Fluoroformyl Hypofluorite, Fluoroformyl Peroxyhypofluorite, and

vibrational frequencies for three rotamers of FC(O)OOC(O)F and bond dissociation enthalpies for the lowest-energy rotamer. We further report enthalpy ...
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11292

J. Phys. Chem. 1996, 100, 11292-11296

Fluoroformyl Hypofluorite, Fluoroformyl Peroxyhypofluorite, and Fluoroformyl Peroxide. A Density Functional Study Michael L. McKee* and Thomas R. Webb* Department of Chemistry, Auburn UniVersity, Auburn, Alabama 36849-5312 ReceiVed: March 5, 1996; In Final Form: April 30, 1996X

Calculations on FC(O)OF, FC(O)OOF, and FC(O)OOC(O)F are reported at the B3LYP/6-31+G(d)//B3LYP/ 6-31+G(d)+ZPC level of theory. All these molecules are predicted to exist as mixtures of rotamers. For FC(O)OF, the calculations reproduce the known structural parameters, the enthalpy difference between the rotamers, the activation energy for their interconversion, and their vibrational frequencies. They predict an O-F bond dissociation energy of 28.7 kcal/mol in the lowest-energy (trans) rotamer. For FC(O)OOF, the calculations predict nonplanar molecules, with O-F and O-O distances of 1.446 and 1.394 Å, respectively, and O-F and O-O bond dissociation energies of 30.8 and 22.3 kcal/mol, respectively, in the trans rotamer. Calculations on FC(O)OOC(O)F reproduce the known structural parameters and vibrational frequencies; they predict an O-O bond dissociation energy of 14.8 kcal/mol. Enthalpy changes for the proposed reactions which lead to the formation of FC(O)OF from FC(O)OOC(O)F and for the proposed decomposition of FC(O)OOF are also calculated.

Introduction Since Cauble and Cady1 first reported the synthesis of fluoroformyl hypofluorite, FC(O)OF, this molecule has been viewed2 as relatively unstable and prone to decomposition. More recently, Argu¨ello et al.3 have reported that the molecule is considerably more stable than originally suggested. They further suggested a possible impurity, fluoroformyl peroxyhypofluorite, FC(O)OOF, responsible for the observed decomposition. They reported that FC(O)OF is present as a mixture of cis and trans rotamers and used gas electron diffraction and theoretical calculations to further probe these rotamers.3b They also proposed a mechanism for the formation of FC(O)OF. The compounds FC(O)OF,3 FC(O)OOF, and FC(O)OOC(O)F4,5 offer an excellent opportunity to survey a series of closely related molecules. Previous calculations on FC(O)OF3b have examined the geometries, vibrational frequencies, and relative energies of the rotamers. We report additionally the geometry of the transition state for the interconversion and the bond dissociation enthalpies of the O-F and C-O bonds in the lowerenergy rotamer. FC(O)OOF has not been previously characterized;3 we have calculated geometries, vibrational frequencies, and relative energies of rotamers as well as transition state geometries, activation barriers, and bond dissociation enthalpies for the lower-energy rotamer. We also report calculated vibrational frequencies for three rotamers of FC(O)OOC(O)F and bond dissociation enthalpies for the lowest-energy rotamer. We further report enthalpy changes for reactions in the proposed mechanism of formation of FC(O)OF and for a suggested decomposition of FC(O)OOF.3a These calculations afford an excellent opportunity for the interplay between theory and experiment which has proven so helpful in other studies from this laboratory in recent years.6 A number of recent studies7 have described hypofluorites and radicals related to these molecules (such as FC(O)O) as species of interest in atmospheric chemistry, synthesis, and theory. Methods Calculations have used the GAUSSIAN94 program system.8 Geometries were fully optimized within the appropriate point X

Abstract published in AdVance ACS Abstracts, June 15, 1996.

