J. Phys. Chem. 1988, 92, 3323-3325 explained by the exchange of adsorbed CO species between on-top and bridge sites, which is induced by the change of surface electronic energy levels. Now we can explain the asymmetric features of EM-IRRAS spectra in light of such an adsorption site exchange. The depression of negatively pointed peak of on-top C O indicated in Figure 2 can be attributable to the decrease in population of this species at negative potential limit E , due to unfavorable interaction of the 5a orbital of C O with substrate Pt atoms, together with the substantial frequency shift upon the electrode potential. On the other hand, the unipolar feature of bridge-bonded C O can be explained by the increase of this species at negative potential limit E , along with no potential-dependent frequency shift. Figure
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5a demonstrates the difference spectrum of TR-IRRAS spectra o f t = 0 and t = 9 depicted in Figure 4a. It can be seen that the spectrum Figure Sa is very similar with the characteristic features of the EM-IRRAS spectrum as shown in Figure 5b. In conclusion, we have directly observed the adsorption site interconversion of C O on Pt(100) electrode surface for the first time. The preference of adsorption site is determined solely by the applied potential, which indicates that the relative position of surface electronic levels compared to the 27r* level of C O strongly affects the C O coordination site. In addition, the characteristic features of EM-IRRAS spectra can be successfully interpreted in the scheme of the adsorption site interconversion induced by the change of electrode potential.
Heat Capacity and Glass Transition Behavior of Amorphous Icet Y. P. Handa and D. D. Klug* Division of Chemistry, National Research Council of Canada, Ottawa, Canada K1 A OR6 (Received: February 22, 1988)
The heat capacity of low-density amorphous ice, made by warming and annealing high-density amorphous ice, has been measured in the range 90-136 K. A glass transition was observed at 124 K for samples that were annealed in the range 124-130 K. The glass transition observed is presumably the same as that reported in the literature for amorphous ice prepared by vapor deposition or rapid quenching of water.
Introduction The properties of low-density amorphous ice are a topic of great current interest,l and in particular, the relationship between low-density amorphous ice and liquid water has intrigued many researchers over the past several years. Some years ago McMillan and Los2 and later Sugisaki et aL3v4reported the existence of a glasslike transition in amorphous ice made by vapor deposition on a cold plate. Recently, Johari et aL5 reported the Occurrence of this behavior in amorphous ice made by a rapid quenching techniques6 They remarked that such behavior is not observed in amorphous ice made from the high-density amorphous ice recently reported by Mishima et al.' It is the purpose of this Letter to demonstrate that such a transition indeed occurs in this material and to discuss its relationship with the transitions observed in low-density amorphous ice prepared by other techniques. It is not required that the low-density amorphous ices prepared by different techniques be structurally identical or closely related to supercooled water. High-density amorphous ice can, for example, be prepared with densities which vary by up to 25%.8 In addition, the amorphous ice prepared by the rapid quenching technique may be several percent denser than the vapor-deposited materiaL9 It is therefore important to establish whether each particular form of low-density amorphous ice undergoes a glass transition to learn if it is or is not closely related to the supercooled liquid. Since the amorphous ice made from high-density amorphous ice is believed to be obtained by a path which is equivalent to melting of the solid to a quenched high-density l i q ~ i done , ~ may postulate that the low-density amorphous ice made from high-density amorphous ice is a relaxed form of the high-density glass and therefore closely related to the supercooled liquid. Experimental Methods and Results Two samples of high-density amorphous ice were prepared, as described by Mishima et by pressurizing 2.7 g of ice Ih in an indium cup in a 12.5-mm-i.d. pistop-cylinder apparatus. The samples were pressurized to 17.5 kbar, which is well beyond the 'Issued as NRCC No. 28983
