J. Phys. Chem. 1992,96, 2051-2059 a purely hydrogen-bonded chain but a more complex liquidlike structure.
Acknowledgment. Acknowledgment is made to the Thomas F. and Kate Miller Jeffress Memorial Trust and to the GrantsIn-Aid program for faculty of Virginia Commonwealth University for partial support of this research. We thank K. E. Shriver for
2051
assistance in the design and construction of the molecular beam machine. L. W. Sieck was supported by the Division of Chemical Science, Energy, Office of Basic Energy Sciences, U S . Department of Registry No. C H 3 0 H , 67-56-1; CD30D, 811-98-3; CD30H, 184929-2.
Photochemistry of H20--F2 Complexes In Solid Argon. Infrared Spectra of HF- -HOF, HO- -HF, and FO- -HF Complexest Thomas C. McInnis and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: May 6, 1991; In Final Form: October 30, 1991 1
Codeposition of H 2 0 and F2 in excess argon on a cold window at 12 f 1 K gave new absorptions for the Lewis acid-base H20--F2 complex. Photolysis of this sample with 220-1000-nm light formed several new products. Ab initio calculations using HONDO 7.0 and the DZP basis set were performed to support the identification of new F2 + H 2 0 product complexes. HF- -HOF, FH- -FOH, and FH- - 0 H F exhibited v,(HF) modes at 3877.9, 3781.3, and 3638.0 cm-I, respectively. The F atom + H 2 0 reaction product, the FH- -OH radical complex, showed further growth upon sample annealing to 31 f 2 K and a u,(HF) absorption at 3631.9 cm-'; the matrix plays a unique role in the stabilization of FH--OH, which will be difficult to observe in the gas phase. In addition, complexes of OF and OF2 with H F were formed on photolysis and showed marked growth upon sample annealing. This work reveals product species in the stepwise reaction of water and fluorine.
Introduction Gas-phase reactions of H 2 0 with F atoms have yielded important information about the rotational distribution of the HF product formed.' The matrix photochemical reaction of H 2 0 and F2in solid N2 reported by Noble and Pimentel gave the first evidence of the hypofluorite molecule, HOF, in a complex with HF.2 Further work prepared HOF separately and verified the matrix assignments.35 In the N2matrix work, a simple structure, HF- -HOF, was elucidated. With a less interacting matrix such as Ar,the possibility of forming different complex structures from the products of this highly exothermic reaction might be enhanced. In addition, the FH- -OH or HF- -HO radical complex might also be formed on diffusion and reaction of F atoms with water in contrast to the gas-phase reaction where the HF and OH products separate. Furthermore, the photochemistry of the H20--F2 complex can be compared with photolysis of the NH3--Fz, CH4--F2, and H2--Fz systems and to work in progress with the H2S--F2complex.@ Experimental Section The vacuum and cryogenic apparatus and techniques have been described previously.lOJ' Spectra in the 4000-400-~m-~region were recorded using a Nicolet 7199 FTIR spectrometer. The matrices were scanned 500-1000 times a t 1-cm-' resolution; the wavenumber accuracy is f0.3 cm-'. Additional spectra in the 480-200-cm-I region were recorded using a Perkin-Elmer 983 infrared spectrophotometer. These matrices were scanned 1-4 times a t 2- or 4-cm-' resolution giving a wavenumber accuracy of i l cm-I. Doubly distilled water, D 2 0 (Aldrich), D2I80(Oak Ridge National Laboratory), and H2I80(Cambridge Isotope Laboratories) samples were transferred to evacuated stainless steel fingers with valves; the use of separate fingers for each isotopic sample minimized exchange. Water vapor was initially deposited with argon on a CsI window at 13 f 1 K; deposition rates were controlled by variations in sample temperatures (0-15 i 2 "C) and 'Taken in part from the M.S. Thesis of T. C. McInnis, University of Virginia, Charlottesville, VA, 1991.
0022-365419212096-205 1$03.00/0
needle valve settings. The water concentrations ranged from approximately lOOO/l to loooO/l when compared to other matrix water experiments.l2-I4 A spectrum of H 2 0 , D20, H2I8O, or D2I80diluted in argon was recorded. Fluorine (Matheson) was diluted with argon to Ar/F2 ratios of 50/1, 100/1, 200/1, or 300/1, and the solution was passed through a U-tube at 77 K at 8-10 mmol/h and condensed with water vapor for up to 7 h, and another spectrum was recorded. The sample was photolyzed with 220-1000-nm light from a high-pressure mercury arc (1000 W, BH6-1B, T.J. Sales, Fairfield, N J ) using a cooled 10-cm water filter for 1 h, and another spectrum was recorded. The matrix was then warmed to 23-28 K for 7-9 min and re-cooled to 13 K, and another spectrum was recorded. An additional 1 h of full arc photolysis was followed by annealing to 29-33 K and recooling, and spectra were recorded after each process. Additional experiments were done with OF2(Ozark Mahoning) and HF and D F in an argon matrix and with HzO and HF and DF in a nitrogen matrix. The DF sample was prepared by reacting F2 and Dz (Air Products) in a stainless steel can as described previou~ly.'~ (1) Agrawalla, B. S.;Setser, D. W. J. Phys. Chem. 1984,88, 657; 1986, 90,2450. (2) Noble, P. N.; Pimentel, G. C. Spectrochim. Acta, Parr A 1968,24,797. (3) Studier, M. H.; Appelman, E. H. J . Am. Chem. Soc. 1971,93,2349. (4) Goleb, J. A.; Claassen, H. H.; Studier, M. H.; Appelman, E. H. Spectrochim. Acta, Part A 1972,28,65. Appelman, E. H.; Kim, H. J. Chem. Phys. 1972, 57, 3272. (5) Appelman, E. H.; Downs,A. J.; Gardner, C. J. J. Phys. Chem. 1989, 93, 598. (6) Andrews, L.; Lascola, R.J . Am. Chem. SOC.1987, 109,6243. (7) Johnson, G. L.; Andrews, L. J . Am. Chem. Soc. 1980, 102, 5736. (8) Hunt, R. D.; Andrews, L. J. Chem. Phys. 1985, 82, 4442. (9) Andrews, L.; McInnis, T. C.; Hannachi, Y. J . Phys. Chem., in press. (IO) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1982, 76, 2875. ( 1 1 ) Johnson, G. L.; Andrews, L. J. Am. Chem. Soc. 1982, 104, 3043. (12) Andrews, L.; Johnson, G. L. J . Chem. Phys. 1983, 79, 3670. For more recent work from this lab that employed a crystal microbalance to measure water deposition, see: Andrews, L.; Burkholder, T. R.J. Phys. Chem. 1991, 95, 8554. (13) Redington, R. L.; Milligan, D. E. J . Chem. Phys. 1962, 37, 2162. (14) Ayers, G. P.; Pullin, A. D. E. Spectrochim. Acta. Part A 1976, 32, 1629. (15) Andrews, L.;Johnson, G. L. J . Phys. Chem. 1984, 88, 425.
