Morphing the Internal Dynamics of Acetylacetone ... - ACS Publications

The rotational spectrum of trifluoroacetylacetone shows that the molecule exists in an enolic Cs form and displays the features of internal rotations ...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/JPCA

Morphing the Internal Dynamics of Acetylacetone by CH3 → CF3 Substitutions. The Rotational Spectrum of Trifluoroacetylacetone Laura B. Favero,† Luca Evangelisti,‡ Biagio Velino,‡ and Walther Caminati*,‡ †

Istituto per lo Studio dei Materiali Nanostrutturati (ISMN), Sezione di Bologna CNR, via Gobetti 101, I-40129 Bologna, Italy Dipartimento di Chimica “G. Ciamician” dell’Università di Bologna, Via Selmi 2, I-40126 Bologna, Italy

‡

S Supporting Information *

ABSTRACT: The rotational spectrum of trifluoroacetylacetone shows that the molecule exists in an enolic Cs form and displays the features of internal rotations of the CH3 and CF3 groups, whose barriers to internal rotation were determined to be V3 = 379 and 30.8 cm−1, respectively. Its internal dynamics appears to be intermediate between those of acetylacetone, where proton tunneling and lowbarrier internal rotation of the two methyl groups make the spectrum quite complex, and hexafluoroacetylacetone, a perfectly “rigid” molecule on the time scale of microwave spectroscopy.

■

INTRODUCTION Acetylacetone (AA) can be considered a prototype molecule for studying keto−enol tautomerization: The titles of 654 scientific articles from the time period 1985−2013 contain the word “acetylacetone”. A variety of both experimental1−9 and theoretical11−16 methods have been applied to determine the shape of AA in the gas phase. They lead to contrasting results as to the relative stability of the ketonic and enolic forms, as well as the symmetry (Cs or C2v) of the enolic tautomer. The internal dynamics of AA is quite complex, and its rotational spectrum has hitherto resisted assignment until a few years ago owing, in the first instance, to the very low barrier to internal rotation of the two methyl groups.10 Further difficulties are related to the estimation of the barrier to proton transfer. The rotational spectrum has been assigned only for the AA substate of the two internal rotations. The defect of inertia is much smaller than that expected for the out-of-plane hydrogens of two methyl groups, setting very low V3 values for the CH3-group internal rotations. The statistical weights of the rotational transitions and the equivalence between two pairs of carbons suggest a C 2v configuration or a low barrier to proton transfer. In hexafluoroacetylacetone (HFAA), the two methyl groups of AA are replaced by two CF3 groups. This double substitution makes the molecule perfectly rigid17 within the resolving power of our microwave (MW) spectrometer, which is described below. What will happen when only one methyl group is replaced in AA with a CF3 group? The obtained molecule is 1,1,1-trifluoro2,4-pentanedione (1,1,1-trifluoroacetylacetone, TFAA). Its symmetry is changed with respect to both AA and HFAA, as shown in Chart 1, in the sense that proton transfer (combined with skeletal relaxation) generates two unequal conformers rather than two equivalent minima. © 2014 American Chemical Society

A few investigations are available on this molecular system. A symmetrical chelated ring with a linear hydrogen bond was reported on the basis of the electron diffraction method.18 Furthermore, semiempirical and ab initio studies were performed on TFAA with theoretical methods (INDO and B3LYP) that cannot produce an accurate description of the system.19−21 Recently, the mechanism of UV-induced conformational changes among the enol types of TFAA was studied.22 Finally, various DFT calculations and experimental FTIR and FT-Raman methods were used to perform a comprehensive vibrational analysis of this molecule.23 A recent theoretical study was focused on the competition between the intramolecular hydrogen bond and π-electron delocalization.24 None of the articles cited above could provide information on the internal dynamics of TFAA. Because TFAA appears intermediate between the extremely floppy AA molecule and the extremely rigid HFAA one, it looks promising to obtain information on the two-dimensional potential energy surface due to the internal rotations of both the CH3 and CF3 rotating tops. For this reason, we decided to investigate the rotational spectrum of TFAA. We present here the assignment of the pure rotational spectrum of the lowest-energy enolic species E1 of TFAA (see Figure 1 for atom numbering) and all of its singly substituted 13C and D isotopologues. All of these spectra present the features of internal rotations of both CH3 and CF3 groups. Their analysis, individually and in combination, provides a wealth of evidence supporting Cs symmetry for the shape of the enolic tautomer. In Received: January 17, 2014 Revised: May 14, 2014 Published: May 21, 2014 4243

