Hydrogen sigma

As described in the case of the 1-adamantyl cation, protosolvation of carbon-carbon and carbon-hydrogen sigma bonds can lead to cationic 2e-3c bonding and the formation of carbonium ion centers. The role of this interaction in the chemistry of superelectrophiles is typihed by protosolvation of the tert-butyl cation (1) in superacid to provide the dicationic... 

However, the chemical properties of an alkene are dramatically affected by the presence of the double bond. Recall that a carbon-carbon pi bond is considerably weaker than a carbon-carbon or carbon-hydrogen sigma bond. It is possible to selectively cause a reaction to occur at a pi bond under conditions that do not affect the sigma bonds. The pi bond is the weak spot of an alkene, and it is there that most chemical reactions occur. This is why unsaturated fats spoil more readily than saturated fats. Their pi bonds provide a place for reaction with oxygen to occur, which leads to spoilage. 

Each of the carbon-hydrogen sigma bonds is formed by overlap of an sp2 hybrid orbital on carbon with the Is orbital of a hydrogen atom. The C—H bond length in ethylene (1.08 A) is slightly shorter than the C—H bond in ethane (1.09 A) because the sp2 orbital in ethylene has more s character (one-third, v) than an sp3 orbital (one-fourth, v). The s orbital is closer to the nucleus than the p orbital, contributing to shorter bonds. 

The transition metals used in catalytic reduction are able to adsorb large quantities of hydrogen onto their surfaces, probably by forming metal-hydrogen sigma bonds. Similarly,... 

ABSTRACT, This chapter presents an overview of recent chemical progress involving molecules with actinide-to-carbon and actinide-to-hydrogen sigma bonds. The synthetic, structural, chemical/mechanistic, thermochemical, and bonding aspects of this chemistry are developing at a rapid rate. 

However, at low temperatures, all of these hydrides react rapidly (l.e., at a rate which is rapid on the nmr time scale by ca. -40 C) and reversibly with CO to yield bright-yellow complexes, which are formulated on the basis of spectroscopic data as dihaptoformyls [82,83] (eq.(64)). There is no infrared spectroscopic evidence for actinide carbonyl complexes. The enedlolate forming reaction is too rapid to allow complete characterization of the Wa-derlved formyl. In regard to thermodynamics, it is found by van t Hoff measurements that for 3Cb, AH - -4.5(9) kcal/mol, AS = -11.7(4.3) e.u. and for Ya—Yb, AH = -5.9(1.5) kcal/mol, AS = -23.9(7.4) e.u.. Migratory Insertion of carbon monoxide into an actlnlde-to-hydrogen sigma bond is clearly exothermic [83]. 

Figure C3.2.7. A series of electron transfer model compounds with the donor and acceptor moieties linked by (from top to bottom) (a) a hydrogen bond bridge (b) all sigma-bond bridge (c) partially unsaturated bridge. Studies with these compounds showed that hydrogen bonds can provide efficient donor-acceptor interactions. From Piotrowiak P 1999 Photoinduced electron transfer in molecular systems recent developments Chem. Soc. Rev. 28 143-50.

Lewin and Cohen (1967) determined the products of dediazoniation of ben-zophenone-2-diazonium salt (10.42, Scheme 10-77) in five different aqueous systems (Table 10-7). About one-third of the yield is 2-hydroxybenzophenone (10.46) and two-thirds is fluorenone (10.45, run 1) copper has no effect (run 2). On the other hand, addition of cuprous oxide (run 3) has a striking effect on product ratio and rate. The reaction occurs practically instantaneously and yields predominantly fluorenone. As shown in Scheme 10-77, the authors propose that, after primary dediazoniation and electron transfer from Cu1 to 10.43 the sigma-complex radical 10.44 yields fluorenone by retro-electron-transfer to Cu11 and deprotonation. In the presence of the external hydrogen atom source dioxane (run 12) the reaction yields benzophenone cleanly (10.47) after hydrogen atom abstraction from dioxane by the radical 10.43. 

Group Branch Extension of Aromaticity Sigma/Rho Fragment Interaction Special ortho effects Crowding Intramolecular Hydrogen Bonding... 

Sobolewski AL, Domcke W, Dedonder-Lardeux C, Jouvet C (2002) Excited-state hydrogen detachment and hydrogen transfer driven by repulsive (l)pi sigma states a new paradigm for nonradiative decay in aromatic biomolecules. J Phys Chem Chem Phys 4 1093—1100... 

The theory of stereoselectivity found in intramolecular hydrogen migration in polyenes was disclosed by Woodward and Hoffmann 51>. The HO—LU interaction criterion is very conveniently applied to this problem 64>. The LU of the C—H sigma part participates in interaction with the HO of the polyene n part. The mode of explanation is clear-cut... 

Since only differences between cr s for the substituents are involved, one may arbitrarily set the absolute value of one sigma value without loss of generality. Normally one chooses a hydrogen atom as a reference substituent for which o is defined as zero. All other rate constants may then be compared to the one characterizing the reaction of the reference substance (k0). In these terms equation 7.4.18 becomes... 

It would appear from the studies on these systems that each metal atom can only accommodate one propagating center. If this were not the case, the metal atom would have attached two sigma-bonded polymer chains with hydrogen atoms on the j8-carbon atoms. The compound Zr(CH2CH2C6HB)4 has been synthesized (19) and found to be unstable above — 80°C, an observation which supports this statement. 

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