Proton Abstraction from Carbon Acids. Ab Initio Molecular Orbital

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J. Phys. Chem. 1996, 100, 3441-3447

3441

Proton Abstraction from Carbon Acids. Ab Initio Molecular Orbital Study on the Proton Abstraction from Acetaldehyde by NH2-, OH-, F-, SiH3-, PH2-, and SHMikael Pera1 kyla1 † Department of Chemistry, UniVersity of Joensuu, P.O. Box 111, FIN-80101, Joensuu, Finland ReceiVed: June 20, 1995; In Final Form: September 15, 1995X

Proton abstraction from acetaldehyde by the first- and second-row hydride anions NH2-, OH-, F-, SiH3-, PH2-, and SH- have been investigated with ab initio quantum mechanical calculations up to the MP2/631+G**//HF/6-31+G* and MP2/6-311+G**//MP2/6-31+G* level. Progress of energies, geometric variables, and charge distributions along the reaction coordinate have been investigated by employing the intrinsic reaction coordinate method. These computations revealed that proton abstractions occur in three essentially separate steps. The main difference between the first- and second-row hydrides is that the carbon, which donates proton in the reaction, changes hybridization from sp3 to sp2 later in the reaction coordinate in the case of the first-row systems. Consequently, the flow of negative charge from the CH2 to CHO end of the developing enolate anion is retarded more in the case of the first-row hydrides. The results showed that the proton abstractions by the first-row hydride anions have more imbalanced transition states than the second-row hydride anions.

Introduction Proton transfer between normal acids, those involving electronegative atoms (e.g., N, O, F, S) as hydrogen acceptors or donors, are usually very fast and essentially diffusion controlled in the thermodynamically favorable direction.1-5 In contrast, proton transfer to or from carbon is usually much slower and has high intrinsic barrier. High intrinsic barriers have been attributed to changes in electron delocalization and hybridization, poor hydrogen bond donor-acceptor properties of carbon, and solvent reorganization in the transition state.1,6-8 Furthermore, proton abstractions from carbon acids producing resonancestabilized anions have been known for long to occur anomalously slow despite favorable reaction thermodynamics.6 The underlying reason giving rise to this phenomenon is that the delocalization of the negative charge, which stabilizes reaction product, lags behind the proton abstraction.9 Thus, the delocalization and proton abstraction are not synchronized in the reaction, there is transition-state imbalance. Transition-state imbalance has been used to describe situations in which processes like bond breaking and forming, charge development and destruction, and solvation and desolvation have progressed unequally at the transition state of the reaction.9-12 Although this phenomenon has been the most thoroughly demonstrated experimentally for proton abstractions from carbon acids, it is a general phenomenon that may occur whenever there is more than one process involved in chemical reaction.9,11,13 Much investigated systems in which the transition-state imbalance is revealed are the deprotonations of arylnitroalkanes (R, R′ ) aryl, Y ) NO2) by bases (B, ν is formal charge) like carboxylates, amines, and hydroxide ion.14-18

resonance on group Y (NO2 in the case of arylnitroalkanes) requires geometrical changes in the structure of the molecule and therefore is lagging behind the proton transfer and charge development on carbon. Theoretical qualitative and quantitative models have been developed to treat the transition-state imbalance.9,19-21 Saunders22 has recently investigated the transition state of the proton transfer between acetaldehyde and its enolate anion in the gas phase and reported results at several levels of molecular orbital theory. Bernasconi and Wenzel10 investigated similar reaction and confirmed that the transition state in the gas phase is characterized by charge imbalance. This phenomenon, which Bernasconi calls the principle of nonperfect synchronization, has been recently reviewed.9 Furthermore, enzyme-catalyzed proton abstractions from carbon acids, which has been a subject of much discussion recently,23-25 may involve transition-state imbalance which may be a significant factor in these reactions as well. Ab initio quantum mechanical model calculations on enzyme-catalyzed proton abstractions from carbon acids have been carried out in our laboratory.26 It has been the purpose of the present work to study in detail the mechanism of the gas-phase proton abstraction from carbon acids by ab initio quantum mechanical calculations. Acetaldehyde was used as a model carbon acid, and the proton abstracting bases NH2-, OH-, F-, SiH3-, PH2-, and SHconsidered span a wide range of proton affinities, producing highly endo- and exothermic as well as nearly thermoneutral reaction profiles. Reactant and product complexes and transition states were calculated, and the intrinsic reaction coordinate (IRC) method27 was utilized to investigate reaction paths of the proton abstractions. The use of the IRC method allowed investigating the progress of geometric and electronic variables along the reaction coordinate. Computational Details

