Horowisz, A. Grabowska, R. Wortmann, and W. Liptay, J. Lumin. 52, 265 (1992). ... J. J. P. Stewart (Frank J. Seiler Laboratory, US Air Force. Academy, CO 80840 ...
Tautomerism in 2,2’-Bipyridyl-3,3‘-diol VENELIN ENCHEV Institute of Organic Chemistry, Bulgarian Academy of Sciences, 2213 Sofia, Bulgaria Received June 22, 1994; revised manuscript received November 21, 1994; accepted December 5, 2994
Three stable tautomeric forms, dienol (DE), ketoenol (KE), and diketo (DK), of 22’bipyridyl-3,3’-diol BP(OH), were found in this study, using the semiempirical AM^ and MNDC-PM3 and ab initio (4-31~basis set) methods. All calculations were carried out without any symmetry restrictions. There is a good agreement between the ab initio calculated and experimentally obtained structural parameters for the DE tautomer. Transition structures, corresponding to the DK KE and KE DE processes have also been found. On the basis of the results from the present work, an asynchronous KE DE mechanism of the IPT reaction in BP(OH), is proposed. (two-step) DK
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-
-
=)
0 1996 John Wiley & Sons, Inc.
Introduction
T
he 2,2’-bipyridyl-3,3‘-diol BP(OH), has been recommended as a lasing dye [1, 21 and fluorescence standard 121. This molecule is also known as a promising candidate for a solar energy collecting material [3]. Upon absorption of uv light, BP(OH), undergoes photoautomerization involving an intramolecular proton transfer ( ~ mreaction ) in the excited state without an energy barrier [4,51. Since the dipole moments of BNOH), have been found to be vanishing in its fluorescent state [61, it has been concluded that the photoprocess is a symmetric double-proton transfer, in which both oxygen-bonded protons move to the closely lying nitrogen atoms and a diketo tautomer is formed (Fig. 1). Very little is known about the back IPT reaction in the ground state and the conclusions by different authors on the mechanism are contradic-
tory. The large Stokes-shifted fluorescence of BP(OH), has been ascribed to its diketo (DK) tautomer [4, 51. On the other hand, no minima corresponding to ketoenol (KE) or DK tautomers have been obtained by ab initio calculations [71. The aim of the present work was to describe the structure and tautomersm of BP(OH), by means of quantum chemical methods. A detailed knowledge of the tautomerism of BP(OH), is a prerequisite for a discussion of the ground-state intramolecular proton-transfer reaction.
Computational Details Both semiempirical MNDO-PM3 [81 and AM^ [91 and ab initio methods were used during the study of the different tautomeric forms of BP(OH),. The semiempirical calculations were carried out with the MOPAC 6.0 program package [lo]. The geome-
International Journal of Quantum Chemistry, Vol. 57,721 -728 (1996) 0 1996 John Wiley & Sons, Inc.
CCC 0020-7608/ 96 / 040721-08
ENCHEV 16
5
DE
15
5
12
’6
O-H
KE d
15
12
5 16
entropies of the tautomers and transition states were calculated at 298 K, according to the classical procedure implemented in MOPAC. Ab initio calculations were performed using the program GAMESS [12]. The standard 4 - 3 1 ~[131 basis set was used at the Hartree-Fock level for two reasons: First, optimized geometries, calculated with split-valence basis sets (3-21~,4-31~, or 6-31~),are in good agreement with the experimentally determined gas-phase structures [14-181. Second, for hydrogen-bonded systems of first-row elements, the 4 - 3 1 ~ basis set is known to yield results at the SCF level that are comparable to those achieved with much more sophisticated methods [ 19-21]. m ~ o - ~ ~ 3 - o p t i m i z egeometries d were used as starting structures for the ab initio calculations. Ab initio geometry optimizations and transition-state searching were perfomed within C, symmetry, using the algorithms I l l , 221 which form the ”standard method” for GAMESS geometry searches. The gradient optimizations were terminated when the gradient length over all optimized parameters was reduced to 1.0 X Hartrees/Bohr. The character of the saddle points was verified by the subsequent calculation of vibrational frequencies.
