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FTIR spectroscopic studies of the matrix photoionization and photolysis products of methylene halides
JOURNAL OF MOLECULAR SPE~TR~G~~W
97, 362-378 (1983)
FTIR Spectroscopic Studies of the Matrix Photoionization and Photolysis Products of Methylene Halides BENUELJ. KELSALLAND LESTERANDREWS Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Matrix photoionization of methylenebromide produced absorptions at 1019, 897, and 788 cm-’ identified previously as CBrl , CHBrl , and CHBrr . High-resolution FTIR spectra revealed overlapping l/2/1 triplets for natural bromine isotopes with individual linewidths near 0.2 cm-‘. New absorptions at 3 121,2897, and 1345 cm-’ are assigned to the (CHrBr+)Br cation complex which yields CHBri on photolysis. A substantially increased yield of the CHCl: species made possible observation of the C-H stretching mode at 3033 cm-’ and the symmetric C-Clz stretching mode at 845 cm-’ along with the previously observed stronger 129 1- and 1044-cm-’ fundamentals. The high resolution and enhanced signal-to-noise capability of the FTIR are clearly demonstrated in this investigation. INTRODUCTION
The molecular spectroscopist often finds shifts in absorption frequencies due to different isotopic substitutions quite useful in characterizing new molecular species. Isotopic splitting patterns can yield information relating to the stoichiometry and symmetry while the frequency shift measurements can give information about the geometry of the molecule. Frequently with heavier atoms, isotope shift data cannot be utilized because the shifts are too small to be resolved. With the advent of commercially available infrared spectrophotometers employing computer-controlled interferometers, the experimentalist has gained a very powerful tool which permits the acquisition of data not readily accessible with conventional prism or grating instruments. Through the use of repetitive scans and the measurement of many transform points, high-resolution spectra (0.06 cm-‘) with good signalto-noise characteristics may be obtained. This new information can be used to explore geometries of molecules containing heavier atoms and, in addition, it can be used to explore small environmental perturbations such as those found in matrices. Matrix isolation studies involving methylene bromide, methylene chloride, and bromochloromethane are described here. These serve to illustrate some of the information which can be obtained from high-resolution infrared spectra and to provide additional information which supplements earlier studies using a grating spectrophotometer (I). EXPERIMENTAL
DETAIL8
The cryogenic and vacuum equipment, and vacuum-uv photoionization source have been described in detail previously (2, 3). Reagent-grade CHzClz and CHzBrz (Aldrich Chemical Company, Inc.), “CH2C12 (90% 13C,Merck, Sharp and Dohme), 0022-2852/83/020362-17$03.00/O Copyri&t Q 1983 by Academic press, Inc. All rights of reproductionin any form reserved.
362
MATRIX STUDIES OF METHYLENE
HALIDES
363
CHzBrCl (Matheson Company, Inc.), and CD#Zlz and CD2Br2 (Merck, Sharp and Dohme) were purified by vacuum distillation and then diluted with argon to give Ar/reagent molar ratios ranging from 400/ 1 to 800/ 1. Diluted methylene halide samples were codeposited at 2 mmole/hr on a lo-25 K CsI plate with a comparable quantity of pure argon introduced through a 3-mm-i.d. quartz discharge tube. Infrared spectra (400-4000 cm-‘) were recorded with a Nicolet 7 199 Fourier transform infrared (FTIR) spectrophotometer operating at 0.12~cm-’ resolution. Spectra were also taken at l.O-cm-’ resolution to minimize Nemst glower and He-Ne laser light irradiation of the sample. Spectra were recorded after sample preparation, diffusion at 22 K, and photolysis with filtered radiation from a high-pressure Hg arc. All of the spectra illustrated in this paper were calculated from the sum of 1000 interferograms. The frequencies reported are accurate to within 0.03 or 0.3 cm-’ depending on the resolution employed. RESULTS
In the first study, argon diluted CH2Br2 (Ar/CH2Br2 = 1600/l) was condensed at 25 K in the absence of argon resonance radiation. Figures 1 and 2 illustrate two groups of absorptions observed with 0.12~cm-’ resolution in the C-Br stretching region. The first figure displays the antisymmetric (b2) C-Br stretching mode after 1.3 hr of sample deposition, while the second figure shows the symmetric (a,) C-Br stretching mode after 20.8 hr. In both cases, the triplets with approximate relative
/ 654
I
652
1
650
1
640
I
646
I
644
WWENUMBER m-11
FIG. 1. FTIR spectrum of methylene bromide v9 fundamental recorded with 0.12-cm-’ resolution from 2 tnmole of ArjCh,Br, = 1600/l sample deposited at 25 K.
364
KELSALL AND ANDREW!3
585
I
I
I
I
584
583
582
581
WRVENUMEER :cnrl)
50
FIG. 2. FTIR spectrum of methylene bromide v3 fundamental recorded at 0.12-cm-’ resolution from 42 mmole of the same sample as in Fig. 1.
intensities of 1:2:1 for natural-abundance bromine isotopes (50.54% 79Br, 49.46% 81Br) indicate that the molecular species giving rise to these absorptions contain two equivalent bromine atoms. The bromine isotopic components were quite sharp; fullwidth at half-maximum (FWHM) was 0.16 -t 0.01 cm-’ for the symmetric mode and about 0.5 cm-’ for the antisymmetric mode. The slight asymmetry which is observed for the antisymmetric C-Br stretch in Fig. 1 is probably due to the presence of a second much weaker multiplet, perhaps a triplet, which is shifted to a slightly lower frequency and underlies the principal absorptions. The low-frequency component is seen in the figure. Table I lists the frequencies and assignments for the absorptions illustrated in Figs. 1 and 2. In a second study, a 1000/l sample was condensed at 25 K, high-resolution spectra were recorded, the sample was cooled to 10 K, and more spectra were recorded. No temperature dependence on linewidth was found. Methylene bromide in argon (Ar/CH2Br2 = 400/l) was condensed on a CsI plate at 10 K while being exposed to the radiation from a microwave-induced argon discharge. The principal product absorptions observed in this experiment were identified in an earlier matrix photoionization study of CH2Br2 (I). Figures 3,4, and 5 illustrate selected regions in the spectra where bromine isotope shifts were resolved using a 0.12~cm-’ resolution. In all of these figures trace (a) was recorded for the freshly prepared matrix sample, trace (b) was taken after sample warming to 22 K for 15 min and cooling to 15 K, and trace (c) was recorded after photolysis by the water
MATRIX STUDIES OF METHYLENE
365
HALIDES
TABLE I Absorption Bands (cm-‘) and Assignments for the Symmetric and Antisymmetric C-Br Stretching Modes of CH2Br2 in a High-Resolution Study
wn-1) a
assignment
582.97
v,(a,)
582.44
v3(a1) CH2 7gBr81Br
581.92
V3kll) CH2 8lBr 2
CH2 7gB,
2
649.41
vg(bl) CH2 79B1.2
648.96
vg(bl) CH2 7gBr81Bl.
648.48
vg(bl) CH2 81B, b
647.93
-1
.
a
Wavenumber accuracy is + 0.03 cm
b
This band is probably due to a second matrix site for CH2 81Br 2'
filtered high-pressure mercury arc (220-1000 nm) for 13 min. The band positions and intensities of the absorptions shown in Figs. 3, 4, and 5, which are attributable to products containing bromine, are listed in Table II.
UAVENLINBER (m-1)
FIG. 3. The FTIR spectrum of CBri produced by vacuum-uv photolysis of Ar/CH2Br2 = 400/l sample during condensation at 10 K is shown in trace (A), spectrum (B) was recorded atter a 1O-22- 15 K thermal cycle, and trace (C) was taken after 220- to lOOO-nm photolysis for 30 min.
366
KELSALL AND ANDREWS
FIG. 4. FTIR spectrum of CHBrr from same sample as in Fig. 3.
781 FIG.
780
778
778 77? WAVENUMBER tdl
778
77b
5. FTIR spectrum of CHBr, from same sample as in Fig. 3.
367
MATRIX STUDIES OF METHYLENE HALIDES TABLE II Product Absorptions (cm-‘), Assignments, and Absorbances Which Show Bromine Isotopic Splittings in a Hi-Resolution Study of CH,Br, Photoionization Products Absorbance
Assignment
11
I3
0.08
0.07
0
(Site A)
0.17
0.15
0
+
(Site A)
0.08e
0.07
+
(Site B)
0.12e
0.11
0.10
C7'Br81Br+
(Site B)
0.22
0.20
0.20
C81Br
(Site B)
C"Br
1019.01
C7'Br81Br+ C81Br
1018.63 c C"Br
2
2 2
+
0
0.11
0.10
0.10
?
0.04
0.04
0.04
?
