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Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
Spectrochimica
Acta,
1982,
Tel.
I&a-red
18, pp.
PressLtd. Printedin NorthernIreland 1231to 1247.Pergairlon
spectra of the fluorinated acetic acids in condensed and vapour phase J. R. BARCIZL~ and C. OTERO Institute of Optics, Madrid, Spain
(deceives 4 July 1961, in revisedform 6 &larch. 1962) Abstract-The infra-redspectraof fluorinatedacetic acids have been measured in different states of aggregation, and an attempt has been made to assign the fundamental vibration frequencies of monomeric and dimerie forms.
IN PREVIOUS work in this laboratory the infra-red and Raman spectra were studied of chloroacetic acids [l, 21 and its deuterated derivatives [3, 41, and the vibration frequencies of these three substances were established. In the present work a study is made of fluoroacetic acids which form a similar group which does not appear to have been studied previously. The study of fluoroacetic acids allows one to observe the influence exerted by the fluorine atom on the molecule, and fluorine is known often to differ from the other halogens in its effects. Moreover we know that carboxylic acids occur as monomers and dimers, the equilibrium between which may be displaced by variations in the temperature. In the liquid state the dimer form predominates, while in the gaseous state and especially at high tem~ratures the monomer form is preferred. Each of these forms shows characteristic absorption bands, which can be distinguished in the spectra of the liquid and vapour. Fluoroacetic acids have a much higher vapour pressure than chloroacetic acids and are therefore more suitable for making comparisons of the spectra in the two phases. The only previous work on these compounds is that of FUSON et al. [5], JOS~EN et al. [6], FUSON and JOSIEN [7] and KAGARISE [8], which dealt with the vibrations of the oarboxyl group in trifluoroacetic acid. In the present work, we have recorded the infra-red spectrum between 4000 and 200 cm-1 for the three Huoroacetic acids, and have attempted a vibrational assignment. EXPERIMENTAL
The fluoroacetio acids used were commercial products (Fluka), purified by distillation before use. Monofluoroacetic acid is solid at room temperature, and [l] M. P. JORGE and J. R. BARCEL~, Anaks real sot. espaii. fcs. quim. (Madrid) 53B, 339 (1957). [2] J. R. BAECEL~and M. P. JORGE, Am& real sx espaii. $8. y pim. (~~~~) 54B, 5B (1958). [3] C. OTERO,J. R. BARCEL~ and F. G~MEZ HERRERA, Anales real sot. espaii.$fia. quim. (Madrid) 55B, 205 (1959). [4] J. R. BARCEL~, M. P. JORGE and C. OTERO, J. Chem. Phys. 28,123O (1958). [5] N. FUSON, M. L. JOSIEN, E. A. JONES and J. R. LAWSON, J. Ghem. Phys. 20, 1627 (1952). [S] M. L. JOSIEN, N. FUSON, E. A. LAWSON and J. I%. JONES, Camp. rend. 238, 1163 (1952). [7] N. Fuson and M. L. JOSIEN,J. Am. Opt. Sot. 43, 1102 (1953). [8] R. E. KAGARISE, J. Chem. Phys. 27, 519 (1957). 1231
J. R. B?LRCEL~ and C. OTERO
1232
difluoroacetic and trifluoroacetic acids are liquids. The vapour pressure of these acids is such that at normal temperature only that of trifluoroacetic acid is sufficient for the spectrum to be obtained in the gaseous state. For the other acids a vapour cell with rock salt or potassium bromide windows was used which could be heated electrically up to 150°C. The spectra of these substances were also studied as solutions in carbon tetrachloride. The spectra of the condensed forms were studied between 4000 and 200 cm-l using LiF, NaCl, KBr and CsBr prisms. The solutions and gases were only measured in the region of 4000-400 cm-l. In this region a Hilger-209 spectrometer was used and for the region 400-200 cm-l a Perkin Elmer 21 instrument. The spectra are given in Figs. 1, 2 and 3. In Tables 1, 2 and 3 the frequencies observed for each acid in the different states are recorded. DISCUSSION In order to assign the frequencies in the chloroacetic acids, we have assumed that these acids have a symmetry plane which, containing the carboxylic group, also contains the G-X bond, so that it is also a symmetry plane of the halogen group. Some studies of the dipole moment [9] and electron diffraction [6] of trifluoroacetic acid seem to indicate that this acid has no symmetry plane. Present ideas [lo], that there is no symmetry plane, appear to be contlrmed by the fact that for monofluoroacetic acid it has been possible to distinguish in the monomer two isomeric forms, the tram and the gauche. If this is so, there will only be vibrations of one class, eighteen for each acid, all active to the Raman and infra-red. For the study and classification of these vibrations we shall follow the principles used in the classification of the frequencies of the chloroacetic acids [l]. We assign the eighteen vibrations thus : six to the carboxylic group, six to the halogenated group, and the remaining six to the skeleton as a whole. Carboxylic acids occur in two forms, monomer and dimer, the latter being formed by union of the carboxylic groups through hydrogen bonds. Each form would be expected to have its own frequencies, which as far as the carboxylic group is concerned has been verified. The remaining frequencies, of the halogenated group and skeleton are less easily identified. As the dimer form predominates in the condensed phase and the monomer in the gaseous phase it was hoped by studying the two phases to establish the frequencies corresponding to each form. Carboxylic group C-O
In the carboxylic group six frequencies are considered, the OH, the C=O, the stretching, the two OH bending frequencies (one in the plane of the carboxylic
group and the other outside it) and the CYo group deformation. ‘0 [Q] J. H. GIBBS and CH. P. SMYTH, J. Am. Chem. Sec. [lo] S. I. MIZUSHIMA, Structure of Molecules and Internal
(1954).
In this connexion
73, 5115 (1951). Rotation. Academic Press, New York
Tnfra-red spectra of the fluorinated acetic acids in condensed and vapour phase
. I . .
1233
1234
J.R.BARcEL~
andC.
OTERO
d
’
.iJ
I
k
I
Infra-red
spectra
Table Liquid 261 (80) 270 (SO] 284 (80) 401 (SO) 435 (9O)L 449 (80) .515 (80) 598 (90) 636 (90)L 675 (90) 700 (90)
of the fluorinated
1. Observed Ccl,
sol.
(80) (30)L (20) (40) (5O)L (90)L (90) (90) (90)
1456 (80) 177F (90)
444 450 519 574
(90) (90) (40) (30)
664 (65)
frequencies
Cold wponr
of trifluoroacetic
Hot rapour
447 (90) 519 (50) 580 (70)
452 (50) 504 (30) 576 (66)
664 (SO) 702 (90)
666 703 767 776 796
797 (75)L 812 (8O)L 822 (90) 890 (40)
1115 1165 1217 1237 1277 1366
(50) (90) (90) (100) (45) (20)
1457 (40) 1700 (40) 1787 (100)
2458 (25) 2582 (45)
2578 (40)
2755 (40)
2735 2851 2918 2957 3144 3498 3668
2935 (60) 3280 (90)
infra-red
and vapour
phase
acid
Assignment Fundamental
SO4 (S’S)
846 962 1027 1060 1068 1124 1162 1207 1234
acetic acids in condensed
(25) (45) (60) (50) (80) (65) (40)
905 (95) 968 (3O)L 1034 (20)
1130 1182 1200 1237 1290 1352 1408 1462
(100) (100) (100) (85) (95) (50) (70) (90)
1781 (100) 2420 2480 2541 2595 2695 277.5 2846
820 857 896 970 1021 1047
(70) (90) (75) (85) (90) (60)L (50)L (70) (3O)L (30) (30)
1137 (100) 1197 (95)
1355 (50) 1417 (65) 1460 (50) 1757 (40) 1843 (95)
(10) y (30) (3O)L (60) (30) > (45) (70)
Fund~nental Fundamental Fundamental Fundamental Fundamental Fu~ldamental 401 + 261 = 662 Fundamental Fundamental 270 + 504 = 774 284 + 504 = 788 F~~nclamental 401 x 2 = 802 401 -+ 449 = Fundamental Fundamental 435 f 598 = 449 f 598 = 270 + 804 = Fundamental Fundamental F~ln~lamental Fundamental Fundamental 401 -+ 962 = Fundamental F~lndanlental 449 + 1234 = Fundamental Fundamental
1035 1047 1074
1363
1683
Submaxima of OH band of the associated form 2889 (90)
2957 (95) , 3141 (100) 3593 (60)
850
3150 (60) 3587 (100)
Fundamentai Fundamental
1235
1236
J. R. BARCEL~ and C.
OTERO
Table 2. Observed infra-red frequencies of difluoroacetic acid Liquid 338 (45) 452 (20) 482 (60) 574 (90) 662 (50) 779 (70)
ccl, sol.