S0022-3654(96)00676-4 CCC: $12.00

group, and frequencies were calculated at the B3LYP/6-31+G(d) level.9,10 Zero-point and heat-capacity corrections (298 K) are included without any weighting factor at the B3LYP/631+G(d) level. The standard “DFT” level is B3LYP/6-31+G(d)//B3LYP/6-31+G(d)+ZPC plus thermal corrections to 298 K. Standard grid sizes were used throughout. The transition vector was inspected for each transition state (8TS, 10TSa, and 10TSb) to ensure that the distortion corresponds to a motion which interconverts reactant and product (torsion about the C-O bond in 8TS, torsion about the O-O bond in 10TSa, and torsion about the C-O bond in 10TSb). In each case the expected distortion was found. With an appropriate choice of gradient correction and modest basis set, density functional theory (DFT) has been shown to give results of near chemical quality.11 In addition, spin contamination does not seem to be as serious for DFT as compared with Hartree-Fock (HF) theory.12 Ventura and Kieninger7h have recently demonstrated the superiority of DFT over conventional ab initio methods for the study of F-O bonds. Results and Discussion Absolute energies (in hartrees) of species of interest are presented in Table 1, reaction energies and enthalpies are presented in Table 2, and structures of species of interest are given in Figure 1. FC(O)OF. Our calculations on FC(O)OF identify two stable species, the trans and cis rotamers, 8a and 8b. We have also located the transition state 8TS for their interconversion. Both 8a and 8b are predicted to be planar. The calculated O-F bond distances in 8a and 8b are 1.425 and 1.441 Å, respectively, which can be compared to gas electron diffraction values of 1.418 and 1.425 Å, respectively.3b These O-F bond distances are similar to those in HOF, OF2, SF5OF, and CF3OF (all between 1.41 and 1.45 Å)13 but not in FOOF (1.575 Å).14 In the transition state 8TS, the calculated O-F distance has lengthened to 1.446 Å (ca. 0.03 Å longer than in either 8a or 8b), which can be rationalized by the diminished interaction between a lone-pair orbital on O and the π* carbonyl orbital. The CdO (carbonyl) and C-F bond distances are quite similar in 8a, 8b, and 8TS. © 1996 American Chemical Society

A DFT Study of Fluoroformyl Derivatives

J. Phys. Chem., Vol. 100, No. 27, 1996 11293 NLDFT/TZVP (nonlocal density functional theory) level3b the order is reversed by 0.7 kcal/mol. Our calculations correctly predict the trans rotamer to be more stable than the cis but underestimate the difference (+0.3 kcal/mol, Table 2). We attribute the different trans/cis stability order calculated by the two density functional methods (NLDFT and B3LYP) to the fact that the B3LYP exchange functional includes HartreeFock exchange. The experimental energy of activation3a for the interconversion of rotamers (8a f 8b), which corresponds to the barrier to rotation about the C-O bond, is 9.6 kcal/mol; we calculate the enthalpy of activation (∆Hq) for this isomerization as 10.9 kcal/mol. Similarly, the calculated vibrational frequencies and intensities for the trans and cis rotamers (Table 3) agree quite well with those reported by Argu¨ello et al.3b The calculated enthalpy changes for the decompositions 8a f 5 + F and 8a f 4 + 1 (Table 2) are the bond dissociation enthalpies for the O-F and C-O bonds in 8a. The calculated O-F bond dissociation enthalpy is 28.7 kcal/mol. This value is on the low side of the wide range of O-F bond energies, from about 20 to 50 kcal/mol.15 It may be compared with estimates of 35 kcal/mol in CF3SO2OF, 43 kcal/mol in CF3OF, and 33 kcal/mol in FSO3F.16 The relatively low value may reflect stabilization of the FC(O)O radical 5 through delocalization of the unpaired electron. The calculations clearly indicate that dissociation of the O-F bond is much more favorable than C-O bond dissociation (81.5 kcal/mol). FC(O)OOF. Our calculations on FC(O)OOF again identify two rotamers, trans and cis (10a and 10b), as the stable forms of the molecule. We also examined two transition states. One, 10TSa, is the transition state for the degenerate interconversion of enantiomers of 10a; the other, 10TSb, is for the interconversion of 10a and 10b. We predict that both 10a and 10b are nonplanar, with COOF dihedral angles of 88.3° and 89.5°, respectively. These dihedral angles are similar to that found

