NMR & ESR• Submitted by

Bhanu Pratap

Singh Choudhary

IIIrd year, F.T.

• Submitted to

Dr. Arun Goyal

Asst. Prof.

Deptt.of Dairy & Food Chemistry.

• Without going into latin or greek, spectroscopy is the study of the interactions between light and matter.

• Here light refers to any sort of electromagnetic radiation, such as visible light, UV, IR, and radiowaves.

• Depending on the frequency or wavelength of the radiation involved we will have different types of interactions with matter (molecules).

• The following chart shows the ranges (wavelengths), for different types of spectroscopies.

• wavelength and frequency are inversely proportional, so higher frequencies mean shorter wavelength.

10-10 10-8 10-6 10-4 10-2 100 102

wavelength ( cm)

-rays x-rays UV VIS IR -wave radio

What is Spectroscopy?

NMR• Nuclear magnetic resonance was first described and

measured in molecular beams by Isidor Rabi in 1938.

• in NMR spectroscopy, a composite system of nuclei in molecules with non-zero nuclear spins ( I ≠ 0 ) is placed in a constant magnetic field.

• Similarly, the NMR spectrum resulting from transitions between different states of nuclear spin system contains a wealth of information regarding chemical environment of the nuclei, and this is used to extract structural information of molecules.

Contd. • Nuclear magnetic resonance (NMR) is based upon the

measurement of absorption of radiofrequency (RF) radiation by a nucleus in a strong magnetic field.

• Absorption of the radiation causes the nuclear spin to realign or flip in the higher-energy direction.

• After absorbing energy the nuclei will re-emit RF radiation and return to the lower-energy state.

The principle of NMR

• nuclei with odd number of protons, neutrons or both will have an intrinsic nuclear spin.

• When a nucleus with a non-zero spin is placed in a magnetic field, the nuclear spin can align in either the same direction or in the opposite direction as the field.

• These two nuclear spin alignments have different energies and application of a magnetic field lifts the degeneracy of the nuclear spins.

• A nucleus that has its spin aligned with the field will have a lower energy than when it has its spin aligned in the opposite direction to the field.

Contd.

• The energy of a NMR transition depends on the magnetic-field strength and a proportionality factor for each nucleus called the magnetogyric ratio.

• The local environment around a given nucleus in a molecule

will slightly perturb the local magnetic field exerted on that nucleus and affect its exact transition energy.

• This dependence of the transition energy on the position of a particular atom in a molecule makes NMR spectroscopy extremely useful for determining the structure of molecules.

• Absorption (or emission) spectroscopy, as IR or UV. Detects the absorption of radiofrequencies (electromagnetic radiation) by certain nuclei in a molecule.

• Unfortunately, some quantum mechanics are needed to understand it (a lot to really understand it…).

• As opposed to the atomic mass or charge, the spin has no macroscopic equivalent. It exists, period...

• Only nuclei with spin number (I) 0 can absorb/emit electromagnetic radiation.

• Even atomic mass & number I = 0 (12C, 16O)• Even atomic mass & odd number I = whole integer (14N, 2H, 10B)• Odd atomic mass I = half integer (1H, 13C, 15N, 31P)

• The spin states of the nucleus (m) are quantized

• Properly, m is called the magnetic quantum number.

m = I, (I - 1), (I - 2), … , -Im = I, (I - 1), (I - 2), … , -I

Background

• For 1H, 13C, 15N, 31P (biologically relevant nuclei with I = 1/2):

• This means that only two states (energy levels) can be taken by these nuclei.

• Another important parameter of each particular nuclei is the magnetic moment (), which can be expressed as:

• It is a vector quantity that gives the direction and magnitude (or strength) of the ‘nuclear magnet’

• h is the Planck constant

• is the gyromagnetic ratio, and it depends on the nature of each nuclei.

• Different nuclei have different magnetic moments.

• The energy of a spin in a magnetic field will depend on the magnetic field, which we call Bo, and :

m = 1/2, -1/2m = 1/2, -1/2

= I h / 2 = I h / 2

Background (continued)

NMR - Instrumentation • There are two types of NMR spectrometers, 1- Continuous-wave (cw) and 2- Pulsed or Fourier-Transform (FT-NMR)

• Cw-NMR spectrometers have largely been replaced with pulsed FT-NMR instruments. However due to the lower maintenance and operating costs, cw instruments, they are still commonly used for routine NMR spectroscopy.

• In low-resolution cw instruments electromagnets are cooled with water and magnets in FT-NMR spectrometers are cooled with liquid helium.