0.05
0.05
0.04
(Site A)
0.11
0.07
0.06
1017.67
1017.30
I2=
(Site A)
1019.59
1018.17
d
b
knl-l)a
2
+
897.43
CH7'Br
896.90
CH7gBr81Br+
(Site A)
0.20
0.17
0.12
CH'lBr
+
(Site A)
0.10e
0.08
0.05
+
(Site B)
0.06e
0.05
0.04
896.44 CH7'Br
2
2 2
+
895.99
CH7gBr81Br+
(Site B)
0.13
0.12
0.08
895.47
CH81Br
(Site B)
0.06
0.05
0.04
778.77
CH7'Br
(Site A)
0.07
0.07
0.08
778.29
CH7'Br81Br
(Site A)
0.14
0.14
0.15
777.83e
CH81Br
(Site A)
0.07
0.08
0.08
777.94e
CH"Br
(Site B)
0.05
0.06
0.06
777.24
CB7gBr81Br
(Site B)
0.08
0.10
0.11
776.98
CH"Br
(Site B)
0.05
0.06
0.06
2
+
2
2 2
2
-1
. E Wavenumber accuracy is + 0.03 cm Absorbance of band in fFeshlv oreoared matrix samole. ' Absorbance of band after s&l& was warmed to 22 k for 15 min and then retooled to 15 K. d Absorbance of band followinq 13 min of exposure to Hg arc radiation (220-1000 run). e Estimated absorbance contribution of indicated species to the observed band.
Figure 3 shows the spectral region of the antisymmetric C-Br stretching mode of CBr:. Of the six major absorptions observed all are symmetric except for the two lower frequency bands which show asymmetry presumably due to an underlying absorption. The bandwidth at half-maximum (FWHM) for the four sharper bands was 0.16 + 0.0 1 cm-‘. No change in band contour was detected after mild warming, but after photolysis, the 1019.6- and 10 19.0~cm-’ bands were destroyed and the intensity of the 1018.7~cm-’ band was decreased to yield a distorted triplet. The
368
KELSALL AND ANDREWS
intensity data in Table II shows that the absorption pattern can be explained on the basis of two overlapping triplets which overlap with a third unidentified multiplet. Figure 4 shows the spectral region for the CHB$ cation absorption (1). In the freshly prepared matrix the CHBr2f absorption consisted of five bands with FWHM of about 0.2 cm-‘. The contours of these five bands did not change significantly on diffusion or photolysis, but the intensity data in Table II show that the bands decreased with both diffusion and photolysis. The bands labeled site A showed a more rapid decrease with diffusion than those of site B, while the site B bands decreased more rapidly with photolysis. The intensity data suggest that there are two overlapping triplets which give rise to the five detected bands. Figure 5 shows the spectra for the major CHBrz radical absorption. In the freshly prepared sample five principal absorptions were detected, the strongest component had a 0.20 f 0.01~cm-’ FWHM. The bands showed no gross changes with sample warming, although the 777.2~cm-’ peak grew slightly with respect to the 777.8 cm-’ peak. The spectrum of the photolyzed sample shows the continuation of the general trend observed in the diffision experiment. This trend results in the appearance of a shoulder on the high-frequency side of the 777.8~cm-i band. The intensity data for the 777.8- and 779.4~cm-’ bands in Table II are corrected for the partial overlap with each other. The corrected data show that two overlapping triplets are present. The symmetric C-Br stretching mode (5) of CBrz centered at 594.6 cm-’ showed no bromine isotope structure even after diffusion and photolysis. The symmetric CBr stretching mode of CH2Br2, however, was resolved into a sharp triplet as reported in Fig. 2. The antisymmetric C-Br stretching mode of CHzBrz and CBr2 were recorded; bands for both species were broad and showed no resolved bromine isotope absorptions before or after diffusion and photolysis. Since the most photosensitive product absorptions reported in the earlier study (I) were not detected in the high-resolution experiments, three more photoionization experiments were performed with CH2Br2 using 1.0~cm-’ resolution for faster data collection. The most photosensitive product absorptions (destroyed by 290- to lOOOnm photolysis) were observed at 684 cm-’ for CH2Br2+and 546 cm-’ for CD2Br2+in this FTIR study, which gave higher signal to noise, particularly above 2000 cm-‘, than the previous grating studies. New observations in the present FTIR study are (1) a group of bands at 3 121, 2896, and 1345 cm-’ (A = 0.11, 0.32, and 0.02, respectively),’ which were 80% destroyed upon 60 min of 290- to 1000~nm photolysis, (2) a group of absorptions produced by this same photolysis at 3133, 2948, 1361 cm-’ (A = 0.05,O. 12,0.02, respectively), (3) sharp 3099 (A = 0.04), 1128 (A = 0.03), and 525 cm-’ (A = 0.02) absorptions decreased 50% by 290- to lOOO-nm photolysis, and (4) a sharp new 3039.8~nm band (A = 0.02), which increased on 290- to lOOOnm photolysis with the 897~cm-’ (A = 0.11) CHBr2+absorption. Irradiation with the full arc (220- 1000 nm) for 80 min destroyed group ( 1) and (3) bands, halved group (2), and reduced group (4) by 20%. Photolysis behavior in the other two experiments substantiated this grouping of bands. In one of these experiments 590~nm photolysis for 30 min substantially reduced the 684~cm-’ band without affecting other absorp’ The 2896-cm-’ band was reported as 2901 cm-’ in Table IV of the grating study (I) without the needed calibration correction.
MATRIX STUDIES OF METHYLENE HALIDES
369
tions, 340~nm radiation for 90 min decreased group (1) and (3) band absorbances (by 25%) and increased group (2) band intensities without affecting group (4) bands, and 220- to 1000~nm radiation for 15 min reduced group (1) bands to 30%, group (3) to 50%, doubled group (2), and increased group (4) fivefold; spectra are illustrated in Fig. 6c for the sample deposited with vacuum-uv radiation and Fig. 6d after the 220- to 1000~nm photolysis in the 2900- to 3 lOO-cm-’ region. In the other experiment 500-to 1000~nm photolysis for 30 min destroyed the 684-cm-’ band without affecting the other product absorptions. Photolysis at 290-1000 nm decreased groups (1) and (3) and increased groups (2) and (4) as described above. In a similar CD2Br2 experiment, the (1) set of bands appeared at 2358, 2127, and 997 cm-’ and were characterized by their lack of sensitivity to red light, slight photolysis with 340 nm, 80% decrease with 290- to 1000~nm radiation and comlete destruction by the full arc. A sharp 2162-cm-’ band (A = 0.05) produced by 290nm photolysis represents the (2) group and 1030- and 506-cm-’ bands 80% destroyed by 290~nm photolysis are due to the (3) set. Similar experiments were performed with CHzClz and 13CH2C12and the spectra were similar to those reported earlier. The absorption near 764 cm-’ and the sharp band at 1193 cm-‘, which photolysed with red light, have been assigned to the parent cation (I). Other less photosensitive product bands at 3 130, 3 108 (A = 0.08), 2904, 2882 (4 = 0.20), 2764 (A = 0.1 l), and 2475 cm-’ (A = 0.03) were reduced about 40% on 290- to 1000~nm photolysis. Photolysis with the Pyrex-filtered mercury arc produced an eightfold growth in the sharp 1044~cm-’ triplet assigned to isolated
3100
3000 WAVENUMBER
2900 ICRI~)
FIG. 6. The 290- to 31OO-cm-’ region from FTIR spectra recorded with 1.0-c& resolution of Ar/ CH& = 400/l samples subjected to vacuum-uv radiation during condensation at 10 K. (a) Spectrum of CHzClz sample, bands that decrease on 290- to lOOO-nmphotolysis identified with an arrow, (b) spectrum of CH&lBr sample, (c) spectrum of CHZBr2sample, and (d) same as (c) spectrum after 200- to lOOO-nm photolysis for 15 min. Precursor absorptions are labeled P.
370
KELSALL
AND ANDREWS
MATRIX STUDIES OF METHYLENE HALIDES
371
CHCl: ; FWHM for the three chlorine isotopic components listed in Table III were 0.35 cm-‘. This substantial growth of the 129 I- and 1044-cm-’ CHCl: cation bands was accompanied by the growth of a sharp new band at 3032.8 cm-’ and a new band system at 845.4 cm-‘, which are illustrated in Fig. 7a. A sharp 9/6/ 1 relative intensity triplet at 1195.40, 1192.78, and 1190.08 + 0.02 cm-‘, which has been assigned to Ccl: (4), remained after photolysis. Photolysis in the ‘3CH2C12experiment was even more dramatic; a 20-fold growth in the 13CHC1: absorptions was observed upon 290to 1000~nm photolysis for 60 min, as shown in Fig. 7b. As a further diagnostic this sample was irradiated with 220- to lOOO-nm light for 60 min; the ‘3CHCl: absorptions decreased about 40% and a triplet absorption at 1156.93, 11.45 18, and 115 1.4 1 + 0.03 cm-‘, illustrated in Fig. 8 and listed in Table IV, increased 120%. The present CD& experiment using 50% microwave power gave an increased yield of the most photosensitive band at 603 cm-’ (A = 0.08) and shoulder at 6 10 cm-’ (.4 = 0.02), p rovided a new sharp 1083-cm-’ (A = 0.065) band and shoulder
,
1303
1280
WAVEtiMBER (CliTll
850
FIG. 7. Regions in the l.O-cm-’ resolution FTIR spectrum for CHCl$ fundamentals produced by photolysis of CH&lz during condensation with excess argon. (a) Spectrum of vacuum-uv irradiated CH2C12 sample and spectrum after 290- to lOOO-nmphotolysis for 1 hr, (b) spectrum of vacuum-uv irradiated “CH&& sample and spectrum after 290- to lOOO-nmphotolysis for 1 hr.
