Investigating CH4 Thermal Activation in Clathrate Nanocages

Solid State Communications, 1992

Basing upon new observations in the near-infrared and some simplifying assumptions, we evaluate the harmonic frequencies and anharmonic coefficients for a CH4 molecule vibrating in the anisotropic crystal field of phase II.

The Journal of Physical Chemistry B, 2006

Molecular dynamics simulations were performed on methane clathrate hydrates at ambient conditions. Thermal expansion results over the temperature range 60-300 K show that the unit cell volume increases with temperature in agreement with experiment. Power spectra were obtained at 273 K from velocity autocorrelation functions for selected atoms, and normal modes were assigned. The spectra were further classified according to individual atom types, allowing the assignment of contributions from methane molecules located in small and large cages within the structure I unit cell. The symmetric C-H stretch of methane in the small cages occurs at a higher frequency than for methane located in the large cages, with a peak separation of 14 cm-1. Additionally, we determined that the symmetric C-H stretch in methane gas occurs at the same frequency as methane in the large cages. Results of molecular dynamics simulations indicate the use of power spectra obtained from the velocity autocorrelation function is a reliable method to investigate the vibrational behavior of guest molecules in clathrate hydrates.

The Journal of Chemical Physics, 1998

The reactions of atomic chlorine with CH 4 and CD 4 were studied at five collision energies ranging from 0.13 to 0.29 eV using resonance-enhanced multiphoton ionization of the CH 3 and CD 3 products. Core-extracted ion arrival profiles were used to determine methyl radical product speed distributions. The distributions contain products that are moving anomalously fast which energetically cannot result from the reaction of ground-state chlorine with ground-state methane. We attribute these products to reaction of ground-state chlorine with methane vibrationally excited in trace quantities into low-energy bending and torsional modes. Measurements of product spatial anisotropy are used to confirm this interpretation and to indicate that the possible reaction of spin-orbit excited chlorine is less important. These low-energy vibrations create large enhancements in reactivity over ground-state molecules, and consequently, vibrationally excited reagents dominate reactivity at low collision energies and contribute substantially at the highest collision energies studied. It is suggested that vibrationally excited reagents play an important role in the thermal kinetics of the reaction of chlorine with methane and may contribute significantly to explain the observed deviation from Arrhenius equation behavior. Scattering distributions of the products of both ground-state and vibrationally excited reactions are reported, and additional measurements of the internal state distributions of the CH 3 and CD 3 products reveal that the methyl radicals contain very little energy in rotation or vibration.

2004

The rovibration partition function of CH4 was calculated in the temperature range of 100-1000 K using well-converged energy levels that were calculated by vibrational-rotational configuration interaction using the Watson Hamiltonian for total angular momenta J = 0-50 and the MULTIMODE computer program. The configuration state functions are products of ground-state occupied and virtual modals obtained using the vibrational self-consistent field

Electron scattering by methane molecule plays a crucial role in understanding the interaction dynamics between electrons and molecules In this research we focus on investigating the elastic and rotational excitation process for electron scattering by CH_4 within the energy range of 530 eVThe main objective is to calculate the differential and total cross sections to provide insights into the scattering behavior and energy transfer mechanisms involved in these collisions To achieve this advanced theoretical model and computational method are employed Accurate potential energy and interaction potentials specific to CH_4 are utilized to ensure reliable and precise calculations The calculated cross sections are compared with experimental data and previous theoretical studies to validate their accuracy and assess the effectiveness of the computational approach The results obtained from the calculations of the differential and total cross sections shed light on the angular distribution of the scattered electrons the potential energy governing the electronmolecule interaction and the rotational dynamics of CH_4 molecule during electron collisions These findings contribute to a comprehensive understanding of electronmolecule interactions and have implications in various scientific disciplines The research provides a comprehensive investigation of electron scattering by CH_4 and offers valuable insights into the underlying physics of elastic and rotational excitations The calculated cross sections serve as fundamental inputs for theoretical models and simulations enhancing our ability to predict and interpret experimental observations The outcomes of this study contribute to the broader field of electronmolecule interactions and pave the way for further advancements in this area of research

Journal of Physical Chemistry C, 2007

We report the results of a rigorous theoretical study of the quantum translation-rotation (T-R) dynamics of one, two, and three H 2 and D 2 molecules confined inside the small dodecahedral (H 2 O) 20 cage of the sII clathrate hydrate. For a single D 2 molecule, o-and p-D 2 , in the small cage, accurate quantum five-dimensional (5D) calculations of the T-R energy levels and wave functions are performed by diagonalizing the 5D Hamiltonian which includes explicitly, as fully coupled, all three translational and the two rotational degrees of freedom of D 2 , while the cage is taken to be rigid. These calculations provide a quantitative description of the quantum dynamics of D 2 inside the small cage and enable comparison with our quantum 5D results for the encapsulated H 2 , p-and o-H 2 , published very recently. The ground-state properties of one, two, and three p-H 2 and o-D 2 molecules in the small cage are calculated rigorously using the diffusion Monte Carlo method, with the emphasis on the quantum dynamics of two confined hydrogen molecules. The guest molecules are found to be effectively excluded from the sizable central region of the cage; they reside within a shell less than 2 bohrs wide and are additionally localized by the corrugation of the H 2 -cage interaction potential. The two H 2 molecules are compressed, their mean distance inside the cage being much smaller than in the free H 2 dimer.

