Nuclear magnetic resonance spectroscopy (NMR)
The most powerful tool available for organic structure determination. It is easily used with a very small sample without ruining the sample
What does the NMR spectrum provide?
Info on the structure of the compound and many structures can be determined using only the NMR spectrum
How is NMR spectroscopy most commonly used ?
With other forms of spectroscopy and chemical analysis to determine the structures of complicated organic molecules
Of the nuclei studyied using NMR, what nuclei are the most useful?
1H, 13C, 15N, 19F, and 31P, of which proton (1H) and carbon-13 (13C) NMR are most useful because H and C are the major components of organic molecules.
What does a nucleus with an odd atomic number or an odd mass number have?
Nuclear spin, which can be observed by the NMR spectrometer
Magnetic moment
A magnetic field generated by a spinning proton which moves like an electric current in a loop of wire
What happens when the proton is placed in the field of a larger magnet?
It twists to align itself with or against the field of the larger magnet
which of the protons alignments is lower energy?
The protons alignments with the field of the magnet is a lower energy than orientation against the field.
Alpha spin state
The lower energy state with the proton aligned with the field, are more common because they are lower in energy than beta spin states
Beta spin state
The higher energy state with the proton aligned against the external magnetic field
How are the protons aligned when there is no external magnetic field?
The proton magnetic moments have random orientations
how does the strength of the magnetic field affect the spin states?
the energy difference between the two states is larger in a strong magnetic field than it is in a weaker field, because the energy difference is proportional to the strength of the magnetic field
what equation describes the relationship between the energy of the spin states and the strength of the magnetic field
deltaE = gamma X (h/2pi) X B[sub]0
where:
delta E = energy difference between the alpha and beta states
h = planks constant
B[sub]0 = strength of the external magnetic field
gamma = gyromagnetic ratio, 26,753 sec^-1 gauss^-1 for a proton
gyromagnetic ratio (gamma)
a constant that depends on the magnetic moment of the nucleus under study, measured in gauss
what is the SI unit for magnetic field?
tesla (T), or 10,000 gauss
when is a nucleus "in resonance"?
when its subjected to the right combination of magnetic field and electromagnetic radiation to flip its spin. its absorption of energy is detected by the NMR spectrometer
what is the equation for a photons energy
E = hv
E = energy
v = frequency of electromagnetic wave
gamma X (h/2pi) X B[sub]0 = delta E = hv
what is the gyromagnetic ratio of a proton?
gamma = 26,753 sec^-1 gauss^-1
where do proton resonance frequencies occur in fields of currently available magnets?
the radio frequency region of the spectrum
how do real protons in organic compounds exist?
they are surrounded by electrons that partially shield them from the external magnet and generate a small induced magnetic field
induced magnetic field
generated by electrons circulating around the protons of a nucleus, oppose the externally applied field, shielding the nucleus
how does shielding of the proton affect its relationship with the magnetic field? What equation is correlated with this relationship?
the effective magnetic field at the shielded proton is always weaker than the external field, so the applied field must be increased for resonance to occur at a given frequency: B[sub]effective = B[sub]external - B[sub]shielding
what would happen if all protons were shielded by the same amount? Does this happen?
they would all be in resonance at the same combination of frequency and magnetic field, but this doesn't happen b/c protons in different chemical environments are shielded by different amounts.
deshielding
some more electronegative atoms in a compound will draw the e-'s away from the proton, deshielding it so it absorbs at a lower field
why are shielding effects of electrons at various positions generally different?
because of the diverse and complex structures of organic molecules
what aspects of the NMR spectrum do we consider?
1. the # diff. absorbtions (also called signals, or peaks) implies how many diff. types of protons are present
2.the amount of shielding shown by these absorbtions implies the electronic structure of the molecule close to each type of proton
3. the intens
what happens in an NMR spectrometer?
protons in the sample are placed in a magnetic field, which they align with or against. they are then subjected to radiation of a frequency they can absorb by changing the orientation of their magnetic moment relative to the field. because of shielding, t
the simplest NMR consists of what 4 parts?