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248

The Journal of Physical Chemistry A

Article

Chart 1. Tautomeric Forms of TFAA

Table 1. MP2/6-311++G(d,p) Values of the Spectroscopic Constants and of the Relative Energies of the Three Tautomeric/Conformational Species of TFAA

Figure 1. Cs canonical enolic form of TFAA, including atom numbering used throughout the text.

addition, the structural parameters of the carbon frame are reported with high precision.

■

EXPERIMENTAL SECTION A commercial sample of TFAA (98%, Aldrich) was used without further purification. The spectra of the 13C isotopologues were measured in natural abundance. The mono- (namely, OD and C3D) and di- (OD−C3D) deuterated isotopologues were generated by passing D2O (99%, Promochem GmbH) over the sample. The rotational spectrum in the 6−18 GHz frequency region was measured using a coaxially oriented beam-resonator arrangement (COBRA) type25 pulsed-supersonic-jet Fouriertransform microwave (FTMW) spectrometer,26 already described elsewhere and recently updated with the FTMW++ set of programs.27 Helium, as the carrier gas, was passed over TFAA at room temperature, at a backing pressure of about 0.2 MPa, and expanded through a pulsed valve (General Valve, series 9, 0.5-mm nozzle diameter) into the Fabry−Perot cavity to about 1 × 10−3 Pa. Helium was used to reduce the conformational relaxation effects and the formation of complexes of TFAA with rare gases. The spectral line position was determined after Fourier transformation of the 8k-data-point timedomain signal, recorded at intervals of 100 ns. Each rotational transition was split by the Doppler effect, enhanced by the coaxial arrangement of the supersonic jet and resonator axes in the COBRA-FTMW spectrometer. The rest frequency was calculated as the arithmetic mean of the Doppler components. The estimated accuracy of frequency measurements was better than 3 kHz, and lines separated by more than 7 kHz were resolvable.

a

Absolute energy = −642.185254Eh.

MP2 calculations, which provide slightly distorted structures with respect to certain expected symmetric species (see, for example, the case of benzene29). In this case, the E1 conformer is slightly distorted with respect to the plausible plane of symmetry (none of the fluorine atoms of the CF3 top lie exactly on the symmetry plane) but with a very low barrier with respect to the symmetric species. For both enolic forms, we also calculated the barriers to the internal rotations of the CH3 and CF3 groups.

■

ROTATIONAL SPECTRA AND ANALYSES OF INTERNAL ROTATIONS We started the spectral search for E1, the predicted most stable species, using the theoretical values of the rotational constants in Table 1 to predict its spectrum. We first scanned the frequency region where the J = 6 ← 5 μa band was predicted.

■

THEORETICAL CALCULATIONS MP2/6-311++G(d,p) ab initio calculations were performed28 to calculate the chemical parameters of interest and to obtain information on the relative energies and geometries of the plausible species of TFAA. The shapes and spectroscopic parameters of the two most stable enolic species and one ketonic species are reported in Table 1. The natures of all stationary points were verified by subsequent analytical secondderivative calculations. The enolic forms were found to be considerably more stable than the keto tautomer, according to their intramolecular hydrogen bonds. The E1 enolic form was found to be slightly distorted with respect to the imposed Cs symmetry, as often happens with

Figure 2. Rotational spectrum of TFAA, showing that each rotational transition is split into five component lines, due to the internal rotations of the CF3 and CH3 groups. See text for labeling of the component lines. 4244

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248

The Journal of Physical Chemistry A

Article

Table 2. Spectroscopic Parameters of the Parent and C3D, O−D, and OD−C3D Deuterated Species of TFAA ERHAM parent A (MHz) B (MHz) C (MHz) DJ (kHz) DJK (kHz) DK (kHz) d1 (Hz) d2 (Hz) V3CF3 (kJ·mol−1) V3CH3 (kJ·mol−1) sCF3 sCH3 ρF ρH βF (deg) βH (deg) ε01 (MHz) ε1−1 (MHz) ε10 (MHz) ε11 (MHz) B002,01 (kHz) B020,10 (kHz) B200,10 (kHz) B002,10 (kHz) B400,11 (Hz) ∠(a,i)CH3 (deg) ∠(a,i)CH3 (deg) IαCF3 (u Å2) IαCH3 (u Å2) Et(0,±1)f (MHz) Et(±1,0)f (MHz) Et(±1,∓1)f (MHz) Et(±1,±1)f (MHz) σg (kHz) Nh

