824
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
L.
Andrews and F. T. Prochaska
Infrared Spectra of the Intramolecular Hydrogen-Bonded Difluorohaloform Anions in Solid Argon Lester Andrews" and Frank T. Prochaska Chemistry Department, University of Virginia. Chariottesville, Virginia 2290 1 (ReceivedAugust 29, 1978) Publication costs assisted by the Petroleum Research Fund
'The compounds CHF,, CHFzC1,CHF2Br,and CHFJ were subjected to argon resonance photoionization during condensation with excess argon at 15 K. In the fluoroform experiments, new absorptions at 3599,1279, 1174, and 603 cm-' are assigned to the electron-captureproduct F-H--(CF2)-,which is supported by C-13 and D isotopic data. In the CHF2Cl and CHF,Br studies both F-H--(CFX)- and X-H--(CF,)- species were characterized by infrared spectra including the C-13 and D substituted molecules. The CHFJ compound produced the analogous F-H--(CFI)- type I11 hydrogen-bonded species, but the iodide elimination product appears to be the type I species I---HCF2.
Introduction Molecular electron capture products have been examined in inert matrices by ESR and IR spectroscopies in a number of recent ~ t u d i e s . l - ~Of particular interest are haloform molecules which can eliminate a halide ion and form an intramolecular hydrogen bonded anion. Two different haloform products have been characterized to date: type I hydrogen bonded species, such as Cl---HCC12718and type I11 hydrogen bonded species, such as F-H--(CClJ or F-H--(CFC1)-.5~6,9Since the degree of proton transfer in the hydrogen bond is governed by the proton affinities of the opposing groups,1o further understanding of this fundamental interaction may be found by comparing infrared spectra of haloform anions formed with different halogens in the hydrogen-bonded position and in the CX2 group, and by deuterium and carbon-13 substitution. We report here a matrix isolation study of the electron-capture products of CHF,, CHF2C1,CHF,Br, and CHF,I; the competing processes of fluoride and chloride, bromide, or iodide elimination can be studied in the latter mixed compounds. Experimental Section The vacuum system, cryogenic apparatus, and discharge photoionization sources have been described previously.11J2 Fluoroform (Peninsular ChemResearch) and CHFzCl (Pennwalt Corp.) were condensed at 77 K and distilled from glass beads in a Pyrex finger to remove volatile impurities. The isotopic fluoroform samples CDF, and 13CHF, were synthesized from CDCl, and I3CHC1, as described previ0us1y.l~ The compounds CDFzCl and 13CHF2C1 were prepared by the reaction of labeled chloroform with an SbF3/SbC15mixture at 40 "C; volatile product was removed by distillation a t -90 "C. Samples of CHF2Br and CDF2Br,prepared by Carver and Andrews from the reaction of CHBr3 with SbF, and Br2,14were distilled from glass beads prior to use in this work. Difluoroiodomethane was synthesized by the Hunsdieker method using CHF2COOH, Ag2C03, and I2 reagents as described by Has~e1dine.l~ After separating the CHFJ and GO2 products near -90 "C, the CHF21 used to prepare matrix samples was distilled from glass beads to remove final traces of volatile impurities. Argon (Air Products, 99.995 70)was used without further purification. Argon matrix gaslreagent gas mixtures of 400/1 to 160011 were deposited a t 2 mM/h with argon gas from the quartz discharge tube a t 2 mM/h for 2 h before a mi0022-3654/79/2083-0824$0 l ,0010
crowave discharge was initiated in the discharge tube with 1-,3-, or 10-mm i.d. orifices. Samples were then codeposited with the discharge operating for 13-20 h; the argon gas from the discharge halved the actual reagent concentration in the matrix. Final expanded-scale spectra were recorded a t 8 cm-l/min on a Beckman IR-12 spectrophotometer, and the matrix samples were photolyzed by a high-pressure mercury arc (Illumination Industries, Inc., BH6-lB, 1000 W) with water and glass (Corning) filters. The spectra were rescanned under expanded-scale conditions after each photolysis to measure any changes in band intensities. Band positions are accurate to better than 1 cm-'.