372
KELSALL AND ANDREWS
tb) r
L
li
zoo
1190
1180
1150
1160
1150
WAVENLltlBERkm-l)
FIG. 8. The FTIR spectrum of the 1150- to 12OOcm-’ region for sample of “CH2C12 subjected to vacuum-uv radiation during condensation of 10 K is shown in trace (a). Spectrum (b) was recorded after photolysis with 290- to lOOO-nm and 220- to lOOO-nm radiation for 1 hr each; the spectrum shows the ‘2CClf and ‘%Clf photoproducts.
at 1088 cm-’ (4 = 0.015), and a reduced yield of the 1195-cm-’ triplet (4 = 0.025). Photolysis with 290- to lOOO-nm radiation caused a 20% increase in chlorine isotopic band systems at 1127.3 (A = 0.03) and 864.8 cm-’ (A = 0.01) and a weak 2234cm-’ absorption (A = 0.005) and a 20% decrease in new 2360- and 2 129-cm-’ bands. However, 220- to 1000~nm radiation increased the 2234, 1127.3, and 864.8 cm-’ bands by a factor of 4, markedly increased the 643.3-cm-’ Ar,D+ band (I) (A = 0.03 to 0.45) and halved the 2360- and 2129-cm-’ absorptions. Another CDICll experiment using 80% microwave power gave a reduced yield of the most photosensitive bands and an increased yield of the 1127-cm-’ (4 = 0.11) and 1195~cm-’ (A = 0.15) bands. A sharp 9/6/ 1 triplet at 1197.0, 1194.4, 119 1.7 cm-’ superimposed with 1195.9-, 1192.7-, 1190. l-cm-’ triplet was destroyed upon photolysis while the 1195-cm-’ triplet remained and the 1127 triplet doubled in absorbance. Two similar experiments were performed with CH&lBr to complement the CH& and CH2Br2 studies. The first, run with the argon discharge excited by 70% diathermy power, gave the same bands as the second, employing 30% power; however, the latter gave substantially more pronounced photochemistry. The strong 993.3-, 988.5-cm-’ doublet (A = 0.15, 0.05) and sharp new bands at 1255.1 (4 = 0.01) and 303.6 cm-’
MATRIX STUDIES OF METHYLENE
HALIDES
373
TABLE IV Infrared Absorptions (cm-‘)’ Assigned to Isotopic Ccl; Species CH2Cl2
Or CD2Cl2 12c35c12+
1195.40
12c35c137c1+
1192.78 1190.08
13
12c3'c12+
CH,Cl,
1156.93
13c35c12+ 13C35C137Clt
1154.18 1151.41
a Wavenumber
accuracy
13c37c12+
is + 0.03 cm
-1
,
(A = 0.04) grew a factor of 20 upon 290- to 1000~nm photolysis for 60 min; these bands were destroyed by a 60-min exposure to the full arc. A strong new 29 14-, 2930cm-’ product doublet (A = 0.04, 0.06) and two weaker bands at 3092, 3 119 cm-’ (4 = 0.02) are shown in Fig. 6b. These absorptions were destroyed by Pyrex-filtered photolysis. Bands at 86 1, 866, 1187, and 1195 cm-’ due to CHClBr radical (I) and a sharp 1116.6-, 1120.6-cm-’ doublet (A = 0.06, 0.02) assigned to CClBr+ (4) increased some with Pyrex- and more with water-filtered photolysis. Two weak new absorptions at 713.8 and 529.6 cm-’ (A = 0.03) were destroyed by Pyrex-filtered photolysis. DISCUSSION
The bromine isotopic splittings and linewidths will be considered followed by new assignments to the parent and two daugher cations.
Bromine Isotopic Splittings and Linewidths The very sharp bromine isotopic splittings observed for CH2Br2 at high dilution in solid argon are of considerable interest. The symmetric C-Br stretching mode v3(u,) (Fig. 1) exhibits a slightly larger isotopic splitting than the antisymmetric CBr stretching mode v&r) (Fig. 2); however, the better resolution of the former is due to substantially sharper linewidths, 0.16 -I-0.0 1 cm-’ as compared to 0.5 +- 0.1 cm-’ for the latter. The lack of a temperature dependence for the linewidth shows that the lines are inhomogeneously broadened probably owing to a collection of unresolved matrix sites. The sharpest lines observed for CH2Br2 in solid argon are comparable to the linewidth (0.19-O-25 cm-‘) found for the Q branch of HCl in solid argon using a high-resolution grating instrument (6), the v3 mode of SO2 in solid argon (0.1-0.2 cm-‘) using a diode laser spectrometer (7), but not as sharp as the split v3 components of SF6 in solid argon (0.03-0.12 cm-‘) using FTIR (8). In all of these cases, many
374
KELSALL AND ANDREWS
of the narrow lines that make up the broader band profile observed with lower resolution are ascribed to “matrix site effects” owing to different orientations of the guest molecule in the matrix or of the matrix atoms around the guest molecule. The bromide isotopic splitting for the cation and radical product species are of perhaps more interest as an additional source of information on matrix sites responsible for inhomogeneous line broadening. In the case of the 10 19-cm-’ absorption assigned earlier (4) to CBr2f (Fig. 3), no change in relative intensities of the several components was observed after warming to 22 K but photolysis destroyed the 10 19.6and 10 19.0-cm-’ bands and decreased the 1018.7-cm-’ band to yield a triplet distorted by unidentified absorption at 10 17.8 cm-‘. The individual lines in the CBri multiplet are exceptionally sharp, 0.16 + 0.0 l-cm-’ FWHM. The intensity data in Table II shows that the original absorption pattern can be explained as two overlapping triplets, each due to CBrzf with a different local matrix environment, which overlap to a lesser degree with a third unidentified absorption. The 1/2/l relative intensity patterns for the triplets indicate that the two bromine atoms in CBr2+are equivalent in both matrix sites, one more photosensitive than the other. The multiplets at 897 and 778 cm-’ (Figs. 4 and 5), assigned previously (I) to CHBr2+and CHBr2, behaved similarly. The components are quite sharp, 0.20-cm-’ FWHM, and decrease upon sample warming and photolysis. The intensity data show that two overlapping triplets give rise to the five observed lines. Since the three isotopic components of each triplet have 1/2/l relative intensities, the two bromine atoms in CHBr$ and CHBrz are equivalent in both matrix sites of each species. The C79Br2+and C*lBr2+isotopic frequencies for the antisymmetric C-Br stretching vibration provide a basis for calculation of the Br-C-Br valence angle. The bromine isotopic frequencies for both matrix sites give a valence angle upper limit (9) of 147”. This may be compared with a similar calculation of 138.0” for the valence angle of Ccl: from the present ‘2C35Clt and 12C3’C12fisotopic frequencies. More extensive data available for Ccl: from carbon-13 substitution gives 137.6” from the ‘3C35C1: and 13C35C1:data, 126.7” from the ‘2C35C1:and ‘3C35C12+ data, and 126.6” from the 12C37C1:and 13C3’C12+ isotopic pairings. The average from the upper and lower limits from terminal and apex atom isotopic substitution, 132 f 6”, represents a reasonable measure of the valence angle in Ccl:. A like consideration for the 147’ upper limit on CBr2+suggests a 140 f 8” valence angle for CBr: . It is reasonable for CBrzf to have a slightly larger valence angle than Ccl,‘, but the present angle determinations are_not sufficiently accurate to make a definitive statement. Parent Cations Some new data were obtained for the CD2C12+parent ion which complements the vibrational picture for the strongly interacting antisymmetric (b2) vs and u9 modes observed for these ions. An increased yield of CD2Cl: in the present 50% microwave power experiment gave 603-, 610-cm-‘, and 1083-, 1087-cm-’ doublets for this species. Since the structure of CH2C1: is not known, rigorous product rule calculations cannot be performed, but a simple check on the vibrational interaction between the two modes in the b2 class can be made by comparing v8 X u9 product ratio for the (O/H) isotopes of the parent cation and the neutral molecule using matrix frequencies
MATRIX STUDIES OF METHYLENE
HALIDES
375
