Semiempirical and ab initio molecular orbital studies on the

Ab Initio Study of Intramolecular Ring Cyclization of Protonated and BF3-Coordinated trans- and cis-4,5-Epoxyhexan-1-ol ... Linda Wong , George R. Pac...
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J . Phys. Chem. 1990, 94, 8161-8168

8161

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Semiempirical and ab Initio Molecular Orbital Studies on the Mechanism of the Acid-Catalyzed Isomerization Benzene Oxide Phenol. 1. Structure of Protonated Benzene Oxidet Philip George,*,$Charles W. Bock,l and Jenny P. Glusker*,s Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104; The Institute f o r Cancer Research, The Fox Chase Cancer Center, Philadelphia, Pennsylvania 191 I I ; Department of Chemistry, Philadelphia College of Textiles and Science, Philadelphia, Pennsylvania 191 44; and American Research Institute, Marcus Hook, Pennsyloania 19061 (Received: February 8, 1990; In Final Form: May 7 , 1990)

-

The relative stabilities of 0-protonated benzene oxide and its related carbonium ions were investigated in order to obtain further insight into the mechanism of the acid-catalyzed isomerization benzene oxide phenol by using AM1 and the 6-31G and 6-3 1 G* basis sets with full geometry optimization, together with single-point energy determinations MP2/6-3 1G* and MP3/6-3 IG* at the RHF/6-31G* geometry. At all levels of computation, three carbonium ions, with a p-quinonoid disposition of the formal double bonds-cis, gauche, and trans with respect to the H-H internuclear distance in the CHOH group-were found to be almost equal in energy and significantly more stable than 0-protonated benzene oxide by 12-14 kcal mol-’. The bond lengths, and the distribution of total atomic charge calculated by using Mulliken population analysis, indicate extensive electron delocalization in the carbonium ions. Frequency analyses and calculation of the potential energy curve for the rotation of the carbonium ion H-0 bond about the C-0 bond show that the gauche and trans rotamers are stable intermediates. But the 0-protonated oxide and the cis rotamer of the carbonium ion are transition states-the former for the interconversion of the P form of the gauche rotamer (with the HO group on C , ) and the M form of the gauche rotamer (with the H O group on the adjacent carbon atom, C,) and the latter for the interconversion of the M and P forms of each gauche rotamer. These results suggest that the 0-protonated oxide is not on the main reaction pathway of the conversion of benzene oxide to phenol but that p-quinonoid carbonium ions are formed directly upon protonation. The rate-determining step in the production of phenol is the subsequent conversion of these carbonium ions to an 0-protonated ketone (C-protonated phenol under another name).

Introduction The formation of an epoxide is the initial step in the major metabolic pathway for the oxidation of benzene, benzene derivatives, and polycyclic aromatic hydrocarbons (PAHs). As a result of this oxidation, the ring to which the oxygen is bonded no longer exhibits aromatic character.’S2 Subsequent isomerization gives a keto intermediate (the NIH shift), followed by a second isomerization which gives a phenol, thereby restoring aromaticity’ (Chart I). Further reactions of the phenol yield metabolic end products such as quinones, glucuronic and sulfate esters, and mercapturic acids.* The production of the phenol occurs spontaneously, and kinetic studies have shown both a pH-independent and an acid-catalyzed process to be In the widely accepted mechanism for the acid-catalyzed isomerization of benzene oxide, protonation of the epoxide oxygen atom is assumed to occur first giving a stable structure in which the epoxide ring is still intact (I, Chart 11). This 0-protonated benzene oxide is analogous to 0-protonated oxirane (11), which molecular orbital (MO) calculations have shown to be a minimum on the potential energy surface and to be the most stable form of protonated ethylene oxide.9 Ring opening to give the carbonium ion (111) is presumed to be the rate-determining step. In a subsequent reaction, migration of a hydrogen atom from C , to C, accounts for the N I H shift and results in the formation of a protonated keto intermediate (V). Proton loss from the carbonium ion or from this protonated keto structure has been considered for the eventual production of the phenoL4 In a study of the reaction sequence for benzene using the semiempirical M I N D 0 / 3 method,1° Ferrell and Loew” found, as one might have expected, the 0-protonated benzene oxide structure, I, to be slightly more stable than a carbonium ion, but only by 0.8 kcal However, in studies of the corresponding and Ford and SmithI3 reactions of ethylene oxide, Nobes et ‘A preliminary account of this work was presented at the 13th Austin Symposium on Molecular Structure, University of Texas at Austin, Austin, TX 78721. Abstract S4, p 90, March 12-14, 1990. f University of Pennsylvania. $The Fox Chase Cancer Center. Philadelphia College of Textiles and Science.

CHART I

hy&ocarbon

phenol

ketone

found that MND0I4 and a b initio calculations using a minimal or a small basis set, e.g., RHF/STO-~CI//RHF/STO-~G, (1) Daly, J . W.; Jerina, D. M.; Witkop, B. Experientio 1972, 28, I 129-1264 and references therein. (2) Harvey, R. G. Arc. Chem. Res. 1981, 14, 218-226 and references therein. (3) Kasperek, G. J.; Bruice, T. C. J . Am. Chem. SOC.1972, 94, 198-202. (4) Kasperek, G. J.; Bruice, T. C.; Yagi, H.; Kaubisch, N.; Jerina, D. M. J . Am. Chem. SOC.1972, 94, 7876-7882. ( 5 ) Kasperek, G. J.; Bruice, P. Y.; Bruice, T. C.; Yagi, H.; Jerina, D. M. J. Am. Chem. SOC.1973, 95, 6041-6046. (6) Keller, J . W.; Heidelberger, C. J . Am. Chem. SOC. 1976, 98, 2328-2336. (7) Bruice, P. Y.; Bruice, T. C.; Dansette, P. M.; Selander, H. G.; Yagi, H.; Jerina, D. M. J . Am. Chem. SOC.1976, 98, 2965-2973. (8) Bruice, T. C.; Bruice, P. Y . Arc. Chem. Res. 1976, 9, 378-384. (9) Nobes, R. H.; Rodwell, W. R.; Bouma, W. J.; Radom, L. J . Am. Chem. SOC.1981, 103, 1913-1922. (10) Bingham, R. C.; Dewar, M. J . S.; Lo, D. H. J . Am. Chem. SOC.1975, 97, 1285-1 293. (11) Ferrell, J. E., Jr.; Loew, G. H. J . Am. Chem. SOC.1979, 101, 1385-1 388. (12) Ferrell and Loew” depict the structure of the carbonium ion with double bonds in “0-quinonoid” positions (111) although it is clear from the geometry given in Table 3 of their supplementary material that the shorter bonds are in “p-quinonoid” positions-a result which we have confirmed.

