Site–site potential function and second virial coefficients for linear molecules

The second virial coefficients for several linear molecules were calculated using the 2CLJ potential including the electrostatic and induction effects with modified mixing rules for unlike pairs. Least squares fits of experimental values for B(T) were used to calculate the energy parameters σ and ε in the LJ core potential for N₂, O₂, Cl₂, F₂, CO, CO₂, NO, N₂O, C₂H₆, C₂F₆ and the strongly polar molecules CH₃Cl, CH₃F, CH₃CF₃, CH₃CHF₂, and CF₃CH₂F. The analysis takes into account rotation of the dipole out of the molecular axis. The calculated results for the second virial coefficient agree well with experimental data. In addition, the effect of the induction terms on the potential for calculating the second virial coefficient is shown to be important only for the molecules with strong dipole or quadrupole moments.     

Spectra and structure of organogermanes: Microwave and Raman spectra of methyltrichlorogermane

The microwave spectrum of methyltrichlorogermane has been investigated in the region 26.5 to 40.0 GHz. The ground state rotational constants, B, were found to be 1602.19, 1601.42, 1601.10, 1600.71, 1600.02, 1537.84, 1537.10, and 1536.36 MHz for the symmetric top molecules CH₃⁷⁰Ge³⁵Cl₃, CH₃⁷²Ge³⁵Cl₃, CH₃⁷³Ge³⁵Cl₃, CH₃⁷⁴Ge³⁵Cl₃, CH₃⁷⁶Ge³⁵Cl₃, CH₃⁷⁰Ge³⁷Cl₃, CH₃⁷²Ge³⁷Cl₃, and CH₃⁷⁴Ge³⁷Cl₃, respectively. For the asymmetric top molecules CH₃⁷²Ge³⁵Cl₂³⁷Cl and CH₃⁷⁴Ge³⁵Cl₂³⁷Cl the ground state rotational constants A, B, and C were found to be 1597.96, 1559.31, 1203 and 1597.17, 1558.59, 1207 MHz, respectively. From the rotational constants the r_s values for the GeCl bond distance of 2.135 ± 0.006 Å and the CGeCl bond angle of 106.0 ± 0.7° were obtained. The centrifugal distortion constant for the CH₃Ge³⁵Cl₃ species was calculated to be 0.35 ± 0.08 kHz. The Raman spectra of methyltrichlorogermane has been recorded in the gas phase and the methyl torsional overtone (Δν = 2) was observed. From the observed frequency shift the barrier to internal rotation has been calculated to be 1.45 kcal/mole.     

Influence of the basis set and convergence in ab-initio valence-bond calculations

Extended and minimal basis set ab-initio valence-bond (VB) calculations have been performed for the following molecules: LiH, CH, ¹CH₂, ³CH₂, CH₃, NH₃ and H₂O. The influence of the choice of the basis sets and of the structures included on the values of a few observables, including energy, geometry, hyperfine coupling constants and dipole moment, has been studied.     

Proton and deuteron magnetic resonance of methanes,silanes and GeH3D dissolved in nematic liquid crystals

The proton and deuteron magnetic resonance spectra of CH₄, CH₃D, CH₂D₂, CHD₃, CD₄, SiH₄, SiH₃D, SiH₂D₂, SiH₃D, SiD₄, GeH₃D, dissolved in nematic liquid crystals, are reported. It was found that these molecules, which are essentially tetrahedral, exhibit anisotropic interactions and are partially oriented in the nematic phase. This effect is presumably due to slight deformations induced by the anisotropic medium. Some of the aspects related to the interpretation of the results are discussed.     

