1016
J. Phys. Chem. 1992,96, 1016-1018
Thermodynamlcs of CB0in Pure 0,, N,, and Ar H. S. Chen,* A. R. Kortan, R. C. Haddon, and D. A. Fleming AT&T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: October 25, 1991; In Final Form: December 13, 1991)
Fcc C , crystals exposed to pure 0 2 are converted to mbon-oxygen adducts at -200 O C . The carbon-oxygen speciea decompose , are determined by calorimetry into CO and C02gases at a higher temperature, -400 OC. The total heats of oxidation of C to be -53 kcal/mol. The activation energy for the oxidation is 58.2 kcal/mol. The Cw powders in inert gases such as N2 and Ar sublime at -600 O C . A method is derived to determine the heats of sublimation from thermogravimetric analysis. The heat of sublimation of Cbo powders in pure N2 or Ar is found to be 2 3 9 kcal/mol.
The CM molecule is found to be exceptionally stable. The stability of the icosahedral Cbomolecule, relative to a graphitic and one may structure, is due to the absence of dangling bond~,l-~ expect higher thermal and oxidative stability of Cw over graphitic layered structures. Although Cw is known to withstand very high temperatures in an inert atmosphere (-900 "C in argon), in open air and oxygen it does oxidize readily at 500 0C44 and is less oxidatively stable than graphite? Kroll et a1.7 reported that, upon exposure to low-energy and high-energy photon sources, films of Cboconverted to carbonyl-like structures even at 20 K. We have previously reported* thermodynamic measurements of Cw powders exposed to oxygen at -250 OC and that fcc Cs0 converts to amorphous carbon-oxygen species with destruction of icosahedral Cw molecular structure. To understand this behavior, we carried out thermodynamic measurements of Cw powders exposed to 02, N2,or Ar at higher temperatures. We observed that Cbopowders readily oxidize in oxygen with an activation energy Q 58.2 kcal/mol. The ratio CO/C02 gases evolved is -3, increasing with temperature of oxidation. The heat of sublimation of c 6 0 in N2 is determined to be -39 kcal/mol. Cbopowders were prepared using procedures published prev i o ~ s l yand ~ ~purified ~ by chromatography. The crystalline powders used in the present study were then sublimed from the extract at 400 OC. The purity of the Cbopowders was determined to be greater than 99% by mass spectrometry. The structure of the Cw powders was identified as fcc. The oxidation and sublimation kinetics of Cw powders were investigated by differential scanning calorimetry (Du Pont 9 12 DSC) and thermogravimetric analysis (Perkin-Elmer TGA7). The samples were exposed in pure 02, Ar, or N, at a constant flow rate of 100 cm3/min. Figure 1 shows DSC and TGA scans at 1.25 OC/min in pure 0,.The DSC scan (Figure la) shows two exothermic peaks at 306 and 430 OC. The correspondingTGA curve (Figure lb) shows the sample weight gain starting at -250 OC and peaking at 305 OC. The sample then lost weight linearly up to 370 OC and more rapidly at higher temperature. We note that the fist exothermic
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( 1 ) Haufler, R. E.; Conccicao, J.; Chibante, L. P. F.; Chai, Y.; Byme, N . E.; Flanagan, S.; Haley, M. M.; OBrien, S . C.; Pan, C.; Xiao, Z.;Billups, W. E.; Ciufolini, M. A.; Hauge, R. H.; Margrave, J. L.; Curl, R. F.; Smalley, R. E. J . Phys. Chem. 1990, 94, 8634. (2) Newton, M. D.; Stanton, R. E. J. Am. Chem. Soc. 1986,108,2469. (3) Luthi, H. P.; AlmlBf, J. J. Am. Chem. Soc. 1987, 135, 357. (4) Haufler, R. E.; Chai, Y.; Chibanto, L. P. F.; Conceicao, J.; Jin, C.; Wang, L. S.;Maruyama, S.;Smalley, R. E. Mater. Res. SOC.Symp. Proc. 1991,204,627. ( 5 ) Milliken, J.; Keller, T. M.; Barohavski, A. P.; McElvany, S. W.; Callahan, J. H.; Nelson, H. H. Chem. Mater. 1991, 3, 386. (6) Vassallo, A. M.; Pang, L. S.K.; Cole-Clark, P. A.; Wilson, M. A. J . Am. Chem. Soc. 1991, 113,7920. (7) Kroll, G. H.; Benning, P. J.; Chen,Y.; Ohno, T. R.; Weaver, J. H.; Chibante, L. P. F.; Smalley, R. E. Chem. Phys. Lett. 1991, 181, 112. (8) Chen, H. S.; Kortan, A. R.; Haddon, R. C.; Kaplan, M. L.; Chen, C. H.; Mujsce, A. M.; Chou, H.; Fleming, D. A. Appl. Phys. Lett. 1991,23,2956. (9) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Hoffman, D. R. Nature 1990, 347, 354.
