Resonance assisted hydrogen bonding and dynamic mechanism for crystal disorder in the enolic form of acetylacetone: a theoretical analysis

Journal of Molecular Structure: THEOCHEM 713 (2005) 59–63 www.elsevier.com/locate/theochem

Resonance assisted hydrogen bonding and dynamic mechanism for crystal disorder in the enolic form of acetylacetone: a theoretical analysis Pablo Campomanes, M. Isabel Mene´ndez, Toma´s L. Sordo* Departamento de Quı´mica Fı´sica y Analı´tica, Universidad de Oviedo, C/Julia´n Claverı´a, 8, 33006 Oviedo, Spain Received 23 September 2004; accepted 3 November 2004

Abstract We report a theoretical analysis of the electronic structure of the enolic form of acetylacetone with the B3PW91, MP2 and CCSD methods using the aug-cc-pVDZ basis set. Although both the GIAO-NICS values and the current density maps display a clear non-aromatic character for cis-2-enol of acetylacetone, C2A, and for the TS for the interconversion of two equivalent cis-2-enols of acetylacetone, TSA, an NBO analysis reveals an important stabilising conjugation stretching all over the skeleton of heavy atoms. The stabilization of TSA by resonance makes possible the dynamic process producing the crystal disorder observed in X-ray diffraction. q 2004 Elsevier B.V. All rights reserved. Keywords: Resonance assisted hydrogen bonding; Acetylacetone; Crystal disorder; Theoretical calculations

1. Introduction O–H/O hydrogen bonds occur in many chemical and biological systems. Gilli et al. [1,2] have proposed the resonance assisted hydrogen bonding (RAHB) model to explain the observed strong hydrogen bonding in the enol forms of diketones. The correlation between the strength of the hydrogen bonding in those systems measured by the distance between the two oxygen atoms and the O–H bond length, and the delocalization of the conjugated double bonds has been interpreted in terms of a mechanism of synergistic interplay of resonance and hydrogen bond formation which constitutes the RAHB model. The molecular structure of acetylacetone has been the goal of many experimental studies raising some controversy. In a gas-phase electron diffraction study it was found that the enol tautomer of hexafluoroacetylacetone has a ˚ and planar symmetric ring with a O/O distance of 2.551 A the enol proton lying in the ring plane [3]. Electron diffraction experiments on acetylacetone have been interpreted by Karle et al. [4] indicating that the enol form of acetylacetone is symmetrical with a very short O/O * Corresponding author. Tel.: C34 98 510 34 75; fax: C34 98 510 31 25. E-mail address: [email protected] (T.L. Sordo). 0166-1280/$ - see front matter q 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2004.11.003

˚ and the H atom in the plane of the ring distance of 2.381 A whereas Shibata’s interpretation [5] rendered an unsym˚) metric enol structure with a longer O/O distance (2.512 A and the H atom clearly out of the ring plane. NMR studies indicate also a Cs symmetry for acetylacetone [6]. The X-ray structure of an organic drug in acetylacetone as solvent showed in the cocrystal an asymmetric H bond in acetylacetone with the H atom out of the molecular plane, ˚ and O–H and O/H and a O/O distance of 2.535 A ˚ distances of 1.03 and 1.46 A, respectively, although this asymmetry of the H bond might be induced by the asymmetry of the crystal environment [7]. More recent X-ray studies have shown that acetylacetone exists in the crystal in the cis-enol form with the central H atom equally distributed over two equivalent positions near the oxygen ˚ ) so that the hydrogen atoms (O/O distance of 2.547 A bond has two separated potential minima. The crystal symmetry of acetylacetone requires assuming that its X-ray structure is a superposition of two nondistinguishable cisenol isomers, although the nature of crystal disorder (static or dynamic) is not clear [8]. A theoretical study [9] of the enolic form of acetylacetone has shown that the intramolecular H-bond combined with the conjugation give rise to an important cooperativity of about 6 kcal/mol. The application of a simple quantum

