J. Phys. Chem. 1992, 96, 5217-5219 tential using scanning tunneling microscopy. For iodine on Au(1 1 l), we observed a (d/5Xd/5)R3Oostructure in the potential range --400 to --15 mV (vs Ag/AgCl) and a 3 X 3 closepacked structure above --15 mV. The adlattice orientation with respect to the substrate lattice is determined by comparing the bare Au( 1 1 1) lattice at low potential with the iodine adlattice or from the images that show both the adlattice and substrate lattice in some tip bias range. While the (d/5Xd3)R30° structure is in agreement with the ex situ LEED experiment, we observed a close-packed structure of iodine at high potentials instead of a semiclose-packed structure identified by the ex situ LEED experiment. In the case of bromine on Au( 1 1 l ) , we observed a (d/5Xd/3)R3O0 structure in the potential range -0 to 100 mV. A close-packed bromine adlattice with a rotation of -20° relative to the Au( 1 1 1) lattice was observed above 100 mV. In the similar potential range, we also observed a hexa onally superperiodic structure with a lattice constant of -9.4 superimposed on the close-packed structure. The superperiodic structure has the same orientation as the close-packed structure as determined from the images that show both the superperiodic structure and the close-packed structure. We found that the contrast of the iodine adlattice depends on both the tip bias and surface potential. The iodine adlattice can be “semitransparent” which allows us to “see” the underlying substrate lattice and, therefore, directly determine the orientation of the adlattice relative to the substrate lattice. We believe that the origin of the phenomenon is electronic, depending on the electronic states of the adlattice, substrate lattice, and the tip. In a similar potential and tip bias range, we did not see the effect on the bromine adlattice. This interesting phenomenon deserves further study. Acknowledgment. We thank H. Song, Y.Li, J. A. DeRose, P. I. Oden, and J. Pan for help in the lab, L. A. Nagahara for important discussions, M. J. Weaver for sharing his results with us prior to publication, and the referees who pointed out a mistake in our potential scale in the previous versions of the paper. Support was received from the N S F (Dir 89-20053), O N R (N00014-
-
-
1
5217
90-J-1455), and the Vice President for Research at ASU. References and Notes (1) Hubbard, A. T. Chem. Reu. 1988,88, 633. (2) Sonnenfeld, R.; Schneir, J.; Hansma, P. K. In Modern Aspects of Electrochemistry; Brockris, J. O M . , Ed.;Plenum: New York, 1990. Manne, S.; Hansma, P. K.; Massie, J.; Elings, V. B.; Gewirth, A. A. Science 1991, 251, 183. (3) Samant, M. G.; Toney, M. F.; Borges, G. L.; Blum, L.; Melory, 0. R. J . Phys. Chem. 1988,92,220. Ocko, B. M.; Wang, J.; Davenport, A.; Isaacs, H. Phys. Rev. Lett. 1990, 65, 1466. (4) Yau, S.-L.; Vitus, C. M.; Schardt, B. C. J . Am. Chem. SOC.1990,112, 3677. (5) Schardt, B. C.; Yau, S.-L.;Rinelli, F. Science 1989, 243, 1050. Vogel, R.; Baltruschat, H. Surf. Sci. 1991, 259, L739. (6) Vitus, C. M.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 7559. (7) Yau, S.-L.;Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Am. Chem. Soc. 1991, 113, 6049. (8) Deakin, M. R.; Li, T. T.; Melroy, 0. W. J . Electroanal. Chem. 1988, 243, 343. (9) Rodriguez, J. F.; Soriaga, M. P. J . Electrochem. Soc. 1988, 135,616. (10) Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P.; Villegas, 1.; Suggs, D. W.; Stickney, J. L. J . Phys. Chem. 1991, 95, 5245. (11) McCarley, R. L.; Bard, A. J. J . Phys. Chem. 1991, 95, 9618. (12) Gao, X. P.; Weaver, M. J. Submitted for publication. (13) DeRose, J . A.; Thundat, T.; Nagahara, L. A,; Lindsay, S. M. Surf. Sci. 1991, 256, 102. (14) Nagahara, L. A,; Thundat, T.; Lindsay, S. M. Reu. Sci. Instrum. 1989, 60, 3128. (15) Takayanagi, K.; Yagi, K. Jpn. Instrum. Methods 1983, 24, 337. Harten, U.; Lahee, A. M.; Toennies, J. P.; Woll, Ch. Phys. Reu. Lett. 1985, 54, 2619. Woll, Ch.; Chiang, S.; Wilson, R. J.; Lippel, P. H. Phys. Reu. 1989, B39, 7988. Barth, J. V.; Brune, H.; Ertl, G.; Behem, R. J . Phys. Reu. 1990, 42. 9307. (16) Nakai, Y.; Zei, M. S.; Kolb, D. M.; Lehmpfuhl, G. Ber. Bunsen-Ges. Phys. Chem. 1984,88, 340. (17) Tao, N . J.; Lindsay, S. M. J . Appl. Phys. 1991, 70, 5141. Gao, X.; Hamelin. A.; Weaver, M. J . Chem. Phys. 1991, 95, 6993. (18) Wang, J.; Davenport, A. J.; Isaacs, H. S.; Ocko, B. M. Science 1992, 255, 1416. (19) Tao, N . J.; Lindsay, S. M. Submitted for publication. (20) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985, I , 66. (21) Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J . Electroanal. Chem. 1987, 222, 305.
