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NUCLEAR MAGNETIC RESONANCE

Teacher Student Dr. K.R.Gupta Prashant. R. Rathod

rprashant85@gmail.com

S.K.B. College of Pharmacy. Kamptee Nagpur

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NMR

1. Chemical Shift. 2. Factors Affecting Chemical Shift. 3. Spin-Spin Coupling. 4. Spin-Spin Decoupling. 5. Application. 6. 13C NMR

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1. CHEMICAL SHIFTIntroduction :

An NMR spectrum is a plot of the radio frequency applied against absorption.

A signal in the spectrum is referred to as a resonance. The frequency of a signal is known as its chemical shift.

The diff in the absorption position of a particular proton from the absorption position of a reference proton is called as Chemical Shift. The most commonly employed reference compound is tetramethylsilane. Nature of the electron cloud about the nucleus will govern the region of the rf portion of the electromagnetic spectrum from which the nucleus absorbs energy.

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Chemical shift depends on the chemical environment of the proton Proton in the different chemical environment - different chemical shift. Proton in the same chemical environment - same chemical shift.

Regions in which the induced field supports or adds to the external field are said to be deshielded, because a slightly weaker external field will bring about resonance for nuclei in such areas.

However, regions in which the induced field opposes the external field are termed shielded because an increase in the applied field is needed for resonance.

Shielded regions are designated by a plus sign, and deshielded regions by a negative sign.

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Deshielded = The induced magnetic field reinforces the applied magnetic field.Shielded = The induced magnetic field opposes the applied magnetic field

Shield shift & Deshield shift in the position of NMR absorption arising from shielding or deshielding of proton are called Chemical shift, and degree of shielding depends on the density of the circulating electrons. A peak at a chemical shift, d, of 10 ppm is said to be downfield or deshielded with respect to a peak at 5 ppm, or if you prefer, the peak at 5 ppm is up field or shielded  with respect to the peak at 10 ppm.

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SHIELD SHIFT DESHIELD SHIFT

Requires a higher applied field strength.

Shifts the absorption up field or higher field

Requires a lower applied field strength.

Shifts the absorption down field or lower field

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The field experienced at a particular atomic nucleus in a molecule H= Ho(1- σ)

Where, σ = Shielding or screening constant of atom Ho = Applied fieldAnd, Resonance frequency for the particular atom

y Ho [ 1- σ ] V = 2 л

Thus, the amount of shielding determines how much the applied field has to be increased in order to attain that field [H] which induced the transition in the nucleus. The field independent scale for che shift (б) & the screening constant is,

б = σ – σ ref =

Chemical shift values are proportional to the applied magnetic field.

H - H ref

H ref

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Under the influence of the magnetic field, electrons bonding the proton tend toprecess around the nucleus in a planeperpendicular to the magnetic field. Thisdevelops a secondary field that opposesthe primary field. So nucleus experiences resultant field that is smaller (I.e., shielded from full effect of primary field). As a consequence, external field must be increased to cause nuclear resonance. The frequency of the precession and the magnitude of the secondary field is a direct function of the external field.

The circulation of electrons around a nucleus in an applied field is called diamagnetic current and the nuclear shielding resulting from it is called diamagnetic shielding.

Diamagnetic Shielding:

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The magnetic field at the nucleus is not equal to the applied magnetic field; electrons around the nucleus shield it from the applied field. The difference between the applied magnetic field and the field at the nucleus is termed the nuclear shielding. Consider the s-electrons in a molecule. They have spherical symmetry and circulate in the applied field, producing a magnetic field which opposes the applied field. This means that the applied field strength must be increased for the nucleus to absorb at its transition frequency. This up field shift is also termed diamagnetic shift.

Electrons in p-orbital's have no spherical symmetry. They produce comparatively large magnetic fields at the nucleus, which give a low field shift. This "deshielding" is termed paramagnetic shift.