TABLE 1: Absolute Energies (hartrees) of C/F/O Relevant Speciesa B3LYP/ PG state 6-31+G(d) F F2 O2 CO2 OF (1) OOF (2) FOF (3) FCO (4) FC(O)O (5) FC(O)F (6) FOOF (7) FC(O)OF trans (8a) FC(O)OF cis (8b) FC(O)OF TS (8TS) FC(O)OO trans (9a) FC(O)OO cis (9b) FC(O)OOF (10a) FC(O)OOF (10b) FC(O)OOF (10TSa) FC(O)OOF (10TSb) FC(O)OOC(O)F s-s (11a) FC(O)OOC(O)F s-a (11b) FC(O)OOC(O)F a-a (11c)

K D∞h D∞h D∞h C∞V Cs C2V Cs C2V C2V C2 Cs Cs C1 Cs Cs C1 C1 Cs C1 C2 C1 C2

-99.730 59 1 + -199.517 29 Σg 3 - -150.327 57 Σg 1 + -188.590 39 Σg 2Π -174.883 12 2A′′ -250.076 75 1A 1 -274.672 84 2A′ -213.110 45 2B 2 -288.348 61 1A 1 -313.028 63 1A -349.834 79 1A′ -388.128 16 1A′ -388.127 42 1A -388.109 57 2A′′ -363.489 10 2A′′ -363.488 04 1A -463.270 78 1A -463.269 13 1A′ -463.262 05 1A -463.256 64 1A -576.726 99 1A -576.724 14 1A -576.721 58 2P

ZPE (NIF)b 0.0 1.48 (0) 2.34 (0) 7.26 (0) 1.58 (0) 3.52 (0) 3.38 (0) 5.08 (0) 7.79 (0) 8.69 (0) 5.55 (0) 10.72 (0) 10.55 (0) 10.19 (1) 10.74 (0) 10.69 (0) 12.66 (0) 12.53 (0) 12.52 (1) 12.16 (1) 19.98 (0) 19.76 (0) 19.53 (0)

Cpc 〈S2〉d 1.48 2.10 2.08 2.24 2.49 2.72 2.58 2.08 2.74 2.68 3.12 3.34 3.38 3.02 3.37 3.38 4.15 4.07 3.65 3.67 4.96 5.01 5.09

0.75 2.01 0.75 0.81 0.75 0.76

0.75 0.75

a A real wave function is used to describe F (3P) and OF (2Π) rather than the required complex wave function. b Zero-point energy (kcal/ mol) and number of imaginary frequencies in parentheses. c Heat capacity (kcal/mol) integrated from 0 to 298 K. d Spin-squared value for open shell systems before spin projection.

The energetics of these species agree quite well with the available experimental data and previous theoretical calculations.3 At the MP2/6-31G(d) level (including zero-point and thermal corrections) and trans-cis enthalpy difference (8a f 8b) is 1.8 kcal/mol (1.2 ( 0.1 kcal/mol, exptl3), while at the TABLE 2: Reaction Energies and Reaction Enthalpies (kcal/mol) F2 f 2F OOF (2) f O2 (3Σg) + F FOF (3) f OF (1) + F FC(O)O (5) f F + CO2 FC(O)F (6) f FCO (4) + F FOOF (7) f 2OF (1) FOOF (7) f F + OOF (2) t-FC(O)OF (8a) f c-FC(O)OF (8b) t-FC(O)OF (8a) f 8TS t-FC(O)OF (8a) f FC(O)O (5) + F t-FC(O)OF (8a) f FCO (4) + OF (1) FC(O)O (5) + F2 f t-FC(O)OF (8a) + F FC(O)OOF (10a) f FC(O)OOF (10b) FC(O)OOF (10a) f 10TSa FC(O)OOF (10a) f 10TSb FC(O)OOF (10a) f FC(O)OO (9a) + F FC(O)OOF (10a) f FC(O)O (5) + OF (1) FC(O)OOF (10a) f FCO (4) + OOF (2) FC(O)OOF (10a) f FC(O)F (6) + O2 (3Σg) FC(O)OOC(O)F (11a) f 2FC(O)O (5) FC(O)OOC(O)F (11a) f FC(O)OOC(O)F (11b) FC(O)OOC(O)F (11a) f FC(O)OOC(O)F (11c) FC(O)OOC(O)F (11a) f t-FC(O)OO (9a) + FCO (4) FC(O)OOC(O)F (11a) f 2FCO (4) + O2 (3Σg) t-FC(O)OO (9a) f c-FC(O)OO (9b) FC(O)OO (9a) + F2 f FC(O)OOF (10a) + F

