Computational investigation by ab initio methods of the spectroscopic characteristics of hafnium monosulfide.

The lowest-lying electronic states of 180Hf32S molecule have been described by using the theoretical ab initio methods, state-averaged full valence complete active space self-consistent field (SA-CASSCF) and multireference configuration interaction with single and double excitation (MRCI-SD). These calculations have predicted 22 electronic states below 40,124 cm-1. The unobserved states [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] have been explored for the first time. The potential energy curves (PECs) have been constructed, and the term energy values at equilibrium Te, vibrational constants ωe, and ωeχe have been calculated for all predicted states. Also, the permanent dipole moments (PDMs) as well as the transition dipole moments (TDMs) have been examined and presented as function of the internuclear distance R. In addition, by employing the spin-orbit coupling in the calculations, 43 Ω± components have been obtained below 40,645 cm-1. The observed states, named A1Σ+ and B1Π, have been assigned respectively as [Formula: see text] and [Formula: see text] of [Formula: see text]. All the calculated spectroscopic constants are found in good agreement with the experiment.

Tracing pore-space heterogeneities in X-type zeolites by diffusion studies.

Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.

129Xe NMR study of the framework flexibility of the porous hybrid MIL-53(Al).

The metal-organic framework MIL-53 exhibits a structural transition between two possible porous structures, so-called large-pore (lp) and narrow-pore (np) forms, depending on the temperature or when guest molecules are adsorbed. (129)Xe NMR has been used to study the lp --> np transition induced by the adsorption of xenon as revealed by the adsorption isotherms. The NMR spectra show that the two structures, characterized by two distinct lines, coexist for xenon pressures above 5 x 10(4) Pa at room temperature, but a complete transformation is achieved when the temperature is decreased. An original interpretation of the NMR results allowed us to quantify the rate of the structural transformation. In particular, at room temperature, we have shown that 28% of the channels remain open. Two possible interpretations of the hysteresis observed in the chemical shift variation versus xenon pressure are proposed.