LOW-TEMPERATURE ABSORPTION SPECTROSCOPY (Cryoabsorption Spectroscopy) FREDERIC I. HOCH Haward Medical School, Boston, Massachusetts
IMPROVEMENTS in instrumentation for the dispersion and measurement of radiations have been continuous over the past few decades. The increasing availability of such apparatus has facilitated the increasing range of application of absorption spectroscopy. Not as much attention has been given to the possibilities of treating the radiation-absorbing materials themselves to produce spectra of increased information value. Reduction of temperature often results in changes in absorption spectra. Spectra of cooled materials may exhibit increased absorption and detail, and chemical structures unstable at ordinary temperatures may cause measurable absorption. Very low temperatures may be produced and maintained by apparatus intrinsically capable of simplification for wide use, and such techniques already have been applied in a number of investigations. Cryoabsorption spectroscopy is the study of the absorption of electromagnetic radiations by materials which are a t low temperatures. The effects of temperature upon the colors of substances-thermochromic effects-were noted by Brewster in 1831 ( I ) , but J. Becquerel in 1907 (8) first reported changes in absorption spectra of bubstances a t low temperatures, dthough Liveing and Dewar had visually observed the spectrum of liquid oxygen earlier (3). Subsequent investigations have dealt with the spectral range from the infrared to the ultraviolet, and recently the vacuum ultraviolet (4) regions. Temperatures have extended from that attainable with solid carbon dioxide--about 195°K.-to that with liquid helium-about 4°K.
varying with the degree of hydration of a solid film or with the solvent used (5, 6). Intramolecular and intermolecular effects upon the absorbing structureq may he considered. The primary effect of the reduction of temperature upon radiation-absorbing systems is an intramolecular one: the depopulation of thermally excited energy levels in accord with the Boltzmann relationship. The lowered probability of such energy transitions reduces the number and absorbance of lines or bands depending on thermal excitation, while the increased population of the lower energy levels causes increased absorbanre in the corresponding spectral bands or lines. This narrowed range of energy levels from which spectral features arise produces a decrease in the width of bands. A further reduction in half-hand width is due to the decrease of the Doppler effect in proportion to the square root of the absolute temperature. Intermolecular effects of reduction of temperature follow upon this primary effect. The depopulation of thermally excited translational, rotational, and vibrational energy levels reduces the randomness of orientation of the molecules of the absorbing system, and thus the randomness of intermolecular electrical and magnetic forces. Such increases in field strengths cause "internal" Stark and Zeeman effects upon absorbing structures; hands or lines are broadened, split, and shifted, and their absorbance is decreased. These spectral alterations are most strikingly seen when intermolecular fields undergo abrupt changes in symmetry of orientation, which occur at characteristic temperatures of transition of state, or a t temperatures PHYSICAL BASIS associated with transitions of crvstal structure in the Reducing the temperature of many substances solid state, However, the effect is recognizable within produces characteristic changes in their absorption a single state of crystal habitus, where thermal conspectra: increase in absorbance of bands, reduction of traction is a physical indication of increasing internal their half-band width, changes in the number of bands field strengths, and where spectral changes are obor lines, and shifts in position of absorption maxima. served. Other intrastate spectral shifts ma,v be due to The first three phenomena increase the informational the displacement of equilibria between molecular utility of these spectra both for qualitative and quanti- isomers of different energy status; as temperature is tative purposes. However, the physical factors pro- lowered, configurations requiring the least energy for ducing such spectral alterations are complex and their maintenance predominate. In crystals, where vibrations of the lattice absorb interrelated, and may act in opposing directions. Not all materials exhibit spectral deviations a t low tem- rharacteristic low-energy frequencies of radiation, a peratures, and the spectra of structurally similar special intermolecular effed is noted in cryozbsorption compounds may show them to varying degrees; a spectra. These vibratory modes map be coupled compound may itself show cryoabsorption effects additively or negatively with intramolecular modes, 469
JOURNAL OF CHEMICAL EDUCATION Effects of Low Temperatures upon Features of Absorption Spectra Changes from spectra observed at ---ordinary temperatures----Band DO& Hall- Band Numher Absorb- 'lion bod. symoj anre d < f t width metry bands
Intramolecular effects Population of ground state alone Reduction of Doppler effect Intermolecular effects Increase in field strength Crystal lattice ahsorption Lattice-molecular coupling zt External effects Christiansen filter effect Multiple internal rcfleetions Contraction of absorbing medium Presence or increase =
+
+
+
-
0
-
-
+ + +
+ +; deerease =
+ +
+
+ -
-; no change = 0.
increasing spectral details. However, reduction of thermal energy to very low values allows population of only the ground energy state of the crystal lattice, and spectral detail may again be reduced. Low temperatures in crystalline substances may thus cause any combination of variations in absorbance, position, number, and contour of bands. In powdered or microcrystalline systems anomalous variations of light scattering with wavelength of iucident radiation may also alter the contours of absorption hands (Christiansen effect) (7), while multiple internal reflections may produce an increase in effective optical depth with resulting increased absorbance (8).