0 1992 American Chemical Society
McInnis and Andrews
2052 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
1
'id00
3650
3400
3650
3800
3j50
3j00
3650
3600
3550
3500
3450
W A V EN UMBERS
Figure 1. Infrared spectra in the 4000-3450-cm-' region for H 2 0 sublimed at 0 O C through a needle valve into an Ar/F2 (100/1) matrix at 12 K: (a) after codeposition; (b) after full arc photolysis; (c) after annealing to 30 i 2 K. L
la
C
*
C
A
n
w
[L
D
U
b
N
A
P
680
480
WfiVENUMBERS Figure 2.
Infrared spectra in the 1480-480-cm-' region for sample shown
in Figure 1.
Results Fluorine samples were diluted with argon and codeposited with water vapor at 13 K. New absorptions for H 2 0 , H2I80,D20,and D2180photochemical reactions with Fz will be presented.
480
300
20'0
WAVENUMBERS Figure 3. Infrared spectra in the 480-200-cm-' region for H20 sublimed at 0 OC into an Ar/F2 (200/1) matrix at 12 K (a) after codeposition; (b) after full arc photolysis; (c) after annealing to 28 i 2 K. H 2 0 + F2 Five codeposition experiments were performed with H 2 0and F2using different combinations of argon dilutions and water vapor pressures such that the F2/H20ratios ranged between 4/1 and 4 0 / 1 . Infrared spectra are illustrated in Figures 1-3, and new product absorptions are listed in Table I. Absorptions labeled W at 3777.0, 3756.4, 3711.5, 3670.0, 3653.6, 3638.0 cm-'
H 2 0 --Fz Complexes in Solid Argon
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2053
2
Figure 4. Infrared spectra in the 4000-3450-cm-' region of H2'*0sublimed at 0 OC into an Ar/F2 (100/1) matrix at 12 K: (a) after deposition; (b) after full arc photolysis; (c) after annealing to 34 f 2 K.
are due to water, and the absorption labeled Wz at 3573.6 cm-l is due to water dimer.14 The water dimer band at 3573.6 cm-' is 4-fold less intense than the strongest monomer band at 3756.4 cm-'. Since hydrogen bonding intensifies the dimer band, the amount of dimer in these samples is substantially less than in the monomer. The bending region reveals the same conclusion: the 1624.2-cm-' monomer band is 1 order of magnitude stronger than the 1610.4- and 1593.0-cm-' dimer bands. Reagent deposition (Figures la, 2a, and 3a) produced sharp, new medium-intensity absorptions labeled C in the antisymmetric stretching region of H 2 0 at 3745.6 and 3717.6 cm-', a broader band at 3727 cm-', and the water dimer region at 3567 cm-'. Additional sharp absorptions at 3638.0 and 1590.0 cm-', nearly coincident with water fundamentals but formed in the presence of added F2, were intensified relative to other water absorptions. In addition, a weak band near the F2fundamental was observed at 877.5 cm-'along with bands for the usual trace impurities (CF,, SiF4,OCF,, O F 3 in commercial fluorine. Absorptions previously characterized include bands at 3555,753,720,635,629,614, and 608 cm-'due to H20--HF and bands due to the N2--HF complex at 3881 and 263 cm-1.12 Photolysis (Figures lb, 2b, and 3b) with 220-nm light dissociated F2 molecules and produced several new absorptions (Table I). In the H F stretching region, noteworthy new bands appeared at 3877.9 (labeled l), 3781.3 (labeled 2), 3638 (labeled 3), 3631.9 (labeled 4), and 3687.0 cm-I (labeled 7). In the OH stretching region, medium-intensity bands were observed at 3537.9 (labeled 4), 3529.6 (labeled 3), and 3511.8 cm-l (labeled 1). The HOF bending region revealed several weak absorptions at 1396.4, 1387.2, 1381.7, and 1345.2 cm-' (labeled 1, 2, 2, and 3, respectively). The OF stretching region revealed new bands at 1036.5 and 886.7 cm-'(labeled 5 and 2, respectively). In the lower region, new absorptions were observed at 694.3, 506.8, 457.5, 394, and 357 cm-', labeled 4, 3, 3, 2, and 2, respectively. Annealing (Figures IC, 2c, and 3c) to the 29-33 K range increased the bands labeled 1 by 10% and decreased the bands labeled 2 and 3 by 10%. Bands labeled 4 increased by 50%, and the band labeled 7 doubled. In the OF stretching region, the 1036.5-cm-' band increased markedly, weak bands appeared at
923.2, 919.9, and 914.3 cm-', and strong absorptions occured at 820.7 and 818.7 cm-'. Finally, a sharp 0 2 F band at 1489 cm-' was Observed after annealing.16 Additional photolysis after annealing further increased the 1, 2, and 3 bands, and a final annealing increased still further the 4, 5 , 6, and 7 bands. Absorption bands assigned to the several product species remained approximately constant within a species during annealing and during a 1 order-of-magnitude change in the F2 H 2 0 concentration ratio. Hzl 0 Fz. Three deposition experiments were performed with H2180and F2 with varying combinations of argon dilution. Infrared spectra are illustrated in Figures 4 and 5. Water F2 complex bands shifted to 3731.4, 3712, 3704.1, and 3558 cm-' and decreased on full arc photolysis. In the bending region, with F2 added, a sharp band at 1583.7 cm-' increased relative to the H2180precursor absorptions. A weak band again appeared at 877.5 cm-'. Red visible photolysis (590-1000 nm) gave no products; however, 290-1OOO-nm radiation gave about 20% of the total yield using full arc irradiation. Full arc photolysis produced sharp new bands at 3518.3 and 3500.3 cm-I in the OH stretching region (labeled 3 and 1, respectively). In the HOF bending region, weak bands appeared at 1392.9, 1384.6, 1378.6, and 1341.6 cm-'. In the OF stretching region, weak bands were observed at 1005.8, 867.4, and 860.1 cm-l. Annealing again increased important new absorptions labeled 4 at 3631.2, 3526.7, and 694.3 cm-' and bands at 889.1 and 793.3 cm-' (labeled 5 and 6). D20 F2 Three deposition experiments were performed with D20and F2 Sharp new bands were observed at 2776.3 and 2657.8 cm-' in the OD stretching region. In the bending regions of HOD and DzO, strong sharp bands were observed at 1398.4 and 1174.6 cm-', respectively. In the Fz stretching region, a perturbed band appeared at 877.5 cm-'. All of the deposition bands behaved in the same manner; these bands decreased on photolysis and increased again upon annealing. Photolysis produced new absorptions in the DF and OD stretching regions at 2843.7, 2775.5, 2669, 2602.1, and 2591.1
/+
+
(16) Arkell, A. J . Am. G e m . SOC.1965,87,4057.
McInnis and Andrews
2054 The Journal of Physical Chemistry, Vol. 96,No. 5, 1992
.1
Is
(D
a
.....
