Raman Spectrometry Ronald E. Hester, Chemistry Department, University of York, England
A
SELECTION
OF
THE
LITERATURE
which has appeared between late 1969 and late 1970 is covered by this review. The growth of activity in the Ramail spectrometry field during this period has far outstripped the overall growth of scientific work, largely as a result of the increased availability of several high quality commercial laser-spectrometers mentioned in the previous review in this series (1). The selection presented here is governed in part by the reviewer’s personal interest, of course, but an attempt is made to cover aspects of the whole field of Raman activity, with emphasis on novel applications of the technique and nonroutine structural studies. Professor Sir Chandrasekhara Venkat%Raman died in Bangalore, S. India, on 21 November 1970, aged 82. It is fortunate that he lived to witness the great resurgence of interest in the effect for which he was awarded the 1930 Nobel Prize in Physics. Although he had many honors bestowed upon him, including the Lenin Peace Prize in 1958 for activities in other spheres, it must have been particularly gratifying to him to see the full potential of his early work realized through the application of lasers in Raman scattering. Perhaps the best scientific tribute to him has been the series of articles published in 1971 by a group of distinguished Indian men of science ( 2 ) . A sign of the maturity of the subject has been the publication of several new books on laser-Raman spect,rometry during the period of this review (3-6). The field also continues to be served well by the Chemical Society Specialist Reports ( 7 ) , and analytical chemists will find a recent review by Hendra and Vear (8)useful. EXPERIMENTAL TECHNIQUE
Among the more spectacular new developments in analytical Raman spectrometry has been the remote determination of pollutants in air a t low concentrations (ppm). Japanese and American groups have developed similar procedures, employing back-scatter of a giant-pulse laser, collecting the radiation with a large diameter telescope mirror, and analyzing the spectra in the normal way. Distance is monitored and controlled by the time elapsed between the pulse generation and signal reception. The method has been applied to CO1 and SO2 determination in atmospheres polluted by oil smoke 490R
plumes and variously to NO, CO, HzS, HzO, Nz, 02, and Hz (9-14). Several reviews which highlight advances in technique have appeared. New developments in detection and amplification of Raman signals and sample illumination and handling have been summarized by Bulkin (16),while representatives of the Jarrell Ash and Cary Instrument Companies have reviewed applications of their commercial iaser Raman spectrometers to aqueous solution analysis, the study of organic molecules, detection of low frequency vibrations, the study of metal bonding in inorganic and organometallic compounds, the study of polymers (16). and have made a comparison with infrared in terms of instrumentation, sample handling, and applications (I?’). A demonstration of the sensitivity of the technique is the determination of 50 ppm of benzene in polluted water using a simple 5 mW He-Ne gas laser source (18). An eficient new German spectrometer has been described ( I @ , giving automatic adjustment to optimum signal-to-noise ratio, wavenumber reproducibi!ity to I/* cm-’ and digital wavenumber output. The peculiar properties of laser radiation have formed the basis for the design of a r.ew Fisher spectrometer (do), and the performance of a high resolution instrument with an 8-meter focal length has been demonstrated through the Ramen spectrum of N 2 0 (21). The use of a tunable spin-flip Raman laser in infrared absorption spectrometry has been shown to yield resolution superior to a conventional grating spectrometer (22), though doubts have been raised about the Iaser linewidth limitations (23). A number of improvements in sampling technique have been made, and methods for dealing with “difficult” samples have been described. Optimization of illumination and light collection for small samples in capillary tubes (24),a jacket for thermostating capillary cells t o 4=0.25”C over the temperature range 5-95 “C @6),and a compensated polarization scrambler, for use in depolarization and intensity measurements on microsamples (%’) have been presented. A new laser sample cell for increasing the intensity of Raman scattering from polycrystalline powders has advantages of simplicity and ease of adjustment (27), while use of a wedgeshaped cell with fiber optics used to obtain good light collection has been shown (68)to give intense, background-