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The chemical shift given by a particular proton is the frequency difference between its absorption and the absorption from tetramethylsilane, TMS, Si(CH3)4. The actual chemical shift depends on the radio frequency used, which in turn depends on the magnetic field strength. The shift can be made field-independent by defining it thus:

The value of d is quoted in parts per million – this means that for a machine operating around 200MHz the shifts correspond to frequency differences of a few hundred Hz.

The chemical shift depends on the environment of the hydrogen atom concerned. The local external magnetic field, the one with which the nucleus interacts, is not the same as the applied external field. This is because the nucleus is shielded from this field to a greater or lesser extent by the other atoms in the vicinity and their electrons.

x 106

vo= (v– vo) x 106

d =vo

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2. FACTOR AFFECTING CHEMICAL SHIFT

1. Electro negativity2. Effect of magnetic anisotropy a) Ethene

b) Ethyne c) Benzene & Aromatic Protons

3. Hydrogen Bond 4. Solvent Effect

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FACTOR AFFECTING CHEMICAL SHIFT 1. Electro negativity :- In organic compound--------proton covalently bonded to C,N,O,

by a sigma bond --------when placed in a strong magnetic field--------electrons of sigma bond circulate-------generate a small induced magnetic field which opposes the applied magnetic field.

Shielding depends on electron density, so near by

electronegative atom withdraws electron density from the neighborhood proton so required small applied field.

Attachment of electronegative atom then Deshielding increase with increase in electro negativity Shielding decrease with increase in electro negativity

Order of electro negativity— I < Br < Cl < F Deshielding of CH3 proton

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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. This is reflected by the plot shown in the graph to the left which is based on the data shown below.

Electro negativity

Chem

ical

Shift

These effects are cumulative, so the presence of  more electronegative groups produce more deshielding and therefore, larger chemical shifts.

compound

σ/ppm

CH4 CH3Cl CH2Cl CHCl3

0.23 5.30 7.273.05

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The word "anisotropic" means "non-uniform". Magnetic anisotropy means that there is a "non-uniform magnetic field". Occurrence of shielding & Deshielding can be determine by the location of proton in the space--------- called Space effect. In a molecule having group or atom with multiple bond character [ bond],interatomic electron circulation is possible & circulation of electrons is only in certain performed direction about the bond, thus such bonds exhibit anisotropy of diamagnetic susceptibility. As a result, the nearby protons will experience 3 fields: the applied field, the shielding field of the valence electrons and the field due to the p system. Depending on the position of the proton in this third field, it can be either shielded (smaller d) or deshielded (larger d), which implies that the energy required for, and the frequency of the absorption will change. .

2. Effect of magnetic Anisotropy / Space Effect :-

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As the nature of carbon moves from SP3 to SP2,attached protons resonates at lower field. This tendency is not adhered to, however since protons on SP hybridized carbon, alkynic protons, resonates up field of alkenic protons & cyclopropane protons which possess some SP2 character are among the most shielded of protons. Localized diamagnetic fields are induced about an individual nucleus by the intra-atomic circulation of its electron cloud. In a molecule having groups of atoms with multiple bond character [ π electron ], interatomic electron circulation is possible. Here the circulation of electrons is only in certain preferred directions about the bond & thus such bond exhibit anisotropy of diamagnetic susceptibility [ Diamagnetic anisotropy ]. The effect, either shielding or deshielding, on a nearby proton depends

a) Upon its distance from the multiple bond &b) Upon its orientation with respect to the bond,

Ethene, Ethyne, & Benzene shows this phenomenon.

CC

C

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A) ETHENE

When its double bond is oriented at right angle to the applied field, ethene has a π electron circulation about the bond. The induced magnetic field reinforce the applied field at the protons, which are consequently deshieled shown in fig (a).

In fig (b) shielding cones (+) & regions of deshielding (-), any proton held in these regions will be shielding & deshielding accordingly. Carbonyl groups [C=O] behave similarly.