482 (60)L 567 (25) 897 (10)
936 (80)
929 (30)
1085 (90) 1122 (90)
1092 (80) 1130 (75)
1217 (40) 1255 (40)
1209 (80) 1242 (90)
1346 (70) 1387 (50)
1337 (45) 1367 (40)
1426 (55)L 1455 (60) 1527 (30) 1537 (30) 1700 (80) 1739 (90) 2488 (50)L 2570 (60) 2660 (50) 2690 (50) 2800 (50) 2932 (70) 2995 (70) 3180 (65)
Hot vapour
482 (60) 571 (80) 666 (75) 782 (90) 905 (70) 1027 (30) 1090 (90) 1120 (90) 1170 (85) 1230 (60)L 1308 (80)L 1335 (80) 1402 (75)
1456 (90) 1698 (50) 1756 (85)
Assignment Fundamental Fundamental Fundamental Fundamental Fundamental 338 + 452 = 780 Fundamental Fundamental Fundamental 388 + 666 = 1004 Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 452 + 936 = 1388 Fundamental 338 + 1085 = 1423 Fundamental
1517 (55) 1693 (60) 1815 (95)
2577 (30) 2670 (35)
452 + 1085 = 1537 452 + 1255 = 1707 Fundamental Fundamental
Submaxima of OH band of the associated form
2857 (90) 2935 (90) 3090 (65) 3520 (90)
2982 (65) 3240 (55) 3587 (90)
Fundamental Fundamental Fundamental
the work of BuAroz et al. (1 I), HADZI and SHEPPARD[ 121 and HADZI and PINTAR [ 131 is interesting. These frequencies are probably little disturbed by the halogenated group, so that they may be similar in the three acids. OH stretching vibrations. The vibration for the monomer form is found between 3500 and 3600 cm-l as a sharp band. This band is either not observed in the condensed form, or is very weak. Under our conditions of measurement it was not observed in any of the three acids in the condensed form. It appears in the solutions and gases, the intensity increasing as the temperature rises. In trifluoroacetic acid, [ll] S. BRATOZ,D. HADZIand N. SHEPPARD, S~~~trochi~.Acta 8, 249 (1956). [12] D. HADZIand N. SHEPPARD, Proc. Roy. Sot. A261, 247 (1953). [13] D. HADZIand M. PINTAR,Spectrochim. Acta 12, 162 (1958).
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1237
in the cold vapour
it appears at 3593 cm-l and in the hot vapour at 3587 cm-l, agreeing well with the results of FUSON et al. [5] and KAGARISE [8]. In the solution in Ccl, it was found at 3498 cm-l, agreeing well with BRATOZ [ll]. Table 3.
Observed infra-red frequencies of monofluoroacetic
Solid 284 294 324 454 483 498 546 646 770 792
(60) (60) (50) (40) (50) (45) (40) (90) (55) (60)
ccl,
sol.
1027 (40) 1076 (70)
1219 1256 1302 1392 1447 1514 1547 1680 1729
(55) (60) (40) (35) (65) (40) (50) (65) (70)
2465 2521 2570 2676 2797 2865 2957
(40) (55)L (60) (55) (60) (80) (95)
3140 (100)
Assignment Fundamental
457 (50) 480 (50) 560 (30) 664 (65)
839 (85) 899 (65)
Hot vapour
acid
664 735 802 851 867
(55) (50) (20) (30) (35)
908 (60) 931 (90) 1095 (70)
1212 (50) 1245 (70) 1277 (40) 1435 1522 1540 1715 1735 1790 2475 2544 2580 2677 2787 2857 2930 3035 3080 3120 3520
(40) (70) (80) (75) (90) (40) (10) (15)L (30) (30) (35) (90) (90) (55) (50) (85) (40)
1025 1075 1128 1157
(40)L (90) (40) (50)
1308 (65) 1386 (60) 1517 (50) 1642 (60) 1746 (95) 1806 (90)
Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 294 + 546 = 840 Fundamental Fundamental Fundamental 483 + 498 = 1029 Fundamental 483 + 646 = 1129 Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 294 + 1219 = 1513 770 x 2 = 1540 294 + 1392 = 1686 Fundamental Fundamental Submaxima of OH band of the associated form
2847 (60) 2920 (75)
Fundamental Fundamental
3310 (50) 3587 (100)
Fundamental Fundamental
With difluoroacetic acid it appears in the hot vapour at 3587 cm-l and in the at 3520 cm-l. For monofluoroacetic the values are 3587 cm-l and 3520 cm-l. These values conform well with those of trifluoroacetic acid. The OH stretching band for the associated form is always very broad and it is solution
1238
J. R. BARGE& and C. OTERO
difficult to fix its absorption maximum. It is also accompanied by satellite bands of lesser intensity. This band is found between 3000 and 3200 cm-l. In trifluoroacetic acid this band appears in all spectra although with unequal intensity. In that of the liquid and cold vapour, it is intense, losing intensity in solution and the hot vapour. In the cold vapour its frequency is at 3141 cm-l and when heated at 3150 cm-l In solution the frequency is at 3144 cm-1 and in agreeing well with previous work. the condensed phase it is a little above 3280 cm-l. For the other acids there is no reference in the literature. In difluoroacetie acid the band appears in the liquid at 3180 cm-l in solution at 3100 cm-l and in the hot vapour at 3240 cm-l in the last two cases with medium intensity. In monofluoroacetic acid the corresponding values are 3140 cm-i, 3120 cm-l and 3310 cm-i. Accompanying this wide band of the associated form there are other bands of lesser intensity and usually at lower frequency, often referred to as submaxima of the associated band. There have been numerous hypothesis on their origin, connecting them with the dimer form. This is confirmed by the present work since their intensity decreased as the equilibrium is displaced towards the monomer, and even in some cases Like di- and monofluoroacetic acids they disappear with the spectrunl of the hot vapour. C=O strefching vibration. The freqpencies for C=O stretching are in the region 1700-l 800 cm-l. For the carboxylic.acids there are two frequencies, the lower for the dimer form and the higher for the monomer. According to FLETT [14] the frequency of the dimer form appears in the aliphatic acids with values greater than 1700 cm-l, this frequency being higher when there are electronegative substituents. For this reason these frequencies in the fluoroacetic acids should be higher than in the chloroacetics [II 21 and in the trifluoroacetic acid higher than in the other two. Besides this band there is another sharper one corresponding to the mono~ler form, above 1800 cm-l. Our assignment for the monomer band of trifluoroacetic acid was 1843 cn-l, a value a little higher than those of FUS~N et al. and KAGARISE. For the associated form in the liquid it was 1776 cm-l, in tbe sclution 1781 cm-l a.nd in the cold va,pour 1781 cm-l, in heated vapour 1757 em-l, more in agreement with FUSON et al. than with KAGARISE. For the other two acids the values are: difluoroacetic monomer form 1815 cm-l in the vapour, and for the dimer form 1739 cm-l and 1756 cm-l respectively in the condensed phase and in solution; for monofluoroacetic monomer 1790 cm-1 in solution and 1806 em-l in the vapour, and for the dimer 1729 cm-l in the liquid, 1737 cm-l in solution and 1746 cm-l in the vapour. Since atmospheric absorption bands could not be eliminated in our equipment the position of the monomer band in the condensed states could not be fixed. BELLANATO and BARCEL~ [15J studied in greater detail the solutions of fluoroacetic acids in Ccl,, using a Perkin-Elmer 112 spectrometer in which the vapour bands of atmospheric water had been eliminated. In these conditions they found in the C=O band of the monofluoroacetic acid five maxima of absorption which are at These five maxima can be interpreted by 1733, 1754, 1761, 1783 and 1805cm-I. -. M. C. FLETT,J. Chem. Sot. 962 (1951). [15] J. BELLANATO and J. R. BARCEL~,&~ctrochim. Acta 16, 1344 (1960). [14]
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1239
assuming that the compound has internal rotation, presenting the two isomers trans and gauche. For these two monomer forms there are three dimers, the trans-trans, the gauche-gauche and the trans-gauche. The two bands of greatest frequency correspond to the monomer forms and the three others to the dimer. Difluoroacetic acid, too, should possess this rotational isomerism but since there may only be a small energy difference between the two forms it may only appear in the spectrum as a slight asymmetry of the band. Theory does not expect rotational isomers for tritluoroacetic acid. C-O stretching vibration. The C-O stretching vibration is recognized now as interfering with the OH vibration in the plane. Deuteration does not give conclusive proof about this. It is clear that these two frequencies correspond to two bands, one around 1466 cm-l and the other around 1200 cm-l, but, their assignment is doubtful. Some authors assign both without distinction, but others have attributed the higher frequency to C-O stretching and the lower to OH. However, HADZI and PINTAR [I31 indicate the possibility that at least in the dimer form the higher frequency and the lower corresponds to OH. We shall assign the higher frequency to C-O to OH. In trichloroacetic acid the C-O vibration of the monomer form is assigned to 1468 cm-1 observed in the cold vapour and 1417 cm-l in heated vapour. The C-O vibration in the dimer form has been observed in all phases at 1456 cm-l in the liquid, at 1457 cm-l in solution, at 1462 cm-l in the cold vapour and at 1460 cm-l in heated vapour. Our assignment in the dimer form agrees well with that of FUSON et al and KAGARISE. For the other acids the assignments are: ditluoroacetic acid monomer form 1402 cm-l only observed in the v&pour, dimer form 1455 cm-l and 1456 en-l observed in liquid and solution respectively. In monofluoroacetic acid the values are a little lower: for the monomer form 1386 cm-l observed in the vapour and for the dimer 1447 cm-l and 1435 cm-l observed in the liquid and solution respectively. OH vibration of deformation. The vibration in the plane, subject to the reservations mentioned in connexion with the C-O vibration, appears in the region of 1200 cm-l while that outside it appears much lower at about 900 en-l. The OH deformation in the plane for trifluoroacetic acid monomer, which appears in all states is taken at 1124 cm-l (liquid) 1115 cm-l (solution), 1130 cm-l (cold vapour) and These assignments agree well with those of FUSON et al. 1137 cm-l (hot vapour). and KAGARISE. The dimer form is assigned to a band which does not appear in the hot gas phase, and in the others lies at 1207 cm-l (liquid), at 1217 cm-l (solution), and at 1200 cm-l (cold vapour). These values are in accord with those given by FUSONet al. but not with KAGARISE, who gives much higher values. In the other two acids the values for both the monomer and the dimer forms are a little higher. In difluoroacetic the monomer form only observable in the hot gas phase appears at 1170 cm-l in the dimer form, at 1255 cm-l in the liquid, and at 1242 cm-l in solution. In the monofluoroacetic that of the monomer form is only observable in the vapour at 1157 cm-l, and of the dimer form in the liquid at 1256 cm-l, and in solution at 1245 cm-i. The OH deformation outside the plane is around 900 cm-l, although in some acids it is found a little lower. For the trifluoroacetic acid we assigned this to the bands at
