Molecular spectra

Type of physical science: Chemistry

Field of study: Chemistry of molecules: structure

Molecular spectra result from the study of the interaction of electromagnetic radiation with matter. The experimental observables, energy and intensity, are dependent upon the excited states that are available for molecular excitation. These energy levels are dependent upon the structure of the molecule. The analysis of the energies of electromagnetic radiation and the strength of that interaction with a particular molecule reveals information about the structure of that molecule.

Overview

Molecular spectroscopy is the study of the energy and magnitude of the interaction of electromagnetic radiation with matter. A molecular spectrum is the record of the change in energy and intensity of electromagnetic radiation after it interacts with a molecule. Although electromagnetic radiation is a continuous spectrum, from radio waves to X rays, molecules have direct interaction only with those photons that possess energies that can match exactly the unique energy levels for that molecule.

In addition to the energy condition for a spectroscopic transition to occur, there are other selection rules, based upon the molecule, that determine whether a molecule will absorb or emit radiation. These changes--the internal motions of a molecule--are caused by the rotational, vibrational, and electronic movements within a molecule. Each of these different motions of a molecule can be changed from its normal ground state. Nevertheless, the excited states that are available to molecular excitation are quantized; that is, they have specific, discrete values. A molecule may possess many energy levels, corresponding to different internal motions of the molecule. Yet, not all the possible levels may be attained by electromagnetic radiation.

The allowed transitions between ground-state and excited-state energy levels are subject to a variety of selection rules. It is important to recognize that the energy levels that are attained by the interaction of electromagnetic radiation and a molecule (a molecular spectrum) are only a portion of the total number of excited states of the molecule. The motion of a molecule around independent axes is called rotational motion. For changes in rotational energy levels, the rule is that only molecules with a permanent dipole moment can make a transition between different states of rotation. Although this includes most molecules, some molecules are nonpolar.

The movement of atoms between the chemical bonds that hold them together is called vibrational motion. In changing vibrational energy levels, only those vibrations that result in a change in the dipole moment of the molecule are allowed. As in rotational motion, only some of the energy levels available in a molecule can be excited by electromagnetic radiation. The movement of electrons within a molecule is electronic motion. Electronic energy levels can be changed only when a change in the dipole moment occurs as a result of electron rearrangement in the molecule.

Some of the electronic energy levels of molecules are more easily excited than others, resulting in varying intensities among different molecules. Each of these various internal motions of a molecule is quantized; that is, it has a discrete and distinct energy level that can be changed only by the loss or gain of an exact amount of energy.

The energies of electromagnetic radiation absorbed by molecules cover a wide range, from the microwave to X ray. Microwaves and radio waves are at the lowest energy level and cause molecules to rotate. Infrared radiation has ten to one hundred times more energy than microwave radiation and causes the atoms in molecules to vibrate. Visible radiation and ultraviolet radiation have ten to one hundred times more energy than infrared and cause electronic redistribution within molecules to occur. Vacuum ultraviolet and X rays have ten to one hundred times more energy than visible light and cause the ejection of electrons from molecules. This separation of the energies and energy levels in molecules enables each of the internal motions to be treated independently. Therefore, the energy of electromagnetic radiation used will determine the molecular motion being excited and the information being gathered.

The change in the energy levels of a molecule is accomplished by the absorption or emission of a photon of the correct energy. This is called a spectroscopic transition. The spectra of molecules, therefore, have two major criteria: They must satisfy the energy condition, and they must satisfy the molecular dipole conditions. The spectrum itself is obtained by focusing electromagnetic radiation of known energies upon the molecule and determining which of the energies are being absorbed. The graph of the amount of absorption or emission versus energy is called a spectrum.

In addition to absorptive and emissive transitions, those exactly satisfying the energy condition, molecules can interact with electromagnetic radiation without absorbing energy directly from the incident radiation. This phenomenon is called scattering and has two major forms. In the first type, Rayleigh scattering, the interaction of the electromagnetic radiation with the molecule is called elastic scattering. In other words, the light is scattered as if it were in an ideal collision; neither the molecule nor the incident radiation experiences any energy loss or gain. Yet, the interaction of nonabsorbing electromagnetic radiation and a molecule can be inelastic. This is called Raman scattering, where a molecule either can give up or can take on energy from a photon. In Raman scattering, it is neither the presence of nor the changing of a dipole that results in absorption; absorption results, rather, from the ability of the light to induce a dipole by the rearrangement of the electronic structure in the molecule.

