Absorption Frequencies of Triple Bonds

Table 7.20 Absorption Frequencies of Single Bonds to Hydrogen Table 7.21 Absorption Frequencies of Triple Bonds Table 7.22 Absorption Frequencies of Cumulated Double Bonds Table 7.23 Absorption Frequencies of Carbonyl Bands...

TABLE 7.21 Absorption Frequencies of Triple Bonds (Continued)... 

The spectra of nitriles (R—C=N) are characterized by weak to medium absorption in the triple-bond stretching region of the spectrum. Aliphatic nitriles absorb near 2260-2240 cm"1. Electron-attracting atoms, such as oxygen or chlorine, attached to the carbon atom a to the C=N group reduce the intensity of absorption. Conjugation, such as occurs in aromatic nitriles, reduces the frequency of absorption to 2240-2222 cm"1 and enhances the intensity. The spectrum of a typical nitrile is shown in Figure 2.31. 

Conjugation Effects. The introduction of a C=C bond adjacent to a carbonyl group results in delocalization of the n electrons in the C=0 and C=C bonds. This conjugation increases the singlebond character of the C=0 and C=C bonds in the resonance hybrid and hence lowers their force constants, resulting in a lowering of the frequencies of carbonyl and double-bond absorption. Conjugation with triple bonds also shows this effect. 

A number of hydrocarbon radicals having multiple bonds at the radical centre have also been trapped in inert matrices and studied by IR spectroscopy. Thus, ethynyl radical was obtained by vacuum UV photolysis (9) of matrix-isolated acetylene (Shepherd and Graham, 1987) as well as when acetylene and argon atoms excited in a microwave discharge were codeposited at 12 K (Jacox and Olson, 1987). An appearance of diacetylene bands was observed when the matrices were warmed up, while the absorptions of the radical C2H disappeared. Detailed isotopic studies of D-and C-labelled ethynyl radicals showed a surprisingly low frequency of the C=C bond stretching vibration at 1846 cm instead of c.2100cm for a true C=C triple bond (the band at 2104 cm was attributed to the... 

In the spectra of those nitrile-boranes having a CN bond order of about 3 the absorption due to the CN triple bond is observed near 2300 cm-1, which is higher than the CN frequency of the free nitriles 19>. This event has been attributed to strict sp-hybridisation and to the nonexistence of canonical forms involving C=N double bonds in these adducts 9>. 

Nitrile triple bond stretching absorptions are at slightly higher frequencies (and usually more intense) than those of alkyne triple bonds. Compare this spectrum of butyronitrile with that of oct-l-yne in Figure 12-8. 

Nitriles Nitriles show a characteristic C = N stretching absorption around 2200 cm-1 in the infrared spectrum. This absorption can be distinguished from the alkyne C=C absorption by two characteristics Nitriles usually absorb at frequencies slightly higher than 2200 cm-1 (to the left of 2200 cm-1), while alkynes usually absorb at frequencies slightly lower than 2200 cm-1 and nitrile absorptions are usually stronger because the C = N triple bond is more polar than the alkyne C=C triple bond. 

The position of an IR absorption is related to both the strength of the bond and to the nature of the two atoms that form the bond. For example, a carbon-carbon triple bond absorbs a higher frequency than a carbon-carbon double bond, which absorbs at a higher frequency than a carbon-carbon single bond. Bonds between two atoms of significantly different mass absorb at higher frequencies than bonds between two atoms of similar mass. 

In a magnetic field, the n electrons of a carbon-carbon triple bond are induced to circulate, but in this case the induced magnetic field opposes the applied magnetic field (BJ. The proton thus feels a weaker magnetic field, so a lower frequency is needed for resonance. The nucleus is shielded and the absorption is upfield. 

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