Using a potential-energy surface obtained in part from ab initio calculations, the H + CH3 → CH4 bimolecular rate constant at T = 300 K is determined from a Monte Carlo classical trajectory study. Representing the CH stretching potential with a standard Morse function instead ofthe ab initio curve increases the calculated rate constant by an order of magnitude. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio stretching potentials.Two properties of the H + CH3 α CH4 potential-energy surface which significantly affect the recombination rate constant are the shape of the CH stretching potential and the attenuation of the H3CH bending frequencies. Ab initio calculations with a hierarchy of basis sets and treatment of electron correlation indicate the latter is properly described [13]. The exact shape of the CH stretching potential is not delineated by the ab initio calculations, since the ab initio calculations are not converged for bond lengths of 2.0–3.0 Å [12]. However, the form of this stretching potential deduced from the highest-level ab initio calculations, and fit analytically by eq. (2), is significantly different from a Morse function. The experimental recombination rate constant is intermediate of the rate constants calculated with the Morse and ab initio CH stretching potentials. This indicates that the actual CH potential energy curve lies between the Morse and ab initio curves. This is consistent with the finding that potential energy curves for diatomics are not well described by a Morse function [12]. 相似文献
The extended maximum overlap approximation (EMOA) method has been applied to a series of fluorine and chlorine derivatives of some alkanes in the present communication. Strictly localized molecular orbitais for 34 molecules have been constructed explicitly from hybrid atomic orbitals on IEHT level of approximation. The calculated heats of formation and dipole moments are in remarkable agreement with experimental data. Results are comparable with MENDO/3 and CNDO/2 calculations, respectively. Charge distributions show that the electron withdrawing effect of the halogen extends throughout the molecular skeleton with decreasing intensity; the inductive effects of the groups +I [CH3—] < +I [CH3CH2-] < +I [CH3CH2CH2-] < +I [(CH3)2CH—] < +I [(CH3)3C—] (positive) and —I [F—] > —I [Cl—] (negative) are perfectly reproduced. The s-electron density on carbon and proton in C—H bond correlates with the experimental carbon-13—proton nuclear spin-spin coupling constant in the series of some chloroethanes. 相似文献
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. 相似文献
Stretching vibration wavenumbers have been estimated for (NH4)2[VO(O2)2F] and (NH4)3[VO(O2)2F2] based on normal coordinate calculations employing a valence force field derived from bond length—force constant correlations. The results bear out the recently suggested re-assignment of the vanadium—ligand stretching vibrations. The effect of ligands on the asymmetry of the group is discussed using structural data for 19 vanadium (V) peroxo complexes. The asymmetry of the group is different in compounds with different coordination numbers. This difference is accompanied by characteristic shifts of the vanadium—peroxo oxygen stretching mode absorptions. 相似文献
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 D4h7 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. 相似文献
In view of existing contradictory assignments of the symmetrical stretching vibrations associated with the formal C-C and C-F bonds of trans/cis oxalyl fluoride, an additional theoretical analysis of the corresponding calculated wavenumbers was preformed on trans-C2O2F2 and cis-C2O2F2 based on previously calculated ab initio scaled force fields at the HF/6-31G computational level and new force fields calculated at the MP2/aug-cc-pVTZ level. This novel analysis included computational data from the isotopic shifts brought about by incorporating 13C and 14C atoms into the structure. A detailed examination of the calculated wavenumbers made it possible to validate the assignments of the ν2 and ν3 wavenumbers in the trans-C2O2F2 and cis-C2O2F2 molecules as the formal C-C bond stretching and the formal C-F bond symmetrical stretching vibrations, respectively. 相似文献
As part of a homologous series of novel polyfluorinated bipyridyl (bpy) ligands, the title compound, C16H14F6N2O2, contains the smallest fluorinated group, viz. CF3. The molecule resides on a crystallographic inversion centre at the mid‐point of the pyridine Cipso—Cipso bond. Therefore, the bpy skeleton lies in an anti conformation to avoid repulsion between the two pyridyl N atoms. Weak intramolecular C—H...N and C—H...O interactions are observed, similar to those in related polyfluorinated bpy–metal complexes. A π–π interaction is observed between the bpy rings of adjacent molecules and this is probably a primary driving force in crystallization. Weak intermolecular C—H...N hydrogen bonding is present between one of the CF3CH2– methylene H atoms and a pyridyl N atom related by translation along the [010] direction, in addition to weak benzyl‐type C—H...F interactions to atoms of the terminal CF3 group. It is of note that the O—CH2CF3 bond is almost perpendicular to the bpy plane. 相似文献
