;\'lATRIX
3235
REACTIONS O F FLUOROHALOMETHANES WlTH ALKALIA;\IETALS
Matrix Reactions of Fluorohalomethanes with Alkali Metals: Infrared Spectrum and Bonding in the Monofluoromethyl Radical by James I. Raymond and Lester Andrews" Chemistry Department, University of Virginia, Charlottesrille, Vcrginia 22901
(Receiaed June '7, 1971)
Publication costs borne completely by The Journal of Physical Chemistry
When CHtFBr diluted in argon is codeposited with a beam of atomic lithium at 15"K, the free radicals CHzF and CH2Br are stabilized in sufficient concentration for observation of several infrared absorptions. Studies utilizing CDzFBr and CHtFC1 help confirm the spectral identity of the monofluoromethyl free radical. The new absorptions are assigned to the symmetric C-F stretching vibration of the CHzF and CDzF radicals, and the DCD bending mode of CDzF. The vibrational potential function giving the best fit of symmetric vibrational frequencies is determined and bonding in CH2F is discussed.
Introduction The monofluoromethyl radical has been postulated as a chemical intermediate. CH2F has been produced for kinetic studies by the reactions of Ka with CH2FCl' and Br with CH3F.2 The esr spectrum3 of CH2F has been observed in krypton and xenon matrices near liquid nitrogen temperature following 2.8-MeV electron irradiation of CHSF. In this work, Fessenden and Schuler3 observed 13C hyperfine splittings which enabled them to determine the pyramidal angle (0) to be about 5" or less. TKOsubsequent4*jmolecular orbital calculations have suggested that the CHzF radical is slightly nonplanar. Recent matrix studies of the vacuum-ultraviolet photolysise of CHSF have yielded species identified as CH2F in addition to C F and HCF. Matrix reactions of alkali metal atoms with CHzClX and CH2BrX precursors in this laboratory7n8 have yielded the CHzCl and CH2Brfree radicals. To understand the bonding and structure in these radicals, further studies of CHzF have been done using the matrix reaction of CH2FX with alkali metals. A detailed description of these experiments follows.
Experimental Section The cryogenic refrigeration system, alkali atom source, and experimental technique have been described in detailss Isotopically enriched samples of lithium metal, 99.99% 'Li and 95.6% 6Li, 4.4% 4Li (ORXL), and sodium metal (J. T. Baker, lump) were used without purification. Bromofluoromethane was synthesized by the Hunsdiecker method as described by Haszeldine.'O The reaction was carried out in iron pipe (3 ft of 0.26 in. pipe connected to 3 in. of 0.5 in. pipe) equipped with a pressure gauge and a needle valve for venting off the gaseous product into a liquid nitrogen cooled trap. The silver salt precursor (silver monofluoroacetate) was prepared by dissolving sodium monofluoroacetate in ice-water and stirring in an
equimolar amount of silver nitrate. The insoluble product was separated by filtration and dried for 24 hr in vacuo. A 11.0-g (0.060 mol) sample of the silver salt was placed in the reactmionapparatus and cooled to liquid nitrogen temperature. Bromine 10.5 g (0.065 mol), was added to the salt, and the vessel was sealed and evacuated. Instantaneous reaction occurred at room temperature, the pressure rising to 160 psi. The gaseous product was vented into the receiver and purified by trap-to-trap distillation using an acetone slush. The yield was less than 10%. The bromofluoromethane-d2 was prepared by deuterating the methylenic carbon of monofluoroacetamide in a basic methanol-d solution, converting back into the silver salt, and repeating the previously described reaction sequence. Mass spectra showed the fractionated product to be a 2 : l : l mixture of the dideuterio-, monodeuterio-, and hydrogen bromofluoromethanes, respectively. Iodofluoromethane was synthesized by reacting stoichiometric quantities of methylene iodide and crystalline mercuric fluoride in a glass apparatus. The reaction mixture was heated to 150" and then recooled quickly to room temperature. The liquid residue remaining in the pot was fractionated on a vacuum line, and samples were prepared immediately from these (1) J. F . Reed and B. S . Rabinovitch, J . P h y s . Chem., 61, 598 (1957). (2) A. hl. Tarr, J. W. Coomber, and E. Whittle, Trans. Faraday Soc., 61, 1182 (1965). (3) R . W. Fessenden and R . H . Schuler, J . Chem. Phys., 43, 2704 (1965). (4) K. Morokuma, L. Pedersen, and M . Karplus, ibid., 48, 4801 (1968). (5) D . L. Beveridge, P. A. Dobosh, and J. A. Pople, ibid., 48, 4802 (1968). (6) M . E. Jacox and D. E . Milligan, ibid., 50, 3252 (1969). (7) L. Andrews and D. W. Smith, ibid., 53, 2956 (1970). (8) D. W. Smith and L. Andrews, ibid., in press. (9) L. Andrews, (bid., 48, 972 (1968). (10) R.N. Haszeldine, J . Chem. Soc., 4259 (1952). T h e Journal of Physical Chemistry, Vol. 76, N o . 21, 1971
JAMES I. RAYMOND AND LESTERANDREWS
3236 80
.-
J
20
FREOUENCY, ( ern-' 1
Figure 1. Infrared spectra in the 1340-1540-, 1100-1200-, and 350-600-cm-1 spectral region for CHsFBr in argon (M/R = 200: I ) deposited at l5'K without alkali metal, deposited with 'Li (matrix/alkali = M/A = 200: 1), deposited with 6Li (M/A = 200: I), and deposited with Na (M/A = 200: 1).
