Journal of Research of the National Bureau of Standards Vol. 48, No. 1, January 1952 Research Paper 2290 Infrared Absorption Spectra of Five Halomethanes Earle K. Plyler and Nicolo Acquista The infrared absorption spectra of five halomethanes, bromodifluoro methane, dibromodifluoro methane, bromochlorodifluoro methane, bromotrifluoromethane, and iodotrifluoromethane have been studied from 2 to 38 microns. Fundamental, combination, overtone, and difference bands have been identified. Many unobserved fundamentals have been predicted by comparisons with related molecules and with Raman spectra. The funda mentals have been classified as to the type of motion and the principal atom involved. Four tables are included to show the relationships among the fundamentals of the molecules compared. 1. Introduction pressure used is indicated on the graph. It is seen that pressures as low as 0.01 cm of Hg were necessary The infrared spectra of bromodifluoromethane, to resolve the structures of the strongest bands. In dibromodifluoromethane, bromochlorodifluorome th some cases, it is possible to tell from the graph ane, bromotrifluoromethane, and iodotrifluorometh whether the bands have P, Q, and R, or only P and ane have been measured from 2 to 38 /*. The R branches. purpose of the measurements was to identify the Figure 2 shows the abosorption spectra of CBrF3 fundamentals and to classify them as to their modes and CIF3 from 2 to 15 /x- The pressures of the gases of vibration. The resulting classification allows a are indicated next to the respective traces. The comparison of the vibrations of these molecules with cell length employed was also 5 cm. The curves in other related substituted methanes studied by other figures 1 and 2 are traces from the original record investigators [l].1 Recently Decius subjected many obtained from the double-beam spectrophotometer. methane derivatives to a normal coordinate treat These graphs have not been corrected for surface ment and quantitatively verified the experimental reflection of the cells or for stray radiation in the assignments [2]. Plyler and Benedict extended the instrument. However, a reflection filter was used calculations of Decius and were able to check some beyond 9.2 ju, and as a consequence, the stray radia of the assignments of the molecules they studied [3]. tion did not exceed 5 percent at any wavelength. In addition, the Raman spectra of the molecules Figure 3 shows the infrared absorption spectra of have been investigated [4]. the five compounds in the region from 14 to 36 /*. These measurements were extended to 38 /x, but no 2. Instrumentation bands were observed beyond 36 M- These com pounds were measured in the gaseous state, using A double-beam Perkin-Elmer model 21 spectro cells of 5, 40, and 130 cm in length. The pressures, photometer measured the compounds from 2 to 15 /*, where measureable, are indicated on the graph. and a single-beam Perkin-Elmer spectrometer ex Attempts were made to examine the spectra of these tended the measurements from 15 to 38 /z. A KBr molecules as a liquid because many of the funda prism was used from 15 to 23/i and a TIBr-I prism mentals did not appear in the gaseous state. How from 23 to 38 M- ever, due to the high volatility of all the compounds For most of the measurements from 2 to 23 ju, a at room temperature, only CBr2F2 could be measured 5-cm gas cell with pressures of 1 atm or less was in this manner. A liquid cell with a thickness 1.5 used. In order to detect some of the weak absorp mm was used for these measurements. The portion tion bands, two cells of 40- and 130-cm lengths were of the curves drawn in broken lines indicates that employed. These two cells could not be evacuated. weak absorption bands could not be observed because They were filled by flowing gases through them. of atmospheric bands. Table 1 gives a list of wave The five compounds were supplied by R. C. lengths of all the bands that appear in the