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Methods and advantages of multiple internal reflection infrared spectroscopy
SpectmUmica Acta, Vol 28A,pp701to706.Pergamon Press1072 Printed in Northern Ireland
Methods and advantages of multiple internal reflection infrared spectroscopy M. H. BROOKER* and D. E. IRISH Department of Chermstry,Umversrty of Waterloo, Waterloo, Ontarro (Received27 October1970) Abstract-The condltlons reqmred to obtam “trans~sslon-hke” spectra from multiple and smgle mternal reflectron 1.r. spectroscopy are discussed. Multiple mternal reflection spectra, properly obtained, are not distorted as are smgle mternal reflectronspectra and provrde the transverse optical frequencies directly. Excellent 1.r. spectra of powdered samples can be obtamed by MIR techmques. Illustrations are provided for NaCIO, showmg the effect of different prism angles, and for K,Ni (NO,),*H,O. INTRODUCTION INTERNAL reflection
spectroscopy is at a state of development where it can now be considered as a practical method for obtaining routine i.r. spectra. In the course of recent i.r. and Raman studies [l, 21 of solid phase ionic nitrates we found multiple internal reflection (MIR) methods to have a decided advantage over conventional sampling techniques, especially in regions of high sample absorptivity. The technique used and aspects of the interpretation are discussed below. A quantitative treatment of the fundamental formulae describing total internal reflection has been given by HANSEN [3] and related to absorptivity and concentration of absorbing species. Internal reflectance spectra have been obtained for aqueous solutions [P6], molten salts [7-S] and powdered samples [l, 2, 9, lo]. In spite of the excellent MIR spectra of SiO, obtained by IFARRICK and RIEDERMAN [9], the application of internal reflection techniques to solid samples has not been widely accepted. Generally, there is concern that reflection spectra will be subject to considerable distortion and hence the band frequencies will be unreliable. Although band distortion can be a problem, especially for single pass internal reflection studies [ 11, 121, this problem can easily be avoided with MIR techniques. Many optical arrangements have been proposed [S, 7, 13, 141. In our studies we have used the Wilks model-9 * Present address: Dept. of Chemrstry, Mount Allison Umverslty, Sackvdle, N.B , Canada. PI M. H. BROOEER and D. E. IRISH,Can. J. Chem. 43, 1183 (1970). PI M. H. BROOKER, D. E. IRISH and G. E. Bovn, J. Chem. Phys. 53, 1083 (1970).
[31 W. N. HANSEN, Spectrochzm. Acta 21, 209 (1965); 21, 815 (1965). [41 B. KATLAPSKY and R. E. KELLER, Anal. Chem. 35, 1665 (1963). 151 C P. MALONEand P. A. FLOURNOY, Spectrochim. Acta 21,1361 (1965). VI J. T. MILLER and D. E. IRISH,Can. J. Chem. 45, 147 (1967). r71 A. BANDY, J. P. DEVLIN, R. BURGER and B. MCCOY, Rev. Scz. In&. 35, 1206 (1964). PI K. WILLIAMSON, P. LI and J. P. DEVLIN, J. C&m. Phys. 43, 3891 (1968). PI N. J. HARRICK and N. H. RIEDERMAN, Spectrochzm. Acta 21, 2135 (1965). Cl01 M. H. BROOEER and D. E. IRISH,Inorg. Chem. 8, 219 (1969). [Ill R. P. J. COONEY,C. THAYER, P. C. LI and J. P. DEVLIN, J. Chem. Phys. 51, 302 (1969). WI J. P. DEVLIN and R P. J. COONEY, J. C&m. Phys. 52, 5495 (1970). Vol. 1. Walks Scientn?c Corp., South Norwalk, Corm. P31 Internal Rejkctzon Spectroscopy, (1965). L-141W. N. HANSEN and J. A. HORTON, Anal. Chem. 36, 783 (1964). 701
702
M. H. BROOKERand D. E. IRISH
