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Keto−Enol Tautomerism and Conformational Landscape of 1,3Cyclohexanedione from Its Free Jet Millimeter-Wave Absorption Spectrum Camilla Calabrese,† Assimo Maris,† Luca Evangelisti,†,∥ Laura B. Favero,§ Sonia Melandri,*,† and Walther Caminati† †

Dipartimento di Chimica “G. Ciamician”, Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy Consiglio Nazionale delle Ricerche, Istituto per lo Studio dei Materiali Nanostrutturati (CNR-ISMN), Via P. Gobetti 101, 40129 Bologna, Italy

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S Supporting Information *

ABSTRACT: The free jet millimeter-wave absorption spectrum of 1,3cyclohexanedione has been investigated in the 59.6−74.4 GHz frequency range, and the rotational spectra of two conformational species, the chairdiketo and boat-diketo, and probably one excited vibrational state belonging to the chair-diketo form have been assigned. Quantum chemical calculations, performed at the B3LYP/6-311++G** and MP2/ 6-311++G** levels, were used to characterize the potential energy surface minima. The potential energy surface related to the interconversion of the observed diketonic species was modeled at the DFT level.

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INTRODUCTION β-Diketones are a family of compounds extensively investigated in chemistry, mainly in relation to the keto−enol equilibrium that can easily take place according to their chemical motif:

impossible (unless suitable substituents are introduced), and this could favor the keto forms with respect to the cases of linear β-diketones for isolated (gas phase) molecules. Let us consider the prototype cyclic β-diketone 1,3 cyclohexanedione (CHD). It has been found that in the solid state CHD exists in the keto−enolic form with monomer units connected by intermolecular hydrogen bonds.7 However, for systems in which the CHD molecules undergo weaker interactions, such as in dilute chloroform solutions, proton NMR spectra are in accord with an equilibrium mixture of enol monomer, enol dimers, and keto monomer.8 Similarly, a FTIR spectroscopic study of CHD in chloroform is consistent with the molecules in the keto form in dilute solution undergoing keto/enol dimer interconversion in more concentrated solutions.9 The IR spectrum of CHD deposited in an Ar cryomatrix shows the presence of only the diketo form, while tautomerization seems to occur in a solid film during the annealing experiment.10 As to the gas phase, only a gas phase electron diffraction study has been performed on CHD. In that work the authors observe a conformational mixture of chair and boat conformation (60/40%) in favor of the chair form and

R−C(O)−CH 2−C(O)−R′ ↔ R−C(OH)CH−C(O)−R′

The enolic form is predominating in the simplest linear diketones, such as malonaldehyde (MA) and acetylacetone (AA). For these molecules plenty of publications are available in the literature, often with contrasting results (see refs 1−5 and the literature cited within). We outline that microwave (MW) investigations supplied precise information on the concentration of species involved in the keto−enol equilibrium and on the internal dynamics of the interconversion of both MA and AA. MA in the gas phase exists nearly 100% as the enolic form, with a double minimum barrier to proton transfer and a tunneling frequency of about 21 cm−1.1−4 In the gas phase also AA is predominantly in the enolic form (either symmetric or with a low barrier to proton transfer) and presents a very low barrier to internal rotation of the two methyl groups.5 As outlined in a recent DFT study on the keto−enol tautomerism in linear and cyclic β-diketones,6 the literature about linear β-diketones is very rich but much more limited for cyclic β-diketones. In the cyclic family of β-diketones, intramolecular H-bonds stabilizing the enolic forms are © XXXX American Chemical Society

Special Issue: Terry A. Miller Festschrift Received: August 5, 2013 Revised: September 6, 2013

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T = 363 K and P = 200 kPa, using the rigid rotor and harmonic oscillator approximations. Anharmonic frequency calculations were performed at the B3LYP level for the assigned conformers. All calculations were performed using the Gaussian 03 suite of programs.14

claim that this is in agreement with the MP2/6-311G** zero point and entropy corrected relative energies.11 No microwave data are available on CHD. Since this technique can supply information on the structure, on conformational equilibrium, and on internal dynamics, we decided to investigate its rotational spectrum.

