12 -- Part 1: INFRARED SPECTROSCOPY AND MASS SPECTROMETRY
To propose structures for possible compounds, use the chemical shifts, splitting patterns, and integrations shown in a proton NMR spectrum.
The number of peaks and their chemical shifts in a 13C NMR spectrum can be used to determine the number of carbon atoms in a compound.
Predict the major features of the 13C NMR spectrum with a chemical structure.
The structures of unknown organic compounds can be determined by combining the information from NMR, IR, and mass spectrums.
The left occipital lobe of a human brain is shown to have a tumor in it.
The largebore, broadband NMR instrument produces a rapidly varying field to obtain spatial information that is used to form an image.
NMR can be used with a very small sample, and it does not harm it.
Many structures can be determined using only the NMR spectrum, which provides a lot of information about the structure of the compound.
More often than not, the structures of complicated organic molecules can be determined with the help of other forms of spectroscopy and chemical analysis.
A wide variety of nuclei, including 1H, 13C, 15N, 19F, and 31P, are studied using NMR.
Nuclear Magnetic Resonance (NREM) was first used to study protons, and the most common is the 1H NMR spectrometer.
Unless a different nucleus is specified, "Nuclear magnetic resonance" is assumed to mean "proton magnetic resonance".
We begin our study with 1H and end it with 13 C.
The nucleus of a protons has an odd atomic number of 1.
The spinning protons can be seen as a sphere of positive charge.
The charge is moving in a loop of wire.
When a small bar magnet is placed in the field of a larger magnet, it twists to align itself with the field of the larger magnet, which is a lower energy arrangement than an orientation against the field.
There have been dom orientations in the absence of an external magnetic field.
There are more spins when the a@spin state is lower.
The Earth's magnetic field is about half a gauss.
The energy difference between the two spin states of a protons is small.
For a strong magnetic field of 25,000 gauss, it is only about 10-5 kcal>mol.
The small energy difference can be detected by the NMR.
The spin of the protons can change from a to b or from a to a when they interact with a photon.
A nucleus aligned with the field can absorb the energy needed to flip.
When a nucleus is subjected to the right combination of magnetic field and electromagnetic radiation to flip its spin, it is said to be "in resonance", and its absorption of energy is detected by the NMR spectrometer.
The RF region of the spectrum contains the fields of currently available magnets.
The radio frequencies needed for resonance are calculated based on the field and the most powerful magnets are usually designed for the most practical price range.
The most common operating frequencies for student spectrometers have been 60 MHz, corresponding to a magnetic field of 14,092 gauss.
Higher-resolution instruments operate at frequencies of 200 to 600 MHz, corresponding to fields of 46,972 to 140,918 gauss.
We have considered the resonance of a naked protons in a magnetic field, but real protons in organic compounds are not naked.
They are protected from the external magnetic field by electrons.
A loop of wire is moved into a magnetic field.
The principle of the electric generator is that the electrons in the wire flow around the loop in the direction shown in Figure 13-4.
The electric current creates a magnetic field.
In a molecule, the electron cloud around each nucleus acts like a loop of wire.
This current is caused by the magnetic field opposing the external field.
If the external field is increased to 70,460 gauss, the effective magnetic field at the proton is increased to 70,459 gauss.
If all protons were protected by the same amount, they would all be in resonance.
protons are protected by different amounts in different environments
A magnetic field can induce a current in a wire.
This current causes its own magnetic wire loop to move in the opposite direction of the applied field.
The nucleus feels a slightly weaker field because of the magnetic field set up by the current.
A protons is protected by electrons.
The hydroxy protons absorb at a lower field than the methyl protons, but still at a higher field than a naked protons.
The shielding effects of electrons at different positions are different because of the diverse and complex structures of organic molecule.
Let's look at what happens in an NMR spectrometer.
The sample compound has particles placed in a magnetic field.
While still in the magnetic field, the protons are subjected to radiation of a Frequency they can absorb by changing the orientation of their magnetic moment relative to the field.
If protons were isolated, they would all absorb the same magnetic field.
The shielding of protons in a molecule depends on their environment.
The radiation at different magnetic field strengths is absorbed by the protons in the molecule.
The original idea was to vary the magnetic field and plot a graph of absorption energy as a function of the magnetic field strength.
The H shielded hydroxy proton appears to be in the left field.
Information about the electronic environment of the protons in a molecule is provided by the NMR spectrum.
The relative strength of the magnetic field that causes the protons to absorb energy from the RF transmitter is what determines each environment.
