J. Phys. Chem. 1995, 99, 1151 - 1155
1151
Theoretical Studies of Proton Transfer in (CH3CHO-H-OCHCH3)+ Chih-Hung Chu and Jia-Jen Ho* Department of Chemistry, National Taiwan Normal University, 88 Sec. 4, Tingchow Road, Taipei, Taiwan 1 1 7, ROC Received: September 12, 1994; In Final Form: November 7, 1994@
The geometric structures as well as the transfer of a proton in the complex of proton-bridged acetaldehyde dimer (CH3CH0)2H+ are studied theoretically by ab initio calculations. Four isomeric Structures of the complex are found, and among them the cis-cis complex is calculated to be the most stable conformation. The energy difference between the least and the most stable conformation is less than 1 kcallmol. A symmetric double well is found for proton transfer on the potential-energy hypersurface. A modified procedure was introduced while the energy barrier of proton transfer was calculated. With the transition structure fully optimized (allowing mutual movement of the two aldehyde moieties during proton transfer) the barrier is calculated to be 1.73 kcallmol, significantly smaller than that (4.15 kcallmol) calculated with a fixed distance between the two aldehyde subunits in the process of transfer. Each equilibrium structure in cis-cis and trans-trans conformations of (CH$HO)zH+ has C, symmetry with angular 0 - H - 0 , whereas the transition structure belongs to point group C 2 h . Comparison of these results with those of the formaldehyde cluster ion (HCHO)zH+ is also presented.
Introduction
HL
In recent experiments many protonated aldehyde cluster ions (CH3CHO),H+, n = 1-1 1, were observed.' These cluster ions were produced from neutral acetaldehyde clusters, prepared in a pulsed-nozzle supersonic expansion and then subjected to multiphoton ionization. Among all observed cluster ions, (CH$ZHO)zH+ and (CH3CH0)4H+ are found to be particularly stable. The experimental results provide insight into stable structures of hydrogen-bonded acetaldehyde cluster ions and the energetics of proton transfer exhibited in these ions. Experiments on proton transfer in gaseous reactions,
C
X
'
L/
R
HR
are hampered because of the large energy evolved in the association step and the small energy barriers (generally less than 5 kcaymol) of proton transfer2-* between oxygen and nitrogen atoms. The entire process of this reaction may have a rate near the collision-controlled in the association step, and the fundamental features of proton transfer would be extremely difficult to isolate. The ab initio molecular orbital method becomes one of the attractive alternatives to investigate the geometry of transient species and the energetics of proton transfer. We reported9 equilibrium structures of the protonated formaldehyde dimer and its proton-transfer energy barriers on potential-energy hypersurfaces. According to previous calculations, the energetics of proton transfer are strongly related to the geometry of (A-H-A)+, and the most stable conformation of the protonated formaldehyde dimer (HCH0)2H+ contains an angular 0 - H - 0 bond. A double well with barriers 3.49 kcaU mol pertained to energetics of proton transfer at a fixed equilibrium inter-oxygen distance. In the present work we extended our calculations to a more complicated molecular ion, (CH3CHO-H-OCHCH3)+, as a preliminary stage of further
* To whom correspondence should be addressed. @
Abstract published in Advance ACS Abstracts, January 1, 1995.
0022-365419512099-1151$09.00/0
Figure 1. General geometry of fully optimized complexes (XCHOH-OHCX)+, X = CH3 or H, obtained without any prior assumption concerning symmetry and with all parameters fully optimized. The left subunit is tagged L, the right R, and the middle proton m.
work on (CH3CHO)J-I+ and (HCH0)4Hf. The calculations focus on the conformations at the minima in the potential-energy hypersurface and the energetics of proton transfer proceeding in the cluster ion.
