7920
J . Phys. Chem. 1989, 93, 7920-7925
some enhancement of y at 602 nm when the lowest lying excited state of a molecule (“band gap”) is lowered by substitution toward the operating wavelength. The experimental determination of the whole dispersion curve for y for all the molecules studied by us is beyond the scope of this work because of experimental difficulties in obtaining tunable femtosecond pulses. One can, however, try to assess the possible effects of dispersion by assuming the major part of the nonlinearity of an extended T-electron system to come from the lowest excited state. In a simple two-state picture the third-order nonlinearity can be accounted for by considering an anharmonic oscillator equation”I2*
mx + wo2x + r i + bx3 = eE(t) = eF(gW‘+ e-’.‘) ( 5 ) where m is the electron mass, e is the electron charge, F is the field amplitude, and w its frequency. wo stands here for the resonance frequency of the oscillator, r is the damping coefficient, and b is the anharmonic coefficient responsible for the third-order nonlinearity. Assuming that the induced dipole moment can be expressed in powers of the field as p =
ex = a E
+ yE3
(6) one can derive the dispersion relation for the hyperpolarizability. For the hyperpolarizability component relating to degenerate four wave mixing, i.e. y(-w;w,--o,o), we obtain
Far from resonance the damping term becomes negligible, and one can write
This relation will certainly overestimate the dispersion effect (20) Prasad, P. N.; Samoc, M.; Perrin, E. J . Cfiem. Phys., in press.
because of the omission of the damping term and because of the assumption that only one excited state contributes to the experimental y. In comparison, a free electron model using the perturbative sum-over-state approach shows that x ( ~depends ) inversely on the sixth power of the band gap.12 We have used, however, eq 8 to extrapolate the experimental values to zero frequency. The results are collected for the derivatized a-terthiophene structures in Table 11. It can be seen that, in this two-level anharmonic oscillator model, the influence of the shift of the “band gap” on the values of y at 602 nm is substantial (although admittedly overestimated, as point out above). While for molecules that absorb in the UV the extrapolated values are not drastically different from those at 602 nm, the dispersion may account for a significant enhancement in some cases. It is, however, comforting to find that the qualitative trend of the substituent effect even with these zero frequency extrapolated values is the same.
Acknowledgment. This research was sponsored by the Directorate of Chemical Sciences of the Air Force Office of Scientific Research, the Polymer Branch of the Air Force Wright Aeronautic Laboratory, and by the Office of Innovative Science and Technology-Defense Initiative Organization under Contract Nos. F4962087C0042 and F4962087C0097. We thank Dr. Bruce Reinhardt of the Polymer Branch of Air Force Wright Aeronautic Laboratory for providing us with the sample of alkoxy derivatized p-pentaphenyl and for helpful discussions. We also thank Dr. Joseph J. Tufariello and Dr. Harry F. King for many helpful discussions. We thank Mr. Z. Zhu for his help in preparing the iodo and nitro derivatives of a-terthiophene. Registry No. PhH, 7 1-43-2;PhPh, 92-52-4;p-PhC6H4Ph,92-94-4; 1 3 5- 7 0-6; 2 ” , 5”- b i s ( d e c y lox y ) P h (p-C,H 4 ) ,P h , 1,1’:4’, 1”:4”,l”‘:4”’, 1””:4””-quinquephenyl, 122964-95-8; thiophene, 110-02-1; a-bithiophene,492-97-7;a-terthiophene, 1081-34-1;cr-tetrathiophene, 5632-29-1; a-pentathiophene,5660-45-7; 1,4-bis(2-thienyl)benzene, 23354-94-I ; 2,5-bis(2-thienyl)pyrrole, 89814-62-0;2,5-bis(5iodo-2-thienyl)thiophene,104499-99-2;2-(2-thienyl)-5-(5-nitro-2-thienyl)thiophene, 122845-17-4; 2,5-bis(5-nitro-2-thienyl)thiophene, 122845-18-5,
Vibrational Spectroscopic Studies of the Phase Transitions in Cyclohexane at High Pressure Julian Haines and Denis F. R. Gilson* Department of Chemistry, McCill University. 801 Sherbrooke St. W., Montreal, Quebec H3A 2K6, Canada (Received: March 20, 1989)
The vibrational spectra (infrared and Raman) of cyclohexane have been obtained as a function of pressure by use of diamond anvil cells. In cyclohexane two phase transitions were detected, at 5.1 and 9.6 kbar, from the splittings in the internal modes and from changes in the slopes on plots of v versus pressure. The high-pressure phase, stable above 9.6 kbar, is identical with the phase obtained at low temperature. The vibrational spectra indicate that the unit cell symmetry changes from Oh to D2, at the first transition and then to C2hat the second transition. Values for the mode Griineisen parameter, yi,range from 0.006 to 0.039 in phase I, from -0.01 to 0.1 in phase 111, and from 0.006 to 0.16 in phase 11.