purified cuts. The infrared spectrum showed the product to be pure iodofluoromethane with small amounts of 1,2-difluoroethane and methylene iodide as the major impurities. Chlorofluoromethane (DuPont, Fluorocarbon 31), methylene fluoride (DuPont, Fluorocarbon 32), and argon (Air Products, 99.995%) were used without purification. Samples of bromofluoromethane, chlorofluoromcthane, iodofluoromethane, and methylene fluoride in argon (matrix/reactant = l I / R = 200 : 1) were codeposited with an atomic beam of lithium or sodium (matrix/alkali = hI/A = 200: 1) on a cesium iodide window maintained at 15°K. Deposition times ranged from 18 to 24 hr. Infrared spectra were recorded during and after deposition using a Beckman IR-12 filter-grating spectrophotometer in the 200-4000-~m-~ spectral region. Frequency accuracy is lrt0.5 cm-l, and spectral slit widths are 0.8 cm-I at 1100 cm-I and 900 cm-l, 0.9 cm-I at 700 cm-I, and 2.1 em-' at 500 cm-'.
Results Bromojluoromethane with Alkali Metals. The gas phase spectrum of CH2FBr, previously unreported, compared favorably with the spectrum of CHzFCl gas" and CH2FBrisolated in an argon matrix. The spectra obtained after reaction of 6Li and 'Li (M/A = 200: 1) with CHzFBr (M/R = 200: 1) were compared with the spectrum of CH2FBr deposited without the metals. The Journal of Physical Chemistry, Vol. 76, N o . 21, 1971
Several new absorptions were present after reaction with lithium which were not present in experiments without Li. Weak new absorptions occurred at 521.2, 504.2, 837.9, and 843.0 cm-1 in the 7Li experiments and at 541.4, 887.6, and 891.ri cm-I in the 6Li experiments. The 521.2- and 504.2-cm-l bands agree with argon matrix values for the 7LiBr monomer and the 541.4cm-I absorption agrees with that of 6LiBr monomer as reported by Schlick and Schncpp.12 The bands a t 837.9 and 843.0 cm-1 in the 'Li runs and the bands at 887.6 and 891.5 cm-1 in the RLiexperiments agree with the observed argon matrix frequencies for ?LiF and 6LiF, respectively.13 The intensities of the LiF and LiBr are approximately comparable in individual experiments. Although precise data are not available concerning the concentration dependence of these intensities, it appears that more bromine than fluorine was abstracted. The very fact that fluorine abstraction competes with bromine abstraction indicates that the CF bond dissociation energy in the precursor may not be as great as empirically expected. Unfortunately, precise data concerning the bond dissociation energies are not available. Other bands occurring for the Li-CHzFBr-Ar SYS(11) E. K. Plyler and M . A. Lamb, J. Res. N a t . Bur. Stand., 45, 204 (1950). (12) S. Schlick and 0. Schnepp, J. Chem. Phys., 41, 463 (1964). (13) M. J. Linevsky, ibid., 38, 658 (1963).
