E. Cohen de Lara

Effect of an electric field on a methane molecule. I. Infrared analysis of methane (CH4–CD4) adsorbed in NaA zeolite in the temperature range 150–20 K

The spectra of methane adsorbed at low temperature in NaA zeolite show two strong effects of the field existing in the cavities: (i) the appearance of the ν1 forbidden band which increases when decreasing the temperature and (ii) the splitting of the ν3 degenerate band. The variation with T of its components lead to the conclusion that the molecule progressively is oriented in a C3V configuration with respect to the field.

Molecular dynamics by numerical simulation in zeolites: Methane in NaA

A molecular dynamical study of one methane molecule in a cavity of NaA zeolite is performed in order to compare calculated to experimental data obtained by infrared spectroscopy and neutron scattering experiments in the temperature range 300–30 K. The calculation shows the trajectory of the molecule in the cavity and then the occupied volume as a function of energy. It allows the calculation of average quantities and correlation functions: (i) the mean field felt by the molecule comparable to the field responsible for the induced infrared band ν1, (ii) the average of the potential energy (to be compared to the heat of adsorption) and of the velocity squared, (iii) the external frequency distribution, and (iv) the position autocorrelation function which is related to the dynamical structure factor seen by neutron scattering.

Effect of an electric field on a methane molecule. II. Calculation of the degeneracy splitting of the ν3 band. Expression of the second derivatives of the CH4 dipole moment and evaluation of the second derivative of the C–H bond polarizability

In order to interpret infrared spectra of methane adsorbed in NaA at low temperature, we have calculated the frequencies of CH4 and CD4 perturbed by an electric field. This calculation needs the expressions of the second derivative of the dipole moment and the polarizability. These later were given by Montero and we have expressed the terms ∂2m/∂Si∂Sj using the same approximations: additivity of the CH bond dipole derivatives. We find for the CH bond derivatives: μ’=−0.7×10−10 esu, μ‘=+1.5×10−2 esu. The magnitude of the field (3×105 esu) has been determined by the intensity of the induced ν1 band. The orientation of the field deduced from the experiments, is along a C3 axis of the molecule. Adjusting theoretical and experimental results, we have calculated the parallel and perpendicular second derivatives of the CH bond polarizability 2β‘+δ‘=−6.4×10−8 cm, δ‘− β‘=+5.3×10−8 cm.

Adsorption sites in Na-A zeolites. Neutron diffraction study of NaA-methane and NaA-acetylene systems*

Neutron diffraction has been used to study the adsorption of methane and acetylene in Na-A zeolites. The experiment has been carried out with a dehydrated powder sample (400°C) and with samples containing one molecule per cavity. The effect of the sorbed gas has been increased by using deuterated molecules. In spite of some dubiousness on the structure of the dehydrated Na-A we have been able to find the region in the zeolitic cavity where the molecule is adsorbed: at room temperature for acetylene and at low temperature (40 K) for methane, molecules are trapped in front of the NaII and NaIII cations. In its equilibrium position, the axis of the acetylene molecules is not parallel to the axis of the window. As the temperature is raised (150 and 300 K) our analysis of the diffraction pattern of the methane Na-A sample shows that the molecule is more and more delocalized. This last result is in agreement with previous quasi-elastic measurements.

Method for the calculation of the vibrational frequency shift of physisorbed molecules. Application to H2 adsorbed in NaA zeolite

The vibrational frequency shift of physisorbed diatomic molecules is related to the interaction with the adsorbent expressed in terms of the internuclear distance ρ. It is calculated by the Schrodinger equation, the perturbation theory, and a simplified method. We show that it is sufficient to calculate the interaction potential for the values of ρ in the ground and first vibrational states in order to get a precision of 10% on the frequency shift. The comparison between the theoretical and experimental frequency shift of H2 adsorbed in NaA zeolite is used to adjust the interaction potential, especially in terms of the ionicity of the crystal.

Electric field effect observed on the infrared spectra of the N2O molecule adsorbed in NaA zeolite

The spectrum of the N2O molecule adsorbed in the cavity of NaA zeolite presents two main components for each of the stretching vibrational modes. It is assumed that they correspond to molecules parallel and antiparallel to the electric field of the inner surface of the zeolite cavity. In order to verify this assumption, the frequency shifts and the intensities of these components have been calculated for the two orientations of the molecule with respect to the field, by applying the model of Bishop for the vibrational Stark effect. These calculations require the knowledge of molecular quantities such as derivatives of permanent electric moments and polarizability.

Effect of an electric field on a methane molecule

In zeolite cavities the strong electric field due to the ionic charges induces forbidden IR bands and splitting of degenerate vibrational modes. This has been observed on the CH4 molecule orientated in tripod configuration in adsorption sites of NaA and CaA zeolites. The two components of the v 3 splitting have similar intensities for a given temperature but evolve differently with the temperature, corresponding possibly to a variable distance of the molecule with respect to the site. We have calculated the infrared intensity, i.e., |∂M/∂Q|2, taking into account the permanent and the induced moment . The calculated intensities for the electric field directed along one of the CH bonds are plotted versus the field amplitude value and compared with the experimental data. For CH4 in NaA this leads to the assignment of each component (A and E symmetry) and to the conclusion that the derivatives of the permanent moment and of the polarizability tensor of CH4 with respect to the normal coordinate Q 3 have opposi...

Semiempirical calculation of the interaction energy of a CH4 molecule and a Na+ cation

We present a semiempirical calculation of the interaction energy of a CH4 molecule and one Na+ cation of the zeolite Na A. This energy depends on the distance, R, between the Na+ nucleus and the carbon nucleus of CH4, and on the orientation of the molecule. Taking in account the anisotropy of the repulsion, the most favourable orientation, at any R, is such that the carbon-Na+ axis is a C 2V axis with three hydrogens pointing to the cation, and the least favourable is such that the axis is still a C 3V axis but with one hydrogen pointing to the cation. The results are in agreement with the ones of Sauer et al. obtained by an ab initio calculation.

Effect of electric field on internal vibration frequencies.

The v 2, v 3 bands and v 1, 2v 2 Fermi diad of CO2 are observed in NaA and CaA zeolites at 306 and 193 K. The frequency shifts are compared with the shifts calculated in the harmonic oscillator approximation by Wilsons method. We consider only the electrostatic effect of the zeolite electric field. This calculation shows the perturbation on the force constants; this effect is not sufficient to account for the observed shifts.

New features about the effect of the thermal treatment on the structure of dehydrated Na—A zeolite

Abstract Conflicting results have recently raised doubt upon the Fm3c cubic structure of Na—A zeolite. Preliminary neutron and high resolution X-ray diffraction results on dehydrated Na—A zeolite emphasize the influence of dehydration temperature. When dehydration is performed at 350°C the lattice remains apparently cubic. If Na—A zeolite is dehydrated at 400°C then the symmetry is no longer cubic at room temperature. The splitting of the lines suggest an orthorhombic distortion. When this distorted lattice is hydrated again at 20°C, complete recovery to the supposed Fm3c structure is observed.