Molecular structure and properties of CH3BeF and CH3MgF

The molecules methylberyllium fluoride and methylmagnesium fluoride have been studied using a priori electronic structure theory. Self-consistent-field wavefunctions have been computed over a double zeta basis set of contracted gaussian functions. The geometrical structure of each molecule has been predicted assuming the three heavy atoms are collinear. For CH₃ BeF, the predicted C-Be and Be-F distances are 1.70 and 1.40 Å. For CH₃MgF, the analogous bond distances are 2.08 Å and 1.78 Å. A number of molecular properties have been predicted including dipole moments, which are 1.75 and 2.38 debye for CH₃BeF and CH₃MgF.     

SCF Deformation densities and electrostatic potentials of purines and pyrimidines

4-31G wave functions have been computed for five purines and pyrimidines. The calculated deformation densities have been partitioned into atomic fragments, which were integrated to yield atomic multipole moments. The transferability of atomic fragments between related molecules was verified by constructing model maps for uracil and guanine from appropriate fragments of cytosine and adenine. Model electrostatic potentials calculated from the moments of model atoms are similar to the corresponding 4-31G potentials. Comparison of 4-31G and 4-31G** deformation densities of cytosine provides simple rules for estimating the effects of polarization functions on the atomic multipole moments of most atom types occurring in the purines and pyrimidines. These rules were applied to the other molecules and yielded reasonable approximations for their molecular dipole moments. Substituting CH₃ for H has little effect on the deformation density beyond the substitution center.     

Isomerization of CH3O+=CHCH3 to CH2=O+CH2CH3

Ab initio calculations establish that CH₃O⁺=CHCH₃ (1) rearranges in gas phase isolation to CH₂=O⁺C₂H₅ (2) directly rather than through CH₃OCH₂CH ₂ ⁺ (3). The reaction is predicted to be antarafacial, in accord with the Woodward-Hoffmann (W-H) predictions. We predict an activation energy of 212.0 kJ/mol for this process at the QCISD(T)/6-311G**//MP2/6-311G** level. We also reinvestigated the degenerate rearrangement of CH₃O=CH ₂ ⁺ by a 1,3-sigmatropic shift. The W-H model is not a good one for the transition state (TS) for the latter reaction because the π bonding has been completely broken off. That TS is stabilized by three-center bonding between the carbons and the hydrogen being transferred. We also examined the questions of the importance of polarization functions on hydrogen and a set of outer valence functions on all the atoms in describing these hydrogen transfer TSs, and whether it is necessary to include these functions in the TS optimization runs. For the rearrangements we studied, polarization functions on hydrogen are crucial only for 1,2 hydrogen shifts. The 6-31G* basis set is adequate and good for the optimization of TSs of other ring sizes. For the 1,3 and 1,4 shifts we examined, a combination of both outer valence functions and polarization functions on hydrogen causes reductions in the computed activation energies ranging from 5.9 kJ/mol for the 1,4 shift at the RHF level to 15.6 kJ/mol for the 1,3 shift at the MP2 level.     

Dipole moments,transition moments,oscillator strengths,radiative lifetimes,and overtone intensities for CH and CH+ as computed by quasi-degenerate many-body perturbation theory

The correlated, size-consistent, ab initio effective valence-shell dipole operator (μ^v) method is used to calculate dipole moments and transition dipole moments of the CH molecule and transition dipole moments of the CH⁺ ion as a function of internuclear distance. The dipole and transition dipole moments computed here compare well with those of other accurate ab initio methods. The transition dipole moments are then used to calculate oscillator strengths and radiative lifetimes for the A → X and B → A transitions of the CH⁺ ion and the A → X transition of the CH molecule. Comparisons are made with the best available theoretical and experimental lifetimes. Finally, the CH ground-state dipole moment function is used to evaluate overtone intensities and to examine simple models of the CH overtone intensities in polyatomic molecules.     

Nonempirical calculations of dipole moments of molecules in semifloating gaussian basis sets

For the example of the calculation of the dipole moments of the HF, HCl, H₂O, NH₃, CO, H₂CO, CH₃F molecules in two-exponent and three-exponent Gaussian basis sets, we have studied the effect of including floating functions in the basis, directly giving the effect of polarization of the electron shell of the atom in the molecule. We have established a weak dependence of the calculated dipole moment on the dimensionality of the basis, the number of floating functions, and also the orbital exponents of the hydrogen atoms. The correction introduced by the floating functions in molecules with polar bonds is considerably greater than the correlation correction. The proposed approach allows us to decrease the dimensionality of the orbital basis by a factor of 1.5–2 without making the agreement with experiment worse.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 481–485, July–August, 1990.     