3237
MATRIXREACTIONS OF FLUOROHALOMETHANES WITH ALKALIMETALS -~
Table I : Infrared Absorptions Resulting from the Deposition of H2CFBr in Argon without Alkali Metal and with ELi, 'Li, and Na" Identification
P B P
CH4 P P P E
HzCFBr-no Li
HzCFRGLi
1460.0 (0.18)
1461.2 (0.089) 1355.9 (0.054) 1312.0(0.81) 1308.0 (0.089) 1277.1 (0.041) 1245 (w) 1227.2 ( 0 . 1 3 ) 1162.9 (0.33) 1159.3 (sh) 1151.6 (0.049)
1311.4(1.2)
.. 1276.3 (0.084) 1245.0 (0.057) 1227.5 ( 0 . 2 3 )
C
U U U (CHzFb P P U
1311.8 (0.68) 1306.5 ( 0 . 1 3 ) 1277.2 (0.050) 1244.6 (0.039) 1227.6 ( 0 . 1 4 ) 1163.0 (0.090)
1151.0(0.055)
1080.5 (0.21) 1059.9 (O%T) 1646.2 ( 0 . 5 1 )
1059.9 (OYOT) 1046.3 (0.88)
6 4 1 . 7 (O%T) 619.4 ( 0 . 1 9 ) ~
f393.4 (0.078) 6 4 2 . 0 (O%T) 620.3(0.080) 5 7 8 . 6 (0.087) 5 4 1 . 4 (0.072)
B
1460.0 ( 0 . 2 0 ) 1356.0 (0.044) 1311.5 ( 1 . 3 ) 1306.0 (0.066) 1277.0 (0.066) 1245 (w) 1227.9 (0.17) 1162.8 ( 0 . 3 5 )
1125.1 (0.026)
U LE
HzCFBr-Na
1460.5 ( 0 . 1 6 )
IlOO.0 (0.034)
LiF
B P P L LiBr
HZCFBr-vLi
525.5 ( 0 . 1 8 ) 368.0(0.23)
NaBr
1078.9 (0.085) 1059.9 (O%T) 1046.4 (0.33) 8 4 3 . 0 (0.067) 837.9 (0.11) 693.5 (0,067) 6 4 2 . 2 (O'%T) 619.6(0.13) 5 6 7 . 3 (0.050) 5 2 1 . 2 (0.078) 5 0 4 . 2 (0.062) 533.7 (0.050) 491.3 (0.12) 367.5(0.10)
1059.9 ( 1 . 2 ) 1046,5 ( 0 . 5 2 )
6 4 2 . 0 (O%T) 620.2(0.15)
2 7 5 . 1 (0.043)
All M/R and M/A = 2 0 0 : l . sh = a Frequencies are expressed in cm-1 and optical densities (OD) are given in parentheses. shoulder, w = weak; U = unidentified absorption; P = parent absorption.
tems are shown for selected spectral regions in Figure 1. All absorptions occurring in the 300-1500-~m-~region along with thcir identifications are listed in Table I. Parent (CH2FBr) absorptions are designated by the symbol P and unidentified absorbers by U. Several absorptions in the 500-600-cm-' region show lithium isotope shifts and are identified as L and LE. It seems plausible that the 525.5-cm-l band in the 6Liexperiment which shifts to 491.3 cm-l in the 7Li experiment is a Li-CH2X species, probably LiCH2F. R!tolecules of this type have previously been described by Andrews. l 4 * l 5 Another set of absorptions labeled L, 578.5 and 567.3 cm-l in the -6 and -7 experiments, respectively, shows a lithium isotopic shift, but their identity has not been deduced. It is clear that bands showing a lithium isotopic shift cannot bc due to an isolated free radical. The bands B, E, and C designated in the figure appeared when either ELi or 7Li (M/A = 200:l) was allowed to react with CH2FBrin argon (M/R = 200 : 1). The B absorptions, which occur at 1355.9, 693.4, 368.0 cm-' and 1356.0, 693.5, 367.5 cm-l in the -6 and -7 runs, respectively, are due to the CH2Br radical.8 The E bands appeared at 1162.9 (@Li)and 1162.8 cm-I ('Li), and the C bands a t 1151.6 (6Li) and 1151.0 cm-1
('Li). I n addition, an absorption at 1306.0 cm-1 was present in both lithium isotope experiments and is the argon matrix frequency of methane.le When CH2FBr was deposited simultaneously with Na, the band designated E was observed a t 1163.0 cm-I although the intensity mas reduced (Figure 1, Table I). Another weak absorption was present only in the Na experiments a t 275.1 cm;l and has been assigned to the argon matrix frequency of the NaBr monomer.* Three other hands exclusive to the sodium-bromofluoromethane experiment were found a t 1125.1, 1100.0, and 1080.5 cm-l and are labeled U in Table I, representing unidentified frequencies. The peaks previously noted as B and C are absent here, while the absorption attributed to methane (1306.5 cm-l) was observed. Sample-warming experiments were carried out for the 'Li- and 6Li-CH2FBr-Ar experiments; the deposited samples were warmed to 35°K and then recooled to 15°K. The intensities of the B, E, and C absorptions decreased or disappeared entirely relative (14) L. Andrews, J . Chem. Phys., 47, 4834 (1967). (15) L.Andrews and T. G . Carver, J . Phys. Chem., 72, 1743 (1968). (16) F.H.Frayer and G . E. Ewing, J . Chem. Phys., 48,781 (1968).