absorption effects among structurally similar compounds. A summary of the determinations of cryoabsorption spectra is presented in the table. APPARATUS
The most immediate practical problem of cryoabsorption spectroscopy is the design of cryoabsorption cells which combine suitable optical properties with an effective cooling system. A number of cells have been designed, their simplicity approximately proportional to the desired absolute temperature. They are of two general classes (see the figure): (a) immersion types in which the absorbing material and the refrigerating medium both lie in the optical light pakh (9); and (b) conduction types in which only the absorbing material is in the light path, and where refrigeration is maintained by thermal conduction (10). Immersion cryoabsorption cells are usually simple, but are applicable only for spectral regions where the absorption of the refrigerant does not interfere. Control of temperature and wider spectral range are assets of conduction cells. Their construction, however, poses more problems, one of which is the nature of the junction between the thermal conductor and the optically clear and flat windows. This union must maintain its optical properties and withstand repeated shifts from low to normal temperatures, differences in contracture of its elements, vacuums, and solvents-a not inconsiderable demand. Specimens have been prepared for cryoabsorption studies in solution or in solid state. Eutectic mixtures which remain fluid or vitreous at low temperatiires have been used as solvents (11). Solid materials may be spread in thin films over clear surfaces or mounted as single crystals directly on holders whirh orient crystal axes for the observation of polarized cryoabsorption spectra (12'). APPLICATIONS
Cryoabsorption spectra have been used in studies of (a) atomic and molecular structure, (b) solid state and crystal structure, (c) monomolecular chemical reactions, and id) ~, unstable molecules and confieurations. The most extensive application has been in the theoretical interpretation of spectra for the elucidation of atomic and molecular structures. The rare earths have been so studied intensively; the shielded 4 j electrons of these elements give rise to sharply defined absorption spectra a t ordinary temperatures, and reduction of temperature with its further removal of external perturbations and the magnification of intermolecular Stark and Zeeman effects produces spectra of detail sufficient for analysis of energy levels. Changes lmmerrian Cell Conduction Cell in the spectra of rare earths at the temperature of liquid air were first reported by J. Becquerel (2), who noted that the increase in resolution produced ahThe physical changes producing cryoabsorption sorption lines comparable in detail with those of metal effects in absorbing systems may thus act in opposing vapors. The same investigator (IS) observed characdirections, the net effect being the resultant of these teristic cryoabsorption effects at the temperature of actions. This may explain the variations in cryo- liquid hydrogen and the effects of temperature upon
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absorbance in the direction of crystal axes. Further studies by Freed ( i d ) , Spedding (15), and others (16) have given information on atomic structure, the influence of neighboring ions, and the magnitude and symmetry of electrical fields in the crystals of the rare earths. The cryoahsorption spectra of molecules may reveal energy transitions obscured in spectra a t ordinary temperatures. Solids of ordinarily gaseous elements (17), salts of heavy metals (18), halogens (19), and organic molecules have been studied. Among the latter group, benzene in its three states was among the first investigated (20); the fundamental absorption frequencies in the vapor a t 93°K. and in the solidstate spectrum a t 93°K. were identical, and the alterations in absorbance, band width, and position of ma.xima were ascribed to molecular interactions. A series of aromatic hydrocarbons (21) a t about 100°K. showed resolution of room-temperature bands into subpeaks; absorptions classified as a-,0-, and parahands by studies of analogous series of molecules exhibited distinctive and characteristic differences in shifts of position at low temperatures (22). Studies on compounds of biological origin have included porphyrins (25, 2 ) and chlorophylls (25, 661, where resolution of hands was sufficient to enable the ronstruction of tentative term diagrams; purines and pyrimidines (6, 27), where cryoabsorption effects are variable and may be dependent on the degree of hydration ( 6 ) ;cholesterol (28), which shows resolution of new bands in the infrared region; and amino acids (67) among which the aromatic acids show cryoabsorption changes. The spectra of proteins at 101temperatures show changes due to their aromatic amino acid components (29,SO) or to prosthetic groups such as hemes (51)or carotenes (10). Amorphous polymeric molecules a t 4'IC show very small spectral changes, although one band of a crystalline polymer polythene was sharply narrowed (52). Intermolecnlar forces and the orientation of molecules in the solid state and in crystals have heenstudied hy these methods. The internal electrical field strength of rare-earth salts a t 53°K. was estimated at. 10%volts per cm. by comparison of cryoahsorption spectra with those produced by external fields (55). Polarized infrared spectra of N-acetylglycine and diketopiperazine a t 88°K. (54) and of ammouium nitrate, thallous nitrate, and plumbous nitrate a t 113°1
