2488
The Journal of Physical Cbemistty, Yo/. 83, No. 19, 1979
water on residual hydrogen-bonded silanol pairs on the silica gel surface; (2) formation of more hydrogen-bonded silanol pairs through hydrolysis of surface siloxanes reached by the growing water aggregates; (3) further water adsorption on these new hydroxyl groups. van Roosmalen, A. J.; Mol, J. C. J . Pbys. Chem. 1978, 82, 2748. Peri, J. 5.J. Pbys. Chem. 1966, 70, 2937. Ryason, P. R.; Russell, B. 6.J. Pbys. Cbem. 1 Little, L. H."Infrared Spectra of Adsorbed Species", Academic Press: London, 1966; pp 258-266. (5) Hair, M. L. "Infrared Spec?roscopy in Surface Chemistry", Marcel Dekker: New 'fork, 1967; pp 84-85. (6) Volkov, A. V.; Kiselev, A. V.; Lygin, V. I. Wuss. J. Phys. Cbem. 1974, 48, 703. (1) (2) (3) (4)
B. W. Keelan and L. Andrews (7) Morrow, 5. A,; Cody, I. A,; Lee, L. S. M. J . Pbys. Chem. 1976, 80, 276 1. (8) Lucchesi, P. J.; Glasson, W. A. J . Am. Chem. Soc. 1956, 78, 1347. (9) Benesi, H. A,; Jones, A. C . J . Phys. Chem. 1959, 63, 179. (10) Davydov, V. 'fa.; Kiselev, A. V.; Zhuravlev, L. T. Trans. Faraday Soc. 1964, 60, 2254. (11) van Roosmalen, A. J., unpublished results. (12) &egg, S.J.; Sing, K. S.W. "Adsorption, Surface Area, and Porosity", Academic Press: New 'fork, 1967; Chapter 111. (13) Elellamy, L. J.; Pace, R. J. J . Specfrocblm. Acta 1966, 22, 525. (14) Mohr, S. C.; Wilk, W. D.; Barrow, 0 . D. J . Am. Chem. SOC. 1965, 87, 3048. (15) Lippincott, E. R.; Schroeder, R. J . Cbem. Phys. 1955, 23, 1099. (16) Armislead, C. G.; Tyler, A. J.; Hambleton, F. H.;Mitchell, S.A.; Hockey, J. A. J. Phys. Cbem. 1969, 73, 3947. (17) Kiselev, A. V.; Lygin, V. I. Russ. Cbem. Rev. 1962, 31, 175. (18) Tursi, A. J.; Nixon, E. R. J . Chem. Phys. 1970, 52, 1521. (19) Magnusson, L. 5. J. Pbys. Chem. 1970, 74, 4221.
~t~a~e Infrared s. Spectra of Parent and onded Parent Anions in Solid Argon at I5 K Brian W. Keeilan and Lester Aradrews" Chemistry Deparintent, University of Virginia, Charloffesvi//e, Virginia 2290 1 (Received March 5, 1979) Publication costs assisted by the Petroleum Research Fund
Dilute samples of dihalofluoromethanes in argon were subjected to argon discharge radiation during condensation at 15 E(,New infrared absorptions were grouped by filtered mercury-arc photolysis and assigned to CFXzradicals, CFX2+cations, (CHFX+)Xdaughter cations, parent cations, and two different types of intramolecular hydrogen-bonded parent anions. The two anions produced upon electron capture by CHFClzshowed no carbon-13 shift for the hydrogen stretching mode, which suggests the F-H--(CCl2)-and Cl-H--(CFCl)- arrangements. The effect of halogen substitution is demonstrated by the observation of similar F-H--(CBrz)- and F-H--(CIJ- and different Rr---HCFBr and I---WCFI intramolecular hydrogen-bonded anions.
Introduction Intramolecular hydrogen-bonded anions of halogenated methanes have been recently studied in argon matrices. A type 111 species, in which the hydrogen-bonding halide ion has a sufficiently high proton affinity to break the carbon-hydrogen bond,I was first observed by Jacox and Milligan (JM) in studies of CHFC12 and CHFzCl prec u r s o r ~ Type . ~ ~ ~1species, in which the hydrogen-bonding halide ion does not have a sufficiently high proton affinity to break the carbon-hydrogen bond, were first identified by Andrews et al. in work on CHGI,, CMBr,, and mixed h a l o f o r r n ~ . ~Wydrogen-bonded ,~ anions exhibit unusual spectroscopic properties; in particular, the effect of different halogens in the halide and carbon-bonded positions on the strength of the hydrogen bond is of interest. The most photosensitive species in recent matrix photoionization studies of chlorofluoro- and brornofluoromethanes were identified as parent cations in infrared and optical spectra.6-8 The photolytically stable daughter cations exhibited unusually high carhon-halogen vibrations which were ascribed to increased K bonding in these planar ~ a t i o n s . ~ , ~ The CI-IFC12,CHFBr,, and CHFB2matrix systems were subjected to argon resonance photoionization, and the infrared spectra were analyzed for both positively and negatively charged products. These studies are described in detail here. 0022-3654/79/2083-2488$0 1.OO/O
Experimental Section The experimental methods and apparatus have been discussed in detai1.5~9J0Samples of dichlorofluoromethane and its deuterium and carbon-13 analogues, dibromofluoromethane and its deuterium analogue, and diiodofluoromethane in argon (Ar/CHFX2 = 250/1, 400/1, or 600/1) were condensed on a 15 K cesium iodide window at approximately 2 mmol/h while exposed to an open argon microwave discharge through a 1-or 3-mm id. orifice for approximately 17 h. Due to codeposition of argon from the quartz discharge tube, the final matrixlreactant ratios in the matrix were double that of the original sample. The CHFClz and CHFBr2spectra were recorded on a Beckman IR-12 infrared spectrophotometer; high resolution scans of regions of interest were taken before and after each filtered high-pressure mercury arc photolysis by using expanded wavenumber scale at 10 cm-l/min in the 2002000-cm-' range, and at 15 cm-l/min in the 2000-4000-~m-~ range. The CHFIz spectra were taken on a Perkin-Elmer 521 spectrophotometer with similar experimental parameters. Dichlorofluoromethane (DuPont, Freon 21) and dibromofluoromethane (Peninsular ChemResearch) were purified before use by glass bead distillation. Deuterated dichlorofluoromethane was synthesized by heating CDC13 (99.8% D) and HgF2 to 90 'C for 30 min and separating the products. Carbon-13 enriched dichlorofluoromethane 0 1979 American Chemical Society
