Perfluoromethyl Fluorocarbonyl Peroxide, CF3OOC(O)F: Structure,Conformations, and Vibrational Spectra Studied by Experimental andTheoretical Methods

Frank Trautner,† Khodayar Gholivand,‡,§ Placido Garcıa,‡ Helge Willner,*,‡ Mauricio F. Erben,|

Carlos O. Della Vedova,*,|,⊥ and Heinz Oberhammer*,†

Institut fur Physikalische und Theoretische Chemie, UniVersitat Tubingen, D-72076 Tu¨bingen,Germany, Fakulta¨ t 4, Anorganische Chemie, Gehard-Mercator-UniVersitat Duisburg, Lotharstr. 1D-47048 Duisburg, Germany, CEQUINOR (CONICET-UNLP), Departamento de Quı´mica,Facultad de Ciencias Exactas, UniVersidad Nacional de La Plata, 47 esq. 115 (1900) La Plata,Republica Argentina, and Laboratorio de SerVicios a la Industria y al Sistema Cientı´fico(UNLP-CIC-CONICET), Camino Centenario, Gonnet, Buenos Aires, Repu´blica Argentina

Received December 11, 2002

The conformational properties and the geometric structure of perfluoromethyl fluorocarbonyl peroxide, CF3OOC-(O)F, have been studied by matrix IR spectroscopy, gas electron diffraction, and quantum chemical calculations(HF, B3LYP, and MP2 methods with 6-311G* basis sets). Matrix IR spectra imply a mixture of syn and anti con-formers (orientation of the CdO bond relative to the OsO bond) with ∆H° ) Hanti° − Hsyn° ) 2.16(22) kcal/mol.At room temperature, the contribution of the anti rotamer is about 3.0%. The OsO bond (1.422(15) Å) is withinthe experimental uncertainties equal to those in related symmetrically substituted peroxides CF3OOCF3 andFC(O)OOC(O)F (1.419(20) and 1.419(9) Å, respectively), and the dihedral angle δ(COOC) (111(5)°) is intermediatebetween the values in these two compounds (123(4)° and 83.5(14)°, respectively).

Introduction

The most interesting structural feature of peroxides X2O2

is the dihedral angleδ(XOOX). For the parent compoundH2O2, an effective dihedral angle (vibrationally averaged val-ue) of 120.0(5)° was derived from rotational constants.1 Theestimated experimental equilibrium value, which correspondsto the minimum of the torsional potential, is 112(1)°.2 Inmany other peroxides, this value is larger: 123(4)° inCF3OOCF3,3 129(2)° in SF5OOSF5,4 135(5)° in CH3OOCH3,5

144(6)° in Me3SiOOSiMe3,6 and 166(3)° in ButOOBut.6 Twotrends are evident from this rather limited number ofexamples: the dihedral angle increases with increasing stericrequirements of the substituents and decreases with theirincreasing electron-withdrawing properties (cf. CF3OOCF3

and CH3OOCH3). Interaction between the lone pairs7 of thetwo oxygen atoms and anomeric effects8 are usually thequalitative explanation for the gauche orientation of substit-uents in peroxides. The latter describes a stabilizing overlapbetween the lone pair of oxygen and theσ* orbital of theopposite O-X bond. Thus, the dihedral angle depends onthe form of the lone pairs, the size of the anomeric effect,and the steric demands of the substituents. According to thismodel, the angle should beg90°. However, unusual struc-tures that do not obey this model are found in FOOF9 and

* Authors to whom correspondence should be addressed. E-mail:carlosdv@quimica.unlp.edu.ar (C.O.D.V.); heinz.oberhammer@uni-tuebingen.de (H.O.).

‡ Gehard-Mercator-Universita¨t Duisburg.§ Permanent address: Tarbiat Modarres University, P. O. Box 14155-

4838, Teheran, Iran.| Universidad Nacional de La Plata.⊥ Laboratorio de Servicios a la Industria y al Sistema Cientı´fico

(UNLP-CIC-CONICET).† Universitat Tubingen.

(1) Redington, R. L.; Olson, W. B.; Cross, P. C.J. Chem. Phys.1962,36, 1311.

(2) Koput, J.Chem. Phys. Lett.1995, 236, 512.(3) Marsden, C. J.; Bartell, L. S.; Diodati, F. P.J. Mol. Struct.1977, 39,

253.(4) Zylka, P.; Oberhammer, H.; Seppelt, K.J. Mol. Struct.1991, 243,

411.

(5) Hass, B.; Oberhammer, H.J. Am. Chem. Soc.1984, 106, 6146.(6) Kass, D.; Oberhammer, H.; Brandes, D.; Blaschette, A.J. Mol. Struct.

1977, 40, 65.(7) Alleres, D. R.; Cooper, D. L.; Cunningham, T. P., Gerratt, J.;

Karadakov, P. B.; Raimondi, M.J. Chem. Soc., Faraday Trans.1995,91, 3357.

