J. Phys. Chem. 1995,99, 17525-17531
17525
IR and Visible Absorption Spectrum of the Fluoroformyloxyl Radical, FCOZ, Isolated in Inert Gas Matrices Gustavo A. ArgiielloJ Hinrich Grothe, Marc Kronberg, and Helge Willner* Institut f i r Anorganische Chemie der Uniuersitat Hannover, Callinstrasse 9, 0-30167 Hannover, G e m n y
Hans-Georg Mack Institut f i r Physikalische und Theoretische Chemie der Uniuersitat Tiibingen, Auf der Morgenstelle 8, 0-72076 Tiibingen, Germany Received: September 14, 1994; In Final Form: April 18, 1995@
Vacuum flash pyrolysis of bis(fluoroformy1) peroxide, FC(O)OOC(O)F, diluted in N2, Ar,or Ne, yields on subsequent quenching of the products in a matrix at 10, 14, or 6 K, respectively, the fluoroformyloxyl radical, FCOi, in the X2B2 ground state. It has been possible to record its infrared spectrum and to identify all bands by their photochemical behavior for the first time. The assignment of the six fundamental vibrations on the basis of C2" symmetry (a, 1475, 960, 519; bl 1098,474, b2 735 cm-I; Ne matrix) was achieved by a normal coordinate analysis taking into account additional data of I8O- or I3C-labeled FCOi. The experimental fundamental vibrations and the structural parameters derived from the vibrational data are compared to the results of ab initio and DFT (density functional theory) calculations. The visible spectrum of F C O i isolated in a neon matrix agrees with the known gas phase spectrum. The origin of the B2A1 X2B2 transition is observed at 13 103 cm-I, and four fundamentals at 1607, 1117, 839, and 611 cm-' are detected due to the excited state species (B2A1). In a fast reaction with NO, the FCO2' radical quantitatively forms FNO and C02. The anticipated intermediate FC(0)ONO is not observed.
-
Introduction Until very recently, studies concerned with the decrease of stratospheric ozone were mainly restricted to reactions where C1 and Br atoms played the principal role. The chemistry of other fragments formed by the photolysis of CFCs remained almost unexplored until it became necessary to find CFC substitutes. In our view, it is desirable to understand their potential detrimental effects on the atmosphere before starting the large scale use of these substitutes. This prompted many laboratories to study oxidation and general degradation mechanisms of various CFCs and HFCs. It is established that stable FC(0)X molecules (X = F, C1, H) are formed by oxidation of halogenated methyl and ethyl radicals in the stratosphere. In fact, the FC(0)F concentration in the stratosphere has been measured, and an increase with time was These species can be photolyzed again in the stratosphere to give FCO' radicals. The formation of these FCO' radicals is not restricted to the stratosphere, and their tropospheric existence is due to chemical attack of OH radicals on FC(O)H, which has been shown to be a degradation product of HFC 134a.3 Information on the physical and spectroscopic properties and on the chemical reactivity of the FC(0)X species is, no doubt, very important for the understanding of atmospheric reactions, particularly, the reaction sequence that originates with the formation of the FCO' radical, viz., FC(0)X followed by
+ hv - FCO' + X (X = H, F, Cl) FCO' + FCO' - FC(0)F + CO FCO' + 0, - FC(0)OO'
(1) (2)
(3) along with the recently reported reaction rate constants and
'
Alexander von Humboldt fellow. Permanent address: INFIQC Departamento de Fisico-Qufmica, Facultad de Ciencias Quimicas, Universidad Nacional de C6rdoba. C.C. 61, SUC.16. 5016 Chdoba, Argentina. Abstract published in Advance ACS Abstracts, July 15, 1995. @
relative concentrations in the which strongly favors the formation of the fluoroformylperoxyl radical, FC(0)OO'. This radical can further react with other atmospheric constituents, such as NO, according to FC(0)OO'
+ NO -FCO,' + NO,
(4)
Very recently5 the rate coefficient for (4) was determined as 2.5 x 10" cm3 molecule-1 s-I. This leaves no doubt that, among the FCO, radicals, FC02' will be one product in the stratosphere. A catalytic cycle for ozone consumption due to the FCO2' radical was po~tulated,~.~ and the first experimental rate constant5 for the reaction FCO,'
+ 0, - products
(5)
imposes an upper limit of 6 x cm3 molecule-' s-' for k5. It was demonstrated very recently that this value is even lower.'O The dissociation energy of the CF bond in FC02' was calculated to be in the range 16.7-20.9 kcal mol-', and kinetic measurements suggested a lifetime on the order of 3 s (298 K) with respect to its unimolecular decomposition.'' From the spectroscopic point of view, little is known about the FCOi radical. The visible absorption spectrum exhibits interesting vibrational fine structure and is assigned to the B2A1 X2B2 transition.I2 Four vibrations in the excited electronic state and one fundamental mode at 520 cm-' in the ground state are observed. Geometric structures and the corresponding vibrational frequencies for the ground and the excited state are obtained by ab initio calculations.I2 Very recently from a photodetachment study of FC02- the heat of formation AfW298(FC02) = -85.2 f 2.8 kcal mol-' and three ground state vibrations 1465(70), 950(70), and 500(70) cm-I have been determined.35 In this study FCO2' radicals and the products of their reaction with NO are trapped in inert gas matrices. This allows us to c o n f i i the visible spectrum of FC02' and to record its complete infrared spectrum for the first time.
