8552
J . Am, Chem. SOC.1991, 113, 8552-8554
1489, and 1431 cm-I, the broad C-O stretching peak at 1278 cm-I, and the broad phenolic 0-H stretching peak centered at 3342 cm-I. The small peaks between 2975 and 2850 cm-l are due to a small amount of hydrocarbon contamination. The top part of Figure 2 shows the spectrum for the 4HTP surface after exposure to the silane. In the high-frequency region, the phenolic 0-H band originally present at 3342 cm-' has disappeared, and stronger absorptions between 2975 and 2850 cm-' indicate the presence of the hydrocarbon portion of the silane coupling agent. In the low-frequency region, the somewhat enhanced aromatic ring stretches are still present at 1585 and 1486 cm-1,8 but there is a new absorption at 918 cm-' arising from the symmetric Siphenoxy stretching mode of the reaction product, Scheme L9 The two overlapping bands a t 1277 and 1260 cm-' result from the asymmetric Si-phenoxy stretch and the symmetric H3C-Si-CH3 deformation, re~pectively.~ Ellipsometric results4 are in accord with the SAW and FTIRERS experiments. The average measured thickness of three vapor-deposited 4-HTP layers is 5.8 f 0.8 A, which increases to 12.5 f 1.2 A after reaction with [CH3(CH2)7](CH3)2SiCl.Similar results are found for surface-confined 4-ATP before (7.4 f 0.9 A) and after (1 2.1 f 1.2 A) exposure to [CH,(CH,),](CH,)$iCI. These data show that both reactants and products are present at approximately monolayer coverage. Since we do not know the orientation of the adsorbates, it is difficult to infer theoretical thicknesses for the organic monolayers, but the trend toward thicker layers is expected. To summarize, we have demonstrated that well-characterized reactions occur between surface-confined monolayers and vapor-phase reactants at atmospheric pressure. These reaction conditions provide an important link between solution and ultrahigh-vacuum studies. Real-time SAW experiments, FTIR-ERS, and ellipsometry demonstrate that the vapor-phase coupling reactions between surface-confined 4-HTP or 4-ATP and [CH3(CH,),](CH,)$iCl result in monolayer coverages of stoichiometric reaction products. Moreover, experiments with other coupling agents, [CH3(CH2),](CH,),SiCI ( n = 0, 2), show appropriate attenuations in mass, methylene stretching absorption intensity, and thickness, further supporting our conclusions. At present, we have no evidence that either the reactants or products are highly organized, but experiments are in progress to extend this study to other vapor-phase coupling reactions that may lead to such structures. Acknowledgment. The excellent technical assistance of Barbara L. Wampler is gratefully acknowledged. Experiments at the University of New Mexico are supported by the Sandia-University Research Program (DOE) and the National Science Foundation (CHE-90146566). R.M.C. gratefully acknowledges a Society of Analytical Chemists of Pittsburgh Starter Grant Award and an Office of Naval Research Young Investigator Award. Research at Sandia National Laboratories is supported by the U S . DOE under Contract No. DE-AC04-76DP00789. Registry NO.Au, 7440-57-5; [CH3(CH2)7](CH3)2SiCI,18162-84-0; 4-HTP, 637-89-8; 4-ATP, 1193-02-8.
Supplementary Material Available: Details of the SAW, ellipsometry, and synthesis and spectral characterization of solution analogues of the surface-confined reactants and products ( 5 pages). Ordering information is given on any current masthead page. (8) Enhancements in ring mode absorptions after reaction are most likely the result of orientationalchanges that are consistent with the surface selection rules for FTIR-ERS; see: (a) Greenler, R. G . J . Chem. Phys. 1966,44, 310. (b) Greenler, R. G . J . Chem. Phys. 1969,50, 1963. (c) Porter, M. D. Anal. Chem. 1988,60, 1143A. (9) The band assignments for surface-confinedHS(C6H,)OSi(CH3)2[(CH2)7CH3] have been confirmed by comparison to an authentic sample of Details of the synthesis and the NMR and (C,H~)OSi(CH3)2[(CH2)7CH3]. FTlR spectral analyses are given in the supplementary material. (a) Anderson, D. R. In Analysis of Silicones; Smith, A. L., Ed.; Wiley: New York, 1974; Chapter 10. (b) Bellamy, L. J. The Infra-red Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975; Chapter 20.