(8) Kirby, J.The Anomeric Effect and Related Stereochemical Effects atOxygen; Springer: Berlin, 1983.

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10.1021/ic0262583 CCC: $25.00 © 2003 American Chemical Society Inorganic Chemistry, Vol. 42, No. 9, 2003 3079Published on Web 04/01/2003

ClOOCl10 with dihedral angles of 88.1(4)° and 81.03(1)°,respectively. Similarly, in a series of peroxides, such asFC(O)OOC(O)F,11 CF3C(O)OOC(O)CF3,12 FC(O)OONO2,13

CH3C(O)OONO2, and CF3C(O)OONO2,14 the dihedral anglearound the O-O bond is also smaller than 90°. Hence,dihedral angles smaller than 90° are observed in peroxideswith two sp2-hybridized substituents (RC(O) or NO2), andif both substituents are sp3-hybridized, the dihedral angleis larger than 120°. An intermediate angle has been ob-served in CF3OONO2 (105.1(16)°),15 which contains onesp3- and one sp2-hybridized substituent. In order to continuethe studies on peroxide structures, we report in this paperthe characterization of CF3OOC(O)F, which contains sp3-and sp2-hybridized carbon atoms, using vibrational spectros-copy, gas electron diffraction (GED), and quantum chemicalcalculations.

Another aspect of this work is an improved preparationof CF3OOC(O)F. The title compound was synthesized forthe first time in 1967 by Cauble and Cady by photochemicalreaction of FC(O)OOC(O)F with fluorine16 with a yield ofabout 5% based upon the quantity of peroxide consumed.Talbot17 and Aymonino18 reported new photochemical reac-tions involving the formation of this peroxide. Anderson andFox19 and DesMarteau20 synthesized CF3OOC(O)F using thereaction between CF2(OF)2 and COF2 catalyzed by CsF.

Experimental Section

Caution! CF3OOC(O)F is potentially explosiVe, especially in thepresence of oxidizable materials. It is important to take safetyprecautions when this compound is handled in the liquid or solidstate. Reactions inVolVing this species should be carried out onlyin millimolar quantities.

General Procedure and Reagents.Volatile materials weremanipulated in a glass vacuum line equipped with two capacitancepressure gauges (221 AHS-1000 and 221 AHS-10, MKS Baratron,Burlington, MA) and three U-traps and glass valves with PTFEsystems (Young, London, U.K.). The vacuum line was connectedto an IR cell (optical path length 200 mm, Si windows 0.5 mmthick) contained in the sample compartment of an FTIR instrument(Nicolet Impact 400D, Madison, WI). This arrangement allowedus to observe the purification processes and to follow the courseof the reactions. Solid materials were handled in a drybox (Braun,Unilab) filled with nitrogen containing less than 1 ppm H2O andO2. The pure products were stored in flame-sealed glass ampules

under liquid nitrogen in a long-term Dewar vessel. The ampuleswere opened and resealed with an ampule key21 on the vacuumline, an appropriate amount was taken out for the experiments, andthen, they were flame-sealed again. For Raman measurements, thesample was transferred into a 4 mm glass capillary. NMRmeasurements were carried out on samples in 3 mm o.d. flame-sealed capillaries, which were centered inside 5 mm NMR tubescontaining CDCl3 (Merck) and CFCl3 (Merck) as lock andreference. Details of the matrix-isolation apparatus have been givenelsewhere.22

FC(O)OOC(O)F was prepared in a flow system according to theprocedure described in the literature by reacting CO, F2, and O2.23

For the synthesis of CF2(OF)2, an 80 mL metal reactor was chargedwith 4 g of fine grounded CsF (p.a. Fluka) inside the drybox. Thereactor was filled with a mixture of 20 mmol of CO2 and 80 mmolof F2. After a reaction time of 3 days at room temperature, thebulb was cooled to-196 °C, and the excess of F2 was pumpedoff. The product was passed in a vacuum through a series of trapsheld at-70,-120, and-196°C, and pure CF2(OF)2 was collectedin the -120 °C trap.24,25

CO (95.5%, Linde, Munich, Germany), F2 (Solvay), and O2(99.999% Linde, Munich, Germany) were obtained from com-mercial sources and used without further purification.