,9092
0
,9574
H2 b Figure 6. (a) HF- - H O with DZP bond lengths and (b) FH--OH with DZP bond lengths. 0
,/
1.3281
i I 1480
1280
1080 680 WRVENUMBERS
a
600
(80
F i 5. Infrared spectra in the 1480-480-cm-l region for sample shown in Figure 4.
cm-I as given in Table I; the band at 2775.5 cm-I dominated the product spectrum. The lower region revealed sharp bands at 1036.9 and 996.4 cm-I and weaker bands at 1033.4,1028.1,898.2, 893.3, and 888.4 cm-'. In the far-infrared region, broad new absorptions appeared at 390 and 295 cm-l, while sharper stronger absorptions grew in at 340 and 270 cm-I. Annealing produced weak product bands at 2868.6, 2832.0, and 2803.5 cm-I and a sharp band at 2607.2 cm-l. The OF stretching region showed familiar bands at 1036.9, 923.0,919.4,914.2,825.8, 820.9, and 817.9 cm-'. The far-infrared region produced bands near OFz at 4652,463.4, and 458.6 cm-'. Finally, the band at 516.6 cm-I increased nearly 75% on annealing. D2180 FP Two experiments were performed with different concentrations of DJ80 and F2 The spectra revealed deposition bands at 2756.3, 2646.7, 1391.8, 1166.6, and 877.5 cm-I. Photolysis products in the OD stretching region included a weak band at 2586.5 cm-' and a stronger band at 2575.2 cm-I. The lower region contained weak bands at 1029.0, 1025.1,992.2, 866.4, and 861.8 cm-I. Annealing revealed only one product in the OD stretching region at 2592.0 cm-'. The OF stretching region yielded a sharp band at 1005.9 cm-', three weak bands at 898.2, 893.3, and 888.4 cm-l, and three stronger bands at 798.3, 790.9, and 794.0 cm-I. OF2 HF/DF. Three experiments were done with OF2 and HF/DF mixtures. New bands were observed at 391 IA3858.4, and 3816.2 cm-' with deuterium counterparts at 2868.6,2832.0, and 2803.5 cm-', respectively. In the DF experiments, the farinfrared region revealed bands at 465.2, 463.4, and 458.6 cm-I. Photolysis produced a sharp, strong 1028.1-cm-' band with a resolved 1034.O-cm-' satellite. Annealing decreased both bands slightly. Calculatiom. Ab initio SCF-HF calculations using the HONDO 7.0 p r ~ g r a m ~ were ~ v ~performed ~ on OH, HzO, HF, OF, F,, HOF,
.3 187 F2
b Figure 7. (a) FH- -FO with DZP bond lengths and (b) FH- -OF with DZP bond lengths. 0 ,952 1 F2
+
1.9972 /
/
/
0 ,9513 2
+
(17) Dupuis, M.;Watts, J. D.; Villar, H. O.;Hurst, G.J. B.Compu?.Phys. Commun. 1989,52,415.
H2
/
0
' zpo55 1
F1
b Figure 8. (a) HF--HOF with DZP bond lengths and (b) FH- -FOH with DZP bond lengths.
OFz, and several complexes involving these molecules (Figures 6-8). Initially, STO-3Gbasis set calculations were run on several
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2055
H 2 0 --F2 Complexes in Solid Argon TABLE I: Product Absorptions (cm-') Formed on Codepition, Wotolysis, and Annealing of H20a d F2 and Selected Isotopes in Solid Argon H 2 0 + F2 H2l8O+ F2 D 2 0 F2 D2I80 F T deposition 2776.3 2756.3 products 3745.6 3731.4 3727 3712 3717.6 3703.0 3638.0 3625.4 2657.8 2646.7 3567 3558 2608 2597 1398.4 1391.8 1590.0 1174.6 1583.7 1166.6 877.5 877.5 877.5 877.5 photolysis products 3877.9 2843.7 3877.6 2843.7 3781.3 3781.1 2775.5 2775.7 3687.0 2701.1 2700.8 3638 3638 2669 2669 3529.6 3518.3 2602.1 2586.5 3511.8 3500.3 2591.1 2575.2 1396.4 1392.9 1033.4 1029.0 1387.2 1028.1 1384.6 1025.1 1381.7 1378.6 1345.2 1341.6 996.4 992.2 898.2 867.4 893.3 866.4 860.1 888.4 886.7 861.8 506.8 390 506.8 457.5 340 457.5 295 394 270 357 2868.6 annealing products 391 1.5 2832.0 3858.4 2827.2 3853.8 3854.0 2803.5 3816.2 2670.6 3631.2 3631.9 2670.6 2607.2 3531.9 3526.7 2592.0 1036.9 1036.5 1005.8 1005.9 898.2 923.0 923.2 893.3 919.4 919.9 9 14.2 889.1 888.4 914.3 798.3 825.8 798.1 825.5 794.0 820.9 793.3 820.7 790.9 817.9 818.7 792.1 516.6 694.3 694.3 5 16.6
+
+
TABLE II: Energies (in brrtrees) of Submolecules and Complexes Obtained from HONDO 7.0 ab Initio Calculations Using STO-3c and DZP Basis Sets molecule STO-3G DZP OH -74.3649 -75.4 102 -14.9660 -76.0468 H20 -99.3977 F -97.9865 HF -98.5729 -100.0478 OF -171.8026 -114.1755 HOF -172.3142 -114.7887 -269.7980 -273.5370 OF2 HF--HO -172,9440 -115.4633 FH- -OH -1 72.945 1 -175.4670 FH--OH2 -1 73.5507 -176.1089 F2 -195.98 16 -198.7393 FH- -FO -270.3774 -274.2250 FH- -OF -270.3777 -274.2252 HOH--F2 -270.9475 -274.7860 H 2 0 --F2 (side) -210.9476 -214.7864 H,O- -F2 (end) -270.9476 -274.7867 FH- -FOH -270.9508 -274.8416 FH- - 0 H F -270.9528 -274.8426 Irum-HF- -HOF -270.9572 -274.8432 FH--FOF -368.3723 -373.5862 FH- -OF2 -368.3725 -373.5850
molecules to obtain optimized geometries (Table 11). Using these optimized geometries, force constant calculations were carried out at the STO-3G level to ensure that the structures and energies obtained were minima. With the geometry output from the STO-3G calculations, the energies and geometries of selected structures were re-calculated using double { with polarization (DZP) basis sets. (Tables I1 and 111). At this level of theory, (18) Dupuis,
M.;Rhys, J.; King, H. F. J . Chem. Phys. 1976, 65, 111.