2735.3426(5) 752.80905(5) 662.67242(4) 0.1183(2) 0.835(2) −0.57(3) −4.0(1) 50.76(7) 0.368c 4.54c 40.0c 31.7c 0.45973(3) 0.01386(1) −5.58(2) 12.77(4) −130.9(1) −0.1322(3) −2.1315(7) −0.1953(3) −0.106(5) 1.37(8) −0.46(3) 0.10(1) −0.33(3) 19.56(7) 39.47(9) 89.71(4) 3.236(5) 393.8(4) 7.377(3) 399.8(4) 399.8(4) 3.5 360

a

XIAM

C3D

OD

OD−C3D

OD

OD−C3D

2695.05(1) 752.2982(2) 660.0066(2) 0.143(2) 0.85(4) [−0.57]b [−4.0] 64.4(7) 0.310c 4.57c 34.4c 32.0c 0.45265d 0.01369d −5.706d 12.885d −125.7(3) −0.38(1) −5.27(1) −0.60(1) −0.20(3) − −1.12(6) 0.37(3) −1.6(5) 19.70d 39.33d [89.71]b [3.236]b 380.0(9) 18.77(5) 394.6(9) 394.0(9) 4.3 75

2695.077(1) 749.2043(1) 657.3251(1) 0.0907(7) 0.55(1) [−0.57] [−4.0] 36.9(3) 0.583c 4.64c 64.0c 32.5c 0.45392d 0.0136d −5.527d 13.051d −115.89(9) − −0.131(2) − −0.05(1) −5(1) − − − 19.19d 39.82d [89.71]b [3.236]b 347.7(3) 0.394(5) 348.1(3) 348.1(3) 2.1 85

2655.308(2) 748.7724(1) 654.6738(1) 0.102(1) 0.59(2) [−0.57] [−4.0] 43.3(4) 0.519c 4.81c 57.7c 33.7c 0.44697d 0.01343d −5.646d 13.176d −111.0(2) − −0.247(2) − − − − −0.04(1) − 19.32d 40.27d [89.71]b [3.236]b 333.0(5) 0.741(5) 333.7(5) 333.7(5) 2.9 85

2695.081(3) 749.754(4) 656.775(4) 0.0907(9) 0.56(2) − −5.8(12) 38.6(4) 0.569(4) 4.641(3) 63.3 32.4

2655.313(4) 749.370(3) 654.076(3) 0.103(1) 0.60(2) − −6.7(2) 45.3(5) 0.524(3) 4.673(4) 57.9 32.6

21.3(7) 39.3(2) 89.08e 3.253e

18.9(5) 39.7(2) 89.08e 3.253e

2.4 85

3.0 85

Errors in parentheses in units of last digits. bNumbers in brackets fixed to the corresponding parent-species values. cValues derived from ERHAMfitted parameters. dFixed to the values derived from the corresponding experimental parameters of the parent molecule, taking into account the different orientations of the internal rotor axes with respect to the principal axis system. eFixed to the experimental values of the moments of inertia relative to the CHF3 or CH3F symmetry axis (from refs 40 and 41, respectively). fTorsional energies (MHz) with respect to ground substate (0,0). g Standard deviation of frequency fit. hNumber of assigned frequencies included in the fit. a

The two tops, despite their considerable inertial differences, were found to produce similar tunnelling splittings, indicating, already at first sight, that V3(CF3) is much lower than V3(CH3). The rotational spectra of molecules with two CH3 rotors have been analyzed with different model Hamiltonians,30−33 and comparisons among the various methods are available.34−38 Although the values of the V3 barriers to the simultaneous internal rotations of a CH3 group and a CF3 group in an asymmetric rotor have already been reported, for the CH3F··· CHF3 molecular complex,39 this is the first time that the V3 barriers of a molecular system containing these two groups have been determined from the splittings derived from the combination of the two motions. We tried first to fit the 360 component lines belonging to 67 rotational transitions of TFAA using the XIAM code (based on the combined axis method, CAM31), but the standard deviation of the fit was too large (∼150 kHz) compared to our experimental uncertainty (∼2 kHz). Similar unsatisfactory results were