1240
J.R.BARcEL~
and C. OTERO
797 cm-l in the cold vapour and 796 cm-l in the hot vapour, while we take the band for the dimer form at 886 cm-l (liquid), 890 cm-l (solution), 905 cm-l (cold vapour) and 896 cm-l (heated vapour). These assignments do not agree with those of FUSON et al. whose values are much higher. KACMRISE only assigns the dimer form band. In the other acids the frequencies are somewhat higher especially for the monomer species. In difluoroacetic acid the monomer gives a band only observable in the heated vapour at 905 cm-l, while the dimer form band is observed in the liquid at 936 cm-l and in solution at 929 cm- l. In monoiluoroacetic acid the monomer gives a band at 867 cm-l in the v&pour, and the dimer at 899 cm-l in the liquid, and 908 cm-l in solution. Ho de ormation vibrations. The lower vibration of those assigned to the car?O f boxy1 group corresponds to the deformation vibration of the
For this frequency we only make one assignment. According to to the monomer band, and the dimer band occurs in a somewhat higher region, but in this region we have placed one of the C---P vibrations in trif-luoroacetic acid, and therefore it is impossible to recognise the band of the carboxylic group to which we refer. In the other acids in this region no absorption appears. It might happen that for this frequency there is no difference in absorption for the monomer and the dimer forms, but this cannot at present be decided. In our assignments we consider that in trifluoroacetic acid the frequency of this group appears at 675 cm-l (liquid) 664 cm-l (solution) 664 cm-l in the cold vapour and 666 cm-l in heated vapour. In difluoroacetic acid this frequency was only observed in the liquid at 662 cm-l and in the heated vapour at 666 cm-l. In monofluoroacetic acid it was observed in the liquid at 646 cm-l in solution at 664 cm-l and in the vapour at 664 cm-l. In connexion with the vibrations of the carboxylic group in the monomer and associated forms, it must be stated that in the assignments made by us, which are generally in agreement with the data in the literature, one must distinguish between two classes of displacement in passing from the monomer form to the dimer. In the case of OH and C=O stretching vibrations, this displacement is towards the lower frequencies, but in the case of C-O vibration and the two deformation vibrations 0 OH, this displacement is towards the high frequencies. In the case of the CR ‘0 deformation, we have not been able to localize more than one frequency for the two forms. In trifluoroacetic acid KAGARISE finds two, and if this is correct, then the monomer-dimer displacement is also towards the higher frequencies. Thus in general it happens that the stretching frequencies give a displacement towards the lower frequencies and the deformation frequencies give one towards the high ones. An exception is the C-O stretching frequency, but it is already known that this frequency has very peculiar characteristics. KACIARISE it corresponds
Halogenated group While the carboxylic group frequencies have been the subject of numerous studies and discussions in the literature this is not the case with the halogenated group. In
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1241
these acids only trifluoroacetic has been studied, but FUSON et al. [5] observed frequencies higher than 700 cm-l only. In practice, there are only the assignments of KAGARISE [8]. For the di- and monofluoroacetics there are no assignments. As a guide for these one must turn to other compounds with these halogenated groups. The studies made on methylfluoroform [16] containing the CF, groups and the pentafluoro derivatives of ethane [17] also containing this group were taken into account. For the groups -CHF, and -CHJ! the studies made on difluoro- and monofluoroethane [ 181 were considered. Likewise the work of HERMAN [I91 on the molecular spectra of the fluoro derivatives. In this halogenated group containing four atoms there are six vibrations in the three acids under study. These vibrations although equal in number, will not have similar frequencies. There will be vibrations of the C-F group (the only ones in the trifluoroacetic acid) for which valency vibrations will appear in the region of 1200 cm-l and the deformation vibrations in the region of 600 cm-*. There will be valency vibrations of the group CH (except in trifluoroacetic acid) around 3000 cm-l and the deformation vibrations around 1300 cm-l, as the presence of fluorine reduces the frequency of the latter vibration with regard to hydrocarbons. CH stretching vibrations. Of these vibrations which should appear around 3000 cm-l, are two for monofluoroacetic acid, one for difluoroacetic acid and none for trifluoroacetic acid. As they coincide with the region of the submaxima of the band OH . . * 0, in the states in which the associated form predominates, the absorption In the heated vapour it is easier to fix these maxima are difficult to determine. frequencies. In monofluoroacetic acid we have assigned them to 2957 cm-l and 2865 cm-l in the liquid, 2930 cm-l and 2857 cm-l in solution and 2920 cm-l and The higher frequency corresponds to the asymmetric 2847 cm-l in the vapour. vibration and the lower to the symmetric. These values are in accord with the 2920 cm-l and 2876 cm-l given by SMITH et al. [18] for monofluoroethane. They are a little lower than those assigned by BARCEL~ and JORGE [2] for monochloroacetic acid. For difluoroacetic acid the frequency CH is put at 2995 cm-l in the liquid, 2935 cm-l in solution and 2982 cm-l in the vapour. CH deformation vibrations. These vibrations occur only in difluoro and monofluoroacetic acids. In the hydrocarbons they lie between 1300 and 1400 cm-l. The presence of fluorine, however, reduces the frequency as observed by HERMAN [19] especially when it is attached to the same carbon atom and hydrogen atom. There are two CH deformation vibrations for each acid. In monofluoroacetic acid one is In difluoroacetic one is in the plane of symmetrical and the other asymmetrical. H-C-C and the other outside the plane. The latter is most influenced by the presence of the fluorine and has the lowest value. The values of these frequencies seem somewhat higher than those for the chloroacetic acids. The region in which these vibrations occur is complex since there are also the CF stretching vibration and some of the carboxylic group vibrations. We have made [16] W. H. THOMPSON and R. B. TEMPLE, J. Chem. Sot. 1428 (1948). [17] 0. RISING and R. C. TAYLOR,~$~&~&~. Acta 12, 1036 (1959). [18] D. C. SMITH, R. A. SAUNDERS,J. R. NIELSEN and E. E. FERGUSOP~,J. Ghem. 847 (1952). [19] M. HERMAN, Ind. chim. Beige 16, 86 (1951).
Phys.20,
J. R. BARCEL~and C. OTERO
1242
the following assignments : difluoroacetic acid, asymmetric deformation 1346 cm-l (liquid) 1337 cm-l (solution) and 1335 cm-l (vapour) ; symmetrical deformation 1122 cm-r (liquid), 1130 cm-i (solution) and 1120 cm-r (vapour). For monofluoroacetic acid the deformation in the plane is 1302 cm-l (liquid) 1277 cm-l (solution) and 1308 cm-l (vapour) ; for the deformation outside the plane the values are put at 1219 em-l (liquid) and 1212 cm-l (solution). In the vapour this frequency does not appear, possibly being too weak, and owing to technical difficulties it was not possible to distinguish it. CF ~~r~~c~~~gv~b~at~o~. CF vibrations appear in all the acids but their number is variable. In trifluoroacetic acid there are three, in difluoroacetic two and in monolluoroacetic one. The spectral region of these frequencies is 1300-1000 cm-l, in which as already stated many other bands occur. We have considered the result of BOWLING et al. [20] and HERMAN [19] who suggest that if there is only one fluorine atom attached to carbon the characteristic band appears between 1020-l 100 cm-l. With two atoms of fluorine one of the frequencies will be of the order described and the other will be somewhat higher, between 1100 and 1260 cm-l. If there are three, there may be frequencies of the order of 1320 cm-l. The values assigned by us are: trifluoroaoetic acid 1162 cm-l (liquid), 1165 cm-l (solution), 1182 cm-l (cold vapour) and 1197 cm-l (hot vapour) for the lowest frequency. For the intermediate frequency the values are 1234 cm-l (liquid) none was observed in the solution, 1237 cm-l (cold vapour) and 1237 cm-l (hot vapour). For the highest frequency, none was observed in the liquid, 1277 cm-r for the solution, 1290 cm-l in the cold vapour and none was observed in the hot vapour. For difluoroacetic acid the lowest frequency appears at 1085 cm-1 in the liquid, 1092 cm-l in the solution and 1090 cm-l in the vapour. For the highest frequency the values are 1217 cm-l in the liquid, 1209 cm-l in the solution and none appears in the vapour. For monofluoroacetic acid the only frequency appears at 1076 cm-l in the liquid, 1095 cm-i in the solution and 1075 cm-l in the vapour. FUSON and JOSIEN [5] and KAGARISE [S] give only two frequencies for these vibrations in trifluoroaoetic acid, which agree well with ours. The frequencies of the fluoro derivatives of ethane in general also agree with ours, altho~~gh in these cases there are higher frequencies than those we have suggested here. C-F deformation vibrations. These vibrations are probably in the region 500600 cm-l though some authors have assigned values outside this range. In pentafluoroethane RISING and TAYLOR [ 171 assign some of these vibrations to frequencies of about 687 cm-i and KAGARISE [8] for trifluoroaoetic acid, between 500 and 600 cm-l, one frequency for the monomer and another for the dimer. The number of vibrations will be-three for trifluoroacetio acid, one for difluoroacetic and another for monofluoroacetic acid. In the latter two cases there are deformation vibrations in which the hydrogen atoms of this group participate and we have included them in the CH vibrations since they appear in regions higher frequency. Our assignments are as follows : for tritluoroacetic acid t!he highest appears at 700 cm-l in the liquid, none was observed in the solution, at 702 cm-l in cold vapour, and at
of
ISOl B. B~WLIIW, R.
GORE:,
R. TV. RTAFFOXD
and V. Z.
I~ILLIAMS,
&al. Ckmr. 20,402 (1948).
