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

 

IR Spectra Interpretation

 IR Spectra Interpretation

IR Spectra Interpretation

In order to understand infrared spectra, absorption bands in the spectrum of an unknown chemical must be compared to known absorption frequencies for various types of bonds.

A molecule's chemical bonds vibrate when it absorbs infrared light. The bonds are elastic and flexible. Infrared spectroscopy is a form of vibrational spectroscopy because of this. Consequently, due to the differences in their structural makeup, different molecules vibrate at various frequencies. This is why infrared spectroscopy allows for the differentiation of molecules.

The first criterion for one chemical compound to absorb infrared light is the presence of a vibration during which there is a non-zero change in the dipole moment with respect to distance. The second prerequisite for infrared absorbance is that there must be an equivalence between the energy of the light intruding on a molecule and the difference in vibrational energy levels within the molecule.

A good spectrum should have a minimum of five characteristics: low noise, little to no baseline offset, a flat baseline, peaks that are on scale, and no spectral distortions. Below is an example of the IR spectrum that has all the characteristics.

In order to interpret the IR spectra, you should know the following factors that affect the vibrational frequency. 

Factors Influencing Vibrational Frequency
1. Coupled Vibration

If a simple C-H bond is present in the compound then its stretching occurs at one frequency if the CH2 group is present, here two stretching’s occur it can be symmetric or asymmetric. This is coupled vibration. Normally stretching shows at a higher frequency than bending. Symmetric stretching causes a lowering of wave number or frequency and asymmetric stretching increases wave number more than normal stretching.

2. Fermi Resonance

when the overtone band and fundamental band overlap, Fermi resonance occurs.

The Overton band is the band that forms when a transition occurs from 0 to 2 transitional levels in the vibrational frequency of molecules.

3. Electronic effects

This effect is produced due to the change of substituents in the neighbor group.

It is of three types

a) Inductive effect (I)

It can be positive and negative (Occur in sigma bond), Position of bonds remains same but pulling of electrons towards more electronegative atoms occurs that create dipoles in the molecules.

+I ………      Decrease wavenumber (alkyl group)

-I  ……      Increase wavenumber (Electronegative atom)

1.

Positive Inductive effect

   

Negative Inductive effect

b)   b) Mesomeric or Resonance effect

Lengthening of bond occur, decrease wavenumber (Occur in pi bond). It has dual effect, if single bond convert to double bond than wavenumber increase and if double bond convert to single bond than wavenumber decrease.

Resonance or Mesomeric effect

c) Field effect

When two atoms interact through space they cause repulsion between them which in turn increases the wave number. Specifically, in those molecules that have ortho group substituents. Cis and trans form interaction produce, in case of cis vibrational frequency or wave number increases and in trans form wave number decreases.

4. Hydrogen Bonding

a) Intramolecular hydrogen bonding

Sharp bands and well-defined bands form a spectrum which in turn increases the wave number.  For example, sharp bands are formed in the case of amines due to the lower electronegativity of Nitrogen as compared to alcohols (oxygen low E. N).

b) Intermolecular hydrogen bonding

Present between the two molecules, forms broad bands in the spectrum, it is concentration dependent, in dilute solutions N-H stretching occurs at 3500 cm-1 but in the condensed phase or after hydrogen bonding it decreases waves number to 3300 cm-1. O-H stretching occurs at 3650 cm-1 in dilute solutions but in concentrated solutions it decreases stretching frequency to 3450 cm-1. So, Hydrogen bonding shifts the wave number to the Lower level.

5. Bond Order

Bond order is directly related to the strength of a bond. As we know bond order is the number of bonds between two atoms. More bonds, the more energy is required to stretch or bend the bond. So, as in triple bond there are three bonds between atoms that are formed by sharing of three pair of electrons between these two atoms its vibrational frequency increase.

In a single bond, there is only one sigma bond.

In a double bond, there is one pi and one sigma bond.

In a triple bond, there are two pi bonds and one sigma bond, so triple bond is strongest bond.

As we know sigma bond is strong than the pi bond but the pi bonds are stiffer and vibrate faster increase the frequency of vibrations so when IR radiation falls on the sample the vibrational frequency for a single bond is lower than the double bond which is lower than the triple bond. 

Bond Angle

The bond angle is also related to bond order. In a single bond, an angle is 109.5^o, more s-character, more strength of the bond, and greater vibrational frequency value. S-character in molecules increases the strength of molecule’s bonds, resulting in shortness of bond. Short bonds are strong bonds that require more force to compress or stretch.

 Now, following in the table that shows the values of ir vibration of specific functional groups, you should have to compare the values by seeing the values from table and seeing the spectra.