B3LYP/6-31+G(d)

+ZPC

+Cp

exptla

35.2 11.7 37.1 17.3 117.7 43.0 17.2 0.5 11.7 30.7 84.4 4.5 1.0 5.5 8.9 32.0 24.5 52.4 -53.6 18.7 1.8 3.4 80.0 142.1 0.7 3.2

33.7 10.5 35.3 16.8 114.1 40.6 15.2 0.3 11.2 27.8 80.3 5.9 0.9 5.4 8.4 30.1 21.2 48.3 -55.2 14.3 1.6 3.0 75.8 134.3 0.6 3.6

34.6 11.3 36.7 17.8 115.0 41.9 15.7 0.3 10.9 28.7 81.5 5.9 0.8 5.9 7.9 30.8 22.3 49.0 -54.6 14.8 1.7 3.1 76.5 135.7 0.6 3.8

37.9 12.7b 17-21c 123.8d 48e 19.9f 1.2g 9.6g

a Unless otherwise noted, reaction enthalpies are computed from heat of formation at 298 K from: Atkinson, R.; Baulch, D. L.; Cox, R. A.; Hampson, R. F., Jr.; Kerr, J. A.; Troe, J. J. Phys. Chem. Ref. Data 1992, 21, 1125. b For an alternative value of 11.7 kcal/mol, see: Pagsberg, P.; Ratajczak, E.; Sillesen, A.; Jodkowski, J. T. Chem. Phys. Lett. 1987, 141, 88. c Theoretical estimate of bond energy. See: Maricq, M. M.; Szente, J. J.; Li, Z.; Francisco, J. S. J. Chem. Phys. 1993, 98, 784. d A heat of formation at 298 K at -145.3 kcal/mol is used for FC(O)F. Schneider, W. F.; Wallington, T. J. J. Phys. Chem. 1994, 98, 7448. e Reference 17. f See: Dixon, D. A.; Andzelm, G.; Fitzgerald, G.; Wimmer, E. J. Phys. Chem. 1991, 95, 9197. g Reference 3.

11294 J. Phys. Chem., Vol. 100, No. 27, 1996

McKee and Webb

Figure 1. Calculated structures of species at the B3LYP/6-31+G(d) level of theory. Experimental values (in parentheses) for FC(O)O (5) are from ref 7n. Gas electron diffraction values for t-FC(O)OF (8a) and c-FC(O)OF (8b) (in parentheses) are from ref 3b, while values for FC(O)OOC(O)F (11a) are from ref 5.

TABLE 3: Calculated (B3LYP/6-31+G(d)) and Observed Vibrational Frequencies (cm-1) for t-FC(O)OF (8a) and c-FC(O)OF (8b) trans-FC(O)OF (8a) sym assign

