Spectroscopy Lecture Notes PDF

Spectroscopy DR. AFSHAN ABDUL SHAKOOR LECTURER LIST  contents Basic definition UV-visible spectroscopy IR spectroscopy Mass spectroscopy  definitions Spectroscopy: It is the branch of science that deals with the study of interaction...

Spectroscopy DR. AFSHAN ABDUL SHAKOOR LECTURER LIST  contents Basic definition UV-visible spectroscopy IR spectroscopy Mass spectroscopy  definitions Spectroscopy: It is the branch of science that deals with the study of interaction of matter with light. OR It is the branch of science that deals with the study of interaction of electromagnetic radiation with matter. Electromagneti c Radiation  Electromagnetic radiation Electromagnetic radiation consist of discrete packages of energy which are called as photons. A photon consists of an oscillating electric field (E) & an oscillating magnetic field (M) which are perpendicular to each other. Electromagnetic Radiation  Frequency and wavelength Frequency :It is defined as the number of times electrical field radiation oscillates in one second. The unit for frequency is Hertz (Hz). 1 Hz = 1 cycle per second Wavelength (λ): It is the distance between two nearest parts of the wave in the same phase i.e. distance between two nearest crest or troughs.  Electromagnetic Radiation The relationship between wavelength & frequency can be written as: c=νλ As photon is subjected to energy, so E=hν=hc/λ  Definitions (continue..) Wave Number: It is the no of waves per unit distance. SPECTRUM: When electromagnetic radiation is passed through the prism, it splits into the regions of different colors. Each region is characterized by the wavelength, frequency, wavenumber and energy. Spectrum is a graph of intensity of absorbed or emitted radiation by sample verses frequency (ν) or wavelength (λ). Spectrometer: is an instrument design to measure the spectrum of a compound.  Types of spectrum Emission spectrum Absorption spectrum 1.Emission spectrum: The excited atom or molecule tends to emit the energy in the form of light. The dispersion of this resulting light by passing through the prism/grading give rises a spectrum called as the emission spectrum.  Types of spectrum 2. Absorption spectrum: it is the spectrum obtained due to the absorption of electromagnetic radiations within certain range of wavelength.  Ultra-violet visible spectroscopy UV-Visible spectroscopy deals with the absorption of electromagnetic radiations in the visible and the U.V region. It is also known as the electronic spectroscopy. It involves the gain of energy by a molecule and the transfer of electrons from lower energy level to higher energy level. UV-Visible region extends from 200nm- 800nm.  Principle of UV-Visible spectroscopy UV-visible being of higher energy than the infrared radiations cause the transition of valence electrons from the ground state to excited state on their absorption by a sample. Near UV Region: 200 nm to 400 nm Far UV Region: below 200 nm Far UV spectroscopy is studied under vacuum condition. The common solvent used for preparing sample to be analyzed is either ethyl alcohol or hexane.  Theory of UV-visible Spectroscopy A molecule possess three types of energy 1. electronic transitional energy (Et) 2. rotational energy (Er) 3. vibrational energy (Ev) Thus the total energy of the molecule is E= Et + Er+ Ev So, Et is energy due to electronic transitions. Er is energy due to the rotation of the molecule Ev is energy due to the vibrations of the atoms within a molecule  continue So, E = Et >Er >Ev when an UV radiation strikes a molecule, it first increase its rotational energy, then vibrational and finally electronic energy. It means electronic transitions are responsible for the absorption spectrum in UV–visible region. Electronic Transition s The possible electronic transitions can graphically shown as:  Electronic transition The absorption of U.V-visible radiations by a sample results in transfer of electrons from one energy states to another energy states is known as the electronic transitions. The possible electronic transitions are 1 σ → σ* transition 2 π → π* transition 3 n → σ* transition 4 n → π* transition 5 σ → π* transition 6 π → σ* transition  1 σ → σ* transition σ electron from orbital is excited to corresponding anti- bonding orbital σ*. The energy required is large for this transition. e.g. Methane (CH4) has C-H bond only and can undergo σ → σ* transition and shows absorbance maxima at 125 nm.  2 π → π* transition π electron in a bonding orbital is excited to corresponding anti- bonding orbital π*. Compounds containing multiple bonds like alkenes, alkynes, carbonyl, nitriles, aromatic compounds, etc undergo π → π* transitions. e.g. Alkenes generally absorb in the region 170 to 205 nm.  3 n → σ* transition Saturated