Accurate equilibrium structures of fluoro- and chloroderivatives of methane

This work is a systematic study of molecular structure of fluoro-, chloro-, and fluorochloromethanes. For the first time, the accurate ab initio structure is computed for 10 molecules (CF₄, CClF₃, CCl₂F₂, CCl₃F, CHClF₂, CHCl₂F, CH₂F₂, CH₂ClF, CH₂Cl₂, and CCl₄) at the coupled cluster level of electronic structure theory including single and double excitations augmented by a perturbational estimate of the effects of connected triple excitations [CCSD(T)] with all electrons being correlated and Gaussian basis sets of at least quadruple-ζ quality. Furthermore, when possible, namely for the molecules CH₂F₂, CH₂Cl₂, CH₂ClF, CHClF₂, and CCl₂F₂, accurate semi-experimental equilibrium (r^SE_e) structure has also been determined. This is achieved through a least-squares structural refinement procedure based on the equilibrium rotational constants of all available isotopomers, determined by correcting the experimental ground-state rotational constants with computed ab initio vibration–rotation interaction constants and electronic g-factors. The computed and semi-experimental equilibrium structures are in excellent agreement with each other, but the r^SE_e structure is generally more accurate, in particular for the CF and CCl bond lengths. The carbon–halogen bond length is discussed within the framework of the ligand close-packing model as a function of the atomic charges. For this purpose, the accurate equilibrium structures of some other molecules with alternative ligands, such as CH₃Li, CF₃CCH, and CF₃CN, are also computed.     

The centrifugal distortion constant DK of symmetric top molecules

The centrifugal distortion constant D_K of a symmetric top molecule has proved a very elusive parameter to estimate by spectroscopic analysis. Values of D_K have been calculated for the molecules BF₃, SO₃, NH₃, PH₃, AsH₃, NF₃, PF₃, AsF₃, CH₃D, SiH₃D, GeH₃D, CH₃F, CH₃Cl, CH₃Br, CH₃I, CH_eCN, CH₃CCH, H₂C₃H₂, cyclo-C₃H₆, and isotopic modifications thereof, in terms of their quadratic force constants. In all cases the force constants used are reliably determined by existing spectroscopic data. Where direct comparison is possible, good agreement with experimental D_K values is found, and the calculated constants are considered reliable to ± 5%. The constants are intended to be of assistance in future spectroscopic and structural studies on the molecules listed. Structural applications to methyl iodide, silane, and cyclopropane are considered.     

Distinct transport properties of O2 and CH4 across a carbon nanotube

It is of fundamental importance to investigate either O₂ or CH₄ molecules across nanochannels in many areas such as breathing or separation. Thus, many researches have focused on such a single type of molecules across nanochannels. However, O₂ and CH₄ can often appear together and crucially affect human life, say, in a mine. On the basis of molecular dynamics simulations, here we attempt to investigate the mixture of O₂ and CH₄, in order to identify their different transport properties in a nanochannel. We take a single-walled carbon nanotube (SWCNT) as a model nanochannel, and find that their transport properties are distinctly different. As the concentration of O₂ increases up to a high value of 0.8, it is always faster for CH₄ molecules to transport across the SWCNT, and the total number of gas molecules transporting across the SWCNT is decreased. Meanwhile, CH₄ molecules are always dominant in the SWCNT, and the total number of O₂ or CH₄ inside the SWCNT is a constant. By calculating the van der Waals interaction between the SWCNT and O₂ or CH₄, we find that the net interaction between CH₄ and the SWCNT is much stronger. Our findings may offer some hints on how to separate CH₄ from O₂, and/or store CH₄ efficiently.     

Atomic level analysis of biomolecules by the scanning atom probe

Utilizing the unique features of the scanning atom probe (SAP) the binding states of the biomolecules, leucine and methionine, are investigated at atomic level. The molecules are mass analyzed by detecting a single atom and/or clustering atoms field evaporated from a specimen surface. Since the field evaporation is a static process, the evaporated clustering atoms are closely related with the binding between atoms forming the molecules. For example, many thiophene radicals are detected when polythiophene is mass analyzed by the SAP. In the present study the specimens are prepared by immersing a micro cotton ball of single walled carbon nanotubes (SWCNT) in the leucine or methionine solution. The mass spectra obtained by analyzing the cotton balls exhibit singly and doubly ionized carbon ions of SWCNT and the characteristic fragments of the molecules, CH₃, CHCH₃, C₄H₇, CHNH₂ and COOH for leucine and CH₃, SCH₃, C₂H₄, C₄H₇, CHNH₂ and COOH for methionine.     