1. a stable magnet, with a sensitive controller to produce a precise magnetic field
2. a radio frequency (RF) transmitter, emitting a precise frequency
3. a detector to measure the samples absorption of RF energy
4. a recorder to plot the output from the
chemical shifts
the difference in parts per million between the resonance frequency of the proton being observed and that of tetramethylsilane. AND/OR the variations in the positions of NMR absorbtions, arising from electronic shielding and deshielding, called chemical s
wy is a reference compound used to compare the magnetic field strength of resonances of the sample and resonance protons?
because its hard to measure the absolute field where a proton absorbs with enough accuracy to distinguish individual protons because the signals differ by so little. determining the value relative to a reference compound is much more accurate
what is the most common NMR reference compound?
tetramethylsilane (TMS), (CH3)4Si. has 12 identical proton absorbtions that have high field strength = upfield of almost all other NMR signals
give the equation for chemical shift (ppm)
chemical shift (ppm) = (shift downfield of TMS (Hz)/total spectrometer frequency (MHz))
what is the most common scale of chemical shifts?
S (delta)
the effect of an e- group on the chemical shift also depends on what?
its distance from the protons: the effect of an electron withdrawing substituent decreases with increasing distance, is negligible after 4 bonds/beta protons
what happens if more than one electron withdrawing group is present?
the deshielding effects are almost additive
how do aromatic rings effect vinyl and aromatic protons?
they produce late deshielding effects. the aromatic ring of pi bonding electrons acts as a conductor and the external magnet induces a ring current, at the center of the ring, the induced field acts to oppose the external field. the induced field lines cu
how do double bonds effect vinyl and aromatic protons?
the pi electrons of an alkene deshield the vinyl protons in the same way that an aromatic ring of electrons deshields the aromatic protons, but the effect is not as large because there is not such a large, effective ring of electrons as there is in benzen
how does deshielding affect acetylene protons?
they absorb less than vinyl protons. the triple bond has a cylinder of electron density surrounding the sigma bond. the orientation of the cylinder can circulate to reduce an induced magnetic field with an axis that the acetylene proton lies on. results i
how does deshielding affect aldehyde protons?
aldehyde protons (-CHO) are deshielded both by the circulation of electrons in the double bond and the inductive electron withdrawing effect of the carbonyl oxygen atom
how does desheilding affect hydrogen bonded protons?
the chemical shifts of O-Hs and N-Hs depend on the concentration; in concentrated solns, the protons are deshielded by H bonding and absorb at a relatively low field. when diluted with a non H-bonding solvent, H-bonding becomes less important, absorb even
how does desheilding affect carboxylic acid protons?
b/c they are bonded to an O next to a carbonyl group, they have considerable positive character, they are strongly deshielded and absorb at higher chemical shifts
what ors the # of NMR signals correspond to?
the # of diff. kinds of protons present in the molecule
chemicaly equivallent
protons in identical chemical environments with the same shielding that have the same chemical shift
accidentally equivalent
protons that are not chemically equivalent but happen to absorb at the same chemical shift
what is the area under a peak proportional to?
the # of H's contributing to that peak
integrators
part of the NMR spectrometer that computes the relative area of the peaks. it draws a 2nd trace that rises when it goes over a peak, the amount the integral rises being proportional to the area of that peak
what happens to the proton in the NMR if there are other protons nearby?
their small magnetic fields also affect the absorption frequencies of the protons being observed
spin-spin splitting
the splitting of signals into multiplets, results when 2 diff types of protons are close enough that their magnetic fields influence each other. it is a reciprocal property: if 1 proton splits anther, the 2nd proton must split 1st
magnetically coupled protons
2 diff types of protons that are close enough that their magnetic fields influence each other
can protons that absorb at the same chemical shift split each other?
no because they are in resonance at the same combination of frequency and field strength
N+1 rule
the multiplicity of an NMR follows the rule: if a signal is split by N neighboring equivalent protons, it is split into N+1 peaks
help w/ pg 580 stuff!
help w/ pg 580 stuff!
will protons on non adjacent carbons be magnetically coupled?
no they are too far away so there is no spin-spin splitting, as it takes place primarily through the bonds of the molecule
geminal protons
protons bonded to the same C atom
when can geminal protons split each other
only if they are nonequivalent
how are multiplets usually not symmetrical?
they usually "lean" upward toward the signal of the protons responsible for splitting
what are the steps to drawing an NMR spectrum?
