Intramolecular proton transfer in 2-(2 0 -hydroxyphenyl)benzoxazole: the reliability of ab initio calculations on simplified structures

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1 Journal of Molecular Structure (Theochem) 489 (1999) Intramolecular proton transfer in 2-(2 0 -hydroxyphenyl)benzoxazole: the reliability of ab initio calculations on simplified structures A. Fernández-Ramos a, J. Rodríguez-Otero a, *, M.A. Ríos a, J. Soto b a Departamento de Química Física, Factultade de Química, Universidade de Santiago de Compostela, E Santiago de Compostela, Galicia, Spain b Departamento de Química Física, Facultad de Ciencias, Universidad de Málaga, E-29071, Málaga, Spain Received 22 January 1999; accepted 8 February 1999 Abstract It is common practice in Computational Chemistry to model the behaviour of bulky compounds from simplified structures. This procedure enables the use of higher computational levels, with generally improved results. The most crucial problem in this methodology is choosing how simple the structures in question should be, in fact, oversimplification in this context can lead to a highly different behaviour relative to the starting one. In order to check how valid this approach is, in this work we modelled intramolecular proton transfer in 2-(2 0 -hydroxyphenyl)benzoxazole in its ground and first excited singlet states from structures simplified to a variable extent. Ab initio calculations at the HF/3-21G level in the ground state and the CIS/3-21G level in the excited state included geometric optimization of potential tautomers and the intervening transition states. The results obtained reveal that the structure of the original compound must be simplified carefully if spurious conclusions are to be avoided Elsevier Science B.V. All rights reserved. Keywords: Proton transfer; Intramolecular hydrogen bond; ESIPT; Ab initio calculations; CIS calculations; HBO 1. Introduction 2-(2 0 -Hydroxyphenyl)benzoxazole (HBO) is a typical example of compound exhibiting excited state intramolecular proton transfer (ESIPT). Barbara et al. [1] have found experimental evidence of ESIPT in other heteroaromatic compounds such as 2-(2 0 - hydroxyphenyl)benzimidazole and 2-(3 0 -hydroxy pyridyl)benzimidazole. Theoretically, HBO can occur in an enol form (N) possessing an N H hydrogen bond or a keto form (T) where the proton is transferred and gives rise to an * Corresponding author. O H interaction (Fig. 1). However, experimental facts suggest that, at least in non-protic solvents, the enol form prevails over the keto form. Excitation of the enol tautomer to its first singlet, S 1, gives rise to an enole form N* that turns into an excited state T* via a proton transfer promoted by stability inversion. Following deactivation to the ground state T, a proton transfer leads back to the starting form N. This process has been studied in experimental [2,3] and theoretical terms [4 9]. Among the latter are the ab initio calculations at the 3-21G level carried out by our group [9], which involved computing data points up to the MP3 level for the ground state and the CIS/3-21G level for the excited state S 1. Although this computational level /99/$ - see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S (99)

2 256 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 1. Proton transfer and excitation-emission processes in a four-level scheme for HBO. can in principle be improved, this is no easy task for a bulky compound such as HBO, which would require a high computational investment. In order to confirm the accuracy of modelling HBO from simplified structures, we conducted an ab initio study of proton transfer in the ground (S 0 ) and first excited singlet state (S 1 ) of structures [2-(2 0 -hydroxy)oxazole] (I), [2-(2 0 - hydroxy)ethyleneoxazole](ii) and (1-amino-3-propenal) (III) shown in Fig Computational aspects All calculations were carried out by the Gaussian 94 software package [10]. The four structures studied (HBO, I, II, and III) were optimized with no geometric constraints in their enol and keto forms, as were the intervening transition states, both in the ground and in the first excited singlet states. The corresponding normal-mode frequencies were calculated in all cases in order to confirm that the minimum or transition-structure condition was satisfied. The ground state computations were performed at the SCF/3-21G level. Those in the excited state considered electron correlation by calculations of interactions between configurations using single-excitation (CIS/3-21G), a level that appears to be reasonably high for exploring potential surface energies for excited states [11]. While modest, the basis set used (3-21G) provided to us acceptable results with compounds involving similar proton transfers [12 14]. Other authors have confirmed this fact, as for instance Sobelewski et al. [15]. 3. Results and discussion 3.1. Ground state (S 0 ) Tables 1 and 2 list the energies for the proton transfer process as well as selected geometric parameters for the hydrogen bond in the structures studied. One immediate inference is that the structures of all the compounds in their three forms (N, T and TS) exhibit C s symmetry. Notwithstanding this coincidence between HBO and its simplified forms, more detailed analysis reveals marked differences that strengthen with increasing simplification. In terms of relative stabilities, both I and II correctly predict that the enol form should be the more stable. However, while the N T energy difference for I is very similar to that for HBO (13.52 versus kcal/mol), that for II is virtually zero (0.49 kcal/mol). By contrast, in structure III, the keto form is much more stable (12.23 kcal) than the enol form. Similar conclusions can be drawn regarding the proton transfer (as inferred from energy barriers), namely: the results are similar for I, rather different for II and very different for III. As the imaginary frequency in the transition state is directly related to the curvature of the reaction pathway at that point, the calculated values of such frequencies support the previous assertions. Thus, both II and III lead to substantially higher frequencies than do HBO and I. This is clearly related to the sharper reaction pathways exhibited by II and III in the transition state region. All these