TABLE III: Bond Distances (in angstroms) of Submolecules and Complexes and HF Fundamentals (cm-') Obtained from HONDO 7.0 ab Initio Calculations Using the DZP Basis Set molecule r(HF) r(OH) r(OF) complex bond OH 0.956 0.944 H20 HF 0.903 OF 1.324 HOF" 0.950 1.370 OF2 1.343 HF- -HO 0.905 0.957 2.100 FH- -OH 0.909 0.957 1.917 FH--OHz 0.914 0.945 1.786 FH- -FO 0.903 1.328 2.234 FH- -OF 0.904 1.319 2.210 HOH--FZ 0.944 3.143 0.944 H 2 0 --F2 (side) 0.989 3.625 0.990 H 2 0 --F2 (end) 0.944 3.015 FH- -FOHb 0.905 0.951 1.375 2.006 FH- -OHFC 0.907 0.951 1.366 2.640 rrunr-HF- -HOFd 0.905 0.952 1.372 1.997 FH- -FOF 0.903 1.347 2.333 1.341 FH--OF2 0.903 1.342 2.299 "Calculated H O F angle = 100.4'; experimental angle = 97.2O. See ref 32. bCalculated H O F angle = 100.2'. 'Calculated H O F angle = 101.2°. dCalculated H O F angle = 100.7'.
agreement with experiment cannot be more than qualitative. When H F was combined with OH, two isomers were considered, HF--HO (Figure 6a) and FH- -OH (Figure 6b); the former showed anti-hydrogen bonding with respect to HF, and the latter showed hydrogen bonding with respect to HF. The hydrogen bond distance is indicative of the stability of the complex. In the reverse complex, the'hydrogen bond distance is 2.100 A while the h drogen bond distance in FH--OH is much shorter at 1.917 In comparison to the isolated fragments, both the H F and OH bond distances are lengthened slightly in each case. The HF- -HO complex is 3.3 kcal/mol more stable than the separate fragments, whereas the FH- -OH complex is more stable than its individual components by 5.6 kcal/mol. Two isomeric complexes containing H F and OF fragments were considered. Both species were hydrogen bonded; one involved bonding to the F atom of OF (Figure 7a) and the other to the 0 atom of OF (Figure 7b). The FH--OF complex is 0.1 kcal/mol more stable than FH- -FO at the DZP level of theory; this difference is not signifcant. As expected, the h drogen bond distance is shorter in the FH- -OF complex, 2.210 , while FH- -OF has a 2.234-A hydrogen bond distance. The stability gained by complexing the submolecules is 1.O kcal/mol for FH- -OF and 1.1 kcal/mol for FH- -OF. Both hydrogen bonding (HOH- -F2) and Lewis acid-base (H20--F2) interactions were considered for the primary waterfluorine complex. An oxygen lone pair-end (u) bonded F2 Lewis acid-base complex is more stable than the sideways (r)bonded F2 arrangement by 0.2 kcal/mol and the hydrogen bonded isomer by 0.5 kcal/mol at the DZP level of theory. Calculations for three complex isomers containing H F and HOF were conducted. The trans HF- -HOF structure (Figure 8a) was calculated to be the most stable arrangement involving the two submolecules. HF- -HOF was found to be 1.O kcal/mol more stable than the reverse arrangement, FH--FOH (Figure 8b). In addition, FH- - 0 H F was more stable than FH- -FOH but was not as stable as HF- -HOF based on DZP energies. In comparisons of the DZP geometries, the hydrogen bond distance for FH--FOH is 2.006 A and is 1.997 A for trans-HF- -HOF. Calculated bond distances are summarized in Table 111. The trans-HF--HOF complex was 4.2 kcal/mol more stable than its separate components. FH- -FOH was found to be 3.2 kcal/mol more stable, and FH- - 0 H F was 3.8 kcal/mol more stable than the isolated components.
1
x
2056 The Journal of Physical Chemistry, Vol, 96, No. 5, 1992 Calculations were also performed on FH- -FOF and FH- -OF2. Although STO-3G calculations showed the oxo hydrogen bonded species to be more stable by 0.1 kcal/mol, DZP calculations gave a more stable geometry for the fluoro hydrogen bonded species by 0.7 kcal/mol. Owing to the well-known difficulties with calculations for O F bonds, the present results suggest comparable energies for both s t r ~ c t u r e s . ' ~
Discussion Products of the water and fluorine codeposition and photochemical reactions will be identified, and reaction mechanisms will be presented. H20--F2. Codeposition of water with fluorine yielded several new product bands (labeled C), which decreased upon full arc photolysis. Since the bands appeared upon deposition, decreased upon photolysis known to dissociate F2, and increased on annealing (except for the 3638.0-cm-1 band which was covered by a photolysis product), the C absorptions are identified as water-fluorine complexes. Further evidence for this supposition is the 877.5-cm-' band in all isotopic water experiments. Matrix Raman studies show the IR-inactive fluorine molecule vibration at 892 cm-1,20 and the 877.5-cm-I band is in the proper region for a perturbed F2 molecule made IR active by interaction with a water molecule. Perturbed bands of water are also evident, and these bands display proper isotopic shifts for water. For example, the band at 3745.6 cm-l shifts to 3731.4 cm-I in the H2I80experiments, giving an 160/'80 ratio of 1.003 80; the 160/'80 ratio for the strong H20 band at 3756.4 cm-' is 1.003 87. The sharp C bands at 3745.6 and 3717.6 cm-', which fall inside of the isolated HzO u3 vibrational-rotational bands at 3756.4 and 371 1.5 cm-I, the sharp perturbed F2band at 877.5 cm-l, and the enhanced symmetric modes at 3638.0 and 1590.0 c d , near the isolated HzO fundamentals, are assigned to the H20--Fzcomplex. The broader C bands at 3727 and 3567 cm-I are assigned to (H20)2--F2 complexes. The two obvious structures for the water-fluorine complex involve hydrogen bonding (HOH- -F2) and Lewis acid-base (HzO- -F2) interactions. Little displacement was observed for the above H20--F2bands relative to isolated HzO and F2. DZP calculated frequencies for submolecules and complexes predicted shifts in the complexes. The shifts predicted for the end-on Lewis acid-base complex, which has the lowest energy at the DZP level, were in better agreement with the observed spectrum than shifts predicted for the sideways and hydrogen bonded structures. The largest red shifts for u, (3 cm-I) and u(F2) (3 cm-') were predicted for the end-on structure. The Fz--HF complex shows a small (4 cm-') red shift for the HF fundamental,z' in accord with the present H20--Fz observation. Finally, does the H 2 0 molecule rotate in the H20--F2 complex as isolated H 2 0 rotates in solid argon? If the sharp C bands shifted 6-10 cm-l inside of the water vibration-rotation bands are due to the same motions, the H 2 0 submolecule in the F2 complex retains rotational freedom about at least one axis. HOF Complexes. Upon full arc photolysis, three distinct new absorptions were produced in the H F stretching region, which are believed to be due to three isomers of the HF and HOF complex. Bands labeled 1 in Figures 1-5 are assigned to HF- -HOF. Those labeled 2 belong to FH- -FOH, while those designated by 3 are attributed to FH- - 0 H F . Absorptions due to these isomers are listed in Table IV. The most stable complex, 1, based on ab initio calculations, increased slightly on annealing while the less stable complexes 2 and 3 decreased slightly. In the HF stretching region, perturbed H F bands were observed at 3877.9, 3781.3, and 3638.0 cm-I. It should be noted that the 3954- and 3962-cm-' bands of isolated HF were unchanged by photolysis. This implies that products 1,2, and 3 were formed by the photochemical reaction of F, and H20 in a complex and not from isolated HF, which was an impurity in the F2sample. D 2 0experiments showed perturbed ~~~~~
(19) Rohlfing, C. M.; Hay, P. J. J . Chem. Phys. 1987, 86, 4578. (20) Howard, W. F., Jr.; Andrews, L. J . Am. Chem. SOC.1973,95, 3045. (21) Hunt, R. D.; Andrews, L. J . Phys. Chem. 1988, 92, 3769
McInnis and Andrews TABLE I V Product Absorptions (cm-I) of the Three Complexes, 1, 2, and 3, Formed between HF and HOF and Selected Isotopes in Solid Argon" DF+ DF+ comulex H F + HOF H F + HISOF DOF D1*OF us (HF/DF stretch)b 1 3877.9 3877.6 2843.7 2843.7 2 3781.3 3781.1 2775.5 2775.5 3 3638.0 3637.7 2669.1 2669.1 ~~
u I (OH/OD stretch)c
1
3511.8
3500.3
2591.1
2575.2
3
3529.6
1 2 3
(bend)' 1396.4 1392.9 1381.7, 1387.2 1378.6, 1384.6 1345.2 1341.6
3518.3
2602.1
2586.5
1033.4 1028.1 996.4
1029.0 1025.1 992.2
898.3 888.4 893.3
861.8 866.4
u2
vl (OF/'*OF stretch)c
1
2 3
886.7
2 in plane
394 357 506.8 457.5
860.1 867.4 vi
2 out of plane
3 in plane 3 out of plane
(HF/DF) 295 270 390 340
"1 is HF--HOF. 2 is FH--FOH. 3 is FH--0HF. * H F fundamental (Q-induced) in solid argon (cm-I): 3919.5; DF 2876.6. See ref 13. CHOF fundamentals in solid argon (cm-I): u l . 3572; u2, 1350; u j , 888. HISOF: u l , -3561; u2, -1347; u3, -860. DOF: uI. -2639; u2, -998; u,, -890. See ref 5 for HOF. Values for H1*0Fand DOF were calculated using solid nitrogen ratios. See ref 2.
