Encapsulation of methane molecules into carbon nanotubes

Methane gas (CH₄) is a chemical compound comprising a carbon atom surrounded by four hydrogen atoms, and carbon nanotubes have been proposed as possible molecular containers for the storage of such gases. In this paper, we investigate the interaction energy between a CH₄ molecule and a carbon nanotube using two different models for the CH₄ molecule, the first discrete and the second continuous. In the first model, we consider the total interaction as the sum of the individual interactions between each atom of the molecule and the nanotube. We first determine the interaction energy by assuming that the carbon atom and one of the hydrogen atoms lie on the axis of the tube with the other three hydrogen atoms offset from the axis. Symmetry is assumed with regard to the arrangement of the three hydrogen atoms surrounding the carbon atom on the axis. We then rotate the atomic position into 100 discrete orientations and determine the average interaction energy from all orientations. In the second model, we approximate the CH₄ molecule by assuming that the four hydrogen atoms are smeared over a spherical surface of a certain radius with the carbon atom located at the center of the sphere. The total interaction energy between the CH₄ molecule and the carbon nanotube for this model is calculated as the sum of the individual interaction energies between both the carbon atom and the spherical surface and the carbon nanotube. These models are analyzed to determine the dimensions of the particular nanotubes which will readily suck-up CH₄ molecules. Our results determine the minimum and maximum interaction energies required for CH₄ encapsulation in different tube sizes, and establish the second model of the CH₄ molecule as a simple and elegant model which might be exploited for other problems.     

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.     

Relative intensities in x-ray photoelectron spectra. Part VI. The spectra of He(I), He(II), Y Mζ and Zr Mζ

The 2s- and 2p-electron photoionization cross-sections at photon energies up to 190 eV have been calculated, using the RPAE method for averaged configurations of the C, N, O and Ne atoms. The RPAE method ensures a more accurate relation between the cross-sections, 2s/2p, than that obtained using the Hartree—Fock method. Within the framework of the Gelius—Siegbahn model, but with the use of theoretical atomic cross-sections, we have calculated the photoionization cross-sections for He(I), He(II), Y Mζ, Zr Mζ for CH₄, C₂H₆, C₃H₈, C₂H₄, C₂H₂, NH₃, H₂O, CN^?, N₂, CO, CO₂, N₂O and NO₂^? molecules. For CO, N₂, CO₂, N₂O and H₂O molecules, a comparison is made between the theoretical and experimental cross-sections for hν < 60 eV. The calculated absolute and relative values of the molecular-orbital cross-sections are in reasonable agreement with experiment, especially at hν ? 40 eV. The calculations correctly reproduce the change in intensities under the transition He(I) → He(II). We have shown that our calculations have a significant advantage over those performed using the PW and OPW approximations. It is shown for NO, N₂, CO, H₂O, CH₄, NH₃ and N₂O molecules that the total photoionization cross-section calculated taking into account the real structure of the molecular orbitals is in better agreement with the experimental photoabsorption cross-section than is the sum of the cross-sections for the atoms in a molecule.     

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.     

Ionization and dissociative ionization of methane molecules

A technique for mass-spectrometric investigation of the yield of positive ions produced by direct and electron-impact dissociative ionization of methane molecules is described, and respective experimental data are presented. Doubly charged C ₂ ⁺ , CH ₃ ²⁺ , and CH ₄ ²⁺ ions, as well as singly charged D ₂ ⁺ , CD ₃ ⁺ , and CD ₄ ⁺ ions, are detected in the mass spectrum of a methane molecule at electron energy U _e = 90 eV for the first time. From ionization efficiency curves, the ionization energy of the parent molecule and the appearance energy of fragment ions are determined. The ionization energy of the CH₄ molecule is found to be 12.62 ± 0.20 eV. Electron-molecular reactions that may take place when a low-energy electron beam interacts with a methane molecule are analyzed. The ionization process and the formation of methane molecule fragments are studied.     

