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Theoretical models for activation of CO2 towards hydration (CO2 + H2O → H2CO3) by cationic binding sites
Cbemic&Pbysics65 (19.32) 107-111 North-fioEand PubIShiig Company
.. ’ ..
FOR-ACT&TION OF CO2 TOWARDS HYDEUTION BY~CATIONiC BINDING SITES : Yves JEAN and Frtiiois VOLATRON -0botitoire de Qi&e 3h&&pe (ERA 549j, Uni?wsiti &is-&d, THE&&I&~
(CO2 + HZ0
MCCDELS
4
H2C03)
L&iment 490. F-91405 G-STY Cedex, Frmee
Received 28 July 1981
Theoretical models are studkd which ikstrare how the hydration reaction CO* + H20 -+S&C03 cm be catalyzed by one or two ationic binding sites &). In the latter, fhe arrangementof the two binding sites is held during the course of the reaction. Simulating a rigid molecukr receptor. Two daerent arrangementi of the binding sites are studied, and ;hek r&t&T
abiiities to lower the activaiion energy of the hydration reaction are studied.
1. Mroduction
Receptor-substrate interactions can lead both to complexation and activation of the substrate. Theoretical studies on hydrogen bonding and sohation of molecules and ions-[i-6] provide theo:etical models for the interaction between 2 substrate and the binding sites df a receptor. Moreover, in some calculations [7,8], not only the binding sites, but the whole receptor molecules have been-taken into account. On the other band, perturbations induced in 2 substrate by intermolecular interactions can either increase or decrease its reactivity towards a given reactant [9-131. The&ore, theoretical models can be investigated for complexation and activation of moiecules by biological or synihetic receptor molecules. In 2 recent p2per 1141, Jean and Lehn reported a study of the stability of various [COz, n&s] complexes (n = 1,2 and 4). They also showed that complexation of CO2 by positive bhding sites (represented by positive charges) makes easier the approach of a water molecule, acting as a nucleophile, towards the carbon atom. in this paper, we report a theoretical model for the overali.C02-H,C0, interconversion, which involves a proton transfer from Hz0 to Co2 moiety. Carbon dioxide is activated by ammonium binding sites, taken +s models for binding with protonated amines. Such ammo&m sites are actually the binding sites of an ar0301-Q1U4/82/0000-0&?0/$02.75
0 2982 North-Holland
tlficial molecuiar receptor, designed for the recognition of linear triatornic m&ecules, which has been recently synthetized [15]. We successively study the catalytic influence of(i) one and (ii) two cationic sites. In the latter, two spatial arrangements of the cationic sites are considered, simulating two different molecular receptors.
2. Method of calctition We performed ab-i&o calculations using the GAUSSIAN 70 series of programs [16], with STO-3G [17] basis set. The limitations of minimal basis set calculationi are well hewn. However, this level of c&uktion is sufficient for our purpose which is to study qualitatively the catalytic role of cationic binding sites ti CO2 hydration reaction. We used for NH: [IS] and CO2 [19] the geometries optimized previously (N-H : 1Lid4 A and C=O : 1.I 88 A); optimization of the reaction product, HzCO,, led to the following set of geometric2I parameters: C=O : 1.219 1Ic,C-O : 1.387 ii, O-H : 0.983 R, LO==C-0: 124’4 and K-O-H : 106’2. :’ The energy is -260.0771 au. In-a prelimikry cakulatio~, we studied the gcataIyzed reaction: The resiction pathway for the CO2 + HZ0 + H2C0, hydr&ion reaction has been already ca& c&ted by ab irdtio methods using 2 double zeta basis
Y. Jean, F. Voknon;Actimtion of CO2 lorrardr /iydmtion eaerpy of 53 kcal/mol for th= hydration reaction (fig. 2, curve (a)], a value close to that computed activation
Fi.