Synthesis of CF3OOC(O)F. To ensure the quality of the finalproduct, we have synthesized CF3OOC(O)F by two differentroutes: (i) in a modified synthesis described by Cauble and Cady16

and (ii) by reaction of CF2(OF)2 with COF2 in the presence of CsF,according to the literature procedure.19 (i) In a typical run, a 1 Lquartz bulb was charged with 50 mbar of FC(O)OOC(O)F, 50 mbarCOF2, and 100 mbar F2. After the bulb was immersed in a waterbath at 20°C, the content was irradiated with a low-pressuremercury lamp (40 W) for 3 h. The excess of F2 was pumped off at-196 °C, and the products of several batches were separated in aseries of traps held at-100,-140, and-196 °C. After repeatedtrap-to-trap distillation, pure CF3OOC(O)F was obtained in the-140 °C trap. (ii) The method reported by Anderson and Fox19

and DesMarteau20 was also used, by mixing in a metal reactor 50mmol COF2 and 20 mmol CF2(OF)2 over treated CsF (CsOCF3).After a reaction time of 18 h at-20 °C, the mixture was passed ina vacuum through a series of traps held at-100,-140, and-196°C, and pure CF3OOC(O)F was collected in the-140 °C trap.

The purity of the products was checked by19F and13C NMRspectroscopy at-70 °C because of the high volatility of thecompound and for safety reasons:δF(CF3) ) -69.2 ppm (doublet),δF(COF) ) -32.6 ppm (quartet),δc(CF3) ) 123.2 ppm (quartet),δc(COF)) 142.7 ppm (doublet),1JCF(CF3) ) 269.2 Hz,1JCF(COF)) 298.1 Hz. The data are in accordance with previous reportedvalues.26

Preparation of Matrixes. Small amounts (<1 mmol) of thesample were transferred into a small U-trap immersed in liquidnitrogen. This U-trap was mounted in front of the matrix supportand kept at-140 °C (isopentane bath). A gas stream (≈3 mmolh-1) of argon (99.999%, Messer Griesheim) was directed over thecold sample in the U-trap, and the resulting gas mixtures werepassed through the heated quartz nozzle (4 mm internal diameter,at the end with a 1 mmorifice and a heated zone of 10 mm) at

(9) Hedberg, L.; Hedberg, K.; Eller, P. G.; Ryan, R. R.Inorg. Chem.1988, 27, 232.

(10) Birk, M.; Friedl, R. A.; Cohen, E. A.; Pickett, H. M.; Sander, S. P.J.Chem. Phys.1989, 91, 6588.

(11) Mack, H.-G.; Della Ve´dova, C. O.; Oberhammer, H.Angew. Chem.,Int. Ed. Engl.1991, 30, 1145.

(12) Kopitzky, R.; Willner, H.; Hermann, A.; Oberhammer, H.Inorg. Chem.2001, 40, 2693.

(13) Scheffler, D.; Schaper, J.; Willner, H.; Mack, H.-G.; Oberhammer,H. Inorg. Chem.1997, 36, 339.

(14) Hermann, A.; Niemeyer, J.; Mack, H.-G.; Kopitzky, R.; Beuleke, M.;Willner, H.; Christen, D.; Scha¨fer, M.; Bauder, A.; Oberhammer, H.Inorg. Chem.2001, 40, 1672.

(15) Kopitzky, R.; Willner, H.; Mack, H.-G.; Pfeiffer, A.; Oberhammer,H. Inorg. Chem.1998, 37, 6208.

(16) Cauble, R.; Cady, G. H.J. Am. Chem. Soc.1967, 89, 5161.(17) Talbott, R. L.J. Org. Chem.1968, 33, 2095.(18) Aymonino, P. J.; Blesa, M. A.An. Asoc. Quı´m. Argent.1970, 58, 27.(19) Anderson, L. R.; Fox, W. B.Inorg. Chem.1970, 9, 2182.(20) DesMarteau, D. D.Inorg. Chem.1970, 9, 2179.

(21) Gombler, W.; Willner, H.J. Phys. E: Sci. Instrum.1987, 20, 1286.(22) Bodenbinder, M.; Ulic, S.E.; Willner, H.J. Phys. Chem.1994, 98,

6441.(23) Russo, A.; DesMarteau, D. D.Inorg. Chem.1995, 34, 6221.(24) Hohorst, F. A.; Shreeve, J. M.J. Am. Chem. Soc.1967, 89, 1809.(25) Cauble, R.; Cady, G. H.J. Am. Chem. Soc.1967, 89, 1962.(26) Huang, Q.; DesMarteau, D. D.Chem. Commun.1999, 1671.

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3080 Inorganic Chemistry, Vol. 42, No. 9, 2003

temperatures between 25 and 400°C within milliseconds and weresubsequently quenched on the matrix support at 14 K. Since thevapor pressure of the sample in the cold U-trap was≈ 10-3 mbarand the pressure of the inert gas stream during the deposition≈1mbar in the U-trap, the resulting sample-to-gas ratio can beestimated to be 1:1000. Matrix IR spectra were recorded on aIFS66v/S FT spectrometer (Bruker, Karlsruhe, Germany) in thereflectance mode with a transfer optic. A DTGS detector with aKBr/Ge beam splitter in the region 4000-400 cm-1 was used. Inthis region, 64 scans were coadded for each spectrum with aresolution of 1 cm-1.

