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Ebook Physical chemistry (4th edition): Part 2

(BQ) Part 2 book "Physical chemistry" has contents: Rotational and vibrational spectroscopy, electronic spectroscopy of molecules, magnetic resonance spectroscopy, statistical mechanics, experimental kinetics and gas reactions, chemical dynamics and photochemistry,.and other contents. | 13 Rotational and Vibrational Spectroscopy 13.1 13.2 13.3 13.4 13.5 13.6 13.7 13.8 13.9 13.10 The Basic Ideas of Spectroscopy Einstein Coefficients and Selection Rules Schrodinger Equation for Nuclear Motion ¨ Rotational Spectra of Diatomic Molecules Rotational Spectra of Polyatomic Molecules Vibrational Spectra of Diatomic Molecules Vibration–Rotation Spectra of Diatomic Molecules Vibrational Spectra of Polyatomic Molecules Raman Spectra Special Topic: Fourier Transform Infrared Spectroscopy Molecular spectroscopy is a powerful tool for learning about molecular structure and molecular energy levels. The study of rotational spectra gives us information about moments of inertia, interatomic distances, and angles. Vibrational spectra yield fundamental vibrational frequencies and force constants. Electronic spectra yield electronic energy levels and dissociation energies. The types of spectroscopic transitions that can occur are limited by selection rules. As in the case of atoms, the principal interactions of molecules with electromagnetic radiation are of the electric dipole type, and so we will concentrate on them. Magnetic dipole transitions are about 10 5 times weaker than electric dipole transitions, and electric quadrupole transitions are about 108 times weaker. Although the selection rules limit the radiative transitions that can occur, molecular collisions can cause many additional kinds of transitions. Because of molecular collisions the populations of the various molecular energy levels are in thermal equilibrium. 13.1 The Basic Ideas of Spectroscopy 13.1 THE BASIC IDEAS OF SPECTROSCOPY When an isolated molecule undergoes a transition from one quantum eigenstate with energy E1 to another with energy E2 , energy is conserved by the emission or absorption of a photon. The frequency of the photon is related to the difference in energies of the two states by Bohr’s relation, h ϵ hc ͉ ס E1 Ϫ E2 ͉ ˜ (13.1) where we have used the symbol ( ) /1 .