ARTICLE pubs.acs.org/JPCA
How Trifluoroacetone Interacts with Water Laura B. Favero,† Luca Evangelisti,‡ Assimo Maris,‡ Alicia Vega-Toribio,§ Alberto Lesarri,§ 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'Universita, Via Selmi 2, I-40126 Bologna, Italy § Departamento de Química Física y Química Inorganica, Facultad de Ciencias, Universidad de Valladolid, E-47011 Valladolid, Spain ‡
bS Supporting Information ABSTRACT: The rotational spectra of five isotopologues of the molecular adduct 1,1,1-trifluoroacetonewater have been assigned using pulsed-jet Fourier-transform microwave spectroscopy. All rotational transitions appear as doublets, due to the internal rotation of the methyl group. Analysis of the tunneling splittings allows one to determine accurately the height of the 3-fold barrier to internal rotation of the methyl group and its orientation, leading to V3 = 3.29 kJ 3 mol1 and — (a,i) = 67.5, respectively. The water molecule is linked to the keton molecule on the side of the methyl group through a OH 3 3 3 O hydrogen bond and a CH 3 3 3 O intermolecular contact, lying in the effective plane of symmetry of the complex.
’ INTRODUCTION 1,1,1-Trifluoroacetone (TFA) is a biologically active and blood pressure reducing substance. Because of its acidity, which is higher than that of acetone, and of the electronegative effects of the CF3 group, relatively mild basic conditions are sufficient to break the CC bonding. TFA is therefore ideal for use as a building block for carbo- and heterocycles containing CF3 groups. This opens up numerous possibilities for synthetic applications. TFA can be used as a hydride-acceptor in the Oppenauer oxidation of secondary alcohols in the presence of diethylethoxyaluminum. The oxidant enables a selective oxidation of secondary alcohols in the presence of primary alcohols.1 Despite its interest, just a few spectroscopic/photochemical investigations are available on TFA, such as (i) a detailed study of the vapor phase photolysis at 3130 Å, reported already more than half a century ago;2 (ii) a measure of the quantum yields of phosphorescence and corresponding intersystem crossing, biacetyl production, and carbon monoxide production;3,4 (iii) a recent study of the rotational spectrum.5 In principle, TFA could exist also in the enolic tautomeric form, as it is the case, for example, for acetylacetone6 and hexafluoroacetylacetone.7 The MW investigation has shown,5 however, that the keto form is predominant in the gas phase of TFA. In solution, TFA can exist as enolate (in basic conditions in solution), providing carbon carbon reactions, and it can be used as a building block in the synthesis of fluorinated molecules. Because of its biological importance, we thought that the way TFA interacts with water could be relevant to understand its activity. r 2011 American Chemical Society
In addition, such a study would allow observing the effect of the interaction with water on the V3 barrier of the methyl group.5 We report here the results of the investigation of TFA with water. From now on, we will label the 1:1 complex as TFAW.
’ EXPERIMENTAL SECTION The rotational spectrum of TFAW was measured in the 618 GHz frequency region using a COBRA-type8 pulsed supersonic-jet Fourier-transform microwave (FT-MW) spectrometer9 described elsewhere10 and updated with the FTMWþþ set of programs.11 A mixture of 2% TFA and 2% of H2O in helium was expanded from ca. 0.3 MPa to about 5 mPa through the solenoid valve (General Valve, Series 9, nozzle diameter 0.5 mm) into the FabryPerot cavity. The frequencies were determined after Fourier transformation of the 8k data points time-domain signal, recorded with 100 ns sample intervals. Each rotational transition is split by Doppler effect, enhanced by the coaxial expansion of the molecular beam along the resonator axes. The rest frequency is calculated as the arithmetic mean of the frequencies of the Doppler components. The estimated accuracy of frequency measurements is better than 3 kHz and lines separated by more than 7 kHz are resolvable.
Special Issue: David W. Pratt Festschrift Received: December 17, 2010 Revised: February 15, 2011 Published: March 15, 2011 9493
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Table 1. MP2/6-311þþG** Relative Energies and Spectroscopic Constants of the Five More Stable Adducts of TFAW*
ΔE0 and ΔECP are the zero point corrected and the counterpoise corrected relative energies, respectively. EB, EB,0, and EB,CP are the corresponding values of the dissociation energy. a Absolute energy 566.187957 Eh 3 b Absolute energy 566.184768 Eh 3 c Absolute energy 566.102194 Eh 3 *
A sample of TFA (purchased from Aldrich) was used without further purification. The spectra of the deuterated and H218O isotopologues were observed with enriched heavy water samples.
