Proton NMR (Nuclear Magnetic Resonance) Spectroscopy

 

Proton NMR (Nuclear Magnetic Resonance) Spectroscopy

Contents

History

Introduction

Sample preparation

Instrumentation

Factors influencing proton NMR

History

Isidor Isaac Rabi, who won the 1944 Nobel Prize in Physics, deserves praise for discovering NMR. The first exact measurements and descriptions of nuclear magnetic resonance were conducted in 1938 by Isidor Rabi. Rabi of Columbia University has successfully detected NMR in molecular beams. His efforts led to him winning the Physics Nobel Prize in 1944. However, it would be another two years before NMR development really got underway. In the late 1940s and early 1950s, the Bloch group at Stanford University and the Purcell group at Harvard University each independently invented NMR spectroscopy. The 1952 Nobel Prize in Physics was split between Edward Mills Purcell and Felix Bloch for their contributions.

History of NMR

Introduction:

An analytical chemistry method known as NMR spectroscopy is used in quality control and research to ascertain a sample's composition and purity as well as its molecular structure. The best method for studying nanomaterials, where the long-range order is typically disturbed by the surface and other defect areas, as well as the short-range order of all the resonant nuclei in the sample under investigation, is NMR spectroscopy.

Spectroscopic techniques, NMR is an essential technique that uses radio frequency pulses to measure an atomic nucleus' resonant frequency range in relation to its chemical and environmental circumstances (for example, the most prevalent stable isotopes 1H, 15N & 13C,). NMR spectroscopy has historically been used to conduct research on atom nuclei rather than electrons.

All nuclei are electrically charged and have many spins, which is the basic idea underpinning NMR spectroscopy. The external magnetic field generates the potential for an energy transfer in this condition. This energy transfer often takes place in a single step, moving from lower to higher energy levels. Using a radio frequency source enables this energy transfer or absorption.

Factors Affecting Radio Frequency

Three factors affect radio frequency, which is necessary for energy absorption. It is a distinctive feature of the nucleus type (e.g., 1H or 13C). The nucleus's chemical environment affects the absorption radio frequency. In the event that the magnetic field is not uniform, it also depends on where typical nuclei are located. The third factor provides the framework for comprehending the concept of magnetic resonance imaging (MRI) for measurements of the self-diffusion coefficient and coherence selection.

Energy is released at the same frequency as the nuclei's spin returns to its starting point. This energy transfer corresponds to a signal, which is then processed to provide the identical signal in the form of the related nucleus's NMR spectrum.

Chemical shift

Chemical shift is the difference between the signal from the reference molecule and the resonance frequency of spinning protons. One of the most important characteristics that can be used to determine molecular structure is nuclear magnetic resonance chemical change.

Resonance Frequency

Similar to electrons, protons have a charge that may spin, which results in a magnetic dipole moment. While a proton is in an external magnetic field, its magnetic dipole moment will align with the field. The proton can spin in one of two ways in any external magnetic field, but it has two magnetic dipole moment orientations and will align with the external magnetic field in one of two ways. One of these orientations that is perpendicular to the magnetic field's direction is referred to as the spin-up (+1/2). This spin state will be more stable and have a lower energy.

The spin-down state (-1/2) is the alternative orientation, which will be along the magnetic field's axis but in the other direction. This will be the more energetic and unstable quantum spin form. The spin-up proton will absorb energy and change (flip) to the spin down state if electromagnetic waves (radiofrequency waves) are presently directed at it with just the proper frequency. It is said to undergo resonance at this point, and this frequency is known as the resonance frequency or chemical shift.

Simply flipping of protons called as resonance.

When we encounter a radio frequency (Rf) radiation nucleus in NMR, the nucleus and its magnetic field turn (or it causes the nuclear magnet to pulse, thus the term NMR).