After the observation of these lines, many more transitions, with values of J up to 12 and of K−1 up to 3, were assigned and measured, including several μb type lines. All transitions appeared as quintuplets, a feature that is typical for molecules containing two different internal rotors with C3v symmetry. The five component lines are labeled according to the values assumed by the quantum numbers σCF3 and σCF3 that define the symmetry species of the torsional sublevels (the rotational transitions occur between sublevels of the same symmetry). Their allowed values are 0, 1, and 2 (or −1),30 so that the label 00 corresponds to the nondegenerate A1 substate, whereas labels 01, 10, 11, and 12 correspond to the doubly degenerate sublevels (0, ±1), (±1, 0), (±1, ±1), and (±1, ∓1), respectively, whose symmetry can be classified as E1, E2, E4, and E3, respectively, of the G18 PI group. Such a pattern is shown in Figure 2 for the 82,6 ← 72,5 transition. Subsequently, we assigned the rotational spectra of all mono-13C species in natural abundance and of the three deuterated species mentioned in the Experimental Section. 4245

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248

The Journal of Physical Chemistry A

Article

Table 3. ERHAM Spectroscopic Parameters of the 13C species of TFAAa,b 13

13

C1

A (MHz) B (MHz) C (MHz) DJ (kHz) ε01 (MHz) ε1−1 (MHz) ε10 (MHz) ε11 (MHz) Et(0,±1)d (MHz) Et(±1,0)d (MHz) Et(±1,∓1)d (MHz) Et(±1,±1)d (MHz) σe (kHz) Nf

13

C2

c

2735.19(3) 750.1156(2) 660.5672(3) 0.118(3) −129(3) −0.15(1) −2.18(1) −0.18(1) 388(8) 7.52(6) 394(8) 394(8) 3.3 35

2733.23(2) 752.844(1) 662.5728(5) 0.114(2) −140(4) −0.13(1) −2.10(1) −0.15(1) 420(13) 7.15(5) 425(13) 425(13) 3.3 40

13

C3

2729.93(2) 751.8784(2) 661.6369(2) 0.119(2) −130(2) −0.14(1) −2.13(1) −0.15(1) 391(8) 7.27(5) 397(8) 397(8) 2.8 33

13

C4

2734.78(3) 747.2923(2) 658.3639(3) 0.115(3) −130(3) −0.12(1) −2.14(1) −0.16(1) 392(8) 7.29(6) 398(8) 398(8) 3.3 34

C5

2717.37(4) 740.1122(3) 651.7698(3) 0.145(3) −133(5) −0.11(2) −2.00(2) −0.13(2) 399(15) 6.70(7) 404(15) 404(15) 4.3 32

a Undetermined parameters were fixed to the corresponding values for the parent species. bV3 barriers are, within the experimental uncertainty, the same as for the parent species. cErrors in parentheses in units of last digits. dTorsional energies (MHz) with respect to ground substate (0,0). e Standard deviation of frequency fit. fNumber of assigned frequencies included in the fit.

Table 4. Experimental (rs) and MP2/6-311++G(d,p) (re, for E1 and E2) Coordinates of the Substituted Atoms of TFAAa,b a (Å)

b (Å) re

rs C1 C2 C3 C4 C5 H14 H15 a

±1.557(1) id ±0.912(2) ±2.2293(7) ±3.3961(5) ±0.620(3) ±1.8217(8) c

re

E1

E2

rs

E1

E2

1.566 0.122 −0.919 −2.234 −3.392 −0.665 −1.748

1.547 0.096 −0.937 −2.308 −3.430 −0.735 −1.016

±0.12(1) ±0.381(4) ±0.605(3) ±0.193(8) ±1.124(2) ±1.6414(9) ±1.7003(9)

−0.143 0.392 −0.596 −0.196 −1.139 −1.648 1.605

0.170 −0.261 0.629 0.123 1.127 1.691 −1.731

See Figure 1. bCoordinate c set to zero by symmetry. cErrors in parentheses are in units of the last digits. dImaginary value.