Each of the different types of spectroscopy requires a different energy of electromagnetic radiation and excites a different type of molecular motion. Each type of spectrometer can be generalized in the following manner. The spectrometer has a source of electromagnetic radiation--a Klystron for microwave, a piece of hot metal (glow bar) for infrared, a tungsten light bulb for visible, a deuterium lamp for ultraviolet, and a high-intensity laser for Raman scattering. The polychromatic (many-wavelength) radiation is focused through an optical system (monochromator) that filters the radiation into smaller units. After passing through the sample, a detector of the radiation compares the amount of light that passes through the sample with the reference intensity. This is displayed in some manner by a recording device.

Applications

The energies of electromagnetic radiation that are absorbed by molecules do more than show the differences in energy between molecular energy levels. For each type of spectroscopy, specific information regarding an aspect of the structure of a molecule can be determined. In some cases, this is qualitative; in others, it is quantitative. In all cases, however, it provides a nondestructive technique for investigating molecules in controllable environments. Some techniques are best for small molecules; other techniques provide information about larger molecules. Taken as a whole, these techniques provide a complete picture about the structure of a molecule.

The lowest-energy quantized molecular motion is rotational motion. The energy required to perturb this motion occurs in the radio and microwave regions of the electromagnetic spectrum. Simple absorption requires a permanent dipole movement for a molecule. In addition, Raman scattering of molecules that do not possess a dipole movement can be performed.

Qualitatively, molecules can be sorted between those with and those without permanent dipole moments. In addition, the separation of the energy levels for rotational motion (absorptive and Raman) depends upon the atomic mass, bond lengths, and overall geometry of the molecule.

Rotational spectra are recorded for compounds of known mass and shape. The analysis of the resulting spectrum yields highly accurate bond-length information. Rotational spectra are analyzed in the exploration of space to determine the composition of some gases in space and in planetary atmospheres.

The next highest energies for molecular excitation occur in the infrared region. Like the rotational levels, the vibrational levels can be either absorptive or Raman-scattered or both. If they are both, however, the overall symmetry of the molecule can be assigned to be markedly different from those where the transitions are exclusively absorptive or Raman. In addition, the energy of the vibration depends upon the masses of the atoms and the strength of the bonds holding the atoms together. Vibrational spectra of molecules possessing similarly bonded atoms reveal the differences in the bonding of these atoms both qualitatively and quantitatively. For bonds that are composed of the same atoms, the strength of the bond can be determined. This is of great interest to chemists as they study new and different molecules and their reactions.

Understanding how different structures and experimental conditions affect bond strength gives an insight into the potential chemistry of similar molecules. The exact nature of a vibrational spectrum is so unique for a molecule that it provides a "fingerprint" spectrum useful for identification purposes. In addition to uses as a technique in space exploration, infrared spectroscopy is heavily used as a means for determining the identity of compounds in many different applications, from air pollution to drug analysis.

Electronic spectroscopy examines the highest energy a molecule can absorb that rearranges the electron density without ejecting the electron from the molecule. The energy required for these transitions occurs in the visible and ultraviolet regions of the electromagnetic spectrum. The effect of the absorption of a photon of the correct energy is to place the molecule in an electronic excited state, in which the molecule's electrons are distributed around the atomic nuclei in a different arrangement from the normal, nonexcited state.

As with other forms of spectroscopy, the visible-ultraviolet spectrum contains information of a qualitative and quantitative nature. The three-dimensional structure and its size determine the energy of and tendency to absorb energy in the visible and ultraviolet regions. The relative magnitude of absorption by a molecule gives a qualitative picture of the possible structures of the molecule. The relative energies of the absorptive transitions give a qualitative view of the size of the portion of a molecule where the electron change is occurring.

In addition to the qualitative aspects, two quantitative aspects of electronic molecular spectroscopy are utilized. The first of these is based upon the quantitative nature of absorptive spectroscopy. In other words, the more of a particular molecule that is present in a sample, the more photons will be absorbed. For any absorbing molecule, a linear relationship between the absorbed light and the number of molecules can be found, and accurate determinations of numbers of molecules in unknown samples can be made.

Besides the macroscopic determination of the amount of a compound, the validity of the equation used to describe it may be checked. Electron energy levels in a molecule are described by mathematical functions called molecular orbitals. An electronic transition is the movement of an electron from one molecular orbital to another. The ability of this process to occur can be predicted using the mathematical functions and can be observed in the spectrum.

This provides a check on the theoretical description of the electronic structure of molecules.

The highest-energy electromagnetic radiation that interacts with molecules results in the ejection of electrons from atoms and molecules. In this technique, vacuum ultraviolet and X rays are focused upon a sample, and the energy levels of the ejected electrons are determined.

When vacuum ultraviolet is used, the electrons are ejected from the energy levels involved in the bonding of the molecules. These energies give the exact values of those electronic states.

Analysis of the shape of the spectrum gives a qualitative picture of the extent of each electron's involvement in bonding within the molecule. When X rays are used, the ejected electrons are from the core of the atoms in the molecule and serve as a qualitative and semiquantitative means of atomic analysis of a compound.

