Hydrogen Bonds Involving Carbon

Experimental interatomic distances in crystalline [Me2P-PMe2-PMe2J [CF3SO3] suggest that weak P-C-H—-O and P-C-H—-F bonding may be present [34,51]. 

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Hydrogen bond involving an acidic hydrogen atom borne by a fluorine-substituted or halogen-substituted carbon seems to contribute to the activity and the selectivity of volatile fluorinated anaesthetics (Table 2). These molecules, although non-functional, can bind stereoselectively with protein targets of the central nervous system [33,34]. 

X-Ray crystallography has been used as a tool to study the structure of 3-amino-2-benzoyl-4-methoxymethyl-6-methylfuro[2,3- ]pyridine, 34 <2004CHE1442>. Four intramolecular contacts exist in the molecule, including an unusual intramolecular hydrogen bond involving a hydrogen atom bonded to an sp -hybridized carbon atom. 

Alkanes, alkenes, and alkynes also have characteristic C — H stretching frequencies. Carbon-hydrogen bonds involving sp3 hybrid carbon atoms generally absorb at frequencies just below (to the right of) 3000 cm-1. Those involving sp2 hybrid carbons absorb just above (to the left of) 3000 cm-1. We explain this difference by the amount of 5 character in the carbon orbital used to form the bond. The s orbital is closer to the nucleus than the p orbitals, and stronger, stiffer bonds result from orbitals with more s character. Even if an alkene s C=C absorption is weak or absent, the unsaturated C—H stretch above 3000 cm-1 reveals the presence of the double bond. 

If on the other hand [434] has the 9-barbaralyl cationic structure [2], the observed shifts of the two signals at — 151°C are as expected the carbon-hydrogen bonds involving C(l), C(2), C(4), C(5), C(6) and C(8) are not equivalent and therefore C-H will preferentially be found in olefinic positions since olefinic C-H-bonds have lower zero-point energy than saturated C-H-bonds. Therefore the C-average chemical shift will be shifted downfield (279). By the same reasoning, only a minor shift is expected for the C(3), C(7)... 

The infrared spectra of a number of hydroxyalkylmetallocenes have been interpreted in terms of hydrogen bonding involving the metal atom. The best example is provided by the epimeric alcohols (XVI and XVII), which exhibit absorption at 3610 and 3561 cm. , respectively, in dilute carbon disulfide solution 23, 24, 59), The low frequency band which, with the absorption near 3600 cm. appears as well in the spectra of acyclic a- and jS-hydroxyalkylferrocenes, has been assigned to the metal-bound hydrogen bond 23, 24, 59, 61),... 

Sammes, M.P. and Harlow, R.L. (1976) Intramolecular hydrogen bonds involving polar carbon hydrogen bonds infrared and H nuclear magnetic resonance spectra of some cyano-methyl and benzyl sulphones, J. Chem. Soc. Perkin Trans. H, 1130 1135. 

The steps involved in an E2 reaction are the breaking of the carbon-hydrogen bond, the carbon double bond formation, and the breaking away of the carbon-halogen bond. 

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