observation of the lowest bending frequencies of carbon suboxide, dicyanoacetylene, diacetylene and...

Spectrochimica Acta, 1965, Vol. 31, pp. 1709 to 1716. Perganlon Press Ltd. Printed in Northern Ireland

Observation of the lowest bending frequencies of carbon suboxide, dicyanoacetylene, diacetylene and dimethylacetylene* t

FOIL A. MILLER, DONALD H. LEMMON and R. E. WITKOWSKI Mellon Institute, Pittsburgh, Pennsylvania

(Received 18 February 1965)

Abstract-Frequencies of the lowest bending modes of three long linear molecules have been observed in the far infrared spectrum for the first time. For carbon suboxide the band is at

63 rt 2 cm-l, and is very weak. A rationalization for its amazing lowness and weakness is offered. For dicyanoacetylene the band is at 107 cm-l and strong, for diacetylene 221.5 cm-1 and strong. For dimethylacetylene (194 cm-‘, strong) some of the features recently reported by KOPELMAN could not be duplicated.

Part I. Carbon Suboxide

THE structure of carbon suboxide, O=C=C=C=O, and the understanding of its vibrational spectrum have been long-standing problems. Recent spectroscopic results have been interpreted on the linear model [l-3]. The evidence for this structure given in Ref. [l] is so convincing that we take the linearity to be estab- lished. There are, however, still some problems with the vibrational assignments [2]. The principal question is the frequency of the lowest infrared-active bending mode (Y, of species rr,). Dr. G. R. WILKINSON examined the infrared spectrum of a very thick film at 100°K from 20 to 250 cm-l and found nothing [4]. We made a careful search of the gas from 70 to 400 cm-l with long path lengths (JI x 1 > 300 torr x 7.5 m), and concluded that Y, is surely outside of this region [Z]. There have been several recent predictions of the frequency. LAFFERTY et al. [l] suggested either 25 or 63 cm-l as approximate values. They were calculated from the equation Q” = 2B2/m, using two possible assignments for the D-R transition at 3189 cm-l. PITZER and STRICKLER [5] proposed a value of about 50 cm-l, and gave some interesting explanations concerning certain difficulties in the assignments. Very recently MCDOUGALL [6] deduced 61.6 f 2.6 cm-l from entropy measurements.

The availability of a new far infrared instrument with performance superior to that of our earlier one has led us to renew the search for v,. It has now been found at 63 f 2 cm-l.

* This work was supported by Grant GP-1628 from the National Science Foundation. t A portion of this paper is from a thesis to be submitted by D. H. LEMMON in partial

fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Pittsburgh.

[l] W. J. LAFFERTY, A. G. MAKI and E. K. PLYLER, J. Chem. Phys. 40, 224 (1964). [2] F. A. MILLER and W. G. FATELEY, Spectrochim. Acta 20, 253 (1964). [3] A. P. ALEKSANDROV, V. I. TYULIN and V. M. TATEVSKII, Opt.Spectr. (USSR) (EnglGh

Tram&.) 17, 19 (1964). [4] G. R. WILKINSON, King’s College, London. Personal communication. [5] K. 6. PITZER and S. J. STRICKLER, J. Chem. Phys. 41, 730 (1964). [6] L. A. MCDOUGALL, Diss.Abstr.25,1603 (1964); L.A. MCDOUGALLCNI~J.E.KILPATRICK,

J. Chem. Phya. 42, 2311 (1965).

1709

1710 F. A. MILLER, D. H. LEMMON and R. E. WITKOWSKI

EXPERIMENTAL

The sample was prepared by the dehydration of malonic acid, and the purity was checked by examining the infrared spectrum from 400-4000 cm-l. The results compared well with our best earlier preparations [2].