In the abstraction the development of negative charge through † X

e-mail address: [email protected]. Abstract published in AdVance ACS Abstracts, January 15, 1996.

0022-3654/96/20100-3441$12.00/0

The ab initio molecular orbital calculations were carried out using Gaussian 92.28 The geometries of the isolated compounds, transition states, and the geometries of the reactant and product ion-dipole complexes were optimized at the HF/6-31+G* and MP2/6-31+G* level. In contrast to the earlier report at lower computational level transition states were found for NH2-, OH-, © 1996 American Chemical Society

3442 J. Phys. Chem., Vol. 100, No. 9, 1996

Pera¨kyla¨

TABLE 1: Relative Energies (kJ mol-1) of Reactant and Product Complexes, Transition States, and Products Calculated at the MP2/6-31+G**//HF/6-31+G* + ∆ZPE and MP2/6-311+G**//MP2/6-31+G* + ∆ZPE Levela MP2/6-31+G**//HF/6-31+G*

MP2/6-311+G**//MP2/6-31+G*

hydride

reactant complex

TS

product complex

product

reactant complex

TS

product complex

product

NH2OHFSiH3PH2SH-

-68.7 (6.0) -77.0 (5.9) -72.3 (2.1) -35.0 (2.4) -39.9 (3.7) -45.3 (3.6)

-86.6 (-5.0) -93.4 (-7.2) -87.2 (-13.8) 5.3 (-11.2) -3.8 (-10.1) 9.9 (-13.2)

-193.4 (13.3) -155.4 (7.6) -123.9 (-3.4) -105.9 (5.1) -34.0 (-2.9) -35.2 (7.6)

-157.2 (6.8) -87.0 (-1.9) 4.9 (-12.5) -43.3 (-4.8) -9.1 (-5.8) 61.5 (-11.9)

-72.0 (6.8) -75.1 (5.7) -71.2 (1.4) -38.1 (4.2) -41.8 (3.7) -47.1 (2.8)

-78.5 (-1.1) -87.1 (-3.5) -97.7 (-12.4) -0.1 (-10.8) -12.9 (-9.5) 1.6 (-10.1)

-192.3 (11.2) -166.5 (5.4) -140.0 (-8.0) -97.2 (4.3) -43.8 (-3.4) -48.7 (3.8)

-155.8 (4.7) -97.5 (-4.2) -15.7 (-14.3) -43.1 (-6.0) -15.1 (-7.3) 52.3 (-12.7)

a

∆ZPEs are in parentheses. Energies are relative to isolated reactants.

TABLE 2: Relative Energies (kJ mol-1) of Reactant and Product Complexes, Transition States, and Products Calculated at the HF/6-31+G* + ∆ZPE Levela 6-31+G* hydride

reactant complex

TS

product complex

product

NH2OHFSiH3PH2SH-

-64.5 -72.5 -71.2 -30.3 -34.4 -39.2

-45.3 -55.1 -48.0 76.5 71.8 92.2

-186.1 -146.2 -92.0 -55.0 12.3 1.9

-155.8 -85.5 22.1 -9.4 29.9 114.8

a

Energies are relative to isolated reactants.

