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Vibrational spectroscopy6.3.7 Absorbant intensity and IR spectroscopyt r a n s m i t t a n c e Fig. 6-18 IR spectrum. s （strong ）、m （medium ）、w（weak ）、vw （very weak ）a b s o r b a n c eVibrational spectroscopy3334 cm -1–OH stretch. Normal range: 3350±150 cm -1. This is a verycharacteristic group frequency. All of the peaks due to the OH group are broad due to hydrogen bonding.Vibrational spectroscopy 3390 cm-1–NH2antisymmetric stretch.Normal range: 3300±100 cm-1. Muchweaker adsorption than the OH stretchin hexanol.Vibrational spectroscopy 3290cm-1–NH2symmetric stretch. 2°amines have only one NH stretch, and3°amines have none.Vibrational spectroscopy Spectral interpretation always starts at the high end, because there are the best group frequencies and they are the easiest to interpret. No peaks appear above 3000 cm-1, the cut-off for unsaturated C-H. the four peaks below 3000 cm-1 are saturated C-H stretching modes.Vibrational spectroscopy 3050 ±50 cm-1 corresponds to the aromatic orunsaturated C(sp2)-H stretch. Always above3000 cm-1. These bands are not assigned tospecific vibrational modes.Vibrational spectroscopy 3080cm-1=CH3080 cm1 2 antisymmetric stretch. An absorption above 3000 cm-1 indicates the presence of an unsaturation (double or triple bond or an aromatic ring).Vibrational spectroscopy 2247 cm-1 C≡N stretch. Normal range:2250±10 cm-1 , lowered 10-20 cm-1when conjugated. Compare to the C ≡Cstretch in 1-heptyne (3000 cm-1).Vibrational spectroscopy 1742 cm-1, -C=O stretch. In small ring esters, this vibration is shifted to higher frequency by coupling to the stretch ofthe adjacent of O-C and C-C bonds. The amount of coupling depends on the O-C(O)-C angle. As with other carbonyl groups, conjugation lowers the frequency.Vibrational spectroscopy 1642 cm-1, C=C stretch. Normalrange:164020cm-1for cis andrange: 1640±20 cm for cis andvinyl, 1670±10 cm-1for trans, tri andtetra substituted. Trans-2-hexene(overlay menu) has only a very weakabsorption, because there is verylittle dipole change when an internaldouble bond stretches (it is nearlysymmetric).Vibrational spectroscopyThe broad peak at approximately 1460 cm-1is actually two overlapping peaks. At1640±10 cm-1, the antisymmetric bend ofthe CH3group absorbs. This is a degenerage bend (one shown).Vibrational spectroscopy At 1375 ±10cm-1, the CH3 symmetric bend (also called the“umbrella” bend) absorbs. This peak is very useful becauseit is isolated from the other peaks. Compare the spectrum ofcyclohexane. The most prominent difference between thetwo spectra is the absence of a CH3 symmetric bend in thecyclohexane spectrum./cm-11715 1800 1828 1928。