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1243
703 cm-l in the hot vapour. That of intermediate frequency is at 598 cm-l in the liquid, 574 cm-l in the solution, 580 cm-l in the cold vapour, and 576 cm-l in the hot vapour. The lowest is at 515 cm-l in the liquid, 519 cm-l in the solution, 519 cm-l in the cold vapour and 504 cm-l in the hot vapour. In difluoroacetic acid the only frequency assigned appears at 574 cm-l in the liquid, at 567 cm-l in the solution and at 571 cm-l in the vapour. In monofluoroacetic acid the only frequency assigned appears at 546 cm-l in the liquid, 560 cm-l in the solution and none was observed in the vapour. External vibrations Of these six vibrations connected with the whole skeleton two correspond to the displacement, of the carboxylic group in its symmetry plane and outside it, another two to similar displacements of the halogenated group, the fifth to the C-C valency vibration and the sixth to the torsion of the two groups around the C-C axis. The frequencies of displacement of the carboxyl group, and of the C-C valency vibration should be similar for the three acids. This will not be true with the other frequencies, since both the displacements of the halogenated group and the torsion are influenced by the variation of mass of this group according to whether it has one, two or three of fluorine. There are two deformations of the carboxylic group, one in the plane of the carboxylic group and another outside this plane. In the chloroacetic acids JORGE and BARCEL~ [l] have placed these frequencies in the region of 400-500 cm-l. One must assume that given the nature of this vibration its frequencies must be very similar in the three fluoroacetic acids and in turn similar to that of the chloroacetic acids. In the chloroacetic acids only one frequency was found for these two vibrations, either due to a coincidence or because one of them was too weak. In the fluoroacetic acids in the condensed phase two frequencies were found, except in difluoroacetic which only showed one. These were assigned as follows: trifluoroacetic acid 449 cm-l and 435 cm-l, difluoroacetic acid 452 cm-l and monofluoroacetic acid 483 cm-l and 498 cm-l. In solution they are: trifluoroacetic, 450 cm-l and 444 cm-l, difluoroacetic, none observed, and monofluoroacetic acid 483 cm-l and 480 cm-l. In the gas phase are found no bands for mono- and difluoroacetic acids and only one for trifluoroacetic acid. The frequency of the latter was 447 cm-l in cold vapour and 452 cm-l in hot vapour. Thus there is no displacement although the monomer form should predominate in the hot vapour. This isolated result seems to indicate that the existence of hydrogen bonds does not exert great influence on these frequencies of displacement of the carboxylic group. C-C stretching vibrations. In principle these frequencies must be very similar in the three acids since they will not be influenced by the substituents. It is usually thought to lie between 800 cm-l and 1000 cm-l. For the chloroacetic acids JORGE and BARCEL~ [l] suggested the following values : 955 cm-l, 920 cm-l and 940 cm-1 respectively for tri-, di- and monochloroacetic acid. RISING and TAYLOR [17] give the values 982 cm-l, 947 cm-l, and 923 cm-l in the pentafluorated derivatives of chloro-, bromo- and iodoethane respectively. For tritluoroacetic acid FUSON and JOSIEN [5] and KAGARISE [8] give a somewhat lower figure of 825 cm-l.
II
I
Vcc) 6tO-H)a rockin#!F
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1245
In our spectra this band was not observed in all states. Our values are as follows : trifluoroacetic acid 962 cm-l in the condensed phase, 968 cm-l and 970 cm-l in the acids it was gas phase ; no observation in solution. In di- and monofluoroacetic observed in solution. The values are : 897 cm-l for ditluoroacetic acid and 931 cm-l for monofluoroacetic acid. The value assigned to difluoroacetic is a little low in relation to the others. In this region this compound only has two bands, that indicated and another previously assigned to the OH deformation outside the plane. Thus there is only the possibility of assigning it in the present way or assuming that its frequency is 936 cm-l and coincides with OH deformation outside the plane. Deformation vibrations of the halogenated group. Although it has been suggested as likely that these compounds do not have a symmetry plane, the presence of the plane containing the group COOH leads one to assume that the deformations of the halogenated group may be of two classes, one in this plane and the other outside it. Naturally these vibrations will not be of similar frequencies in the three acids, owing to the variation of mass of this halogenated group. We consider that they vary from 300 cm-l in the trifluoroacetic acid to 800 cm-r in the monofluoroacetic acid. SMITH et al. [ 181 for the radical CFH, in monofluoroethane suggest values of 8 11 cm-r and 872 cm-l, for CF,H in difluoroethane 471 cm-r and 571 cm-l and for CF, in trifluoroRISING and TAYLOR for CF, give frequencies of 260 cm-l and ethane 365 cm-i. 440 cm-l. We have observed the lower frequency range of the spectrum only in the condensed phase, so that except for one of the frequencies we do not have any data for the gas phase. The values obtained for frequencies are : trifluoroacetic acid 284 cm-l and 401 cm-l, ditluoroacetic acid 338 cm-l and 462 cm-l and monofluoroacetic acid 770 cm-l and 792 cm-l. This last frequency has a value of 802 cm-l in the gas phase. Torsional vibration. This low frequency must be influenced by the mass of the halogenated group, being lower in trifluoroacetic and higher in monofluoroacetic acid. In trifluoroacetic acid there is a band at 261 cm-l which is assigned to this vibration; in monofluoroacetic acid we assign it to the band at 284 cm-l. In this region no band appears on difluoroacetic acid but would be expected at an intermediate frequency around 275 cm-l. Other bands. After the assignments have been made of the eighteen vibrations and the overtones and combinations indicated in Tables 1, 2 and 3, there still remain some bands unassigned. These bands cannot be considered as combinations or overtones in view of their low frequency or high intensity. The only explanation for them would be the presence of impurities as a result of the actions of these acids on the windows of the containing cell. These bands lie at 270 cm-l in trifluoroacetic, 780 cm-l in difluoroacetic and 294 cm- r, 324 cm-l and 454 cm-l in monofluoroacetic acid. The strength of these acids prevented us from observing the spectrum at high temperatures of the vapour of monofluoroacetic acid at frequencies lower than 600 cm-l. In Tables 4, 5 and 6 there is a summary of the fundamental frequencies of the three acids (Y’ refers to the associated form), the assignment, the frequency in cm-l and the phase in which they were observed (condensed, vapour or solution). In Fig. 4 the frequencies are given for the carboxyl group in the two forms, monomer and dimer, and in Fig. 5 the frequencies assigned to the three acids in the condensed form.
J. R. BAR.CEL~
1246 Table 4.
Fundamental
and C. OTERO
frequencies of triffuoroaeetie acid Assignment
261 284 401
Mode _-. Torsion Rock. CF, Rock. GF,
G
435 449 515 598
Rock. COOH Rock. COOH 8(CF,) S(CFs)
c
675
,C<”
700 796 886 962
6!CF$ SW--H), 8(0-H), P(C--c)
1137 1162 1207
8(0-H), @L-F) d(O-H), v(C--F) v(C--F)
94
T’
1234 1290
1'
1417
v(C-O),
%
1456 1776 1843 3280 3587
Y(C-O), v(C=O), v(C=O), Y(O-H + * . 0),
%
Y(O-H)m
1%
Phase __...._ c C G C c c
c P
c c
I’ c
(7 c
c
c T' c
V
Frequency
Table 5. Fundamental Phase c C
897
V(C-Oc)
905
W-H)
C c
1085 1122 1170
c
c I'
C
G F' c c
F’
1346 1402 1455 1739 1815 2995 3180 3587
%41
v7 v5 66 85' %3
w2 9)2
PI
Assignment
VS
c
1217 1255
014
v1O
1'
C
2'15
W=,) KC0
1'
Z’S
%l
562
936
,%l
?JlO
Torsion Rock. CF, 2 Rock. COOH Rock. CF,
c
C
2116
@8
Mode
c
c
%2
frequencies of difluoroacetic acid
Frequency (275) 338 452 482 574
c
918 %?
W-H),
Vu18 vu17
v12 and
"8
m
%5 815 I
v(C-F)
V7
WH) W-R), v(C--F) W-H),
2'14
&C--H) Y(C-o)m
2’5
Y(C--O)a Y(C==O& Y(c=o)m Y(C-H) v(O-H . . . 0),
v4’
q’
r(O--H),
Vu1
W6 013
UC? , ('4
4' 9J3 9J2
U16
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase Table 6. Fundamental Phase
c
frequencies of monofluoroacetic Mode
Freauencv
acid
Assignment
c
284 483 498 546
Rock. COOH Rock. COOH 6(C-F)
G
646
,C<”
%o
C
770 792 867 899 931 1076 1157 1219 1256 1302 1386 1447 1729 1806 2865 2957 3140 3587
Rock? (CF) Rock. (CF)
219
C C
c V
c C C
V
c c c V
c C V G
c c V
6(0-H) 6(0-H), V(C-C) V(C-F)
m
015 % 2'7 '6 2114 , '6
fi(CH,) r(C-0), r(C-O)a r(C=O), r(C=O),.
y(O--H)m
2116 %5
6(0-H), tw (CH,) 6( O-H)
r(CHa) WH,) r(O-H
1247
2'5 v4
I
w4
I w-3 v3 212
. . . 0),
%3 2'1' WI
After this paper had been written we heard privately of the work done by TAYLOR (University of Michigan) and ROBINSON on the Raman spectrum of trifluoroacetic acid and its frequency assignments. Although in principle the results are similar to ours, the assignments do not always agree. One important conclusion can be drawn from the work of TAYLOR and ROBINSON, since these authors find polarized and depolarized lines, indicating the existence of elements of symmetry in this molecule. Acknowledgment-This work has been done in part with the help of a grant from the Juan March Foundation to one of the authors (C. 0.). The spectra between 400 and 200 cm-l were measured with equipment of the Laboratoire de Recherches Physiques de la Sorbonne (Paris), through the kindness of Dr. A. HIDALGO.
10
Acta,
1982,
Tel.
I&a-red
18, pp.