and F-.29 All the minima and transition states were characterized by calculating vibrational frequencies at the HF/6-31+G* or MP2/6-31+G* level. Zero-point vibration energy changes (∆ZPE) reported are calculated from unscaled ZPEs as listed in the output of Gaussian program. Energies were further calculated up to the MP2/6-31+G**//HF/6-31+G* and MP2/ 6-311+G**//MP2/6-31+G* level. The intrinsic reaction coordinate (IRC) 27,30 reaction path following computations were done at the HF/6-31+G* level for all the hydrides. Additional reaction path calculations were carried out at the MP2/6-31+G* level for proton abstractions by OH- and F- in order to study the reliability of the computations at the HF level. IRC pathways were computed in mass-weighted Cartesian coordinates with a step size of 0.1 amu1/2 bohr. Results and Discussion Geometries of the Stationary Points. Energies for the reactant and product complexes, transition states, and isolated products relative to the isolated reactants are listed in Tables 1 and 2. The structures of the reactant and product complexes, the structure of the transition state, and at the structures at points s ) -0.5 and s ) 0.5 from the IRC computations of the proton abstraction by OH- are presented in Figure 1. Selected geometric parameters of the transition states of all the studied hydrides are listed in Table 3. Geometries shown in Figure 1 for the proton abstraction by OH- at various points of the reaction are in general terms similar to those of the other hydrides excluding three exceptions. First, SiH3- forms a covalent pentacoordinated product complex with a Si-O1 distance of 2.009 Å (MP2/6-31+G*). Second, an additional stationary point was found for the proton abstraction by F-. In this complex FH was bonded through hydrogen to C2 carbon which had partly preserved pyramidal structure with CH2 angle (Figure 2) of 149.9° at the HF/6-31+G* and 139.4° at the MP2/6-31+G* level. The energy of this structure was only 1.6 kJ mol-1 (7.1 kJ mol-1 without ∆ZPE) under the energy of the transition state at the MP2/6-311+G**//MP2/6-31+G* + ∆ZPE level. Third, in the product complex of SH- the anionic oxygen of enolate abstracted proton from S producing a complex between neutral enol and SH-. At the HF/6-31+G*

level a stationary point for a complex between SH2 and enolate anion was located as well. The energy of this complex was -6.9 kJ mol-1 at the MP2/6-31+G**//HF/6-31+G* + ∆ZPE level. It can be seen from the geometries of the transition states (Table 3) that the first-row systems have earlier transition states than the second-row systems. This is revealed by longer C1C2 and shorter C1-O1 and C2-H2 distances of the first-row systems as compared to the second-row systems. Also, CH2 angles, which can be taken as a measure of hybridization of C2 (CH2 angle is 120° for sp3 carbon and 180° for sp2 carbon), are smaller at the transition states of the first-row systems. Thus, hybridization of C2 is less advanced toward sp2 at the transition states of the first-row hydrides. The geometries of the transition states are reasonably similar at both the HF and MP2 level. The only notable differences are in the C1-C2-S angle which is 104.4° at the HF level and 98.8° at the MP2 level and in the cases of F- and SH- in the CH2 angles which are 4° smaller at the HF than MP2 level. Energies of the Stationary Points. Complexation energies of the reactant and product complexes of the first-row hydrides are more negative than the energies of the second-row hydrides (Table 1). Relative to the separated gas-phase species, the reactant complexes of NH2-, OH-, and F- have energies between -71.2 and -75.1 kJ mol-1 and the corresponding product complexes between -140.0 and -192.3 kJ mol-1 at the MP2/6-311+G**//MP2/6-31+G* + ∆ZPE level. Energies for SiH3-, PH2-, and SH- are -38.1 to -47.1 kJ mol-1 for the reactant complexes and -43.8 to -97.2 kJ mol-1 for the product complexes. MP2/6-31+G**//HF/6-31+G* energies can be seen to be in agreement with the MP2/6-311+G**// MP2/6-31+G* energies. The more negative complexation energies of the first-row hydrides and the decrease of the energies as the rows are traversed to the right reflect the changes in the electronegative nature of the elements and in the ionic character of the bonds the elements make. As discussed recently by Gronert,31 this is seen additionally in the lower intrinsic proton-transfer barriers for the first-row hydrides as compared to the second-row hydrides. The observations of that work are valid for the proton abstractions of the present work as well. Moreover, Del Bene has reported detailed studies on the effects of the basis sets and correlation treatments on geometries and complexation energies of the first- and second-row hydrides.32-34 Proton abstractions by NH2-, OH-, and F- have early transition states (HF/6-31+G*) which disappear when electron correlation and zero-point energy contributions are included. The energies of the transition states relative to the energies of the reactant complexes are -6.5, -12.0, and -26.5 kJ mol-1 for NH2-, OH-, and F-, respectively, at the MP2/6-311+G**/ /MP2/6-31+G* + ∆ZPE level. Transition state energies are overestimated at the HF level10,22,35-37 and are 30.2 kJ mol-1 for NH2-, 30.5 kJ mol-1 for OH-, and 39.0 kJ mol-1 for Frelative to the energies of reactant complexes. The secondrow systems have much higher transition-state energies but

