Overview of Molecular Spectroscopic Techniques

 

Molecular Spectroscopy

Molecular spectroscopy

Molecular spectroscopy is the study of how electromagnetic waves interact with materials. How electromagnetic radiation affects matter. In molecular spectroscopy, matter is typically a whole molecule that exhibits changes in response to electromagnetic radiation. The foundation of spectroscopy is electromagnetic radiation (EMR), and IR-Spectroscopy is the study of how IR radiations interact with matter. The sample is exposed to radio waves in 1H-NMR, which are used to determine, recognize, and analyses the material's structure. Gerhard Herzberg made the discovery of molecular spectroscopy.

Principle

“In molecular spectroscopy, electromagnetic radiation interacts with materials to create an absorption pattern, or spectrum, from which structural or compositional details can be determined.”

Molecules show transitions from ground level to higher or excited state/level. Molecular spectroscopy comprises various techniques that we have discussed earlier, here we review all these spectroscopic techniques again. It includes 

1. UV-Visible Spectroscopy
2. Infra-red Spectroscopy
3. Proton Nuclear Magnetic Resonance Spectroscopy
4. RAMAN Spectroscopy

Now we will discuss,

UV-visible spectroscopy

We have discussed earlier, here we explain some points related to ultraviolet and visible spectroscopy. The radiation ranges between 10 to 800 nm. Below 200 nm the region is known as the vacuum region. When UV light falls on the sample then different types of electronic transitions occur.

The energy required to bring these transitions tells the structure or kind of bonds either single, double, or triple bond or hetero atoms present in a sample, for example, 10 to 190 or 200 nm absorption signals show that in our sample there are single bonds and transitions occurs from sigma to sigma*. There is a total of six types of electronic transitions, four are allowed and two are forbidden transactions.

UV light is more powerful than IR and Radio radiation so, they cause the transition of electrons by changing loosely bonded (pi-bond) or non-bonding electrons in samples thus changing their distribution.

It is used both in quantitative and qualitative analysis.

Alternate double and single bonds also called conjugate bonds explained by using UV Visible spectroscopy.

The energy difference between the HOMO and LUMO shows the amount of UV light utilized.

Light Source

• Tungsten lamp
• Xenon lamp
• Deuterium lamp

Solvents

• Water
• Chloroform
• Acetonitrile
• Rectified spirit

IR Spectroscopy

In 1900 Williams Weber Coblentz discover infrared spectroscopy.

It is usually used for determining the kinds of functional groups in molecules.

IR radiation ranges from 0.8 to 200 u. 0.8 to 2.5 u is near IR region, 2.5 to 15 u region is the infrared region or ordinary region and this region is mostly used by chemists because molecular vibrations detection and measurement is possible in this region.

The region between 15 to 200 u is called as far IR region. In general IR region ranges between 4000 to 667cm-1. This is further subdivided into functional group region (4000-1300) and fingerprint region (1300-667) cm^-1 for comparing the structure of organic molecules with the existing molecule’s structure.

Units for IR absorption radiation are wave number, microns (u) or wavelength, cm-1 but IR spectra usually plotted percentage transmission versus wave number (u). For changing units wave number to wavelength, we use the formula as follows,

As we know

                        Wave number =1/ wavelength in cm

                         1 u = 10-4 cm

                        Wave number in cm-1 =10000/wavelength in microns

We get wavelength cm-1 units.

The intensity of IR radiation is expressed as absorbance or transmittance, so the relationship between these two is as follows

                        A = log 10 (1/T)

When IR Radiation falls on a sample, absorption of light occurs that corresponds to the vibrational energy of molecules in our sample, (different vibrational and rotational changes in the energy of molecules occur). The obtained IR value tells the type of functional groups in the sample. Most values lie in 4000-1300 cm^-1 as this region is a functional group region of IR.

• A large number of bands in the spectrum tell the structure of molecules.
• Standard IR value table was also discussed in previous lectures.
• Spectrum forms when a transition occurs between two energy levels.

                         E _vib = ( ν +1/2) hν

For linear and non-linear molecules, we can determine the vibrational degree of freedom. Every vibrational degree of freedom tells the fundamental mode of vibration and these modes of vibration form bands in the spectrum.