Rotational analysis of vibrational polyads in tetrahedral molecules: Line parameters of the infrared spectrum of 12CH4 in the range 2250–3260 cm−1—Theory versus experiment

Experimental and theoretical line parameters of the infrared spectrum of ¹²CH₄ in the range 2250–3260 cm^?1 covering the pentad ν₁, ν₃, 2ν₂, ν₂+ν₄, and 2ν₄ are reported. The individual line strengths are reproduced with a relative precision of 12% comparable to the experimental accuracy. In all, 6499 transitions have been calculated in the spectral region 2250–3260 cm^?1. Their intensities range from 2.5 to 213 000 × 10^?24 cm/molecule. Virtually all the absorptions of ¹²CH₄ in this range are satisfactorily reproduced.     

Geometric shielding corrections for total cross section calculations of electron scattering by CH4, C2H6, C2H3F3, C2H4, C2F4, C2Cl4 and C2Cl2F2 from 30–5000?eV

To quantify the changes in the geometric shielding effect in a molecule as the incident electron energy varies, an empirical fraction, which represents the total cross section contributions of shielded atoms in a molecule at different energies, is presented. Using this empirical fraction, the total cross sections for electron scattering by CH₄, C₂H₆, C₂H₃F₃, C₂H₄, C₂F₄, C₂Cl₄ and C₂Cl₂F₂ are calculated over a wide energy range from 30 to 5000 eV by the additivity rule model at the Hartree-Fock level. The quantitative total cross sections are compared with those obtained by experiment and other theories where available. Good agreement is attained above 100 eV.     

Screening effects in the ionization of molecules by positrons

Theoretical DWBA calculations for the ionization of N₂, CO, CO₂ and CH₄ molecules are presented. Previous calculations are improved by calculating the wavefunctions of the ejected electron and of the positron in the spherically averaged, screened field of the molecule. Results for N₂ and CO are in very good agreement with the experimental data, while those for CO₂ and CH₄ need more improvement.     

Inner-shell excitation and EXAFS-type phenomena in the chloromethanes

Small angle inelastic scattering of 2.5 keV electrons was used to study the inner-shell excitation of CH₄, CH₃Cl, CH₂Cl₂, CHCl₃, CCl₄ and C₂H₅Cl in the regions of carbon 1s, chlorine 2p and chlorine 2s excitation. Structure observed below the carbon 1s ionization threshold in each molecule is assigned to promotions of a carbon 1s electron to unoccupied valence and Rydberg orbitals. Trends in the distribution of spectral intensities through the series of chloromethane carbon 1s spectra are discussed in terms of the growth of a potential barrier. Broad features are observed in the chlorine 2p continua of CH₂Cl₂, CHCl₃ and CCl₄ and the carbon 1s continuum of CCl₄ which are assigned as the energy loss equivalent of extended X-ray absorption fine structure (EXAFS).     

Nitromethane pyrolysis in shock tubes and a micro flow reactor with a controlled temperature profile

Nitromethane has many applications, such as in racing, as a gasoline fuel additive, and as a monopropellant. Despite a large number of studies and the small size of the molecule, the combustion chemistry of nitromethane is still not well understood. To improve models, the pyrolysis of nitromethane (CH₃NO₂) was investigated experimentally in shock tubes and in a micro flow reactor with a controlled temperature profile (MFR), under dilute conditions. Several spectroscopic diagnostics were used in the shock tubes to follow the concentration time histories of CO, H₂O (both using IR laser absorption), and CH₃NO₂ (UV light absorption). A quadrupole mass spectrometer was used to measure CH₃NO₂, NO₂, CH₄, C₂H₄, and C₂H₂ at various temperatures with the MFR. These unique experimental results were compared to modern, detailed kinetics models from the literature, and no mechanism was able to reproduce these data over the wide range of conditions investigated. Predictions for the CO and H₂O levels were generally inaccurate, and the CH₄, C₂H₄, and C₂H₂ predictions were poor in most cases for the MFR data. Importantly, all models largely differ in their predictions. A numerical analysis was performed to identify ways to improve the next generation of nitromethane models. Results indicate that nitromethane decomposition needs to be improved below 1050 K, and that hydrocarbon-NOx interactions still need to be further investigated.     

