Potential energy distributions and normal coordinates of methyl,methyl-d3 and partially deuterated methyl chloride,bromide and iodide

The potential energy distributions and normal coordinates (L^?1 matrices) for twelve methyl halides, CH₃X, CH₂DX, CD₂HX and CD₃X (X = Cl, Br, I) have been calculated from known structural data. General harmonic force fields for methyl chloride, bromide and iodide previously determined from the most complete available isotopic frequency, Coriolis and centrifugal distortion data were used. The vibrational modes of these molecules are compared and discussed.     

Correlations between fundamental frequencies and physical parameters of some R3SbX2 compounds

For molecules of the type R₃SbX₂, where R is methyl or phenyl, and X is a halogen, certain correlations involving the fundamental frequencies of vibration have been established. These frequencies were found to be dependent on the atomic mass and electronegativity of the halogen and the molecular weight and moment of inertia of the molecule.     

The rotational barrier in dimethyl acetylene and related molecules

The rotational barriers in dimethyl acetylene (CH₃CCCH₃), diamino acetylene (H₂NCCNH₂), dihydroxy acetylene (HOCCOH), methyl trifluoro methyl acetylene (CF₃CCCH₃), silyl methyl acetylene (SiH₃CCH₃), propene, cis and trans 2-butene and ethane were studied by ab initio molecular orbital methods using various basis sets. The eclipsed structure for dimethyl acetylene and its CF₃ and SiH₃ analogs was found to be the most stable, as has been inferred from experimental work, and the barrier height for these compounds has been predicted. The barriers in the OH and NH₂ substituted acetylenes, propene, butene and ethane were studied in order to more clearly understand the important influences in determining the barrier mechanism; specifically, the delocalized molecular orbital and Pauling VB model have been compared.     

Geminal interactions in ClZ(CH3)2X molecules (Z = C, Si, Ge) according to ab initio calculations

The structure and electronic parameters of ClZ(CH₃)₂X molecules (Z = C, Si, Ge, X = CH₃, OCH₃) were calculated by the RHF/6–31G(d) and RHF/6–311G(d,p) methods with full geometry optimization; calculations of ClZ(CH₃)₂OCH₃ molecules were also performed by the RHF/6–31G(d) method with partial geometry optimization. The ³⁵Cl NQR frequencies calculated from the populations of less diffuse 3p constituents of valence p orbitals of chlorine [RHF/6–31G(d)] were in agreement with the experimental values. The ³⁵Cl NQR frequencies for molecules with X = OCH₃ are lower than those for molecules with X = CH₃ (the Z atom being the same), due mainly to direct through-field polarization of the Z-Cl bond, induced by the effect of unshared electron pair of the oxygen atom in the trans position with respect to that bond. The difference in the ³⁵Cl NQR frequencies decreases in going from Z = C to Z = Si, Ge, in parallel with variation of the Z-Cl bond polarization as the size of Z increases.     

Microwave and infrared spectra of deuterium-substituted germyl halides and the structures of the germyl halides

We have obtained microwave spectra of fully and partially deuterated germyl halides GeD₃X and GeHD₂X (X = F, Cl, Br, I) and analysed the infrared spectra in the GeH stretching region of the asymmetric species. The resulting A_o and B_o values have been combined with existing data on GeH₃X to give improved structures for the germyl halides. The values of the HGeX angles obtained show a correlation with GeH stretching frequencies, as in methyl compounds, but there appears to be no such correlation between stretching frequencies and GeH bond lengths.     

Electron spin relaxation in liquids : edited by L.T. Muus and P.W. Atkins,Plenum Press,New York, 1972, pp. xiv + 537, price £29.50

The infrared and Raman spectra of trimethylarsine sulfide, trimethylarsine selenide, and their perdeuterated analogues have been recorded. An assignment of 22 of the 24 fundamental vibrational frequencies (exclusive of the methyl torsions) has been made for each molecule. Assignments were made on the basis of symmetry selection rules and comparison with structurally similar molecules.A normal coordinate analysis for each of these molecules was carried out to reinforce the assignments. Assuming a valence force field for each molecule of C_3v symmetry, a set of 24 force constants was refined to give a least squares fit of the calculated frequencies to the observed frequencies. Calculations of the potential energy distribution for each molecule show that there exists little coupling between the different modes.     

