C sp - H... O Interactions

As already mentioned, the acetylenic =C-H...O interaction had been well established from crystal structures and IR spectroscopic data [36]. For example, the crystal structures of o-chloro and o-bromo-benzoylacetylene [44], show distinctive = C - H. .. O contacts in which the molecules are interlinked by twofold screw symmetry, as shown in 1 for the o-bromo derivative. 

Desiraju and Murty [45] have correlated =C-H... O distances, ranging from 3.2 A to 3.4 A, with infrared C-H stretching frequencies from 3190 to 3280cm , demonstrating the attractive nature of the C-H...O interaction. The molecule with the shortest C-H...O distance and lowest C-H frequency was N-methylpropiolamide, the one molecule in the list containing an amido oxygen, the 

The 1 1 complex of benzoquinone with / -chlorophenol 5 also displays C-H. .. O(carbonyl) and C... O(hydroxyl) interactions. Energy calculations [40] have shown that Coulomb forces play a decisive role in fixing the offset between neighboring chains in the layer structure of benzoquinone and the other two complexes. 

Pyrazine carboxamide crystallizes in four modifications, a,P,y,S [57], of which the structures of the a,p, and S forms have been reported. The a-form [58] contains two types of cyclic hydrogen-bonded pairs with N - H. .. O distances of 2.90 A and N-H. .. N(aromatic) distances of 3.14 A. The and S form exhibit a different 

Dunitz and coworkers reported convincing experimental and theoretical evidence for C - H. . . O hydrogen bonding in the hydrated crystalline form of tricyclic or- 

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Alkenyl (vinyl), aryl, and alkynyl carbocations are particularly unstable with respect to alkyl carbocations. Compare the isopropyl cation with the iso-propenyl cation. In the latter the central C has two a bonds, one tr bond, and one empty orbital, so it is sp-hybridized (linear). Both ions are stabilized by the C(sp )—H (T bonds of the CH3 group on the right. In the isopropyl cation there is an additional interaction with C(sp )—H <7 bonds on the left, whereas in the isopropenyl cation there is an additional interaction with C(sp )—H a bonds on the left. Since C(sp ) orbitals are lower in energy than C(sp ) orbitals, the C(sp2)—(7 bonds are lower in energy than the C(sp )—H o-bonds. 

First, the interaction of a Lewis acidic palladium 11) complex with a C(sp —H bond of an electron-neutral or electron-rich (het)arene might result in its electro-phiUc palladation (173 175 Scheme 7.42), a fundamental process later referred to as electrophilic C—H bond activation. In contrast, the oxidative addition of a C(sp )—X bond to palladium(O) is favored for electron-poor (het)arenes, thus providing an orthogonal entry into Mizoroki-Heck chemistry. After C—H activation, convenhonal alkene insertion (175 176) and P-hydride elimination (176 177) eventually lead to the formation of catalytically inactive palladium(O). In order to achieve catalytic turnover, however, paUadium(O) must be reoxidized to palladium(II). 

Because the EHT-MO calculations on the model cation [Ru(ti5-C5H5) (C=C=CH2 (C0)(PH3)]+ indicate that the C and Cp atoms of the allenylidene unit are electrophilic and nucleophilic centers, respectively, and the H-O hydrogen atoms of water and alcohols are electrophilic, it has been proposed that the transition states for the above mentioned additions require heteroatom-C interactions, which labilize the O-H bonds, favouring the migration of the H-O hydrogen atoms to the Cp atom of the allenylidene. Thus, the lower nucleophilicity of the H-C(sp)carbon atom of phenylacetylene and H-C(sp ) carbon atoms of methane and acetone could explain why the additions of the latter substrates to the allenylidene ligand are kinetically disfavored processes [23]. 

We wish to discuss the reason why the CN group also decreases the activation barrier. Because the CN group possesses the tt orbital, the charge-trans-fer (CT) interaction between the doubly occupied d orbital of the Pd center and the n orbital of CN is formed in addition to the CT interaction between the C-H o orbital and the occupied d orbital of the Pd center [20], as shown in Scheme 5A. This interaction induces additional stabilization energy, to lower the activation barrier. The oxidative addition of the sp C-H o-bond of benzene... 

Thus, the bonding in metal compounds and complexes has traditionally been viewed as ionic (positive metal center interacting with negative ions HO, O2-, Cl", AcO ) and coordinate donor [Lewis acid-base interactions positive metal center interacting with negative ions and electron-pair Lewis bases ( NH3, PPh3, HOH)]. Examples of ionic versus covalent bonding illustrate the tradition H+Cl-versus H-Cl (ls-3p), C4+(C1-)4 versus C(-C1)4 [Tsp -lSpU], Fe3+(C1")3 versus Fe(-Cl)3 [3d sp -(3p)3 ], H+ OH versus H-OH (ls-2p), C4+(O2-)2 versus O=C=O... 

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