J. Phys. Chem. 1992, 96, 1705-1708
varies only from 12.77 to 13.04 eV for the four rotamers. The average IP (12.96 eV) of the two equilibrium conformations is approximately 7.5% above the reported experimental value (12.05 eV),** which is the expected deviation for an IP estimated from Koopman's theorem.29 The dependence of calculated dipole moments on the basis set for each of the four conformations is shown in Figure 1. It may be seen that values of p calculated with the three polarized basis sets are approximately equal and, coincidentally, close to those obtained using the much lower level, 3-21G basis. Significantly, dipole moments obtained with the 6-31G and D-95 basis sets are substantially greater (by approximately a factor of 2), illustrating the well-established importance of polarization functions to adequately characterize the electron distribution.
*1
T
GT
G GG' A
E
i
D
E
1705
F
Basis Set
Figure 1. Basis set dependence of the molecular dipole moment in TCTFE: (A) 3-21G; (B) 6-31G; (C) D95;(D) 6-31G(d); (E) 6-311G(d); (F) 6-311G(2df).
markedly (by -20%) in the G T and GG' conformations, which agrees with observed variations in the bond angles. The latter trend is manifested by the substantial increases in frequency of the two lowest modes (excluding the torsional mode), which are due, predominantly, to C-C-F and C-C-Cl bond deformations. Zero point vibrational energies [ZPVE] and thermal contributions to the enthalpy [H(T) - H(O)],calculated from the scaled frequencies using standard formulas,26 are given in Table 111. As noted above, there is no correction to the equilibrium energy difference, M(G-T), whereas the two saddle point energies require a correction of -0-4 kcal/mol. Electronic Properties. The fluorines' Mulliken charges are, of course, negative, ranging from -0.24 to -0.35 (dependent upon the basis set), whereas q(C1) for both chlorines is close to zero. Both carbons retain a positive charge, with the relative magnitudes dependent upon the number of bonded fluorine atoms.27 The approximate ionization potentials [IP = -e(HOMO)] exhibit a comparatively small dependence upon the basis set; e.g., for the T conformer, the IP ranges from 12.94 to 13.44 eV with the six basis sets used here. Neither do they change substantially with conformation. For instance, with the 6-31 1G(d) basis, the IP (26) Hill, T. L. An Introduction to Statistical Thermodynamics; Addison-Wesley: Reading, MA, 1960; p 167. (27) For example, the following Mulliken charges were obtained for the T conformer using the 6-311G(d) basis set: q(F,) = q(F2) = -0.25; q(C1,) = -0.02; q(C12) +0.03; q(C,) = +0.12; q(C2) = +0.59.
Summary and Conclusions The geometries and relative energies of the equilibrium- and transition-state conformations of lI1,2-trichloro- 1,2,2-trifluoroethane were investigated by ab initio calculations using a number of basis sets ranging from 3-21G to 6-311G(2df). Bond lengths and angles obtained with the 6-31G(d) and 6-31 1G(d) bases were in close agreement to each other and to experimental results. The calculated equilibrium energy difference, M(G-T), obtained by MP2 calculations with the polarized basis sets, was of the same sign and of similar magnitude to those from experiment. It was found that the GT saddle point energy is greater than the energy of the GG'transition state, in agreement with expectations based upon chlorintchlorine steric repulsions. Scaled vibrational frequencies calculated with the 6-31G(d) basis set were in satisfactory agreement with experimental data for the equilibrium conformers, particularly for vibrations below 1000 cm-l. The results of this investigation provide evidence that the 631G(d) basis set appears to be sufficient to yield an adequate characterization of the geometries, relative energies, and vibrational spectra in chlorofluoroalkanes.
Acknowledgment. We thank Dr. David A. Dixon for helpful discussions regarding this work. This research was sponsored in part by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract F49620-90-C-0076. Registry NO. TCTFE, 76-13-1 (28) Doucet, J.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1975,62,355. (29) Schwartz, M. E. In Applications of Electronic Structure Theory; Schaefer, H. F., 111, Ed.; Plenum Press: New York, 1977; p 357.
