How does IR work?

Choose the bond in each pair that you expect to vibrate at the higher wavenumber.

(b) C―H vs. C―O

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

(a)

(b)

(c)

(d)

Justify the carbonyl stretching frequencies indicated for 3-chlorobenzaldehyde and 3-fluorobenzaldehyde.

There are three carbon–oxygen bonds in methyl acetate.

a. What are their relative bond lengths?

b. What are the relative infrared (IR) stretching frequencies of these bonds?

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