1912
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
F. Goetz, T. B. Brill, and J. R. Ferraro
Pressure Dependence of the Raman and Infrared Spectra of 6-Octahydro-l,3,5,7-tetranitro-l ,3,5,7-tetrazocinet
CY-,
6-, y-, and
F. Goetz, T. 6. Brill," Deparfment of Chemistry, University of Delaware, Newark, Delaware 19711
and J. R. Ferraro Chemistry Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received April 1 1, 1978) Publication costs assisted by the Air Force Office of Scientific Research and Argonne National Laboratory
The Raman and infrared spectra are reported of the a-,p-, y-, and &polymorphs of HMX (octahydro1,3,5,7-tetranitro-1,3,5,7-tetrazocine) under pressure using a diamond anvil cell (DAC). The stability to pressure or the pressure-induced phase transitions were determined for each polymorph. Sample pressures up to 10 kbar were used in the Raman experiments; infrared spectra were recorded at pressures up to 54 kbar. The observed behavior of the polymorphic forms is as follows: 0-HMX is stable up to 54 kbar and a-HMX is stable up to 42 kbar. y-HMX converts to 0-HMX at 5.5 kbar, and 6-HMX converts to a mixture of a- and 0-HMX at pressures below 0.5 kbar. No new polymorphs were produced under pressure. An inverse relationship exists between temperature and pressure effects;the polymorph most stable at low temperature and having the smallest volume (0-HMX) is the most stable at high pressure. The infrared spectra (600-1700 cm-') of the solid HMX polymorphs at ambient pressure are reported for the first time without matrix effects. The -CH2- and -NOz vibrational regions (>1200 cm-') in the absence of the KBr matrix, and with the increased resolution obtained in the DAC, show minor differences from the KBr spectra reported in the literature.
Introduction Recently we have been concerned with the solid state behavior of the four known polymorphs of HMX at elevated temperatures.' The diagnostic technique used in these studies has been laser Raman spectroscopy. In order to more completely elucidate the behavior of polycrystalline HMX under extreme conditions, a study of polymorph behavior under static pressure was undertaken. It was our hope to compare the stabilities of the various polymorphs under nonambient temperature and pressure conditions. HMX refers to the monopropellant, octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine. Its skeletal structure is shown in Figure 1. The stabilities of the four HMX polymorphs a t 300 K are known to be P > a > y > P-HMX contains a ring conformation such that the NO2 groups adopt a chairlike arrangement. This gives the entire molecule a center of ~ y m m e t r y .The ~ ~ ~ring conformation of cy-, y-, and 6-HMX is such that all of the NOz groups are positioned on one side of the m01ecule.l~~~~ The densities of the polymorphs follow the trend P > a > y > 6.3
The present vibrational study of pressure-induced transformations in HMX was carried out for three reasons. First, we wished to determine whether a polymorph more stable than 4-HMX exists under high pressure. To this end, P-HMX was examined by the Raman technique up to 10 kbar and in the infrared up to 54 kbar. In general an inverse relationship between temperature and pressure exists as far as phase transformations are concerned. For example, the application of a few kbar pressure at 300 K has roughly the same effect on a crystal lattice as cooling it to 77 K,6assuming pressure and temperature dependent interactions to be similar. Hence any polymorphs more 'Work performed under the auspices of the Division of Basic Energy section of the U.S. Department of Energy (ANL) and supported by a grant from the Air Force Office of Scientific Research (University of Delaware). 0022-365417812082-1912$01.OO/O
stable than P-HMX should be observable. Second, the high pressure information aid in characterizing the polymorph behavior for pressures at and well above those found in combustion chambers. Combustion chamber pressures are typically 0.1-1 kbar. Third, composite mixtures of HMX and organic binding agents show a change in their burning rate as a function of pressure in the 0.2-kbar pressure range.' At present, no definite explanation of this phenomenon is available. Information on the pressure-induced polymorph transformations of HMX is useful to have if high temperature-stable polymorphs, such as 6-HMX, undergo phase transitions to other polymorphs in the 0.1-1-kbar pressure range. If transformations do occur, then one polymorphic form may be burning at one pressure while another form burns at another pressure.
