1262
A. M. Snider, P. F. Krause, and F. A. Miller
Vibrational Spectra and Assignments for Tetrachloro- and Tetraiodoallenela,b A. Monroe Snider, Jr., Paul F. Krause, and Foil A. Miller"1c Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received December 22, 1975)
Infrared and Raman spectra were obtained for solid ClZC=C=CC12 and I&=C=CI2. A complete vibrational assignment is presented for each compound, except for one mode of C314.The observed spectra are completely compatible with D 2 d symmetry. The values of the fundamentals are as follows for the solids: C3C14 al: 1312,425,252; bl: 91(?); bz: 1977,651,221;e: 859,305,202,141 cm-'; C3I4 al: 1226,179,98;bl: -; bz: 1912, 424, 200; e: 682, 275, 133, 84(?) cm-l.
I. Introduction This work is part of a study in our laboratory of various perhalo allenes and butatrienes. Althodgh tetrafluoro-, tetrachloro-, tetrabromo-, and tetraiodoallene have all been prepared, there has been no study of their vibrational spectra.2a Only infrared survey spectra, obtained for characterization purposes, are available. This alone made the study of these allenes seem worthwhile. In addition it was thought that a knowledge of their fundamentals would be helpful in interpreting the spectra of the corresponding butatrienes. Because all four of these compounds are unstable, they are not easy to work with. We report here our results for tetrachloroand tetraiodoallene.
11. Tetrachloroallene
A. Experimental. 1.Preparation of the Sample. The following reactions were used: NaOH
CHC12-CC12-CHC12 --+ I CHCly-CCl=CC12 I1
KOH, CaO + 150-170
O C
Cl&=C=CC12 I11
Compound I was obtained from K & K Laboratories. The first reaction has been described by Primzb and the second by Pilgram and Korte,2cwho were the first to isolate 111. For the second step it was necessary to remove the water normally present in reagent grade KOH by preheating it in vacuo until its melting point exceeded 200 "C. The grinding of the dry KOH, the mixing of it with CaO, and the loading of the pyrolysis tube were done in a glove box in order to maintain dryness. I11 was collected in a trap maintained a t -78 "C. Tetrachloroallene is a white solid. When warmed to about 10 "C, it melts and dimerizes spontaneously to form another solid which melts at 93 oC.2cBoth the monomer and the dimer have considerable vapor pressure below their melting points. 2. Spectroscopic Procedures. Because of the low stability of tetrachloroallene, infrared and Raman spectra were obtained only on the polycrystalline solid a t about 100 K. An exception was one fairly strong Raman band observed in CCL solution. The infrared spectrum was measured from 35 to 4000 cm-l with Beckman IR-11 and IR-12 spectrophotometers. The spectral slit width was less than 2 cm-l throughout the entire range. A low temperature cell of conventional design, equipped with KBr or polyethylene windows, was used.3 The Raman spectrum was obtained with a Spex Ramalog unit which has been described el~ewhere.~ Briefly, it uses a 90" illumination-to-viewing arrangement, a Spex 1401 double The Journal of Physical Chemistry, Voi. 80,No. 11, 1976
monochromator with two 1200 line/mm gratings blazed at 500 nm, and an ITT FW 130 detector with S-20 spectral response. Excitation was with 488.0-nm radiation from a Spectra Physics Model 164 Ar+ laser. Because of the low stability of tetrachloroallene, it was studied only a t low temperature and in the solid state. Consequently no depolarization ratios were obtained, with the exception of the one band a t 252 cm-l which was also measured in solution. The Raman cold cell was a modified version of the infrared one. The usual infrared cell frame, which is attached to the refrigerant reservoir and hangs in vacuo, was replaced by a stainless steel wedge with a mirror finish. The sample was deposited from a vapor jet onto a face of the liquid-nitrogen-cooled wedge which is tilted 15" relative to the incoming vertical laser beam. Infrared wavenumbers. are believed to be accurate to f 1 cm-l and Raman ones to f 2 cm-l except for the one band marked "circa". 3. Results. The observed infrared and Raman wavenumbers of tetrachloroallene are given in Table I. Additional bands at 164 and 390 cm-l in both infrared and Raman spectra were identified as impurities by their variation in intensity in different samples. Similar evidence suggests that an impurity band overlaps the sample band a t 203 cm-l. B. Discussion of the Results. 1. General Comments. I t is unfortunate that Raman depolarization ratios and infrared vapor phase band contours are not available. Consequently band assignments are based only on selection rules, group frequencies, and analogy with similar molecules. Tetrachloroallene is expected to have Dzd symmetry, with one CClz twisted 90" relative to the other, by analogy with allene.5 The number of fundamental vibrations in each symmetry species, and their spectroscopic activity, is given in Table 11. Table I11 compares our assignments for tetrachloroand tetraiodoallene with those for analogous bands of tetrachloro- and tetraiodoethylene. 2. Species al. These three fundamentals are only Raman active. The C=C=C symmetric stretch is expected around 1100 cm-l. (It is 1073 cm-l in allene,j 1095-1075 cm-l in chloro-, bromo-, and iodoallene,6 and has similar values in several other derivatives.6) There are two possible candidates in perchloroallene, 866 and 1312 cm-l. Unhappily, each is about 200 cm-' from the expected position, with no apparent reason for a displacement such as there is in allene-d4.5 The 866-cm-' band is appreciably more intense. On the other hand, it can be explained as a sum tone, whereas 1312 cm-l cannot. Since the corresponding band in tetraiodoallene is certainly a t 1226 cm-l, 1312 cm-l rather than 886 cm-l is adopted for VI.
