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.     

Calculation of the frequencies and intensities in the infrared and raman spectra of cyclopropenone

By using the values of the vibrational frequencies of normal and deuterated cyclopropenone (II-d₀ , II-d₂ ) and ¹⁶ O-and ¹⁸ O-substituted dimethylcyclopropenone (III) as -well as the infrared and Raman intensities of II a consistent set of force constants has been derived for the cyclopropenone skeleton. The derived values show that the zwitterionic form makes a substantial contribution to the electronic ground state of the molecule. The combined frequency and intensity calculation - simulation of the infrared and the Raman spectrum - is shown to be a good method for making a proper assignment of calculated and observed vibrations and deriving realistic sets of force constants.     

The harmonic force field and absolute infrared intensities of butyne-2

The frequencies, harmonic force field and absolute IR intensities for butyne-2-d₀ and butyne-2-d₆ are reported. The final set of “harmonized” fundamental frequencies for butyne-2-d₀ and butyne-2-d₆ obeys the Teller—Redlich product rule very well. Starting values for the force constants were obtained from the harmonic force field of propyne, and diagonal force constants were adjusted in order to reproduce the experimental “harmonized” frequencies for the d₀ and d₆ compounds.The integrated IR intensities were measured according to the Wilson—Wells—Penner—Weber method, using nitrogen as a broadening gas. Thirteen sets of ?μ/?S values were obtained from the experimental intensities, using an iterative least-squares fitting procedure. This number could be reduced to one by use of several selection criteria. The signs of the remaining set appeared to be in complete agreement with the best set for propyne as reported both by Kondo and Koga and by Bode et al. The final ?μ/?S parameters were transformed into atomic polar tensors. Both kinds of intensity parameters are discussed and compared with corresponding parameters for related molecules.     

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.     

Theoretical prediction of vibrational spectra. The a priori scaled quantum mechanical (SQM) force field and vibrational spectra of pyrimidine

The complete harmonic force field of pyrimidine has been computed at the ab initio Hartree—Fock level using a 4–21 Gaussian basis set. In order to compensate the systematic overestimations of the force constants at the aforementioned level of quantum mechanical approximation, the theoretical force constants were empirically scaled by using nine scale factors. (The values of all these scale factors were previously determined by fitting the theoretical force field of benzene to the observed vibrational spectra of benzene.) The resulting a priori scaled quantum mechanical (SQM) force field is regarded as the most accurate and physically the most correct harmonic force field for pyrimidine. This force field was then used to predict the vibrational spectra of pyrimidine-h₄ and pyrimidine-d₄. On the basis of these a priori vibrational spectra uncertain assignments have been confidently resolved. After a few reassignments, the mean deviations between the experimental and calculated frequencies are below 9 and 18 cm⁻¹ for the non-CH stretching in-plane and the out-of-plane vibrations, respectively. Computed IR intensities are generally in agreement with experiments at a qualitative level.     

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.     

Infrared and Raman spectra and normal coordinate analysis of trans-1,2-dicyanocyclopropane and its deuterated isotopomers

Infrared and Raman spectra are presented for trans-1,2-dicyanocyclopropane and its 1,2-d₂- and 3,3-d₂-isotopomers. Most of the d₀ assignments agree with previous assignments of Strumpf and Dunker encompassing 26 of the 27 fundamentals. The 1,2-d₂ assignments are similar, but for the 3,3-d₂ species only 11 bands could be assigned. A complete ab initio quantum mechanical force field has been calculated for this molecule at the 6-31G* basis set level. This force-field was scaled and least-squares optimized using eight parameters for functionally related diagonal force constants and their geometric mean for off-diagonals. Both theoretical minimum energy and “experimentally corrected” geometries were used with no significant difference in results. An earlier calculation with a 4-31G basis set gave a similar frequency fit but different scaling factors. As an alternative approach, the scaled ab initio force field, was also used as a starting point for a more conventional refinement of 27 force constants which had a significant influence on the potential energy distribution. The remainder were fixed at the scaled ab initio values. This empirical force field resulted in a fit to 65 fundamentals of d₀, 1,2-d₂ and 3,3-d₂ to < 1% average error.     