Experimental Section Instrumentation. Spectra of HMX under high pressure were obtained using a diamond anvil cell which has been described previously.s In the Raman experiments, 180° backscattering geometry similar to the system of Adams et al? was employed. The relatively poor Raman scattering ability of y-HMX necessitated the use of a thin metal gasket to allow a greater thickness of y-HMX to be retained between the diamond surfaces. This produced an increase in the amount of Raman scattering. Raman spectra were recorded using a Spex Model 1401 double monochromater spectrometer with either a Coherent Radiation Model 52 argon ion or krypton ion laser source, and a photon counting detection system. The argon ion laser was tuned to 488.0 nm with a power output of 0.7 W at the sample. The krypton ion laser was tuned to 647 nm with a power output of 0.38 W a t the sample. The krypton ion laser was used because of the fluorescence present in one of the diamond cells.g A spectral bandpass of 5 cm-l was used. The spectrometer was calibrated with the 218-, 314-, and 459-cm-l bands of CC4. Vibrational frequencies are accurate to within f l cm-l in the relative 0 1978 American Chemical Society
Pressure Dependence of
HMX Vibrational Spectra
TABLE I: Pressure Induced Effects on the Raman Active Vibrations of CY-,p-, 7-,and 6-HMX
"02 H2C' 02N-N
/
"CH2
\
N-NO2
\
The Journal of Physical Chemjstty, Vol. 82, No. 17, 1978 1913
polymorph
B
I
I NO2 Figure 1. Structural formula of HMX.
sense and f 2 cm-l in the absolute sense. Infrared spectra were recorded using a Beckman IR-12 spectrophotometer equipped with a 6X beam condenser as described previously.1° Pressures were determined by calibrating the spring of the DAC and determining the area of the diamond anvil contact and by observing known pressure phase transitions. Metal gaskets were used for these experiments. The small area of the laser beam allowed us to focus onto the center of the solid phase and record its spectrum in lieu of a second phase. Thus, we consider the spectra obtained to be quasi-hydrostatic. We estimate the pressure to be accurate to k0.5 kbar. Polymorph Preparation. Samples of colorless granular P-HMX were obtained from T. L. Boggs of the Naval Weapons Center, China Lake, Calif. In an attempt to remove any residual RDX (hexahydro-1,3,5-trinitro-striazine), the 6-HMX was heated in a vacuum oven a t 140 "C for 24 h. The resulting tan product was extracted with acetone. The purified P-HMX was obtained by slow evaporation of the acetone solution. The crystals were found to contain less than 0.1% RDX by mass spectral analysis.ll Samples of a-HMX were prepared by the recrystallization of HMX from hot nitric acid as reported by Cady.12 Upon cooling the hot nitric acid solution, fine crystalline needles of a-HMX precipitated. Samples of y-HMX were prepared according to Cady's method12 by pouring an acetone solution of HMX into a large excess of water. The solution was then filtered to remove the resulting y-HMX powder. The powder was air dried and stored in a desiccator over Pz06. Small amounts of 6-HMX were prepared for immediate use by immersing a partially filled capillary tube of granular P-HMX in an oil bath set a t 185 "C.' An immersion time of 3-5 min gave complete conversion to 6-HMX with the least amount of sample degradation. The capillary tube was then withdrawn from the oil bath; the 6-HMX was removed from the tube and used in the diamond anvil cell. X-ray powder patterns of the above produced samples of a-,P-, y-, and 6-HMX were compared to those reported by Cady12 to ensure their identity.
Results and Discussion All of the pressure induced phase transitions observed in this investigation were found to be irreversible. Although spectroscopic studies at high pressure often deal with reversible pressure-induced phase transition^,'^," the irreversible nature of the transitions in the HMX system may be explained by the fact that the polymorph with the smallest molecular volume, P-HMX, is also thermodynamically the most stable polymorph a t 300 K. Thus, any pressure-induced phase transition to the 0-polymorph might be expected to be irreversible. R a m a n Spectra. Recent work on the Raman spectra of the four known polymorphs of HMX are to be published.'
