Lecture 3 – NMR Basics

• Magnetic non-equivalence

• Chemical shift and the scale used in 1H NMR

• Effect of electronegativity and pi bonds (shielding/deshielding)

• Typical chemical shifts for organic compounds – Table 13.1

• Examples of typical spectra

• Line shape – effect of neighbouring magnetic nuclei

• Splitting patterns and information garnered from them

NMR Spectrum Characteristics

downfield upfield

Position of signal is the chemical shift

Non-equivalent 1H atoms exist in slightly different magnetic environments, therefore it takes a different amount of energy (and therefore a different frequency of radiation) to make each type go against the external magnetic field.

When a certain type of proton within a molecule absorbs energy and is promoted to the higher energy state (i.e. against the external field), it is said to resonate.

The horizontal axis on NMR spectrum represents the change in the frequency of the radiation applied (Rf) and is therefore related to energy.

Chemical shift (δδ)

= position of signal – position of TMS peak x 106

spectrometer frequency

Enables us to use same scale for different size spectrometers (60 MHz, 400 MHz, 850 MHz, etc.)

TMS = (CH3)4Si, signal appears at 0 Hz on spectrum, therefore used as reference

Chemical shifts are reported as ppm (parts per million) relative to TMS and usually occur in the 0-12 ppm range for 1H spectra

Shielding and Deshielding

Electrons around atoms are also magnetic and therefore will have an effect on the magnetic environment around a particular atom.

When a nucleus is surrounded by lots of electron density, it is said to be shielded. When the electron density is perturbed (e.g. by an adjacent electronegative atom), the nucleus of interest is deshielded.

Deshielding causes a signal to shift downfield on NMR spectra.

Table 13.1 – Typical 1H NMR Chemical Shifts

Effect of structure on 1H Chemical Shift

CH3F CH3OCH3 (CH3)3N CH3CH3

4.3 3.2 2.2 0.9

i.e. electronegativity or other atoms plays a role in shift

H

H

H

H

H

HH H

HHCH3CH3

7.3 5.3 0.9

Pi electrons reinforce external field and signals show downfield

Wednesday:

What happens when a proton has H neighbours on an adjacent atom?

Labs – work on the problem set (ACD software), start collecting spectra on the unknown sample.