calcda

exptlb

cis-FC(O)OF (8b) ∆

calcda

exptlb

∆

a′ FCdO str 1970 (420) 1932 (83) 38 1926 (383) 1906 (91) 20 a′ C-F str 1178 (370) 1191 (100) -13 1261 (328) 1261 (100) 0 a′ C-O str 1031 (40) 992 (10) 39 952 (20) 936 (9) 16 a′ O-F str 945 (41) 910 (11) 35 899 (60) 861 (8) 38 a′′ C o-o-p 729 (32) 743 (16) -14 726 (35) 737 (23) -11 a′ FCdO 650 (8) 655 (4) -5 625 (22) 626 (5) -1 a′ bend 502 (2) 505 (1) -3 534 (2) c (0.7) a′ bend 304 (3) 307 (4) -3 310 (4) 317 (0.5) -7 a′′ tors 187 (0) 186 (0) 1 145 (1) 150 (0.1) -5 a Intensity for calculated frequencies (in parentheses) given in km/ mol. b Measured IR frequencies in gas phase and relative integrated band intensities in Ar matrix (ref 3b). c Not observed in gas phase; 530 cm-1 in Ar matrix.

in FOOF (ca. 88°).14 The calculated O-F distances in 10TSa and 10TSb are 1.408 and 1.463 Å, which are shorter and longer, respectively, than the O-F distance in 10a (1.446 Å). The shorter O-F distance in 10TSa (relative to 10a) is accompanied by a longer O-O distance (O-O 1.463 Å 10TSA; 1.394 Å 10a), while the longer O-F distance in 10TSb (relative to 10a) is accompanied by a very slightly shorter O-O distance (O-O 1.391 Å 10TSb; 1.394 Å 10a). The O-O distances in all FC(O)OOF species are considerably longer than the O-O distance in FOOF (1.219 Å).14 The C-O bond lengths change in parallel with the O-F bonds (from 1.386 and 1.384 Å in 10a and 10b to 1.355 Å in 10TSa and 1.408 Å in 10TSb). The CdO and C-F bond distances in the fluoroformyl moiety are again essentially constant. The trans rotamer 10a is again the lowest-energy form, preferred over the cis 10b by only 0.8 kcal/mol. The small enthalpy difference indicates that this compound should exist as a mixture of rotamers. Transition state 10TSa for interconverting enantiomers of 10a (rotation about the O-O bond) is

5.9 kcal/mol above 10a. Transition state 10TSb for interconverting 10a and 10b (rotation about the C-O bond) is 7.9 kcal/ mol above 10a. In comparison, the rotational barrier about the C-O bond in FC(O)OF (8a f 8TS) is about 10 kcal/mol, as noted above. Calculated vibrational frequencies and intensities of 10a and 10b are given in Table 4. Argu¨ello et al.3a noted that the IR spectra of FC(O)OF and the impurity which they proposed as FC(O)OOF are very similar. The calculated vibrational frequencies in Table 3 (FC(O)OF, 8a) and Table 4 (FC(O)OOF, 10a) bear out this observation. However, the calculated frequency at 824 cm-1 for FC(O)OOF, which is predicted to be of medium to strong intensity (151 km/mol), does not have a close counterpart in the spectrum of FC(O)OF. We suggest looking for a band in the IR near this position. Corresponding bond dissociation energies for FC(O)OOF again point out the importance of the delocalized FC(O)O radical 5 as a dissociation product. The calculated dissociation enthalpy for the O-F bond in FC(O)OOF is 30.8 kcal/mol, again toward the low end of the range of O-F bond enthalpies, and similar to that calculated for FC(O)OF above. However, this is not the most favorable dissociation; the dissociation of the O-O bond is much more favorable (22.3 kcal/mol). This is much lower than the O-O bond dissociation enthalpy estimated for FOOF (48 kcal/mol)17 from thermochemical data. Besides reflecting the formation of 5 in the dissociation of the O-O bond of FC(O)OOF, the O-O bond dissociation energies also reflect the difference in the O-O bond distances noted above; the O-O bond is much shorter in FOOF than in FC(O)OOF. Dissociation of the C-O bond is again less favorable; the calculated dissociation enthalpy is 49.0 kcal/mol. Although not a bond cleavage reaction, we also calculated the enthalpy change for the decomposition of FC(O)OOF (10a) to FC(O)F and O2 (3Σg ). While we did not calculate the activation barrier for this process, the large exothermicity (-54.6