frequency, cm-I dv/dPa stability range 1.36 stable 312b3C 0.26 3 5gd 41ZblC - 0 432bJ -0 0.26 834c 0.26 881d 0.26 9 50'
CY
400d 450d 846c 87gd 92gd 945c
0.76 stable 0.43 0.32 0.32 0.21 0.53
7
402b 458b)C 485bpC 840d 847d 87gd 92gd 940b
0.68 stable below 5.5 kbar; 0.61 transforms to p-HMX 0.61 0.91 0.68 0.91
6
-0
0.91 stable at low pressure; transforms to an CYand p-HMX mixture below 0.5-kbar pressure
392 446 473 846 870 882 930 943
a Calculations based on frequency shift over a pressure range of 0-3.3 kbar for p-HMX, 0-9.4 kbar for CY-HMX, and 0-4.4 kbar for y-HMX. Not calculated for 6 -HMX due to the low pressure of conversion. Broadens under pressure. Weakens in intensity under pressure. Small pressure effect in intensity or half-band width.
The spectral regions involving ring motion were found to be the most useful for identifying the polymorphs. The unique appearance of the spectrum for each polymorph in the 300-500-~m-~region and the intensity of the vibrations in the 700-1050-~m-~ region permitted unambiguous identification in each case.l These spectral regions in the Raman experiment are shown in Figure 2. Attention is focused on the spectral changes occurring in the 300-550-cm-' region for the identification of polymorph changes occurring as the sample is subjected to pressure. Table I summarizes the effect of pressure on the Raman frequencies in the 300-1050-~m-~ range. The vibrations at 312,412 and 432 cm-' in P-HMX broaden and decrease in intensity with increasing pressure. These vibrations become indistinguishable above 3.3 kbar from the background noise. Vibrations at 834 and 950 cm-l also weaken and broaden somewhat as the pressure is increased. The 358- and 881-cm-l modes are least affected by pressure, and, in fact, they become relatively more intense as the pressure is increased. Lattice modes were not monitored because of the experimental difficulty in observing these modes close to the laser exciting line. 0-HMX crystallizes in the P 2 J c (Ca5) space group such that the factor group analysis of the internal modes gives 39Ag + 39Au + 39Bg + 39Bu for the number and symmetry of the modes.lJ5 Only the 39A, 39B, modes are Raman active. However, one observes only about 40 internal modes in 0-HMX even though the factor group analysis suggests about twice as many might exist. Iqbal et al.15 assigned the Raman spectrum of 0-HMX as arising from degenerate A, + B, vibrations. The relative intensity of
+
1914
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
F.
Goetz, T. B. Brill, and J. R.
Ferraro
urlo
870 882
39 2 446 473
930 943
O h (AFTER l2hrs AT AT 5.5kba)
5.5h (AFTER ONE WR)
5 5 ktar
4.4 ktar
3.3kbar
0ktci
W ' CM"
'
500
' 4do '
C M"
300
'
Figure 2. The ring motion regions of a-,P-, 7-, and 6-HMX used for polymorph identification.
the A, and B, motions comprising each band cannot be obtained from symmetry alone. Symmetric vibrations tend to be the most pressure sensitive and commonly exhibit a decrease in intensity as well as a broadening with increasing pressure, although a pressure gradient can also induce broadening.13 The A, and B, vibrations of the factor group are symmetric and antisymmetric, respectively, about a mirror plane perpendicular to the unique b axis of the unit cell. The 312-, 432-, 834-, and 950-cm-l modes are found to markedly broaden and decrease in intensity with increasing pressure. Hence this could suggest that these modes are A, vibrations. The 358- and 881-cm-' vibrations show little change with pressure and could result from motion involving B, modes. In a pictorial sense, the strongly pressure sensitive bands are ring motion vibrations which involve large changes in the molecular volume. The less sensitive bands are ring motions that may involve little change in molecular volume and therefore would be less sensitive to compression of the crystal. However, we emphasize that these assignments are only suggested, and in a complicated molecule such as HMX these considerations might be an oversimplification. The pressure dependent Raman modes of a-HMX show a shift to higher energy as pressure is applied. The most sensitive modes appear to be the 400-, 846-, and 945-cm-' vibrations. A large pressure induced shift to higher energy is exhibited by the 400-cm-' vibration, although there is little effect on the breadth or the relative intensity of the band. Conversely, the two vibrations in the ring stretch region (846 and 945 cm-l), which have a large dvf dP ratio, lose intensity in relation to the other modes. These two vibrations may be assigned as being primarily Ai symmetric ring vibrations of the CZupoint group characteristic of C Y - H M X . ~cy-HMX ~~ does not convert to any other polymorph up to the 10-kbar pressure limit of the Raman experiments. A y-to P-HMX polymorph transition is observed at a pressure of 5.5 kbar. Figure 3 shows the spectrum of y-HMX as pressure is applied; Table I lists the frequencies
7-HMX IN CAPILLARY TUBE
I
I
,
I
1030 900
I
I
1
I,,
800 700"
cm-l
''
~
500 400 300
Figure 3. Pressure induced spectral changes in y-HMX. A transition occurs at 5.5 kbar.