Concerted Hydroxyl Ion Attack and Pseudorotation in the Basecatalyzed Hydrolysis of Methyl Ethylene Phosphatet Carmay Lim* and Philip Tole Department of Molecular and Medical Genetics, Department of Chemistry and Department of Biochemistry, University of Toronto, I King’s College Circle, Toronto, Ontario M5S IA8, Canada (Received: March 4, 1992; In Final Form: April 16, 1992)
The gas-phase and solution free energy profiles for the base-catalyzed hydrolysis of methyl ethylene phosphate (MEP) were obtained using ab initio molecular orbital calculations and continuum dielectric methods. The correlation energy is estimated with second-order Mdler-Plesset theory and the 6-3 1+G* basis using fully optimized 3-21+G* geometries. In vacuum and in solution, OH- attack on MEP is concerted with pseudorotation to form intermediate 2a, which undergoes ring opening faster than exocyclic cleavage of intermediate 2b (Scheme 11); the apical attack of OH- and the apical departure of the ring oxygen in 2a are in accord with an in-line mechanism. This new mechanism is consistent with the experimental observation that MEP hydrolyzes exclusively with ring opening in dilute alkaline solution (pH 8-1 1).
Introduction Pseudorotation in pentacovalent species is defined as an intramolecular process where a trigonal bipyramid (TBp), which may be short-lived, is converted into another by deforming angles This work was supported by the Protein Engineering Network Center of Excellence.
so that the final TBP appears to have performed a 90° rotation relative to the initial state.’-3 The pseudorotation concept has been used to explain the rapid exocyclic cleavage with ring retention of phosphate esters and other kinetic data for phostonate and phosphinate hydrolyse^.^,^ A classic example of its application is in rationalizing the PH-Prduct profile for the hydrolysis of methyl ethylene phosphate (MEP)? In particular, the mechanism
0022-365419212096-5217$03.00/00 1992 American Chemical Society
5218 The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 SCHEME I
Letters
(PV) were treated classically. Thus, the entropy change between two states is given by
As = Astrans +
+ Asvib
(1)
+ AErot + AEvi, + PAV
(2)
and the enthalpy change is given by MEP
121
111
1
1
AH = AE + AEt,,,
A
I
\
o\p/o o” \ methyl hydroxyethylphoophele
ethylene phosphate
SCHEME 11
where AE is the MP2/6-31+Gz//HF/3-21+G* electronic energy change. The entropy and enthalpy changes are combined to give the free energy. To estimate how the gas-phase free energy profile is modified by solvent, the solvation free energies of the reactants, transition states, intermediates, and products were estimated by the continuum dielectric method described by Lim et ale9The 3-21+G* geometries and Mulliien atomic charges and the CHARMM version 221° van der Waals radii were employed with an internal and solvent dielectric constant of 2 and 80, respectively. Poisson’s equation was solved by the finite-difference method using a rough and fine 71 X 71 X 71 grid with 1.0- and 0.25-A spacing, respectively, centered on the phosphorus atom. The free energy change, AG,,,, from A to B in solution can be calculated from the thermodynamic cycle A(e4 JAW)
A(sln) n
I \ o\p/o o” \
ethylene phosphate
-J A%
-
B(wd WE)
B(sln)
A cih
where AGgs is the gas-phase free energy change from A to B and the AG, are solvation free energies, i.e. AG,,, = AG,,,
+ AG,(B) - AG,(A)
MeOH
in Scheme I involving slow pseudorotation of a phosphorane intermediate 1 was proposed to account for the observation that MEP hydrolyzes exclusively with ring opening in dilute alkali (pH 8-1 1); Le., endocyclic cleavage of 1 was assumed to be much faster than pseudorotation of 1 to Z4 Here, we report an alternative mechanism for the base-catalyzed hydrolysis of MEP (Scheme 11) that is consistent with experiment; viz., OH- attack on MEP is concerted with pseudorotation to form intermediate h,which undergoes ring opening faster than exocyclic cleavage of 2b. Thus, in contrast to Scheme I, 1 is not a stable intermediate nor does it determine the observed product distribution for the hydrolysis of MEP in dilute alkaline solution.