Introduction Cyclohexane is a well-known example of a compound that undergoes an order-disorder transition in the solid state. The transition occurs at 186.1 K,’ and an X-ray crystallographic investigation2 indicated that the low-temperature phase I1 has a monoclinic structure, space group C2/c (C2h6), with four molecules ( 1 ) Aston, J. G.; Szasz, G.J.; Finke, H. L. J . Am. Cfiem.Soc. 1943, 65, 1135. (2) Kahn, R.; Fourme, R.; AndrC, D.; Renaud, M. Acta Crystallogr. 1973, 29B. 131.
0022-3654/89/2093-7920$01 .50/0
per unit cell, changing to a face-centered cubic structure, phase I, space group Fm3m (O:), with four molecules per unit cell. The transition has been subject to numerous vibrational spectroscopic investigation^.^-^ Most of these are consistent with the crys(3) Dows, D. A. J. Mol. Spectrosc. 1965, 16, 302. (4) Le Roy, A. C. R . Hebd. Seances Acad. Sci. 1965, ZbOB, 6079. (5) Ito, M. Spectrochim. Acta 1965, 21, 2063. (6) Obremski, R. J.; Brown, C. W.; Lippincott, E. R. J . Chem. Pfiys. 1968, 49, 185. (7) Sataty, Y . A.; Ron, A. Chem. Phys. Lett. 1974, 25, 384. (8) Rohrer, U.; Falge, H. J.; Brandmiiller, J. J . Raman Spectrosc. 1978, 7, 15.
0 1989 American Chemical Society
Phase Transitions in Cyclohexane at High Pressure
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7921
1
tallographic data, except that Obremski and co-workers6 have proposed an orthorhombic structure with DZhsymmetry for phase 11. Order-disorder transitions occurring at low temperature can, in many cases, be induced through the application of high press u r e ~ .Added ~ information can be obtained from high-pressure experiments due to the differences between temperature and pressure as experimental variables. Increasing pressure causes a direct reduction in interatomic distances, while cooling primarily reduces molecular motion with some consequential effects on interatomic distances. In the case of cyclohexane, previous high-pressure differential thermal analysis (DTA), differential scanning calorimetry (DSC), thermal conductivity, heat capacity, and compression studies’*Is have provided significant information that was not obtainable from variable-temperature experiments, notably, the presence of a third, intermediate, phase not observed at atmospheric pressure. A metastable phase can be obtained at atmospheric pressure for samples subjected to uncontrolled cooling and is found to transform irreversibly to phase I1 upon annealing.I6 This phase is likely to be different from the stable high-pressure phase 111. In addition, Burns and Dacol have investigated the Raman spectra of glassy crystalline cyclohexane, formed by quenching phase I in liquid nitrogen.” Obremski and co-workers6 have obtained infrared spectra of single crystals of two of the solid phases of cyclohexane, grown in a diamond anvil cell (DAC), and proposed crystal symmetries based on their results. The pressure at which these spectra were recorded was not reported. Additional high-pressure studies include proton N M R relaxation measurements of lattice diffusion in phase I.’*