The influence of the substituents to the carbonyl group on the CO stretching frequency of organic carbonyl compounds was investigated by CNDO/2 calculation and analysis of the force constant and localized orbitals of the CO bond. The substituent-induced changes in the CO force constant could be split up in two additive contributions: an indirect, geometry-dependent influence and a direct, geometry-independent influence. From localized orbital calculations a relationship was found between the ionic characters of the carbonyl σ- and π-bonds and the CO bond length. 相似文献
The intrinsic bond strength of C2 in its 1Σg+ ground state is determined from its stretching force constant utilizing MR‐CISD+Q(8,8), MR‐AQCC(8,8), and single‐determinant coupled cluster calculations with triple and quadruple excitations. By referencing the CC stretching force constant to its local counterparts of ethane, ethylene, and acetylene, an intrinsic bond strength half way between that of a double bond and a triple bond is obtained. Diabatic MR‐CISD+Q results do not change this. Confinement of C2 and suitable reference molecules in a noble gas cage leads to compression, polarization, and charge transfer effects, which are quantified by the local CC stretching force constants and differences of correlated electron densities. These results are in line with two π bonds and a partial σ bond. Bond orders and bond dissociation energies of small hydrocarbons do not support quadruple bonding in C2. 相似文献
The equilibrium C-H bond length has been determined up to now for about 40 polyatomic molecules. These data are used to demonstrate the existence of quantitative correlations betweenre(C-H), isolated C-H bond stretching frequency and average distancerg. It is also shown that ab initio calculations are often reliable to calculate the absolute value ofre(C-H), if an empirical correction is made. Some other correlations are also discussed. Finally, accuratere(C-H) values are predicted for simple molecules. 相似文献
The asymmetric unit of O,O′‐dimethyl [(2,3,4,5,6‐pentafluorophenyl)hydrazinyl]phosphonate, C8H8F5N2O3P, is composed of two symmetry‐independent molecules with significant differences in the orientations of the C6F5 and OMe groups. In the crystal structure, a one‐dimensional assembly is mediated from classical N—H…O hydrogen bonds, which includes R22(8), D(2) and some higher‐order graph‐set motifs. By also considering weak C—H…O=P and C—H…O—C intermolecular interactions, a two‐dimensional network extends along the ab plane. The strengths of the hydrogen bonds were evaluated using quantum chemical calculations with the GAUSSIAN09 software package at the B3LYP/6‐311G(d,p) level of theory. The LP(O) to σ*(NH) and σ*(CH) charge‐transfer interactions were examined according to second‐order perturbation theory in natural bond orbital (NBO) methodology. The hydrogen‐bonded clusters of molecules, including N—H…O and C—H…O interactions, were constructed as input files for the calculations and the strengths of the hydrogen bonds are as follows: N—H…O [R22(8)] > N—H…O [D(2)] > C—H…O. The decomposed fingerprint plots show that the contribution portions of the F…H/H…F contacts in both molecules are the largest. 相似文献
A procedure is described which allows analysis of ab initio SCF—MO results in the framework of an OEMO model, in terms of total energy and a PMO quantitative approach. The procedure is applied to the analysis of the effects of non-bonded interactions upon the conformational preferences of molecules of the type CH3—X (with X = CH3, NH2, OH). The use of a basis of fragment localized MO's allows discussion of the energy effects in terms of bond—bond and bond—lone pair repulsions and conjugative stabilizations. It is found that the non-bonded interactions are responsible for the conformational preference and the rotational barrier in these molecules: in all cases the optimum conformation is characterized by the smallest bond—bond repulsion and the largest conjugative stabilization. The factors determining the asymmetries of the methyl groups in methylamine and methanol have also been investigated. 相似文献