0022-3654/88/2092-3323$01.50/0
transition pressure of 10.5 kbar, to ensure complete conversion to high-density amorphous ice. The samples were recovered in their indium cups at 77 K and placed in an automated Tian-Calvet heat-flow calorimeter (Setaram Model BT)'O that had been cooled to 77 K. High-density amorphous ice transforms to low-density amorphous ice at about 114 K," and consequently the low-density amorphous ice required for study in this work was obtained by heating the high-density amorphous ice samples as prepared to temperatures higher than 114 K. The detailed thermal history of the samples is given below. One sample of high-density amorphous ice was heated to 124 K, and the low-density amorphous ice thus obtained was annealed for 2 h at this temperature, cooled to 77 K, and then scanned at the rate 10 K h-I to 125 K. The low-density amorphous ice was then warmed and annealed at 129 K for 2 h, cooled to 77 K, and then scanned at 10 K h-' to room temperature. The other sample of high-density amorphous ice was converted into low-density amorphous ice as described above, then annealed for 2 h at 130 K before cooling to 77 K, and then scanned to room temperature. Blank runs with indium from the two samples were performed from 80 K to room temperature at 10 K h-l, and heat capacities of the samples were obtained as described previously.1° The heat capacities of low-density amorphous ice are shown in Figures 1 and 2, in addition to that of an unannealed sample of high-density amorphous ice" and the heat capacity of ice Ih.lo (1) Sceats, M. G.; Rice, S. A. In Water, A Comprehensive Treatise; Franks, F., Ed.; Plenum: New York, 1982; Vol. 7, p 83. (2) McMillan, J. A.; Los, S. C. Nature (London) 1965, 206, 806. (3) Sugisaki, M.; Suga, H.; Seki, S. Bull. Chem. Soc. Jpn. 1968,41,2591. (4) Sugisaki, M.; Suga, H.; Seki, S. In Physics of Ice; Riehl, N., Bullemer, B., Engelhardt, H., Eds.; Plenum: New York, 1969; p 329. ( 5 ) Johari, G. P.; Hallbrucker, A.; Mayer, E. Nature (London) 1987,330, 552. (6) Mayer, E. J. Appl. Phys. 1985, 58, 663. (7) Mishima, 0.;Calvert, L. D.; Whalley, E. Nature (London) 1984, 310, 391. (8) Klug, D. D.; Mishima, 0.; Whalley, E. J . Chem. Phys. 1987,86, 5323. (9) Mayer, E., private communication. (10) Handa, Y. P.; Hawkins, R. E.; Murray, J. J. J. Chem. Thermodyn. 1984, 16, 623. (11) Handa, Y. P.; Mishima, 0.;Whalley, E. J . Chem. Phys. 1986, 84, 2766.
0 1988 American Chemical Society
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The Journal of Physical Chemistry, Vol. 92, No. 12, 1988 301
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Figure 1. Heat capacity, C,, of low-density amorphous ice (Ida), annealed for 2 h at the temperatures indicated. The sample labeled lda(129 K) was previously annealed at 124 K for 2 h (see text). The heat capacity of high-density amorphous ice (hda) is also shown. The curves labeled hda and Ida( 130 K) have been shifted upward by 2 J K-I mol-'.
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Letters TABLE I: Heat Capacities, Cp,of Low-Density Amorphous Ice Samples Annealed for 2 h at the Temperatures Indicated' C,, J K-I mol-' T, K Ida( 124 K) Ida( 129 K) Ida( 130 K) 90 14.95 15.00 14.88 92 15.18 15.26 15.12 94 15.58 15.59 15.46 96 15.92 15.92 15.75 98 16.18 16.17 16.10 100 16.43 16.48 16.38 102 16.62 16.62 16.70 104 16.94 16.91 17.00 17.20 106 17.22 17.30 108 17.43 17.47 17.59 17.79 110 17.74 17.89 112 18.05 18.08 18.12 114 18.29 18.32 18.39 18.50 1I 6 18.54 18.66 118 18.82 18.80 18.96 120 19.07 19.09 19.26 122 19.37 19.38 19.55 19.65 124 19.72 19.97 126 20.17 20.41 128 20.61 20.85 130 20.65 21.23 132 20.12 21.33 134 18.47 20.83 136 15.02 19.32 160 21.70 21.23 165 22.41 21.90 170 23.21 22.70 175 24.02 23.56 180 24.88 24.38 185 25.60 25.19
The samples labeled Ida( 124 K) and Ida( 129 K) belong to the same high-density amorph. The values of Cpthrough the large exotherm in Figure 1 are omitted. the heat capacity of low-density amorphous ice through the endothermic transition region. The heat capacity then drops by about 2 J K-' mol-' after the transformation to ice IC.