Matrix Photoionization of Dihalofluoromethanes
The Journal of Physical Chemisfty, Vol. 83, No. 19, 1979 2489
TABLE I: Absorptions Observed upon 1 5 K Deposition of Ar/CHFCI, = 400/1, Ar/CDFCl, = 400/1, and Ar/WHFCI, (90% 13C)= 600/1 Samples during Exposure to Open Argon Microwave Discharge Radiation and Band Intensities (Absorbance Units) of the Latter before and after Filtered High-pressure Mercury Arc Photolysisu absorption, cm-l CHFCl,
CDFCI,
425
316
458 692 698 788 811
459 464 512 517 797 812
919
919
964 969 1075 1144 1147 1180
1079 1144 1147 1180
1197 1281 1283, 1277
1197 1279 961
1313 1353 1363
1313 1353 1363
1420 2518 2762 3308 3478
1395 1849 2017 2462 2624
%HFCl, 424 452 457 691 698 767 788 811 890 904 919 942 947 1049 1062 1075 11 16 1120 1152 1158 1180 1197 1251 1285 1292 1283 1318 1328 1328 1420 2518 2754 3307 3477
I,
I2
0.07 0.08 0.14 0.05 0.07 0.15
0.04 0.05 0.08 0.05 0.09 0.17
0.00 0.04 0.00 0.12 0.11 0.18
0,51 0.07 0.33 0.00
0.63 0.08 0.33 0.00
0.61 0.09 0.36 0.01
0.015 0.05 0.03 0.5 0.17 0.05 0.28 0.16 1.5
0.015 0.05 0.04 0.9 0.16 0.11 0.28 0.16 0.78
0.00
0.16
0.17
0.09
0.07
0.03
0.03
0.00 0.00 1.4 0.07 0.16 0.31 0.16 0.05 0.13 0.05 0.02 0.03
0.07 0.005 0.02 0.05 0.03 0.12 0.01 0.39 0.03 0.40 0.09
0.10 0.01 0.02 0.06 0.03 0.17
0.09 0.01 0.02 0.06 0.03 0.17
0.00 0.00 0.05 0.06 0.03 0.04
0.37 0.03 0.28 0.07
0.21 0.03 0.32 0.08
0.01 0.00 0.34 0.09
IO
0.07 0.07 0.14 0.05 0.12 0.22 (shY 0.71 0.09 0.35 0.00 0.04 0.01 0.03 0.03 0.4 0.17 0.04 0.29 0.16 1.5
I3
assignment B B, 13Pb B DCl; A A, HCl; B A ',A CFC1, Ar,H' "CFC1, C C C 13A'
? 12A'
CFC1, CFCl B 13cci; IZB
"eel; P' CHFCl I3C 12C A' CFC1,' CFC1; I3C l2C B Cl---HCFCl A A
a I o , before filtered high-pressure mercury arc photolysis; I , , after 30-min 420-600-nm photolysis; I , , after 30-min 340-600-nm photolysis; I,, after 30-min 220-1000-nm photolysis. Parent. Shoulder.
was made by fluorinating 13CHC13(90% 13C)with SbF,, activated by a catalytic amount of SbC16, at 0 "C for 90 min, and separating the products by gas chromatography, as described previously.ll Deuterated dibromofluoromethane was synthesized by allowing CDBr, (99% D) to react with SbF3/SbFS = 4 / 1 a t 25 "C for 1 h. Diiodofluoromethane was made by mixing CHI, (2 g) and HgFz (1 g) and warming to 50 "C for 15 min.
Results Dichlorofluoromethane. One experiment with CHFClz employed the l-mm i d . orifice discharge tube and an Ar/CHFC12 = 400/1 sample. The spectrum was similar to that reported by JM2 except the product yield was 2 to 4 times greater in the present longer experiment. An identical experiment was conducted with the 3-mm i.d. orifice discharge tube, and the product absorptions, listed in Table I, were generally stronger with the larger orifice. For example, the CFCl2+cation absorptions6 were sixfold stronger, but the C bands were half as intense with the 3-mm tube as compared to the present l-mm tube experiments. A number of product absorptions, identified in Table I, are due to the Ar,H+, CFC12,and CFCl species observed in earlier w ~ r k . ~ J ~ - ' * Examination of the photolysis behavior of these absorptions allows grouping of bands which are due to the same species. Absorptions at 3478,3308,811,698, and 692 cm-l, hereafter denoted as species A, were slightly de-
creased by 420600-nm light, and were increased somewhat on each subsequent photolysis. Bands at 2518,1180,788, 458, and 425 cm-l, hereafter attributed to species B, were relatively unaffected by 420-600-nm radiation, decreased substantially by 340-600-nm light, and virtually destroyed by full photolysis. Absorptions at 1420,1292,969, and 964 cm-l, hereafter denoted as C absorptions, increased with 420-600-nm radiation, were unaffected by 340--600-nm light, and were destroyed by full photolysis. The broad band at 2762 cm-', unaffected by the two filtered photolyses, was destroyed by the full arc. A weak, broad band at 1281 cm-' was markedly reduced by 420-1000-nm triplet was slightly photolysis. An 1197,1194,1191-~m-~ increased on full arc photolysis. One Ar/CDFCI2 = 400/1 experiment was performed with the 3-mm orifice tube, and the product absorptions are listed in the second column of Table I. A number of absorptions were relatively unaffected by 420-600-nm radiation, but were substantially increased by 34C-600-nm and full photolyses: 2624, 2558, 2462, 812, 517, and 512 cm.'l; this behavior is similar to that exhibited by the A absorptions. Several absorptions were relatively unaffected by 420-600-nm light, but were substantially reduced by 340-600-nm radiation, and were destroyed by the full arc: 1849, 1180, 797, 459, 332, and 316 cm-'; this behavior is identical with that shown by species B in the natural isotope experiments. A band at 1395 cm-l was increased with 420-600-nm light, unaffected by 340-600-nm radi-
2490
The Journal of Physical Chemistty, Voi. 83, No. 19, 1979
B. W. Keelan and L. Andrews
v
v I
I
1
n
'3ccl;
13C H F C ,I I
"
&XJ 3440 3380 3320 3260"
I
I
I
I
1
.
I
c
2720 2660 2800 2540 2480 '1390'
I ,
I
I
.