1. Schen??ticrepresentation
CO2 + Hz0 - HICOJ hydiation
of the
reaction coordinate reaction (see ref. [tOI)_
for
set [20] . We did not try to recompute the reaction path with our own basis set. We simply used the reaction cuorcjinate reported by Jonsson et al. [ZO] , which is mainly characterised, at the transition state, by a mixing between II,0 approach (C_..O, distance) and proton +mfer from Hz0 to CO, moiety (03-H distance) (fig. I). Using this reaction coordinate, we found an t E(KCAUMOL.)
at the SCF level in ref. [20] (56 kcalfmol). The transition state is found for a CL..03 distance of 1.71 a, instead of 1.68 A in ref. [20].. Now, we study how this potential energy curve is modified when CO2 interacts with cationic binding sites. In alI calcularions, the reaction coordinate associated with the approach of Hz0 towards CO2 is that used for the uncatalyzed reaction_ Only the orientation of the [CO2..H,O] supermolecule with respect to the cationic binding site(s) has been varied.
3. Activation of CO, by one eatio&e bittdiq site: [CO,,NH$] +H20+ [H2COs, NH$]
Depending on which cxygen atom the proton migrates, there are two structures for *he reaction product {H&O,) m] (fig. 3). Keeping constant the geome&tiesof monomers, we optimized the intermolecular geometrical parameters,RN...O distance and NOC angle (3) depicted in fig. 3. In these calcu!ations, the N-H .._Ohydrogen bond is assumed to be linear. Structure 1 is found to be more stable than 2 by 8.3 kcal/mo!.
Ibis result can be readily rationahzed by noticing that H2COs is stabilized by a donation from OH groups to the & orbital of carbonyl group. Complexation of the car-bony1 oxygen atom 0, by NH$ (1) increases this 71donation and leads to the most stable complex. On the contrary, the n-donor character of 02 in 2 is decreased by its interaction with @. The stabilization energges associated with the formation of complexes 1 2nd 2 aie -37 and -28.7 kcaI/mol reqectively. ‘Ibere-
HIO i
Bc\o/H CL t _: a.?.‘3
2.A236 ;
2. Potential energy curves for CO2 + HI0 -c H&O3 hy-dqtionreaction. (a) Unutalyzed reaction; (b) CO2 activated by one nnmoniumbind@ site; (c) COI, activated by Tao ammonium binding sites, ia the arrangement depicted in fii. 5 (receptor I); (d) CO, actiwted by two smmonium binding sites, in the armenent depicted in fig. 5 (receptor II). Fii.
I
+i3
1 Fig. 3. Optimum structuresfor [H2C03, ti energiesare -316.0047 au (I) and -3159916
2
I complexes.lXe au (2).
Fk. 4. Schematic representation of the reaction pztb
for [CO*,
NH: j
+ i-I20 +
[HsCOs, NH:] reaction [see fs.
2, CUTE@) and
table 11.
fore, e is more efficient for stabilizing the reaction product thau the reactant, the stabilization energy associated witha liuear CO2 . ..NH$ complex being only
16.9 kcal/rnoL Among the two mechanisms for proton migration, we only studied that leading to ‘he most stable product 1 (fig. 4). At each point of the reaction path, both the distance (RRN.__,r)and the orientation ($=NOl C) of NH$ with respect to the reacting molecules (CO2 and H20) have been optimized. The results are reported in table 1 and fz_ 2 [curve (b)] . The cataIytic activity of the cationic binding site Nii results in a lowering of the activation energy from 53 to 38.8 kczljmol. Table 1 Optimum intermolecular parameters(RN,..Q and Q = LNOrC) as a function of reaction coordinate in [CO*, NI$] + H20 + [HeCOs. NH;] reaction (see else fii. 4). ~&given withrespect to reactants [CO,, NH:] and Ha0 to infinity, is reported in fs. 2 [cmve @)I