Methods of Calculation All calculations were carried out with the ab initio Gaussian 92 set of computer programs." The polarized split valence 4-31G* basis set was used for the fact that it was demonstrated to yield satisfactory agreement with experiment^^^-^^ and that there existed convenient comparison of our calculated results, as this set was widely used for calculation of energy barriers for proton transfer in similar ~ y ~ t e m ~ . ~Correlation ~ - ~ ~ J ~ - ~ ~ effects were included via second-order Moller-Plesset perturbation theory (MP2). The basis set superposition error (BSSE) inherent in computation of molecular interaction energies was corrected via the Boys-Bemardi counterpoise technique." The molecular arrangement is illustrated in Figure 1. All atoms in the left subunit except X (X = CH3) are labeled L, right R, and middle m. The representation of parameters is similar to that of our previous work.9 The subunits (XCHO) of the complexes were investigated first, followed by calculations of optimized 0 1995 American Chemical Society
Chu and Ho
1152 J. Phys. Chem., Vol. 99, No. 4, 1995 H 1 081 \IO,
8
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Figure 2. Optimized geometries of acetaldehyde and its trans- and cis-protonated counterparts (bond length in angstroms and angles in degrees). S6F and M62 energies are given in atomic units (hartrees).
structures of protonated complexes. There are four geometric isomers of the (CH3CHO-H-OCHCH3)+ complex. The one shown in Figure 3A, which has a central proton Hm located at the same side as hydrogen atom HL with respect to the CL-OL axis and also on the same side as HR with respect to the CRORaxis, is designated the cis-cis isomer. Similar nomenclature is applied to conformations in parts B, C, and D of Figure 3; they are designated cis-trans, trans-cis, and trans-trans complexes, respectively. When the fully optimized equilibrium structure of each isomer was determined, a procedure modified from our previous work was undertaken to calculate the potential-energy surface for proton transfer. We obtained the energy profile by calculating the energy of the system as a function of r, without the constraint of fixed R at the equilibrium length; we considered the possibilities of mutual movements of the two subunits at each step of transfer of the proton in the complex. The new results that yield a much smaller energy barrier for transfer on the potential-energy surface are discussed in later sections. The various R values were employed to investigate the energy barriers over a wide range of lengths.
Results and Discussion The calculated results of the protonated subunit, (CH3CHOH)+, at both HF and MP2 levels, are presented in Figure 2. Two protonation positions are shown and designated cis and trans subunits according to the nomenclature described above. The cis conformation is slightly more stable than the trans one, at both levels of calculations. A more enhanced steric effect exists between proton Hm and the methyl group in the trans conformation; a larger angle of O(CC0) (125.7") in the trans form relative to that in the cis form (120.1") is good evidence of this effect. The C - 0 bond length of each protonated acetaldehyde increases from 1.185 to 1.242 A in trans and to 1.243 A in cis. The newly formed 0 - H bond acquires some
TABLE 1: Protonation Enerev (kcaVmol) of XCHO" ~~
HF
species
uncorr
MP2
cod
uncorr
corr
exptl'
HCHO 182.68 181.74 175.21 172.89 183.0 (177.2)d CH3CHO (cis) 195.16 194.26 188.17 185.88 194.5 (188.9)d CHiCHO (trans) 194.80 193.77 187.84 185.49
X represents the substituent H or CHi; see text for the definition of protonation position. Corrected for basis set superposition error (BSSE). Experimental proton affinity, corrected for computed zeropoint vibrational energy and contributions from translational and rotational terms. Values in parentheses precede these conditions. "See ref 19.
