**8. Interpretation of spectra**

Identification of a molecular structure from the IR spectrum can be realized using information from correlation tables and absorbances from the functional group region of the spectrum and comparison of the obtained spectrum with those of known compounds or obtain a known sample of a suspected material.

A preliminary examination of a spectrum use requires the examination of two important spectrum areas: functional group region (**4000-1300 cm-1**) and the **909-650** cm-1 region. The characteristic stretching frequencies for important functional groups such as OH, NH, and C=O occur in this portion of the spectrum. The absence of absorption in the assigned ranges for the various functional groups can usually be used as evidence for the absence of such groups from the molecule. The absence of absorption in the **1850-1540** cm-1 region excludes a structure containing a carbonyl group.

Strong skeletal bands for aromatics and heteroaromatics fall in the **1600-1300** cm-1 region of the spectrum. These skeletal bands arise from the stretching of the carbon-carbon bonds in the ring structure. The lack of strong absorption bands in the **909-650** cm-1 region generally indicates a aliphatic structure. Aromatic and heteroaromatic compounds display strong outof-plane C-H bending and ring bending absorption bands in this region. The intermediate portion of the spectrum, **1300-909** cm-1 is usually correspond to the fingerprint region. The

It is well known that molecules absorb a unique set of IR light frequencies, because the frequency of vibration involved depends on the masses of atoms involved, the nature of the bonds and the geometry of the molecule. A molecule absorbs only those frequencies of IR light that match vibrations that cause a change in the dipole moment of the molecule. Each organic molecule, with the exception of enantiomers, has a unique infrared spectrum. This is because symmetric structures and identical groups at each end of one bond will not absorb in the IR range. The spectrum has two regions. The *fingerprint* region is unique for a molecule and the

The entire spectral pattern is unique for a given compound. The bands that appear depend

In a complicated molecule many fundamental vibrations are possible, but not all are observed movements which do not change the dipole moment for the molecule or the those

IR is usually preferred when a combination of qualitative and quantitative analysis is required. It is often used to follow the course of organic reactions allowing the researcher to

For the analysis, the samples can be liquids, solids, or gases. The only molecules transparent to IR radiation under ordinary conditions are monatomic and homonuclear molecules such as Ne, He, O2, N2, and H2. One limitation of IR spectroscopy is that the solvent water is a

Computerized spectra data bases and digitized spectra are widely used today especially in

Identification of a molecular structure from the IR spectrum can be realized using information from correlation tables and absorbances from the functional group region of the spectrum and comparison of the obtained spectrum with those of known compounds or

A preliminary examination of a spectrum use requires the examination of two important spectrum areas: functional group region (**4000-1300 cm-1**) and the **909-650** cm-1 region. The characteristic stretching frequencies for important functional groups such as OH, NH, and C=O occur in this portion of the spectrum. The absence of absorption in the assigned ranges for the various functional groups can usually be used as evidence for the absence of such groups from the molecule. The absence of absorption in the **1850-1540** cm-1 region excludes a

Strong skeletal bands for aromatics and heteroaromatics fall in the **1600-1300** cm-1 region of the spectrum. These skeletal bands arise from the stretching of the carbon-carbon bonds in the ring structure. The lack of strong absorption bands in the **909-650** cm-1 region generally indicates a aliphatic structure. Aromatic and heteroaromatic compounds display strong outof-plane C-H bending and ring bending absorption bands in this region. The intermediate portion of the spectrum, **1300-909** cm-1 is usually correspond to the fingerprint region. The

*functional group* region is similar for molecules with the same functional groups.

on the types of bonds and the structure of the molecule.

which are so much alike that they coalesce into one band.

characterize the products as the reaction proceeds.

very strong absorber and attacks NaCl sample cells.

research, chemistry, medicine, criminology, etc

obtain a known sample of a suspected material.

structure containing a carbonyl group.