obtained for the fittings of all other isotopologues, except for the OD monodeuterated and OD−C3D dideuterated species. In only these two cases was the standard deviation of the XIAM fit satisfactory (2.4 kHz). In contrast, the ERHAM code (based on an effective rotational Hamiltonian30) led to satisfactory fits for all isotopologues. For both programs, we chose Watson’s S reduction and the Ir representation.42 The obtained results are reported in Table 2 for the parent and C3D, OD, and OD−C3D deuterated species. In Table 3 (synthesized form) and Table S4 (extended form) (Supporting Information) are the values of the parameters of the 13C species, for which only a few transitions have been measured. The meanings of all parameters are described in refs 30 and 31. As stated above, ERHAM, unlike XIAM, does not directly provide the V3 barriers. However, for each internal top, through the relation w1 = −(Δ0/F)(8/27), we calculated the values of the perturbation coefficients w1, and from them, we estimated the s reduced barriers.43 Δ0 is EE − EA, the difference in energy between the E and A sublevels for each rotor. This was calculated in the approximation for which each top was considered as a single top, but using the F numbers calculated for the two-top case. The same equation was applied replacing Δ0 with ΔE01,00 [Et(0,±1) − Et(0,0)] or ΔE10,00 [Et(±1,0) − Et(0,0)] and F with FH or FF, respectively, where FF = 1/2(h/2π)2IrαH/[IrαFIrαH − (ΣgIgρgFρgH)2] and FH = 1/2(h/2π)2IrαF/[IrαFIrαH − (ΣgIgρgFρgH)2] and IrαF and IrαH are the reduced moments of inertia of the tops referred to their symmetry axes. All of these parameter were

calculated by the ERHAM program. From s, we then obtained V3 from the relation V3 = 9/4Fs. The V3 values are collected in Table 2. For the OD and OD−C3D isotopologues, by comparing the V3 values provided directly by the XIAM fittings to those estimated from the ERHAM fittings as described above (see rows 9 and 10 of Table 2), one can note that the results are in fairly good agreement. As a consequence, one can assume that the V3 parameters estimated for all of the other isotopologues by ERHAM are also reliable values. The two-dimensional potential energy surface for the two internal rotations for the parent species is graphically shown in Figure 3. The values of the two V3 barriers are very close to the ab initio values obtained for conformer E1, strongly supporting the assignment of the observed spectrum to this species.

■

STRUCTURAL INFORMATION We used the rotational constants of the parent species and of the 13C- and D-substituted isotopologues (ERHAM data) to determine the substitution coordinates44 of the substituted atoms. The obtained values are compared in Table 4 to the corresponding ab initio data. One can note that rs and re coordinates are generally in better agreement for conformer E1. However, for the hydroxyl hydrogen, the differences between the rs and re values are about 0.1 Å. These discrepancies are plausibly related to the Ubbelohde effect,45 a vibrational effect that induces a change in the H-bond lengths upon H → D substitution.46,47 4246

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248

The Journal of Physical Chemistry A

Article

complete set of 13C isotopologue parameters. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: +39-051-2099456. Phone: +39-051-2099480. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank the Italian MIUR (PRIN Project 2010ERFKXL_001) and the University of Bologna (RFO) for financial support.

■

Figure 3. Two-dimensional potential energy surface for the internal rotations of the CF3 and CH3 groups of the parent species of TFAA.