A Beckman IR-11 spectrophotometer was used to study the range 35 to 500 cm-l. The frequency accuracy for sharp bands is f06 cm-l. Spectral slit widths were typically l-2 cm-l. The sample was examined at 10 meter path length

Table 1. Observed low-frequency infrared bands

cm-’ Intensity Assignment

I. Carbon suboxide 63 & 2

214 f 5 282 f 3 439

VW* vvw*, b vvw*, b XV*

% (T”)

%30(R) - 550(I) = 280 Impurity (Ref. 121)

II. Dicyanoecetylene 107 vs vu (Tk)

213 & 4 w 472(I) - 263(R) = 209 692(R) - 472(I) = 220

316 VW Real? 3 x 107(I) = 321 363 YW Reel? 263(R) + 107(I) = 370’ 394 402

m 504(R) - 107(I) = 397

III. Diacetylene ~213 P

221.5 Q ~8 -229 R I

272 276 277.5

IV. Dimethylacetylene ~184

194 Q 203 R 1

voh)

?

Vl!A%“)

w, m, B = weak, medium, strong. v = very. b = broad. R = Raman active. I = infrared active * Intensities relative to rest of spectrum.

and a pressure of 80 torr in a multiple reflection cell with polyethylene windows. The results are shown in Table 1 and Fig. 1. The most interesting finding is the very weak band at 63 & 2 cm-l. No structure was observed with a spectral slit width of 1.8 cm-l. Other broad and even weaker bands were observed at 214 f 5 and 282 -J= 3 cm-l. The latter was reported in our earlier paper [2]. The 214 cm-l band is so weak that it verges on the edge of credulity.

All three bands are in fact extremely feeble. This may be realized by noting that a p x 1 of 1000 torr-cm is sufficient for the rest of the spectrum, whereas 80 times this was used for Fig. 1. Another way to emphasize the weakness is to note that the p x I used is equivalent to about 3 mm of liquid or solid C&O,--far more than is usually required. This may be the reason WILKINSON did not find the band; his thickness was probably well below 1 mm.

Observation of the lowest bending frequencies

%T - I

c302

%T

C

I-

*

I-

cm-l

I I I I I I-.! L I I I I I

290 260 230 200 170 cm-l

220 190 160 13(

Fig. 1. Low-frequency infrared bands. (Re-traced)

I. CsO, gas. 80 torr and 10 meter path. Resolution 1.8 cm-l at each of the bands. The next higher band is 439 cm-*.

II. Diacetylene gas in 10 cm cell. A. Empty cell. R. 13 torr. C. -700 torr. Resolution at 220 cm-‘: 1.8 cm-l.

III. Dimethylacetylene gas at 1 meter path. A. Empty cell. D. -1 torr. E. 5 torr. F. 28 torr. Resolution 1.0 cm-*.

DISCUSSION

There is no doubt in our minds that v, is 63 cm-l, in spite of its weakness, because a value very close to this was predicted from two completely independent types of experiments [l, 61. * We have been unable to devise an explanation for 214 cm-l, but it is so weak that this is not a serious matter. The 439 cm-l band.is due to an impurity [2], and is weak at normal sample thickness although extremely intense under the present conditions.

Because of the low value of 63 cm-l for v7, many upper states are well populated. Table 2 gives some relative populations at 100’ and 300°K. Clearly the distribution is favorable for difference bands. It would also emphasize the anharmonicity in this vibration. The extent of the anharmonicity is not yet clear, which is unfortunate

* The result supports LAFFERTY et d's assignment [l] for the II-Il transition at 3189 cm-l.


Table 2. Relative population of several states of the mu bending vibration of 63 cm-l, assuming it to be harmonic

Relative energy Relative population

V (cm-‘) 100°K 300°K

0 0 1 .oo 1.00 1 63 0.81 1.48 2 126 0.49 1.64 3 189 0.26 1.62 4 252 0.13 1.49

because it may be a key to understanding the peculiarities of the spectrum. There is no evidence for unusual anharmonicity in the appearance of the band. No subsid- iary peaks due to upper stage transitions are observed, and the 63 cm-l absorption is not even unusually broad. (Its half width appears to be about 35 cm-l in the gas.) On the other hand LAFFERTY et al. found clear evidence for a long series of hot, bands involving Y, in the complex absorption around 3200 cm-l. Furthermore the parallel infrared fundamentals at 1573 and 2258 cm-l do not show the expected doublet separation of 11 cm-l, and it has been suggested that this is because numerous hot bands have smeared the contour. Both of these bits of evidence suggest that v, is quite anharmonic.

