Chapter 16 - Chapter 16 An Introduction to Infrared...

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Unformatted text preview: Chapter 16 An Introduction to Infrared Spectrometry The infrared region of the spectrum encompasses radiation with wavenumbers ranging from about 12,800 to 10 cm-1 or wavelengths from 0.78 to 1000 m. The infrared spectrum is divided into near-, mid-, and far- infrared radiation. THEORY OF INFRARED ABSORPTION SPECTROMETRY The ordinate is linear in transmittance. The abscissa is linear in wavenumbers with units of reciprocal centimeters. Modern instruments utilize a microcomputer capable of producing a variety of other output formats, such as transmittance versus wavelength and absorbance versus wavenumber or wavelength. Dipole Changes During Vibrations Infrared radiation is not energetic enough to bring about electronic transitions. Absorption of infrared radiation is thus confined largely to molecular species that have small energy differences between various vibrational and rotational states. In order to absorb infrared radiation, a molecule must undergo a net change in dipole moment as a consequence of its vibrational or rotational motion. continued The dipole moments is determined by the magnitude of the charge difference and the distance between the two centers of charge. No net change in dipole moment occurs during the vibration or rotation of homonuclear species such as O 2 , N 2 , or Cl 2 ; consequently, such compounds cannot absorb in the infrared. Vibrational Transitions: Vibrational energy levels are quantized, and for most molecules the energy differences between quantum states correspond to the mid-infrared region. Types of Molecular Vibrations: Vibrations fall into the basic categories of stretching and bending. A stretching vibration involves a continuous change in the interatomic distance along the axis of the bond between two atoms. Bending vibrations are characterized by a change in the angle between two bonds and are of four types: scissoring, rocking, wagging, and twisting . Vibrational Frequency : The natural frequency of the oscillation is m = natural frequency m = mass of the attached body k = force constant of the spring m k m = 1 2 continued The equation may be modified to describe the behavior of a system consisting of two masses m 1 and m 2 connected by a spring. Here, it is only necessary to substitute the reduced mass for the single mass m where Thus, the vibrational frequency for such a system is given by = + m m m m 1 2 1 2 m k k m m m m = = + + 1 2 1 2 1 2 1 2 ( ) continued Quantum Treatment of Vibrations The radiation in wavenumbers, E h h k m = = 2 E h E h h k rad iation m = = = = 2 = = 1 2 53 10 c k k ....
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Chapter 16 - Chapter 16 An Introduction to Infrared...

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