0022-3654/90/2094-8 161%02.50/0 0 1990 American Chemical Society

8162 The Journal of Physical Chemistry, Vol. 94, No. 21, I990 CHART I1

George et al. A

-+0’ H +;H *5

H. ,O-H

111

I

H

7-

7

-

IV

O7 HI

Figure 1. ( A ) 0-protonated benzene oxide with the numbering system used to identify the bond lengths and angles. (B) Cross section of the structure in the plane of symmetry passing through the H7-07 group and the midpoints of the lines joining C, and C 2 and C, and C5.

V

VI

split-valence basis set, e.g., RHF/3-2lG//RHF/3-2lG, give stability relationships that are vastly different from those obtained if more extended basis sets, including polarization functions, are employed, e.g., RHF/6-31G**//RHF/4-3IG and RHF/631G*//RHF/6-31G*, or if the effects of electron correlation are taken into account, e.g., MP2/6-3lG**//RHF/4-3lG and MP2/6-31G**//RHF/6-31G.1s In particular, the 2-hydroxyethyl cation (IV) (the structure analogous to the carbonium ion ( I l l ) ) is a genuine minimum on the MNDO,]) STO-3G,93-21G,I3 and 4-31G9 potential surfaces, the force constant matrix having all positive eigenvalues.16 However, at the higher levels of computation, it lies many kcal mol-] above the I-hydroxyethyl cation (VI) (0-protonated acetaldehyde, the analogue of the 0-protonated keto intermediate (V)),9313and the force constant matrix has a single negative eigenvalue showing IV is the transition state for the conversion of I1 into V I . ’ 3 I n order to gain a better understanding of the mechanism of the acid-catalyzed benzene oxidation reaction in which phenol is produced, it is therefore essential to determine the exact nature, at higher levels of calculation, of the relationship between 0protonated benzene oxide and the carbonium ion. In this paper we report results of optimizations using the 6-31G” and 6-31G*I8 basis sets, with single-point energy determinations taking electron correlation into account,19 MP2/6-3lG*//RHF/6-31G* and MP3/6-3lG*//RHF/6-31G*. Parallel calculations using the semiempirical AM1 modelZohave also been carried out to see how well the results agree with those obtained a b initio, since for larger molecules, and especially PAHs of carcinogenic interest, full a b (13) Ford,G. P.; Smith, C. T. J . Am. Chem. Soc. 1987,109, 1325-1331. (14) Dewar. M. J . S.: Thiel, W. J . Am. Chem. Soc. 1977.99.4899-4907. ( 1 5 ) These notations indicate respectively the level at which the total molecular energy was computed and the optimized geometry employed. See ref 9 and 13 for details of these calculations. (16) (a) Mclver, J. W . , Jr.; Kormornicki, A. J . Am. Chem. Soc. 1972, 94, 2625-2633. (b) Pople, J. A.; Krishnan, R.; Schlegel, H. B.; Binkley, J. S. Inr J . Quantum Chem., Quanrum Chem. Symp. 1979, 13,225-241, (c) Schlegel, H.B. In New Theoretical Concepts for Understanding Organic Reactions; Bertrln, J., Csizmadia. I. G., Eds., Kluwer Academic Publishers: Dordrecht, 1989; pp 33-53 (17) Hehre, W J.; Ditchfield, R.; Pople. J. A. J . Chem. Phys. 1972, 56, 2257-2261, (18) Hariharan, P. C.; Pople, J. A . Theor. Chim. Acta 1973, 28, 213-222. (19) (a) Mdler, C.; Plesset, M. S. Phys. Reo. 1934, 46, 618-622. (b) Binkley, J. S.; Pople, J. A. Int. J . Quantum Chem. 1975, 9, 229-236. (c) Pople, J. A.; Binkley, J. S.; Seeger, R. Int. J . Quantum Chem. 1976, IOS, 1-19. (20) Dewar, M. J. S.; Zoebisch, E. G.: Healy, E. F.; Stewart, J. J. P. J . Am. Chem. Soc. 1985, 107, 3902-3909.

c iH7) vans Figure 2. (A) Carbonium ion structure for protonated benzene oxide with the numbering system used to identify the bond lengths and angles. (B) Cross section of the structure in the trans and cis rotamers in the plane of symmetry passing through H7, 07,H I , C , , C,, and H,.

H1

Hy7

P -gauche

cis

H7B

dH7 H1

6 H7

M -gauche

trans

Figure 3. View down the chiral axis 07-CIin the p-quinonoid carbonium ion structures. The two enantiomeric forms of the gauche structure are characterized as P-synclinal and M-synclinal, respectively, in accord with the clockwise twist needed to make 07-H7 eclipse Cl-H, in the case of the gauche structure at the left and the anticlockwise twist needed in the case of the gauche structure at the right.*’ For brevity these two structures will be referred to as P-gauche and M-gauche.

initio studies, are not yet practicable. The major finding from these calculations is that 0-protonated benzene oxide, C,symmetry (see Figure I A ) , is a transition state on the potential energy surface, rather than a local minimum. In addition, relaxation of the symmetry constraint leads directly to a carbonium ion with asymmetric p-quinonoid-type bonding, Le.,

Structure of Protonated Benzene Oxide

The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8163

TABLE I: AH, Values (kcal mol-') Calculated Using the AM1 Model, Total Molecular Energies, ET(in au), Calculated Using the 6-31C and 6-31C* Basis Sets,a and the Energies of the Cis and Trans Rotamers of the Carbonium Ion Relative to That for the Gauche Rotamer (in kcal mol-') ~~~