A set of phosphine complexes of the type W(CO)3(PX3)2(CH2CH2) (X=H, CH3, F, Cl, Br, and I) were investigated by density functional theory method (BP86) to examine the effect of the substituent X on the orientation of C-C vector of the ethylene ligand with respect to one of the metal-ligand bonds as well as the donation and the backdonation in the bonding ligands of phosphine and ethylene. When X=CH3, H, F, and Cl, the ethylene C-C vector prefers to be coplanar with metal-phosphine bonds, while for the ethylene complexes containing PBr3 and PI3 ligands, the structural preference is coplanarity of the ethylene and the metal-carbonyl bonds. The molecular orbital calculations and natural bond orbital analysis were used to examine the structural consequences derived from these complexes. It can be concluded that the structural preferences in the complexes have a clear relation to electronic effects of phosphine ligands. Our calculations for halide phosphine complexes, particularly for PBr3 and PI3, allow us to conclude that in addition to electronic effects, steric factors can also affect the orientation of the ethylene ligand in complexes. 相似文献
In the crystal structure of O,O′‐diethyl N‐(2,4,6‐trimethylphenyl)thiophosphate, C13H22NO2PS, two symmetrically independent thiophosphoramide molecules are linked through N—H…S and N—H…π hydrogen bonds to form a noncentrosymmetric dimer, with Z′ = 2. The strengths of the hydrogen bonds were evaluated using density functional theory (DFT) at the M06‐2X level within the 6‐311++G(d,p) basis set, and by considering the quantum theory of atoms in molecules (QTAIM). It was found that the N—H…S hydrogen bond is slightly stronger than the N—H…π hydrogen bond. This is reflected in differences between the calculated N—H stretching frequencies of the isolated molecules and the frequencies of the same N—H units involved in the different hydrogen bonds of the hydrogen‐bonded dimer. For these hydrogen bonds, the corresponding charge transfers, i.e. lp (or π)→σ*, were studied, according to the second‐order perturbation theory in natural bond orbital (NBO) methodology. Hirshfeld surface analysis was applied for a detailed investigation of all the contacts participating in the crystal packing. 相似文献
In order to provide additional data for the relative lengths of methyl-C? H bond distances in acetyl derivatives, which are difficult to determine accurately by the conventional tools of structural chemistry, the geometries of CH3COH, CH3COF, CH3COCH3, CH3COOH, and CH3CONH2 were determined by ab initio SCF gradient optimization at the 5-31G** level and compared with previous 4-21G results. For acetaldehyde 6-311G4* calculations were also performed and the correlated methyl-C? H stretching potential energy functions were determined. It is found that the calculated differences between the in-plane and out-of-plane methyl-C? H bonds are practically independent of the computational scheme. The calculated results are in contrast to relative bond lengths obtained by some vibrational overtone spectroscopic studies, but are in perfect agreement with C? H bond length differences determined from isolated C? H stretching frequencies of partially deuterated compounds. The reliability of the latter, and other spectroscopic data concerning the assignment of the methyl-C? H vibrations are critically analyzed. On the basis of the available evidence we conclude: (1) the methyl groups of the CH3C(?O)X systems here discussed contain one strong (in-plane) and two weak (out-of-plane) C? H bonds; (2) intensities of C? H local mode spectra do not provide a reliable basis for assignment to individual bonds. 相似文献
The structures, energies, atomic chaiges and IR spectra of complexes (CH2)2O…XY (X, Y = H, F, Cl, Br, and I) have been examined by means of ab initio molecular orbital theory at the second-order level of Moller-Plesset perturbation correction. It is found that the hydrogen bond O…H-X is non-linear. The attraction between X and the H atoms in oxirane ring causes O…H-X bond bending. The hydrogen bond slighdy weakens the bond strength of C-O, and leads the bending and stretching mode of IR to shift to the red. The calculation results show that there is no evidence of a significant extent of proton transfer to give (CH2)2OH …X- in the isolated complexes. 相似文献
To enable a comparison between a C—H…X hydrogen bond and a halogen bond, the structures of two fluorous‐substituted pyridinium iodide salts have been determined. 4‐[(2,2‐Difluoroethoxy)methyl]pyridinium iodide, C8H10F2NO+·I−, (1), has a –CH2OCH2CF2H substituent at the para position of the pyridinium ring and 4‐[(3‐chloro‐2,2,3,3‐tetrafluoropropoxy)methyl]pyridinium iodide, C9H9ClF4NO+·I−, (2), has a –CH2OCH2CF2CF2Cl substituent at the para position of the pyridinium ring. In salt (1), the iodide anion is involved in one N—H…I and three C—H…I hydrogen bonds, which, together with C—H…F hydrogen bonds, link the cations and anions into a three‐dimensional network. For salt (2), the iodide anion is involved in one N—H…I hydrogen bond, two C—H…I hydrogen bonds and one C—Cl…I halogen bond; additional C—H…F and C—F…F interactions link the cations and anions into a three‐dimensional arrangement. 相似文献