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
free spectra, and avoids decomposition of powdered samples through excessive radiation. Two cells suitable for molten salt spectrometry have been constructed and described in good detail by Quist (89). At the detection end of the Raman spectrometer, digital evaluation of spectra has led in various ways to improvements in signal-to-ncise ratios obtainable (30-32). Multi-scan-averaging by computer has enabled the intensity of the 992 cm-1 band of benzene to be measured from a solution with a concentration of only 2.3 X lo-’ mole cm+ (SO), and the v1 band of CB7CL in natural CCld has been enhanced by computer treatment of the basic measurements (SI). Curve analysis of overlapping bands, band intensity integration, and spectral sensitivity corrections to relative band intensities all are easily and automatically achieved with digitized spectra. A radically different detector system has been used for recording the extremely weak hyperRaman spectra, this involving an image intensifier, an isocon camera tube, and a multichannel analyzer (33). The depolarization ratios of Raman bands are of great value in making assignments, so that the loss of polarization information in scattering from polycrystalline powdered solids has always been a severe handicap. A significant advance, therefore, is the development of a method for recovering this lost information by immersion of the powdered sample in a liquid of the same refractive index, n, as the solid. The technique has been illustrated by application to benzophenone immersed in a liquid of n = 1.60, a t 9OC-1700 cm-1 ( 3 4 , to ion-exchange resins immersed in benzene and silica gel adsorbed species immersed in hexane and heptane (39, and to the simple salts Ba(NO& and N a2S04,this final powder giving trouble due to birefringence effects (36). It further has been demonstrated that the polarized Raman spectra of transparent polycrystalline solids provide results essentially identical with those obtainable from single crystal studies (37). Increased attention has been paid to the effects of high pressures on Raman spectra of both solids and liquids, and several procedures and results reported (38-40). An area which has seen surprisingly little activity is that of rapid Raman spectrometry, the methods developed by Delhaye and his coworkers having been put to use on a very limited
number of chemical systems this far (41-43). Further X-ray Raman scattering results have been obtained, using chromium K a radiation scattering by solids of light elements, Li, Be, B, and
c (44).
INTENSITIES
The bulk of the Raman intensity work remains dependant on Placzek’s polarizability theory, though some further elaboration of a quantum-mechanical theory has been presented (46). Experimental and theoretical studies of absolute intensities and depolarizations of bands from several small organic and organo-germanium molecules have been made, C-0, C-H, C-C, Ge-C, and Ge-C1 bond polarizability derivatives (E’) being determined (46, 47). New quantum mechanical calculations of bond polarizability derivatives for pyridine and some deuteropyridines have produced results in good agreement with those from semi-empirical calculations (48). Correlation of bond polarizability derivatives with bond order have been made with many different systems, among them dimethyl sulfoxide (49), a number of hydrides (60),cyclohexane and cyclohexane-d12 (61), the perbromate ion, BrOr- (511, group TV tetrahalides (63), and the hydroxide, chloride, and iodide of trimethylplatinum (64). Bond properties in meta,l carbonyls and their derivatives also have been studied through Raman intensity measurements (56,66). Some of the difficulties associated with measurement of absolute intensities have been eliminated by a rotating-sector-disk null method of cornparing laser-excited Raman scattering with blackbody radiation (67). Relative inteneities a,re comparatively easy to obtain, but angular dependence of intensity scattered by random fluids still presents problems (68, 69). The dependence of the intensity of Raman bands on concentration in binary s01u.tions has been re-examined, and further complicating effects concerned with the polarity of the components of the solutions have been explored (60). Absolute intensities of bands from a series of halogen-substituted hydrocarbons have been compared for the gaseous arLd liquid phases (61). The wide variety of intensity changes a t the phase transition, coupled with many significant shifts in band frequencies, indicate scope for stil! more work in this area. Similar complicated patterns of behavior have been eyhibited for liquid-solid phase transitiom (52j. Anot,her interesting development in this poorly-understood area of Raman intensities is the demonstration that’ scattering from optically active molecules is slightiy diberent, in right and left circuiarly polarized incident iight (63)