Results Infrared studies with the four CHF2X precursors will be described. CHF,. Fluoroform was recently examined by matrix photoionization methods in this laboratory13 with particular emphasis on the strong CF3+product absorption a t 1667 cm-l. The 500- and 3500-cm-l regions contained very strong new absorptions which exhibited large deuterium shifts and are the subject of the present study. Table I lists the new absorptions and intensities observed in an Ar/CHF, = 800/1 study using the 10-mm open discharge tube, and the spectrum is shown in Figure la. Of particular interest are the strong doublet at 3562.5 (7 cm-l fwhm) and 3598.5 cm-l with weaker bands a t 3532, 3578, and 3630 cm-l, a new 1 2 7 9 - ~ mabsorption -~ on the side of a strong 1274-cm-' CF4 band, a strong doublet a t 1173.8 and 1167.2 cm-l, and strong new absorptions a t 562, 567, 572 and 603 cm-' labeled A in the figure. A sharp doublet was observed a t 1588.0, 1585.0 cm-l, which is shown in Figure 2b of ref 13. Additional strong absorptions due to CF2,CF3,and CF4 and site-split bands due to OCFz were also ob~erved.l~$~' This sample was warmed to 30-35 K over 10 min and recooled to 14 K, and another expanded-scale spectrum was recorded. In addition to the expected decrease of CF2 and CF3 bands with growth of C2F6and CF4 absorptions, the 3630-cm-l band increased, the 3598-cm-l feature increased and the sharpened peak shifted to 3603.0 cm-l, while the 3562-cm-l band decreased. The 1279-cm-l band decreased, and the very strong 1174-cm-l band decreased while the 1167-cm-' band slightly increased. In the lower frequency region the 572and 603-cm-' bands decreased while the 562- and 567-cm-l bands increased; band absorbance units following sample 0 1979 American Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 7, 7979 825
IR Study of Difluorohaloform Anions in Solid Argon
I 3600
I6
35&
I
I*
2660 2&
I
1300
I
r
I
*
I
1220 TI40" 600 W A V E N U M B E R S (cm-1)
I
500
4( 3
Flgure 1. Infrared spectra of fluoroform samples subjected to argon resonance photoionization during condensation at 15 K: (a) Ar/CHF3 = 800/1, (b) Ar/I3CHF3= 800/1, 90% C-13, and (c) Ar/CDF, = 1600/1, dashed traces show changes caused by warming to 30-35 K and recooling to 15 K.
warming are also given in Table I. These product band intensities were substantially increased in another experiment using a more concentrated Ar/CHF3 = 200/1 sample. Carbon-13 enriched fluoroform (90%) was examined after condensation of an Ar/l3CHF3 = 800/1sample with argon discharge radiation from the 10-mm open tube, and the spectrum is shown in Figure lb. The higher frequency absorptions were observed at 3561.0,3596.5,and 3629 cm-l, a sharp new 1241-cm-l band appeared on the side of the 13CF4band a t 1234 cm-', a very strong doublet was observed at 1144.0 and 1137.5 cm-', and strong new bands were found in the lower frequency range at 562, 566, 572, and 603 cm-l. The sharp doublet was shifted to 1550.5, 1547.5 cm-' ( A = 0.05,0.02) and the 1580-1598-crn-' region was free of absorption. A temperature cycling operation to 30-35 K like that described above for the natural isotopic sample caused the same changes in the new C-13 product band intensities. An additional cycle to 35-40 K continued the trend and further reduced 3561.0 cm-l to A = 0.07, shifted 3596.5 to 3602.0 cm-l and increased it to A = 0.55, reduced 1241 cm-l to A = 0.2, markedly reduced 1144 cm-l to A = 0.2 and slightly decreased 1137 cm-l to A = 0.8, markedly reduced 572 and 603 cm-I and increased 562 and 566 cm-'. Another experiment with an Ar/13CHF3 = 200/l sample doubled the product yield; 2 h of 220-1000-nm photolysis on this matrix sample caused slight changes in band intensities in the same direction as sample warming. A more dilute Ar/CDF, = 1600/1 experiment was performed using the open discharge tube and the infrared spectrum is contrasted in Figure IC.The higher frequency
multiplet labeled A shifted to 2607, 2626, 2636, 2653, and 2662 cm-l, the intermediate bands were found at 1278.0, 1174.8, and 1168.5 cm-l, and the lower frequency multiplet at 421,429,439,449, and 465 cm-l, and Ar,D+ at 644.5 cm-l ( A = 0.16)." In addition, the CF3+ absorption a t 1666.5 cm-l and the 1588.0-,1585.0-cm-' doublet ( A = 0.04,0.02) were unshifted from CHF, studies. Thermal cycling of this sample to 30-35 K over 20 min to allow diffusion caused analogous increases and decreases in the A product absorptions which are shown in dashed traces in Figure IC. Particularly noteworthy are the reversal in the 2626- and 2653-cm-' band intensities, loss of the 1278-cm-' absorption, decrease in the 1175-cm-' band, and the intensity reversal of the 421- and 449-cm-I absorptions. The CDF, precursor absorptions also broadened and slightly reduced, while the 1666.5-cm-' CF3+absorption was reduced from A = 0.10 to 0.03 and the 1588.0,1585.0 cm-' doublet was reduced substantially ( A = 0.02,O.OO). Another experiment was done with an Ar/CDF, = 400/1 sample; the same bands and relative intensities were observed with yields increased three-to-fourfold; 2 h of 220-1000-nm photolysis only slightly affected this spectrum. A similar study was performed with an Ar/CHF3/CDF3 = 800/1/1 sample; the spectrum was identical with those described above for the separate CHF, and CDF, experiments. No new absorptions were observed between the strong hydrogen and deuterium product absorptions in the higher and lower frequency regions. Sample warming again caused the same changes described above. An Ar/CHF3 = 400/1 sample was codeposited with Na atoms from a 220 "C source for 20 h. No NaF or CHFz product was observed; the only new feature was a broad
826
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
L. Andrews and F. T. Prochaska
Figure 2. Infrared spectra of difluorohaloform samples deposited at 15 K with simultaneous argon discharge photoionization: (a) Ar/CHF3 = 80011, (b) Ar/CHF,CI = 40011, (c) Ar/CHF,Br = 40011, and (d) Ar/CHF,I = 40011.