’ RESULTS AND DISCUSSION A. Theoretical Calculations. Before collecting the rotational spectra, we performed theoretical calculations using the Gaussian03 package of programs.12 The MøllerPlesset terms up to second order (MP2/6-311þþG**) were taken into account. In a second stage, we refined these calculations further, by correcting for basis-set superposition error using the counterpoise method.13 We report here only the data for the keto form of TFA. This because the enolic tautomer, which is predicted to be 48.4 kJ mol1 higher in energy for the monomer,5 could originate three stable adducts with water (their shapes and energies are given as Supporting Information), but even the most stable of these adducts was calculated to be at least 32.0 kJ mol1 higher in energy than the adduct of water with the keto form. Five stationary points were found with their configurations and molecular constants reported in Table 1. The geometries of the optimized structures are given as Supporting Information. On the basis of the optimized structures, we calculated the vibrational frequencies within the harmonic approximation and verified that all five stationary points are real minima. Also zero point corrections to the electronic energies were estimated, together with quartic centrifugal distortion constants. The various kinds of energy and dissociation energy values are also reported in Table 1. B. Assignment of the Spectra. As suggested by the values of the relative energies, we started the search of the μa type transitions of
species I and II. We could identify only one spectrum. First, μa-R type transitions with Ka = 02 and with J ranging from 4 to 9 were easily identified and assigned. Later on several μb-R type transitions were also measured. All transitions were split into two A/E components, according to the tunneling effects expected for the hindered internal rotation of a single methyl group. The measured transitions are given in the Supporting Information. The computer program XIAM (based on the combined axis method, CAM)14 has been used to fit simultaneously the A and E component lines, according to Watson’s semirigid Hamiltonian15 in the S reduction and Ir representation. Synthetically, the Hamiltonian for XIAM can be expressed as14,16 H ¼ Hrot þ Htor þ HCD
ð1Þ
where Hrot (the rotational part) includes a single set of rotational constants, corresponding to the values for the infinite barrier limit, HCD includes set of centrifugal distortion parameters common to both states, and Htor is the torsional part14 Htor ¼ Feff ðp FPa Þ2 þ
V3 ð1 cos 3RÞ 2
ð2Þ
The fitting results are a “rigid” limit set of rotational and first order centrifugal distortion constants, common to A and E sublevels, together with the parameters describing the internal rotation of the methyl group. These include the V3 barrier, the IR moment of inertia of the top, the angle — (a,i) that the methyl group internal rotation axis forms with the a-principal axis and one internal rotation-overall rotation interaction distortion constant (Dpi2J).14 Due to the Cs symmetry of the molecule, the angle — (b,i) is the complement of — (a,i), while — (c,i) is 90. The determined parameters are reported in Table 2. 9494
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Table 2. Experimental Spectroscopic Constants of the Observed Isotopologues of TFAW (S-Reduction, Ir Representation) TFA-H2O
TFA-H2O18
A/MHz
3551.803(1)a
3551.355(2)
B/MHz
971.8069(1)
919.2831(2)
955.7346(1)
934.6758(1)
920.1127(1)
C/MHz
888.2278(1)
844.1237(1)
874.0495(1)
856.6399(1)
843.7391(1)
DJ/kHz
0.2304(5)
0.215(2)
0.2304b
0.2304b
DJK/kHz
2.68(9)
2.60(7)
2.68b
2.68b
d1/kHz
0.0169(6)
0.0169b
0.0169b
0.0169b
0.0169b
d2/kHz
0.0158(5)
0.0158b
0.0158b
0.0158b
0.0158b
0.0961 3.293(8)
0.0961 3.283(1)
0.0961 3.255(1)
0.0961b 3.109(1)
0.0961(9) 3.290(9)
Dpi2J/MHz V3/kJ 3 mol1
IR/uÅ2
— (a,i)/
c
σ/kHzd Ne
TFA-DOH
TFA-HOD
3539.94(3)
b
TFA-D2O
3542.09(6)
b
3530.661(1)
0.2034(8) 2.44(2)
b
3.28(1)
3.28b
3.28b
3.28b
3.28b
67.47(3)
67.58(3)
67.6(1)
67.82(3)
58.81(1)
2
2
3
5
4
57
22
18
20
39
Paa/uÅ2
473.364
503.075
482.113
493.988
502.547
Pbb/uÅ2
95.611
95.628
96.092
95.967
96.429
Pcc/uÅ2
46.677
46.678
46.673
46.711
46.711
Error in parentheses in units of the last digit. b Fixed to the value of the parent species. c The values of the — (b,i) and of the — (c,i) parameters are the complement to 90 and 90 (from the planarity of the mainframe), respectively. d Root-mean-square deviation of the fit. e Number of lines in the fit. a
Table 3. Experimental and Calculated Substitution Coordinates of the Water Atoms in the Complex
Table 4. Structural r0 Parameter of TFAW fitted parameters
derived parameters
calc water atom
coorda
O Hbond Hfree a
obs
conformer I
conformer V
a/Å b/Å
(3.8798(4) (0.10(2)
þ3.859 0.103
3.962 0.168
a/Å
(2.9575(5)
þ3.110
3.043
b/Å
(0.698(2)
þ0.696
0.447
a/Å
(4.5463(3)
þ4.613
4.435
b/Å
(0.644(3)
0.609
0.942
The c coordinate is assumed to be zero.