RESONANCERAMAN EFFECTS (RRE)
The selective enhancement of some band intensities under irradiation conditions which approximate those appropriate to electronic absorption has been studied with a variety of gaseous and solid samples as well as with liquid and solution species. The condition where the frequency used for excitation of spectra of gases is high enough t o reach the region of continuous absorption has been treated theoretically by Behringer (64), one unusual result which emerges being the prediction of enhanced intensity of overtones. The difference between resonance Raman and resonance fluorescence effects has been well iilustrated by a series of studies with halogen and interhalogen gases (65). I n this work, overtone sequences up to the 14th harmonic were observed in the case of a strong RRE, with an argon laser used to excite the spectra. Similar eflects have been obtained using a quasicontinuous ruby laser with bromine gas at ca. 150 Torr pressure (86). Results obtained from iodine in solution in chloroform differ in some respects from those from the gas, but have satisfactorily been accommodated by Mortensen’s theory (67). This seeks to establish a relation between R R E and the molecular absorption spectrum, and has been used also to derive depolarization ratios for RRE (68,69). The RRE in permanganate ion, MnOa-, has been excited with the argon laser 5145 d line, and several overtone and combination tones observed from a 10-3 molar solution (70). Information on coordination geometry in strongly coloured thiocyanate complexes of some transitioii metals has been derived from their RRE spectra ( 7 l ) , and a number of reports of RRE in carotenoid pigment molecules have appeared (‘2, 7.9). Solid state RREs have been dernonqtrated in a number of semiconductors (74-78), and also from the Sa- ior. doped in a sodium chloride single crystal (79). It s e e m clear, however, that theoretical developments Ere not keeping psce with experimental advances in this area. SOLVENT EFFEClS A N D BAND SHAPES
Many experimental stildies of solvent effects on Raman spectra have been made, and only a few representative ones can be mentioned here. There has been no major breskthrollgh in the theoretical treatment of solvent effects on band intensities and shapes, though some progress E m been made through the consideration of weak intermolecular interactions (80-81). The v 1 band half-width in SO2 dissolved in the solvents CS2, CCl4, and CHCh has been used as an indicator of strength of
solvent interaction, with rotational contributions to line width being distinguished (83). The behavior of strongly anisotropic molecules such m CHBCN and CS2 has been interpreted iD terms of a modification of the dielectric polarization theory of solvent effects (84, and the influence of changing solvent, concentration, and temperature has been examined for PhCl and CHCla mixtures (86), for picolines in water, methanol, and ethanol (86), and for a series of silanes in hexane, CHCL, CClr, and acetone (87). The relationship between band shape, particularly band width, and the kinetics of fast processes in solution has been used to study proton transfer reactions in sulfate-bisulfatewater systems (88). A number of other studies have used band prcfiles to obtain rotational and vibrational correlation times in liquids, these including CHaI, CzHsI, and (CH3)aCCl (89), HC1 in solution (go), and various organic liquids (91). Molecular motion in liquid and solid HCl has been characterized through temperature-dependent studies of the spectral band width, shape, and depolarization ratio, from which it has beec suggested that nearly free tumbling of the HC1 molecule occurs in the cubic solid at temperatures above 320 O K (92). Correlation functions for liquid CHCL based on the R.G. Gordon almost-free rotation model have been evaluated, it being apparent that mechanisms other than molecular rotation must be included for the depolarized Itayleigh line (93). Low frequency Raman data have been used (94) to yield information cn molecular motions in