Chapter 9, Problem 22P

Interpretation Introduction

Interpretation:

The structure for an alcohol with molecular formula ${\text{C}}_5{\text{H}}_{12}\text{O}$ having given ${}^1\text{H NMR}$ spectrum is to be proposed and chemical shifts and splitting pattern are to be specified.

Concept introduction:

Few elements such as ${}^{13}{\text{C and }}^1\text{H}$ have nuclei behaving as magnets about an axis. When placed in magnetic field irradiated with electromagnetic energy of specific frequency, the nuclei tend to absorb energy via magnetic resonance. NMR spectrum is a graph between intensities of NMR peaks versus magnetic field (frequency). An NMR spectrum is used to predict the structure of molecules.

There are four features of NMR spectra that help in predicting the structure:

The number of signals in the spectra tells us the number of different types of protons present in the molecule.

Chemical shift is the measurement on $\delta$ scale (in ppm) where the peaks occur when the nuclei absorb energy through magnetic resonance. This chemical shift position of a set of protons tells us the magnetic environment around those protons, which depends upon electron density around them.

Area under the peaks tells us the number of similar protons.

The splitting pattern of a signal tells us about the number of protons present on adjacent carbon.

NMR signal of a particular proton generally splits into N + 1 peaks, where N is the number of hydrogen atoms on the adjacent atoms. If there is no hydrogen present on the adjacent atom, then resonance will be singlet. If there is only one hydrogen present on adjacent carbon, then signal will split into two peaks or have doublets. Two hydrogens on adjacent atoms split the signal into three peaks, a triplet, and so on.