&...O, (-4) RN...01(A) 2.550
0 (ded
2.63
2.515
2.43 2.23 2.03 1.83 1.73
2.507 2.498 2.489 2.480 2.459
1.63
2.446
1.53 1.46 1.387
2.441
180 152.7 150.0 147.2 144.4 141.6 138.9 132.9 1295 128.6
2.432
125.5
2.423
122.3
3.0
2.523
0 -
6.0
-
1.5
-
7.0
-
2.3 8.2 31.8 38.8 27.2 -0.9 -33.7 -46.9
4. Actimtion of CO2 by two cationic binding sites:
[CC5 2N$]+
H20’
&CO3,2NH$]
In this section, we study a theoretical model for an activation of CO2 by receptor molecules with two binding sites. Only the binding sites (NH$) are explicitly taken into account in our calculations, while in an actual receptor mo!ecule covalent bonds would hold them in a given arrangement. The relative position of the binding sites is held during the course of the reaction, sirmdating a rigid receptor molecule. On the basis of previous calculations on [CO,, 2iT$J complexes [14], we selected two arrangements (I and II) fig. 5) for the binding sites, which should be rather efficient for CO2 activation. They are such that, at the beginning of the reaction (C...03 = -), CO, can interact with the receptor through linear N-K.0 hydrogen bonds making angles of 60” (I) and 90” (II) with the NJ axis (fig. 5). In our calculations, we kept the 01...02 and N..N axis parallel (fig. 6) and we adjusted the position of the supermolecule [C02, H,O] with respect to the binding sites through optimization of the parameters R and r defmed in fig. 6. The results of these optimizatiom are reported in table 2, and the corresponding energy curves are drawn in fig. 2 [curves (c) and (d) for receptors I and II respectively] . Both molecular receptors I and II catalyze the hydration reaction more efficiently than does an unique &ionic binding site [cuve (b)] . Furthermore, the efficiency of receptor II is much greater than that of receptor I, the activation ene:.$es being 20.8 and 1.3 kcal/moI (for I and II reapectivey). This is due to the fact that the arrangement of bin&g sites in II is not very efficient for stab&&on of CO2 molecule [14], which is the reactant, while it is
110
Fig.
5.
;bns
Geometriesof “mo~ecukr receptors”
I and II. Only the binding sites 0%)
;LIe explicitly
taker? into account in the c&oki-
(see text).
ideal for the product H2C03. Indeed, if we super“\o ‘ -. pose the two (H2C03,Ni) compIexes depicted in fig. , 3, it can be seen that the ideal arrangement for two binding sites (whose electrostatic energy is neglected) is characterized by a N...N distance of 2.702 A (instead of 2.376 A in II, fig. 5) and by L.N..M angles of 82”6 (1) I R aad 88”I (2) (instead of 90’ in LI, fig_ 5). On the con4 tmy, configuration 1 for the cationic binding sites is H_$$ !-I ....““...~...“‘.......... /it better for stabilizing the reactant 1141, but less efficient % for the product H,CO, (N..N = 5.046 a, and LN...NH H *$ $.I4 = 60”). These theoretical models illustrate the imporFig. 6. Definition of the intemobculax parameters, R al?d r, ;ance of the spatial arrangement of binding sites in a optIn%?& in the reactions ca+aIyzed by two bind& sites. ThGs receptor molecule. Both receptors I and II have two @rire correspondsto the reaction catalyzed by the molecular cation&zbinding sites. However, receptor II is more efa!rnost
receptor I (see f& 5).
