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Integrated absorption intensities of haloethanes and halopropanes
Spectrochimica AC&, Vol.5OA,No. 13,pp. 2223-2231, 1994 Copyright @ 1994 Elsevier Science Ltd printed in Great Britain. All rights reserved 0584-8539(93)30027-T 05&t-8539&l $7.00 + 0.00
Pergamon
Integrated absorption intensities of haloethanes and halopropanes MARILYN
P. OLLIFF and
GAD
FISCHER*
Departmentof Chemistry, Facultyof Science, Australian National University, Canberra 0200, Australia (Receiued 12 August 1993; in revised form and accepted 1 December 1993) Abstract-The infrared absorption intensities of the chlorofluorocarbons C+&FY, (x+ y = 6); the hydrofluorocarbons G,H,F,, (x+y=6); and a number of hydrochlorofluorocarbons, including some members of the propane series, have been measured. Absorption intensities have been obtained by integration over specified ranges of frequencies. The ranges used include the atmospheric window (1250-833 cm-‘), 3500-450 cm-‘, 1300-700 cm-‘, and those for selected individual absorption bands. Comparisons of the results have been made with published work where available, and attention is drawn to possible sources of error in the measurement of band areas. The spectra of the halopropanes have been included for the range 3500-150 cm-‘. A preliminary study has been made of the relation between the number of fluorine atoms in the molecule and the intensity of absorption of the C-F stretching vibrations.
THE ADVERSE environmental
effects of chlorofluorocarbons (CFCs) are well known. The search by manufacturers for direct substitutes for CFCs that will cause less damage to the ozone layer has led to the commercial introduction of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). While these substances are less stable in the troposphere and are therefore less likely to penetrate the stratosphere to damage the ozone layer, they are still efficient absorbers of infrared radiation and will continue to contribute to global warming. Halocarbons absorb most strongly in the region of the ‘atmospheric window’. The atmospheric window is that part of the electromagnetic spectrum between approximately 8-12 urn (1250-833 cm-‘) through which about 25% of the thermal radiation from the Earth’s surface escapes into space [l]. This region may be defined as the range of infrared wavelengths not absorbed by naturally occurring atmospheric gases. Calculations of global warming predictions make use of infrared absorption intensities pertaining to a particular choice of wavelength range corresponding to the atmospheric window. Hence, it is important that the exact wavelength range over which intensities are measured is specified and fixed for all molecules, so that meaningful comparisons can be made of the greenhouse warming potentials (GWP) for different molecules. In the published work results have been reported for a number of different frequency ranges, including 1535-440 cm-’ [2], 1250-833 cm-’ [3], 1500-600 cm-’ [4] and 1200-800 cm-’
151. Another factor that deserves consideration in the calculation of GWPs is the temperature at which the absorption intensities have been measured. Air temperature decreases rapidly with altitude in the troposphere, so it can be argued that measurements should be made at low temperatures, or over a range of temperatures. CAPPELLANI and RESTELLI [4] observed only a weak temperature dependence of the absorption intensities of the HFCs and HCFCs they investigated, suggesting that in the calculations of the GWPs, corrections for room temperature measurements may be less than other errors incurred in the calculations, such as those associated with variations in the choice of spectral regions. In this work all measurements were made at room temperature. Measurements of the infrared absorption intensities for series of CFCs, HCFCs and HFCs are reported in this work, and where available are compared to previously reported values [6-91. Results are given in terms of the total intensity, the intensities of individual bands, and the intensities for the window regions 1250-833 cm-’ and 1300-700 cm-‘. The former window region is included because it is the range that has * Author to whom correspondence
should be addressed. 2223
2224
M. P. OLLIFF and G. FISCHER
been used in most of the published work and is the one accepted by the majority of research workers as defining the atmospheric window. Results are also given for the latter region because it encompasses all the strong bands of the halocarbons, including those that fall between 833 and 700 cm-‘, and in the vicinity of 1250 cm-‘. The total intensity measurements cover the mid-infrared range 3500-450 cm-’ and are included as they allow for comparisons of the overall absorption strengths of different molecules and are necessary, for example, in investigations of the relationship between the total intensity and the number and position of fluorine atoms. Where band overlap has not been too severe, absorption intensities for individual bands have been measured, since observed intensities due to a particular vibrational mode are necessary for comparisons with calculated band intensities in the determination of intensity parameters. Spectra are also reported for the wavenumber range 500-150 cm-’ for the fluoropropanes studied, and the band absorption intensities have been measured (although not included in this work) for them and for the other halocarbons. Because of strong absorption by water vapour present in the atmosphere, absorption by halocarbons in this latter region is not important in climate modelling. One of the problems associated with the measurement of absorption intensities concerns the difficulty with which reproducible results are obtained. This applies particularly to results obtained by different research groups. For this reason close attention was directed to possible sources of errors in the measurements. In this work several additional experiments were carried out to identify and, where possible, to eliminate sources of errors. These included choice of baseline, investigations of the change in areas under the spectrum due to a misalignment of the cell in the sample compartment, the effects of a change in spectrometer resolution, alternative apodization functions and comparisons of results obtained on different spectrophotometers.
EXPERIMENTAL Infrared absorption intensities were determined using a Perkin-Elmer 1600 Fourier Transform Infrared (FTIR) spectropbotometer with a resolution of 2.0 cm-‘, and some measurements were also conducted on Perkin-Elmer 1800 FTIR and Bio-Rad F60 FTIR spectrophotometers with resolutions of 0.2 and 0.1 cm-‘, respectively. Two glass cells, of lengths 3.415 f 0.005 cm and 10.429 f 0.005 cm, fitted with 40 mm diameter KBr windows were used. For the spectra recorded in the XJO-150cm-’ range cells with CsI or polythene windows were used. All spectra were recorded at room temperature. The Beer-Lambert law gives the absorbance of a substance at a particular wavelength as
= lcpl where IO= incident radiation intensity, I = emergent radiation intensity, K= absorbance coefficient in atm-’ cm-‘, p =pressure in atmospheres and 1=pathlength in centimetres. For the absolute intensity of a band or region, it is necessary to integrate the absorbance coefficient across all wave numbers in the range of interest, to give the band strength
where v is the wave number of the radiation in cm-‘. Vapour pressures of the halocarbons investigated were measured using a Baratron differential pressure head of range O-100 Torr (1 Torr = 0.1333 kPa). The manufacturers’ error range was given as 0.3% of the full scale reading, that is 0.3 Torr. In order to minimize errors in pressure measurements, pressures of between 5 and 50 Torr were used, as for lower pressures much larger relative errors would be incurred. To minimize instrumental errors the combinations of vapour pressures and cell path lengths were chosen so that the absorbance maximum at the centre of the band fell in the range 0.1-3.0 absorbance units. Prior to undertaking measurements of the
Integrated absorption intensities of haloethanes and halopropanes
2225
absorption intensity, several preliminary spectra were recorded for each compound, in order to select both a suitable pressure range and the cell with the appropriate pathlength. The spectral data for each compound was ratioed with the previously recorded background spectrum obtained for the evacuated cell. The resultant spectrum therefore showed only the absorbance due to the vapour within the cell. However, the baseline obtained could still have been affected by some instrumental problems, as discussed below, so the line of least absorption used in measuring areas was obtained by inspection of the spectrum. For these relatively heavy molecules with correspondingly small rotational constants, it was believed that the vapour pressures employed were sufficient to ensure broadening of the rotational fine structure of the individual bands, enabling the absorption intensities to be determined accurately [6]. This was confirmed by the consistent results obtained for a number of molecules investigated, including the lightest HFC161 (fluoroethane), using resolutions ranging from 2 to 0.2 cm-‘. Two different types of absorption areas are discussed in this work. Band areas refer to the absorption due to one vibrational mode and usually encompass a single band, starting at the point where absorbance exceeds approximately 0.01 units, continuing across the band as the maximum is reached, and terminating at a point where the absorbance is again less than 0.01 units. Where bands overlap, more than one band may be included in the area calculation if no point with absorbance less than approximately 0.01 units can be found between the bands. The second type of integrated absorption intensities concerns specific wavelength regions and these are of importance in climate modelling. Region