Vibrational Spectra for Substituted Allenes
1263
TABLE I. Observed Infrared and Raman Bands of Polycrystalline C12C=C=CC12 at ca. 100 K
Infrared cm-I
Raman cm-l
Intensity"
89
Assignment
Intensityb
vw, SP
93 100 140 202 221 253 305 427
Lattice mode u4 (torsion)? Lattice mode Lattice mode
91
1000
ca. 143 204 222 252 305 425 464 515 652
790 ? 10 40 15 70 5 10 5
866 1312
10
2
VI
2178
5
1312
vw, SP vw, SP W
m vs W W
vw
651 859
S
vs
1977
0 1 u10
+ impurity (see text)
u7 u3
p = 0.0'
UQ
v2
221 221
+ 253 = 474 + 305 = 526
u6
va
221
vs
+ 651 = 872
v5
+ 866 = 2178
" w, m, s = weak, medium, strong; v = very; sp = sharp. Relative peak intensities on a scale of 0-1000, uncorrected for instrument response. cc14 solution. TABLE 11: Fundamental Vibrations of XzC=C=CX2 ( DZ,JSymmetry)
Assignment, cm-' Species
Activity
No.
Schematic description
c12c=c=cc11
IaC=C=CI2
a1
NP),-
1 2
R, R, ir
e
R, ir
1312 425 252 91? 1977 651 221 or 202 859 305 202 or 221 141
1226 179 98
bi bz
C=C=C sym stretch CX2 sym stretch CX2 scissors Torsion C=C=C antisym stretch CX2 sym stretch CX2 scissors CX2 antisym stretch CX2 wag and rock CX2 wag and rock C=C=C bend
3 4 5 6 7 8 9 10 11
TABLE 111: Comparison of the Fundamentals (in cm-') for Tetrachloroethylene and Tetrachloroallene, Tetraiodoethylene and Tetraiodoallene
Schematic description
c2c14" c~c14 C Z I ~ ~
CX2 antisym str CX2 antisym str CX2 sym str, in phase CX2 sym str, out of phase CX2 scissoring, in phase CXp scissoring, out of phase CXp wag
447 777 237 310 288
CXp wag rock CX2 rock Torsion
5121 347 176 110
859 425 651 252 221
; 91?
:::\ 183 525 106 129 218
;I
682 179 424 98 200
2 133
53?
" Reference 7 . Reference 11. For the CCl2 symmetric stretch (up) there are four possible candidates: 886, 515, 464, and 425 cm-'. The last one, 425 cm-', is selected because it is by far the most intense of the four. It is similar t o the wavenumber of the corresponding
?