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B) ETHYNE This is a linear molecule with an axis of symmetry passing through the triple bond.Orientation of the ethyne molecule with its longitudinal axis parallel to the mag field causes a diamagnetic circulation of π – electrons in such a way that its

protons experience an induced magnetic field opposing the applied field. The protons are therefore shielded by the anisotropy of the triple bond & resonate at 2.6 σ .

Induced local magnetic field of the pi electrons is against the applied field, it requires a greater applied field to bring an acetylenic hydrogen into resonance

Induced flow of electrons in the pi system of alkyne

1.

2.

1

2

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C) BENZENE & AROMATIC PROTONS

At the aromatic protons ------------- induced magnetic field ----- reinforces the applied field ---------- therefore resonates at lower field.The proton lying above the plane of an aromatic ring------ will be shielded.The proton in the plane of an aromatic ring----------------- will be Deshieled.

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.

Hydrogen bonding decrease the electron density around the proton, causing deshielding. If the temperature of the sample increase, weaken the hydrogen bonding-------causes up shield shift.The Chemical shift of

a) Intermolecular hydrogen bonded protons are dependent of temperature & solute concentration.

b) Intramolecular hydrogen bonded protons are independent of temperature & solute concentration.

Protons that are involved in hydrogen bonding (this usually means -OH or -NH) are typically observed over a large range of chemical shift values.  The more hydrogen bonding there is, the more the proton is deshielded and the higher its chemical shift will be. However, since the amount of hydrogen bonding is susceptible to factors such as solvation, acidity, concentration and temperature, it can often be difficult to predict.

3. Hydrogen Bonding :-

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4. SOLVENT EFFECTS

Aromatic solvents often induce large “ Solvent Shift ” effect, particularly in proton groups adjacent to a carbonyl or other polar function. Polar compounds often exhibit good solvent shift effect with Benzene, Pyridine.

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SPIN – SPIN COUPLING / SPIN – SPIN SPLITTING Many PMR spectra have resonance bands split into doublets,

triplets, quartets etc.This phenomenon is known as Spin coupling or spin-spin splitting. The interaction of the spin of a proton with that of another proton or protons attached to an adjacent carbon is the main cause of spin spin coupling.The interaction involves the spins of the bonding electrons of all three bonds, H-C, C-C, O-H. Consider two protons on adjacent carbons

R gr not a protons & do not bear protons on nuclei adjacent to those under consideration.

Then provided that the R groups are not all the same, the signal for each proton will appear as a

doublet.

H H

R”’ C C R

R” R’

(A) (X)

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.

JAX is identical for each signal & independent of field strength. An increase in field strength ---- VA & VX to shift further apart -----------but JAX is constant. Proton A can have either a spin parallel or ant parallel with respect to that of X,& likewise proton X is aware of two spin forms of A. Each proton being aware of two states of its neighbor experiences two effective fields from it, & generates two effective field from it, & generates two corresponding resonance signals. Since the populations in each spin state are the same, resonance lines of equal intensity result.

H H

R”’-C--C-R

R” R’

A X

VA VX

JAx

H

JAx

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J COUPLING

The spacing between the peaks as a result of spin-spin splitting

is called as J coupling of unit cycles/sec or Hertz [Hz]. J value is independent of field strength. J is the coupling constant between two protons & J values give further information on molecular structure & stereo chemical feature.

Thus e.g.- J value between adjacent axial hydrogen in a cyclohexane is 10-13 Hz, while J between axial & equatorial or between two equatorial hydrogen's is 3-5 Hz.

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.

Nuclei which are close to one another could cause an influence on each other'seffective magnetic field. If the distance between non-equivalent nuclei is less than or equal to three bond lengths, this effect is observable. This is called spin-spin coupling or J coupling.