Experimental Section Cyclohexane (Aldrich Chemical Co. HPLC grade, purity 99.9%) was used without further purification. Raman and ruby fluorescence spectra were recorded on an Instruments S.A. spectrometer with a Jobin-Yvon U-1000 1. O m double monochromator and equipped with a Nachet optical microscope. The 514.532-nm line of a Spectra Physics Model 165 argon ion laser (50 mW at the sample) was used for excitation. The resolution was typically 4 cm-I. A Diacell Products DAC (Leicester, England) was used for the Raman pressure studies. The sample, together with a ruby chip as a pressure ~ a l i b r a n t , was ’ ~ placed in the 300-pm hole of a 400-pm thick stainless steel gasket. The cell was assembled and mounted on an x-y stage under the 32X objective of the microscope. Infrared spectra (2- or 4-cm-’ resolution) were acquired on a Nicolet 61 99 FT-IR spectrometer equipped with a liquid nitrogen cooled mercury cadmium telluride (MCT) detector. A 100-pmthick stainless steel gasket was used with a High Pressure Diamond Optics Inc. (Tuscon, AZ) DAC. A layer of 0.14% w/w sodium nitrate in sodium bromide, prepared according to the method of Klug and Whalley,*O and a drop of cyclohexane were placed into the 400-pm-wide gasket hole. The pressure was calibrated with respect to the frequency shift of the asymmetric stretch of the nitrate ion. The cell was mounted on an optical bench with f4 condensing optics in the sample chamber of the spectrometer. For the C-H region, which is much more intense than the rest of the spectrum, a thicker layer of calibrant was used. All experiments were performed at 20 OC. Lengthy equilibration time was required (9) Ferraro, J. R. Vibrational Spectroscopy at High External Pressures: The Diamond Anvil Cell; Academic Press: Orlando, FL, 1984. (IO) Wurflinger, A. Eer. Bunsen-Ges. Phys. Chem. 1975, 79, 1195. ( I 1) Arntz, H.; Schneider, G. M. Faraday Discuss. Chem. SOC.1980,69, 139. (12) Wisotzki, K. D.; Wurflinger, A. J . Phys. Chem. Solids 1982.43, 13. (1 3) Sandrock, R.; Schneider, G. M. Eer. Bunsen-Ges Phys. Chem. 1983, 87, 197. (14) Figuitre, P.; Guillaume, R.; Szwarc, H. J. Chim. Phys. 1971, 68, 124. (15) Andersson, P. J . Phys. Chem. Solids 1978, 39, 65. (16) Kahn, R.; Fourme, R.; AndrB, D.; Renaud, M. C. R . Hebd. Seances Acad. Sci., Ser. E 1970, 1078. (17) Burns, G.; Dacol, F. H. Solid State Commun. 1984, 51, 773. (18) Folland, R.; Ross, S. M.; Strange, J. H. Mol. Phys. 1973, 26, 27. (19) Barnett, J. D.; Block, S.; Piermarini, G. J. Reu. Sci. Instrum. 1973, 44, 1.
(20) Klug, D. D.; Whalley, E. Rev. Sci. Instrum. 1983, 54, 1205.
A
A
4
3030
2975
2950
1
33C9
2800 2675 WAVENUYBER
2925
2850
A
2975
2950
2625
B
2925
2900
2675
2850
2h25
L4A VE NU b1BE R
1
Figure 1. Infrared spectrum (3000-2825 cm-’) of cyclohexane a t (A) 3.8, (B) 5.7, and (C) 22.0 kbar.
I
I
A
C W P
I
4
1930
1290
iiso
idlo
670 WAVENUMBER
$30
$so
lis0
Figure 2. Infrared spectrum (1430-450 cm-I) of cyclohexane at (A) 3.9, (B) 5.1, and ( C ) 20.2 kbar.
after passing through the 111
-
I1 transition.