Basic Instrumental Layout for NMR

A continuous-wave NMR instrument consists of the following units:

• a magnet to separate the nuclear spin energy states; • at least two radiofrequency channels, one for field/frequency stabilization

and one to furnish RF irradiation energy; • a sample probe containing coils for coupling the sample with the RF field;• a detector to process the NMR signals;• a sweep generator for sweeping either the magnetic or RF field through the

resonance frequencies of the sample;• a recorder to display the spectrum.

• The spectrum is scanned by two methods. 1) In the frequency-sweep method, the magnetic field is held

constant, which keeps the nuclear spin energy levels constant, then the RF signal is swept to determine the frequencies at which energy is absorbed.

2) In the field sweep method, the RF signal is held constant, then the magnetic field is swept, which varies the energy levels, to determine the magnetic field strengths that produce resonance at fixed resonance frequency.

NMR Signals• The number of signals shows how many

different kinds of protons are present.

• The location of the signals shows how shielded or deshielded the proton is.

• The intensity of the signal shows the number of protons of that type.

• Signal splitting shows the number of protons on adjacent atoms. =>

The NMR Graph

=>

Magnetic Shielding

• If all protons absorbed the same amount of energy in a given magnetic field, not much information could be obtained.

• But protons are surrounded by electrons that shield them from the external field.

• Circulating electrons create an induced magnetic field that opposes the external magnetic field.

Shielded Protons

Magnetic field strength must be increased for a shielded proton to flip at the same frequency.

=>

Protons in a Molecule

Depending on their chemical environment, protons in a molecule are shielded by different amounts.

=>

ESR• EPR was first observed in Kazan State University by a Soviet physicist

Yevgeny Zavoisky in 1944

• In ESR spectroscopy, molecules in a state containing unpaired electrons, .i.e., with nonzero spin-angular momentum (molecules in non-singlet states, S ≠ 0 ) are placed in constant magnetic field.

• The ESR spectrum resulting from transitions between molecular states with different S M components contains chemically relevant information.

• For example, the ESR spectrum of unpaired electrons in transition metal complexes contains information on how ligands are arranged around the metal ion

• Fine structure results from the interaction between electrons and nuclear spin and this can serve as a fingerprint for molecular, nuclear and electron spin density

Principle• The basic physical concepts of ESR are analogous to

those of nuclear magnetic resonance (NMR), but it is electron spins that are excited instead of spins of atomic nuclei.

• Because most stable molecules have all their electrons paired, the ESR technique is less widely used than NMR.

• However, this limitation to paramagnetic species also means that the ESR technique is one of great specificity, since ordinary chemical solvents and matrices do not give rise to ESR spectra.

Electronic Quantum Spin

Electrons usually occupy electronic shells in atoms as pairs.

In such pairs the electrons have spins opposite one another so the associated magnetic fields cancel.

In some ions, free radicals or paramagnetic materials, however, a single electron may occupy an orbital.

Placing such an atom into a strong magnetic field will tend to cause the electrons to align with the field.

Adding radiofrequency energy to the system in this situation can cause the electron spins to “flip” so they oppose the magnetic field.

This energy absorption can be detected by monitoring the energy input as radiofrequencies are scanned.

• ΔE = geμBB0, where ge is the electron's so-called g-factor (see also the Landé g-factor) and μB is the Bohr magneton

• This equation implies that the splitting of the energy levels is directly proportional to the magnetic field's strength, as shown in the diagram

• An unpaired electron can move between the two energy levels by either absorbing or emitting electromagnetic radiation of energy ε = hν such that the resonance condition, ε = ΔE, is obeyed. Substituting in ε = hν and ΔE = geμBB0 leads to the fundamental equation of EPR spectroscopy: hν = geμBB0.

• Experimentally, this equation permits a large combination of frequency and magnetic field values, but the great majority of EPR measurements are made with microwaves in the 9000 – 10000 MHz (9 – 10 GHz) region, with fields corresponding to about 3500 G (0.35 T).

Because the exact energy absorption frequency of the unpaired electrons are sensitive to their molecular surroundings, the ESR spectra provide information on the nature of these surroundings.

This is provided both by the position of major absorbance frequencies & by the splitting of these absorbances into hyperfine spectral features.

While a free radical might have a characteristic ESR spectrum as a pure isolated entity, it may demonstrate specific spectral features when placed into a particular solvent or biological system.

ESR Spectral Information

Trace levels of Mn2+ ions in dolomite, CaMg(CO3)2

http://www.chem.memphis.edu/faculty/lloyd/lloyd.htm