-
0022-3654/95/2099-17525$09.00/0 0 1995 American Chemical Society
Arguello et al.
17526 J. Phys. Chem., Vol. 99, No. 49, 1995
TABLE 1: Vibrational Wavenumbers (cm-') of FCOz', FC'*Oz', and F13CO$ Isolated in Ne, Ar, and N2 Matrices FC02' 3472.6 3393.2 3753.3 2597.5 2515.5 2233.0 21 16.2 1962.3 1917.3 1612.1 1488.5 1452.7 1477.3 1097.7 968.9 954.3 943.7 735.2 518.5 474.3 a
1 }
Ar matrix
Ne matrix I" FC"02'
Fi3CO2*
FC02'
FCi802'
1.3 5.3 2.6 0.9 2.1 8.3 1.2 2.4
3412.7 3320.9 2703.5 2559.8 2462.2 2185.7 2086.9 1928.3
3499.0 3427.6 2786.9 2627.5 2536.3 2262.1 2139.7 1968.7 1929.4 1632.7 1494.5 1477.2 1451.3 11 13.6 975.1 969.4 957.8 730.6 521.6 482.5
3419.2 3349.4 2718.4 2555.9 2484.2 2208.4 2079.7 1921.7 1898.2 1591.0 1461.4 1429.8
6.1 100 46.8 41.5 9.3 9.3 9.1
3389.9 3312.0 2684.5 5230.7 2462.2 2179.0 2056.3 1911.9 1885.6 1571.1 1425.1 1443.5 1465.7 1070.2 942.6 925.2 922.2 729.5 497.3 463.5
1585.3 1478.7 1440.4 1415.1 1076.2 964.2 944.9 933.9 712.0 516.6 468.8
Nz matrix FC02' FC"02'
FI3CO2' 3353.4 2735.6 2590.1 248 1.4 2213.2 2109.4 1955.0 1913.7 1604.6
3503.6 3432.7 2789.8 262 1.7 2542.8 2265.9 2133.7
3424.4 3354.7 2724.3 2549.6 2490.9 2212.2 2076.0
1632.4 1496.6 1478.1 1449.2 11 16.5 975.5 971.6 953.8 728.5 522.8 481.3
1462.6 1431.4 1088.9 948.9 943.9 929.7 722.8 502.1 469.8
1442.7 1407.4 1091.4 969.9 960.6 949.3 707.6 519.7 477.3
1186.1 948.9 940.2 935.6 724.8 500.7 471.3
assignment CzUsym ~ v I + v ' ?AI
Integrated relative infrared intensites I ( v I + ( v ~ + v ~ ) += ~ v100. ~)
Results and Discussion Fluoroformyloxyl radicals, FCO2*,were generated by vacuum flash pyrolysis of the precursor FC(O)OOC(O)F diluted in inert gases (Nz, Ar, Ne) and stabilized in the respective matrices after quenching the products at low temperatures. At a pyrolysis temperature of 250 "C about 90% of the precursor was converted into FCO2' and some C02. The deep blue colored matrices were investigated by infrared and visible absorption spectroscopy. The Infrared Spectrum of FCOz'. A typical infrared spectrum of FCO2*isolated in a N2 matrix is presented in Figure 1. The figure shows a difference spectrum of the pyrolysis products of FC(O)OOC(O)F before and after photolysis. In this way only the bands of the photolabile FCOi radical and of its photolysis product, C02, are present. Red light of the wavelength >620 nm is sufficient to bleach the matrix until it is colorless. The negative bands at 662.5 and 2348 cm-' (not shown here) clearly demonstrate that COZis the sole IR active photolysis product of FCOZ. One FCOi radical forms one C02 molecule; therefore, the relative band intensities of both species can be compared: Z(C02, 662.5 cm-'):Z(FCO2', 728.5 cm-') = 1.44:l. Similar spectra are recorded using Ne and Ar as matrix materials and FC(O)OOC(O)F fully labeled with I80and I3C. All observed band positions, together with their relative integrated intensities and assignments, assuming C2" symmetry are collected in Table 1. Corresponding to the selection rule for the CzVpoint group,