areas refer to the total absorption between specified limits such as 1300-700 cm-‘, and may include several bands, or parts of a band if the bands overlap the limits of the region. Absorption areas were obtained from the FMR spectrophotometer by defining the limits of the wavelength range of the region in cm-‘, and selecting a suitable baseline. Areas for each band or region were obtained for a range of pressures for each compound. Large differences in intensities have been found in the results obtained by different research groups as can be seen in the data reported in the paper by FISHER et al. [2]. Some of the discrepancies may be attributed to variations in the choice of wavelength regions over which the absorption intensities are integrated. In this respect our own results are relatively correct, allowing us to make valid comparisons between the values for different compounds. Another, and important cause for discrepancies concerns the choice of baseline, that is the precise location of the zero absorption line. The possibility had to be entertained that individual bands could extend beyond the ends of the chosen wavelength region and so the baseline was not constructed using the region limits but from a point corresponding to no absorbance whether it occurred within the frequency range of the band or not. This was carried out by examining the preliminary spectra for each substance for sections of the spectrum where no absorbance was seen to occur. A horizontal line was drawn by the instrument through the selected point and used as the baseline for all subsequent area calculations for that compound. This line of least absorption was usually close to the ratioed baseline of the spectrum but it could differ slightly due to the instrumental drift in the absorbance at the 3500 cm-’ end of the spectrum and an increase in noise at frequencies below 5OOcm-‘. Fluctuations in the ratioed baseline appear only in the 3500-3OOOcm-’ region and may be as large as +0.0020 absorbance units about the zero absorbance line. Although the cause of this has not been conclusively determined, it may be due to some instability of the optical compartment of the instrument. It does not appear to be directly associated with changes in temperature or humidity in the laboratory. Noise in the spectrum causes very small changes in the absorbance at the selected baseline point, but by close inspection of repeated recordings of the same spectrum, the variation in area measurements was found to be not greater than 0.5%. Apart from the care exercised in the choice of baseline a number of further precautions were followed in the measurement of intensities. They included purging of the sample compartment with dry nitrogen gas and the construction of a special cell holder in order to eliminate errors that may be introduced by slightly different alignments of the cell in the sample compartment. Apodization functions are used by the instrument to reduce the side lobes of the band which are introduced during Fourier transformation. A reduction in side lobes is desirable when selecting baseline points, so apodization is used although it has the effect of slightly broadening the bands. A choice of two apodization functions is available on the Perkin-Elmer 1600, weak or strong Norton Beer. The difference in areas resulting from using the two functions was found to be in the order of 0.2%. Weak Norton Beer was used throughout, and the possible error associated with this selection taken into consideration when presenting an overall error value. Additional experiments were carried out for one set of data using a Perkin-Elmer 1800 FTIR spectrophotometer with resolution of 0.2 cm-‘, and a Bio-Rad F60 FTIR spectrophotometer with
2226
M. P. OLLIFFand G. FWHER
resolution of 0.1 cm-’ not normally available for use. The largest difference between the results for all these instruments for the region 1300-7OOcm-’ was in the order of +l.O%. CFC113 (99.9%), CFC113a (99%), CFCllS (98%), CFC116 (98%) were obtained from the Aldrich Chemical Company. Ail other compounds were obtained from PCR, with the exception of HCFC244ca and HCFC235cb which were synthesized by us and vacuum distilled prior to use. The compounds were used as supplied by the manufacturer. In reporting data on the absorption intensities for halocarbons it is important to know the degree of purity of the compounds. However, it can be argued that when using region intensity measurements in climate modelling, spectra obtained using the same purity of substance as that commercially available are appropriate. It was decided not to attempt to correct the data using the percentage purity given by the suppliers as it has been found that the impurities are most likely to be other halocarbons [3] which absorb in similar regions, thus distorting the bands of interest to a very small extent. Nevertheless, the presence of small amounts of impurities is expected to contribute to the overall error in the absorption intensities. For one or two compounds it was found [3] that due to the greater volatility of the impurity the amount of impurity collected in the sample cell varied considerably from run to run until the impurity was exhausted. As a consequence discrepancies in a spectrum due to a significant amount of impurity could usually be observed by the appearance of unexpected peaks or changes in area, and the spectrum discarded. When all the possible sources of error are taken into account, the overall error for the total absorption intensity (over the range 3500-450 cm-‘) was estimated to be between +2% and It4%. The variation in error arises mainly from the difference in the distribution of the bands in the spectrum. For a spectrum with many bands of similar maximum intensities, measurements may be taken over a wide range of pressures. However, when the spectrum includes both very strong and very weak bands, total absorption intensity can only be measured for a small range of pressures, or bands must be measured individually and the results added for a region, introducing more errors in the results. The errors for the window regions are smaller than those for the total area, as an error in baseline selection is smaller for the narrower regions which contain most of the bands. A slight
change in the position of the baseline point due to instrumental noise causes a small change in area measurement. This is relatively very small when the area is taken over a few hundred wavenumbers, and the intensity in that region is quite large. The difference becomes significant for the total range of 3050 cm-’ where there is only a small increase in absorption intensity compared to that in the selected window regions.
RESULTS AND DISCUSSION
The compounds selected have been divided into four groups. The first group consists of haloethanes possessing increasing numbers of fluorine atoms. Group 2 consists of the same haloethanes in so far as the arrangement of fluorine atoms is concerned, but with the chlorine atoms replaced by hydrogen. Group 3 includes those HCFCs which are replacements for CFCll, CFC12 and CFC114 that are used as refrigerants and propellants [lo], and group 4 includes halopropanes suggested as replacements for the solvent CFC113 [ll]. The total absolute integrated absorption intensities determined for these compounds for the range 3500-450 cm-‘, and for selected ranges corresponding to two choices of the atmospheric window are listed in Table 1. Units of cm-* atm-’ are used throughout. These values are the average values obtained from spectra recorded at lo-12 different vapour pressures for each compound. The intensities calculated did not vary from the average by more than 2%) except in some cases for the largest range, 3500-450 cm-‘, where errors were increased by slight variations in the baseline. In particular, the baseline errors became relatively more significant in those wavelength regions where little absorption occurred. For some of the compounds listed in Table 1 integrated intensities have been previously reported and these are included for the purposes of comparison. However, direct comparisons are difficult, as frequently different spectral regions have been used. An illustration of how the choice of different windows may significantly affect the magnitude of the measured absorption intensities can be obtained by a consideration of the spectra of HFC152a and HFC134a, Figs 1 and 2. Lines have been drawn on the spectra in the figures to show ‘window A’ in the position of 1250-833 cm-‘, ‘window B’ in
Integrated absorption intensities of haloethanes and halopropanes
2227
Table 1. Absolute integrated absorption intensities (cm-* atm-‘) of selected regions (cm-‘). Total refers to the range 3500-450 cm-’ Total
1250-833
1300-700
926 1975 2053 2616 2514 3577 3107 3867 2640
1946 2579 2639 3289 3143 3836 3707 4190 4965
CFCl 10 CFClll CFC112 CFC112a CFCl13 CFCl13a CFCl14 CFC114a CFCl15 CFCll6
Ccl&F3 CClF,CClF, CCl,FCF, CCIF2CF3 CF,CF,
2015 2708 2622 3402 3177 3979 3803 4588 5049
HFC170 HFC161 HFC152 HFC152a HFC143 HFC143a HFc134 HFC134a HFC125
CH3CH3 CH,CH2F CH2FCH2F CH,CHF, CH2FCHF, CH,CF, CHF2CHF2 CH,FCF, CHF,CF,
1064 N/A 1746 2043 3210 2802 3481 4224
507
510
1392 1557 2252 2343 2010 3159
1398 1591 2750 2400 2703 3522
HCFC14lb HCFC142b HCFC123 HCFC124
CH,CC12F CH,CClF, CHCI&F, CHCIFCF,
1941 2717 3145 3641
1199 2261 2026 2469
1761 2281 2745 3171
HCFC244ca HCFC235cb HCFC225ca HCFC225cb FC218
CHF2CF2CH2Cl CF&F,CH,CI CF3CF,CHC12 CF,CICF,CFHCI CF,CF,CF,
2788 3893 4379 4196 5887
2011 2846 3122 3027 2330
2218 3446 3824 3710 5460
(a) (b) (c) range
CCI,CCl~ CCI$CI,F CCI,FCCIIF CCl,CClF, CCI,FCCIF,
(a)
(b)
(c) 1781;
3401
3126
4141
3507t 3937t
4678 5327 793’
1648
1719 3401
3272
3169 3908
1912 2577 2859
1732 2474 2552 4043
3261
2643 3160
Reference [7]. Reference [8]. Reference [4] range 1500-6OOcm-‘; otherwise * reference [9] calculated values for the 3500-200 cm-‘; t reference [6] range 1300-700 cm-‘.