Proton Abstraction from Carbon Acids

J. Phys. Chem., Vol. 100, No. 9, 1996 3443

Figure 1. Structures of the reactant and product complexes, transition state (TS), and structures at points s ) -0.5 and s ) 0.5 from the IRC computations of the proton abstraction by OH-. Selected bond distances at the HF/6-31+G* and MP2/6-31+G* (in parentheses) level are shown in the figure.

TABLE 3: Selected Geometric Parameters of the Transition States for the Proton Abstractions by the Hydride Anions at the HF/6-31+G* and MP2/6-31+G* Levela hydride -

NH2

OHFSiH3PH2SHa

6-31+G* MP2/6-31+G* 6-31+G* MP2/6-31+G* 6-31+G* MP2/6-31+G* 6-31+G* MP2/6-31+G* 6-31+G* MP2/6-31+G* 6-31+G* MP2/6-31+G*

C1-O1

C1-C2

C2-H2

C2-X

H2-X

C1-C2-H2

C2-H2-X

CH2 angle

1.211 1.244 1.212 1.246 1.215 1.256 1.222 1.262 1.222 1.263 1.228 1.270

1.452 1.459 1.450 1.454 1.440 1.429 1.422 1.421 1.420 1.419 1.410 1.405

1.286 1.200 1.301 1.221 1.404 1.422 1.515 1.464 1.540 1.489 1.660 1.728

2.783 2.877 2.651 2.734 2.558 2.578 3.349 3.297 3.242 3.199 3.174 3.197

1.502 1.685 1.352 1.518 1.155 1.159 1.839 1.838 1.706 1.712 1.516 1.474

109.2 109.1 108.0 108.0 108.3 107.4 107.3 107.9 107.5 107.6 104.4 98.8

173.6 171.3 175.3 172.8 175.6 174.3 173.6 173.7 174.6 175.9 175.8 174.1

127.5 126.1 127.3 126.8 129.3 133.0 139.6 139.1 140.0 139.7 145.3 149.0

Distances in angstroms and angles in degrees.

Figure 2. Atomic numbering of acetaldehyde. The CH2 angle measures the angle between the C1-C2 axis and H3-C2-H4 bisector.

follow the similar trend in location of the transition states as the first-row systems: SiH3- has a central transition state; it is slightly later for PH2-, and in the transition state SH- has almost completely abstracted the proton from acetaldehyde. The energies of the separated products are highly overestimated for F- and the second-row hydrides at the HF level. This is due to the failure of the HF/6-31+G* basis set to reproduce the proton affinities of these hydrides and acetaldehyde.10,22,31,38

Also, this is seen in the energies of the product complexes which in a similar way are overestimated at the HF/6-31+G* level. Energetic and Structural Changes in the IRC Calculations. The IRC method, which can be used to trace a steepestdescent minimum-energy path from the transition state toward reactant and product, was employed to investigate the progress and timing of structural and electronic changes in the proton abstractions. Variation of energy along the intrinsic reaction coordinate s is shown in Figure 3, variation of C2-H2 and C2-X (X is the heavy atom of the base) in Figure 4, and variation of CH2 angle, which measures the hybridization of C2, in Figure 5 for the proton abstraction by OH- and F- at the HF/6-31+G* and MP2/6-31+G* level. Energy profiles show that the proton-transfer barriers of OHand F- are 20 kJ mol-1 lower at the MP2/6-31+G* than at the HF/6-31+G* level. MP2 transition state for the proton abstraction by OH- is earlier and that of F- slightly later than at the

3444 J. Phys. Chem., Vol. 100, No. 9, 1996

Figure 3. Variation of energy along the intrinsic reaction coordinate s for proton abstraction by OH- and F- at the HF/6-31+G* and MP2/ 6-31+G* level. The stationary point for F- was reached in the HF/631+G* computation at point s ) 1.93 and in the MP2/6-31+G* computation at point s ) 0.58.

