Mueller Fundamentals of Quantum Chemistry

Molecular Spectroscopy and Modern Electronic Structure Computations
1. Auflage 2007
ISBN: 978-0-306-47566-5
Verlag: Springer US
Format: PDF
Kopierschutz: 1 - PDF Watermark

E-Book, Englisch, 278 Seiten, eBook

As quantum theory enters its second century, it is fitting to examine just how far it has come as a tool for the chemist. Beginning with Max Planck’s agonizing conclusion in 1900 that linked energy emission in discreet bundles to the resultant black-body radiation curve, a body of knowledge has developed with profound consequences in our ability to understand nature. In the early years, quantum theory was the providence of physicists and certain breeds of physical chemists. While physicists honed and refined the theory and studied atoms and their component systems, physical chemists began the foray into the study of larger, molecular systems. Quantum theory predictions of these systems were first verified through experimental spectroscopic studies in the electromagnetic spectrum (microwave, infrared and ultraviolet/visible), and, later, by nuclear magnetic resonance (NMR) spectroscopy. Over two generations these studies were hampered by two major drawbacks: lack of resolution of spectroscopic data, and the complexity of calculations. This powerful theory that promised understanding of the fundamental nature of molecules faced formidable challenges. The following example may put things in perspective for today’s chemistry faculty, college seniors or graduate students: As little as 40 years ago, force field calculations on a molecule as simple as ketene was a four to five year dissertation project.

Zielgruppe

Research

Autoren/Hrsg.

Mueller, Michael P.

Mueller, Michael

Inhaltsverzeichnis

Classical Mechanics.- Fundamentals of Quantum Mechanics.- Rotational Motion.- Techniques of Approximation.- Particles Encountering a Finite Potential Energy.- Vibrational/Rotational Spectroscopy of Diatomic Molecules.- Vibrational and Rotational Spectroscopy of Polyatomic Molecules.- Atomic Structure and Spectra.- Methods of Molecular Electronic Structure Computations.

Leseproben

Chapter 6
Vibrational/Rotational Spectroscopy of Diatomic Molecules (p. 113-114)

This chapter focuses on applying the fundamentals of quantum mechanics developed in the previous chapters to interpreting the vibrational and rotational transitions that occur within diatomic molecules in infrared spectroscopy. Analysis of an infrared spectrum of a diatomic molecule results in structural information about the molecule and the energy differences between the molecule’s vibrational and rotational eigenstates.


6.1 FUNDAMENTALS OF SPECTROSCOPY

Molecular spectroscopy is a means of probing molecules and most often involves the absorption of electromagnetic radiation. The absorbed electromagnetic radiation results in transitions between eigenstates of a molecule. The type of eigenstates involved in a transition depends on the energy of the radiation absorbed. Figure 6-1 shows an electromagnetic spectrum along with the relative energies, wavelengths, and frequencies associated with each type of radiation. Absorbed ultraviolet and visible radiation generally results in transitions amongst electronic eigenstates. Absorbed infrared radiation results in changes in vibrational and rotational eigenstates. Absorbed microwave radiation results in changes in rotational eigenstates.

The specific wavelengths of radiation that are absorbed in each region of the electromagnetic spectrum depend on the energy difference between the eigenstates of a molecule. As an example, a diatomic molecule with a "stiff" bond will absorb at a higher energy photon (shorter wavelength) than another diatomic molecule with a less "stiff" bond.

The absorbed radiation in a spectrum provides information on the energy differences amongst various eigenstates of a molecule; however, it does not provide any information on the actual eigenstates involved in the transitions. Quantum mechanics is needed in order to analyze a spectrum in terms of assigning an absorption in a spectrum to a specific transition in eigenstates of a molecule. The energy of a photon of electromagnetic radiation is inversely proportional to its wavelength