the position of 1300-700 cm-‘, and ‘window C’ in the position 1500-6OOcm-‘. In the case of HFC152a there are no areas of absorbance in window B that do not occur in window A. This is in good agreement with the results in Table 1 which show that the window B area is only 6 cm-* atm-’ larger than that for window A. This small difference may be due to the tail of the band centred at 868 cm-‘. If the range is enlarged to that of window C, areas of absorption incorporating the bands between 1500 and 1300 cm-’ are included, which helps to account for the differences between the results reported by Refs [4, 7, 81 and those reported in this work. Figure 1 also shows the entire mid-infrared range of the spectrum from 3500 to 450 cm-‘, illustrating the point that the total intensity is increased by the inclusion of the bands around 3000,570 and 470 cm-‘. Similarly, Fig. 2 illustrates how differences in intensity measurements may be related to the window chosen. In this case there is a considerable difference between windows A and B due to the band at about 1300 cm-‘. The absence of hydrogen atoms in CFCs excludes absorbance around 3000 cm-’ due to the CH stretching vibrations, and there is little or no absorbance in the range 2900-15OOcm-‘. As a consequence, the integrated intensity for the total range can be expected to be very close to that for window C. There is some discrepancy between the results for the total range reported in this work and those from MAGID [7] over the range 1535-440 cm-‘, although the maximum difference is in the order of 4%, which is not large for this type of work. The results show an increase in intensity with an increase in the number of fluorine atoms in the molecule for both the CFCs and the HFCs. A comparison of the total
M. P. OLLIFFand G.
2228
FISCHER
2.702 2A
,
B-
+--A
4
m %
-.
I 9
N
.
2
cm-l 500
Fig. 1. Infrared spectrum of HFC152a in the range 3500-450 cm-’ showing the positions of the window regions A (1250-833 cm-‘), B (1300-700 cm-‘) and C (1500-600 cm-‘).
2.784IO A m 8
I I
-
-c-
--I
-B-
cm-' 500
Fig. 2. Infrared spectrum of HFC134a in the range 3500-450 cm-’ showing the positions of the window regions as in Fig. 1.
intensity for HFC170, 793 cm-* atm-’ [9]; CFCllO, 1781 cm-* atm-’ [9]; and CFC116, 5049 cm-* atm- ’ illustrates the large change in intensity as the atoms attached to the carbons change from hydrogen to chlorine and then to fluorine. Table 2 lists the band intensities for the haloethanes. It was not always possible to identify individual bands due to overlapping areas, so the absorbance maximum, the band limits and the intensity for each range are shown in Table 2. It must be noted that
Integrated absorption intensities of haloethanes and halopropanes
2229
the errors in band intensities for weak bands are significantly larger than for strong bands. This is due to the relatively low maximum value and the effects of noise on the spectra leading to fluctuations in the baseline. For a band intensity less than Table 2. Band intensities for the haloethanes investigated in this work. Band maxima and ranges are given in cm-‘, intensities in atm-’ crne2 Intensity
Max.
Band range
Intensity
1448 1396 1120 1061 880
3080-2820 1552-1433 1433-1343 1224-1110 1110-985 930-825
437 25.5 89.7 73.3 361 69.4
2975 1412 1139 943 868 569 468
3100-2910 1500-1300 1210-1020 1005-900 900-830 610-530 510-450
195 344 1093 261 33.8 24.3 48.8
HFC143
3005 1433 1379 1319 1249 1107 911 753 577 476
3055-2866 1503-1408 1408-1345 1345-1290 1290-1214 1214-1033 945-832 777-727 613-545 545-450
186 70.1 68.8 41.8 56.1 1369 135 9.22 15.9 73.3
727 1278 134 873 126 39
HFC143a
3034 1407 1280 1233 973 830 603
3068-2999 1474-1319 1319-1252 1252-1113 1050-928 862-796 643-570
27.7 354 488 1837 408 18.8 77.7
1312-1253 1253-1216 1216-1079 1079-1019 972-902 902-862 862-820 758-713 698-653 636-586
191 138 1847 546 437 151 479 16.0 16.7 44.6
HCFC134
2995 1391 1309 1205 1133 905 779 541
3059-2938 1420-1369 1369-1262 1257-1180 1180-1089 938-873 802-738 570-505
128 15.7 175 124 2145 21.6 35.7 41.6
1355-1266 1266-1206 1206-1160 1160-1070 1070-1024 955-861 861-822 756-713 603-575 575-543
456 1354 124 483 57.4 1100 51.5 128 12.2 16.7
HFC134a
2984 1464 1428 1301 1191 1105 973 843 666 549
3100-2750 1490-1447 1447-1397 1344-1243 1243-1133 1133-1027 1027-920 873-800 700-594 594-500
80.4 34.1 79.8 1030 1366 370 220 62.3 132 38.6
Max.
Band range
1114 1018 911 856 810 731
1155-1088 1057-980 940-870 870-834 834-762 762-700
CFCl12
1121 1032 844 788 627 484
1225-1063 1063-995 973-810 810-725 645-612 498-463
678 207 1160 579 4.72 10.5
HFC152
n/a
CFC112a
1171 1036 856 783 627
1215-1150 1056-985 922-812 812-723 650-610
821 446 718 575 37.5
HFC152a
CFC113 [3]
1180 1118 1042 910 816
1237-1138 1138-1078 1078-997 954-847 846-765
841 486 461 765 665
CFC3lla
1256 1225 909 858 713 561
1290-1238 1238-1200 945-880 880-825 750-690 590-525
CFc114
1271 1231 1185 1052 922 887 847 735 678 616
CFCl14a
1294 1226 1195 1110 1052 920 847 735 589 560
CFClll
[3]
287 105 187 336 706 316
HFC161
M. P. OLLIFF and G. FISCHER
2230
Table 2. (Co&wed) Max.
Band range
Intensity
Max.
Band range
Intensity
CFCl 15
1349 1239 1184 1131 982 762 647 560
1378-1314 1273-1211 1211-1156 1156-1066 1025-931 781-738 666-628 580-538
220 1851 497 744 891 96.7 43.3 16.8
HFC125
3001 1444 1357 1308 1209 1146 867 727 578 523
3040-2960 1476-1412 1412-1337 1337-1255 1255-1171 1171-1051 915-824 756-693 605-557 549-496
51.2 14.7 37.2 588 1981 969 167 128 55.3 25.4
CFCll6
1328 1250 1206 1139 1115 714 519
1359-1295 1277-1222 1220-1186 1157-1134 1134-1084 737-692 542-495
91.4 3757 33.4 41.5 1011 130 29.3
HCFC14lb
3013 2954 1445 1387 1161 1102 927 762 593
3054-2985 2985-2914 1475-1417 1417-1359 1207-1137 1137-1050 960-880 800-700 621-560
25.1 8.19 18.5 65.7 336 575 259 555 79.1
HCFC142b
3021 2961 1447 1395 1192 1134 967 904 682 543
3077-2991 2991-2936 1474-1423 1423-1356 1268-1159 1159-1060 1001-932 932-865 718-641 574-510
25.5 6.22 19.5 136 995 671 200 389 174 53.5
HCFC123
3011 1324 1279 1195 1146 1107 1064 999 842 770 672 633 559 527
3031-2972 1342-1303 1303-1256 1256-1172 1172-1119 1119-1082 1082-1040 1019-966 893-800 800-731 691-647 647-614 578-542 542-505
13.2 279 557 844 656 51.5 30.3 37.8 493 71.6 96.2 9.58 12.4 16.0
HCFC124 1377 1286 1215 1166 1101 885 818 697 573 531
3026-2970 1433-1324 1324-1254 1254-1187 1187-1125 1125-1071 925-849 849-786 720-671 590-552 552-506
19.0 179 605 977 774 395 289 147 130 13.8 24.8
50 cm-’ atm-’ the error may be in the order of 50%. Small spectral features caused by very weak bands, that is, those with a maximum absorbance of less than 0.1 absorbance units have been omitted, therefore addition of the individual band intensities for a compound will be less than the regional intensities shown in Table !. Some difficulty was encountered in the selection of the band limits in certain cases due to shoulders of overlapping bands. For the halopropanes the spectra in the range 3500-450 cm-’ are shown in Figs 3-7, with the band ranges and intensities indicated on the figures. Figures 8-12 show the halopropane spectra in the range 500-150 cm-‘. These spectra have been included here as little information on the infrared spectra of many of the halopropanes has been previously reported, and moreover, they serve to show the relative smallness of the absorption below 450 cm-‘. For all these halocarbon molecules absorption below 450 cm-’ is usually due to CF3 or CC& rocking modes and torsion about the C-C bond, and therefore is expected to be weak [12-141. This is confirmed by our measurements of the absorption spectra in the range 500-150 cm-‘, and hence changes to the absorption intensity over the range 3500-150 cm-’ because of truncation of the spectra at 450 cm-’ will be relatively very
