Concept of Chromophore, types of Chromophore and Auxochrome in UV-Visible spectroscopy

Otto Witt, a German chemist, proposed the chromophore and auxochrome theory to explain how a substance's color and composition are related. Chromophores and auxochromes are the two major categories of molecular elements in charge of colour in molecules.

This hypothesis states that:

The presence of specific unsaturated groups, or groups with numerous bonds, in an organic substance, causes it to seem colored. These multi-bonded groups are known as chromophores.

Chromogens are substances that include the chromophore group. The number of chromophores or the degree of conjugation raises the intensity of the color.

For instance, while the molecule CH₃ - (CH = CH)₆ - CH₃ is yellow, ethene (CH2 = CH2) is colorless.

The existence of certain groups that, although not chromophores as such themselves, deepen the colour of the chromogen. These auxiliary groups are referred to as auxochromes.

Concept of Chromophore

In UV-Visible spectroscopy, we have discussed different types of transitions that occur due to the movement of electrons from one level to another when molecules absorb light. Those compounds that show absorbance in the 400-800 mu (visible) region appeared colored to our eyes. So, Chromophores are the groups that impart color to the compounds. The exact color depends upon the wavelength type that absorbs by the molecule. Generally, the Nitro group is yellowish in color. When in any compound if a nitro group is present it cause a yellow color to the overall molecule, so the Nitro group is a chromophore because it gives characteristics yellow color to the compound.

“Chromophore is defined as any isolated covalently bonded group that shows a characteristics absorption in the ultra-violet and visible region.” 

Important chromophore includes ethylenic, carbonyls, acetylenic, nitriles, esters, acids group, etc. Among all, Carbonyl is the most important chromophore.

Types of chromophore

There are two types

i)  Those chromophores that contain pi electrons and undergo pi to pi* transitions, these include acetylenes, ethylene’s, etc.

ii) Chromophores that contain pi electrons as well as non-bonding (n) electrons. Such type of chromophores shows two types of transitions л to л* and n to л*. Examples include nitriles, carbonyls, azo/ nitro compounds, etc.

Not any specific rule is present that provide the identification of chromophore. Changing the position and intensity of absorption contribute to various factors. Following are the keys that help to determine chromophore

Points to identify chromophore

a) In the spectrum absorption band near 270 to 350 mu with low intensity and _ᵋmax at 10-100 due to the n to pi* carbonyl transitions.

b) A band near to 300 mu may consist of two or three conjugated units (chromophores).

c) Aromatic system is shown by the absorption with _ᵋmax between 1000-10,000.

d) The high value of _ᵋmax from 10,000 to 20,000 shows the presence of simple conjugated chromophores like alpha-beta-unsaturated ketones.

Substitution of an aromatic nucleus with other groups can extend chromophore, and the absorption possibly occurs at a higher extinction coefficient (_ᵋmax).

From the above discussion, it is concluded that the presence of functional groups is possible by UV-Visible spectroscopy but when the conjugation (alternate double and single bond) is absent in molecules and when different compounds with the same functional groups show absorption at the same extinction co-efficient, it is not possible to detect functional groups in compounds. So, we say that functional groups or compounds can be determined by other spectroscopic techniques.

The following figure shows the different chromophores, transitions, and absorption values.

Types of Chromophore

Auxochrome

The Greek term auxochrome means "to increase the color."

It is a group that, by itself, does not function as a chromophore, but when bound to a chromophore, it shifts adsorption towards a wavelength that is longer while also intensifying it.

When combined with the chromophores in an organic molecule, they do not produce the color on their own but rather enhance the color of the chromogen (it is the substance that bears chromophore group).

Auxochromes can act as electron donors or acceptors. Both varieties of auxochromes can be found in nitro phenylenediamine compounds. Auxochrome is a collection of atoms that is joined to a chromophore and alters how well it can absorb light. Auxochromes all include one or more electron pairs that are not bonded.

Auxochrome examples include -OH, -OR, -NH2, −NH2, −NHR, −NR2 are auxochromes. etc. The absorption maxima in this kind shift to longer wavelengths.

 

Auxochrome Example

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