Rotational diffusion of carbon monoxide in various simple liquids

Infra-red spectra of CO dissolved in liquid N₂, O₂, Ar, Xe and CH₄ have been recorded both in the fundamental and the first overtone. From the band profiles, rotational correlation functions, band moments and intermolecular mean-square torques have been calculated. With the help of Gordon's rotational diffusion model [1] we computed the theoretical correlation functions in the ‘M-diffusion’ (including or not a distribution of angular momentum correlation times) and ‘J-diffusion’ limits. The validity of the Gordon and Debye diffusion models in the various dense gases is discussed.     

Core photoelectron spectroscopy of some acetylenic molecules

Carbon 1s binding energies have been measured for CH₃CCH, CH₃CCCH₃, CF₃CCH and CF₃CCCF₃ and compared to a verified value for acetylene. Assignments are based on the application of a CNDO potential model with relaxation corrections which is quite successful in predicting binding energy shifts and upon qualitative considerations. Substitution of CF₃ groups shifts the acetylenic C 1s binding energy from 291.2 (HCCH) to 292.2 in CF₃CCH and 292.7 eV in CF₃CCCF₃. The unequal substitutional shifts are probably due to a saturation of substituent effect expected in competitive situations. With reservations arising from uncertainties in assignment due to lack of resolution, it appears that acetylenic C 1s binding energies decrease [to 290.7 (av.) in CH₃CCH and to 290.1 eV in CH₃CCCH₃] upon replacement of H by CH₃ groups. Although the decrease in acetylenic binding energies agrees with the chemical notion that CH₃ groups are electron donating with respect to unsaturated portions of the molecule, theoretical calculations available in the literature indicate that actual electron withdrawal or donation does not occur in these differently substituted molecules. The shifts of apparent binding energy correlate reasonably well with a ground state potential model which accounts for the effect of the charge on the adjacent atoms as well as on the photoionized atom. Even better correlation is obtained if the atomic potentials are corrected for electronic redistribution (relaxation) effects which occur during the photoionization process, and it is suggested that relaxation effects make a significant contribution to shifts of apparent binding energies. Surprisingly ground state potential and relaxation corrected potential calculations with the CNDO method suggest a large difference in C 1s binding energies of the two acetylenic carbon atoms in CH₃CCH which is not verified experimentally nor mirrored by calculations on CF₃CCH. The CH₃ binding energies are 291.8 eV in CH₃CCH and 291.3 eV in CH₃CCCH₃, both higher than values assigned to CH₄ or C₂H₆.     

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.     

Total cross-sections for positron scattering by a series of molecules

The additivity rule is employed to obtain the total (elastic+inelastic) cross-sections for positron scattering from molecules including a number of diatomic, polyatomic molecules (H₂, N₂, HCl, CO₂, NH₃, SF₆, CH₄, C₂H₄ and C₃H₈) over an incident energy range of 10-1000 eV. The total cross-sections (TCS) of the constituent atoms of molecules are obtained by employing a complex optical model potential (composed of static, polarization and absorption potential). The present results are compared with experimental data and other theoretical calculations, good agreement is obtained in intermediate- and high-energy region. Received: 11 November 1997 / Revised: 23 March 1998 / Accepted: 16 June 1998     

Vibrational assignments for styrene-β-D2 from the infrared and Raman spectra

The molecule styrene-β-D₂ has been prepared. The liquid-phase infrared spectrum in the region 400 to 3500 cm^?1 and the laser Raman spectrum have been recorded. Vibrational assignments for this molecule have been made largely by comparison with those of Condirston and Laposa (2) for C₆H₅CHCH₂, C₆H₅CDCD₂, C₆D₅CHCH₂, and C₆D₅CDCD₂.     