DF absorptions which displayed similar behavior at 2843.7,2775.5, and 2667.3 cm-l. The H / D ratios for these bands are 1.3637, 1.3624, and 1.3630 as compared with the H/D ratio of 1.3625 for isolated hydrogen fl~0ride.l~ Note that the HF fundamental of 1 is the highest since the hydrogen is not bonded and that this fundamental for 3 is the lowest, slightly above the v, mode for FH- -OHz. Other absorptions are associated with these bands by photolysis and annealing behavior. The displacements of these fundamentals with respect to isolated HOF values further support the identities of species 1, 2, and 3. In the OH stretching region, bands grew in at 3511.8 and 3529.6 cm-I. The former exhibited constant relative intensity with the 3877.9-cm-' band and the latter, with the 3638.0-cm-' band. These bands are due to 0-H stretching vibrations since they exhibit both H / D and 160/'80 shifts. The band labeled 1 at 3511.8 cm-'shifted to 3500.3 cm-l in the HZl80experiments, and the 3529.6-cm-' band (labeled 3) shifted to 3518.3 cm-' in the ratio for 1 is 1.0033, and the same experiments. The 160/180 ratio is 1.0032 for 3. For HOF, the 160/'80 ratio is expected to be approximately 1.0031.'~5 For H 2 0 , the 160/180 ratio for uI in solid nitrogen is 1.0021 and for u3 is 1.0038.22 For the diatomic OH oscillator, the 160/180 ratio is 1.0031.23 The possibility that the two bands are due to perturbed water is eliminated because the 160/180 ratios do not match. If the complexes involved only HF perturbed OH, then there should be no other absorptions assoCiated with the bands previously described, but such is not the case. The 351 1.8- and 3529.6-cm-' bands are due to u,(OH stretch) vibrations of HOF in complexes with H F as is shown by the observation of other perturbed HOF modes in the complexes. In the bending region of HOF,new 1396.4-, 1387.2-, 1381.7-, and 1345.2-cm-' bands appear. The isolated H O F bending fundamental is 1350 cm-l in solid argon.5 The 1396.4-cm-I band belongs to HF- -FOH; the band at 1387.2 cm-' belongs to HF- -HOF and has a site splitting at 1381.7 cm-I. Finally, a red-shifted band at 1345.2 cm-l can be attributed to FH- -0HF. The H/Dand 160/180 ratios are appropriate for the (22) Fredin, L.; Nelander, B.; Ribbegard, G.J . Chem. Phys. 1977, 66, 4065. (23) Cheng, B. M.; Lee,Y. P.; Oglivie, J. F. Chem. Phys. Lett. 1988, 151, 109.
H20--F2 Complexes in Solid Argon
The Journal of Physical Chemistry, Vol. 96, No. 5, 1992 2057
TABLE V: Product Absorptions (cm-I) of the Complex, 4, Formed between HF and OH and Selected Isotopes in Solid Argon"
HFt OH
HFt I80H
DF+ OD
DFt lSOD
(HF/DF stretch) 3631.9 3631.2 2670.6 2670.6 3526.7 2607.2 OH/OD stretchb 3537.9 2592.0 694.3 516.6 694.3 516.6 uI(HF/DF lib) "The complex, 4, is FH--OH. bOH fundamental in solid argon (cm-I): OH, 3548.2; ISOH,3537.1; OD, 2616.1; IsOD, 2600.3. See ref Y,
23.
v2 mode of HOF in the complexes. The v3 (OF stretch) fundamental of HOF occurs at 888 cm-l in solid argon? In the D 2 0 experiments, absorptions at 898.2,888.4, and 893.3 cm-l can be assigned to complexes 1, 2, and 3, respectively. The librational region of HF shows absorptions for 2 and 3 but not for 1 since the H F perturbation is too weak, and this mode is expected below 200 cm-l. Two absorptions for each complex are seen, one for the in-plane and another for the out-of-plane librational mode. Absorptions at 394 and 507 cm-' belong to the in-plane motions of 2 and 3, respectively. The 357- and 457.5-cm-l bands are assigned to the out-of-plane motions of 2 and 3, respectively. The librational modes of HF are dependent upon the strength of the HF bond. The HF stretching mode of N2--HF absorbed at 3881 cm-l, while the librational mode of HF in the complex appeared at 262 cm-I.l3 In the H20--HF complex, the H F stretch was observed at 3555 cm-I. The in-plane mode for the H20--HF complex, the HF stretch was observed at 3555 cm-I. The in-plane mode for the H20--HF complex comes at 753 cm-l and the out-of-plane mode at 635 cm-I.12 The HF stretch for 2 was found at at 3781.3 cm-I and for 3 at 3638.0 cm-I. The librational modes of these complexes fall between the librational modes of H20--HF and N2--HF. It is of interest to note that in a nitrogen matrix HF- -HOF was the only HOF-containing product formed,2v4while in an argon matrix FH- - 0 H F and FH- -FOH were also produced. The Nz host contains lone pairs which strongly interact with the trapped molecule. This is evidenced by the fact that HF in an N2matrix absorbs at 3880 cm-l while in an Ar matrix H F absorbs at 3919 cm-1.13924 In addition, the N2matrix interacts more with H O F than the Ar matrix does, 3536 cm-l for the former versus 3572 cm-l for the latter.5 This might suggest that the free HF interaction with the N2matrix is more of a stabilizing effect than the interaction of HF with HOF in the FH- -0HF and FH- -FOH complexes since they were not produced in the N2 matrix. A possible explanation for the formation of HF--HOF in an N2 matrix may be that it is the only complex which is more stable than the combined energies of N2--HF and N2--HOF. The present a b initio calculations show that the HF- -HOF complex is the most stable of the three complexes containing HOF. In other words, N2--HF- -HOF is probably more stable than N2-
-HOF- -HF. OH Complex. A series of new absorptions appear on photolysis and grow further upon annealing. These bands, labeled 4 in the figures and summarized in Table V, are assigned to FH- -OH formed in the F atom-water reaction. In the HF stretching region, a perturbed band was observed at 3631.9 cm-I, red-shifted 287.6 cm-' from the Q-induced fundamental of HF. The OH band associated with this was observed at 3537.9 cm-I. In deuteriated experiments, the D F stretch shifted to 2670.2 cm-' and the OD stretch was shifted to 2607.2 cm-I. In the D2Is0 experiments, the IsOD absorption shifted to 2592.0 cm-I. The absorption for isolated O H has been observed at 3548.2 cm-I with that for @OH at 3537.1 cm-l; the isolated OD fundamental is 2616.1 cm-I with that for IsOD a t 2600.3 cm-l. The I60H/'*OH ratio for the isolated diatomic is 1.0031.23 The perturbed molecule gives a value of 1.0032 for this isotopic ratio. The 160D/1sODratio is 1.0061 for the isolated diatomic and 1.0059 for the perturbed molecule. The OH/OD ratio for the unperturbed diatomic is 1.3563 versus (24) Bowers, M. T.; Kerley, G. I.; Flygare, W. 45, 3399.
