NMR Spectroscopy and Stereo Chemistry

By
Srinivasa K

Introduction

Nuclear Magnetic Resonance Spectroscopy is a sophisticated analytical technique which has applcations in many fields. After its discovery it was mainly used for structural determination of molecules. Now it has become an indispensable tool in diverse fields which include Chemistry, Life sciences, Physiology and Medicine. ......., some structural data and stereo chemistry of the compound can be very easily assessed from the spectrum directly. For instance an absorption in the range 7 to 8 ppm can be almost certain to be due to an aromatic compound.

Basically there are four types of information that can be got by examining a NMR spectrum.

• The Chemical shift.
• Integral of the signal
• Multiplicity
• Coupling constant

Chemical shift

In ethanol there are six hydrogen atoms. The electronic environment around the nuclei of these hydrogens is not the same. Oxygen is more electronegative hence the hydrogen bonded to it has lesser electron density around its nucleus compared to the two hydrogen atoms bonded to the next carbon. Similarly the three methyl hydrogen atoms have the same electron density around their nuclei but different from the methylene hydrogens and different from the hydrogen bonded to oxygen. In short in ethanol there are three different types of hydrogen atoms, different in terms of the electron density around their nucleii.

Sheilding and Deshielding:

The protons which have least electron density around them, are more exposed in other words least shielded or most deshielded. The greater the deshielding more is its chemical shift value(shift away from TMS, also called downfield, towards TMS would be upfield).

The NMR spectrum of ethanol will have three signals corresponding to three different types of hydrogen atoms.

In the spectrum the distance between peaks is expressed in Hz or parts per million (ppm, δ). This difference is also expressed in comparison with an internal standard which is normally Tetramethylsilane ( TMS ) and is called Chemical shift. (δ)

Why an Internal Standard?

A UV Spectrum is plotted with absorbance (log I_o / I ) vs wavelength in nm. So the peak position termed λ _max is expressed in nm (nano meters). Similarly in an IR spectrum the peak position is in wave numbers (cm^-1). In NMR technique the variable in the X-axis is the strength of external magnetic field. Hence the peak position should be expressed in units of magnetic field strength. Though this is possible in principle in practice it is not. Instruments operate at different field strengths ( 1.4, 2.3, 5.2 Tesla and so on.)and the resonance frequency varies with field strenth, that is a signal of a particular proton would appear at different field strengths in different instruments. Also the accurate determination of field strength (one part in 100,000,000)or the resonance frequency is technically difficult to measure.

Hence the position of the signal is expressed relative to that of a reference called internal standard, one such compound is tetramethylsilane (TMS)

Why TMS ?

1. It has twelve identical protons and a very small amount ( less than a drop) would give a sharp and intense signal. Its position is taken as zero δ(ppm=parts per million). ppm δ = (position of the signal in Hz- position of TMS in Hz) / spectrometer frequency in MHz.

2. It is highly volatile can be eliminated from the sample readily after the experiment.

3. Soluble in most orgnic compounds. 2-sila-2,2-dimethylpentanesodium sulfonate is useful when polar solvents are used.

4. Protons of most organic compounds appear at a greater δ than TMS.