Structural Information Infrared Fingerprints Structural Information The most useful part of the infrared spectrum for the detection and determination of organic species is from 2.5 to 15 m in wavelength which corresponds to a wavenumber range of 4000 to 667 cm-1. 3 1

Infrared Chemists tend to use wavenumbers not wavelengths. Wavenumbers are directly proportional to energy. A higher wavenumber corresponds to a higher energy. 3 2

Infrared Infrared Absorption Molecules excited to a higher energy state A quantised process A molecule only absorbs selected frequencies Corresponds to energy changes of the order 8-40 kJ/mole 3 4

Infrared Infrared Absorption The absorbed energy corresponds to the VIBRATIONAL frequencies of the molecule. Not all bonds in a molecule are capable of absorbing infrared energy. Only those bonds which contain a DIPOLE MOMENT. A bond must therefore present an electrical dipole which is changing as a function of time at the same frequency as the incoming radiation. 3 5

Infrared Vibrational Modes Simplest types Many vibrational modes Stretching Bending 3 8

Infrared 3 8

Infrared 3 8

Infrared Bond Properties A diatomic molecule can be considered as two vibrating masses connected by a spring. The bond distance continually changes but an equilibrium or average bond distance can be defined. This behaviour is described as harmonic oscillation. The natural frequency of vibration of a bond is derived from Hooke’s law for vibrating springs. 3 9

Infrared Bond Properties The total amount of energy is proportional to the frequency of the vibration and for a harmonic oscillator is determined by the force constant (K) of the spring and the masses m1 and m2 of the two bonded atoms. The reduced mass,  of the system is given by:  = m1m2 / m1 + m2 3 10

Infrared Stronger bonds have a larger force constant and vibrate at higher frequencies. 3 12

Infrared Bonds between atoms of higher masses vibrate at lower frequencies. 3 12

Infrared Instrumentation Sample Preparation Dispersive Spectrometers Fourier Transform Spectrometers Sample Preparation Correlation Charts and Tables 3 13

Spectrum Analysis The method of spectral analysis is dependent upon the information you have available. General Rules Identification of Functional Groups Molecular Formulae and Hydrogen Deficiency Full Spectral Interpretation 3 15

Spectrum Analysis 3 15

Spectrum Analysis How to approach the analysis of an infrared spectrum. Or - what you can tell at a glance! Looking for functional groups. 3 15

DO NOT WORRY ABOUT SUBTLETIES. Spectrum Analysis Look for a few major functional groups - C==O, OH, NH, C==C and C==C - which are conspicuous. Do not try to make a detailed analysis of the CH absorptions near 3000cm-1. DO NOT WORRY ABOUT SUBTLETIES. 3 15

Spectrum Analysis 1. Is a carbonyl group present? A strong absorption in the region 1820-1660 cm-1 Often the strongest in the spectrum. You can’t miss it! 2. If a carbonyl is present check the following types, if it is absent go to 3. 3 15

Spectrum Analysis ACIDS: is OH also present? broad absorption near 3400-2400 cm-1. AMIDES: is NH also present? medium absorption near 3400 cm-1. ESTERS: is C--O also present? strong intensity absorption near 1300- 1000 cm-1. 3 15

Spectrum Analysis ANHYDRIDES:two C==O near 1810 and 1760 cm-1. ALDEHYDES: is aldehyde C--H present? two weak absorptions near 2850 and 2750 cm-1. KETONES: the proceeding five choices have been eliminated. 3 15

Spectrum Analysis 3. If carbonyl absent then check: Alcohols and phenols broad absorption near 3400-2400 cm-1 and strong intensity absorption near 1300-1000 cm-1. Amines medium absorption near 3400 cm-1. Ethers strong intensity absorption near 1300-1000 cm-1. 3 15

Spectrum Analysis 4. Double Bonds A weak absorption near 1650 cm-1. 5. Triple Bonds A weak, sharp absorption near 2150 cm-1. 6.Hydrocarbons None of the preceeding found Major absorptions in CH region near 3000 cm-1. 3 15

RESIST THE IDEA OF TRYING TO INTERPRET EVERY PEAK. Spectrum Analysis RESIST THE IDEA OF TRYING TO INTERPRET EVERY PEAK. IT IS NOT POSSIBLE! 3 15

Spectrum Analysis Molecular Formulae Ethane Derived from Empirical Formulae Ethane Empirical formula CH3 Molecular mass 30 Molecular formula CH3CH3 3 15

Spectrum Analysis The Index of Hydrogen Deficiency The number of  bonds or rings a molecule contains. From a comparison of the molecular formula and that of a corresponding acyclic saturated compound. The difference in numbers of hydrogens between these formulae divided by two gives the index of hydrogen deficiency. 3 15

Spectrum Analysis Hydrogen Deficiency Alkane: CnH2n+2 Alkene or cycloalkane: CnH2n Alkyne: CnH2n-2 N, P, As, etc: +1 O, S, Se, Te: no change Halides: -1 3 15

Spectrum Analysis The Index of Hydrogen Deficiency One - a double bond or a ring but not both. Two - a triple bond, 2 double bonds, two rings or a combination of both. Four - a ring and three double bonds for example benzene. 3 15

Spectrum Analysis Other examples Chloral Hydrate Nicotine 3 15

How to approach the analysis of an infrared spectrum. Spectrum Analysis How to approach the analysis of an infrared spectrum. A more detailed look. 3 15