Kinetics of the Reaction of Chlorine Atoms with Dimethyinitramine Yannis C.Lazarou, Cbrysostomos Michael, and Panos Papagiannakopoulos* Department of Chemistry, University of Crete, 71409 Heraklion, Crete, Greece (Received: April 23, 1991; In Final Form: October 8, 1991)
-
The kinetics of the reaction CI + (CH3)*NN02 HC1 + CH2N(CH3)N02have been studied over the temperature range 273-353 K by using the very low pressure reactor technique. The absolute rate constant at 303 K was found to be k = (1.86 0.54) X lo-]] cm3 molecule-' s-], and from the range of temperature measurements (273-353 K), k = 1.44$',;? X exp(-1200 370/RT) cm3 molecule-' s-I (R expressed in cal mol-] K-I). Transition-state (TS) models of the reaction rule out a TS geometry with collinear C1-H-C bonds in favor of a bent TS geometry, and the estimated A factor was in excellent cm3 molecule-] 8 . agreement with the experimental value 10-9.84
*
*
Introduction The gas-phase reactions of chlorine atoms with organic species have received considerable interest due to the importance of such reactions a remmal processes for chlorine atoms and non-methane hydrocarbons in the atmo~phere.I-~In particular, the reaction (1) Chameides,
W. L.; Cicerone, R. J. J. Geophys. Res. 1978, 83, 947.
(2) Cronn, D.; Robinson, E.Geophys. Res. Lett. 1979, 6, 641.
of chlorine atoms with nitramines can be significant to the catalytic decomposition of nitramines in the combustion process of nitramhe propellant^.^ Such reactions are highly exothermic leading to HC1 and unstable radicals, which are consequently decomposed (3) Singh, H. B.; Kasting, J. F. J. Atmos. Chem. 1988, 7, 261. (4) Boggs, T. L. In Fundamentals of Solid Propellant Combustion; Summerfield, M., Kuo, K., Eds.; AIAA: New York, 1984. Chapter 3; p 121.
0022-3654/92/2096-1705$03.00/0 0 1992 American Chemical Society
Lazarou et al.
1706 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
through scission of the weaker N-N bond. The simplest nitramine is dimethylnitramine, and it is related to the cyclic nitramines RDX (hexahydro- 1,3,5-trinitro-1,3,5-triazine) and HMX (octahydro1,3,5,7-tetranitr0-1,3,5,7-tetrazocine); both of these are known to be powerful propellants. In present study we measured the rate constants for the exothermic reaction CI
+ (CH3)2NN02
+
HCl
+ CH2N(CHj)N02
PHO = -8 kcal/mol
over the temperature range 273-353 K using the very low pressure reactor (VLPR) t e c h n i q ~ e . ~This technique has been shown to produce reliable rate constants for a number of chlorine atom reactions, by working in the milliTorr pressure regime where complications from secondary reactions are not observedS6 Experimental Section The theory of the very low pressure reactor (VLPR) has been discussed in great detail by Benson et al.’ The main characteristics of the system are as follows: The bimolecular reaction is taking place in a Knudsen cell at steady-state pressures less than 5 mTorr. Both reactants are introduced into the reactor through two separate capillary inlets and are allowed to react for a short period of time. Consequently, both reactants and products are discharged through a small aperture in the reactor to the first stage of a differentially pumped system. Therefore, a continuous molecular flow is maintained leading to a collimated molecular beam that is sampled with a quadrupole mass spectrometer that is mounted in the second stage vacuum chamber. The molecular beam is modulated with a tuning fork chopper at the entrance of the second vacuum chamber, so that the weak mass spectrometric signal can be magnified almost 1000 times with a lock-in amplifier. Chlorine atoms were generated by flowing 5% C12 in helium (ultrahigh purity) through a quartz tube coated with phosphoric acid and enclosed in a 2.45-GHz microwave cavity operating at 30 W. The complete