Microwave spectrum of two top molecules in the excited states dimethyl ether, dimethyl sulfide, and dimethyl silane

Microwave spectra of dimethyl ether, dimethyl sulfide, and dimethyl silane in the torsionally excited states have been measured. The methyl internal rotations of these molecules were analyzed from the observed multiplets and from the reported multiplets of transitions. The method developed for ethyl silane in the previous paper was extended to equivalent two top molecules. For equivalent two top molecules, apparent barriers of methyl internal rotations obtained from the experiments were corrected by the kinetic and potential cross terms. They are V₃=2645.8±10.0, 2632.4 ± 42.9, 2146.0 ± 13.8, 1651.5 ± 10.1, 1648.0 ± 13.7, and 1649.9 ± 11.8 cal/mol for (CH₃)₂O, (CD₃)₂O, (CH₃)₂S, (CH₃)₂SiH₂, (CH₃)₂SiD₂, and (CH₃)₂SiHD, respectively. The potential cross terms, V₁₂(1−cos3α₁)(1−cos3α₂) terms are negligible for the three molecules, while V^′₁₂sin3α₁sin3α₂ terms are also very close to zero except those for (CH₃)₂O and (CD₃)₂O which are small but not negligible (V^′₁₂=−124.4,−158.0 cal/mol). The investigations were extended to those of non-equivalent two top species and the corrected barriers of the methyl tops, V₃, are obtained to be 2615.6 ± 8.6 and 2155.0 ± 15.2 cal/mol for CH₃OCD₃ and CH₃SCD₃. The corrected barrier, V₃(CD₃) of CH₃OCD₃, is obtained to be 2634.4 ± 7.1 cal/mol, while that of CH₃SCD₃ cannot be solved due to the lack of the data available.     

NO-methanol interaction on the surface of Pt(3 3 2)

The interaction between NO and CH₃OH on the surface of stepped Pt(3 3 2) was investigated using Fourier transform infra red reflection-absorption spectroscopy (FTIR-RAS) and thermal desorption spectroscopy (TDS). At 90 K, pre-dosed CH₃OH molecules preferentially adsorb on step sites, suppressing the adsorption of NO molecules on the same sites. However, due to a much stronger interaction with Pt, at 150 K and higher, the adsorption of NO molecules on step sites is restored, giving rise to peaks closely resembling those of NO molecules adsorbed on clean Pt(3 3 2) surface. Adsorbed CH₃OH is very reactive on this surface, and is readily oxidized to formate in the presence of O₂, even at 150 K. In contrast, reactions between CH₃OH and co-adsorbed NO are slight to non-existent. There are no new peaks in association with intermediates resulting from CH₃OH-NO interactions. It is concluded that the reduction of NO with CH₃OH on Pt(3 3 2) does not proceed through a mechanism of forming intermediates.     

Structures and thermochemistry of methyl ethyl sulfide and its hydroperoxides: HOOCH2SCH2CH3, CH3SCH(OOH)CH3, CH3SCH2CH2OOH,and radicals