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RESULTS AND DISCUSSION Ab Initio Calculations. Molecular geometries of the five most stable conformers of both diketo and ketoenol tautomers of CHD were already available (see Figure 1).10,11

EXPERIMENTAL METHODS The Stark modulated free jet millimeter-wave absorption (FJMMWA) spectrometer used in this study has already been described previously,12,13 but it has recently been updated. Briefly, the apparatus consists of the radiation source, a variable attenuator, a horn−dielectric lens system (Montech-Clayton, Australia), which focuses the radiation in the central part of the chamber with a waist of about 2 cm, and a Schottky diode detector. We use a Millitech DXW10 for frequencies above 60 GHz or a Millitech 4731 4H-1111 to work below 60 GHz. To generate the millimeter-wave radiation, we start from a radiofrequency source in the 2−18.6 GHz region (HP 8672A) which is fed to an amplifier multiplier chain (Virginia diode AMC WR12 quadrupler). The nominal input frequency range of the multiplier is 15−22.5 GHz (10 dBm typical input power), while the nominal output frequency is 60−90 GHz (13 dBm typical output power). The MW radiation and the free jet are arranged perpendicularly, and a high voltage Stark modulation (electric field up to 750 V cm−1 at a frequency of 33 kHz) is applied to the molecular sample. After detection, the modulated signal is fed to a lock-in amplifier. The spectrometer is computer controlled via a National Instrument GPIB board. The GPIB controller communicates with the HP synthesizer in order to guide the frequency scan step by step and with the lock-in amplifier through which the signal provided by the detector is recovered, amplified, and then collected according to the chosen gate time. The control software of the spectrometer has been developed on the LabView graphical programming platform. Globally the system works in the 52−74.4 GHz region, and the accuracy of the frequency measurements is better than 50 kHz. Two different conditions were used: the first experiment was run using a gas mixture of CHD (∼1.5%) in argon expanded from a pressure of 20 kPa to about 0.05 kPa, while a second one was run using a mixture of CHD (∼0.6%) in helium expanded from a pressure of 50 kPa to about 0.1 kPa. The mixtures were expanded through a nozzle with a diameter of 0.35 mm at 363 K. Previous experiments12,13 have shown that the average rotational temperature reached in the sampled volume of the free jet is about 10 K and the rotational distribution is Boltzmannian.

Figure 1. Five most stable geometries of CHD and their relative energies (kJ mol−1) B3LYP/MP2 with 6-311++G** basis set.

Reference 10 focuses on the tautomeric equilibrium and the stabilizing forces, and in that work four geometries (chair and boat diketo and cis- and trans-ketoenol) were optimized using density functional methods (B3LYP and M05-2X) and MP2 with the 6-311++G** basis set. In ref 11 only the diketo forms were taken into consideration and three geometries were found to be stable, namely, the chair, boat, and twist diketo forms that were optimized by B3LYP and MP2 with the 6-311G** basis set. The five geometries of CHD reported in these papers were reoptimized giving the rotational constants, dipole moment components, relative energies, relative zero point corrected energies and free energies. All these data are reported in Table 1. In Table 1, the general picture obtained from the theoretical calculations is different if we rely on B3LYP or MP2 results or if we compare purely electronic energies to free energies. Considering the B3LYP results, if we take into account the purely electronic energies, there are three stable species (chair and boat diketo and cis-ketoenol) below 8 kJ mol−1, while if we take into account the ΔG values at this level of theory, only the chair and boat diketo forms remain below 12 kJ mol−1 and the energy difference between chair and boat diketo is lowered from 5.23 (ΔE) to 1.40 kJ mol−1 (ΔG). According to the MP2 electronic energies, only the first two conformations remain at low energies, while the cis-ketoenol rises to ∼14 kJ mol−1, the twist-diketo remains around 16 kJ mol−1, and the transketoenol reaches 24 kJ mol−1. The MP2 relative free energies increase with respect to the purely electronic ones for all conformations but the diketo-boat (ΔE = 7.46 kJ mol−1 ΔG = 2.65 kJ mol−1). These variations are significant and would result in a very different population distribution of the conformers in the gas phase which we should be able to discern with the experimental observations. Besides the characterization of the energy minima, the possible pathways to interconversion were analyzed. For the

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THEORETICAL CALCULATIONS In order to have an overview of the energies of the conformers and the possible pathways and transition states for interconversion and also to estimate some useful spectroscopic parameters, we analyzed the possible minima and transition states with the B3LYP/6-311++G** method. All stationary points were characterized by normal mode vibrational frequency calculations in order to ascertain their nature. The minima found on the potential energy surface were then optimized with MP2 and the same basis set. Thermal corrections to the Gibbs free energies have been computed at B

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Table 1. DFT (B3LYP/6-311++G**) and Ab Initio (MP2/6-311++G**) Calculated Energies, Rotational Constants, Quartic Centrifugal Distortion Constants, and Electric Dipole Moment Components of the Five Most Stable Species of CHD