T&k 2 Opthun intermolecolar pameters (R and r, see fg. 6 for deftition) as a function of reaction mordinate in the hydration reaction utalyzcd by mobcti receptors I and II. AJ?, given with respect to reactants [Cc)_. 2NH?,] and Hz0 to Winity, is reported in fq. 2 (c) and (d) for molecular receptors I and II respeetivelyj
[curves
k...o,
@)
AE tkal/mol)
r (A)
R W I
II
I
XI
I
II
2.312
2.800
1.188
ISSS
0
3.0
2.312
2.800
1.188
1.185
-9.7
-10.2
2.63
2.230
2.43 2.23
2.236 2.222
2.728 2.693
I .188 1.181
1.188 i.189
-12.5 -14.0
-13.4 -18.4
2.658
1.174
1.189
-11.9
2.03
2.209
2.623
1.189
-4.5
I.83
2.19s
2.588
1.167 1 .lS9
1.190
17.6
1.73
2.167
2.518
1.143
1.191
26.8
-0.7
1.63
2.159
2.477
1.128
1.194
10.0
--13.3
153 1.46
2.172
2.465
1.114
I.198
-16.1
2.179
2.460
1.107
1.200
-44.7
-72.4
1.357
2.186
2.455
I.!00
1.203
-57.3
-91.8
0
-2p.1 -16.8 1.3
-39.0
ficient than receptor I for lowerin& the activation energy because it fits better the reaction prcduct. Consequently, a stronger driving force towards the reaction product is induced which makes the rektion easier j211.
A&nowiedgement Professor J&¶. Lehr is gratefully acknowiedged for smesting this study and for critical readings of the manuscript.
References [l] PA KoIIman, Accounts Cbem. Res. 10 (1977) 365; in: Modern theoretical chemistry, Vol4, ed. H.F. Schaefer III (Plenum Press, New York, 1977) p. 109. [Zj K. Mcrckuma, Accounts Chem. Res. 10 (1977) 294. 13 ] W. Kdos, Theoret. Chim. Acta 54 (1980) 187. 14) P. Sehuter, W. Hkubeti and W. Me&s, Topics Current Chem. 60 (1975) 1. [5] L. Radom. in: Modern theoretical chemistry, VoL 4, ed. H.F. Schaefer III (Plenum Press, New York, 19X’) p. 333. [6] A. PuIbnan and A.M. Armbruster, Chem. Phys. Letters 36 (1975) 558. [7] A. Pulhnan, C. Giessner-Prettre and Yu.V. Kruglyzk, Chem. Phys. Letters 35 (1975) 156.
[8] _ T. Yamabe, K. Hori, K. .A&+$ and K. F&I& Tetrahedron 35 (1979) 1065. [S] P.L. Huyskens. J. Am. Chem. Sac. 99 (1977) 2578. [IO] A. Johansson, P. KaIIman, S. Rothenberg and J. McKeivey, J. Am. Chem. Sot. 96 (1974) 3794. [ll] Y.C. TseandM.D. Newton, J. Am.Chem. Sot. 99 (1977) 612. I121 (a) I.H. Whims. D. Spangler, D.A. Femec, G.M. Ma@on and R.L. Schowen, J. Am. Chem. Sot. 102 (1980) 6619; (b) LH. Warns, G.M. Maggiora and R.L. Schowen, I. Am. Chem. Sot 102 (1980) 7831. [13] Ft. Lmery, M. de Oliveira and B. Pulhnz, Int J. Quantum Chem., Q::atum Biol. Symp. 6 (1979) 459. [14] Y. Jean sod J.&f. Lehn, Chem. Phys. 39 (1979) 111. [15] J.M. L&n, E. Sonveau and A. Willard, J. Am. Chem. Sot. 100 (1978) 4914. [16] W.J. Hehre, W.A. Lath%& R. DItctieId, M.D. Newton and J.A. Pople. Quantum Chemistry Program Exchange, Program no. 236, Indiana University, Bloomington, In1171 %?&ehze, RF. Stew& and J.A. Pople, J. Chem. Phys. 51 (1970) 2657. [IS] W.A. Lathan, L.A. Curtiss, WJ. Helue, J.B. Lisle aad J.APop& Progr. whys. Crg. Chem. ll(1974) 175. [19] M.D. Newton, W.A. Lathen,W.J. Hehro and J.A. PopIe, J. Chem. Phys. 52 (1970) 4064. [ZO] B. Znsson, G. KarIstrstriim,H. Wennerstrdm. S. For& B. Roos 2nd J. Almliif, J. Am. Chem. Sot. 99 (1977) 4628. [21] W.P.Jencks, in: Catalysis in chemistry and enzymology (McGraw-Hill, New York, 1969j ch. 5.