Integrated absorption intensities of haloethanes and halopropanes
2231
2.654 lA
Range cm-l Band i!aensily cm-2am-l
:iEz 1370-1295 ‘%iE 805-750 z-z
128 108 72.5 1986 150 64.3 90.2 119
-0.00’ 5-f3500
3000
2500
2000
1500
1000
cm-l
500
Fig. 3. Infrared vapour spectrum of HCFC244ca in the range 3500-450 cm-’ (12 Torr and 10 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
small (for example, HCFC225cb has absorption intensity in the range 3500-450 cm-’ of 4196 cm-* atm-‘, yet in the range 500-180 cm-’ of only 47 cm-* atm-‘). For this reason, and also because the errors in the measurements of small absorption intensities are relatively large (see above), and because instrumental noise is larger in this region, the contributions of the absorption intensities in the low frequency region have been omitted from the total integrated absorption intensities listed in Table 1. The strongest bands are found in the region 1300-lOOOcm-’ and are typically due to carbon-fluorine bond stretches [12-141. The intensity of an infrared band is proportional to the transition dipole moment squared and since the electronegativities of the atoms involved are in the order of F > Cl > H, the total band intensities may be expected to increase when H is replaced by Cl and then by F. This trend, as noted for the CFCs and I-WCs can also be seen in the HCFCs. The difference between HCFC14lb, with a total intensity of 1941 cmm2atm-‘, and HCFC142b, with a total intensity 2717 cmm2atm-’ is due to the replacement of one chlorine atom with one fluorine atom. Similarly, HCFC123 and HCFC124 have intensities 3145 cm-* atm-’ and 3641 cm-* atm-‘, respectively. In the case of HCFC244ca and HCFC235cb, one hydrogen is replaced by one fluorine, resulting in the larger difference of 1105 cmm2atm-‘. Comparisons between the total intensities of isomeric pairs of molecules, as in CFC112 and CFCl12a; CFC113 and CFC113a; and CFC114 and CFCl14a, show a small increase for those isomers where the fluorines are attached to both carbons. This is in direct contrast with the results for the HFCs where the molecules with more evenly distributed fluorines show a large decrease in total intensity. Unfortunately the unavailability at present of a sample of HFC152 leaves a gap in this part of the work. A comparison of band intensities in the region of approximately 1350-1050 cm-’ with the number of fluorine atoms present is shown in Fig. 13. C-F stretching vibrations occur in this region for all the molecules, and although other vibrations may be included, the strength of the C-F stretch is such that most of the intensity can be attributed to this vibration. Figure 13 illustrates the increase in band intensity with the increase in number of fluorine atoms for both CFCs and HFCs. Similar comparisons may be drawn for the isomeric series of both types of halocarbons investigated here. Theoretical calculations of
M. P. OLLIFF and G. FISCHER
2232
Bandintensity cur2 atm-’
Range cd 3020-2935 1465-1415 1415-1328 1328-1160 1160-1082 1082-loo0 845-750
2500
21.3 39.0 2z E 136 133 51.9 37.9
2000
1500
1000
cm-l
Fig. 4. Infrared vapour spectrum of HCFC235cb
in the range 3500-450 cm-’ (5 Torr and 10 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
2.9366A
Bandhensitycm-2atm-1
Range cm-’
3050-2965 1420-1320 1324%1100 ‘%% 780-738 738-695
3500
MOO
2500
2000
12.7 236
In
yz
YE 339 53.3 216
lu
I
I
1500
1000
cm-’ 500
Fig. 5. Infrared vapour spectrum of HCFCZ25ca in the range 3500-450 cm-’ (9 Torr and 10 cm cell). Average band intensities in cm-’ atm-’ are given for selected bands.
Integrated absorption intensities of haloethanes and halopropanes
2233
2.7701A
z%E
46.4 12.1
%zO$
3500
3000
2500
2710 423 785 70.8 38.5
2000
1500
1000
cr
500
Fig. 6. Infrared vapour spectrum of HCFC225cb in the range 3500-450 cm-’ (11 Torr and 10 cm cell). Average band intensities in cm-’ atm-’ are given for selected bands.
2.9581 A Raagc cm-’ 1287-1227 1415-1329 1227-1180 ‘fE 75s705 m-520
Band imasitycm-* am-1 3% 590 E 191 49.6
m i
L
1000
Q
8
0
P I
cm- 500
Fig. 7. Infrared vapour spectrum of HCFC218 in the range 3500-450 cm-’ (7 Torr and 3 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
M. P. OLLIFF and G. FECHI?R
2234
0.9 *
0.7
0.S
0.3
0.1
-0.1 so0
.a0
400
350
300
230
200
cm -I
Fig. 8. Infrared vapour spectrum of HCFC244ca for the range 500-150 cm-’ (127 Torr and 10 cm cell).
0.9 A
0.7
SO0
410
400
320
300
230
200
C. -1
Fig. 9. Infrared vapour spectrum of HCFC235cb for the range 500-150 cm-’ (82 Torr and 10 cm cell).
2235
Integrated absorption intensities of haloethanes and halopropanes
-
0.s *
0.7
0.5
0.3
0.1
-0.1
502
a0
400
900
350
2w
200
lw
“4
Fig. 10. Infrared vapour spectrum of HCFC225ca for the range 500-150 cm-’ (175 Torr and 10 cm cell).
0.5
0.3
0.1
-0.i
wo
450
400
wo
300
250
220
C. -I
Fig. 11. Infrared vapour spectrum of HCFC22Scb for the range SO-150 cm-’ (180 Torr and 10 cm cell).
120
M. P. OLLIFFand G. FISCI-IER
2236
so0
400
450
300
3so
2so
200
em -i
Fig. 12. Infrared vapour spectrum of HCFC218 for the range NO-150 cm-’ (152 Torr and 10 cm cell).
so00
w
+ 0
+
OtXI
0
+WC
0 ;t -.0
I.
I.
1
2
I.,.,.,.
3
4
5
6
7
no. of fluorines Fig. 13. The increase in band intensity for the region encompassing the C-F stretching vibrations with the increase in the number of fluorine atoms present in the molecule for the ethane series of CFCs and I-WCs.
150
Integrated absorption intensities of haloethanes and halopropanes
2237
intensities are expected to offer more insight into the effects of the positions of the fluorine atoms in these molecules, and are currently in progress. Acknowledgements-One of us (G.F.) acknowledges support for this project from the Australian Research Council. We also acknowledge the help given by Mr John Csaki and Mr Denes Bogsanyi, and the use of the facilities in the ResearchSchool of Chemistry at the Australian National University, Canberra.
REFERENCES [l] [2] [3] [4] [5] [6] [7] [S] [9] [lo] [ll]
R. E. Dickinson and R. J. Cicerone, Nature 319, 109 (1986). D. A. Fisher, C. H. Hales, W. C. Wang, M. K. W. Ko and N. Dak Sze, Nature 344,513 (1998). M. P. Olliff and G. Fischer, Spectrochim. Acta 48A, 229 (1992). F. Cappellani and G. Restelli, Spectrochim. Actu 48A, 1127 (1992). A. C. Brown, C. E. Canosa-Mas, A. D. Parr and R. P. Wayne, Atmos. Enuir. 24A, 2499 (1990). P. Varanasi and S. Chudamani, J. Geophys. Res. 93, 1666 (1988). H. Magid, personal communication reported in ref. 2. D. G. Gehring, personal communication reported in ref. 2. K. Tanabe and S. Saeki, Bull. Chem. Sot. Japan 45,32 (1972). ATOCHEM, The New FORANF Runge. Atochem SA, France (1992). C. C. Dudman, D. G. Hey, P. G. Johnson, D. C. McBeth, N. J. Mime and N. Winterton, Solvent Cleaning: KY’s New Product Development, Baltimore (1990). [12] J. Rud Nielsen, C. Y. Liang, R. M. Smith and D. C. Smith, J. Chem. Phys. 21,383 (1953). [13] J. Rud Nielsen, C. Y. Liang and D. C. Smith, J. Chem. Phys. 21, 847 (1953). [14] J. Rud Nielsen, C. Y. Liang, D. C. Smith and Morris Alpert, J. Chem. Phys. 21, 1070 (1953).