Physical Review B, 2007

We have performed high-resolution inelastic neutron scattering studies on three samples of hydrogenated tetrahydrofuran-water clathrates, containing either H2 at different para/ortho concentrtion, or HD. By a refined analysis of the data, we are able to assign the spectral bands to rotational and center-of-mass translational transitions of either para- or ortho-H2. The H2 molecule rotates almost freely, while performing a translational motion (rattling) in the nanometric-size cage, resulting a paradigmatic example of quantum dynamics in a non-harmonic potential well. Both the H2 rotational transition and the fundamental of the rattling transition split into triplets, having different separation. The splitting is a consequence of a substantial anisotropy of the environment with respect to the orientation of the molecule in the cage, in the first case, or with respect to the center-of-mass position inside the cage, in the second case. The values of the transition frequencies and band intensities have been quantitatively related to the details of the interaction potential between H2 and the water molecules, with a very good agreement.

Scientific Reports, 2016

In porous materials the molecular confinement is often realized by means of weak Van der Waals interactions between the molecule and the pore surface. The understanding of the mechanism of such interactions is important for a number of applications. In order to establish the role of the confinement size we have studied the microscopic dynamics of molecular hydrogen stored in the nanocages of clathrate hydrates of two different dimensions. We have found that by varying the size of the pore the diffusive mobility of confined hydrogen can be modified in both directions, i.e. reduced or enhanced compared to that in the bulk solid at the same temperatures. In the small cages with a mean crystallographic radius of 3.95 Å the confinement reduces diffusive mobility by orders of magnitude. In contrast, in large cages with a mean radius of 4.75 Å hydrogen molecules displays diffusive jump motion between different equilibrium sites inside the cages, visible at temperatures where bulk H 2 is solid. The localization of H 2 molecules observed in small cages can promote improved functional properties valuable for hydrogen storage applications.

Chemical Physics Letters, 2011

A new potential energy surface of methane is constructed using extended ab initio CCSD(T) calculations at 19 882 points. Its analytical representation is determined through an expansion in symmetry adapted products of orthogonal coordinates involving 276 parameters up to 8th order with the equilibrium bond r e = 1.08601 ± 0.00004 Å and four quadratic parameters scaled to experimental fundamental vibration frequencies. Variational calculations give RMS (obs.-calc.) deviations of 0.085 and 0.25 cm À1 for vibration levels of the pentad and octad. Rotational energies up to J = 10 are calculated using potential expansion in normal coordinate tensors with maximum errors of 0.0007 and 0.0003 cm À1 for 12 CH 4 and 13 CH 4 .

The Journal of Physical Chemistry A, 2007

Higher-lying five-dimensional translation-rotation (T-R) eigenstates of a single p-H 2 and o-D 2 molecule confined inside the small dodecahedral (5 12 ) cage of the structure II clathrate hydrate are calculated rigorously, as fully coupled, with the cage assumed to be rigid. The calculations cover the excitation energies up to and beyond the j ) 2 rotational level of the free molecule, 356 cm -1 for H 2 and 179 cm -1 for D 2 . It is found that j is a good quantum number for all the T-R states of p-H 2 , j ) 0 and j ) 2, considered. The same is not true for o-D 2 , where a number of T-R states in the neighborhood of the j ) 2 level show significant mixing of j ) 0 and j ) 2 rotational basis functions. The 5-fold degeneracy of the j ) 2 level of p-H 2 is lifted completely due to the anisotropy of the cage environment, as is the 3-fold degeneracy of the j ) 1 level of o-H 2 studied by us previously. Pure translational mode excitations with up to four quanta display negative anharmonicity, which was observed earlier for the translational fundamentals and their first overtones. The issues of assigning the combination states of p-H 2 with excitations of two or all three translational modes, and of the strength of the mode coupling as a function of the excitation energy, are studied carefully for a range of quantum numbers. The average T-R energy of the encapsulated p-H 2 is calculated as a function of temperature from 0 to 150 K. † Part of the "Giacinto Scoles Festschrift".

2018

Los efectos de confinamiento cuantico, entendidos como los cambios en la estructura y la dinamica de una molecula al pasar de un entorno libre a una cavidad con alguna longitud caracteristica del orden del nanometro, representan un reto y una oportunidad. Un reto, porque todavia queda trabajo para poder comprenderlas y modelarlas correctamente. Una oportunidad, porque ofrecen los medios para ajustar propiedades moleculares como la adsorcion, la difusion o incluso la reactividad. La presente tesis doctoral se centra en el estudio teorico y computacional del sistema consistente en una sola molecula de H2 (o bien de D2) atrapada en la cavidad interna de un nanotubo de carbono estrechos de una sola pared. Desde que Dillon y coautores sugirieron la existencia de efectos de confinamiento cuantico como explicacion la inesperadamente alta adsorcion de H2 en nanotubos de carbono, este tema ha recibido mucha atencion desde puntos de vista teoricos y experimentales. La intencion 'de esta T...