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mechanical model of a particle in a ring has also shown that the conjugation lowers the energy of the system [10]. The question has been raised as to whether the enol of acetylacetone and of related compounds may be considered as 6p-electron systems displaying aromatic bond delocalization so that this large cooperativity effect might be attributed to aromaticity. A topological analysis of the charge density in the intramolecular O–H/O hydrogen bond of 1-phenyl-1,3-butadione [11] has given evidence of extensive p-delocalization in the ketone-enol group. Large formal charges have been found in the oxygen atoms and the enol hydrogen, which impart polar character to the hydrogen bond, so that the hydrogen position is stabilized by both electrostatic and covalent bonding contributions at each side of the hydrogen atom. These findings have led to a modified RAHB model in which the RAHB can be described as a feedback mechanism that drives the charges in the ring toward symmetry [12]. However, a theoretical investigation of tautomers of acetylacetone using the ‘nucleus-independent chemical shifts’ (NICS) of ringshaped chelated and of the interconversion transition structure has excluded cyclic bond delocalization in these systems [13]. In the present work we undertake a computational study of the intramolecular hydrogen bond in the enolic form of acetylacetone with the aim of getting a deeper understanding of the interplay between hydrogen bonding and conjugation in RAHB. The origin of the important cooperativity associated with RAHB in acetylacetone will be investigated using NICS values, the current-density flow, and an NBO analysis. Given that aromaticity is a multidimensional property requiring the use of several criteria for its determination [14,15] the analysis of the current density flow will help interpreting NICS values. In the light of this theoretical analysis the nature of the crystal disorder found in the more recent X-ray study will be addressed.

2. Methods We performed DFT calculations using the B3PW91 hybrid functional, which combines Becke’s 3-parameter functional [16] with the non-local correlation provided by the Perdew–Wang expression [17], and MP2 calculations [18] with the aug-cc-pVDZ basis set. Stable structures were fully optimized using the GAUSSIAN 98 series of programs [19]. Standard integration grids were employed in the DFT calculations. Vibrational frequencies of all structures were calculated at both theory levels to characterize them as minimum energy structures and obtain the zero point vibrational energies (ZPVE). CCSD/aug-cc-pVDZ single point calculations were carried out on the MP2 optimised geometries of the most stable enolic form of acetylacetone and the transition state (TS) for the interconversion of two such energy minima.

Table 1 NICS in ppm evaluated at the B3PW91/aug-cc-pVDZ theory level, and relative energies (including the ZPVE correction) and cooperativity, in kcal/mol, calculated at both B3PW91/aug-cc-pVDZ and MP2/aug-ccpVDZ levels Structure

DE

NICS

Cooperativity B3PW91/MP2

C2A T2A C1A T1A TSA

0.9/0.0 13.4/11.1 18.4/14.1 23.5/18.0 0.0/0.2

K0.98

7.4/7.2

K1.33

8.3/7.0

The B3PW91/aug-cc-pVDZ electron density was analysed by means of the ‘atoms in molecules’ theory of Bader [20] using the AIMPAC package [21]. We computed the nucleus-independent chemical shifts (NICS) along the axis perpendicular to the cyclic moiety and passing through the (3, C1) ring critical point of electron density [22] using the GIAO method at the B3PW91/aug-cc-pVDZ theory level. Negative NICS denote aromaticity whereas positive NICS indicate antiaromatic character. The current density vector field was computed at the HF/6-31G* theory level on the B3PW91/aug-cc-pVDZ optimized geometry using the continuous gauge formulation denoted by the acronym CTOCD-DZ (for continuous transformation of origin of current density with diamagnetic current set to zero) by Lazzeretti et al. [23]. In this formulation the current density at each point is computed with that point as gauge origin. As a consequence of this choice of gauge origin the diamagnetic component of the density vector field that describes the classical circulation of charge about the gauge origin is formally set to zero, being replaced by a term that depends on the first-order wave function obtained by treating the linear momentum operator as a perturbation. The most significant practical consequence is that this choice is known to give realistic current densities with even quite small basis sets. The SYSMO program was used [24].

Scheme 1. Structures considered in the present work.

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Table 2 ˚ and deg) for C2A and TSA Most important computed and experimental geometrical parameters (A Experimental

O7–H O6–H C4–O C2–O C2–C3 C3–C4 C1–C2 C4–C5 O–O O–H–O angle (deg)

Theoretical

Shibata [5] (el. diff.)

Karle [4] (el. diff.)

Bauer [3] (el. diff.)

Boese [8] (X-ray, 110 K/210 K)

C2A (DFT/MP2)

TSA (DFT/MP2)

1.626 1.049 1.243 1.319 1.382 1.430 1.493 1.525 2.512 137.0

1.190 1.190 1.315 1.315 1.416 1.416 1.497 1.497 2.381 175.0

1.276 1.276 1.259 1.259 1.406 1.406 1.546 1.546 2.552 175.5

1.78/1.74 0.89/0.91

1.554/1.627 1.018/1.006 1.252/1.258 1.320/1.339 1.376/1.379 1.439/1.453 1.491/1.497 1.506/1.512 2.498/2.553 151.7/150.7

1.202/1.207 1.202/1.207 1.285/1.296 1.285/1.296 1.406/1.412 1.406/1.412 1.497/1.503 1.497/1.503 2.361/2.373 158.0/158.8

An NBO analysis [25] was performed using the version implemented in the JAGUAR program [26] at the B3PW91/ aug-cc-pVDZ theory level. These methods have proved adequate for the analysis of RAHB in systems similar to acetylacetone [11,27].

