Interaction between probe molecules and zeolites.

Cumulative coordinates for approximations of high‐order atomic multipole moments in aluminosilicate and aluminophosphate sieves*

“…Simple one‐ and two‐dimensional approximate analytical forms were indeed proposed for the Si and O atomic charges with respect to the internal geometry of the framework atoms. The Si and O charge values of the silicalite/STO‐3G and all‐siliceous mordenite/6‐21G* obtained via the respective analytical dependences 14–16 coincide with those computed by periodic Hartree–Fock (PHF) calculations 17, 18. Similarly, through other low‐order moments, the O dipole moments were approximated with respect to the bond distances and angles for a series of all‐siliceous zeolites, aluminosilicates, and aluminophosphates 19, 20.…”

“…The transferability of the lowest MM, i.e., the Mulliken atomic charges, on the contrary, has been clearly verified for a series of all‐siliceous zeolites, which generally present a narrow interval for the Si–O bond length and a wide Si–O–Si angle variation 14–16. Simple one‐ and two‐dimensional approximate analytical forms were indeed proposed for the Si and O atomic charges with respect to the internal geometry of the framework atoms.…”

“…A simple sine function was found to be appropriate for a qualitative representation of the dipole dependences for the O atoms through all the systems at different basis set levels. Together with approximations of the Mulliken charges of the Si and O atoms already shown 14–16, these dependences of the MMs could thus provide an easy way to calculate the electrostatic field within any arbitrary zeolite framework even if it would be untractable through direct PHF computations.…”

Probing zeolites by vibrational spectroscopies

“…to the average charge of the first-shell oxygen atoms. In this regard, the theoretical works of Larin et al 259,260 are also worth mentioning together with the subsequent and more refined works of Nachtigall et al [274][275][276][277][278][279][280][281] Other minor spectroscopic features are present in the spectra shown in Fig. 8 are an indirect measure of the basicity of the zeolitic framework (see Section 4).…”

“…29,226,[228][229][230][258][259][260] The spectroscopic evidence for the polarization of the CO molecule is an upward shift (blue shift) in the stretching frequency of C-O bond with respect to theñ 0 (CO) of the gas (2143 cm À1 ), 261 owing to an increase in the force constant of the C-O bond and a decrease in the bond length. CO molecule has been the most widely used probe for characterizing cationic sites in zeolites.…”