MIR attachment and the Beckman attenuated total reflection (ATR) unit described previously [6] and now called SIR (single internal reflection). The reflectivity, R, at a plane boundary between two isotropic homogeneous phases, one of which is the transparent prism, is a function of. (a) polarization of the incident light, (b) angle of incidence, 6, to the plane boundary in the transparent phase, (c) n, the ratio of the real part of the refractive index of the absorbing sample, n,, divided by that of the transparent prism, n, and (d) the attenuation index, K, of the absorbing phase. For a given spectrophotometer, the ratio of parallel to perpendicular light is constant. The angle of incidence can be maintained at a prescribed value np and K are specific intensive properties of the sample; the product n,K is called the extinction coefficient. For frequencies at which the sample absorbs radiation, n, and K alter rapidly with resulting changes in R. When an i.r. spectrophotometer is used with either MIR or SIR attachments, reflectivity is given by the ratio of light intensity reaching the detector with and without a sample present, R/R, In transmission work, the transmittance is given by a similar ratio, T/T,. Since spectrophotometers usually give transmittance directly, reflectivity will also be measured directly. A spectrum can be obtained which closely resembles a transmission spectrum. If at any frequency the angle of incidence fails to exceed the critical angle 0,, total internal reflection will not occur and a distorted spectrum will be obtained. It is necessary to maintain 0 at sufficiently high angles to allow for variations in n. It is useful to choose a prism so that n, - w, > 0.5. AgBr and KRS-5 are suitable Intensity of a vibraprism materials for most organic and inorganic compounds tional band observed by internal reflectance techniques decreases with increasing 19 (above 13,)and decreasing n. Compensation can be achieved by increasing the number of internal reflections. Intensity of vibrational bands can also be controlled by varying the amount of sample in contact with the MIR plate Polishing procedures Transmission of the prisms is a critical factor and should be maximized in a If transmission non-absorbing region of the sample before a spectrum is recorded falls below 15 per cent the prism should be repolished. Spectra should also be recorded at slower scan speeds than conventional transmission spectra, a consequence of the lower energy. Polishing of MIR plates is more difficult than transmission plates, since a higher degree of excellence is required. The polishing procedure for KRS-5 was similar to that described [13]; however, we have found that treatment of the KRS-5 plate with hot water (in an ultrasonic cleaner if available) was superior to the usual grinding procedures. KRS-5 is slightly soluble in hot water and the outer layer may be uniformly dissolved off together with any embedded impurities. Grindmg may further press impurities into the plate. Polishing was performed on an extremely soft felt polishing cloth onto which liberal The plate was rinsed in quantities of methanol and ceric oxide had been placed methanol and buffed lightly to a high gloss on soft flannelette. It was usually necessary to repeat the polishing procedure several times. Because of its softness, AgBr was more difficult to polish than KRS-5. Embedded particles were removed by treatment of the AgBr prism with a 5% solution of
Methods and advantages of multiple mternal reflection Infrared spectroscopy
703
n-butylamine in ethanol in an ultrasonic cleaner. AgBr is soluble in n-butylamine and impurities are removed along with a thin layer of AgBr. Grinding with 600 grit abrasive could be carried out as described for KRS-5, but this was seldom necessary. The AgBr prism was polished with a soft felt cloth soaked in a 25% solution of n-butylamine in ethanol. As polishing proceeds, ethanol was added to the cloth to retard the dissolving power of the n-butylamine. Finally, the plate was rinsed in ethanol and lightly buffed on soft flannelette The polishing procedure was repeated several times. MIR
us SIR