TABLE I : New Product Absorptions (em-') and Intensities (Absorbance Units) in Ar/CHF, = 800/1Matrix Photoionization Experiment and following a Temperature Cycle t o 35 I 2 K absorption Io after warm assign 562 567 572 603 679 770-774 899 96 7 984 1040 1086 1102 1167 1174 1187 1222 1240 1245 1252 1274 1279 1306 1316 1585-1588 1667 1900-1943 3049 3532 3562 3578 3598 3630
0.08 0.11 0.18 0.65 0.05 0.04 0.02
0.01 0.04 0.04 0.22 0.43
1 2
0.1 5 0.10 0.10 0.08
0.24 0.24 0.06 0.30
A A A A
0.05
?
0.04 0.02 0.02
OCF,
0.06 0.04 0.14 0.24
>1 1 0.10 0.06 0.1 2 0.15
1 0.6
2 0.5 0.5 0.01 0.06 0.04 0.15
0.2 0.03 0.04 0.01-0.03 0.08
0.1 0.30 0.04 0.90 0.04 0.30 0.02
0.1 0.22 0.00 0.48 0.07 0.50 0.06
?
OCF, ? 0 3
CF 3 CF, A A ?
CF, OCF, C2F6 CF, CF, A ?
CHF, CF,' CF,' OCF, CHF, A site A A site A A site
band peaked at 2940 cm-l ( A = 0.08) below the parent mode at 3049 cm-l. This matrix sample was photolyzed by an AH-4 lamp for 45 min, by a BH-6 lamp for 40 min
using 290-1000-nm filters, and by the full 220-1000-nm output of the BH-6 arc for 40 min; no changes were produced in the spectrum. One sample of Ar/CHF3 = 400/1 was subjected to 2-keV proton irradition3 for 5 h during deposition at 15 K. The following bands and intensities were observed: 3602 c m ( A = 0.07), 1667 (0.05), 1317 (0.01), 1274 (0.14), 1252 (0.51), 1222 (0.05), 1174 (0.05), 1167 (0.35), and 562, 567 (0.02). CHF2C1. Chlorodifluoromethane and the deuterium substituted compound have been studied by Jacox and Milligan (JM)6 and the present observations are in excellent agreement. The infrared spectrum of an especially productive CHFzCl experiment using a 3-mm orifice tube and an Ar/CHF2Cl = 400/1 sample is contrasted in Figure 2b with the other CHFzX spectra; the new bands are labeled A and B and intensities before and after photolysis are given in Table 11. The photolysis behavior is noteworthy: a 30-min exposure to 420-1000-nm high-pressure mercury arc light had no effect; a similar exposure to 340-600-nm radiation slightly reduced the A bands at 636, 824,1190, and 3456 cm-l and increased the B absorptions at 1143, 1253, and 2688 cm-l; a like exposure to 2901000-nm light continued this trend and grouped a new 357-, 367-cm-' doublet with the 1143-, 1253-, and 2688-cm-l absorptions, and 220-1000-nm photolysis reversed this trend and increased the former and decreased the latter set of bands. In addition, product bands due to CHF2, CF,Cl, and CF2C1+,and Si0 from the discharge tube, are listed in the table.13,14r18,'9 A sample of 90% 13CHF2C1containing a small amount of 13CHF3,I3CO2,and SiF4impurities was synthesized and the spectrum from an experiment using Ar/13CHF2C1 800/1 is shown in Figure 3a in the regions of interest. New bands appeared at 357,367,622,635,801,1117,1162,1223, 2687,3456,3478, and 3626 cm-l; absorptions due to 13CF2C1 at 741,1120, and 1176 cm-l and to l3CFzC1+at 1379 and 1476 cm-l were also observed. Although the spectrum
The Journal of Physical Chemistry, Vol. 63,No. 7, 1979 027
IR Study of Difluorohaloform Anions in Solid Argon
WAVE N u M BERS (cm-1) Figure 3. Infrared spectra of isotopic chlorodifluoromethane samples following matrix photoionization during condensation at 15 K: (a) Ar/13CHF2CI = 800/1, 90% C-13, (b) Ar/CDF,CI = 40011.
TABLE 11: New Product Absorptions (cm-') and Intensities (Absorbance Units) in Ar/CHF,CI = 400/1 M Matrix Photoionization ExDeriment absorption 276 357 367 623 636 761 824 1040 1143 1147 1165 1175 1190 1208 1222 1227 1253 1330 1415 1517 2688 3456 3478 3626
I, 0.04 0.09 0.08 0.17 0.80
hv,
hv,
assign
01.35
0.20
sh 2 0.07 0.12 0.58
sht 2 0.07 0.12 0.13
0.04 0.11 0.09 0.09 0.07 0.50 0.25b 0.00 sh 2 0.07 0.12 0.23 1.3 0.14 0.12 1.1 0.10 0.18 0.09 0.75 0.14 0.15 0.02
impurity B B
0.15 0.24
0.04 0.13 0.11 0.08 0.04
0.04
0.50
0.40 0.09 shC 2 0.07 0.12 0.65 1.3 0.14 0.12 1.0 0.10 0.20 0.10 0.65 0.57 0.32 0.3.0
1.1
1.4
0.68 0.45 0.26 0.08
0.90 0.07 0.11 0.02
A A CF,C1 A 0, B CF,C1 CHF, CHF,
A CF,CI CF, Si0 B C,F, CF,Cl+ CF,Cl+ B A A Asite
a hv I is 30 rnin of 340-600-nm high-pressure mercury arc photolysis, hu, is 30 min of 290-1000 nm, and h v , is 30 min of 220-,1000 nm. Contains weak CHF,Cl absorption. Shoulder.
