Infrared and Raman Spectroscopy

Principles and Spectral Interpretation

Peter Larkin

• Publisher: Elsevier
• Year: 2011
• eBook ISBN: 9780123870186
• Topic: Physical Sciences
• Subtopic: Analytic Chemistry

Infrared and Raman Spectroscopy: Principles and Spectral Interpretation explains the background, core principles and tests the readers understanding of the important techniques of Infrared and Raman Spectroscopy. These techniques are used by chemists, environmental scientists, forensic scientists etc to identify unknown chemicals. In the case of an organic chemist these tools are part of an armory of techniques that enable them to conclusively prove what compound they have made, which is essential for those being used in medical applications.The book reviews basic principles, instrumentation, sampling methods, quantitative analysis, origin of group frequencies and qualitative interpretation using generalized Infrared (IR) and Raman spectra. An extensive use of graphics is used to describe the basic principles of vibrational spectroscopy and the origins of group frequencies, with over 100 fully interpreted FT-IR and FT-Raman spectra included and indexed to the relevant qualitative interpretation chapter. A final chapter with forty four unknown spectra and with a corresponding answer key is included to test the readers understanding. Tables of frequencies (peaks) for both infrared and Raman spectra are provided at key points in the book and will act as a useful reference resource for those involve interpreting spectra.This book provides a solid introduction to vibrational spectroscopy with an emphasis placed upon developing critical interpretation skills. Ideal for those using and analyzing IR and Raman spectra in their laboratories as well as those using the techniques in the field.- Uniquely integrates discussion of IR and Raman spectra- Theory illustrated and explained with over 100 fully interpreted high quality FT-IR and FT-Raman spectra (4 cm-1 resolution)- Selected problems at the end of chapters and 44 unknown IR and Raman spectra to test readers understanding (with a corresponding answer key)

Chapter 1. Introduction

Infrared and Raman Spectroscopy

Vibrational spectroscopy includes several different techniques, the most important of which are mid-infrared (IR), near-IR, and Raman spectroscopy. Both mid-IR and Raman spectroscopy provide characteristic fundamental vibrations that are employed for the elucidation of molecular structure and are the topic of this chapter. Near-IR spectroscopy measures the broad overtone and combination bands of some of the fundamental vibrations (only the higher frequency modes) and is an excellent technique for rapid, accurate quantitation. All three techniques have various advantages and disadvantages with respect to instrumentation, sample handling, and applications.

Vibrational spectroscopy is used to study a very wide range of sample types and can be carried out from a simple identification test to an in-depth, full spectrum, qualitative and quantitative analysis. Samples may be examined either in bulk or in microscopic amounts over a wide range of temperatures and physical states (e.g., gases, liquids, latexes, powders, films, fibers, or as a surface or embedded layer). Vibrational spectroscopy has a very broad range of applications and provides solutions to a host of important and challenging analytical problems.

Raman and mid-IR spectroscopy are complementary techniques and usually both are required to completely measure the vibrational modes of a molecule. Although some vibrations may be active in both Raman and IR, these two forms of spectroscopy arise from different processes and different selection rules. In general, Raman spectroscopy is best at symmetric vibrations of non-polar groups while IR spectroscopy is best at the asymmetric vibrations of polar groups. Table 1.1 briefly summarizes some of the differences between the techniques.

TABLE 1.1 Comparison of Raman, Mid-IR and Near-IR Spectroscopy

^∗True for FT-Raman at 1064 nm excitation.
	Raman	Infrared	Near-IR
Ease of sample preparation	Very simple	Variable	Simple
Liquids	Very simple	Very simple	Very simple
Powders	Very simple	Simple	Simple
Polymers	Very simple^∗	Simple	Simple
Gases	Simple	Very simple	Simple
Fingerprinting	Excellent	Excellent	Very good
Best vibrations	Symmetric	Asymmetric	Comb/overtone
Group Frequencies	Excellent	Excellent	Fair
Aqueous solutions	Very good	Very difficult	Fair
Quantitative analysis	Good	Good	Excellent
Low frequency modes	Excellent	Difficult	No

Infrared and Raman spectroscopy involve the study of the interaction of radiation with molecular vibrations but differs in the manner in which photon energy is transferred to the molecule by changing its vibrational state. IR spectroscopy measures transitions between molecular vibrational energy levels as a result of the absorption of mid-IR radiation. This interaction between light and matter is a resonance condition involving the electric dipole-mediated transition between vibrational energy levels. Raman spectroscopy is a two-photon inelastic light-scattering event. Here, the incident photon is of much greater energy than the vibrational quantum energy, and loses part of its energy to the molecular vibration with the remaining energy scattered as a photon with reduced frequency. In the case of Raman spectroscopy, the interaction between light and matter is an off-resonance condition involving the Raman polarizability of the molecule.

The IR and Raman vibrational bands are characterized by their frequency (energy), intensity (polar character or polarizability), and band shape (environment of bonds). Since the vibrational energy levels are unique to each molecule, the IR and Raman spectrum provide a “fingerprint” of a particular molecule. The frequencies of these molecular vibrations depend on the masses of the atoms, their geometric arrangement, and the strength of their chemical bonds. The spectra provide information on molecular structure, dynamics, and environment.

Two different approaches are used for the interpretation of vibrational spectroscopy and elucidation of molecular structure.

1) Use of group theory with mathematical calculations of the forms and frequencies of the molecular vibrations.

2) Use of empirical characteristic frequencies for chemical functional groups.

Many empirical group frequencies have been explained and refined using the mathematical theoretical approach (which also increases reliability).

In general, many identification problems are solved using the empirical approach. Certain functional groups show characteristic vibrations in which only the atoms in that par...

Table of contents

• Cover image
• Table of Contents
• Front Matter
• Copyright
• Dedication
• Preface
• Chapter 1. Introduction
• Chapter 2. Basic Principles
• Chapter 3. Instrumentation and Sampling Methods
• Chapter 4. Environmental Dependence of Vibrational Spectra
• Chapter 5. Origin of Group Frequencies
• Chapter 6. IR and Raman Spectra-Structure Correlations
• Chapter 7. General Outline and Strategies for IR and Raman Spectral Interpretation
• Chapter 8. Illustrated IR and Raman Spectra Demonstrating Important Functional Groups
• Chapter 9. Unknown IR and Raman Spectra
• Appendix. IR/Raman Correlation Charts
• Index