Chem II - IR Spectroscopy Theory

What is a stretch?

when bonds expand and contract

What is measured on the x axis of an IR spectrum?

wave number (cm^-1)

What is measured on the y axis of an IR spectrum?

% transmittance

When lower % transmittance, ____

more energy is absorbed

Peaks indicate ___

vibrations

The stronger the bond and the lighter the atoms, ____

the higher wavenumber the stretching peak will be

The intensity of the peak is related to ___

the polarity of the bond

The more polar the bond, the more intense/strong the peaks will be because ___

their bond dipoles change more significantly when they vibrate

What effect does hybridization have in an IR spectrum?

greater s character → orbital extends less → shorter, stronger bond → higher wave number

What effect does resonance have in an IR spectrum?

more resonance → more single bond character → weaker bond → lower energy frequency C=O stretch

What effect does shape (broadness) have on an IR spectrum? 

h-bonding is a dynamic process that causes weakening in N-H and O-H bonds → we see average of different bond strengths

• O-H will be broader than N-H

• O-H for carboxylic acids are very broad

Difference between primary and secondary amine on IR spectrum?

Primary amine has 2 stretches: symmetric and asymmetric with 2 N-H bonds

Secondary amine only has 1 stretch w/ only 1 N-H bond