(1) Lowrey, A. H.; George, C.; D’Antonio, P.; Karle, J. Structure of Acetylacetone by Electron Diffraction. J. Am. Chem. Soc. 1971, 93, 6399−6403. (2) Andreassen, A. L.; Bauer, S. H. The Structures of Acetylacetone, Trifluoroacetylacetone and Trifluoroacetone. J. Mol. Struct. 1972, 12, 381−403. (3) Iijima, K.; Ohnogi, A.; Shibata, S. The Molecular Structure of Acetylacetone as Studied by Gas-Phase Electron Diffraction. J. Mol. Struct. 1987, 156, 111−118. (4) Harris, R. K.; Rao, R. C. Gas-Phase NMR Studies of Chemical Equilibria: 1Methodology. Org. Magn. Reson. 1983, 21, 580−586. (5) Folkendt, M. M.; Weiss-Lopez, B. E.; Chauvel, J. P., Jr.; True, N. S. Gas-Phase Proton NMR Studies of Keto−Enol Tautomerism of Acetylacetone, Methyl Acetoacetate, and Ethyl Acetoacetate. J. Phys. Chem. 1985, 89, 3347−3352. (6) Hush, N. S.; Livett, M. K.; Peel, J. B.; Willett, G. D. VariableTemperature Ultraviolet Photoelectron Spectroscopy of the Keto− Enol Tautomers of Pentane-2,4-Dione. Aust. J. Chem. 1987, 40, 599− 609. (7) Tayyari, S. F.; Zeegers-Huyskens, Th.; Wood, J. L. Spectroscopic Study of Hydrogen Bonding in the Enol Form of β-DiketonesII. Symmetry of the Hydrogen Bond. Spectrochim. Acta 1979, 35A, 1289− 1295. (8) Srinivasa, R.; Feenstra, J. S.; Park, S. T.; Xu, S.; Zewail, A. H. Direct Determination of Hydrogen-Bonded Structures in Resonant and Tautomeric Reactions Using Ultrafast Electron Diffraction. J. Am. Chem. Soc. 2004, 126, 2266−2267. (9) Caminati, W.; Grabow, J.-U. The C2v Structure of Enolic Acetylacetone. J. Am. Chem. Soc. 2006, 128, 854−857 and references therein. (10) Dannenberg, J. J.; Rios, R. Theoretical Study of the Enolic Forms of Acetylacetone. How Strong Is the Hydrogen Bond? J. Phys. Chem. 1994, 98, 6714−6718. (11) Ishida, T.; Hirata, F.; Kato, S. Thermodynamic Analysis of the Solvent Effect on Tautomerization of Acetylacetone: An Ab Initio Approach. J. Chem. Phys. 1999, 110, 3938−3945. (12) Bauer, S. H.; Wilcox, C. F. On Malonaldehyde and Acetylacetone: Are Theory and Experiment Compatible? Chem. Phys. Lett. 1997, 279, 122−128. (13) Sharafeddin, O. A.; Hinsen, K.; Carrington, T., Jr.; Roux, B. Mixing Quantum-Classical Molecular Dynamics Methods Applied to Intramolecular Proton Transfer in Acetylacetone. J. Comput. Chem. 1997, 18, 1760−1772. (14) Mavri, J.; Grdadolnik, J. Proton Potential in Acetylacetone. J. Phys. Chem. A 2001, 105, 2039−2044. (15) Grabowski, S. J. π-Electron Delocalisation for Intramolecular Resonance Assisted Hydrogen Bonds. J. Phys. Org. Chem. 2003, 16, 797−802. (16) Chou, Y.-C. Group-Theoretical Investigation of the Tunneling Splitting Patterns of Enolic Acetylacetone. J. Mol. Spectrosc. 2010, 263, 34−43.

The experimental and ab initio values of the rotational constants are in good agreement with each other (maximum deviation is 0.7% for A), so we did not attempt any structural improvement. The ab initio geometry is given in the Supporting Information.

■

DISCUSSION AND CONCLUSIONS We could assign the rotational spectrum of enolic form E1 only. Although we can attribute the failure to observe the keto form to its high energy, it appears more difficult to explain why we did not observe any line of the second enolic form, E2, whose energy was estimated to be just 200 cm−1 above that of E1. Probably, a conformational relaxation48 involving mainly a proton transfer can take place upon supersonic expansion. Although, at first sight, AA, HFAA, and TFAA appear to be very similar to each other, their internal dynamics are quite different and generate completely different features in their rotational spectra. AA and HFAA are at the opposite extremes. The MW spectrum of AA is dominated by the effects of largeamplitude motions, such that, so far, only the assignment of the rotational spectrum of the AA torsional sublevel is possible. In contrast, HFAA, appears to be a rigid molecule, and no splittings due to these motions have been observed. In the case of TFAA, however, we were able to obtain full information on the internal rotation of the two symmetric tops. The moderately high V3 barrier of the CH3 top and the low barrier of the CF3 top correspond to similar values of the s reduced barriers, which, in turn, generate similar internal rotation splittings of the rotational transitions. The ERHAM code appeared to be more flexible than XIAM in fitting the experimental data and led to satisfactory fits for all isotopologues. An interesting experimental observation is the effect of H → D substitution of the hydroxyl hydrogen on the V3 barrier. Probably, according to the reverse Ubbelohde effect,46 this isotopic substitution is accompanied by a shortening of the O···O distance and a decrease of the steric repulsion between the H13 and H14 hydrogen atoms, resulting in an increase of both V3 barriers. Then, the coupling between the two V3 barriers can also change, and the internal rotation splittings can be interpreted with a simpler model such as XIAM: Small structural changes generate considerable variations in the coupling between the two internal rotations.