Instrumentation. (A) Gas Electron Diffraction. Electrondiffraction intensities for CF3OOC(O)F were recorded with aGasdiffraktograph KD-G227 at 25 and 50 cm nozzle-to-platedistances and with an accelerating voltage of ca. 60 kV. The samplereservoir was cooled to-80°C, and the inlet system with the nozzlewas at room temperature. The photographic plates were analyzedwith the usual procedures28 and averaged molecular intensities inthe ranges 2-18 and 8-35 Å-1 (s ) 4π/λ sin ϑ/2, λ electronwavelength,ϑ scattering angle) are shown in Figure 1.

(B) Vibrational Spectroscopy.Gas-phase infrared spectra wererecorded with a resolution of 2 cm-1 in the range 4000-400 cm-1

on the FTIR instrument Bruker IFS 66v. FT-Raman spectra of liquidCF3OOC(O)F were recorded with a Bruker RFS 100/S FT-Ramanspectrometer. The sample in a 4 mmglass capillary was excitedwith 500 mW of a 1064 nm Nd:YAG laser (ADLAS, DPY 301,Lubeck, Germany).

(C) NMR Spectroscopy.The 19F and13C NMR spectra of theneat liquid sample were recorded with a Bruker MSL 200spectrometer and a19F/1H dual head (for19F) or a multinuclearprobe head (for13C) operating at 188.31 and 50.33 MHz,respectively. For each19F NMR spectrum, four scans wereaccumulated in a 64-kB memory, with a delay time of 2.2 s betweenscans. For the13C NMR spectrum, 132 scans of the same samplewere recorded with a delay time of 60 s. Low-temperaturemeasurements (-70°C) were carried out by using a Bruker variable-temperature controller with a copper-constantan thermocouple.

Quantum Chemical Calculations

CF3OOC(O)F can adopt, in principle, two differentconformations with the CdO bond of the FC(O) group eithersynperiplanar or antiperiplanar with respect to the OsO bond(Chart 1). The molecular geometries of both conformers wereoptimized with the B3LYP hybrid method and HF and MP2

approximations, using 6-311G* basis sets. All three methodspredict the anti conformer to be higher in energy, by∆E )1.14 kcal/mol (B3LYP), 2.58 kcal/mol (HF), and 2.24 kcal/mol (MP2). According to the B3LYP method, the enthalpydifference ∆H° ) 1.00 kcal/mol and Gibbs free energydifference∆G° ) 0.93 kcal/mol, only slightly smaller thanthe energy difference derived by this method. If thesecorrections between∆E, ∆H°, and∆G° are applied to theMP2 result, we obtain∆H° ) 2.10 kcal/mol and∆G° )2.03 kcal/mol. Figure 2 shows the potential function forinternal rotation around the OsO bond as derived with theB3LYP/6-311G* method, which possesses a minimum atδ(COOC)) 113.4°.

Vibrational wavenumbers and IR and Raman intensitieswere calculated with the B3LYP/6-311G* method and areincluded in Table 1. All quantum chemical calculations wereperformed with the Gaussian 98 software package.29 Vibra-tional amplitudes for the low-energy conformer were derivedfrom theoretical (B3LYP/6-311G*) Cartesian force constantsusing the program ASYM40.30

Vibrational Analysis

As far as we know, there has been no detailed vibrationalstudy of CF3OOC(O)F. Although the IR spectrum wasreported,16,17 it has been used only as a purity criterionwithout assignment of the bands. The observed band posi-

(27) Oberhammer, H.Molecular Structure by Diffraction Methods; TheChemical Society: London, 1976; Vol 4, p 24.

(28) Oberhammer, H.; Gombler, W.; Willner, H.J. Mol. Struct. 1981, 70,273.

(29) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B. G.;Chen, W.; Wong, M. W.; Andres, J. L.; Head-Gordon, M.; Replogle,E. S.; Pople, J. A.Gaussian 98, revision A.7; Gaussian, Inc.:Pittsburgh, PA, 1998.

(30) Hedberg, L.; Mills, J. M.J. Mol. Spectrosc. 1993, 160, 117.

Figure 1. Averaged molecular electron diffraction intensities for long (top)and short (bottom) nozzle-to-plate distances and residuals.

Figure 2. Potential energy curve for CF3OOC(O)F as a function of theCO-OC dihedral angle calculated with the B3LYP/6-311G* approximation.

Chart 1. Representation of the Syn (left) and Anti (right) Conformersof CF3OOC(O)F

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tions in the IR (gas phase and matrix) and Raman spectra,together with calculated wavenumbers (B3LYP), are col-lected in Table 1. The 21 normal modes of vibration in theC1 symmetry group are all active in both IR and Ramanspectra. The agreement between calculated and observedwavenumbers is satisfying and is illustrated in Figure 3 bycomparing the simulated and the experimental spectrumobtained in an Ar matrix at 14 K.