dissociation of C12 was checked by mass spectrometry. Our sensitivity to C12 was less than 1OIomolecules cm-) so that the ratio [C12]/[Cl] after discharge was zero. Dimethylnitramine (DMNA) was synthesized by nitration of the corresponding dialkylformamide* and was further purified by subsequent degassing. In order to avoid sticking on the surfaces, and assist in the uniform flow of DMNA since the vapor pressure of DMNA at 293 K is less than 500 mTorr, dimethylnitramine was diluted in helium (2% DMNA/He mixtures) and was introduced to the reactor. Flow rates of all gases were determined by following the pressure drop (measured on Validyne Model DP 15-30 and DP 15-28 transducers) in a known volume (700 cm3) as the gases flowed through a 1-mm X 20-cm capillary. The determinations of [Cl] and [DMNA] concentrations were done by measuring the actual intensities of the mass spectrum peaks Icl( m / z = 35) and I D M N A ( m l z = 90). Thus, accurate calibration curves of 1, versus [MI were obtained for both species. The uncertainty in I M measurements was *5%, and therefore the ratios R1 = [Cl],/[Cl] and R2 = [DMNA],/[DMNA] were determined with an accuracy of f7%. The reaction cell was mounted on a stainless steel flange containing a 5-mm aperture. The interior surfaces of the cell ( V = 103 cm3) were coated with halocarbon wax in order to inhibit wall recombination. The escape constant of the cell was measured by following the first-order decay curve (monitored by the mass spectrometer) for various gases after a fast halt of the flow. k,,, was found to be 2.88 ( T/M)1/2s-l, where T is the absolute temperature and M is the molecular weight. (5) Golden, D. M.; Spokes, G. N.; Benson, S . W. Angew. Chem., Inr. Ed. Engl. 1913, 12, 534. (6) Heneghan, S. P.; Knoot, P. A.; Benson, S. W. I n f . J . Chem. K i n e f . 1981, 13, 617. (7) Benson, S. W.; Baghal-Vayjooee, M. H.; Colussi, A. J. J . Am. Chem. SOC.1978, 100, 3214. ( 8 ) Robson, J. H. J . Am. Chem. SOC.1955, 77, 107.
The temperature of the reactor was held constant by circulating a thermostat4 liquid (water or ethanol) through an outer jacket surrounding the reactor. The temperature was controlled and monitored by a refrigerated bath circulator (Haake Model F3) with an accuracy f l K. The molecular beam was chopped at 200 Hz and monitored by a quadrupole mass spectrometer (Balzers QMG 5 11) with a cross-beam analyzer. The signal was further amplified with a lock-in amplifier ( N F Model 570). The electron energy of the ionizer was kept low at 19 eV, where the fragmentation of HC1 ( m / e 36) to C1+ ( m / e 35) was less than 1%. Therefore, the formation of HCl reaction product did not interfere with the monitoring of C1 atom concentration (mass peak m / z = 35). Our new VLPR system was initially tested by measuring the rate constant of the well-known reaction of C1 with CHI. The Arrhenius parameters obtained over the temperature range 263-303 K were log A = -10.96 f 0.25 cm3 molecule-l s-l and E , = 2.8 f 0.3 kcal/mol, which are in good agreement with previously reported value^.^-^ Results The mass spectrometric analysis of the reaction products reveals the formation of HC1 (mass peak m / e = 36), NO2 (mass peak m / e = 46), and CH3N=CHz (mass peak m / e = 43). Therefore, the chemical reaction under study is
+
2
C1 + (CH3)2NN02 HCl CH2N(CH3)NO2 (1) followed by the exothermic decomposition CH2N(CH3)N02
CH3N=CHz
+ NO2
AHD = -14 kcal/mol (2)
of the primary reaction product and the secondary reaction C1+ CH3N=CH2
A HCl + CH2”N%H2
AHo = -18 kcal/mol (3)