for CHCl: (vg = 1193 cm-‘, u9 = 764 cm-‘) and CD&l; (vs = 1083 cm-‘; u9 = 603 cm-‘). The cation ratio is 0.7 16 as compared to 0.732 for the molecule. This general agreement is supportive of the parent cation assignments. The 684- and 546-cm-’ antisymmetric C-Br stretching mode bands for CH2Br: and CDzBrt have been observed without detecting the wagging modes of the same symmetry. A new less photosensitive (50% destroyed by 290 photolysis) species has been observed at 3099, 1128, and 525 cm-’ in the CH2Br2 experiments and at 1030 and 506 cm-’ with CD2Br2. This group (3) absorption species cannot be identified from the present data. Daughter Cations The CHClt daughter cation was first identified in CHCl3 matrix photoionization experiments (10); later work, however, showed this to be the (CHC@Cl species (II) and gave slightly different absorptions for the isolated CHCl: cation prepared from CHClz (1). The present studies produced an unusually large yield of CHCl: upon 290- to lOOO-nm photolysis, which enabled two weaker important vibrational fundamentals to be observed, as shown in Fig. 7. The increased signal-to-noise capability of the FTIR technique, particularly in the higher-frequency region where matrix samples are more scattering, enabled a sharp new 3032.8cm-’ band and a sharp new 9/6/l triplet absorption beginning at 845 cm-’ to be observed. The analogous 13CHC1: bands shifted to 3023.0 and 825.5 cm-‘, which indicates assignment of these bands to the C-H and symmetric C-Cl;! stretching modes of the CHCl: carbocation. These weaker new bands can be assigned to 13CHCll with confidence because they increased with the more intense bands on 290- to lOOO-nm photolysis and decreased with the more intense bands upon 220- to 1000~nm photolysis. Growth of CDCl: upon photolysis was much less extensive; a weak new band at 2234 cm-’ could be due to the C-D stretching mode in CDCl:. Chemical support for observation of the C-H stretching mode of CHCI: at 3033 cm-’ is found with the bromine-substituted precursors. In the CH$lBr experiments, 290-nm photolysis increased the strong 993.3-, 988.5-cm-’ CHClBr+ doublet and produced weak bands at 1255 and 3036 cm-‘. In CH2Br2 studies a weak 3039.8cm-’ band appeared on photolysis with growth in the stronger 897-cm-’ CHBrf absorption (I). These C-H fundamentals may be compared with CHC13, CHC12Br, and CHBr3 at 3054, 3059, and 3065 cm-’ in solid argon (II). It is readily seen that the C-H stretching mode in the carbocation is not significantly changed from the C-H mode in the saturated hydrocarbon, which demonstrates that the C-H bond is not affected by the cation center. This, however, is. not the case for the chlorine atoms, which readily conjugate with the vacant p orbital on the carbocation and substantially increase the C-Cl stretching modes. The two C-Cl stretching modes for CHClt (1045, 845 cm-‘) are higher than the isoelectronic molecule BHClz (892, 740 cm-‘) (12) where some ?r bonding is probably also involved, and the saturated molecule CHzClz (748, 710 cm-‘) where no ‘lrbonding is possible. High-resolution spectra gave 0.35 f 0.03-cm-’ FWHM for the chlorine isotopic
376
KELSALL
AND
ANDREWS
triplets at 1045 and 1019 cm-’ for CHCl: and ‘3CHCl: ; additional much weaker sites were observed at 1050 and 1025 cm-’ for each carbon isotope. The CHBrz species gave sharper lines with 0.18 f 0.02-cm-’ FWHM and two sites separated by 0.9 cm-‘. It is perhaps surprising for CHBr2+ to exhibit sharper lines than CHC12f. If these linewidths are due to a collection of matrix sites, then CHBr2+ occupies more uniform sites than CHCQ, which may be due to specific local packing arrangements between the guest ion and the host matrix. It was proposed in the earlier study with methylene halides that the CHC12+daughter cation was produced upon hydrogen photodetachment from the other daughter cation complex (CH,Cl’)Cl thought to be a decomposition product of the CH&l: parent cation (I). The present studies provide evidence for the (CH&‘)X species produced and trapped in the original photoionization process, presumably from decomposition of the parent cation. The uv spectra of the parent ions clearly show a larger yield for CH2Br: than CH$I: . Accordingly, the (CH2Br+)Br species is probably produced in a correspondingly larger yield than (CH#Y)Cl, and intense new bands in the CH2Br2 experiments are considered for assignment to the (CH2Br+)Br species. The strongest product band in methylene bromide experiments in the C-H stretching region at 2897 cm-’ was accompanied by bands at 3 121 and 1345 cm-’ which were 80% destroyed on 290- to lOOO-nm photolysis that produced a similar new trio at 3133, 2948, and 1361 cm-’ (Fig. 6). Methylene bromide-d* counterparts of the original set at 2358,2127, and 977 cm-’ were replaced by the strongest d2 counterpart at 2 162 cm-’ upon photolysis; the weaker d2 counterpart was probably obscured by CO* absorption in this region. The observation of two new bands in the C-H stretching region (2897 and 3 121 cm-‘) that photolyse together is indicative of a two-hydrogen species. The corresponding d2 counterparts at 2 127 and 2358 cm-’ are reasonable isotopic shifts for C-H(D) stretching vibrations (2897 cm-‘/2 127 cm-’ = 1.362; 3 12 1 cm-‘/2358 cm-’ = 1.324). Similar ratios were observed for CH2Br2/CD2Br2 in solid argon (vl, 3006 cm-‘/2205 cm-’ = 1.363; vg, 3073 cm-‘/23 17 cm-’ = 1.328). The isotopic frequency ratios suggest that the more intense 2897-cm-’ band is the symmetric C-H2 stretching mode and that the sharper, weaker 3 12 l-cm-’ band is the antisymmetric C-H2 stretching mode. The new 1345-cm-’ band and its d2 counterpart at 997 cm-’ are near the symmetric scissor bending modes of CHtBr radical at 1355 cm-’ and CDzBr at 1076 cm-’ (IS), which is su=estive of a similar type of vibrational mode. On the basis of the above vibrational data, the 3 12 1-, 2897-, and 1345-cm-’ bands are assigned to the CH2Br+ daughter ion in the (CH2Brf)Br complex. It is perhaps surprising that the C-Br stretching mode for this species was not detected particularly in view of the fact that the symmetric C-Br stretch and scissor bend are of comparable intensity for CH*Br radical (13). Photolysis with 290- to lOOO-nm radiation produced the CHB$ daughter cation absorbing at 897 cm-’ (by hydrogen atom elimination) and another species with sharp absorptions at 3 133,2948, and 136 1 cm-‘. The latter set of absorptions maps the above set assigned to the CH*Br+ complex, which suggests assignment to another structural form of the complex or the isolated CH2Br+ daughter cation itself. The observation of two sets of similar bands in the CH&lBr experiment at 3 119, .
MATRIX STUDIES OF METHYLENE
HALIDES
377
3092,2930, and 29 14 cm-’ supports identification of the daughter ion complex; two possibilities exist, (CH&l+)Br and (CH*Br+)Cl. In the CHzClz experiments, an analogous set of bands was observed at 3 108 and 2882 cm-‘; ‘%H$l2 shifted these bands to 3096 and 2875 cm-’ and CD&l2 produced the stronger band at 2 129 cm-‘. These absorptions are believed to be due to the (CHJJl+)Cl complex, which yields CHCl: on 290- to 1000~nm photolysis. The weaker 3130- and 2904cm-’ bands in CH2Clz experiments (Fig. 6b) are probably counterparts of the stronger 3108- and 2882-cm-* bands due to a different arrangement of the (CH$l+)Cl complex: (CH&l+)Cl + h~(290-1000 nm) - CHCl: + H. Finally, it should be mentioned that the growth of CC12+on full arc (220- 1000 nm) photolysis at the expense of CHCl: indicates H photodissociation from the latter: CHClz + hv(220-12 000 nm) - Ccl: + H. The major photochemical processes have been discussed in the earlier report (I). The present experiments gave a larger increase in the CHCl$ cation yield on 290- to 1000~nm photolysis, which made possible the detection of two weaker fundamentals for this daughter ion. In general the mercury arc photochemistry of these samples is more dramatic when less intense vacuum ultraviolet radiation is used to prepare the original sample. CONCLUSIONS
FTIR spectra of matrix samples provide high resolution and increased signal-tonoise capability. In the case of bromocarbon species, bromine isotope splittings were resolved and linewidths of 0.16 + 0.0 1 cm-’ were observed for the v3 mode of CH2Br2 and the v3 mode of CBr2+.New photosensitive absorptions at 3 12 1, 2897, and 1345 cm-’ are assigned to the (CHzBr{)Br complex, which eliminates H to give CHBr$. A markedly increased yield of CHCl,’ allowed observation of the C-H stretching mode at 3033 cm-’ and the symmetric C-Cl2 stretching mode at 845 cm-‘; the vibrational spectrum of CHCl: reveals a normal C-H bond and unusually strong C-Cl bonds which indicates conjugation of chlorine to the positive carbon center. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Foundation under Grant of CHE 79-10966. RECEIVED:
August 23, 1982 REFERENCES
1. 2. 3. 4.
L. ANDREWS,F. T. PROCHASKA,ANLIB. S. AULT, J. Amer. Chem. Sot. 101,9-15 (1979). F. T. PROCHASKAAND L. ANDREWS,J. Chem. Phys. 67, 1091-1098 (1977). L. AMIREWS,D. E. TEVAULT, AND R. R. SMARDZEWSKI, Appl. Specfrosc. 32, 157-160 (1978). L. ANDREWSAND B. W. KEELAN, J. Amer. Chem. Sot. 101, 3500-3504 (1979).
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5. L. ANDREWS ANLI T. G. CARVER, J. Chem. Phys. 49, 896-902 (1969). 6. M. ALLAVENA, H. CHAKRO~N, AND D. WHITE, “Molecular Spectroscopy of Dense Phases-Proceedings of the 12th European Congress on Molecular Spectroscopy,” Strasbourg, France, 1975, p.
365-368. 7. M. DUBS AND Hs. H. GUNTHARD, Chem. Phys. Las. 47,421-425 (1977). 8. B. I. SWANSONAND L. H. JONES,J. Chem. Phys. 74, 3205-3215 (1981). 9. M. ALLAVENA, R. RYSNIK, D. WHITE, V. CALDER, AND D. E. MANN, J. Chem. Phys. 50,3399-3410 (1969). IO. M. E. JACOX AND D. E. MILLIGAN, J. Chem. Phys. 54, 3935-3950 (1971). 11. L. ANDREWS,C. A. WIGHT, F. T. PROCHASKA,S. A. MCDONALD, AND B. S. AULT, J. Mol. Spectrosc. 73, 120-143 (1978). Subtract calibration of 4 cm-’ at 3000 cm-‘. 12. J. H. MILLER AND L. ANDREWS,J. Amer. Chem. Sot. 102,4900-4906 (1979). 13. D. W. SMITH AND L. ANDREW& Chem. Phys. 5295-5303 (1971).