Because of the small rotational constants of the CF3OOC-(O)F molecule, the infrared bands in the gas phase show nodistinct contours. The three bands at 1920, 1245, and 613cm-1 in the IR gas phase spectrum are composed of at leasttwo unresolved modes, which are shown in the Ar matrixspectrum. In the simulated spectrum, one CO stretching modeoccurs, and therefore, the additional band at 1932 cm-1 isassigned to a combination, intensified by Fermi resonance.The assignments of the observed bands are performed bycomparison with the calculated spectra, and the approximatedescription of modes is based on the potential energydistribution derived from the calculated force constants, aswell as on comparison with spectra of related molecules.Many vibrations are strongly coupled in this compound.

Recently, Man-Hua et al.31 reported a simple correlationfor per/poly-fluorodiacyl peroxides RFC(O)OOC(O)RF be-tween theνO-O stretching mode and the inductive effect ofthe RF group. Taking into account the electronic propertiesof the substituents attached to the peroxide bond, we canexpect the wavenumber of theνO-O stretching mode forCF3OOC(O)F between the corresponding values for thesymmetrically substituted peroxides CF3OOCF3 (821cm-1) and FC(O)OOC(O)F (910 cm-1). TheνO-O mode of

CF3OOC(O)F is seen at 888 cm-1 as the most intense Ramanband of the liquid.

Matrix Infrared Spectra

When the Ar/CF3OOC(O)F mixture is passed through theheated spray-on nozzle and the heated mixture is subse-quently deposited as a matrix, some IR matrix bands increasein their relative intensity. These bands are assigned to thehigher energy anti conformer of CF3OOC(O)F and are alsolisted in Table 1.

Two groups of bands are suitable to study the conforma-tional distribution: (i) the CdO stretching bands near 1900cm-1 and (ii) the C-F stretching bands at 1264/1240 cm-1,because the syn/anti conformational change involves sig-nificant variations in the bond strengths of the CdO andCsF bonds. The change in relative intensities within thegroup of IR matrix CF bands with increasing temperatureof the spray on nozzle is shown in Figure 4.The area ratiosof the isolated pairs of bands belonging to both conformersare determinated and plotted on a logarithmic scale as afunction of the reciprocal absolute temperature. These ratiovalues correspond closely to the concentration ratios of thetwo conformers. Such van’t Hoff plots forνCdO (1897/1911+1932 cm-1) (Figure 5) andνC-F (1264/1240 cm-1)band pairs yield experimental enthalpy differences of 2.16and 2.17 kcal/mol, respectively, and a mean value of∆H°) 2.16(22) kcal/mol. A wholly experimental∆G° valuecould not be determined because of the high dispersion foundfor the integrated absorption coefficient ratios of the two

(31) Man-Hua, S.; Yu, Z.; Yan-Ling, Q.; Zi-Ying, Z.; Cheng-Xue, Z.J.Fluorine Chem.2001, 109, 129.

Table 1. Observed and Calculated Vibrational Data for CF3OOC(O)F

experimental

Ar matrixc calculatedd

IR gasa Ramanb syn anti syn antiassignment/approxdescription of mode

3817 (0.27) 2ν1

1920 (32) 1906 s 1932 (22) 1897 (3.0) 1977 (46) 1954 (50) ν5 +ν9

1911 (65) ν1/ν (CdO)1300 (39) 1292 (47) 1288 (40) 1287 (30) ν2/νas(CF3)1245 (63) 1233 w 1240 (59) 1264 (2.2) 1231 (37) 1252 (48) ν3/ν (FsCO)

1230 (39) 1214 (21) 1230 (41) ν4/ν (OsCF3)1203 (2.9) 2ν12

1172 (100) 1155 vw 1165 (100) 1169e 1155 (100) 1155 (75) ν5/νas(CF3)1005 (5.4) 1005 m 1003 (7.3) 1016 (4.1) 960 (0.70) ν6/νs (CF3)930 (3.4) 930 vs 929 (5.5) 899 (0.01) 945 (4.6) 904 (4.7) ν7/ν (OsC(O)F)

888 vs 888 (0.26) 881 (0.39) ν8/ν (OsO)751 (4.0) 744 vw 749 (6.7) 740 (0.01) 755 (3.6) 748 (3.4) ν9/γ (FC(O)sO)

692 s 693 (0.16) 686 (0.31) ν10/δ (OsCdO)690 (0.56) 687 (0.70) 675 (1.0) ν11/δs (CF3)613 (3.9) 613 m 613 (2.0) 610 (1.13) 613 (2.3) ν12/δas(CF3)

607 (4.3) 602 (0.98) 603 (0.51) ν13/δas(CF3)528 (0.30) 527 w 528 (0.01) 525 (0.51) 532 (0.17) ν14/F (CF3)