contains many bands, the 10% C-12 absorptions were observed in the middle region with approriate relative intensity to indicate the vibration of a single carbon atom; for example, the 1223-1220-~m-~doublet is the C-13 counterpart of the 1253-1251-cm-' doublet. The highpressure mercury arc photolysis behavior was identical with the natural isotopic experiment. An experiment was performed with an Ar/CDF2Cl = 400/1 sample; as shown in Figure 3b new product absorptions were observed at 272,462,472, and 482 cm-', in addition to a strong 819-825-cm-l doublet, and intense bands at 1144,1191,1254,1952,2557,2572, and 2720 cm-'. The CFzCl and CF2C1+ absorptions and the 644-cm-' Ar,D+ band were also observed. Photolysis with 340-600and 290-1000-nm light for 30-min periods had no effect on the spectrum; however, the 220-1000-nm photolysis
TABLE 111: New Product Absorptions (cm-I) and Intensities (Absorbance Units) in Ar/CHF,Br = 400/1 Matrix Photoionization Experiment absorption
I,
hula
hv,
hv,
assign
367-362 590 615 625 646 685 723 822 967 1076 1103 1137 1143 1161 1165 1175 1188 1193 1197 1222 1227 1250 1307 1317 1368 1484 1910-1943 2406 3444 3460 3502 3614
0.06 0.01 0.20 0.14 0.06 0.30 0.07 0.05 0.04 0.20 0.65 2 2 0.13 0.08 0.15 0.2sh 0.7sh 1.5 0.21 0.16 0.85 0.00 0.03 0.50 0.14 0.07 0.32 0.47 0.3sh 0.02 0.06
0.06
0.06
? ?
0.20 0.14 0.06
0.18 0.12 0.06
0.07 0.06
0.06 0.08
0.20
0.24
0.06 0.03 0.21 0.15 0.10 0.30 0.09 0.09 0.07 0.37 0.75 2 1.6 0.13 0.07 0.15 0.3 0.8 1.5 0.23 0.15 0.72 0.05 0.03 0.32 0.10 0.09 0.27 0.56 0.35 0.08 0.07
2 2
2 2t
0.08 0.15 0.2sh 0.7sh 1.5
0.08 0.15 0.2sh 0.7sh 1.5
0.85
0.90
0.32 0.47 0.3sh 0.02 0.06
0.35 0.47 0.3sh 0.04 0.07
A A ?
CF,Br
A ?
OCF, ?
CF, CF,Br B ?
CHF, CHF, A A CF,Br CF, Si0 B ?
CHF, CF,Br+ CF,Br+ OCF, B
A A A ?
a hu, is 30 min of 340-600-nm photolysis, hu, is 2901000 nm, photolysis, and h v , is 220-1000-nm photolysis.
markedly increased the 462-, 472-, 482-cm-' set, the 819-825-~m-~ doublet, the 1191-cm-' band, and the 2557-, 2572-, and 2720-cm-' bands, while markedly decreasing the 272-, 1144-, 1254-, and 1952-cm-l absorptions. CHF2Br. A thorough study of bromodifluoromethane and the deuterium isotope was performed with the 1-,3-, and 10-mm orifice discharge tubes. The spectrum from the 3-mm tube study with Ar/CHF2Br = 400/1 is shown in Figure 2c and the absorptions and photolysis behavior are listed in Table 111. Prominent new bands of especial interest were observed at 615, 625, 1143, 1193, 1250, 2406, and 3444 cm-l, in addition to a weaker band at 723 cm-'.
828
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
L. Andrews and F. T. Prochaska
:-fy
m2 c
1 A
I
I
2700
2600
I.
1
25d6 1760
I
.I
1720 1500
1 ,
1300
1200
1106 740
700
" 480
440
WAVENUMBERS (crn-7)
Figure 4. Infrared spectrum of Ar/CDF,Br = 400/1 sample after argon resonance photoionization during condensation at 15 K.