I
A
,
'
I
U I
1280" 1170 114O"95Oc 790 760 WAVENUMBERS (cm-')
1
730 700
I
"460 440
Flgure 1. Portions of the infrared spectrum of a 90% I3C-enrichedAr/l3CHFCI2= 600/1 sample subjected to argon resonance emission during condensation on a 15 K window (a) before high-pressure mercury arc photolysis, (b) following 30-min 420-600-nm photolysis, and (c) following 30-min 220-1000-nm photolysis.
ation, and destroyed by full photolysis, allowing assignment to the deuterium analogue of C. A band at 2017 cm-l was unaffected by filtered photolyses, but was destroyed by the full arc, and is thus the deuterium counterpart of the 2762-cm-l band. A weak, broad 1279-cm-l absorption was virtually destroyed by 420-1000-nm photolysis. CFCl,, CFC12+,and CFCl were also observed, as was the matrix-solvated deuteron species, Ar,D+, at 644 cm-1.6,9,12 One experiment was performed on 90% carbon-13 enriched Ar/CHFC12 = 600/1 sample by employing a 3-mm orifice discharge tube. The absorption positions and intensities are listed in Table I; the important features are shown in Figure 1. The 3477-, 3307-, 788-, 698-, and 691-cm-l absorptions were decreased by 420-600-nm radiation, and were increased by 340-600-nm light and the full arc to near their original intensities; this photolytic behavior is characteristic of species A. Another set of bands, 2518, 1152, 767, 457, 452, and 424 cm-l, was unaffected by 420-600-nm radiation, nearly halved in intensity by 340-600-nm photolysis, and virtually destroyed by the full arc; these absorptions are due to the carbon-13 analogue of B. Three bands, at 1388,1285, and 947 cm-l, displayed behavior analogous to that of the C species; 420-600-nm light caused an increase in band intensities, the 340-600-nm light had little effect, and the full arc destroyed the absorber. A weak band at 2754 cm-l was unaffected by filtered photolyses and destroyed by the full arc, and a weak, broad 1251-cm-l absorption was substantially reduced by 420-600-nm photolysis. The ultraviolet-visible spectrum of an Ar/CHFCl, = 400/1 sample subjected to discharge photolysis for 4 h exhibited a broad continuous absorption, recorded by using techniques described previously.8 This absorption appeared from 440 to 320 nm with superposed structure between 340 and 380 nm due to CFCl and weak structure between 440 and 510 nm appropriate for CC12, similar to the spectrum described by JM.2 The continuous ab-
sorption area centered near 380 nm was markedly reduced by 500-1000-nm photolysis. Pyrex-filtered photolysis reduced absorption in this area slightly leaving the structured CFCl and CClz absorptions. Dibromofluoromethane. Three discharge experiments each were performed on CHFBr, and CDFBr2, as well as sample blanks for each species; in addition a sodium reaction was done with CHFBrz to identify radical and anionic products, using techniques described previ0us1y.l~ Data from an Ar/CHFBr2 = 400/1 experiment, employing a 3-mm orifice, are depicted in Figure 2 and presented in Table 11,including the CFBr2, CFBr2+,CBrz, CHFBr, and HBr,- products reported p r e v i ~ u s l y . ' ~ ~ ~ - ' ~ As in the dichlorofluoromethane study, analysis of photolytic behavior of the product absorptions allows classification of bands by species. Bands denoted as I11 absorbers at 3441,3287, and 673 cm-' decreased somewhat upon 420-600- and 340-600-nm photolysis, and were virtually destroyed with 290-1000-nm radiation. Absorptions attributed to species I at 2782,1091, and 634 cm-' were decreased slightly by 420-600-nm light and virtually destroyed by 340-600-nm radiation. Bands at 1393 and 1286 cm-' tripled in intensity upon exposure to 420600-nm radiation, were decreased by 340-600-nm light, and were destroyed by 290-1000-nm photolysis; these are denoted as species C absorptions. A band at 1020 cm-' was relatively unaffected by 420-600- and 340-600-nm photolyses, but was halved in intensity by 290-1000-nm radiation. The new absorption at 1197 cm-l was virtually destroyed by 420-600-nm light. An Ar/CHFBr2 = 600/1 experiment was performed with a 1-mm orifice; the results were similar to the previously described experiment except for different yields of product species. Absorbances of the I, C, and 1197-cm-l bands were 2-3 times greater in the l-mm experiment whereas species 111 and CFBr2+were produced in threefold greater yield in the 3-mm experiment.
The Journal of Physical Chemistry, Vol. 83,No. 10, 1979 2491
Matrix Photoionization of Dihalofluoromethanes
I
I
I
"
"
I
I
I
1
I
I
I
I
V
I
"
650
I
1/ I m
1
CH FBr, 1
I
1
3400
3300
"2800
1
.
"
1350
I
I
1
I
1200 1300 1250 WAVENUMBERS (cm")
1
1150
I
1100
I
1000
1050
I
*
Figure 2. Infrared spectrum of an Ar/CHFBr, = 400/1 sample exposed to argon resonance radiation during deposition on a 15 K cold window (a) before high-pressure mercury arc photolysis, (b) after 30-min 420-600-nm photolysis, and (c) after 30-min 290-600-nm photolysis.
TABLE 11: Absorptions and Intensities (Absorbance Units) Observed upon 1 5 K Deposition on Ar/CHFBr, = 400/1Sample during Exposure t o Open Argon Microwave Discharge Emission, and Following Filtered High-pressure Mercury Arc Photolysisa absorption, cm-' 626 634 641 673 710 7 28 737 819 822 897 988 1020 1066 1091 1127 1136 1149 1158 1.177 1.197 1226 1286 1309 1393 1402 2782 3049 3287 3441
I,
I,
12
I,
0.05 0.07 0.37
0.03 0.08 0.28
0.00 0.08 0.18
0.00 0.09 0.03
0.07 0.04 0.19 0.08 0.04 0.11 0.28
0.07 0.04 0.20 0.09 0.04 0.11 0.27
0.07 0.04 0.20 0.08 0.04 0.10 0.25
0.08 0.03 0.21 0.08 0.05 0.10 0.13
0.12 0.03 0.12 0.05 0.03 0.06 0.06 0.01 0.04 0.04 0.04 0.07 0.02 0.21 0.07
0.11 0.04 0.13 0.05 0.04
0.01 0.06 0.04 0.04 0.11 0.04 0.05
0.02 0.04 0.13 0.05 0.04 0.01 0.06 0.02 0.04 0.07 0.05 0.00
0.00 0.04 0.13 0.03 0.04 0.01 0.06 0.00 0.04 0.01 0.05 0.00
assignment
P I CBr, 111 P (BrHBr)CZH, ? ? ?
CFBr: CBr: P I ?
CFBr, CHFBr CFBr P
P+(?) Si0 C CFBr: C ?
I P 0.17 0.13 0.03 I11 0.05 0.04 0.01 111 a I,, initial intensity; I , , after 30-min 420-600-nm photolysis; I,, after 30-min 340-600-nm photolysis; i,,after 30-min 290-1000-nm photolysis.