■

REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Complete ref 28, table with the MP2/6-311++G(d,p) geometry of the complex, and tables of transition frequencies and 4247

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248

The Journal of Physical Chemistry A

Article

(17) Evangelisti, L.; Tang, S.; Velino, B.; Giuliano, B. M.; Melandri, S.; Caminati, W. Hexafluoroacetylacetone: A “Rigid” Molecule with an Enolic Cs Shape. Chem. Phys. Lett. 2009, 473, 247−250. (18) Anderssen, A. L.; Zebelman, D.; Baure, S. H. Hexafluoroacetylacetone and Hexafluoroacetic Anhydride. J. Am. Chem. Soc. 1971, 93, 1148−1152. (19) Gordon, M. G.; Kobo, R. D. An INDO Investigation of the Structure and Bonding of Acetylacetone and Trifluoroacetylacetone. J. Am. Chem. Soc. 1973, 95, 5863−5867. (20) Buemi, G. Ab Initio DFT Study of the Hydrogen Bridges in Hexafluoro-Acetylacetone, Trifluoro-Acetylacetone and Some 3Substituted Derivatives. J. Mol. Struct. (THEOCHEM) 2000, 499, 21−34. (21) Sliznev, V. V.; Lapshina, S. B.; Girichev, G. V. Ab Initio Study of the Structure of Enolic and Ketonic Forms of β-Diketones with the General Formula R″COCH2COR′ (R′ and R″ = H, CH3, CF3). J. Struct. Chem. 2006, 47, 220−231. (22) Minura, Y.; Nagashima, N.; Kudoh, S.; Nakata, M. Mechanism of UV-Induced Conformational Changes among Enol-Type Isomers of (Trifluoroacetyl)acetone Studied by Low-Temperature Matrix-Isolation Infrared Spectroscopy and Density Functional Theory Calculation. J. Phys. Chem. A 2004, 108, 2353−2362. (23) Raissi, H.; Nowroozi, A.; Roozbeh, M.; Farzad, F. Molecular Structure and Vibrational Assignment of (Trifluoroacetyl)Acetone: A Density Functional Study. J. Mol. Struct. 2006, 787, 148−162. (24) Nowroozi, A.; Roohi, H.; Ghoogheri, M. S. S.; Sheibaninia, M. The Competition between the Intramolecular Hydrogen Bond and πElectron Delocalization in TrifluoroacetylacetoneA Theoretical Study. Int. J. Quantum Chem. 2011, 111, 578−585. (25) Grabow, J.-U.; Stahl, W.; Dreizler, H. A Multioctave Coaxially Oriented Beam-Resonator Arrangment Fourier-Transform Microwave Spectrometer. Rev. Sci. Instrum. 1996, 67, 4072−4084. (26) Balle, T. J.; Flygare, W. H. Fabry−Perot Cavity Pulsed Fourier Transform Microwave Spectrometer with a Pulsed Nozzle Particle Source. Rev. Sci. Instrum. 1981, 52, 33−45. (27) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; Lopez, J. C.; Mata, S. Molecular Beam Fourier Transform Microwave Spectrum of the Dimethylether−Xenon Complex: Tunnelling Splitting and 131 Xe Quadrupole Coupling Constants. Chem. Phys. Lett. 2004, 392, 1−6. (28) 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.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.01; Gaussian Inc.: Pittsburgh, PA, 2003. (29) Moran, D.; Simmonett, A. C.; Leach, F. E.; Allen, W. D.; Schleyer, P. v. R.; Schaefer, H. F. J. Popular Theoretical Methods Predict Benzene and Arenes to be Nonplanar. J. Am. Chem. Soc. 2006, 128, 9342−9343. (30) Groner, P. J. Effective Rotational Hamiltonian for Molecules with Two Periodic Large-Amplitude Motions. J. Chem. Phys. 1997, 107, 4483−4498. (31) Hartwig, H.; Dreizler, H. The Microwave Spectrum of trans-2,3Dimethyloxirane in Torsional Excited States. Z. Naturforsch. A 1996, 51, 923−932.