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TABLE I: T d t h T~mpe" To,TI,d Tz ("C), ud tbe Heats of Oxidatioa A H 0 (kd/mol) at Variorrr Heating Rates a ("C/min)
a
TO
TI
T2
m 0
0.62 1.25 2.5 5.0 10.0 20.0
256 272 295 317 349 37 1
284 306 332 361 393 42 1
427 440 455 460
57.0 53.1 53.1 52.2 50.7
-
peak at 306 OC in the DSC scan is higher by 16 OC than the peak maximum -290 OC in the first derivative of the corresponding TGA scan. It suggests that in the temperature range 280-320 OC, there exists two chemical reactions: one resulting in weight gains caused by carbon-oxygen bond formation and the other, weight loss via decomposition of the carbon-oxygen species into CO and C02. In the temperature range 320-370 OC, the rate of weight loss is nearly constant. The higher temperature reaction is similar in both DSC and TGA scans. With increasing scan rate a, the two exothermic peaks tend to merge together indicating a higher activation energy for the higher temperature reactions. The onset and peak temperatures To, T I ,and T2and total heat of reaction AHo are listed in Table I. Using the modified Ozawa-Chen method for Q / R T >> 1l0or the method of Augis and Bennet" for isochromal transformation kinetics such that d In ( T / a ) / d In (1/7') = Q / R (1) we determined from the In T / a vs 1/T plots shown in Figure 2 the activation energies Qo = Q1 = 19.0 kcal/mol and Q2 = 58.2 kcal/mol. The activation energies for the first reaction Qo = QI (=19.0 kcal/mol.) are in fair agreement with that for carbonoxygen bond formation, Q = 17.2 kcal/mol, reported previous1y.S The total heat of oxidation AHo as shown in Table I tends to decrease from 57.0 to 50.7 kcal/mol with increasing scanning rate or increasing temperature of reaction. Taking the heats of formation to be 600,* -26.4, and -94.1 kcal/mol12 respectively for the reactions 60C (graphite) c 6 0 molecule
--
c + %Oz(g) c + O,(g)
CO(g)
(2)
C02(B) we arrived at the ratio of C02to total gas product (C02and CO) r = (AHo- 600/60 - 26.4)/(94.1 - 26.4). The r value decreases from 0.30 to 0.21, or the ratio of CO/CO2 increases from 2.3 to 3.4, with increasing temperature of oxidation. These values are (10) Yinnon, H.; Uhlmann, D. R. J . Non-Cryst. Solids 1983, 51, 253.
(11) Augis, J. A.; Bennett, J. E. J. Thermal Anal. 1978, 13, 283.
(12) Kinoshita, K. Carbon, Electrochemical and Physiochcmical Properties; Wiley and Sons: New York, 1988; p 177.
0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 3, 1992 1017
Letters
t
2o 01 200
i \ 1
I
300
400
\.---- I
500
600
v 700
T ('C)
Figure 3. TGA s a n s of Cm in pure N2and 0,.Scan rate a = 1.2 "Clmin.
-21 200
I
1
I
I
I
I
I
I
1
I
I
I
I
I
5
400
300
TEMPERATURE 'C
Figure 1. DSC and TGA scans of Cm in pure 0,.The scan rate a = 1.2 OC/min.
1.10
1.20
1.40
1.60
1 0 3 1(K) ~ 1.4
1.5
1.6
1.7
1.8
1.9
Figure 4. Fraction of weight loss, x , of Cm in N2at various heating rates Q = 0.3, 1.2, and 5 OC/min. X: new batch of Cm a = 0.3 OC/min.
