Page 341 - Instant notes

I5
APPLIED VIBRATIONAL
SPECTROSCOPY

Key Notes
Transitions between molecular vibrational energy levels are
associated with absorption or emission of radiation in the infrared
−1
portion of the electromagnetic spectrum, 100–10000 cm .
Infrared spectroscopy is widely used to identify molecular
vibrations characteristic of specific types of bonds.
A normal mode is a collective vibrational displacement in which
all atoms move in phase. For small displacements, normal modes
are independent of each other, and can often be identified with
particular types of bond vibration, for example C−H or C=O
stretches. A non-linear molecule of N atoms has 3N−6 normal
modes; a linear molecule has 3N−5 modes.
Vibrational Raman spectroscopy requires a change in the
polarizability of the molecule during the vibration. The specific
selection rule allows only transitions of ∆υ=±1. The Stokes and
anti-Stokes vibrational transitions occur at frequencies v ex −v and
v ex +v, respectively, where v is the frequency of the vibrational
oscillator and v ex is the frequency of the incident excitation
radiation.
The normal mode of a molecule with a center of symmetry
cannot be both infrared and Raman active (and may be neither).

Related topics General features of Practical aspects of
spectroscopy (I1) spectroscopy (I2)

Vibrational spectroscopy (I4)

Infrared spectroscopy

Allowed transitions between two harmonic oscillator vibrational energy levels requires
electromagnetic radiation of the same frequency as the bond vibration,
, where k is the force constant of the bond and µ is the reduced mass
(see Topic I5). Example vibrational data for a number of diatomic molecules containing
atoms of specific isotopes are given in Table 1.