91 IO
J . Am. Chem. Soc. 1990, 112, 91 10-91 15
for oscillation has even found its way into widely used textbooks of ohvsical c h e m i ~ t r v . ~It~ is imDortant to bear in mind that self-iihibition providcs another foim of feedback that can lead to chemical oscillation. While this is the first self-inhibitory chemical oscillator of which we are aware, it is likely that there (37) Atkins, p. W. Physical Chemistry, 3rd ed,; Freeman: N~~ york, 1986: p 728.
are many more oscillatory reactions based on self-inhibition that are waiting- to be discovered.
Acknowledgment. This work was supported by the National Science Foundation (Grant CHE-8800169) and by a U S Hungarian cooperative grant from the NSF and the Hungarian Acadkmy of Sc'iences. We thank Patrick De Kepper and Harry Swinney for useful discussions and for sharing their results with us prior to publication.
Role of Ion-Neutral Complexes during Acid-Catalyzed Dehydration of Ethanol in the Gas Phase Guy Bouchoux* and Yannik Hoppilliard Contribution from the Dgpartement de Chimie, Laboratoire des Mbcanismes Rbactionnels, Ecole Polytechnique, 91 128 Palaiseau Cedex. France. Received April 23, 1990. Revised Manuscript Received July 6, I990
Abstract: Acid-catalyzed dehydration of ethanol in the gas phase has been studied both theoretically and experimentally. Molcculnr orbital calculations have been done at the MP3/6-3 1 G*//6-3 IG* level with correction of the zero-point vibrational energy. Protonated ethanol, 1 is predicted to isomerize easily into the C2H4-.H-.0H2+ complex, 2 (activation energy 120
k.l/mol). This result is in agreement with the observation of a near statistical hydrogen exchange preceding the dehydration rcuction 1 --+ C2H4+ H 3 0 + . In the case of the water-solvated ion C2HSOH2.-OH2+,5, isomerization into a proton-bound coniplcx C2H4.-HSO2+,6, needs a larger amount of energy (ca. 180 kJ/mol). Again the calculations agree with experiments: thc important activation energy for the process 5 6 prevents hydrogen exchanges and ethene elimination. Extension of thcsc conclusions to higher systems is discussed.
-
I ,2 elimination reactions are of fundamental interest in organic chcmistry and arc ubiquitous processes in gas-phase chemistry of cationic or anionic species. For such reactions, some mechanistic proposals presently encountered in the literature involve ion-neutral complexes, i.e., species in which noncovalent interactions retain closc togcthet two entities able eventually to react unimolecularly or bimolecularly. as key intermediate~.I-~In the case of a protonatcd spccics, a suggcsted dissociation mechanism implies formation of an clusive proton-bound complex as summarized in Schcmc I . Starting from CH,CH2XH+, the direct C-X bond elongation leads to thc products C2H5' and XH. During this process, interaction between these two products may give rise to a protonbound intcrmcdiatc which, in turn, decomposes into C2H4 XH2+. Dchydratian of protonated alcohols is a common type of elimination reaction whose description may enter into this frame. The purpose of the present work is to test the validity of Scheme I in providing a reasonable view of the potential energy surface associatcd with dchydration of protonated alcohols in the gas phase. Therefore, investigations on the prototype systems C2HsOH2+ (C2HSOH2+.H 2 0 and C2H50H2+,C 2 H 5 0 H )have been done by means of molecular orbital calculations and comparison has been madc with mass spcctromctry experiments.
+
1. Dehydration of Protonated Ethanol In the gas-phase C2H50H2+,1, may be produced by protonation of ethanol under chemical ionization condition^^-'^,'^-^',^^ or by ( I ) McAdoo. D. J. Mass Spectrom. Rev. 1988. 7. 363.
(2) Bouchoux, G.Adc. Mass Spectrom. 1989,11, 812. (3) (a) Burgcrs. P. C.: Terlouw, J. K. In Specialist Periodical Reports: Mass Spectrometry; Rose. M. E., Ed.; The Royal Society of Chemistry: London, 1989: Vol. IO, Chapter 2. (b) Heinrich, N.; Schwarz, H. In Ion and Cluster Ion Spectroscopy and Strucfures: Maier, P., Ed.; Elsevier: Amsterdam. 1989: p 329. (c) Tuma, W.: Foster. R. F.: Brauman, J. 1. J . Am. Chem. SOC.1988. 110, 2714. (4) Colorimo, M.; Branacdleoni, E. Org. Mass Spectrom. 1982. 17, 286. ( 5 ) Jarrold. M. F.: Illics. A. J.; Kirchna, N. J.; Bowers, M. T. Org. Mass Spectrom. 19R3,18. 388.