**8. Interpretation of spectra** 

absorption pattern in this region is complex, with bands originating in interacting vibrational modes. Absorption in this intermediate region is probably unique for every molecular species. For example, in the cases of hydrocarbons, organic compounds classified as saturated or unsaturated based on the absence or presence of multiple bonds, the energy of the infrared light absorbed by a C-H bond depends on the hybridization of the hybrid orbital, in the order of sp3>sp2>sp. The sp3-hybridized C-H bonds in saturated hydrocarbons absorb in the 2850-3000 cm-1 region. The sp2-hybridized C-H bonds from alkenes absorbs at **3080** cm-1. A sp-hybridized C-H bond in a molecule, alkyne absorbs infrared at 3320 cm-1. Another classification of hydrocarbons can be made based on absorptions due to the carboncarbon bond. Carbon-carbon bond strength increases in the order of single>double>triple. Saturated hydrocarbons all contain carbon-carbon single bonds that absorb in the 800-1000 cm-1 region. But, unsaturated hydrocarbons also contain carbon-carbon single bonds that absorb in this same region. So, this interval can not be considered as fingerprint region because most organic compounds have carbon-carbon single bonds.

The alkanes give an IR spectrum with relatively few bands because there are only CH bonds that can stretch or bend.


The next table present the characteristic group frequencies of organic molecules.

Organic Compounds FT-IR Spectroscopy 163

P.L. Polavarapu: "Vibrational Spectra: Principles and Applications with Emphasis on

Y. Wang, R. Tsenkova, M. Amari, F. Terada, T. Hayashi, A. Abe, Y. Ozaki: 'Potential of

D. Baurecht, U.P. Fringeli: "Quantitative Modulated Excitation Fourier Transform Infrared

M-J. Paquet, M. Laviolette, M. Pézolet, M. Auger: "Two-Dimensional Infrared

Y. Kauppinen, J. Partanen: "Fourier Transforms in Spectroscopy", Wiley-VCH Verlag

J.M. Chalmers, P.R. Griffiths (Editors): "Handbook of Vibrational Spectroscopy. Theory and

P.W. Atkins, Physical Chemistry. 2nd Ed. San Francisco: W.H. Freeman and Company, 1982.

B.W. Cook, K. Jones. A Programmed Introduction to Infrared Spectroscopy. New York: Heyden & Son Inc., 1972. Excellent resource for the beginning spectroscopist. R. T. Morrison, R. N. Boyd. Organic Chemistry. 5th Ed. Boston: Allyn and Bacon, Inc., 1987.

R. L., Shriner, R. C., Fuson, D. Y., Curtin, T. C. Morrill. The Systematic Identification of

R. M., Silverstein, G. C., Bassler, T. C. Morrill. Spectrometric Identification of Organic

A. Lee Smith,. Applied Infrared Spectroscopy: Fundamentals, Techniques, and Analytical

G. Socrates, Infrared Characteristic Group Frequencies. 2nd Ed. Chichester: Wiley, 1994. A

Provides a brief description of spectroscopy. Includes relevant IR spectra for each

Organic Compounds. 6th Ed. New York: Wiley, 1980. Contains a brief section on IR spectroscopy. Mainly a text for identification of compounds by chemical

Compounds. 4th Ed. New York: Wiley, 1981. Description of mass spectrometry, IR spectrometry, 1H NMR spectrometry, 13C spectrometry, and UV

Problem-Solving. New York: Wiley, 1979. Comprehensive treatment of IR spectroscopy. Includes history, instrumentation, sampling techniques, qualitative

Discussion of vibrational spectra from a quantum mechanical view.

R.N. Bracewell: "The Fourier Transform and Its Applications", McGraw Hill, 2000

Spectroscopy", Review of Scientific Instruments, 2001:72, 3782-3792

Two-Dimensional Correlation Spectroscopy in Analysis of NIR Spectra of Biological Fluids. I. Two-Dimensional Correlation Analysis of Protein and Fat Concentration-Dependent Spectral Variations of Milk", Analusis Magazine,

Correlation Spectroscopy Study of the Aggregation of Cytochrome C in the Presence of Dimyristoilphosphatidylglycerol", Biophysical Journal, 2001:81,

E.O. Brigham: "The Fast Fourier Transform", Prentice-Hall, Inc., 1974

Optical Activity", Elsevier Science B.V. 1998;

Instrumentation", John Wiley & Sons Ltd., 2002 M. Cho: "Two-Dimensional Optical Spectroscopy", CRC Press, 2009

family of organic compounds.

and quantitative applications.

comprehensive reference of correlation tables.

**9. References** 

1998:26, M64-M69

305-312

tests.

spectrometry.

Gmbh, 2001


Table 1. Schematic representation of the Infrared Group Frequencies classification