It is instructive to inquire whether 63 cm-l gives observable binary combina- tions with any of the three Raman-active fundamentals which, in the liquid, are at 577, 830, and 2200 cm-l.

(1) 577 + 63 = 640 mrl. A strong infrared band is observed at 636 cm-l. There is a serious difficulty with this assignment, however, for 636 cm-l is completely gone at 100°K or lower. We therefore thought that 636 had to be a difference tone. PITZER and STRICKLER [5] kept the sum tone, and explained the disappearance at low temperature as follows. At room temperature a large fraction of the molecules are in excited states of v, (Table 2). Suppose that the vibration becomes strongly anharmonic with increasing vibrational quantum number v. Since the intensity of combination tones arises solely from anharmonic terms, the intensity of the sum tone could be considerable at room temperature. On cooling to 100°K the population shifts into states with lower vibrational quantum numbers, the effective anharmonicity is smaller, and consequently 636 cm-l is no longer observed. This is an ingenious explanation, and unfortunately one which is hard to test experimentally.

(2) 577 - 63 = 514 cm- 1. Nothing is observed near here at room temperature. Even though this is on the tail of the strong 550 cm-l band, we would expect it to be detected readily. If a sum tone is observed, the corresponding difference tone should have nearly the same intensity except for adjusting by the BOLTZMANN factor [7]. The sum tone at 636 cm-l is strong, and Table 2 shows that the

[7] G. HERZBERG, Infrared and Ranmn Spectra, p. 266. van Nostrand, New York (1945).

Observation of the lowest bending frequencies 1713

BOLTXMANN factors are quite favorable at 300°K for many values of v. The difference tone should therefore be seen. Its absence is a serious difficulty for which we have no solution.

(3) 830 + 63 = 893 cm-l. A medium-intensity band with much fine structure is observed at -900 cm-l. It disappears at low temperature like the 636 cm-l one does. The same explanation may be given.

(4) 830 - 63 = 767 cm- l. A medium-intensity band with fine structure is observed at ~780 cm-l; its lowest-frequency component is 773.6 cm-l. The values should not disagree this much, but the Raman value is for the liquid so the difference may not be real. This band also disappears at 100°K. One would not expect this on the basis of the BOLTZMANN factor alone (Table 2), but if the anharmonicity is markedly smaller in the low-v states as postulated, the two effects together would account for the observation.

(5) 2200 + 63 = 2263 cm-l. This would be hidden by the very strong funda- mental at 2258 cm-l.

(6) 2200 - 63 = 2137 cm-l. There is a very weak shoulder at 2140 cm-l, but it is far from certain that this is the correct explanation.

The value of 63 cm-l for v, thus provides a better explanation for 636, ~780 and -900 cm-l than was available before, but all difficulties have not been removed. The explanation of why 636 and 900 cm-l disappear at low temperature needs experimental support, and the non-detection of 514 cm-l is not understood.

Two other problems concerning v, have been most puzzling: why its frequency is so low and why its intensity is so weak. These can now be rationalized by using a suggestion of PITZER and STRICKLER’S which was made to explain the lowness, but can readily be extended to explain also the weakness of v7. Consider the frequency first. They pointed out the possibility of writing resonance forms B and C:

The -C= in these is isoelectronic with -N=, and would have a bent configur- ation at lowest energy. These resonance forms, if important, would therefore make

it easier to bend the molecule at the central atom. The h=C- portion is still linear, so bending at the outer carbons is not affected. Of the three bending modes, only v, is mainly a bending at the central atom (Fig. 2). Therefore only the frequency of v, is abnormally low; the frequencies of v5 and vs remain about where expected, at 577 and 550 cm-l respectively. (Nearly 10 times higher than v,!)

