Analytical Instrumentation: NMR Spectroscopy

Nikhilbinoy.CAssistant ProfessorICE DepartmentAnalytical Instrumentation:NMR Spectroscopy

BasicsNuclear Magnetic Resonance Spectroscopy.Most powerful technique for chemical analysis.Accurate and non-destructive method.

Principle:Nuclear SpinThe nuclii of certain isotopes have an intrinsic spinning motion around their axis.Nuclear spin is the total nuclear angular momentum quantum number, characterized by I.Spin quantum number I may be integral, half-integral, or zero.If the number of neutrons and the number of protons in a nucleus are even, the spin (I) would be zero. Nuclii of this type do not give rise to an NMR signal, neither do they interfere with an NMR signal from other nuclii. Eg: - C12 , O16.Nuclii having either the number of protons or the number of neutrons as odd have half integral spin. Eg: - H1, B11, P31, etc.Nuclii which have both the number of neutrons and the number of protons as odd, would have integral spin. Eg: - H2, and N14.

Principle:Nuclear Spin

Principle:Spinning Charge in an External Magnetic Field

Principle:Spinning Charge in an External Magnetic FieldAll spinning nucleus under the influence of external magnetic field are placed at origin of the three coordinate system.Nuclii with half integral spin I=1/2 behave like charged, spinning spherical bodies and give the best resolved spectra.Nuclii with spin greater than I=1/2 behave like non-spherical, charged rotating bodies.

Principle:Spinning Charge in an External Magnetic FieldAll spinning nucleus under the influence of external magnetic field are placed at origin of the three coordinate system.

Principle:Spinning Charge in an External Magnetic FieldAll spinning nucleus under the influence of external magnetic field are placed at origin of the three coordinate system.

Principle:Spinning Charge in an External Magnetic FieldQuantum mechanics tells that, for net absorption of radiation to occur, there must be more number of nuclii in lower energy state than in the higher energy one.All magnetic moments are represented by using single moment.

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Principle:Resonance Condition

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Principle:Resonance Condition

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Principle:Resonance ConditionQ) Resonance?A) Because the frequency of radio signal is tuned to the larner frequency (a resonance condition is created)

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Principle:Relaxation MechanismWhen the nuclii under external magnetic field is super imposed by the radio frequency transmitted by the rf coil, resonance occurs, and the nuclii is excited.This state is the unstable state.The nuclii returns to orginal state by releasing the energy that it absorbed after very small time.This energy is detected by another rf coil, placed perpendicular to both transmitter rf coil and external magnetic field.Receiver rf coil is placed perpendicular to transmitter rf coil and external magnetic field to reduce the interference between them.

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Proton NMR SpectroscopyImportant and well established NMR spectroscopy.Proton (Hydrogen) have high gyroscopic ratio, and so the magnetic moment.

Two types:Continuous NMR Spectroscopy.Fourier Transform NMR Spectroscopy.

cwNMR SpectroscopyContinuous Wave NMR Spectroscopy

cwNMR SpectroscopyThere are two ways:A constant frequency, which is continuously on, probes the energy level while the magnetic field is varied.A constant magnetic field, while the frequency is varied.For this two methods, the instrumentation is same.EH

Instrumentation

Instrumentation:Magnetic FieldMagnetic field produced must be stable and homogeneous.Stability is achieved by continuously compensating for small rapid fluctuations in magnetic field with coils wound around the pole face and by controlling the temperature of the magnet to minimize thermal fluctuations.Inhomogeneity is compensated with small magnetic fields produced by passing DC current through small electrical coils, and spinning the sample to effectively average out field gradients.

Instrumentation:Magnetic FieldThe stability of field-frequency can be conveniently achieved by a pair of coils located parallel to the magnet face.Alternate effective field.The magnet is placed in a thermostated oven and surrounded by heavy thermal insulation to compensate the effect of temperature.The magnet used is permanent magnet or electromagnet.Permanent magnet is less expensive, but doesnot allow the resonance of different nuclii.

Instrumentation:Magnetic FieldCommercial magnets range from 1.4T to 16.4T.The magnet is so large.