97, 362-378 (1983)
FTIR Spectroscopic Studies of the Matrix Photoionization and Photolysis Products of Methylene Halides BENUELJ. KELSALLAND LESTERANDREWS Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 Matrix photoionization of methylenebromide produced absorptions at 1019, 897, and 788 cm-’ identified previously as CBrl , CHBrl , and CHBrr . High-resolution FTIR spectra revealed overlapping l/2/1 triplets for natural bromine isotopes with individual linewidths near 0.2 cm-‘. New absorptions at 3 121,2897, and 1345 cm-’ are assigned to the (CHrBr+)Br cation complex which yields CHBri on photolysis. A substantially increased yield of the CHCl: species made possible observation of the C-H stretching mode at 3033 cm-’ and the symmetric C-Clz stretching mode at 845 cm-’ along with the previously observed stronger 129 1- and 1044-cm-’ fundamentals. The high resolution and enhanced signal-to-noise capability of the FTIR are clearly demonstrated in this investigation. INTRODUCTION
The molecular spectroscopist often finds shifts in absorption frequencies due to different isotopic substitutions quite useful in characterizing new molecular species. Isotopic splitting patterns can yield information relating to the stoichiometry and symmetry while the frequency shift measurements can give information about the geometry of the molecule. Frequently with heavier atoms, isotope shift data cannot be utilized because the shifts are too small to be resolved. With the advent of commercially available infrared spectrophotometers employing computer-controlled interferometers, the experimentalist has gained a very powerful tool which permits the acquisition of data not readily accessible with conventional prism or grating instruments. Through the use of repetitive scans and the measurement of many transform points, high-resolution spectra (0.06 cm-‘) with good signalto-noise characteristics may be obtained. This new information can be used to explore geometries of molecules containing heavier atoms and, in addition, it can be used to explore small environmental perturbations such as those found in matrices. Matrix isolation studies involving methylene bromide, methylene chloride, and bromochloromethane are described here. These serve to illustrate some of the information which can be obtained from high-resolution infrared spectra and to provide additional information which supplements earlier studies using a grating spectrophotometer (I). EXPERIMENTAL
DETAIL8
The cryogenic and vacuum equipment, and vacuum-uv photoionization source have been described in detail previously (2, 3). Reagent-grade CHzClz and CHzBrz (Aldrich Chemical Company, Inc.), “CH2C12 (90% 13C,Merck, Sharp and Dohme), 0022-2852/83/020362-17$03.00/O Copyri&t Q 1983 by Academic press, Inc. All rights of reproductionin any form reserved.
362
MATRIX STUDIES OF METHYLENE
HALIDES
363
CHzBrCl (Matheson Company, Inc.), and CD#Zlz and CD2Br2 (Merck, Sharp and Dohme) were purified by vacuum distillation and then diluted with argon to give Ar/reagent molar ratios ranging from 400/ 1 to 800/ 1. Diluted methylene halide samples were codeposited at 2 mmole/hr on a lo-25 K CsI plate with a comparable quantity of pure argon introduced through a 3-mm-i.d. quartz discharge tube. Infrared spectra (400-4000 cm-‘) were recorded with a Nicolet 7 199 Fourier transform infrared (FTIR) spectrophotometer operating at 0.12~cm-’ resolution. Spectra were also taken at l.O-cm-’ resolution to minimize Nemst glower and He-Ne laser light irradiation of the sample. Spectra were recorded after sample preparation, diffusion at 22 K, and photolysis with filtered radiation from a high-pressure Hg arc. All of the spectra illustrated in this paper were calculated from the sum of 1000 interferograms. The frequencies reported are accurate to within 0.03 or 0.3 cm-’ depending on the resolution employed. RESULTS
In the first study, argon diluted CH2Br2 (Ar/CH2Br2 = 1600/l) was condensed at 25 K in the absence of argon resonance radiation. Figures 1 and 2 illustrate two groups of absorptions observed with 0.12~cm-’ resolution in the C-Br stretching region. The first figure displays the antisymmetric (b2) C-Br stretching mode after 1.3 hr of sample deposition, while the second figure shows the symmetric (a,) C-Br stretching mode after 20.8 hr. In both cases, the triplets with approximate relative
/ 654
I
652
1
650
1
640
I
646
I
644
WWENUMBER m-11
FIG. 1. FTIR spectrum of methylene bromide v9 fundamental recorded with 0.12-cm-’ resolution from 2 tnmole of ArjCh,Br, = 1600/l sample deposited at 25 K.
364
KELSALL AND ANDREW!3
585
I
I
I
I
584
583
582
581
WRVENUMEER :cnrl)
50
FIG. 2. FTIR spectrum of methylene bromide v3 fundamental recorded at 0.12-cm-’ resolution from 42 mmole of the same sample as in Fig. 1.
intensities of 1:2:1 for natural-abundance bromine isotopes (50.54% 79Br, 49.46% 81Br) indicate that the molecular species giving rise to these absorptions contain two equivalent bromine atoms. The bromine isotopic components were quite sharp; fullwidth at half-maximum (FWHM) was 0.16 -t 0.01 cm-’ for the symmetric mode and about 0.5 cm-’ for the antisymmetric mode. The slight asymmetry which is observed for the antisymmetric C-Br stretch in Fig. 1 is probably due to the presence of a second much weaker multiplet, perhaps a triplet, which is shifted to a slightly lower frequency and underlies the principal absorptions. The low-frequency component is seen in the figure. Table I lists the frequencies and assignments for the absorptions illustrated in Figs. 1 and 2. In a second study, a 1000/l sample was condensed at 25 K, high-resolution spectra were recorded, the sample was cooled to 10 K, and more spectra were recorded. No temperature dependence on linewidth was found. Methylene bromide in argon (Ar/CH2Br2 = 400/l) was condensed on a CsI plate at 10 K while being exposed to the radiation from a microwave-induced argon discharge. The principal product absorptions observed in this experiment were identified in an earlier matrix photoionization study of CH2Br2 (I). Figures 3,4, and 5 illustrate selected regions in the spectra where bromine isotope shifts were resolved using a 0.12~cm-’ resolution. In all of these figures trace (a) was recorded for the freshly prepared matrix sample, trace (b) was taken after sample warming to 22 K for 15 min and cooling to 15 K, and trace (c) was recorded after photolysis by the water
MATRIX STUDIES OF METHYLENE
365
HALIDES
TABLE I Absorption Bands (cm-‘) and Assignments for the Symmetric and Antisymmetric C-Br Stretching Modes of CH2Br2 in a High-Resolution Study
wn-1) a
assignment
582.97
v,(a,)
582.44
v3(a1) CH2 7gBr81Br
581.92
V3kll) CH2 8lBr 2
CH2 7gB,
2
649.41
vg(bl) CH2 79B1.2
648.96
vg(bl) CH2 7gBr81Bl.
648.48
vg(bl) CH2 81B, b
647.93
-1
.
a
Wavenumber accuracy is + 0.03 cm
b
This band is probably due to a second matrix site for CH2 81Br 2'
filtered high-pressure mercury arc (220-1000 nm) for 13 min. The band positions and intensities of the absorptions shown in Figs. 3, 4, and 5, which are attributable to products containing bromine, are listed in Table II.
UAVENLINBER (m-1)
FIG. 3. The FTIR spectrum of CBri produced by vacuum-uv photolysis of Ar/CH2Br2 = 400/l sample during condensation at 10 K is shown in trace (A), spectrum (B) was recorded atter a 1O-22- 15 K thermal cycle, and trace (C) was taken after 220- to lOOO-nm photolysis for 30 min.
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KELSALL AND ANDREWS
FIG. 4. FTIR spectrum of CHBrr from same sample as in Fig. 3.
781 FIG.
780
778
778 77? WAVENUMBER tdl
778
77b
5. FTIR spectrum of CHBr, from same sample as in Fig. 3.
367
MATRIX STUDIES OF METHYLENE HALIDES TABLE II Product Absorptions (cm-‘), Assignments, and Absorbances Which Show Bromine Isotopic Splittings in a Hi-Resolution Study of CH,Br, Photoionization Products Absorbance
Assignment
11
I3
0.08
0.07
0
(Site A)
0.17
0.15
0
+
(Site A)
0.08e
0.07
+
(Site B)
0.12e
0.11
0.10
C7'Br81Br+
(Site B)
0.22
0.20
0.20
C81Br
(Site B)
C"Br
1019.01
C7'Br81Br+ C81Br
1018.63 c C"Br
2
2 2
+
0
0.11
0.10
0.10
?
0.04
0.04
0.04
?
