15. Analytical Techniques:IR, NMR, Mass Spect

a. Which occurs at a larger wavenumber:

3. a C–N stretch or a C=N stretch?

4. a C=O stretch or a C–O stretch?

Which of the following characteristic absorption bands would you expect to observe in the IR spectrum of isopentyl acetate?

Rank the following acetophenone derivatives on the basis of the carbonyl stretching frequency (1 = highest ; 4 = lowest ).

(a)

(b)

(c)

(d)

Would it be possible to differentiate between $C_2{H}_5\mathrm{OH}$ (ethanol) and ${\mathrm{CH}}_{3}O{\mathrm{CH}}_3$ (dimethyl ether) using infrared (IR) spectroscopy?

Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber.

(f) C―S vs C=O

Which of the following correctly matches a characteristic infrared absorption peak with the corresponding functional group in aspirin?

An IR spectrum shows a strong, broad absorption around $3300-3500 \mathrm{cm}-1$ and a sharp absorption near $1700 \mathrm{cm}-1$. Which of the following molecules is most likely to produce this spectrum?

Justify the carbonyl stretching frequencies indicated for benzaldehyde and 4-methoxybenzaldehyde.

Which of the following correctly identifies the major absorption bands in the IR spectrum of triphenylmethanol?

Assuming that the force constant is approximately the same for C–C, C–N, and C–O bonds, predict the relative positions of their stretching vibrations in an IR spectrum.

Based on Hooke's law, choose the bond in each pair that you expect to vibrate at a higher wavenumber.

(e) C=N vs C≡N

Calculate the reduced mass for the following bonds.

(a) C―H

A C-D (carbon–deuterium) bond is electronically much like a C-H bond, and it has a similar stiffness, measured by the spring constant, k. The deuterium atom has twice the mass (m) of a hydrogen atom, however.

(a) The infrared absorption frequency is approximately proportional to $\sqrt{\frac{k}{m}}$ , when one of the bonded atoms is much heavier than the other, and m is the lighter of the two atoms (H or D in this case). Use this relationship to calculate the IR absorption frequency of a typical C-D bond. Use 3000 cm^–1 as a typical C-H absorption frequency.

(b) A chemist dissolves a sample in deuterochloroform (CDCl₃) and then decides to take the IR spectrum and simply evaporates most of the CDCl₃. What functional group will appear to be present in this IR spectrum as a result of the CDCl₃ impurity?

Convert the following infrared wavelengths to cm^-1. (a) 6.24 𝜇m, typical for an aromatic C=C (b) 3.38 𝜇m, typical for a saturated C-H bond (c) 5.85 𝜇m, typical for a ketone carbonyl

For each pair, choose the molecule that you expect to have the highest wavenumber for its C=O stretch.

(d)