The steady-state concentration of dimethylnitramine molecules is given by the expression [DMNAIO kl IC11 =1+R2 = (1) [DMNA] ~ , D M N A where km,DMNA is the escape constant of dimethylnitramine and [DMNA], is the concentration of dimethylnitramine in the absence of chlorine atoms. Furthermore, the steady-state concentration of chlorine atoms is given by the expression (1 a)kl[DMNA] R l = - [Cllo -1+ (11) [ell kesc,Cl where [Cl], is the concentration of chlorine atoms in the absence of dimethylnitramine, a = k3[Cl]/(k3[C11+ kcsc,IM).and,k,,cl and keoc,lM are the escape constants of chlorine atoms and imine, respectively. The above expression was obtained by assuming the steady-state concentration of CH2N(CH3)N02radical, and its concentration is given by the relation k, [Cl] [DMNA] kl [Cl] [DMNA] [CH2N(CH3)N02] = N kcsc.R k2 k2 where R = CH2N(CH3)NOZ.The approximation k2 >> km,Ris valid since k,,,(303) = 5.31 s-I and kz = 10” s-l (D,(N-N) = 6 kcal/molI2 and A = 1015,5s-I l o ). Expression I can be written in the form (R2 - ~ ) R I ~ ~ , D=MkiN [Cllo A and a plot of (R, - l)Rlkesc,DMNA versus [Cl], should yield a
+
(9) DeMore, W. B.; Molina, M. J.; Watson, R. T.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; Ravishankara, A. R. Chemical Kinetics and Photochemical Data for Use in Stratospheric Modelling. JPL Publication 90- I ; Jet Propulsion Laboratory, California Institute of Technology: Pasadena, CA, 1990. (10) Benson, S. W. Thermochemical Kinerics, 2nd ed.; Wiley-Interscience: New York, 1976.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1707
Reaction of Chlorine Atoms with Dimethylnitramine
Y
1 -f
Y C
Y
rl
/' -
-2s
,
U
I
0
1
1 ~ 1 1 x
loi2
I
I
2
3
Figure 1. Plot of (R2- l)Rlksee,DMNA versus [Cl], at 323 K. Symbol size
reflects the propagated errors (2a). TABLE I: Measured Values of Rate Constant k, at Various Temperatures
T,K 213 303 323 353
ki (*2a),
lo-" cm3 molecule-' s-I 1.60i 0.30 1.86 i 0.54 2.35 f 0.52 2.57 f 1.06
kW,CI(R, -
1)
kl [ DMNA]
I
6
2.9
3.5
3.2
( K-i
(1000/T)
I 3.8
)
Figure 2. Arrhenius plot of In k , versus 1/T.Symbol size reflects the propagated errors (2a). H C',, ',170
no. of expts 10 10 10 10
straight line with a slope equal to k , and a zero intercept. Least-squares fits of the data yield straight lines that were constrained to go through the point (0,O). A typical plot at 323 K is shown in Figure 1 . Experiments were performed at four different temperatures, 273,303,323, and 353 K, and the rate constants kl obtained are presented in Table I. The precision of the k l rate constant measurements was about 30% (2u). Expression I1 can also be written as a=
2.8
( m o ~ e c u ~ e s / c m) 3
-1
and the quantity a is calculated for each experiment, using the previously calculated k , ; the values obtained were in the range 0 . 3 . Furthermore, from the definition of a the expression ~wc,IMRI ~a) / (=~ k,[Cl]o is derived, and therefore a plot of kW,lMRIa/(l - a)versus [Cl], should yield a straight line with a slope equal to k3. Due to the high uncertainty in the values of a (about 30%), the data are scattered and reliable values for the rate constant k3 could not be extracted. However, the value for k, at 303 K has been found to be approximately 1.5 X cm3 molecule-l s-l. An Arrhenius plot for kl is presented in Figure 2. Linear least-squares analysis of the kl temperature-dependence data yields the activation energy and the Arrhenius A factor for reaction 1 . E , = 1200 f 370 (2u) cal/mol A = 1.442,# X 1O-Io (2u) cm3 molecule-l s-]
Discussion The thermochemical kinetics version of conventional transition-state theory has been applied to reaction 1, assuming a transition-state geometry.1° The entropy change AS$for forming a mole of transition-state complex from the two reactants is given by hs* = SO(comp1ex) - So(DMNA) - SO(C1) = ASO(difference) - SO(C1) where ASo(difference) is the difference in entropies between transition-state and reactant DMNA and includes changes in translation, vibration, rotation, internal rotation, electronic, sym-
'0
Figure 3. Bent model of transition state for CI + (CH&NN02.
