Spectroscopy

 Spectroscopy is the study of the interaction of electromagnetic radiation with matter. There are many forms of spectroscopy, each contributing useful information to identify substances and to determine various characteristics of their structure.  When a beam of electromagnetic radiation of intensity I o is passed through a substance, it can be either absorbed or transmitted, depending upon its frequency, ν, and the structure of the molecule it encounters.  Electromagnetic radiation is energy; thus when a molecule absorbs radiation it gains energy as it undergoes a quantum transition from one energy state (E initial ) to another (E final ).  Electromagnetic waves travel at the speed of light c, and are characterized by frequency (ν)or wavelength (λ) and amplitude, c= νλ

 The frequency of the absorbed radiation is related to the energy of the transition by Planck's law: E final - E initial = ∆E = hν = hc/λ.  If a transition exists that is related to the frequency of the incident radiation by Planck's constant, the radiation can be absorbed.  If the frequency does not satisfy the Planck expression, then the radiation will be transmitted.  The type of absorption spectroscopy depends upon the frequency range of the electromagnetic radiation absorbed.

 Microwave spectroscopy involves a transition from one molecular rotational energy level to another.  radiation from the microwave portion of the electromagnetic spectrum.  Vibrational spectroscopy (or infrared spectroscopy) measures transitions from one molecular vibrational energy level to another, and requires radiation from the infrared portion of the electromagnetic spectrum.  Ultraviolet-visible spectroscopy (also called electronic absorption spectroscopy) involves transitions among electron energy levels in the molecule, which require radiation from the UV- visible portion of the electromagnetic spectrum.

 The main goal of IR spectroscopic analysis is to determine the chemical functional groups in the sample.  Infrared radiation spans a section of the electromagnetic spectrum having wavenumbers 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

 IR absorption positions are generally presented as either wavenumbers (ν ) or wavelengths (λ).  Wavenumber defines the number of waves per unit length. Thus, wavenumbers are directly proportional to frequency, as well as the energy of the IR absorption.  The wavenumber unit (cm –1, reciprocal centimeter).  In the contrast, wavelengths are inversely proportional to frequencies and their associated energy. At present, the recommended unit of wavelength is μm (micrometers), but μ (micron) is used in some older literature.

 IR radiation does not have enough energy to induce electronic transitions as seen with UV. Absorption of IR is restricted to compounds with small energy differences in the possible vibrational and rotational states.  If the frequency of the radiation matches the vibrational frequency of the molecule then radiation will be absorbed, causing a change in the amplitude of molecular vibration.

 Molecular vibrations The positions of atoms in a molecules are not fixed; they are subject to a number of different vibrations. Vibrations fall into the two main categories of stretching and bending.

 Two atoms joined by a covalent bond may undergo a stretching vibration in this type the two atoms move back and forth as if joined by a spring  This type is subdivided into symmetric and a symmetric  This type of vibration arise because of stretching and contracting bonds without change in bond angle.

 Bending: Change in angle between two bonds. There are four types of bend:  Rocking the two atoms move in the same direction and the same plane  Scissoring two atoms approaches each other in the same plane  Wagging the two atoms move up and down the plane with respect to the central atom  Twisting one atom moves up and the other moves down

 1- Mass of atom light element vibrate at higher frequency  2- Strength of the bond triple bond vibrate at higher frequency than double bond  3- Arrangement of various atom in the molecules N-H, C-H, O-H occur at higher frequency  N.B. No two compounds except enantiomers can have similar IR spectra.

Bond frequency  C-H cm -1  N-H cm -1  O-H cm -1  C=C cm -1  cm -1  it is advisable to start from high frequency of the spectra and use finger print for confirmation  Since different functional group absorb IR radiation at different wave length so their presence in molecules can be determined

 Liquids may examined neat or in solution. - Neat liquids are examined between salt plates this is by pressing a liquid sample between flat plates produces film. - Volatile liquids are examined in sealed cells. - Silver chloride may be used for samples that dissolve sodium chloride plates. - Solutions are handled by using cell containing pure solvent, is placed in the reference beam.  Solids are usulally examined as mull( using mulling oil as Nujol), pressed disc ( using dry KBr)or as a deposited glassy film

 The two important areas for preliminary examination of a spectrum are cm -1 and cm -1  The high frequency portion of the spectrum is called functional group region.  cm -1 referred to as the “finger-print” region. The absorption pattern in this region is frequently complex. This portion is valuable when examined in reference to the other region.  Absorption in this intermediate region is uniqe for every molecular species.