IR, NMR, Chromatography

IR spectroscopy (Study Material)
NMR spectroscopy (Study Material)
Chromatography (Study Material)

UV-Vis and IR spectroscopy

UV-Visible spectroscopy (Study Material)

IR spectroscopy (Study Material)

Colorimetry and Beer's-Lambert's Law

Beer's-Lambert's Law & Colorimetry (Study Material)

Assignment 02 (Class Work)

Assignment 01 (Lab Work) 

Practice Sheet 01 (Lab Work) 

Answer Keys -Assignment 01 (Lab Work)

Understanding Spectroscopy: Important Links...

http://www.chem.ualberta.ca/~inorglab/spectutor.htm gives basic idea about spectroscopic principles through animation . The Spectroscopy tutorial requires Macromedia Flash Plug-In (minimum version 4.0). If you have not installed it download Flash.

http://orgchem.colorado.edu/hndbksupport/irtutor/tutorial.html  is very good reference for understanding IR spectroscopy and interpretation of IR/NMR spectra. 

Other good sites for introductory IR are

http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/InfraRed/infrared.htm
http://teaching.shu.ac.uk/hwb/chemistry/tutorials/molspec/irspec1.htm
 

from wikipedia (http://en.wikipedia.org/wiki/Infrared_spectroscopy) you may get a basic idea about the instrumental techniques.

IR ABSORPSION

INTRO, IR Conti.....

You may have come to think of a molecule as having rigid bond lengths and bond angles, this is not the actual case, since bond lengths and angles represent the average positions about which atoms vibrate. A molecule is not a rigid assemblage of atoms. A molecule can be said to resemble a system of balls of varying masses corresponding to atoms and springs of varying strengths (force constants ) corresponding to the chemical bonds of a molecule.

There are two types of molecular vibrations, stretching and bending. A stretching vibration is a rhythmical movement along the bond axis such that the inter-atomic distance is increasing or decreasing. A bending vibration may consist of a change in bond angle between bonds with a common atom or the movement of a group of atoms with respect to the remainder of the molecule without movement of the atom in the group with respect to one another.

Only those vibrations that result in a rhythmical change in the dipole moment of the molecule are observed in the IR. Various stretching and bending vibration of a molecule occur at certain quantized frequencies. When IR light of that same frequency is incident on the molecule, energy is absorbed and the amplitude of that vibration is increased. The frequency of the vibration remains unchanged. When the molecule reverts from the excited state to ground stale the absorbed energy released as heat.

Each atom has three degrees of freedom, corresponding to motions along any of the three cartesian coordinate axes (x, y, z). A polyatomic molecule of n atoms has 3n total degrees of freedom. However, 3 degrees of freedom are required to describe translation, the motion of the entire molecule through space. Additionally, 3 degrees of freedom correspond to rotation of the entire molecule. Therefore, the remaining 3n – 6 degrees of freedom are true, fundamental vibrations for nonlinear molecules. Linear molecules possess 3n – 5 fundamental vibrational modes because only 2 degrees of freedom are sufficient to describe rotation.

INTRO, IR Spectroscopy

Infrared (IR) spectroscopy is one of the most common spectroscopic techniques used by organic and inorganic chemists. Simply, it is the absorption measurement of different IR frequencies by a sample positioned in the path of an IR beam. The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample. Different functional groups absorb characteristic frequencies of IR radiation. Hence it is used to identify chemical compounds or monitor changes occurring in the course of a chemical reaction. For example, the spectra of two related molecules, 1-propanol (an alcohol), and propanoic acid (a carboxylic acid) are given below. Their spectra are considerably different and tell about important features of a molecule. Using various sampling accessories, IR spectrometers can accept a wide range of sample types such as gases, liquids, and solids. Thus, IR spectroscopy is an important and popular tool for structural elucidation and compound identification.

Infrared refers to that part of the electromagnetic spectrum between the visible and microwave regions. Electromagnetic spectrum refers to the seemingly diverse collection of radiant energy, from cosmic rays to X-rays to visible light to microwaves, each of which can be considered as a wave or particle traveling at the speed of light.  

Infrared radiation spans a section of the electromagnetic spectrum having wave numbers from roughly 13,000 to 10 cm^–1 , or wavelengths from 0.78 to 1000 μm. It is bound by the red end of the visible region at high frequencies and the microwave region at low frequencies.

Of greatest practical use to the organic chemist is the limited portion of this region, the mid IR range 4000–400 cm^–1. An increase in wave number corresponds to an increase in energy. This is a convenient relationship for the organic chemist 

Infrared radiation in the range from about 10000-100 cm^-1 is absorbed by organic molecules and converted into energy of molecular vibration. This absorption is quantized but vibrational spectra appear as bands rather than as lines because a single vibrational energy change is accompanied by a number of rotational energy changes. it is with these vibrational-rotational bands, particularly those occurring between 4000-400 cm^-1 that an organic chemist concerned. In IR spectroscopy, an organic molecule is exposed to infrared radiation. When the radiant energy matches the energy of a specific molecular vibration, absorption occurs. The frequency of absorption depends on the-

1. Relative masses of atoms 
2. Force constants of the bonds and
3. Geometry of the molecule

In a typical IR spectrum the band positions are presented as wave numbers. The wave number, plotted on the X-axis, is proportional to energy; therefore, the highest energy vibrations are on the left. Band intensities can be expressed either as transmittance (T) or absorbance (A). Transmittance, T, is the ratio of radiant power transmitted by the sample (I) to the radiant power incident on the sample (I₀). Absorbance (A) is the logarithm to the base 10 of the reciprocal of the transmittance (T). The percent transmittance (%T) is plotted on the Y-axis. Absorption of radiant energy is therefore represented by a “trough” in the curve. Zero transmittance corresponds to 100% absorption of light at that wavelength. Organic chemist usually report intensity in semi-quantitative terms (s = strong, m = medium, w = weak).