y-to P-HMX
and pressure dependence of the vibrations. The phase transition exhibits some hysteresis at 5.5 kbar. The presence of the 402-cm-l vibration indicates that the sample remains mostly as the 7-HMX polymorph when a pressure of 5.5 kbar is first applied. However, by the end of the scan, approximately 30 min later, the sample has converted predominantly to P-HMX. The spectrum shown in Figure 3, labeled as "5.5 kbar after 1h", was recorded after the sample was subjected to this pressure for 1 h before the spectral scan was begun. It shows complete conversion to the 0-HMX structure, although the very strongly pressure sensitive vibrational mode a t 312 cm-l in P-HMX was not observed. The top spectrum in Figure 3 was recorded on a sample immediately after release of 5.5 kbar pressure which had been held for 12 h. The spectrum was found to be the same as that of a sample of 6-HMX at ambient pressure and includes the 312-cm-l vibration which disappeared under pressure. Samples of 0-HMX formed by transformation of y-HMX were monitored for 78 h a t atmospheric pressure and showed no evidence of conversion to the original r-HMX. Figure 3 also illustrates the differences we noted in the spectral quality as a function of pressure. We believe that the general decrease in intensity of the spectrum with increasing pressure results mainly from the pressure gradient existing in the DAC since, upon release of the pressure, the original intensity of the spectral lines returns. The pressure dependence of 6-HMX is illustrated by the spectra in Figure 4. The individual vibrational frequencies are listed in Table I. Values of dv/dP are not listed for 6-HMX because they could not be accurately obtained using a diamond anvil cell. The transformation from 6-HMX to an a- and P-HMX polymorph mixture occurs between 0.0 and 0.5 kbar pressure. Transformation of 6-HMX to an cy- and P-HMX mixture at this comparatively low pressure may play a role in the pressure dependence
Pressure Dependence of HMX Vibrational Spectra
The Journal of Physical Chemistry, Vol. 82,No. 17, 1978 1915
m-m 8-HMX
7-HMX
a-HMX
Flgure 4. Pressure induced spectral changes in 6-HMX. A transformation of 6- to an a-and p-HMX mixture occurs less than 0.5 kbar.
1
1
0 1600
1
1
1400
1
1
1200
1
1
loo0
i
1
800
1
1
600
cm-1 Figure 6. Infrared spectra (600-1700 cm-I) of a-,(3-, y-, and 6-HMX. Spectra were recorded on neat samples in a diamond anvil cell. Figure 5. A summary of the temperature- and pressure-inducedphase transitions of the a-,@-, 7-, and 6-polymorphs of HMX. The solid lines are transitions with slow heating (ref 1) and the dashed lines are transitions with pressure. (R) indicates reversibility with cooling.