Possible fundamental bands for Linear and non-linear molecules are as follows:

• Linear molecules =3n-5
• Non-linear molecules =3n-6
• "n" is the number of atoms

In space there are three coordinates (x, y, Z) to locate a molecule, each atom can move in these three coordinates only so there are always 3 translational degrees of freedom for every molecule. The rotational degree of freedom for linear molecules is two and for nonlinear molecules is 3. So,

Total degree of freedom =3n

Translational degree of freedom =3

Rotational degree of freedom =2 for linear

                                              =3 for non-Linear

                                                  = 3n-3-2

Vibrational degree of freedom = 3n-5

                                                  = 3n-3-3

Vibrational degree of freedom = 3n-6

For example, in the case of Co₂, there are three atoms in the Molecule and this is a linear molecule, so

               3n-5

              3(3)-5

               9-5

              4 vibrational degrees of freedom.

Kinds of vibrations

There are two fundamental kinds of vibrational motion

A. Stretching

• Symmetric stretching
• Asymmetric stretching

B. Bending

• Scissoring
• Rocking
• Wagging
• Twisting

Important solvents used in IR spectroscopy are

• Chloroform
• Carbon tetrachloride
• Carbon disulfide

H2O cannot be used because it shows absorbance in different IR regions.

Samples in solution forms are more convenient to use in IR.

Light Source

• Nernst Glower (rod of a sintered mixture of Zirconium, Ytterium, and Erbium)
• Rod of silicon carbide when heated electrically also produced IR light also called Globar.

Nuclear Magnetic Resonance Spectroscopy (NMR)

NMR is a very useful technique for determining the complete information of a compound.

There are two most common NMR techniques which are proton nmr used for determining the number and type of protons and carbon NMR utilized for identifying the carbon atoms.

Proton NMR is the mostly used and easier technique.

Proton NMR Spectroscopy we have already been discussed about it.

Isidor Isacc Rabi is the pioneer of NMR Spectroscopy who was awarded the noble prize in 1944 in physics.

• NMR signals or peaks give an idea about the amount of energy required to bring protons in resonance. A number of signals tell the number of protons.
• Intensity of peaks or signals in the NMR spectrum shows the number of protons of that type.
• Position of peaks shows the environment of protons that are either shielded or deshielded.
• Splitting of signals tells the number of protons attached to the adjacent carbon.

H-NMR Spectrum

Solvents

• Deutrochloroform, CDCL3
• Carbontetrachloride, CCl4
• Carbon disulfide, CS2
• Hexachloroacetone, (CCl3)2CO

Source of light

Radiofrequency source operating at 60 megacycles per second and varying field strength because all nuclei cannot resonate at this frequency.

RAMAN Spectroscopy

In most cases, we observe that when incident light strikes a substance, some of the light is absorbed by the substance and some of the light is scattered; this scattered light has lower energy or frequency than the incident light. However, in some substances, the opposite phenomenon occurs, with a frequency of scattered light being higher than the incident light; this effect is known as RAMN scattering.

Only some chemicals exhibit this.

The polarizability of molecules is the criterion for RAMAN spectroscopy; RAMAN is used to identify compounds that cannot be recognized by IR because every molecule exhibits polarization.

The only prerequisite for IR is a dipole moment.

Mass spectrometry

It is not a form of molecular spectroscopy. Since light and matter interact in spectroscopy, spectrometry measures light radiations after they are absorbed by molecules to obtain useful fragments and fragmentation patterns used for analyzing molecules.

The molecular weight and elemental makeup of a chemical can be ascertained using this analytical instrument. The main idea is to generate ions with an ion source, then segregate these ions based on their m/Z ratio and detect them both qualitatively and quantitatively.

By being attacked with electrons in the gas phase, the sample produces molecular ions. Additional information related to the mass spectrometry technique was previously detailed.

In the gas phase sample is bombarded with electrons producing molecular ions. Other fragments also formed in the mass spectrometry process that we already discussed.

It also distinguishes between isotopes of the same elements.

It has several applications, in drugs, mass spectrometry determines the structure of drugs and metabolites in a drug sample.

Purpose

Used for measuring the mass-to-charge ratio of one or more molecules in samples. The exact molecular weight of sample components can often be calculated.

Spectrum of MS