Question

Distinguish between intermolecular and intramolecular hydrogen bonding using ir spectroscopy

Answer 1

A good way to examine the infrared spectra of molecules for hydrogen bonding is to compare and contrast two molecules, one with intramolecular hydrogen bonding, and another with intermolecular bonding.  The pair of 4-carbon dicarboxylic acid molecules, fumaric and maleic acid, is a good example.  Maleic acid is the cis-isomer, while fumaric acid is the trans-isomer.  In maleic acid, the two carboxylic acid groups can be close enough to interact internally, producing intramolecular H-bonding.  The trans orientation of fumaric acid prevents it from performing any intramolecular interactions, but in the solid and liquid state, it can experience intermolecular interactions.

To start things off, we first look at fumaric acid, the trans-isomer.  I calculated the frequencies and IR spectra of four different molecular systems in the gas phase, using density functional theory to model the atoms and orbitals needed for the computations.  Maleic acid and fumaric acid were two of the four systems, and acrylic acid (missing one of the two carboxylic acid units) is the representative in the other two systems (monomer and intermolecular dimer).  These models let us compare and contrast the effects of hydrogen bonding on infrared spectroscopy.