Electronic Structure and Proton Transfer in Ground-State

May 27, 2010 - To whom correspondence should be addressed. E-mail:[email protected]. Telephone: (203) 432-3975. Fax: (203) 432-6144., †...
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Electronic Structure and Proton Transfer in Ground-State Hexafluoroacetylacetone Chandrima Chatterjee, Christopher D. Incarvito, Lori A. Burns,† and Patrick H. Vaccaro* Department of Chemistry, Yale UniVersity, P.O. Box 208107, New HaVen, Connecticut 06520-8107 ReceiVed: February 8, 2010; ReVised Manuscript ReceiVed: April 26, 2010

˜ 1A1) of hexafluoroacetylacetone (HFAA) has been subjected to synergistic The ground electronic state (X experimental and theoretical investigations designed to resolve controversies surrounding the nature of intramolecular hydrogen bonding for the enol tautomer. Cryogenic (93K) X-ray diffraction studies were conducted on single HFAA crystals grown in situ by means of the zone-melting technique, with the resulting electron density maps affording clear evidence for distinguishable O1-H and H · · · O2 bonds that span an interoxygen distance of 2.680 ( 0.003 Å. Such laboratory findings have been corroborated by a variety of quantum chemical methods including Hartree-Fock (HF), density functional [DFT (B3LYP)], Møller-Plesset perturbation (MPn), and coupled cluster [CCSD, CCSD(T)] treatments built upon extensive sets of correlationconsistent basis functions. Geometry optimizations performed at the CCSD(T)/aug-cc-pVDZ level of theory predict an asymmetric (Cs) equilibrium configuration characterized by an O · · · O donor-acceptor separation of 2.628 Å. Similar analyses of the transition state for proton transfer reveal a symmetric (C2V) structure that presents a potential barrier of 21.29 kJ/mol (1779.7 cm-1) height. The emerging computational description of HFAA is in reasonable accord with crystallographic measurements and suggests a weakening of hydrogenbond strength relative to that of the analogous acetylacetone molecule. I. Introduction The concerted proton-transfer and hydrogen-bonding processes that characterize the cis-enol tautomers of simple β-diketones long have served to guide the interpretation of analogous phenomena taking place in substantially larger complexes. The presence of a strong intramolecular hydrogen bond that adjoins hydroxylic (proton-donating) and ketonic (proton-accepting) oxygen centers markedly stabilizes such species,1,2 causing them to dominate over their diketo counterparts under ambient, isolated-molecule conditions. Examples of key concepts that have been elaborated on through detailed consideration of this structural motif include low-barrier hydrogen bonding (LBHBing),1,3 which is distinguished by great strength, short distance, and vanishingly small impediment to hydron migration, and resonance-assisted hydrogen bonding (RAHBing),4 whereby delocalization of π-electron density about a quasi-aromatic (chelated) ring induces a synergistic enhancement of donor-acceptor interactions. As the progenitor of the β-diketo enol family, malonaldehyde (MA) affords a model system for unraveling the vibrational and electronic specificity of proton-transfer dynamics;5,6 however, technical difficulties associated with the synthesis and handling of this labile compound have limited the scope of laboratory studies. The greater stability realized by terminal methylation has made acetylacetone (AA) the target of numerous investigations conducted in both gas-phase7,8 and condensed-phase9,10 environments. Although remnants of controversy still exist regarding the geometry of AA,11 the vast majority of experimental8,9 and theoretical12,13 endeavors concur in asserting the ground electronic state to support a symmetric double-minimum well along the O · · · H · · · O reaction coordinate, with two equivalent mini* To whom correspondence should be addressed. E-mail: [email protected]. Telephone: (203) 432-3975. Fax: (203) 432-6144. † Present Address: Center for Computational Molecular Science and Technology, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA 30332-0400 USA.

mum-energy configurations of Cs symmetry being separated by a potential barrier of finite height. The present work focuses on the related hexafluoroacetylacetone (HFAA) molecule, where the introduction of electron-withdrawing fluoromethyl substituents is expected to weaken intramolecular hydrogen bonding, a prediction corroborated, in part, by infrared/Raman spectral patterns,14,15 NMR chemical shifts,16 and photofragment productstate distributions.17 In particular, cryogenic X-ray diffraction measurements and high-level ab initio calculations will be used to elucidate the equilibrium geometry and potential surface topography of this substituted β-diketo enol. Although various experiments have examined the groundstate geometry of HFAA, the conclusions reached by these efforts have been a subject of controversy. An early investigation of gas-phase electron diffraction by Andreassen and coworkers18 reported the O · · · H · · · O moiety to be symmetric, with the hydrogen atom positioned midway between two oxygen centers separated by 2.551 Å. However, their structural refinement was based on the assumption of an encompassing C2 point group that had an angular deviation of roughly 6° between planes defined by the C3dC1-O1 and C3-C2dO2 moieties (cf. Figure 1 for numbering scheme). The model employed for this study also considered the fluoromethyl substituents to be rigid entities, leading to an O1-C1-C4-F1 dihedral angle of τO1C1C4F1 ) 46.7°. The subsequent electron diffraction work of Iijima et al.19 lifted such restrictions by treating the -CF3 groups as flexible rotors characterized by low effective barriers to internal rotation, thereby yielding a value of 60.0° for τO1C1C4F1. In keeping with the previous findings of Andreassen, these authors argued that their analyses failed to converge for an asymmetric enol tautomer. Instead, they proposed a symmetric (C2V) molecular framework that placed the shuttling hydron in the plane of the six-membered chelate ring, where it bisected the 2.606 Å interoxygen distance (θOHO ) 180.0°) and gave a structural motif commensurate with the phenomenon of LBHBing.

