ropics in..
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Chemical Instrumentation Edited by GALEN W. EWING, Setan Hall University, So. Orange, N. J. 07079
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These articles are intended to serve the readers of THISJOURNAL by calling attention to new developments i n the theory, & e n , or availability of chemical laboratory instrumentatim, m by presenting useful insights and ezplanations of topics that are of practical importance lo those who use, m leach the use of, modem inslrumenlalim and instmmental lechnipues. The editor invites correspondence frmn prospective cmtribulors.
XLVIII.
Rarnan Spectroscopy-Part
One
Bernard J. Bulkin. Hunter College of the City University of New York, New York, N . Y. 1'0021 WHY A RENAISSANCE? Raman spectroscopy, for many years merely a textbook phenomenon and not a laboratory reality t o mast chemists, is emerging from the shadow of its complementary technique, infrared iipectroscopy. This renaissance, largely stimulated by t,he development of continuaus gas Irtsem, encouraged by progress in photomultiplier tube technology, has been espped by the revival of some old ideas in optics and some new thoughts about sample handling. Tho result is that the Raman spectrum of CCI, shown in Figure 1-obtained in 3 minut,es exposure time with a mercury arc (plm about 25 minutes of film processing) on 30 ml of sample has been replaced with the CCI, spectrum shown in Figure 2taken in 3 minnt,es with 3 microliters of CCI, using He-Ne laser excitation a t 632.8 nm. I n this article we wish t o discuss the current statna of Raman instrumentation from the viewpoint of components-lasers, monochromators, detectors, amplifying systems, and sample handling. The theory of the llaman effect, adequately treated in many monographs (1) will not be discussed here. To understand t,he design features of contemporary R.aman instruments, i t is best t o briefly reomsider the problems of the older Raman instruments. One of these, the Cary Model 81, has actually made the transition from mercury arc excitation 1.0 Insel sources. I t is fair t o say that most of the problems of Raman stem fmm the need for several
intense, monochromatic souroes of radiation, preferably but not necessarily in the visible region of the spectrum (so that glass optics and cells can be used). These sources should be stable over long periods of time, easily turned on and off, and readily incorpor&ed into an optical system. The Toronto mercury arc, closest approximation t o these goals for many years, had numerous shortcomings. I n general the arcs were made of glass and an int,ense line a t 435.8 nm was used. The arcs were difficolt t o start, usually required eontinuous pumping t o achieve the high vacuums needed, and frequently involved a mass of water lines to cool t,he electrodes and rheostat,s. They were not particnlady stable sources, making any sort of integration over long time periods such as that required for measurement of depolarization rst,ios, rather difficult unless il ratio recording method of same sort was employed. Many colored compoundii absorb radiation a t 43.5.8 nm and t,heir spectra could not be obt,ained. This problem severely limited the application of Raman spect,roscopy t o the problems of inorganic chemistry-problems which i t is often uniquely powerful in solving-for many years. Same valiant att,empts t o overcome this difficuky using Cd, Na, Rb, He, and other arcs, both dc and radio frequency or microwave excited, were made by several groups and the spectra of numerous inorganics obtained this way, but other difficulties of prepsrat,ian and lifetime thwarted the developments in this 8RB.
Figure 1 . Roman lpectrum of liquid CCI,, recorded photographically in o 3-min expowre using 3 0 to left and right ml of v m p l e . Hg 4 3 5 . 8 nm excitation. Excitation by other wovelengths con be ~ e e n of 4 3 5 . 8 nm spectrum. Calibration spectrum oppeorr above the Roman spectrum.
Bernard J. Bulkin is :mistant p ~ d e s s o r ,f chemistry nt Hunter College of the
Zity IJni~ersityof New York. I'rior tc oining the Hunter College staff in 1967. 3r. Bulkin was a. ~ostdoctoralfellow nt he Swiss Federal Institute of Teehr~ology n Zurich. Switzerland. He received the 3.S. degree from the Polytechnic Institute ,f B~.ooklynin 1962 and the 1'h.D. de. :me from Purdue University i n 1966 3r. Bulkin's research interests include tudy of intermolecu1;m forces using inrared and Raman spectroscopy. ns well IS synthesis and speetm of orennonetnllic compounds.