The Journal of Chemical Physics, 2004

The effects of two nearly isoenergetic C-H stretching motions on the gas-phase reaction of atomic chlorine with methane are examined. First, a 1:4:9 mixture of Cl 2 , CH 4 , and He is coexpanded into a vacuum chamber. Then, either the antisymmetric stretch (3 ϭ3019 cm Ϫ1) of CH 4 is prepared by direct infrared absorption or the infrared-inactive symmetric stretch (1 ϭ2917 cm Ϫ1) of CH 4 is prepared by stimulated Raman pumping. Photolysis of Cl 2 at 355 nm generates fast Cl atoms that initiate the reaction with a collision energy of 1290Ϯ175 cm Ϫ1 ͑0.16Ϯ0.02 eV͒. Finally, the nascent HCl or CH 3 products are detected state-specifically via resonance enhanced multiphoton ionization and separated by mass in a time-of-flight spectrometer. We find that the rovibrational distributions and state-selected differential cross sections of the HCl and CH 3 products from the two vibrationally excited reactions are nearly indistinguishable. Although Yoon et al. ͓J. Chem. Phys. 119, 9568 ͑2003͔͒ report that the reactivities of these two different types of vibrational excitation are quite different, the present results indicate that the reactions of symmetric-stretch excited or antisymmetric-stretch excited methane with atomic chlorine follow closely related product pathways. Approximately 37% of the reaction products are formed in HCl(vϭ1,J) states with little rotational excitation. At low J states these products are sharply forward scattered, but become almost equally forward and backward scattered at higher J states. The remaining reaction products are formed in HCl(vϭ0,J) and have more rotational excitation. The HCl(vϭ0,J) products are predominantly back and side scattered. Measurements of the CH 3 products indicate production of a non-negligible amount of umbrella bend excited methyl radicals primarily in coincidence with the HCl(vϭ0,J) products. The data are consistent with a model in which the impact parameter governs the scattering dynamics.

Journal of The American Chemical Society, 1972

The energy of formation for the methane molecule (in the Hartree-Fock approximation) is analyzed by (1) building up the electronic configuration starting from the nuclear field and adding one pair of electrons at a time; (2) by variation of the C-H distances; (3) by removing a single electron from each orbital (single ionization). The energy and electronic densities for CH4*+, CH46+, CH44+, CH42+, CH4+, and CH4 are discussed using two methods. The first technique uses Hartree-Fock atomic data and a very simple physical model (requiring essentially no computations). With this method, the total energies and the orbital energies of CHIS+, CH46+, and CHa'+ are computed to about the same accuracy as obtained from the Hartree-Fock molecular computations (the second technique). The Hartree-Fock energies are analyzed with the bond energy analysis technique, and the Hartree-Fock densities are analyzed with the electron population analysis technique. The study of the ionization potentials of a single electron (from each of the occupied orbitals) brings about a clear indication of the large amount of reorganization which follows ionization. This effect was pointed out in paper VI of this series for valency electron; it is now stressed for inner shell electrons. It is noted that the 100% agreement between the computed and the experimental ionization potential for the inner shell indicates that the correlation corrections are affected by the rearrangement to about the same per cent as the Hartree-Fock energies. A discussion on the hybridization for methane would predict (in a Hartree-Fock model) a s2p2 hybridization; however, mainly because of charge-transfer effects, the hybridization is about ~1 .~~2 . 6 ; clearly, the hybridization is a function of the C-H distance and of the degree of ionization of the species (and these effects are discussed).

ChemPhysChem, 2018

In this paper we present real space analyses of the nature of the dihalogen‐water cage interactions in the 512 and 51262 clathrate cages containing chlorine and bromine, respectively. Our Quantum Theory of Atoms in Molecules and Interacting Quantum Atoms results provide strong indications that halogen bonding is present even though the lone pairs of water molecules are already engaged in hydrogen bonding interactions.

The Journal of Physical Chemistry A, 2002

Thermodynamic state function (enthalpy, entropy, and heat capacity) were calculated for several types of silicon hydrides taking into account the strongly anharmonic character of some of the molecular vibrations (internal rotation, inversion, and pseudorotation). The anharmonic motions were treated as one-dimensional motions taking place along the harmonic normal coordinates, neglecting anharmonic coupling terms. Partition functions were calculated from the idealized numerical eigenvalue spectrum in the case of pseudorotation; for the other types of large amplitude motions, we used quantum-corrected classical partition functions. Following the work of Knyazev and Tsang, we derived a novel partition function for an asymmetric double well potential. We then used the data to calculate enthalpies, entropies and free energies of reaction for several types of chemical reactions among silicon hydrides, at both the harmonic and the anharmonic level. Differences arising from the inclusion of anharmonicity are discussed.