infrared. McHarness of the Jackson Laboratories. Tests In addition, there are listed bands that do not appear made by the Bureau's Mass Spectrometry Section either because their intensity is too weak or because indicated minute traces of foreign halogenated- they fall outside the wavelength region studied. methanes in four of the compounds. Only CBr2F2 These frequencies are taken either from the Raman had impurities in slight excess of 3 percent. These spectra or from overtone and combination bands. impurities were not observed in the CBr2F2 spectrum, Some of these frequencies have also been predicted probably because they were masked by the strong by comparison with analogous halomethanes. When absorption bands of the molecule. a band has been sufficiently resolved by the prism spectrometers, the P, Q, and R, or P and R branches 3. Results have been listed separately. The intensities are indicated adjacent to the respective band, and these Figure 1 shows the absorption spectra of CHBrF2, are denoted by abbreviations of very weak, weak, CBr2F2, and BrClF2 in the region from 2 to 15 /*. medium, strong, and very strong. The funda The cell length was 5 cm for all measurements. The mentals have been designated by the numbering i Figures in brackets indicate the literature references at the endfof this paper. system proposed by Plyler and Benedict [3]. Using 92 this notation, it is possible to see the regularities The number of fundamentals expected in these that exist among related molecules of different molecules can be predicted, as is well known, from symmetry. Another way of denoting the funda symmetry considerations. The five compounds can mentals, which was also used, is to characterize be put into three point groups. CBrClF2 and them by the molecular motion involved. In this CHBrF2 have only one plane of symmetry (the system, vx represents the stretching vibration be CBrCl and CHBr plane, respectively), and thus tween the central carbon and the X atoms, and bx belong to the point group Cs, whereas CBr2F2 has signifies the bending of the X atom against the rest two symmetry planes, placing it in C2v. The mole- of the molecule. cules t)BrF3 and CIF3 have a threefold axis of sym-
WAVE NUMBERS IN CMJ 5000 3000
7 8 9 WAVELENGTH IN MICRONS
FIGURE 1. Infrared absorption spectra of bromodifluoro-methanet bromochlorodifluoromethane, dibromodifluoromeihane from 2 to 15 i*. 93 TABLE 1. Classification of the infrared hands of hromodi- TABLE 1. Classification of the infrared bands of hromodi- fluoromethane, hromochlorodifiuoromethane, dihromodifiuoro- fiuoromethane, hromochlorodifiuoromethane, dibromodifluoro- methane, hromotrifiuoromethane, and iodotrifluoromethane methane, hromotri fiuoromethane, and iodotrifluoromethane— Continued
X V I Assignments X V I Assignments
CHBrF2
C3rCl?2
M Cm-i 3.33 3003 S «i 9.07 1102 VS 001 7.40 R 1351 8.69 1150 oog 7.44 1344 S 008 11. 42 R 876 vs 7.48 P 1337 11.47 Q 872 006 7.76 R 1289 11.55 P 866 vs 7.81 Q 1280 006 15.48 648 003 7.85 P 1274 s" 25.0 » 400 vs 004 8.81 1135 S 003 26.3 « 380 007 8.90 R 1123 22.7 440 009 9.02 Q 1108 S 004 33.3 a 300 vw 005 9.10 P 1099 •50.0 a 200 002 13.82 R 724 4.35 2298 2oos 13.93 Q 718 007 4.46 2241 wM &>8+ooi 14.12 P 708 s . 4.56 2192 2ooi 17.11 R 584 5.09 1964 w 001+006 17.34 Q 577 S oo» 5.73 1745 vMw 001+003 17.48 P 572 6.58 1519 W 006+003 31.0 323 005 6.93 1443 VW 008+005 41.6 «240 vw &?2 7.49 1335 VW 008+002 2.36 4237 CO6-|-C«JI 7.68 1302 M 006+009, 2oo3, 001+002 2.45 4080 w W3M1 9.90 1110 W 003+009 2.69 3716 w 007+001 10.23 977 M 007+003 3.06 3267 vw C02+C01 10.49 953 VW 003+005 3.76 2659 vw 2oo 8 12.83 779 VW 001—005 3.83 2610 vw 008+006 13.82 R 724 3.96 2525 vw 2o06 13.95 0 716 004+005 '• 4.10 2438 vw 008+009 14.17 P 706 w 4.25 2352 vw 2oo3 4.44 2252 vw 003+009 4.55 2197 Mw 2oog CBrF3 5.03 1988 007+006 5.19 1926 vw 008+004 5.97 1675 w 004+009,005+008 9.21 1085 S 001 6.25 1600 w 002+008, 005+006 8.29 1206 VS 0068 6.97 1434 vw 2oo7 13.04 R 767 w 13.14 Q 761 003 8.44 1185 2oo4 s 9.68 1033 wM 005+007 13.26 P 754 10. 