1. determine how many types of protons are present and their proportions
2. estimate the chemical shifts of the protons
3. determine the splitting patterns
4. summarize each absorption in order, from lowest died to highest
5. draw the spectrum, using the
what happens when 2 magnetically coupled protons have equal effects on each other?
the splittings are equal
coupling constant
the distance between the peaks of a multiplet, measured in Hz, represented by J so that the fouling constant between H^a and H^b is J[sub]ab
what causes multiplets to have the same coupling constants?
adjacent groups of protons that split each other
what does the magnetic affect that one proton has on another depend on?
the bonds connecting the protons, but not on the strength of the external magnetic field, so it doesn't vary wight he field strength of the spectrometer
what is the most commonly observed coupling constant?
the 7 Hz splitting of protons on adjacent carbon atoms in freely rotating alkyl groups
what do splitting patterns and their coupling constants help to do?
distinguish among possible isomers of a compound and distinguish stereoisomers
what do unusually large coupling constants in the vinyl region usually indicate?
stereochemistry around a double bond
see pg 585 for a table of typical proton coupling constants
see pg 585 for a table of typical proton coupling constants
complex splitting
where signals are split by adjacent protons of more than 1 type with diff coupling constants
doubly of doublets
a doublet in which both peaks has split again so there are 4 total peaks
how can a doublet of doublets be analyzed?
a diagram called a splitting tree
what often result from stereochemical differences?
different chemical shifts for different protons on the same carbon atom
how do we know if similar appearing protons are equivalent: HELP WITH THIS!!!!
mentally substitute another atom for each of the protons in question; if the same product is formed, the protons are chemically equivalent.
what happens if a different product is formed in which the replaced atoms are diastereomers
the protons are called diastrereotopic protons, which appear at diff. shifts and can split each other
what happens if a different product is formed in which the replaced atoms are enantiomers?
the protons are enantiotopci, can not be distinguished by NMR = "equivalent by NMR
what is the most stable form of diastereotopic protons?
when they exist in diff. chemical environments; they experience diff chemical environments and are nonequivalent by NMR and split each other
what happens when a molecule contains an asymmetric C atom?
the protons on any methylene groups are usually diastereotopic and may or may not be resolved in the NMR
does the NMR give an instantaneous picture of a molecule?
no, certain functional groups affect absorbtion
what are the two kinds of protons in the chair conformation?
axial and equitorial protons, which switch during chair-chair interconversions.
how does temp effect chair-chair interconversions?
at room temp, the chair-chair interconversions are fast on an NMR time scale and show only one, averaged peak
at low temps, the chair-chair interconversions are retarded, and show two nonequivalent types of protons that split each other, giving 2 broad ba
freezing out
the technique of using low temps to atom conformational interconversions
at what rate does proton exchange occur in most alcohols and carboxylic acids, adni in most amides and amines?
fast, producing one sharp, averaged signal. if it is slow, splitting occurs. if only moderately slow, we may see a broadened peak that is neither cleanly split nor cleanly averaged.
what kind of signal do protons on N show?
a broadened signal, because of the moderate rate of exchange and b/c of the magnetic properties of nitrogen in the nucleus. may give sharp and cleanly split absorbtions, sharp and unsple=it, or broad and shapless
what do the proton shifts of O-H and N-H depend on?
the concentration of the solvent
what can be used to simplify the complex splitting patterns of O-H and N-H?
deuterium- is invisible in H NMR so the resulting spectrum shows the loss of a signal from adjacet H's
when interpreting H NMR spectra, look for structural info based on what?
# absorbtions
chemical shifts
areas of peaks
spin-spin splitting
what groups are invisible in H NMR?
carbonyls ans internal alkynes
the development of what made C NMR possible?
the development of Fourier transform NMR spectroscopy and high field superconducting spectrometers
what does the CNMR determine?
the magnetic environments of C atoms
why did C NMR take loger to become a regular technique than H NMR?
because C NMR signals are much weaker than proton signals
why dosnt C12 (the C in 99% of molecules) give a signal?
b/c it has an even # of protons and an even # of neutrons, so it has no magnetic spin and can't give off a signal
why is C NMR less sensitive than NMR?
