Electronic structure of silane hydride anion (SiH5-) and model studies of inter- and intramolecular exchange in pentacoordinate silicon species. Ab initio investigation

Abstract

behavior. However, <j>(F) almost has a monotonic behavior, the calculated value for NF2 being 0.003 au too small. The same problem occurs for , with only the calculated NF2 value preventing a monotonic increase across the series.In summary, then, most of the theoretical properties follow one of two patterns: (a) a monotonic increase or decrease across the series or (b) a potential curve-like behavior with maximum or minimum at CF2. The simplest understanding of these two patterns may be in the fact that the electron distribution may appear to behave differently, depending on the expectation value through which we observe it. For example, inspection of the (1/>a) values in Table V implies that the average distance of electrons from nu-cleus A decreases monotonically across the series. This behavior might be interpreted to imply that the "size" of the molecules decreases monotonically from BeF2 to OF2. Thus, the (1/>A) description of the electron distribution is consistent with properties of type a above. However, the calculated values of (r1 2) with respect to the center of mass show a different pattern, in which the molecular "size" decreases from BeF2 to CF2, but then increases at NF2 and again at OF2. We see that this picture of the electron distribution is harmonious with those properties following pattern b described above. We conclude that a major factor in determining the two patterns of periodic behavior for BeF2 through OF2 is the ambiguity involved in the concept of molecular size.

Differentiating Isomeric Deprotonated Glucuronide Drug Metabolites via Ion/Molecule Reactions in Tandem Mass Spectrometry

“…The electron withdrawing nature of the chloro-substituents in HSiCl 3 enhances the electrophilicity of the silicon atom, which promotes hypervalency and also enhances the reactivity of the Si−Cl bonds. 32–34 In the second step, a chloride anion cleaves off from the silicon atom and forms a hydrogen bond with the 4-OH group (as indicated by O in Figure 2) of the O -or N -methylglucuronide, concerted with formation of a Si−O bond. Elimination of an HCl molecule yields the ionic product [M − H + HSiCl 3 − HCl] − (Figure 2).…”

“…On the basis of the calculations, the reactions are initiated by binding of HSiCl 3 to the carboxylate group of the deprotonated glucuronic acid, generating a covalently bound pentacoordinated silicon anion that is in close proximity to the 4-OH group (Figure ). The electron withdrawing nature of the chloro-substituents in HSiCl 3 enhances the electrophilicity of the silicon atom, which promotes hypervalency and also enhances the reactivity of the Si–Cl bonds. − In the second step, a chloride anion cleaves off from the silicon atom and forms a hydrogen bond with the 4-OH group (as indicated by O in Figure ) of the O - or N -methylglucuronide, concerted with formation of a Si–O bond. Elimination of an HCl molecule yields the ionic product [M – H + HSiCl 3 – HCl] − (Figure ).…”

New Insights into the Remarkable Difference between CH5– and SiH5–

“…It has long been known that the electronic structures of the isoelectronic CH 5 – and SiH 5 – anions are remarkably different. − The carbon anion is a transition state, [CH 5 – ] ‡ , for the H – + CH 4 → CH 4 + H – exchange reaction and lies nearly 50 kcal/mol above the reactants and products. The silicon anion, SiH 5 – , is a stable molecule that is bound by more than 20 kcal/mol relative to SiH 4 + H – .…”

“…− and SiH 5 − molecules are isoelectronic, the early calculations of Wilhite and Spialter, 1 Baybutt, 2 and Keil and Ahlrichs 3 indicated that these anions are very different species. CH 5 − is the high-energy saddle point for the S N 2 exchange reaction…”

The Mechanisms of Nucleophilic Substitution at the Main Group Elements and Design of Intramolecular Tautomeric Systems

“…Data of ab initio calculations of the reaction of the SiHs formation, when silane is attacked by a hydride ion [43][44][45], and of the serniempirical MNDO calculation of the SN2 reaction pathway (R3SiX + X-; X = halide, R = HI Alkyl) (46) pre~ict a backside approach for the electronegative nucleophilic groups Y with respect to the group X being substituted. The preferability of the trigonal-bipyramidal bond configuration of the pentacoordinate silicon atom XIXa is also predicted.…”