The Journal of Physical Chemistry, Vol, 76, No. $1, 1971
3238
JAMES I. RAYMOND AND LESTERANDREWS
1 u 2o
1200
1160
FREQUENCY,
I120
1080
(cm-ll
Figure 2. Infrared spectra in the 1080-1210-cm-~ region for the reaction of ?Li (M/A = 200: 1 ) with the following substituted methanes in argon (M/R = 200: 1 ) deposited a t 15°K: a, CHIFBr; b, CDZFBr; c, CHZFCl; and d, CHZFI.
to the unchanged parent absorptions. Owing to scattering conditions no quantitative data could be obtained. No new absorptions were observed after the diffusion experiments. Chlorojluoromethane with Alkali Metals. When CH2FCI (M/R = 200:l in argon) reacted with the two lithium isotopes, several new absorptions appeared which were common to both expriments. Figure 2 shows the 10S0-1210-cm-1 spectral region for the 7Li-CH2FC1-Ar system and Table I1 lists absorptions of interest for this reaction system, which occurred at 1390.8, 826.3, 820.4, and 396.6 cm-‘. In addition, absorptions at 1162.7 and 1149.3 cm-’ appeared which correspond to the E and C bands of the Li-CH2FBr The Journal of Physical Chemistry, Vol. 76, No. $1, 1971
experiments. Weak unidentified absorptions (labeled U) are present at 1073.2 and 682.0 cm-l for both 6Li and ?Li reactions with CH2FC1. Two other sets of medium intensity peaks occurred at 842.3 and 837.4 em-’ in the 7Li experiment and at 891.5 and 887.0 cm-l in the eLi experiment. These correspond to the reported frequencies for ?LiF and eLiF, respectively.1a No LiCl absorptions were detected due to the low yield of this reaction. When N a was deposited with a sample of CH2FCl in argon, the only new absorption that appeared was a weak band at 1162.8 em-’ (E), which is common to all of the preceding alkali metal-CH2FBr and -CH2FC1 experiments. Iodofluoromethane with Alkali Metals. When CH2FI in argon (M/R = 200: 1) was codeposited with 7Li metal (Table 11) an intense absorption appeared a t 447.5 cm-l, which is exclusive to this experiment; its assignment to 7LiI follow^.^' The E and C absorptions at 1162.7 and 1148.5 cm-l were present, but the E absorption was slightly more intense here than in the previous experiments utilizing the other halofluoromethanes as precursors. The E and C bands, at 1162.6 and 1149.2 cm-l, respectively, were also found in the spectrum of the 6Li-CH2FI-Ar system. Methylene Fluoride with Lithium. Difluoromethane in argon (M/R = 200 : 1) was deposited simultaneously with 7Li (M/A = 200: 1) for 20 hr. An analogous run was carried out using 6Li (M/A = 200: 1). No new absorptions appeared which could be attributed to lithium fluoride, the CH2F radical, or other known species. Bromofluoromethane-dz with Lithium. Several experiments were carried out depositing CD2FBr (M/R -- 200: 1) in argon without Li, and with 6Li (M/A = 200:l) and ?Li (M/A = 200:l). The spectrum of CD2FBr, though unreported in the literature, appears reasonable in comparison to the spectrum of its analog, CH2FBr. The major impurities in the series of experiments were attributed to the incomplete deuteration of the bromofluoromethane. CHzFBr and CDHFBr were present in equivalent amounts comprising a total of 50% of the starting material used to prepare the samples. Hereafter the use of the symbol CD2FBr will refer to this 2: 1: 1 mixture just discussed. Figure 2 shows the 1080-1210-~m-~spectral region for the reaction of ?Li with the CD2FBr, and Table I1 lists particular absorptions of interest for the 7Li-CD~FBr-Ar system. When CD2FBr (M/R = 200: 1) in argon was codeposited with a beam of 7Li (M/A = 200: 1))the 7LiF and 7LiBr bands were again present in addition to two major new absorptions at 1191.4 and 1093.0 cm-’, labeled El” and E2”, respectively. The band designated E in past experiments was apparent here as an (17) L. Andrews and G. C. Pimentel, J . Chem. Phys., 47,3637 (1967).
3239
MATRIXREACTIONS OF FLUOROHALOMETHANES WITH ALKALIMETALS Table 11: Infrared Absorptions in the 1080-1210-cm-1 Region Plus Other Bands of Interest for the Matrix Reaction of Lithium-7 (M/A = 200: 1) with HzCFBr, D2CFBr, H2CFCI, and H2CFI in Argon (M/R = 200: 1) at E o K a Identi-
Identification
u,
cm-1
HsCFBr 1356.0 1162.8 1151.0
( E:: 693.5 { 491.3 E:: 367,s
OD
(0.044) (0.35) (0.055 (0.067) (0.11) (0.067) (0.078) (0.062) (0.12) (0.10)
fication
u,
D&FBr* 1193.8 1191.4 1162.8 1161.2 1117.2 1112.0 (sh) 1108.7 (sh) 1093.0 1016.2
i
(Ea“) (DzCBr) (TLiF) (’LiBr ) (LE, LE‘, LE”) (B1 (DHCBr ) (DzCBr)
i
HzCFCl 1390.8 1162.7 1149.3 1126.8 1118.8 1073.2 842.3 837.4 826.3 820.4 682.0 396.6
cm-1
{ 504.2 E:; 490.0 367.0 315 4 2 262.6
OD
(0.15) (0.19) (0.10) (0.12)
(O%V (0.14) (0.040) (0.11) (0.13) (0.11) (0.23) (0.048) (0.14) (0.11)
HzCFI
{ti::
(0.074) (0.12) (0.028) (0.063) (0.023) (0.18) (0.25) (0.23) (0.23) (0.089) (0.029) (0.53)
1162.7 1148.5 1135.0 1124.2 1116.1 1108.0 1091.8 1078.5 447.5
Y are given in cm-1; optical densities (OD) and band identification are given in parentheses. tively, of DaCFBr-DHCFBr-HZCFBr.
