CH fragments

The CH fragment which is linked to the OH group (Sh = 5.45 ) can easily be located in the H and NMR spectra. The chemical shift values Sc =74.2 for C and Sh = 3.16 for //are read from the CH COSY plot. The H signal at S,i = 3.16 splits into a triplet (11.0 Hz) of doublets (4.0 Hz). The fact that an antiperiplanar coupling of 11 Hz appears twice indicates the diequatorial configuration (trans) of the two substituents on the cyclohexane ring 5. If the substituents were positioned equatorial-axial as in 4 or 5, then a synclinal coupling of ca 4 Hz would be observed two or three times. 

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

Surface methyls have also been synthesized by the collision-induced dissociation of methane physisorbed on Ni(lll) surfaces.[7, 8] This approach avoids the effects of coadsorbates other than hydrogen, and a number of aspects of Ae reaction and decomposition of CH3 and CH fragments on Ni(lll) have been determined.[9] However, the method is relatively complex and best suited for study of low coverages. 

More recently, the dissociation of ethanol was studied by SERS [Lai et al., 2008]. By employing isotopically labelled ethanol, it was found that C—C bond breaking already occurs at low potentials, resulting in chemisorbed CH and CO. Upon oxidation the CH fragments are converted to CO at a potential below that of CO oxidation, suggesting that, at least on platinum, the potential limiting step in the oxidation of the adsorbed C species is the oxidation of CO. 

Figure 4.10 Secondary ion intensities of ethylidyne, =CCH3, on platinum(l 11) during reaction with D2 at 383 K. Curves a-d are the measured SIMS intensities of CH + fragments at 15-18 amu, respectively. Curves e-h represent a kinetic simulation for a consecutive reaction via two intermediates (adapted from Creighton et al. [30]).

An analogous procedure can be applied to the dicyanoethylene. We can dissect the molecule into an NC—CN fragment and a HC=CH fragment. The appropriate interaction diagrams are shown in Fig. 37. A consideration of these diagrams leads to the following conclusions. 

The carborane 64c was characterized by a consistent set of NMR data, ab initio MO calculations, and by X-ray structure analysis [88]. The synthetic potential of 64c is indicated by its reaction with Fe(CO)3 fragments to give 69 [87]. The di-ferracarborane 69 is isostructural with nido-decaborane(14), because two BH and four BH(H) fragments were replaced by two isolobal Fe(CO)3 and four CH fragments, respectively. 

Fig. 3.1 Born-Oppenheimer vibrational potentials for a diatomic molecule corresponding to the CH fragment. The experimentally realistic anharmonic potential (solid line) is accurately described by the Morse function Vmorse = De[l — exp(a(r — r0)]2 with De = 397kJ/mol, a = 2A and ro = 1.086 A (A = Angstrom = 10 10m). Near the origin the BO potential is adequately approximated by the harmonic oscillator (Hooke s Law) function (dashed line), Vharm osc = f(r — ro)2/2. The harmonic oscillator force constant f = 2a2De...

The last reaction in Scheme 13 becomes possible when the sulfide has a CH2 or CH fragment, adjacent to the sulfur atom whose protons are getting more acid when the molecule is oxidized. Electron-withdrawing groups (EWGs) increase the ease of deprotonation of this site even more. Thus, a-MeO-, a-AcO-, and a-F-sulfides were prepared (Eq. 9) [67-70]. When the nucleophihc attack is sterically hindered or a-protons are absent, products acetoxylated in the phenyl ring are... 

CH fragment neighboring the nitrogen [RiR CH-N(R )-0 ] (Janzen and Blackburn 1969). This extends the identification possibilities of nitrone as a spin trap. 

As seen from Scheme 7.2, the epoxy-ring cleavage and nickel oxidation proceed simultaneously. The nickel-oxygen bond is formed. This results in the formation of the carbon-nickel biradical in which Ph-CH fragment can rotate freely. The cleavage of the (NiO)-C bond leads to the formation of a mixture of styrenes. At early reaction stages (30 min), cis and trans olefins are formed in 50 50 ratio. After a prolonged contact (30 h), when all possible transformations should be completed, the trans isomer becomes the main product and cis trans ratio becomes 5 95. Such enrichment of the mixture with the trans isomer follows from the formation of the di-P-(trimethylsilyl)styrene anion-radical and its isomerization. The styrene formed interacts with an excess of the nickel complex. 

The substitution of a —CH= fragment by heteroatoms of type X (277-electrons) and Z (0 7r-electrons) (see Table VI) involves a change in the number of the 7r-electrons in a ring. 1,4-Diboracyclohexadiene (23), iso-electronic with the 477-electron benzene dication, and 1,4-dihydropyrazine... 

Dehydrogenation of methane on a zeolite cluster has also been proposed to proceed via interaction of a CH fragment with the deprotonated zeolite lattice. DFT calculations performed with a 3T-atom cluster (248) and HF calculations with a lT-atom cluster (254) gave very similar results. The calculated transition state determined from the DFT calculations (248) that leads to dehydrogenation is shown in Fig. 17. 

Vicinal coupling constants in HC—CH fragments are related to the dihedral angle by the relationship... 

Isolobal considerations suggest that it should be possible to replace the CH fragments of letnihedmnc. C R . with ML fragments. What ML., fragments would you suggest0 Draw structures of the complexes. 

Figure 29.7 Mechanism of hyperfine interactions by spin polarization in the CH fragment.

Alzbeta Krutosikova and Tibor Gracza present in the sixth chapter recent studies of heteroanalogs of pentalene dianion in which the CH fragment is replaced by O, NH, S, Se, and Te atoms. Mostly the syntheses of these kinds of compounds or their derivatives are presented, but the distinction between particular representatives is well outlined. 

Pyrolysis of acetylene to a mixture of aromatic hydrocarbons has been the subject of many studies, commencing with the work of Berthelot in 1866 (1866a, 1866b). The proposed mechanisms have ranged from formation of CH fragments by fission of acetylene (Bone and Coward, 1908) to free-radical chain reactions initiated by excitation of acetylene to its lowest-lying triplet state (Palmer and Dormisch, 1964 Palmer et al., 1966) and polymerization of monomeric or dimeric acetylene biradicals (Minkoff, 1959 see also Cullis et al., 1962). Photosensitized polymerization of acetylene and acetylene-d2 and isotopic analysis of the benzene produced indicated involvement of both free-radical and excited state mechanisms (Tsukuda and Shida, 1966). 

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