The results of an Ar/CDFBr2 = 400/1 experiment, employing a 3-mm orifice, are tabulated in Table I11 and shown in Figure 3. Bands at 2451,697,508, and 503 cm-l were relatively unaffected by 420-600- and 340-600-nm photolyses, but were decreased by full photolysis, which is analogous to the behavior of species 111 in the natural isotope experiments. Absorptions at 2096,1176, and 595 em-l were decreased slightly by 420-600-nm light and destroyed by 340-600-nm radiation; this behavior is appropriate for the deuterium analogue of I. The deuterium counterpart of the 1393-cm-l absorption was observed at
TABLE 111: Absorptions and Intensities (Absorbance Units) Observed upon 1 5 K Deposition of an Ar/CDFBr, = 400/1Sample during Exposure to Open Argon Microwave Discharge Emission, and Following Filtered High-pressure Mercury Arc Photolysisa absorption, cm-' 498 503 508 592 595 674 697 894 988 1020 1067 1126 1136 1176 1188 1198 1226 1249 1309 1366 1402 2096 2261 2277 2451 2596
I,
I,
12
I,
0.09 0.10 0.09
0.09 0.09 0.09
0.10 0.09 0.08
0.11 0.08 0.07
0.03
0.02
0.00
0.00
0.65
0.63
0.61
0.39
111
P CFBr;( CBr,' P
0.07 0.27
0.07 0.28
0.06 0.26
0.05 0.10
0.03 0.11 0.08 0.06 0.13 0.09 0.08 0.05 0.05 0.03 0.03
0.03 0.11 0.06 0.03 0.08 0.08 0.09 0.05 0.11 0.03 0.02
0.03 0.11 0.00 0.00 0.07 0.09 0.08 0.04 0.09 0.03 0.00
0.04 0.11 0.00 0.00 0.11 0.10 0.07 0.09 0.01 0.03 0.00
0.29 0.03
0.30 0.03
0.29 0.03
0.23 0.02
assignment;
DBr; I11 111 P I P
?
CFBr, I P' ?
S io ?
CFBr,+ C ?
I P P I11 I11
a I,, initial intensity; I,, after 30-min 420-600-nm photolysis; I,, after 30-min 340-600-nm photolysis; I,, aft_er 30-min, 220-1000-nm photolysis.
1366 em-l; it was doubled by 420-600-nm photolysis, decreased slightly by 340-600-nm radiation, and destroyed by full photolysis. The 1020-cm-l absorption was observed unshifted in the deuterium experiments with the same photolysis behavior. An 1 1 8 8 - ~ m band - ~ was halved by 420-600-nm photolysis and destroyed by 340-600-nm photolysis. In one experiment, an Ar/CHFBr2 = 200/1 sample was codeposited with sodium atoms to identify radical and anionic products. A new 1149-cm-' absorption has been assigned to the CHFBr radical.17 Weak bands at 3285 and 673 cm-l ( A = 0.03 and 0.02, respectively) grew in following 1h of unfiltered medium-pressure mercury arc photolysis,
2492
The Journal of Physical Chemistry, Vol. 83, No. 19, 7979 7
1
I
I
2600
2500
V
I
I
'2100
^v
1
1
B. W. Keelan and L. Andrews
I
I
r
I
I
1350
1300
I
nV
I
V
V
I
"
500
I
I
CDFBr2 I
1
1250
1200 W A V E N U M B E R S (cm-')
l
'
1020
A
"
I
700
L
1
Figure 3. Infrared spectrum of an ArlCDFBr, = 40011 sample subjected to argon resonance emission during condensation at 15 K (a) before high-pressure mercury arc photolysis, (b) following 30-min 420-600-nm photolysis, and (c) following 30-min 220-1 000-nm photolysis.
WAV E NU M BER S (cm-')
Figure 4. Portions of the infrared spectrum of an Ar/CHFI, = 250/1 sample exposed to argon resonance radiation during deposition at 15 K (a) before high-pressure mercury arc photolysis and (b) after 30-min 290-1 000-nm photolysis.
which readily ionizes sodium and provides a source of electrons for anion formation.2 These absorptions were destroyed by unfiltered high-pressure mercury arc photolysis. The ultraviolet-visbile spectrum of Ar/CHFBr2 = 400/ 1 sample after codeposition with argon discharge radiation for 3 h showed a broad absorption ( A = 0.10) centered at 370 nm. Photolysis with 420-1000-nm light destroyed the 360-440-nm red half of the original broad band, leaving a weak broad absorption peaked at 360 nm which resisted pyrex-filtered photolysis. Diiodofluoromethane. Four experiments were performed with CHF12, including a sample blank to identify precursor and impurity absorptions. The results from an Ar/CHF12 TZ: 300/ 1 experiment subjected to discharge radiation from a l-mm orifice tube are given in Table IV and Figure 4. Absorptions at 1256 and 1138 cm-' have been assigned to the CHFI free radicalall The new product absorptions a t 914 and 1270 cm-l were reduced slightly,
the 735- and 1149-cm-' bands were halved, and the 2865and 3158-cm-l absorptions were destroyed by 290-1000-nm photolysis.
Discussion New molecular ions and free radicals will be identified and characterized, and hydrogen bonding in haloform electron-capture products will be discussed. (8'-H)--(CX2)-. The A and B absorptions were observed by Jacox and Milligan following argon discharge and alkali metal-mercury arc photolysis experiments.2 The alkali metal studies provide a sound basis for the characterization of A and B as molecular anions. The evidence presented here is in agreement with the J M identification of A as a (F-H--CCl,)- species. The present carbon-13 experiment employed a 90% enriched sample, and the broad 3478- and 3308-cm-' absorptions exhibited 1f 1cm-l carbon-13 shifts, which confirm that these absorptions are not due to a C-H stretching mode,
The Journal of Physical Chemistty, Vol. 83, No. 19, 1979 2493
Matrix Photoionization of Dihalofluoromethanes
TABLE IV: Absorptions and Intensities (Absorbance Units) Observed upon 1 5 K Deposition of an Ar/CHFI, 3 0 0 / 1 Sample during Exposure to Open Argon Microwave Discharge Radiation and Following 2 2 0 - 1 0 0 0 - n m High-pressure Mercury Arc Photolysisa absorption, cm'l 531 63 7 735 914 1041 1059
I,
0.04 0.03
I, 0.02 0.02
species
P P CFI, CFI,'
P P 0.03 0.12
0.00 0.09 0.06 0.05 0.08 0.05 0.03 0.05
0.03 0.07 0.07 0.09 0.05
CHFI CFI, ?