Figure 4. Variation of C2-X (X ) O, F) and C2-H2 distances along the intrinsic reaction coordinate s for proton abstraction from acetaldehyde by OH- and F- at the MP2/6-31+G* and HF/6-31+G* level.

HF level. The geometries of the transition states reported in Table 3 indicate similar differences in the locations of the transition states at the MP2 and HF levels. However, the timing of the geometric changes are essentially similar at both levels. It can be concluded from the comparison of the IRC computations and the geometries of the transition states that for the purpose of the present work the main features of the reactions are sufficiently well described at the HF level. However, computations at the higher levels would be needed in order to get quantitative complexation energies and barrier heights. Variation of energies along the intrinsic reaction coordinate s for all the hydride anions at the HF/6-31+G* level is shown in Figure 6. Variation of C2-X (X ) N, O, F, Si, P, and S) and C2-H2 distances is shown in Figure 7, and variation of CH2 angles is shown in Figure 8. It can be seen from the figures that there are clear similarities in the relative timing of geometric changes in the proton abstractions and, regardless of their endo- or exothermicity, can be characterized by three relatively well separated steps: (1) Approach of hydride anion and acetaldehyde as characterized by contraction of the C2-X distance, (2) movement of hydrogen

Pera¨kyla¨

Figure 5. Variation of CH2 angle along the intrinsic reaction coordinate s for proton abstraction from acetaldehyde by OH- and F- at the MP2/ 6-31+G* and HF/6-31+G* level.

Figure 6. Variation of energy along the intrinsic reaction coordinate s for proton abstraction from acetaldehyde by NH2-, OH-, F-, SiH3-, PH2-, and SH- at the HF/6-31+G* level.

between the heavy atoms, and (3) separation of the product species with notable changes in the geometry of acetaldehyde. The proton abstraction sequence starts by the approach of the carbon, where the proton to be abstracted is bonded and the anionic proton abstracting atom to the distance at which the actual proton transfer will occur. This distance is 2.78 Å (NH2-), 2.65 Å (OH-) and 2.56 Å (F-) for the first-row and 3.35 Å (SiH3-), 3.24 Å (PH2-), and 3.17 Å (SH-) for the second-row systems at the HF/6-31+G* level. As compared to the reactant complexes, energies rise in this step by 20 kJ mol-1 (HF/6-31+G*) for the first-row systems and 60 kJ mol-1 for the second-row systems. However, relative to the transition states the energies rise comparably for both the systems. The end point of step one is located at about s ) -0.5 for the firstrow systems and at about s ) -1.0 for the second-row systems. In the second step the proton is transferred from the carbon (C2) to the proton abstracting atom while the distance between the heavy atoms remains essentially constant. Transition states of the proton abstractions are located, and the main energetic changes take place in this step. A clear difference between the first- and second-row systems in this step is that the values of the CH2 angles increase considerably (∼20°) in the case of the

Proton Abstraction from Carbon Acids

Figure 7. Variation of C2-X (X ) N, O, F, Si, P, and S) and C2H2 distances along the intrinsic reaction coordinate s for proton abstraction from acetaldehyde by NH2-, OH-, F-, SiH3-, PH2-, and SH- at the HF/6-31+G* level.