crystalline p CeH4Br2, p-C6H4BrC1, and a- and 8phases of p-CsH4C12,mean square amplitudes of molecular librations and the intermolecular adjustment constants being calculated. The comparison of low frequency Raman bands from 28 different organic liquids with their far-infrared absorption spectra has revealed no correlaticn between the two sets of data (96). It has been concluded that the Ramrtn ecattering and infrared absorption arise from the formation of transient complexes between two or more molecules, with the Raman scattering arising from the symmetric vibration and translation of the complex, while the low-frequency infrttred absorption is the direct consequence of hindered rotatiosal oscillations of the molecules, for example, a benzene ring in a complex. Very sharp (width ca. 1 cm-1) hies in the depolarized Eayleigh wing of liquid toluene and benzene have been reported and attributed to shortlived, sniali amplitude molecular orientrttiona! vibrations (95). The nature of the liquid state has been probed a t a fundamental level through Raman scattering experiments with liquid helium,
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
*
491 R
the results being interpreted variously in terms of roton pairs and correlation functions (97-99). SOLID STATE
The physics of the solid state has been studied extensively through Raman scattering. However, much of the work is of but little interest to analytical chemists and only brief mention will be made here of a few of the advances which have no immediate chemical interest. The spectrum of V K centers (free Ch-) in KCl crystals doped with 0.5 mole % AgCl has been generated after X-irradiation of the crystals a t 77 OK. The intensity of the characteristic peak a t 235 cm-' diminishes rapidly with continued 5145 A Ar+-laser irradiation owing to recombination of photochemically reduced Ago electrons with V K centers (100). Ag+ ions up to 3.5 mole % in NaCl crystals induce Raman scattering also, and peaks a t 85 and 171 cm-I have been reported (101). Similar studies of LiF doped with MnZ+and Mg2+ (IO$), gold doped alkali halides (103), S I - and Sa- in sulfur-doped alkali halides, and N I - in potassium halides (104)are in the literature. Other species studied as impurities in alkali halides include NOz- (106), CN- (106), and Ti+ (10'7). A theoretical treatment has been presented for the first-order Raman effect in insulating crystals in which the true electromagnetic modes of the crystal (polaritons) are scattered by the lattice vibrations (108). Experimental results on the temperature dependence of Raman scattering in silicon ( l o g ) , and on freecarrier spin-flip Raman scattering from the wide-band-gap semiconductors CdS and ZnSe and indium-doped ZnSe and CdS have been reported (110). Electric (111) and magnetic (118) fieldinduced Raman effects in crystals, and stress-dependent effects (115, 114) also have been studied. Raman spectra of several metallic tungsten bronzes have been obtained (116). Cubic Na0.81WOa exhibits three weak features in the 200400 cm-I region which are correlated with WOa spectra, hexagonal R b . 8 WOa displays a single broad feature a t 655 cm-1, while tetragonal Nh,, WOa has a rich spectrum of eleven features. The obvious advantages in terms of structure analysis of single crystal laser-Raman studies have been thoroughly exploited during the period under review. Typical examples among the many materials examined by this method have been graphite (116), NaBF4 (117), KC103 (118), and The(Moo33 (119). Translational modes containing intermolecular N-H . . . N hydrogen-bond vibrations have been characterized in the spectrum of the imidazole single crystal (180), and low frequency external vibrations in urea 492 R
*
have been used for calculation of mean square amplitudes of librational oscillations, the results agreeing well with Xray dsraction values (131). Further studies of HgIl single crystals have been reported (122, 133). Assignments of modes in a number of coordination complexes have been made possible through single crystal studies, these including tetrakis (thiourea)nickel(II) dichloride (134), trans-hydridochlorobis(triethy1phosphine)platinum (186), Cs2LiCo(CN)e (136), and M ~ Z ( C Oand )~~ Rel(Co)lo (137). The internal modes of the octahedral PCl6- anion and tetrahedral PCl4+ cation have been assigned from an oriented single crystal study of PCl6 (138),and a technique for studying orientation effects on nearly opaque single crystals with laser Raman spectrometry has been described in some detail (129). Reports of Raman spectra of several liquid crystals have been made. Comparison of crystal, nematic, and isotropic phases of p,p'-azoxyanisole has enabled pseudo-lattice modes in the liquid crystal to be characterized, and the conclusion has been drawn that the spectrum is affected only by shortrange ordering (130, 131). Low frequency spectra of cholesteryl propionate, nonanoate, and palmitate also have been shown to be very sensitive to the degree of short range ordering (133). GASES