A DFT Study of Fluoroformyl Derivatives

J. Phys. Chem., Vol. 100, No. 27, 1996 11295

TABLE 4: Calculated (B3LYP/6-31+G(d)) Vibrational Frequencies (cm-1) and Intensities (km/mol) for syn-syn, syn-anti, and anti-anti FC(O)OOC(O)F, t- and c-FC(O)OOF, t- and c-FC(O)OO, and FC(O)Oa syn-syn 11a a b a b a b a a b b a b a b a a b a

1969 (324) 1938 (449) 1218 (38) 1173 (967) 1036 (9) 962 (60) 934 (17) 748 (30) 732 (26) 717 (7) 647 (6) 576 (19) 400 (0) 337 (6) 335 (0) 97 (0) 97 (1) 63 (0)

anti-anti 11c

syn-anti 11b 1952 (446) 1941 (388) 1237 (286) 1178 (661) 992 (37) 940 (30) 889 (28) 747 (31) 724 (26) 703 (3) 634 (9) 587 (22) 414 (1) 334 (1) 323 (3) 95 (0) 80 (1) 51 (1)

a b a b a b a a b b a b a a b b a a

1944 (2) 1928 (962) 1262 (241) 1191 (596) 937 (6) 923 (92) 863 (2) 741 (39) 725 (19) 671 (2) 630 (9) 594 (31) 429 (0) 323 (0) 316 (2) 80 (2) 78 (2) 26 (0)

t-FC(O)OOF 10a

c-FC(O)OOF 10b

1966 (381) 1175 (437) 999 (16) 931 (41) 824 (151) 745 (23) 675 (11) 565 (24) 426 (1) 326 (2) 127 (0) 98 (0)

1952 (442) 1208 (353) 976 (13) 886 (49) 801 (135) 739 (26) 655 (9) 576 (23) 434 (2) 337 (1) 121 (1) 80 (1)

t-FC(O)OO 9a a′ a′ a′ a′ a′′ a′ a′ a′ a′′

1964 (363) 1176 (234) 1128 (229) 890 (70) 700 (35) 675 (7) 512 (2) 326 (3) 144 (0)

FC(O)Ob 5

c-FC(O)OO 9b 1944 (379) 1199 (287) 1114 (94) 860 (52) 699 (37) 661 (15) 546 (3) 337 (2) 118 (1)

a1 b2 a1 b1 a1 b2

1516 (333) 1198 (138) 979 (100) 738 (41) 529 (12) 490 (9)

a The mode symmetry is given to the left for C , C , and C symmetry species. The IR intensity is given in parentheses. b Experimental frequencies s 2 2V (cm-1) in Ne matrix (ref 7n) are 1475, 1098, 960, 735, 519, and 474.

TABLE 5: Assignment of syn-syn FC(O)OOC(O)F (11a) IR Spectra with Assistance of Calculated (B3LYP/ 6-31+G(d)) Vibrational Frequencies (cm-1) and Intensities (km/mol) exptl (intensity)a,b

calcd (intensity)

∆

symm/assignment

1940 (vs) 1900 (vs) 1225 (m) 1170 (vs) 1005 (m) 950 (s) 905 (m) 740 (s)

1969 (324) 1938 (449) 1218 (38)c 1173 (957) 1036 (9) 962 (60) 943 (17) 748 (30) 732 (26)

29 38 -7d 3 31 12 29 8

a CdO stretch b CdO stretch a C-F stretch b C-F stretch a O-O stretch b C-O stretch a C-O stretch a O-CdO bend b O-CdO bend

a vs ) very strong; s ) strong; m ) medium. b From ref 4. c The s/a conformer is only 1.7 kcal/mol higher in energy than the s/s conformer. The s/a band at 1237 cm-1 is predicted to be very intense (286 km/mol). d The difference is 12 cm-1 with respect to the s/a band.