H.J . Chem. Phys. 1966,
TABLE VI: Product Absorptions (cm-I) of the Complex, 5, Formed between HF and OF and Selected Isotopes in Solid Argon"
OF stretchb
HF/DF + OF
HF/DF + l80F
1036.5/1036.9
1005.8/1005.9
" 5 is FO--HF. bOF fundamentals in solid argon (cm-I): OF, 1028.6; l80F, 997.7. See refs 25 and 26. Isolated OF was observed at 1028.1 cm-' in the present work.
1.3570 for the OH stretch of FH- -OH. The isotopic ratios of the unperturbed and perturbed OH radicals are in excellent agreement. Finally, the 694.3-cm-I band is assigned to an HF librational mode in the FH- -OH complex. The lack of I8Oshift, the deuterium shift (694.3/516.6 = 1.344), and appearance in the range of this motion for the H20--HF complexI2 all support this assignment. The absorptions were attributed to FH- -OH and not to HF-HO for three reasons. In FH- -OH, the HF stretch is perturbed greatly since the hydrogen of HF is involved in bonding with the OH fragment. On the other hand, the OH stretch in FH- -OH is not perturbed much since the hydrogen of O H is not involved in bonding. The exact opposite would be expected for HF- -HO, a weakly perturbed H F stretching mode and a strongly perturbed OH stretching mode. Observations reveal evidence for FH- -OH as the HF stretch is perturbed by 287.6 cm-I and the OH stretch is red-shifted by only 10.3 cm-l. Furthermore, calculations show FH--OH to be 2.3 kcal/mol more stable than HF- -HO. Finally, the observation of the OH fundamental in FH- -OH a t 3537.9 cm-I lends credence to the recent reassignment of the O H fundamental at 3548.2 cm-' in solid argon.23 When considering matrix effects on the FH- -OH complex, it should be noted that the complex was not produced in solid N2. This suggests that there is less efficient F atom diffusion in an N2matrix. In order for this complex to be produced on photolysis, diffusion of an F atom out of the matrix cage is required. Furthermore, an additional yield of FH--OH is produced on annealing in solid argon. The basicity of the O H radical relative to that of H 2 0 is an interesting question. The HF fundamentals of these complexes show that HzO is more basic as the HF fundamental is displaced 364 cm-I in HzO--HF and only 288 cm-' in HO--HF. DZP calculations substantiate this order of HF fundamentals. If the calculated H F fundamentals are scaled by the 3555.4-cm-l matrix fundamental for HzO--HF, the value 3628 cm-l is predicted for HO- -HF, just 3 cm-' below the observed value. OF Complex. Similar to the OH complex with HF are absorptions due to OF-containing complexes. These absorptions, labeled 5 and collected in Table VI, were present on photolysis and grew further upon annealing. It is suspected that these absorption are due to FH- -OF and/or (FH)2--OF. Unfortunately, no perturbed H F stretching motions could be associated with the perturbed OF stretching motions, If the complex does indeed involve HF, it seems that a complex with the hydrogen of HF involved in hydrogen bonding to the oxygen atom of O F would be favored over a complex with hydrogen bonding to the fluorine, FH-mOF. There is strong evidence for perturbed OF stretching modes. Isolated OF in solid argon appeared a t 1028.6 cm-I with l80F at 997.7 cm-1.25-27In H20experiments, an absorption appeared at 1036.5 cm-l, a blue shift of 7.9 cm-I from the isolated diatomic radical. The absorption was shifted to 1005.8 cm-I for perturbed ISOF, a blue shift of 8.1 cm-l. D 2 0 experiments show a slight change in the absorptions; perturbed 160Fabsorbs at 1036.9 cm-l and the l80Fat 1005.9 cm-I. The 160F/'80F ratio for the isolated species is 1,0319 while the perturbed species has a comparable ratio of 1.0305.27 The location of the perturbed band and the isotopic ratios give strong evidence for a perturbed OF radical complex. Mechanistic arguments, which will be discussed later, and previous assignments (25) Arkell, A. J . Phys. Chem. 1969,73, 3877. (26) Andrews, L.; Raymond, J. I. J . Chem. Phys. 1971, 55, 3078. (27) Andrews, L. J. Chem. Phys. 1972, 57, 51.
