Infrared Spectroscopy Practice Problems
A student synthesized a compound whose molecular formula was found to be C6H6O and its IR spectrum is given below. What chemical shift would be observed for its signals in 1H NMR?
Consider the IR spectrum shown below:
Determine the functional group found in the molecule if its chemical formula is C6H12O.
Identify the bond in each pair that absorbs at a higher wavenumber.
i) N–H stretch or N–H bend
ii) C–F stretch or C–I stretch
Identify the bond in each pair that absorbs at a lower wavenumber:
i) the C=O stretch in butan-2-one or the C=O stretch in but-3-en-2-one
ii) the C=O stretch of a ketone or the C=O stretch of an ester
Explain why the C–N absorption of an amine occurs at 1020 cm–1 while the C–N absorption of an amide occurs at 1400 cm–1.
Rank the C=O absorption of the following compounds from highest to lowest frequency.
Given that the force constant is identical for C–C, C–F, C–O, and C–Br bonds, arrange the stretching frequency of the bonds from lowest to highest.
Consider the C–N bonds in N-methylacetamide. Rank them by i) increasing bond length and ii) increasing stretching frequency.
Identify which of the following bonds would show IR-active vibrations and which would not.
Calculate the wave numbers in cm-1 corresponding to the following infrared wavelengths.
(a) 8.33 ?m for C—N stretching
(b) 9.09 ?m for C—O stretching
(c) 3.33 ?m for C—H stretching
An O—H bond, electronically similar to an O—D bond, has an equivalent spring constant value, k. Use the formula for infrared absorption frequency f given below:
Where k is the spring constant, and μ is the reduced mass, to calculate the IR absorption frequency of an O—D bond. Use the O—H IR frequency value as 3300 cm−1.
If you run an IR spectrum by dissolving the compound in a solvent with D2O as an impurity, state the functional group whose vibrational frequency can be present in the same region as the O—D stretch.