6484
J. Phys. Chem. 1987, 91, 6484-6490
Protonation of Three-Membered Ring Heterocycles. An ab Initio Molecular Orbital Study 0. M6, J. L. G. de Paz, and M. Yiiiiez* Departamento de Qujmica, Facultad de Ciencias, C-XIK Universidad Autdnoma de Madrid, Cantoblanco, 28049-Madrid, Spain (Received: April 8, 1987)
The protonation of three-membered ring heterocycles was studied by using basis sets ranging from minimal to split valence plus polarization quality. The structures of neutral and protonated species are analyzed. Split-valence bases predict very loosely bound structures for sulfur-containing compounds. As a consequence, their basicities are underestimated at this level of accuracy. This failure of split-valence bases is corrected when polarization functions on the heavy atoms are included. Geometrical changes upon protonation are explained in terms of interactions between the MO's of the base and the orbitals of the incoming hydrogen, although antiaromatic systems behave as quite rigid systems. Theoretical results adequately reproduce the experimental sequence of basicities. We have found that an adequate descriptionof the protonated forms of sulfur-containing compounds requires an appropriate polarization of the valence shell of sulfur. The antiaromatic systems azirene, oxirene, and thiirene are predicted to be more basic than azirane, oxirane, and thiirane, respectively. Correlation energy contributions at second order (MP2) lead to a decrease of absolute protonation energies whereas third-order corrections yield a slight increase. Relative protonation energies are not too much affected by correlation corrections.
1. Introduction
The chemistry of small ring compounds has been, for many years, the focus of scientific attention from both the experimental and the theoretical point of view. In particular three-membered ring heterocycles have been heavily investigated during the past 10 years' because some of them are typical "antiaromatic" systems2 and have been proposed as intermediates in a variety of reactions. It seems now well established, for instance, that oxirene is an intermediate in the Wolff rearrangement3d of a-diazoketenes, that thiirene is the primary adduct in the S('D) acetylene or that azirene appears in the thermolysis of benzotriazoles9 and is the intermediate in the ring contraction reaction of phenylnitrenes.1° Moreover, although oxirene and azirene have, a t present, only a transient existence, they may exist as stable species in interstellar space, while thiirene has been recently prepared and most of its chemical properties investigated."J2 The corresponding saturated counterparts, azirane, oxirane, and thiirane, are stable compounds and well characterized and also present a very versatile ~ h e m i s t r y . ' ~ All this has stimulated considerable theoretical work on these systems, mainly devoted to the study of the structure and relative stability of their different isomeric However, it is surprising to realize that the
+
(1) Torres, M.;Lown, E. M.; Gunning, H. E.; Strausz, 0. P. Pure Appl. Chem. 1980,52, 1623. (2) Breslow, R.Pure Appl. Chem. 1971,28, 111; Arc. Chem. Res. 1973, 6, 393. (3) Jones, Jr., M.; Ando, W. J. Am. Chem. SOC.1968, 90, 2200. (4) Csizmadia, I. G.; Font, J.; Strausz, 0. P. J. Am. Chem. SOC.1968, 90, 7360. (5) (6) 11; J. (7)