The extinction coeflicient, nrc, for carbon tetrachloride has maxima at 762 and 786 cm-l with the intensity of the latter greater. The nK curve for benzene has a symmetric maximum at 675 cm-l [ 151. Few compounds have larger nK values. If no distortion occurs in the internal reflection spectra of benzene and carbon tetrachloride, then no distortion is expected in spectra of a sample obtained under the same conditions. In our spectra of carbon tetrachloride and benzene using a small surface area centrally located on a AgBr or KRS-5 MIR plate, no band distortion or frequency shifts occurred. These results seemed contradictory to previous authors [8, 151 who obtained distorted spectra for carbon tetrachloride and benzene with a single-pass SIR, KRS-5 prism. To investigate the apparent discrepancy, we recorded spectra for carbon tetrachloride and benzene with a single reflection at 62” angle of incidence to a KRS-5 SIR prism. The recorded spectra now showed both frequency shifts and band distortion in the form of an intensity reversal for the carbon tetrachloride doublet (Fig. 1). It appears that SIR spectra are subject to more distortion than MIR spectra. In single reflection spectra, dispersion of incident light may result in some components of the light striking the surface with less than the prescribed angle of incidence. In MIR compensation for dispersion occurs at each reflection, which tends to concentrate the light rays at the single prescribed angle which decreases the possibility of distortion [ 161. MIR
spectra of solid samples
Conventional sampling techniques for obtaining transmission spectra for solids can be troublesome and unreliable for a variety of reasons: (a) If organic mulling agents are used, overlap of sample bands with those of the mulling agent can occur. Although it is possible to choose “windows” by using a variety of mulling agents, band overlap often remains a problem, especially in the 700 and 1400 cm-l regions. (b) If alkali halide pellets are employed, ion exchange between the sample and the host lattice often gives rise to extraneous bands and frequency shifts. (c) Anomalous dispersion can occur in spectra of solids deposited mechanically on window materials (Christiansen Effect). (d) Single crystal transmission studies often require impractically thin samples. Of these problems, only ion exchange is a problem in MIR experiments, and it may usually be overcome. For instance, we have found that [153 A. C. GILBY, J. BURR, JR., W. KRUEUER and B. CRAWPORD, JR., J. Phys. Chem. (1966). [16] P. A. WILKS, JR., AppZ. Spectry 22, 782 (1968).
70, 1526
704
M. H. BROOEER
\
and D. E. IRISH
MIR
AgBr
SIR
KRS
Sd
5
62’
Fig. 1. Comparison of multiple and smgle mternal refleotlon. Photograph of spectra obtained for the extremely strong absorption doublet of carbon tetrechlorlde obtamed by MIR snd SIR techniques.
KRS-5 exchanges with AgNO, and anhydrous Cd(NO,),, but neither salt exchanges with AgBr. Photographs of MIR i r. spectra for powdered NaClO, and K,Ni(NO,),.H,O pressed against KRS-5 prisms are shown in Figs. 2 and 3,together with transmission spectra obtained for the same samples mulled in nujol, to illustrate the clarity that can be obtained by MIR techniques. The only preparatory step was the grinding of the samples into very fine powders to improve the sample-prism contact and prevent scratching of the MIR plate. Sodium chlorate has recently been exhaustively studied by Raman [17] and i.r. spectroscopy [18, 191. Since the crystal structure for NaClO, is non-centrosymmetric, Raman and i r. bands should be coincident. The [I77 C. M. HAFWWIQ,D. L. ROUSSEAUand S. P. S. PORTO, Phys. Rev. 188, 1328 (1969). [18] J. L. HOLLENBERO and D. A. Dows, S~ectrochzm. Acta 16,1165 (1960). [19] G. ANDERMANNand D. A. Dows, J. Phys. Chews. Sol& 28, 1307 (1967).
Methods and advantages of multiple internal reflectron infrared spectroscopy
I
1200
I
I
*
I
1000
I
I1
1,
BOO
I
t 8 600
I.