Results and Discussion
x
15
FIGURE 1. Tautomers of 2,2’-bipyridyl-3,3’-diol (BP(OH),). DE, dienol; KE, ketoenol; DK, diketo. DK is a zwitterionic structure with a zero dipole moment.
tries of the investigated tautomers were completely optimized using a procedure called eigenvector following [ll](option EF). The option TS was applied to determine transition structures. The keyword PRECISE was used to reach a mean gradient value lower than 0.01 kcal mol-’ k ’ for all calculations. The transition structure corresponding to the synchronous double-proton transfer was located using a gradient norm minimization method (options NLLSQ and GNORM = 0.00. Finally, the stationary points of the energy surfaces were characterized by vibrational analysis. The
722
Three local minima, corresponding to the tautomers DE, KE, and DK, were found in the potential energy surface of BP(OH),. Table I lists the 4-31~,MNDO-rM3, and AM1 energies of the tautomers (Fig. 1) with respect to the most stable structure, which was found to be in agreement with available experimental data: the DE form. Recently, AM^ calculations of the DE and DK tautomers, constrained to be planar, were reported [231. Hess et al. [71, using STO-3~ and 3 - 2 1 ~ basis , that they have sets and symmetry D Z h reported not found KE and DK tautomers of BP(OH),. Possible reasons for this might be the symmetry constraints and the size of the basis set used. For example, studying the enolimine-ketoenamine tautomerism of aromatic azomethines. H o h a n n et al. [241 showed that the STO-3G basis set predicts only the existence of an enolimine tautomer, while the 3-21c basis set predicts the existence of both tautomers, the enolimine being more stable by 16.4 kJ/mol. Similar results were also obtained for the tautomers of o-hydroxybenzaldehyde. Nagaoka VOL. 57, NO. 4
TAUTOMERISM IN 2,2'-BIPYRIDYL-3,3'-DlOL
TABLE I Relative stabilities (in kcal / moll for tautomers of BP(OH), shown in Figure 1. Tautomers
DE
KE
DK
0.00 0.00 0.00
16.29 15.32 12.03
21.48 17.82 19.68
0.00
Not found
11.23
Computationallevel AM 1
MNDO-PM3 4-31G 1 / 4-31G (SCF + MBPT(2)) 16-31G** +O.g*ZPE 13-21Ga aAccording to [271.
and Nagashima [25], using the STo-3G basis set, have not found the keto tautomer, while Rios et al. [26], using the 3-21c basis set, showed that the keto tautomer can exist but it is more unstable than is the enol tautomer by 76.9 kJ/mol. Quite recently, Mordzinski et al. [27] reported to have found DE and DK tautomers of BHOH), in the ground state, using 3 - 2 1 ~and 6-31~**basis sets and fully optimized equilibrium geometries. However, they have not found a nonsymmetric KE tautomer. The relative stabilities of BP(OH), tautomers, given in Table I, show that there is a qualitative agreement between the computational levels used. However, the structural features are specific. The ab initio and M N D C - P M ~ calculations yield planar conformations for all three tautomers. The planar conformation of the DE tautomer is in agreement with the experimental results [7]. The AM^ Hamiltonian produces a nonplanar equilibrium conformation for DE and DK with calculated dihedral angles between the two 3-hydroxypyridine rings of 62 and 13", respectively. The planar conformation for DE, calculated by AM^, was found to be 1.84 kcal/mol more unstable than is the nonplanar one. In the last case, the molecule was constrained to be planar. A nonplanar conformation was also found by the MNDO-PM3 method, using full geometry optimization. This conformation is 6.47 kcal/mol more unstable than is the planar one and the torsion angle between the two 3-hydroxypyridine rings was calculated to be 56". The ab initiocalculated structural data for the tautomers of BP(OH), are given in Table 11. There is a good agreement with the experimental data 171. It was reported in [28] that the geometries of the DE tautomer, obtained by the DZV basis set and those by Hess et al. [71, are quite close. The calculated bond distances and angles at the 4 - 3 1 ~level for tautomers DE and DK presented in this article are