Conformational stability and force field of chloromonophosphazenes: MNDO calculations,vibrational spectra and normal coordinate analyses of Cl3PNP(X)Cl2 (X = O,S) and [Cl3PNPCl3][Y] (Y = Cl,PCl6)

The potential energy around the PN bonds for the Cl₃PNP(X)Cl₂ (X = O, S) molecules and [Cl₃PNPCl₃]⁺ cation have been derived from MNDO (modified neglect of diatomic overlap) calculations. The most stable conformations are two s-trans isomers in nearly eclipsed forms. The calculated structural parameters agree well with the X-ray experimental data. Barriers of 6 and 1.5 kJ/mol for the rotations of the POCl₂ and PCl₃ groups are predicted. In addition, the Raman spectra and the qualitative depolarization measurements for these molecules in the liquid phase have been obtained. All the data indicate that the molecules exist as a mixture of two rotamers in the molten phase. These two conformers are stabilized in the crystal packing of Cl₃PNP(O)Cl₂. The observed frequencies are in good agreement with the calculated values obtained by normal coordinate analysis. The MNDO calculation of the harmonic force field is in reasonable agreement with the experimental values. The force-constant values assigned to the torsional modes around the PN bonds correspond to low barriers for the internal rotations. These easy internal rotations around the PN and PN bonds can explain the flexibility of the phosphazene backbone and the elastomeric properties of the polyphosphazene polymers.     

Scaled Quantum Mechanical Force Fields for the Isoelectronic Molecules CF3X (X = SiH3, PH2, SH,Cl )

The vibrational properties of the CF₃X (X = SiH₃, PH₂, SH, Cl) series of molecules were studied by means of density functional theory (DFT) techniques. After obtaining the optimized geometrical parameters and conformations, the frequencies corresponding to the normal modes of vibration and the associated force constants were calculated. The original force fields in Cartesian coordinates were transformed to local symmetry coordinates and subsequently scaled to reproduce the experimental frequencies. Some trends observed in the experimental data could be explained on the basis of the calculated atomic charges.     

Microporous Magnesium and Manganese Formates for Acetylene Storage and Separation

Acetylene sorption of microporous metal formates M(HCOO)₂ (M=Mg and Mn) was investigated. Measurements of acetylene sorption at 196, 275, and 298 K showed a Type I isotherm with quick saturation at low pressures, and 50–75 cm³ g^?1 uptake at 1.0 atm. The single‐crystal X‐ray structure analysis of the acetylene‐adsorbed metal formates revealed that acetylene molecules occupy two independent positions in the zigzag channels of the frameworks with a stoichiometry of M(HCOO)₂?1/3C₂H₂, which is consistent with the gas sorption experiments. No specific interaction except van der Waals interactions between the adsorbed acetylene molecules and the walls of the frameworks was found. Sorption properties of other gases, including CO₂, CH₄, N₂, O₂, and H₂, were also investigated. When the temperature was increased to 298 K, the amount of adsorbed acetylene was still above 60 cm³ g^?1 for Mg(HCOO)₂ and 50 cm³ g^?1 for Mn(HCOO)₂, whereas the uptake of other gases decreased substantially. The microporous metal formates may thus be useful not only for the storage of acetylene but also its separation from other gases at room or slightly higher temperatures.     

Intermedions in catalysis,spectroscopy and electron distribution: Part II. Acetylene and dicarbon

Using the intermedion approach to catalysis, the acetylene molecule is considered as either two CH groups or as a C₂ molecule with hydrogens attached. By establishing relationships between CH and C₂ molecules and the acetylene molecule as electrons are added or removed, the electron distribution is found to be decreased from 5.0 to 4.5 for each CH group and increased from 8.0 to 9.0 for the C₂ group. Equations which describe intermedions by relating vibrational frequencies and numbers of electrons are presented for acetylene, dicarbon, ortho and para hydrogen molecules, hydrogen atoms and negative hydrogen molecules. Catalysts predicted for the hydrogenation of acetylene and dicarbon include those used experimentally and commercially and a few others not reported. Numbers accurate to ±0.0002 identify the catalytic metal, its oxidation state and details of the reaction mechanism. Some of the unanswered questions which arise from this treatment of catalysis are presented.     

Accurate theoretical IR and Raman spectrum of Al2O2 and Al2O3 molecules

The accurate harmonic vibration frequencies together with the infrared (IR) and Raman intensities of the most stable conformers of Al₂O₂ and Al₂O₃ molecules have been calculated by the density functional theory (DFT) method with B3LYP exchange–correlation potential and using a set of the augmented correlated consistent basis sets up to quintuple order. The anharmonic vibration frequencies of the non-linear Al₂O₂ molecule have also been calculated. The obtained equilibrium geometrical parameters, harmonic and anharmonic vibration frequencies along with the IR and Raman intensities good converge to their limits with increasing the size of the used basis set. A comparison of the calculated harmonic and anharmonic vibrational frequencies with the available experimental ones points out that the small differences between the calculated harmonic and experimental frequencies can be further substantially reduced when calculations of the anharmonic vibrational frequencies will be available for all types of molecular geometries.     