Table 1.

Functional Groups   C-H Alkanes (stretch) -CH3 (bend) -CH2- (bend)	Frequency (cm-1)   3000-2850 cm-1 1450 & 1375 cm-1 1465 cm-1
C=C	1680-1600cm1
Aromatic	1600 & 1475 cm-1
C ☰ C	2260-2100 cm-1
C=O                                             Aldehyde	1740-1720 cm-1
Ketones	1725-1705 cm-1
Carboxylic acid	1725-1700 cm-1
Ester	1750-1730 cm-1
Amide	1700-1640 cm-1
Anhydride	1810 and 1760 cm-1
Acid chlorides	1800 cm-1
C-O	1300-1000 cm-1
O-H Free H-Bonded	3650-3600 cm-1 3400-3200 cm-1
N-H	3100-3500 cm-1
C-N amines	1350-1000 cm-1
C=N Imines and oximes	1690-1640 cm-1
Nitriles	2260-2240 cm-1
X=C=Y	2270-1940 cm-1
N=O Nitro	1550 and 1350 cm-1
S-H Mercapto	2550 cm-1
S=O	1375-1300 cm-1
C-X
Fluorides	1400-1000 cm-1
Chlorides	785-540 cm-1
Bromides	< 667 cm-1
Iodides	< 667 cm-1

Detailed process to understand Spectra 

• First, when you want to interpret the spectrum, not every peak needs to be examined. Rather, IR is excellent for identifying a few distinct functional groups, such as alcohols and carbonyls. In this way, it’s complementary to other techniques (like NMR) which don’t yield this information as quickly.

• Two specific regions of the spectrum—3200-3400 cm-1 and 1650-1800 cm-1—provide 80% of the data that is most pertinent to our needs.

• The instrumental resolution, sampling strategy, and presence or absence of spectral manipulation (such as baseline adjustment, smoothing, and subtraction) should all be known before viewing a spectrum.

• Strongly IR-absorbing atmospheric gases like water vapor and carbon dioxide have ambient concentrations that are high enough for their peaks to show up in mid-IR spectra.

• The group wavenumbers presented in Table I should be noted whether they are present or absent when you read the spectrum from left to right like a sentence in a book. In order to quickly evaluate whether or not certain significant functional groups are present in a sample, you scan the spectrum from left to right and use the peak ranges in Table I. The easiest bands to see are the most powerful, but these peaks are typically the ones that are most helpful for diagnosis. Less intense peaks still matter, but it is advisable to take care of the larger ones first because they are simpler to see and allocate.

• Peaks with lower intensity are known as secondary bands for a specific functional group. There are two reasons why secondary bands need to be found. First off, a lot of functional groups have numerous peaks in the mid-IR, thus finding as many peaks as you can for each peak for a functional group increases the likelihood of a proper interpretation. Second, these bands must be assigned to prevent the mistaken assumption that they are the result of a functional group that might not actually exist.

• Write down the functional groups you believe to be present as the peaks are allocated during your interpretation process. Put the pieces of the functional groups you've discovered together like a puzzle piece to create real chemical structures. Next, check to see if the structure you have sketched is compatible with the spectrum. For instance, the OH may be joined to the methylene or the ring in a sample that includes an OH group, CH2, and benzene ring. Your true choice between these two options can be determined by looking at the spectrum.

• The factors that are discussed above are the main reason for increasing or decreasing the frequency of IR. For example, carbonyl stretching frequency that given in table is different for different groups containing carbonyls like aldehydes, ketones, etc. From the above discussion you will also be able to give specific frequency value to specific functional groups in molecules by considering the above factors. 

 

 IR Spectroscopy

IR spectroscopy

History

William Weber Coblentz is widely regarded as the pioneer of infrared (IR) spectroscopy, having published the results of a large study of compounds in 1905. His work demonstrated that chemical functional groups had specific and characteristic IR absorptions, and he was able to accurately record the IR spectra of 135 compounds - a feat that still stands today, some 60 years later.

The method of IR spectroscopy gained traction during World War II, when a method for characterizing synthetic rubber formulations was needed for the war effort. This led to the development of the first commercial instruments from Beckman and Perkin Elmer, and in 1957, Perkin Elmer introduced the first low-cost IR spectrophotometer, the Model 137, priced at just $3800.

The Coblentz Society was formed shortly after to educate early practitioners in the art, and the method was used widely. However, it experienced a significant resurgence in the sciences with the advent of Fourier transform IR (FT-IR) instruments in the late 1960s and early 1970s. These instruments could collect spectra in a matter of seconds, and, by signal averaging, spectra of very high quality could be measured.

