Let Us Break the Words Down in Nuclearmagneticresonance (NMR) Spectroscopy

H-NMR Notes:

The molecular formula and functional groups are determined using the mass spectrum and the IR spectrum. The H NMR spectrum is used to put the entire structure together.

Let us break the words down in NuclearMagneticResonance (NMR) spectroscopy.

Nuclear is for the nucleus and we will use 1H and13 C, notice that 1 and 13 are odd numbers. We must always use odd numbers if we are to get the nucleus to spin. Even numbers of protons and neutron will cancel their spins out. Deuterium 2H is even 1 (ne) proton spin cancels one neutron spin, and does not have a net spin and no discernable magnetic field (ok, so this is a little lie, they have fields but just very small, small enough so we say none)

Magnetic fields are created by moving charges. As the positive nucleus spins it creates a magnetic field (recall the Orsted’s experiment which proved when you move and electron it creates a magnetic field).So it is with this nucleus, this little guy creates a tiny magnetic field of its own. We then place it in an external magnetic field using a very powerful man made magnet.

Resonance, is when one objects vibrations cause a second object to vibrate.This is like a guitar string that vibrates back and forth, this then causes the air and wood (second object, and yes air can be an object) of the guitar to vibrate - at the same frequency.Another example of resonance is the familiar sound of the sea that is heard when a seashell is placed up to your ear. Even in anquiet room background noise fills the seashell, causing vibrations within the seashell. Like an empty hole of a guitar the seashell will vibrate at certain frequencies. If one of the frequencies in the room forces air within the seashell to vibrate at its natural frequency, a resonance situation is created. A sound wave in the room – which you may not even be able to hear – results in a big vibration - a loud sound. The next time you hear the the sea in a seashell, you are hearing is one of the frequencies in the room.

We want to get the already spinning nucleus to vibrate like the sea shell so we direct many different radio waves at it. One of those waves will surely be the right one and it will cause the magnetic proton to begin to vibrate. Now it will absorb that radio wave. More than that, there are many protons(H+) on say propane (C3H8). Each proton is different and will absorb different frequencies of radio waves.Hydrogen’s that are near polar atoms requires more energy to start vibrating (or vibrate at greater frequencies). Protons near polar atoms have their electron pulled away and are thus naked or de-shielded. Electrons have their own spin and thus a magnetic field just like the nucleus does and this magnetic electron field protects the nucleus from the pull of our external magnet. With the electron protection removed by the polar proton,they are held in the external magnetic field more rigidly, or more strongly, and need a real jolt to get them to resonate.

The hydrogen atom's environment

Electrons near protons cut down thesize of the external magnetic field. Suppose you were using a radio frequency of 90 MHz, and you aimed it at a proton and it began to resonant (vibrate at 90 MHz, BBC Radio 4 is found at 90 MHz). If electrons were introduced - they have their own magnetic fields -the proton would stop resonating, the electrons are protecting or shielding it.

How would you bring it back into the resonance condition again? You would have to use a different radio frequency. It is now shielded so we would have to increase the frequency to get at the proton, maybe 92MHz.

Protons that are in the same environment are identical in every way so the same radio wave will force them all to resonate at the exact same frequency, creating overlapping signals. These signals will be stronger due to the overlap (thus they will be higher) but they have the same frequencies (energy).

However, if they are different protons (not in the same environment) then they of course send out different signals (as they absorb different frequencies (energy to resonate). Realize that in principle, a peak will be observed for every magnetically distinct nucleus in a molecule.

The area under each signal is proportional to the amount of radio energy and number of equivalent protons that give rise to that signal.

Spin-spin coupling or Splitting or Multiplicity:

As with humans, couples cause splitting (just as couples can split). What is a couple? Well, you can’t hold your own hand and you ought not to go out on a date with anyone in your own family, so you cannot couple with your same protons or equivalent protons.

Equivalent protons are protons in the same environment and do not couple – you do not hold hands with yourself. These same protons (equal or equivalent) being equal send the same signal so their signals overlap ( as they are in the same “environment”). A hydrogen cannot couple with itself (or protons that are equivalent, the same, to it), meaning that the methyl group hydrogens cannot couple with each other as they all have the same magnetic environment and so send out 1 collective similar signal as they are the same.

