibrational constants for gas-phase heteronuclear diatomics and atoms adsorbed on surfaces

An empirical relationship is presented for predicting vibrational constants for gas-phase heteronuclear diatomic molecules and atoms adsorbed on surfaces. Excellent agreement with experimental values is obtained. The method is used for predicting the unknown vibrational constants of AgS and CTe.     

Proton magnetic resonance studies of cyclic compounds—III cis- and trans-2,6-dimethylmorpholines

Commercial 2,6-dimethylmorpholine has been separated into cis- and trans-isomers by vapour phase chromatography. The PMR spectra of the bases, analysed as ABXK₃ systems (J_AK = J_BK = 0), show that the most abundant isomer has the cis-configuration, consisting probably of a single conformation in which both methyl groups are equatorial. The trans-isomer is probably a mixture of rapidly inverting and energetically equivalent conformations, since the vicinal coupling constants deduced from the PMR spectrum are averages of those expected for the two conformations.     

Vibrational contributions to indirect spin-spin coupling constants calculated via variational anharmonic approaches

Zero-point vibrational contributions to indirect spin-spin coupling constants for N2, CO, HF, H2O, C2H2, and CH4 are calculated via explicitly anharmonic approaches. Thermal averages of indirect spin-spin coupling constants are calculated for the same set of molecules and for C2X4, X = H, F, Cl. Potential energy surfaces have been calculated on a grid of points and analytic representations have been obtained by a linear least-squares fit in a direct product polynomial basis. Property surfaces have been represented by a fourth-order Taylor expansion around the equilibrium geometry. The electronic structure calculations employ density functional theory, and vibrational contributions to indirect spin-spin coupling constants are calculated employing vibrational self-consistent-field and vibrational configuration-interaction methods. The performance of vibrational perturbation theory and various approximate variational calculations are discussed. Thermal averages are computed by state-specific and virtual vibrational self-consistent-field methods.     

Vibrational frequencies,force constants,coriolis constants,mean amplitudes and shrinkage effects for CH3CC-CCX (X = H,Cl, Br an

Normal coordinate analyses for methyldiacetylene and its 1-halo derivatives, CH₃CC-CCX (X = H, CI, Br or I), were performed on the basis of experimental vibrational frequencies not published previously. The molecules belong to a pyramidal-axial model of C_3v symmetry. Coriolis coupling constants of the E_a × E_b type are given. Also the mean amplitudes of vibration, perpendicular amplitude correction coefficients and shrinkage effects were calculated. The mean amplitudes are discussed in particular, and characteristic values for certain types of interatomic distances are deduced.     

Coriolis coupling constants of cis and trans-1,2-dichloroethylene

Coriolis coupling constants and the inertia defect for cis- and trans-1,2-dichloroethylene have been calculated, using force constants obtained by the iterative consistency method. The results obtained are compared with those of previous workers and it is shown that two sets of force constants which reproduce the vibrational frequencies satisfactorily lead to completely different values for the Coriolis coupling constants.     

Tautomeric equilibrium of ketonic and cis- and trans-enolic forms of β-dicarbonyl compounds in solution

In this paper the tautomeric equilibrium of ketonic and cis- and trans-enolic forms of β-dicarbonyl compounds is considered as an acid-base protolytic one. The application of the Brønsted-Izmaylov theory has enabled to suggest a general equation K_TS = for keto-enol equilibrium constant, depending on the solvent, the well known Meyer's equation being included in it as a particular case. Equilibrium constants have been bromometrically determined in a number of solvents of keto-enols of three types, i.e. “cis-fixed”, “trans-fixed” and that involving both stereo-isomeric forms. A method for quantitative determination of cis-trans-enol equilibrium had been proposed.     

Studies in the pinacol rearrangement—V The rearrangements of cis- and trans-1,2-diphenyl-1,2-ditolylethylene oxides

The intermediate formation of the two epoxides was observed in the perchloric acid-catalysed rearrangement of meso-2,2′-dimethylbenzpinacol (P_meso) to 2-methylbenzoyl tolyl diphenyl methane (K) Two kinds of epoxides were separated with preparative TLC and the configuration (cis and trans) of these epoxides was inferred in the light of UV, NMR and IR spectra, dipole moment and kinetic data. The rearrangement rates of these epoxides were measured in HClO₄-anhydrous acetic acid. Linear correlations of logarithm of the rate constants (log k) with the Hammett acidity function of the medium (H₀) were obtained, which supports the A-1 mechanism, together with the large positive values of the entropy of activation. It is shown that the perchloric acid-catalysed rearrangement of meso-2,2′-dimethylbenzpinacol (P_meso) occurs via such many kinetically distinguishable routes as the following where C and T are cis- and trans-epoxide respectively. The mechanisms of the rearrangements were discussed from the data of kinetics.     