0002-7863/91/1513-8552$02.50/0
Molecular Orbital Theory Calculations of Aqueous Solvation Effects on Chemical Equilibria Christopher J. Cramer* US.Army Chemical Research Development and Engineering Center Aberdeen Proving Ground, Maryland 2101 0-5423 Donald G. Truhlar* Department of Chemistry, Supercomputer Institute, and Army High- Performance Computing Research Center University of Minnesota Minneapolis, Minnesota 55455-0431 Received July 16, 1991 Molecular modeling techniques1 have advanced to the point where computational chemistry can predict the relative energies of many interesting structures, intermediates, and possible reaction products. Parametric models2+ based on semiempirical molecular orbital theory are especially useful for treating substituent effects and evaluating competing structures; for reactions in aqueous solution, though, there is considerably uncertainty about the applicability of the calculated results since the computational models do not include the solvent. One way to improve on this situation is to combine these models with the local-field S C F a p p r o a ~ h . ~ In this spirit, we have recently proposed and calibrated a new parameterized model,6 called AM1-SM1, in which an aqueous "solvation model" (SM1) is added to the Fock operator from neglect-of-diatomic-differential-overlap7 semiempirical molecular orbital theory using the Austin model 1 (AMI), parameterization for the solute. SMI treats the solvent as a bulk continuum with a generalized Born modeI8s9with dielectric screening for the polarization energy (we use a model in which the solute cavity from which dielectric is excluded is composed of superimposed spheres9p10) and with surface tension terms" (based on the solvent-accessible surface areal2) for cavity and dispersion effects. Parameters are available6 for 298 K for solutes containing H, C, N, 0, F, S, C1, Br, and I. The theory is especially promising because it requires considerably less in the way of computational resources than simulations with explicit inclusion of a large number of water molecule^,'^ yet at the same time it allows for solventinduced changes in the solute charge distribution. Here we report the first tests of AM1-SM1 for the effect of solvation on reactive equilibria, in particular for acid-base proton transfer reactions, prototropic tautomerizations, and the rotameric isomerization of the peptide linkage. We define AAGO,,,, = AGO,, - AGO, (1) where AGO is the standard-state (1 M) free energy change for (1) Naray-Szabo, G.; Surjan, P. R.; Angyan, J. G.Applied Quantum Chemistry; Reidel: Dordrecht, 1987. (2) Dewar, M. J. S.; Thiel, W. J. Am. Chem. SOC.1977, 99,4899,4907. (3) (a) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J . Am. Chem. SOC.1985, 107, 3902. (b) Dewar, M.J. S.;Dieter, K. M. J. Am. Chem. SOC.1986, 108, 8075. (4) Stewart, J. J. P. J . Comput. Chem. 1989, 10, 209, 221. ( 5 ) For reviews, see: (a) Tapia, 0. In Quantum Theory of Chemical Reactions; Daudel, R., Pullman, A., Salem, L., Viellard, A., Eds.; Reidel: Dordrecht, 1980; Vol. 2. p 25. (b) Tomasi, J.; Alagona, G.; Bonaccorsi, R.; Ghio, C. In Modelling of Structure and Properties of Molecules; MaksiE, 2. B., Ed.; Horwood: Chichester, 1987; p 330. (6) Cramer, C. J.; Truhlar, D. G. J . Am. Chem. SOC.,in press. (7) (a) Pople, J. A.; Santry, D. P.; Segal, G. A. J. Chem. Phys. 1965.43, 5129. (b) Pople, J. A.; Beveridge, D. L. Approximate Molecular Orbital Theory; McGraw-Hill: New York, 1970. (8) (a) Hoitjink, G. J.; de Boer, E.; van der Meij, P. H.; Weijland, W.P. Reel. Trau. Chim. Pays-Bas 1956, 75,487. (b) Peradejordi, Cah. Phys. 1%3, 17, 393. (9) Still, W. C.; Tempczak, A,; Hawley, R. C.; Hendrickson, T. J . Am. Chem. SOC.1990, 112, 6127. (10) (a) Miertius, S.;S c r m , E.; Tomasi, J. J. Chem. Phys. 1981,55, 117. (b) Bonaccorsi, R.; Cimiraglia, R.; Tomasi, J. J . Comput. Chem. 1983,4, 567. (1 1) Hermann, R. B. J . Phys. Chem. 1972, 76, 2754. (12) Lee, B.; Richards, F. M. J . Mol. Biol. 1971, 55, 379. (13) (a) Watts, R. 0.; Clementi, E.; Fromm, J. J . Chem. Phys. 1974,61, 2550. (b) Beveridge, D. L.; DiCapua, F. M. Annu. Reo. Biophys. Biophys. Chem. 1989, 18,431. (c) Jorgensen, W. L. Chemtracts: Org. Chem. 1991, 4. 91.