Pergamon
Integrated absorption intensities of haloethanes and halopropanes MARILYN
P. OLLIFF and
GAD
FISCHER*
Departmentof Chemistry, Facultyof Science, Australian National University, Canberra 0200, Australia (Receiued 12 August 1993; in revised form and accepted 1 December 1993) Abstract-The infrared absorption intensities of the chlorofluorocarbons C+&FY, (x+ y = 6); the hydrofluorocarbons G,H,F,, (x+y=6); and a number of hydrochlorofluorocarbons, including some members of the propane series, have been measured. Absorption intensities have been obtained by integration over specified ranges of frequencies. The ranges used include the atmospheric window (1250-833 cm-‘), 3500-450 cm-‘, 1300-700 cm-‘, and those for selected individual absorption bands. Comparisons of the results have been made with published work where available, and attention is drawn to possible sources of error in the measurement of band areas. The spectra of the halopropanes have been included for the range 3500-150 cm-‘. A preliminary study has been made of the relation between the number of fluorine atoms in the molecule and the intensity of absorption of the C-F stretching vibrations.
THE ADVERSE environmental
effects of chlorofluorocarbons (CFCs) are well known. The search by manufacturers for direct substitutes for CFCs that will cause less damage to the ozone layer has led to the commercial introduction of hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs). While these substances are less stable in the troposphere and are therefore less likely to penetrate the stratosphere to damage the ozone layer, they are still efficient absorbers of infrared radiation and will continue to contribute to global warming. Halocarbons absorb most strongly in the region of the ‘atmospheric window’. The atmospheric window is that part of the electromagnetic spectrum between approximately 8-12 urn (1250-833 cm-‘) through which about 25% of the thermal radiation from the Earth’s surface escapes into space [l]. This region may be defined as the range of infrared wavelengths not absorbed by naturally occurring atmospheric gases. Calculations of global warming predictions make use of infrared absorption intensities pertaining to a particular choice of wavelength range corresponding to the atmospheric window. Hence, it is important that the exact wavelength range over which intensities are measured is specified and fixed for all molecules, so that meaningful comparisons can be made of the greenhouse warming potentials (GWP) for different molecules. In the published work results have been reported for a number of different frequency ranges, including 1535-440 cm-’ [2], 1250-833 cm-’ [3], 1500-600 cm-’ [4] and 1200-800 cm-’
151. Another factor that deserves consideration in the calculation of GWPs is the temperature at which the absorption intensities have been measured. Air temperature decreases rapidly with altitude in the troposphere, so it can be argued that measurements should be made at low temperatures, or over a range of temperatures. CAPPELLANI and RESTELLI [4] observed only a weak temperature dependence of the absorption intensities of the HFCs and HCFCs they investigated, suggesting that in the calculations of the GWPs, corrections for room temperature measurements may be less than other errors incurred in the calculations, such as those associated with variations in the choice of spectral regions. In this work all measurements were made at room temperature. Measurements of the infrared absorption intensities for series of CFCs, HCFCs and HFCs are reported in this work, and where available are compared to previously reported values [6-91. Results are given in terms of the total intensity, the intensities of individual bands, and the intensities for the window regions 1250-833 cm-’ and 1300-700 cm-‘. The former window region is included because it is the range that has * Author to whom correspondence
should be addressed. 2223
2224
M. P. OLLIFF and G. FISCHER
been used in most of the published work and is the one accepted by the majority of research workers as defining the atmospheric window. Results are also given for the latter region because it encompasses all the strong bands of the halocarbons, including those that fall between 833 and 700 cm-‘, and in the vicinity of 1250 cm-‘. The total intensity measurements cover the mid-infrared range 3500-450 cm-’ and are included as they allow for comparisons of the overall absorption strengths of different molecules and are necessary, for example, in investigations of the relationship between the total intensity and the number and position of fluorine atoms. Where band overlap has not been too severe, absorption intensities for individual bands have been measured, since observed intensities due to a particular vibrational mode are necessary for comparisons with calculated band intensities in the determination of intensity parameters. Spectra are also reported for the wavenumber range 500-150 cm-’ for the fluoropropanes studied, and the band absorption intensities have been measured (although not included in this work) for them and for the other halocarbons. Because of strong absorption by water vapour present in the atmosphere, absorption by halocarbons in this latter region is not important in climate modelling. One of the problems associated with the measurement of absorption intensities concerns the difficulty with which reproducible results are obtained. This applies particularly to results obtained by different research groups. For this reason close attention was directed to possible sources of errors in the measurements. In this work several additional experiments were carried out to identify and, where possible, to eliminate sources of errors. These included choice of baseline, investigations of the change in areas under the spectrum due to a misalignment of the cell in the sample compartment, the effects of a change in spectrometer resolution, alternative apodization functions and comparisons of results obtained on different spectrophotometers.
EXPERIMENTAL Infrared absorption intensities were determined using a Perkin-Elmer 1600 Fourier Transform Infrared (FTIR) spectropbotometer with a resolution of 2.0 cm-‘, and some measurements were also conducted on Perkin-Elmer 1800 FTIR and Bio-Rad F60 FTIR spectrophotometers with resolutions of 0.2 and 0.1 cm-‘, respectively. Two glass cells, of lengths 3.415 f 0.005 cm and 10.429 f 0.005 cm, fitted with 40 mm diameter KBr windows were used. For the spectra recorded in the XJO-150cm-’ range cells with CsI or polythene windows were used. All spectra were recorded at room temperature. The Beer-Lambert law gives the absorbance of a substance at a particular wavelength as
= lcpl where IO= incident radiation intensity, I = emergent radiation intensity, K= absorbance coefficient in atm-’ cm-‘, p =pressure in atmospheres and 1=pathlength in centimetres. For the absolute intensity of a band or region, it is necessary to integrate the absorbance coefficient across all wave numbers in the range of interest, to give the band strength
where v is the wave number of the radiation in cm-‘. Vapour pressures of the halocarbons investigated were measured using a Baratron differential pressure head of range O-100 Torr (1 Torr = 0.1333 kPa). The manufacturers’ error range was given as 0.3% of the full scale reading, that is 0.3 Torr. In order to minimize errors in pressure measurements, pressures of between 5 and 50 Torr were used, as for lower pressures much larger relative errors would be incurred. To minimize instrumental errors the combinations of vapour pressures and cell path lengths were chosen so that the absorbance maximum at the centre of the band fell in the range 0.1-3.0 absorbance units. Prior to undertaking measurements of the
Integrated absorption intensities of haloethanes and halopropanes
2225
absorption intensity, several preliminary spectra were recorded for each compound, in order to select both a suitable pressure range and the cell with the appropriate pathlength. The spectral data for each compound was ratioed with the previously recorded background spectrum obtained for the evacuated cell. The resultant spectrum therefore showed only the absorbance due to the vapour within the cell. However, the baseline obtained could still have been affected by some instrumental problems, as discussed below, so the line of least absorption used in measuring areas was obtained by inspection of the spectrum. For these relatively heavy molecules with correspondingly small rotational constants, it was believed that the vapour pressures employed were sufficient to ensure broadening of the rotational fine structure of the individual bands, enabling the absorption intensities to be determined accurately [6]. This was confirmed by the consistent results obtained for a number of molecules investigated, including the lightest HFC161 (fluoroethane), using resolutions ranging from 2 to 0.2 cm-‘. Two different types of absorption areas are discussed in this work. Band areas refer to the absorption due to one vibrational mode and usually encompass a single band, starting at the point where absorbance exceeds approximately 0.01 units, continuing across the band as the maximum is reached, and terminating at a point where the absorbance is again less than 0.01 units. Where bands overlap, more than one band may be included in the area calculation if no point with absorbance less than approximately 0.01 units can be found between the bands. The second type of integrated absorption intensities concerns specific wavelength regions and these are of importance in climate modelling. Region areas refer to the total absorption between specified limits such as 1300-700 cm-‘, and may include several bands, or parts of a band if the