0-protonated benzene oxide p-quinonoid-type carbonium ion cis gauche trans gauche cis gauche trans

--

a

AMI

RHF/6-31G// RHF/6-31G

RHF/6-3 1G*// R H F / 6 - 3 1G *

MP2/6-31G*// RHF/6-3 1G*

MP3/6-3 I G * / / RHF/6-31G*

+208.4

-305.686 87

-305.81 2 44

-306.735 89

-306.77028

-305.728 17 -305.728 23' -305.726 93 +o. 1 +0.8

-305.848 03 -305.848 14' -305.847 55 +o. 1 +0.4

-306.755 29 -306.755 66 -306.756 29' +0.2 -0.4

-306.792 44 -306.792 71 -306.792 96' +0.2 -0.2

+ 174.8 + 174.5 +173.8' +0.3 -0.7

For the a b initio calculations the listing gives the respective levels: energy computation//geometry employed.

the shorter bonds are located at opposite sides of the ring as in p-benzoquinone, and not in a conjugated sequence, -C=CC=C-, as in &benzoquinone. To help understand why a gauche structure is favored, further exploration of the potential energy surface has been carried out, including the investigation of cis and trans structures, C, symmetry (see Figures 2 and 3).2' Energy values and frequency analyses for the 0-protonated oxide and the three carbonium ion structures are presented first, followed by a study of the potential energy curve for rotation of the H7-07 group about C i a 7 in the carbonium ion. Geometrical parameters are then listed, together with the salient features of the distribution of total atomic charge calculated by using Mulliken population analysis.z2 In the case of the carbonium ion structures these data provide clear evidence for significant electron delocalization. Finally, the mechanism of the acid-catalyzed reaction is discussed in the light of these results-recognizing that the conclusions apply specifically to what would be the gas-phase reaction and that, for substituted benzenes and PAHs, the relationships between the carbonium ions and the 0-protonated oxide may be different. In the second part of this study the results of similar calculations on C-protonated phenol will be reported, the C2-protonated derivative being the 0-protonated ketone under another name.

'Lowest energy.

+10

0 6AHf

&ET -10

kcalmd

-20

-30

-40

I AM 1

I RHF16-31G'// RHF16-310'

I MP3E-31 W// RHF16-31G'

Computational Methods

The molecular orbital calculations were performed with the program developed by Pople and his c o k a g ~ e on s~~ VAX 1 1 /780and Cray XMP computers. All structures were fully optimized at the semiempirical RHF/6-3IG,I7 and RHF/6-3 IG*'* levels of computation. Electron correlation was included in the a b initio calculations by using Moiler-Plesset perturbation theoryI9 at the MP2, MP3/6-31G*//RHF/6-31G* (frozen core) levels. Vibrational frequencies were obtained from analytical second derivativest6calculated at the RHF/6-31G//RHF/6-3 IG level to ensure that the computed structures corresponded to either true energy minima or transition states. Vibrational frequencies were also obtained by using AMI ,20

GAUSSIAN 86

Results and Discussion

A . Energy. AHrvalues for 0-protonated benzene oxide and the cis, gauche, and trans structures of the carbonium ion (see Figure 3) calculated by using A M I , and the total molecular energies, ET,calculated by using the 6-31G and 6-31G* basis sets, together with single-point energy determinations, MP2/6-3 1G* and MP3/6-31G* at the RHF/6-31G* geometry, are listed in Table I. The energies of the cis and trans structures of the carbonium ion differ by less than 1 kcal mol-' from that for the (21) Cahn, R. S.;Ingold, C.; Prelog, V. Angew. Chem., Int. Ed. Engl. 1966, 5, 385-415, especially p 407. (22) Mulliken, R. S. J . Chem. Phys. 1955, 23, 1833-1840, 1841-1846, 2338-2346. (23) 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. GAUSSIAN86; Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984.

Figure 4. Schematic plot of the energy change for the conversion of 0-protonated benzene oxide into the gauche rotamer of the carbonium ion, and benzene oxide into oxepin, at various levels of calculation.

gauche structure, showing the potential energy surface to be very nearly flat in this region. In sharp contrast, all three carbonium ion structures are significantly more stable than the 0-protonated oxide at all three levels of calculation. In Figure 4,the schematic plot shows that, just as for the benzene oxide-oxepin valence t a ~ t o m e r i s m ,the ~ ~ energy change reaches an approximately constant value with the inclusion of electron correlation, MP2 and MP3 using the 6-31G* geometries, with the gauche pquinonoid-type carbonium ion more stable than the 0-protonated oxide to the extent of 12-14 kcal mol-'. B. Frequency Analysis. Complete listings of the vibrational frequencies calculated by using AM1 and the 6-31G basis set are available as supplementary material, Tables IS-IVS. (See paragraph at end of paper regarding supplementary material.) At each level the eigenvalues of the force constant matrix for both the gauche and trans conformers of the carbonium ion are positive, showing these structures to be local minima on the potential energy surface and therefore stable intermediates. On the other hand, there is a single negative eigenvalue of the force constant matrix for 0-protonated benzene oxide and for the cis conformer of the carbonium ion. Hence, these structures are transition states, the former for the interconversion of two gauche structures, one with the H O group on C , and the other with the H O group on C1, and the latter for the interconversion of object and mirror image forms of each gauche structureZ5(see Figure 3). (24) Bock, C. W.; George, P.; Stezowski, J. J.; Glusker, J. P. Stmct. Chem. 1989, I, 33-39.

8164 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990

175.5

t

175.0 174.5 md"

174.0

173.5

0

5

0

l

m

l

5

0

m

m

3

w

~

(bo7c1 C2) degees

Figure 5. Potential energy curve for the rotation of the H7-07 group

about the Cl-07 bond in the carbonium ion structure, calculated using

AMI.

