Fields of molecular spectroscopy
Microwave spectroscopy
For diatomic molecules the rotational constants for all but the very lightest ones lie in the range of 1–200 gigahertz (GHz). The frequency of a rotational transition is given approximately by ν = 2B(J + 1), and so molecular rotational spectra will exhibit absorption lines in the 2–800-gigahertz region. For polyatomic molecules three moments of inertia are required to describe the rotational motion. They produce much more complex spectra, but basic relationships, analogous to those for a diatomic molecule, exist between their moments and the observed absorption lines. The 1–1,000-gigahertz range is referred to as the microwave region (airport and police radar operate in this region) of the electromagnetic spectrum. Microwave radiation is generated by one of two methods: (1) special electronic tubes such as klystrons or backward-wave oscillators and solid-state oscillators such as Gunn diodes, which can be stabilized to produce highly monochromatic radiation and are tunable over specific regions, and (2) frequency synthesizers, whose output is produced by the successive multiplication and addition of highly monochromatic, low-frequency signals and consists of a series of discrete frequencies with small separations that effectively provide a continuous wave signal (e.g., 6 hertz separations at 25 gigahertz).
Types of microwave spectrometer
There are two types of microwave spectrometer in use. In the conventional Stark-modulated spectrometer, the sample is contained in a long (1- to 3-metre, or 3.3- to 9.8-foot) section of a rectangular waveguide, sealed at each end with a microwave transmitting window (e.g., mica or Mylar), and connected to a vacuum line for evacuation and sample introduction. The radiation from the source passes through a gaseous sample and is detected by a crystal diode detector that is followed by an amplifier and display system (chart recorder). In order to increase the sensitivity of the instrument, signal modulation by application of a high-voltage square wave across the sample is used. The second type is the Fourier-transform spectrometer, in which the radiation is confined in an evacuated cavity between a pair of spherical mirrors and the sample is introduced by a pulsed nozzle that lowers the temperature of the sample to less than 10 K. The sample is subjected to rotational energy excitation by application of a pulsed microwave signal, and the resulting emission signal is detected and Fourier-transformed to an absorption versus frequency spectrum. In both instruments the energy absorbed or emitted as the molecules undergo transitions from one quantized rotational state to another is observed. The Fourier-transform instrument has the advantage of providing higher resolution (1 kilohertz [kHz] relative to 30 kHz) and of exhibiting a much simpler spectrum due to the low sample temperature that insures that the majority of the molecules are in the few lowest energy states.
For observation of its rotational spectrum, a molecule must possess a permanent electric dipole moment and have a vapour pressure such that it can be introduced into a sample cell at extremely low pressures (5–50 millitorr; one millitorr equals 1 × 10−3 millimetre of mercury or 1.93 × 10−5 pound per square inch). The spectra of molecules with structures containing up to 15 atoms can be routinely analyzed, but the density and overlapping of spectral lines in the spectra of larger molecules severely restricts analysis.
Molecular applications
The relationship between the observed microwave transition frequency and the rotational constant of a diatomic molecule can provide a value for the internuclear distance. The quantitative geometric structures of molecules can also be obtained from the measured transitions in its microwave spectrum. In addition to geometric structures, other properties related to molecular structure can be investigated, including electric dipole moments, energy barriers to internal rotation, centrifugal distortion parameters, magnetic moments, nuclear electric quadrupole moments, vibration-rotation interaction parameters, low-frequency vibrational transitions, molecular electric quadrupole moments, and information relative to electron distribution and bonding. Microwave spectroscopy has provided the detailed structure and associated parameters for several thousand molecules.
The use of Fourier-transform spectrometers has provided a method for studying many short-lived species such as free radicals (i.e., OH, CN, NO, CF, CCH), molecular ions (i.e., CO+, HCO+, HCS+), and Van der Waals complexes (i.e., C6H6―HCl, H2O―H2O, Kr―HF, SO2―SO2). There is a special relationship between microwave spectroscopy and radio astronomy. Much of the impetus for the investigation of the microwave spectra of radical and molecular ions stems from the need for identifying the microwave emission signals emanating from extraterrestrial sources. This collaboration has resulted in the identification in outer space of nearly 200 species, including the hydroxyl radical, methanol, formaldehyde, ammonia, and methyl cyanide.
