15. Analytical Techniques:IR, NMR, Mass Spect

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?

Which of the bonds shown in red are expected to have IR-active stretching frequencies?

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

Which of the following compounds has a vibration that is infrared inactive?

1-butyne, 2-butyne, H₂, H₂O, Cl₂, ethene

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.

Why is the C–O absorption band of 1-hexanol at a smaller wavenumber (1060 cm^–1) than the C–O absorption band of pentanoic acid (1220 cm^–1)?

Which occurs at a larger wavenumber:

a. the C–O stretch of phenol or the C–O stretch of cyclohexanol?

b. the C=O stretch of a ketone or the C=O stretch of an amide?

c. the C–N stretch of cyclohexylamine or the C–N stretch of aniline?

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?

Rank the following compounds from highest wavenumber to lowest wavenumber for their C=O absorption bands:

b.

Calculate the reduced mass for the following bonds.

(a) C―H

Calculate the reduced mass for the following bonds.

(c) C―Cl

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

(d)

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

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