0.05
0.05
0.04
(Site A)
0.11
0.07
0.06
1017.67
1017.30
I2=
(Site A)
1019.59
1018.17
d
b
knl-l)a
2
+
897.43
CH7'Br
896.90
CH7gBr81Br+
(Site A)
0.20
0.17
0.12
CH'lBr
+
(Site A)
0.10e
0.08
0.05
+
(Site B)
0.06e
0.05
0.04
896.44 CH7'Br
2
2 2
+
895.99
CH7gBr81Br+
(Site B)
0.13
0.12
0.08
895.47
CH81Br
(Site B)
0.06
0.05
0.04
778.77
CH7'Br
(Site A)
0.07
0.07
0.08
778.29
CH7'Br81Br
(Site A)
0.14
0.14
0.15
777.83e
CH81Br
(Site A)
0.07
0.08
0.08
777.94e
CH"Br
(Site B)
0.05
0.06
0.06
777.24
CB7gBr81Br
(Site B)
0.08
0.10
0.11
776.98
CH"Br
(Site B)
0.05
0.06
0.06
2
+
2
2 2
2
-1
. E Wavenumber accuracy is + 0.03 cm Absorbance of band in fFeshlv oreoared matrix samole. ' Absorbance of band after s&l& was warmed to 22 k for 15 min and then retooled to 15 K. d Absorbance of band followinq 13 min of exposure to Hg arc radiation (220-1000 run). e Estimated absorbance contribution of indicated species to the observed band.
Figure 3 shows the spectral region of the antisymmetric C-Br stretching mode of CBr:. Of the six major absorptions observed all are symmetric except for the two lower frequency bands which show asymmetry presumably due to an underlying absorption. The bandwidth at half-maximum (FWHM) for the four sharper bands was 0.16 + 0.0 1 cm-‘. No change in band contour was detected after mild warming, but after photolysis, the 1019.6- and 10 19.0~cm-’ bands were destroyed and the intensity of the 1018.7~cm-’ band was decreased to yield a distorted triplet. The
368
KELSALL AND ANDREWS
intensity data in Table II shows that the absorption pattern can be explained on the basis of two overlapping triplets which overlap with a third unidentified multiplet. Figure 4 shows the spectral region for the CHB$ cation absorption (1). In the freshly prepared matrix the CHBr2f absorption consisted of five bands with FWHM of about 0.2 cm-‘. The contours of these five bands did not change significantly on diffusion or photolysis, but the intensity data in Table II show that the bands decreased with both diffusion and photolysis. The bands labeled site A showed a more rapid decrease with diffusion than those of site B, while the site B bands decreased more rapidly with photolysis. The intensity data suggest that there are two overlapping triplets which give rise to the five detected bands. Figure 5 shows the spectra for the major CHBrz radical absorption. In the freshly prepared sample five principal absorptions were detected, the strongest component had a 0.20 f 0.01~cm-’ FWHM. The bands showed no gross changes with sample warming, although the 777.2~cm-’ peak grew slightly with respect to the 777.8 cm-’ peak. The spectrum of the photolyzed sample shows the continuation of the general trend observed in the diffision experiment. This trend results in the appearance of a shoulder on the high-frequency side of the 777.8~cm-i band. The intensity data for the 777.8- and 779.4~cm-’ bands in Table II are corrected for the partial overlap with each other. The corrected data show that two overlapping triplets are present. The symmetric C-Br stretching mode (5) of CBrz centered at 594.6 cm-’ showed no bromine isotope structure even after diffusion and photolysis. The symmetric CBr stretching mode of CH2Br2, however, was resolved into a sharp triplet as reported in Fig. 2. The antisymmetric C-Br stretching mode of CHzBrz and CBr2 were recorded; bands for both species were broad and showed no resolved bromine isotope absorptions before or after diffusion and photolysis. Since the most photosensitive product absorptions reported in the earlier study (I) were not detected in the high-resolution experiments, three more photoionization experiments were performed with CH2Br2 using 1.0~cm-’ resolution for faster data collection. The most photosensitive product absorptions (destroyed by 290- to lOOOnm photolysis) were observed at 684 cm-’ for CH2Br2+and 546 cm-’ for CD2Br2+in this FTIR study, which gave higher signal to noise, particularly above 2000 cm-‘, than the previous grating studies. New observations in the present FTIR study are (1) a group of bands at 3 121, 2896, and 1345 cm-’ (A = 0.11, 0.32, and 0.02, respectively),’ which were 80% destroyed upon 60 min of 290- to 1000~nm photolysis, (2) a group of absorptions produced by this same photolysis at 3133, 2948, 1361 cm-’ (A = 0.05,O. 12,0.02, respectively), (3) sharp 3099 (A = 0.04), 1128 (A = 0.03), and 525 cm-’ (A = 0.02) absorptions decreased 50% by 290- to lOOO-nm photolysis, and (4) a sharp new 3039.8~nm band (A = 0.02), which increased on 290- to lOOOnm photolysis with the 897~cm-’ (A = 0.11) CHBr2+absorption. Irradiation with the full arc (220- 1000 nm) for 80 min destroyed group ( 1) and (3) bands, halved group (2), and reduced group (4) by 20%. Photolysis behavior in the other two experiments substantiated this grouping of bands. In one of these experiments 590~nm photolysis for 30 min substantially reduced the 684~cm-’ band without affecting other absorp’ The 2896-cm-’ band was reported as 2901 cm-’ in Table IV of the grating study (I) without the needed calibration correction.
MATRIX STUDIES OF METHYLENE HALIDES
369
tions, 340~nm radiation for 90 min decreased group (1) and (3) band absorbances (by 25%) and increased group (2) band intensities without affecting group (4) bands, and 220- to 1000~nm radiation for 15 min reduced group (1) bands to 30%, group (3) to 50%, doubled group (2), and increased group (4) fivefold; spectra are illustrated in Fig. 6c for the sample deposited with vacuum-uv radiation and Fig. 6d after the 220- to 1000~nm photolysis in the 2900- to 3 lOO-cm-’ region. In the other experiment 500-to 1000~nm photolysis for 30 min destroyed the 684-cm-’ band without affecting the other product absorptions. Photolysis at 290-1000 nm decreased groups (1) and (3) and increased groups (2) and (4) as described above. In a similar CD2Br2 experiment, the (1) set of bands appeared at 2358, 2127, and 997 cm-’ and were characterized by their lack of sensitivity to red light, slight photolysis with 340 nm, 80% decrease with 290- to 1000~nm radiation and comlete destruction by the full arc. A sharp 2162-cm-’ band (A = 0.05) produced by 290nm photolysis represents the (2) group and 1030- and 506-cm-’ bands 80% destroyed by 290~nm photolysis are due to the (3) set. Similar experiments were performed with CHzClz and 13CH2C12and the spectra were similar to those reported earlier. The absorption near 764 cm-’ and the sharp band at 1193 cm-‘, which photolysed with red light, have been assigned to the parent cation (I). Other less photosensitive product bands at 3 130, 3 108 (A = 0.08), 2904, 2882 (4 = 0.20), 2764 (A = 0.1 l), and 2475 cm-’ (A = 0.03) were reduced about 40% on 290- to 1000~nm photolysis. Photolysis with the Pyrex-filtered mercury arc produced an eightfold growth in the sharp 1044~cm-’ triplet assigned to isolated
3100
3000 WAVENUMBER
2900 ICRI~)
FIG. 6. The 290- to 31OO-cm-’ region from FTIR spectra recorded with 1.0-c& resolution of Ar/ CH& = 400/l samples subjected to vacuum-uv radiation during condensation at 10 K. (a) Spectrum of CHzClz sample, bands that decrease on 290- to lOOO-nmphotolysis identified with an arrow, (b) spectrum of CH&lBr sample, (c) spectrum of CHZBr2sample, and (d) same as (c) spectrum after 200- to lOOO-nm photolysis for 15 min. Precursor absorptions are labeled P.
370
KELSALL
AND ANDREWS
MATRIX STUDIES OF METHYLENE HALIDES
371
CHCl: ; FWHM for the three chlorine isotopic components listed in Table III were 0.35 cm-‘. This substantial growth of the 129 I- and 1044-cm-’ CHCl: cation bands was accompanied by the growth of a sharp new band at 3032.8 cm-’ and a new band system at 845.4 cm-‘, which are illustrated in Fig. 7a. A sharp 9/6/ 1 relative intensity triplet at 1195.40, 1192.78, and 1190.08 + 0.02 cm-‘, which has been assigned to Ccl: (4), remained after photolysis. Photolysis in the ‘3CH2C12experiment was even more dramatic; a 20-fold growth in the 13CHC1: absorptions was observed upon 290to 1000~nm photolysis for 60 min, as shown in Fig. 7b. As a further diagnostic this sample was irradiated with 220- to lOOO-nm light for 60 min; the ‘3CHCl: absorptions decreased about 40% and a triplet absorption at 1156.93, 11.45 18, and 115 1.4 1 + 0.03 cm-‘, illustrated in Fig. 8 and listed in Table IV, increased 120%. The present CD& experiment using 50% microwave power gave an increased yield of the most photosensitive band at 603 cm-’ (A = 0.08) and shoulder at 6 10 cm-’ (.4 = 0.02), p rovided a new sharp 1083-cm-’ (A = 0.065) band and shoulder
,
1303
1280
WAVEtiMBER (CliTll
850
FIG. 7. Regions in the l.O-cm-’ resolution FTIR spectrum for CHCl$ fundamentals produced by photolysis of CH&lz during condensation with excess argon. (a) Spectrum of vacuum-uv irradiated CH2C12 sample and spectrum after 290- to lOOO-nmphotolysis for 1 hr, (b) spectrum of vacuum-uv irradiated “CH&& sample and spectrum after 290- to lOOO-nmphotolysis for 1 hr.
