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Rotational and Vibrational Spectroscopy

Infrared (IR) Spectroscopy: – First real IR spectra measured by Abney and Festing in 1880’s – Technique made into a routine analytical method between 1903- 1940 (especially by Coblentz at the US NBS) – IR spectroscopy through most of the 20th century is done with dispersive (grating) instruments, i.e. monochromators – Fourier Transform (FT) IR instruments become common in the 1980’s, led to a great increase in sensitivity and resolution Raman Spectroscopy: – In 1928, C. V. Raman discovers that small changes occur the frequency of a small portion of the light scattered by molecules. The changes reflect the vibrational properties of the molecule – In the 1970’s, lasers made. | VILLANOVA UNIVERSITY Rotational and Vibrational Spectroscopy Lecture Date January 30th 2008 Vibrational and Rotational Spectroscopy Core techniques - Infrared IR spectroscopy - Raman spectroscopy - Microwave spectroscopy 1 The History of Infrared and Raman Spectroscopy Infrared IR Spectroscopy - First real IR spectra measured by Abney and Festing in 1880 s - Technique made into a routine analytical method between 19031940 especially by Coblentz at the US NBS - IR spectroscopy through most of the 20th century is done with dispersive grating instruments i.e. monochromators - Fourier Transform FT IR instruments become common in the 1980 s led to a great increase in sensitivity and resolution Raman Spectroscopy - In 1928 C. V. Raman discovers that small changes occur the frequency of a small portion of the light scattered by molecules. The changes reflect the vibrational properties of the molecule - In the 1970 s lasers made Raman much more practical. Near-IR lasers 1990 s allowed for avoidance of fluorescence in many samples. W. Abney E. R. Festing Phil. Trans. Roy. Soc. London 1882 172 887-918. 2 Infrared Spectral Regions IR regions are traditionally sub-divided as follows Region Wavelength A pm Wavenumber v cm-1 Frequency v Hz Near 0.78 to 2.5 12800 to 4000 3.8 x 1014to 1.2 x 1014 Mid 2.5 to 50 4000 to 200 1.2 x 1014to 6.0 x 1012 Far 50 to 1000 200 to 10 6.0 x 1012to 3.0 x 1011 After Table 16-1 of Skoog et al. Chapter 16 What is a Wavenumber Wavenumbers denoted cm-1 are a measure of frequency - For an easy way to remember think waves per centimeter Relationship of wavenumbers to the usual frequency and wavelength scales Converting wavelength A to wavenumbers 9.93x1O 1B 1.99 X1010 9.93 X 10-2 J photon J photon J photon ENERGY PHOTON 1 1 I hji J photon 50 000 cm 10 000 cm 5 000 cm 1 WAVENUMBER 1 1 cm 1 200 nm 1 000 nm 2 000 nm WAVELENGTH 1 1 1 1 1 1 1 1 1 1 X nm 1 000 nm - 10 000 cm 1 2x10 19 J photon Image from www.asu.edu