There is a difference between the resonance field of the protons being observed and that of TMS.
CH3 is enough to distinguish individual protons because the signals often differ by a few thousandths of a gauss.
The methyl groups of TMS are electron-rich, and their protons are protected.
One strong absorption is given by the 12 protons in TMS.
A small amount of TMS is added to the sample, and the instrument measures the difference in magnetic field between where the protons in the sample are absorbed and where the TMS are absorbed.
The chemical shift of the protons is what determines the distance downfield of TMS.
The chemical shift in a sample is measured by a constant magnetic field by newer spectrometers.
The horizontal axis of the NMR spectrum is adjusted to show the chemical shift between the signals of a protons and TMS.
A chemical shift in parts per million can be calculated by dividing the shift inhertz by the frequencies measured in millions ofhertz.
In a 300-MHz spectrum, 1 part per million is 300 part per million.
The use of absorptionless chemical shifts standardizes the values of all NMR spectrometers.
The d scale is the most common scale for chemical shifts.
The d scale goes to the left of the spectrum when most protons are deshielded.
The spectrum is adjusted in two different ways.
Downfield toward 60 MHz more deshielded protons.
The chemical shift of a A 300-MHz spectrometer records a protons absorption at a 2130 Hz downfield, which is the same in any shielding from TMS.
Determine the chemical shift.
The fraction spectrometer is the chemical shift.
The chemical shift is not changed at 60 MHz.
There are two signals from methanol and a reference peak in the 300-MHz NMR spectrum.
The protons are absorbed by the methyl protons.
We say that the methyl protons absorb at d 3.4 because of the chemical shift.
It has a chemical shift of d 4.8.
The deshielding effects of the oxygen atom are shown by the hydroxy protons and the methyl protons.
The chemical shift in an alkane is about d 0.9.
The methyl protons are protected by an additional 2.5 parts per million.
Similar deshielding effects can be produced by other eronegative atoms.
The table compares the chemical shifts of the two compounds.
The chemical shift of the protons depends on the electronegativity of the substituent.
The chemical shift depends on the distance from the protons.
The chemical shift of the hydroxy protons is d 4.8, and it is separated from oxygen by one bond.
The chemical shift of the protons is 3.4 and they are separated from oxygen by two bonds.
The effect of an electron-withdrawing substituent decreases with increasing distance, and the effects are usually negligible on protons that are separated from the electronegative group by four or more bonds.
The decreasing effect can be seen by comparing the chemical shifts of the various photos.
The metal container at the back has a negative shielding effect.
In 1-bromobutane, protons on the magnet, cooled a carbon, and b protons are all protected by a liquid bath.
The b protons are protected by a negligible used to control the spectrometer and amount.
The third chlorine moves the chemical shift to d 7.2 for chloroform.
The peak is slightly less moved by each additional chlorine.
The chemical shift of the remaining methyl protons is changed by the addition of chlorine atoms.
The chemical shift of a protons is determined by its environment.
The reasons for some of the more interesting and unusual values can be found in the table of representative chemical shifts.
There is a table of chemical shifts in Appendix 1.
Predict the chemical shifts of the protons using Table 13-3
The carbonyl group in acetic acid is next to the methyl group.
COOH should absorb between d 10 and d 12.
The table shows how aromatic rings and double bonds affect the vinyl and aromatic protons.
The same type of circulation of electrons that shields the nucleus from the magnetic field is what causes these deshielding effects.
The induced field is at the center of the ring.
Most aromatic protons absorb in the range of d 7 to d 8.
The chemical shift for its protons is an average of all the possible orientations because benzene is tumbling in the solution.
If we were able to hold benzene problems.
All chemical shifts are affected by neighboring substituents.
The numbers assume that alkyl groups are the only other substituents present.
Appendix 1 has a more complete table of chemical shifts.
The one with the benzene ring edge-on to the magnetic field would absorb at a higher field.
The figure shows the spectrum of toluene.
The aromatics absorb protons d 7.2.
The methyl protons are absorbed by d 2.3.
The aromatic ring of electrons protects the aromatic protons from the pi electrons of an alkene.
The effect in the alkene is smaller than in benzene because there is not a large ring of electrons.
The applied field at the middle of the double bond is opposed by the magnetic field generated by the motion of the pi electrons.
The vinyl protons are on the edge of the field, where the field bends around and reinforces it.
Most vinyl protons absorb in the range of d 5 to d 6.
The applied magnetic field along the axis of the induced field ring is opposed by the magnetic field of the circulating aromatic electrons.