0 1991 American Chemical Society
Communications IO the Editor
J. Am. Chem. SOC.,Vol. 113, No. 22, 1991
Table 1. Proton Transfer Free Energy Changes on Solvation AAG -aq, kcal /mol
-- + -- + -- + - ++ - +-
reaction N H A ++ aniline N H I + aniline.H+ NH? MeNH, NH, MeNH3+ N H 3 pyridineH+ NH4' + pyridine NH4+ + M e 2 N H N H 3 + Me2NH2+ NH4+ Me,N N H 3 Me,NH+ NH4+ PhCO2NH, + PhCO2H NH4+ AcON H 3 AcOH NH, PhOH NH4+ PhONH, CpH* NH4+ + Cpaniline MeNH,+ ani1ine.H' + MeNH, aniline pyridine.H+ aniline.H+ pyridine aniline.H+ Me,NH aniline Me2NH2+ aniline + Me,NH+ aniline.H+ + Me,N aniline + PhC0,H aniline.H+ + PhCOT aniline AcOH aniline.H+ AcOaniline + PhOH aniline.H+ + PhOaniline + C p H aniline.H+ + CpMeNH, + pyridine.H+ MeNH3+ + pyridine MeNH, + Me2NH2+ MeNH,+ + Me2NH Me,NH+ MeNH3+ Me3N MeNH, PhC0,MeNH, PhC0,H MeNH,' MeNH, AcOH MeNH3+ + AcOMeNH, PhOH MeNH3+ + PhOMeNH, CpH MeNH,' + Cppyridine.H+ Me2NH pyridine Me2NH2+ pyridine Me,NH+ pyridine.H+ Me,N pyridine PhC0,H pyridine.H+ PhC0,pyridine + AcOH pyridine.H+ + AcOpyridine + PhOH pyridine.H+ + PhOpyridine + CpH pyridine.H+ + CpM e 2 N H + Me3NH+ Me2NH2++ Me,N Me2NH2+ PhC0,Me2NH + PhC02H Me2NH + AcOH Me2NH2++ AcOMe2NH + PhOH Me2NH2++ PhOMe,NH + CpH Me2NH2++ CpMe,NH+ PhCOY Me3N P h C 0 2 H Me3N AcOH Me3NHt + AcOMe3N PhOH Me3NHt + PhOMe,N CpH Me3NH+ + CpPhCO2H + AcOPhCO2- AcOH PhC02H + PhO- -m PhCO2- PhOH PhC02- C p H PhC02H + CpAcOH PhOAcO- PhOH AcO- CpH AcOH + CpPhOH + CD- PhO- + C D H " From refs I5 and 17. * C p = cyclopentadienyl
+
+ + + +
-- +++ -+ - + -- + + + --- ++ + - -+ + + + -- ++ + ---+ -- + + - ++ - ++ + + + --- ++ + +
-+
expt"
AMISMI
12.9 9.6 25.1 17.6 23.7 37.3 46.9 41.1 37.6 9.6 25.1 17.6 23.7 24.4 34.0 28.2 24.7 15.5 8.0 14.1 27.7 37.3 31.6 28.0 -7.6 -1.4 12.2 21.8 16.0 12.5 6.2 19.7 29.3 23.5 21 .o 13.7 23.2 17.4 14.9 9.4 3.7 1.3 -5.7 -8.1 -2.4
19.0 11.3 25.9 20.6 28.8 146.4 151.7 143.0 137.5 11.3 25.9 20.6 28.8 127.4 132.6 124.0 118.5 14.6 9.4 17.4 135.2 140.4 131.7 126.2 -5.3 2.9 120.6 125.8 117.1 111.6 8.2 125.8 131.1 122.4 116.9 117.6 122.9 114.3 108.7 5.3 -3.5 -8.9 -8.6 -14.2 -5.5
(CsH5).
the equilibrium in question. In all cases AGO,, was calculated by optimizing the structures of reactants and products in aqueous solution with AMI-SMI, and AGO, was computed from gas-phase optimized structures by AMI .I4 Vibrational contributions were assumed to cancel, Le., to change negligibly upon solvation. A value implies that the illustrated equilibrium positive AAGO,,, shifts to the left on solvation, and a negative value implies the opposite. The results for acid-base equilibria are in Table I, where they are compared to experiment. The latter were obtained by corproton affinities or deprotonation recting the e~perimentall~ enthalpies using AMI calculated absolute entropiesI6 to obtain AGO, values and using standard pK, differences" to obtain AGO,,.
#*,
(14) All calculations were carried out using AMSOL version 1 .O, Cramer, C. J., Truhlar, D. G., Quantum Chemistry Program Exchange program no. 606, which is based on AMPAC version 2.1: Liotard, D. L.; Healy, E. F.; Ruiz, J. M.; Dewar, M. J. S . QCPE Bull. 1989, 9, 123. See: Cramer, C. J.; Truhlar, D. G. QCPE Bull. 1991, 11, 57. (15) (a) Meot-Ner, M.; Sieck, L. W. J. Am. Chem. Soc. 1991, 113. 4448. (b) Lias, S. G.: Liebman, J. F.; Levin, R. D. J. Phys. Chem. Ref. Dara 1984, 13, 695. (16) Dewar, M. J. S.; Ford, G. P. J. Am. Chem. Soc. 1977, 99, 7822. ( I 7 ) (a) Handbook ofchemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, 1988. (b) Streitwieser, A., Jr.; Nebenzahl, L. L. J. Am. Chem. Soc. 1976, 98, 2188.
8553
Table 11. Isomerization Free Energy Changes on Solvation
==== Mh
1:
5:
reaction 1 2 3 a
fi
AAG'g*q, kcal / mol expt" AMI-SMI >4.0