Absorptions due to the CFzBr radical at 685 and 1137 cm-l and the CF2Br+ cation a t 1368 and 1484 cm-l were also o b s e r ~ e d Photolysis . ~ ~ ~ ~ ~with 340-600-nm radiation had no effect on the bands; however, 290-1000- and 220100Q-nm photolysis separated the new product absorptions into two groups: the 615-,625-, 723-, 1193-, and 3444-cm-' bands labeled A clearly increased with the final photolysis, whereas the 1143-, 1250-, and 2406-cm-l bands, denoted B, first increased with 290-nm photolysis and then decreased with 220-nm light. An experiment with CHFzBr using the 1-mm orifice discharge lamp gave 70-80% of the band intensities as the 3-mm experiment detailed here, and a similar study with the 10-mm orifice discharge lamp gave 12@130%. All of these studies showed the same photolysis behavior which associate the 615-, 6 2 5 , 723-, 1193-, and 3444-cm-l bands, and the 1143-, 1250-, and 2406-cm-' absorptions with two different molecular species. Three experiments were done with CDF,Br; infrared spectra from the 3-mm tube experiment after 13 h of sample photoionization are illustrated in Figure 4 and the new bands and intensities are given in Table IV. New absorptions of particular interest were observed at 466 cm-l, a triplet at 713, 724, 738 cm-l, a very strong 1144-cm-' band, 1189- and 1193-cm-l shoulders on a very intense 1197-cm-' CF,Br band, a sharp, strong 1251-cm-l absorption, and broader bands at 1733 and 2550 cm-'. Photolysis with 340-600-nm light caused no change in band intensities, while 290-1000-nm radiation produced only slight changes as given in Table IV. However, exposure to the full 220-1000-nm arc increased absorptions labeled A at 466,713, 724,1189,1193,2550,and 2560 cm-' while bands labeled B a t 1144,1251, and 1733 cm-' were decreased. A temperature cycle to 35 f 2 K altered the relative intensities of site splittings in the A set of bands and slightly decreased the B group of absorptions. Other experiments using the 10-mm tube for 9 h and the 1-mm tube for 18 h produced comparable yields of the same product bands with the same photolysis behavior described for the 3-mm tube study, although growth of the 466-, 724-, 1189-, 1195-, and 2550-cm-' bands was more dramatic in the 1-mm tube study. Sodium atoms were codeposited with an Ar/CHF2Br = 400/1 sample for 20 h and the spectrum was similar to that reported by Carver and Andrew~:'~ no NaF was produced, but NaBr was observed at 285 cm-' ( A = 0.03) as was CHFz a t 1165, 1175, and 1317 cm.-l ( A = 0.09, 0.35, and 0.03, respectively). After 23 min of full-light AH-4 photolysis, sharp, new bands appeared a t 1143 and 1250 cm-' ( A = 0.13 and 0.05) in addition to a broad 2406-cm-l band ( A = 0.02). These bands increased 2570 following 15 min of 290-1000-nm BH-6 photolysis. The sample was then exposed to 220-1000-nm BH-6 light for 20 min: the CHFz bands were sharpened and reduced IO%, the 1143-, 1250-,
TABLE IV: New Product Absorptions (cm-l) and Intensities (Absorbance Units) in Ar/CDF,Br = 400/1 Matrix Photoionization Experiment
-
absorption
I,,
hv,a
hvZb
cyc
367-362 457 466 685 713 7 24 738 822 934 1103 1137 1144 1189 1195 1197 1217 1222 1227 1251 1307 1330 1368 1484 1733 2550 2560 2590 2714
0.06 0.01 0.17 0.19 0.07 0.37 0.12 0.02 0.02 0.57 1 2 0.2shd 0.6sh 1.5 0.09 0.10 0.10 1.1 0.03 0.04 0.38 0.09 0.31 0.51 0.2sh 0.02 0.03
0.06 0.01 0.16
0.06 0.04 0.21 0.19 0.19 0.45 0.12 0.05 0.02 0.70 1 1.5 0.4 0.8 1.5 0.10 0.13 0.11 0.7 0.09 0.04 0.27 0.06 0.19 0.69 0.3 0.11 0.04
0.06 0.10 0.15
0.14 0.35 0.12 0.04 0.02 1 2 0.2sh 0.6sh 1.5
1.1
0.31 0.51 0.2sh 0.04 0.03
0.32 0.22 0.06 0.08 0.27 0.50 1.2 0.1 1.0 0.9 0.06 0.05 0.03 0.5 0.06 0.10 0.13 0.45 0.5 0.05 0.04
assign ?
A A CF,Br A A A ?
CDF, CF, CF,Br B A
A CF,Br CDF, CF, Si0 B ?
C,F, CF,Br* CF,Br+ B A A A ?
a h v , is 30 rnin of 340-600-nm photolysis followed by
h v , is 30 min of 30 min of 290-1000-nm photolysis, cy is a temperature cycle 15 220-1000-nm photolysis. to 35 * 2 to 1 5 K over 10 min. sh denotes a shoulder.
and 2406-cm-' bands remained the same within 2070, and a new 615-, 625-cm-' doublet ( A = 0.02)) a sharp 1195-cm-' band ( A = 0.13) with an 1189-cm-' shoulder ( A = 0.01), and a broad 3444-cm-' band ( A = 0.07) appeared. CHFJ. Two experiments were done with CHFJ using the 1-mm discharge lamp; infrared spectra from the first study are illustrated in Figure 2d and the new bands and intensities are given in Table V. New absorptions were observed a t 1178,2874, and 3380 cm-' in addition to the CFzI radical a t 627 and 1126 cm-' and the CFJ' cation13 at 1321 and 1434 cm-l. Full arc photolysis halved the 1178and 3380-cm-' absorptions and destroyed the 2874-cm-l band; a temperature cycle to 30 K restored most of the 1178- and 3380-cm-' band intensities while no 2874-cm-l absorption was produced. In the second experiment, a similar spectrum was observed. Photolysis with 420-1000and 340-600-nm radiation had no effect on the spectrum; however, exposure to 290-1000-nm light for 30 rnin destroyed the 2874-cm-' band and halved the 1178- and 3380-cm-I absorptions. Another CHFzI experiment was
IR
The Journal of Physical Chemistry, Vol. 83, No.