448 vw 444 (0.05) 447 (0.05) ν15/F (CF3)369 m 370 (0.06) 382 (0.17) ν16/δ (OdCsF)271 m 286 (0.07) 275 (0.09) ν17/δ (OsOsCF3)

264 (0.22) 264 (0.14) ν18/δ (OsOsC(O)F)85 vs 96 (0.02) 85 (0.06) ν19/τ (FCOsO)

69 (0.01) 68 (0.02) ν20/τ (CF3sO)52 (0.05) 52 (0.01) ν21/τ (OsO)

a In parentheses, relative absorbance at band maximum.b Liquid, room temperature, band intensities: vs) very strong; s) strong; m) medium; w)weak; vw) very weak.c Band position at the most intensive matrix site. In parentheses, relative integrated absorbance of the rotamers deposited with thespray on nozzle held at room temperature.d B3LYP(6-311G*). In parentheses, relative band strength of both rotamers; 100≡ 867.5 km mol-1. e Overlappedby the respective syn absorption.

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3082 Inorganic Chemistry, Vol. 42, No. 9, 2003

conformers. This value of 3.1(18) for the ratio between synand anti forms precludes the use of the value 1 which couldbe thought as a usual approximation. This value would alsobe suggested taking into account ab initio calculations forwhich the ratio is 0.92 when the B3LYP/6-311G* ap-proximation is used. Therefore, a plausible approximationto calculate∆G° is by using the calculated (B3LYP/6-311G*)entropy difference of∆S° ) 0.23 cal K-1 mol-1. A value of∆G° ) 2.09(22) kcal/mol is derived, and a contribution of3.0% of the anti rotamer is obtained at room temperature.

At temperatures higher than 300°C, new bands ofdecomposition products appeared in the IR matrix spec-trum. These bands were unambiguously assigned to CF3O•

and FC(O)O• radicals.32 No bands belonging to either of theperoxy radicals CF3OO• and FC(O)OO• have been ob-served.33 This behavior is in agreement with the calcu-lated (B3LYP) bond enthalpies:D(CF3O-OC(O)F) )21.3 kcal/mol,D(CF3-OOC(O)F) ) 75.5 kcal/mol, andD(CF3OO-C(O)F) ) 70.2 kcal/mol.34

Structure Analyses

The radial distribution function (RDF), which was derivedby Fourier transformation of the molecular electron diffrac-tion intensities, is shown in Figure 6. The experimental RDFis reproduced much better with the syn than with the anti

conformation. A preliminary structural model derived fromthe RDF was refined by least-squares fitting of the molecularintensities.

(32) Arguello, G. A.; Willner, H.J. Phys. Chem. A2001, 105, 3466.(33) Sander, S.; Pernice, H.; Willner, H.Chem. Eur. J.2000, 6, 3645.

(34) Note: Bond enthalpies were calculated as the difference of enthalpiesof products and reactives at 298 K and 1 atm (H ) Eelec + ZPE +Evibr + Erot + Etrans + RT). Thus,DRO-OR, for example, ROORf2RO•, is given byDRO-OR ) 2H(RO•) - H(ROOR). Schlegel, H. B.J. Phys. Chem.1984, 88, 6254.

Figure 3. Ar matrix IR spectrum of CF3OOC(O)F at 300 K (A), andsimulated infrared spectrum forsyn-CF3OOC(O)F from B3LYP/6-311G*calculation (B).

Figure 4. IR spectra in the 1280-1220 cm-1 region for CF3OOC(O)Fisolated in Ar matrixes at 14 K using different nozzle deposition temper-atures.

Figure 5. van’t Hoff plot using the ratios of integrated band areas of theIR matrix bands at 1897/(1911+ 1932) cm-1 obtained after quenching therotamer equilibria ofsyn- and anti-CF3OOC(O)F at different nozzletemperatures.

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Inorganic Chemistry, Vol. 42, No. 9, 2003 3083

The following assumptions, which are based on the resultsof the MP2 calculation, were made in this refinement (seeTable 2): (1) There is planarity of the FC(O)-O moietyandC3V symmetry of the CF3 group with a possible tilt anglebetween theC3 axis and the O2-C2 bond direction (for atomnumbering see Figure 7). (2) Differences between the twoO-C bond lengths, between the mean C2-F bond lengthof the CF3 group and C1-F1, and between the two O-O-Cangles were constrained to calculated values. (3) The O1-C1-F1 angle and the torsional angles of the FC(O) and CF3

groups around the O-C bonds were not refined. Attemptsto refine these geometric parameters lead to very largestandard deviations. (4) Vibrational amplitudes were refinedin groups. Such amplitudes, which either caused largecorrelations between geometric parameters or were poorlydetermined by the GED intensities, were set to calculatedvalues. Attempts to refine amplitudes for the very closelyspaced 1,3-nonbonded distances resulted in very largecorrelations between bond angles. If these amplitudes werevaried by(0.003 Å, the refined angles changed by less thanthe error limits given in Table 3. With these assumptions,nine geometric parameters (p1 to p9) and five vibrationalamplitudes (l1 to l5) were refined simultaneously. The