Results and Discussion Significant changes are observed in the vibrational spectra at pressures of 5.1 and 9.6 kbar, including splitting of the internal
1922 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989
Haines and Gilson
TABLE I: Infrared Data for Cyclohexane"
cm-l 2932 vs u,
2913 sh 2904 sh 2853 s 2793 m 2660 s 1450 s
1352 w
1257 m
phase I dv/dP, d In v/dP, cm-'/kbar kbar-I 0.85 0.00029
yi 0.01 1
cm-' 2934 vs
2914 sh 2904 sh 2855 s
0.53
0.00019
0.008
0.31 0.44
0.00011 0.00016
0.004 0.006
0.22
0.21
0.000 17
0.000 16
0.007
0.006
1042 w
0.35
0.00033
0.013
1019 w
1 .oo
0.00099
0.039
904 s
0.19
0.00021
0.008
864 s
0.72
0.00083
0.033
525 w
u,
phase 111 du/dP, d In u/dP, cm-l/kbar kbar-I 0.88 0.000 30
yi
0.03
0.56
0.000 20
0.02
2791 m 2658 s 1451 s 1449 s 1438 s
0.35 0.06
0.000 13 0.0
0.01 0.0
1351.5 s
0.06
1262 sh 1252 w
1043 w 1032 sh 1018 w 901 sh 895 m 865 s
-0.10
0.00005
-0.00008
0.006
-0.01
0.15
0.000 15
0.02
0.04
0.000 04
0.004
-0.30
-0.000 35
0.06
0.00006
-0.04 0.007
519 w
cm-' 2971 sh 2960 vs 2944 vs 2937 sh 2925 sh 2910 sh 2866 sh 2865 s 2797 m 2699 s 1457 s 1453 s 1448 s 1444 s 1364 sh 1360 m 1353 s 1345 m 1271 m 1266 sh 1263 sh 1254 m 1125 w 1108 w u,
1030 sh 1024 m 911 sh 904 m 898 m 889 w 878 m 870 s 864 sh 528 m
phase du/dP, cm-'/kbar 1.60 1.64 0.98
I1 d In v/dP, kbar-l 0.000 54 0.000 58 0.000 33
y,
assigntb
0.100 0.107
0.86
0.000 30
0.056
0.49 0.55 0.48 0.61
0.000 17 0.000 19 0.000 15 0.000 23
0.03 1 0.035 0.028 0.043
Y I ~
}
Y26
Y14, "27
0.78 0.48 0.18 0.30 0.51 0.04
0.000 57 0.000 35 0.000 13 0.000 23 0.000 36 0.000 03
0.106 0.065 0.024 0.043
0.09
0.000 08
0.015
0.41
0.000 37
0.069
0.18
0.000 18
0.033
0.20 0.17
0.000 22 0.000 17
0.73 0.41
0.000 84 0.000 48
I
y28
0.006
V8 ~g
1
sI'
0.156
y16
"For Y between 3000 and 2825 cm-I, the pressure of phases I, 111, and I1 is 3.8, 5.7, and 22.0 kbar, respectively. For Y between 2800 and 500 cm-I, the pressure of phases I, 111, and I1 is 3.9, 5.1, and 20.2 kbar, respectively. bFollowing ref 6.
TABLE 11: Raman Data for Cyclohexane phase I11 (6.9 kbar)
phase I ( 3 . 5 kbar) u, cm-I 2942 s 2920 sh
du/dP, cm-l/kbar
d In u/dP, kbar-I
yi
2896 w 2855 w 1444 w 1273 w
u, cm" 2950 s 2926 m
du/dP, cm-l/kbar
d In u/dP, kbar-I
yi
2900 w 0.87
0.00030
0.012
1.22
0.00096
0.038
2857 m 1442 w 1280 w 1266 w
1031 m
1034 m
805 s
807 s
'Following ref
-0.02 1.12 0.59
0.0
0.0
0.00088 0.00047
0.10 0.05
u,
cm-I
2964 s 2942 w 2934 w 2915 m 2910 sh 2867 w 1451 w 1293 w 1279 w 1270w 1049 s 1042 m 819 s
phase I1 (20.3 kbar) du/dP, d In u/dP, cm-l/kbar kbar-I
yi
assignt' VI
} } 0.75
0.00026
0.048
0.87
0.00067
0.124
0.31
0.000 25
0.046
U17 U18
u2 "19
]
y22
6.