1912 424 200 682 280,270 137, 130 84?
mode in tetrachloroethylene, 447 cm-' (ref 7 and Table 111). The fact t h a t it also appears as a very weak band in the infrared spectrum could well be due to a lowering of symmetry in the solid state, and is not regarded as a serious obstacle to the assignment. The lowest a1 mode is a CC12 scissors ( ~ 3 ) .The polarized band a t 252 cm-l is the obvious choice. It too appears weakly in the infrared spectrum, and the same reason given above is suggested for this. In CZC14 the analogous wavenumber is 237 cm-l.7 3. Species bl. The only mode in this species is the torsion, and the only candidate which fits the selection rules is 91 cm-'. This is the strongest band in the entire Raman spectrum. Torsional bands are usually weak in the Raman effect; that is certainly the case in allene and allene-~id.~ However in our molecule the mode involves motions of highly polarizable chlorine atoms, so it could be intense. The torsion in allene is 865 cm-', in tetrachloroethylene about 110 ~ m - l Therefore .~ 91 cm-I seems reasonable for the torsion. This value for the solid is probably considerably higher than the vapor wavenumber would be.s Another possibility is that 91 cm-l is a lattice mode. This could be tested by seeing whether the Raman band is still present in solution, but unfortunately a The Journal of Physical Chemistry, Vol. 80, No. 11, 1976
1264
A. M. Snider, P. F. Krause, and F. A. Miller
satisfactory solution spectrum could not be obtained. We believe the infrared band at 93 cm-l is a lattice mode because of its similarity in appearance to the ones a t 89 and 100 cm-l. 4. Species b2 and e. These modes are both infrared and Raman active. There is no experimental criterion for telling them apart, so less convincing evidence must be used. I t is certain that v5, the C=C=C antisymmetric stretch, is responsible for the very strong infrared band a t 1977 cm-’. Although it is also allowed in the Raman spectrum, it is not observed there. This is not surprising, for the corresponding Raman band of allene is very weak, and that of allene-d4 was not observed a t a1l.j The next lower unassigned fundamental is the CClz antisymmetric stretch (US), followed by the CC12 symmetric stretch (v6). We put the antisymmetric mode higher by analogy with CzC14, numerous 1,l-dichloroethylenes, CzF4, CzBr4,7 and C214.9-11They are assigned to the strong infrared bands a t 859 and 651 cm-l, respectively. The lowest of the remaining fundamentals, v 1 1 , is assigned to the very strong Raman band near 141 cm-l. This is close to the values for the two corresponding C=C=C bends in 1,l-difluoroallene,151 and 167 cm-l.12 I t was difficult to determine the wavenumber accurately because the band is on the side of the very strong 91-cm-I Raman band, and in the infrared it is weak. This leaves three more fundamentals: one out-of-phase CClz scissoring (v7) and two wag-rock modes (vg and ~ 1 0 )The . three obvious bands which are available are 202,221, and 305 cm-l. I t is probable that v g and v10 are well separated because they are in the same species. This is true for allene (1015 and 842 cm-1) and allene-d4 (830 and 667 cm-l).j Therefore vg is probably 305 cm-l. We arbitrarily assign 202 to v10 and 221 to v7, but these could be interchanged. 5 . Other Bands. The three weak, sharp infrared bands observed a t 89,93, and 100 cm-I are attributed to lattice modes, but without any real evidence. Four other bands are accounted for as sum tones. There is no observed band that does not have an explanation. 111. Tetraiodoallene
A. Experimental. 1. Preparation of the Sample. The method of Kai and Seki was used.13 I t is as follows: HC=CCH2Br
(1)12. KOH
-+
(2) KI
I2,KOH
ICzCCH2I --+ IZC=C=CHI
12, KOH
+ I2C=C=CI2
The starting material was from Aldrich Chemical Co. Tetraiodoallene is a pale yellow crystalline solid that melts a t 93-94 “C. I t is sufficiently stable to be studied spectroscopically at room temperature if air is excluded. However decomposition at room temperature, even in the absence of air, is sufficiently rapid to give considerable discoloration from released iodine in a few days. 2. Spectroscopic Procedures. The instrumentation was the same as that already described, with some minor additions. In the infrared the 1 0 0 4 5 0 - ~ m region -~ was also scanned with a Digilab FTS-14 Fourier transform interferometer. Samples were in the form of polyethylene or KBr pressed disks, and for the mid-infrared Nujol mulls between KBr plates were also used. Unfortunately the region below 100 cm-l could not be studied in the infrared. The low-frequency limit of the interferometer was 100 cm-l, and sample decomposition prevented use of the IR-11 with its longer scan time. The Journal of Physical Chemistry, Vol. 80, No. 11, 1976