Each spin now seems to has two energy ‘sub-levels' depending on the state of the spin it is coupled to:

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EXAMPLE, OF COUPLING IN H NMR THE SPECTRA OF 1,1-DICHLOROETHANE SHOWN BELOW

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The methine -CH can adopt two alignments with respect to the applied field. As a result, the signal for the adjacent methyl -CH3 is split in two lines, of equal intensity, a doublet.

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The proximity of  "n" equivalent H on neighboring carbon atoms, causes the signals to be split into "n+1” lines. This is known as the multiplicity or splitting Or coupling pattern of each signal. Equivalent protons (or those with the same chemical shift) do not couple to each other. If the neighbors are not all equivalent, more complex patterns arise. To a first approximation, protons on adjacent sp3 C tend to behave as if they are equivalent.

The methyl -CH3 protons give rise to 8 possible combinations with respect to the applied field. However,some combinations are equivalent & there are 4 magnetically different effects. As a result, the signal for the adjacent methine -CH is split into four lines, of intensity ratio 1:3:3:1, a quartet

Now consider the effect of the -CH3 group has on the adjacent -CH . 

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Ha = no neighboring proton, so n + 1 = 0 + 1 = Singlet

Hb = 3 neighboring protons, so n + 1 = 3 + 1 = Quartet. It is the furthest downfield due to proximity to the oxygen ! Hc = 2 neighboring proton, so n + 1 = 2 + 1 = Triplet

Hb is the most deshielded (highest ppm) becoz the proton are adjacent to the “0” & the C=0 which are both electron withdrawing, leaving the Hb protons deshieled. Ha protons are more deshielded than Hc protons becoz they are next to the C=0.

Splitting = n + 1,

Where, n = neighboring protons. Must be within 3 bonds or less.

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SPIN DECOUPLING

If the long rang coupling is present or if complex absorption gives multiplets then it becomes very difficult to determine structure. A proton spin couples with neighboring proton because it has sufficient life time in a given spin state. If life time of a spin is reduced i.e. if the exchange betn spin state of nuclei is speeded up then little information about the neighboring nuclei will be obtained.

C C

H H

Ha Hb

Ha & Hb are in different environment. 2 Doublets are observed.

If Ha is strong irradiated with strong correct RF ---- so that the rate of its transition betn the two energy states becomes larger ------ then the life time of this nucleus in any one spin state will be too short to resolve coupling with Hb. So the Hb proton will have 1 time average view of Ha & hence Ha will come to resonance only once & Hb will appear as a singlet & not doublet.

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Time dt is needed to resolve the two lines of a doublet which is related to J. Thus information of doublet is possible if each spin state of Ha has a life time greater than dt.Due to double irradiation life time becomes still less & thus coupling is not possible, so it result in a singlet by spin-spin decoupling.

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APPLICATION

1. Structure elucidation :- NMR spectrum indicates ---- The no of main NMR signals ------ No of equivalent proton. The che shift -------- what type of hydrogen atoms are present. The spin-spin splitting ------- possible arrangement of groups. From the area of peak --------- no of hydrogen nuclei present in each group. The distinction betn the following isomer easily made by NMR spectra.

CH3 - CH – CH3

Cl

CH3 - CH2 - CH2 - Cl

Three signal Two signal Three proton triplet (CH3 --) Two proton sextet (-- CH2--) Two proton triplet (--CH2 Cl)

Doublet (6H) -- Up field Septet (1H) -- Downfield

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Intermolecular hydrogen bonding shifts the absorption for a concerned

proton downfield. Intramolecular hydrogen bonding also shifts the absorption downfield. The extent of hydrogen bonding varies with the solvent,

concentration of solution & the temperature. Here two types of hydrogen bonding can be distinguished as the intramolecular hydrogen bonding is not concentration dependent.3. Detection of Aromaticity

Proton attached to the Benzoyl, polneuclear & heterocyclic compds whose π electrons follow Huckel’s rule are extremely deshieled due to the circulating sextet (ring current) of π electron. As a result , the signal for the aromatic protons appear at a very low field than that observed even for benzene.From this, the aromatic character of the compound under investigation can be predicted

2. Detection of hydrogen bonding

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The Cis & Trans isomer can be distinguished as the concerned protons have different values of the chemical shifts as well as the coupling constant.