McInnis and Andrews
2058 The Journal of Physical Chemistry, Vol. 96, No. 5, 1992
strongly support the identification of an OF radical perturbed by HF. In addition, there is a slight change in the OF stretching frequency when the OF diatomic is perturbed by H F versus DF. The blue shift observed for the OF stretch in the complex is also expected since blue shifts were observed for similar complexes: N,- -HF, OC- -HF, and NO- -HF.28-30 OF2 Complexes. Absorptions labeled 6 are due to complexes of H F with OF,. Isolated 160F2was present upon deposition as an impurity in the fluorine sample at 825.5 cm-I. The I60F2 fundamentals in solid argon occur at 920.0 cm-' for u1, 826.0 cm-l for u3, and 466.0 cm-l for u ~ New . ~perturbed ~ bands also appear near the OF2 fundamental modes. The band at 923.2 cm-I near the uI region, the absorption at 820.7 cm-' near the u3 region, the band at 463.4 cm-I near the u2 region, and the 391 1.5-cm-' band can be attributed to the FH- -OF2 complex. The u,(HF) and u2 modes of the complex were obtained from experiments codepositing OF2 with H F or DF. The perturbation of the H F stretching motion is very slight, showing a red shift of only 8.0 cm-I. This is indicative of a very weak complex as was expected and is substantiated by DZP calculations. Other weak complexes include FH- -02,which absorbs at 3917 ad,and FH- -F2,which absorbs at 3915 cm-l in solid argon. The us mode H F complexed with OF2 is expected near the us mode of H F complexed with the very weak bases O2 and F2. In FH--OF,, the symmetric stretch was blue-shifted by 3.3 cm-l but the antisymmetric stretch and symmetric bend were red-shifted by 5.3 and 2.6 cm-I, respectively. Isotopic data for isolated OF, yields 160F2/180F2 ratios of 1.0302 and 1.0338 for u1 and u3, r e ~ p e c t i v e l y . ~Experimental ~+~~ data for the FH- -OF2 complex gives '60F2/'80F2ratios of 1.0278 and 1.0345 for uI and u3, respectively. A second set of perturbed OF, bands grew in on photolysis and increased upon annealing. These bands (also labeled 6) belong to HbF--H,F- -OF2. The q, u3, and u2 modes of the OF2 submolecule were observed at 914.3, 818.7, and 458.6 cm-'. The us modes of H,F and HbFwere determined to be 3858.4 and 3816.2 cm-', respectively, from OF2 experiments. Deuteriated experiments gave H/D ratios of 1.3624 and 1.3612 for the respective us modes. The 160F2/180F2 ratios for uI and u3 were 1.0283 and 1.0336, again in reasonable agreement. The (HF), and F2--(HF), complexes can be used as models to rationalize the u,(HF) values. The dimer (HF),, H,F- -HbF, has the us motion of H,F occuring at 3896 cm-I and the us motion of HbF at 3825.5 cm-'. In F2--H,F- -HbF, the us motion of the H,F submolecule is expected to show a red shift while the us mode of the HbF submolecule is shifted to 3817 cm-1.21 In F 2 0 - H,F- -HbF, the H,F motion is shifted to 3858.4 cm-' and the HbF mode is red-shifted 3816.2 cm-I. Higher Order Complexes. The bands labeled 7 in Figures 1 and 6 are tentatively assigned to the FH- -0FH- -OHz complex arising from photolysis of the dimer complex (H20),- -F2. The only identifiable feature of this complex is an H F stretch at 3687 cm-I that shifts to 2701 cm-' in deuterium experiments. The breadth of the band and the H / D (1,3650) ratio verify that the absorption is due to the stretching motion of perturbed HF. Since the perturbation of H F is somewhat large, a reasonably strong base must be involved. Also,the product was formed on photolysis and increased upon annealing, which suggests formation by association of another water molecule with a primary complex, presumably 3. (28) Andrews, L.; Davis, S.R. J . Chem. Phys. 1985, 83, 4983. (29) Andrews, L.; Arlinghaus, R. T.; Johnson, G. L. J . Chem. Phys. 1983, 78, 6347. (30)Davis, S.R.; Andrews, L.; Trindle, C. 0. J . Chem. Phys. 1987, 86, 6027. (31) Andrews, L.; Withnall, R.; Hunt, R.D. J . Phys. Chem. 1988,92,78. (32) Kim, H.; Pearson, E. F.; Appelman, E. H. J . Chem. Phys. 1972,56, 1. (33) Dizdaroglu, M.; von Sonntag, C.; Schulte-Frohlinde,D. J . Am. Chem. SOC.1975, 97, 7. (34) Beesk,F.; Dizdaroglu, M.; Schulte-Frohlinde,D.; von Sonntag, C. Int. J . Radial. Biol. 1979, 36, 565.
The u,(HF) mode of FH--OH2 occurs at 3555 cm-l.IZ The u,(HF) mode of FH- -OH occurs at 363 1.9 cm-', and for FH- OHF, the absorption occurs at 3638.0 cm-l. If seems that the complex involves hydrogen bonding of the hydrogen of H F to an oxygen atom of either OH or HOF with the addition of another submolecule present in the sample. Water can diffuse through the argon matrix, so the FH- - 0 F H - -OH2 complex is suggested. Mechanisms. The mechanisms of formation of the various products from these experiments will be discussed. The first evidence of product formation came upon deposition, which gave [(H,O)- -(F,)] and higher order versions of this complex, (HzO)2- -(FJ 1 [(Hz0)- 4F2) 21, and [ 2- 4F2) 21 Complexes 1-7 are all formed upon photolysis, but complexes 4-7 show significant growth on annealing whereas complexes 1-3 do not. Photolysis allows for the dissociation of F2 in the complex or isolated in the matrix. As a result, the water molecules in the matrix can react with one F atom to form H F and OH or with two F atoms to form HOF and HF, depending primarily upon whether or not the F2 is trapped adjacent to H 2 0 in the complex or is isolated in the matrix. In the latter case, annealing aids the diffusion and reaction of such F atoms. The OH radical can react with another F atom to form HOF. Complexes 1-3 are just isomeric arrangements of H F and HOF in the same matrix cage that originally contained one H 2 0 molecule and one F2 molecule. Finally, insufficient changes of complexes 1,2, and 3 were observed on annealing to support the involvement of a second water molecule in their formation. 9
H20+ F' 'OH
-
+ F'
HF
+ *OH
HOF
AH = -15 kcal/mo12 (1)
AH = -54 kcal/mol (est),
(2)
In the same matrix cage, it is reasonable to expect all three structural isomers since calculations show that all three have energies within 1 kcal/mol of each other. It is further expected that the most stable product would be favored, but the matrix can absorb excess energy and the HF- -HOF complexes trapped may be those that are formed first. As can be seen in eq 1, complex 4, FH- -OH, forms a hydrogen bond to give a product more stable than its separate components. Annealing, however, does increase the absorptions for the most stable arrangement at the expense of absorptions for the less stable arrangements. In this case, the remaining F atom must have enough energy to diffuse out of the matrix cage and react elsewhere in the matrix. When complexes 5 and 6 are considered, a matrix cage containing one H 2 0 molecule and two F2 molecules will give a more highly fluorinated product. From eq 2, the HOF product can react with an additional F atom to form H F and OF; calculations show this reaction to have an estimated AH of -45 kcal/mol. The formation of FH- -OF is probable if excess F atoms escape the matrix cage. In addition, though, there may be another H F molecule in the matrix cage from reaction 1, and it is therefore possible to form FO- -(HF),. Annealing allows for better diffusion of F atoms and H F molecules and the formation of the OFcontaining complexes. Complex 6, FH- -OF,, is probably formed by reaction of another F atom with complex 5, FH- -OF. HOF