metry, and optical isomerism entropies. It is reasonable to assume that the reaction proceeds through a tight transition state in which the C-H and H-Cl bond distances are elongated by 0.4 AIo relative to the normal covalent lengths." The C-H-Cl angle may extend at different intervals depending on the approach of the C1 atom relative to the plane of DMNA molecule. Those angle intervals are determined by the van der Waals radii of the adjacent atoms. An approach of C1 on the DMNA plane from the side of the 0 will form a C-H-Cl angle (relative to the C-H bond and away from 0)that may vary from 150' to 140'. Another approach of C1 on the DMNA plane from the side of the other methyl radical will form an angle (away from methyl) that may vary from 140' to 188'. A third approach of C1 on a vertical plane relative to the DMNA plane will form an angle that may vary from 120' to 220'. Thus, the average transition-state geometry will have an angle Cl-H-C about 1 30°, as shown in Figure 3, and the detailed entropy calculations are presented in Table 11. The main contribution to the entropy difference is due to the one-dimensional internal rotation about the C-H bond, which was calculated from the equation
Sf= 4.6 + R In
(Zf'/2/u)
+ R/2 In (T/298)
where Z, is the moment of inertia of the rotor and u = 1 is the symmetry number. The internal rotations are treated as free rotations.' From the ASl(303) value (Table 11), the A factor for a bimolecular reaction can be calculated by using the expression A = 10-5.72(T/298)2exp(hsf/R)
The calculated values for A are 10-11.98 cm3 molecule-l s-I for the linear TS and 10-lo.oocm3 molecule-' s-' for the bent TS. The experimental value of A obtained by our Arrhenius plot (Figure 2) is 10-9.84 cm3 molecule-' S-I, which is in good agreement with ( 1 1 ) Politzer, P.; Sukumar, N.; Jayasuriya, K.; Ranganathan, S. J . Am. Chem. SOC.1988, 110, 3425.
J . Phys. Chem. 1992, 96, 1708-1718
1708
TABLE II: Estimation of the AS'(298) for Reaction 1 Using the Bent TS Model (Figure 3) ASo(diff), degrees of freedom eu translational: AS,,= 3 / 2 RIn ( M ' / M ) = 3 / 2 RIn (125/90) 1 .o rotational: ASr,, = '12RIn ((IAIBZc)*/IAIBZc)= 1/2R In (19.5) 3.0 1.4 electronic: ASel = R In (2s + 1) = R In 2 1.4 symmetry: ASo = R In ( u / u * ) = R In 2 internal rotations: about C-..H bond, Sf ( I , = 50.8 amu A2) 8.5 3.1 about C-N bond, ASf = I/2RIn (Ir*/Z,) about N-N bond, ASf = In (Ir*/Zr) 0.3 vibrational: v(C-H), 3000 cm-' v(C.-.H), 2100 cm-' rc vb(HeC*N). 1050 cm-I v ~ ( H - ' * ~ * N )750 , cm-' 0.3
in most reactions of C1 with oxygenated hydrocarbons having C-H bond strengths less than 95 kcal/mol there is no activation energy.13 Since our reaction has an activation energy of 1.2 kcal/mol, there is strong evidence that the C-H bond strength in DMNA is higher than 95 kcal/mol and most probably has a value between methane (104 f 1 kcal/mol) and ethane (98 f 1 kcal/mol).14 Thus, the estimated value of 95 kcal/mol obtained by theoretical calculation^'^ is rather low. The secondary reaction of C1 with CH3N=CH, has been found to have a rate constant of 1.5 X cm3 molecule-' s-I at 303 K, an order of magnitude lower than that of k l , and presented no complications to our experiments. The analogous reaction of C1 with CH3CH=CH2 has a much higher rate constant of 2.68 X cm3 molecule-' s-I at 295 K.I6 This is probably due to the fact that the imine molecule possesses a rather large negative charge around the nitrogen atom that repels the approaching C1 atom and reduces the reactive cross section. Finally, this fast reaction of C1 with dimethylnitramine can play a significant role in the stability of nitramines. Small concentrations of C1 atoms over nitramine samples will decompose them to NO2 and the corresponding imine. This chemical decomposition of nitramines has been also observed in the infrared multiphoton decomposition of dimethylnitramine in the presence of C12molecules. l
-
(2)Vb(HeC'H),
1450 Cm-'
-o
(2)V,.,(H"*c'H),
1000 cm-i
0.2
-
(2)Vb(C'**H''cl), 700 cm-'
0.6
--t
v(H-.Cl), 2100 cm-'
AS'(298) = 19.8 - 39.5 = -19.7 eu
0
total 19.8
the conventional transition-state theory (bent model). Therefore, our experimental findings suggest that the approach of C1 atoms relative to the C-H bond involves a bent structure. The nonlinear approach of C1 may be normal considering that the incoming C1, while approaching a methyl group, is attracted by all three H atoms and is repelled by the 0 and N atoms. Therefore, it is oriented in such a way (inclined to the C-H axis) that it can form a covalent bond with one H atom, compensating for the repulsion by the 0 and N atoms and the attraction by the other H atoms. The A factor obtained ( cm3 molecule-' s-') is very close to the value 10-9.76cm3 molecule-' s-I, reported for the reaction of C1 with dimethyl ether.12 Furthermore, it has been found that
Acknowledgment. Y.L. acknowledges the financial support by the Institute of Electronic Structure and Laser, F.O.R.T.H. Registry No. Chlorine (atomic), 22537-15-1; dimethylnitramine, 41 64-28-7. (13) Michael, J. V.; Nava, D. F.;Payne, W. A.; Stief, L. J. Chem. Phys.