of the burning rate of HMX. It is known that the burning rate of composite mixtures of HMX and organic binders 89 a function of pressure has a slope change in the 0.2-kbar pressure rangea7 Of course the temperature of the burning material is considerably above the 300 K where our pressure measurements were made. The 6-HMX which might be present as a high temperature-stable polymorph may be partially or completely converted to P-HMX under the pressure loads during combustion. In order to solve this problem more completely, further work on the combined effect of pressure and temperature on 6-HMX, as well as the kinetics of the phase transitions is needed. The spectrum of the a- and P-HMX mixture at 1:l kbar shown in Figure 4 is typical of the spectra of all samples obtained at pressures as high as 7.7 kbar. No apparent shift in the relative intensities of the a-and P-HMX bands is observed as the pressure on the sample is increased. Upon release of pressure, the spectrum of the mixture remains unchanged. The difference in intensity between the 300-500-crn-' and 700--1050-~m-~ regions in the spectrum at ambient pressure in Figure 4 is due to a different setting for the photon counting sensitivity as each region was scanned. The temperature dependence of the phase transitions of the HMX polymorphs was previously determined under slow heating conditions.' The results are summarized in Figure 5. The pressure dependence of these same phase transitions as determined in this work are also indicated in Figure 5. The pressure stability of the polymorphs can be seen to be the inverse of the temperature stability. That is, the &polymorph is the most stable of the four at low temperature and is the most stable at high pressure. The
6-polymorph is the most stable a t high temperature and is the least stable at high pressure. The factors which influence or control the different degrees of stability toward pressure in the HMX polymorphs have not been established at the present time. Infrared Spectra. The infrared spectra (600-1700 cm-l) of the four polymorphs of HMX recorded in a diamond anvil cell at zero pressure are shown in Figure 6. Table I1 lists the vibrational frequencies of the polymorphs and the assignments proposed by Iqbal et al.15for P-HMX. In general, our frequencies for /3-HMX are similar to those reported by Iqbal et a l . , I 5 Bedard,16and Werbin,17although some significant differences between all of the reported spectra appear in the vibrations above 1200 cm-l. The spectra of the remaining polymorphs appear similar to those reported by CadyI2 and Bedard.16 The differences between spectra reported here and those previously reported lie mostly in band shifts of several cm-' magnitude, differences in intensities of certain vibrations, and the absence of several weak modes mentioned in the earlier reports. In the specific case of P-HMX, the Raman frequencies reported by Iqbal et al.I5 and by Goetz and Brill' are the same, but the infrared frequencies differ for some modes. The greatest difference is found in the -NOz and -CH2vibrations above 1200 cm-'. We believe the discrepancy may be explained chiefly by the difference in the sample matrix. All of the previously reported IR spectra were recorded on HMX in KBr pellets, while the spectra in this study were recorded using a neat sample in the diamond anvil cell. The differences in the spectra, therefore, may be due to matrix effects of the KBr. To our knowledge the neat infrared spectra of a-,p-, y-, and 6-HMX have not been reported before. The infrared spectrum of 0-HMX under pressure exhibits the expected positive slope of dv/dP. The major
1918
The Journal of Physical Chemistry, Vol. 82, No. 17, 1978
F. Goetz, T. B. Brill, and J. R. Ferraro
TABLE 11: Infrared Active Vibrational Frequencies (cm-' ) and Probable Assignmentsa of Modes in Crystalline HMX Polymorphs
s
Q
Y
assignment
6
603 605
B
ring motion and 7 (NO,)
oi
Y
1019 1040 1088 1090
625
1090 1110
1090 1110 1112
655
1146
6 58
1148
660 668
1149 1150 670
1205 687
1215
712 713
1220
714 6
and y ( N 0 , )
1240
740
1255
742
1270
7 50
1280 1320
7 51
1280 1320
1280 1320
752
1325 1340
753 1348
7 58 765 768
1370
768
1370
6(CH,)
1375
769 827
1393 1394
ring stretch 1397
845 848 862
1398
848 1416 866
1420
871
1422 1432
910 1433
918
1454
920 925 940
1455 1465
941
1468
945 961
965
vs(NO,) and vs(N-N)
1221 7 37
945
assignment
1032
622 625 648 651
6
965
965 1016
1540
1550
vas(NOz)
1550 1560
1570
1570
a Reference 15. vas and v s represent antisymmetric and symmetric stretching motions; S represents deformation motions; represents deformation involving one ring and one nonring bond.