Increased availability of high quality Raman instrumentation has resulted in a massive increase in the number of papers reporting spectra of gases. A particularly striking illustration of the sensitivity achievable is the use of Raman scattering for precise measurement of atmospheric oxygen balance (133). I n this work measurement of O/N and C02/N concentration ratios under ambient atmosphere conditions were determined to a precision of 0.3 and 0.006 ppm total atmosphere respectively, by means of scattered photon counting over a 174-hour integration period. High resolution techniques have been further developed, using both conventional dispersion and interferometry (134-156) , one interesting result being the observation of spin fine structure in the vibration-rotation Raman spectrum of oxygen. The pure rotational Raman spectrum of NO under high resolution conditions has yielded rotational constants Bo = 1.6961(4) and Do = 5.46 X (158). Features due to low energy electronic transitions in the NO spectrum also have been characterized (158, 139). Rotational constants for W 1 2 and a5Cla7Cl ( I @ ) , for CS2 (141), and for an excited state of C02 generated in a low pressure electric discharge (142) have
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
been reported, and hot bands in the Raman spectra of molecular chlorine gas and CClr vapor have been used to determine anhannonicity constants (I&).
Molecular structure work with larger molecules is aided by gas phase studies, where the intermolecular effects of condensed phases are absent. Several papers treating spectra from phosphorus, arsenic, and sulfur vapors have appeared (144, 146), the species P,, P2, AS,, As2, Asap, AS&, ASPS,Ss, S7, and 86 being identified. Halides of carbon, silicon, germanium, tin (146, 147), selenium (148), tellurium (149, 160), gallium, indium, mercury, zinc (161), aluminum (161, 168), and phosphorus (163) have been studied in the gas phase. Comparative examinations of gas, liquid, and solid states for the compounds Si~Cls (164) and Re207(166) have been reported. MATRIX ISOLATION
Although infrared spectrometry of molecular species isolated a t low temperatures in inert gas solid matrices has been well-developed for many years, it has been only very recently that good quality Raman spectra from matrix isolated species have been obtained. Spectra to within 200 cm-l of the exciting line have been reported for SF6 in Ar at 4K, with a matrix:sample ratio of 500, using a platinum mirror as the matrix support (166). The same technique resulted in good spectra from CHCL in Ar at mole ratio dilutions of 100-200. A single band a t 253 cm-I has been reported as arising from Xe-C1 stretching of linear XeC12 matrix-isolated in Xe (167),and vibrational modes of SeO2 monomer and dimer have been characterized using C02 as the matrix (168). A still more unusual matrix material is SiMe4 which has been used for studies of TiC14.PHa (169). Related to the above results, though clearly differing in some essential details, is a study of Raman spectra of S02, C2H2, H2S, and HCl as guest molecules in 8-quinol clathrates (160). Frequencies here are closer to the gas-phase frequencies than to those of the solid or liquid-state, and indicate that the guest molecules are isolated monomers. The basic techniques of matrix-isolation at low temperature have been applied to an impressive study of pure solid Cc14 and isotopically enriched a6CC14 under high resolution conditions (161). SURFACE ADSORPTION
Acetone adsorbed on y-alumina has been investigated by Raman spectrometry, and bands a t 1575 and 1625 cm-l identified as characteristic of an electron donor-acceptor complex be-
tween the carbonyl group of acetone and the adsorbent surface (168). Similarly, a band in the region 1685-1703 cm-' is attributed to an H-bridge bond formed between the carbonyl and a hydroxyl group at the surface of the alumina. Raman spectra of nitrobenzene on alumina, and also on other sorbents, e.g., porous glass and Aerosil, have been reported as arising from coverings ca. 1 4 monolayers thick (163).