Metallomethanes: XIII. Vibrational spectra and force fields of cyanomercuriomethanes

Vibrational spectra with assignments and results of normal coordinate calculations for cyanomercuriomethanes CH_4−n(HgCN)_n (2 ≤ n ≤ 4) are discussed. The valence force constants of the central CHg bonds are 2.149, 1.944, and 1.798 N cm⁻¹,while those of the HgCN bonds are 2.204, 2.123, and 2.162 N cm⁻¹, for n = 2, 3 and 4, respectively. All these force constants are lower than the corresponding constants for methylmercury cyanide (2.445 and 2.379 N cm⁻¹). The overall behaviour of these force constants as a function of the degree of mercuration n is quite similar in both the cyanomercuriomethanes CH_4−n(HgCN) and methylmercuriomethanes CH_4−n(HgCH₃)_n series with the difference that there are variations in the constants at higher values in the former series. The potential energy distributions indicate that the valence vibrations of the CH, CN, and HgCN bonds are almost independent of all other vibrations, which in turn are more or less strongly coupled.     

Pseudo-exact force constants by the high and low frequency separation method

A method based on the “Approximate Separation of High and Low Frequencies” by Wilson et al. is developed and applied to molecules with three vibrations in a single species. It is shown that for a number of ZXY₃ (C_3v), ZXY₂ (C_2v), and ZXY (C_s) type molecules, for which exact force constants and isotopic data are available, this method—when used in conjunction with the isotopic data-reproduces the exact force constants of the reduced secular determinant. The high and low frequency separation seems to be a good method for determining the force constants in many n = 3 cases, where the highest and/or lowest frequency vibrations are rather characteristic and where—as in most cases—isotopic frequency data alone are insufficient for the determination of the exact force field. Extension to higher order secular determinants should also be possible.     

Ab initio SCF computation of force constants for CO2

Force constants for CO₂ have been evaluated using SCF wave functions. The effect ofd basis functions and geometry are investigated. Comparison with experimental values shows that a large error, due to neglect of electron correlation, occurs for theK ₁₂ interaction stretch force constant.     

A general quadratic force field for the pyridazine molecule: out-of-plane B1 normal modes

On the basis of Raman and infrared data for pyridazine-d_o′, −3, 6-d₂, and −d₄ we are carrying out a normal coordinate treatment using a general quadratic force field (GQFF) on a non-redundant basis. The BB^* matrix has been set up and diagonalized to obtain the linear redundancy relations. Afterwards, using the Schmidt's process, we constructed the independent coordinates used in our calculations. To date we have completed the refinement of the calculations of the force constants for the B₁ specie.     

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.     

Composes isomorphes MeX2O4E2: I. Etude vibrationnelle de MnSb2O4 entre 4 et 300 K: champ de force et tenseur élastique

A vibrational study of the tetragonal antimony oxides (Mn, Ni, Zn) Sb₂O₄E₂ in which E is a lone pair has been carried out using Raman and infrared spectroscopy between 4 and 300 K. A force field calculation has been performed using previous results (J. P. Vigouroux et al., Spectrochimi. Acta A 38, 393, 1982) and a new structural evolution approach using models and results of X-ray and neutron diffraction experiments (R. Chater and J. R. Gavarri, J. Solid State Chem. 59, 123, 1985). This approach delivers mean force constants F_a, F_c, elastic constants C_ij, and Grüneisen parameters. Using these results it has been possible to propose initial force constants which have been refined. For MnSb₂O₄ the following results are obtained: (1) In MnO₆ octahedra, the six MnO bond force constants are about 60 N m⁻¹. (2) In SbO₃E tetrahedra, the three SbO force constants are about 200 N m⁻¹ while OSbO angle force constants are in the range 50–70 N m⁻¹. (3) SbE · · · SbE interactions are evaluated in the three Mn, Zn, Ni isomorphous compounds: the interaction is strongly connected with the SbSb distances found in these structures. It is shown that the force field calculation is in agreement with the results of the structural evolution approach: the values F_a = 49 N m⁻¹, F_c = 70 N m⁻¹ can be compared qualitatively with the various averaged force constants.     

