IR Spectroscopy CHM 411 Suroviec

I. Theory of IR Spectroscopy Frequency of absorbed radiation is the molecular vibration frequency responsible for absorption process – which is directly proportional to wavenumber

A. Dipole Moment HCl Charge distribution around molecule is not symmetrical and therefore has a dipole moment

B. Rotation and Vibrational Transitions Rotational Vibrational

C. Types of Molecular Vibrations The relative positions of atoms in a molecule are not fixed but fluctuate continuously as a consequence of a multitude of different types of vibrational and rotational allowed states around the bonds in the molecule

II. Vibrational Modes It is possible to deduce the number and kinds of vibrations in simple diatomic and triatomic molecules

A. Motion of a molecule Need to consider: 1.Motion of a molecule through space (translational) 2.Rotation around center of gravity 3.Individual vibrations

B. Vibrational Coupling Energy of vibrations and wavelength of absorption is influenced by several factors: 1.Strong coupling between stretching vibrations only occurs when there is an atom in common to the 2 vibrations 2.Interaction between bending vibrations requires a common bond between vibrational groups 3.Coupling between stretching and bending vibrations can occur if stretching bond forms one side of an angle that varies 4.No interaction between groups separated by 2 or more bonds

Example: CO 2

Example: H 2 O

III. Instrumentation A. ATR

B. FTIR