Study of Difluorohsloform Anions in Solid Argon
TABLE V: New Product Absorptions (cm-') and Intensities (Absorbance Units) in Ar/CHF,I = 400/1 Matrix Photoionization Experiment absorption
I,
hva
cyb
assign
0.06 0.08 ? 0.02 590 0.1 0.1 CFJ 0.1 627 C C CHF,I c 632 0.04 0.03 OCF, 0.02 967 CHF~I C C c 1078 CHF,I C C C 1109 CF,I 0.5 0.5 0.5 1126 ? 0.01 0.3 0.3 1162 CHF, 0.1 0.1 0.1 1165 CHFJ 0.5 0.5 0.5 1172 A 0.3 0.5 0.6 1178 OCF, 0.10 0.04 0.12 1239 CHF~I c C C 1248 CHF 0.02 0.02 0.02 1317 CF,I+ 0.10 0.06 0.04 1321 0.08 0.08 0.02 1330 C,FA CF,P 0.02 0.01 (3.03 1434 OCF, 0.12 0.10 1914-1942 0.04 (I---HCF,) 0.00 0.00 0.02 2874 CHFJ 0.08 0.07 3021 0.08 0.24 0.11 0.18 A 3380 0.04 0.10 A 0.02 3420 0.04 0.04 A 3460 0.00 A site 0.02 0.04 0.04 3550 a hv is 30 min of 220-1000-nm photolysis. cy is temperature cycle to 30 K and back to 14 K in 5 min. Completely absorbing CHFJ precursor absorptions.
run without the argon discharge to measure precursor absorptions, which are also listed in Table V, and any impurities in the sample.
7, 1979 829
1274 cm-l also exhibited this photolysis and sample warming behavior. The very strong doublet at 1167, 1174 cm-' is almost coincident with CHF, at 1165, 1175 cm-l; however, the CHF, absorption at 1316 cm-' is too small for a significant CHF, contribution to this strong doublet. Carbon-13 substitution produced a strong doublet at 1137,1144 cm-' which is different from 13CHF2radical at 1142,1153 cm-l?l and deuterium substitution gave a strong 1168-, 1175-cm-l doublet. The final band of this species appeared at 1279 cm-l on the side of CF4 at 1274 cm-l which may obscure another component of this feature; C-13 substitution shifted the latter new band to 1241 cm-', and deuterium moved the band to 1278 cm-l. The large carbon-13 shifts for the 1167-, 1174-cm-l doublet and the 1279-cm-l band are consistent with C-F stretching vibrations. The above new infrared absorptions labeled A in Figure 1 are assigned to the type I11 intramolecular hydrogenbonded fluoroform molecular anion F-H-- (CF,)-. Although this species was not formed on mercury arc photolysis of Ar/Na/CHF3 samples, the analogous F-H--(CFCl)- and F-H--(CFBr)- species to be discussed below were formed in sodium experiments and their infrared spectra are similar enough to characterize the common parts of these molecular anions. Dissociative electron capture by fluoroform probably requires more energetic electrons, since the C-F bond dissociation energy exceeds the electron affinity of fluorine. It is postulated that electrons liberated in the matrix photoionization of fluoroform and its photolysis products13 are captured by fluoroform, which, under the argon discharge radiation, eliminates fluoride ion, which in turn abstracts the proton to form the type I11 species given in reaction 1.