372
KELSALL AND ANDREWS
tb) r
L
li
zoo
1190
1180
1150
1160
1150
WAVENLltlBERkm-l)
FIG. 8. The FTIR spectrum of the 1150- to 12OOcm-’ region for sample of “CH2C12 subjected to vacuum-uv radiation during condensation of 10 K is shown in trace (a). Spectrum (b) was recorded after photolysis with 290- to lOOO-nm and 220- to lOOO-nm radiation for 1 hr each; the spectrum shows the ‘2CClf and ‘%Clf photoproducts.
at 1088 cm-’ (4 = 0.015), and a reduced yield of the 1195-cm-’ triplet (4 = 0.025). Photolysis with 290- to lOOO-nm radiation caused a 20% increase in chlorine isotopic band systems at 1127.3 (A = 0.03) and 864.8 cm-’ (A = 0.01) and a weak 2234cm-’ absorption (A = 0.005) and a 20% decrease in new 2360- and 2 129-cm-’ bands. However, 220- to 1000~nm radiation increased the 2234, 1127.3, and 864.8 cm-’ bands by a factor of 4, markedly increased the 643.3-cm-’ Ar,D+ band (I) (A = 0.03 to 0.45) and halved the 2360- and 2129-cm-’ absorptions. Another CDICll experiment using 80% microwave power gave a reduced yield of the most photosensitive bands and an increased yield of the 1127-cm-’ (4 = 0.11) and 1195~cm-’ (A = 0.15) bands. A sharp 9/6/ 1 triplet at 1197.0, 1194.4, 119 1.7 cm-’ superimposed with 1195.9-, 1192.7-, 1190. l-cm-’ triplet was destroyed upon photolysis while the 1195-cm-’ triplet remained and the 1127 triplet doubled in absorbance. Two similar experiments were performed with CH&lBr to complement the CH& and CH2Br2 studies. The first, run with the argon discharge excited by 70% diathermy power, gave the same bands as the second, employing 30% power; however, the latter gave substantially more pronounced photochemistry. The strong 993.3-, 988.5-cm-’ doublet (A = 0.15, 0.05) and sharp new bands at 1255.1 (4 = 0.01) and 303.6 cm-’
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TABLE IV Infrared Absorptions (cm-‘)’ Assigned to Isotopic Ccl; Species CH2Cl2
Or CD2Cl2 12c35c12+
1195.40
12c35c137c1+
1192.78 1190.08
13
12c3'c12+
CH,Cl,
1156.93
13c35c12+ 13C35C137Clt
1154.18 1151.41
a Wavenumber
accuracy
13c37c12+
is + 0.03 cm
-1
,
(A = 0.04) grew a factor of 20 upon 290- to 1000~nm photolysis for 60 min; these bands were destroyed by a 60-min exposure to the full arc. A strong new 29 14-, 2930cm-’ product doublet (A = 0.04, 0.06) and two weaker bands at 3092, 3 119 cm-’ (4 = 0.02) are shown in Fig. 6b. These absorptions were destroyed by Pyrex-filtered photolysis. Bands at 86 1, 866, 1187, and 1195 cm-’ due to CHClBr radical (I) and a sharp 1116.6-, 1120.6-cm-’ doublet (A = 0.06, 0.02) assigned to CClBr+ (4) increased some with Pyrex- and more with water-filtered photolysis. Two weak new absorptions at 713.8 and 529.6 cm-’ (A = 0.03) were destroyed by Pyrex-filtered photolysis. DISCUSSION
The bromine isotopic splittings and linewidths will be considered followed by new assignments to the parent and two daugher cations.
Bromine Isotopic Splittings and Linewidths The very sharp bromine isotopic splittings observed for CH2Br2 at high dilution in solid argon are of considerable interest. The symmetric C-Br stretching mode v3(u,) (Fig. 1) exhibits a slightly larger isotopic splitting than the antisymmetric CBr stretching mode v&r) (Fig. 2); however, the better resolution of the former is due to substantially sharper linewidths, 0.16 -I-0.0 1 cm-’ as compared to 0.5 +- 0.1 cm-’ for the latter. The lack of a temperature dependence for the linewidth shows that the lines are inhomogeneously broadened probably owing to a collection of unresolved matrix sites. The sharpest lines observed for CH2Br2 in solid argon are comparable to the linewidth (0.19-O-25 cm-‘) found for the Q branch of HCl in solid argon using a high-resolution grating instrument (6), the v3 mode of SO2 in solid argon (0.1-0.2 cm-‘) using a diode laser spectrometer (7), but not as sharp as the split v3 components of SF6 in solid argon (0.03-0.12 cm-‘) using FTIR (8). In all of these cases, many
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of the narrow lines that make up the broader band profile observed with lower resolution are ascribed to “matrix site effects” owing to different orientations of the guest molecule in the matrix or of the matrix atoms around the guest molecule. The bromide isotopic splitting for the cation and radical product species are of perhaps more interest as an additional source of information on matrix sites responsible for inhomogeneous line broadening. In the case of the 10 19-cm-’ absorption assigned earlier (4) to CBr2f (Fig. 3), no change in relative intensities of the several components was observed after warming to 22 K but photolysis destroyed the 10 19.6and 10 19.0-cm-’ bands and decreased the 1018.7-cm-’ band to yield a triplet distorted by unidentified absorption at 10 17.8 cm-‘. The individual lines in the CBri multiplet are exceptionally sharp, 0.16 + 0.0 l-cm-’ FWHM. The intensity data in Table II shows that the original absorption pattern can be explained as two overlapping triplets, each due to CBrzf with a different local matrix environment, which overlap to a lesser degree with a third unidentified absorption. The 1/2/l relative intensity patterns for the triplets indicate that the two bromine atoms in CBr2+are equivalent in both matrix sites, one more photosensitive than the other. The multiplets at 897 and 778 cm-’ (Figs. 4 and 5), assigned previously (I) to CHBr2+and CHBr2, behaved similarly. The components are quite sharp, 0.20-cm-’ FWHM, and decrease upon sample warming and photolysis. The intensity data show that two overlapping triplets give rise to the five observed lines. Since the three isotopic components of each triplet have 1/2/l relative intensities, the two bromine atoms in CHBr$ and CHBrz are equivalent in both matrix sites of each species. The C79Br2+and C*lBr2+isotopic frequencies for the antisymmetric C-Br stretching vibration provide a basis for calculation of the Br-C-Br valence angle. The bromine isotopic frequencies for both matrix sites give a valence angle upper limit (9) of 147”. This may be compared with a similar calculation of 138.0” for the valence angle of Ccl: from the present ‘2C35Clt and 12C3’C12fisotopic frequencies. More extensive data available for Ccl: from carbon-13 substitution gives 137.6” from the ‘3C35C1: and 13C35C1:data, 126.7” from the ‘2C35C1:and ‘3C35C12+ data, and 126.6” from the 12C37C1:and 13C3’C12+ isotopic pairings. The average from the upper and lower limits from terminal and apex atom isotopic substitution, 132 f 6”, represents a reasonable measure of the valence angle in Ccl:. A like consideration for the 147’ upper limit on CBr2+suggests a 140 f 8” valence angle for CBr: . It is reasonable for CBrzf to have a slightly larger valence angle than Ccl,‘, but the present angle determinations are_not sufficiently accurate to make a definitive statement. Parent Cations Some new data were obtained for the CD2C12+parent ion which complements the vibrational picture for the strongly interacting antisymmetric (b2) vs and u9 modes observed for these ions. An increased yield of CD2Cl: in the present 50% microwave power experiment gave 603-, 610-cm-‘, and 1083-, 1087-cm-’ doublets for this species. Since the structure of CH2C1: is not known, rigorous product rule calculations cannot be performed, but a simple check on the vibrational interaction between the two modes in the b2 class can be made by comparing v8 X u9 product ratio for the (O/H) isotopes of the parent cation and the neutral molecule using matrix frequencies
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for CHCl: (vg = 1193 cm-‘, u9 = 764 cm-‘) and CD&l; (vs = 1083 cm-‘; u9 = 603 cm-‘). The cation ratio is 0.7 16 as compared to 0.732 for the molecule. This general agreement is supportive of the parent cation assignments. The 684- and 546-cm-’ antisymmetric C-Br stretching mode bands for CH2Br: and CDzBrt have been observed without detecting the wagging modes of the same symmetry. A new less photosensitive (50% destroyed by 290 photolysis) species has been observed at 3099, 1128, and 525 cm-’ in the CH2Br2 experiments and at 1030 and 506 cm-’ with CD2Br2. This group (3) absorption species cannot be identified from the present data. Daughter Cations The CHClt daughter cation was first identified in CHCl3 matrix photoionization experiments (10); later work, however, showed this to be the (CHC@Cl species (II) and gave slightly different absorptions for the isolated CHCl: cation prepared from CHClz (1). The present studies produced an unusually large yield of CHCl: upon 290- to lOOO-nm photolysis, which enabled two weaker important vibrational fundamentals to be observed, as shown in Fig. 7. The increased signal-to-noise capability of the FTIR technique, particularly in the higher-frequency region where matrix samples are more scattering, enabled a sharp new 3032.8cm-’ band and a sharp new 9/6/l triplet absorption beginning at 845 cm-’ to be observed. The analogous 13CHC1: bands shifted to 3023.0 and 825.5 cm-‘, which indicates assignment of these bands to the C-H and symmetric C-Cl;! stretching modes of the CHCl: carbocation. These weaker new bands can be assigned to 13CHCll with confidence because they increased with the more intense bands on 290- to lOOO-nm photolysis and decreased with the more intense bands upon 220- to 1000~nm photolysis. Growth of CDCl: upon photolysis was much less extensive; a weak new band at 2234 cm-’ could be due to the C-D stretching mode in CDCl:. Chemical support for observation of the C-H stretching mode of CHCI: at 3033 cm-’ is found with the bromine-substituted precursors. In the CH$lBr experiments, 290-nm photolysis increased the strong 993.3-, 988.5-cm-’ CHClBr+ doublet and produced weak bands at 1255 and 3036 cm-‘. In CH2Br2 studies a weak 3039.8cm-’ band appeared on photolysis with growth in the stronger 897-cm-’ CHBrf absorption (I). These C-H fundamentals may be compared with CHC13, CHC12Br, and CHBr3 at 3054, 3059, and 3065 cm-’ in solid argon (II). It is readily seen that the C-H stretching mode in the carbocation is not significantly changed from the C-H mode in the saturated hydrocarbon, which demonstrates that the C-H bond is not affected by the cation center. This, however, is. not the case for the chlorine atoms, which readily conjugate with the vacant p orbital on the carbocation and substantially increase the C-Cl stretching modes. The two C-Cl stretching modes for CHClt (1045, 845 cm-‘) are higher than the isoelectronic molecule BHClz (892, 740 cm-‘) (12) where some ?r bonding is probably also involved, and the saturated molecule CHzClz (748, 710 cm-‘) where no ‘lrbonding is possible. High-resolution spectra gave 0.35 f 0.03-cm-’ FWHM for the chlorine isotopic
376
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ANDREWS
triplets at 1045 and 1019 cm-’ for CHCl: and ‘3CHCl: ; additional much weaker sites were observed at 1050 and 1025 cm-’ for each carbon isotope. The CHBrz species gave sharper lines with 0.18 f 0.02-cm-’ FWHM and two sites separated by 0.9 cm-‘. It is perhaps surprising for CHBr2+ to exhibit sharper lines than CHC12f. If these linewidths are due to a collection of matrix sites, then CHBr2+ occupies more uniform sites than CHCQ, which may be due to specific local packing arrangements between the guest ion and the host matrix. It was proposed in the earlier study with methylene halides that the CHC12+daughter cation was produced upon hydrogen photodetachment from the other daughter cation complex (CH,Cl’)Cl thought to be a decomposition product of the CH&l: parent cation (I). The present studies provide evidence for the (CH&‘)X species produced and trapped in the original photoionization process, presumably from decomposition of the parent cation. The uv spectra of the parent ions clearly show a larger yield for CH2Br: than CH$I: . Accordingly, the (CH2Br+)Br species is probably produced in a correspondingly larger yield than (CH#Y)Cl, and intense new bands in the CH2Br2 experiments are considered for assignment to the (CH2Br+)Br species. The strongest product band in methylene bromide experiments in the C-H stretching region at 2897 cm-’ was accompanied by bands at 3 121 and 1345 cm-’ which were 80% destroyed on 290- to lOOO-nm photolysis that produced a similar new trio at 3133, 2948, and 1361 cm-’ (Fig. 6). Methylene bromide-d* counterparts of the original set at 2358,2127, and 977 cm-’ were replaced by the strongest d2 counterpart at 2 162 cm-’ upon photolysis; the weaker d2 counterpart was probably obscured by CO* absorption in this region. The observation of two new bands in the C-H stretching region (2897 and 3 121 cm-‘) that photolyse together is indicative of a two-hydrogen species. The corresponding d2 counterparts at 2 127 and 2358 cm-’ are reasonable isotopic shifts for C-H(D) stretching vibrations (2897 cm-‘/2 127 cm-’ = 1.362; 3 12 1 cm-‘/2358 cm-’ = 1.324). Similar ratios were observed for CH2Br2/CD2Br2 in solid argon (vl, 3006 cm-‘/2205 cm-’ = 1.363; vg, 3073 cm-‘/23 17 cm-’ = 1.328). The isotopic frequency ratios suggest that the more intense 2897-cm-’ band is the symmetric C-H2 stretching mode and that the sharper, weaker 3 12 l-cm-’ band is the antisymmetric C-H2 stretching mode. The new 1345-cm-’ band and its d2 counterpart at 997 cm-’ are near the symmetric scissor bending modes of CHtBr radical at 1355 cm-’ and CDzBr at 1076 cm-’ (IS), which is su=estive of a similar type of vibrational mode. On the basis of the above vibrational data, the 3 12 1-, 2897-, and 1345-cm-’ bands are assigned to the CH2Br+ daughter ion in the (CH2Brf)Br complex. It is perhaps surprising that the C-Br stretching mode for this species was not detected particularly in view of the fact that the symmetric C-Br stretch and scissor bend are of comparable intensity for CH*Br radical (13). Photolysis with 290- to lOOO-nm radiation produced the CHB$ daughter cation absorbing at 897 cm-’ (by hydrogen atom elimination) and another species with sharp absorptions at 3 133,2948, and 136 1 cm-‘. The latter set of absorptions maps the above set assigned to the CH*Br+ complex, which suggests assignment to another structural form of the complex or the isolated CH2Br+ daughter cation itself. The observation of two sets of similar bands in the CH&lBr experiment at 3 119, .
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3092,2930, and 29 14 cm-’ supports identification of the daughter ion complex; two possibilities exist, (CH&l+)Br and (CH*Br+)Cl. In the CHzClz experiments, an analogous set of bands was observed at 3 108 and 2882 cm-‘; ‘%H$l2 shifted these bands to 3096 and 2875 cm-’ and CD&l2 produced the stronger band at 2 129 cm-‘. These absorptions are believed to be due to the (CHJJl+)Cl complex, which yields CHCl: on 290- to 1000~nm photolysis. The weaker 3130- and 2904cm-’ bands in CH2Clz experiments (Fig. 6b) are probably counterparts of the stronger 3108- and 2882-cm-* bands due to a different arrangement of the (CH$l+)Cl complex: (CH&l+)Cl + h~(290-1000 nm) - CHCl: + H. Finally, it should be mentioned that the growth of CC12+on full arc (220- 1000 nm) photolysis at the expense of CHCl: indicates H photodissociation from the latter: CHClz + hv(220-12 000 nm) - Ccl: + H. The major photochemical processes have been discussed in the earlier report (I). The present experiments gave a larger increase in the CHCl$ cation yield on 290- to 1000~nm photolysis, which made possible the detection of two weaker fundamentals for this daughter ion. In general the mercury arc photochemistry of these samples is more dramatic when less intense vacuum ultraviolet radiation is used to prepare the original sample. CONCLUSIONS
FTIR spectra of matrix samples provide high resolution and increased signal-tonoise capability. In the case of bromocarbon species, bromine isotope splittings were resolved and linewidths of 0.16 + 0.0 1 cm-’ were observed for the v3 mode of CH2Br2 and the v3 mode of CBr2+.New photosensitive absorptions at 3 12 1, 2897, and 1345 cm-’ are assigned to the (CHzBr{)Br complex, which eliminates H to give CHBr$. A markedly increased yield of CHCl,’ allowed observation of the C-H stretching mode at 3033 cm-’ and the symmetric C-Cl2 stretching mode at 845 cm-‘; the vibrational spectrum of CHCl: reveals a normal C-H bond and unusually strong C-Cl bonds which indicates conjugation of chlorine to the positive carbon center. ACKNOWLEDGMENT The authors gratefully acknowledge financial support from the National Science Foundation under Grant of CHE 79-10966. RECEIVED:
August 23, 1982 REFERENCES
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L. ANDREWS,F. T. PROCHASKA,ANLIB. S. AULT, J. Amer. Chem. Sot. 101,9-15 (1979). F. T. PROCHASKAAND L. ANDREWS,J. Chem. Phys. 67, 1091-1098 (1977). L. AMIREWS,D. E. TEVAULT, AND R. R. SMARDZEWSKI, Appl. Specfrosc. 32, 157-160 (1978). L. ANDREWSAND B. W. KEELAN, J. Amer. Chem. Sot. 101, 3500-3504 (1979).
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5. L. ANDREWS ANLI T. G. CARVER, J. Chem. Phys. 49, 896-902 (1969). 6. M. ALLAVENA, H. CHAKRO~N, AND D. WHITE, “Molecular Spectroscopy of Dense Phases-Proceedings of the 12th European Congress on Molecular Spectroscopy,” Strasbourg, France, 1975, p.
365-368. 7. M. DUBS AND Hs. H. GUNTHARD, Chem. Phys. Las. 47,421-425 (1977). 8. B. I. SWANSONAND L. H. JONES,J. Chem. Phys. 74, 3205-3215 (1981). 9. M. ALLAVENA, R. RYSNIK, D. WHITE, V. CALDER, AND D. E. MANN, J. Chem. Phys. 50,3399-3410 (1969). IO. M. E. JACOX AND D. E. MILLIGAN, J. Chem. Phys. 54, 3935-3950 (1971). 11. L. ANDREWS,C. A. WIGHT, F. T. PROCHASKA,S. A. MCDONALD, AND B. S. AULT, J. Mol. Spectrosc. 73, 120-143 (1978). Subtract calibration of 4 cm-’ at 3000 cm-‘. 12. J. H. MILLER AND L. ANDREWS,J. Amer. Chem. Sot. 102,4900-4906 (1979). 13. D. W. SMITH AND L. ANDREW& Chem. Phys. 5295-5303 (1971).