rvib = 3a,(IR, Rap) + 2b,(IR, Ra dp) + b,(IR, Ra dp) all six fundamentals will be IR and Raman active. Four single sharp bands at 481.3, 523.8, 729.5, and 1116.5 cm-' and the two groups of bands at around 970 and 1450 cm-' are attributed to the six fundamental modes of the FCOz' radical isolated in a NZ matrix. No further band below 480 cm-' is observed, and all bands at higher wavenumbers are of low intensity. Comparison of experimental and theoretical l6'I8O and I2/l3Cisotopic shifts allows the unambiguous assignment of all bands. To calculate the isotopic shifts with the normal coordinate program NCA,I3 structural parameters for the FC02' radical are needed as input data. Ne matrix data were used throughout because they usually resemble gas phase wavenumbers. In Table 2 geometric parameters as obtained by various ab initio
1
1500
1
~
'
1300
1
'
(
~
1
*
1100 900 wavenumber cm"
~
'
1
700
I'
7
'
1
500
Figure 1. Difference IR spectrum of FCOI' and its photolysis products COz
+ F isolated in a Nz matrix.
TABLE 2: Structural Parameters (pm, deg) for FC02' r(C0) r(CF) 120" would be expected. The "experimental" bond angle and estimated bond lengths derived from force constants (discussed later) are also collected in Table 2. The bands at 518.5 and 474.3 cm-' must belong to the lowest al(v3) or bi(~5)mode, and again it is possible to distinguish them by their isotopic shifts. For the OCO bending mode v3 a large 16/180 and a small l2/l3C isotopic shift are expected, whereas for the OCO rocking mode v5, the shifts should be smaller and larger, respectively, as observations prove 31'( = 518.5 and v5 = 474.3 cm-I). The next highest unassigned groups of bands around 970 cm-] must correspond to a stretching mode. This spectral region is expanded and shown in Figure 3. For this stretching vibration as well as others at 1450 and 1100 cm-', it is important to note that the band positions are red-shifted when going from a N2 to a Ne matrix; the opposite is generally true for most molecules. The complex band pattern, presented in Figure 3, depends on matrix material and isotopomers and must be the result of an anharmonic resonance of the fundamental with an overtone. The weakest of the three bands in each spectrum is assigned to 2v5 (AI)
is in accordance with the assignment v4 = 1097.7 and v5 = 474 cm-', because the experimental wavenumber ratios for l6/I8O,1.0496, and I2/l3C,1.0319, are in excellent agreement with calculated ratios of 1.0494 and 1.0326, respectively. (ii) The complex band pattern in the region around 1475 cm-' is probably due to Fermi resonance of three a1 modes (vl(a1) with the A1 modes ( ~ 2 + ~ 3and ) 2%) rather than of two bl modes (v4(bl) being in Fermi resonance with v2fv5 (BI)), which would result in a simpler spectral pattern. (iii) The "best" theoretical result (B3P86/3-21G, see below) for infrared band positions, intensities, and isotopic shifts is in full agreement with this assignment (Table 