Absolute energy values B3LYP/MP2 −384.015965/−382.944178 hartree. bData already reported by B. Bandyopadhyay et al. in J. Phys. Chem. A 2012, 116, 3836−3845. cAbsolute energy values B3LYP/MP2 −383.885124/−382.811728 hartree. dValues relative to the experimental preexpansion conditions: T = 363 K, P0 = 500 kPa. Absolute energy values B3LYP/MP2 −383.927937/−382.854496 hartree. eThe values in parentheses represent the relative free energies calculated excluding the three lowest lying vibrational modes. a

following discussion the atom numbering we use is that reported in Scheme 1. For six-membered rings three puckering coordinates are needed for the search of the possible minima of the potential energy surface (for a general treatment of puckering coordinates see ref 15). In our case some bond distances, bond angles, and dihedral angle were maintained fixed and equal, namely, C4X0 = C6X0, C4X0C2 = C6X0C2 = C5X0C2 =

90°, and C4X0C2C6 = 180° (X0 is the midpoint between C4 and C6, and X0, C2, C4, C6 form a reference plane). In this way the three “wing” dihedral angles (dih1 = C1C2C6X0, dih3 = C3C2C4X0, and dih5 = C5X0C6C2) define the position of C1, C3, and C5 with respect to the reference plane, and the values of three dihedral angles allow for the definition of all conformations (see Scheme 1b). C

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Scheme 1. Structure and Atom Numbering of CHDa

a

eventually allow relaxation of the population of the boat conformation over the chair in an Ar expansion. The pseudo-rotation-like motion allows for the interconversion among boat and twist configurations generated in the three-dimensional space described by dih1, dih3, dih5. Figure 2 depicts all possible configurations on a two-dimensional section (dih1, dih3) of the potential energy surface where the calculated stationary points are plotted with black circles and the values of the energies and of the third coordinate (dih5) are indicated for each point. Among the possible configurations the boat Cs and the twist C2 forms are minima while the boat C1 form is a high energy transition state. The twist C1 form has all the harmonic vibrational frequencies positive, but a free optimization of the structure converges on the nearest boat Cs form. The values of the calculated dih1 and dih3 dihedral angles may be reproduced (white squares) as functions of the pseudorotation-like coordinate φ in the following way:

X0 is the midpoint between C4 and C6.

The interconversion between the boat and chair minima is possible through a ring puckering motion that involves the motion of the two “wing” dihedral angles dih1 and dih3 through a half-chair transition state. The energies and geometries of the optimized stationary points are reported in Table 2. Note also that there are two series of equivalent Table 2. Calculated Energies and Selected Geometrical Parameters of Boat and Chair Diketo CHD

dih1 = 0.90 cos φ + 0.38 sin φ

(1)

dih3 = 0.90 cos φ − 0.38 sin φ

(2)

Rotational Spectrum. Theoretical calculations suggest the two most stable conformations of the diketo form (chair and boat) to be enough populated to allow detection in the jet plume (especially considering their sizable μb and μc dipole moment components), while the twist-diketo and ketoenol forms are probably too high in energy and their spectra too weak to be detected. Moreover, the low barrier to interconversion of the boat to the chair form suggests that the search for the higher energy conformer could necessitate the use of He as the carrier gas to prevent relaxation. The jet-cooled rotational spectrum, recorded using Ar as the carrier gas, showed the presence of a few high J (10−13), high Ka (10 and 11) lines resulting from the coalescing of four transitions. These lines consist of two μc R-type and two μb Rtype overlapped rotational transitions. It was then possible to measure transitions with lower Ka (J from 13 to 23 and Ka from 9 to 1), resolved in quartets made of two μc- and two μb-R-type lines. The μb-components were stronger than the respective μc ones, resulting in an intensity ratio of ∼2, which is in agreement with the calculated μb2 over μc2 ratio of 2.6 (see Table 1). Figures 3 and 4 illustrate this behavior. The spectra reported in the figures are those recorded in He for reasons that will become clear after the following paragraph; nevertheless the line pattern is maintained independently of the carrier gas used. The transition frequencies, given as Supporting Information, were fitted using Watson’s semirigid Hamiltonian in the “S” reduction and Ir representation,16 obtaining the spectroscopic parameters reported in Table 3, species I. Subsequently a spectrum using He as the carrier gas was recorded where two other series of lines which were analyzed similarly to the previous one were observed. The spectroscopic parameters related to these spectra are also reported in Table 3 (species II and III). The two new series of lines (II, III) were much weaker than the first one (∼10% for species II and 5% for species III), so fewer lines were measured (see, for example, Figure 4 where two transitions belonging to different species are reported). Also, the comparison of the intensities between μb and μc component lines belonging to species II is in favor of a higher value of the μb dipole moment component, while for species III it was not possible to measure separate μb and μc component

configurations (lines 1 and 2 of the same table), but interconversion between equivalent forms through the ring puckering motion involves a high energy planar form that is not a transition state, since the normal mode calculation gave three negative frequencies. These results show that the barrier between the boat and chair conformations (8.4 kJ mol−1 in Table 2) is low enough to D

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Figure 2. Calculated energies and selected geometrical parameters of boat and twist diketo CHD.