Hétérocycles fluorés—I : Addition du diazométhane et du phenylazide au bis (trifluorométhyle)-3,3 acrylate de méthyle et a ses dérivés. Obtention de cyclopropanes et d'aziridines fluorés

Title olefins are prepared by Wittig reactions between hexafluoroacetone and the methylenephosphoranes Ph₃PCHX (X  CN, CO₂Me, COPh). They react readily with diazomethane or phenylazide affording pyrazolines or triazolines which are photodecomposed into the corresponding fluorocyclopropanes or aziridines. The latter compounds are potential azomethine ylides and react with methyl acetylene dicarboxylate giving pyrrolines (X  CN, CO₂Me). When X  COPh, rearrangement into 4-oxazoline is observed.     

Theoretical study of isomerism of compounds of N2 and N2H2 molecules with aluminum clusters Al13 and Al12Ti

Segments of the potential energy surfaces corresponding to successive elementary stages of multistep fragmentation of nitrogen and diimine molecules upon their reaction with the aluminum cluster Al₁₃ and its doped analogue Al₁₂Ti have been calculated by the density functional theory method. The minimum energy pathways of these reactions have been calculated for the stages of physisorption, chemisorption, and N₂ and N₂H₂ fragmentation with different ways of insertion of fragments into the Al₁₃ and Al₁₂Ti cages. Relative energies, structural characteristics, and vibrational frequencies of coordinated and fragment isomers have been calculated, the barriers separating these isomers have been evaluated, and molecular diagrams of the reactions have been constructed. The effect of doping on the relative energies of intermediates and activation barriers has been considered. A conclusion has been drawn that doping with titanium should facilitate the reactions of molecular nitrogen with aluminum clusters. Unusual isomers with a five- and six-coordinate nitrogen atom N* have been localized. The results are compared with the data of analogous previous calculations of the PES of isomers corresponding to stepwise fragmentation of an acetylene molecule in related Al₁₃C₂H₂ and Al₁₂TiC₂H₂ derivatives.     

Substitution methyl group structures in symmetric top molecules

In the absence of a direct experimental determination of A₀ for a symmetric top methyl group molecule, the relations used to determine the off-axis hydrogen coordinates by the mono-substitution technique are shown to lead to the prediction of abnormally long CH bonds. The molecules methyl iodide and methane, for which A₀ constants for the CH₃I and CH₃D species have been determined directly, and for which the ground and equilibrium state geometries are well known, are used as test cases. Methyl chloride, methyl cyanide, and methyl acetylene, for which reliable ground state geometries are known, are also considered.In order to obtain the A₀ rotational constant for CH₂DI required in this study, an analysis of the rovibration spectrum, of CH₂DI was carried out. The results of this work are given in the Appendix.     

Studies on metal—acetylene complexes : VI. Crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)-platinum(II), [(C2H5)2B(N2C3H3)2](CH3)Pt(C6H5CCCH3)

The crystal and molecular structure of [diethylbis(1-pyrazolyl)borato]methyl(1-phenylpropyne)platinum(II), ([(C₂H₅)₂B(N₂C₃H₃)₂](CH₃)Pt(C₆H₅CCCH₃)), has been determined by a single crystal X-ray diffraction study using diffractometer techniques. The compound crystallizes in the monoclinic space group P2₁/c with a = 13.239(6), b = 11.077(5), c = 15.619)7) Å and β = 114.53(2)°. The observed density of 1.71(2) g cm^?3 agrees well with the calculated value, 1.687 g cm^?3, assuming four molecules in the cell. A conventional agreement factor of 0.036 was obtained by least-squares refinement on F using 3289 observations and 194 variables. The coordination about the platinum atom is square planar, if the acetylene is assumed to occupy one coordination site. The substituents of the acetylene are cis-bent away from the Pt atom, the methyl substituent by 17.7(1.0)°, and the phenyl substituent by 21.2(9)°. The coordinated triple bond length is 1.227(10) Å. These results indicate that the acetylene is moderately perturbed on coordination, consistent with the observation that Δν(CC) is 211 cm^?1. The conformation of the ring formed by the bidentate polypyrazolylborate ligand is that of a “shallow” boat. One of the methylene H atoms on the ethyl substituents on the polypyrazolylborate ligand when placed in an idealized position is 2.65 Å distant from the Pt atom.     