bands overlap the limits of the region. Absorption areas were obtained from the FMR spectrophotometer by defining the limits of the wavelength range of the region in cm-‘, and selecting a suitable baseline. Areas for each band or region were obtained for a range of pressures for each compound. Large differences in intensities have been found in the results obtained by different research groups as can be seen in the data reported in the paper by FISHER et al. [2]. Some of the discrepancies may be attributed to variations in the choice of wavelength regions over which the absorption intensities are integrated. In this respect our own results are relatively correct, allowing us to make valid comparisons between the values for different compounds. Another, and important cause for discrepancies concerns the choice of baseline, that is the precise location of the zero absorption line. The possibility had to be entertained that individual bands could extend beyond the ends of the chosen wavelength region and so the baseline was not constructed using the region limits but from a point corresponding to no absorbance whether it occurred within the frequency range of the band or not. This was carried out by examining the preliminary spectra for each substance for sections of the spectrum where no absorbance was seen to occur. A horizontal line was drawn by the instrument through the selected point and used as the baseline for all subsequent area calculations for that compound. This line of least absorption was usually close to the ratioed baseline of the spectrum but it could differ slightly due to the instrumental drift in the absorbance at the 3500 cm-’ end of the spectrum and an increase in noise at frequencies below 5OOcm-‘. Fluctuations in the ratioed baseline appear only in the 3500-3OOOcm-’ region and may be as large as +0.0020 absorbance units about the zero absorbance line. Although the cause of this has not been conclusively determined, it may be due to some instability of the optical compartment of the instrument. It does not appear to be directly associated with changes in temperature or humidity in the laboratory. Noise in the spectrum causes very small changes in the absorbance at the selected baseline point, but by close inspection of repeated recordings of the same spectrum, the variation in area measurements was found to be not greater than 0.5%. Apart from the care exercised in the choice of baseline a number of further precautions were followed in the measurement of intensities. They included purging of the sample compartment with dry nitrogen gas and the construction of a special cell holder in order to eliminate errors that may be introduced by slightly different alignments of the cell in the sample compartment. Apodization functions are used by the instrument to reduce the side lobes of the band which are introduced during Fourier transformation. A reduction in side lobes is desirable when selecting baseline points, so apodization is used although it has the effect of slightly broadening the bands. A choice of two apodization functions is available on the Perkin-Elmer 1600, weak or strong Norton Beer. The difference in areas resulting from using the two functions was found to be in the order of 0.2%. Weak Norton Beer was used throughout, and the possible error associated with this selection taken into consideration when presenting an overall error value. Additional experiments were carried out for one set of data using a Perkin-Elmer 1800 FTIR spectrophotometer with resolution of 0.2 cm-‘, and a Bio-Rad F60 FTIR spectrophotometer with
2226
M. P. OLLIFFand G. FWHER
resolution of 0.1 cm-’ not normally available for use. The largest difference between the results for all these instruments for the region 1300-7OOcm-’ was in the order of +l.O%. CFC113 (99.9%), CFC113a (99%), CFCllS (98%), CFC116 (98%) were obtained from the Aldrich Chemical Company. Ail other compounds were obtained from PCR, with the exception of HCFC244ca and HCFC235cb which were synthesized by us and vacuum distilled prior to use. The compounds were used as supplied by the manufacturer. In reporting data on the absorption intensities for halocarbons it is important to know the degree of purity of the compounds. However, it can be argued that when using region intensity measurements in climate modelling, spectra obtained using the same purity of substance as that commercially available are appropriate. It was decided not to attempt to correct the data using the percentage purity given by the suppliers as it has been found that the impurities are most likely to be other halocarbons [3] which absorb in similar regions, thus distorting the bands of interest to a very small extent. Nevertheless, the presence of small amounts of impurities is expected to contribute to the overall error in the absorption intensities. For one or two compounds it was found [3] that due to the greater volatility of the impurity the amount of impurity collected in the sample cell varied considerably from run to run until the impurity was exhausted. As a consequence discrepancies in a spectrum due to a significant amount of impurity could usually be observed by the appearance of unexpected peaks or changes in area, and the spectrum discarded. When all the possible sources of error are taken into account, the overall error for the total absorption intensity (over the range 3500-450 cm-‘) was estimated to be between +2% and It4%. The variation in error arises mainly from the difference in the distribution of the bands in the spectrum. For a spectrum with many bands of similar maximum intensities, measurements may be taken over a wide range of pressures. However, when the spectrum includes both very strong and very weak bands, total absorption intensity can only be measured for a small range of pressures, or bands must be measured individually and the results added for a region, introducing more errors in the results. The errors for the window regions are smaller than those for the total area, as an error in baseline selection is smaller for the narrower regions which contain most of the bands. A slight
change in the position of the baseline point due to instrumental noise causes a small change in area measurement. This is relatively very small when the area is taken over a few hundred wavenumbers, and the intensity in that region is quite large. The difference becomes significant for the total range of 3050 cm-’ where there is only a small increase in absorption intensity compared to that in the selected window regions.
RESULTS AND DISCUSSION
The compounds selected have been divided into four groups. The first group consists of haloethanes possessing increasing numbers of fluorine atoms. Group 2 consists of the same haloethanes in so far as the arrangement of fluorine atoms is concerned, but with the chlorine atoms replaced by hydrogen. Group 3 includes those HCFCs which are replacements for CFCll, CFC12 and CFC114 that are used as refrigerants and propellants [lo], and group 4 includes halopropanes suggested as replacements for the solvent CFC113 [ll]. The total absolute integrated absorption intensities determined for these compounds for the range 3500-450 cm-‘, and for selected ranges corresponding to two choices of the atmospheric window are listed in Table 1. Units of cm-* atm-’ are used throughout. These values are the average values obtained from spectra recorded at lo-12 different vapour pressures for each compound. The intensities calculated did not vary from the average by more than 2%) except in some cases for the largest range, 3500-450 cm-‘, where errors were increased by slight variations in the baseline. In particular, the baseline errors became relatively more significant in those wavelength regions where little absorption occurred. For some of the compounds listed in Table 1 integrated intensities have been previously reported and these are included for the purposes of comparison. However, direct comparisons are difficult, as frequently different spectral regions have been used. An illustration of how the choice of different windows may significantly affect the magnitude of the measured absorption intensities can be obtained by a consideration of the spectra of HFC152a and HFC134a, Figs 1 and 2. Lines have been drawn on the spectra in the figures to show ‘window A’ in the position of 1250-833 cm-‘, ‘window B’ in
Integrated absorption intensities of haloethanes and halopropanes
2227
Table 1. Absolute integrated absorption intensities (cm-* atm-‘) of selected regions (cm-‘). Total refers to the range 3500-450 cm-’ Total
1250-833
1300-700
926 1975 2053 2616 2514 3577 3107 3867 2640
1946 2579 2639 3289 3143 3836 3707 4190 4965
CFCl 10 CFClll CFC112 CFC112a CFCl13 CFCl13a CFCl14 CFC114a CFCl15 CFCll6
Ccl&F3 CClF,CClF, CCl,FCF, CCIF2CF3 CF,CF,
2015 2708 2622 3402 3177 3979 3803 4588 5049
HFC170 HFC161 HFC152 HFC152a HFC143 HFC143a HFc134 HFC134a HFC125
CH3CH3 CH,CH2F CH2FCH2F CH,CHF, CH2FCHF, CH,CF, CHF2CHF2 CH,FCF, CHF,CF,
1064 N/A 1746 2043 3210 2802 3481 4224
507
510
1392 1557 2252 2343 2010 3159
1398 1591 2750 2400 2703 3522
HCFC14lb HCFC142b HCFC123 HCFC124
CH,CC12F CH,CClF, CHCI&F, CHCIFCF,
1941 2717 3145 3641
1199 2261 2026 2469
1761 2281 2745 3171
HCFC244ca HCFC235cb HCFC225ca HCFC225cb FC218
CHF2CF2CH2Cl CF&F,CH,CI CF3CF,CHC12 CF,CICF,CFHCI CF,CF,CF,
2788 3893 4379 4196 5887
2011 2846 3122 3027 2330
2218 3446 3824 3710 5460
(a) (b) (c) range
CCI,CCl~ CCI$CI,F CCI,FCCIIF CCl,CClF, CCI,FCCIF,
(a)
(b)
(c) 1781;
3401
3126
4141
3507t 3937t
4678 5327 793’
1648
1719 3401
3272
3169 3908
1912 2577 2859
1732 2474 2552 4043
3261
2643 3160
Reference [7]. Reference [8]. Reference [4] range 1500-6OOcm-‘; otherwise * reference [9] calculated values for the 3500-200 cm-‘; t reference [6] range 1300-700 cm-‘.