.. ’ ..
FOR-ACT&TION OF CO2 TOWARDS HYDEUTION BY~CATIONiC BINDING SITES : Yves JEAN and Frtiiois VOLATRON -0botitoire de Qi&e 3h&&pe (ERA 549j, Uni?wsiti &is-&d, THE&&I&~
(CO2 + HZ0
MCCDELS
4
H2C03)
L&iment 490. F-91405 G-STY Cedex, Frmee
Received 28 July 1981
Theoretical models are studkd which ikstrare how the hydration reaction CO* + H20 -+S&C03 cm be catalyzed by one or two ationic binding sites &). In the latter, fhe arrangementof the two binding sites is held during the course of the reaction. Simulating a rigid molecukr receptor. Two daerent arrangementi of the binding sites are studied, and ;hek r&t&T
abiiities to lower the activaiion energy of the hydration reaction are studied.
1. Mroduction
Receptor-substrate interactions can lead both to complexation and activation of the substrate. Theoretical studies on hydrogen bonding and sohation of molecules and ions-[i-6] provide theo:etical models for the interaction between 2 substrate and the binding sites df a receptor. Moreover, in some calculations [7,8], not only the binding sites, but the whole receptor molecules have been-taken into account. On the other band, perturbations induced in 2 substrate by intermolecular interactions can either increase or decrease its reactivity towards a given reactant [9-131. The&ore, theoretical models can be investigated for complexation and activation of moiecules by biological or synihetic receptor molecules. In 2 recent p2per 1141, Jean and Lehn reported a study of the stability of various [COz, n&s] complexes (n = 1,2 and 4). They also showed that complexation of CO2 by positive bhding sites (represented by positive charges) makes easier the approach of a water molecule, acting as a nucleophile, towards the carbon atom. in this paper, we report a theoretical model for the overali.C02-H,C0, interconversion, which involves a proton transfer from Hz0 to Co2 moiety. Carbon dioxide is activated by ammonium binding sites, taken +s models for binding with protonated amines. Such ammo&m sites are actually the binding sites of an ar0301-Q1U4/82/0000-0&?0/$02.75
0 2982 North-Holland
tlficial molecuiar receptor, designed for the recognition of linear triatornic m&ecules, which has been recently synthetized [15]. We successively study the catalytic influence of(i) one and (ii) two cationic sites. In the latter, two spatial arrangements of the cationic sites are considered, simulating two different molecular receptors.
2. Method of calctition We performed ab-i&o calculations using the GAUSSIAN 70 series of programs [16], with STO-3G [17] basis set. The limitations of minimal basis set calculationi are well hewn. However, this level of c&uktion is sufficient for our purpose which is to study qualitatively the catalytic role of cationic binding sites ti CO2 hydration reaction. We used for NH: [IS] and CO2 [19] the geometries optimized previously (N-H : 1Lid4 A and C=O : 1.I 88 A); optimization of the reaction product, HzCO,, led to the following set of geometric2I parameters: C=O : 1.219 1Ic,C-O : 1.387 ii, O-H : 0.983 R, LO==C-0: 124’4 and K-O-H : 106’2. :’ The energy is -260.0771 au. In-a prelimikry cakulatio~, we studied the gcataIyzed reaction: The resiction pathway for the CO2 + HZ0 + H2C0, hydr&ion reaction has been already ca& c&ted by ab irdtio methods using 2 double zeta basis
Y. Jean, F. Voknon;Actimtion of CO2 lorrardr /iydmtion eaerpy of 53 kcal/mol for th= hydration reaction (fig. 2, curve (a)], a value close to that computed activation
Fi.