Methodology The gas-phase reaction profile for the addition of OH- to MEP was explored at the Hartree-Fock level with a 3-21+G* basis set which included diffuse functions (s and p orbitals) on the nonhydrogen atoms and polarization functions (3d orbitals) on the phosphorus atom. The 3-21+G* geometries of tetravalent MEP and HOC2H4P042-5were found to be in good agreement with e~periment.~” The correlation energy is estimated with secondorder Mdler-Plesset theory and the 6-3 l+G* basis using fully optimized 3-21+G* geometries. The reaction coordinate used for the nucleophilic attack of OH- on the phosphorus atom of MEP was the hydroxyl 0 to P distance; that for endocyclic and exocyclic cleavage of 2 was the apical ring 0 to P distance and methoxy 0 to P distance, respectively. Unless stated otherwise, the extrema on the reaction pathway were fully optimized; Le., the maximum force and displacement were simultaneously less than 0.000 45 and 0.0018, respectively. To determine the gas-phase free energy profile, 3-21+G* vibrational frequencies were computed for the reactants, transition states, intermediates, and products. The entropy and vibrational energy (&,) were calculated from the frequencies and geometries according to standard statistical mechanical formulas.* The rotational (Erot)and translational energy (Et,,,) and the work term
Results and Discussion As the hydroxyl ion approaches MEP, a long-range ion-dipole minimum [3] (Figure la) and transition state [4] (Figure lb) are formed at a P-05’(H) distance of 5.35 and 2.92 A, respectively. Relative to the MEP + OH- reactants at infinity, the free energy of the long-range ion-dipole minimum is -9.7 kcal/mol and the free energy difference between 3 and 4 is 3.3 kcal/mol. As the P-05’(H) decreases to 1.69 A, 1 was not found to be a stable intermediate in vacuum, but a true transition state [5]16(Figure 2a) with only one negative 3-21+G* vibrational frequency (-142.6 cm-I). Using the program MOLVIB,’’ an analysis of the 3-21+G* negative frequency shows that it has pseudorotation-like character with contributions from the angle bends, 01-P-02 and 01-P03‘, and the torsions, H-05’-P-0lI02’-P-O3’-C3’, and C20 2 - P a l . The 3-21+G* optimization of 5 following the intrinsic reaction coordinatei2 led to a rapid decrease of the energy and the 01-P-02 and 01-P-03’ angles. This was characteristic of the pseudorotation process where the hydroxyl hydrogen, which is 2.33 %I from 0 1 in 5, rotates away from 0 1 toward 03’ as the P-03’ lengthens leading to a phosphorane intermediate [2a] (Scheme 11) with 03’ apical and the hydroxy hydrogen 1.96 8, from 03’. The 3-21+GS optimization of 5 where the H-05’P-01 dihedral angle of -12’ was changed to initial values of -60’ and -120’ also resulted in pseudorotation toward 2a. To confirm the 3-21+G* results, a larger basis set was employed. The 631+G* optimization of 1 with the smallest overall 3-21+G* force led initially to a structure where the maximum force and displacement were 0.000 78 and 0.2841, respectively; continued optimization of the latter with analytic force constants computed at every point resulted in pseudorotation. The 6-31+G* results confirmed the 3-21+G* result that 1 is not a minimum. A corollary of the latter is that although OH- attack of MEP is concerted with pseudorotation to form 2,elimination of methoxide from 2b is not concerted with pseudorotation since 1 is not a stable intermediate and is higher in energy than 2. Note however that addition of methoxide to MEP is also not concerted with pseudorotation since a stable intermediate with an axial and equatorial
The Journal of Physical Chemistry, Vol. 96, No. 13, 1992 5219
Letters
f
t
:::‘2.92
\\
Y Figure 1. (a) Fully optimized 3-21+G* geometry of the long-range ion-dipole minimum 3 for gas-phase nucleophilic addition of OH- to MEP. (b) Fully optimized 3-21+G* geometry of the transition state 4 for gas-phase nucleophilic addition of OH- to MEP.