Figure 3. Rotational transitions for both conformers chair and boat of CHD (experimental in black, simulation after the fit in red and blue, respectively).

lines. Moreover, only one centrifugal distortion constant (DK) was determined for species III, and it is very similar to that of the chair-diketo form. On the bases of the relative intensities of the rotational transitions, the ratio of the dipole moment components, and the fact that the two new series of lines were observed only in the spectrum with He, we can assign the first spectrum to the global minimum indicated by the calculations, the chair-diketo form, while the other two series must belong to a higher energy conformation or to a vibrational excited state.

The identification of the species generating the two new series of lines is more difficult. From the rotational constants we can assert that they must belong to the diketo form, since the ketoenol tautomer has significantly different values. Moreover the energy barrier for tautomerization is very high (∼120 kJ mol−1),6 which excludes the possibility of relaxation of the higher energy tautomers to the lower one during the gas expansion, suggesting that species II and III, which were not observed in Ar but only in He, cannot originate from the ketoenol tautomer. Also the twist-diketo form should be E

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between the chair and boat diketo forms (see Table 1) while it is very different from the relative zero point corrected energies or the relative Gibbs free energies calculated at the preexpansion conditions also reported in Table 1. The vibrational analysis shows that the greatest contribution to the thermal corrections to the Gibbs free energies comes from the calculated entropy which in turn is influenced to a great extent by the low lying vibrational states See Table 1 where we report the Gibbs free energies calculated excluding these vibrations and Tables S2 and S3 of the Supporting Information for the list of vibrational wavenumbers and vibrational−rotational interaction constants. These large amplitude vibrations are the most likely to be affected by anharmonicity. Anharmonic calculations at the B3LYP level show in fact that the zero point corrected relative energy between chair and boat forms is higher than the harmonic one: ΔE0,harm= 3.69 kJ mol−1 and ΔE0,anharm = 4.19 kJ mol−1, the latter value being closer to the experimental estimated energy difference of 7 kJ mol−1. The conformational composition of CHD, with the presence of the boat form, is different from that of cyclohexane where the chair form is significantly more stable.19 This effect is due to the replacement of the CH2 groups in cyclohexane with the two CO groups (C1O and C3O) in CHD. While in the boat form of cyclohexane the C−H bonds in C1 and C6 and C3 and C4 are eclipsed, in CHD the C−H bonds in C6 and C4 point each toward one oxygen atom with which it can positively interact in both the chair and boat conformations.

Figure 4. Experimental rotational line 11(11)−10(11) (only Ka is given) for both conformers chair (67845.10 MHz) and boat (67399.45 MHz) of CHD. The lines result from the coalescing of four transitions 11(11,1)−10(10,0), 11(11,0)−10(10,1), 11(11,0)−10(10,0), 11(11,1)−10(10,1) due to near prolate degeneracy.

Table 3. Experimental Spectroscopic Parameters (SReduction, IR Representation) and Derived Planar Moments of Inertia of CHD I, chair-diketo A / MHz B / MHz C / MHz DJ / kHz DJK / kHz DK / kHz d1 / kHz d2 / kHz σb / MHz Nc Ni / NI Paa / uÅ2 Pbb / uÅ2 Pcc / uÅ2

3153.407(4) 1896.562(3) 1304.058(4) 0.328(5) −0.48(2) 1.24(2) −0.063(1) −0.0162(4) 0.07 60 1 246.87 140.67 19.60

a

II, boat (diketo)

III, chair-diketo, v = 1

3133.15(2) 1883.14(4) 1276.76(5) 0.53(7) −0.8(1) 1.7(1)

3128.500(8) 1885.56(66) 1293.07(64)

0.07 16 0.1 251.46 144.38 16.92

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1.23(4)