10.1021/jp101224e  2010 American Chemical Society Published on Web 05/27/2010

Ground-State Hexafluoroacetylacetone

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Figure 1. Structure and Dynamics in HFAA. The proton-transfer process in enolic HFAA is depicted schematically, illustrating the role of the shuttling hydron (H1) that mediates interconversion of two equivalent asymmetric (Cs) conformers through a symmetric (C2V) transition state. Each atom has been distinguished by affixing a numerical subscript to the elemental symbol, with all tabulated structural parameters referring to the labeling scheme appropriate for the leftmost minimum-energy configuration.

In contrast to electron diffraction studies, X-ray photoelectron measurements by Brown20 have provided strong evidence for a nonsymmetrical HFAA equilibrium configuration. Two prominent peaks of equal amplitude were found to appear in the O1s ionization region, implying that the oxygen atoms experience different chemical environments consistent with inequivalent hydrogen bonding (i.e., O-H · · · O rather than O · · · H · · · O). Tayyari et al.15,21 have reinforced these conclusions by exploiting infrared (IR) absorption and Raman scattering techniques to examine a series of enolic β-diketones under both gas-phase and condensed-phase conditions. The red shift in frequency and enhancement in spectral broadening noted by these authors for the IR-active O-H stretching mode of HFAA, as well as related effects induced by deuterium isotopic labeling, were argued to be indicative of an underlying double-minimum potential surface. Introduction of fluoromethyl substituents also produced systematic changes in the pattern of Raman intensities for C-C stretching degrees of freedom, an observation interpreted in terms of the reduced π-electron conjugation expected for a chelated Cs equilibrium geometry. Additional support for this assertion follows from the mid-IR vibrational features reported by Méndez De Leo and co-workers22 for isolated HFAA molecules physisorbed on a Si(100) surface, which were reproduced best by theoretical predictions built upon an asymmetrical (Cs) structure for the enol tautomer. More recent analyses of the gaseous microwave spectrum by Evangelisti et al.23 have suggested HFAA to be a “rigid” species of Cs symmetry, where large effective barriers to internal rotation of the massive -CF3 groups impede experimental detection of tunneling-induced splittings (vide infra). In support of the aforementioned experimental work, a wide variety of theoretical efforts have been reported for the ground electronic state of HFAA. Early geometry optimization studies by Burk and Koppel24 exploited semiempirical AM1 and PM3 methods to explore all possible enolic and ketonic forms of the molecular framework. This work found the most stable species to be a cyclic enol tautomer distinguished by a planar sixmembered ring that clearly exhibited an unsymmetrical placement of the pivotal hydrogen atom. The O · · · O donor-acceptor distance (rO · · · O), an often-cited metric for the efficacy of proton transfer and the nature of hydrogen bonding, was predicted to be 2.86 Å by AM1 and 2.65 Å by PM3. Based upon such results, these authors postulated the intramolecular dynamics of HFAA to be governed by an effective double-minimum potential surface in which two equivalent and asymmetric (Cs) equilibrium configurations are separated by a symmetric (C2V) transitionstate structure that presents a reaction barrier of 101 kJ/mol (8430 cm-1) or 115 kJ/mol (9580 cm-1) height, as calculated by the AM1 or PM3 schemes. Buemi25 was the first to examine HFAA by means of density functional theory (DFT), with B3LYP/6-31G** optimization showing the global minimum-energy configuration to be an