the position of 1300-700 cm-‘, and ‘window C’ in the position 1500-6OOcm-‘. In the case of HFC152a there are no areas of absorbance in window B that do not occur in window A. This is in good agreement with the results in Table 1 which show that the window B area is only 6 cm-* atm-’ larger than that for window A. This small difference may be due to the tail of the band centred at 868 cm-‘. If the range is enlarged to that of window C, areas of absorption incorporating the bands between 1500 and 1300 cm-’ are included, which helps to account for the differences between the results reported by Refs [4, 7, 81 and those reported in this work. Figure 1 also shows the entire mid-infrared range of the spectrum from 3500 to 450 cm-‘, illustrating the point that the total intensity is increased by the inclusion of the bands around 3000,570 and 470 cm-‘. Similarly, Fig. 2 illustrates how differences in intensity measurements may be related to the window chosen. In this case there is a considerable difference between windows A and B due to the band at about 1300 cm-‘. The absence of hydrogen atoms in CFCs excludes absorbance around 3000 cm-’ due to the CH stretching vibrations, and there is little or no absorbance in the range 2900-15OOcm-‘. As a consequence, the integrated intensity for the total range can be expected to be very close to that for window C. There is some discrepancy between the results for the total range reported in this work and those from MAGID [7] over the range 1535-440 cm-‘, although the maximum difference is in the order of 4%, which is not large for this type of work. The results show an increase in intensity with an increase in the number of fluorine atoms in the molecule for both the CFCs and the HFCs. A comparison of the total
M. P. OLLIFFand G.
2228
FISCHER
2.702 2A
,
B-
+--A
4
m %
-.
I 9
N
.
2
cm-l 500
Fig. 1. Infrared spectrum of HFC152a in the range 3500-450 cm-’ showing the positions of the window regions A (1250-833 cm-‘), B (1300-700 cm-‘) and C (1500-600 cm-‘).
2.784IO A m 8
I I
-
-c-
--I
-B-
cm-' 500
Fig. 2. Infrared spectrum of HFC134a in the range 3500-450 cm-’ showing the positions of the window regions as in Fig. 1.
intensity for HFC170, 793 cm-* atm-’ [9]; CFCllO, 1781 cm-* atm-’ [9]; and CFC116, 5049 cm-* atm- ’ illustrates the large change in intensity as the atoms attached to the carbons change from hydrogen to chlorine and then to fluorine. Table 2 lists the band intensities for the haloethanes. It was not always possible to identify individual bands due to overlapping areas, so the absorbance maximum, the band limits and the intensity for each range are shown in Table 2. It must be noted that
Integrated absorption intensities of haloethanes and halopropanes
2229
the errors in band intensities for weak bands are significantly larger than for strong bands. This is due to the relatively low maximum value and the effects of noise on the spectra leading to fluctuations in the baseline. For a band intensity less than Table 2. Band intensities for the haloethanes investigated in this work. Band maxima and ranges are given in cm-‘, intensities in atm-’ crne2 Intensity
Max.
Band range
Intensity
1448 1396 1120 1061 880
3080-2820 1552-1433 1433-1343 1224-1110 1110-985 930-825
437 25.5 89.7 73.3 361 69.4
2975 1412 1139 943 868 569 468
3100-2910 1500-1300 1210-1020 1005-900 900-830 610-530 510-450
195 344 1093 261 33.8 24.3 48.8
HFC143
3005 1433 1379 1319 1249 1107 911 753 577 476
3055-2866 1503-1408 1408-1345 1345-1290 1290-1214 1214-1033 945-832 777-727 613-545 545-450
186 70.1 68.8 41.8 56.1 1369 135 9.22 15.9 73.3
727 1278 134 873 126 39
HFC143a
3034 1407 1280 1233 973 830 603
3068-2999 1474-1319 1319-1252 1252-1113 1050-928 862-796 643-570
27.7 354 488 1837 408 18.8 77.7
1312-1253 1253-1216 1216-1079 1079-1019 972-902 902-862 862-820 758-713 698-653 636-586
191 138 1847 546 437 151 479 16.0 16.7 44.6
HCFC134
2995 1391 1309 1205 1133 905 779 541
3059-2938 1420-1369 1369-1262 1257-1180 1180-1089 938-873 802-738 570-505
128 15.7 175 124 2145 21.6 35.7 41.6
1355-1266 1266-1206 1206-1160 1160-1070 1070-1024 955-861 861-822 756-713 603-575 575-543
456 1354 124 483 57.4 1100 51.5 128 12.2 16.7
HFC134a
2984 1464 1428 1301 1191 1105 973 843 666 549
3100-2750 1490-1447 1447-1397 1344-1243 1243-1133 1133-1027 1027-920 873-800 700-594 594-500
80.4 34.1 79.8 1030 1366 370 220 62.3 132 38.6
Max.
Band range
1114 1018 911 856 810 731
1155-1088 1057-980 940-870 870-834 834-762 762-700
CFCl12
1121 1032 844 788 627 484
1225-1063 1063-995 973-810 810-725 645-612 498-463
678 207 1160 579 4.72 10.5
HFC152
n/a
CFC112a
1171 1036 856 783 627
1215-1150 1056-985 922-812 812-723 650-610
821 446 718 575 37.5
HFC152a
CFC113 [3]
1180 1118 1042 910 816
1237-1138 1138-1078 1078-997 954-847 846-765
841 486 461 765 665
CFC3lla
1256 1225 909 858 713 561
1290-1238 1238-1200 945-880 880-825 750-690 590-525
CFc114
1271 1231 1185 1052 922 887 847 735 678 616
CFCl14a
1294 1226 1195 1110 1052 920 847 735 589 560
CFClll
[3]
287 105 187 336 706 316
HFC161
M. P. OLLIFF and G. FISCHER
2230
Table 2. (Co&wed) Max.
Band range
Intensity
Max.
Band range
Intensity
CFCl 15
1349 1239 1184 1131 982 762 647 560
1378-1314 1273-1211 1211-1156 1156-1066 1025-931 781-738 666-628 580-538
220 1851 497 744 891 96.7 43.3 16.8
HFC125
3001 1444 1357 1308 1209 1146 867 727 578 523
3040-2960 1476-1412 1412-1337 1337-1255 1255-1171 1171-1051 915-824 756-693 605-557 549-496
51.2 14.7 37.2 588 1981 969 167 128 55.3 25.4
CFCll6
1328 1250 1206 1139 1115 714 519
1359-1295 1277-1222 1220-1186 1157-1134 1134-1084 737-692 542-495
91.4 3757 33.4 41.5 1011 130 29.3
HCFC14lb
3013 2954 1445 1387 1161 1102 927 762 593
3054-2985 2985-2914 1475-1417 1417-1359 1207-1137 1137-1050 960-880 800-700 621-560
25.1 8.19 18.5 65.7 336 575 259 555 79.1
HCFC142b
3021 2961 1447 1395 1192 1134 967 904 682 543
3077-2991 2991-2936 1474-1423 1423-1356 1268-1159 1159-1060 1001-932 932-865 718-641 574-510
25.5 6.22 19.5 136 995 671 200 389 174 53.5
HCFC123
3011 1324 1279 1195 1146 1107 1064 999 842 770 672 633 559 527
3031-2972 1342-1303 1303-1256 1256-1172 1172-1119 1119-1082 1082-1040 1019-966 893-800 800-731 691-647 647-614 578-542 542-505
13.2 279 557 844 656 51.5 30.3 37.8 493 71.6 96.2 9.58 12.4 16.0
HCFC124 1377 1286 1215 1166 1101 885 818 697 573 531
3026-2970 1433-1324 1324-1254 1254-1187 1187-1125 1125-1071 925-849 849-786 720-671 590-552 552-506
19.0 179 605 977 774 395 289 147 130 13.8 24.8
50 cm-’ atm-’ the error may be in the order of 50%. Small spectral features caused by very weak bands, that is, those with a maximum absorbance of less than 0.1 absorbance units have been omitted, therefore addition of the individual band intensities for a compound will be less than the regional intensities shown in Table !. Some difficulty was encountered in the selection of the band limits in certain cases due to shoulders of overlapping bands. For the halopropanes the spectra in the range 3500-450 cm-’ are shown in Figs 3-7, with the band ranges and intensities indicated on the figures. Figures 8-12 show the halopropane spectra in the range 500-150 cm-‘. These spectra have been included here as little information on the infrared spectra of many of the halopropanes has been previously reported, and moreover, they serve to show the relative smallness of the absorption below 450 cm-‘. For all these halocarbon molecules absorption below 450 cm-’ is usually due to CF3 or CC& rocking modes and torsion about the C-C bond, and therefore is expected to be weak [12-141. This is confirmed by our measurements of the absorption spectra in the range 500-150 cm-‘, and hence changes to the absorption intensity over the range 3500-150 cm-’ because of truncation of the spectra at 450 cm-’ will be relatively very