706
I
400
cm-’
Fig. 2. Infrared spectra of NaClO, at room temperature. (a) MIR agamst KRS-5, 0 = 60”. (b) MIR agamst KRS-5, 0 = 45’. (c) MIR agamst KRS-5, 0 = 30’. (d) Nujol mull between AgBr plates. Spectra were recorded under slmllar instrumental condltlons on a P.E. 621 1.1. spectrometer. For the N&IO,-KRS-5 mterface, B. w 37’. MIR frequencies for 8 equal to 60” and 45’ (482, 624, 930, 936, 962, 970, 988, 995 cm-l) are within a one cm-l experimental error of accepted Raman and i.r. values [17, 191. The improved resolution in the 900-1000 cm-l region of the MIR spectra reveals bands at 962, 970 and 995 cm-l which have not previously been reported. We will not attempt to assign these bands, but it may be noted that this region may be complicated by overtones of the ca. 480 cm-l bands and 37C1isotopic species. An increase in the MIR band distortion with decreased angle of incidence is also obvious (Fig. 2) although the frequencies for 8 equal to 45’ are still reliable. From Figs. 2 and 3, it is clear that MIR bandwidths are narrower than bandwidths for samples mulled in nujol. The MIR frequencies for K,Ni(NO,)B - H,O (Fig. 3), 433,510,811,830,1260,1318 and 1345 cm-r, are also in excellent agreement with the
M. If. B~~OOEER and D. $1, TRIS~:
706
I.
I.
1400
b.
k.
1200
I
‘
f.
too0
1
s
11
800
1.
1.
600
“1
400
cm-’ Fig. 3. Tnfrarcd spectra of K,NI(NO~), H,O at room temperature. Top, MIR 0 = 60’; Bottom, nujol mull between AgBr plates.
literature values [lo, 201. The clarity of the 1400 cm-l region is an added bonus for this compound It may be seen from Fig 2 that there is no band ca 1030 cm-l present in the MIR spectra of NaClO,. A longitudinal optical mode has been observed at this frequency by Raman and speoular reflection techniques [17-191. It has been shown elsewhere 1211that with proper experimental conditions, muItiple internal reflectance spectra will give only the transverse optical frequencies, i.e. the frequencies usually measured in ideal transmission experiments. Ao~~owledgements-This work was supported by a grant from the National Research Council of Canada One of us (M. H. BROOKER) gratefully acknowledgesa N.R.C. postdoctoral followhp. [20] D. M. L. %ODGAME and M. A HITCEXAN, Inorg. Chsn. 6, 813 (1967). [Zl] M. H BROOKER, J. Chem. Phys 53,410O (1970).
Methods and advantages of multiple internal reflection infrared spectroscopy M. H. BROOKER* and D. E. IRISH Department of Chermstry,Umversrty of Waterloo, Waterloo, Ontarro (Received27 October1970) Abstract-The condltlons reqmred to obtam “trans~sslon-hke” spectra from multiple and smgle mternal reflectron 1.r. spectroscopy are discussed. Multiple mternal reflection spectra, properly obtained, are not distorted as are smgle mternal reflectronspectra and provrde the transverse optical frequencies directly. Excellent 1.r. spectra of powdered samples can be obtamed by MIR techmques. Illustrations are provided for NaCIO, showmg the effect of different prism angles, and for K,Ni (NO,),*H,O. INTRODUCTION INTERNAL reflection
spectroscopy is at a state of development where it can now be considered as a practical method for obtaining routine i.r. spectra. In the course of recent i.r. and Raman studies [l, 21 of solid phase ionic nitrates we found multiple internal reflection (MIR) methods to have a decided advantage over conventional sampling techniques, especially in regions of high sample absorptivity. The technique used and aspects of the interpretation are discussed below. A quantitative treatment of the fundamental formulae describing total internal reflection has been given by HANSEN [3] and related to absorptivity and concentration of absorbing species. Internal reflectance spectra have been obtained for aqueous solutions [P6], molten salts [7-S] and powdered samples [l, 2, 9, lo]. In spite of the excellent MIR spectra of SiO, obtained by IFARRICK and RIEDERMAN [9], the application of internal reflection techniques to solid samples has not been widely accepted. Generally, there is concern that reflection spectra will be subject to considerable distortion and hence the band frequencies will be unreliable. Although band distortion can be a problem, especially for single pass internal reflection studies [ 11, 121, this problem can easily be avoided with MIR techniques. Many optical arrangements have been proposed [S, 7, 13, 141. In our studies we have used the Wilks model-9 * Present address: Dept. of Chemrstry, Mount Allison Umverslty, Sackvdle, N.B , Canada. PI M. H. BROOEER and D. E. IRISH,Can. J. Chem. 43, 1183 (1970). PI M. H. BROOKER, D. E. IRISH and G. E. Bovn, J. Chem. Phys. 53, 1083 (1970).