Hydroperoxides and the corresponding peroxy radicals are important intermediates during the partial oxidation of methyl ethyl sulfide (CH₃SCH₂CH₃) in both atmospheric chemistry and in combustion. Structural parameters, internal rotor potentials, bond dissociation energies, and thermochemical properties (ΔH_f^o, S^o and C_p(T)) of 3 corresponding hydroperoxides CH₂(OOH)SCH₂CH₃, CH₃SCH(OOH)CH₃, CH₃SCH₂CH₂OOH of methyl ethyl sulfides, and the radicals formed via loss of a hydrogen atom are important to understanding the oxidation reactions of MES. The lowest energy molecular structures were identified using the density functional B3LYP/6‐311G(2d,d,p) level of theory. Standard enthalpies of formation (ΔH_f^o₂₉₈) for the radicals and their parent molecules were calculated using the density functional B3LYP/6‐31G(d,p), B3LYP/6‐31 + G(2d,p), and the composite CBS‐QB3 ab initio methods. Isodesmic reactions were used to determine ?H_f^o values. Internal rotation potential energy diagrams and rotation barriers were investigated using the B3LYP/6‐31G(d,p) level theory. Contributions for S^o₂₉₈ and C_p(T) were calculated using the rigid rotor harmonic oscillator approximation based on the structures and vibrational frequencies obtained by the density functional calculations, with contributions from torsion frequencies replaced by internal rotor contributions. The recommended values for enthalpies of formation of the most stable conformers of CH₃SCH₂CH_2, CH₂(OOH)SCH₂CH₃, CH₃SCH(OOH)CH₃, and CH₃SCH₂CH₂OOH are ?14.0, ?33.0, ?37.2, and ?32.7 kcal/mol, respectively. Group additivity values were developed for estimating properties of structurally similar and larger sulfur‐containing peroxides. Groups for use in group additivity estimation of sulfur peroxide thermochemical properties were developed.     

Flame spectra from the interaction of fluorine and some halogen-containing compounds

The cold flames generated by the interaction of gaseous fluorine and CH₃I, C₂H₅I, CH₂Cl₂, CHCl₃, and CCl₄ are described. The spectra of these flames were determined. The spectroscopic results are interpreted from the point of view of the occurrence of branching in fluorination chain reactions, when superequilibrium concentrations of chemically excited molecules are formed, the latter subsequently decaying into a radical and an atom (as in the case of the F₂+CH₃I system) or into a biradical and a saturated molecule (F₂+CH₂CI₂ system).In conclusion, the author wishes to express his gratitude to Academician V. N. Kondrat'ev, A. E. Shilov, and A. M. Chaikin for a discussion of the results and assistance in the work.     

Pressure broadened line widths of BrCN in the microwave region

The line widths of cyanogen bromide (BrCN) have been measured at room temperature (305 K) by using a double modulation microwave spectrograph. The self-broadening of two quadrupole hyperfine lines of the transition J = 3 → 4 has been measured. The foreign gas broadening by OCS, CO₂, N₂, CH₃CN, CH₃I, HCHO and CH₃CHO molecules has been measured only for the intense line at 32·957 GHz. These measured line widths have been compared with the calculated line widths using Anderson [6] as well as Murphy and Boggs [8] theories of pressure broadening.     

A study on the GVFF of CHF3, CH2F2, and CH3F

The complete GVFF of CHF₃, CH₂F₂, and CH₃F has been calculated from self-consistent-field ab initio energies, using a 4–31 G basis set. The larger part of the interaction force constants is close to those of the best available force fields from experimental data. Only one interaction term in CH₃F and the interaction force constants of the A₁ species in CH₂F₂ differ appreciably from the experimental ones. Using constraints from the ab initio studies we have improved the GVFF of CH₃F and CH₂F₂. It is shown that all comparable stretch-stretch interaction terms are of the same order of magnitude in the three molecules. The sign of all stretch/bend force constants are in accordance with those predicted by the hybrid orbital force field.     

Electro-optical parameters and Raman intensities of CH4, CH3D,CH2D2, CHD3 and CD4

The absolute Raman intensities and the depolarization ratios of the vibrational bands of gaseous CH₄, CH₃D, CH₂D₂, CHD₃ and CD₄ have been computed here using a compact formulation of the bond polarizability theory, in its zero and first-order approximations. The agreement with experimental values taken from the literature is very good for the first-order approximation, although the difference between both approximations is not very large for these molecules. The derivatives of the polarizability with respect to the symmetry coordinates of methane are given with signs that are physically meaningful.     