Theoretical study of the static Jahn-Teller effect—I. Two-mode vibronic coupling in octahedral halocomplexes with doubly degenerate electronic states

A two-mode E_g-(a_1g+e_g) vibronic coupling is analyzed for octahedral systems. Analytic formula for the adiabatic potential surface (APS) is obtained considering quadratic vibronic terms and anharmonicities of normal vibrations as well. Potential constants, viz. five elastic force constants and three vibronic constants, are evaluated from the numerical map of the APS applying the non-linear regression analysis. Numerical values are obtained for hexahalocomplexes on the CNDO/INDO level of total energy calculations.     

Untersuchungen an molekülkomplexen von n-σ-TYP MITT. I. über freie enthalpie—kraftkonstantenkorrelationen von molekülkomplexen des jods m

Linear correlations have been found between the ΔG⁰ values of the molecular complexes R₁R₂R₃PO/I₂, R₁R₂SO/I₂ and R₁R₂SeO/I₂ and the PO, SO and SeO valence force constants, respectively. The nature of the correlation is determined by the ZO donor bond and not by the donor atom, where Z is P, S or Se. The change on ΔG⁰ values for an equal change in the ZO valence force constants increases in the order R₁R₂SO/I₂?R₁R₂R₃PO/I₂ < R₁ R₂SeO/I₂. Seleninyl complexes with I₂ are more stable than the analogous thionyl complexes. From ΔG⁰—f_zo correlation deductions can be made about the nature of the ZO donor bond and ΔG⁰ values can be evaluated from vibrational spectra. A linear correlation exists between the ΔG⁰ values of corresponding thionyl and seleninyl complexes which is of the same form as the correlation between the valence force constants of analogous thionyl and seleninyl compounds.     

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.     

Metallomethanes: XII. Vibrational spectra and force fields of methylmercurimethanes

The vibrational spectra data, the assignments, and the results of normal coordinate calculations for CH_{4 − n}(HgCH₃)_n molecules (2 ⩽ n ⩽ 4) are reported. The central CHg valence force constants are 1.870, 1.653, and 1.582 N cm⁻¹ while the terminal ones are 2.121, 2,101, and 2.160 N cm⁻¹ for n = 2, 3 and 4, respectively. The latter values are 12, 21, and 27% higher than the central CHg force constants, but all of them are substantially lower than those in dimethylmercury (2.379 N cm⁻¹). These findings can be accounted for in terms of increasing shift of electron density towards the periphery of these molecules and increasing non-bonded metal-metal interaction. The nature of the normal modes is discussed.     

Force constants in molecular crystals of methyl-and perdeuteriomethylmercury(II) halides

Force constants for the internal vibrations involving the metal and for the lattice vibrations of Hg(CH₃)X and Hg(CD₃)X (X = Cl, Br or I) are calculated on the basis of a D_4h⁷ layer structure. The internal HgX stretching force constants are much lower than for these molecules in solution, but HgC stretching force constants are slightly higher. The HgX and longitudinal translatory force constants within the lattice layer are close in value to the strong and weak HgX bond stretching force constants respectively in the unsymmetrical [Hg(CH₃)X₂]^? complex ions.     

Ab initio study of the structure of enolic and ketonic forms of β-diketones with the general formula R″COCH2COR′ (R′ and R″ = H, CH3, CF3)

Geometric parameters, values of force constants and vibration frequencies for different geometric configurations of enolic and ketonic forms of molecules of β-diketones, R″COCH₂COR′ (R″ and/or R′ are H, CH₃, CF₃) have been computed by the ab initio method MO LCAO SCF using wide bases of Cartesian Gaussians. The enolic form of the considered molecules is most energetically favorable. Values of isomerization energy, the height of internal rotation barriers, and energies of the intramolecular hydrogen bond were obtained for it. The stabilization of the enolic form was shown to be caused by the presence of the intramolecular hydrogen bond in the chelate ring. The effect of a substituent on the computed values of molecular parameters was examined. The comparison with the available experimental data testifies to a sufficiently high reliability of the data obtained.