Lecture 3 - IR Spectroscopy PDF

INFRA RED SPECTROSCOPY  INFRA RED SPECTROSCOPY Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond. As a result, the bonds vibrate at different frequencies The frequency of vibration can be found by detecting when the...

INFRA RED SPECTROSCOPY  INFRA RED SPECTROSCOPY Different covalent bonds have different strengths due to the masses of different atoms at either end of the bond. As a result, the bonds vibrate at different frequencies The frequency of vibration can be found by detecting when the molecules absorb electro-magnetic radiation. Various types of vibration are possible. Examples include... STRETCHING and BENDING SYMMETRIC BENDING ASYMMETRIC STRETCHING STRETCH Molecular Vibrations Covalent bonds vibrate at only certain allowable frequencies. Hooke's Law  IR-Active and Inactive A polar bond is usually IR-active. A nonpolar bond in a symmetrical molecule will notabsorb in IR (IR-inactive).  Factors Affecting IR absorption 1- Force Constant (K or f) By increasing the bond force constant, the wave number will increase. The force constant K increases due to increasing the number of the bonds.  Factors Affecting IR absorption 2- Hybridization increase of S% increases the frequency υ’ due to electron in S is near to the nucleus so, bond length decreases and also K will increase and υ’ will increase.  Factors Affecting IR absorption 3- Resonance The presence of conjugated system increases the probability of resonance to occur which transfer the double bond to single bond so, the bond length will increase and so the vibration frequency υ’will decrease (bond is weak).  Factors Affecting IR absorption 4- Dipole moment Any absorption needs dipole moment to occur. The difference in electronegativity between the atoms make higher dipole moment so higher absorption.  Factors Affecting IR absorption 5- Hydrogen Bond Due to Hydrogen Bond, the bond length of O-H increase, then the bond length increase and so the (K) decrease and υ’ decrease.  Factors Affecting IR absorption 6- Mass of Atoms by increasing the mass of atoms, υ’ will decrease. Modes of Vibrations SYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES ASYMMETRIC STRETCHING BENDING AND STRETCHING IN WATER MOLECULES BENDING  INFRA RED SPECTRA - USES IDENTIFICATION OF The presence of bonds such as O-H and PARTICULAR BONDS C=O within a molecule can be confirmed IN A MOLECULE because they have characteristic peaks in identifiable parts of the spectrum. IDENTIFICATION OF The only way to completely identify a COMPOUNDS BY compound using IR is to compare its DIRECT COMPARISON spectrum with a known sample. The part OF SPECTRA of the spectrum known as the ‘Fingerprint Region’ is unique to each compound.  INFRA RED SPECTRA - INTERPRETATION IR Spectrum: It is a curve representing the relation between wave number (4000-650 cm-1) and % transmission and divided into: A- finger print region: is the region from 650-1500 cm-1 and is specific for each compound, it contains many absorption caused by bending vibrations as well as stretching vibrations. B- functional group region: from 4000-1500 cm-1, it includes absorptions due to stretching and used for identification of different types of functional groups present in the molecule. FINGERPRINT REGION  IR SPECTRUM OF A CARBONYL GROUP carbonyl compounds show a sharp, strong absorption between 1800 and 1650 cm-1 this is due to the presence of the C=O bond  IR SPECTRUM OF AN OH GROUP alcohols show a broad absorption between 3200 and 3650 cm-1 this is due to the presence of the O-H bond  IR SPECTRUM OF A CARBOXYLIC ACID carboxylic acids show a broad absorption between 3200 and 3600 cm-1 this is due to the presence of the O-H bond they also show a strong absorption around 1700 cm-1 this is due to the presence of the C=O bond  CHARACTERISTIC ABSORPTION FREQUENCIES Bond Class of compound Range / cm-1 Intensity C-H Alkane 2960 – 2850 (stretching) medium C-H Alkane 1470 – 1350 (bending) medium C-H Alkyne 3300 strong C-H Alkene 3080-3020 variable C=C Alkene 1680 - 1620 variable C=C Aromatic 1600 and 1500 variable C=O Carbonyl Compounds 1800 - 1650 strong O-H Alcohol 3650 - 3200 variable, broad N-H Amine, Amide 3500 (approx) medium CC Alkyne 2100 - 2250 medium EXAMPLES OF IR SPECTRA EXAMPLES OF IR SPECTRA EXAMPLES OF IR SPECTRA EXAMPLES OF IR SPECTRA EXAMPLES OF IR SPECTRA