electron density from the weakened C - 0 bond. The C-C bond lengths decrease from 1.503 to 1.468 A in trans and to 1.464 A in the cis form. These characteristics are similar to those in the protonated formaldehyde c ~ m p l e x . ~ Table 1 presents protonation energies of (XCHO), X = H, and CH3, with and without BSSE corrections calculated at HF and MP2 levels. These corrections are about 1 kcal/mol at the HF level, and about 2 kcaVmol at MP2, but have no effect on the relative energetics at both levels. The electron-releasing character of the methyl group in acetaldehyde increases the protonation energy about 13 kcaYmol at either level of calculation relative to that of formaldehyde, which causes the 0 - H bond length in protonated acetaldehyde to become shorter by 0.004 A. The cis protonation is slightly preferable to that at trans. The experimental proton affinities18 corrected with computed zero-point vibrational energies and with translational and rotational contributions appear in the last column of Table 1. There is satisfactory agreement, especially with HF values uncorrected for BSSE, but MP2 values are slightly underestimated. The relative values of experimental data are in good accord with calculated ones. Such agreement enables us to employ these theoretical procedures to examine properly the energetics of-proton transfer. The four calculated equilibrium geometric isomers of
J. Phys. Chem., Vol. 99,No. 4, 1995 1153
Studies of Proton Transfer in (CH3CHO-H-OCHCH3)'
(B) HF = -305.895019
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1231
..
\
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1.231
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i
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Figure 3. Optimized geometries of (CH3CHO)*H+(bond length in angstroms and angles in degrees). The notations A, B, C, and D designate ciscis, cis-trans, trans-cis, and trans-trans conformations, respectively, See the text for explanation. HF energies are given in atomic units (hartrees).
(CH3CHO-H-OCHCH3)' are shown in Figure 3. The left subunits of the complexes in Figure 3A,B are similar, all in cis conformation, whereas at the right subunit of the complex the cis conformation pertains to Figure 3A but is trans in Figure 3B. As the cis-protonated acetaldehyde in Figure 2 is favored energetically, the cis-cis complex in Figure 3A is predicted to have smaller energy than the cis-trans complex in Figure 3B. Likewise, as the left subunits in Figure 3C,D have trans conformations but the right is cis in part C and trans in part D, the trans-cis complex in Figure 3C is expected to be more stable than the trans-trans complex in Figure 3D. Our calculated results are in satisfactory agreement with the above argument that the trans-trans isomer is energetically the least stable conformation. The energy difference between cis-cis and trans--trans conformations is only about 1 kcal/mol. As a result, these four geometric isomers probably transform readily from one form to another on rotation of the two C - 0 bonds of the complex. It may be also noted from the calculated results that the smaller R is in the complex in the equilibrium conformation, the more stable energetically the complex is. In this case the cis-cis complex has the smallest value of R (2.516 A), whereas R in the trans-trans counterpart has the largest value (2.541
A). The binding energies designated AE = E(XCH0-HOCHX)' - E(XCH0-H)' - E(XCHO), X = CH3 or H, appear in Table 2, with the BSSE correction. The magnitude of binding energies of the four isomeric acetaldehyde complexes follows the same trend of stabilities as the calculated HF energies listed in Figure 3, the cis-cis complex having the most stable conformation. The counterpoise corrections amount to about 1.5 kcal/mol at the HF level; these values exceed those
TABLE 2: Binding Energies (kcaVmo1) of Protonated Aldehyde DimeP HF/4-31G* species uncorr cod (HCHO-H-OHCH)' 29.13 27.48 cis-cis complex' 29.93 28.30 cis-trans complex 29.64 27.88 trans-cis complex 29.61 27.83 trans-trans complex 29.28 27.39 a Binding energy AE = E(XCH0-H-OCHX)+-E(XCHO-H)'E(XCH0). X = CH3 or H. Corrected for basis set superposition error (BSSE). Complex represent (CH3CHO-H-OCHCH3)+; see text for the definition of the substitutional position.
