P-xylene Molecules Research Articles

The silicalite/p-xylene system: part II - the correct choice for structural and theoretical parameters in computer simulations of non-bonded sorbent/sorbate interactions

Diffusion of p-xylene in silicalite is simulated by using the Buckingham exp-6 model. The best agreement with experimentally determined activation energies for the diffusion (25–30 kJ/mol) is obtained when the framework atoms are relaxed (24±2 kJ/mol). Study of the interaction energy versus the rotation angle of p-xylene along its long molecular axis (010 direction) shows the existence of a single minimum corresponding to a stable orientation of the molecule at the channel intersection (site I). The importance of including the Si-H interactions between the sorbent and the sorbate is emphasized : neglecting these interactions (the widely used Lennard-Jones model, where “ the crucial contact is the O…H distance”) yields overestimated sorption energies and two possible orientations for the p-xylene molecule at site I. A significant simplification of the computer simulations might be achieved by considering only the Si⋯hydrocarbon interactons : in that case the calculated energies have to be multiplied by ≈2.5. Accordingly, as already shown by 29Si CP-MAS nmr, the crucial contact seems rather to be Si…H.

Molecular Complexes in Dilute Liquid Binary Solutions

Abstract Highly diluted solutions of dipolar liquid toluene in nondipolar benzene as well as in nondipolar p-xylene show local extrema of the magnetooptical and light scattering effects. The observed local extremum of the degree of depolarization of unpolarized incident light D n , the Cotton-Mouton constant C M and the Verdet constant V (Faraday effect) coincide with the mole concentration f 2 ∼ 0.015 and f 2 ∼ 0.03 of toluene in solutions. The local extreme of D n , C M and V values are interpreted on the grounds of molecular interaction between polar (toluene) and nonpolar (benzene, p-xylene) molecules.

Molecular tunnelling inp-tert-butylcalix[4]arene(2:1)p-xylene

Inelastic neutron scattering has been used to study molecular tunnelling in the p-tert-butylcalix[4]arene(2:1)p-xylene complex. At low temperature the measured spectra show several bands between 0·63 and 2·6 meV which are interpreted as being due to transitions between tunnel-split librational states of the p-xylene methyl groups. Effects of coupling between a CH3 group and the whole p-xylene molecule have been observed. The rotational barrier provided by the host lattice is very small and the methyl groups behave as almost free quantum rotors.

A study of Dianin's inclusion compound NMR and energy calculations

A study of static and dynamic properties of p-xylene and o-xylene guest molecules in Dianin's inclusion compound is presented. Molecular interaction energy calculations were performed to obtain the minimum energy positions and the energy barriers experienced by the guest molecules in the Dianin's cages. Complementary, solid state 2H NMR spectra were measured from partially deuterated guest and host molecules in order to determine the dynamic modes of the guests as a function of the temperature. From the lineshape and spin-lattice relaxation time measurements activation energies and rates for the detected motions are obtained. Agreements between these activation energies and the calculated energy barriers were not found. A correlated motion of the possible dynamic processes of the host and the guest molecules is suggested.

Polarized Fourier transform infrared microscopy as a tool for structural analysis of adsorbates in molecular sieves

Using FTIR microscopy with polarized IR radiation on silicalite I single crystals fully loaded with p-xylene, the existence of an ordered adsorbate could be proven for the first time by IR spectroscopy. By analyzing the polarized absorption bands the orientation of the p-xylene molecules relative to the host structure could be determined. The results agree well with structural data obtained from X-ray diffraction experiments. These first results suggest that polarized IR microscopy could develop into a powerful tool for the analysis of adsorbate structures, assisting in complete structure resolution by diffraction techniques

Location of para‐Xylene in Yb‐Faujasite (Zeolite Y) by Neutron Diffraction

Rietveld analysis of neutron powder photographs of zeolite Y containing p-xylene revealed an interaction of the methyl groups of p-xylene with the oxygen atoms of the zeolite cage. The p-xylene molecules occupy two crystallographically different positions in the supercage, one of which is shown in the picture on the right. Owing to symmetry, each type of position is present four times.

Caracterisation et localisation de la molecule de para-xylene dans une zeolithe pentasil : Cas des complexes formes par deux boralites avec le para-xylene

Precise X-ray powder diffraction investigations on pentasil type zeolites having respectively the H 1.4B 1.36Al 0.02Si 94.6O 192 and H 3.2B 3.2Al 0.02Si 92.8O 192 compositions reveal that the visual modifications observed on the step-scan profiles can be interpreted by localizing the centroid of the p-xylene molecule on the mirror plane of the 4c site in the Pnma space-group. Among the three possible orientations of the p-xylene molecule, the best fit is obtained for the solution where the line joining the methyl groups is parallel to the straight channel. Several trial and error calculations show that the organic molecule might present a statistical disorder with two simultaneous orientations in the channels.

Calorimetric study on the state of aromatic molecules sorbed on silicalite

Differential heats of sorption have been determined calorimetrically for benzene, toluene, ethylbenzene, and p-xylene on silicate as a function of pore filling. In all cases abrupt changes in the state of the sorbed molecules are observed when the amount sorbed exceeds 1 molecule per 1/4 unit cell. While in the case of benzene, toluene, and ethylbenzene sorbate-sorbate interaction occurs only above 1 molecule per 1/4 unit cell, the sorbed p-xylene molecules interact with each other below this loading.