Integrated absorption intensities of haloethanes and halopropanes
2231
2.654 lA
Range cm-l Band i!aensily cm-2am-l
:iEz 1370-1295 ‘%iE 805-750 z-z
128 108 72.5 1986 150 64.3 90.2 119
-0.00’ 5-f3500
3000
2500
2000
1500
1000
cm-l
500
Fig. 3. Infrared vapour spectrum of HCFC244ca in the range 3500-450 cm-’ (12 Torr and 10 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
small (for example, HCFC225cb has absorption intensity in the range 3500-450 cm-’ of 4196 cm-* atm-‘, yet in the range 500-180 cm-’ of only 47 cm-* atm-‘). For this reason, and also because the errors in the measurements of small absorption intensities are relatively large (see above), and because instrumental noise is larger in this region, the contributions of the absorption intensities in the low frequency region have been omitted from the total integrated absorption intensities listed in Table 1. The strongest bands are found in the region 1300-lOOOcm-’ and are typically due to carbon-fluorine bond stretches [12-141. The intensity of an infrared band is proportional to the transition dipole moment squared and since the electronegativities of the atoms involved are in the order of F > Cl > H, the total band intensities may be expected to increase when H is replaced by Cl and then by F. This trend, as noted for the CFCs and I-WCs can also be seen in the HCFCs. The difference between HCFC14lb, with a total intensity of 1941 cmm2atm-‘, and HCFC142b, with a total intensity 2717 cmm2atm-’ is due to the replacement of one chlorine atom with one fluorine atom. Similarly, HCFC123 and HCFC124 have intensities 3145 cm-* atm-’ and 3641 cm-* atm-‘, respectively. In the case of HCFC244ca and HCFC235cb, one hydrogen is replaced by one fluorine, resulting in the larger difference of 1105 cmm2atm-‘. Comparisons between the total intensities of isomeric pairs of molecules, as in CFC112 and CFCl12a; CFC113 and CFC113a; and CFC114 and CFCl14a, show a small increase for those isomers where the fluorines are attached to both carbons. This is in direct contrast with the results for the HFCs where the molecules with more evenly distributed fluorines show a large decrease in total intensity. Unfortunately the unavailability at present of a sample of HFC152 leaves a gap in this part of the work. A comparison of band intensities in the region of approximately 1350-1050 cm-’ with the number of fluorine atoms present is shown in Fig. 13. C-F stretching vibrations occur in this region for all the molecules, and although other vibrations may be included, the strength of the C-F stretch is such that most of the intensity can be attributed to this vibration. Figure 13 illustrates the increase in band intensity with the increase in number of fluorine atoms for both CFCs and HFCs. Similar comparisons may be drawn for the isomeric series of both types of halocarbons investigated here. Theoretical calculations of
M. P. OLLIFF and G. FISCHER
2232
Bandintensity cur2 atm-’
Range cd 3020-2935 1465-1415 1415-1328 1328-1160 1160-1082 1082-loo0 845-750
2500
21.3 39.0 2z E 136 133 51.9 37.9
2000
1500
1000
cm-l
Fig. 4. Infrared vapour spectrum of HCFC235cb
in the range 3500-450 cm-’ (5 Torr and 10 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
2.9366A
Bandhensitycm-2atm-1
Range cm-’
3050-2965 1420-1320 1324%1100 ‘%% 780-738 738-695
3500
MOO
2500
2000
12.7 236
In
yz
YE 339 53.3 216
lu
I
I
1500
1000
cm-’ 500
Fig. 5. Infrared vapour spectrum of HCFCZ25ca in the range 3500-450 cm-’ (9 Torr and 10 cm cell). Average band intensities in cm-’ atm-’ are given for selected bands.
Integrated absorption intensities of haloethanes and halopropanes
2233
2.7701A
z%E
46.4 12.1
%zO$
3500
3000
2500
2710 423 785 70.8 38.5
2000
1500
1000
cr
500
Fig. 6. Infrared vapour spectrum of HCFC225cb in the range 3500-450 cm-’ (11 Torr and 10 cm cell). Average band intensities in cm-’ atm-’ are given for selected bands.
2.9581 A Raagc cm-’ 1287-1227 1415-1329 1227-1180 ‘fE 75s705 m-520
Band imasitycm-* am-1 3% 590 E 191 49.6
m i
L
1000
Q
8
0
P I
cm- 500
Fig. 7. Infrared vapour spectrum of HCFC218 in the range 3500-450 cm-’ (7 Torr and 3 cm cell). Average band intensities in cm-* atm-’ are given for selected bands.
M. P. OLLIFF and G. FECHI?R
2234
0.9 *
0.7
0.S
0.3
0.1
-0.1 so0
.a0
400
350
300
230
200
cm -I
Fig. 8. Infrared vapour spectrum of HCFC244ca for the range 500-150 cm-’ (127 Torr and 10 cm cell).
0.9 A
0.7
SO0
410
400
320
300
230
200
C. -1
Fig. 9. Infrared vapour spectrum of HCFC235cb for the range 500-150 cm-’ (82 Torr and 10 cm cell).
2235
Integrated absorption intensities of haloethanes and halopropanes
-
0.s *
0.7
0.5
0.3
0.1
-0.1
502
a0
400
900
350
2w
200
lw
“4
Fig. 10. Infrared vapour spectrum of HCFC225ca for the range 500-150 cm-’ (175 Torr and 10 cm cell).
0.5
0.3
0.1
-0.i
wo
450
400
wo
300
250
220
C. -I
Fig. 11. Infrared vapour spectrum of HCFC22Scb for the range SO-150 cm-’ (180 Torr and 10 cm cell).
120
M. P. OLLIFFand G. FISCI-IER
2236
so0
400
450
300
3so
2so
200
em -i
Fig. 12. Infrared vapour spectrum of HCFC218 for the range NO-150 cm-’ (152 Torr and 10 cm cell).
so00
w
+ 0
+
OtXI
0
+WC
0 ;t -.0
I.
I.
1
2
I.,.,.,.
3
4
5
6
7
no. of fluorines Fig. 13. The increase in band intensity for the region encompassing the C-F stretching vibrations with the increase in the number of fluorine atoms present in the molecule for the ethane series of CFCs and I-WCs.
150
Integrated absorption intensities of haloethanes and halopropanes
2237
intensities are expected to offer more insight into the effects of the positions of the fluorine atoms in these molecules, and are currently in progress. Acknowledgements-One of us (G.F.) acknowledges support for this project from the Australian Research Council. We also acknowledge the help given by Mr John Csaki and Mr Denes Bogsanyi, and the use of the facilities in the ResearchSchool of Chemistry at the Australian National University, Canberra.
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R. E. Dickinson and R. J. Cicerone, Nature 319, 109 (1986). D. A. Fisher, C. H. Hales, W. C. Wang, M. K. W. Ko and N. Dak Sze, Nature 344,513 (1998). M. P. Olliff and G. Fischer, Spectrochim. Acta 48A, 229 (1992). F. Cappellani and G. Restelli, Spectrochim. Actu 48A, 1127 (1992). A. C. Brown, C. E. Canosa-Mas, A. D. Parr and R. P. Wayne, Atmos. Enuir. 24A, 2499 (1990). P. Varanasi and S. Chudamani, J. Geophys. Res. 93, 1666 (1988). H. Magid, personal communication reported in ref. 2. D. G. Gehring, personal communication reported in ref. 2. K. Tanabe and S. Saeki, Bull. Chem. Sot. Japan 45,32 (1972). ATOCHEM, The New FORANF Runge. Atochem SA, France (1992). C. C. Dudman, D. G. Hey, P. G. Johnson, D. C. McBeth, N. J. Mime and N. Winterton, Solvent Cleaning: KY’s New Product Development, Baltimore (1990). [12] J. Rud Nielsen, C. Y. Liang, R. M. Smith and D. C. Smith, J. Chem. Phys. 21,383 (1953). [13] J. Rud Nielsen, C. Y. Liang and D. C. Smith, J. Chem. Phys. 21, 847 (1953). [14] J. Rud Nielsen, C. Y. Liang, D. C. Smith and Morris Alpert, J. Chem. Phys. 21, 1070 (1953).