difference between the spectrum at 30 kbar and that at ambient pressure appears to be an intensification of the 758-cm-' mode and a shift of this mode to 761 cm-l. The vibrations at 965 and 945 cm-' reverse in intensity and both experience a 5-cm-' shift to higher energy when the pressure is increased from 0 to 30 kbar. Although no polymorph transition is observed in aHMX when subjected to pressures as high as 42 kbar, some reversible intensity changes do occur. Tentative assignments of the vibrations of a-HMX have been made.' Modes in the ring motion region (700-1100 cm-') as assigned in p-HMX15 also appear in a-HMX and no doubt result from ring torsions and stretching. Vibrations at 714 and 742 cm-' decrease in intensity and increase in frequency by 4 cm-l at 42 kbar, while the vibration at 768 cm-' is unchanged in both frequency and intensity. The band at 848 cm-l loses intensity as pressure is applied. The vibration at 920 cm-l increases by 10 cm-l with no change in intensity at 42-kbar pressure. The 945-cm-' mode increases in frequency by 3 cm-l. Vibrations at 1032 and 1090 cm-' exhibit a large decrease in intensity, but only the 1032-cm-' mode shows a pressure-induced frequency increase. Those vibrations showing a large decrease in intensity as pressure is increased may be assigned tentatively to ring modes having a high degree of symmetric
character. The nitro group and (N-N) vibrations at 1280 and 1320 cm-l exhibit no frequency shift, but their pressure-induced broadening indicates considerable symmetric character in these vibration^.'^ The infrared spectrum at ambient pressure of y-HMX is shown in Figure 6 and the vibrational frequencies are listed in Table 11. The spectrum of this polymorph recorded at 42 and 54 kbar indicates complete conversion to P-HMX. The y-to 8-HMX conversion was found to be irreversible upon release of the pressure. This result is in accordance with the Raman results mentioned earlier. The spectrum of 6-HMX at ambient pressure is also shown in Figure 6. Vibrational frequencies of the polymorph are summarized in Table 11. At 42 kbar the spectrum appears to be a mixture of P-HMX with either some unaffected 6-HMX or some a-HMX present. The infrared spectra of a- and 6-HMX are so similar in the region studied that a definite assignment of the second component of the mixture is impossible. The results, however, are in accordance with the pressure-induced transition of these polymorphs observed in the Raman spectra.
Acknowledgment. T.B.B, gratefully acknowledges the support of this research by the Air Force Office of Sci-
ESR Study of Os(II1)- and Ru(II1)-Ammine Complexes
entific Research (AFOSR-76-3055). F.G., as part of a “Thesis Parts Appointment”’ the support of the Argonne Center for Educational Affairs funded by the Department of Energy. The authors also acknowledge With and appreciation the aid Of Dr* J. Basile of Argonne National Laboratory in the highpressure Raman experiments.
References and Notes (1) F. Goetz and T. B. Brill, to be submitted for publication. (2) W. C. McCrone, Anal. Chem., 22, 1225 (1950). (3) H. H. Cady, A. C. Larsqn, and D. T. Cromer, Acta Cvstallogr., 16, 617 (1963). (4) C. S. Choi and H. Boutin, Acta Crystalbgr., Sect B, 26, 1235 (1970). (5) R. E. Cobbledick and R. W. H. Small, Acta Crysfallogr., Sect. B , 30, 1918 (1974). (6) C. N. R. Rao and J. R. Ferraro in “Spectroscopy in Inorganic Chemistry”, Academic Press, New York, N.Y., 1971, p 70.
The Journal of Physical Chemisty, Vol. 82, No. 17, 1978
1917
(7) K. P. McCarty, AFRFL-TR-7659, Edwards Air Force Base, Calif., 1976, and references therein. (8) C. E. Wier, E. R. Lippincott, A. Van Valkenberg, and E. N. Bunting, J . Res. Natl. Bur. Stand., ( U . S . ) , Sect. A , 63, 55 (1959). (9) D. M. Adams, S. J. Payne, and K. Martin, Appl. Spectrosc., 27, 377 (1973). (10) C. Postmus, J. R. Ferraro, and S. S. Mitra, Inorg. Nucl. Chem. Lett, 4. 155 (1968): J. R. Ferraro and L. J. Basile.. ADD/. 28. . . SDectrosc.. . 505 (1974). ’ ’ (11) B. B. Goshgarian, Air Force Rocket Propulsion Laboratory, personal communication. (12) H. H. Cady and L. C. Smith, LAMS-2652, Los Alamos Scientific Laboratory, May 3, 1962. (13) C. Postmus, K. Nakamoto, and J. R. Ferraro, Inorg. Chem., 6, 2194 (1967). (14) J. R. Ferraro and G. J. Long, Acc. Chem. Res., 8, 171 (1975). (15) 2 . Iqbal, S. Bulusa, and J. R. Autera, J. Chem. Phys., 60, 221 (1974). (16) M. Bedard, H. Huber, J. L. Meyers, and G. F. Wright, Can. J. Chem., 40, 2278 (1962). (17) A. Werbin, UCRL-5078, University of California Radiation Laboratory, Dec. 31, 1957.