3 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 2. Enol (N) and keto (T) forms of the structures studied. conclusions are apparent from Fig. 3, which shows an outline of the curves for the proton transfer in the four compounds studied. Similar conclusions can be drawn with regard to geometric reproduction, the most significant expression for which is the hydrogen bond moiety (see Table 2). The similarity between HBO and I in this respect is extremely good for the enol form and somewhat worse for the keto form. In contrast, the geometries of II and III are markedly different from those of HBO. One other highly interesting basis for comparison of proton transfer is the location of the transition state in the reaction pathway. For HBO and I, the transition state is closer to the keto form, with differences between the O H and N H distances of and Å, respectively for both the compounds. For II and III, the differences are and Å, respectively, which suggests a virtually centred transition state in the former case and one closer to the enol form in the latter. Despite these divergences, the distance between the two electronegative atoms in the transition state is fairly similar in all the four compounds. The shortened N O distance in the transition state, relative to its value at minima, is seemingly consistent in all the cases with a previously postulated mechanism [9] by which the N and O approach each other, the proton transfer takes

4 258 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Table 1 Relative energies (kcal/mol) of the different structures studied in their ground state HBO I II III Enol form (N) Keto form (T) Transition state (TS) Imaginary frequency of TS (cm 1 ) Table 3 Relative energies (kcal/mol) of the different structures studied in their first excited singlet HBO I II III Enol form (N*) Keteo form (T*) Transition state (TS*) Imaginary frequency of TS* (cm 1 ) Table 2 Selected geometric parameters (Å) for the hydrogen bond in the different structures studied in their ground state HBO I II III d(oh) Enol form (N) d(nh) d(no) d(oh) Keto form (T) d(nh) d(no) d(oh) Transition state (TS) d(nh) d(no) place in the transition state and the two atoms depart afterwards Excited state (S 1 ) Tables 3 and 4 show the energies for the proton transfer process as well as selected geometric parameters for the hydrogen bond in the structures studied. As can be seen from Table 3, the relative stability of HBO in the excited state is inverted relative to its ground state; thus, the keto form is 7.25 kcal/mol more stable than the enol form and the energy barrier for intramolecular proton transfer in this excited state is only 3.73 kcal/mol. Despite the computational constraints, all this accounts accurately for the experimentally observed ESIPT process. The simplified structure I reproduces this behaviour very well: the keto form is 8.03 kcal/mol more stable than the enol form and the energy barrier is only 2.68 kcal/mol. Structure II and, especially, III, give much worse results; in any case, both predict a more stable keto form in S 1. However, the keto enol stability difference for II is too large (15.13 kcal/mol), whereas its Table 4 Selected geometric parameters (Å) for the hydrogen bond in the different structures studied in their first excited singlet state HBO I II III d(oh) Enol form (N*) d(nh) d(no) d(oh) Keto form (T*) d(nh) d(no) d(oh) Fig. 3. Sketch of the reaction path for the four structures studied in their ground state S 0. Transition state (TS*) d(nh) d(no)