The zero-point vibrational energies of the 0-protonated oxide and the three carbonium ion structures differ by less than 0.5 kcal mol-]. C. Potential Energy Curve for the Rotation of the H7-07 Group about the C,-O7 Bond in the Carbonium Ion Structure.26 The rotational potential energy curve, calculated by A M I , confirms these relationships between the three carbonium ion rotamers and, in addition, characterizes the transition state for the conversion of the trans into the gauche conformer (see Figure 5). The barrier height is 1.7 kcal mol-' in the forward and 0.9 kcal mol-I in the reverse direction, and for the interconversion of the P and M forms of the gauche structure via the cis structure transition state, the barrier height is only 0.3 kcal mol-I. D. Geometry. ( I ) 0-Protonated Benzene Oxide. The structure of 0-protonated benzene oxide has H7-07 pointing away from the ring as shown in Figure 1B, Le., in a "trans" position. Attempts (25) Eliel, E. L.; Allinger, N. L.;Angyal, S.J.; Morrison, G. A. Conformafional Analysis; Interscience: New York, 1967; Chapter I . For a more detailed discussion, see: Bassingdale, A. The Third Dimension in Organic Chemistry; Wiley: New York, 1984; Chapter 5 . (26) As in the rotation of the H-O group about the C-O bond in phenolz7 and o-fluorophenoL2* rotation about the central bond in 1,3-b~tadiene,*~ 2-aza-l,3-butadiene, and 2,3-diaza-I,3-b~tadiene,~~ and rotation about the Cphcnyl-Cvinyl bond in ~ t y r e n e , ~the ' nuclear repulsion energy, V,,, passes through a maximum for stable states and a minimum for transition states. A stable gauche structure is thus more "compact" than a neighboring cis transition-state structure. (27) Bock, C. W.; Trachtman, M.; George, P J . Mol Sfrucf.(THEO-

--.

C H E M.I 1986. 139. 63-74 I

~

George et al.

at locating the corresponding "cis" structure using AMI were unsuccessful. The values calculated for the bond lengths and angles are listed in Table 11. In general, there is satisfactory agreement between the AMI and a b initio values with respect to the position of the shorter and longer bonds, the smaller and larger bond angles, the shape of the six-carbon ring, and the bonding of the hydrogens. The greatest numerical discrepancies are in the 6-31G values for C l - 0 7 and LH707Cl compared to the AMI and 6-31G* values: but this is not unexpected since it is well-known that the 6-31G basis set treats bonding around oxygen rather poorly.32 In common with the unprotonated molecule, the -C3=C4Cs=C6- segment in 0-protonated benzene oxide resembles the However, bonding in the ring nechain in ~is-1,3-butadiene.~~ cessitates a decrease in LCCC of about 5' and there are compensatory changes in the bond lengths. The formal double bond is longer in both the unprotonated and the protonated molecule, whereas the formal single bond is shorter in the unprotonated molecule but longer in the protonated one. At the 6-31G* level of computation the value for the folding angle 0 in the cross section of the structure in the symmetry plane (see Figure 1B) is 178.9', showing that the six-carbon ring system deviates from coplanarity to an even lesser extent than in the unprotonated molecule where the corresponding angle is 174.4°.24 The oxygen atom, 0 7 , again lies well below this nominal ring plane with the folding angle (Y = 102.0', compared to 106.1' for the unprotonated molecule. The summation of the bond angles around each of the C3 carbon atoms34deviates from the ideal value of 360' for perfect coplanarity by only a few hundredths of a degree (see Table 11). And in the three-membered epoxide ring, even though LC2C,O7 is some 47' less than the ideal value for C4 carbon34of 109.5', there are compensatory increases in the remaining bond angles around C , and the summation is only 16' less than the ideal value of 656.8'. The geometrical feature most affected by the 0-protonation is the shape of the epoxide ring. In the unprotonated molecule the apical angle is greater than the base angle(s) in the isosceles triangle, whereas it is less in the protonated molecule. As with other oxonium ions the disposition of the bonds around the oxygen is that of a trigonal pyramid. At the 6-31G* level, O7 is 0.586 8, above the ClC2H7plane, and the angles that this plane makes with H7-07 and C l - 0 7 are 37.7' and 22.3', respectively. (2) Carbonium Ion Structures. The calculated bond lengths and angles for the gauche structure, and for the trans and cis

(28) George, P.: Bock, C. W.; Trachtman, M. J. Mol. Srrucr. (THEO-

CHEW 1987, 152. 35-53. (29) Bock, C. W.; George, P.; Trachtman, M. Theor. Chim. Acta 1984, 64, 293-3 1 I .

(30) Bock, C. W.; George, P.; Trachtman, M. J. Comput. Chem. 1984,5, 395-41 0. (31) Bock, C. W.: Trachtman, M.; George, P. Chem. Phys. 1985, 93, 431-443.

(32) Boggs, J. E.; Cordall, F. R. J. Mol. Sfruct. (THEOCHEM)1981,76, 329-347. (33) Bock, C. W.: Trachtman, M.; George, P. Unpublished calculations. (34) C3 and C4 denote the connectivity, Le., carbon atoms with three and four nearest-neighbor atoms, respecti~ely.'~ (35) Pople. J . A.; Gordon, M. J . Am. Chem. Soc. 1967,89,4253-4261.

Structure of Protonated Benzene Oxide

The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8165

TABLE 111: Calculated Bond lengths (in A) for the Trans, TS, Gauche, and Cis Rotamers of the p-Quinonoid-TypeCarbonium Ion' AMI 6-3 1G 6-31G* parameter trans TS gauche cis trans gauche cis trans gauche 1.4849 1 ,370, 1.4113 1.411, 1.370, 1.4849 1.4067 0.989, 1.1635 1.1073 1.107, 1.109, 1.1075 1.1073

1.4872 1.3740 1.4059 1.4158 1.3672 1.482, 1.412, 0.9688 1.162, 1.1077 1.107, 1.109, 1.107, 1.1064

1.4897 1.3706 1.409, 1.4134 1.3678 1.489, 1.4117 0.969, 1.1530 1.107, 1.1076 1.109, 1.1073 1.1059