Instrumentation:Super Conducting MagnetModern NMR spectrometers use superconducting solenoid magnets.The magnets consists of a main field coil made of superconducting Nb/Sn or Nb/Ti wire with a dozen or more superconducting shim coils wound around the main coil to improve homogeneity.The super conducting coil must be submerged in liquid helium.The magnet and liquid helium reservoir are encased in a liquid nitrogen reservoir to decrease the evaporative loss of more expensive liquid helium.The sample probe is mounted in the bore along with a set of room temperature shim coils.These coils are adjusted with every new sample placed in the probe to compensate for sample composition, volume and temperature.

Instrumentation:The Radio Frequency TransmitterThe transmitter is a 60MHz crystal controlled oscillator.The coil that transmits the radio frequency is made in two halves.To allow insertion of sample holder.The two halves are placed in the magnetic gap.

Instrumentation:The Signal Amplifier and DetectorThe detector coil consists of a few turns of wire.Placed at right angles to the source coil and the stationary field to minimize pickup from these fields.The coupling between transmitting and receiving coils cannot be completely eliminated and some leakage is always known to be present.Solved to some extend by using devices called paddles, which acts as inductors mutually coupled to both receiver and transmitter.The voltage change is quite small and must be amplified in a radio frequency amplifier.

Instrumentation:The Sample HolderSample holder consists of a glass tube (0.5mm outer diameter).The tube must be transparent to RF radiation, durable, and chemically inert.Microtubes for smaller sample volumes are also available.Glass or Pyrex tubes are commonly used.These are study, practical and cheap.

Instrumentation:The Sample ProbeThe sample chamber into which the sample holder is placed is called the probe.Also contains the sweep source and detector coils.Ensures reproducible positioning of the sample.Modern NMR probe uses a single wire coil to both excite the sample and detect the signal.The sample probe is also driven with an air driven turbine for rotating the sample tube along its longitudinal axis at several hundred rpm.For maximum sensitivity, a fixed frequency probe is needed.Means separate probe is required for each nucleus.

Fourier Transform NMR SpectroscopyPulsed NMR Spectroscopy

Operation PrincipleIrradiating a nuclii with one frequency component in turn will take hours to complete the analysis.For high resolution spectrum, this must be done slowly.Irradiate all the nuclii at the same time, followed by mathematical analysis of the complex rf resonance frequency emitted by nuclii as they relax back to the equilibrium state.Proved much more efficient.A pulse signal is used to excite the entire nuclii.Pulse signal is the combination of a number of frequency components.As the relaxation mechanism is a first order process, which is different for different nuclii.The rf signal emitted by the sample will decay exponentially, which is known as a Free Induction Decay (FID) signal.Instead of analysing the intensity of emitted signal, the FID signal is used to analyse the components in the sample.

Chemical Shift

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Chemical shift, which is positive

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Chemical shift, which is positive

Spin-Spin Coupling

Why the methyl peak is split into triplet?

Why the methylene peak is split into quartet?

Coupling ConstantThe spacing between the peaks of the methyl triplet are equal to the spacing between the peaks of the methylene quartet.This spacing is measured in Hz and is called coupling constant J, and is the characteristic of the efficiency of the spin interaction.

Summarizing General RulesNuclii having the same chemical shift (called isochronous) do not exhibit spin-splitting. They may actually be spin coupled, but the splitting cannot be observed directly.Nuclii seperated by three or fewer bonds will usually be spin-coupled and will show mutual spin-splitting of the resonance signal (same Js), provided they have different chemical shifts. Longer-range coupling may be observed in molecules having rigid configuration atoms.The magnitude of the observed spin-splitting depends on many factors and is given by the coupling constant J. J is the same for both partners in a spin-splitting interaction and is independent of the external magnetic field strength.The splitting pattern of a given nucleus (or set of equivalent nuclii) can be predicted by the n+1 rule, where n is the number of neighbouring spin-coupled nuclii with the same (or very similar) Js.

Summarizing General RulesThe intensity ratio is given by the numbers in Pascals triangle.

Summarizing General RulesIf a given nuclii is spin-coupled to two or more sets of neighbouring nuclli by different J values, the n+1 rule doesnot predict the entire splitting pattern. Instead, the splitting due to one J set is added to that expected from the other J sets.

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