3). The remaining bands of the spectrum in the region 36001550 cm-' are assigned on the basis of the above-mentioned band positions of vl-vg and their isotopic shifts. Excellent agreement between observed and calculated band positions and isotopic shifts exists only for the strongest combination v3+v4 and 2 ~ 4 .All further bands are more or less perturbed like V I and ~ 2 . The General Valence Force Field of FC02'. Using the program NCAI3 with an ab initio force field as starting parameters (Table 4), with the structural parameters from Table 2, column "a", and the vibrational data collected in Table 3, the 10 inner coordinate force constants are calculated. The results are given in Table 4 together with the potential energy distribution. By this force field all vibrational data are reproduced within their experimental uncertainties. It is interesting to compare the bonding properties of FCO2' with those of CF2 and FCO'. In FCOi the CO/CO' interaction force constant is unusually large, which is due to the rare case that v1(v,CO2) is observed at higher wavenumbers than v4(vasC02)and causes an unusual potential energy distribution for v4 and ~ 5 . The same situation exists for CF2 (VI = 1225.0843, v3 = 1114.4426 cm-'),I4 and the calculated force constants using l2/l3Cisotopic shifts aref, = 6.1O,fr = 1.52,fa = 2.32,La = 0.66 lo2 N m-I . I 5 The CF bond strengths of CF2 and FCO2' are very similar, although in FC02' the CF bond dissociation is very low energy (20 kJ mol-')". In the FCO' radical the CF bond (4.9 lo2 N m-I)l6 is weaker and the CO bond (14.4 lo2 N m-I)l6 is stronger than the respective bonds in FC02'. This demonstrates that the single antibonding electron in FCO' is more localized in the CF, and in FCO2* in the C02 bonding region. Force constant-bond length correlations of CO and CF bonds are presented in Figure 4. The slope for CO bonds is based on
17528 J. Phys. Chem., Vol. 99, No. 49, 1995
Arguello et al.
TABLE 3: Fundamental Wavenumbers (cm-l) and Their Isotopic Shifts for FC02' DFT"
Ne matrix 1475 ( 100)b 960 (42) 519 (9) 1098 (47) 474 (9) 735 (9) a
AV('~/~~O)
Av(l2/l3C)
2 0 f 10 27.5 f 1 21.2 f 0.1 27.5 f 0.1 10.8 f 0.1 5.68
35 f 10 7 f 2 1.9 f 0.1 21.5 f 0.1 5.5 f 0.1 23.14
Av( l6/I8O) 1494 (235) 983 (44) 521 (17) 1047 (175) 491 (10) 731 (37)
34 30 20 25 12 6
Av(I~/'~C) 36 4
9 23 5 24
assignment V I al v2 al v3 a1 v4 bl VJ bi V6 b2
B3P86/3-21G, intensities (km mol-') in parentheses, unscaled values. Relative integrated intensities.
TABLE 4: Force Constants (103 N m-l, Normalized on 100 pm Bond Length) and Potential Energy Distribution (PED) of FCOi force constants exutl 6.09 8.28 fWP 0.565 foCF 1.70 fctico 0.94 2.79 fco/co, fCWoCF 0.52 ~ C O / ~ C F 0.55 fCO/oCF -0.85 foCF/OCF 0.81 fCF
fco
PED
calc"
vz
VI
v3
v4
v5
v6
6.53 0.350 0.688 7.77 0.616 0.149 2.090 0.439 0.62 1.Ooo 0.602 0.210 2.996 1.70 0.66 -0.174 0.120 -0.750 -0.148 3.25 0.208 0.64 -0.194 -0.338 0.23 -0.301 -0.521 -0.99 0.287 -1.431 0.81
B3LYP/3-21 G.