Figure 8. Variation of CH2 angle along the intrinsic reaction coordinate s for proton abstraction from acetaldehyde by NH2-, OH-, F-, SiH3-, PH2-, and SH- at the HF/6-31+G* level.

second-row systems (Figure 8), indicating considerable hybridization of C2 toward sp2, while in the case of the first-row systems the values change only little (∼5°). This difference probably originates from the differences in the magnitudes of the interactions between the transferring hydrogen and the anionic C2. For the first-row hydrides this interaction is stronger, favoring more sp3 like carbon and more localized anion. This is simply a manifestation of the stronger hydrogen bond donor capacities of the first-row atoms as compared to the second-row atoms. In other words, because the bonds between hydrogen and the first-row atoms are polarized in such a way that hydrogens carry more positive charge than when bound to the second-row atoms, and because this difference is preserved in the transition states as well (Table 4),31 the flow of negative charge from and hybridization of C2 is retarded more in the case of the first-row hydrides. This difference is clearly seen in the development of the charge distributions of the product enolate anion along the reaction coordinate as well (see below). In the third step after the proton has been transferred to the hydride, the heavy atoms are again separated. Here, after the transition states have been passed the geometry

J. Phys. Chem., Vol. 100, No. 9, 1996 3445 of the carbon, which has lost the proton, continues changing hybridization to sp2. In the case of the first-row systems the main part of the hybridization takes place in this step. In contrast, hybridization of C2 from sp3 to sp2 is well advanced for the second-row systems already at the end of step 2. Similar stepwise proton transfers have been reported for the transfers in (H2O)2H+ and (H2O)2Li+ systems36 and in formic acid dimer39 and C2H7-.40 Rearrangements of Electron Distributions in the IRC Calculations. Changes in the charge distribution of acetaldehyde along the intrinsic reaction coordinate of the proton abstraction were monitored by calculating the charges assigned to the atoms in CHO (atoms C1, H1, and O1) and CH2 (atoms C2, H3, and H4) groups of acetaldehyde. Group charges obtained from the natural bond orbital (NBO) analyses41,42 are shown for the proton abstractions by NH2-, OH-, and F- in Figure 9 and by SiH3-, PH2-, and SH- in Figure 10 (HF/631+G*). Charges from the Mulliken43 and NBO analyses of CHO and CH2 groups, the bases, and the transferred hydrogen at the transition states are listed in Table 4 (MP2/6-311+G**/ /MP2/6-31+G*). Group charges of acetaldehyde and its enolate anion are listed in the table as well. It can be seen from Figures 9 and 10 that the most prominent changes in the charge distributions occur in step 2 where the proton is transferred between the heavy atoms. The starting point for the movement of the proton (Figure 7), the point where the heavy atoms have reached the minimum distance (Figure 7), and the point where the group charges start decreasing (Figures 9 and 10) clearly coincide. Excluding the changes in the charges of CHO and the values of CH2 angles, this distinctly stepwise behavior is true for the end point of step 2 as well. In step 3 CHO groups continue gaining negative charge. This is due to the continuing hybridization of C2 from sp3 to sp2 hybridized carbon and concomitant development of resonance stabilized product enolate anion. This shows that the charge distribution through resonance, which requires geometrical changes to take place before it is fully developed, is lagging behind the proton abstraction. Thus, there is transition state imbalance in the proton abstractions.9 The comparison of the group charges at the transition state and the corresponding charges of acetaldehyde and its enolate anion (Table 4) confirm the same observation. In addition, group charges of CHO are much less developed at the transition states than the charges of CH2, which in some cases are even more negative at the transition state than in the product enolate anion. As compared to the first-row systems, CHO charges develop earlier in the proton abstractions by the second-row hydrides telling about earlier geometrical changes at C2 and, thus, earlier development of resonance. In the case of NH2- and OH- the CH2 group charges have a clear well locating exactly in the end point of step 2. The values of the group charges of CH2 in the case of NH2-, OH-, and F- are similar at the minima of the wells and are -0.68 (Mulliken) and -0.57 (NBO). In contrast, the CHO charges at that point increase steadily from -0.28 (Mulliken) and -0.33 (NBO) to -0.20 (Mulliken) and -0.25 (NBO) in the series NH2-, OH-, and F-. A similar trend is observed for the second-row hydrides as well. However, in this case the CH2 group charges are larger and CHO group charges smaller at the end point of step 2 due to the earlier hybridization of C2. These results indicate that the development of CHO group charges are clearly lagging behind the development of CH2 group charges in the proton abstractions and that this imbalance is larger for the first-row hydrides than for the second-row hydrides. At the transition state the charges on the bases are more negative, and the charges on the transferring hydrogen