kcal/mol, Table 2) suggests that the reaction may proceed quite readily, in agreement with the experimental observations.3a FC(O)OOC(O)F. The calculated distances for the syn-syn isomer of FC(O)OOC(O)F (11a) agree well with those determined in the electron diffraction study and lower-level calculations.5 We reproduce the trends in energy differences between the syn-syn (11a), syn-anti (11b), and anti-anti (11c) isomers in the earlier calculations, although our calculated energy differences are consistently smaller than those previously calculated at the HF/6-31G* level.5 Thus, at our level of theory, 11b (syn-anti) is predicted to lie 1.7 kcal/mol above 11a (vs 3.2 kcal/mol earlier), and 11c (anti-anti) is predicted to lie 3.1 kcal/mol above 11a (vs 6.4 kcal/mol earlier). The calculated vibrational frequencies of 11a (Tables 4 and 5) agree well with those reported previously.4 Most of the bands with predicted high intensity in the calculated spectra of 11b and 11c resemble those of 11a in frequency and intensity. However, the calculated intensities of the C-F stretching mode near 1225 cm-1 differ markedly. Because this band is weak in the spectrum of 11a but is predicted to be more intense in the spectrum of 11b and 11c, we expect absorption in this region to be quite sensitive to the amounts of 11b and/or 11c present. This intensity difference could provide the basis for a variable-temperature infrared study to determine the rotamer populations and the enthalpy changes accompanying interconversion of rotamers.

The dissociation of FC(O)OOC(O)F may proceed symmetrically (FC(O)O as product) or unsymmetrically (FC(O)OO and FCO as products). The calculations clearly favor the symmetrical cleavage path (14.8 vs 76.5 kcal/mol). This difference is reflected in the dominant formation of FC(O)OF over FC(O)OOF in the fluorination of FC(O)OOC(O)F.3a The symmetrical cleavage gives the O-O bond enthalpy, which is lower than the values for FOOF and for FC(O)OOF. Again, the long O-O bond (1.436 Å) in 11a (compared to 1.394 Å in 10a and 1.219 Å in FOOF) reflects the different bond enthalpies (14.8 kcal/mol in 11a, 22.3 kcal/mol in 10a, and 48 kcal/mol in FOOF17). FC(O)O and FC(O)OO. The two radicals FC(O)O (5) and FC(O)OO (9) have been calculated to evaluate decomposition mechanisms. The experimental structure (Figure 1) and frequencies (Table 4) are available for FC(O)O7n (5) but not for FC(O)OO (9).18 We compared experiment and theory for 5 and report the structure (Figure 1) and vibrational frequencies19 (Table 4) for 9 in the hopes that they may aid identification. We predict the trans rotamer (9a) to be 0.6 kcal/mol more stable than the cis rotamer (9b). Formation of FC(O)OF. Argu¨ello et al.3a proposed that two main reactions are responsible for the formation of FC(O)OF: the photolysis of the peroxide FC(O)OOC(O)F (eq 1) and the reaction of the product FC(O)O with F2 (eq 2).

FC(O)OOC(O)F f 2 FC(O)O

(1)

FC(O)O + F2 f FC(O)OF + F

(2)

Our calculations indicate that both of these reactions are endothermic. For eq 1, the calculated enthalpy change is +14.8 kcal/mol; for eq 2, it is +5.9 kcal/mol. It is not surprising that the preparation of FC(O)OF is a photochemical process; UV photolysis provides more than enough energy for these reactions. Conclusion This study suggests that DFT (specifically B3LYP/6-31+G(d)) can be a very useful tool in examining a number of properties of electron-rich systems. In this study, these properties include molecular geometries, conformational preference, reaction enthalpies and enthalpies of activation, vibrational