-1 90
100
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T/K Figure 2. Expanded plot of the heat capacity of low-density amorphous ice shown in Figure I . Tgis the glass transition temperature. The heat capacities of lda(124 K) and of ice Ih are also shown. The curves labeled 124, 129, and 130 K have been shifted upward by 1,3, and 5 J K-' mol-',
respectively. The heat capacities are listed in Table I and are accurate to f1%.l0 There are several features apparent in the plots of heat capacity versus temperature. There is a large exothermic peak due to the transformation of high-density amorphous ice to low-density amorphous ice. A part of this transition is shown by the curve labeled hda in Figure 1, and this transformation is complete at about 1 2 0 K.738*" As seen in Figure 1, the heat capacity of low-density amorphous ice, curves labeled Ida( 129 K) and Ida( 130 K), exhibits two features in the range 90-185 K. There is an endothermic peak in the range 124-135 K, which is characteristic of a glass transition, followed by a large exothermic peak in the range 135-160 K, which has been shown to be due to the transformation to ice IC." There is an additional transformation of ice IC to Ih that occurs over the range 180-220 K, and this transformation has been discussed elsewhere."*'* The heat capacity of low-density amorphous ice is about 2 J K-' mol-' higher than that of ice Ih in the range 90-122 K. The heat capacity rises by about 0.7 J K-' mol-' above the base line obtained by extending (12) Handa, Y . P.; Klug, D.D.; Whalley, E. Can. J . Chem., in press.
Discussion The heat capacity results for low-density amorphous ice are shown in detail in Figure 2. The heat capacities for the replicate runs and for the two samples (Table I) were found to be within 0.2 J K-' mol-' and are within 0.4 J K-' mol-' of the values reported by Sugisaki et al.394for their sample as deposited at 120 K. The heat capacities are about 2 J K-' mol-' higher than the values for ice Ih'O in the same temperature range, and this is due in part to the slightly weaker hydrogen bonding in low-density amorphous ice. The frequencies of uncoupled 0-H and 0-D vibrations in D 2 0 and H20, respectively, measured in the Raman spectrum*,13of low-density amorphous ice show that the firstneighbor hydrogen-bonded oxygens in this phase are about 0.0 1 A farther apart than in ice ICor Ih, and the hydrogen bonding is therefore somewhat weaker. The heat capacities below 124 K of the low-density amorphous ice samples (Table I) remain the same after annealing at different temperatures which indicates that any enthalpy relaxation due to conversion from high-density to low-density amorphous ice (see Figure 1) or due to the structural relaxation of low-density amorphous ice has been completed before the samples were scanned through the glass transition region. At a glass transition, an increase in the heat capacity is expected with increasing temperature and an overshoot of the heat capacity above the equilibrium value for the supercooled liquid is often observed. As is ~ e l l - k n o w n ,this ' ~ gives rise to a peak in the heat capacity which can be amplified by annealing at long times near the glass transition temperature. A large jump of 20-35 J K-' mol-' was, (13) Sivikumar, T. C.; Rice, S.A,; Sceats, M . G. J . Chem. Phys. 1978, 69, 3468. (14) Das, G. C.; Bever, M. B.; Uhlmann, D.R.; Moss, S. C. J . Non-Crysf. Solids 1972, 7. 251.