asymmetric (Cs) enol that possessed two distinct O-H bonds (viz., rO1-H1 ) 1.002 Å and rO2 · · · H1 ) 1.670 Å) and was located 10.08 kJ/mol (842.6 cm-1) below a transition state of C2V character. Predicted structural parameters were in reasonable accord with those derived from prior semiempirical efforts; however, the latter were found to have significantly overestimated the proton-transfer barrier height. In a subsequent investigation, Tayyari et al.26 argued that Buemi’s work excluded adjustment of dihedral angles for the -CF3 moieties (presumably due to convergence difficulties), instead constraining them at orientations eclipsed and staggered with respect to the carbonyl and hydroxyl oxygen atoms. To remedy this shortcoming, these authors invoked tight convergence criteria for full DFT optimizations that exploited the hybrid B3LYP and B3P86 correlation-exchange functionals in conjunction with various basis sets. The resulting HFAA equilibrium geometries were of C1 symmetry, reflecting a slight rotation of the F4-C5-C2dO2 dihedral angle (by 32.4 and 5.3° for B3LYP/6-31G** and B3LYP/6311++G** treatments) that eliminated the molecular plane. While the O · · · O separation of 2.551 Å from B3LYP/6-31G** calculations was in excellent agreement with the electron diffraction measurements,18 improvement of basis set quality to 6-311++G** extended the donor-acceptor distance to 2.592 Å. Nevertheless, such B3LYP/6-311++G** analyses support a unimolecular reaction that proceeds by way of a C2V transition state residing 10.24 kJ/mol (860 cm-1) above two equivalent equilibrium tautomers. Sliznev et al.27 have performed an extensive computational investigation of potential surface topography in substituted β-diketo enols, with the influence of electron-electron interactions in ground-state HFAA being revealed by successive application of the Møller-Plesset perturbative expansion taken to second (MP2), third (MP3), and fourth [MP4(SDQ)] orders. Building upon stationary point geometries optimized at the Hartree-Fock (HF) level of theory through use of polarizationaugmented Huzinaga-Dunning basis sets of triple-ζ quality, the proton-transfer barrier height of 12.4 kJ/mol (1040 cm-1) predicted from single-point MP2 calculations was found to be substantially lower than those deduced from analogous MP3 (2460 cm-1) and MP4 (2270 cm-1) treatments. Such observations reflect known shortcomings of the MP2 scheme,28 including its tendency to overcompensate for electron correlation effects. All ab initio results reported by these authors concur in asserting an equilibrium configuration of Cs symmetry, with MP2 optimization suggesting the crucial O · · · O donor-acceptor distance to be 2.591 Å. Interestingly, both Tayyari26 and Sliznev27 noted that “averaging” of the structural parameters computed for the two equivalent minimum-energy forms of enolic HFAA gave a “hybrid” framework in remarkable agreement with that deduced from gas-phase electron diffraction measurements (a finding that must be considered to be fortuitous, at best).

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TABLE 1: Crystallographic Parameters for HFAAa parameter

value

empirical formula formula weight (g mol-1) crystal color, habit crystal system space group a (Å) b (Å) c (Å) β (deg) V (Å3) Z Dcal (g cm-3) µ (cm-1) F000 2θmax (deg) number of measured reflections

C 5 H 2 F 6O 2 208.06 colorless, prism monoclinic P21 (#4) 5.7364(3) 5.3479(3) 11.5262(6) 96.319(3) 351.45(4) 2 1.966 23.062 204.0 133.0 total: 3895 unique: 949 Friedel pairs: 318 0.051 123 7.72 0.0375 0.0955 1.088 -0.18, 0.21

Rint number of parameters reflection/parameter ratio residuals: R1 (I > 2σI) residuals: wR2 (all reflections) goodness-of-fit (GOF) δFmin, δFmax (e Å-3)

a Parameters related to crystallographic analyses of HFAA are tabulated, including the linear absorption coefficient for Cu KR radiation (µ), the Bragg angle where maximum diffraction is recorded (θmax), and the total number of electrons per unit cell (F000). The unit cell has dimensions specified by lengths a, b, and c (parentheses denote one standard deviation uncertainties), where the angle between a and c is given by β (the other two angles being fixed at 90° by the monoclinic crystal system), with the resulting occupancy (Z) and volume (V) allowing for calculation of the crystal density (Dcal). The internal R value (Rint) gauges the agreement among symmetry-related reflections and affords a traditional indicator for the quality of X-ray data sets. The unweighted residual (R1) describes deviations between intensity-scaled diffraction measurements and predictions emerging from a crystallographic model, as determined by R1 ) ∑(|Fobs| - |Fcal|)/∑|Fobs|, where the sums are taken over all reflections, with Fobs and Fcal denoting observed and calculated structure factors, respectively. The corresponding weighted 2 residual (wR2) is defined in a similar fashion, wR2 )[∑w(Fobs 2 2 2 2 1/2 Fcal ) /∑w(Fobs ) ] , where the selected weighting scheme (0 e w e 1) imposes greater significance to reflections appearing at higher angles. The goodness-of-fit (GOF) metric describes how well a statistical model fits a set of observations, with the ideal result being equal to unity. The minimum (maximum) value of residual electron density found in the final electron density difference Fourier map is specified by δFmin (δFmax).