11296 J. Phys. Chem., Vol. 100, No. 27, 1996 frequencies and assignments, and bond dissociation processes leading to various possible product radicals. Where experimental data (distances, vibrational frequencies, relative energies, and barriers) are available, the calculations reproduce these values quite well. We are therefore confident that these calculations predict reliable values for quantities which have not yet been measured, such as the unknown bond distances, vibrational frequencies, relative energies and barriers of FC(O)OOF, and the bond dissociation energies in all three molecules. Acknowledgment. Computer time for this study was made available by the Alabama Supercomputer Network. References and Notes (1) Cauble, R. L.; Cady, G. H. J. Am. Chem. Soc. 1967, 89, 5161. (2) Russo, A.; DesMarteau, D. D. Angew. Chem., Int. Ed. Engl. 1993, 105, 956. (3) (a) Argu¨ello, G. A.; Balzer-Jo¨llenbeck, G.; Ju¨licher, B.; Willner, H. Inorg. Chem. 1995, 34, 603. (b) Argu¨ello, G. A.; Ju¨licher, B.; Ulic, S. E.; Willner, H.; Casper, B.; Mack, H.-G.; Oberhammer, H. Inorg. Chem. 1995, 34, 2089. (4) Arvia, A. J.; Aymonino, P. J.; Schumacher, H. J. Z. Anorg. Allg. Chem. 1962, 316, 327. (5) Mack, H.-G.; Della Vedova, C. O.; Oberhammer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 1145. (6) (a) Ekern, S.; Illies, A.; McKee, M. L.; Peschke, M. J. Am. Chem. Soc. 1993, 115, 12510. (b) Hung, M.-L.; McKee, M. L.; Stanbury, D. M. Inorg. Chem. 1994, 33, 5108. (c) Deng, Y.; Illies, A. J.; James, M. A.; McKee, M. L.; Peschke, M. J. Am. Chem. Soc. 1995, 117, 420. (d) Webb, T. R.; Pollard, J. D.; Goodloe, G. W.; McKee, M. L. Inorg. Chim. Acta 1995, 229, 127. (e) Tang, H. R.; McKee, M. L.; Stanbury, D. M. J. Am. Chem. Soc. 1995, 117, 8967. (f) Nicovich, J. M.; Wang, S.; McKee, M. L.; Wine, P. H. J. Phys. Chem. 1996, 100, 680. (7) (a) Maricq, M. M.; Szente, J. J.; Khitrov, G. A.; Francisco, J. S. J. Chem. Phys. 1993, 98, 9522. (b) Maricq, M. M.; Szente, J. J.; Li, Z.; Francisco, J. S. J. Chem. Phys. 1993, 98, 784. (c) Cobos, C. J.; Croce, A. E.; Castellano, E. Chem. Phys. Lett. 1995, 239, 320. (d) Francisco, J. S.; Sander, S. P. Chem. Phys. Lett. 1995, 241, 33. (e) Sehested, J.; Sehested, K.; Nielsen, O. J.; Wallington, T. J. J. Phys. Chem. 1994, 98, 6731. (f) Li, Z.; Friedl, R. R.; Sander, S. P. J. Phys. Chem. 1995, 99, 13445. (g) Ventura, O. N.; Kieninger, M. Chem. Phys. Lett. 1995, 245, 488. (h) Apeloig, Y.; Albrecht, K. J. Am. Chem. Soc. 1995, 117, 9564. (i) Mo¨rs, V.; Argu¨ello, G. A.; Hoffmann, A.; Malms, W.; Ro¨th, E. P.; Zellner, R. J. Phys. Chem. 1995, 99, 15899. (j) Alleres, D. R.; Cooper, D. L.; Cunningham, T. P.; Gerratt, J.; Karadakov, P. B.; Raimondi, M. J. Chem. Soc., Faraday Trans. 1995, 91, 3357. (k) Russo, A.; DesMarteau, D. D. Inorg. Chem. 1995, 34, 6221. (l) Dibble, T. S.; Francisco, J. S. J. Phys. Chem. 1994, 98, 11694. (m) Arnold, D. W.; Bradforth, S. E.; Kim, E. H.; Neumark, D. M. J. Chem. Phys. 1995, 102, 3493. (n) Argu¨ello, G. A.; Grothe, H.; Kronberg, M.; Willner, H.; Mack, H.-G. J. Phys. Chem. 1995, 99, 17525. (8) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski,

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