Ozone generation in oxygen-fed wire-to-cylinder ozonizer: effect of CH2Cl2, CHCl3 and CCl4 diluents

The inhibition effect of methylene chloride CH₂Cl₂, chloroform CHCl₃ and carbon tetrachloride CCl₄ on the ozone generation process from oxygen by negative corona discharge was experimentally investigated. The experiments were performed in a system of coaxial cylindrical electrodes at total gas pressure of 900 mbar and ambient temperature of gaseous mixtures. The rate of ozone generation as well as ozone concentration apparently decreases with a rising number of substituted chlorine atoms in the methane molecule at constant specific energy consumption dissipated in the discharge and at constant concentration of gaseous impurities in oxygen. In addition to experimental results, the paper presents theoretical considerations aimed at identifying the main process responsible for inhibition of ozone generation. The consumption of considerable fraction of oxygen atoms by CH _x Cl _y–1 radicals formed in discharge, is likely the most important mechanism responsible for the deleterious effect of such compounds on the efficiency of ozone production.     

IR studies of hydrogen bonded complexes formed by CH donors in cryogenic solutions

We have studied the vibrational spectra of weak hydrogen-bonded complexes formed by C₂H₂, CH₃CCH and their deuterated derivatives (C₂D₂, C₂HD, CH₃CCD) as proton donors with (CH₃)₂O and (CH₃)₃N as proton acceptors. The normal coordinate analysis has been carried out for all isotopic species of the acetylene-trimethylamine complex. Calculated frequencies and relative band intensities for the acetylene molecule bound to (CH₃)₃N are in accord with the experimental data.     

Analysis of the CH3D Nonad from 2000 to 3300 cm

As part of the simultaneous analysis of line positions and intensities of the first two polyads of monodeuterated methane, the results achieved for the region 3-5 μm are reported. It involves the three highest fundamentals, (ν₁, ν₂, ν₄), overlapped by overtone (2ν₃, 2ν₅, 2ν₆) and combination (ν₃+ν₆, ν₃+ν₅, ν₅+ν₆) bands. The theoretical model was based on the global tensorial model implemented in the MIRS package. Some 10 000 line positions and 2400 line intensities have been modeled to ±0.000 88 cm⁻¹ and ±3.6% respectively, using measurements obtained at 0.0056 and 0.011 cm⁻¹ resolution with the Fourier transform spectrometer at National Solar Observatory located at Kitt Peak. The strongest band in this polyad is ν₄(E) at 3016.7 cm⁻¹ with a strength of 6.3×10⁻¹⁸ cm⁻¹/(molecule cm⁻²) at 296 K; the weakest band is 2ν₃(E) at 2597.7 cm⁻¹ with a strength of 1.9×10⁻²⁰ cm⁻¹/(molecule cm⁻²) at 296 K. The total calculated absorption arising from the CH₃D nonad is 8.95×10⁻¹⁸ cm⁻¹/(molecule cm⁻²) at 296 K.     

Calculated polar substituent effects on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations: a DFT study

The effects of substituents on the stability of 3‐substituted(X) bicyclo[1.1.1]pent‐1‐yl cations (3) and 4‐substituted(X) bicyclo[2.2.1]hept‐1‐yl cations (4), for a set of substituents (X = H, NO₂, CN, NC, CF₃, CHO, COOH , F, Cl, HO, NH₂, CH₃, SiH₃, Si(CH₃)₃, Li, O^?, and NH₃⁺) covering a wide range of electronic substituent effects were calculated using the DFT theoretical model at the B3LYP/6‐311 + G(2d,p) and B3LYP/6‐31 + G (d) levels of theory, respectively. Linear regression analysis was employed to explore the relationship between the calculated relative hydride affinities (ΔE, kcal/mol) of the appropriate isodesmic reactions for 3/4 and polar field/group electronegativity substituent constants (σ_F and σ_χ, respectively). The analysis reveals that the ΔE values for both systems are best described by a combination of both substituent constants. The result highlights the importance of the σ_χ dependency of charge delocalization in these systems. Copyright © 2012 John Wiley & Sons, Ltd.     

以Ne原子为联合原子的系列分子里德伯能级结构

本文在独立电子近似的基础上,利用多重散射自洽场理论方法,系统地计算了含10个电子的分子B₂,HF,H₂O,NH₃,CH₄的里德伯能级结构,通过分析这些分子量子数亏损的变化规律,阐明了其激发态的电子能级结构,根据分子电子组态的联合原子极限,确定了各里德伯初始态的主量子数,量子数亏损的计算值与已有的实验值符合良好。 关键词：