P
0.07
CHFJ CHFI CFI:
0.00
P I
0.03
P
0.00
I11
0.05
Cl'--HCFCl CI---HC@l, Cl---HCClBr Cl---HCClI Cl---HCBr, Br---HCFBr Br---HCCl, Br---HCCIBr Br---HCBr, I---HCF, I---IICF~ I---HCCl,
a I,, initial intensity; I,,after 30-min 290-1000-nm pho. tolysis.
and suggest an H-F vibration as assigned by JM. The H / D ratio 3308/2462 = 1.3436 is slightly lower than the HF/DF diatomic ratio 3962/2896 = 1.3681 in solid argon,lg which indicates more anharmonicity in the 3308-cm-l vibration than in HE' itself. J M proposed that species A is formed upon electron capture by CHFCl, eliminating a fluoride ion which attaches to the hydrogen position giving a C12C--H-F- structure. Our findings support the J M mechanism. However, the unusually high proton affinity of fluoride ion, 370 kcal/mol,20 suggests proton abstraction and an electron distribution more like F-H--CC12-, a type I11 hydrogen-bonded species. It can also be reasonably expected that CC12has a higher electron affinity than HF, in support of the present F-H--CC12model. The 3 3 0 8 - ~ m absorption -~ is, therefore, due to an H-F vibration reduced from the isolated HF value of 3962 cm-' by hydrogen bonding to CC1,-, which is known to broaden and red-shift stretching vibrations.21 The 3478-cm-l band is probably due to the H-F vibration of this species in a different matrix site or structural arrangement. The 698,693-cm-l A feature exhibited no carbon-13 shift and a large deuterium shift to 517,513 cm-l which is appropriate for a hydrogen deformation, in agreement with JM. The strong 811-cm-' absorption shifted to 788 cm-l with carbon-13 substitution, and curiously to 812 cm-l with deuterium. It is interesting to note that this antisymmetric C-C1, stretching mode at 811 cm-' for the CCL- group in F-H--CC12- appears just above the neutral CCl, value a t 746 cm-1.22 The sodium-mercury arc photolysis experiment with dibromofluoromethane produced the 3287- and 673-cm-' absorptions, which demonstrate the anionic nature of species 111. The 3287-cm-l absorption shifted to 2451 cm-l upon deuteration. The proximity of the hydrogen fluoride stretching modes at 3308 and 3478 cm-' in the CHFClz experiments and the 3287- and 3441-cm-l bands in the CHFBr, experiments indicates that the latter bands are due to the bromine analogue of species A, F-H--(CBr,)-; deuterium data support this assignment. An absorption a t 673 cm-', which shifted to 508,503 cm-I upon deuteration, exhibits appropriate behavior for the hydrogen deformation. The (CBr,)- antisymmetric stretch was probably obscured by CHFBr, parent absorption near 700 cm-l; however, this mode was observed at 697 cm-l in the
absorptions, cm-'
Type 111 F-H--(CF,)F-H--( CFC1)F-H--(CCl,)F-H--(CFBr)' F-H--( CBr,)F-H--( CFI)' F-H--(CI,)CI-H--( CF,)CI-H--(CFC1)Br-H--( CF,)-
assignment
P
1100 1138 1149 1164 1178 1245 1256 1270 1281 2865 3037 3158
TABLE V: Comparison of Hydrogen-Stretching Frequencies for Intramolecular Hydrogen-Bonded Anions Identified in This and Other Haloform Studiesa
a
Type 1
3562, 3 5 9 9 3456 3308, 3 4 7 8 3444 3287, 3 4 4 1 3380 3158 2688 2518 2406 2762 2723 2681 2670 2640 2782 2795 2764 2730 2874 2865 2863
References 2-5 and 22.
CDFBr, experiments. Again the additional electron has the effect of increasing the C-Br, fundamental from the 641-cm-l neutral CBr, value.15 An absorption at 3158 cm-I in the diiodofluoromethane experiments is attributed to the H F stretching mode of the analogous F-H--(C12)- anionic species. The H-F stretching fundamentals of the F-H--(CX2)species decrease from 3308 to 3287 to 3158 cm-' for X = C1, Br, and I, demonstrating an increase in hydrogen bond strength in the series. As shown in Table V, a like trend has been observed by Andrews and Prochaska for the F-H--(CFX)- species.23 Apparently, the heavier halogens are less effective in stabilizing the extra electron density at the dihalocarbene, which allows a stronger interaction with hydrogen in the bond to HF. (X-H)--(CFX)-. The strong broad 2518-cm-l absorption of species B in the CHFCl, experiments is deuterium shifted to 1849 cm-l, indicating a hydrogen stretching mode. In the present 90% 13C-enriched CHFCl, experiment, there was no measurable carbon-13 shift for this band, in disagreement with the conclusion of JM from a 55% enriched sample;2 this indicates that the hydrogen stretching vibration does not involve carbon. The 788-cm-l band was displaced to 767 cm-l in the carbon-13 experiment; the position and shift are appropriate for a C-Cl stretching mode. The 1180-~m-~ absorption was carbon-13 shifted 28 cm-l; the position and shift are appropriate for a C-F stretching mode. The 425-cm-l absorption deuterium shifted to 316 cm-l, indicating a hydrogen deformation mode. The 454-cm-l absorption is probably due to the (CFC1)- bending mode. The 9/1 intensity ratio of the 1152-cm-' to 1180-cm-l bands in the carbon-13 experiment shows that the absorber contains only one carbon. As reported earlier, the appearance of the B absorptions on photolysis of sodium-doped samples denotes an anion., The hydrogen stretching mode at 2518 cm-l is not appropriate for an H-F vibration, but is reasonable for a perturbed H-C1 stretching fundamental below the H-Cl monomer value of 2888 cm-l in solid argon.24 By analogy with species A, species B is identified as Cl-H--(CFCl)-, produced upon electron capture, chloride
2494
B. W. Keelan and L. Andrews
The Journal of Physical Chemistry, Vol. 83, No. 19, 1979
ion elimination, and proton abstraction in the same matrix cage, reaction 1. The low hydrogen deformation frequency CHFClz + e-
-
-
(CHFClZ-) Cl-H--(CFCl)- and F-H--(CC12)- (1)
and the great intensity and breadth of the hydrogenstretching absorption are readily explained by the hydrogen bonding in this species. Jacox and Milligan have identified the B absorber as the CHFC1- anion;z this identification is clearly incorrect as the hydrogen-stretching frequency shows no measurable carbon-13 shift. The hydrogen chloride stretching frequency, 2518 cm-l, is below that reportedz3 for C1-H-(CFz)- at 2688 cm-', indicating a stronger interaction with (CFC1)- as compared to (CFJ-. X---HCFX. In the CHFClz experiment, a weak, broad band was observed at 2762 cm-l, which was shifted 8 cm-l lower upon carbon-13 substitution. The position and carbon-13 shift suggest a carbon-hydrogen stretching mode in a hydrogen-bonded species; the deuterium counterpart a t 2017 cm-' is consistent with this identification. The C-H stretching mode of CHFCl is expected near 3000 cm-l, and hydrogen bonding to chloride ion would red-shift this vibration.21 The 2762-cm-l absorption is assigned to the C-H stretching mode in the type I anion Cl---HCFCl. It should be noted that Cl---HCFCl differs from C1-H-(CFC1)- only in the type of hydrogen bonding. The identification of the type I species Cl---HCFCl is supported by the observation of the strongest absorption of the analogous C1---HCClZspecies a t 2723 cm-1.495 In the