[31 W. N. HANSEN, Spectrochzm. Acta 21, 209 (1965); 21, 815 (1965). [41 B. KATLAPSKY and R. E. KELLER, Anal. Chem. 35, 1665 (1963). 151 C P. MALONEand P. A. FLOURNOY, Spectrochim. Acta 21,1361 (1965). VI J. T. MILLER and D. E. IRISH,Can. J. Chem. 45, 147 (1967). r71 A. BANDY, J. P. DEVLIN, R. BURGER and B. MCCOY, Rev. Scz. In&. 35, 1206 (1964). PI K. WILLIAMSON, P. LI and J. P. DEVLIN, J. C&m. Phys. 43, 3891 (1968). PI N. J. HARRICK and N. H. RIEDERMAN, Spectrochzm. Acta 21, 2135 (1965). Cl01 M. H. BROOEER and D. E. IRISH,Inorg. Chem. 8, 219 (1969). [Ill R. P. J. COONEY,C. THAYER, P. C. LI and J. P. DEVLIN, J. Chem. Phys. 51, 302 (1969). WI J. P. DEVLIN and R P. J. COONEY, J. C&m. Phys. 52, 5495 (1970). Vol. 1. Walks Scientn?c Corp., South Norwalk, Corm. P31 Internal Rejkctzon Spectroscopy, (1965). L-141W. N. HANSEN and J. A. HORTON, Anal. Chem. 36, 783 (1964). 701
702
M. H. BROOKERand D. E. IRISH
MIR attachment and the Beckman attenuated total reflection (ATR) unit described previously [6] and now called SIR (single internal reflection). The reflectivity, R, at a plane boundary between two isotropic homogeneous phases, one of which is the transparent prism, is a function of. (a) polarization of the incident light, (b) angle of incidence, 6, to the plane boundary in the transparent phase, (c) n, the ratio of the real part of the refractive index of the absorbing sample, n,, divided by that of the transparent prism, n, and (d) the attenuation index, K, of the absorbing phase. For a given spectrophotometer, the ratio of parallel to perpendicular light is constant. The angle of incidence can be maintained at a prescribed value np and K are specific intensive properties of the sample; the product n,K is called the extinction coefficient. For frequencies at which the sample absorbs radiation, n, and K alter rapidly with resulting changes in R. When an i.r. spectrophotometer is used with either MIR or SIR attachments, reflectivity is given by the ratio of light intensity reaching the detector with and without a sample present, R/R, In transmission work, the transmittance is given by a similar ratio, T/T,. Since spectrophotometers usually give transmittance directly, reflectivity will also be measured directly. A spectrum can be obtained which closely resembles a transmission spectrum. If at any frequency the angle of incidence fails to exceed the critical angle 0,, total internal reflection will not occur and a distorted spectrum will be obtained. It is necessary to maintain 0 at sufficiently high angles to allow for variations in n. It is useful to choose a prism so that n, - w, > 0.5. AgBr and KRS-5 are suitable Intensity of a vibraprism materials for most organic and inorganic compounds tional band observed by internal reflectance techniques decreases with increasing 19 (above 13,)and decreasing n. Compensation can be achieved by increasing the number of internal reflections. Intensity of vibrational bands can also be controlled by varying the amount of sample in contact with the MIR plate Polishing procedures Transmission of the prisms is a critical factor and should be maximized in a If transmission non-absorbing region of the sample before a spectrum is recorded falls below 15 per cent the prism should be repolished. Spectra should also be recorded at slower scan speeds than conventional transmission spectra, a consequence of the lower energy. Polishing of MIR plates is more difficult than transmission plates, since a higher degree of excellence is required. The polishing procedure for KRS-5 was similar to that described [13]; however, we have found that treatment of the KRS-5 plate with hot water (in an ultrasonic cleaner if available) was superior to the usual grinding procedures. KRS-5 is slightly soluble in hot water and the outer layer may be uniformly dissolved off together with any embedded impurities. Grindmg may further press impurities into the plate. Polishing was performed on an extremely soft felt polishing cloth onto which liberal The plate was rinsed in quantities of methanol and ceric oxide had been placed methanol and buffed lightly to a high gloss on soft flannelette. It was usually necessary to repeat the polishing procedure several times. Because of its softness, AgBr was more difficult to polish than KRS-5. Embedded particles were removed by treatment of the AgBr prism with a 5% solution of