PressLtd. Printedin NorthernIreland 1231to 1247.Pergairlon
spectra of the fluorinated acetic acids in condensed and vapour phase J. R. BARCIZL~ and C. OTERO Institute of Optics, Madrid, Spain
(deceives 4 July 1961, in revisedform 6 &larch. 1962) Abstract-The infra-redspectraof fluorinatedacetic acids have been measured in different states of aggregation, and an attempt has been made to assign the fundamental vibration frequencies of monomeric and dimerie forms.
IN PREVIOUS work in this laboratory the infra-red and Raman spectra were studied of chloroacetic acids [l, 21 and its deuterated derivatives [3, 41, and the vibration frequencies of these three substances were established. In the present work a study is made of fluoroacetic acids which form a similar group which does not appear to have been studied previously. The study of fluoroacetic acids allows one to observe the influence exerted by the fluorine atom on the molecule, and fluorine is known often to differ from the other halogens in its effects. Moreover we know that carboxylic acids occur as monomers and dimers, the equilibrium between which may be displaced by variations in the temperature. In the liquid state the dimer form predominates, while in the gaseous state and especially at high tem~ratures the monomer form is preferred. Each of these forms shows characteristic absorption bands, which can be distinguished in the spectra of the liquid and vapour. Fluoroacetic acids have a much higher vapour pressure than chloroacetic acids and are therefore more suitable for making comparisons of the spectra in the two phases. The only previous work on these compounds is that of FUSON et al. [5], JOS~EN et al. [6], FUSON and JOSIEN [7] and KAGARISE [8], which dealt with the vibrations of the oarboxyl group in trifluoroacetic acid. In the present work, we have recorded the infra-red spectrum between 4000 and 200 cm-1 for the three Huoroacetic acids, and have attempted a vibrational assignment. EXPERIMENTAL
The fluoroacetio acids used were commercial products (Fluka), purified by distillation before use. Monofluoroacetic acid is solid at room temperature, and [l] M. P. JORGE and J. R. BARCEL~, Anaks real sot. espaii. fcs. quim. (Madrid) 53B, 339 (1957). [2] J. R. BAECEL~and M. P. JORGE, Am& real sx espaii. $8. y pim. (~~~~) 54B, 5B (1958). [3] C. OTERO,J. R. BARCEL~ and F. G~MEZ HERRERA, Anales real sot. espaii.$fia. quim. (Madrid) 55B, 205 (1959). [4] J. R. BARCEL~, M. P. JORGE and C. OTERO, J. Chem. Phys. 28,123O (1958). [5] N. FUSON, M. L. JOSIEN, E. A. JONES and J. R. LAWSON, J. Ghem. Phys. 20, 1627 (1952). [S] M. L. JOSIEN, N. FUSON, E. A. LAWSON and J. I%. JONES, Camp. rend. 238, 1163 (1952). [7] N. Fuson and M. L. JOSIEN,J. Am. Opt. Sot. 43, 1102 (1953). [8] R. E. KAGARISE, J. Chem. Phys. 27, 519 (1957). 1231
J. R. B?LRCEL~ and C. OTERO
1232
difluoroacetic and trifluoroacetic acids are liquids. The vapour pressure of these acids is such that at normal temperature only that of trifluoroacetic acid is sufficient for the spectrum to be obtained in the gaseous state. For the other acids a vapour cell with rock salt or potassium bromide windows was used which could be heated electrically up to 150°C. The spectra of these substances were also studied as solutions in carbon tetrachloride. The spectra of the condensed forms were studied between 4000 and 200 cm-l using LiF, NaCl, KBr and CsBr prisms. The solutions and gases were only measured in the region of 4000-400 cm-l. In this region a Hilger-209 spectrometer was used and for the region 400-200 cm-l a Perkin Elmer 21 instrument. The spectra are given in Figs. 1, 2 and 3. In Tables 1, 2 and 3 the frequencies observed for each acid in the different states are recorded. DISCUSSION In order to assign the frequencies in the chloroacetic acids, we have assumed that these acids have a symmetry plane which, containing the carboxylic group, also contains the G-X bond, so that it is also a symmetry plane of the halogen group. Some studies of the dipole moment [9] and electron diffraction [6] of trifluoroacetic acid seem to indicate that this acid has no symmetry plane. Present ideas [lo], that there is no symmetry plane, appear to be contlrmed by the fact that for monofluoroacetic acid it has been possible to distinguish in the monomer two isomeric forms, the tram and the gauche. If this is so, there will only be vibrations of one class, eighteen for each acid, all active to the Raman and infra-red. For the study and classification of these vibrations we shall follow the principles used in the classification of the frequencies of the chloroacetic acids [l]. We assign the eighteen vibrations thus : six to the carboxylic group, six to the halogenated group, and the remaining six to the skeleton as a whole. Carboxylic acids occur in two forms, monomer and dimer, the latter being formed by union of the carboxylic groups through hydrogen bonds. Each form would be expected to have its own frequencies, which as far as the carboxylic group is concerned has been verified. The remaining frequencies, of the halogenated group and skeleton are less easily identified. As the dimer form predominates in the condensed phase and the monomer in the gaseous phase it was hoped by studying the two phases to establish the frequencies corresponding to each form. Carboxylic group C-O
In the carboxylic group six frequencies are considered, the OH, the C=O, the stretching, the two OH bending frequencies (one in the plane of the carboxylic
group and the other outside it) and the CYo group deformation. ‘0 [Q] J. H. GIBBS and CH. P. SMYTH, J. Am. Chem. Sec. [lo] S. I. MIZUSHIMA, Structure of Molecules and Internal
(1954).
In this connexion
73, 5115 (1951). Rotation. Academic Press, New York
Tnfra-red spectra of the fluorinated acetic acids in condensed and vapour phase
. I . .
1233
1234
J.R.BARcEL~
andC.
OTERO
d
’
.iJ
I
k
I
Infra-red
spectra
Table Liquid 261 (80) 270 (SO] 284 (80) 401 (SO) 435 (9O)L 449 (80) .515 (80) 598 (90) 636 (90)L 675 (90) 700 (90)
of the fluorinated
1. Observed Ccl,
sol.
(80) (30)L (20) (40) (5O)L (90)L (90) (90) (90)
1456 (80) 177F (90)
444 450 519 574
(90) (90) (40) (30)
664 (65)
frequencies
Cold wponr
of trifluoroacetic
Hot rapour
447 (90) 519 (50) 580 (70)
452 (50) 504 (30) 576 (66)
664 (SO) 702 (90)
666 703 767 776 796
797 (75)L 812 (8O)L 822 (90) 890 (40)
1115 1165 1217 1237 1277 1366
(50) (90) (90) (100) (45) (20)
1457 (40) 1700 (40) 1787 (100)
2458 (25) 2582 (45)
2578 (40)
2755 (40)
2735 2851 2918 2957 3144 3498 3668
2935 (60) 3280 (90)
infra-red
and vapour
phase
acid
Assignment Fundamental
SO4 (S’S)
846 962 1027 1060 1068 1124 1162 1207 1234
acetic acids in condensed
(25) (45) (60) (50) (80) (65) (40)
905 (95) 968 (3O)L 1034 (20)
1130 1182 1200 1237 1290 1352 1408 1462
(100) (100) (100) (85) (95) (50) (70) (90)
1781 (100) 2420 2480 2541 2595 2695 277.5 2846
820 857 896 970 1021 1047
(70) (90) (75) (85) (90) (60)L (50)L (70) (3O)L (30) (30)
1137 (100) 1197 (95)
1355 (50) 1417 (65) 1460 (50) 1757 (40) 1843 (95)
(10) y (30) (3O)L (60) (30) > (45) (70)
Fund~nental Fundamental Fundamental Fundamental Fundamental Fu~ldamental 401 + 261 = 662 Fundamental Fundamental 270 + 504 = 774 284 + 504 = 788 F~~nclamental 401 x 2 = 802 401 -+ 449 = Fundamental Fundamental 435 f 598 = 449 f 598 = 270 + 804 = Fundamental Fundamental F~ln~lamental Fundamental Fundamental 401 -+ 962 = Fundamental F~lndanlental 449 + 1234 = Fundamental Fundamental
1035 1047 1074
1363
1683
Submaxima of OH band of the associated form 2889 (90)
2957 (95) , 3141 (100) 3593 (60)
850
3150 (60) 3587 (100)
Fundamentai Fundamental
1235
1236
J. R. BARCEL~ and C.
OTERO
Table 2. Observed infra-red frequencies of difluoroacetic acid Liquid 338 (45) 452 (20) 482 (60) 574 (90) 662 (50) 779 (70)
ccl, sol.
482 (60)L 567 (25) 897 (10)
936 (80)
929 (30)
1085 (90) 1122 (90)
1092 (80) 1130 (75)
1217 (40) 1255 (40)
1209 (80) 1242 (90)
1346 (70) 1387 (50)
1337 (45) 1367 (40)
1426 (55)L 1455 (60) 1527 (30) 1537 (30) 1700 (80) 1739 (90) 2488 (50)L 2570 (60) 2660 (50) 2690 (50) 2800 (50) 2932 (70) 2995 (70) 3180 (65)
Hot vapour
482 (60) 571 (80) 666 (75) 782 (90) 905 (70) 1027 (30) 1090 (90) 1120 (90) 1170 (85) 1230 (60)L 1308 (80)L 1335 (80) 1402 (75)
1456 (90) 1698 (50) 1756 (85)
Assignment Fundamental Fundamental Fundamental Fundamental Fundamental 338 + 452 = 780 Fundamental Fundamental Fundamental 388 + 666 = 1004 Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 452 + 936 = 1388 Fundamental 338 + 1085 = 1423 Fundamental
1517 (55) 1693 (60) 1815 (95)
2577 (30) 2670 (35)
452 + 1085 = 1537 452 + 1255 = 1707 Fundamental Fundamental
Submaxima of OH band of the associated form
2857 (90) 2935 (90) 3090 (65) 3520 (90)
2982 (65) 3240 (55) 3587 (90)
Fundamental Fundamental Fundamental
the work of BuAroz et al. (1 I), HADZI and SHEPPARD[ 121 and HADZI and PINTAR [ 131 is interesting. These frequencies are probably little disturbed by the halogenated group, so that they may be similar in the three acids. OH stretching vibrations. The vibration for the monomer form is found between 3500 and 3600 cm-l as a sharp band. This band is either not observed in the condensed form, or is very weak. Under our conditions of measurement it was not observed in any of the three acids in the condensed form. It appears in the solutions and gases, the intensity increasing as the temperature rises. In trifluoroacetic acid, [ll] S. BRATOZ,D. HADZIand N. SHEPPARD, S~~~trochi~.Acta 8, 249 (1956). [12] D. HADZIand N. SHEPPARD, Proc. Roy. Sot. A261, 247 (1953). [13] D. HADZIand M. PINTAR,Spectrochim. Acta 12, 162 (1958).