C13 has an odd # neutrons, giving the same spin as a proton. b/c only 1% of all compounds have a magnetic C13, the 13 C NMR has a sensitivity decreased by a factor of 100, plus the gyromagnetic ration (and therefore the resonance frequency) of 13 C is onl
besides decreased sensitivity, why is 13C NMR not a good way to obtain a spectrum?
b/c its a continuous wave spectrometer that produces weak signals that get lost in the noise= ing and tedious process to compensate for the noise
what happens when magnetic nuclei are placed in a uniform magnetic field and irradiated with a pulse of radio frequency close to their resonant frequencies
they produce a complex signal that decays as the nuclei lose the energy they gained from the pulse
free induction decay (or transient)
a complex signal that decays as the nuclei lose the energy gained from the pulse, produced by the precession of many nuclei at slightly different frequencies. contains all info needed t calculate a spectrum
FID- free induction decay
can be recorded by a radio receiver and a computer in 1 to 2 seconds and averaged in a few inures. a computer converts tha avg transients into a spectrum
fourier transform
mathmatical technique used to compute the spectrum from free induction decay
fourier transform spectroscopy
the technique of using pulses and collecting transients; requires sophisticated electronics capable of generating precise impulses and accurately receiving the complicated transients
are C13 signals subject to shielding?
yes, they are deshielded by electron withdrawing substituents
how does the size of C NMR chemical shifts compare to those of H NMR?
the C NMR shifts are usually 15 to 20 times larger (b/c the C atom is the one closer to a shielding/deshielding group than its attached H)
how does the resonance frequency of C NMR compare to H NMR?
13C is 1/4 that of the proton, as is the gyromagentic ratio
how do the peak areas of C NMR compare to H NMR?
the areas of 13C NMR aren't proportional to the # of C's: C's with 2 or 3 protons attached give the strongest signals, C's with no protons give weak signals
how does the spin-spin splitting of C NMR compare to H NMR?
only 1% of 13C NMR sample are magnetic, so theres only a small probability that an observed 13C nucleus is adjacent to another, therefore C-C splitting can be ignored, but C-H coupling is common
how are C NMR spectra recorded to simply it?
pby using proton spin decoupling
proton spin decoupling
where protons are continuously irradiated with a broadband "noise" proton transmitter, resulting in all protons in resonance and rapidly flip their spins. the carbon nuclei see an average of the possible combinations of proton spin states. each C signal a
off resonance decoupling
simplifies the spectrum but allows some of the splitting info to be retained. only the 13C nuclei are split only by the protons directly bonded to them. the N+1 rule applies. this method is recognized by the appearance of TMS as a quartet at 0 ppm, split
what is the best procedure for obtaining a C NMR spectrum?
to run the spectrum twice: the singlets in the broadband decoupled spectrum indicate the # of non equivalent types of C atoms and their chemical shifts. the multiplicities of the signals in the off resonance decoupled spectrum indicate the number of hydro
DEPT (Disortionless Enhanced Polarization Transfer) def, and advantages:
a more recent technique that provides the same info as off resonance decoupling, but is easier to run on modern, computer controlled Fourier transform spectrometers and gives better sensitivity and avoids overlapping multiplets b/c all the peaks remain de
What occurs during DEPT?
each 13C nucleus is magnetically coupled to the protons bonded to it, which, under the right circumstances, allows the transfer of polarization from the protons to the C nucleus. the # of protons bonded to the 13C nucleus determines how this polarization
what kind of peaks are produced by the methyl groups (C's,CH, CH2, and CH3) in the normal spectra, DEPT-90 spectrum, and DEPT-135 spectrum?
C's: appears only in the normal spectrum, but not in either DEPT spectrum
Methine C's (CH): give normal pos. peaks in all 3 spectra
Methylene (CH2):give normal peaks in normal spectrum, no peaks in DEPT-90, and neg. peaks in the DEPT-135 spectrum
Methyl (
How can 13C NMR spectra be interpreted? What info is used?
using the same principles as interpreting H NMR spectra, often its easier, using the following info:
1. the # diff. signals implies how many diff types of C atoms are present
2. the chemical shifts of those signals suggest what types of functional groups
what is the goal of NMR spectroscopy?
to get the most uniform magnetic field possible ad to have the sample behave as if it were all at a single point in the magnetic field, with very molecule subjected to exactly the same external magnetic field
nuclear magnetic resonance imaging, or magnetic resonance imaging (MRI)
uses the same physical efforts as NMR, but its goals are almost pop.
what happens in nuclear magnetic resonance imaging?
a heterogeneous sample (body) is placed in a magnetic field of a large bore superconducting magnet. the magnetic field is purposely uniform, with a gradient that allows just the protons in one plane of the sample to be in resonance at any 1 time. use of c
relaxation times
makes the technique even more useful: in strong magnetic field, slightly more proton spins are aligned with the field (lower energy state) than against it. a radio frequency pulse of just the right duration inverts some spins, increasing the # of spins or
see pgs 612-613 for review of solving spectroscope problems
see pgs 612-613 for review of solving spectroscope problems