0002-7863/90/ I5 12-9 I 10$02.50/0
t H
XH2'
_._.__ ....
CHI
rr
+
XH;
CHZ
condensation reaction between C2H5' ions (coming, for example, from C2H5Br)and a water molecule.8 (6) Dawson, P. H. Int. J. Mass Spectrom. Ion Phys. 1983,50, 287. (7) Jarrold, M. F.; Kirchner, N. J.: Lin, S.; Bowers, M. T. J . Phys. Chem. 1986,90, 78. (8) Harrison, A. G.Org. Mass Spectrom. 1987,22, 637. (9) Siek, L. W.; Abramson, F. P.; Futrell, J. H. J . Chem. Phys. 1966,45, 2859. (IO) (a) Beauchamp, J . L. J . Am. Chem. SOC.1969, 91, 5925. (b) Beauchamp, J. L.; Case, M. J . Am. Chem. SOC.1972,94. 2638. (c) Beauchamp, J . L.; Caserio. M.; McMahon, T. 8. J . Am. Chem. SOC.1974. 96, 6243. ( I I ) (a) Bohme, D. K.; Mackay, G. I. J . Am. Chem. Soc. 1981, 103,2173. (b) Collyer, S. M.;McMahon, T. B. J. Phys. Chem. 1983,87, 909. (12) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J . A. Ab Inifio Molecular Orbital Theory; John Wiley & Sons: New York, 1986. (13) Frisch, M. J.; Binkley, J. S.;Schlegel,, H. B.; Raghavachari, K.; Melius, C.F.; Martin, R. L.; Stewart, J. J . P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.: Seeger, R.; Whiteside, R. A.; Fox, D. J.; Fleuder, E. M.; Pople, J. A. Gaussian 8 6 Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984. (14) (a) Jones, W. H.: Mezey, P. G.: Csizmadia, I. G.J . Mol. Sfrucf. ( T H E O C H E M ) 1985, 121, 8 5 . (b) Cao, H. Z.: Allavena, M.: Tapia, 0.; Evleth. E. M. Chem. Phys. Lett. 1983,96, 458. (IS) Reiner. E. J.; Poirier, R. A.; Peterson, M. R.; Csizmadia, I . G.: Harrison, A . G.Can. J . Chem. 1986.64, 1652. (16) See, for example: Matsuura, K.; Nunome. K.; Toriyama, K.; Iwasaki, M. J. Phys. Chem. 1989.93, 149.
0 I990 American Chemical Society
Acid-Catalyzed Dehydration of Gas- Phase Ethanol
J . A m . Chem.Soc., Vol. 112, No. 25, 1990 9111
s) dissociate unimolecLong-lived ions I (lifetime of ca. ularly by elimination of an ethylene molecule following reaction a: C2H5OH2' H 3 0 + + C2H4 (a)
-
This fragmentation is accompanied by a very low kinetic energy relcase (To,,= 2.5 meV;' To,s= 3.2 meV, this work), suggesting no appreciable energy barrier for the reverse reaction. The temperature dependence of the reaction rate supports this c o n c l ~ s i o n . ~ ~ Partially deuterated species C2HSOHD+7and C2HSOD2+* give H2DO+/H30+(ratio of metastable peak height 25/75; statistical ratio 29/71) and HD20+/H2DO+/H30+(ratio of metastable peak height 12/43/45; statistical ratio 14/57/29), respectively. Our H results from protonated CH3CD20H and CD3CH20H give H D 2 0 / H 2 D O / H 3 0of 10/42/48 (statistical ratio 14/57/29) and H--O , . D 3 0 / H D , 0 / H 2 D O / H 3 0 of 4/25/46/25 (statistical ratio 3/ , '. ! '..,2.030 TS 34/51/1 I ) , rcspcctivcly. A substantial H / D scrambling is thus 3.352 / associatcd with proccss a when ions 1 of low internal energy are 112 'H samplcd. H.,,*, e3.'r/1.136 The CID (collision-induced dissociations) spectrum of 1 has collision energy. I n both been studied under low6q8 or H/cTiT-c expcrimcnts, it is observed that the loss of a water molecule 'HH (process b) competes with reaction a. Figure I . Optimized (6-31C*) structures of C 2 H 7 0 + ions. ClHjOH2" C2Hj' HZO (b) I
.