close to those obtained at the 3 - 2 1 ~and 6-31~** levels, recently published in [271. A comparison of the optimized geometries of the tautomers DE, KE, and DK (Table 11) shows that the difference in the respective C o C and C-N bond distances do not exceed 0.04 A. There are substantial changes in the $--0 bond lengths, which become shorter by 0.07 A in the KE and DK forms than in the DE form. The calculated $arbon-oxygen bond lengths in KE and DK (1.28 A) are close to the C = O double bond. The central C2-C8 bond becomes slightly shorter in KE and DK tautomers (1.443 and 1.4047 A, respectively) than in the DE tautomer (1.476 A), preserving in this way the single-bond character. The N ..-0 distanc: in the DE tautomer was calculated to be 2.633 A, i.e., shorter than the typical value (2.88 A [29]) for strong O-H...N intramolecular bydrogen bonds. The calculated value is only 0.033 A higher than is experimental one [7]. In the KE and DK tautomers, the $... I 0 distance was found to be 2.508 and 2.545 ,A, respectively, also shorter than the typical (2.93 A [29]) value for N' -H .-.0 intramolecular hydrogen bonds. It has been shown that the ab initio-calculated relative total energy of the tautomeric compounds including the electron correlation effects [30-331 agrees well with the corresponding experimental value. The magnitude of these effects depends on the basis set and the post-Hartree-Fock method used [33-351. The ab initio results, recently reported in [27], are also presented in Table I for comparison. The calculations at the (SCF + MBFT(~)) / 6-31~** level predict a difference of 11.23 kcal/mol in the relative stabilities of the DE and DK tautomers, which is 8.45 kcal/mol lower than that calculated at the 4-31c / / 4 - 3 1 ~level (Table I). On the basis of the above-mentioned results, two IPT mechanisms are considered: (i) a two-step transfer, in which the hydrogen atoms move asyn-
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723
ENCHEV TABLE II Ab initio (4-31 G basis set) structure parameters of tautomers DE, KE, and DK.'
Parameter
DE
KE
DK
TS1 (DK * KE)
TS2 (KE = DE)
Bonds (A) N1-C2 N1-C7 C2-C3 c3-c4 C4-C6 C6-C7 C3-05 05-H16 C2-C8 C8-C9 C8-N 10 N10-Cl4 c9-c11 C11-C13 C13-Cl4 c9-012 012-H15 N1-H15 N10-Hl6 N1-012 610-05
Exptl. 1.3368 (1.338) 1.3202 (1.334) 1.3975 (1.405) 1.3851 (1.377) 1.3753 (1.362) 1.3827 (1.367) 1.3515 (1.343) 0.9662 (1.OOO) 1.4756 (1.467) 1.3975 (1.405) 1.3368 (1.338) 1.3202 (1.334) 1.3851 (1.377) 1.3753 (1.362) 1.3827 (1.367) 1.3515 (1.343) 0.9662 (1.OOO)
1.3410 1.3405 1.4113 1.3693 1.3960 1.3598 1.3497 0.9706 1.4433 1.4174 1.3512 1.3021 1.4246 1.3563 1.4085 1.2842
1.3427 1.3243 1.4211 1.4120 1.3727 1.3814 1.2836
1.3350 1.3376 1.4162 1.3874 1.3888 1.3685 1.3137 1.2790 1.4425 1.4130 1.3480 1.3096 1.4225 1.3641 1.3977 1.2803
1.3389 1.3304 1.4001 1.3761 1.3886 1.3696 1.3506 0.9684 1.4552 1.4079 1.3411 1.3113 1.4075 1.3658 1.3983 1.3082 1.3258 1.1562
Bond angles (") N1-C2-C3 c2-c3-c4 C3-C4-C6 C4-C6-C7 C6-C7-N1 C7-Nl -C2 N1-C2-C8 C2-C3-05 C3-05-Hl6 N1O-C8-C9 C8-C9-C11 C9-C11 -C13 C11-C13-C14 C13-Cl4-NlO C14-NlO-C8 N1O-C8-C2 C8-C9-012 C9-012-Hl5 C2-N1 -H15 C8-Nl0- -H16
1.8218 1.8218 2.6328 (2.60) 2.6328 (2.60) 119.6 (120.7) 118.7 (118.2) 120.1 (120.4) 118.5 (118.9) 121.1 (122.0) 122.1 (120.0) 117.2 (117.1) 124.0 (122.0) 111.7 (105.0) 119.6 (120.7) 118.7 (118.2) 120.1(120.4) 118.5 (118.9) 121.1 (122.0) 122.1 (120.0) 117.2 (117.1) 124.0 (122.0) 111.7 (105.0)
1.6441 1.4471 1.4211 1.3427 1.3243 1.4120 1.3727 1.3814 1.2836
1.5890
1.6441
1.6942
1.0419 1.8154
1.0341 1.0341
1.0294 1.1987
2.5078 2.6375
2.5446 2.5446
2.5736 2.3994
117.0 119.1 121.o 118.5 119.4 124.8 117.8 122.1 111.7 122.2 115.2 120.7 119.8 120.8 121.3 117.2 122.6
120.1 115.2 121.9 119.8 118.6 124.4 117.4 121.4 120.1 115.2 121.9 119.8 118.6 124.4 117.4 121.4
119.8 116.8 121.0 119.8 118.9 123.8 118.9 119.0 105.4 121.8 114.6 121.4 120.1 119.2 122.9 115.7 121.8
112.1
112.8 112.8
113.0 107.5
1.8591 2.4004 2.6733 118.4 118.7 120.6 118.7 119.7 123.8 116.3 122.4 112.0 122.4 116.2 120.1 119.8 120.9 120.7 118.1 121.o 104.2 108.5
aAll structures were found to be planar. For numbering of the atoms, see Figures 1 and 2. The experimental data are taken from 171.