DoMinh, Th.; Strausz, 0. P. J. Am. Chem. Sor. 1970, 92, 1766. M a t h , S. A.; Sammes, P. G. J. Chem. SOC.,Chem. Commun. 1968, Chem. SOC.,Perkin Trans. I 1972, 2623; 1973, 2851. Strausz, 0. P.; Font, J.; Dedio, E. C.; Kebarle, P.; Gunning, H. E. J. Am. Chem. Soc. 1967,89, 4805. (8) Strausz, 0. P. Pure Appl. Chem. 1971, 4, 165. (9) Gilchrist, T. L.; Gymer, G.; Rees, C. W. Pure Appl. Chem. 1971, 2, 275; J. Chem. Sor., Perkin Trans. I 1975, 1. (IO) Th€taz, C.; Wentrup, C. J. .Am. Chem. SOC.1976.98, 1258. (11) Krantz, A.; Laureni, J. J. Am. Chem. Sor. 1977, 99, 4842. (12) Torres, M.; Clement, A.; Bertie, J. E.; Gunning, H.E.; Strausz, 0. P. J. Org. Chem. 1978, 43, 2490. (1 3) See for instance: Comprehensive Heterocyclic Chemistry; Katritzky, A. R., Rees, C. W., a s . ; Pergamon: New York, 1984; Vol. 7. (14) Bonaccorsi, R.;Petrongolo, C.; Scrocco, E.; Tomasi, J. Jerusalem Symp. Quantum. Chem. Biochem. 1970, 2, 181. (15) Clark, T. Jerusalem Symp. Quantum Chem. Biochem. 1970,2,238. (16) Stohrer, U. D.; Hoffmann, R.Angew. Chem. hi.Ed. Engl. 1972, 11, 825. (17) Rohmer, M. M.; Roos, B. J. Am. Chem. Sor. 1975, 97, 2025
0022-3654/87/2091-6484$01.50/0
studies on the protonation of these compounds are rather scarce14*19*22 even though protonation of this kind of systems possesses some challenging problems as to knowing which electronic and structural changes take place upon protonation, and whether these changes affect ring strain or strengthen the C-C bond.16 Furthermore, these systems are well-suited benchmark cases where to test the accuracy of theoretical calculations, because their small size allows the use of rather extensive basis sets and to go beyond the Hartree-Fock level. This would permit to have a more realistic idea of the effects that the usual restrictions imposed on the theoretical treatments employed to investigate much larger molecules have on the magnitude of the protonation energies calculated. In fact, it has been proved in a systematic study of azolesz7 and azinesZ8that not only absolute protonation energies but also relative ones depend on the quality of the basis set used to expand the corresponding wave functions. It has also been foundz7that the inclusion of polarization functions in the basis leads to a decrease in absolute protonation energies which depends on the degree of anisotropy of the charge distribution at the basic center. Unfortunately, the size of organic bases such as azoles or azines prevents, for instance, rigorous investigation of electron correlation effects, or even structural changes induced by polarization effects. We aim in this paper at presenting a systematic study of the protonation of some significant threemembered ring heterocycles: azirane, azirene, oxirane, oxirene, thiirane, and thiirene. Although for some of the neutrals highly accurate calculations there is an almost have been reported in the literature,17~18~23~2s~26 (18) Strausz, 0. P.; Gosavi, R.K.; Denes, A. $3.; Csizmadia, I. G. J. Am. Chem. SOC.1976, 98, 4784. (19) Catalln, J.; Ylfiez, M. J . Am. Chem. SOC.1978, 100, 1398. (20) Strausz, 0. P.; Gosavi, R.K.; Bernardi, F.; Mezey, P. G.; Goddard, J. D.; Csizmadia, I. G. Chem. Phys. Letr. 1978, 53, 211. (21) Hess, Jr., B. A,; Schaad, L. J.; Ewig, C. S. J. Am. Chem. SOC.1980,
102, 2507. (22) Aue, D. H.; Webb, H. M. Davidson, W. R.; Vidal, M.; Bowers, M.