absorption at 1161.2 cm-l with a less intense side band a t 1162.8 cm-1. One of the previously noted B absorptions (367.0 cm-l) occurred again but reduced in intensity. I n addition to El” and E,”, several bands appeared a t 1016.2, 490.0, 315 f 2, and 262.6 cm-lJ which were absent from the alkali metal-CHzFBr runs. The intense, broad 490-cm-1 absorption is labeled in Table I1 as LE, LE’, LE”. This absorption is close in frequency to the 491.3-cm-1 (LE) band noted in the ’Li-CHzFRr-Ar system, and it is tentatively assigned to the substituted methyllithium compound (LiCH2F). The corresponding broad absorption here is probably due to the three possible species, i.e., LiCH2F, LiCHDF, and LiCDzF. The 1016- and 263-cm-l bands are due to CDzBr and the 315-cm-I feature arises from CHDBr.8 When 6Li (M/A = 200: 1) was deposited with CD,FBr (M/R = 200:1), the absorptions a t 1191.4 (El”) and 1093.0 cm-l (E,”) were observed in addition to both lithium-6 halide monomers. All of the other previous new absorptions noted for the 7Li-CDzFBr-Ar system were present, with the exception of the 490.0cm-l band which was shifted to 525.0 cm-1.
(0.11)
(sh)
I, Actually
(0.36) (0.033) (0.031) (0.47) (0.034) (0.12) (0.070) (0.54) (0.20) a 2: 1: 1 mixture, respec-
Discussion The task a t hand is to identify absorptions which can be attributed to the monofluoromethyl free radical and use these data to provide information about its structure and vibrational potcntial function. Monojluoromethyl Identity. The observation of weak bands corresponding to LiBr in the Li-CH2FBr and -CD2FBr experiments indicates that the frer radicals CHzF and CDZF are likely products. The absorption designated E (1162.8 cm-I), which is found in all the experiments utilizing CHzFBr, CH2FCI, and CH2FI as precursors, shows no shift when the t a o lithium isotopes are used. The band is also observed in the two lithium-6 and -7 experiments with CDzFBr, though the intensity of the absorption is reduced by three-quarters. (It should be rccalled that the CDzFBr sample used was in actually a 2 : 1: 1 mixture, respectively, of CDzFBr, HCDFBr, and CH,FBr.) Similarly, when sodium is substituted for lithium and allowed to react with CHzFBr and CHZFCI, the 1163-cm-I (E) absorption again appears though less intensely than in the case of the lithium isotopic experiments. Thus, the absorber The Journal of Physical Chemistry,Vol. 76, N o . 21, 1071
3240 responsible for E likely contains no alkali metal atom since the position of the band is invariant with the mass of the alkali metal used in the experiment. I t seems reasonable therefore to suggest that the CHZF free radical is responsible for the absorption designated E. I n addition, the observation of an absorption at 1079 cm-1 in the 7Li-CHzFBr experiment identified as (CHzF)zis consistent with production of CHzF by the matrix reaction. Jacox and i\Iilligan,6 using vacuum ultraviolet photolysis of methyl fluoride in argon, have in fact observed a band at 1163 cm-l and assigned i t to the free radical CHzF. The weak absorptions labeled C which occur when 6Li and ’Li are deposited with CHzFBr, CHzFCl, and CHzFI appear in close proximity to the absorption assigned to CHZF. The C bands maintain approximately constant relative intensity with the radical absorptions. An absorption in this region is totally lacking in the ?;a experiments with thcse precursors. I t seems reasonable that the bands are likely dur to an LiBr-CHzF complex in the case of the CHzFBr reactant, and LiC1-CHzF in the case of CHzFCl, and an LiI-CHzF complex for thc CHzFI precursor. The small spectral shift (ca. 10 cm-l) betn een the radical fundamrntal and the complex frequency is probably due to the fact that the lithium halide perturbation is small and produces only a small change in the frequency for this motion. When CDzFBr is codeposited with Li two identical bands labeled El” and Ez” appear in each isotope experiment at 1191.4 and 1093.0 em-’, respcctively. Assignment of these absorptions to the CDZFfree radical in the present work confirms the prcvious6 choicc of assignments to C-F modes of DCF (1183 cm-’) and DzCF (1191 cm-I) based on an argon-nitrogcn matrix shift argument. Vibmtional Assignment. Thc hyperfine esr spectrum of CHZY has been interpreted by Fessenden and Schuler3 to indicate that the pyramidal angle 0 is about 5” or less. Recent molecular orbital calculat i o n ~have ~ , ~suggested that the CHzFradical is slightly nonplanar. Group thcoretical notations for the slightly nonplanar C, structurc will bc used. CHzF has six infrared active vibrational modes for thc C, or CZv structure. Thc symmetric C-H stretch vl(a’) has not been observed here, probably due to its low intensity and greater samplc scattering in the highfrequency spectral region. The symmetric HCH valence angle bending mode vz(a’) has also escaped detection in spite of the careful search in thc 1400-1600cm-1 region. The CF stretching mode va(a’) is the assignment for the intense 1162.8-cm-l feature observed hcre and previously.6 The out-of-plane bending modcs for CHzCl and CH,Br, which are believed to be planar radicals17~* are observed at 397 and 367 cm-l. If CHzF were planar, its out-of-plane mode could reasonably be expected in the 400-500-~m-~region. An intensive study of the The Journal of Physical Chemistry, Val. 76, Yo. 21, 1071