modes (Figures 1 and 2) and some significant intensity changes. The slopes of plots of u versus P (Figures 3 and 4) were found to shift abruptly at these pressures. The infrared results on compression and decompression indicate the absence of any significant pressure hysteresis in either transition. This is in contrast to the thermal conductivity and heat capacity results of Andersson,I5 who observed a pronounced hysteresis in the 111 I1 transition. This hysteresis might result from the rather sluggish nature of the transition. The observation of two phase transitions with increasing pressure is consistent with the results obtained from high-pressure DSC, DTA, and PVT measurement^.'^^^ In fact, the phase diagram reported by Wiirflinger,lo if extrapolated to higher pressures, indicates that two phase transitions will occur
-
at approximately 5 and 9 kbar at 293 K. The spectra of phase I are broad and featureless, which is characteristic of a disordered phase. Upon transition to phase 111, v , and ~ ~ 3 0 at , 1257 and 904 cm-', respectively, in phase I, split into two components (Table I). These vibrations, which are both of e, symmetry, correspond to a CH2 twist and a CH, rock, respectively. In the Raman spectrum, vZl of eg symmetry also splits into two components (Table 11). These changes are accompanied by slight shifts in frequency and a narrowing of the bands. The splitting of these modes can be attributed to the site group splitting of the degenerate eg and e, vibrations. The transition to phase I1 results in significantly more splitting, as the three modes mentioned above all split into at least three components. The CH,
Phase Transitions in Cyclohexane at High Pressure
The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 7923
m ,
.. ,
,
*/
Ell5 0
a
a
B
IS
ID
In0
111
/* im
f
i
im
Y
*
Ill0
1
a
II
10
I
a
n
4 0
1
IO
IS
m
I
n
R€S¶RE/UBAR
pIEsmE/I(uR
Figure 3. Pressure dependence of selected infrared bands of cyclohexane.
wag, u28, which had not previously split in phase 111, splits into four components. The I R spectrum of this phase confirms it to be identical with phase I1 obtained a t low t e m p e r a t ~ r e . ~ ? ~ The C-H stretching region of the infrared spectrum also exhibits pronounced changes at the phase transitions (Figure 1). A significant sharpening of the bands is observed a t the I I11 transition. At the 111 I1 transition, v2s splits into at least four components while uz6 splits into two. These bands show significant shifts to higher frequency with increased pressure. Many intensity changes are observed, especially at the I 111 transition. In particular, vz8 in the I R spectrum exhibits a significant increase in intensity while ~ 2 and 9 uW decrease in intensity. These two vibrations however regain intensity at the 111 I1 transition. Two weak vibrations at 1108 and 1125 cm-' appear
-
-+
-
-
in phase 111, while the combination u5 + uj2 loses much of its intensity a t the phase transition and becomes indistinguishable from the background above 15 kbar. The band at 1108 cm-' is of a,, symmetry and is IR-inactive in the free molecule but becomes active when placed in a crystal site of lower symmetry. The changes in intensity and the splitting of the internal modes reflect changes in crystal symmetry at the two phase transitions. The structures of phases I and I1 are known to be Fm3m (Oh5) and C2/c (ea6) from X-ray crystallography.2 The infrared spectra reported here for phases I and I1 are consistent with those reported for these phases at low temperature4" except that the frequencies are shifted by pressure, thereby confirming that the phase obtained by the application of high pressure has the same structure as that produced at low temperature. The infrared spectrum of the
7924 The Journal of Physical Chemistry, Vol. 93, No. 23, 1989 2870
28%
2845
-___
I
~
O
5
IC
I5
20
25
15
20
25
PRESSURE/KBIR 1295
1275
1 */
0
Haines and Gilson obtained for the direct transformation of phase I1 to phase I, which agrees with the change in overall order from 4 to 48. Thus, the change in order can be directly related to the configurational contribution to the change in entropy, expressed as R In (h,/h2), where h, and h, are the respective orders of the factor groups for the two phases. Pronounced changes in slope are observed on the plots of u versus P at the two phase transitions (Figures 3 and 4). These shifts in slope result from the changes in crystal structure which affect the pressure dependence of a given mode. For many bands, dv/dP is lower in phase 111 than in the other two phases (Tables I and 11), indicating that the molecules in phase I11 experience less