1. Schen??ticrepresentation
CO2 + Hz0 - HICOJ hydiation
of the
reaction coordinate reaction (see ref. [tOI)_
for
set [20] . We did not try to recompute the reaction path with our own basis set. We simply used the reaction cuorcjinate reported by Jonsson et al. [ZO] , which is mainly characterised, at the transition state, by a mixing between II,0 approach (C_..O, distance) and proton +mfer from Hz0 to CO, moiety (03-H distance) (fig. I). Using this reaction coordinate, we found an t E(KCAUMOL.)
at the SCF level in ref. [20] (56 kcalfmol). The transition state is found for a CL..03 distance of 1.71 a, instead of 1.68 A in ref. [20].. Now, we study how this potential energy curve is modified when CO2 interacts with cationic binding sites. In alI calcularions, the reaction coordinate associated with the approach of Hz0 towards CO2 is that used for the uncatalyzed reaction_ Only the orientation of the [CO2..H,O] supermolecule with respect to the cationic binding site(s) has been varied.
3. Activation of CO, by one eatio&e bittdiq site: [CO,,NH$] +H20+ [H2COs, NH$]
Depending on which cxygen atom the proton migrates, there are two structures for *he reaction product {H&O,) m] (fig. 3). Keeping constant the geome&tiesof monomers, we optimized the intermolecular geometrical parameters,RN...O distance and NOC angle (3) depicted in fig. 3. In these calcu!ations, the N-H .._Ohydrogen bond is assumed to be linear. Structure 1 is found to be more stable than 2 by 8.3 kcal/mo!.
Ibis result can be readily rationahzed by noticing that H2COs is stabilized by a donation from OH groups to the & orbital of carbonyl group. Complexation of the car-bony1 oxygen atom 0, by NH$ (1) increases this 71donation and leads to the most stable complex. On the contrary, the n-donor character of 02 in 2 is decreased by its interaction with @. The stabilization energges associated with the formation of complexes 1 2nd 2 aie -37 and -28.7 kcaI/mol reqectively. ‘Ibere-
HIO i
Bc\o/H CL t _: a.?.‘3
2.A236 ;
2. Potential energy curves for CO2 + HI0 -c H&O3 hy-dqtionreaction. (a) Unutalyzed reaction; (b) CO2 activated by one nnmoniumbind@ site; (c) COI, activated by Tao ammonium binding sites, ia the arrangement depicted in fii. 5 (receptor I); (d) CO, actiwted by two smmonium binding sites, in the armenent depicted in fig. 5 (receptor II). Fii.
I
+i3
1 Fig. 3. Optimum structuresfor [H2C03, ti energiesare -316.0047 au (I) and -3159916
2
I complexes.lXe au (2).
Fk. 4. Schematic representation of the reaction pztb
for [CO*,
NH: j
+ i-I20 +
[HsCOs, NH:] reaction [see fs.
2, CUTE@) and
table 11.
fore, e is more efficient for stabilizing the reaction product thau the reactant, the stabilization energy associated witha liuear CO2 . ..NH$ complex being only
16.9 kcal/rnoL Among the two mechanisms for proton migration, we only studied that leading to ‘he most stable product 1 (fig. 4). At each point of the reaction path, both the distance (RRN.__,r)and the orientation ($=NOl C) of NH$ with respect to the reacting molecules (CO2 and H20) have been optimized. The results are reported in table 1 and fz_ 2 [curve (b)] . The cataIytic activity of the cationic binding site Nii results in a lowering of the activation energy from 53 to 38.8 kczljmol. Table 1 Optimum intermolecular parameters(RN,..Q and Q = LNOrC) as a function of reaction coordinate in [CO*, NI$] + H20 + [HeCOs. NH;] reaction (see else fii. 4). ~&given withrespect to reactants [CO,, NH:] and Ha0 to infinity, is reported in fs. 2 [cmve @)I
&...O, (-4) RN...01(A) 2.550
0 (ded
2.63
2.515
2.43 2.23 2.03 1.83 1.73
2.507 2.498 2.489 2.480 2.459
1.63
2.446
1.53 1.46 1.387
2.441
180 152.7 150.0 147.2 144.4 141.6 138.9 132.9 1295 128.6
2.432
125.5
2.423
122.3
3.0
2.523
0 -
6.0
-
1.5
-
7.0
-
2.3 8.2 31.8 38.8 27.2 -0.9 -33.7 -46.9
4. Actimtion of CO2 by two cationic binding sites:
[CC5 2N$]+
H20’
&CO3,2NH$]