in Table 1 (about 1 kcal/mol), as more basis functions were employed in the calculations on the complexes. The structure of subunits in the proton-bridged acetaldehyde dimer deviates slightly from that of the protonated monomer in Figure 2. The lengths r(OL-Hm) and r(CL-C) in the left subunit of the cis-cis complex increase to 1.011 and 1.471 8, from 0.962 and 1.464 8, of the cis-protonated monomer, respectively. The structure of the right subunit of the cis-cis complex is similar to that of acetaldehyde, but r(CR-OR) is greater (1.205 vs 1.185 8,) and r(CR-C) is smaller (1.486 vs > *that ~ of Scheiner16 the 1.503 A). In our previous w ~ r k ~and angles a and /3 in the complex were strongly related to the direction and magnitude of the electric dipole moment of the subunit (XCHO). Another factor (steric effect) to be considered in these four isomers is that the cis conformation has a smaller steric effect in forming the complex. Therefore, the angle a of the cis conformation in the left subunit of the complexes is smaller than that of the trans conformation. For instance. a in
1154 J. Phys. Chem., Vol. 99, No. 4, I995
Chu and Ho
TABLE 3: Geometries of (CH3HCO-H-OCHCH3)during Proton TransfeP Y
1.011 1.061
R r(ORHm) r(CLOL) YiCROR) r(CLHL) r(CRHR) r(CLC) Y(CRC)
2.516 1.523 1.231 1.205 1.080 1.087 1.471 1.487 a 114.2 132.2 P 6 8.3 B(HLCLOL) 118.1 Q(HRCROR) 119.5 B(OLCLC) 121.3 Q(ORCRC) 123.5 E"(kcallmol)b 0.00
2.473 1.418 1.227 1.208 1.081 1.087 1.473 1.485 115.5 128.3 5.0 118.5 119.4 121.6 123.2 0.46
1.111
1.190 1.311 1.423 1.523
2.424 1.315 1.222 1.211 1.083 1.086 1.475 1.483 117.0 124.4. 2.5 118.7 119.3 121.9 122.9 1.21
2.379 1.190 1.217 1.217 1.084 1.084 1.479 1.479 120.2 120.2 0.0 119.0 119.0 122.5 122.5 1.73
2.393 1.084 1.210 1.223 1.086 1.082 1.482 1.475 125.6 116.9 -2.3 119.3 118.6 123.0 121.9 1.04
2.451 1.037 1.207 1.227 1.087 1.081 1.485 1.472 129.5 115.2 -3.9 119.4 118.3 123.3 121.6 0.26
2.516 1.011 1.205 1.231 1.087 1.080 1.487 1.471 132.3 114.7 -5.9 119.5 118.1 123.5 121.3 0.00
"All distances in angstroms: angle in degrees. See text for the definition of parameters. E# = energy difference between the most stable structure and the structure during proton transfer.
Figure 3A,B of the cis conformation in the left subunit at 114.2" and 113.9", respectively, are smaller than a in Figure 3C,D of the trans counterparts (1 17.3"). A similar argument applies to the angle p of the cis conformation in the right subunit of the complexes. As a consequence, the cis-cis conformation becomes the most compact among the four isomeric complexes. There are two equivalent complex structures located at the local minima of the potential surface of proton transfer for each isomeric complex. We cite only one isomer, the cis-cis complex, as an example to describe the energetics of proton transfer. Proton transfer proceeded when the central proton Hm moved along the 0-0 axis until the other local minimum in the potential surface was reached. The calculated data are listed in Table 3. It is very interesting to notice the variation of R values, which decreased at first with the increase of r until a transition structure was reached, at which r = 1.190 and R = 2.379 A, the smallest value in the whole transfer process, and then increased until the other equivalent complex structure was attained. In contrast, the relative energy E#, presented in the last row of Table 3, for every calculated structure at varied r increased with increasing r, attained a maximum (1.73 kcaV mol) at the transition structure, and then decreased to the other local minimum. Similar results were obtained when the protonated formaldehyde complex (HzCO-H-OCH2)+ was treated according to the same process. When proton transfer proceeded in (CH3CHO-H-OCHCH3)+, r(CL-OL) decreased from 1.231 to 1.205 A, whereas r(CROR) increased from 1.205 to 1.231 A, listed in the fourth and fifth row of Table 3. The variation of other parameters, r(CLHL),r(CR-HR), B(HLCLOL),B(HRCROR), etc., follows the same trend. It is equivalent to interchange the left and right subunits within the structure. The change of angle 6 from positive 8.3" to negative 5.9" indicates that the central Hm moves from beneath the 0-0 axis to above that axis during the transfer of a proton. The transition structure has the central proton Hm situated right in the middle of the 0-0 axis at which 6 equals 0". To observe the effect of energy barrier of proton transfer with respect to the fixed interdistance of the two subunits, we selected two other values of R (one fixed at equilibrium Re,, the other at Re, +0.2 A) for further calculations. Each geometry at each value of r in the process of transfer was fully optimized, subject only to the constraint of fixed R. The association energy of