:
+
+
At vcry low collision cncrgies (less than 5 eV) H 3 0 + formation
(process a) dominates, in agreement with its lowest enthalpy requirement (see later). The increase in collision energy is followed by the increase of C2H5' ion abundance until reaching a stationary situation. At 8 kcV of collision energy the ratio of peak height The CID spectra of variously deuC2HS+/H30+attains 1 terated ions 1 obtained at IO eV collision energy demonstrate the following trends: for process a, a pronounced H / D exchange is observed but equilibration is not complete and H 3 0 + ions bring one hydrogen preferentially from the original methyl group; for proccss b. H / D cxchange is also important but less than for reaction a, a preference is observed for the direct C-0 bond cleavage. Information regarding the proton-bound complex CH2CH2.H30+, 2, is less abundant. The formation of a cluster ion like 2 has bccn proposed to compete with the proton-transfer reaction: C2H4 + H 3 0 + C2H5' + H2O." The cluster, produced by chemical ionization of a mixture of C2H4 and H 2 0 in a buffer bath of Ar, gives a CID spectrum very close to that of protonated ethanol.' This observation suggests partial loss of structural identity for 1 and 2 below their fragmentation threshold of lowest energy. In summary, and as suggested by the available experimental L
5
q
7
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(17) Smith. D.; Adams. N . G.:Henchman, M. J. J. Chem. Phys. 1980. 72, 4951, (18) Schwarz, H.; Stahl, D. In!. J . Mass Spectrom. /on Phys. 1980, 36, 285. (19) Kcbarlc, P. Annu. Rec. Phys. Chrm. 1977, 28, 445. (20) Hiraoka, K.; Takimoto. H.; Morise, S.K. J. Am. Chem. Soc. 1986, 108. 5683. (21) (a) Iraqi, M.: Lifshitz, C. Inf. J. Muss Specrrom. /on Phys. 1989,88. 45. (b) Morgan, S.; Keesee, R. G.; Castelman. A. W. Jr. J. Am, Chem. SOC. 1989, 1 1 1 , 3841. (22) Raghavachari, K.; Chandrasekhar, J.; Burnier, R.C. J. Am. Chem. Soc. 1984, 106, 3124. (23) McMahon, T. B.; Beauchamp, J. L. J . Phys. Chem. 1977,81, 593. (24) Graul, S . T.; Squires, R. R. I n f . J. Mass Specfrom. Ion Proc. 1987, 81. 183. (25) (a) Audicr, H. E.; Montciro, C.; Mourgues, P.; Robin, D. Commun. Mass Spectrom. 1989, 3. 84. (b) Audier, H. E.; Monteiro, C.: Robin, D. Org. Mass Specfrom. 1989. 24. 146. (c) Audier, H. E.; Monteiro, C.; Mourgues, P.: Robin, D.Org. Mass Specfrom. 1989, 24, 289. (26) Hiraoka, K.; Takimoto, H.; Yamabe, S. J. Phys. Chem. 1986, 90, 59 IO. (27) Mori, Y.; Kitayawa, T. Chem. Phys. Left. 1986, 128. 383, 389. (28) Morton, T. H. Tetrahedron 1982, 38, 3195. (29) Karpas, Z.; Mcot-Ner (Mautner), M. J. Phys. Chem. 1989, 93, 1859. (30)Sheldon, J. C.: Currie. G. J.; Bowie. J. H. J. Chem. Soc. Perkin Trans. 2 1986. 941. (31) Meol-Ncr (Mautner), M. Sieck, L. W. /nf. J. Mass Spectrom. /on Processes 1989, 92, 123. (32) Curtiss. L A.; Blonder. M. Chem. Rei-. 1988, 88, 827.
Table 1. Total Energies (Hartree) and Corrected Zero-Point Vibrational Energies (ZPVE, kJ/mol) of C2H70+Ions"
MP2/6-31G*// 6-31G*l/6-31G*b 6-31GIb ZPVE 1 C2HSO+H2(anti) -154.387 157 (0) -154.822406 (0) 234 2 C,H,.*.HOH2 -154.349409 (99) -154.791382 (81) 222 TS 1-2 -154.344915 ( 1 1 1 ) -154.768508 (142) 212 C2HS+(class.) -78.3 1 1 227 -78.542888 I52 CIHS+ (bridged) -78.309943 -78.551840 154 H 20 -76.010746 -76.195960 54 3 C2H5+ (b) + H2O -1 54.320689 ( 1 75) -1 54.747800 (196) 208 -78.284345 129 C2H4 -78.031 719 -76.473822 87 H3O+ ( C d -76.289 338 4 CZH4 + H10+ -154.321057 (174) -154.758167 (169) 216 'Optimized 6-31G* geometries. bNumbers in parentheses, relative energies (kJ/mol). species
Table II. Calculated and Experimental Relative Energies of C z H 7 0 t Ions (kJ/mol) species
AHexpO (AHfo)b 0 (507)
4e1,Cd:
1 C2H50HZ+
2 C2H4**.HOH2+ 3 CZHS' + H 2 0 4 CZH4 + H,O+ 1/2 TS
0 69 1 7OC
153 (660) 132 (643)