compounds containing atoms with lone pair of electrons like O, N, S and halogens are capable of n → σ* transition. These transitions usually requires less energy than σ → σ* transitions. The number of organic functional groups with n → σ* peaks in UV region is small (150  4 n → π* transition An electron from non-bonding orbital is promoted to anti- bonding π* orbital. Compounds containing double bond involving hetero atoms (C=O, C≡N, N=O) undergo such transitions. n → π* transitions require minimum energy and show absorption at longer wavelength around 300 nm.  5 σ → π* transition 6 π → σ* transition These electronic transitions are forbidden transitions & are only theoretically possible. Thus, n → π* & π → π* electronic transitions show absorption in region above 200 nm which is accessible to UV-visible spectrophotometer. The UV spectrum shows only a few band of absorption. Terms used in UV / Visible Spectroscop y  Chromophore The part of a molecule responsible for imparting color, are called as chromophore. OR The functional groups or a molecule containing multiple bonds capable of absorbing radiations above 200 nm due to n → π* & π → π* transitions.(isolated functional groups capable of absorption of UV-Visible radiations). e.g. NO2, N=O, C=O, C=N, C≡N, C=C, C=S, etc  Auxochrome The functional groups attached to a chromophore which modifies the ability of the chromophore to absorb light , altering the wavelength or intensity of absorption. OR The functional group with non-bonding electrons that does not absorb radiation in near UV region but when attached to a chromophore alters the wavelength & intensity of absorption.  Auxochrome e.g. Benzene λmax = 255 nm OH Phenol λmax = 270 nm NH2 Aniline λmax = 280 nm Absorption & Intensity Shifts 1 Bathochromic Shift (Red Shift) 2 Hypsochromic Shift (Blue Shift) 3 Hyperchromic Effect 4 Hypochromic Effect  1 Bathochromic Shift (Red Shift) When absorption maxima (λmax) of a compound shifts to longer wavelength, it is known as bathochromic shift or red shift. The effect is due to presence of an auxochrome or by the change of solvent. e.g. An auxochrome group like – OH, -OCH3 causes absorption of compound at longer wavelength.  1 Bathochromic Shift (Red Shift) In alkaline medium, p-nitrophenol shows red shift. Because negatively charged oxygen delocalizes more effectively than the unshared pair of electron. O + N O - O N O + - - OH Alkaline OH medium O - p-nitrophenol λmax = 255 nm λmax = 265 nm  2 Hypsochromic Shift (Blue Shift) When absorption maxima (λmax) of a compound shifts to shorter wavelength, it is known as hypsochromic shift or blue shift. The effect is due to presence of an group causes removal of conjugation or by the change of solvent.  3 Hyperchromic Effect When absorption intensity (ε) of a compound is increased, it is known as hyperchromic shift. If auxochrome introduces to the compound, the intensity of absorption increases. N N CH3 Pyridine 2-methyl pyridine λmax = 257 nm λmax = 260 nm ε = 2750 ε = 3560  4 Hypochromic Effect When absorption intensity (ε) of a compound is decreased, it is known as hypochromic shift. CH3 Naphthalene 2methylnaphthalene ε = 19000 ε = 10250  Shifts and Effects Hyperchromic shift Blue Red Absorbance ( A ) shift shift Hypochromic shift λmax Wavelength ( λ ) Beer’s Lambert Law  Beer’s Law When a beam of monochromatic radiation is passed through an absorption medium, then the decrease in the intensity of radiation will be directly proportional to the thickness of the solution. Which mean that absorbance is directly proportional to l= length A= El E= molar absoptivity L= path length (1 cm)  Lambert Law When a beam of monochromatic light is passed through the absorbing medium then the decrease in the intensity of the radiation will be directly proportional to the concentration of the solution. A= Ec E= molar absorptivity (L/mol-cm) C= concentration  WhenBeer’s Lambert Law a beam of monochromatic radiation is passed through the absorbing medium then the decrease in the intensity of the radiation will be directly proportional to the thickness as well as concentration of the solution  Limitations of the Beer- Lambert law The linearity of the Beer-Lambert law is limited by chemical and instrumental factors. Causes of nonlinearity include :deviations in absorptivity coefficients at high concentrations (>0.01M) due to electrostatic interactions between molecules in close proximity scattering of light due to particulates in the sample fluoresecence or phosphorescence of the sample changes in refractive index at high analyte concentration shifts in chemical equilibria as a function of concentration non-monochromatic radiation, deviations can be minimized by using a relatively flat part of the absorption spectrum such as the maximum of an