An Electron Spin Resonance Study of Osmium(II1)- and Ruthenium(111)-Ammine Complexes Shigeyoshi Sakaki, Nobuo Hagiwara, Yukio Yanase, and Aklra Ohyoshi Department of Industrial Chemistry, Faculty of Engineering, Kumamoto University, Kurokami, Kumamoto 860 Japan (Received November 29, 1977; Revised Manuscript Received April 18, 1978)
An electron spin resonance study was carried out on [MX(NH3)5]Clz(M = Ru(II1) or Os(II1);X = C1, Br, or I). The g tensor theory proposed by Hill was slightly modified in this work and used to analyze ESR spectra. These complexes have an % ground state under the condition that the spin-orbit coupling interaction is neglected. The metal-halide covalency increases in the order C1 < Br < I and Ru 5 Os. Both ruthenium and osmium complexes exhibit a similar degree of tetragonal distortion with exception of X = I.
Introduction The effects of various ligands and metal ions on metal-ligand covalency, d-orbital splitting, and distortion of the complex have been of considerable interest in the field of coordination chemistry. To investigate these effects, an ESR study of low-spin d5 complexes seems informative, since g values of such complexes vary widely owing to the structure of the complex and the degree of metal-ligand covalency. Therefore, many ESR studies have been carried out on low spin d5 complexes, such as iron(III), ruthenium(III), and iridium(IV), etc.l Although the osmium(II1) ion has a low spin d5 electron configuration, few ESR studies have been carried out on this complex; there have been reported only four ESR works of osmium(II1) complexes as far as the authors are a ~ a r e . ~It- is~ surprising that typical Werner type complexes, O S X ( N H J ~ ~(X + = halide), have never received ESR study, and that their physicochemical properties are little known without IR6 and UV data.7 In this work, an ESR study was carried out on osmium(II1)-ammine complexes and their ruthenium analogues, [MX(NH3)5]C12(M = Ru(II1) or Os(II1); X = C1, Br, or I). A comparison of the metal-ligand covalency between ruthenium(II1) and osmium(II1) complexes is presented. A discussion is also presented on the d-orbital splitting and distortion of these complexes.
Experimental Section [MX(NH&,IC12 (M = Co(III), Ru(III), or Os(II1); X = C1, Br, or I) was prepared by literature methods.8-10 The 0022-3654/78/2082-1917$01.00/0
Duritv of these comdexes was examined bv UV and visible speciroscopy and/br elemental analysis: [CoC1(NH3),]C12 was doped with about 4 mol % of [MX(NHJ5]ClZ(M = Ru(II1) or Os(III)),on which ESR measurements were carried out with a JEOL JES-3BSX spectrometer. A liquid nitrogen cavity was used with 100-kHz field modulation. The magnetic field was calibrated by using an NMR probe and by placing Mn2+ (doped in MgO) in the cavity. g Tensor Theory The g tensor theory of Hill3 was employed for analysis of ESR spectra, after it had been slightly modified as described in this work. The concise formalism is described here. The d-orbital splitting induced by the ligand field is shown in Figure 1,together with a coordinate system for the examined complexes. It was assumed that the lowsymmetry ligand field has eigenstates, dxy,d,, and d,, with eigenvalues, A, V/2,and -V/2,respectively. The Kramers doublets are described as follows: \k = aI2T2 1 - 1) bI2T2 - 50) 1 + cI2T, + 1 + 1) 2 2 (1) 1 1 1 \k* = aI2T2- - + 1) + bI2Tz + -0) + cI2T2 - - - 1) 2 2 2 where the basis wave functions are given in Table 2 of ref 3. The matrix elements of the low symmetry ligand field and the spin-orbit coupling interaction are shown in eq 2 (Table 3 of Tef 3):
+
0 1978 American
+
Chemical Society
