Introduction

The absorption of electromagnetic radiation, whether in the ultraviolet, visible or infrared range, allows chemists to determine not only the structures of atoms and molecules but also, under certain conditions, their concentration in solution. Each type of electromagnetic radiation tells a different story about the composition of matter.

Concepts

• Electromagnetic radiation

• Planck’s law
• Absorption and emission
• Electron energy levels
• UV/Vis spectroscopy
• Molecular vibrations
• Infrared spectroscopy
• Beer’s law
• Quantitative analysis

Background

Spectroscopy is a general term referring to methods of instrumental analysis based on the interaction of electromagnetic radiation with matter. Depending on the type or energy of electromagnetic radiation, the interaction of light with matter may lead to a variety of different types of transitions in atoms and molecules. The absorption of ultraviolet (UV) or visible (Vis) radiation results in electronic transitions, in which electrons are promoted or excited to higher energy levels, while the absorption of infrared (IR) radiation excites vibrational transitions in molecules. A spectrum is produced by measuring the intensity of radiant energy absorbed by a substance as a function of the wavelength of light from an external energy source. Electronic and vibrational spectra may be used to obtain information concerning the identity of elements and compounds, their atomic and molecular structures, and the concentration of a substance in solution.

Electromagnetic (EM) radiation has a dual wave-particle nature, behaving and exhibiting properties of both particles and waves. The wave properties of EM radiation are characterized or defined by its velocity (c, the speed of light), wavelength (λ, lambda), and frequency (v, nu). See Equation 1. Different types of absorption spectroscopy are usually identified by the wavelength of light involved, namely, X-ray, UV, visible, infrared, microwave or radiofrequency.

The quantum theory or nature of light describes electromagnetic radiation in terms of mass-less particles, called photons, each possessing a quantum or packet of energy (E), where the energy is directly proportional to the frequency (v) of radiation (see Equation 2, known as Planck’s law). Planck’s law defines the relationship between EM radiation as a wave and as a photon of energy.

The proportionality constant h, known as Planck’s constant, is equal to 6.626 × 10⁻³⁴ J • s. Combining Equation 1 with Planck’s law, we know that c is a constant (the speed of light), and therefore wavelength and frequency are inversely proportional. Thus, photons with higher energies have shorter wavelengths and photons with lower energies have longer wavelengths. The energy of EM radiation in UV spectroscopy is measured in wavelength, while IR spectra are typically plotted in terms of wavenumber, which is a frequency unit. (See Parts A and B.)

The quantum nature of light finds a direct parallel in the quantization of electron energy levels within an atom or molecule. The interaction of EM radiation with matter is best described as the increase in energy of an atom or molecule caused by the absorption of a photon. At the molecular or atomic level, these increases in energy are quantized. Electronic and vibrational spectroscopy are due to the following types of absorption.

• Promotion of an electron in an atom or molecule (A) from a lower energy state to a higher energy, excited state, which is denoted A*. See Figure 1. The wavelength of radiation needed to cause electronic transitions is in the range of ultraviolet and visible light, 10 nm to 700 nm (1 nm = 1 nanometer = 1 × 10⁻⁹ m).

• Change in vibrational energy levels associated with stretching and bending chemical bonds—see Figure 2. Like electronic energy levels, vibrational energy levels are also quantized. The wavelength of radiation needed to cause vibrational energy transitions occurs in the infrared region, 700−1000 nm.

The absorption of electromagnetic radiation may be represented using an energy level diagram (Figure 3). E₀ represents the ground state, or at-rest, energy level of an electron in a molecule. The possible excited energy levels for the electrons are quite numerous, since each electron energy level has multiple vibrational energy levels. (Note that there are also rotational energy levels embedded within the different vibrational levels.)

Experiment Overview

The purpose of this guided-inquiry learning activity is to investigate the principles and applications of electronic and vibrational spectroscopy. The activity is divided into three parts:

1. Electronic energy levels and UV-Vis spectroscopy.
   
   
2. Vibrational energy levels and IR spectroscopy.
   
   
3. Beer’s law and quantitative analysis.