Methods and advantages of multiple mternal reflection Infrared spectroscopy
703
n-butylamine in ethanol in an ultrasonic cleaner. AgBr is soluble in n-butylamine and impurities are removed along with a thin layer of AgBr. Grinding with 600 grit abrasive could be carried out as described for KRS-5, but this was seldom necessary. The AgBr prism was polished with a soft felt cloth soaked in a 25% solution of n-butylamine in ethanol. As polishing proceeds, ethanol was added to the cloth to retard the dissolving power of the n-butylamine. Finally, the plate was rinsed in ethanol and lightly buffed on soft flannelette The polishing procedure was repeated several times. MIR
us SIR
The extinction coeflicient, nrc, for carbon tetrachloride has maxima at 762 and 786 cm-l with the intensity of the latter greater. The nK curve for benzene has a symmetric maximum at 675 cm-l [ 151. Few compounds have larger nK values. If no distortion occurs in the internal reflection spectra of benzene and carbon tetrachloride, then no distortion is expected in spectra of a sample obtained under the same conditions. In our spectra of carbon tetrachloride and benzene using a small surface area centrally located on a AgBr or KRS-5 MIR plate, no band distortion or frequency shifts occurred. These results seemed contradictory to previous authors [8, 151 who obtained distorted spectra for carbon tetrachloride and benzene with a single-pass SIR, KRS-5 prism. To investigate the apparent discrepancy, we recorded spectra for carbon tetrachloride and benzene with a single reflection at 62” angle of incidence to a KRS-5 SIR prism. The recorded spectra now showed both frequency shifts and band distortion in the form of an intensity reversal for the carbon tetrachloride doublet (Fig. 1). It appears that SIR spectra are subject to more distortion than MIR spectra. In single reflection spectra, dispersion of incident light may result in some components of the light striking the surface with less than the prescribed angle of incidence. In MIR compensation for dispersion occurs at each reflection, which tends to concentrate the light rays at the single prescribed angle which decreases the possibility of distortion [ 161. MIR
spectra of solid samples
Conventional sampling techniques for obtaining transmission spectra for solids can be troublesome and unreliable for a variety of reasons: (a) If organic mulling agents are used, overlap of sample bands with those of the mulling agent can occur. Although it is possible to choose “windows” by using a variety of mulling agents, band overlap often remains a problem, especially in the 700 and 1400 cm-l regions. (b) If alkali halide pellets are employed, ion exchange between the sample and the host lattice often gives rise to extraneous bands and frequency shifts. (c) Anomalous dispersion can occur in spectra of solids deposited mechanically on window materials (Christiansen Effect). (d) Single crystal transmission studies often require impractically thin samples. Of these problems, only ion exchange is a problem in MIR experiments, and it may usually be overcome. For instance, we have found that [153 A. C. GILBY, J. BURR, JR., W. KRUEUER and B. CRAWPORD, JR., J. Phys. Chem. (1966). [16] P. A. WILKS, JR., AppZ. Spectry 22, 782 (1968).
70, 1526
704
M. H. BROOEER
\
and D. E. IRISH
MIR
AgBr
SIR
KRS
Sd
5
62’
Fig. 1. Comparison of multiple and smgle mternal refleotlon. Photograph of spectra obtained for the extremely strong absorption doublet of carbon tetrechlorlde obtamed by MIR snd SIR techniques.