Figure 1. Observed conformer, global minimum, of the TBAwater adduct. One relatively strong OH 3 3 3 O interaction and one weak CH 3 3 3 O contact stabilize this form.
To better locate water in the complex, we also investigated the spectra of some enriched isotopic species, namely TFAH218O, TFADOH, TFAHOD, TFAD2O. The corresponding spectra were assigned and fitted with the same procedure described above, and the line frequencies are
Ow 3 3 3 O /Å — Ow 3 3 3 OC / — HwOw 3 3 3 O/
2.915(3) 109.37(5) 21(2)
Hw 3 3 3 O/Å — OwHw 3 3 3 O/ — Hw 3 3 3 OC /
149 119
2.044
Ow 3 3 3 HC/Å — Ow 3 3 3 HC/
143
2.411
also available as Supporting Information. No splittings due to the internal rotation of the CF3 group have been observed, as expected from the larger moment of inertia of this internal rotor. C. Conformational and Structural Data. The values of the planar moments of inertia are reported at the bottom of Table 2. Pcc does not change among the five isotopologues, showing that the water unit lies in the plane of symmetry of TFA. By comparing the experimental rotational constants (Table 2) to the theoretical values of Table 1, one can easily see that they can match only Conformers I and V. Since the calculated energy of the latter species one is much higher, it is plausible to assign the observed spectrum to the former one. This hypothesis is confirmed by the information obtained from the spectra of the isotopic species. From these data we calculated the rs coordinates17 of the substituted atoms, that is, the water oxygen and hydrogens. These data are compared to the ab initio values of species I and V in Table 3. The errors of the substitution structure were calculated according to Costain rule.17b The errors given for the r0 parameters are one standard deviation. One can note that the experimental values agree quite well with those of Conformer I. In addition, the experimental values of the centrifugal distortion constants are in good accord and confirm this assignment. The shape of the adduct is then that described by form I, which is shown in Figure 1, together with some structural parameters relevant to the intermolecular H-bond. One can see that the leading motifs of the preferred conformation are a relatively strong Ow-Hw 3 3 3 O H-bond and a weak CH 3 3 3 Ow contact, in 9495
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Table 5. Change of the Methyl Group V3 Barriers of Some Organic Molecules upon Complexation with One Water Molecule mol
TFA
dimethoxymethane
diacetyl
N-methylformamide 0.67
V3(Mol)/kJ mol1
3.10
6.62
3.81
V3(Mol-W)/kJ mol1
3.29
6.83
4.11
reference
this work
conjunction with an effective planar shape of the heavy atoms skeleton. This is confirmed also by the fact that the rotational constant A does not change for the water 16Of18O, which also implies the water oxygen atom to lye along the a-axis. The 15 available rotational constants have been used to obtain, besides the substitution coordinates given above, a partial r0 geometry of the molecule, limited to the determination of the Ow 3 3 3 O distance and of the — Ow 3 3 3 OC and — Hw-H-bondOw 3 3 3 O angles, while keeping the remaining structural parameters fixed to the r0 values of TFA.5 The obtained values are given in Table 4. From the fitted parameters, the values of the structural parameters relative to the OH 3 3 3 O H-bond and to the CH 3 3 3 O intermolecular contact have