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1237
in the cold vapour
it appears at 3593 cm-l and in the hot vapour at 3587 cm-l, agreeing well with the results of FUSON et al. [5] and KAGARISE [8]. In the solution in Ccl, it was found at 3498 cm-l, agreeing well with BRATOZ [ll]. Table 3.
Observed infra-red frequencies of monofluoroacetic
Solid 284 294 324 454 483 498 546 646 770 792
(60) (60) (50) (40) (50) (45) (40) (90) (55) (60)
ccl,
sol.
1027 (40) 1076 (70)
1219 1256 1302 1392 1447 1514 1547 1680 1729
(55) (60) (40) (35) (65) (40) (50) (65) (70)
2465 2521 2570 2676 2797 2865 2957
(40) (55)L (60) (55) (60) (80) (95)
3140 (100)
Assignment Fundamental
457 (50) 480 (50) 560 (30) 664 (65)
839 (85) 899 (65)
Hot vapour
acid
664 735 802 851 867
(55) (50) (20) (30) (35)
908 (60) 931 (90) 1095 (70)
1212 (50) 1245 (70) 1277 (40) 1435 1522 1540 1715 1735 1790 2475 2544 2580 2677 2787 2857 2930 3035 3080 3120 3520
(40) (70) (80) (75) (90) (40) (10) (15)L (30) (30) (35) (90) (90) (55) (50) (85) (40)
1025 1075 1128 1157
(40)L (90) (40) (50)
1308 (65) 1386 (60) 1517 (50) 1642 (60) 1746 (95) 1806 (90)
Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 294 + 546 = 840 Fundamental Fundamental Fundamental 483 + 498 = 1029 Fundamental 483 + 646 = 1129 Fundamental Fundamental Fundamental Fundamental Fundamental Fundamental 294 + 1219 = 1513 770 x 2 = 1540 294 + 1392 = 1686 Fundamental Fundamental Submaxima of OH band of the associated form
2847 (60) 2920 (75)
Fundamental Fundamental
3310 (50) 3587 (100)
Fundamental Fundamental
With difluoroacetic acid it appears in the hot vapour at 3587 cm-l and in the at 3520 cm-l. For monofluoroacetic the values are 3587 cm-l and 3520 cm-l. These values conform well with those of trifluoroacetic acid. The OH stretching band for the associated form is always very broad and it is solution
1238
J. R. BARGE& and C. OTERO
difficult to fix its absorption maximum. It is also accompanied by satellite bands of lesser intensity. This band is found between 3000 and 3200 cm-l. In trifluoroacetic acid this band appears in all spectra although with unequal intensity. In that of the liquid and cold vapour, it is intense, losing intensity in solution and the hot vapour. In the cold vapour its frequency is at 3141 cm-l and when heated at 3150 cm-l In solution the frequency is at 3144 cm-1 and in agreeing well with previous work. the condensed phase it is a little above 3280 cm-l. For the other acids there is no reference in the literature. In difluoroacetie acid the band appears in the liquid at 3180 cm-l in solution at 3100 cm-l and in the hot vapour at 3240 cm-l in the last two cases with medium intensity. In monofluoroacetic acid the corresponding values are 3140 cm-i, 3120 cm-l and 3310 cm-i. Accompanying this wide band of the associated form there are other bands of lesser intensity and usually at lower frequency, often referred to as submaxima of the associated band. There have been numerous hypothesis on their origin, connecting them with the dimer form. This is confirmed by the present work since their intensity decreased as the equilibrium is displaced towards the monomer, and even in some cases Like di- and monofluoroacetic acids they disappear with the spectrunl of the hot vapour. C=O strefching vibration. The freqpencies for C=O stretching are in the region 1700-l 800 cm-l. For the carboxylic.acids there are two frequencies, the lower for the dimer form and the higher for the monomer. According to FLETT [14] the frequency of the dimer form appears in the aliphatic acids with values greater than 1700 cm-l, this frequency being higher when there are electronegative substituents. For this reason these frequencies in the fluoroacetic acids should be higher than in the chloroacetics [II 21 and in the trifluoroacetic acid higher than in the other two. Besides this band there is another sharper one corresponding to the mono~ler form, above 1800 cm-l. Our assignment for the monomer band of trifluoroacetic acid was 1843 cn-l, a value a little higher than those of FUS~N et al. and KAGARISE. For the associated form in the liquid it was 1776 cm-l, in tbe sclution 1781 cm-l a.nd in the cold va,pour 1781 cm-l, in heated vapour 1757 em-l, more in agreement with FUSON et al. than with KAGARISE. For the other two acids the values are: difluoroacetic monomer form 1815 cm-l in the vapour, and for the dimer form 1739 cm-l and 1756 cm-l respectively in the condensed phase and in solution; for monofluoroacetic monomer 1790 cm-1 in solution and 1806 em-l in the vapour, and for the dimer 1729 cm-l in the liquid, 1737 cm-l in solution and 1746 cm-l in the vapour. Since atmospheric absorption bands could not be eliminated in our equipment the position of the monomer band in the condensed states could not be fixed. BELLANATO and BARCEL~ [15J studied in greater detail the solutions of fluoroacetic acids in Ccl,, using a Perkin-Elmer 112 spectrometer in which the vapour bands of atmospheric water had been eliminated. In these conditions they found in the C=O band of the monofluoroacetic acid five maxima of absorption which are at These five maxima can be interpreted by 1733, 1754, 1761, 1783 and 1805cm-I. -. M. C. FLETT,J. Chem. Sot. 962 (1951). [15] J. BELLANATO and J. R. BARCEL~,&~ctrochim. Acta 16, 1344 (1960). [14]
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1239
assuming that the compound has internal rotation, presenting the two isomers trans and gauche. For these two monomer forms there are three dimers, the trans-trans, the gauche-gauche and the trans-gauche. The two bands of greatest frequency correspond to the monomer forms and the three others to the dimer. Difluoroacetic acid, too, should possess this rotational isomerism but since there may only be a small energy difference between the two forms it may only appear in the spectrum as a slight asymmetry of the band. Theory does not expect rotational isomers for tritluoroacetic acid. C-O stretching vibration. The C-O stretching vibration is recognized now as interfering with the OH vibration in the plane. Deuteration does not give conclusive proof about this. It is clear that these two frequencies correspond to two bands, one around 1466 cm-l and the other around 1200 cm-l, but, their assignment is doubtful. Some authors assign both without distinction, but others have attributed the higher frequency to C-O stretching and the lower to OH. However, HADZI and PINTAR [I31 indicate the possibility that at least in the dimer form the higher frequency and the lower corresponds to OH. We shall assign the higher frequency to C-O to OH. In trichloroacetic acid the C-O vibration of the monomer form is assigned to 1468 cm-1 observed in the cold vapour and 1417 cm-l in heated vapour. The C-O vibration in the dimer form has been observed in all phases at 1456 cm-l in the liquid, at 1457 cm-l in solution, at 1462 cm-l in the cold vapour and at 1460 cm-l in heated vapour. Our assignment in the dimer form agrees well with that of FUSON et al and KAGARISE. For the other acids the assignments are: ditluoroacetic acid monomer form 1402 cm-l only observed in the v&pour, dimer form 1455 cm-l and 1456 en-l observed in liquid and solution respectively. In monofluoroacetic acid the values are a little lower: for the monomer form 1386 cm-l observed in the vapour and for the dimer 1447 cm-l and 1435 cm-l observed in the liquid and solution respectively. OH vibration of deformation. The vibration in the plane, subject to the reservations mentioned in connexion with the C-O vibration, appears in the region of 1200 cm-l while that outside it appears much lower at about 900 en-l. The OH deformation in the plane for trifluoroacetic acid monomer, which appears in all states is taken at 1124 cm-l (liquid) 1115 cm-l (solution), 1130 cm-l (cold vapour) and These assignments agree well with those of FUSON et al. 1137 cm-l (hot vapour). and KAGARISE. The dimer form is assigned to a band which does not appear in the hot gas phase, and in the others lies at 1207 cm-l (liquid), at 1217 cm-l (solution), and at 1200 cm-l (cold vapour). These values are in accord with those given by FUSONet al. but not with KAGARISE, who gives much higher values. In the other two acids the values for both the monomer and the dimer forms are a little higher. In difluoroacetic the monomer form only observable in the hot gas phase appears at 1170 cm-l in the dimer form, at 1255 cm-l in the liquid, and at 1242 cm-l in solution. In the monofluoroacetic that of the monomer form is only observable in the vapour at 1157 cm-l, and of the dimer form in the liquid at 1256 cm-l, and in solution at 1245 cm-i. The OH deformation outside the plane is around 900 cm-l, although in some acids it is found a little lower. For the trifluoroacetic acid we assigned this to the bands at
1240
J.R.BARcEL~
and C. OTERO
797 cm-l in the cold vapour and 796 cm-l in the hot vapour, while we take the band for the dimer form at 886 cm-l (liquid), 890 cm-l (solution), 905 cm-l (cold vapour) and 896 cm-l (heated vapour). These assignments do not agree with those of FUSON et al. whose values are much higher. KACMRISE only assigns the dimer form band. In the other acids the frequencies are somewhat higher especially for the monomer species. In difluoroacetic acid the monomer gives a band only observable in the heated vapour at 905 cm-l, while the dimer form band is observed in the liquid at 936 cm-l and in solution at 929 cm- l. In monoiluoroacetic acid the monomer gives a band at 867 cm-l in the v&pour, and the dimer at 899 cm-l in the liquid, and 908 cm-l in solution. Ho de ormation vibrations. The lower vibration of those assigned to the car?O f boxy1 group corresponds to the deformation vibration of the