The mercury 435.8 nm exciting line was $so uncomfortably close t o another intense line a t 404.1 nm, and it was frequently difficult, even with good filtering, to determine which exciting line was responsible for an observed Raman line. For example, studies of the 1640 om-' vibration of liquid water were not feasible using 435.8 nm excitation because the region of interest was overlapped by the 0-H stretching vihmbian cxciled by H g 404.1 nm. I t was impossible, using mercury arcs and anything hut the most sophisticated, homemade, high redudion monochl.ornators, to observe Raman lines close to the exciting line itself. This is one of the most interesting regions of the spectrum, however, especially as it is also difficult to look a t by infrared techniques. While d l of t,he instrumental problems of Raman spectroscopy have not been solved, t h e problems discussed above, which arose from the lack of trnly mitable sources of exciting radiation, have been reduced t o t h e point where they are of no concern t o the user of a contemporary Raman spectromet,er.
(Continued on page A782)
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Chemical lnsfrumenfafion
already adequately performed by t h e myriad of infrared, nmr, mass spectrometers, ete.? We will rtttempt here only the
Roman spectrum of liquid CCI,, scanned p h o t ~ e l e c t ~ i c o l layt 500 rm-'jmin, wing 3 pl of sample, He-Ne 632.8 nm excitation. Figure 2.
APPLICATIONS What functions can Raman serve in the modern chemical laboratory whieh are not
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briefest outline of Raman applications, before passing on the details of t,he instrumentat,ion. 1 . Center of symmetry: There is a selec-
tion rule involving allowed and forbidden infrared and R a m m transitions which is known as the rule of mutual exclusion. This states that if a molecule has a center of symmetry (i.e., belongs to a point group whieh contains the inversion operation, i, as one of its symmetry operations) then there are no transitions which are allowed in both the infrsred and the Raman spectrum. Frequent use has been made of this rule to distinguish between two possible structures or conformations, one of which has a center of symmetry. Same examples: puckered versus planar cyclobutanes, various types of photodimers, cis-trans isomerism. This rule applies strictly only in the gas phase and holds up fairly well in solution., I t should never be applied to solids using the symmetry properties of the isolated molecule. 2. Totally symmetric vibrations: There may be a change in polarizability even when there is no change in dipole moment, so that a band which is forbidden in the infrared is allowed in t,he Raman spectrum. An obvious application comes in studying bond strengths in homonuclear diatomic molecules, whichgive good, strong, ltaman spectra. One of the oldest and most important proofs of the nature of univalent mercury comes from the measurement of the Raman band of HgPt a t 169 cm-' by Woodward in 1934. There have been many other recent applications of this type, pxticularly important ones being those involving metal-metal bonding. There are undoubtedly some important biological applications of this sort as well. (Continued on page A784)
Chemical Instrumentation Under highly symmetric vibrations we should $so mention the importance of studying the vibrations of carbon-carbon double w d triple bonds by Raman spectroscopy. These are quite weak in the infrared, even in relatively unsymmet,rical compounds, but are very intense in t,he Raman spectrum. The vibrational frequencies are also quite sensitive to small changes in the eledran density of the pi system and to strain in the molecule. 3 . Solids: Current Raman instrumentation offers several new avenues regarding the vibrational spectra, of solids. I t is quite easy to run spectra of powdered solids using Raman instruments, with no sample preparation or matrix necessary. Thus the KBr pellet and the Nujol mull are not needed for Raman. The technique is also nondestructive and needs only afew micragrams of sample. Intriguing possibilities in the study of single crystals by Rsman spectroscopy are now opening up. By taking advant,age of the highly polarized nature of the laser beam, and space group theory predictions about the polariaation of the scattered Raman light, much information about, forces and st,ntcture in crystals can he ahtsined. The first work on ionic crystals has been appearing in the last few years, and initial studies an organic crystals are being undertaken. Not only can internal vibrat,ions be observed in these spectra, lattice vibrations are also seen in many cases; t,hese can be used to study intermolecular forces, solid-solid phase transitions, etc. 4 . Inlermoleeularvihralionsand perlurbations: Because it is possible to observe low frequency vib~.nt,ionsin the Raman spectrum without too much difficulty (at, least this appears to be easier in many cases than making comparable measurements in the far infrared spectrum), we should be able to study t,he vibrat,ions of hydrogen bonds themselves, rather than relying on their perturbation of other bonds in the malemle. Some far infrared work in this area, has