CONCLUSIONS The rotational spectra of two conformers of CHD and one excited vibrational state have been assigned in the free jet broadband millimeter-wave spectrum. From comparison of the precise sets of spectroscopic data, including centrifugal distortion constants to the calculated rotational constants, dipole moment component values, and variation of planar moments of inertia, we could determine that the most intense spectrum belongs to the global minimum which is the chairdiketo form. Other observed rotational lines belong to the boatdiketo form and plausibly to one excited vibrational state originating from the global minimum. The higher energy conformer, namely, the diketo-twist form and two stable ketoenol forms, could not be observed in agreement with their calculated lower population. From the strong interplay of experiment and theoretical calculations we reached a complete picture of the conformational landscape of 1,3-cyclohexanedione and showed that in the isolated molecule the lowest energy forms are stabilized by weak C−H···O favorable interactions which are globally more stabilizing than the O−H···O interaction occurring in the ketoenol forms.

0.07 7 0.05 248.66 142.18 19.36

a

Error expressed in units of the last decimal digit. bStandard deviation of the fit. cNumber of transitions of the fit.

excluded because of the calculated very high relative energy. These facts together with the inspection of the values of the planar moments of inertia reported in Table 3, where we can see that the values for species III lie between those of species II and I, suggest that species II is probably the boat-diketo form while species III is probably a vibrational satellite originating from the global minimum. This assignment is also in agreement with the observed intensity of the lines for species II (see consideration on the energies stated below) and with the value of the distortion constant DK for species III, which is very similar to that of the chair-diketo form. The observation of excited vibrational states in supersonic expansions as already been reported as cited in ref 17. The rotational spectra give information also on the relative energies of the observed species (see, for example, ref 18). According to the intensity ratio of the transition lines normalized by the calculated dipole moment components, the population ratio of the boat species to the chair one in the supersonic expansion is about 0.1. If we assume no conformational cooling effect, the energy difference between them can be estimated to be 7 kJ mol−1. Although the error on the estimated energies can be estimates to be up to 30%, this value is very close to the calculated MP2 electronic energy difference

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ASSOCIATED CONTENT

S Supporting Information *

Complete list of experimental transition frequencies for the three observed species of CHD (Table S1), theoretical (B3LYP/6-311++G**) vibrational−rotational interaction coefficients and vibrational wavenumbers of the chair (Table S2) and boat (Table S3) conformers of CHD, and complete reference 14. This material is available free of charge via the Internet at http://pubs.acs.org. F

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(13) Melandri, S.; Maccaferri, G.; Maris, A.; Millemaggi, A.; Caminati, W.; Favero, P. G. Observation of the Rotational Spectra of van der Waals Complexes by Free Jet Absorption Millimeter Wave Spectroscopy: Pyridine-Argon. Chem. Phys. Lett. 1996, 261, 267−271. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; et al. Gaussian 03, revision B.01; Gaussian, Inc.: Pittsburgh, PA, 2003. (15) Cremer, D.; Pople, J. A. A General Definition of Ring Puckering Coordinates. J. Am. Chem. Soc. 1975, 97 (6), 1354−1358. (16) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1977; Vol. 6, p 1. (17) Melandri, S.; Maris, A.; Favero, P. G.; Favero, L. B.; Caminati, W.; Meyer, R. Millimeter-Wave Absorption Free Jet Spectrum, Barriers to Internal Rotation, and Torsional Relaxation in pAnisaldehyde. J. Mol. Spectrosc. 1997, 185, 374−383. (18) Melandri, S.; Evangelisti, L.; Maris, A.; Caminati, W.; Giuliano, B. M.; Feyer, V.; Prince, K. C.; Coreno, M. Rotational and Core Level Spectroscopies as Complementary Techniques in Tautomeric/ Conformational Studies: The Case of 2-Mercaptopyridine. J. Am. Chem. Soc. 2010, 132, 10269−10271. (19) Durig, J. R.; Zheng, C.; El Defrawy, A. M.; Ward, R. M.; Gounev, T. K.; Ravindranath, K.; Rajeswara Raob, N. On the Relative Intensities of the Raman Active Fundamentals, r0 Structural Parameters, and Pathway of Chair−Boat Interconversion of Cyclohexane and Cyclohexane-d12. J. Raman Spectrosc. 2009, 40, 197−204.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +39 051 2099502. Fax: +390512099456. Present Address ∥

L.E.: Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, VA 22904-4319, U.S.

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the Italian MIUR (PRIN Grant 2010ERFKXL_001) and the University of Bologna (RFO) for financial support. L.E. was supported by a Marie Curie International Outgoing Fellowship within the 7th European Community Framework Programme (FP7-PEOPLE-2012-IOF) under Grant PIOF-GA2012-328405.

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REFERENCES

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