Chapter 15, Problem 15.55P

Interpretation Introduction

Interpretation:

The type of stretching mode associated with the strong, sharp IR peak for sodium acetate in the given spectrum is to be identified. It is to be explained why its frequency is different from that of an ester.

Concept introduction:

IR absorption causes excitations in the vibrational motions of molecules. There are two basic types of vibrational modes: stretching and bending. In the stretching mode, the distance between two atoms in a bond grows longer and shorter. Two types of stretching modes are there: symmetric stretch and asymmetric stretch. Symmetric stretching is the simultaneous vibration of two bonds, in which the bonds elongate and contract together. In an asymmetric stretch, one bond elongates while one contracts. For each type of vibration, only certain amounts of energy levels are allowed; thus, only photons of certain energies are absorbed. The frequency at which a peak appears in an IR spectrum is often the same as the frequency of the vibration that is responsible for the absorption of the photon.

For ionic compounds, the molecule exists as an ion and is in the free form. Resonance structures for ions with an equal contribution to the hybrid have weaker bonds and, as a result, appear at a lower frequency (wavenumber) in the IR. On the other hand, resonance structures for molecules with an unequal contribution to the hybrid have stronger bonds and, as a result, appear at a higher frequency in the IR. The $\text{C=O}$ absorption for esters is a strong and sharp peak that appears ${\text{~1730-1750 cm}}^{\text{-1}}$. The frequencies of absorption are directly proportional to the strength of the bond.