1.4914 1.3688 1.411, 1.41 I 3 1.3688 1.4914 1.4126 0.9689 1,1490 1.1065 1.1074 1.109, 1.107, 1.106,

1.4865 1.3565 1.412, 1.4126 1.356, 1.4865 1.4098 0.9540 1.103, I .O733 1.0701 1.074, 1.070, 1.0733

1.4867 1.355, 1.411, 1.4148 1.353, 1.490, 1.4116 0.9529 1.102, 1.072, 1.0698 1.0742 1.0699 1.073,

1.489, 1.354, 1.413, 1.413, 1.354, 1.489, 1.4124 0.9525 1.102, 1.0727 1.0698 1.0742 1.0698 I .O727

1.4882 1.3526 1.4115 1.411, 1.3526 1.4882 1.3780 0.9502 1.104, 1.0755 1.0728 1.076, 1.0728 1.075,

1.488, 1.3518 1.410, 1.413, 1.350, 1.492, 1.3608 0.9492 1.1046 1.074, 1.072, 1.076, I .O727 1.0754

cis 1.491, 1.3508 1.4120 1.4120 1.3518 1.491, 1.3822 0.948, 1.103, 1.0750 1.0726 1.0762 1.0726 1.0750

'For the numbering system see Figure 2A. TABLE IV: Calculated Bond Angles (in deg) for the Trans, TS, Gauche, and Cis Rotamers of the p-Quinonoid-TypeCarbonium Ion' AM 1 6-31G 6-31G* parameter trans TS gauche cis trans gauche cis trans gauche 114.9, 1 20.60 120.50 120.9, 120.50 1 20.60 718.10 108.32 114.5, 114.52 104.04 104.0, 102.7, 654.77 I 16.86 116.86 122.49 122.49 120.5, 120.53 1 18.9, 1 18.94 1 19.4, 1 1 9.49 359.9, 359.9, 359.9, 359.9, 359.9,

115.00 120.39 120.5, 120.97 1 20.62 120.58 718.16 109.0, 109.69 117.1, 103.21 104.09 106.07 655.2, 1 16.7, 1 17.48 122.8, I 21 I 20.2~ 120.58 119.1, 1 1 8.77 119.7, 119.3, 359.9, 359.9, 359.98 359.9, 359.92

114.76 120.6; 120.69 120.98 120.53 120.8, 7 18.4, 108.04 108.56 I 13.52 104.30 104.78 I 10.43 656.35 1 16.59 117.1, 122.7, 121.98 I 20.2~ 1 20.60 1 19.0, 1 18.82 119.5, 119.4, 359.96 359.9, 359.9, 359.97 359.9,

114.5, 120.9; 120.6, 120.9, 120.6, 120.92 718.5, 108.50 110.42 110.42 104.72 104.7, I 1 1.8, 656.64 116.7, 116.70 122.37 122.37 120.44 120.44 1 18.8, 1 18.89 1 19.4, 1 1 9.49 359.99 359.9, 359.9, 359.9, 359.99

115.27 121.1, 118.96 123.1, I 18.9, 121.18 718.66 115.8, I 15.09 1 1 5.0, 102.55 102.55 103.6, 654.14 116.7, 116.7, 121.97 121.9, 121.35 121.35 119.6, 119.6, 118.4, 118.4, 359.9* 359.95 359.9, 359.9, 359.98

114.7, 121.54 118.86 123.1, 1 18.93 121.39 718.63 116.2, 109.4, 113.4, 103.29 103.89 11 1.4, 656.22 116.30 117.1, 122.1, 121.4, 121.3, 121.4, 119.7, 119.5, 1 1 8.45 1 1 8.36 359.97 359.9, 359.9, 359.9, 359.92

114.7, 121.4, 1 18.89 123.1, 1 18.89 121.49 7 18.64 116.37 11 1.3, 11 1.3, 103.69 103.69 1 1 1.6, 656.32 1 16.6, 116.6, 121.8, 121.8, 121.4, 121.40 119.6, 119.62 118.40 1 18.4, 359.9, 359.9, 359.96 359.9, 359.9,

115.0n 121.3, 118.56 123.76 118.5, 121.39 7 18.66 1 1 1.9, 1 15.60 1 15.60 101.02 101.02 105.6, 653.87 1 16.57 116.5, 121.89 121.89 121.6, 121.65 119.7, 119.7, 118.1, 118.1, 359.85 359.9, 359.98 359.9, 359.8,

114.54 121.6, 1 18.49 123.74 118.57 121.57 718.60 1 12.4, 1 10.66 114.54

I O 1 .52 102.45 112.08 655.79 1 16.2, 116.9, 122.0, 121.36 121.6, 121.74 1 19.80 119.67 118.15 1 18.08 359.9, 359.9, 359.9, 359.92 359.8,

cis 114.49 121.6; 1 18.49 123.76 118.49 121.67 718.56 11 2.56 1 12.50 I 12.50 102.08 102.08 1 12.2, 655.87 116.49 116.49 121.74 121.74 121.7, 121.7, 119.7, 119.73 118.1, 118.1, 359.90 359.9, 359.9* 359.9, 359.90

For the numbering system see Figure 2A structures, obtained by imposing a plane of symmetry passing are listed in Tables 111 and IV. through H,, 07,H I , C , and H4, Again there is satisfactory agreement between the AMI and ab initio values for the location of the shorter and longer bonds and the smaller and larger bond angles. The type of ring system in all three structures is clearly p-quinonoid and not o-quinonoid, in that the shorter bonds are located at opposite sides of the ring as in a p-benzoquinone and not in a conjugated sequence, -C= C-C=C-, as in o-benzoquinone. The folding angles in the ring are almost identical in the trans and cis structures. At the 6-31G* level, fi = 168.9' and 168.7' and y = 179.7' and 179.6', respectively (see Figure 2B). The two structures differ significantly only in the angles relating to the C H O H group: a = 143.5' and 135.0' and 6 = 105.6' and 112.2', respectively. The differences, about 7.5', reflect an inplane rotation of c,-o7 and H,-C, in unison about c,-C,-c2, such that the O7--C2 and 07-.C6 internuclear distances are decreased, Le., O7becomes more overshadowed by the ring, and the H ,.-C, and H ,-.C, internuclear distances are increased. The values are listed in Table V S of the supplementary material.