--I\
20
l8I\
16
iio
ii5
iio
lis
130
135
bond length (pm)
Figure 4. CF and CO force constants as a function of their respective bond lengths in different compounds: (a) FCN, refs 18 and 19; (b) FC, ref 20; (c) CF2, refs 15 and 21; (d) CF4,refs 22 and 23; (e) CF3C1, ref 24; (f) CF3Br, ref 24; (g) FCO, ref 16. 40 individual data points.I7 For CF bonds a similar slope is not available, but a literature search demonstrates that 2-foldcoordinated CF compounds give a good correlation (filled points, Figure 4). The tetrahedral molecules CF4, CF3C1, and CF3Br (open points, Figure 4) do not fit on this slope. From these slopes the CO and CF bond lengths (Table 2) are estimated to be rco = 128.8 and rCF = 130.2 pm, respectively. In comparison with the ab initio results, these CO and CF bond lengths are too long and too short, respectively. This may be due to unusual nonbonding forces between 0/0and O/F leading to CF and CO force constants which do not correlate to their respective bond length in the usual manner. Determination of the FCO2' molecular structure in the gas phase is highly desirable. Theoretical Calculations. For small molecules, high-level ab initio calculations can predict reliable geometric parameters, vibrational frequencies, etc. But in the case of the open shell FC02' species, the published results for the vibrational data
(UMP2/6-31G*)lZare unreliable and not usable in the assignment of the experimental infrared spectrum. To find the appropriate theoretical method which reproduces the experimental data best, various quantum chemical models are tested. All calculations are performed with the GAUSSIAN 92/DFT programz5 and a Convex C3860 computer (ZDV, Universitat of Tubingen, Germany). The ab initio LCAO-MO methodz6 (in the UHF and OCSD(T) approximation) and DFT (density functional theory),27applying 3-21G and 6-31 lG* basis sets, are used for the study of the title compound. In the case of the DFT calculations, the following approaches are employed. (i) B3P86: hybrid method, which includes a mixture of Hartree-Fcck exchange with DFT exchange-correlation, Becke's three-parameter functionalz8combined with the nonlocal (gradient-corrected) correlation potential provided by the Perdew 86 expre~sion.~~ (ii) B3LYP: hybrid method, Becke's three-parameter functional with the nonlocal correlation functional of Lee, Yang, and Parr. (iii) SVWN: local spin density approximation (LSDA), Slater exchange functional3" with the Vosko, Wilk, and Nusair correlation fun~tional.3~ (iv) BLYP: LSDA combined with the nonlocal functionals of Becke (exchange)32and of Lee, Yang, and Parr (correlation). For all geometry optimizations, CZ,symmetry is assumed for FC02'. Some of the predicted structures are collected in Table 2 and are discussed above. In Table 5 vibrational frequencies (unscaled values) and intensities as obtained by various theoretical approximations are presented along with the experimental data. The best overall agreement with experiment is obtained by the B3P863-21G calculations. All four DFT hybrid methods (as well as the BLYP/6-31lG* approach) reproduce the observed sequence of frequencies and the corresponding assignment. The BLYP/321G calculations predict the wavenumbers for the OCO rocking mode bl(Y5) (486 cm-I; expt 474 cm-I) to be higher than that of the OCO bending vibration a~(vg)(480 cm-I; expt 519 cm-I). Analogous results for these two vibrations are obtained by the LSDA approximations (SVWN/3-21G, SVWN/6-311G*) and by the ab initio methods (UHF/3-21G, UHF/6-311G*, UMP2/ 6-31G*). Even more striking, the latter five approaches also predict the symmetric CO stretching vibration v~(al)(vsC02) to be lower in frequency than the asymmetric CO stretch Y4(bl) (vasC02). This is in contrast to the experimental observations ( V I = 1475 and v4 = 1097.7 cm-I) and to the predictions of the DFT (nonlocal and hybrid) calculations. The ab initio approaches result in extremely bad values for the wavenumber of the asymmetric CO stretching mode (expt v4 = 1097.7 cm-I; UMP2/6-31 lG* v4 = 3401 cm-I), and, compared to experiment and DFT calculations, they predict vd(b1) to be more intense than vl(al). The Visible Spectrum of FCOz'. The visible spectrum of FCOi isolated in a neon matrix is presented in Figure 5. It is in good agreement with the gas phase spectrum recorded by