been derived and are shown in the right part of Table 4. Their values are typical of the two kinds of noncovalent interactions. D. Discussion. When water forms a 1:1 adduct with molecules with a plane of symmetry with oxygen atoms and containing methyl groups, such as TFA, three effects can be easily observed or determined from the rotational spectrum. They are (i) the loss or not of the original symmetry; (ii) tunnelling splittings produced by the internal motion of the water moiety; and (iii) changes of the potential energy function describing the internal rotation of the methyl group. The results of MW investigations of adducts of water with molecules containing a methyl group are available for dimethoxymethanewater18 and for diacetylwater19 and trans N-methylformamidewater.20 Similarly to the case of diacetylwater, TFAW preserves the symmetry elements of the bare organic molecule. Vice versa, in dimethoxymethanewater, the C2 symmetry of dimethoxymethane is lost in the complex. As to the water tunnelling effects, none of these three considered complexes exhibits features due to the internal motions of water. This is in accord with the typology of the complex with water as the proton donor forming a strong H-bond with low symmetry molecules, as described in a recent perspective article on the complexes of water with organic molecules.21 Instead, in some molecules containing a methyl group it has been measured the change in the V3 barrier of the internal rotation of the methyl group upon interaction with water. In the examples reported in Table 5, including TFAW, the water molecule interacts through an OH 3 3 3 O H-bond with the ether or keto oxygen, and through a CH 3 3 3 Ow contact with the methyl group. One can see that the CH 3 3 3 Ow linkage always produces an increase of the methyl group V3 barrier, corresponding to a stabilization of the energy minimum. For acetic acid, the interaction with one and two molecules of water induces a linear decrease of the V3 barrier with the number of linked molecules of solvent, but in this case water interacts with the carboxylic group and the declining trend is suggested to be correlated with the strength of the hydrogen bond.22 Assuming that the stretching motion between the centers of mass of the two constituent molecules takes place along the inertial a-axis of the complex, one can evaluate the dissociation energy with the approximate pseudiatomic model. Within this
18
4.18
19
20
approximation, the stretching force constant (ks) can be estimated by considering the complex as made of two rigid parts, and using the following equation23 ks ¼ 16π4 ðμRÞ2
½4B4 þ 4C4 ðB CÞ2 ðB þ CÞ2 hDJ
ð3Þ
where μ is the pseudodiatomic reduced mass, DJ is the centrifugal distortion constant, and R is the distance between the centers of mass of the monomers. The value ks = 8.3 N m1 was obtained. Then, assuming a Lennard-Jones type potential, the dissociation energy has been estimated by applying the approximate formula24 EB ¼
1 ks R 2 72
ð4Þ
from which a value EB = 14.1 kJ/mol was calculated. At a first sight, this value of EB appears too low for a OH 3 3 3 O H-bond, but it is in good agreement with the theoretical EB,0 value of Table 1 (16.4 kJ/mol). However, even small couplings of the stretching motion of water (with respect to TFA) to the bending or internal rotation motions of water can make the approximation quite crude.