Vibration-rotation transition probabilities in CH+ and CD+

Vibrational dipole matrix elements and radiative transition probabilities have been evaluated from electric dipole moment functions for the X¹Σ⁺ states of CH⁺ and CD⁺, which were calculated from highly correlated electronic wavefunctions. The dipole moments in ν = 0 amount to 1.679 D (CH⁺) and 1.313 D (CD⁺), respectively. In comparison to other molecular ions the infrared transition probabilities are found to be rather small. For instance, the Einstein coefficient of spontaneous emission A¹₀ amounts to 1.63 s⁻¹ (CH⁺) and 0.19 s⁻¹ (CD⁺). Dipole moment functions of the neutral CH species in the X²Π and a⁴ Σ⁻ states have also been calculated and are compared with previous theoretical functions.     

Dimethylarsenazid (CH3)2AsN3, Dimethylwismutazid (CH3)2BiN3 und Dimethylthalliumzid (CH3)2TiN3

The reaction of (CH₃)₂AsJ and AgN₃ yields (CH₃)₂AsN₃; a colourless liquid (b. p. 136°C) which dissolves as a monomeric in benzene. (CH₃)₂BiN₃ is precipitated in form of colourless needles (dec. temp. 150°C) from an etherical solution of Bi(CH₃)₃ and HN₃. According to its vibrational and mass spectra the molecules are not associated although the (CH₃)₂BiN₃ is not soluble; dipole association of this polar molecules is assumed for the crystal structure. (CH₃)₂TlN₃ can be obtained from TI(CH₃)₃ and ClN₃ as well as from (CH₃)₂TlOH and HN₃ in form of colourless needles and leaves (dec. temp. 245°C). According to its vibrational spectra it has an ionic structure, (CH₃? Tl? CH₃)⁺N^?₃.     

Valence electronic structure of CH3F and CH3Cl: electron momentum distributions and separation energies

Fluoromethane (CH₃F) has been studied by binary (e,2e) coincidence spectroscopy at 1200 eV using non-coplanar symmetric kinematics. Separation energy spectra have been determined in the energy range up to 50 eV at azimuthal angles of 0° and 9°. The separation energy spectra and electron momentum distributions measured for the valence orbitals of CH₃F and CH₃Cl are compared with the results of calculations employing SCF wavefunctions and outer valence as well as extended 2ph—TDA Green function methods. Electron density and momentum density maps have been generated for all valence orbitals of both molecules using the SCF wavefunctions and are used to explain differences in the bonding properties of the halomethanes investigated here.     

DFT calculations of static dipole polarizabilities and hyperpolarizabilities for the boron clusters Bn (n=3–8,10)

The static electric dipole polarizabilities and second‐order hyperpolarizabilities of several bare boron clusters have been calculated with density functional theory. The average second‐order hyperpolarizability γ_av reaches a saturation limit of about 50,000 a.u. already with B₅ for a given type of structure. The average polarizability per atom shows overall a decrease with increasing cluster size, while the average second hyperpolarizability per atom first increases from B₃ to B₆, and then starts to decrease. For the noncentrosymmetric clusters dipole moments and first‐order hyperpolarizabilities are reported. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 78: 131–135, 2000     

TDDFT-SOS Studies on the Polarizabilities and Second-order Hyperpolarizabilities of Zn_3O_3 II-VI Semiconductor Clusters

1 INTRODUCTION II-VI semiconductor materials with ZnO as the representative possess wide forbidden gaps and high exciton binding energy, which have paved the way for the study of exciton characters and short wave photo-conducting devices. They present wide appli- cations, such as ultraviolet laser transmitting device, piezoelectric device, optical waveguild and high efficient quantum dot luminous apparatus[1～8]. Re- cently, II-VI semiconductor cluster materials with nano-structure ha…     

Geometry optimization and electronic structures of molecules H_3AXAH_3

The geometries of molecules H_3AXAH_3(X=O,S,Se and A=C,Si)have been optimizedusing STO-3G ab initio calculations and gradient method and the results are in good agreement withreported experimental values.From the STO-3G optimized geometries,we have also calculated theelectronic structures of these molecules using 4-31G and 6-31G basis sets to obtain the MO energies.atomic net charges and dipole moments.The ionization potentials calculated by 6-31G basis set are ingood agreement with experimental values.     

Gaussian wave functions for CH3 and NH

Ground state single determinant LCAO-MO-SCF wave functions, using a large contracted Gaussian basis set (6^s, 2^p, 1^d/3^s, 1^p), have been computed for the 9 electron molecular systems of CH₃ and NH. The minimum energies obtained using Roothaan's open shell SCF procedure for the planar equilibrium geometries were ?39.5703 Hartree for CH₃ and ?55.8945 Hartree for NH. Additional properties such as electron populations and multiple moments were calculated from the planar wave functions.     