TABLE 6: Transition Energies (em-') for Various Progression from the Origin to v3 = 10 of FCOz', F18COt*, and F13C02' Isolated in a Ne Matrix
lg (l,N
0,s -
assignment
0 4-
0302
v2
v3
v6
obs
0
0 0 0
0
0 0 1 0 0 1 0 0 0
13 103 13717 13941 14222 14327 14546 14710 14824 14939
4 3 2 3 5 4 3 4 6 5 4 5 7 6 5 6 8 7 6 7 9
0 1 0 0 0 1 0 0 0 1 0 0 0 1 0 0 0 1
8
1
7
0 0 0
15 549 15 751 15 912 16 029 16 165 16 355 16 520 16 633 16 776 16 965 17 116 17 236 17 390 17 585 17 716 17 852 17999 18 190 18320 18457 18618 18800 18923 19067 19227 19408 19524 19675
0 0 0 0 0
-
0,1 -
0,o -
Maricq et a l . I 2 However, due to matrix effects, two features are different: (i) the vibrational bands are split by about 30 cm-I; (ii) a spectrum simpler than the gas phase spectrum is obtained (that is, without hot bands, because in a Ne matrix only ground state radicals are involved). Table 6 collects the mean band centers of FCOi, FCI802', and FI3CO2' along with their assignments. A vibrational mode at 2820 cm-' in the excited state mentioned in ref 12 seems to be an artifact because the highest fundamental vibration in the ground state possesses a frequency of 1475 cm-I. Indeed, in our spectrum, the corresponding vibration (1607 cm-I) is observed as a shoulder at 14 710 cm-] (better resolved for FI3CO2, not shown here). The observed transition energies are calculated using the secondorder expression
i
+ +
i kzi
where w? = oI xII 'IzCI&k and x l k o = Xlk in secondorder appr~ximation.~~ A preliminary assignment of the transition is achieved on the basis of the experimental band intensities of the respective progressions. Finally, the parameters YOO, wl, and Xtk are simultaneously fitted decreasing the weight factors for higher quantum vI numbers until the deviations between observed and calculated transitions were < l o cm-l. Table 7 shows the resulting vibrational fundamental frequencies and anharmonicity constants. The anharmonicity constants xIIshould be viewed with caution, except for ~ 3 3 which , is indeed very small. The uncertainty of the vibrational wavenumbers is estimated to be f 5 cm-l, except for the 1600 cm-' band, where it is assumed to be f 1 0 cm-I. Going from the gas phase to neon matrix the band origin (13 150 cm-I)l2 is slightly red-shifted (47 cm-I), and the difference in YOObetween FC02*/FC'*02' and FC02*/F13C02' may be due to the difference in the zero-
FC02'
VI
1
1
0 0 0
0 0
1 0
0 0
0 0
1 0 0
0 1 0
0
0
1 0 0
0 1 0
0
0
1 0
0 1
0 0
0 0
1 0
0 1
0 0 1 0 0 0
0 0 0 1 0 0 0
1 0
1
0 0 2 1 0 1 3
1
8
1
0 0 0
0
1
10 9 8 9
0 0
0 0 0
1
0 0
FCI802' a
0
obs
FI3C02' a
0
0 0 -1
13093 13686 13932 14 162 14276 14514 14600 14746 14870
4 -4 -6
1 9
15 456 15 681
0 0
-3 0 -2 -1 1
-3 0
0 -3
0
0 5 -2 12 -7 7 0 9 -1 10
0 -3 1
0 5
0 0 2
0 -1
0 -1 6
15 902 16 049 16 265 16 350 16 492 16 649 16 845 16 926 17 069 17 228 17 438 17 511 17 654 17832 18029 18092 18242 18428 18612 18675 18833 19017
3
0 1 -6 -4 -6 6
0 2 9 -1 -2 1 0 -5
0 -2 0
0 1 -7 8
0 2 -1
19258 19419
3 -7
obs
a
13084 13686 19908 14 189 14298 14506 14685 14791 14910
0 5 -2 0 0 1 0 -1 -4
15515 15703 15877 15991 16123 16303 16480 16589 16740 16911 17071 17 192 17336 17523 17669 17803 17945 18 134 18269 18411 18561 18739 18867 19013 19 177 19335 19467 19624
-1 7 0 2 0 10 -6 7 -7 5 0 7 7 -3 0 0 9 -10 -1 -3 4 -10 0 0 0 0 0 -6
Calc - obs.
point vibrations. The assignment of the four fundamental vibrations is straightforward comparing the isotopic shifts to those of the corresponding fundamentals in the ground state. For example, the main progression (611 cm-I) is due to the OCO scissor mode w3O, because it shows a large 16'180 and a small 12'13Cisotopic shift. It is interesting to note that in all observed modes mainly the CO2 group is involved, which exhibits an increase in bond strength upon excitation. The Reaction of FCO$ with NO. The kinetics and
Arguello et al.