Sample preparation

NMR uses carbon tetrachloride because it lacks a proton. As a result, it doesn't affect 1H-NMR absorption. In a magnetic field, place the sample. Production of NMR signals occurs when excite the sample nuclei into nuclear magnetic resonance by using radio waves.  With the use of sensitive radio receivers, these NMR signals are found. The intramolecular magnetic field around an atom in a molecule alters atoms resonance frequency. This provides information on a molecule's electrical structure and specific functional groups. Monomolecular organic chemical identification can be done with certainty using nuclear magnetic resonance spectroscopy.

Sample size and Types

The NMR sample may be solid, liquid, or gaseous in nature. The size threshold for NMR is roughly 35 kDa. In order to obtain proton NMR spectra of organic compounds (with the exception of polymers), 5 to 25 mg of material is needed. At very low concentrations, spectra can be produced from lesser amounts. Certain samples require degassing or the removal of oxygen. Provide as much material as will result in a saturated solution because 13C spectra is 6,000 times less sensitive than 1H spectra.

Due to the difference in magnetic susceptibilities between solid particles and solutions, solid particles cause the homogeneity of the magnetic field to be distorted. Hence, a sample with suspended particles has a field homogeneity distortion all the way around each individual particle. Broad lines and fuzzy spectra result from this, which cannot be fixed. Please ensure that your samples don't include any solid particles. The primary field direction in the magnet runs vertically along the length of the sample. The spectrometer's shutter settings are used to rectify the significant distortion of the field homogeneity that either end of the sample causes. Every sample undergoes a brief adjustment that takes a few minutes. With a high grade test sample, a full correction takes many hours.

Solvents must be used to prepare samples that substitute deuterium for hydrogen. The spectrometer uses the NMR lock, a signal from the deuterium nuclei, to stabilize itself. The stockroom has a diverse collection of deuterated solvents.

Normally the sample size is determined by the experiment you're running. The amount of material needed is typically 1–10 mg for proton NMR spectra of organic compounds with a molecular mass less than 600.

Instrumentation

A superconducting magnet, a sample probe, a radio frequency (RF) transmitter, a receiver, and a computer for instrument control and data processing make up the nuclear magnetic resonance (NMR) spectrometer.

Instrumentation

1.Sample holder  

The sample tube is the name given to the sample holder in NMR, which is typically tube-shaped. The tube needs to be robust, chemically inert, and RF radiation transparent. Most often, Pyrex or glass tubes are utilised. Because these are affordable, reliable, and lasting. They typically include a plastic cover to keep the material contained and measure 6–8 cm long and 0.3–0.5 cm in diameter. The spectra of large samples and solutions are obtained using this kind of tube.

2.Permanent Magnet

A NMR instrument may use an electromagnet or a permanent magnet. The magnetic field should be stable and uniform, with no point-to-point changes in its strength or direction. A extremely high strength field, say 20,000 Gauss (G), is required. since the chemical alterations depend on the field's intensity. Usually the diameter of magnet is 15 inches.

 3. Magnetic coil

The resonance frequency of the nucleus and the strength of the magnetic field surrounding the sample are correlated.

Relationship, V = Constant × H_o

The applied RF radiation's frequency must match the nucleus' precessional frequency for the nucleus to resonate. The precessional frequency is fixed if Ho is constant.

4. Radio frequency generator

Radio frequency oscillators are used to produce radio frequency radiation. The coil of the oscillator would be placed around the sample container to maximise the contact of the RF radiation with the sample. The sample is exposed to RF radiation from the oscillator. The oscillator coil is parallel to the magnetic field that is being applied.

 5. Radio frequency receiver

It is positioned such that it is parallel to the oscillator coil and magnetic field. It is set to the transmitter's frequency. The nucleus creates an electromagnetic field (emf) in the detection coil when the precession frequency matches the RF radiation. This signal is amplified and delivered to the readout device.

6. Read out device

The readout mechanism provides a spectrum as a plot of the strength of the magnetic field on the X-axis and the strength of the resonance signal on the Y-axis. The quantity of nuclei resonating at a given field strength directly proportionally affects the strength of the resonance signal.

NMR Instrument