T.; Foldwhite, H.; Vertal, L. E.; Douglas, J. E.; Kollman, P. A.; Kenyon, G. L. J. Am. Chem. SOC.1980, 102, 5151. (23) Tanaka, K.; Yoshimine, M. J. Am. Chem. SOC.1980, 102, 7655. (24) Gosavi, R. K.; Strausz, 0. P. Can. J. Chem. 1983, 61, 2596. (25) Cirsky, P.; Hess, Jr., B. A,; Schaad, L. J. J . Am. Chem. Sor. 1983, 105, 396. (26) Siegbahn, P. E. M.; Yoshimine, M.; Pacansky, J. J . Chem. Phys. 1983, 78, 1384. (27) Mb, 0.;de Paz, J. L. G.; Yifiez, M. J. Phys. Chem. 1986,90, 5597. (28) Mb, 0.; de Paz, J. L. G.; Yifiez, M. J. Mol. Struct. (THEOCHEM) 1987, 150, 135.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 26, 1987 6485
Protonation of Three-Membered Ring Heterocycles
TABLE I: 6-316** Optimized Geometries for Three-Membered Ring Heterocycles and Their Protonated Forms
Neutral Species oxirane Bond Lengths, A
azirane
azirene
c-x c-c
1.448 (1.475)" 1.472 (1.481) 1.078 (1.084) 1.000 (1.016)
1.490 (1.523)b 1.256 (1.263) 1.064 (1.056) 1.009 (1.009)
1.402 (1.431)' 1.453 (1.466) 1.078 (1.085)
cxc xcc
61.0 (60.3) 59.5 (59.9) 115.2 118.6 114.4 (115.7) 64.6(67.5) 1.92 (1.89)a
49.9 (48.8) 65.1 (65.6) 137.6 (138.6)
62.4 (61.6) 58.8 (59.2) 115.3
C-H X-H
XCH XCH' HCH Yg P, D
c-c
C-H X-H
cxc xcc
XCH XCH' HCH Y8
thiirane
thiirene
1.467 (1.465)'' 1.245 (1.250) 1.062(1.065)
1.811 (1.815)e 1.472 (1.484) 1.075 (1.083)
1.840 (1.830)' 1.256 (1.258) 1.065 (1.071)
50.2 (50.6) 64.9 (64.7) 132.5 (132.7)
48.0 (48.3) 66.0 (65.9) 115.2
39.9 (40.2) 70.0 (69.9) 137.0 (140.7)
Bond Angles, deg
aziraneH'
c-x
oxirene
115.2(116.6) 69.2 2.29
2.27 (1.89)'
azireneH'
1.488 1.459 1.073 1.003
1.485 1.255 1.068 1.005
58.7 60.6 113.6
49.0 65.0 134.0
116.3 56.6
65.6
114.9 (1 15.8) 2.81
Protonated Species oxiraneH' Bond Lengths, A 1.496 1.446 1.072 0.956
2.32 (l.84)c
oxireneH'
thiiraneH'
2.65
thiireneH'
1.521 1.245 1.068 0.959
1.866 1.450 1.076 1.328
1.828 1.256 1.070 1.335
48.3 65.8 127.1
45.7 67.2 110.2 112.7 116.2 80.8
40.2 69.9 133.2
Bond Angles, deg 57.8 61.1 109.3 112.8 117.4 59.4
64.8
60.0
'Experimental values taken from ref 38. b4-31Goptimized values taken from ref 25. 'Experimental vajues taken from ref 39. dDZ+Poptimized values taken from ref 23. eExperimentalvalues taken from ref 39.40. fDZ+P optimized values taken from ref 26. gy is the angle between the X-H bond and the plane of the ring. complete lack of data on their protonated forms.14J9 We shall pay special attention to investigate the effects of systematically enlarging the basis set from a minimal to a polarized-split-valence quality on the structure of all species involved (section 3) and on their gas-phase basicities (section 4). Those compounds where sulfur is the heteroatom will deserve a particular consideration since, as has been shown b e f ~ r e , * polarization ~-~~ effects play a special role in sulfur-containing compounds. At the end of section 4 we shall discuss the correlation effects on both absolute and relative protonation energies, computed by Merller-Plesset perturbation theory at second (MP2) and third (MP3) ~ r d e r , ~respectively. "~~ Since the photoelectron spectra of these compounds are very interesting tools in the analysis of a number of chemical properties, especially of their gas-phase basicities, we shall present in section 5 the corresponding orbital energies for all compounds included in this study. We shall show that, in some cases, the quality of the basis influences the assignment of bands of the corresponding spectra, namely when they are very close in energy.
2. Computational Details The geometries of the three-membered ring heterocycles included in this study (1-6 in Figure 1) and their corresponding protonated forms (7-12 in Figure 1) have been fully optimized (29)Stramberg, A.; Wahlgren, U.; Petterson, L.; Siegbahn, P. E. Chem. Phys. 1984, 89, 323. (30) Magnusson, E. J . Comput. Chem. 1984, 6, 612. (31) Cruickshank, D.W. J.; Eisenstein, M. J . Mol. Struct. 1985,130, 143. (32) Honda, M.; Tajima, M. J . Mol. Struct. (THEOCHEM) 1986, 136, Q?