1 0 3 1 (K) ~
Figure 2. In T / a vs 1/T plots for the transition temperatures (K) To, T I ,and T2at various a = 1.2 to 20 OC/min.
close to those found for the oxidation of artificial graphite and char.I3 The author reported the ratio of CO/C02 ranging from 1.6 to 3.5 at temperatures from 575 to 675 "C. When Csopowders were exposed to pure nitrogen at a heating rate of 1.2 OC/min, the sample weight remained constant up to 450 "C and then decreased more or less exponentially (Figure 3). The sample lost most of its weight at -700 "C with a small amount of 5% residue remaining up to 800 "C. For comparison, the corresponding TGA scan in pure oxygen is also shown in Figure 3. There is no indication of weight gain as observed in the case of oxidation in the temperature range 250-320 "C. The initial onset weight of 1% is an instrumentation characteristic observed upon heating. We observed that the TGA scans in pure N2and Ar are identical. The weight loss curve shifts to higher (or lower) temperature by -80 "C, when the scan rate increases to 5.0 OC/min (or decreases to 0.3 OC/min). All curves indicate a nearly exponential weight loss. We attribute the observed weight loss to sublimation of the C6,, powders.
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-
(13) Arthur, J.
R. Trans. Furuduy Sm. 1951,47,
164.
According to the kinetic theory of ideal gases, the mass of substance m, evaporating from a surface area a, in a time t , at a pressure p (dyn/cm2), and temperature T (K) is given by m = pat(M/2rRT)'l2 (3) Herep = poexp(-AH,/RT), M is the molecular weight, R is the gas constant, and AHs is the heat of sublimation. In the present open-flow experiments in pure N2 and Ar, we suppose that the fraction of weight loss x is proportional to m given by eq 3. For first approximation, ignoring the weak T'J2 dependence and a being nearly constant at the early stage of sublimation (x 5 0.4), eq 1 reduces to x = Kt exp[-MS/RT] (34 where K is the proportional constant. Equation 3a describes the "isothermal" transformation kinetics. In a tempering experiment with a constant heating rate dT/dt = a,eq 3a is modified such that
J. Phys. Chem. 1992, 96, 1018-1021
1018
Integrating eq 3, with the aid of the integral Ei(-x) = -Jmx(e-x/x) = e-x(x-2 - x - l ) for x >> 1, yields
--() ,
x = K RT
exp[
-%I
...
The heat of sublimation AHs is determined by plotting the TGA data as shown in Figure 4. From the slopes of In ( x / T ) versus 1/T curves, we obtain AHs = 30, 35, and 39 kcal/mol for a = 0.3, 1.2, and 5 OC/min, respectively. Also from In (a/T)vs 1/T plots for a given x, e.&, 0.1, we obtain AHs = 40 kcal/mol. The heat of sublimation of Cm powders has been reported previously to be 40 kcal/mol.l4 O u r result of AHs = 39 kcal/mol, determined at a scan rate of 5.0 OC/min, is in a good agreement with the reported data. The cause of the lower values, e.g., 30 and 35 kcal/mol obtained at lower rates of 0.3 and 1.2 OC/min, is not clear. Haufler et al.’ reported a much lower value of AHs -22 kcal/mol. It has been ~uggested’~ that the low values of AHs observed may be due to the less structurally ordered Ca powders which undergo a change during the experiment. Alternatively, we proposed here that the presence of nonuniform defective microstructure, such as stacking fault and grain boundary, is ~~
~
(14) Pan, C.; Sampson, M. P.; Chai, Y.; Hauge, R. H.; Margrave, J. L. J . Phys. Chem. 1991, 95,2944.