Discussion The CF,, CF2C1,CF2Br,and CF21free radicals and the CF3+,CF2C1+,CF,IBr+, and CF21+cations have been obCHF, + eF-H--(CF2)(1) served in earlier matrix-isolation s t ~ d i e s ; ~ ~ abJ~J*~~~ The strong doublet at 3562, 3599 cm-l is assigned to us, sorptions due to these species were also found in this work the F-H stretching mode shifted to lower frequency by on CHFB,CHF2C1, CHF,Br, and CHFJ. Of particular hydrogen bonding to CF2-. Hydrogen fluoride forms the interest are the molecular electron capture products in strongest of hydrogen bonds, and the displacement of the these studies, which will be identified and their relative H-F stretching mode from the 3962-cm-' argon matrix stabilities and methods of formation discussed. valuez2 to 3562, 3599 cm-' is consistent with hydrogen Fluoroform. Experiments with fluoroform were charbonding.23 The small 1-2-cm-l carbon-13 shift indicates acterized by two strong bands at 3562.5 and 3598.5 cm-l. that carbon is not directly involved in the vibration; it, These bands exhibited large deuterium shifts to 2626 and 2653 cm-' and small carbon-13 shifts to 3561.0 and 3596.5 however, does suggest that carbon is weakly bound to the cm-l. Since the C-H stretching mode on 12CHF3at 3049 hydrogen and is the source of the "hydrogen bonding" cm-l shifts to 3037 cm-l for 13CHF3,a C-H mode at 3600 perturbation on the H-F vibration. The very large cm-' is expected to exhibit a carbon-13 shift of approxideuterium shifts to 2625, 2653 cm-l confirm this assignmately 14 cm-l; clearly the 3562.5- and 3598.5-cm-l bands ment. The ratios 3562.512625 = 1.3571 and 3598.5/2653 are due to a hydrogen-stretching mode in a carbon= 1.3564 are slightly lower than the HFIDF = 3962/2896 containing species but carbon is not directly involved in = 1.3681 argon matrix ratio consistent with the greater this vibration. The mixed CHF,/CDF, experiment procubic anharmonicity expected for the hydrogen bonded duced each isotopic species with no intermediate comspecies. ponent which shows that this mode involves a single The 562-, 567-, 572-, 603-cm-l multiplet is assigned to hydrogen atom. In a similar manner, a family of bands the hydrogen deformation Vb split by minor differences in a t 562, 567, 572, and 603 cm-' in CHF3 studies exhibited matrix environment or molecular geometry. The small no measurable carbon- 13 shift and were displaced to (0-1 cm-l) carbon-13 and large deuterium shifts are 421-465-cm-' upon deuteration. The mixed experiment supportive of this assignment. again shows that this vibration involves a single hydrogen. The very strong 1167-, 1174-cn1-~doublet is assigned to These new product bands were stable to prolonged the antisymmetric C-F2 stretching mode and the 1279-cm-' 220-1000-nm high-pressure mercury arc photolysis. On band is attributed to the symmetric C-F2 stretching mode thermal cycling the matrix sample to 35 f 2 K, intensity in F-H--(CFz) . The carbon-13 and deuterium substitution reversals were caused in each band group; for example, the data and comparison to the analogous 1103- and 1222-cm-' 3599-cm-' band almost doubled while the 3563-cm-' band modes16 of CF2 support these assignments, which are was halved, and the 562-, 567-cm-l bands more than collected in Table VI. Sample warming increases the 562-, doubled while the 5 ' 2 - , 603-cm-' bands were decreased in 567-, 1167-, and 3599-cm-' absorptions and decreases the like proportion. A very strong doublet at 1167, 1174 cm 672-, 603-, 1174-,1279-,and 3 5 6 3 - c d bands which verifies and a strong 1.279-cm-' band above the CF4 absorption at that these two groups of absorptions are due to different
The Journal of Physical Chemistry, Vol. 83, No. 7, 1979
830
TABLE VI: Vibrational Assignments to Type I11 Hydrogen-Bonded Molecular Anions Involving (CFX)and HF (Species A) vs Vb
"F s V F ~
vs Vb VF VCI
vs
Vb VF
VBr a
3562,3599 567, 603 a , 1279 1166,1174
3561,3597 566,603 a, 1241 1137,1144
2625, 2653 421,449 a, 1278 1168,1175
F-H--(CFCl)-
F-H-(l3CFC1I-
F-D--(CFClI-
3456 623,636 1190 824
3456 622,635 1162 801
2557 462,472,482 1191 819-825
F-H--(CFBrY
F-D--(CFBrl-
3444 615,625 1188,1193 723
2550 457,466 1189,1195 724
F-H-lCFI)3380 1178
Obscured by CF,.
matrix environments or molecular geometries of the same species. Proton radiolysis preferentially produced the former set of F-H--(CF,)- absorptions. The sharp doublet at 1588.0, 1585.0 cm-l in CHFBexperiments is unshifted with CDF3 and shifted to 1550.5, 1547.5 cm-l with WHF,, which is appropriate for a C-F stretching vibration in a species containing no hydrogen. The sharp doublet was unchanged on full arc photolysis and was diminished on sample warming to allow diffusion of trapped species at a rate comparable to CF3+.The sharp 1588-, 1585-cm-l doublet is tentatively assigned to the antisymmetric C-F stretching mode of CF2+. This dihalocarbene cation is probably produced by H F photoelimination from fluoroform parent cation. Ionization of CHF, requires 13.8 eV in the gas phase,24which can be done with 13-15-eV radiation from the open argon discharge tube.25 Chlorodifluoromethane. In a recent study of CHF2C1 and CDF2C1,Jacox and Milligan proposed the molecular anions F--H--CFCl and CHF2-to account for two groups of new bands6 An important diagnostic experiment in the J M work was the production of these bands upon mercury arc photolysis of Ar/Na/CHF,Cl samples which verifies the anion and eliminates a possible cation identification for these new species. The present study includes a 90% C-13 enriched 13CHF2C1 sample and the compound CHF2Br, both of which confirm the fluoride species identification, which we prefer to write F-H--(CFCl)-, and show that the other anionic species is the analogous chloride compound C1-H--(CF2)-. The new carbon-13 data confirm the J M assignments of the 3456-cm-l band to an H-F stretching mode, the 623-, 636-cm-l absorptions to a hydrogen deformation, the 824-cm-l band to a C-C1 stretch, and 1190 cm-l to a C-F stretching mode of the fluoride species. The data on F-H--(CFBr)-, to be discussed in the next section, also strongly support these assignments which are given in Table VI. These same experiments, however, show that the J M identification of CHF2- is not correct. The 2688-cm-l absorption, assigned by J M to the C-H stretch of CHF2-, exhibits a 1-cm-' carbon-13 shift whereas approximately 8 cm-l is expected for a C-H mode; the 2688-cm-' band was displaced to 2406 cm-I in the CHFzBr study, an observation that requires chlorine and bromine, respectively, in this vibrational mode. The ratio 2688/2406 = 1.1172
