[wizrak]

Hi

I learnt that in NMR the larger the chemical shift (diff between resonance frequency and that of reference substance/ operating frequency of spectrometer), the larger the frequency i.e.

-------------------------------------
                                               0 ppm
                                                = increasing frequency

Also, we observe deshielding when the electron cloud near the H1 or C13 isotopes are withdrawn by electronegative atoms as they more easily "flipped" by the external magnetic field (due to its larger impact)

HOWEVER, E=hf (or hv). DOESNT THIS MEAN THAT DESHIELDED H1 OR C13 SHOULD HAVE "FLIPPED" AT THE RIGHT SIDE OF 0PPM MARK (of the diagram above), AS IT REQUIRES LESS ENERGY FROM MAGNETIC FIELD AND THUS LOWER FREQUENCY?

[fledarmus]

Your question is really "what is 0ppm on the NMR scale"

See, once you get all of the spins lined up by a strong magnet, it will take energy to move them. The energy required depends on the strength of the magnet, and NMR machines are generally designated by the approximate energy required to move the proton - for example, 60 MHz (very useful when I was in grad school), 90 MHz, 200 MHz, 400 MHz (what I use now), 600 MHz (useful for things like proteins), and so on. The differences in energy required to move protons in different fields are very tiny compared to the energy required to move a proton - that is the "ppm" part, or parts per million. Since the machines are rated in megahertz, or million hertz, it is easy to see that parts per million will be on the order of hertz.

So what we've done is taken a compound that is the most shielded readily available material we have, and defined that as the zero point of our scale. This makes everything else somewhat deshielded. The compound is tetramethylsilane, or TMS. There are a few exceptions, compounds which will have a chemical shift less than zero, but almost everything we work with in an organic lab will have a positive chemical shift compared to TMS.

For any other question of shielding or deshielding, you have to define what you are comparing it to. The calculations for protons on benzene rings are pretty straightforward - on unsubstituted benzene, the chemical shift is 7.36 ppm. As you change substituents, you can shield or deshield the protons, and they will change chemical shift up or down compared to benzene. But they will all still be positive chemical shifts compared to your zero point, which is TMS.

I hope this made any sense at all.

[wizrak]

Well that "kinda" made sense, but my question was

why does electronegative groups such as Oxygen in R-CO2 - H
cause the proton's ppm of 9~13?

=> *shouldnt the oxygens near the hydrogen pull electrons away from the sigma bonds between hydrogen and oxygen, causing the hydrogen (proton) to be more easily flipped by the "external" magnetic field (and thus absorb less energy). BUT THE TEXTBOOK SAID HIGHER THE PPM = HIGHER THE FREQUENCY, AND SINCE E=hv DOESNT THAT MEAN IT REQUIRED MORE EXTERNAL ENERGY FROM THE MAGNETIC FIELD TO FLIP THE PROTON'S DIPOLE?

[fledarmus]

No, the magnetic field is what is holding the proton in place, keeping it from flipping. It is the radio signal that is actually doing the flipping. When you deshield the proton, that exposes more of the positive charge and means that it is more strongly held by the magnet, reguiring a higher energy radio signal to flip.

[wizrak]

@fledarmus

THANK YOU