1H

Experiment name:

PROTON
PROTON128

Discussion:

This is the simplest, most straight-forward NMR experiment, taken right out of your Carbon Compounds textbook.

In this experiment, a short radio-frequency pulse centered on the Larmor precession frequency of protons in the static magnetic field created by the cryomagnet causes a net transverse magnetization of the protons. This transverse magnetization is monitored as the sample decays exponentially towards its equilibrium state, providing a Free Induction Decay, or FID. The FID contains all of the spectral information in the time domain. Subjecting the FID to a fast Fourier transform produces the spectrum in the frequency domain. Before it can be read, the spectrum must be phased. Subsequent processing allows for integration and peak picking.

Note that you can select PROTON (16 scans) or PROTON128 (128 scans). More scans buy you better signal-to-noise (S/N), but take longer to execute. Since it takes roughly 5 min for the spectrometer to lock and achieve a good shim, the extra 5 min required to move from 8 scans to 128 scans isn’t really that excessive…

Recall that S/N ratio is propotional to the square root of the number of scans you take; doubling the number of scans increases your S/N by a factor of 21/2. More advanced users may wish to set the NS (number of scans) parameter manually.

In this page and following pages, a series of spectra are presented for ethyl benzene:

EtBzpro

The triplet at 1.25 ppm with integral of 3 corresponds to the protons Ha in the methyl group, whereas the quadruplet at 2.65 ppm with integral of 2 corresponds to the methylene protons, Hb. The multiplets at 7.25 ppm with integral of 5 correspond to the aromatic protons Hc.  The singlet at 0 ppm is due to the internal standard, tetramethyl silane (TMS).

Note that the chemical shifts, when expressed in ppm of applied magnetic field, do not change with field strength. The spin-spin coupling (J-constants) are not field-dependent, either.

 

Spectrum of Ethyl Benzene:

(Click on the spectrum to obtain a magnified view.)

t-proton