17530 J. Phys. Chem., Vol. 99, No. 49, 1995
TABLE 7: Wavenumbers (cm-l) and Anharmonicity Constants of the Fundamentals of FC02', FC1*02*,and F13C02' in the Excited B2At State
Ne matrix'
FCO2 16OOb 1110 610 840
FC"02
FCO2
FI3CO2
1511 (1521) 1069 (1078) 589 (602) 839 (846)
1607 (1612) 1117 (1120) 61 1 (622) 839 (840)
1601 (1612) 1105 (1110) 606 (617) 822 (828)
anharmonic constants
assignment a1
XI1
= 0.0
m0al
XI2
= -0.0
w0a]
X13
= -10.5
W0b2
XI6
= -5.3
X22
= 0.0
X23
226
= -6.8
~ 3 = 3 0.2
= 0.4 X 3 6 = -5.3
X66
= -0.3
Gas phase, ref 12. Literature value 2820 cm-', reassigned. w values in parentheses, derived from fit.
mechanisms of FC02' radical reactions are the subject of intense research, and the development of a sound data base is currently persued in several laboratories. Reliable information is so far scarce. There are, to the best of our knowledge, only two experimental determinations of the rate constant for the fast reaction between FCOi and N0.5.'0 In both studies no attempts are made either to look for the possible FC(0)ONO intermediate (which according to ab initio calculations is believed to be highly unstable34)or to search for reaction products. Mors et a l . I o have followed the extinction of FCOi radicals by time-resolved laser absorption measurements, thus precluding the determination of any other species. Wallington et aL5 have fitted their timeresolved visible spectra with a mechanism involving, among others, the reaction between FCOi and NO, for which it is assumed that the only products were FNO and C02. To study this reaction in more detail, mixtures of FC(0)OOC(0)F and NO, diluted in Ar,were vacuum flash pyrolized and subsequently quenched in a matrix. In the resulting infrared spectra, the only new detectable products were FNO, CO2, and FCO2'. This confirms the conclusion that the FC(0)ONO intermediate is very short-lived under our experimental conditions and dissociates into FNO and C02. Conclusion Thermal decomposition of FC(O)OOC(O)F appears to proceed in two steps: FC(O)OOC(O)F FCO,'
-
F
-
2FC0,'
(6)
+ CO,
(7) Under proper experimental conditions, it has been possible to stabilize FCOi radicals in an inert gas matrix in high yield. When the pyrolysis is carried out at higher temperatures, the concentration of C02 increases, while the FCOi concentration decreases. This implies that the activation energy of reaction 7 is on the same order as that for reaction 6. From the infrared spectra of normal and isotopic-enriched FCO2' radicals it has been possible to assign the six fundamental vibrations and to deduce structural parameters. The vibrational assignment and the bonding description are supported by ab inito and DFT calculations. The visible spectrum shows vibrational fine structure with the main progression of 61 1 cm-I and three additional fundamentals in the excited, electronic state. Its high absorption cross section is responsible for the fast photolysis of FC02' into C02 and F atoms upon irradiation with red light. Hence, only dark reactions of the FCO2' radical can play a role in atmospheric chemistry. Experimental Section CAUTION: The reactant mixture for the synthesis of FC(O)OOC(O)F and the peroxide itself is potentially explosive,
