While I was reading "Introduction to spectroscopy"(written by Pavia) to teach other friends, I found that Aldehyde C-H absorption occurs at lower frequency than other C-H stretch.
That was so complicated because carbonyl carbon is sp2 hybridized.
And sp3 hybridized C-H absorption occurs at 3000~2900cm-1.
On my experience, sp2 has more s character than sp3, so aldehyde C-H absorption will be occured above of 3000cm-1.
I'm so curious about these problem. Why are they do that?
The vibrational frequency of a stretching mode is closely dependent on the force constants of the bond involved in the vibration. In an aldehyde, the oxygen atom bound to the carbon atom where hydrogen is linked weakens the force constant of the vicinal C-H bond by inductive effect and, consequently, the frequency of the C-H stretching mode is lowered.
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