following correlation coefficients had values larger than|0.7|: p1/p2 ) -0.83,p1/p4 ) 0.77,p2/p4 ) -0.87, andp7/p8 ) -0.72. The agreement factors for the intensities ofthe long and short nozzle-to-plate distances wereR50 )3.92% andR25 ) 9.87%. Additional least squares analyseswere performed for mixtures of syn and anti conformers.The geometric parameters of the anti form were tied to thoseof the syn conformer with the calculated (MP2) differences,and vibrational amplitudes obtained with the B3LYP methodwere used. The agreement factorR50, which is more sensitivetoward conformational mixtures, decreased very slightly toR50 ) 3.88% for a contribution of 4(8)% of the anti form.The error limit is obtained by Hamilton’s test based on 1%significance.35 This corresponds to a Gibbs free energydifference of∆G° ) 1.9(12) kcal/mol. Since the conforma-tional composition is determined much more accurately fromthe matrix IR spectra, this value (3.0% anti conformer, whichcorresponds to∆G° ) 2.09(22) kcal/mol) was used in thefinal least squares refinement. The geometric parameters andvibrational amplitudes are listed together with calculatedvalues in Tables 2 and 3, respectively, and the atomnumbering is shown in Figure 7.

Discussion

The dihedral angle in CF3OOC(O)F (δ(COOC) )111(5)°) with one sp2-hybridized and one sp3-hybridizedcarbon substituent is intermediate between those found inFC(O)O-OC(O)F (83.5(14)°)11 with two sp2-hybridizedcarbon substituents and in CF3O-OCF3 (123(4)°)3 with two

(35) Hamilton, W.Acta Crystallogr.1965, 18, 502.

Figure 6. Experimental radial distribution function and difference curvefor CF3OOC(O)F. Important interatomic distances of the syn structure areindicated by vertical bars.

Table 2. Experimental and Calculated Geometric Parameters for SynConformer of CF3OOC(O)F

GEDa MP2b B3LYPb

O1sO2 1.422(15) p1 1.432 1.440O1sC1 1.376(11) p2 1.369 1.368O2sC2 1.393(11)c p2 1.386 1.391CdO 1.186(8) p3 1.177 1.172(C2sF)mean 1.319(4) p4 1.322 1.329C1sF1 1.319(4)c p4 1.322 1.328O1sO2sC2 106.7(9) p5 106.3 107.2O2sO1sC1 108.5(9)c p5 108.2 108.9O1sC1dO3 129.6(13) p6 129.8 130.0O1sC1sF1 103.4d 103.4 103.6FsC2sF 110.0(9) p7 110.2 110.1tilt (CF3)e 4.4(14) p8 4.9 5.0δ(O2sO1sC1dO2) -2.8d -2.8 -2.9δ(O1sO2sC2sF2) 180.7d 180.7 180.8δ(CsOsOsC) 111.2 (48) p9 108.7 113.4

a ra values in Å and deg. Error limits are 3σ values. For atom numbering,see Figure 7.b 6-311G* basis sets.c Difference to previous parameter fixedat MP2 value.d Not refined.e Tilt angle betweenC3 axis of CF3 group andO2-C2 bond direction away from O-O bond.

Table 3. Interatomic Distances and Experimental and CalculatedVibrational Amplitudes (Å) for Syn Conformer of CF3OOC(O)F

atom pair distanceamplitude

GEDaamplitudeB3LYPb

CdO 1.17 0.036c 0.036CsF 1.32 0.044(3) l1 0.045OsC 1.38-1.39 0.047(3) l1 0.048O1sO2 1.42 0.047(3) l1 0.048O1‚‚‚F1 2.12 0.057c 0.057O2‚‚‚F 2.15-2.24 0.058c 0.058F1‚‚‚F2 2.16 0.056c 0.056O3‚‚‚F1 2.23 0.050c 0.050O1‚‚‚C2 2.26 0.063c 0.063O2‚‚‚C1 2.27 0.067c 0.067O1‚‚‚O3 2.30 0.051c 0.051O2‚‚‚O3 2.61 0.093c 0.093O1‚‚‚F3 2.64 0.124(13) l2 0.130C1‚‚‚F4 3.05 0.255(99) l3 0.250C1‚‚‚C2 3.14 0.124(13) l2 0.130O3‚‚‚F4 3.19 0.400c 0.400O2‚‚‚F1 3.39 0.061(9) l4 0.060O1‚‚‚F2 2.39 0.061(9) l4 0.061C2‚‚‚O3 3.45 0.172(48) l5 0.215C1‚‚‚F3 3.81 0.165c 0.165F1‚‚‚F4 3.99 0.255(99) l3 0.250C2‚‚‚F1 4.16 0.124(13) l2 0.121C1‚‚‚F2 4.17 0.124(13) l2 0.133O3‚‚‚F2 4.23 0.255(99) l3 0.230O3‚‚‚F3 4.41 0.172(48) l5 0.204F1‚‚‚F3 4.53 0.172(48) l5 0.201F1‚‚‚F2 5.32 0.109c 0.109

a Error limits are 3σ values. For atom numbering, see Figure 7.b 6-311G*basis sets.c Not refined.