Since C, is no longer constrained in the epoxide ring, the summation of the bond angles is much closer to that for idealized bonding around C 4 carbon (see Table IV). The formal double bonds in these p-quinonoid-type structures are appreciably longer (-+0.03 A) than isolated double bonds, and the formal single bonds between the C3 carbon atoms are appreciably shorter ( - 4 . 0 6 A) than typical bonds of this kind; e.g., the 6-31G* values for the double bonds in ethylene and trans-l,3-butadiene are 1.317 and 1.323 A, respectively, and for the center single bond in trans-l,3-butadiene, 1.467 Alterations in bond length of this magnitude indicate significant electron delocalization, and in terms of classical valence bond concepts, the structure can be visualized as a hybrid (I, Chart 111) of the individual structures 11, 111, and IV. Comparison of the internuclear distances H,.-C2 and HI-C6, 07--C2 and 07-.C6, and H7-.C2 and H7--C6 (Table VS) shows that in the stable gauche structure H I and O7 are displaced to (36) Bock, C. W.;George, P.; Trachtman, M. J . Mol. Strucf. (THEOC H E W 1984, 109, 1-16.

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The Journal of Physical Chemistry, Vol. 94, No. 21, 1990

------

C I -Carboniumlons

---,

I

C2 - Carbonium Ions

I

I I I

I

I

P -gauche C2

I

M-gauche. Cl I

--I.---

TS

M -gauche. CI

I

I

I - - - - - - - - - I

0-protonated benzene oxide Figure 6. Stereoselective pathways interconnecting the various rotamers of the carbonium ion via the 0-protonated oxide transition state. See Figure 3 for structural details. CHART 111

90

I

I1

A/

\

I

111

IV

one side of the plane of symmetry passing through C , and C4 in the trans and cis structures, while H7 is displaced to the other side. Comparison of the H7-H1 distances shows the stable gauche structure to be more akin to the cis, and taking the torsion angle H 7 0 7 C , H Iin the cis structure as 0' reference, the angular displacement of H7-07 is about 4 4 O . j ' In the case of the transition state for the conversion of the trans into the stable gauche structure, a comparison of the H,-H, distances shows this gauche transition-state structure to be more (37) Having determined the torsion angles for these gauche structures, it is now possible to reclassify them in terms of the more detailed scheme set out in the lUPAC recommendations3* and subsequent review^'^ in which names are assigned to the particular range in which the torsion angle (8)falls. Thus the stable gauche structure, for which 8 = 44', is 'synclinal" (sc), and the transition state gauche structure, for which 8 108'. is 'anticlinal" (ac). (38) Cross, L. C.; Klyne, W. Pure Appl. Chem. 1976, 45, 11-30. Rule E-5.6: synperiplanar (sp), synclinal (sc),anticlinal (ac), or antiperiplanar (ap) according as the torsion angle is within &30° of Oo, 160°, &120°, or &180°, respectively. (39) See, for example: Oki, M. Recent Advances in Atropisomerism. In Topics in Srereochemisiry; Allinger. N. L., Eliel, E. L., Wilen, S. H., Eds.; Interscience: New York, 1983; Vol. 14, pp 1-81. Especially Figure I , p 7: sp +30° to -30°, +sc + 30' to +90°, s c -30' to -90°, +ac +90° to + I SOo, -ac -90' to -150°, and ap +150° to -150'.

akin to the trans; and taking the same Oo reference, the angular displacement of H7-07 is about 108°.37 E . Distribution of Total Atomic Charge. Even though values obtained by using Mulliken population analysis22are basis set dependent and subject to further modification when electron correlation is i n c l ~ d e d , ~and @ ~different ~ still if a semiempirical method such as AMI is used in which only valence-shell electrons are taken into account, they nevertheless provide qualitative insight into three important aspects of the protonation process. The data are given in Tables VIS and VIIS of the supplementary material. Firstly, in order to bond the proton, electronic charge is, of necessity, drawn from other parts of the molecule. The data show that the conversion of the epoxide oxygen into the HO group in both the 0-protonated oxide and the gauche structure of the carbonium ion is accompanied by a gain of 0.6-0.8 units of electronic charge. Secondly, there is the question as to where the positive charge is located on the p-quinonoid-type carbonium ion structures. The summation of the atomic charges on H2CZ,H3C3,H&, H5C5, and H6C6ranges from +0.8 to +1.0 unit, which leaves little doubt that the charge is located almost entirely on the ring. Moreover, there is a clear indication that the charge alternates around the ring, the values for the groups "ortho" and "para" to the C H O H group being consistently more positive than those for the "meta" groups. Thirdly, this alternation in charge has its origin mainly in an alternation in charge on the carbon atoms themselves. The charge on the "para" carbon atom, C4,is relatively more positive (or less negative) than the charge on the "ortho" carbon atoms, C2 and C,, while the charge on the "meta" carbon atoms is significantly more negative. This distribution is precisely what would be expected if the structures are visualized as a hybrid of the individual valence bond structures 11, 111, and IV (Chart 11) and can be regarded as additional evidence for electron delocalization. (40) Ermler. W . C.; Mulliken, R. S.; Clementi, E. J. Am. Chem. SOC. 1976, 98, 388-394. (41) Ermler, W . C.; Mulliken, R. S. J . A m . Chem. SOC.1978, 100, 1647-1 653. (42) Reed, A. E.; Weinstock, R. B.; Weinhold, F. J . Chem. Phys. 1985, 83. 735-746 and references therein.

Structure of Protonated Benzene Oxide

The Journal of Physical Chemistry, Vol. 94, No. 21, 1990 8167 C,

- Carbonium ions

0-protonated C, -ketone

C2 -Catboniumions

0-protonated C2 -ketone

Figure 7. Overall reaction pathway.