Figure 6. Sectional view of the matrix cryostat. (a) Inlet tube with heated end orifice. (b) Matrix support, copper with two mirror surfaces. The whole matrix support can be turned through 360". Deposition of the matrix sample, 0"; recording of IR matrix spectra, 180": photolysis of the matrices, 270"; recording of UV-vis spectra, 90". (c) Quartz glass fiber. especially in the presence of oxidizable materials, and should be handled with proper safety precautions and only in millimolar quantities ! For the synthesis of FC(O)OOC(O)F and its isotopic labeled species an evacuated 250 mL glass bulb was filled with a mixture of 50 mbar CO (or I3CO, CI8O > 99%, Ventron) and 80 mbar 0 2 (or 1 8 0 2 > 99%, Ventron). Into this mixture 55 mbar F2 was slowly introduced within 5 min. After a reaction time of 1 h the bulb was cooled to -196 "C and the excess of starting material was pumped off. The evaporated residue of the bulb was passed in vacuum through a series of traps held at -70, -120, and -196 "C, and pure FC(O)OOC(O)F was collected in the -120 "C trap. The experimental setup for matrix isolation of FCOi radicals is shown in Figure 6. Prior to use, the vacuum line and transfer capillary were treated with FC(O)OOC(O)F to passivate the system and to remove traces of water. Mixtures of FC(0)OOC(0)F with N2, Ar, or Ne (1:1000),FC(O)OOC(O)F/NO/Ar (1:10:1000), and FC(O)OOC(O)F/NO (1:lOOO) were prepared in a stainless steel high-vacuum line and transferred to the heated tube (a) via a stainless steel capillary. Usually, a gas flow rate of 3 mmol h-I was set, and the mixture passed the heated zone (typically 250 "C) of the 6 mm diameter quartz glass tube through the end orifice of 1 mm. A residence time of about 10 ms inside the heated volume (0.1 mL) could be estimated. The molecular beam was quenched as a N2, Ar, or Ne matrix at 10, 14, or 6 K, respectively. After rotation of the matrix support IR or UV-visible spectra could be taken or photolysis of the matrix sample could be carried out. IR spectra were recorded with a Bruker IFS 66v FTIR spectrometer in the range 5000-400 cm-l; 128 scans were co-
Fluoroformyloxyl Radical Isolated in Inert Gas Matrices added for each spectrum using an apodized resolution of 1 cm-’. Visible spectra were recorded with a 1024 diodide array spectrometer (SI) equipped with a tungsten-halogen lamp. Measurements with a 300 grooves mm-’ grating allowed a spectral resolution of 0.8 nm. Ne, Kr, and Hg pen lamps (Oriel) were used for wavelength calibration. Photolysis experiments were done using a tungsten-halogen lamp in combination with various cutoff filters (Schott).
Acknowledgment. G. A. Arguello is indebted to the Alexander von Humboldt Stiftung for a fellowship that he held while the experimental work was carried out. Financial support by the Deutsche Forschungsgemeinschaft and by the Fonds der Chemischen Industrie is gratefully acknowledged. References and Notes (1) Rinsland, C. P.; Zander, R.; Brown, L. R.; Farmer, C. B.; Park, J. H.; Norton, R. H.; Russell, J. M.; Raper, 0. F. Geophys. Res. Lett. 1986, 13, 769. (2) Wilson, S.R.; Crutzen, P. J.; Schuster, G. D.; Griffith, W. T.; Helas, G. Nature 1988, 334, 689. (3) Tuazon, E. C.; Atkinson, R. J . Atmos. Chem. 1993, 16, 301. (4) Matti Maricq, M.; Szente, J. J.; Kitrov, G. A,; Francisco, J. S. Chem. Phys. Lett. 1992, 199, 7 1. ( 5 ) Wallineton. T. J.: Ellermann. T.: Nielsen. 0. J.: Sehested. J. J . Phvs. Chem.‘ 1994, 91s, 2346. (6) Matti Marica. M.; Szente, J. J.; Kitrov, G. A,; Francisco, G. S. J . Chem.’ Phys. 1993, 68, 9522. (7) Behr, P.; Goldbach, K.; Heydtmann, H. Int. J . Chem. Kinet. 1993, 25, 957. (8) Francisco, J. S.;Goldstein, A. N. Chem. Phys. 1988, 127, 73. (9) Francisco, J. S.; Goldstein, A. N.; Li, Z.; Chao, Y.; Williams, I. H. J . Phys. Chem. 1990, 94, 4791. (10) Mors, V.; Hoffmann, A.; Argiiello, G. A.; Zellner, R. To be published. (111 Wallinaton, T. J.: Hurley. M. D.; Matti Marica, M. Chem. Phys. Lett. 1993, 205; 62. (12) Matti Maricq, M.; Szente, J. J.; Li, Z.; Francisco, J. S. J . Chem. Phys. 1993, 98, 784.
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