So only non equivalent protons (different environments or on a neighboring thus different carbon) can split, or mess up, signals. Different kinds of protons (adjacent carbon protons) can couple for they are close enough for their magnetic fields to influence on another, but different enough tohave slightly different signals. The spin of one nucleus influences the spin of another nucleus. Protons usually only couple, hold hands with, with protons attached to adjacent carbons, now they are different here so send different signals and mess up or split the signal.The "n + 1 rule", which says that if a proton Ha has n equivalent protons on neighboring carbons, then the signal for Hawill be split into n + 1 peaks. For example, a CH3peak will split any "next door" proton signals into 4 peaks, called a quartet.

For example, consider bromoethane (structure given below).

Bromoethane has two different types of hydrogen environments. One at a greater shift away from TMS as it is nearer the electronegative bromine, so we expect two absorptions in the NMR spectrum. One absorption corresponds to the two hydrogens that are closest to the halogen atom.

The hydrogens closer to the bromine will appear as a quartet because they are near three different hydrogens (the hydrogens on the methyl group). Those adjacent hydrogens are communicating their presence to the hydrogens being flipped. They are saying, "We are your neighbors and there are three of us." The reason they are able to communicate their presence is that they are little magnets and as such, they either add to or subtract from the external magnetic field depending on their orientation. Since there are many protons in a sample, the following are the possibilities for the neighboring hydrogens during excitation:

Now if you understand why the "a" hydrogens give a quartet can you figure out why the "b" hydrogens give a triplet? Try to work it out using vectors as done in the above diagram.For simple systems like bromoethane, n+1 peaks will be observed for a given absorption, where n = the number of neighboring, but different hydrogens. This formula can be useful when interpreting simple spectra.

Symmetry:

Below you see a symmetrical molecule, so even though the protons of CH2 both have neighbours, the neighburs are identical because they are on a symetrical molecule. there is no splitting here as the protomns of both CH2 are in the sam envirommnet due to being symetrical. Then we change a Cl for Br and destroy symmetry. The molecule now contains different atoms at each end, the hydrogens are no longer all in the same environment. The CH2 groups will be different and send out different signals, actually the signal will be split 3 times – a triplet (n+1, where n is numbers of H+ on a neighbor)

Practice Question:

1. For the high resolution 1H NMR data below, work out the structure of the molecules concerned. You will find a short table of useful chemical shifts at the end of the questions – this is for C4H8O

Answer

chemicalshift (ppm) / 2.449 / 2.139 / 1.058
Number of carbons / 2 / 3 / 3
splitting / quartet / singlet / triplet

Thesingleton is shown to have 3 hydrogensattached is a CH3group butsinglets (no splitting= no neighboring protons) so must be attached to the C=O. So you have a-COCH3group.

The 2 hydrogens (on a carbon) must have 3 neighbor protons because it has 4 peaks (peaks= n+1 rule going backwards is number of hydrogens in a neighbor is

n = peaks -1). The CH2group attached to C=O.The factthat itis a quartet means thatit isnext door to acarbon with 3 hydrogensattached - a CH3group.That CH3group on the other end, is a triplet, and so must be attached to a carbon with 2 hydrogens on it.The combination of these two peaks is typical of an ethyl group, CH3CH2-. We have 2-butanone

Alcohols: Where is the -O-H peak?

This is very confusing! Different sources quote totally different chemical shifts for the hydrogen atom in the -OH group in alcohols - often inconsistently. The problem seems to be that the position of the -OH peak varies dramatically depending on the conditions - for example, what solvent is used, the concentration, and the purity of the alcohol - especially on whether or not it is totally dry.

The lack of splitting with -OH groups

Unless the alcohol is absolutely free of any water, the hydrogen on the -OH group and any hydrogens on the next door carbon don't interact to produce any splitting. The -OH peak is a singlet and you don't have to worry about its effect on the next door hydrogens.

You do not need to bother about this next bit!