Concluding Remarks In the mechanism for the acid-catalyzed conversion of benzene oxide into phenol it has been assumed that 0-protonated benzene oxide is a stable intermediate and that opening of the epoxide ring to give a carbonium ion with an o-quinonoid-type structure is the rate-determining step.3 However, in sharp contrast, we find (at both the A M I and the 6-31G levels of computation) that the 0-protonated oxide is not a stable intermediate, but a transition state on the potential energy surface, even though the geometry is in keeping with that of an ordinary molecular structure. Instead, a carbonium ion with a p-quinonoid-type (and not an oquinonoid-type) structure is found to be a stable intermediate. Moreover, at all three levels of computation, this p-quinonoid carbonium ion is found to be significantly more stable than the 0-protonated oxide. Although relaxation of the symmetry constraint used to locate the 0-protonated oxide leads directly to a p-quinonoid-type carbonium ion with a gauche structure at the AM1 level, it would be difficult, as far as the reaction mechanism is concerned, to make a distinction between this rotamer and the trans and cis rotamers because they differ so little in energy-be it in terms of AHf, ET, or the energies at the zeroth vibrational level. For the AM1 potential energy surface the pathway connecting the “Clnand “Czn carbonium ions via the 0-protonated oxide transition state is shown in Figure 6 together with the interconversion of the gauche, trans, and cis rotamers. The breaking of C i a 7 and C2-07 is represented as leading respectively to the formation of the M form of gauche.C2 and the P form of gaucheC,. This choice has been based on the observation that breaking a C - 0 bond in the 0-protonated oxide produces an unsymmetrical structure. On breaking C,-07 the 0 7 - H ~ bond is to the right of the H2C2O7 plane, whereas on breaking C2-07 the 07-H7 bond is to the lefr of the H , C , 0 7 plane43 (see Figure 3). With regard to the mechanism of the overall acid-catalyzed reaction, if the initial protonation is assumed to be a rapidly established equilibrium, then the rate-determining step in the production of phenol is probably the N I H shift in which the carbonium ion is converted into the protonated ketone (see Figure 7). It is to be noted that the 0-protonated ketone is, in fact, C,-protonated phenol under another name. Inspection of the molecular models suggests that two independent protonation steps may be operative. The approach of H 3 0 + underneath the six(43) I n principle, similar stereoselective pathways would be appropriate for the reactions of symmetrically substituted oxides, such as 1,4-dimethylbenzene 2,3-oxide and naphthalene 2,3-oxide, and unsymmetrically substituted oxides, such as 4-methylbenzene 1.2-oxide and naphthalene 1.2-oxide. But the TI” and *C2” carbonium ions in the latter case would be positional isomers“ and not equivalent structures, and thus fission of the C - 0 bonds would be regioselective as well as stereoselective. (44) Preliminary calculations on naphthalene 1.2-oxide, using AMI, find the C , and C2 isomers to be stable intermediates with gauche structures and the C , to be more stable than the C2 isomer by about 3.3 kcal mol-’.

carbon ring in the oxide would favor the formation of the trans rotamer of the carbonium ion (H7 trans in Figure 2B), whereas an approach end-on toward the epoxide oxygen would favor the formation of the gauche rotamer (see Figure 3). It should perhaps be emphasized at this point that the calculations pertain to molecular species in isolation and that, in aqueous solution, solvation might alter the energy relationships significantly. Nevertheless, the reaction pathways set out in Figures 6 and 7 account just as well as the widely accepted mechanism for the kinetics of the acid-catalyzed reaction and the observed N I H shift. The present results help explain the contrast in reaction products from ethylene oxide and benzene oxide, namely, the preferential hydrolysis giving ethylene glycol (instead of isomerization giving acetaldehyde) in contrast to isomerization giving phenol (instead of hydrolysis giving dihydroxycy~lohexadiene)!~~The deciding factor would appear to be the reversal in the relative energies of the 0-protonated derivative and the carbonium ion. For example, the 6-3 l G * data give

A

H

IV (Chart II)

I1 (Chad II) H

+o‘

H

H

0-H

q -y&

AET = -22 kcal mol-’

48

I (Chad 111, gauche rotamer ) I (Chart 11) These energy differences are reflected in the roles the 0-protonated derivative and the carbonium ion play on the potential energy surface. With ethylene oxide the former is a stable intermediate while the latter is a transition state,I3 whereas with benzene oxide the former is a transition state while the latter is a stable intermediate. Hence, reaction of the 0-protonated derivative with H 2 0 would be expected to predominate in the case of ethylene oxide, but isomerization of the carbonium ion in the case of benzene oxide. The difference in the relative energies of the 0-protonated derivative vis-Cvis the carbonium ion is attributable to the presence of the conjugated sequence of carbon atoms in the formal structure of the p-quinonoid-type ion, by virtue of which electron delo(45) Winstein, S.; Henderson, R. B. In Heterocyclic Compounds; Elderfield, R. C., Ed.: Wiley: New York, 1950 Volume 1, Chapter 1 and references therein. (46) Pritchard, J . G.; Long, F. A. J . Am. Chem. Sac. 1956,78,6008-6013. (47) Long, F. A.; Pritchard, J. E.: Stafford, F. E. J . Am. Chem. Sac. 1957, 79, 2362-2364. (48) Calculated from the ET values in Table I .

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calization occurs, thereby exerting a stabilizing effect. Acknowledgmenr' We grateful1y the generous grant of computer time on a VAX 1 1 /780 provided by the computer center of the Philadelphia College of Textiles and Science. We thank the Advanced ScientificComputing Laboratory, NC1-FCRF, for providingtime On the XMP and This work was also in part by grants CA-06927 from the National Institutes of Health and BC-242 from the American Cancer Society and by an appropriationfrom the Commonwealth of Pennsylvania.

Supplementary Material Available: Tables IS-IVS, vibrational frequencies calculated using AMI and the 6-31G basis set for

0-protonated benzene oxide, and the trans, gauche, and cis rotamers of the p-quinonoid-type carbonium ion; Table VS, calculated values for internuclear distances between H I , H7, and O7 and the adjacent carbons in the ring, C2 and C6, in the trans, TS, gauche, and cis rotamers of the p-quinonoid-type carbonium ion; Table VIS, the loss of electronic charge on the ring carbons and ring hydrogens, and the gain as the epoxide oxygen bonds the proton i n the formation of O-protonated benzene oxide and the gauche rotamer of the p-quinonoid-type carbonium ion; Table VIIS, total atomic charges on H2C2through H,C6 and H707,and the atomic charges on c, through c6 and 07,in the trans, gauche, and cis rotamers of the p-quinonoid-type carbonium ion (8 pages). Ordering information is given on any current masthead page.