responsible for the lower heats of sublimation derived from TGA measurements. Ca molecules at such defective structures sublime at lower temperature than does those in a bulk crystal grain, and in TGA scan it contributes to the weight loss in the low-temperature regime. This results in the drastic reduction in the slope of x/ T vs 1/ T curve and thus in the apparent heat of sublimation determined. To test this viewpoint, we prepared a new batch of CWpowders sublimed from a purified Ca extract at a slower rate. The new sample showed sharp X-ray diffraction lines,indicating a good crystallinity. The new sample shows no detectable structural relaxation in TGA scan up to 450 OC. The TGA curve at 0.3 OC/min of this new sample diverges starting 5400 OC and merges again -620 OC with that of the less ordered structural sample (see Figure 4). Although the deviation is small, the heat of sublimation of the new sample, -35 kcal/mol, is higher by 5 kcal/mol. We may conclude that the heats of sublimation obtained are of the lower limiting values. The AHs 40 kcal/mol of c 6 0 powders is low as compared with that of graphites (AHH, 170 kcal/mol). This reflects the weak van der Waals bonding among c 6 0 molecules in the fcc ~ t r u c t u r e . ~ ~ J ~
-
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(15) Fischa, J. E.; Heiney, P. A.; McGhie, A. R. Science 1991,252,1288. (16) Fleming, R. M.; Hesen, B.; Kortan, A. R.; Siegriest, T.; Marsh, P.; Murphy, D. W.; Haddon, R. C.; Tycho, R.; Dabbagh, G.; Mujsce, A. M.; Kaplan, M. L.; Zahurak, S.M.Mater. Res. Soc. Symp. h o c . 1991,206,691.
Nltrosyl Hypofluorite: Local Denslty Functional Study of a Problem Case for Theoretlcal Methodst David A. Dixon*st and Karl 0. Christe* Central Research and Development Department, E. I. du Pont de Nemours and Company, Inc., Experimental Station, Wilmington, Delaware 19880, and Rocketdyne, A Division of Rockwell International Corporation, Canoga Park, California 91 303 (Received: October 21, 1991; In Final Form: December 17, 1991)
Local density functional (LDF) theory can successfully reproduce the previously published vibrational spectra and the salient geometrical parameters derived from them for FONO for which conventional ab initio methods (CISD/6-31G*) fail. LDF theory was used to calculate the geometries, vibrational spectra, force fields, and charge distributions for the three isomers cis-FONO, trans-FONO, and FN02. It is shown that FNOz is 40.8 kcal mol-’ more stable than cis-FONO, which in tum is favored by 25.2 kcal mol-’ over the trans isomer. It is shown that the previously published approximate mode descriptions for FONO are correct but that the observed spectra must be due to cis-FONO and not to the trans isomer as previously proposed. The bonding in cis-FONO is best rationalized in terms of an F atom loosely bonded throu h an oxygen atom to an NO2 molecule, resulting in the following structural parameters: rF4 = 1.673 A, r m N = 1.216 rN-0 = 1.190 A, LFON = 118.0°, and LON0 = 135.2’.
8,
Introduction Although much progress has been made during the past 20 years in the field of ab initio calculations of the geometries and energetics of molecules, there have been a number of cases which have been extremely difficult to compute with conventional methods. A classic example for such a “problem” molecule is FOOF, for which very high level calculations (CAS/CCI or CCSD(T)) with a large basis set were required to duplicate the experimental geometry.’* An even more difficult test is the duplication of the observed vibrational spectra. In a recent study,lb it has been shown that density functional theory (which is an ab initio method) in the local density approximation (LDF)* can predict both the structure and harmonic frequencies of FOOF. +ContributionNo. 6018. *E. I. du Pont de Nemours and Co., Inc. I Rocketdyne Division, Rockwell International Corp.
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TABLE I: Geometry Panmeten for FNOj’ param
N-F N-O L( F-N-O) L(0-N-O)
LDF 1.487 1.190 111.8 136.4
expP 1.467 1.180 112 136
Bond distances in angstroms, bond angles in degrees.
Noble has recently called attention3 to another similar “problem” molecule, FONO, or nitrosyl hypofluorite. This ~~
~~
(1) (a) See: Scuseria, G. E. J . Chcm. Phys. 1991, 94,442, for the most recent ab initio molecular orbital calculations. (b) See: Duon, D. A.; Andzelm, J.; Fitzgerald, G.; Wimmer, E. J . Phys. Chcm. 1991,95,9197, for the LDF calculations on FOOF and a discussion of previous calculations.
8 1992 American Chemical Society