For this frequency we only make one assignment. According to to the monomer band, and the dimer band occurs in a somewhat higher region, but in this region we have placed one of the C---P vibrations in trif-luoroacetic acid, and therefore it is impossible to recognise the band of the carboxylic group to which we refer. In the other acids in this region no absorption appears. It might happen that for this frequency there is no difference in absorption for the monomer and the dimer forms, but this cannot at present be decided. In our assignments we consider that in trifluoroacetic acid the frequency of this group appears at 675 cm-l (liquid) 664 cm-l (solution) 664 cm-l in the cold vapour and 666 cm-l in heated vapour. In difluoroacetic acid this frequency was only observed in the liquid at 662 cm-l and in the heated vapour at 666 cm-l. In monofluoroacetic acid it was observed in the liquid at 646 cm-l in solution at 664 cm-l and in the vapour at 664 cm-l. In connexion with the vibrations of the carboxylic group in the monomer and associated forms, it must be stated that in the assignments made by us, which are generally in agreement with the data in the literature, one must distinguish between two classes of displacement in passing from the monomer form to the dimer. In the case of OH and C=O stretching vibrations, this displacement is towards the lower frequencies, but in the case of C-O vibration and the two deformation vibrations 0 OH, this displacement is towards the high frequencies. In the case of the CR ‘0 deformation, we have not been able to localize more than one frequency for the two forms. In trifluoroacetic acid KAGARISE finds two, and if this is correct, then the monomer-dimer displacement is also towards the higher frequencies. Thus in general it happens that the stretching frequencies give a displacement towards the lower frequencies and the deformation frequencies give one towards the high ones. An exception is the C-O stretching frequency, but it is already known that this frequency has very peculiar characteristics. KACIARISE it corresponds
Halogenated group While the carboxylic group frequencies have been the subject of numerous studies and discussions in the literature this is not the case with the halogenated group. In
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1241
these acids only trifluoroacetic has been studied, but FUSON et al. [5] observed frequencies higher than 700 cm-l only. In practice, there are only the assignments of KAGARISE [8]. For the di- and monofluoroacetics there are no assignments. As a guide for these one must turn to other compounds with these halogenated groups. The studies made on methylfluoroform [16] containing the CF, groups and the pentafluoro derivatives of ethane [17] also containing this group were taken into account. For the groups -CHF, and -CHJ! the studies made on difluoro- and monofluoroethane [ 181 were considered. Likewise the work of HERMAN [I91 on the molecular spectra of the fluoro derivatives. In this halogenated group containing four atoms there are six vibrations in the three acids under study. These vibrations although equal in number, will not have similar frequencies. There will be vibrations of the C-F group (the only ones in the trifluoroacetic acid) for which valency vibrations will appear in the region of 1200 cm-l and the deformation vibrations in the region of 600 cm-*. There will be valency vibrations of the group CH (except in trifluoroacetic acid) around 3000 cm-l and the deformation vibrations around 1300 cm-l, as the presence of fluorine reduces the frequency of the latter vibration with regard to hydrocarbons. CH stretching vibrations. Of these vibrations which should appear around 3000 cm-l, are two for monofluoroacetic acid, one for difluoroacetic acid and none for trifluoroacetic acid. As they coincide with the region of the submaxima of the band OH . . * 0, in the states in which the associated form predominates, the absorption In the heated vapour it is easier to fix these maxima are difficult to determine. frequencies. In monofluoroacetic acid we have assigned them to 2957 cm-l and 2865 cm-l in the liquid, 2930 cm-l and 2857 cm-l in solution and 2920 cm-l and The higher frequency corresponds to the asymmetric 2847 cm-l in the vapour. vibration and the lower to the symmetric. These values are in accord with the 2920 cm-l and 2876 cm-l given by SMITH et al. [18] for monofluoroethane. They are a little lower than those assigned by BARCEL~ and JORGE [2] for monochloroacetic acid. For difluoroacetic acid the frequency CH is put at 2995 cm-l in the liquid, 2935 cm-l in solution and 2982 cm-l in the vapour. CH deformation vibrations. These vibrations occur only in difluoro and monofluoroacetic acids. In the hydrocarbons they lie between 1300 and 1400 cm-l. The presence of fluorine, however, reduces the frequency as observed by HERMAN [19] especially when it is attached to the same carbon atom and hydrogen atom. There are two CH deformation vibrations for each acid. In monofluoroacetic acid one is In difluoroacetic one is in the plane of symmetrical and the other asymmetrical. H-C-C and the other outside the plane. The latter is most influenced by the presence of the fluorine and has the lowest value. The values of these frequencies seem somewhat higher than those for the chloroacetic acids. The region in which these vibrations occur is complex since there are also the CF stretching vibration and some of the carboxylic group vibrations. We have made [16] W. H. THOMPSON and R. B. TEMPLE, J. Chem. Sot. 1428 (1948). [17] 0. RISING and R. C. TAYLOR,~$~&~&~. Acta 12, 1036 (1959). [18] D. C. SMITH, R. A. SAUNDERS,J. R. NIELSEN and E. E. FERGUSOP~,J. Ghem. 847 (1952). [19] M. HERMAN, Ind. chim. Beige 16, 86 (1951).
Phys.20,
J. R. BARCEL~and C. OTERO
1242
the following assignments : difluoroacetic acid, asymmetric deformation 1346 cm-l (liquid) 1337 cm-l (solution) and 1335 cm-l (vapour) ; symmetrical deformation 1122 cm-r (liquid), 1130 cm-i (solution) and 1120 cm-r (vapour). For monofluoroacetic acid the deformation in the plane is 1302 cm-l (liquid) 1277 cm-l (solution) and 1308 cm-l (vapour) ; for the deformation outside the plane the values are put at 1219 em-l (liquid) and 1212 cm-l (solution). In the vapour this frequency does not appear, possibly being too weak, and owing to technical difficulties it was not possible to distinguish it. CF ~~r~~c~~~gv~b~at~o~. CF vibrations appear in all the acids but their number is variable. In trifluoroacetic acid there are three, in difluoroacetic two and in monolluoroacetic one. The spectral region of these frequencies is 1300-1000 cm-l, in which as already stated many other bands occur. We have considered the result of BOWLING et al. [20] and HERMAN [19] who suggest that if there is only one fluorine atom attached to carbon the characteristic band appears between 1020-l 100 cm-l. With two atoms of fluorine one of the frequencies will be of the order described and the other will be somewhat higher, between 1100 and 1260 cm-l. If there are three, there may be frequencies of the order of 1320 cm-l. The values assigned by us are: trifluoroaoetic acid 1162 cm-l (liquid), 1165 cm-l (solution), 1182 cm-l (cold vapour) and 1197 cm-l (hot vapour) for the lowest frequency. For the intermediate frequency the values are 1234 cm-l (liquid) none was observed in the solution, 1237 cm-l (cold vapour) and 1237 cm-l (hot vapour). For the highest frequency, none was observed in the liquid, 1277 cm-r for the solution, 1290 cm-l in the cold vapour and none was observed in the hot vapour. For difluoroacetic acid the lowest frequency appears at 1085 cm-1 in the liquid, 1092 cm-l in the solution and 1090 cm-l in the vapour. For the highest frequency the values are 1217 cm-l in the liquid, 1209 cm-l in the solution and none appears in the vapour. For monofluoroacetic acid the only frequency appears at 1076 cm-l in the liquid, 1095 cm-i in the solution and 1075 cm-l in the vapour. FUSON and JOSIEN [5] and KAGARISE [S] give only two frequencies for these vibrations in trifluoroaoetic acid, which agree well with ours. The frequencies of the fluoro derivatives of ethane in general also agree with ours, altho~~gh in these cases there are higher frequencies than those we have suggested here. C-F deformation vibrations. These vibrations are probably in the region 500600 cm-l though some authors have assigned values outside this range. In pentafluoroethane RISING and TAYLOR [ 171 assign some of these vibrations to frequencies of about 687 cm-i and KAGARISE [8] for trifluoroaoetic acid, between 500 and 600 cm-l, one frequency for the monomer and another for the dimer. The number of vibrations will be-three for trifluoroacetio acid, one for difluoroacetic and another for monofluoroacetic acid. In the latter two cases there are deformation vibrations in which the hydrogen atoms of this group participate and we have included them in the CH vibrations since they appear in regions higher frequency. Our assignments are as follows : for tritluoroacetic acid t!he highest appears at 700 cm-l in the liquid, none was observed in the solution, at 702 cm-l in cold vapour, and at
of
ISOl B. B~WLIIW, R.
GORE:,
R. TV. RTAFFOXD
and V. Z.
I~ILLIAMS,
&al. Ckmr. 20,402 (1948).