formaldehyde complex (HCHO-H-OHCH)' is calculated to be 29.13 kcaVmo1, whereas the barrier to proton transfer is only 1.43 kcal/mol, with the transition structure fully optimized. As the zero-point energy of the equilibrium structure is 2.79 kcaY mol, which is greater than the energy barrier for transfer, the system is similar to that described by GornetZ3as a doublewell potential with a barrier below the ground-state vibrational level. The proton is presumably free to move between the two oxygens. If transfer proceeds at a fixed equilibrium distance, R = 2.5 1 A, the calculated energy barrier is 3.49 kcallmol, about 2.4 times as much as that of the previous unfixed case. Then the barrier would be only slightly above the ground-state vibrational level. As the amount of heat released is significant, experiments on the energetics of the proton-transfer process are obviously difficult. When the two subunits were separated 2.71 A from the equilibrium conformation, the energy of each local minimum increased only 1 kcal/mol, but the transfer energy barrier increased to 11.30 kcal/mol, indicating the increased difficulty of proton transfer in an elongated configuration. Similar properties were found in the case of proton-bridged acetaldehyde dimer. The calculated data show correspondingly greater barriers to transfer energy (1.73, 4.15, and 12.42 kcaVmol) and larger association energy (29.93 kcaVmol) than the formaldehyde counterparts. The greater proton affinity of acetaldehyde, due to the electron-releasing methyl group in the molecule which appears more basic toward the proton, is responsible for the calculated results.
Conclusion Among all four geometric isomers of proton-bridged acetaldehyde the trans-trans complex structure is calculated to be the least stable; however, the energy difference between any two of them is less than 1 kcaYmol. Presumably, they can facilely interchange from one to another by rotating the C - 0 single bonds in the subunits. The observed (CHsCH0)2H+ ion' could have any of the four structures discussed, but the cis-cis complex is the global minimum energetically. The large amount of energy released in the association step allows the proton to overcome the transfer barrier. The proton in (AHA)+ may be delocalized between the two oxygens when the proton-bridged complex is formed from the association of AH' and A, where A represents an aldehyde molecule. Our calculations have illustrated the most stable structure of proton-bridged acetaldehyde complexes and the energy barriers of proton transfer that can provide valuable information for spectral investigation of proton transfer in cluster ions.
Acknowledgment. We are grateful to the computer center at National Taiwan Normal University, where the Gaussian package and the computer time were provided. Support for this research from the National Science Council of the Republic of China is also gratefully acknowledged. We are also indebted to the referees for helpful suggestions concerning the manuscript. References and Notes (1) Tzeng. W. B.: Wei. S.: Castleman. A. W., Jr. Chem. Phys. Lerr. 1990. 168, 30. (2) Squires, R. R.; Bierbaum, V. M.: Grabowski. J. J.; DePuy. C. H. J . Am. Chem. SOC. 1983, 105, 5185. (3) Moylan. C. R.; Brauman. J. I. J . Phys. Chem. 1984, 88, 3175. Fameth. W. E.: Brauman, J. I. J. Am. Chem. SOC. 1976, 98, 7891. (4) Henchman, M.; Hierl, P. M.; Paulson. J . F. J. Am. Chem. SOC. 1985, 107.2812. Bohme. D. K.: Mackay, G. I.; Tanner, S . D. J . Am. Chem. SOC. 1979, 101. 3724. Ausloos, P.: Lias, S . G. J . Am. Chem. SOC. 1981, 103. 3641. Bohme. D. K.; Rakshif. A. B.; Mackay. G. I. J . Ani. Chem. SOC. 1982, 104. 1100.
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