On a possible substitution of p-xylene by toluene in p-xylene crystals. The crystal structure of p-xylene, C8H10, at 180 K

Mr=106.17, monoclinic, P21/n, a=5.806(2), b= 5.023 (1), c=11.215 (2)/~, fl=100.48(2) ° , V= 321.61 (14) A3, Z = 2, Dx = 1.096 gcm -3, A(Mo Ka)=0.71069/~,, /x =0.66cm -1, F(000) = 116, T= 180 K, R = 0.045 for 926 reflections [I > 2.5o-(I)]. The cell dimensions are confirmed by powder diffraction. No phase transition is observed between 110 and 273 K. The crystal data reported by Biswas [Indian J. Phys. (1960), 34, 263-271] are incorrect. The bond lengths and bond angles in p-xylene are equal to the corresponding values in toluene within experimental error. The aromatic ring is essentially planar and slightly deformed at the substituent side. The centre of symmetry in p-xylene coincides with the space-group centre. The p-xylene molecules are packed into chains along b by nearly equal van der Waals contacts between the six ring C atoms and one methyl H of a neighbouring molecule. The chains are held together by van der Waals forces. Because of the great resemblance of packing in p-xylene and a-toluene crystals, the possibility of the existence of solid solutions of toluene in p-xylene is proposed. This proposal is verified by measuring the p-xylenerich part of the phase diagram of the toluene/p-xylene system. Separation of pure p-xylene crystals from a mixture containing toluene seems unlikely.

INCLUSION COMPLEXES OF 1,1,2,2-TETRAPHENYLETHANE-1,2-DIOL AND 1,1,2,2-TETRAPHENYLETHANE

Abstract The title compounds were found to be good host molecule for various organic guests. In the crystal structures of the 2:1 p-xylene inclusion complexes of the title host molecules, each p-xylene molecule is surrounded by four phenyl groups of the two neighboring host molecules. Application of such inclusion phenomena to the separation of isomers was also reported.

Crystal structure of a sodium complex, bis-[NN′-ethylenebis(salicylideneiminato)copper(II)]perchloratosodium–p-xylene

A purple compound is formed between sodium perchlorate and two molecules of the copper complex of the Schiff's base NN′-ethylenebis(salicylideneimine)(salen); the p-xylene solvate has been subjected to three-dimensional X-ray crystal structure analysis. The structure was solved by Patterson and Fourier methods and refined by full-matrix least squares to R 0·070 for 1083 diffractometer observations. Z= 4 in a monoclinic unit cell having a= 24·44 ± 0·01, b= 11·283 ± 0·006, c= 14·766 ± 0·004 Å, β= 101·22 ± 0·03°, space group C2/c. The sodium ion lies on a two-fold axis and is co-ordinated approximately octahedrally by oxygen atoms; two, at 2·55 Å, from a perchlorate ion also situated on the two-fold axis, and four from the two salen molecules, so that the sodium ion shares these oxygen atoms with a copper atom, Na–O 2·36, Cu–O 1·90 Å. The copper is four-co-ordinated, Cu–N 1·94 Å, and the salen ligand as a whole is nearly planar apart from the carbon atoms of the ethylene-diamine entity which correspond to a gauche conformation about the C–C bond. Axial contacts to copper are to an oxygen in the other salen ligand (3·43 Å) and to a copper atom in another molecule (3·42 Å). The p-xylene molecule occupies a centre of symmetry and fills a space in a loosely packed structure; its nearest carbon atom neighbour is at 3·93 Å.

Dielectric studies. Part XX. Molecular relaxation of some heterocyclic amines in dilute solution

Dielectric constant and loss data have been obtained at microwave frequencies for acridine, 4-methyl-pyridine, phthalazine, quinoline, and isoquinoline in both cyclohexane and p-xylene solution. The data have been used to calculate relaxation times and apparent dipole moments. For phthalazine, quinoline, and isoquinoline in cyclohexane at 50 °C the distribution coefficient is zero and their relaxation times are very similar. Although the axes about which these three molecules may relax lead to different volumes being swept out, no variation in relaxation behavior has been detected, and each system can be characterized by one relaxation time. The relaxation times for all the heterocyclic molecules except quinoline and acridine in p-xylene are appreciably longer than in cyclohexane. Relaxation time values appear a sensitive means of detecting the weak molecular interaction between the amine and the p-xylene. The difference in behavior between the quinoline and acridine as opposed to isoquinoline could be attributed to a more appreciable steric effect in the former two, hindering the approach of the π-electrons of the p-xylene molecules to the hybridized lone pair on the nitrogen atom. No interaction is, in fact, detectable in the case of quinoline and acridine. The importance of allowing for weak intermolecular forces, even in dilute solution, when relaxation values are being anticipated, is emphasized.

Luminescence Quenching and Decay Processes in Multicomponent Systems: n-Nonane+p-Xylene+CCl4

A monophoton technique to measure fast luminescence decay and utilizing pulsed uv excitation has been employed to determine specific rates of excitation quenching in systems containing n-nonane+p-xylene+CCl4, with n-nonane serving as an optically inert solvent, p-xylene as scintillator, and CCl4 as quencher. The decay-time data are employed to evaluate the characteristic rate parameters kq for quenching of p-xylene luminescence by CCl4. Values so obtained are in good agreement with values obtained from steady-state studies. The variation in kq with p-xylene concentration is consistent with the view that energy migration as such via p-xylene molecules occurs in addition to material diffusion of excited molecules. Dynamic and steady-state measurements of self-quenching in p-xylene+n-nonane systems indicate that, if excimer formation in p-xylene occurs, it is followed by a very rapid nonradiative deactivation of the excimer. Furthermore, differences in the self-quenching behavior as determined with steady-state and decay-time techniques suggest some form of static quenching in p-xylene. This static quenching can be explained on the basis of clustering in the ground state.