CHEMICAL SHIFT UNITS
(PPM)
We already considered that the nuclei, immersed in a strong magnetic field, allign with or against it leading to two energy levels, whose difference is equivalent to a radiofrequency with which resonance can be attained. This is true either for 1H on one hand or 13C on the other.
For a magnet of 1.81 Tesla, the 1H and 13C resonances happen at ca. 80,000,000 Hz (80 MHz) and 20,000,000 Hz (20 MHz), respectively, because the "gyromagnetic constant" of 13C is ca. a fourth of 1H's.
Within those ranges, the various 1H and 13C of a given molecule resonate at slightly different frequencies.
Let's consider a couple of simple numbers and calculations.
Let's engulf the CH3CHClCH3 molecule in a magnetic field of 1.81 Tesla and irradiate it with a frequency range of +/- 1000 Hz within a main radiofrequency of 80,000,000 Hz. We observe that the molecule gives us back two 1H signals centered at:
80,000,317.9 Hz
80,000,123.6 Hz
Can you imagine handling such big numbers in a chemical shift scale?
Let's assume now that we have another NMR instrument at our disposal, one bearing a 7.5 Tesla magnet.
1H resonances take place now at ca. 300,000,000 Hz (300 MHz).
Let's irradiate the CH3CHClCH3 molecule this time with a frequency range of +/- 3000 Hz within a main radiofrequency of 300,000,000 Hz. We observe that the molecule gives us back again two 1H signals but this time centered at:
300,001,192.1 Hz
300,000,463.5 Hz
We are in deep trouble with the"chemical shift" scale:
1) Such big numbers are cumbersome to handle.
2) They are different depending on the instrument.
We can hit the two targets with one shot.
Don't you remember that the chemical shift scales came in "ppm" (PARTS PER MILLION)?
The 1H of our examples resonated at a:
1.81 Tesla magnet
80,000,317.9 Hz
80,000,123.6 Hz
We can simplify by saying that the resonances occurred at:
317.9 Hz
123.6 Hz
in 80,000,000 Hz
In short, they resonate at a given value of Hz as compared to a base value of MHz, i.e. PARTS PER MILLION.
Let's divide the values of 317.9 Hz 123.6 Hz by 80 MHz.
What's the result?
317.9 / 80 = 3.97 ppm
123.6 / 80 = 1.55 ppm
7.5 Tesla magnet
300,001,192.1 Hz
300,000,463.5 Hz
We can simplify by saying that the resonances occurred at:
1,192.1 Hz
463.5 Hz
in 300,000,000 Hz
In short, they resonate at a given value of Hz as compared to a base value of MHz, i.e. PARTS PER MILLION.
Let's divide the values of 1,192.1 Hz and 463.5 Hz by 300 MHz.
What's the result?
1,192.1 / 300 = 3.97 ppm
463.5 / 300 = 1.55 ppm
This way we ended up with much smaller numbers (3,97 ppm and 1.55 ppm) but, what is much more, they are the very same in the two NMR equipments of 1.81 and 7.5 Tesla.
Chemical Shifts in the PPM scale, regardless of whether it is 1H, 13C or any other nuclei, are made independent of the magnetic field strength.
This is paramount for comparison purposes.
It could be useful in some instances to get the chemical shifts back to the Hz scale.
Nothing easier!!! Take the chemical shift in ppm and multiply it by the working frequency of the NMR instrument.
Example:
We have 1H chemical shifts of 3.97 ppm and 1.55 ppm and want to convert it to the Hz scale for an instrument of 15 Tesla working at 600 MHz for 1H.
The result is:
3.97 · 600 = 2,382 Hz
1.55 · 600 = 930 Hz