Similarly, the Axial & equitorial position of the proton or group can be distinguished from their different values of the coupling constants.

C=CHA HB

C=C

HA

HB

4. Distinction betn cis-trans isomers &conformers

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5. Detection of electronegative atom or group :- The presence of an electronegative atom or group in the neighborhood of the proton causes deshielding & the signal is shifted downfield. Greater the electro negativity of the adjacent atom, smaller is the tau value of absorption for the concerned proton.

6. Detection of some double bond character due to Resonance :- In some compound, the molecule acquires a little double bond character due to resonance. Due to this, two signals can be expected for apparently equivalent protons. E.g. Consider NN- dimethyl formamide,

F > C > N & so on.

Downfield Shift

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For structure (a), two signals (singlet's) expected with peak area 6:1 as the two methyl's are exactly equivalent.

In structure (b), the presence of double bond restricts rotation & now the two methyl groups remain no longer equivalent. For this structure, two signals appear for two methyl groups.

C-NCH3

CH3O

H

═..

C=NCH3

CH3O

H

═.... .. +

b

a

c

b

a

a

7. Important in quantitative analysis

In the NMR spectrum of pure ethanol, a triplet is formed for the OH proton but when water is added in alcohol, then due to proton exchange, the triplet collapses to a singlet. The position of this singlet depends upon the water content in alcohol. From the value of chemical shift, the ratio of water & alcohol can be estimated by comparing with the known results.

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FACTOR AFFECTING CHEMICAL SHIFT

1. Electro negativity2. Effect of magnetic anisotropy a) Ethene

b) Ethyne c) Benzene & Aromatic Protons

3. Hydrogen Bond 4. Solvent Effect

There are two major factors that influence chemical shifts (a) deshielding due to reduced electron density (due electronegative atoms) & (b) anisotropy (due to magnetic fields generated by p bonds).

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One method of solving this problem is to report the location of an nmr signal in a spectrum relative to a reference signal from a standard compound added to the sample. Such a reference standard should be chemically unreactive, and easily removed from the sample after the measurement. Also, it should give a single sharp nmr signal that does not interfere with the resonances normally observed for organic compounds. Tetramethylsilane, (CH3)4Si, usually referred to as TMS, meets all these characteristics, and has become the reference compound of choice for proton and carbon nmr.

Reference Compound ;-

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REFERENCE COMPOUND ;- TETRA METHYL SILANE

Advantage :-

Chemical inert, magnetically isotopic, soluble in most of organic solvent. Highly volatile [B.P.-27oC] so easily removed, All hydrogen atom remain in identical environment. More strongly shielded than the protons in any pure organic compound. Low electronegative of silane & four electron releasing CH3 produce

maximum electron density around all equivalent protons hence powerful shielding effect.

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13C NMR

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INTRODUCTION

In this type of NMR spectroscopy, one of the isotopes of carbon, 13C is used. The isotope 13C makes up only 1.1% of naturally occurring carbon, but the sensitivity of modern spectrometers makes this level adequate for the measurement of CMR spectra. One of the major practical problem in CMR spectroscopy is the splitting of signal. Too much splitting would be too complicated to interpret easily. The unwanted splitting is removed by decoupling the 13C spin from that of the proton. This can be done by two principle way, depending upon the frequency of the radiation used in the double resonance.

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.

CH3-CH2-CH-CH3

Br

a c d b

1. One method of decoupling gives a completely proton-decoupled spectrum. This spectrum shows no splitting at all, it consist of a set of single peaks, one for each carbon- or each set of equivalent carbons - in the molecule. There are 4 carbons in this molecule, all are different – that is non-

equivalent in the spectrum 4 peak, one for each the four carbons.