3. Results and discussion Table 1 collects the B3PW91/aug-cc-pVDZ and MP2/ aug-cc-pVDZ relative energies for the cis-2-enol of acetylacetone, C2A, the trans-2-enol, T2A, the cis-1-enol, C1A, the trans-1-enol, T1A, and the TS for the interconversion of two equivalent C2A structures, TSA (see Scheme 1), as well as the largest NICS absolute values obtained along the perpendicular axis passing through the ring critical point for C2A and TSA, and the cooperativity values for C2A and TSA. Table 2 collects the geometrical parameters obtained by different experimental techniques as well as the corresponding theoretical values. Table 3 displays the electron density, rb, and the ellipticity, 3, evaluated at the corresponding bond critical points for C2A and TSA. Unless otherwise stated we will give in the text the relative electronic energies plus the relative ZPVE corrections. When the ZPVE is included the most stable structure for acetylacetone at the B3PW91/aug-cc-pVDZ level is TSA, which is 0.9 kcal/mol more stable than C2A. However at

1.291/1.283 1.402/1.397 1.497/1.486 2.547/2.541 155/147

MP2/aug-cc-pVDZ level the most stable structure is C2A by only 0.2 kcal/mol. Trying to elucidate the relative stability of these two species we found with the CCSD/aug-cc-pDVZ method including the MP2 ZPVE correction that C2A is 2.2 kcal/mol more stable than TSA (4.8 kcal/mol without ZPVE). We evaluated the stabilising contributions of H-bonding, conjugation and their cooperative effects in C2A and TSA by comparing the B3PW91 and MP2 energies and structures of the corresponding cis and trans, and 2- and 1-enols. The difference in energy between C1A and T1A, which provides an estimate of the H-bonding energy in the absence of conjugation, is 5.1 (B3PW91) and 3.9 (MP2) kcal/mol. The B3PW91 hydrogen bonding energy for acetylacetone is similar to the value reported for ice (4.8 kcal/mol) and larger than MP2 value, which is more similar to that for water dimer (3.6 kcal/mol) [28]. The difference in energy between T1A and T2A, 10.1 (B3PW91) and 6.9 (MP2) kcal/mol, provides an estimate of the contribution of conjugation to the stability of the cis-2-enol forms. The cooperativity of H-bonding and conjugation evaluated as [E(X)K E(T2A)]K[E(C1A)KE(T1A)] where XZC2A or TSA, amounts to 7.4 (B3PW91) and 7.2 (MP2) kcal/mol for C2A, and 8.3 (B3PW91) and 7.0 (MP2) kcal/mol for TSA. The difference in energy between T2A and C2A would correspond to the hydrogen bonding energy in the presence of conjugation and is according to our calculations 12.5

Table 3 Computed electron density (rb) and ellipticity (3) in atomic units for C2A and TSA at the B3PW91/aug-cc-pVDZ level Bond

O6–H O7–H C2–O C4–O C2–C3 C3–C4 C1–C2 C4–C5

C2A

TSA

rb (a.u.)

3 (a.u.)

rb (a.u.)

3 (a.u.)

0.3005 0.0695 0.3178 0.3745 0.3160 0.2855 0.2622 0.2567

0.0103 0.0101 0.0532 0.0115 0.3216 0.1882 0.0594 0.0506

0.1740 0.1740 0.3460 0.3460 0.3020 0.3020 0.2600 0.2600

0.0087 0.0087 0.4320 0.4320 0.2498 0.2498 0.0545 0.0545

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Fig. 1. sCp Current density maps evaluated at 1 bohr above the molecular plane of C2A and TSA.

(B3PW91) and 11.1 (MP2) kcal/mol which corresponds to the bare hydrogen bonding energy plus the cooperativity energy, to compare with the value calculated by Gilli et al. [29] using a semiempirical model (12.8 kcal/mol) and with the SCF value calculated by Frisch et al. [30] for malonaldehyde including the ZPVE (11.6 kcal/mol). In general, the optimised geometrical parameters for TSA agree reasonably with those experimentally measured by Karle et al. [4] and Bauer et al. [3] the most appreciable discrepancy corresponding to the O–H–O angle (see Table 2). The keto-enol group is symmetric. The rb value corresponding to the two equivalent O–H bonds is roughly twice that for the O/H interaction and half that for the O–H bond in C2A, and their ellipticity is very small reflecting a practically pure s character. The C2–O and C4–O bond lengths and ellipticities indicate an important double bond character. The bond length and rb and 3 values for the C2–C3 and C3–C4 bonds are similar to those in benzene. On the other hand, the C1–C2 and C4–C5 bonds present a bond length a little shorter than a typical C–C single bond as

Fig. 2. sCp Current density map evaluated at 1 bohr above the molecular plane of benzene.