compression. Bands in the C-H stretching region of the spectrum are among those with the highest du/dP values in all three phases, as these vibrations are located on the “outside” of the molecule, and are affected to a greater degree by the decreased interatomic and intermolecular distances as the pressure is increased. Other modes that show a strong dependence on pressure are u , ~and ujl, a CH2 twist and a C-C stretch, respectively. These results are in agreement with the observations reported by Ferraro: that stretching vibrations are more greatly affected by pressure than are bending vibrations, as the force constants involved depend to a greater extent on interatomic distance. For a majority of the bands that split in phase 11, one component, usually the highest frequency component, exhibits a much greater dependence on pressure than the other components, as compression not only results in a shift to higher frequency but also increases the factor group splitting. The logarithmic pressure derivatives and mode Griineisen parameters for selected modes of cyclohexane are listed in Tables I and 11. The mode Griineisen parameter is defined as follows y i = -d In vi/d In V = ( l / K ) ( d In vi/dP),
PRESSURE/KBAR
Figure 4. Pressure dependence of selected Raman bands of cyclohexane.
intermediate phase, phase 111, was found to be very similar to that of the high-pressure phase obtained in a DAC by Obremski and co-workers9 (phase I1 in their nomenclature). On the basis of infrared polarization measurements, they concluded that this phase was orthorhombic with D2hsymmetry. The single crystal of this phase was obtained by slowly increasing pressure on phase I until the phase transition was observed. It is probable that they were in fact recording the spectrum of the intermediate phase 111. The changes in crystal symmetry at 293 K can be summarized as follows; a cubic crystal with oh symmetry, phase I, transforms at 5.1 kbar to an orthorhombic crystal with DZhsymmetry, phase 111, which then transforms at 9.6 kbar to phase 11, which is monoclinic with C2h symmetry. The corresponding changes in site group are from o h to C2, to Ci.This is consistent with the trend to higher density and lower symmetry with increase in pressure. These structural changes are in agreement with the splittings in the infrared spectra. Factor group analysis predicts that the al,, a2”, and e, modes of the free molecule would split into 1, 2, and 3 components, respectively, in phase 111 and 2, 2, and 4 components, respectively, in phase 11. Although in some cases the predicted splitting is not observed, no band split into more components than predicted. Sandrock and SchneideF obtained transition entropies for the two phase transitions from high-pressure DSC measurements up to 3 kbar.13 They divided the entropy of transition into contributions from volume change and configurational change; the latter should be an indication of the disordering that arises from the number of distinguishable positions the molecule can take up in the high-temperature phase. These entropy changes are equivalent to R In 2 and to a value between R In 6 and R In 8 for the I1 I11 and I11 I transitions, respectively. Thus, a 2-fold disordering process, which the authors proposed was related to ring inversion, is occurring at the former, and a 6- to 8-fold process at the latter transition. These values can be compared to the changes in relative order of the factor group of each phase; h = 4 for 8 for DZh, and 48 for 0,. This implies a 2-fold increase in symmetry at the I1 I11 transition and a 6-fold increase at the 111 I transition. A configurational entropy change of R In 12 to R In 14 was
-
-
-
-
(1)
where K is the isothermal compressibility of the crystal. The compressibility values used were calculated from the results of Bridgeman2’ and of Wisotzski and Wiirflinger12and are 0.026 and 0.0054 kbar-I for phases I and 11, respectively. The compressibility of phase I11 is estimated to be 0.009 kbar-l from calculations based on the available PVT data. Values for y i range from 0.006 to 0.039 in phase I, from -0.01 to 0.1 in phase 111, and from 0.006 to 0.16in phase 11. The y ivalues for phase I are, in most cases, lower than those for phase 11. As phase I1 has a higher density, the application of pressure causes a greater reduction in internal bond lengths. For many vibrations, the y i values for the modes in phase 111 are lower than in the other two phases, again demonstrating that these modes are experiencing the effects of compression to a lesser extent. One striking feature of the y i values, especially those for phases I1 and 