Transcript Infrared
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. Infrared Chemists tend to use wavenumbers not wavelengths. Wavenumbers are directly proportional to energy. A higher wavenumber corresponds to a higher energy. 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 840 kJ/mole 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. Infrared Vibrational Modes Many vibrational modes Simplest types Stretching Bending Infrared Infrared 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. 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. 1 K v 2c The reduced mass, of the system is given by: = m1m2 / m1 + m2 Infrared Stronger bonds have a larger force constant and vibrate at higher frequencies. Infrared Bonds between atoms of higher masses vibrate at lower frequencies. Infrared Instrumentation Dispersive Spectrometers Fourier Transform Spectrometers Sample Preparation Correlation Charts and Tables 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 Spectrum Analysis Spectrum Analysis How to approach the analysis of an infrared spectrum. Or - what you can tell at a glance! Looking for functional groups. 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. Spectrum Analysis 1. Is a carbonyl group present? A strong absorption in the region 18201660 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. 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 13001000 cm-1. 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. 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. 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. Spectrum Analysis RESIST THE IDEA OF TRYING TO INTERPRET EVERY PEAK. IT IS NOT POSSIBLE! Spectrum Analysis Molecular Formulae Derived from Empirical Formulae Ethane Empirical formula CH3 Molecular mass 30 Molecular formula CH3CH3 Spectrum Analysis The Index of Hydrogen Deficiency 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. The Spectrum Analysis Hydrogen Deficiency Alkane: CnH2n+2 Alkene or cycloalkane: CnH2n Alkyne: CnH2n-2 Hydrogen Deficiency N, P, As, etc: +1 O, S, Se, Te: no change Halides: -1 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. Spectrum Analysis Other examples Chloral Hydrate Nicotine Spectrum Analysis How to approach the analysis of an infrared spectrum. A more detailed look.