CHFBr, experiments, species I absorbed at 2782 cm-l and shifted to 2096 cm-l upon deuteration, indicating a hydrogen stretching mode. The intensity and position of this band, relative to that of the parent C-H stretching mode as a model for the HCFBr radical, suggest the type I hydrogen-bonded species, Br---HCFBr, as does the proximity of the Br---HCCl, absorption at 2795 cm-1.415As is expected, type I bromides absorb at higher frequencies than type I chlorides because the bromide ion is a weaker hydrogen-bonding species than the chloride ion. The identification is supported by the observation of species I absorptions at 1091 and 634 cm-l in the natural isotope experiments which may reasonably be assigned to C-F and C-Br stretching modes, respectively. The 634-cm-' band is shifted down to 595 cm-l upon deuteration, as expected; however, the 1091-cm-l band is shifted up to 1197 cm-l upon deuteration, probably due to interaction and mode mixing with the deuterium deformation. In the CHFIz experiments, species I absorbs at 2865 cm-l; the intensity and position relative to the parent C-H mode suggest assignment to the C-H stretching mode in the type I hydrogen-bonded species I---HCFI. This compares very favorably with the analogous 2863-cm-l assignment to I---HCClZand the 2874-cm-' observation for I---HCF2. These fundamentals are substantially higher than the similar type I bromides, as expected. The C-H stretching fundamentals of a number of type I hydrogen-bonded anions are given in Table V. Halogen Effects on the Nature of the Hydrogen Bond. In hydrogen-bonded anions of the haloform stoichiometry, two types of halogens may be distinguished: those which participate in the actual hydrogen bonding, and those which remain bound directly to the carbon atoms. Both types of halogens profoundly influence the nature of the hydrogen bond. The type of hydrogen bond, I or 111, is determined by the magnitude of the proton affinities of the halide ion and carbene ani0n.l The fluoroform anion is a type I11 species because fluoride ion has a greater
proton affinity than (CFz)-,hence the C-H bond is broken in the process of hydrogen bonding, and a species in which (CFJ- is hydrogen bonded to hydrogen fluoride is prod ~ c e d . , ~In contrast, the chloroform anion is a type I species because the proton affinity of (CClZ)-is greater than that of chloride ion, hence the C-H bond is not broken by hydrogen bonding. The smaller halide ions have greater proton affinities than the larger ones.zo Consequently, smaller hydrogen-bonding halogens will favor type I11 species, e.g., the hydrogen-bonding fluorides are type I11 species, while Br-H--(CFz)- is the only known type I11 hydrogen-bonding bromide. The proton affinity of a (CX& fragment increases with increasing halogen size; thus, smaller carbene halogens favor type I11 species; e.g., compare Br-H--(CF2)and Br---HCC12. The H-F stretching mode of F-H--(CClz)- at 3308 cm-l is below t,his vibration for F-H--(CF2)at 3562 cm-l, which indicates a higher proton affinity for (CClZ)-as compared to (CF,)-. This relationship has been observed for the neutral carbenes with proton affinities for CClz and CFz of 208 f 2 and 172 f 2 kcal/mol, respectively.25,z6 Perhaps the most unusual anion studied here is the hydrogen-bonding chloride of CHFC12,which exists as both type I and type I11 species, although the latter dominates. This suggests that the proton affinities of chloride ion and (CFC1)- are comparable. Other Anion Absorptions. New bands were observed as shoulders at 1075 and 1313 cm-l on stronger parent absorptions. These absorptions increased on full arc photolysis and were labeled A' by JM2 Since they also appeared on photolysis of sodium-doped samples, these absorptions are due to other anionic species. The carbon-13 experiment shows that these absorptions are due to single carbon entities and the magnitude of the carbon-13 shifts, from 1313 to 1283 cm-l and from 1075 to 1049 cm-l, denotes C-F vibrations. Hence, these absorptions must be due to different anionic species. The 1313-cm-' absorption is tentatively assigned to the C-F stretching mode in the photolysis product (HCl,-)(CF); its position above the isolated C-F radical2' fundamental at 1279 cm-l is in accord with the obsewation of C-C1 in the analogous (HClL)(CCl)photolysis product.4P The HClF mode conceivably is obscured by the strong A absorption in the 690-705-cm-' region. It is suggested that one mechanism for the photolysis of species B is reaction 2. Cl-H--(CFCl)- hv (220-1000 nm) (HCl,-)(CF) (2) The 1075-cm-l absorption grows substantially on photolysis while species B is destroyed; the deuterium shift to 1079 cm-l demonstrates that the product contains deuterium whose vibration interacts with the 1075-cm-l C-F stretching mode. A reasonable possibility for the 1075-cm-l absorption is the (CHFCl)(Cl-) photolysis product. The C-F vibration may be reduced from the 1151-cm-l value for the isolated CHFCl free radical17due to perturbation by a non-hydrogen bonded C1- in the same matrix cage. Diiodofluoromethvl Radical and Cation. In CHFC1, and CHFBr2 experLments, CFClz and CFBrz radical; respectively, were produced. The CXz antisymmetric stretching modes of these species are 919 and 782 cm-l, re~pectively;'~'~ consequently, the 735-cm-l absorption in the CHFIz experiments can be reasonably assigned to the C-I2 antisymmetric stretching mode of the free radical CF12. This assignment is supported by the observation of this mode at 717 cm-l for the CHI2free radical.,* The C-F stretching modes for CFClz and CFBr, are 1144 and 1136
+
-
Matrix Photoionization of Dihalofluoromethanes
cm-l, respectively; the 1149-cm-' absorption follows the 735-cm-l band on photolysis and the former is tentatively assigned to the CFI, free radical. In the CHFClz and CHFBr, experiments, respectively, CFClZt and CFBr2+were observed, which provide a source of electrons for capture to form anions. The C-F stretching modes of these species are 1353 and 1311 cm-l, respect i ~ e l y ;consequently, ~,~ the 1270-cm-l absorption in the CHFIz experiments may be assigned to the C-F stretch of CF12+. The 914-cm-l absorption is tentatively assigned to the C12 antisymmetric stretch of this species, as it follows the trend established by 1142- and 991-cm-' CX2 stretches of CFC1,' and CFBr2+,respectively.6!7 The CFIz+ cation is probably produced by photoionization of the CFIz radical during sample condensation. Parent Cations. Previous matrix photoionization work with CFC1, and CFBr3 showed that the parent cation is the most photosensitive species produced and trapped.6-8 The ultraviolet-visible absorption spectra recorded after matrix photoionization of CHFCl, and CHFBr, samples revealed broad absorptions from 320 to 440 nm and from 360 to 440 nm, respectively, which were destroyed by photolysis with visible light. These absorptions are appropriate for the parent cations. The photoelectron spectrum (PES) of