J. Phys. Chem. 1988,92, 3325-3335 in fact, observed in the heat capacity of vapor-deposited amorphous ice in the range 126-132 K by Sugisaki et al.3,4using an adiabatic calorimeter. Their samples were deposited at 120 K, which is rather high, and some annealing may therefore have occurred. In a scanning calorimeter such a peak could be greatly broadened. The transition enthalpies for low-density amorphous ice to ice IC for the samples that had been annealed 2 h at 129 or 130 K agreed within experimental error with the value for the unannealed sample." This demonstrates that the additional degrees of freedom available after warming amorphous ice to 129 or 130 K are not sufficient to allow detectable growth of ice ICwithin 2 h. There are several possible reasons for this. Samples prepared as described above from high-density amorphous ice do not have any detectable amount of ice I C , ~and therefore the nucleation sites are not abundant. In addition, crystal growth rates may be small due to slow rotational and translational diffusion of water molecules. This seems to be the case for samples prepared by rapid quenching of water that contain ice I C . ~In addition, our samples apparently have very low surface area as a result of their confinement in indium cups. This is indicated by the relatively low values for the transition enthalpies for ice IC to Ih119'29'5measured for our samples; high values for this transition are the result of high surface areas which are present in the vapor-deposited material.16 Although no quantitative heat capacity data are available from the measurements of McMillan and Los2 on vapor-deposited amorphous ice or Johari et alG5on samples made by rapid
3325
quenching of water, the glass transition reported here (Figure 2) closely resembles the glass transitions reported by these authors. The glass-transition temperature of 124 K obtained in this work is 12-15 K lower than that reported in ref 2 and 5 and is no doubt due to our slower scanning rate of 10 K h-' as compared to their scanning rate of 1200-1800 K h-l. The glass-transition temperature of 126 K for vapor-deposited amorphous ice determined by Sugisaki et al.334using an adiabatic calorimeter is 2 K higher than that reported here. It is interesting to note that the samples of McMillan and Los were not annealed and the samples of Johari et al. were annealed at 130 K. GhormleyI6 has suggested that the measurements of McMillan and Los are possibly incorrect since proper annealing is required to remove the effects of enthalpy relaxation which may occur in the low-density amorphous ice. MacFarlane and Angell" did not observe a glass transition; however, they did not specify their exact annealing times and temperatures. The samples made from rapidly quenched water . ~ the effect of this on the glass contain about 5% ice I C , ~and transition is not clear. The crystallization of ice IC has prevented all investigators, including us, from observing a smooth transition of amorphous ice to the liquid at room temperature. In conclusion, we have shown that a glass transition has been observed calorimetrically in low-density amorphous ice made from high-density amorphous ice at 124 K which is similar to that observed in amorphous ice prepared by vapor deposition or rapid quenching of water. Registry No. H20, 7732-18-5.
(15) Handa, Y. P.; Klug, D. D.; Whalley, E. J . Chem. Phys. 1986, 84, 7009. (16) Ghormley, J. A. J . Chem. Phys. 1968, 48, 503.
(17) MacFarlane, D. R.; Angell, C. A. J . Phys. Chem. 1984, 88, 759.
FEATURE ARTICLE Some New Ideas in the Theory of Intermolecular Forces: Anisotropic Atom-Atom Potentials A. J. Stone* and S. L. Price University Chemical Laboratory, Lensfield Road, Cambridge CB2 I E W, England (Received: July 22, 1987; In Final Form: February I, 1988)
The macroscopic properties of molecular fluids and solids, and the structures and properties of van der Waals complexes, are controlled primarily by the interaction potential between the molecules. A great deal has been learned about the molecular basis for the macroscopic behavior of polyatomic systems by means of statistical mechanics or simulations based on crude model potentials, but much better potentials are needed if we are to understand the quantitative properties of particular systems. Considerable progress has been made, but recent theoretical and empirical studies have shown that both of the traditional approaches to the design of model potentials for polyatomic molecules, namely, the anisotropic central potential and the isotropic atom-atom model, are unsuitable for accurate model potentials, and we describe here a much more powerful anisotropic site-site model. The anisotropies that have to appear in such a model are directly related to the nonspherical features of the atoms or groups of atoms comprising the molecule. These include lone pairs and .rr electrons, or the hydrogen atoms in a unit like a methyl group. These features lead to anisotropies in the local charge distribution, which can be described by a distributed multipole analysis of an ab initio wave function, and to anisotropies in the repulsive wall around the atom.
