Question

Organic Chemistry : HNMR QUESTIONS

What is the relationship between the # of scans and the # of signals?

What are limitations to HNMR?

What are "lock signals" in HNMR?

Answer 1

1) Number of signals : It is determined based on the number of nonequivalent proton group in case 1H-NMR/ different carbon atoms in case of 13C-NMR in a molecule.

Number of scans (n) : These are the number of transients that depends on the quality of spectrum and amount of time of spectrometer to be handled in case FT (Fourier transform NMR using proton (1H)).

For the improvement of S/N ratio multiple scans or signal averaging has to be done. In multiple scanning, the signal increases linearly to that of number of scans whereas noise increases as square root of number of scans.

More scans less noise.

For HNMR the number of scans should be multiple of 4. It may vary from 4-128 scans based on the concentration of sample in order to obtain good spectrum with a straight and flat base line.

2) Limitations to HNMR (proton NMR) :

A) Low porosity results in long experiment

B) Proton NMR cannot predict the relative position of methylene's (-CH₂) in hydrogen bonded structures. The magnitude of effects of substituents on spectral shifts in proton NMR fall off dramatically which are beyond the alpha positions.

C) Usage of H NMR in the series of compounds like compound and its derivatives may appear closely related and sometimes misinterpreted as identical groups.

D) Sometimes there might be change in spectral shifts for the water and N-H peaks in proton NMR.

3) Lock signals : High resolution NMR may require special field - frequency stabilisation in order to allow the stabilisation and to accumulate the signals which were separated by 1hz.

In order to hold resonance condition by NMR which runs parallel to the observed channel. Ex : Deuterium is usually used to hold the resonance in deuterated solvent in order to provide NMR lock signal.

Local signal can be digitalized where it is fed directly into ADC and can be observed in the computer.