In this section, we study a theoretical model for an activation of CO2 by receptor molecules with two binding sites. Only the binding sites (NH$) are explicitly taken into account in our calculations, while in an actual receptor mo!ecule covalent bonds would hold them in a given arrangement. The relative position of the binding sites is held during the course of the reaction, sirmdating a rigid receptor molecule. On the basis of previous calculations on [CO,, 2iT$J complexes [14], we selected two arrangements (I and II) fig. 5) for the binding sites, which should be rather efficient for CO2 activation. They are such that, at the beginning of the reaction (C...03 = -), CO, can interact with the receptor through linear N-K.0 hydrogen bonds making angles of 60” (I) and 90” (II) with the NJ axis (fig. 5). In our calculations, we kept the 01...02 and N..N axis parallel (fig. 6) and we adjusted the position of the supermolecule [C02, H,O] with respect to the binding sites through optimization of the parameters R and r defmed in fig. 6. The results of these optimizatiom are reported in table 2, and the corresponding energy curves are drawn in fig. 2 [curves (c) and (d) for receptors I and II respectively] . Both molecular receptors I and II catalyze the hydration reaction more efficiently than does an unique &ionic binding site [cuve (b)] . Furthermore, the efficiency of receptor II is much greater than that of receptor I, the activation ene:.$es being 20.8 and 1.3 kcal/moI (for I and II reapectivey). This is due to the fact that the arrangement of bin&g sites in II is not very efficient for stab&&on of CO2 molecule [14], which is the reactant, while it is
110
Fig.
5.
;bns
Geometriesof “mo~ecukr receptors”
I and II. Only the binding sites 0%)
;LIe explicitly
taker? into account in the c&oki-
(see text).
ideal for the product H2C03. Indeed, if we super“\o ‘ -. pose the two (H2C03,Ni) compIexes depicted in fig. , 3, it can be seen that the ideal arrangement for two binding sites (whose electrostatic energy is neglected) is characterized by a N...N distance of 2.702 A (instead of 2.376 A in II, fig. 5) and by L.N..M angles of 82”6 (1) I R aad 88”I (2) (instead of 90’ in LI, fig_ 5). On the con4 tmy, configuration 1 for the cationic binding sites is H_$$ !-I ....““...~...“‘.......... /it better for stabilizing the reactant 1141, but less efficient % for the product H,CO, (N..N = 5.046 a, and LN...NH H *$ $.I4 = 60”). These theoretical models illustrate the imporFig. 6. Definition of the intemobculax parameters, R al?d r, ;ance of the spatial arrangement of binding sites in a optIn%?& in the reactions ca+aIyzed by two bind& sites. ThGs receptor molecule. Both receptors I and II have two @rire correspondsto the reaction catalyzed by the molecular cation&zbinding sites. However, receptor II is more efa!rnost
receptor I (see f& 5).
T&k 2 Opthun intermolecolar pameters (R and r, see fg. 6 for deftition) as a function of reaction mordinate in the hydration reaction utalyzcd by mobcti receptors I and II. AJ?, given with respect to reactants [Cc)_. 2NH?,] and Hz0 to Winity, is reported in fq. 2 (c) and (d) for molecular receptors I and II respeetivelyj
[curves
k...o,
@)
AE tkal/mol)
r (A)
R W I
II
I
XI
I
II
2.312
2.800
1.188
ISSS
0
3.0
2.312
2.800
1.188
1.185
-9.7
-10.2
2.63
2.230
2.43 2.23
2.236 2.222
2.728 2.693
I .188 1.181
1.188 i.189
-12.5 -14.0
-13.4 -18.4
2.658
1.174
1.189
-11.9
2.03
2.209
2.623
1.189
-4.5
I.83
2.19s
2.588
1.167 1 .lS9
1.190
17.6
1.73
2.167
2.518
1.143
1.191
26.8
-0.7
1.63
2.159
2.477
1.128
1.194
10.0
--13.3
153 1.46
2.172
2.465
1.114
I.198
-16.1
2.179
2.460
1.107
1.200
-44.7
-72.4
1.357
2.186
2.455
I.!00
1.203
-57.3
-91.8
0
-2p.1 -16.8 1.3
-39.0
ficient than receptor I for lowerin& the activation energy because it fits better the reaction prcduct. Consequently, a stronger driving force towards the reaction product is induced which makes the rektion easier j211.