CHFCl, exhibited bands at 12.0, 12.2, 12.5, and 13.1 eV due to ionization of the nonbonding electrons of chlorine and a strong band with resolved peaks at 14.6 and 14.8 eV which are probably associated with removal of a (C-C1) bonding electron.29 The energy difference from the adiabatic first ionization of CHFCl, at 11.8 eV and the 14.8-eV photoelectron band corresponds to an optical absorption at 413 nm. This PES band difference is in good enough agreement with the photosensitive absorption observed here to assign this continuous 440-320-nm absorption to CHFC12+in solid argon. The transition to a state with ionization of a (C-C1) bonding electron suggests visible photodissociation of CHFC12+eliminating a chlorine atom, reaction 3. The CHFC12+ hv (420-600 nm) (CHFClf)(C1) (3) difference between the appearance potentiaPO of CHFCl+ from CHFC12, 12.7 eV, and the ionization potential of CHFC12, 11.8 eV, is also consistent with the visible photoelimination of C1 from the parent ion. The optical absorption centered at 400 nm and destroyed by 420-1000-nm photolysis in the CHFBr, experiment is assigned to the CHFBr2' parent cation. A small red shift in the transition for CHFBr2+as compared to CHFC12+ is in agreement with earlier work in this laboratory on the CH2Br2+and CHzClz+parent cation^.^' New infrared absorptions destroyed by 420-1000-nm photolysis were observed at 1281 and 1197 cm-' in the CHFCl, and CHFBrz experiments, respectively. The 1281-cm-l absorption shifted to 1279 cm-l with CDFClz and to 1251 cm-' with 13CHFC1z,which is indicative of a C-F stretching mode in a hydrogen-containing molecule. The 1197-cm-l band exhibited a possible CDFBr, counterpart at 1188 cm-l. The photosensitive 1281- and 1197-cm-l absorptions are assigned to C-F stretching modes of the CHFC12+ and CHFBr2+ parent cations. These absorption energies may be compared with 1214and 1160-crn-' assignments to the CFC13+ and CFBr3+ parent cation^.^,^ Ionization of CHFCl,, which requires 11.8 eV in the gas phase, may proceed directly with 13-15-eV radiation from the argon discharge3, or with the more intense 11.6-11.8-eV radiation owing to an expected 1-2 eV solvent shift33in the ionization by the matrix. The ionization energy of CHFBr, is probably about 0.8 eV less than CHFC12which
+
-
The Journal of Physical Chemistry, Vol. 83, No. 79, 7979 2495
provides for direct photoionization of the former by the more intense argon resonance lines. The yield of parent cation absorption was twofold greater in the CHFBr, experiments than in CHFCl, studies, which points to a higher photoionization cross section for the former. Daughter Cations. A set of new absorptions, labeled C, exhibited an extremely unusual photolysis behavior, namely, an increase on 420-600-nm photolysis followed by destruction with 220-1000-nm light. These absorptions, 1420, 1292, and 964 cm-l, were produced with twofold greater intensity in the l-mm diameter orifice tube experiment, while all other product yields decreased. The C absorptions with CHFCl, a t 1420 and 1292 cm-l and a deuterium counterpart of the upper band a t 1395 cm-' were observed by JM, and along with another 1084-~rn-~ absorption, and were tentatively assigned to the CHFC1,' parent cation., The present 420-600-nm photolysis separates the 1084-cm-l band from the other C absorptions, and the carbon-13 isotopic shifts, from 1420 to 1388 cm-l and from 1084 to 1061 cm-', show that both of these bands are due to C-F vibrations. The 1084-cm-' absorption is tentatively assigned to the C-F vibration of another cation species, possibly an aggregate, as yet unidentified. The present carbon-13 isotopic shift for the 1292-cm-l C band to 1285 cm-l denotes a hydrogen deformation mode, whose deuterium counterpart, expected near 1000 cm-', was not observed. The 964-cm-' C band shifted to 947 cm-l with 13CHFC12and exhibited a 942-cm-l satellite with a 3/1 relative intensity due to chlorine isotopes in natural abundance. The isotopic data indicate a C-Cl stretching mode of a single chlorine and carbon. The probable stoichiometry of species C is (CHFCI). The unique photochemical growth of species C with 420-600-nm light which destroys only the parent cation indicates that C is the photolysis product of reaction 3, the CHFCF daughter cation, perturbed slightly by a C1 atom in the same matrix cage. In support of this assignment, the isolated CHFCl+ cation produced from CHzFCl absorbs at 1436 and 1296 cm-l.16 A similar relationship has been observed for the dichloromethyl cation CHC12+produced from CHZClz,1292 and 1045 cm-l, and for (CHC12+)C1formed on matrix photoionization of CHC13, 1291 and 1037 cm-1.5931 The bromine-substituted C species (CHFBr+)Brabsorbs a t 1393 and 1286 cm-l, w,ith a slight bromine shift for the C-F stretching and hydrogen deformation modes, respectively. The isolated CHFBr+ cation absorptions a t 1407 and 1290 cm-' in CH,FBr workI6 are in accord with this assignment, as are the dibromomethyl cation absorptions for CHBr2+ at 1229 and 897 cm-' and for (CHBrz+)Brat 1226 and 900 cm-', re~pectively.~J~ The final destruction of species C absorptions on 220-1000-nm photolysis in CHFCl, studies is accompanied by growth of a triplet absorption at 1197,1194,1191 cm-l which has been assigned to the dichlorocarbene cation,34 reaction 4. Another photodissociation process for species (CHFCl+)Cl+ hu (220-1000 nm)
-
+
CC12+ HF
(4)
C is reaction 5 , also observed in haloform ~ t u d i e s ,as ~?~ (CHFCl+)Cl+ hv (220-1000 nm)
CFClz + Ar,H+ (5)
determined by increase in CFCl, radical and Ar,H+ absorptions on p h o t o l y ~ i s . ~ JThe ~ J ~analogous CBr2+cation was observed at 1020 cm-I in CHFBr2 experiments. The spectroscopy of CC12*and CBr2+,also produced in CHzC12 and CHzBr2studies,31 has been discussed in a separate report.34
2496
The Journal of Physical Chemistty, Vol. 83,No. 79, 7979
I. G. Plotzker and G. J. Exarhos
Conclusions Samples of CHFC12,CHFBr2, and CHFIz subjected to argon discharge radiation during condensation with excess argon at 15 K revealed infrared absorptions which were separated into groups by filtered high-pressure mercury arc photolysis. The new absorptions have been assigned to CFXz free radicals, CFX2+ cations, parent cations, (CHFX+)X daughter cations, and two different intramolecular hydogen-bonded parent anions for each precursor. The hydrogen-stretching vibrations for the two anions produced upon electron capture by CHFClz exhibited no carbon-13 shift, which demonstrates proton abstraction from carbon and suggests the F-H--(CC12)- and Cl-H--(CFCl)- arrangements for these two anions. The effect of halogen substitution in both halide and carbon-bonded positions is demonstrated by the observation of similar type I11 hydrogen-bonded species for F-H-(CBr2)-and F-H--(CIz)- with increasing hydrogen bonding strength and by the different type I hydrogen bonded species for Br---HCFBr and I---HCFI with decreasing hydrogen bonding strength.