2. A second method of decoupling (called off-resonance) gives a spectrum which shows splitting of the carbon signal only by protons attached to that carbon itself. this is , we see only 13C-H coupling & not 13C -C-C-H coupling, this kind of spectrum as a proton-coupled spectrum. For each carbon, then, the multiplicity of the signal depends upon how many protons are attached to it. No proton-Singlet 1 P-Doublet, 2 P-Triplet, 3 P- Quartet.

C C

CC

H

H

H

H

H

H

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CHEMICAL SHIFT

The 13C chemical shift normally range between σ 0-200. Alkyl carbons display themselves in the most up field position. Carbon attached to an electronegative element such as oxygen are shifted downfield, the carbons of an aromatic ring appear further downfield & so on. The carbon of carbonyl groups are easily identified becoz they appear farthest downfield. The changes within these regions can be correlated with structure variations in different chemical classes.The order as follow,

C=C &Aromatic

C Ξ CC –OC – N etc

=

0 σ220 200 180 160 140 120 100 80 60 40 20

TMS

C 0

Alkyl

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The guiding principle, the carbon atom shows a chemical shift which is 15-20 times as great as that of the protons attached to it. The factors that control the chemical shift of hydrogen's e.g.

. The electro negativity of nearby atoms have similar effects on the chemical shift of carbon signal.

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H & 13 C NMR

1. The signal for the NMR of a 13C nucleus is 10-4 times weaker than the signal for a hydrogen nucleus2. A signal for a 13C nucleus is only about 1% as intense as that for 1H because of the magnetic properties of the nuclei, and at the "natural abundance" level only 1.1% of all the C atoms in a sample are 13C (most are 12C)

3 13C signals are spread over a much wider range than 1H signals making it easier to identify and count individual nuclei.

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1. Both give us information about the number of chemically nonequivalent nuclei (nonequivalent hydrogen's or nonequivalent

carbons)

2. Both give us information about the environment of the nuclei (hybridization state, attached atoms, etc.)

1H 13C

Nuclear spin

Resonance frequency at 235 telsa

Natural Abundance

Sensitivity for an equal number of nuclei

Shift Range

1/2 1/2

25.2 MHz100 MHz

1.1 %99.9 %

0.016 %1.00 %

600 ppm20 ppm

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INTERPRETING 1H-NMR SPECTRA

How many types of H ? Indicated by how many groups of signals there

are in the spectra. What types of H ?  Indicated by the chemical shift of each group How many H of each type are there? Indicated by the integration (relative area) of the signal for each group. What is the connectivity ? Look at the coupling patterns. This tells you what is next to each group

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C13-NMR INTERPRETATION

1. No of lines in NMR spectrum indicates, how many types of carbons are present.

2. Signal Height/Size a. Carbons without any attached H’s are short. This is common for carbonyls

(aldehydes are the only carbonyl carbons that have hydrogen's attached) and for substituted carbons in a benzene ring. b. Symmetry duplication multiplies signal height (if you have two copies of

a carbon, the line will probably be taller than normal!)

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3. Aromatics, Symmetry, and C-13 Signals. Most aromatics have symmetry, and both the number of aromatic lines and the splitting of the aromatic lines can be indicative of the substitution pattern on a benzene.Mono- and Para disubstituted benzenes have symmetry. 4 lines, d, d, d Monosubstituted benzene. (Has symmetry). 4 lines, s, d, d Para-disubstituted benzene. (Has symmetry). 6 lines, s, d, d, d, d Ortho- or meta-disubstituted benzene. (Has no symmetry).

4. C13 NMR’s are often acquired as “decoupled” spectra, in which each carbon signal appears as a singlet. It is also possible to get “coupled” C13 NMR’s with splitting. These splitting values are very useful, and follow the N+1/N-1 rules (the number of lines is one greater than the number of attached H’s). Quartet (q) CH3 Triplet (t) CH2 Doublet (d) CH Singlet(s) C (no attached hydrogen's).

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Thank You