Total cross sections of electron scattering by molecules NF3, PF3, N(CH3)3, P(CH3)3, NH(CH3)2, PH(CH3)2, NH2CH3 and PH2CH3 at 30–5000 eV

Total cross sections of electron scattering by eight molecules NF₃, PF₃, N(CH₃)₃, P(CH₃)₃, NH(CH₃)₂, PH(CH₃)₂, NH₂CH₃ and PH₂CH₃, which have some structural similarities, are calculated at the Hartree-Fork level by the modified additivity rule approach [D.H. Shi, J.F. Sun, Z.L. Zhu, H. Ma, Y.F. Liu, Eur. Phys. J. D 45, 253 (2007); D.H. Shi, J.F. Sun, Y.F. Liu, Z.L. Zhu, X.D. Yang, Chin. Opt. Lett. 4, 192 (2006)]. The modified additivity rule approach takes into considerations that the contributions of the geometric shielding effect vary as the energy of incident electrons, the dimension of target molecule, the number of electrons in the molecule and the number of atoms constituting the molecule. The present investigations cover the impact energy range from 30 to 5000 eV. The quantitative total cross sections are compared with those obtained by experiments and other theories. Excellent agreement is observed even at energies of several tens of eV. It shows that the modified additivity rule approach is applicable to carry out the total cross section calculations of electron scattering by these molecules at intermediate and high energies, in particular over the energy range above 80 eV or so. It proves that the microscopic molecular properties, such as the geometrical size of the target and the number of atoms constituting the molecule, are of crucial importance in the TCS calculations. The new results for PH(CH₃)₂ and PH₂CH₃ are also presented at energies from 30 to 5000 eV, although no experimental and theoretical data are available for comparison. In the present calculations, the atoms are still represented by the spherical complex optical potential, which is composed of static, exchange, polarization and absorption terms.     

Coupled cluster investigation on the thermochemistry of dimethyl sulphide,dimethyl disulphide and their dissociation products: the problem of the enthalpy of formation of atomic sulphur

By means of coupled cluster theory and correlation consistent basis sets we investigated the thermochemistry of dimethyl sulphide (DMS), dimethyl disulphide (DMDS) and four closely related sulphur-containing molecules: CH₃SS, CH₃S, CH₃SH and CH₃CH₂SH. For the four closed-shell molecules studied, their enthalpies of formation (EOFs) were derived using bomb calorimetry. We found that the deviation of the EOF with respect to experiment was 0.96, 0.65, 1.24 and 1.29 kcal/mol, for CH₃SH, CH₃CH₂SH, DMS and DMDS, respectively, when ΔH_f,0 = 65.6 kcal/mol was utilised (JANAF value). However, if the recently proposed ΔH_f,0 = 66.2 kcal/mol was used to estimate EOF, the errors dropped to 0.36, 0.05, 0.64 and 0.09 kcal/mol, respectively. In contrast, for the CH₃SS radical, a better agreement with experiment was obtained if the 65.6 kcal/mol value was used. To compare with experiment avoiding the problem of the ΔH_f,0 (S), we determined the CH₃–S and CH₃–SS bond dissociation energies (BDEs) in CH₃S and CH₃SS. At the coupled cluster with singles doubles and perturbative triples correction level of theory, these values are 48.0 and 71.4 kcal/mol, respectively. The latter BDEs are 1.5 and 1.2 kcal/mol larger than the experimental values. The agreement can be considered to be acceptable if we take into consideration that these two radicals present important challenges when determining their EOFs. It is our hope that this work stimulates new studies which help elucidate the problem of the EOF of atomic sulphur.     

