Carbon-hydrogen bonds infrared spectra

Hydrocarbons contain only carbon-carbon bonds and carbon-hydrogen bonds. An infrared spectrum does not provide enough information to identify a structure conclusively (unless an authentic spectrum is available to compare fingerprints ), but the absorptions of the carbon-carbon and carbon-hydrogen bonds can indicate the presence of double and triple bonds. 

The infrared absorption spectrum thus showB that o-chlorophenol in solution in carbon tetrachloride consists of about 91 percent cis molecules and 9 percent trans molecules. The cis molecules are more stable than the trans molecules by a standard free-energy difference of about 1.4 kcal/mole (calculated from the ratio of the areas of the peaks). This is presumably the difference in free energy of the cis molecule with its intramolecular hydrogen bond and the trans molecule with a weaker hydrogen bond with a solvent molecule. 

The infrared spectra of carboxylic acids provide clear evidence for the hydrogen bonding discussed in the preceding section. This is illustrated in Figure 18-2, which shows the spectrum of ethanoic acid in carbon tetrachloride solution, together with those of ethanol and ethanal for comparison. 

The broad, intense absorption at 3300 cm-1 is attributable to a hydroxyl group. Although both phenol and benzyl alcohol are possibilities, the peaks at 2800-2900 cm-1 reveal the presence of hydrogen bonded to -hybridized carbon. All carbons are sp2-hybridized in phenol. The infrared spectrum is that of benzyl alcohol. 

The association constant of pyridazine with ethanol was found to be 4.9 (from electronic absorption spectra) and 6.8 (infrared absorption spectra), and the corresponding values for the strength of the hydrogen bond are 4.2 and 4.6 kcal. The hydrogen-bonded form of P3n-idazine was considered to comprise one alcohol at one azine-nitrogen at small mole ratios of alcohol to azine and to involve the second nitrogen at high mole ratios (an additional shift in the electronic spectrum. The association constants (3.1-3.8) of pyridine, quinoline, and isoquinoline with methanol in carbon tetrachloride have been determined by infrared spectroscopy. 

The infrared spectrum indicated the presence of double bonds. The hydrogenation experiment showed one double bond for every 19.8 carbon atoms. TTie gas chromatograph of the pyrolysed product is shown in Fig. 3. 

The presence in the molecule of two kinds of boron-hydrogen bond is indicated both by its Raman spectrum and by the chemical evidence that four only of the hydrogen atoms are replaceable by methyl groups. Electron diffraction leads to B-H 1.19 A, B-H, 1.33 A, B-B 1.77 A, ZHBH 121.5° and Z.H, BHjj 100°. Raman and infrared spectra of the tetramethyl compound suggest an absence of terminal hydrogen atoms, and electron diffraction shows the four carbon atoms and two boron atoms to be coplanar. 

Mass spectrum The molecular ion of m/z 117 suggests the presence of an odd number of nitrogens. Infrared spectrum No NH or OH appears. Hydrogens bonded to both sp and sp carbon are indicated around 3000 cm" The characteristic C=N peak appears at 2250 cm" and aromatic C=C is suggested by the peak at 1600 cm" ... 

Protic solvents always have more complex infrared spectra because of the presence of hydrogen bonding in the liquid state. In methanol, this involves interaction of the acidic proton on the OH group in one molecule with the oxygen atom in an adjacent molecule (fig. 5.15). The infrared spectrum shows a wide band centered at 3346 cm which is due to the -OH stretch. When methanol is dissolved as a dilute solute in carbon tetrachloride, this band is sharp and appears at 3644 cm . An -OH bending mode appears at 1449 cm. Another broad band due to -OH out-of-plane deformation is centered at 663 cm. The other features of the methanol spectrum are due to the vibrational modes of the CH3- group or to skeletal vibrations [27]. 

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