(32) Kleiner, I. Asymmetric-Top Molecules Containing one MethylLike Internal Rotor: Methods and Codes for Fitting and Predicting Spectra. J. Mol. Spectrosc. 2010, 260, 1−18. (33) Pickett, H. M. The Fitting and Prediction of Vibration-Rotation Spectra with Spin Interactions. J. Mol. Spectrosc. 1991, 148, 371−377. (34) Gerhard, D.; Hellweg, A.; Merke, I.; Stahl, W.; Baudelet, M.; Petitprez, D.; Wlodarczak, G. Internal Rotation and Chlorine Nuclear Quadrupole Coupling of o-Chlorotoluene Studied by Microwave Spectroscopy and ab Initio Calculations. J. Mol. Spectrosc. 2003, 220, 234−241. (35) Kisiel, Z.; Pszczółkowski, L.; Białkowska-Jaworska, E.; Charnley, S. B. The Millimeter Wave Rotational Spectrum of Pyruvic Acid. J. Mol. Spectrosc. 2007, 241, 220−229. (36) Ouyang, C.; Howard, B. J. The Monohydrate and Ddihydrate of Acetic Acid: A High-Resolution Microwave Spectroscopic Study. Phys. Chem. Chem. Phys. 2009, 11, 366−373. (37) Evangelisti, L.; Favero, L. B.; Maris, A.; Melandri, S.; VegaToribio, A.; Lesarri, A.; Caminati, W. Rotational Spectrum of Trifluoroacetone. J. Mol. Spectrosc. 2010, 259, 65−69. (38) Favero, L. B.; Evangelisti, L.; Feng, G.; Spada, L.; Caminati, W. Conformation and Internal Motions of Dimethyl Sulfate: A Microwave Spectroscopy Study. Chem. Phys. Lett. 2011, 517, 139−142. (39) Caminati, W.; Grabow, J.-U.; López, J. C.; Alonso, J. L. Weak CH···F Bridges and Internal Dynamics in the CH3F−CHF3 Molecular Complex. Angew. Chem., Int. Ed. 2005, 44, 3840−3844. (40) Meerts, J.; Ozier, I. Avoided-Crossing Molecular-Beam Experiments on Fluoroform (CF3H) and Fluoroform-d (CF3D). J. Chem. Phys. 1981, 75, 596−603. (41) Papousek, D.; Hsu, Y. C.; Chen, H. S.; Pracna, P.; Klee, S.; Winnewisser, M. Far Infrared Spectrum and Ground State Parameters of 12CH3F. J. Mol. Spectrosc. 1993, 159, 33−41. (42) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1977; Vol. 6, pp 1−89. (43) Herschbach, D. R. Calculation of Energy Levels for Internal Torsion and Over-All Rotation. III. J. Chem. Phys. 1959, 31, 91−108. (44) Kraitchman, J. Determination of Molecular Structure from Microwave Spectroscopic Data. Am. J. Phys. 1953, 21, 17−25. (45) Ubbelohde, A. R.; Gallagher, K. J. Acid−Base Effects in Hydrogen Bonds in Crystals. Acta Crystallogr. 1955, 8, 71−83. (46) See, for example: Snow, M. S.; B. J. Howard, B. J.; Evangelisti, L.; Caminati, W. From Transient to Induced Permanent Chirality in 2Propanol upon Dimerization: A Rotational Study. J. Phys. Chem. A 2011, 115, 47−51 and references therein. (47) See, for example: Gou, Q.; Feng, G.; Evangelisti, L.; Caminati, W. Conformers of Dimers of Carboxylic Acids in the Gas Phase: A Rotational Study of Difluoroacetic Acid-Formic Acid. Chem. Phys. Lett. 2014, 591, 301−305 and references therein. (48) Ruoff, R. S.; Klots, T. D.; Emilson, T.; Gutowski, H. S. Relaxation of Conformers and Isomers in Seeded Supersonic Jets of Inert Gases. J. Chem. Phys. 1990, 93, 3142−3150.

4248

dx.doi.org/10.1021/jp5005727 | J. Phys. Chem. A 2014, 118, 4243−4248