Raman studies of tetraiodoallene were complicated by three factors. (1)T o our surprise, it is a poor Raman scatterer, and high laser power was required to give a usable signal. We had thought that the iodine atoms, the double bonds, and the nearby electronic transition would make it an unusually intense scatterer. (2) Because of the yellow color, every available exciting line from the Ar+ laser was absorbed to some extent. This led to problems with heating and fluorescence. A rotating sample cell similar to that described by Kiefer and Bernstein was very h e 1 p f ~ l . (3) l ~ The low solubility of the compound resulted in incomplete solution spectra. With the exception of four bands observed in methanol solution, all data are for a powder sample. Only one solution band was observed to be polarized (173 cm-l), but the result for it is certain. 3. Results. The observed bands are listed in Table IV. Infrared wavenumbers are believed to be accurate to f l cm-l and Raman ones to f 2 cm-l except for bands marked “shoulder”. B. Discussion of the Results. Again D P d symmetry is assumed. The assignments are included in Tables I1 and IV. 1. Species a1 and bl. These modes are only Raman active. The highest is v 1 , the C=C=C symmetric stretch, which is expected around 1100 cm-l for reasons outlined earlier. I t is certainly the moderately intense band at 1226 cm-l. The next lower is the CI2 symmetric stretch, which in 12C=CIz is 182 cm-I and the strongest Raman band (refs 9-11 and Table 111). In tetraiodoallene the band is almost identical: 179 cm-l, the strongest Raman band in the spectrum, and polarized. The CI2 scissors ( V Q ) and the torsion (vq) must be lower than 179 cm-l. The only possibilities for which infrared counterparts were not observed are 98 and 84 cm-l. Unfortunately this region was not examined in the infrared because of problems with sample decomposition, so it is not known whether the bands are inactive in the infrared or not. The 98-cm-l band was observed in solution and is therefore not a lattice mode, but the 84-cm-l band could not be tested because in solution it is overlain by the broad Rayleigh scattering. If the torsion is 91 cm-l in C3C14,it must be even lower in C314.This would eliminate 98 cm-l and leave only 84 cm-I as a candidate. The very high intensity of 91 cm-I in C3C14 and of 84 cm-l in ‘2314 suggests that they have the same origin. Nevertheless we believe that 84 cm-I is better assigned to the C=C=C bend v11. (See later.) The torsion is left unassigned, and 98 cm-I is attributed to the CI2 scissors ( u g ) . This is close to the value for the alg scissors in CzI4, 106 cm-l.g-ll 2. Species bz and e. These modes are permitted in both the infrared and Raman spectra. There is no doubt that the antisymmetric C=C=C stretch ( u j ) is a t 1912 cm-l. The next lower mode will be the antisymmetric CI;?stretch (vg). In CzI4 the analogues of us are 780 and 638 cm-l, and that of V 6 is 525 cm-1.9-11 In tetraiodoallene the strong band a t 682 cm-l is a good candidate for US, and the strong one a t 424 cm-’ for V6. This leaves four bending vibrations to be assigned: a CI2 scissors (q), two CI2 wag and rock modes ( v g and vm),and the C=C=C bend (~11). Since their frequencies must be lower than those for the stretching modes, the only candidates are 280,270,261,200,137,and 130 cm-l. The 261-cm-l band is not observed in the infrared, and is well explained as 2 X 130 cm-1. The fact that two doublets are observed (at 280 and 270 cm-1 and at 137 and 130 cm-l) suggests that these are due to the two unassigned e modes and that their degeneracy has been removed in the crystal. This was supported by the fact that the 137-130-cm-l doublet collapsed to a single band in solution, a t 112 cm-l. Unfortunately the other pair was too weak to be observed in solution. We therefore assign the
1265
Vibrational Spectra for Substituted Allenes
TABLE IV: Observed Infrared and Raman Bands of I2C=C=C12
Infrared Nujol Mull cm-I
KBr disk cm-' Intensitya Not studied 130 137
422
681 693
Raman
m vw
202
W
269 277 423
m m ms
608 682 693 700
W S
m w, sh
m m
1910
S
em-'
Solid Intensity b
84 98 130 137 179 199 261 271 283 426 439
720 29 490 83, sh 1000 33 9, sh 44 61 190 130
447 466 560 611 682 695 705
1, sh 9 9 16 270 4, sh 1,sh
722 846 872 1226 1914
6 10 17
CH30H solution cm-I
Assignment VI'?