Some of the experimental methods found useful for obtaining spectra from surface adsorbed species have been described, and spectra from a range of adsorbates including halogens, hydrocarbons and halogenated hydrocarbons, nitriles, and aldehydes at a variety of oxide surfaces have been discussed (164). The spectra enable clear distinctions to be made between physical adsorption and chemisorption, the Raman method having some advantages over infrared absorption in terms of the nature of the background and intrinsic activity of the vibrational modes. However, sensitivity (signal/noise ratio) is distinctly inferior to that obtainable in infrared absorption in most of the work reported. Some other systems which have been studied are pyridine on oxide surfaces (166), styrene on silica (166), and pyridine and 2-chloropyridine on silica gel (167). Closely related to this work is a study of propylene sorbed on Azeolites (168). I n this the v(C=C) frequency of CaHa was found to be raised to 1717 or 1750 cm-l in the adsorbed state, with a saturation level of 170 mg CaHs/g zeolite, corresponding to 6-7 CaHa molecules per void, being achieved. Raman spectra also have been used to identify the complex ions AuC14- and AuBr4- and some anionic complexes of In(II1) sorbed on anion exchange resins (169). INORGANIC COMPOUNDS
The utility of the Raman method of studying molecular structure and bonding in inorganic systems is so widely recognized, and the necessary instrumentation so widely available by now, that it seems unnecessary to mention more than a small number of the more important or novel applications here. Aqueous solutions of sodium xenate(X1) have been studied and shown to contain the HXeO4- ion as the dominant species (170). Symmetric XeO84- ions were suggested from the spectra of aqueous solutions of cesium perxenate(VIII), though subsequent work has shown the presence of the HXe06'- ion in alkaline perxenate solutions (171). Detailed spectra have been obtained for the noble gas compounds XeFz, XeF4, and XeOF4 in the vapor phase (172). Halogen-oxygen compounds of a variety of forms have been studied, including
BrOaF and C 1 0 9 as gases (173), ClaO
(174, and OFZ (176),and OZFZ(176), and a number of interhalogen and polyhalide species (177,178). The spectra of IFe+AsFs- in liquid H F (179),of the GeH,+ and GeD,+ ions in liquid NH, (180), and of HC1, DC1, and HBr in liquid SF6 and C2Fa (181) illustrate the possibilities for non-aqueous solution studies. The latter work revealed features corresponding to the gas-phase rotational lines of the hydrogen halides. Concentrated perchloric acid in the form of the dihydrate, H&+C104-, has been studied a t low temperature, the spectra being consistent with a centrosymmetric HzO-H-OHZ structure of trans configuration for the cation (182). Transition metal compounds of a wide variety of types have been studied, including a long series of gas phase hexafluorides (183). The Raman spectrum has been useful in determining the nature of the complex [(Ru("a)& (Nz)] [BF4]4, showing the N r N stretching frequency at 2100 cm-' (184). Metal isotope effects have been used in assigning Zn-N modes in the species [Zn(NHa)r]Iz (186,186). Spectra from the complexes Ni(PF&, Pd(PFa)r, and Pt(PF& have been assigned (187). Other phosphine complexes studied have been Ni(PCla)r, HCo(PF&, and KCo(PF8)r (189), and mixed carbonylphosphines in the series Ni(CO)r-.(PX,),, with X being F, CHa, and CHaO (189, 190). Unusual square-pyramidal coordination geometry has been established for the pentacoordinate ions Ni(CN)ss- (191) and InCls2- (192). The special advantages of Raman spectrometry for determining low frequency symmetric modes of vibration have been further exploited in studies of compounds with metal-metal bonds. Polymetallic carbonyls of a wide range of types, including Mnz(CO)lo, Rez(CO)lo, ReCo(CO)9, and ReFeMn(CO)l4, have yielded low frequency bands assignable to metal-metal bond vibrations, enabling force constants to be calculated (193, 194). Metal "cluster" modes in the species PbaO(OH)s4+ (196), NbaO&, and TasO198- (196), Bis(oH)~d+,Pb4(OH)b4+,and T14(0Et)4 (197) have been determined. Metalmetal bond strengths have been derived for PhsSnz and PhaPbz (198), and for Me&, MesGez, MeaSnz, and MeePbz (199).