Trautner et al.

3084 Inorganic Chemistry, Vol. 42, No. 9, 2003

sp3-hybridized carbon substituents. It is, however, larger thanthe angle found for the isoelectronic peroxide CF3O-ONO2

(105.1(16)°)15 where one substituent is an sp3-hybridizedcarbon atom and the other an sp2-hybridized nitrogen atom.Because of the large error limit for the dihedral angle inCF3O-OC(O)F, this difference is not so evident from theexperimental data, but it is more clearly shown by quantumchemical calculations. The B3LYP/6-311G* method predictsdihedral angles of 102.6° for CF3O-ONO2 and 113.4° forCF3O-OC(O)F.

The syn orientation of the CdO double bond relative tothe OsO single bond is preferred in CF3OOC(O)F. The MP2value (∆G° ) 2.03 kcal/mol) is in excellent agreement withthe experimental∆G° value of 2.09(22) kcal/mol derivedfrom the matrix IR experiment, while the B3LYP method(∆G° ) 0.93 kcal/mol) underestimates this difference. Inmolecules containing the FC(O)O- moiety, the syn form ispreferred mainly because the stabilization due to the anomericinteraction lpσ(O) f σ*(CdO) present in the syn form isgreater than the stabilization due to the anomeric interactionlpσ(F) f σ*(CsF) present in the anti form.36 The preference

for the syn form was found experimentally also in fluoro-carbonyl disulfides, such as FC(O)SSC(O)F,37,38FC(O)SSC-(O)Cl,39 FC(O)SSCH3,40,41and FC(O)SSCF3.42 On the basisof a natural bond orbital (NBO) analysis, a quantitativeexplanation for the syn preference in FC(O)SSCH3 wasgiven, taking into account the contributions of both theanomeric and mesomeric effects.43

The O-O bond length in CF3O-OC(O)F (1.422(15) Å)is within experimental uncertainties equal to those inCF3O-OCF3 (1.419(20) Å), CF3O-ONO2 (1.414(8) Å), andFC(O)O-OC(O)F (1.419(9) Å). These bonds are shorterthan in the parent compound HO-OH (1.475(4) Å)2 or inCH3O-OCH3 (1.457(12) Å).3

Acknowledgment. The authors thank ANPCYT-DAAD(PROALAR) for the German-Argentinean cooperation awards.The Argentinean authors also thank the Consejo Nacionalde Investigaciones Cientı´ficas y Tecnicas (CONICET) andthe Comisio´n de Investigaciones Cientı´ficas de la Provinciade Buenos Aires (CIC), Repu´blica Argentina. They areindebted to the Facultad de Ciencias Exactas, UniversidadNacional de La Plata, Repu´blica Argentina for financialsupport. M.F.E. would like to express his gratitude to HolgerPernice, Mike Finze, and Stefan von Ahsen for theirfriendship and valuable help in the laboratory work duringhis stay in Duisburg.

IC0262583

(36) Hermann, A.; Trautner, F.; Gholivand, K.; von Ahsen, S.; Varetti, E.L.; Della Vedova, C. O.; Willner, H.; Oberhammer, H.Inorg. Chem.2001, 40, 3979.

(37) Ulic, S. E.; Della Ve´dova, C. O.; Aymonino, P. J.J. Raman Spectrosc.1989, 20, 655.

(38) Mack, H.-G.; Della Ve´dova, C. O.; Oberhammer, H.J. Phys. Chem.1992, 96, 9215.

(39) Della Vedova, C. O.J. Raman Spectrosc.1989, 20, 581.(40) Della Vedova, C. O.Spectrochim. Acta, Part A1991, 11, 1619.(41) Fantoni, C. Della Ve´dova, C. O.J. Mol. Spectrosc.1992, 154, 240.(42) Hermann, A.; Ulic, S. E.; Della Ve´dova, C. O.; Mack, H.-G.;

Oberhammer, H.J. Fluorine Chem.2001, 112, 297.(43) Erben, M. F.; Della Ve´dova, C. O.HelV. Chim. Acta,in press.

Figure 7. Molecular models and numbering of the syn conformer ofCF3OOC(O)F.

Perfluoromethyl Fluorocarbonyl Peroxide

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