111, is that the values are significantly higher for the C-H stretching vibrations than for many of the other modes. This is in contrast to the trend reported by that y i is inversely proportional to the square of the vibrational frequency, as weaker bonds exhibit a greater relative susceptibility to compression. The deviation from this trend reported here could be related to the hydrocarbon structure in which the C-H vibrations are located on the outer surface of the molecule and are, therefore, affected to a greater extent by interaction with the neighboring molecules. It should be noted that, while the vibrational modes for phase 111, in general, do not exhibit large changes in frequency with increasing pressure, the compressibility for this phase lies between that of the other two phases. This indicates that, for phase 111, the intermolecular contribution to the compressibility accounts for a larger portion of the total compressibility than for the other two phases. There is a general trend to the effect that y i values increase with decreasing frequency in phases I and 11, but there are many exceptions. Similar observations were reported for adamar~tane,~ (21) Bridgeman, P. W. Proc. Am. Acad. Arts Sei. 1949, 77, 129. (22) Zallen, R. Phys. Rev. B: Solid State 1974, 89, 4485. (23) Zallen, R.; Slade, M . L. Phys. Reu. B: Condens. Matter 1978, B18, 5115.
(24) Burns, G.; Dacol, F. W.; Weller, B. Solid State Commun. 1979, 32, 151.
J . Phys. Chem. 1989, 93, 7925-7931
7925
will have different compressibilities along each crystal axis, as is the case for phases I1 and 111. Cyclohexane, in common with both adamantanez4and adama n t a n ~ n e exhibits ,~~ high yi values for modes involving C-C stretching, indicating that these modes are particularly pressure sensitive. As expected, the y i values are higher in the ordered, high-pressure phases.
and a d a m a n t a n ~ n e . Exceptions ~~ are expected as the Griineisen law was envisaged for relatively simple systems. The systematic variation of y i with vi, reported by Zallen,zl*zzwas based on the study of crystals with intramolecular bonds of similar strengths. The case is very different for more complex polyatomic molecules such as cyclohexane, where both carbon-hydrogen and carboncarbon bonds occur, and is further complicated by the variety in the types of vibrations occurring such as stretching, bending, and twisting. Another cause for this deviation from systematic behavior occurs in the case of lattices of lower than cubic symmetry, which
Acknowledgment. This research was supported by grants from the Natural Sciences and Engineering Research Council of Canada. J.H. acknowledges the award of a scholarship from NSERC. Registry No. Cyclohexane, 110-82-7.
(25) Harvey, P. D.; Butler, I. S.; Gilson, D. F. R. J . Phys. Chem. 1986, 90, 4546.
Resonance Raman Characterization of the Heme c Group in N-Acetyl-microperoxidase-8: A Thermal Intermediate Spin-High Spin State Mixture Jim-Shyan Wang and Harold E. Van Wart* Department of Chemistry and Institute of Molecular Biophysics, Florida State University, Tallahassee, Florida 32306 (Received: March 29, 1989)
Microperoxidase-8 (MP-8), the heme octapeptide from cytochrome c, is a potential water-soluble model for peroxidases. It contains His as a proximal ligand and a sixth coordinationsite accessible for reaction with hydrogen peroxide. The aggregation of MP-8 in neutral aqueous solution which complicates its use as a model is shown to be abolished upon acetylation of the N-terminal Cys residue. Resonance Raman spectroscopy has been used to study the structure of the heme c group of Ac-MP-8. At neutral pH, Ac-MP-8 exhibits two sets of porphyrin skeletal stretching frequencies in the 1400-1700-~m-~region at 296 K. Only the high-frequency member of each set persists at 77 K. The low-frequencyset matches those of the fluoride complex of Ac-MP-8, indicating that they arise from a six-coordinate, high-spin form which has water as the sixth ligand and an expanded porphyrin core. The high-frequency set corresponds to that of an intermediate- or low-spin heme with a contracted core. Since there are no strong field ligands available to the sixth site, the species responsible for the latter set of bands is an intermediate-spin (either pure intermediate (S = 3/2) or quantum admixed intermediate (S = 3/2)-high (S = 5 / 2 ) spin) form of Ac-MP-8. Thus, Ac-MP-8 exists as a thermal mixture of high- and intermediate-spin species. The stabilization of the intermediate-spin state is attributed to a weak axial field from the His-18 residue, which is apparently unable to form a strong bond to the iron atom.