Context

Molecular spectroscopy has developed into a powerful tool for learning about molecular structure and molecular energy levels. Building on the development of modern quantum mechanics in the early twentieth century, molecular spectroscopy has achieved the status of being one of the primary means of investigation of chemical systems, along with thermodynamics and kinetics. The scope of spectroscopy is very extensive and is the primary source of information regarding detailed molecular structure. Because of the unique ability to focus on different parts of the molecule, or on the molecule as a whole, spectroscopy is used to study molecular parts, molecular concentration, molecular size, and molecular interactions in qualitative, semi-quantitative, and quantitative ways. Spectroscopy has become an indispensable tool in the study of molecular structure from various points of view. In one form or another, molecular spectroscopy is used in most studies of chemical systems today and will continue to be in the future.

Principal terms

ABSORPTION: energy from a photon of electromagnetic radiation is added to a molecule in a low energy level, which results in a change to a higher energy level of the molecule

DIPOLE: a condition that exists when electron density is not evenly distributed throughout a molecule or bond; results in partial positive and negative changes in different parts of the molecule or bond

ELECTROMAGNETIC RADIATION: the continuous energy spectrum, commonly called light, which includes radio waves, microwaves, infrared, visible light, ultraviolet, and X rays

EMISSION: the energy from a photon of electromagnetic radiation is released by a molecule in a high energy level, which results in a change to a lower energy level of the molecule

ENERGY LEVEL: a quantized state of energy for a molecule that depends upon the motions and positions of atoms and the electrons in that molecule

INFRARED RADIATION: medium-wavelength electromagnetic radiation (also known as thermal or heat radiation), which interacts with the vibrational levels of a molecule

MOLECULE: a three-dimensional arrangement of atomic nuclei held together by covalent (electron sharing) bonds between the nuclei

PHOTON: the carrier of electromagnetic radiation; in quantized units, its energy is proportional to the wave number and frequency of the radiation

RADIO and MICROWAVES: long-wavelength electromagnetic radiation that can interact with the rotational levels of a molecule

SPECTRUM: a record of the interaction of electromagnetic radiation with a molecule; the observables are intensity and energy

ULTRAVIOLET-VISIBLE LIGHT: short-wavelength electromagnetic radiation that has observable color (visible) or is known as black light (ultraviolet) and interacts with the electronic levels of a molecule

X RAYS: the short-wavelength electromagnetic radiation that ejects electrons from the electronic levels and may destroy the molecule

Bibliography

Atkins, Peter W. PHYSICAL CHEMISTRY. New York: W. H. Freeman, 1986. This book has become a classic in physical chemistry, especially molecular spectra. Separate sections are devoted to the explanation of background and interpretation of each type of molecular spectroscopy. The mathematics is pitched at a high level; however, clear and complete explanations of molecular phenomena accompany the equations. Contains excellent explanations of molecular phenomena.

Barrow, Gordon M. INTRODUCTION TO MOLECULAR SPECTROSCOPY. New York: McGraw-Hill, 1962. This intermediate text guides the reader through the elucidation of molecular structure through the use of molecular spectroscopy. Especially good at spectra-structure correlations. Good pacing of information.

Barrow, Gordon M. THE STRUCTURE OF MOLECULES. New York: W. A. Benjamin, 1963. A clearly written introduction to molecular spectra. Geared for college undergraduates and uses very little mathematics. Excellent introduction of all types of spectroscopy.

Flurry, Robert L., Jr. QUANTUM CHEMISTRY: AN INTRODUCTION. Englewood Cliffs, N.J.: Prentice-Hall, 1983. This intermediate-level text stresses the application of quantum mechanics to molecular structure. Emphasis is on applications to chemical bonding and molecular spectra. Use and understanding with real systems are stressed; mathematics and model systems are used sparingly.

Guillory, William A. INTRODUCTION TO MOLECULAR STRUCTURE AND SPECTROSCOPY. Boston: Allyn & Bacon, 1977. An advanced textbook in molecular spectroscopy. Nevertheless, only the results and utilization of the mathematics and models are presented. Good explanations of advanced and elementary concepts.

Herzberg, Gerhard. "Molecular Spectroscopy: A Personal History." ANNUAL REVIEW OF PHYSICAL CHEMISTRY 36 (1985): 1-30. Discusses Herzberg's contributions to the field of molecular spectroscopy. Herzberg, the 1971 Nobel laureate in chemistry, also includes descriptions of other contributors to the field.

Chemical Bond Angles and Lengths

Quantum Mechanics of Chemical Bonding

Quantum Mechanics of Molecules

Photon Interactions with Molecules

The Interpretation of Quantum Mechanics