C-H stretching frequencies

The axial C—H bonds are weaker flian the equatorial C—H bonds as can be demonstrated by a strongly shifted C—H stretching frequency in the IR spectrum. Axial C-2 and C-6 methyl groins lower the ionization potential of the lone-pair electrons on nitrogen substantially more than do equatorial C-2 or C-6 methyl groups. Ehscuss the relationship between these observations and provide a rationalization in terms of qualitative MO theory. 

From the presence of bands in the IR-spectra characteristic of the 0 0 and OH groups, as well as of aliphatic and aromatic C— H stretching frequencies, Hurd et al. concluded that 2-thienol was a... 

CH3C1 and CHjBr, are 1.06 and 1.01 times larger than the respective experimental harmonic vibrational frequencies. One of the most interesting properties of the vibrational frequencies is the increase in the C-H stretch frequencies in going from the reactants (or products) of the SN2 reaction to the central barrier. 

Schafer, L., and K. Siam. 1988b. Comment on Accuracy of Ab Initio C-H Bond Length Differences and Their Correlation with Isolated C-H Stretching Frequencies. J. Chem. Phys. 88, 7255-7256. 

Example The C = O stretching frequency is about 1700 cm-1 whereas the C—H stretching frequency is about 3000 cm 1 and both of them are almost independent of the rest of the molecule as depicted in Table 22.1. 

In applying the LH model to the above set of kinetic data, the vibrational frequencies associated with the transferring proton in the reactant and product states must be specified to determine Cmpjtr(Q) in Eq. (11). The C—H stretching frequency in the reactant state is set to 2700 cm 1 and the O—H stretching frequency in the product state is set to 3100 cm-1. The numbers were derived from AMI calculations. The transition state frequency o> is set to 2500 cm-1 based on the work of... 

Eischens and Pliskin have interpreted the infrared spectra of ethylene chemisorbed on nickel dispersed on silica 32). When introduced to a surface previously exposed to hydrogen, ethylene gave rise to absorption bands which correspond to the C—H stretching frequencies of a saturated hydrocarbon (3.4-3.5 p) and a deformation associated with a methylene group (6.9 p). A weak band at 3.3 p was attributed to an ole-finic C—H. Treatment of the chemisorbed ethylene with hydrogen caused the spectrum to change to one which was interpreted as due to an adsorbed ethyl radical. Apparently in the presence of hydrogen most of... 

Almost all organic compounds contain C-H bonds and this means that there is invariably an absorption band in the IR spectrum between 2900 and 3100 cm at the C-H stretching frequency. 

The IR spectra of 2,1,3-benzothiadiazole and its derivatives show a well-defined aromatic system, evidenced by the correlation of the C—C and C—H stretching frequencies of 2,1,3-benzothiadiazole and its 4- and 5-substituted derivatives (Me, Cl, and Br) with the corresponding frequencies for benzene derivatives <69CHE180, 69KGS235>. 

In applying the LH model to the above set of kinetic data, the vibrational frequencies associated with the transferring proton in the reactant and product states must be specihed to determine in Eq- (1 ) The C—H stretching frequency in... 

The infrared spectra of the alkanes show clearly absorptions corresponding to the C—H stretching frequencies at 2850 cm 1 to 3000 cm-1. The C—C stretching absorptions have variable frequencies and are usually weak. Methyl (CH3—) and methylene (—CH2—) groups normally have characteristic C—H bending vibrations at 1400 cm-1 to 1470 cm-1. Methyl groups also show a... 

The change in the ionic contribution in the M-O bonding in M(OMe)2 and M(OEt)2 series for the Group II elements is clearly illustrated by the comparison of their IR spectra. It is to be noted that these are the C-H stretching frequencies and not the C-O ones, as one might suppose, that appear to be most sensitive to the electronegativity of metal. Thus from Be to Ba the v(E) Me decrease with nearly 200 cm 1, supposedly due to the induction effect along the M-O-C-H chain [681]. 

Contrary to the weak cage response, the guests properties experience considerable changes. In the encapsulated CH4 molecule, the C-H bonds are 0.014 A shorter than in the free molecule (Table 1). This entails a considerable increase in the C-H stretching frequencies a and t (by 140-150 cm-1) and a small increase in the deformation frequencies e and l (by 10-12 cm-1) (Table 2). For the stretching frequencies, similar shift values (138 cm-1 for symmetrical and 152 cm-1 for antisymmetrical modes) were calculated for the CH4 C60 cluster in Ref. [39] within the MP2 approximation. 

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