(6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25)
Acknowledgment. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the National Science Foundation, Grant CHE 76-11640, for financial support of this research. We also thank Mi-. J. Houston Miller for performing the optical absorption experiments.
(26) (27) (28) (29) (30)
References and Notes (1) Ault, B. S.; Steinback, E.; Pimentel, G. C., J . Phys. Chem. 1975, 79,615. Jacox, M. E.; Milligan, D. E. Chem. Phys. 1976, 16, 195. Jacox, M. E.; Milligan, D. E. Chem. Phys. 1976, 76,381. Andrews, L.; Prochaska, F. T. J. Am. Chem. SOC.1979, 101, 1190. Andrews, L.; Wight, C. A,; Prochaska, F. T.; McDonald, S. A.; Auk,
(31)
(2) (3) (4) (5)
(32) (33) (34)
B. S. J. Mol. Spectrosc. 1978, 73,120. Prochaska, F. T.; Andrews, L. J . Chem. Phys. 1978, 68,5568. Prochaska, F. T.; Andrews, L. J . Phys. Chem. 1978, 82, 1731. Andrews, L.; Prochaska, F. T. J . Phys. Chem. 1979, 83,368. Wight, C. A.; Auk, 8.S.; Andrews, L. J . Chem. Phys. 1976, 65, 1244. Prochaska, F. T.; Andrews, L. J . Chem. Phys. 1977, 67, 1091. Andrews, L.; Willner, H.; Prochaska, F. T. J . Fluorine Chem. 1979, 73, 273. Milliaan. D. E.: Jacox. M. E. J . Mol. Soectrosc. 1973. 46. 460. Milligan; D. E.; Jacox, M. E.; McAuley, j. H.; Smith, C. 'E. J : Mol. Spectrosc. 1973, 45,377. Smith, C. E.; Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1971, 54, 2780. Andrews, L.; Carver, T. G. J. Chem. Phys. 1969, 49,896. Prochaska, F. T.; Andrews, L., to be submitted for publication. Prochaska, F. T.; Keelan, B. W.; Andrews, L. J . Mol. Spectrosc. 1979, 78, 142. Milligan, D. E.; Jacox, M. E. J . Chem. Phys. 1971, 55, 2550. Mason, M. G.; VonHalle; W. G.; Robinson, D. W. J . Chem. Phys. 1971, 54,3491. Beauchamp, J. L. In "Interactions between Ions and Molecules", Ausloos, P., Ed., Plenum: New York, 1975. Pimentel, G. C.; McCleilan, A. L. "The Hydrogen Bond"; W. H. Freeman: San Francisco, 1960. Andrews, L. J. Chem. Phys. 1988, 48, 979. Andrews, L.; Prochaska, F. T. J . Phys. Chem. 1979, 83, 824. Barnes, A. J.; Hallam, H. E.; Scrimshaw, G. F. Trans. Faraday SOC. 1969, 65,3150. Levi, B. A,; Taft, R. W.; Hehne, W. J. J . Am. Chem. SOC. 1977, 99,8454. Vogt, J.; Beauchamp, J. L. J . Am. Chem. SOC.1975, 97,6682. Jacox, M. E.; Milligan, D. E. J . Chem. Phys. 1989, 50, 3252. Smith, D. W.; Andrews, L. J . Phys. Chem. 1972, 76,2718. Doucet, J.; Sauvageau, P.; Sandorfy, C. J. Chem. Phys. 1973, 58, 3708. Hobrock, D. L.; Kiser, R. W. J . Phys. Chem. 1964, 68, 575. The 12.4-eV ionization potential for CHFCI, reported here is probably too high based on the PES in ref 29 and comparison with CHCI, and CHF,CI. Andrews, L.; Prochaska, F. T.; Auk, 6.S. J. Am. Chem. SOC.1979, 101, 9. Andrews, L.; Tevault, D. E.; Smardzewski, R. R. Appl. Spectrosc. 1978, 32, 157. Gedanken, A.; Raz, B.; Jortner, J. J . Chem. Phys. 1973, 58,1178. Andrews, L.; Keelan, B. W. J. Am. Chem. SOC.1979, 701, 3500.
Spectroscopic Studies of Ammonia Reduction of Amorphous AgP03 Irene G. Plotzker and Gregory J. Exarhos* Department of Chemistry, Harvard University, Cambridge, Massachusetts 02 138 (Received April 16, 1979) Publication costs assisted by the National Science Foundation
Silver metaphosphate glasses undergo surface oxidation-reduction reactions with a variety of gaseous reducing agents (H2,Hg, K) at moderate temperatures. Anhydrous ammonia has been found to be an effective reducing agent as well, leading to formation of atomic silver in the glass, followed by atom clustering,and eventual surface metal film growth. Vibrational spectra were measured during surface reduction of amorphous AgP03 under a variety of conditions. Spectroscopic results are used to formulate a model for the gas-solid reduction phenomenon.
Introduction A variety of gaseous reducing agents have been shown to be effective in reducing ionic silver in certain metal oxide glasses of the metaphosphate composition.l Vibrational investigations of such glasses reduced by hydrogen under relatively mild conditions ( T < 200 "C) reveal incorporation of H+ into the glass structure, metal cation reduction, and atom clustering followed by surface metal film growth without alteration of the basic metaphosphate backbone structure. One of the more intriguing reducing agents examined is anhydrous NH3 which reduces Ag+ in 0022-3654/79/2083-2496$01 .OO/O
amorphous Ag20.P205with surprising ease under the mild conditions of 60 "C and a partial pressure of ammonia initially equal to 85 torr. The facility of this reaction and curiosity as to the products formed prompted this spectroscopic investigation. Experimental Section Sodium-free silver metaphosphate glass films of ca. 5 pm thickness were blown from a slowly cooling melt of the stoichiometric crystal (AgPO,). The crystalline isomorph from which the glass was prepared was grown from a melt 0 1979 American
Chemical Society