Electrolyte solution studies have yielded structural information on the aluminate ion in water, the high pH species appearing to be Al(OH)r-, condensing to AlzO(OH)az- with increasing solution concentration (200). There is, however, contrary evidence, favoring polymers of the type [(OH)2A1(0H)4ln, with two highly reactive OH- ions (201). Aqueous metal nitrate solutions have continued to attract much
attention, further evidence being accumulated for a quasilattice interpretation of spectra from very concentrated solutions (802, 203), though distinct ion-pair and ion-solvent interactions have been proposed in non-aqueous solutions of metal nitrates (804-906). Ionization of the st,rong acids HClOl and HReO, in aqueous solution have been studied (207,808), and the effect of electrolytes on the structure of water itself has been subjected to further examination through Raman studies (809). The enigma of polywater (anomalous water, or superwater) remains with us. The Raman spectrum has been cited as offering a better means than infrared absorption of distinguishing the fake from the gen-line article @IO), and ice-type water clusters have been proposed to account for the spectra (211 ) . Raman work with molten salts has ranged from straightforward attempts to identify complex ions such as CdClaand C d C P (2129, MgI4'- (%IS), or Ah&- and AlZC1,- (214), to the more subtle attempts a t probing the quasilattice structure and dynamics of metal nitrate systems (217-223). Spectra of molten metal carbonates (224, chlorates (226), and tetrafluoroborates (226) have been recorded, and the relationships between glassy solids and melts have been studied (227). ORGANOMETALLICS
Compounds of methyl-tin (228), -indium(III) (229), -germanium ($SO), -platinum(IV) (231), and gold(II1) (232) are among the organometallic species investigated. Metal-carbon vibrational modes have been characterized and force constants determined for these, as for a number of metallocenes and their derivatives (233-836). Assignments based on local symmetry for cpPtMes (cp = cyclopentadienyl) (237), and a normal coordinate analysis of complexed benzene in dibenzene chromium (238) have been made. The difficulties arising in assigning modes within molecules wherein free rotation of groups is possible have been discussed in connection with the ethane-like molecules Hg(CHa)z and Hg(CDa):, (239). From the spectra of ether solutions of MgBrz and MgI2, and of the corresponding solid dietherate of MgI2, fairly complete assignments have been deduced for the etherate complexes (240). ORGANIC COMPOUNDS
Among the more interesting simple organic systems which have been studied are the benzenehalogen chargetransfer complexes (241). Structural information also has been derived from the much more complex spectra obtained from steroids (242, 243). The
ANALYTICAL CHEMISTRY, VOL. 44, NO. 5, APRIL 1972
493R
Raman spectra of steroids show clearly the nonpolar multiple bonds and the arrangement of conjugated systems, and allow predictions of the linkage of the A and B rings, making the Raman method more suitable than infrared absorption for the identification of steroids. Studies of adamantane and some of its derivatives provide further examples of the utility of Raman spectra in analysis of complex organic molecules (244,246). An analysis of spectra from bicyclo [3.3.0] octane has been made, the alternation of vibrations and the number of polarized lines determined being consistent with a half-chair trans conformation with C2h symmetry (246). By using different laser polarization geometries] 58 lines have been observed in the spectrum of benzil single crystals, enabling a detailed analysis to be made (247). Other investigations have treated a wide variety of problems, from the skeletal modes of simple C(CN)h (248) or the torsional vibrations of isobutylene (249) to the characterization of spectra from terpenes (260) and even coals (261). The techniques for obtaining good quality Ramen spectra from synthetic polymers have by now become routine and much good work has appeared during the period reviewed. Other detailed reviews of this field have recently been made (262, 255). The work of Koenig and his school has been particularly comprehensive, ranging from the spectra of vulcanized rubbers (264), through studies of a conformational transition in syndiotactic pcly(methylasrylic acid) in water (255) and of thermal defects in poiy(tetrafluoroethylene) (666), to the characterization of the fiber surface of graphite and