For a polyatomic molecule, which is characterized by three moments of inertia, the microwave spectrum of a single molecular species provides insufficient information for making a complete structure assignment and calculating the magnitude of all bond angles and interatomic distances in the molecule. For example, the values of the three moments of inertia of the 12CH281Br12C14N molecule will depend on eight bond parameters (four angles and four distances), hence it is not possible to obtain discrete values of these eight unknowns from three moments. This problem can be circumvented by introducing the assumption that the structure of the molecule will not significantly change if one or more atoms are substituted with a different isotopic species. The three moments of an isotopically substituted molecule are then derived from its microwave spectrum and, since they depend on the same set of molecular parameters, provide three additional pieces of data from which to obtain the eight bond parameters. By determining the moments of inertia of a sufficient number of isotopically substituted species, it is possible to obtain sufficient data from which to completely determine the structure. The best structural information is obtained when an isotopic species resulting from substitution at each atom site in the molecule can be studied.
Infrared spectroscopy
This technique covers the region of the electromagnetic spectrum between the visible (wavelength of 800 nanometres) and the short-wavelength microwave (0.3 millimetre). The spectra observed in this region are primarily associated with the internal vibrational motion of molecules, but a few light molecules will have rotational transitions lying in the region. For the infrared region, the wavenumber (ν̄, the reciprocal of the wavelength) is commonly used to measure energy. Infrared spectroscopy historically has been divided into three regions: the near infrared (4,000–12,500 inverse centimetres [cm−1]), the mid-infrared (400–4,000 cm−1), and the far infrared (10–400 cm−1). With the development of Fourier-transform spectrometers, this distinction of areas has blurred and the more sophisticated instruments can cover from 10 to 25,000 cm−1 by an interchange of source, beam splitter, detector, and sample cell.
Infrared instrumentation
For the near-infrared region a tungsten-filament lamp (6,000–25,000 cm−1) serves as a source. In the middle region the standard source is a Globar (50–6,000 cm−1), a silicon carbide cylinder that is electrically heated to function as a blackbody radiator. Radiation from a mercury-arc lamp (10–70 cm−1) is employed in the far-infrared region. In a grating-monochromator type instrument, the full range of the source-detector combination is scanned by mechanically changing the grating position. In a Fourier-transform instrument, the range available for a single scan is generally limited by the beam-splitter characteristics. The beam splitter functions to divide the source signal into two parts for the formation of an interference pattern. In the near-infrared region either a quartz plate or silicon deposited on a quartz plate is used. In the mid-infrared region a variety of optical-grade crystals, such as calcium fluoride (CaF2), zinc selenide (ZnSe), cesium iodide (CsI), or potassium bromide (KBr), coated with silicon or germanium are employed. Below 200 cm−1 Mylar films of varying thickness are used to cover narrow portions of the region. Thermal detection of infrared radiation is based on the conversion of a temperature change, resulting from such radiation falling on a suitable material, into a measurable signal. A Golay detector employs the reflection of light from a thermally distortable reflecting film onto a photoelectric cell, while a bolometer exhibits a change in electrical resistance with a change in temperature. In both cases the device must respond to very small and very rapid changes. In the Fourier-transform spectrometers, the entire optical path can be evacuated to prevent interference from extraneous materials such as water and carbon dioxide in the air.
A large variety of samples can be examined by use of infrared spectroscopy. Normal transmission can be used for liquids, thin films of solids, and gases. The containment of liquid and gas samples must be in a cell that has infrared-transmitting windows such as sodium chloride, potassium bromide, or cesium iodide. Solids, films, and coatings can be examined by means of several techniques that employ the reflection of radiation from the sample.
The development of solid-state diode lasers, F-centre lasers, and spin-flip Raman lasers is providing new sources for infrared spectrometers. These sources in general are not broadband but have high intensity and are useful for the construction of instruments that are designed for specific applications in narrow frequency regions.