Introduction The heme group is the active cofactor in a variety of oxidative enzymes. This includes almost all of the hydroperoxidases, which utilize hydrogen peroxide as an oxidant, and certain mono- and dioxygenases, which utilize molecular oxygen as the electron acceptor.' In order to study the mechanism by which hydrogen peroxide and molecular oxygen are activated by the heme group in these reactions, it is of interest to have appropriate heme models for study. In particular, there is currently an intense interest in studying the structures of high-valent forms of the heme group obtained by oxidation of the porphyrin and/or iron atom as models for catalytic intermediates of heme enzymes. The factors that influence the structures and reactivity of such intermediates may also reveal clues to the manner in which the protein controls the function of the heme group in the respective enzymes. There are numerous criteria for the ideal heme enzyme model which have proven to be difficult to satisfy completely.2 Restricting discussion to models for the peroxidases, a water-soluble heme species that is stable in the ferric state with His as the fifth ligand and a vacant or easily exchanged sixth ligand is desired. The model should exist as a monomer in aqueous solution in the absence of stabilizing agents such as detergents or organic cosolvents, and it should not form M-oxodimers. Other important features are that the model be readily available and, if possible, soluble in a variety of other solvents at concentrations useful for
* Author to whom correspondence should be addressed. 0022-3654/89/2093-7925$01.50/0
spectroscopic studies. If the model is to be used in conjunction with resonance Raman (RR) studies, it is preferable that the methine carbons not be substituted (e& not a tetraphenylporphyrin (TPP) derivative), so that the R R spectra can be compared directly with those of the enzymes. Two potential heme model species for the peroxidases can be obtained from proteolytic digestion of cytochrome c. Digestion of cytochrome c with pepsin3v4gives microperoxidase-1 1 (MP-I 1), a heme-containing undecapeptide which retains residues 1 1-2 1 (Val-Glu-Lys-Cys-Ala-Gln-Cys-His-Thr-Val-Glu) of the protein. The heme c group remains linked through thioether bonds from the a-carbon atoms of the saturated vinyl groups of two adjacent pyrrole moieties to Cys- 14 and Cys- 17, while His- 18 serves as a fixed proximal ligand.5,6 The sixth coordination site can be occupied by a number of exogenous ligands.',* MP-11 is also a good candidate for R R studies, since the methine bridges are not substituted. The saturation of the two vinyl moieties in the (1) Malmstrom, B. G. Annu. Reu. Biochem. 1982, 51, 21-59. (2) Morgan, B.; Dolphin, D. In Srrurrure and Bonding, Buchler, J. W.,
Ed.; Springer-Verlag: Berlin, 1987; Vol. 64, pp 115-203. (3) Tsou, C. L. Biorhem. J . 1951, 49, 362-367. (4) Tsou, C. L. Biochem. J . 1951, 49, 367-374. (5) Tuppy, H.; Paleus, S. Arra Chem. Scand. 1955, 9, 353-364. (6) Paleus, S.; Ehrenberg, A,; Tuppy, H. Acta Chem. Scand. 1955, 9, 365-374. (7) Harbury, H. A,; Loach, P. A. J. Biol. Chem. 1960,235, 3640-3645. (8) Harbury, H. A,; Loach, P.A. J . Biol. Chem. 1960,235, 3646-3653.
0 1989 American Chemical Society