An ab initio quantum chemical study of reaction mechanisms in the C2H2/CH3OH/KOH/DMSO system

An ab initio quantum chemical study (MP2/6-311++G**//B3LYP/6-31+G*) of a number of possible interactions is performed for the gas phase system of acetylene—potassium hydroxide-dimethylsulfoxide(DMSO)—methanol and with regard to the solvent effect within the continuum model. Key structures in the vinylation reaction are shown to be methoxide ion complexes with the alkali metal hydroxide and acetylene molecules. The formation of these complexes results in the activation of the acetylene molecule and an increase in the nucleophilicity of the methoxide ion. In the C₂H₂/CH₃OH/KOH/DMSO reaction system, a proton exchange between the acetylene molecule and the anionic nucleophile ([OH]^- and [CH₃O]^-) is freely performed with the formation of systems with ethynideions, whereas the thermodynamically preferable formation of vinyl alcohol or methyl vinyl ether is determined by a barrier of 20 kcal/mol.     

Energies of Association of Carbenium and Silylium Cations with Oxygen-Containing Molecules

The energies of association of Me_nH_3-n X⁺ cations (n = 0-3) with molecules of oxygen-containing bases (water, methanol, dimethyl ether) were calculated by the B3LYP/6-31G(d) method. These energies decrease with an increase in the number n of methyl groups at the X atom; this trend is more pronounced for X = C, which is due to a lower degree of charge transfer in the cation from the occupied orbitals of the methyl group to the vacant orbital of X in the case of X = Si. An increase in the association energy with an increase in the number of methyl substituents at the oxygen atom is due to an increase in the energy level of lone electron pairs of oxygen upon methyl substitution. As a result, the energy gap between the electronic levels of the unoccupied orbital of the cation and orbitals of the oxygen lone electron pairs becomes narrower, which makes the interaction between the unoccupied and occupied orbitals more efficient.__________Translated from Zhurnal Obshchei Khimii, Vol. 75, No. 5, 2005, pp. 754–756.Original Russian Text Copyright © 2005 by Ignat’ev, Kochina.     

Study of the interaction between aniline and CH3CN, CH3Cl and CH3F

A computational study of dimers formed by aniline and one or two CH₃X molecules, X being CN, Cl or F, was carried out to elucidate the main characteristics of the interacting systems. Two different structures were found for each of the dimers, depending on the relative location of the CH₃X molecule with respect to the NH₂ hydrogen atoms. The most stable complex is formed with acetonitrile, with a complexation energy amounting to ?27.0?kJ/mol. Methyl chloride and methyl fluoride form complexes with complexation energies amounting to ?18.1 and ?17.5?kJ/mol, respectively, though the structural arrangement is quite different for both structures. In most complexes, the leading contribution to the stabilization of the complex is dispersion, though the electrostatic contribution is almost as important. Three different minima were obtained for clusters containing two CH₃X molecules depending on the side they occupy with respect to the phenyl ring. The complexation energies for these structures amount to ?58.5, ?38.6 and ?36.3?kJ/mol for acetonitrile, methyl chloride and methyl fluoride, respectively.     

Ab initio study of the ground and lower-lying excited electronic states of NiX2 and FeX2 (X=F,Cl, Br,I) molecules

The ground and lower-lying excited electronic states of FeX₂ and NiX₂ (X=F, Cl, Br, I) molecules are systematically investigated by ab initio method at the complete active space self-consistent field (CASSCF) and multiconfigurational quasi-degenerate second-order perturbation (MCQDPT2) levels of theory. It is concluded that the dynamic electron correlation has to be taken into account in the prediction of the properties for such kind of molecules. The equilibrium bond lengths r_e(M–X), force constants and harmonic vibrational frequencies are calculated for the ground and lower-lying excited electronic states. The spin-orbit coupling (SOC) effects are analysed.     

Structure of proton disolvates formed in the acid hydrolysis of ethyl formate and methyl acetate

By the density functional method (B3LYP/6-31++G(d,p)) optimal structures of proton hetero and homo disolvates involving water molecules, ethyl formate, methyl acetate and products of their hydrolysis are calculated. The data on the structure of these ions and the strength of their H bonds are analyzed together with the results of a similar calculation previously performed for methyl formate. It is shown that in proton solvation by two molecules present in the solution during the hydrolysis of ethyl formate, methyl acetate, and methyl formate stable (X…H…X)⁺ or (X…H…Y)⁺ particles form. Structural and energy parameters of their O…H…O bridges obey the same regularities and are mainly determined by a difference in the proton affinity of X and Y molecules. Calculation results are compared to the data of a number of experimental studies of the acid hydrolysis of esters.