already appeared, and it is likely that R a m m studies are now in progress. This may prove especially useful in hialogiesl systems. Other work on biological systems perturbed by intermolecular effects, such as base pairing and stacking, etc., has already shown Raman to he usefnl as a complementary technique t,o infrared and nmr. 5 . Aqwous solulias: It is well known that Raman spectra are obtained in glass or quarts cells, and not subject to any sampling limitat,ians doe to water. Fnrther, the Ramnn speettum of liquid water is rather weak, and many studies can be carried out. on aqneoos solut,ions. This is proving quite useful in current st,udie on various squeona equilibria for a large number of m d a l ion systems. A further application in water has been the study of the effect of dissolved electrolytes on the spectrum of water itself, giving ameasure of the relative strength of hydrogen bonding between anions and water. 6. Group frequencies: I t is not really known yet whether a comparahle number
(Conlin~~d on page A7.96)
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Circle NO. 123 on Readers' Sewice Card 4
Chemical Instrumentation of useful group frequencies will be obtained from Rsmsn spectra such as are now widely used in infrared spectroscopy. However, a t the 1969 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy there were a few encouraging reports in this direction. There are some interesting group frequencies for substit,uted benzenes, and certain nuclei such as sulfur provide a very characteristic set of frequencies when bound in different ways. 7. Polymers: Several groups have been working on a. number of polymer chemistry problems using Raman spectroscopy with v e w interesting results. These studies have usually made use of the polarizat,ion properties of Raman lines. Some work on biopolymers has also been reported, and i t seems as if conformational data in polypeptides may be obtainable from Raman ipictra. The above is merely a sketch of some leadine Raman aoolicatians, which makes .. no p;tense of completeness. I t is illtended only t o show some areas of current interest and to give an idea of the diversiby of applications for these instruments.
LASERS Although pulsed, high-power lasers were used t o obtain Raman spedra in the early 1960's, arid led to the discovery and development of the laser-Raman effect, this work is pvimarily of interest t,o physicisis. I t should not be confused with what is usnslly discussed as Raman spectroscopy, and which is best referred t o as laser excited Reman spectroscopy. Helium-Xea: The most commonly used laser exciting line is 632.8 nm from a IIo-Ne Ixscr. These lasers, now cammercially available with powers ranging from less than a milliwatt np to shout 1110 mw, fulfill mast of the requirements of n good source of monochromatic radiation for llamsn spectroscopy. A power of ahout, 50 mw proves adequate for ohtaining spect,rs of a wide variety of samples. These lasers appear t o be extremely stable, withuwal short-term peak to peak flnctuatians stated by manufacturers as less than 1%. The laser beam, easily visible in a lit room, can be used for a rapid alignment, of spectrometer components. We have been able 1.0 hring a. lhser, monorhmnator, et,c., into our laboratory, set them on a. big table, and do d l the necessary alignments so that spectra could be obt,ainr?d at, near peak performance in s. few hours. The technology of I I d e laser eanst~r~letion is very fat. advanced, and in our experience few adjust~mendsare necessary on the laser as i t arrives from the mauufact,urer. There are several nonlasing helium and neon lines emitted. These are of milch lower int,ensity than the laser line and are usually eliminated by use of a spike filter which costs sbaut $200. These are available from many companies such as Corion, Thin Film Products, Oriel, etc. Alte!.. natively, one can make use of the fact that, the nonlasing lines diverge much more rapidly than the lasing line; thus if the
(Continued on peg? A788)
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Chemical instrumentation distance from the laser to the entrance slit of the manochmmator is large the nonlasing lines can be eliminated or greatly a.t,tenuated by interposing a small hole (e.g., an iris diaphragm which may be ppened and closed). An important property of the laser lines when used as Raman sources is their polarization. The lines are completely polarized, and the axis of polarization can be turned canvenie\tly by mirrors or half wave plates. This greatly facilitates measurements of depolarization ratios to high accuracy. I t also makes alignment of the spectrometer, sample, and laser axes quite easy. This is -usually accomplished with one or more small pieces of Polaroid and visual observations. The polarization of the laser beam also makes possible the very elegant studies now in progress in several laboratories on oriented single crystals, Much information about lattice vibrations is emerging from these studies, which lie on the border between solid-state physics and chemistry. Helium-neon lasers have been extremely valuable in the study of colored comoounds. Manv of the comoounds of intere..!, whir ti ab-