carbon fibers (257). Accordianlike motions in polymer chains have been associated with Rarnan scattering in the 10-40 cm-1 region from single crystals of polyethylene (658). The study of biopolymers by Raman spectrometry has attracted a great deai of attention, there being obvious advantages for the technique in this field because of the minimal interference given by the prime biological solvent, water. ‘Fhe spectrum of native lysczyme in aqueous solution has been partially interpreted with the help of spectra from its constituent amino acids (659), and similar work with ribosomai RNA has demonstrated intensity effects which may be due to its tertiary stl-uctcre (260). Raman intensities also have been used for the characterization of average ccnforniation in a number of polynucleotides and for studying order-disorder changes in these systems (261). The phencmenon of Raman hypochroism in the spectra of polyadenylic acid has been reported (262), and qualitative analyses of spectra from poly(adeny!ic acid) 494R
and poly(cytidy1ic acid) have been made (868). Many other biomolecules have been studied, including some vitamin A type molecules ($64),a metalloporphyrin (,%%), some ATP-metal ion complexes (866) and visual pigment in intact bovine retinas ($67),as well as a series of synthetic polypeptides ($68, 869). ELECTRONIC, STIMULATED, HYPERA N D INVERSE RAMAN EFFECTS
Tables of selection rules for the electronic Raman effect have been presented, including all possible crystal symmetries and state functions (270). I n addition, the Raman-Zeeman effect has been discussed for electronic systems] and criteria for assessing the usefulness of electronic Raman effects have been discussed. More experimental data have been reported on electronic Raman scattering from the trivalent lanthanides ( 2 7 l ) , making observations in the series almost complete. Intensities are greatest from ions in the middle of the lanthanide series, and an odd number of 4f electrons appears to be associated with the most profound differences of polarization features of electronic transitions and phonons. Electrocic transitions of Coz+ have been observed (272, 6 7 3 ) , and a!so of excited mercury atoms (274), of nitric oxide (275), and of substitutional donor and acceptor impurities in gallium phosphide (276). The use of stimulated Raman scattering t,o provide secondary sources of intense light has reached the stage of a tunable system, uti!izing the spin-flip process of conduction electrons in indium antimonide (277). With a COz laser a t 10.6 p as the pump, Raman laser radiation can be tuned from ca. 11.7 to 13.0 p by varying the magnetic field impinging on the InSb from cas 48 to 100 kG. Raman laser power output of ca. 1W peak, with linewidth rictiveand yet broader. Our intention has been to cover that, part, of the literature which is, or may be, of particular value to the analytical chemist, but not to limit this coverage to purely statistical matters. This aim is reflected, for example, in the relative magnitudes of the sections on “Curve Fitting” and “Statistical Control”. One indication of the relative amoufit of activity in these two areas by analytical chemists as opposed to chemical engineers is given in the most recent IMPLIED BY THE TITLE,
National Register of Scientific and Technical Personnel (468) which shows that the principal specialities of analytical chemists (55%) are spectroscopy and chromatography, whereas the principal speciality of chemical engineers j13y0)is quality control. This review covers the four year period: October 1967 to October 1971. Coverage is principally of U.S. origin with limited review of the foreign literature. Our effort has been concentrated on surveying recent journal artic!es and books-with less emphasis given to 1imitedCistribution documents. Considerable selectivity has been
exercised in this review inasmuch as an exposition of fundamental statistical principles and currently useful statistical methodology supported by a number of illurninatring references from the literature was considered preferable to a complete categorized enumeration of the chemical-statistical literature. Roping not to offend the more sophisticated of our readers, we have nevertheiess attempkd to avoid highly technical temrirdogy in introducing the various topics, so that the review may be of some value to those whose background is primarily in analytical chemistry rather than in mathematica! sthstics.
ANALYTlCAL CHEMISTRY, VOL. 4.4,NO. 5 , APRIL 1972
497R
