Chapter 21, Problem 5Q

Interpretation Introduction

Interpretation:

The IR spectra of biphenyl, $4\text{-methyl}\text{benzaldehyde}$, $4\text{-ethyl}\text{propanoate}$, benzophenone and acetanilide should be assigned,

Concept introduction:

IR spectroscopy is spectroscopic method to identify the structure of organic compounds. When IR is passed through the sample, different bonds vibrate at different frequencies of IR by absorbing IR light. In IR spectrum energy in terms of wavenumber ( ${\text{cm}}^{\text{-1}}$ ) is the horizontal axis and intensity is the vertical axis.

As different molecules have different bonds of varying strength so they absorb IR light of different frequencies. This results in different absorption bands in the IR spectrum characteristic to that bond.

IR region is divided into 3 regions, near, mid and far IR region. The mid-IR region extends from $4000-600{\text{cm}}^{\text{-1}}$ and it is the most important region as approximately all organic compounds absorb in this region.

The allowed vibrational energy levels for IR transition is as follows:

  $E=h{\nu }_{\text{ο}}\left(\nu +\text{1}\right)$

Where,

• $E$ is IR energy absorbed by a molecule.
• $h$ is Planck's constant.
• ${\nu }_o$ is zero point vibrational level.
• $\nu$ is vibrational level with integer value of 1,2,3,4.....