New NMR machines fix the magnetic field and vary the radio frequency – older ones it was the opposite). So be careful how you read explanations as they are confusing or opposite. Do you need to worry about this? No. What matters is that you can interpret the resulting NMR spectra.

Think about TMS, the standard which has the maximum amount of magnetic shielding of the hydrogen atoms by the electrons. That reduces the strength of the magnetic field experienced by the hydrogen nuclei more than in any other organic compound. TMS will need a lower frequency than any other organic compound to bring it into resonance.

It comes down to the fact that, if you are varying the radio wave frequencies, TMS will need the lowest frequency (E=hv, so least energy) to reach the resonance condition. (We don’t do this anymore, but, if you are varying magnetic field, TMS will need the greatest external magnetic field to reach the resonance condition).

Reference Point or Zero Mark (TMS)

Before we can explain what the horizontal scale means, we need to explain the fact that it has a zero point - at the right-hand end of the scale. The zero is where you would find a peak due to the hydrogen atoms intetramethylsilane- usually calledTMS.Everything else is compared with this.

You will find that some NMR spectra show the peak due to TMS (at zero), and others leave it out.

TMS is chosen as the standard for several reasons. The most important are:

1) It has 12 protons in exactly the same environment. They are joined exactly the same way, so produces a single peak and strong peak (because there are lots of hydrogen atoms).

2) The electrons in the C-H bonds are closer to the hydrogen’s in this compound than in almost any other compound. Why? Well, because Si actually has less of an electronegative value than carbon, thus all the electrons possible are pushed towards the protons. That means that these proton nuclei are the most shielded from the external magnetic field. They are held loosely in by the field and low frequencies (energy) are needed to get them to resonate.

Other reasons to use TMS is that it is :

-Non toxic

-Cheap

-Inert

-Volatile so boils off and does not stay in the sample

The net effect of this is that TMS produces a peak on the spectrum at the extreme right-hand side (lowest energy). Almost everything else produces peaks to the left of it.

Solvents for NMR spectroscopy

It is important that the solvent itself doesn't contain any hydrogen atoms, because they would produce confusing peaks in the spectrum.

There are two ways of avoiding this. You can use a solvent such as tetrachloromethane, CCl4, which doesn't contain any hydrogen, or you can use a solvent in which any ordinary hydrogen atoms are replaced by its isotope, deuterium - for example, CDCl3instead of CHCl3.

Deuterium atoms have sufficiently different magnetic properties from ordinary hydrogen that they don't produce peaks in the area of the spectrum that we are looking at

Chemical Shifts:

Now consider a hydrogen near a halogen as in bromoethane. This type of hydrogen is in a

magnetically altered situation as compared to the hydrogen in plain old ethane.

The frequency and hence the chemical shift (/ppm) will change depending on the electron density around the proton. Electronegative groups attached to the C-H system decrease the electron density around the protons, and there is less shielding (i.e.deshielding) so the chemical shift increases, more energy is required from EMR to jolt the proton now more strongly held in the magnetic filed. So the presence ofmore electronegative atomshave more deshielding and larger chemical shifts.

Due to its electronegativity, the halogen atom pulls electrons away from the hydrogens in the molecule. Therefore, the hydrogens experience less of the local field and more of the external field. So hydrogens near an electronegative atom should require a higher frequency to resonate. Therefore, they should appear at a higher ppm in the spectrum. Hydrogens like those in bromoethane should appear from 2.5-4.0 ppm in the NMR spectrum downfield from TMS. This is all based on polarity, the more polar is shifted away from TMS (0).Desheild = Downfield (hey! it kind of rhymes)

POLAR NOT POLAR

Peak Integration.

Is the height of the peaks compared to each other, this gives a reflection of the area under the peaks, which equals the numbers of hydrogens for within each peak. An integrator trace crosses a peak or group of peaks, it gains height. The height gained is proportional to the area under the peak or group of peaks. For example, if the heights were 0.7 cm, 1.4 cm and 2.1 cm, the ratio of the peak areas would be 1:2:3. That in turn shows that the ratio of the hydrogen atoms in the three different environments is 1:2:3.