Scheme 2. Most important resonance structures and their weight for C2A.

well as a rb value a little larger. These characteristics could be caused by the action of a certain degree of hyperconjugation of the two methyl groups on C2 and C4 with the p system. This phenomenon would be consistent with an ellipticity not as large as when p conjugation takes place but a little larger than for a C–C single bond. All these features clearly indicate that there is an electron delocalization over the whole system except the bridged H atom. In contrast, the optimised geometrical parameters for C2A agree with those reported by Shibata et al. [5] and particularly with the structure obtained by Boese et al. [8]. In effect, the average value of the theoretical bond lengths ˚ ; C2–C3/ C4–O/C2–O, 1.286 (B3PW91) and 1.298 (MP2) A ˚ C3–C4, 1.407 (B3PW91) and 1.416 (MP2) A; and C1–C2/ ˚ , as well as the C4–C5, 1.498 (B3PW91) and 1.504 (MP2) A O–O distance and the O–H–O angle are in good accord with the values reported by Boese et al. [8]. Strikingly, the ellipticity values for C2–O and C4–O are much smaller for C2A than for TSA. This difference stems from the curvature of the s bond paths because of the larger O–O distance in C2A. Therefore, our theoretical results support clearly the Cs symmetry rendered by NMR experiments, and the X-ray structure obtained with the central H atom equally distributed over two equivalent minima. Moreover, as the energy barrier separating these two potential minima is low (2.2 kcal/mol) our calculations suggest the interpretation of crystal disorder as a dynamic process. To further investigate the origin of the cooperativity we calculated the NICS for C2A and TSA. The NICS values obtained for these two structures in the present work, K0.98 and K1.30, are larger in absolute value than those previously reported (K0.28 and K0.024) [13]. So, although NICS of aromatic compounds are lower than about K3 ppm, the question could be raised as to whether the more negative NICS obtained by us could be indicative of some quasi-aromatic character or not. In order to answer this question we computed the CTOCD-DZ current density vector for C2A and TSA.

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Scheme 3. Most important resonance structures and their weight for TSA.

Fig. 1 displays the corresponding sCp current density maps. For comparison we also computed the sCp current density map for benzene (see Fig. 2). From Fig. 1 it is clear that C2A and TSA are non-aromatic species. However, the current density maps reveal an important conjugation stretching all over C2A and TSA excluding the bridged H atom in agreement with the structural analysis above. An NBO analysis shows that there are four resonance structures for C2A and 8 for TSA (see Schemes 2 and 3). These structures show clearly the presence of an important conjugation in agreement with the current density maps above. In the case of TSA the contribution of a larger number of resonance structures of roughly similar weights than in C2A might be responsible for a more important stabilization of that species thus determining a low energy barrier for the interconversion of the two equivalent C2A structures, and making possible the dynamical mechanism for the experimentally detected [8] crystal disorder. In summary, although current density maps show that C2A and TSA are non-aromatic, they reveal an important conjugation stretching all over the skeleton of heavy atoms in these systems. An NBO analysis corroborates this important conjugation determining a large resonance energy, particularly in TSA. The stabilization of TSA by resonance makes possible the dynamic process producing the crystal disorder observed in X-ray diffraction. References [1] G. Gilli, F. Bellucci, V. Ferretti, V. Bertolasi, J. Am. Chem. Soc. 111 (1989) 1023. [2] V. Bertolasi, P. Gilli, V. Ferretti, G. Gilli, J. Am. Chem. Soc. 113 (1991) 4917. [3] A.L. Andreassen, D. Zebelman, S.H. Bauer, J. Am. Chem. Soc. 93 (1971) 1148. [4] A.H. Lowry, C.G.P. D’Antonio, J. Karle, J. Am. Chem. Soc. 93 (1971) 6399. [5] K. Iijima, A. Ohnogi, S. Shibata, J. Mol. Struct. 156 (1987) 111. [6] L.J. Altman, D. Laungani, G. Gunnarsson, H. Wennerstro¨m, S. Forse´n, J. Am. Chem. Soc. 100 (1978) 8264. [7] A. Camerman, D. Mostopaolo, N. Camerman, J. Am. Chem. Soc. 105 (1983) 1584. [8] R. Boese, M.Y. Antipin, D. Bla¨ser, K.A. Lyssenko, J. Phys. Chem. B 102 (1998) 8654.

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