It is worth noting that the discovery of infrared radiation predates Coblentz's work by almost a century. In 1800, the German-born British-astronomer William Herschel conducted a simple experiment in which he dispersed sunlight through a prism and placed a thermometer at the location of each colour, thus discovering infrared radiation.

Introduction

Infrared Spectroscopy is an analytical technique used to identify organic compounds by determining the intensity and wavelength of light absorbed. This versatile tool measures vibrations in molecules which help to distinguish different types of molecules based on their physical and chemical properties. From identifying or analyzing minor components in a mixture, confirming a compound's identity, and monitoring reactions, to studying the behavior of a sample under various conditions.

The infrared section of the electromagnetic spectrum, or light with a longer wavelength and a lower frequency than visible light, is the subject of IR spectroscopy, sometimes known as infrared spectroscopy. The study of a molecule's interaction with infrared light is known as infrared spectroscopy. Three methods can typically be used to investigate the idea of IR spectroscopy: reflection, emission, and absorption measurements. Finding the functional groups of molecules—relevant to both organic and inorganic chemistry—is the main use of infrared spectroscopy.

IR Spectroscopy identifies infrared light frequencies that are absorbed by molecules. Due to the fact that these particular light frequencies match the frequency of the bonds in molecules, molecules have a tendency to absorb them. Infrared radiation contains the energy needed to excite molecular bonds and cause them to vibrate more violently. However, only polar bonds will interact with electromagnetic infrared radiation. A molecule can be excited by the electromagnetic wave's electric field component because it has distinct regions of partial positive and negative charge.

The dipole moment of the particular molecule changes in parallel with the change in vibrational energy. The polarity of the bond affects how much energy is absorbed. Symmetrical non-polar bonds in N=N and O=O are unable to interact with an electric field; hence they cannot absorb IR radiation.

 

 

IR Region values

The region between 4000 cm-1 and 1600 cm-1 has bands that identify the functional group that is present. It is possible to recognize their bands and utilize them to ascertain the functional group of an unidentified molecule.

The region between 1600 cm-1 and 667 cm-1, known as the fingerprint region, has bands that are specific to each molecule and resemble a fingerprint. These bands are used to compare the spectra of different compounds.

IR Value 

Principle

The IR spectroscopy theory is based on the idea that molecules have a tendency to absorb particular light frequencies that are unique to the corresponding structure of the molecules. The energies depend on the atomic masses, the related vibronic coupling, and the geometry of the molecular surfaces.

For instance, the molecule may be able to absorb the energy present in the incident light, which will cause it to rotate more quickly or vibrate more loudly.

Sample preparation

Infrared spectroscopy can be used with samples that are solid, liquid, or gaseous.

Solid sample

By crushing the sample with an oily-textured mulling agent, solid samples can be created. This mull can now be spread thinly on a salt plate for measurement.

Liquid sample

Since salt plates are transparent to IR light, liquid samples are often held between two of them while being analyzed. Sodium chloride, calcium fluoride, or even potassium bromide can be used to make salt plates.

Gaseous sample

Gaseous samples can have concentrations measured in parts per million, the sample cell must have a comparatively long path length, meaning light must travel a relatively significant distance inside the sample cell.

Sensitivity

A sample as little as 1 to 10 grams can now be identified using infrared spectroscopy. Infrared spectroscopy may be used to analyze almost all organic molecules as well as certain inorganic ones. It has numerous uses and can be utilized in both qualitative and quantitative analysis.

Why water is not used as a solvent in IR

Water cannot be used as a solvent for IR spectroscopy because it has two high infrared absorption peaks. Additionally, alkali halide discs, which are widely used in IR, can be dissolved in water because it is a polar solvent.

For a molecule or sample to exhibit an infrared spectrum, a change in the electric dipole moment of the functional group must occur during the vibration based on the selection rule for IR transitions.

IR light Source

An infrared spectroscopy thermal light source called a Globar is used. It is a silicon carbide rod that has been electronically heated to a temperature of 1,000 to 1,650°C and has a diameter of 5 to 10 mm and a length of 20 to 50 mm (1,830 to 3,000 degrees Fahrenheit).

 

 

 

Solvents used in IR spectroscopy

The two most common solvents are carbon tetrachloride (CCl4) and carbon disulfide (CD). Solvents for polar materials include chloroform, methylene chloride, acetonitrile, and acetone. Solids reduced to fine particles can be analyzed as a thin paste or mull.

 IR Spectra

An IR spectrum is a graph plotted with the infrared light transmitted on the Y-axis against frequency or wave number on the X-axis.

 You may like difference between FTIR and IR 

Analyzing Molecular Fingerprints :A Comprehensive Guide to FTIR Spectroscopy