JAMES I. RAYMOND AND LESTERANDREWS 400-600-~m-~range reveals no symmetric bending mode vd(a’). I n previous studies7 of lithium matrix reactions with CHzCIBr, the LiBr product absorption is approximately as intense as the out-of-plane mode of CHzC1. I n the lithium reaction with CHzFBr, the out-of-plane mode of CHzBr is about twice as intense as the LiF product. Hence, the out-of-plane bend for a planar CHzF is reasonably expected to be at least comparable in intensity with the product LiBr absorption (0.08 OD in several CHzFBr experiments). The fact that no such band is observed for CHzF may suggest that it is slightly nonplanar which should affect the position, nature, and intensity of this normal mode. No othet features are produced by the alkali metal reaction which could be due to the antisymmetric (a”) modes of CHzF. The 940-1020-~m-~spectral region is free of absorption in these experiments. The weak band at 996 cm-1 tentatively assigned to CHzFby Jacox and Milligan6 must be due to some other photolysis product or impurity in their sample. The 1163cm-l absorption of these workers6 appears to be about 0.12 OD, a somewhat lower yield than the 0.36 OD produced in the present work. Of the two new absorptions observed following the reaction of CDzFBr with lithium atoms, the 1191-cm-’ feature is assigned to va(a’) the C-F stretch and the 1093-em-’ band is assigned to vz(a’)the D-C-D valence angle bend. This latter assignment is supported by the observation of the analogous modes7~*of CDzC1, CD2Br, and CD21 a t 1045.0, 1016.0, and 994.5 cm-l. The shift of the C-F mode to higher frequency following dwteration is due to interaction between vz and v8 whose frequency ordering reverses following deuteration. This phenomenon has been observed for difluoromethylla and fluoroform.lg No other bands are observed which can be assigned to CDZF presumably due to their low intensity relative to the bands observed here. Force Constant Calculations. One of the purposes for this work mas to determine the C F stretching force constant of CHzF. A reasonably good approximate C-F force constant can be determined using the FG matrix method with symmetry coordinates and the following assumptions. For the nonplanar radical the a’ symmetry block contains four modes. The highest frequency C-H mode is clearly separable to an excellent approximation. The lowest frequency vibration v4 is of a different symmetry if the radical is planar and is rigorously separable under this condition. However, for the slightly nonplanar radical interaction of v 4 with v 2 and v 3 should be approximately negligible due to the almost planar geometry and the expected lower frequency of v4 (which may fall between CH, (611 cm-l) and CHzCl(397cm-I). (18) T. G. Carver and L. Andrews, J . Chem. Phys., 50, 5100 (1969). (19) 1’. Galasso, G. de Alti, and G. Costa, Spectrochim. Acta, 21, 699 (1965).
MATRIX REACTIONS OF FLUOROHALOMETHANES WITH ALKALIMETALS
3241
Calculations were done using the program FADJ for the a' symmetry block using 120" bond angles, where v 4 is rigorously separable, C-H bond lengths of 1.079 8 and a C-F bond length of 1.36 8. Since the symmetric H-C-H mode of CHzCl is observed a t 1391 cm-', this provides a lower limit for v 2 of CHZF. The ratio of this mode for (CHzC1/CDzC1) is 1.332 which predicts that vz of CHzF is 1458 cm-', based on v2 of CDzF (1093 cm-l). However, this latter mode is shifted to lower frequency by interaction with v 3 of CD2F, hence a higher value of vz of CH2F is expected, perhaps near 1485 cm-l. Calculations were performed for estimated CH2F vz frequencies of 1485, 1515, 1545, 1575, and 1605 cm-'. Symmetry coordinate force constants FIX,Fu, FU were fixed a t 5.38, 0.0, and 0.0 mdyn/A as in the calculations for CHZCl.' For this range of CHzF v2 frequencies, the average difference between calculated and observed frequencies is 18.6, 12.7, 6.8, 1.0, and 6.8 cm-'. Clearly the 1605-cm-' estimate of v2 is too high since the frequency fit is poorer and frequency departure from harmonic is not in the correct direction for a cubic anharmonic term. For all of these calculations the C-F force constant varies from 6.73 to 6.46 mdyn/A. We feel that 1515 cm-l represents the best estimate of vz for CHzF. Table I11 lists these potential constants withlimits influenced by the estimate of v 2 for CH2F.