Lett. 1981, 77, 110.
(14) Golden, D. M.; Benson, S. W. Chem. Reu. 1969, 69, 125. (15) Melius, C . F.; Binkley, J. S. Thermochemistry of the Decomposition of Nitramines in the Gas Phase. In Proceedings of rhe 21st Symposium (Inrernariond) on Combustion;The Combustion Institute: Pittsburgh, 1986; p 1953. (16) Wallington, T. J.; Skewes, L. M.; Siegl, W. 0. J . Phorochem. fhorobiol. A 1988, 45, 167. (17) Lazarou, Y. G.; Papagiannakopoulos, P. J . Phys. Chem. 1990, 94, 7114.
(12) Michael, J. V.; Nava, D. F.; Payne, W. A.; Stief, L. J. J . Chem. Phys. 1979, 70, 3652.
Kinetic and Mechanistic Study of X 4- ClOCl over the Temperature Range 240-373 K
-+
Products (X = Br, CI, F, 0, OH, N)
Philip S. Stevens*,+and James G. Anderson Department of Chemistry and Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138 (Received: May 8, 1991; In Final Form: October 24, 1991)
-
The rate constants for the reactions of X + ClOCl products for X = Br, CI, F, 0, OH, and N have been measured over the temperature range 230-400 K. The rate constants are (in units of cm3 molecule-l s-I) as follows: (2.1 f 0.2) X lo-" exp[(-425 f 3 0 ) / q for Br + CIOCI; (6.0 f 0.6) X lo-'' exp[(127 f 3 0 ) / a for C1+ CIOCI; (1.5 f 0.5) X 10-'0exp[(-47 f 94)/T] for F + ClOC1; (1.3 f 0.8) X lo-'' exp[(-510 f 3 0 ) / a for 0 + CIOCI; (1.7 f 0.8) X 10-12exp[(420f 170)/T] for N + ClOC1. The rate constants for X = Br, C1, F, and N are found to correlate for OH + CIOCI; and k298< 6 X with the electron affinity of the attacking radical, suggesting that the mechanism for these reactions involves the partial transfer of an electron from ClOCl to X, and the activation energy for reaction is determined by the ability of the transition state to accommodate the shift in electron density. This trend is similar to that found for a number of non-hydrogen abstraction C12),where the reactivity scales with the quantity IP(molecu1e) - EA(radical), where IP refers reactions (X + ClNO, 03, to the ionization potential and EA the electron affinity. The reactions of 0 and OH with ClOCl are significantly faster than predicted by the trend, suggesting that the electron-transfer mechanism is not the only driving force in these reactions, which may involve long-range attractive forces leading to stable intermediates. Introduction In recent years, the thermal rate constants for a number of bimolecular reactions have been shown to display trends with
observable properties of the reactants.'-3 By examining the nature of these observables, it was shown that the height of the potential
Present address: Department of Meteorology, The Pennsylvania State University, University Park, PA 16802.
(1) Abbatt, J. P. D.; Toohey, D. W.; Fenter, F. F.; Stevens, P. S.; Brune, W. H.; Anderson, J . G.J . fhys. Chem. 1989, 93, 1022.
0022-3654/92/2096-1708%03.00/0 0 1992 American Chemical Society
