Calculation of ¹⁹F and ²⁹Si NMR Shifts and Stabilities of F^- Encapsulating Silsesquioxanes

Abstract

Hartree-Fock and density functional theory techniques have been used to calculate the structures, the stabilities in aqueous and toluene solutions, and the 19 F and 29 Si NMR shifts of Fencapsulating double-ring geometry silsesquixoanes DnR with n ) 3-6. We find for the fluorides that the encapsulation free energy is most negative for the double four-ring (D4R) cage, in which the most stable geometry has Fin the center. For n ) 5, 6 (and for an opened version of the double five-ring, D5R, cage), the most stable geometry has Fbonded to a single apex Si atom, with a bond distance around 1.76 Å. The 19 F NMR is somewhat deshielded relative to that of the Si(OH) 4 Fmonomer for all of the apex bonded F -DnR species and is very strongly deshielded for the central Fencapsulating DnR species with n ) 3, 4. The Si is slightly deshielded by the presence of Ffor the DnR with central Fions, while in the apex-bonded larger double rings, n ) 5, 6, the Si directly bonded to Fis substantially shielded. The energetic results help to explain why fluorides, as mineralizing agents, increase the yield of zeolites and mesoporous silicas with D4R rings, and the NMR results provide a means for ascertaining double-ring size and the position of Fwithin the double-ring cage.

A Theoretical Study of the Insertion of Atoms and Ions into Titanosilsequioxane (Ti-POSS) in Comparison with POSS

“…This is also different with the fact that the F − is encapsulated in the center of the silicon analogue, T 8 [9]. Nevertheless, Tossell has found that in the larger POSS such as T 10 and T 12 with the double-decker structure F − is bound at the Si atom in a corner of the cage so the present result resembles that [12].…”

“…The structures of the complexes and reaction paths of the halogen anions investigated here (especially F − ) are significantly different with those of the rare gas or cationic elements in the preceding sections. Incidentally, there are many comprehensive studies for the encapsulation of F − into POSS, and it is well known that the endohedral complex F − @T 8 has been observed experimentally [7,8,12].…”

“…For example, there are systematic and comprehensive studies for the encapsulation of various atomic or ionic species into T 8 and T 10 by Hossain et al [9,13,15,16]. Furthermore, incorporation of molecular species such as H 2 , N 2 , and O 2 inside T n (n = [6][7][8][9][10][11][12] that also been investigated by our group [5,18,19]. These studies must give useful information for the future design of the molecular sieve or better drug delivery.…”

“…), referred to as T n , have been the focus of considerable experimental and theoretical interest because of their wide variety of practical uses for many years [1][2][3][4]. Among a huge number of investigations for these diverse polyhedral compounds, the approach to make use of their cavity for various purposes, such as a container of atomic or molecular species [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19], medical supplies [20,21], molecular sieves, or a reaction filed for H 2 formation [22], is one of the extremely attractive research areas of the POSS chemistry. In addition to many interesting experimental observations, theoretical studies have been making considerable contributions in this area.…”

The silica hypothesis for homeopathy: physical chemistry

“…Alkali metals from Na + to Cs + stabilized different oligomers, with Li + -H 2 O interactions further enhan-cing polymerization in the case of Li + . Tossell 20 has explained the role of fluoride ion F À in promoting the formation of D4R cubes. A comparative study of substituted ammonium, NA 4 + , shows markedly different results depending on whether the alkyl group A is methyl, ethyl, or propyl.…”