reaction with CHzFBr yielded only CHzF (0.07 OD). A contrasting piece of data is the earlier reaction of CC1,F with lithium atoms in this laboratory,20 which yielded only LiCl and CClzF with no detectible LiF or CC1, radical. The reaction of CHzClBr with lithium and sodium yielded almost completely CHzC1 and the alkali metal bromide. The lack of fluorine abstraction by lithium from CHZFZ is not surprising due to the strength of C F bonds particularly when two fluorines %rebonded to the same carbon (C-F length is 1.368 A).21 In spite of the weaker C-F bond in CClsF (C-F length is 1.40-1.44 f 0.04 8))the chlorines are apparently irresistible to lithium. I n view of these facts, it is perhaps surprising hhat lithium abstracts fluorine from CHzFCl (C-F length is 1.378 8)and CHzFBr. Using single fluorine, chlorine, and bromine carbon bond dissociation energies of 107, 80, and 67 kcal/mol and alkali metal halide bond energies from Vedeneyev, et a L j Z 2the lithium abstraction reactions of F, C1, and Br are each exothermic by 33 =k 3 kcal/mol. However, the abstraction of F by Na is less exothermic than C1 or Rr abstraction by about 12 kcal/mol, which justifies the preference of C1 or Br abstraction over F abstraction by sodium . Perhaps accurate C-F and C-C1 bond dissociation energies for the CH2FCl molecule will yield a slight thermodynamic preference for decomposition of a
Table I11 : Potential Constants for the Symmetric Block of CHzF Assuming a Planar Structure
H2C::
--Frequenoies, CHzF vz va
1615 i 30 1162.8
om-'-
CDzF
Potential constantsa
1093.0 1191.0
5.38 Fit F1a = 0.0 Fzz = 6.64 zk 0.15 Fz3 = -0.57 i 0.02 F33 = 0.76 zk 0.05 F11
= =
,F ',
a,':Li activated
complex into CHzCl
+
LiF.
This is suggested by bond lengths in CH2FC1 where C-C1 is 1.759 f 0.003 8, shorter than in CHzClz and C-F in 1.378 0.006 8, (1.7724 i 0.0005 longer than CHzFz(1.358 f 0.001 A). Also pertinent is the fact that CHzCl may exhibit more electronic stabilization (4 =k 1 kcal/mol) than CHzF ( 2 f 1 kcal/mol), which may enhance the rate of decomposition of the transition state HzCFCl. .Li into CHzCl and LiF as opposed to CHzF and LiC1. Clearly major factors affecting the decomposition of the HzCXY .AI transition state are the alkali-X (or Y )bond strengths relative to carbon-X (or Y )and the electronic stabilization of the radical CHaX (or Y). We suggest that the electronic stabilization of CHzF is about 2 kcal/mol less than CHzCl and CHzBr Fvhich is influencial in fluoride abstraction from the CHzClF and CHzBrF molecules.
*
5
a
Units: Fu, FZZmdynlh;,
FZ3
mdyn/rad,
Fa3
mdyn i / r a d 2 .
Precumor Reactivity. It is of chemical interest to compare the reactivities of the precursors CHzF2, CHzFC1, CH2FBr,and CHzFI with lithium and sodium atoms. The matrix reaction of lithium with CHzFz produced no detectible CHzF or LiF. Lithium atoms reacted preferentially with the fluorine in CHzFCl producing an intense LiF band (0.25 OD), and no detectable LiCl although some CHzF was observed in addition to an excellent yield of CHzCl. For the CHzFBr reaction with lithium atoms, comparable LiBr and LiF absorptions (0.08 OD) were observed along with CHzF and CHzBr radicals. Reaction of lithium with CHZFIyielded only LiI and CHzF. No CHZI or LiF were detected with thc CHzFI reaction. The sodium atom reaction with CHzFC1yielded only a weak (0.02 OD) CH2F band. Similarly, the sodium
--
Conclusions The potential constants of monofluoromethyl were determined for their reflections on the bonding in this free radical. (20) L. Andrew, unpublished results. (21) Bond lengths from L. E. Sutton, Chem. Soc. (London), Spec. Publ., No. 11 (1958). (22) V. I. Vedeneyev, L. V . Gurvick, V. N. Kondrat'yev, V . A. Medvedev, and Y . L. Frankevich, "Bond Energies," E. Arnold, London, 1966.