Except at very, very low temperatures, all molecules are in motion in some manner. Molecules translate (move from place to place), they rotate in space, and, importantly for this experiment, they vibrate. At temperatures above absolute zero, all the atoms in molecules are in continuous vibration with respect to each other. When the frequency of a specific vibration is equal to the frequency of the IR radiation directed on the molecule, the molecule absorbs the radiation.

Types of electronic transitions

Ultraviolet-visible spectrum

Ultraviolet-visible spectrum:

 

An ultraviolet-visible spectrum is essentially a graph of light absorbance versus wavelength in a range of ultraviolet or visible regions. Such a spectrum can often be produced by a more sophisticated spectrophotometer. Wavelength is often represented by the symbol λ. Similarly, for a given substance, a standard graph of extinction coefficient ε vs. wavelength λ may be made or used if one is already available. For the given substance, the wavelength at which maximum absorption in the spectrum occurs is called λ_max, pronounced "Lambda-max".

Beer's and Lambert's Law

Important terms

 

Chromophore: A covalently unsaturated group responsible for electronic absorptions or any group of atoms that absorbs light whether or not a color is thereby produced.. For example C=C, C=O, NO₂ etc. A compound containing chromophore is called chromogen. There are two types of chromophore

1. Independent chromophore: Single chromophore is sufficient to import color to the compound eg. Azo group .

2. Dependent chromophore: When more then one chromophore is required to produce color. eg acetone having one ketone group is colorless where as diacetyl having two ketone group is yellow.

Auxochrome: A saturated group with nonbonded electrons which, when attached to a chromophore, alters both the wavelength and the intensity of the absorption or A group which extends the conjugation of a chromophore by sharing of nonbonding electrons e.g. –OH, -NH₂ –Cl etc..

 

Bathochromic group: The group which deepens the colour of chromophore is called bathochromic group. e.g. primary, secondary and tertiary amino groups.

Bathochromic shift: The shift of absorption to a longer wavelength due to substitution or solvent effect is termed as bathochromic shift. This is also known as red shift.

Hypsochromic shift: The shift of absorption to a shorter wavelength is termed as hypsochromic shift. This is also known as blue shift.

Hyperchromic shift: An increase in absorption intensity

Hypochromic shift: An decrease in absorption intensity

Ultra Violet Spectroscopy

Ultraviolet spectroscopy is primarily used to measure the multiple bond or aromatic conjugation by measurement of energy absorbed when electrons are promoted to higher energy level. On passing electromagnetic radiation in the UV and Vis resign through a compound with multiple bonds; a portion of the radiation is normally absorbed by the compound. The amount of absorption depends on the wavelength of the radiation and the structure of the compound. The absorption of radiation is due to the subtraction of energy from the radiation beam when electrons in orbitals of lower energy are excited into orbitals of higher energy. The ultraviolet spectrum is simply a plot of wavelength of light absorbed verses the absorption intensity (absorbance or transmittance) and is conveniently recorded by plotting molar absorptivity (e) against wavelength (nm). Since (e) values range, in practice from as low as 10 to as high as 10,000, it is convenient to use log e  as the abscissa of UV spectrum.

A molecule contains electronic, vibrational and rotational energy levels. Each electronic level, within a molecule, is associated with a number of vibrational levels with less energy separation and each vibrational level in turn is associated with a set of rotational levels with even less energy separation. Due to the relatively larger amounts of energy associated with the ultraviolet radiation, they are capable of electronic excitations and induce transition in the electronic, vibrational and rotational energy levels of a molecule. Thus, the ultraviolet spectrum of a molecule results from transition between electronic energy levels accompanied by changes both in vibrational and rotational states.

The strength of absorption i.e. molar absorptivity or e_max is define by combined Beer’s- Lamberts law-

Log (I₀/I) = e.C. l = A

The principal characteristics of an absorption band are its position and intensity. The position of absorption corresponds to the wavelength of radiation whose energy is equal to that required for an electronic transition. The intensity of absorption is largely dependent on two factors:

1. The probability if interaction between the radiation energy and the electronic system
2. the difference between the ground state an excited state

 The molar absorptivity e is a constant for an organic compound at a given wavelength, and is reported as e_max (molar absorptivity at an absorption maximum). It may be mentioned that e is not dimensionless, but is correctly expressed in unit of 10^-2 m2 mol2 but the unit are by convention never expressed. The intensity of absorption is directly proportion to the transition probability. A fully allowed transition will have  e value greater than 10000 and it is called high intensity absorption. The transition with low transition probability and the  e value less than 100 are called low intensity transitions. Transitions with low intensity are due to forbidden transitions.

Spectroscopy:

Spectroscopy is a method of analysis based on the interaction of electromagnetic radiation and matter. In other words, spectroscopy is the study of interaction of light with matter. Modern experimental chemistry uses many spectroscopic techniques viz. ultraviolet and visible spectroscopy, infrared spectroscopy, nuclear magnetic resonance spectroscopy, and electron spin resonance spectroscopy. All depend in some manner on the absorption of energy by a collection of molecules, the energy involved being in different regions of the electromagnetic spectrum.

The visible spectrum constitutes a small part of the total radiation spectrum. Most of the radiation that surrounds us cannot be seen, but can be detected by dedicated sensing instruments. This electromagnetic spectrum ranges from very short wavelengths (including gamma and x-rays) to very long wavelengths (including microwaves and broadcast radio waves). The following chart displays many of the important regions of this spectrum, and demonstrates the inverse relationship between wavelength and frequency.