II. Experimental Methods A. Cryogenic X-ray Diffraction. Single crystal X-ray diffraction data were collected at a temperature of 93 K by executing a series of ω scans at different values of φ on a Rigaku R-Axis Spider diffractometer that utilized monochromated Cu KR radiation (λ ) 1.54817 Å). A summary of relevant experimental and crystallographic information has been compiled in Table 1. All intensities were corrected explicitly for Lorentz and polarization effects, with an empirical adjustment also being applied for X-ray absorption processes. To elucidate the nature of intramolecular hydrogen bonding in HFAA, total electron density and difference Fourier maps were constructed by means of the WinGX Program29 (ver. 1.80.05). The molecular framework was solved through use of direct methods30 and optimized by performing full-matrix least-squares regression on the observed structure factor (F2) as implemented in the

Chatterjee et al. SHELXL-97 software suite.31 While heavy atoms were refined anisotropically, the hydrogen atom bound to the β-carbon (H2) was adjusted in an isotropic fashion under the constraints imposed by the riding model.32,33 The crucial shuttling hydron (H1) also was treated isotropically; however, to locate its position as precisely as possible, the associated displacement parameters were allowed to vary during the refinement. Hexafluoroacetylacetone, which has a melting point of 177 K and therefore exists as a liquid under ambient conditions, was obtained from a commercial source (Matrix Scientific; 98%) and purified by vacuum distillation prior to use. Requisite single crystals of this substance were grown in situ by exploiting a variant of the zone-melting technique9,34 in which an electrically heated tungsten filament was translated slowly along the length of a sample-containing quartz capillary (0.1 mm diameter) that had been mounted on the diffractometer and cooled continuously by a cryogenic (93 K) stream of nitrogen vapor. X-ray data acquired for the initially frozen HFAA showed clear evidence of polycrystalline behavior; however, even a single pass of the heated filament produced a marked improvement in the diffraction pattern, with repeated application of this procedure ultimately enabling a monocrystalline phase to be isolated. B. Computational Techniques. The present theoretical treatments of hexafluoroacetylacetone utilized quantum chemical methods available in the GAUSSIAN35 and ACES II36 software packages, with a small subset of analyses being performed through use of the parallel successor to the latter known as CFOUR.37 Geometry optimization procedures were built upon DFT (B3LYP) and HF-referenced [MPn, CCSD, and CCSD(T)] schemes that exploited extensive correlation-consistent basis sets, including cc-pVDZ, cc-pVTZ, and their augmented counterparts (viz., aug-cc-pVDZ and aug-cc-pVTZ). A computationally expedient admixture of these bases, denoted as {aug}-cc-pVDZ, also was defined by placing diffuse functions on all atomic centers except those constituting the fluoromethyl groups. Tight convergence criteria yielding final root-meansquare (rms) forces of 3000 cm-1), analogous quantities emerging from DFT and MP2 calculations are deemed to be much too low (typically 70%) by inclusion of electron correlation at the MP2 and DFT levels of theory; however, the resulting reaction impediments are deemed to be much too small, reflecting the known tendencies of MP2 and DFT schemes to overcompensate for such interactions. Similar trends have been observed in the case of tropolone,28 where direct comparison with refined spectroscopic data has demonstrated that calculations based upon the coupled cluster paradigm afford a more robust description of intramolecular proton-transfer systems. All of these findings illustrate the sensitivity of potential surface topography to electron correlation effects and emphasize the need to exploit advanced quantum chemical methods for reliable elucidation of structure and dynamics in HFAA. Acknowledgment. This work was performed under the auspices of a grant provided by the Experimental Physical Chemistry Program in the Directorate for Mathematical and Physical Sciences of the United States National Science Foundation (CHE-0809856). One of the authors (L.A.B.) acknowledges the generous support of the NSF for a Graduate Research Fellowship. Significant portions of this study were made possible by the Yale University Faculty of Arts and Sciences High Performance Computing facility and staff. Supporting Information Available: Tables S1, S2, and S3, which respectively contain interatomic distances, bond angles, and dihedral angles for enolic HFAA as optimized at various levels of quantum chemical theory and measured by means of cryogenic X-ray diffraction studies. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Perrin, C. L.; Nielson, J. B. Annu. ReV. Phys. Chem. 1997, 48, 511. (2) Emsley, J. The Composition, Structure and Hydrogen Bonding of the β-Diketones. In Structure and Bonding, Vol. 57: Complex Chemistry; Clarke, M. J., Goodenough, J. B., Ibers, J. A., Jørgensen, C. K., Mingos, D. M. P., Neilands, J. B., Reinen, D., Sadler, P. J., Weiss, R., Williams, R. J. P., Eds.; Springer: Berlin, Germany, 1984; p 147.

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