methyl group [6] (Figure 2b) was found at the HF/3-21+G1 level. Thus,in vacuum, OH- attack on neutral MEP is concerted with pseudorotation to form a monoanionic phosphorane intermediate [2a] (Scheme 11); in contrast, gas-phase OH- attack on ethylene phosphate anion is concerted with ring opening to yield a dianionic phosphate intermediate.” The lowest energy pathway for endocyclic and exocyclic cleavage occurs from 2a and 2b, respectively, where the hydroxy hydrogen is within 2 A of the departing oxygen. (Exocyclic cleavage of 2a was computed to proceed via a higher pathway than that of 2b.) Since the activation free energy for endocyclic cleavage of 2a (2.8 kcal/mol) is less than that for rotation of the hydroxy group in 2a to 2b (9.0 kcal/mol) and for exocyclic cleavage of 2b (9.9 kcal/mol), ring opening is favored over ring retention in vacuum. The apical attack of OH- and the apical departure of the ring oxygen in 2a are in accord with an in-line mechanism. The most significant effect of solvation on the gas-phase free energy profile is to increase the gas-phase free energy barrier from 3 to 4 (by 11.6 kcal/mol) so that formation of 2a is the ratelimiting step. Solvation also increases the exocyclic cleavage barrier (by 6.6 kcal/mol) but has a small effect on the endocyclic cleavage barrier (which is decreased by 0.5 kcal/mol) and a negligible effect on the free energy barrier from 2a to 2b. (The errors involved in calculating the solvation free energies of TBP phosphorane transition states and intermediates are probably similar and are likely to cancel in evaluating the relative solution activation free energy from one TBP phosphorane to another.) Thus, the calculated free energy changes in solution show that, as in vacuum, ring opening of 2 is favored over ring retention and the mechanism in Scheme I1 accounts for the observed product distribution. The results do not support exclusive ring opening due to a S,2(P) mechanism or due to the mechanism in Scheme
Figure 2. (a) Fully optimized 3-21+G* geometry of the transition state 5. (b) Fully optimized 3-21+G* geometry of phosphorane intermediate 6.
I involving intermediate 1. Details of the a b initio and dielectric calculations and the p H dependence of the reaction profiles for the hydrolysis of MEP in vacuum and solution are presented in subsequent papers. Acknowledgment. We thank Professor Ronald Kluger for stimulating discussions. The calculations were performed using GAUSSIAN on a Stardent PS3030 and Cray X M P at the Ontario Center for Large Scale Computation. References and Notes (1) Berry, R. S.J. Chem. Phys. 1960, 32, 933. 491
(2) Westheimer, F. H. Acc. Chem. Res. 1968, I , 70. (3) Luckenbach, R. Dynamic Stereochemistry of Pentaco-ordinated Phosphorus and Related Elements; Georg Thieme Publishers: Stuttgart, 1973. (4) Kluger, R.; Covitz, F.; Dennis, E.;Williams, L. D.; Westheimer, F. H. J . Am. Chem. SOC.1969, 91,6066.
(5) Lim, C.; Karplus, M. Manuscript in preparation. (6) Chiu, Y. H.; Lipscomb, W. N. J. Am. Chem. SOC.1969, 91, 4150. (7) Jones, P. G.; Sheldrick, G. M. Acta Crystallogr. 1984, CIO,547. (8) Hehre, W. J.; Radom, L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory; John Wiley and Sons: New York, 1986. (9) Lim, C.; Bashford, D.; Karplus, M. J. Phys. Chem. 1991, 95, 5610-5620. (10) MacKerell, Jr., A. D.; Wiorkiewicz,-Kuczera, J.; Karplus, M. All-
hydrogen empirical parametrization of nucleic acids for molecular modeling, minimizations and dynamics simulations. (1 1) Kuczera, J.: Kuczera, K. Proaram MOLVIB; Harvard University: Cambridge, 1988. (12) Frisch, M. J.; Head-Gordon, M.; Trucks, G.W.; Foresman, J. B.; Schlegel, H. B.; Raghavachari, K.; Robb, M.: Binkley, J. S.;Gonzalez, C.; Defrecs. D. J.: Fox, D. J.: Whiteside. R. A.: Seeaer. R.: Melius. C. F.: Baker. J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. f;Topiol, S.;Pople, J. A. In Gaussian 90; Gaussian Inc.: Pittsburgh, PA 15213. (13) Lim, C.; Karplus, M. J. Am. Chem. SOC.1990, 112, 5872. (14) Lim, C.; Tole, P. Manuscript in preparation. (15) Tole, P.; Lim, C. Manuscript in preparation. (16) 1 was confirmed as a transition state with a tight option in GAUSSIAN 90.