L. Andrews and
F. T.
Prochaska
TABLE VII: Vibrationd Assignments to Type I11 Hydrogen-Bonded Molecular Anions Involving (CF,)- and HCl or HBr (Species B) Cl-H--(CF,)VS
"b V F ~ V F ~
Cl-D-C1-H--(I3CF2)- (CF,)2687 357, 367 1223 1117
2688 357,367 1253 1143
1952 27 2 1254 1144
Br-H--(CF,)-
Br-D--(CF,)-
2406 1250 1143
1733 1251 1144
VS
VFs VFa
is appropriate for an HC1 to HBr shift, in close agreement with the actual HCl/HBr = 2886/2559 = 1.1278 ratio. The 1143- and 1253-cm-' bands shift to 1117 and 1223 cm-l upon C-13 substitution consistent with their assignment to antisymmetric and symmetric C-F, stretching modes. The final new absorption observed here at 357,367 cm-l showed no C-13 shift and a deuterium shift to 272 cm-l which characterizes a hydrogen deformation mode. The 2688-cm-l band is assigned to vS, the C1-H stretching mode in C1-H--(CF2)-,shifted to lower frequency from the 2886-cm-l HC1 value by hydrogen bonding to CF2-;other assignments to this species are compiled in Table VII. The C1-H--(CF2)-/C1-D--(CF2)- = 2688/1952 = 1.3770 ratio is appropriate for a hydrogen-chlorine stretching vibration, which may be compared to the HCl/DCl = 2886/2091 = 1.3802 ratio. The 357-, 367-cm-' doublet is assigned to Vb, the hydrogen deformation mode. Note that Vb for the chloride compound is considerably lower than the 623-, 636-cm-' value for the fluoride species. The 1143and 1253-cm-l bands are assigned to the two C-F2 modes in CI-H--(CF,)- which are slightly lower than the 1175- and 1279-cm-l values for F-H--(CF2)-. The photolysis behavior of the two groups of bands in the CHF2C1studies show that the two anionic species are interconvertable, reaction 2, which confirms the present identification of the chloride species. F-H--(CFCV
290-1000 nm +220-1000 nm'
CI-H-- (CF2)-
(2)
Rromodifluoromethane. The sodium experiment with CHF,Br produced CHF, and NaBr upon sample condensation; however, on medium-pressure mercury arc photolysis new bands appeared at 1143, 1250, and 2406 cm-l, and on high-pressure mercury arc photolysis, another set of new bands was observed at 614,625,1189, 1195, and 3446 cm-l. These two groups of bands are attributed to two different molecular anions in the CHFzBr system. Both groups of bands were an order of magnitude more intense in the photoionization studies and they correlate with the analogous A and B absorptions in the above CHF2C1work, which can be seen in Tables VI and VII. In species A, bromine substitution for chlorine has little effect except for the carbon-halogen mode; in species B however, the H-X stretching mode is shifted appropriately and the hydrogen bending mode at 357, 367 cm-l in the H-C1 species is probably too low to be observed in the H-Br species. In addition, the full arc photolysis decreases the B species and increases the A species showing that the latter can be made from the former, reaction 3. 220-1000 nm
Br-H--(CF2)F-H--(CFBr)(3) Zododifluoromethane. Experiments with CHFJ produced two new absorptions at 1178 and 3380 cm-' which correlate with the lighter halogen A species and are appropriate for C-F and H-F stretching modes in F-H-+
IR Study of Difluorohaloform Anions in Solid Argon
(CF1)-. The hydrogen deformation mode for this species was either too weak to be observed or obscured by the strong precursor absorption a t 632 cm-'. The other band of interest in these experiments a t 2874 cm-l is probably due to the v, mode of the type I iodide species I---HCF2. The C-H mode of CHF2is expected near the 3049-cm-l CHF3 value and a weak hydrogen bond by iodide ion would displace this mode somewhat to lower frequency. 'Unlike the lighter halide ions with higher proton affinities,%the iodide ion does not abstract a proton to give the type I11 complex, which would adsorb below HI a t 2230 cm-l. The C-H frequency of the present I---HCF2compound a t 2874 cm-l may be compared to this vibration for the similar I---HCC12 species a t 2863 cm-lS8 Bonding Trends. The F-H--(CFX)- complexes exhibit perturbed H-F stretching modes from 3599 to 3380 cm-l, below the H F diatomic fundamental frequency of 3962 cm-l in solid argon. It is interesting to consider the location of the anion electron in this species. Since the electron affinity of CF2 (20.2 eV) is larger than the electron affinity of H F (