37 964 vw 002+007 28.7 348 vw 004 11.20 893 004+"5 18.17 550 w 0079 11.42 875 vw 55.5 bl80 0025 12.14 R 824 vw 4.18 2392 M 2oo68 12.22 Q 818 002+004 4.38 2282 M 0068+001 12. 34 P 810 w 4.62 2164 VW 2coi 1 15. 52 644 M 2»5 5.10 1960 W 0068+003 5.30 1886 W 20079+003 5.42 1844 W 001+003 5.70 1754 W OO68+0079 CBr2F2 6.15 1626 W 001+0079 : 6.64 1507 W 2003 6.80 1470 W 20079+004 9.15 R 1093 7.01 1426 001+004 9.19 1088 001 7.23 1383 w OO68+0025 9.21 P 1086 vs 7.66 1305 wM 003+0079 8.69 Q 1150 008 8.52 1173 M ? 12.06 Q 829 vs 006 8.92 1121 M 003+004, 2o079 16.03 R 624 vs 10.53 949 VW 0025+003 16.10 621 003 10.92 915 VW 001 — 0025 16.18 P 618 vs 11.77 849 W 0068 — 004 29.4 M40 004 11.99 P 833 27.2 ^367 009 12.05 Q 830 W OO68 — OO4 30.2 "331 007 12.11 P 826 35. 5 b28? vw" 005 13.68 735 _.M 001 — 004, 0025+0079 60.6 M65 002 14.25 700 M 2004 4.37 2288 2oog 14. 50 689 M 4.50 2222 w 008+001 ? w 21.3 470 VW ? 4.60 2173 w 2ooi 1 5.85 1709 vw 001+003 6.05 1652 vw 2oo6 CF3I 1 6.90 1449 vw 006+003 7.00 1428 vw 007+001 8.00 1250 w 2oo3 9.26 R 1080 8.91 1122 006+005 9.32 Q 1073 VS 001 10.10 990 w 3oo7, 002+006 w 9.36 P 1068 10.45 R 957 8.44 Q 1186 0068 10.50 Q 952 M 007+003 . 13.40 R 746 vs 10.55 P 948 13.50 Q 740 003 10.96 912 005+003 s vw 13.61 F 735 11.37 R 880 35.2 284 004 11.43 Q 875 M cos—005 18.17 550 wM 0079 11.49 P 870 61.3 a 163 0025 12.49 800 M 003+002 2.86 3495 VW _ 2oo68+ooi 001—u)i 13.19 758 2.94 3400 2ooi+oo«8 w 001 — 009 \ 13. 75 727 3.45 2898 vw 20068+0078 14. 54 688 vw 2oo ' 4 4.10 2438 vw 003+0079+0068 15.10 662 vMw 2oo7 4.28 2336 vw 2ooe8 OO6—007, 002+007 20.90 478 VW 4.45 2246 Mw 001+0068 4. 55 2197 VW 3o03 4.67 2141 w 2ooi 4.76 2100 vw 2oo79+ooi See footnotes at end of table. 5.22 1915 w 003+0068 5. 50 1818 vw 001+003 | 5.81 1721 w 0068+0079 94 TABLE 1. Classification of the infrared bands cf bromcdi- Six methods were used to identify and classify the fluoromethane, bromochlorodifluoromethane, dibromcdifluoro- fundamental vibrations of the five halomethanes, as methane, bromotrifluoromethane, and icdctriflucromethane— Continued follows: (1) comparison with available Raman spectra; (2) comparison with the calculated values of X V I Assignments Stepanov [5]; (3) comparison with similar molecules, that is, isotope effect; (4) calculation based on over CF3I— Continued tone and combination bands; (5) intensity considera
6.23 1605 W W1+W79 tions; and (6) consideration of band envelopes. 6.93 1443 vw 2c«J3, C04+W68 In table 2 are shown the nine infrared fundamental 7.52 1329 w W68+W25 8.02 1246 M W1+W25 frequencies of CHBrF2. These are compared with 8.97 Q 1114 2c>)79 9. 70 R 1031 w the calculated values of Stepanov and with the 9. 75 Q 1025 M W3+04 9.80 P 1020 M assigned fundamentals of CHC1F2. Of eight funda 10. 91 916 OJl — C025, W25 —OJ3 mentals found in the infrared, seven give rise to 12.18 820 vw C04+C079 12. 87 776 w OJl — a>4 strong bands. Stepanov estimates that his calcu vw -1 14.05 711 W25+W79 lated values are accurate to within 30 to 50 cm . 22. 30 448 vw W4+W15 w Six of the nine fundamentals fall within this accuracy. » Predicted from combination and overtone bends, The classification of CHBrF2, as to the modes of b Raman value. 0 Liquid. motion and the main atom involved, is based on the assignments of the related molecule CHC1F2. Six metry and three planes of symmetry. Therefore of the fundamentals should have almost the same they belong to the point group C3v. Molecules in the value for the two molecules, as they involve pri point group C3v have six different active funda marily the motions of the same atoms. The other mentals, whereas those in C2v and Cs have nine, one three vibrations, vBr, 8FBT, and 5Br, should all occur at of which co5, is theoretically inactive in the infrared, smaller wave numbers than the corresponding vibra but which is actually found in this type of tion vC\, 5FCi, and 8Ci. This is the case for the molecule.