+ F'
-
HF
F'
+ 'OF + *OF
-
AH = -45 kcal/mol
(3)
OFz
(4) Lastly, the formation of FH- -0FH--OH2,complex 7, is possible if two HzO molecules are present in the same matrix cage as one F2 molecule. The F atom must only react with one H 2 0 molecule to form FH--0HF. The remaining water molecule can then hydrogen bond to the HOF fragment via the hydrogen, FH-OFH- -OH2. Since complex 7 grew upon annealing, it is possible that a water molecule diffused into the same matrix cage as an FH- - 0 H F complex molecule since there is evidence of diffusion of H 2 0 on annealing. Conclusion Matrix experiments studying photochemical reactions between H 2 0 and Fz clusters in solid argon were performed. Codeposition
J. Phys. Chem. 1992, 96, 2059-2065 formed Lewis acid-base complexes, H20--F2and (H20)2- - F 2 , by close association in the same matrix cage. Photolysis with 220-1000-nm light allowed for the dissociation of F2molecules and produced three major products, which were isomeric arrangements of HF and HOF. FH- -FOH showed v,(HF) mode at 3781.0 an-'.The FH- -0HF molecule exhibited a v,(HF) mode at 3638.0 an-',and the v,(HF) mode for the most stable complex, HF- -HOF, occured at 3877.9 cm-I. In an N 2 matrix, only the HF- -HOF complex was ~ b s e r v e d .There ~ was no evidence for a cyclic HOF--HF complex although cyclic HSF--HF was a major product in analogous H2S/F2 experiment^.^ Unique to this work was the observation of the radical complexes, FH- -OH and FH- -OF, which were characterized by I8O shifts in the radical fundamentals. The OH radical complex formed by F atom attack on H20during matrix annealing showed a v,(HF) at 363 1.9 cm-' and a perturbed v(0H) stretching mode at 3537.9 cm-I. The OH radical is less basic than H20as deduced
2059
from the observed HF fundamentals and a b initio calculations. Hydroxyl radicals are believed to be the active agents in hydrogen abstraction reactions following ionizing r a d i a t i ~ n ,and ~ ~ .the ~~ basicity of OH will affect its interactions with reactive substrates in biological systems. Further reaction with fluorine gave an OF radical complex, FH- -OF, FH- -OF2and (FH),- -OFz. The very weak base, OF2, supported a v,(HF) mode at 3911.5 cm-' for FH--OF2and the v,(HF) modes a t 3858.4 and 3816.2 cm-' for (FH),--OF2. The argon matrix provides a vehicle for reactions of successive fluorine atoms with water leading first to FH--OH, then to HF- -HOF and FH- -OF, and finally to (FH),- -OF2,which were characterized by infrared spectroscopy. Acknowledgment. We gratefully acknowledge financial support from N.S.F. Grant C H E 88-20764, helpful discussions with R. D. Hunt,and Y.Hannachi for performing an ab initio calculation.
Fluorescence Spectra of Reaction Intermedlates In the Photolyses of Phenylsilane and Phenylmethylsllanes at 77 K: Phenyldimethylsilyl Radicals Hiroshi Hiratsuka,* Tom Masatomi, Kenichi Tonokura, Mitsumasa Taguchi, and Haruo Shizuka Department of Chemistry, Gunma University, Kiryu, Gunma 376, Japan (Received: August 28, 1991; I n Final Form: October 31, 1991)
Fluorescence spectra of the photolyzed phenylsilane and its methyl derivatives have been measured in 3-methylpentane at 77 K. Broad emission bands have been observed around 400 nm for the photolyzed phenylsilane and phenylmethylsilanes. In addition to the broad band, sharp fluorescence bands have been observed for the photolyzed phenyldimethylsilane, phenyltrimethylsilane, and phenylpentamethyldisilane. These sharp emissions are essentially identical with each other and are attributable to phenyldimethylsilyl radicals. Excitation spectra observed for the sharp emissions are well interpreted by the results of CNDO/S-CIcalculations on the phenyldimethylsilyl radical. Fluorescence lifetimes of these emission bands have been determined to be ca. 1 ps by means of the single photon counting technique, which is comparable with those of benzyl radicals. Polarization degree curves have been determined for the emission and excitation spectra of phenyldimethylsilyl radicals, and tentative assignment of electronic transition bands has been carried out.
Introduction The silyl radical is one of the important intermediates as well as silylene and silene in the photochemical reactions of organosilicon molecules and In recent years, much attention has been paid to the optical absorption and ESR spectra of phenylsilyl Though vacuum-UV laser flash photolysis of phenylsilane in the gas phase generates only silylene and phenylsilylene as reaction intermediates,"-I3 phenylsilyl radicals (1) Sakurai, H. J . Organomet. Chem. 1980, 200, 261. (2) Ishikawa, M.; Fuchikami, T.; Kumada, M. J. Orgummet. Chem. 1976, 118, 155. (3) Raabe, G.; Michl, J. Chem. Rev. 1985, 85,419. (4) Hu,S.-S.; Weber, W. P. J . Organomet. Chem. 1986, 369, 155. (5) Trefonas P.,111; West, R.; Miller, R. D. J . Am. Chem. Soc. 1985,107,
2737. (6) Chatgilialoglu, C.; Scaiano, J. C.; Ingold, K. U. Organometallics 1982, I, 466. (7) Chatgilialoglu, C.; Ingold, K. U.; Lusztyk, J.; Nazran, S.U.; Scaiano, J. C. Organometallics 1983, 2, 1332. (8) Mochida, K.; Wakasa, M.; Sakaguchi, Y.; Hayashi, H. J. Am. Chem. Soc. 1987, 109,7942. (9) Sakurai, H.; Umino, H.; Sugiyama, H. J. Am. Chem. SOC.1980, 102, 6837. (10) Gwffroy, M.; Lucken, E. A. C. Helv. Chim. Acta 1970, 53, 813.
0022-3654/92/2096-2059$03.00/0
have also been produced in the condensed phase.'-69 Sakurai et al. studied the photolysis of aryldisilanes and showed that silyl radicals are involved in primary photochemical processes. He pointed out that every photochemical reaction of aryldisilane could be explained in terms of phenylsilyl radicals though the reactions did not necessarily proceed via the free radical processes. In their work, substituted phenylsilyl radicals were chemically trapped by 1,l-di-tert-butylethyleneto give adduct radicals. These adduct radicals were detected by ESR measurement. Chatgilialoglu et al. studied the transient absorption spectra of phenylsilyl radicals produced by hydrogen abstraction of phenylsilane, phenyldimethylsilane, diphenylsiine, and triphenylsiiane using tert-butoxyl radicals produced from di-tert-butylperoxide by UV irradiati~n.~.' Mochida et al. measured transient absorption spectra of the intermediates generated by photoejection of the phenyldimethylsilyl anion, the diphenylmethylsilyl anion, and the triphenylsilyl anion in tetrahydrofuran at rmm temperature! Thew absorption spectra were ascribed to the corresponding phenylsilyl radicals. It is noted (1 1) Blitz, M. A.; Frey, H. M.; Tabbutt, F. D.; Walsh, R. J . fhys. Chem. 1990, 94, 3294. (12) Inoue, G.; Suzuki, M. Chem. f h y s . Lett. 1984, 105, 641.
(13) Baggott, J. E.; Frey, H. M.; Lightfoot, P. D.; Walsh, R. Chem. fhys. Lett. 1986, 125, 22.
0 1992 American Chemical Society