Perturbation of the internal vibrations of (CH3)2S and (CH3)2Se in matrix-isolated complexes with H2O and HCl: Indirect evidence for the trans lone pair effect in (CH3)2X molecules

The formation of hydrogen-bonded complexes (CH₃)₂X ? HA is characterized by small frequency perturbations of the internal vibrations of the base molecules in a low-temperature matrix. Apart from the ν_CXC vibrations the largest shifts are measured in the ν_CH3 spectral region. The unexpected large blue shifts, Δν_CH, are interpreted as an indirect evidence for the existence of the trans lone pair effect in methyl chalcogenides. Additional support is obtained from a comparison between the complexes of HCl and H₂O with (CH₃)₂O, (CH₃)₂S, and CH₃OH.     

Structural and thermochemical properties of methyl ethyl sulfide alcohols: HOCH2SCH2CH3, CH3SCH(OH)CH3, CH3SCH2CH2OH,and radicals corresponding to loss of H atom

Sulfide alkoxy radicals are important intermediates during the partial oxidation of alkyl sulfides in atmospheric chemistry and in combustion. The atmospheric reaction sequence to formation of the alkoxy radicals includes (1) initial reaction with OH to create a radical on a carbon site, (2) the carbon radical then associates with ³O₂ to form a peroxy radical, and (3) an NO radical reacts with the peroxy radical to form an alkoxy radical (RO?) plus NO₂. This study determines structural parameters, internal rotor potentials, bond dissociation energies, and thermochemical properties (Δ_fH°, S°, and C_p(T)) of 3 corresponding alcohols HOCH₂SCH₂CH₃, CH₃SCH(OH)CH₃, and CH₃SCH₂CH₂OH of methyl ethyl sulfides studied in order to characterize the thermochemistry of the respective alkoxy radicals. The lowest energy molecular structures were calculated using the B3LYP density functional level of theory with the 6‐311G(2d,d,p) basis set. Standard enthalpies of formation (Δ_fH°₂₉₈) for the radicals and their parent molecules were calculated using B3LYP/6‐31 + G(2d,p), CBS‐QB3, M062x/6‐311 + g(2d,p), and G3MP2B3 methods. Isodesmic reactions were used to determine ?_fH° values. Internal rotation potential energy diagrams and rotation barriers were investigated using the B3LYP/6‐31 + G(d,p) level theory. The contributions for S°₂₉₈ and C_p(T) were calculated using the rigid rotor harmonic oscillator approximation based on the structures and vibrational frequencies obtained by CBS‐QB3 calculations, with contributions from torsion frequencies replaced by internal rotor contributions. Group additivity and hydrogen bond increment values were developed for estimating properties of structurally similar and larger sulfur‐containing peroxide molecules and their radicals.     

An XPS—AES study of gaseous xanthates and related sulfur-containing compounds

Core XPS spectra for carbon, oxygen, and sulfur and KLL Auger spectra for sulfur in CH₃OCS₂CH₃, (CH₃OCS₂)₂, CS₂, and OCS have been measured and relaxation shifts determined for the sulfur atoms by combining the S 1s measurement with the S KLL (¹D₂) measurement.Relaxation shifts of the sulfur atoms were also estimated from CNDO results for the neutral and core-ionized molecules using the equivalent cores approximation. The results are in qualitative agreement with the measurements, but exaggerate the relaxation by about 100%.The results show that the bonding of the (CH₃O-) group in the two xanthate compounds is very similar. The ionization energies of the S and -S- atoms within the xanthate molecules differ from each other by 1.5 eV; this difference arises almost entirely from the initial-state charge distribution rather than from final-state relaxation. However, the ionization energies of similarly bonded sulfur atoms are nearly the same. The effect of the oxygen atom on the bonding of the carbon and -S- atoms in the (-CS₂-) group in the xanthate compounds is to increase the (C 1s-S 2p_{$\text{3}\text{2}$}) ionization energy difference from the value reported for aliphatic disulfides.