Analysis of absorption spectra
The absorption of infrared radiation is due to the vibrational motion of a molecule. For a diatomic molecule the analysis of this motion is relatively straightforward because there is only one mode of vibration, the stretching of the bond. For polyatomic molecules the situation is compounded by the simultaneous motion of many nuclei. The mechanical model employed to analyze this complex motion is one wherein the nuclei are considered to be point masses and the interatomic chemical bonds are viewed as massless springs. Although the vibrations in a molecule obey the laws of quantum mechanics, molecular systems can be analyzed using classical mechanics to ascertain the nature of the vibrational motion. Analysis shows that such a system will display a set of resonant frequencies, each of which is associated with a different combination of nuclear motions. The number of such resonances that occur is 3N − 5 for a linear molecule and 3N − 6 for a nonlinear one, where N is the number of atoms in the molecule. The motions of the individual nuclei are such that during the displacements the centre of mass of the system does not change. The frequencies at which infrared radiation is absorbed correspond to the frequencies of the normal modes of vibration or can be considered as transitions between quantized energy levels, each of which corresponds to excited states of a normal mode. An analysis of all the normal-mode frequencies of a molecule can provide a set of force constants that are related to the individual bond-stretching and bond-bending motions within the molecule.
When examined using a high-resolution instrument and with the samples in the gas phase, the individual normal-mode absorption lines of polyatomic molecules will be separated into a series of closely spaced sharp lines. The analysis of this vibrational structure can provide the same type of information as can be obtained from rotational spectra, but even the highest resolution infrared instruments (0.0001 cm−1) cannot approach that of a Fourier-transform microwave spectrometer (10 kilohertz), and so the results are not nearly as accurate.
Because of the anharmonicity of the molecular vibrations, transitions corresponding to multiples (2νi, 3νi, etc., known as overtones) and combinations (ν1 + ν2, 2ν3 + ν4, etc.) of the fundamental frequencies will occur.
The normal-mode frequencies will tend to be associated with intramolecular motions of specific molecular entities and will be found to have values lying in a relatively narrow frequency range for all molecules containing that entity. For example, all molecules containing a carboxyl group (C=O) will have a normal vibrational mode that involves the stretching of the carbon-oxygen double bond. Its particular frequency will vary, depending on the nature of the atoms or groups of atoms attached to the carbon atom but will generally occur in the region of 1,650–1,750 cm−1. This same type of behaviour is observed for other entities such as the oxygen-hydrogen (O―H) stretching motion in the hydroxyl group and the C=C stretching motion in molecules with carbon-carbon double bonds. This predictable behaviour has led to the development of spectral correlation charts that can be compared with observed infrared spectra to aid in ascertaining the presence or absence of particular molecular entities and in determining the structure of newly synthesized or unknown species. The infrared spectrum of any individual molecule is a unique fingerprint for that molecule and can serve as a reliable form of identification.
Raman spectroscopy
Raman spectroscopy is based on the absorption of photons of a specific frequency followed by scattering at a higher or lower frequency. The modification of the scattered photons results from the incident photons either gaining energy from or losing energy to the vibrational and rotational motion of the molecule. Quantitatively, a sample (solid, liquid, or gas) is irradiated with a source frequency ν0, and the scattered radiation will be of frequency ν0 ± νi, where νi is the frequency corresponding to a vibrational or rotational transition in the molecule. Since molecules exist in a number of different rotational and vibrational states (depending on the temperature), many different values of νi are possible. Consequently, the Raman spectra will consist of a large number of scattered lines.
Most incident photons are scattered by the sample with no change in frequency in a process known as Rayleigh scattering. To enhance the observation of the radiation at ν0 ± νi, the scattered radiation is observed perpendicular to the incident beam. To provide high-intensity incident radiation and to enable the observation of lines where νi is small (as when due to rotational changes), the source in a Raman spectrometer is a monochromatic visible laser. The scattered radiation can then be analyzed by use of a scanning optical monochromator with a phototube as a detector.
The observation of the vibrational Raman spectrum of a molecule depends on a change in the molecules polarizability (ability to be distorted by an electric field) rather than its dipole moment during the vibration of the atoms. As a result, infrared and Raman spectra provide complementary information, and between the two techniques all vibrational transitions can be observed. This combination of techniques is essential for the measurement of all the vibrational frequencies of molecules of high symmetry that do not have permanent dipole moments. Analogously, there will be a rotational Raman spectrum for molecules with no permanent dipole moment that consequently have no pure rotational spectra.