3446 J. Phys. Chem., Vol. 100, No. 9, 1996

Pera¨kyla¨

TABLE 4: Charges of CHO and CH2 Groups, Base, and the Transferred Proton at the Transition States Obtained from the Mulliken and NBO Population Analyses at the MP2/6-311+G**//MP2/6-31+G* Level CHO

CH2

base

H2

hydride

Mulliken

NBO

Mulliken

NBO

Mulliken

NBO

Mulliken

NBO

NH2OHFSiH3PH2SHacetaldehyde enolate anion

-0.186 -0.204 -0.261 -0.309 -0.339 -0.438 0.049 -0.675

-0.119 -0.135 -0.252 -0.307 -0.315 -0.390 0.019 -0.540

-0.308 -0.396 -0.552 -0.279 -0.325 -0.460 -0.213 -0.326

-0.398 -0.409 -0.473 -0.402 -0.403 -0.431 -0.233 -0.460

-0.908 -0.794 -0.560 -0.464 -0.446 -0.310

-0.838 -0.829 -0.727 -0.373 -0.423 -0.382

0.401 0.394 0.373 0.051 0.110 0.209 0.164

0.354 0.372 0.452 0.083 0.140 0.203 0.214

Conclusions

Figure 9. Variation of CHO and CH2 group charges along the reaction coordinate s obtained from NBO analyses at the HF/6-31+G* level for the proton abstractions by NH2-, OH-, and F-.

Proton abstraction from a model carbon acid acetaldehyde by the first- and second-row hydride anions NH2-, OH-, F-, SiH3-, PH2-, and SH- was investigated with ab initio quantum mechanical calculations up to the MP2/6-31+G**//HF/6-31+G* and MP2/6-311+G**//MP2/6-31+G* level. Reactant and product ion-dipole complexes and transition states were determined for the proton abstractions, and the intrinsic reaction coordinate (IRC) reaction path following calculations were used to study the change in energy, geometric variables, and charge distributions along the reaction coordinate. Proton abstractions were found to occur in three essentially separate steps. The first step involves approach of the anionic hydride and the carbon where the proton to be abstracted is located. Proton is transferred and transition states are located in the second step. In the case of the first-row systems the geometric variables, excluding the moving proton, are essentially constant in this step. In contrast, for the second-row hydrides the proton donating carbon starts changing hybridization from sp3 to sp2 in this step. This allows the flow of negative charge from the CH2 to CHO end of the developing enolate anion to be more advanced in the case of the second-row hydrides. This is probably due to the less favorable interactions between the proton donating, at the transition state negatively charged carbon, and the leaving to the base bonded hydrogen in the case of the second-row systems. In the third step the heavy atoms separate again, and the carbon, which has donated proton in the reaction, continues hybridizing to sp2 and the CHO end of the enolate anion gaining negative charge. Thus, due to the nonsynchronous development of proton transfer and the changes of the geometric parameters and the charge distribution of the developing enolate, clear transition-state imbalance is observed in the studied proton abstractions. This imbalance is more prominent for the firstrow hydrides than for the second-row hydrides. References and Notes

Figure 10. Variation of CHO and CH2 group charges along the reaction coordinate s obtained from NBO analyses at the HF/6-31+G* level for the proton abstractions by SiH3-, PH2-, and SH-.

are more positive in the case of the first-row than the secondrow hydrides. This difference is due to the more ionic nature of the bonds between the first-row atoms and hydrogen. Comparison between the transition-state energies and the polarities of the transition states (charges on CH2, transferring hydrogen, and base) suggests that a more ionic transition state leads to a smaller transition-state energy. It has been earlier observed that for the first- and second-row hydrides there exists a correlation between the charge on the transferring hydrogen and the intrinsic proton-transfer energies.31

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