5432
J. Phys. Chem. 1991,95, 5432-5434
Temperature Dqmdence of the afbbs Energy Orderlng of Isomers of C1202 Z d d k S M M * *and ~ Mllp UWk2 Max-Planck-Institut fur Chemie (Otto-Hahn-Institut), 0-6500 Mainz, Federal Republic of Germany (Received: September 13, 1990; In Final Form: February 20, 1991)
A computational evaluation of relative stabilitiesof ClOOCl (C2symmetry). ClClO2 (C,),and ClOClO (C,) isomers of CI202
in ideal gas phase has been carried out on the basis of recent quantumchemical data. In the low-temperature region the C2isomer represents a prevailing component of the equilibrium isomeric mixture. However, at high temperatures the relative stabilities of C2and C, (and finally of C2and C,) are interchanged. The relative stability approaching and interchanging is reflected in the overall thermodynamicfunctiom of the system and in their isomerism contributions, especiaUy in the temperature dependence (with a rather pronounced maximum) of the heat capacity terms. The results represent a good example of possible effects of enthalpy-entropy interplay in isomeric systems.
1. Introduction
TABLE I: k e y of the Relative Eaylctia (W/md) in the
Various mechanisms have been suggested for Antarctic ozone d e p l e t i ~ n ,among ~ ’ ~ them catalytic cycles with participation of the C10 radical, and also of its dimer C1202,this being an impetus to a deeper interest in the system. As, however, experimental characterization of the dimer has so far been incompkte or partial (for a survey of the observations, see ref 14) computational treatmentslcl6 represent a natural alternative for information enhancement. The computations revealed three local energy minima on the dimer potential hypersurface. However, the related species stabilities were treated in t e r m of the relative energy terms only. Thus, this report aims a t completing the computational studies14J5 with a proper thermodynamical relative stability treatment.
SystenP
2. Description of C1204 Isomeric System Following fragmentary experimental fdig~lQ”-~ on a possible CI2O2isomerism, the multiplicity presumption was employed in the potential-hypersurface stationary-point search.14J5 Three structures (all of them being nonplanar and singlet electronic states) were identified and characterized14J5 as local energy minima. Their energetics was evaluated by an isodesmic-reaction ( I ) Permanent and re rintsrder addreas: The J. Heyrovskj Institute of Phynical chmrlrtry and &ctroc-, C m Acedemy of Scienas, DoIe)&ova 3, CS-18223 Prague 8-Kobylisy, Czech and Slovak Federal Reaublic. ’ (2) Undergraduate fellow. Faculty of Science, Charles University, Prague 2, Czech and Slovak Federal Republic. (3) Callis, L. B.; Natarajan, M. J . Gcophys. Res. 1986, 91, 10771. (4) Solomon, S.;Garcia, R. R.; Rowland, F. S.;Wuebblu, D. J. Nature
1986.321.755. ( 5 ) McElroy, M.B.;Salawitch, R. J.; Wofsy, S.C.; Logan, J. A. Nature 1986,321, 759. (6) Tung, K. K.; KO,M. K. W.; Rodriguez, J. M.;Szc, N.D.Nature 1986, 322, 811. (7) Crutzcn, P.; Arnold, F. Nature 1986, 324, 651. (8) Stolanki, R. S.;Schoeberl, M.R. Geophys. Res. Lett. 1986,13, 1210. (9) Mahlman. J. D.; Fels, S . B. Geophys. Res. Lett. 1986, 13, 1316. (10) Molina, L. T.; Molina, M. J. J. Phys. Chem. 1987, 91, 433. (11) Molina, M. J.; Tso, T. L.; Molina, L. T.: Wang, F. C. Y . Science 1987,238, 1253. (12) Barrett, J. W.; Solomon, P. M.;de