Calculation of vibrational constants of diatomic molecules

We apply Wigner's effective-harmonic-oscillator (EHO) function with the Morse potential to the calculation of vibrational constants of diatomic molecules. A one-particle system is described by the operator representation of Wigner's function. We find the EHO parameters both for a single molecule and for an equilibrium ensemble of molecules. We obtain numerical values of the constants for the iodine molecule.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 24, No. 4, pp. 485–487, July–August, 1988.     

Constrained force constants for octahedral molecules

A set of constrained force constants has been derived from experimental vibrational frequency data for eighty three octahedral molecules. Superimposing the condition that the larger value for f_d—f_dd′ be used when f_dα—f_dα″ is a maximum on the six equations relating vibrational frequencies to force constants generates a seventh. This provides a uniform set of results for all 83 molecules. The values of the force constants have a simple rationale in terms of chemical bonding theory. Some preliminary calculations for SF₆ show that these force constants are suitable for use in generating reliable molecular dynamical trajectory data.     

Microwave spectra, geometries, and hyperfine constants of OCAgX (X = F, Cl, Br)

A pulsed jet cavity Fourier transform microwave spectrometer has been used to measure the rotational spectra of OCAgX (X = F, Cl, Br) in the frequency range 5-22 GHz. Metal atoms were generated via laser ablation and were allowed to react with CO and a halide precursor, prior to stabilization of the products within a supersonic jet of argon. These are the first experimental observations of OCAgF and OCAgBr, and the first high resolution spectroscopic study of OCAgCl. All three molecules are linear. Accurately determined rotational constants have been used to evaluate the various internuclear distances, which are found to be consistent with trends established for OCAuX and OCCuX species. The C-O distances are short, and the M-C distances are significantly longer than those in other molecules containing a metal-carbonyl bond. Precise values of centrifugal distortion constants and halogen nuclear quadrupole coupling constants have also been determined. The coupling constants are compared with the results of previous studies of OCCuX and OCAuX and are used to infer trends in the electron distributions of the molecules. Ab initio calculations have been performed and employed to predict the geometries, vibrational frequencies, and Mulliken valence orbital populations of the various species.     

An ab initio potential energy surface and spectroscopic constants for the XΣg state of NO2

A three-dimensional potential energy function has been calculated for the X¹Σ⁺_g state of NO⁺₂ from ab initio MRD-CI data. With this PE function, converged vibrational calculations have also been performed for ten vibrational states, with the aid of a computer program developed in the present work for this purpose. The calculated harmonic frequencies, vibrational term values and rotational constants are in good agreement with experimental data.     

Large configuration interaction calculations of nuclear spin-spin coupling constants: III. Vibrational effects in ammonia

Large configuration interaction calculations of the proton—proton coupling constant for several geometrical configurations of the ammonia molecules are reported. The analytical expressions for the energy surface and the coupling constant as functions of two cartesian displacement coordinates are fitted to the calculated values. The potential is used for the calculation of the vibrational wavefunctions for ¹⁵NH₃ and ¹⁵ND₃ species and the vibrational averaging of the coupling constant is carried out using these functions. Though the value of the coupling constants shows a very strong geometry dependence, the vibrational corrections are found to be small. A possible correlation of the proton—proton coupling constant with an angular parameter in the NH₂ group in RNH₂ compounds is indicated.     

Molecular constants of substituted ethylene-type molecules: IV. Inertia defects and rotational constants of vinyl halides and their deuterated derivatives

The inertia defects of some vinyl halides and their deuterated derivatives have been calculated on the basis of the general formulation developed by Oka and Morino. The initial sets of Urey-Bradley force constants, based on published data, were adjusted by a least squares procedure until they reproduced the fundamental vibrational frequencies satisfactorily. The Coriolis coupling constants which satisfy the sum rules derived were evaluated and used to calculate the inertia defect in the ground vibrational state. The theoretical values of the inertia defect are used to determine the rotational constant A which cannot otherwise be accurately determined from an “a” type transition.     

Molecular vibrational constants of some simple polyatomic molecules: trihalogenomethanes,CHX3 and trihalogenosilanes,SiHX3 (X = F,Cl, Br)

Force constants and compliance constants for the CHX₃ and SiHX₃ molecules have been obtained, using the vibrational frequencies of CHX₃ and CDX₃ (SiHX₃ and SiDX₃) molecules only. The reliability of the resulting force field has been checked by calculation of Coriolis coupling constants, centrifugal distortion constants and mean amplitudes of vibration and by comparison with the available experimental data.