The accurate calculation of dipole moments and dipole polarizabilities using Gaussian-based density functional methods

Dipole moments and static dipole polarizabilities have been calculated for a number of small molecules using the linear combination of Gaussian-type orbitals–local spin density method. The effect of augmenting standard orbital basis sets with polarization functions has been investigated. A set of optimum ζ_d, for use in calculating polarizabilities, has been derived for the first-row atoms C, N, O, and F. The results of this optimized doubly polarized double-zeta basis set compare well with results obtained using a double-zeta basis set augmented by four even-tempered ζ_d polarization functions. The results of the optimized basis set, and a basis set augmented with only a single ζ_d polarization function derived from it, compare very favorably with those obtained from Møller–Plesset perturbation theory and with experimental data. They show a marked improvement on results obtained using standard Hartree–Fock self-consistent-field molecular orbital methods where no treatment of electron-correlation is included.     

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.     

Dipole moments of (CH3)3MX compounds with bonds between halogens and group IV a elements: evidence for pX → dM bonding

The electric dipole moments of the two series of molecules (CH₃)₃MX with M  C, Si, Ge, Sn and Pb, and X  Cl and Br have been determined. The two carbon compounds show behaviour different from that of the others, and this is interpreted as indicating that p_X → d_M back-bonding is present for the other elements of the series, and it is suggested that the extent of this back-bonding decreases with an increase in the atomic number of M.     

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.     

Correlated dipole polarizabilities and dipole moments of the halides HX and CH3X (X=F,Cl and Br)

Summary We report values of the correlated dynamic dipole polarizability for the halides HX and CH₃X (X = F, Cl and Br). The polarizabilities are calculated within the second-order polarization propagator approximation (SOPPA). The correlated results are in much better agreement with the available experimental results, compared to RPA. We also report the second-order dipole moments using both the relaxed and unrelaxed MP2 density matrices. The relaxed results are in better agreement with experiment.     

Cross sections and spin polarizations of electrons elastically scattered from oriented molecules (CH3I)

Elastic differential cross sections and spin polarizations for electrons elastically scattered from CH₃I are calculated using the independent atom model. Three molecular orientations with respect to the incident electron wavevector are considered — first, the molecule is oriented randomly, second, the electron wave front and molecular bond are parallel, and third, the wavefront and the bond axis are perpendicular. It will be seen to what extent orientational averaging weakens features of the cross section and spin polarization. The calculations show that cross section and spin polarization measurements are a possible tool for determining the degree of molecular orientation. There is no degeneracy betweenI–C andC–I in cross section and spin polarization measurements. The results presented here for 200 eV and 600 eV electrons scattered by CH₃I should be considered as a case study and it should be possible to find molecules and electron energies for which even more dramatic differences between the various orientations between the molecules and the electrons can be expected.     

Gas phase structure and vibrational spectra of dimethoxysulfane (CH3O)2S

The vapor phase structure of (CH₃O)₂S [1] has been investigated by electron diffraction and ab initio MO calculations, which both result in a C₂ symmetry for the most stable geometry, the C_s conformer being less stable by about 12 kJ/mol. The torsional barrier for rotation about one SO bond was calculated as 37 kJ/mol (trans barrier). The geometrical parameters (electron diffraction) of the C₂ conformer are: d_SO = 162.5(2), d_CO = 142.6(3), d_CH = 110.5(7) pm angles OSO = 103(1)°, SOC = 115.9(4)°, HCH = 109(1)°, torsional angle COSO = 84(3)°. Geometrical data calculated with 6-31G^* basis set agree well with the diffraction data; calculated dipole moments 1.1 D (C₂) and 3.3 D (C_s). The infrared spectrum of gaseous (CH₃O)₂S and the Raman spectra of liquid and solid (CH₃O)₂S are reported and have been almost fully assigned to the 27 fundamental vibrations.     

Deactivation of I(2P1/2) by CH3Cl,CH2Cl2, CHCl3, CCl3F,and CCl4

Collisional deactivation of I(²P_1/2) by the title compounds was investigated through the use of the time-resolved atomic absorption of excited iodine atoms at 206.2 nm. Rate constants for atomic spin-orbit relaxation by CH₃Cl, CH₂Cl₂, CHCl₃, CCl₃F, and CCl₄ are 3.1±0.3×10⁻¹³, 1.28±0.08×10⁻¹³, 5.7±0.3×10⁻¹⁴, 3.9±0.4×10⁻¹⁵, and 2.3±0.3×10⁻¹⁵cm³ molecule⁻¹ s⁻¹, respectively, at room temperature (298 K). The higher efficiency observed for relaxation by CH₃Cl, CH₂Cl₂, and CHCl₃ reveals a contribution in the deactivation process of the first overtone corresponding to the C(SINGLEBOND)H stretching of the deactivating molecule (which lies close to 7603 cm⁻¹) as well as the number of the contributing modes and certain molecular properties such as the dipole moment. It is believed that, for these molecules, a quasi-resonant (E-v,r,t) energy transfer mechanism operates. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 799–803, 1998