KRS-5 exchanges with AgNO, and anhydrous Cd(NO,),, but neither salt exchanges with AgBr. Photographs of MIR i r. spectra for powdered NaClO, and K,Ni(NO,),.H,O pressed against KRS-5 prisms are shown in Figs. 2 and 3,together with transmission spectra obtained for the same samples mulled in nujol, to illustrate the clarity that can be obtained by MIR techniques. The only preparatory step was the grinding of the samples into very fine powders to improve the sample-prism contact and prevent scratching of the MIR plate. Sodium chlorate has recently been exhaustively studied by Raman [17] and i.r. spectroscopy [18, 191. Since the crystal structure for NaClO, is non-centrosymmetric, Raman and i r. bands should be coincident. The [I77 C. M. HAFWWIQ,D. L. ROUSSEAUand S. P. S. PORTO, Phys. Rev. 188, 1328 (1969). [18] J. L. HOLLENBERO and D. A. Dows, S~ectrochzm. Acta 16,1165 (1960). [19] G. ANDERMANNand D. A. Dows, J. Phys. Chews. Sol& 28, 1307 (1967).
Methods and advantages of multiple internal reflectron infrared spectroscopy
I
1200
I
I
*
I
1000
I
I1
1,
BOO
I
t 8 600
I.
706
I
400
cm-’
Fig. 2. Infrared spectra of NaClO, at room temperature. (a) MIR agamst KRS-5, 0 = 60”. (b) MIR agamst KRS-5, 0 = 45’. (c) MIR agamst KRS-5, 0 = 30’. (d) Nujol mull between AgBr plates. Spectra were recorded under slmllar instrumental condltlons on a P.E. 621 1.1. spectrometer. For the N&IO,-KRS-5 mterface, B. w 37’. MIR frequencies for 8 equal to 60” and 45’ (482, 624, 930, 936, 962, 970, 988, 995 cm-l) are within a one cm-l experimental error of accepted Raman and i.r. values [17, 191. The improved resolution in the 900-1000 cm-l region of the MIR spectra reveals bands at 962, 970 and 995 cm-l which have not previously been reported. We will not attempt to assign these bands, but it may be noted that this region may be complicated by overtones of the ca. 480 cm-l bands and 37C1isotopic species. An increase in the MIR band distortion with decreased angle of incidence is also obvious (Fig. 2) although the frequencies for 8 equal to 45’ are still reliable. From Figs. 2 and 3, it is clear that MIR bandwidths are narrower than bandwidths for samples mulled in nujol. The MIR frequencies for K,Ni(NO,)B - H,O (Fig. 3), 433,510,811,830,1260,1318 and 1345 cm-r, are also in excellent agreement with the
M. If. B~~OOEER and D. $1, TRIS~:
706
I.
I.
1400
b.
k.
1200
I
‘
f.
too0
1
s
11
800
1.
1.
600
“1
400
cm-’ Fig. 3. Tnfrarcd spectra of K,NI(NO~), H,O at room temperature. Top, MIR 0 = 60’; Bottom, nujol mull between AgBr plates.
literature values [lo, 201. The clarity of the 1400 cm-l region is an added bonus for this compound It may be seen from Fig 2 that there is no band ca 1030 cm-l present in the MIR spectra of NaClO,. A longitudinal optical mode has been observed at this frequency by Raman and speoular reflection techniques [17-191. It has been shown elsewhere 1211that with proper experimental conditions, muItiple internal reflectance spectra will give only the transverse optical frequencies, i.e. the frequencies usually measured in ideal transmission experiments. Ao~~owledgements-This work was supported by a grant from the National Research Council of Canada One of us (M. H. BROOKER) gratefully acknowledgesa N.R.C. postdoctoral followhp. [20] D. M. L. %ODGAME and M. A HITCEXAN, Inorg. Chsn. 6, 813 (1967). [Zl] M. H BROOKER, J. Chem. Phys 53,410O (1970).