94 112 173, P 187
u3
I
u10
ua u7
2 X 130 = 260
I
lJg
u6
+ +
2 X 130 179 = 439 2 X 84 271 = 439
+ 271 = 450 + 271 = 470 2 X 283 = 566 2 X 130 + 98 = 620 271 + 426 = 697 283 + 426 = 709 283 + 439 = 722 179 199
U8
2 X 426 = 852 2 X 429 = 878
100
01
14
u.5
w, m, s = weak, medium, strong; v = very; sh = shoulder; p = polarized. Relative peak intensities on a scale of 0-1000, uncorrected for instrument response.
doublets to the wag and rock modes: 280 and 270 cm-l to ug and 137 and 130 cm-l to ~ 1 0 (The . e mode a t 682 cm-I may also be split, but the weaker component a t 694 cm-l can be explained as a sum tone (see Table IV)). The singlet a t 200 cm-' must be due to u7, the CI:! scissors. I t is too high for the only other possibility, u11, which is 141 cm-I in C3C14 and is certainly lower in C&. For u11, the C=C=C bend, 84 cm-l is a good candidate. Both it and its counterpart in C&14 (141 cm-l) are very intense. I t must be admitted, though, that it is not known whether 84 cm-l is due to u11, to the torsion, or to a lattice mode. 3. Remaining Bands. Eleven other bands were observed that cannot be used as fundamentals. All of them can be assigned reasonably as sum tones as shown in Table IV. IV. Conclusions for Both Molecules
The spectra of both tetrachloro- and tetraiodoallene can be interpreted very satisfactorilyon the basis of D2d symmetry. A little consideration convinces one that no other reasonable symmetry would be acceptable. The antisymmetric C=C=C stretch drops only 3.3% on going from tetrachloro- to tetraiodoallene. The symmetric stretch drops just twice as much, or 6.6%. This is compatible with one's qualitative ideas about these modes. In the antisymmetric stretch the central carbon atom has most of the amplitude, and the C-X bonds are scarcely affected by the vibration. In the symmetric stretch the outer carbon atoms have most of the amplitude, and the C-X bonds are com-
pressed a little when the C=C ones are lengthened. Hence this mode is more sensitive to the nature of X.
Acknowledgments. Dr. G. L. Carlson of Mellon Institute, Carnegie-Mellon University, very kindly made the Digilab instrument available to us. This work was supported by the National Science Foundation under Grant No. GP-40836. References and Notes (1) (a) Dedicated to Professor Richard C. Lord on his 65th birthday and retirement. (b) The portion on tetrachloroallene is from a thesis submitted by A. Monroe Snider, Jr., in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Pittsburgh, 1974. (c) Ph.D. degree with Professor Richard C. Lord, Johns Hopkins University, 1942. (2) (a) We have just learned that a paper on tetrafluoroallene by J. R . Durig, Y. S. Li, J. D. Witt. A. P. Zens, and P. D. Ellis is being submitted to Spectrochim. Acta. (b) H. J. Prins, Red. Trav. Chim. Pays-Bas, 68, 901 (1949); (c) K. Pilgrim and F. Korte, Tetrahedron Lett., No. 19, 883 (1962). (3) R. C. Lord, R. S. McDonald, and F. A . Miller, J. Opt. SOC.Am., 42, 149 (1952). (4) F. A. Miller, B. M. Harney, and J. Tyrrell, Spectrochim. Acta, Pat? A, 27, 1003 (1971). (5) R . C. Lord and P. Venkateswarlu, J. Chem. Phys., 20, 1237 (1952). (6) F. R. Dollish, W. G. Fateley, and F. F. Bentley, "Characteristic Raman Frequencies of Organic Compounds", Wiley. New York, N.Y., 1974, p 143. (7) T. Shirnanouchi, "Tables of Molecular Vibrational Frequencies", Consolidated Vol. 1, U S . Government Printing Office, Washington, D.C., (1972). No. C13.48:39. (8) W. G. Fateley, I. Matsubara, and R . E. Witkowski, Spectrochim. Acta, 20, 1461 (1964). (9) R. Forneris and D. Bassi, J. Mol. Spectrosc., 26, 220 (1968). (10) E. J. Fluorie and W. D. Jones, Spectrochim. Acta, Part A, 25, 653 (1969) (11) R. Forneris and M. Uehara, J. Mol. Struct., 5, 441 (1970). (12) J. R . Durig, Y. S. Li, C. C. Tong, A. P. Zens, and P. D. Ellis, J. Am. Chem. SOC.,96, 3805 (1974). (13) F. Kai and S. Seki, Chem. Pharm. Bull., 14, 1122 (1966). (14) W. Kiefer and H. J. Bernstein, Appl. Spectrosc., 25, 609 (1971).
The Journal of Physical Chemistry, Vol. 80, No. 1 1 , 1976