T h e Journal of Physical Chemistry, Vol. 75, LVo. 21, 1971
3242
Potential Constants. Bending Force Constants. The H-C-H symmetric bending force constant for CHzF is 0.76 0.05 X lod1‘ erg/rad2, which is slightly higher than that for CH2Cl (0.62 f 0.01) and near values for other CH, groups.’ The C-F stretch, H-C-H bending interaction force constant (-0.57 f. 0.02) X dyn/rad is somewhat higher than that for CHzCl (-0.32 3t O.O2),’ which is no surprise since these modes interact more for CHzF and CDzF due to their close proximity. The negative sign has been rationalized in terms of hybridi~ation.~ C-F Force Constant. The C-F force co9stant deduced here for CHZF(6.64 f 0.15 mdyn/A) exceeds that for CH3F (5.79 m d ~ n / A ) . Comparison ~~ to methylene fluoridez4and f l u o r ~ f o r mis~less ~ meaningful dur to the well-known strengthening of C-F bonds as the number of such bonds to a single carbon is increased from one to four. Since CHzFis nearly planar, fluorine bonded to an sp2-hybridized carbon is expected to have a higher force constant than its sp3 counterparts. The possibility of additional strengthening of the C-F bond in CHzFwill be discussed below. Bonding in Fluoromethyl Radicals. Comparison of the bond dissociation energies and ionization potentials in Table IV indicates that no electronic stabilization is present in CF3 relative to CH3, in direct contrast to the observations for cc13. I n spite of difficultieszbin calculating reliable potential functions for AX3 molecules, thp reported C-F force constantz6for CF3 is consistent with thc absence of elcctronic stabilization as discussed earlicr.18 The data of Table I V for CHFZ suggest a small amount of rlectronic stabilization; however, the C-F forcc constant18 for CHF, is not sufficiently accurate to attest this point. The bond dissociation rnergies in Table IV suggest that 1-2 kcal/mol of electronic stabilization may be attributed to CH2F. Some stabilization is indicated by the lower ionization potential of CHzF rrlative to CH3. Prrhaps this small amount of electronic stabilization in C H 2 is~ responsible for the higher C-F force constant in CHzF than in CH3F. For a planar or very nearly planar CHZF radical, the p orbitals on carbon and fluorine perpendicular to the molecular plane are of appropriate symmetry to form
The Journal of Physical Chemistry, Vol. 75, N o . 21, 1971
JAMES I. RAYMOND AND LESTERANDREWS Table IV : Ionization Potentials and Bond Dissociation Energies Involving Fluoromethyl Radicals R
CH3 CH,F CHFz CFa
DR-H, kcal/mol
104.2 =k 0.70 5 102,8b < 103.3b 106.7~0.5~
9 * 95c 9.35 9.45 10.1
a J. C. Amphlett and E. Whittle, Trans. Faraday Soc., 64, 2130 (1968). A. M. Tarr, J. W. Coomber, and E. Whittle, ibid., 61, 1182 (1965). c F. P. Lossing, P. Kebarle, and J. B. DeSousa, “Ionization Potentials of Alkyl and Halogenated Alkyl Free Radicals” in “Advances in Mass Spectroscopy,” Pergamon Press, London, 1959. See also R. S.Neale, J. Phys. Chem., 68, 143 (1964).
bonding and antibonding r-molecular orbitals. Three electrons occupy these R’I.O.’s, two are bonding and one is antibonding, giving a net of one n-bonding electron for the C-F bond in addition to the CT bond. Such n bonding probably occurs in the isoeoectronic OF radical whose force constant (5.40 mdyn/A) is higher than in OF2 (4.0 m d y ~ ~ / A )Hence, . ~ ~ K bonding in CHzF could account for the electronic stabilization and increased C-F force constant. Clearly, departure from planarity decreases overlap of fluorine and carbon 2p orbitals for a fluoromethyl radical. Hence, the pyramidal8 CF3 radical (F-C-F = 111’) cannot be effectively stabilized by K bonding. Furthermore, the higher electronegativity of fluorine may reduce its participation in K bonding to a free radical carbon 2p orbital.
Acknowledgments. The authors gratefully acknowledge financial support for this research by the National Science Foundation under Grant GP-8687. J. I. R. acknowledges fellowship support by the National Defense Education Act. (23) J. Aldous and I. M. AIills, Spectrochim. Acta, 19, 1567 (1963). (24) A . G. Meister, J. M. Dowling, and A. J. Bielechi, J. Chem. Phys., 25, 941 (1956). (25) I , w. Levin, {bid., 47, 4685 (1967). (26) D. E. Milligan and M. E. Jacox, ibid., 48, 2265 (1968). (27) L. Andrews and J. I. Raymond, ibid., in press.