Zafra, R. L.; Jaramillo, M.; Emmons, L.; Parrish, A. Nature 1988,336,455. (1 3) Watson, R. T. In &one Depletion, Health and Envfronmental Consequences; Jones. R. R., Wigley, T., ma.;Wiley: Chichuter, U.K., 1989. (14) McGrath, M.P.; Clemitshaw, K. C.; Rowland, F. S.;Hehre, W. J. J . Phys. Chem. 1990,94,6126. (15) McGrath, M. P.; Clemitshaw, K. C.; Rowland, F. S.;Hehre, W. J. Gcophys. Res. Lett. 1988, 15, 883. (16) Jsnren, F.; Oddenhede, J. J. Phys. Chem. 1990, 94, 2235. (17) Rochkind, M.M.; Pimentel, G. C. J . Chem. Phys. 1967, 46,4481. (18) Alcock, W. G.: Pimentel, G. C. 1. Chem. Phys. 1968, 48, 2373. (19) Andrews, L.; Raymond, J. 1. J. Chem. Phys. 1971,55, 3087. (20) Chi, F. K.; Andrews, L. J. Phys. Chem. 1973, 77, 3062. (21) Bhatia, S.C.; Hall Jr., J. H.Inorg. Chem. 1981, 20, 629. (22) Loupoc, R. C.; Potier, J. J. Chfm.Phys. 1983, 80, 449. (23) Cox, R. A.; Hayman, G. D. Nature 1988, 332,796.
isomer (symmetry) CloOcI (C,) ClC102 (C,) ClOClO (C,)
AEP
W a f
0.0
0.0 5.02
0.24 3 1.69
OAccording to ref 14; related to chlorine peroxide CIOOCI. Potential-energy difference with respect to the CloOcI structure. eGround-state energy difference with respect to the CIOOCl structure (i.e., AE,corrected for zero-point vibrations). refinement of the data computed in the highest energy approximation. Chlorine peroxide, ClOOCl (C2 point group of symmetry), was found to be the lowest species in the ground-state energy scale (Le., in thermochemical terms, the zero-temperature enthalpy scale). The second isomer, hypervalent chloryl chloride ClC102 (C, symmetry), lies in the scale quite close to the peroxide structure. The third species is unsymmetrical (Cl), hypervalent chlorine chlorite, ClOClO. The relative ground-state energies are surveyed in Table I (the corresponding potential energy terms are included for comparison). The energetics together with 63 1G*/MP2 qualityi4J5 geometries and harmonic vibrational frequencies represent an input for the relative-stability treatment.
3. Temperature Isomeric Interplay A simple way of incorporating temperature effects into relative stability reasoning consists in application of the so-called simple Boltzmann factors2e26 exP[-Md (RT)I W,’
=
(1)
/L 1P [ - A E , / ( R n I
where AE,are the potential energy terms belonging to the individual isomers in their n-membered set, related to a common reference energy level (e.g., the energy of one of the isomers chosen as the reference structure). However, these Boltzmann factors prevent an interchange in isomer stabilit order with changing temperature. A correct approach considers5; rotational-vibrational motions of the individual isomers by means of their partition functions q,, yielding the weight factors Q, e x P [ - m O J / ( R n l w, = (2) exp[-~o,/(RT)I
5%
I-
I
where Mood denotes the ground-state energy terms related to one of the isomers as a reference structure. We shall follow a (24) Slanina, Z. Int. J. Quantum Chem. 1919, 16, 79. (25) Slanina, Z. Thermochfm.Acta 1984, 78, 47. (26) Slanina, Z. Contemporary Theory of Chemical Isomerism; Academia: Prague, 1986.
0022-3654191I2Q95-5432SQ2.5QlQ , Q 1991 American Chemical Society I
,
30.96
The Journal of Physical Chemistry, Vol. 95, No. 14, 1991 5433
Gibbs Energy Ordering of Isomers of ClZOz
TABLE Ik CluractcriPtiOnof Sow Db-
Poirb. d tk ClA
IntcrplrY
ic I?
-------
‘