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1243
703 cm-l in the hot vapour. That of intermediate frequency is at 598 cm-l in the liquid, 574 cm-l in the solution, 580 cm-l in the cold vapour, and 576 cm-l in the hot vapour. The lowest is at 515 cm-l in the liquid, 519 cm-l in the solution, 519 cm-l in the cold vapour and 504 cm-l in the hot vapour. In difluoroacetic acid the only frequency assigned appears at 574 cm-l in the liquid, at 567 cm-l in the solution and at 571 cm-l in the vapour. In monofluoroacetic acid the only frequency assigned appears at 546 cm-l in the liquid, 560 cm-l in the solution and none was observed in the vapour. External vibrations Of these six vibrations connected with the whole skeleton two correspond to the displacement, of the carboxylic group in its symmetry plane and outside it, another two to similar displacements of the halogenated group, the fifth to the C-C valency vibration and the sixth to the torsion of the two groups around the C-C axis. The frequencies of displacement of the carboxyl group, and of the C-C valency vibration should be similar for the three acids. This will not be true with the other frequencies, since both the displacements of the halogenated group and the torsion are influenced by the variation of mass of this group according to whether it has one, two or three of fluorine. There are two deformations of the carboxylic group, one in the plane of the carboxylic group and another outside this plane. In the chloroacetic acids JORGE and BARCEL~ [l] have placed these frequencies in the region of 400-500 cm-l. One must assume that given the nature of this vibration its frequencies must be very similar in the three fluoroacetic acids and in turn similar to that of the chloroacetic acids. In the chloroacetic acids only one frequency was found for these two vibrations, either due to a coincidence or because one of them was too weak. In the fluoroacetic acids in the condensed phase two frequencies were found, except in difluoroacetic which only showed one. These were assigned as follows: trifluoroacetic acid 449 cm-l and 435 cm-l, difluoroacetic acid 452 cm-l and monofluoroacetic acid 483 cm-l and 498 cm-l. In solution they are: trifluoroacetic, 450 cm-l and 444 cm-l, difluoroacetic, none observed, and monofluoroacetic acid 483 cm-l and 480 cm-l. In the gas phase are found no bands for mono- and difluoroacetic acids and only one for trifluoroacetic acid. The frequency of the latter was 447 cm-l in cold vapour and 452 cm-l in hot vapour. Thus there is no displacement although the monomer form should predominate in the hot vapour. This isolated result seems to indicate that the existence of hydrogen bonds does not exert great influence on these frequencies of displacement of the carboxylic group. C-C stretching vibrations. In principle these frequencies must be very similar in the three acids since they will not be influenced by the substituents. It is usually thought to lie between 800 cm-l and 1000 cm-l. For the chloroacetic acids JORGE and BARCEL~ [l] suggested the following values : 955 cm-l, 920 cm-l and 940 cm-1 respectively for tri-, di- and monochloroacetic acid. RISING and TAYLOR [17] give the values 982 cm-l, 947 cm-l, and 923 cm-l in the pentafluorated derivatives of chloro-, bromo- and iodoethane respectively. For tritluoroacetic acid FUSON and JOSIEN [5] and KAGARISE [8] give a somewhat lower figure of 825 cm-l.
II
I
Vcc) 6tO-H)a rockin#!F
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase
1245
In our spectra this band was not observed in all states. Our values are as follows : trifluoroacetic acid 962 cm-l in the condensed phase, 968 cm-l and 970 cm-l in the acids it was gas phase ; no observation in solution. In di- and monofluoroacetic observed in solution. The values are : 897 cm-l for ditluoroacetic acid and 931 cm-l for monofluoroacetic acid. The value assigned to difluoroacetic is a little low in relation to the others. In this region this compound only has two bands, that indicated and another previously assigned to the OH deformation outside the plane. Thus there is only the possibility of assigning it in the present way or assuming that its frequency is 936 cm-l and coincides with OH deformation outside the plane. Deformation vibrations of the halogenated group. Although it has been suggested as likely that these compounds do not have a symmetry plane, the presence of the plane containing the group COOH leads one to assume that the deformations of the halogenated group may be of two classes, one in this plane and the other outside it. Naturally these vibrations will not be of similar frequencies in the three acids, owing to the variation of mass of this halogenated group. We consider that they vary from 300 cm-l in the trifluoroacetic acid to 800 cm-r in the monofluoroacetic acid. SMITH et al. [ 181 for the radical CFH, in monofluoroethane suggest values of 8 11 cm-r and 872 cm-l, for CF,H in difluoroethane 471 cm-r and 571 cm-l and for CF, in trifluoroRISING and TAYLOR for CF, give frequencies of 260 cm-l and ethane 365 cm-i. 440 cm-l. We have observed the lower frequency range of the spectrum only in the condensed phase, so that except for one of the frequencies we do not have any data for the gas phase. The values obtained for frequencies are : trifluoroacetic acid 284 cm-l and 401 cm-l, ditluoroacetic acid 338 cm-l and 462 cm-l and monofluoroacetic acid 770 cm-l and 792 cm-l. This last frequency has a value of 802 cm-l in the gas phase. Torsional vibration. This low frequency must be influenced by the mass of the halogenated group, being lower in trifluoroacetic and higher in monofluoroacetic acid. In trifluoroacetic acid there is a band at 261 cm-l which is assigned to this vibration; in monofluoroacetic acid we assign it to the band at 284 cm-l. In this region no band appears on difluoroacetic acid but would be expected at an intermediate frequency around 275 cm-l. Other bands. After the assignments have been made of the eighteen vibrations and the overtones and combinations indicated in Tables 1, 2 and 3, there still remain some bands unassigned. These bands cannot be considered as combinations or overtones in view of their low frequency or high intensity. The only explanation for them would be the presence of impurities as a result of the actions of these acids on the windows of the containing cell. These bands lie at 270 cm-l in trifluoroacetic, 780 cm-l in difluoroacetic and 294 cm- r, 324 cm-l and 454 cm-l in monofluoroacetic acid. The strength of these acids prevented us from observing the spectrum at high temperatures of the vapour of monofluoroacetic acid at frequencies lower than 600 cm-l. In Tables 4, 5 and 6 there is a summary of the fundamental frequencies of the three acids (Y’ refers to the associated form), the assignment, the frequency in cm-l and the phase in which they were observed (condensed, vapour or solution). In Fig. 4 the frequencies are given for the carboxyl group in the two forms, monomer and dimer, and in Fig. 5 the frequencies assigned to the three acids in the condensed form.
J. R. BAR.CEL~
1246 Table 4.
Fundamental
and C. OTERO
frequencies of triffuoroaeetie acid Assignment
261 284 401
Mode _-. Torsion Rock. CF, Rock. GF,
G
435 449 515 598
Rock. COOH Rock. COOH 8(CF,) S(CFs)
c
675
,C<”
700 796 886 962
6!CF$ SW--H), 8(0-H), P(C--c)
1137 1162 1207
8(0-H), @L-F) d(O-H), v(C--F) v(C--F)
94
T’
1234 1290
1'
1417
v(C-O),
%
1456 1776 1843 3280 3587
Y(C-O), v(C=O), v(C=O), Y(O-H + * . 0),
%
Y(O-H)m
1%
Phase __...._ c C G C c c
c P
c c
I’ c
(7 c
c
c T' c
V
Frequency
Table 5. Fundamental Phase c C
897
V(C-Oc)
905
W-H)
C c
1085 1122 1170
c
c I'
C
G F' c c
F’
1346 1402 1455 1739 1815 2995 3180 3587
%41
v7 v5 66 85' %3
w2 9)2
PI
Assignment
VS
c
1217 1255
014
v1O
1'
C
2'15
W=,) KC0
1'
Z’S
%l
562
936
,%l
?JlO
Torsion Rock. CF, 2 Rock. COOH Rock. CF,
c
C
2116
@8
Mode
c
c
%2
frequencies of difluoroacetic acid
Frequency (275) 338 452 482 574
c
918 %?
W-H),
Vu18 vu17
v12 and
"8
m
%5 815 I
v(C-F)
V7
WH) W-R), v(C--F) W-H),
2'14
&C--H) Y(C-o)m
2’5
Y(C--O)a Y(C==O& Y(c=o)m Y(C-H) v(O-H . . . 0),
v4’
q’
r(O--H),
Vu1
W6 013
UC? , ('4
4' 9J3 9J2
U16
Infra-red spectra of the fluorinated acetic acids in condensed and vapour phase Table 6. Fundamental Phase
c
frequencies of monofluoroacetic Mode
Freauencv
acid
Assignment
c
284 483 498 546
Rock. COOH Rock. COOH 6(C-F)
G
646
,C<”
%o
C
770 792 867 899 931 1076 1157 1219 1256 1302 1386 1447 1729 1806 2865 2957 3140 3587
Rock? (CF) Rock. (CF)
219
C C
c V
c C C
V
c c c V
c C V G
c c V
6(0-H) 6(0-H), V(C-C) V(C-F)
m
015 % 2'7 '6 2114 , '6
fi(CH,) r(C-0), r(C-O)a r(C=O), r(C=O),.
y(O--H)m
2116 %5
6(0-H), tw (CH,) 6( O-H)
r(CHa) WH,) r(O-H
1247
2'5 v4
I
w4
I w-3 v3 212
. . . 0),
%3 2'1' WI
After this paper had been written we heard privately of the work done by TAYLOR (University of Michigan) and ROBINSON on the Raman spectrum of trifluoroacetic acid and its frequency assignments. Although in principle the results are similar to ours, the assignments do not always agree. One important conclusion can be drawn from the work of TAYLOR and ROBINSON, since these authors find polarized and depolarized lines, indicating the existence of elements of symmetry in this molecule. Acknowledgment-This work has been done in part with the help of a grant from the Juan March Foundation to one of the authors (C. 0.). The spectra between 400 and 200 cm-l were measured with equipment of the Laboratoire de Recherches Physiques de la Sorbonne (Paris), through the kindness of Dr. A. HIDALGO.
10