5 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 4. Sketch of the reaction path for the four structures studied in their first excited singlet state S 1. proton transfer barrier is too low (1.11 kcal/mol). These errors are inverted in III, with a keto enol difference and energy barrier of 4.29 and kcal/ mol, respectively. Fig. 4 outlines the transfer process and provides a visual depiction of the above-described energy considerations. The magnitude and location of the different singular points affects the imaginary frequency in the transition state. Thus, the frequencies for I and HBO are very similar (1344 and 1436 cm 1, respectively); however, those for II and III are rather different: too low for II (356 cm 1 ) and too high for III (2089 cm 1 ). As a result, II exhibits little curvature and III a strongly curved reaction pathway in the transition state region. Geometric differences between the enol form of HBO in its S 0 and S 1 states suggest that N! N* electron excitation strengthens its hydrogen bond as the O H distance is lengthened while the N H and N O distances are substantially shortened in the process. Fig. 5 shows the effect of excitation on bond distances in the pseudo-aromatic ring. As can clearly be seen, the double bonds are markedly lengthened and the single bonds shortened. All this considerably favours the proton transfer and hence the formation of the keto form, the bonding scheme for which (Fig. 1) is much better correlated with the new bond lengths. The analysis of the results for the N! N* processes in the simplified structures reveals that both I and II undergo the above-described changes for HBO (viz. strengthening hydrogen bonds and a favoured proton transfer leading to the keto form); however, structure I is obviously much more similar to HBO than is structure II. In contrast, III exhibits none of these changes, which is seemingly consistent with its low tendency to undergo ESIPT as a result of its high energy barrier and the identical energies of its tautomers in the S 1 state (Fig. 4). In addition, the N* tautomer of III is slightly non-planar, only by 13.4 (in the HOCC dihedral) at the most, with the exception of the imine proton which is perpendicular to the ring. The origin of the geometric differences introduced by the S 0! S 1 excitation in the enol form can be explained, at least in qualitative terms, by analysing in detail the results of the configuration interactions (CIS) adopted for the excitated state study. Thus, the excitation process in all the structures studied involves the HOMO! LUMO transition, which corresponds to a pp* excited singlet state. The shapes of these orbitals (Fig. 6) allow one to interpret the lengthening and shortening changes in the pseudo-aromatic ring. Thus, when the HOMO! LUMO transition involves the loss of the bonding character of a bond (or the gain of anti-bonding character), the bond concerned is lengthened; the inverse transformation leads to a shortened bond. This qualitative analysis is not so revealing for structure III as, for example, it does not account for the lengthened CyN bond. This error may be the result of structure III being markedly non-planar in its S 1 state, which would further decrease the already limited capacity of these qualitative considerations to predict geometric changes arising from electron excitation. Regarding the geometric reproduction of the keto tautomer T*, from Table 4 it follows that the similarity between HBO and I is extremely good and, contrary to the ground state, better than for the enol form. In contrast, II and III exhibit geometries that differ considerably from those of HBO. The differences are even greater than in the ground state: the structures contain very long hydrogen bonds (with N O distance of Å in II and Å in III). Another distinguishing factor for the keto form in these two structures is the fact that the S 1 state results

6 260 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 5. Effect of electron excitation (S 0! S 1 ) on bond lengths in the pseudo-aromatic ring. The excited state values are given in brackets. largely from the HOMO-1! LUMO transition, which corresponds to a np* excited singlet state. The differences between the O H and N H distances in the excited transition state of HBO and structure I are and Å, respectively. This suggests that the transition state is closer to the enol form, in a similar position, in both cases. This difference for structure II ( Å) is suggestive of a purely qualitative similarity. For the III structure the value of that geometrical difference (0.005 Å) shows a different fact with a transition state virtually centred in the reaction pathway. As can be seen

