The spectra below highlight how hydrogen bonding can affect the position and the
ID: 1051491 • Letter: T
Question
The spectra below highlight how hydrogen bonding can affect the position and the appearance of the IR absorption band for a hydroxyl functional group. How does hydrogen bonding apparently influence the broadness of the absorption for a hydroxyl functional group? Also, how does hydrogen bonding influence the relative position or wavenumber of both absorption bands? Use the following equation derived from Hooke's law in your explanations: wavenumber (1/lambda) = k Squareroot f/m lambda = the wavelength of light, k = a constant, f = bond strength, m = mass of atoms.Explanation / Answer
A) The given spectrums clearly reveals that the presence of H-bonding between molecules broadens the IR absorption.
In spectrum A, there is no H-bonding due to the presence of two bulky ter-butyl groups adjacent to the -OH group and hence the O-H peak is narrow.
In spectrum B, there is a H-bonding between O-H groups of adjacent molecules and broadens the O-H absorption peak.
B) Due to H-bonding, there is a molecular associations between the molecules that increases the reduced mass of the compound. According to Hooke's law, the frquency of absorption is inversely proportional to reduced mass, thus the H-bonded molecules vibrate at lower frequencies.
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