chronously; and (ii) a one-step mechanism, in which they move synchronously. The AM^ and MNDO-PM3 methods were used to calculate the geometry and the vibrational force field for the transition structures along the reaction paths. The
724
transition structures are shown on Figure 2. All transition structures were found to be planar. The calculated activation enthalpy AH', activation entropy AS#, and free energy of activation AG' at 298 K are listed in Table 111. The values of the VOL. 57, NO. 4
TAUTOMERISM IN 2,2’-BIPYRIDYL-3,3’-DlOL
TSI (DK-KE) 0
15
12
TS2(KE-D E)
15
12
TS (DK-DDE)
15
12
FIGURE 2. Transition structures.
activation parameters vary significantly with the method used. The MNDO-rM3 predicts higher values than those by AM^. Activation entropies AS’ for both mechanisms are small and the free energies of activation AG’ almost coincide with the activation enthalpies. The asynchronous mechanism involves an intermediate KE tautomer which has a lower energy than that of the DK tautomer (see Table I), and for this reason, the process is exothermic. The synchronous mechanism is also exothermic but the barrier is higher (see Table 111). The synchronous transition state corresponds to a saddle point with two imaginary frequencies (Table 111).
During recent years, quantum chemical studies (at AM^ [36-401 and ab initio [37] levels) on the potential energy surface, corresponding to the synchronous and asynchronous mechanisms of the double-proton transfer reaction in asophenine [371, free-base porphirins [381, 2,5-dihydroxy-l,4-benz@ quinone [39], and guanine-cytosinebase pairs [40], have been carried out. Holloway et al. [371 showed that the potential barriers for the systems, obtained by ab initio calculations, were lower by a factor of 2 (3-21c / / 3-21~)or 3 (MPZ / 3-216 / / 3-21~) than those predicted by the AM^ method. Still, the obtained results are qualitatively consistent. For this reason, ab initio calculations were performed
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ENCHEV
TABLE 111 and MNW- p~halculated activation parameters of the ground-state intramolecular proton transfer (IPT) reaction of BP(OH), at 298 K in the vapor phase.
AM+
AG'
AS'
AH# Process
V#
(kcal mol - ')
(cal rnol-' K-'1
(kcal rnol - ')
(cm - '1
11.29 8.88
- 3.59 - 4.32
12.36 10.17
1769.251 1733.211
24.03
- 6.31
25.91
1985.68i 1749.801
18.04 12.07
- 1.29 - 1.22
18.42 12.43
2555.531 2461.741
33.16
- 1.96
33.74
2632.251; 2437.531
AMI
-
Asynchronous DK 3 KE KE DE Synchronous DK * DE MNDO-PM3 Asynchronous DK * KE KE 3 DE Synchronous DK * DE
AHX, activation enthalpy; ASX, activation enthropy; AGX, free energy of activation; interconverting reactants and products.