Vibronic Excitation of Oxygen-Atom Transfer from NO, to Ethylene by Long-Wavelength Visible Light in a Cryogenic Matrix Munetaka Nakata,+ Kazuhiko Shibuya,t and Heinz Frei* Chemical Biodynamics Division, Lawrence Berkeley Laboratory, University of California, Berkeley, California 94720 (Received: February 14, 1990)

Chemical reaction has been observed upon irradiation of ethylene.N02 pairs in solid Ar with continuous-wave dye laser light at visible wavelengths as long as 574 nm, well below the 398-nm dissociation limit of isolated NO2. Reaction products were acetaldehyde, ethylene oxide, NO, and ethyl nitrite radical, as established by FT infrared spectroscopy. Kinetic analysis of product absorbance growth showed that acetaldehyde is the prevalent final oxidation product of direct, single-photon photolysis of ethylene.N02pairs, while ethylene oxide is exclusively produced by yellow and shorter wavelength induced photodissociation of trapped ethyl nitrite radical. Experiments with frans- and cis-CHD=CHD yielded nitrite radical CHD-CHD-ON0 under retention of stereochemistry but with scrambling in the epoxide product. On the basis of these results and those of our earlier work on NO2 photooxidation of cis- and trans-2-butene, a reaction path is proposed that involves 0-atom transfer from NO2 to the C=C bond to give a short-lived singlet oxirane biradical. The photolysis wavelength dependence of the reaction quantum efficiency indicates that NO2 reactant vibrational excitation plays an essential role and opens a reaction path that is unique to visible light induced alkene + NO2 chemistry in a matrix.

I. Introduction We have recently reported oxidation of trans- and cis-2-butene by N O 2 excited at red or yellow wavelengths in an Ar matrix.IV2 These long-wavelength photons excite NO2 to vibronic levels tens of kilocalories per mole below the dissociation limit of the isolated reactant. Absorption of a single photon by reactant pairs led to exclusive production of 2,3-epoxybutane (plus NO) with a high degree of stereochemical retention. Concurrently a butyl nitrite radical was trapped, and its conformation established by FT infrared spectroscopy. Although optical absorption of NO2 at long visible wavelengths is mainly due to the 2B2 excited state, the molecule prepared by excitation in this spectral region has predominantly the character of a highly vibrationally excited 2A, ground-state species because of very strong coupling of the 2B2 and 2A, state^.^ Indeed, our results of a photolysis wavelength dependence study of the reaction quantum efficiency of 2-butene.N02 were consistent with reaction of vibrationally unrelaxed NO2. An observed correlation between the stereochemistry of the trapped butyl nitrite radical and the epoxide indicated that these products have a common transient precursor, most probably an oxirane biradicaL2 This would imply that the initial reaction step is 0-atom transfer from N O 2 to the C C double bond. We report here photooxidation of ethylene by selective vibronic excitation of C2H4.N02, cis-CHD=CHD.N02, and transCHD=CHD.N02 pairs in solid Ar with a continuous-wave (cw) 'Present address: Tokyo University of Agriculture and Technology, 3-5-8 Saiwai-cho, Fuchu, Tokyo 183. 'Tokyo Institute of Technology, Ohokayama, Meguro, Tokyo I52 *To whom correspondence should be addressed.

0022-3654/90/2094-8 168$02.50/0

dye laser at wavelengths as long as 575 nm. A study of this prototype alkene NO2 photoreaction in a matrix is of special interest for several reasons. First, since long-wavelength visible photons prepare NO2 essentially as a highly vibrationally excited ground-state species, this system offers an opportunity to explore the effect of reactant vibrational excitation on bimolecular reactivity. Second, while vibronic excitation of alkene.N02 "sustained collisional pairsn4 in the matrix initiates 0-atom transfer, thermal reaction of NO2 with ethylene and higher alkenes * ~ ~ to initial formation of a C-N in the gas phases7 or s ~ l u t i o n leads bond to give nitroalkyl radicals. This difference in the chemical reaction path is particularly intriguing since in both the case of the matrix and the thermal gas phase (or solution) experiment the reacting NO2 has predominantly 2Al ground-state character. A third interesting point is the interpretation of the photolysis

+

(1) Nakata, M.; Frei, H.J . Am. Chem. SOC.1989,1 1 1 , 5240. (2)Nakata, M.; Frei, H. J . Phys. Chem. 1989,93,7670. (3) Hsu,D.K.; Monts, D. L.; Zare, R. N. Spectral Atlas of Nitrogen Dioxide; Academic Press: New York, 1978. (4)Frei, H.;Pimentel, G. C. In Chemistry and Physics of Matrix Isolated Species; Andrews, L., Moskovits, M., Eds.; Elsevier: Amsterdam, 1989;p 139. (5) Cottrell, T. L.; Graham, T. E. J . Chem. SOC.1953,556;J . Chem. SOC. 1954,3644. (6)Atkinson, R;Aschmann, S. M.; Winer, A. M.; Pitts, Jr., J. N. I n r . J . Chem. Kinet. 1984,16,697.See also: Sprung, J. L.; Akimoto, H.; Pitts, Jr., J. N. J . Am. Chem. SOC.1971,93,4358; J . Am. Chem. SOC.1974.96.654, (7)Ohta, T.;Nagura, H.;Suzuki, S . I n r . J . Chem. Kiner. 1986,18, 1. (8) Shechter, H.Rec. Chem. Prog. 1964,25, 55. (9)Giamalva, D.H.;Kenion, G. B.; Church, D. F.; Pryor, W. A. J . Am. Chem. SOC.1987, 109,7059. In the case of the reaction in solution, an additional path involving abstraction of allylic hydrogen is observed.

1990 American Chemical Society