7 8 9 10 WAVELENGTH IN MICRONS FIGURE 2. Absorption spectra of bromotrifluoromethane and iodotrifluoromethane from 2 to 15 M- 95 WAVE NUMBERS IN CM'1 700 600 500 450 400 350 300 100 57cm Hg 5.0cm CELL 80 130cm CELL CHBrF2 60
40
20
100 40cm CELL 80 '5.0cm CELL 57cm Hg 130cm CELL
CCIBrF2 60
40
20
100
0 80 1.5mm CELL CBr2 F2 1 60 CO z g 40
20
£100 UJ o- 80
60
40
20 CF,I 100
80
60
40
20 20 22 24 26 36 0 WAVELENGTH IN MICRONS FIGURE 3. Infrared absorption spectra of bromodifluoromethane, bromochlorodifluoromethane, dibromodifluoromethane, bromotrifluoromethane, and iodotrifluoromethane from 14 to 36 n.
-1 assignments in table 2. The frequency, 240 cm , The six fundamentals of CBrF3 and CF3I have which was outside the TIBr-I region, was determined been accounted for by a comparison with the cal -1 from the combination bands. The value 240 cm culated values of CBrF3 of Stepanov and with the for a>2 seems more probable for the bending vibration fundamental frequencies of the related symmetric 1 of bromine than the calculated value of the 320 cm— top molecule CC1F3 previously studied. Five of the when related molecules are considered. fundamentals of CBrF3 and CF3I fall within the spectral region measured. The sixth fundamental, TABLE 2. Infrared fundamentals of bromodifluoromethane a>25, was determined from the calculated value for compared with the fundamentals of chlorodifluoromethane and with the calculated values of Stepanov CBrF3, and for CF3I it was determined from the combination bands. The vFe, *>Fa, and 5Fa vibrations of the three molecules have almost the same fre Classifica CHC1F2, in CHBrF2, in CHBrF2, tion frared gas, v frared gas, v calculated, v quencies. It is somewhat unusual that the vibra tions j>Ci, vBr, and V! are almost equal. This effect cm~\ cm-1 cm-1 may be produced by the small C-F distances in these oi 3023, VH 3003, »>H 3030 C08 1347, 5H 1344, SK 1350 molecules. The intensities of w4 and co79, the 6F for 606 1311, 8K 1280, 5H 1300 I C03 1178, VFS 1135, i>F<, 1110 both compounds, are very weak. This appears to 0>4 595, 5Fs 576, 8FS 550 be typical of this vibration and is found to occur in C07 809, PCI 718, vBt 660 W9 1116, J>Fa 1108, 1>F* 1070 similar fluoromethanes. The selection of these two C05 365, 5FCI 323, 5FBr 380 a «2 422, 5c 1 240, 5 Br 320 frequencies as fundamentals seems justifiable on the basis of the overtone and combination bands. Table a Predicted from combination and overtone bands. 3 also shows that the effect of replacing a chlorine by 96 1 TABLE 3. Comparison of the fundamentals of bromotrifluoro- 1150, 872, and 648 cm" . The frequencies w4,