’ CONCLUSIONS In summary, we have assigned the rotational spectra of five isotopologues of the complex TFAW in a jet-cooled supersonic expansion. We observed only one conformer which is the global minimum configuration of the complex. The failures in observing other forms of the adduct can be due to their higher energies or, for the lowest in energy not observed species, to a conformational relaxation upon supersonic expansion.25 It appears that water prefers to have a secondary interaction after the OH 3 3 3 O H-bond with the methyl rather than with the CF3 group. Similarly to other complexes, such a water 3 3 3 CH3 interaction stabilizes the minimum of the 3-fold barrier, producing an increase of the V3 parameter with respect to the isolated molecule. The pseudiatomic molecule model to calculate the dissociation energy from the centrifugal distortion DJ parameter supplies a relatively low EB value (14.1 kJ/mol), which is, however, in relatively good agreement with the theoretical value (16.4 kJ/mol). ’ ASSOCIATED CONTENT
bS
Supporting Information. Complete ref 12; three tables of transition frequencies; table with MP2 optimized geometries for the five observed conformers; and table with the shapes and energies of the adducts of water with the enolic form. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. 9496
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’ ACKNOWLEDGMENT We thank the Ministero dell’Istruzione, dell’Universita e della Ricerca (MIUR, PRIN 2008) and the University of Bologna (RFO) for financial support. A.V.-T. and A.L. thank the Spanish MICINN for funding (CTQ2009-14364-C02-02) and the MEC for a doctoral FPU grant for A.V.-T ’ REFERENCES (1) Mello, R.; Martínez-Ferrer, J.; Asensio, G.; Gonzalez-Nu~ nez, M. E. J. Org. Chem. 2007, 72, 9376. (2) Sieger, R. A.; Calvert, J. G. J. Am. Chem. Soc. 1954, 76, 5197. (3) Beavan, S. W.; Phillips, D.; Brown, R. G. Chem. Phys. Lett. 1975, 36, 542. (4) Gandini, A.; Hackett, P. A. J. Am. Chem. Soc. 1977, 99, 6195. (5) Evangelisti, L.; Favero, L. B.; Maris, A.; Melandri, S.; Vega-Toribio, A.; Lesarri, A.; Caminati, W. J. Mol. Spectrosc. 2010, 259, 65. (6) Caminati, W.; Grabow, J.-U. J. Am. Chem. Soc. 2006, 128, 854. (7) Evangelisti, L.; Tang, S.; Velino, B.; Giuliano, B. M.; Melandri, S.; Caminati, W. Chem. Phys. Lett. 2009, 473, 247. (8) (a) Grabow, J.-U.; Stahl, W. Z. Naturforsch., A: Phys. Sci. 1990, 45, 1043. (b) Grabow, J.-U. Doctoral Thesis, Christian-AlbrechtsUniversit€at zu Kiel, Kiel, Germany, 1992. (c) Grabow, J.-U.; Stahl, W.; Dreizler, H. Rev. Sci. Instrum. 1996, 67, 4072. (9) Balle, T. J.; Flygare, W. H. Rev. Sci. Instrum. 1981, 52, 33. (10) Caminati, W.; Millemaggi, A.; Alonso, J. L.; Lesarri, A.; Lopez, J. C.; Mata, S. Chem. Phys. Lett. 2004, 1, 392. (11) Grabow, J.-U. Habilitationsschrift, Universit€at Hannover, Hannover, Germany, 2004. http://www.pci.uni-hannover.de/∼lgpca/ spectroscopy/ftmw (12) Frisch, M. J. et al. Gaussian03, revision B.01; Gaussian Inc.: Pittsburgh, PA, 2003. (13) Boys, S. F.; Bernardi, F. Mol. Phys. 1970, 19, 553. (14) Hartwig, H.; Dreizler, H. Z. Naturforsch., A: Phys. Sci. 1996, 51, 923. (15) Watson, J. K. G. In Vibrational Spectra and Structure; Durig, J. R., Ed.; Elsevier: New York, 1977; Vol. 6, pp 189. (16) Gerhard, D.; Hellweg, A.; Merke, I.; Stahl, W.; Baudelet, M.; Petitprez, D.; Wlodarczak, G. J. Mol. Spectrosc. 2003, 220, 234. (17) (a) Kraitchman, J. Am. J. Phys. 1953, 21, 17. (b) Van Eijck, B. P. J. Mol. Spectrosc. 1982, 91, 348. (18) Favero, L. B.; Giuliano, B. M.; Melandri, S.; Maris, A.; Caminati, W. Chem.—Eur. J. 2007, 13, 5833. (19) Favero, L. B.; Caminati, W. J. Phys. Chem. A 2009, 113, 14308. (20) Caminati, W.; Lopez, J. C.; Blanco, S.; Mata, S.; Alonso, J. L. Phys. Chem. Chem. Phys. 2010, 12, 10230. (21) Evangelisti, L.; Caminati, W. Phys. Chem. Chem. Phys. 2010, 12, 14433. (22) Ouyang, B.; Howard, B. J. Phys. Chem. Chem. Phys. 2009, 11, 366. (23) Millen, D. J. Can. J. Chem. 1985, 63, 1477. (24) Novick, S. E.; Harris, S. J.; Janda, K. C.; Klemperer, W. Can. J. Phys. 1975, 53, 2007. (25) Ruoff, R. S.; Klots, T. D.; Emilson, T.; Gutowski, H. S. J. Chem. Phys. 1990, 93, 3142.
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