7 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 6. HOMO and LUMO for the enol forms of the structures studied. from Table 4, the geometric reproduction of the transition state is very good for structure I, with differences of a few hundredths of an angstrom relative to HBO, but very bad for II and III and worsened by the fact that both are considerably non-planar. This is not surprising for the transition state of structure III as, as noted earlier, its enol form is also non-planar. In addition, the widest dihedral angle (13.2 for the HOOC torsion) is very similar to that for the enol form, where the imine proton also lies roughly normal to the molecular plane. However, the loss of planarity in the transition state of structure II, which amounts to 28.6 for the HOOC dihedral, is more surprising as its enol and keto form are both absolutely planar. Accounting for this phenomenon is seemingly no easy task; it may be closely related to a possible crossing of electronic configurations in the S 1 state of the structure. Thus, while the electron excitation for the enol form and the transition state corresponds to the HOMO! LUMO transition, that for the keto form corresponds essentially to the HOMO-1! LUMO transition. This fact was fully confirmed by Sobolewski and Domcke [16] at higher-level of calculation (CASSCF and MRCI). The electron state produced by the HOMO! LUMO transition in this keto form of structure II is basically the second excited singlet, S 2. Geometric optimization of this state lead to a nonplanar structure that is very similar to the transition state (see Fig. 7). Thus, the out-of-plane atom displacement is similar for the transition state S 1 and the keto form S 2, with OCCC dihedral angles of 25.0 and 30.0, respectively. 4. Conclusions Computations on HBO and its simplified derivatives I, II and III in the ground and first excited singlet states reveal that great care should be exercised in modelling intramolecular proton transfer in the parent compound from the simplified structures. The results indicate that structure I can effectively

8 262 A. Fernández-Ramos et al. / Journal of Molecular Structure (Theochem) 489 (1999) Fig. 7. Optimized geometries for structure II: transition state for the excited singlet S 1 (left) and keto form of the excited state S 2 (right). Coefficients of CI indicate that in both cases the electron transition corresponds essentially to the HOMO! LUMO transition. replace the parent compound in both, the qualitative and quantitative terms. The processes undergone by this structure in its S 0 and S 1 states faithfully reproduce those that occur in HBO; geometric and energetic deviations between the two are negligible. This structure can therefore be an appropriate substitute for HBO with a view to employing higher-level computations (viz. extended basis sets, electron correlation). In contrast, structure II reproduces only partly, and mostly in a qualitative manner, some of the properties of the parent molecule. Finally, structure III is very poorly consistent with HBO. Although it preserves the fundamental structure of the parent compound, it is oversimplified and leads to a markedly distorted behaviour. This suggests that the aromatic rings in HBO play a crucial role in its proton transfers. Acknowledgements This work was partially supported by the Xunta de Galicia Project No. XUGA20903A98. References [1] P.F. Barbara, P.K. Wals, L.E. Brus, J. Phys. Chem. 93 (1989) 29. [2] D.L. Williams, A. Heller, J. Phys. Chem. 74 (1970) [3] Th. Arthen-Engeland, T. Bultman, N.P. Ernsting, M.A. Rodríguez, W. Thiel, Chem. Phys. 163 (1992) 43. [4] L. Lavtchieva, V. Enchev, Z. Smedarchina, J. Phys. Chem. 97 (1993) 306. [5] V. Enchev, Indian J. Chem. 33B (1994) 336. [6] K. Das, N. Sarkar, A.K. Ghosh, D. Majumdar, D.N. Nath, K. Bhattacharyya, J. Phys. Chem. 98 (1994) [7] S. Nagaoka, A. Itoh, K. Mukai, U. Nagashima, J. Phys. Chem. 97 (1993) V. Guallar, M. Moreno, J.M. Lluch, F. Amat-Guerri, A. Douhal, J. Phys. Chem. 100 (1996) [9] M. A Ríos, M.C. Ríos, J. Phys. Chem. 99 (1995) [10] M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, N.G. Johnson, M.A. Robb, J.R. Cheeseman, T. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Stefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, V. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Difrees, J. Baker, V. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, Gaussian 94, Revision C.3, Gaussian, Inc., Pittsburgh PA, [11] J.B. Foresman, M. Head-Gordon, J.A. Pople, M.J. Frisch, J. Phys. Chem. 96 (1992) 135. [12] A.M. Graña, M.A. Ríos, J. Rodríguez, Struct. Chem. 2 (1991) 575. [13] M.A. Ríos, J. Rodríguez, J. Comput. Chem. 13 (1992) 860. [14] M.A. Ríos, J. Rodríguez, Can. J. Chem. 71 (1993) 303. [15] A.L. Sobolewski, C. Woywod, W. Domcke, J. Chem. Phys. 98 (1993) [16] A.L. Sobolewski, W. Domcke, Chem. Phys. Letters 211 (1993) 82.