,'Y
frequency of imaginary vibration
initio-calculated SCF barriers are fairly invariant to the basis set [35, 431. The strongest effect arises when electron correlation is included into the total energies [35, 37, 42-45]. However, such a task exceeds our present computational capabilities. The structures of the transition states TS1 and
for the two-step mechanism only. For this mecha~ / 4 3 1 ~activation barriers are nism, the 4 - 3 1 / much smaller than those by AM^ and MNDO-PM~ (see Tables I11 and IV). This is not surprising, since AM^ and MNDO-PM~ appear to overestimate the activation barriers involving IPT [37,41,42]. The ab TABLE IV
Ab initio total energies at stationary points, barrier height, and imaginary frequency for two-steptransfer reaction DK 3 KE * DE. A. Total energies (in au)
Species
4-31G 114-31G
DE KE DK TSI (DK * KE) TS2 (KE * DE)
- 641.03593195 - 641.01676283 - 641.00456321 -641.00054116 - 641.01 522357
(SCF + MBPT(2))16-31G** -I-O.g*ZPE 13-21Ga
- 643.818143 not found - 643.800134
B. 4-31G I14-31G calculated barriers Process
-
DK * KE KE DE
A€:,, (kcal mot - '1
(crn - ')
2.524 0.966
1432.851 1165.601
V#
Assignment N10-HI6 stret.; H16-NIO-C8 bend N1 -H15stret.; H15-NI -C2bend.
Data from [27]. 1 au = 627.52 kcal rnol- .
a
726
'
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TAUTOMERISM IN 2,2’-BIPYRIDYL-3,3’-DlOL
TS2 (Fig. 2), which were found to be planar, are shown in Table 11. It can be seen that for the DK * KE process there are substantial changes-the C3-05 bond is elongated while the C2-C3-05 and C8-NlO-Hl4 bond angles are diminished. Similarly, in the KE 3 DE process, the C9-012 bond is elongated and the C8-C9-012 and C2-Nl-Hl3 bond angles are decreased. The classical barrier heights for the proton transfer from DK to KE and KE to DE are calculated to be 2.52 and 0.97 kcal/mol, respectively (Table IV). These values are very low; therefore, the two-step proton transfer is very fast. The calculated imaginary frequencies are given in Table IV. These modes describe the in-plane motion of the transferring proton.
10. J. J. P. Stewart (Frank J. Seiler Laboratory, US Air Force Academy, CO 80840, 1990); MOPAC 6.0, QCPE program 455, Bloomington, IN, 1990. 11. J. Baker, J. Comput. Chem. 7, 385 (1986). 12. M. W. Schmidt, K. K. Baldridge, J. A. Boatz, S. T. Elbert, M. S. Gordon, J. H. Jensen, S. Koseki, N. Matsunaga, K. A. Nguyen, S. J. Su, T. L. Windus, M. Dupuis, and J. A. Montgomery, J. Comput. Chem. 14, 1347 (1993). 13. R. Ditchfield, W. J. Hehre, and J. A. Pople, J. Chem. Phys. 54, 724 (1971). 14. T. Oie, G. H. Loew, S. K. Burt, J. S. Binkley, and R. D. MacElroy, Int. J. Quantum Chem., Quantum Bid. Symp. 9, 223 (1982). 15. L. Schafer, C. van Alsenoy, and J. N. Scarsdale, J. Mol. Struct. 86, 349 (1982). 16. V. J. Klimkowski, J. D. Ewbank, C. van Alsenoy, J. N. Scarsdale, and L. Schafer, J. Am. Chem. SOC.104, 1476 (1982). 17. L. Schafer, J. Mol. Struct. 100, 51 (1983). 18. J. E. Boggs and Z . Nui, J. Comp. Chem. 6, 46 (1985). 19. S. Scheiner, J. Chem. Phys. 77, 4039 (1982).
Conclusion Three tautomeric forms (DE, KE, and DE) of BP(OH), have been found by means of semiempirical AM^ and MNDO-PM~) and ab initio quantum chemical calculations. Transition structures, corresponding to the DK 3 KE (TSl), KE DE (TS21, and DK DE (TS) processes (Fig. 2), have been also located. The results obtained suggest that the ground-state proton transfer reaction of BNOH), takes place in two steps, each involving transfer of a single proton. The ab initio 4 - 3 1 ~ / / 4-31~ calculated barriers (Table IV) have values too low and this explains the very fast tautomerization process in the ground state.
-
-
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