A&nowiedgement Professor J&¶. Lehr is gratefully acknowiedged for smesting this study and for critical readings of the manuscript.
References [l] PA KoIIman, Accounts Cbem. Res. 10 (1977) 365; in: Modern theoretical chemistry, Vol4, ed. H.F. Schaefer III (Plenum Press, New York, 1977) p. 109. [Zj K. Mcrckuma, Accounts Chem. Res. 10 (1977) 294. 13 ] W. Kdos, Theoret. Chim. Acta 54 (1980) 187. 14) P. Sehuter, W. Hkubeti and W. Me&s, Topics Current Chem. 60 (1975) 1. [5] L. Radom. in: Modern theoretical chemistry, VoL 4, ed. H.F. Schaefer III (Plenum Press, New York, 19X’) p. 333. [6] A. PuIbnan and A.M. Armbruster, Chem. Phys. Letters 36 (1975) 558. [7] A. Pulhnan, C. Giessner-Prettre and Yu.V. Kruglyzk, Chem. Phys. Letters 35 (1975) 156.
[8] _ T. Yamabe, K. Hori, K. .A&+$ and K. F&I& Tetrahedron 35 (1979) 1065. [S] P.L. Huyskens. J. Am. Chem. Sac. 99 (1977) 2578. [IO] A. Johansson, P. KaIIman, S. Rothenberg and J. McKeivey, J. Am. Chem. Sot. 96 (1974) 3794. [ll] Y.C. TseandM.D. Newton, J. Am.Chem. Sot. 99 (1977) 612. I121 (a) I.H. Whims. D. Spangler, D.A. Femec, G.M. Ma@on and R.L. Schowen, J. Am. Chem. Sot. 102 (1980) 6619; (b) LH. Warns, G.M. Maggiora and R.L. Schowen, I. Am. Chem. Sot 102 (1980) 7831. [13] Ft. Lmery, M. de Oliveira and B. Pulhnz, Int J. Quantum Chem., Q::atum Biol. Symp. 6 (1979) 459. [14] Y. Jean sod J.&f. Lehn, Chem. Phys. 39 (1979) 111. [15] J.M. L&n, E. Sonveau and A. Willard, J. Am. Chem. Sot. 100 (1978) 4914. [16] W.J. Hehre, W.A. Lath%& R. DItctieId, M.D. Newton and J.A. Pople. Quantum Chemistry Program Exchange, Program no. 236, Indiana University, Bloomington, In1171 %?&ehze, RF. Stew& and J.A. Pople, J. Chem. Phys. 51 (1970) 2657. [IS] W.A. Lathan, L.A. Curtiss, WJ. Helue, J.B. Lisle aad J.APop& Progr. whys. Crg. Chem. ll(1974) 175. [19] M.D. Newton, W.A. Lathen,W.J. Hehro and J.A. PopIe, J. Chem. Phys. 52 (1970) 4064. [ZO] B. Znsson, G. KarIstrstriim,H. Wennerstrdm. S. For& B. Roos 2nd J. Almliif, J. Am. Chem. Sot. 99 (1977) 4628. [21] W.P.Jencks, in: Catalysis in chemistry and enzymology (McGraw-Hill, New York, 1969j ch. 5.