Actually vs may also be slightly lowered by this effect. In the bending of a linear chain of identical coupled oscillators, the mode that has the largest number of loops or nodes in the chain has the highest frequency, the one with the secondlargest number has the second highest frequency, and so on down to the mode where the chain vibrates as a single loop which has the lowest frequency of all [S]. Assuming that carbon suboxide approximates such a system, one would expect vs > vg > v, (Fig. 2). Actually, vg > vg > v7. Why the reversal of vs and va? Since vs does not involve a bend at the central carbon, it would not be affected by forms B and C.

[S] J. C. SLATER and N. H. FRANK, Afechmics, pp. 151 ff. McGraw-Hill, New York (1947).


In contrast, vg does involve some bending there, as well aa some at the outer carbons. Forms B and C could therefore lower the frequency of vg slightly, placing it out of the expected order in the sequence. (Another factor is that vs is a liquid value whereas vg and v, are for the gas, but it is not certain whether this has moved vg up or down. In any event this change-of-phase shift is probably less than 20 cm-l.)

We turn now to the question of why v, is so weak. It was pointed out in [2] that analogy with other molecules indicates that v, should be very intense. We regarded the vibration as the motion of two large bond moments moving at the end of long lever arms, so that Sp/SQ was expected to be very large. Why is it not? An answer to this, too, is found in forms B and C. Normally in a C=O group the oxygen would be the negative end of the dipole. In B and C the positive charge counteracts this,

577 cm-’ (Liq.1

-rr, v, &+ 550

Fig. 2. The bending modes of carbon suboxide.

thus sharply reducing the bond moment and making Sp/SQ nearly zero. The large drop in intensity is further evidence that forms B and C are quite important.

It probably could not have been predicted a priori that forms B and C would make important contributions to the final structure. The observations indicate that they do. Perhaps it is better to say that the assumption that forms B and C are important is one way of rationalizing the low frequency and low intensity. There are very few linear molecules for which forms analogous to B and C can reasonably be written. PITZER and STRICKLER cite the methylene wagging in ketene (H&&&O), and the bending in C,, as other examples. We have been unable to find any other known linear molecules where this postulate can be tested against experiment.

The finding of v, does not by any means solve all the problems in the infrared spectrum of carbon suboxide. For example: 1. There are the problems concerning 636, 514 and 900 cm-l already discussed. 2. The particular combination giving rise to the 3200 cm-l band system analyzed by

LAFFERTY et al. [l] is unknown. The first hot band is 20 cm-l higher than the band origin (3189 VS. 3169 cm-l). Why is the shift so large-nearly l/3 the value of v,? Why to higher frequency S We suspect that the latter results from a potential well with a rather flat minimum and steeply rising sides. Resonance forms B and C would have two potential minima for the bending v,, with a maximum in the center. Hence if these forms are important-and the evidence is that they are-such a well would be understandable.

3.

4.

Observation of the lowest bending frequencies 1715

This same 20 cm-l spacing is found between the 820 and 840 cm-l bands, t,he first members of the complex absorption centered near 900 cm-l. At 2500 cm-l, the initial spacing is 7 cm-r. Why!

The components in the 1500 cm-l absorption run to lower frequency, whereas in

the other four regions of resolvable fine structure they run to higher frequencies [2]. Why!

Part II. Dicyanoacetylene In our earlier work on dicyanoacetylene, N=C-CrC-C=N, there was good

evidence from several sum and difference tones that the lowest bending mode (Ye, Z-,) is at 107 cm-l [9]. This reports the confirmation of the value by direct observation.

EXPERIMENTAL

The sample was prepared as in [9]. A small single beam far infrared spectrom- eter was used [lo]. The cell length was 7.5 m., and the pressure <l torr. A very strong band was observed at 107 cm-l. It is at least 100 times as intense as the 63 cm-l band of C&O,. Several additional weak bands were observed at 7.5 m and 8 torr, and their explanations are given in Table 1.

DISCUSSION

We had originally expected the lowest bending frequency in dicyanoacetylene (DCA) to be lower than that in C&O, because: (1) DCA contains one more atom. The longer the chain, the lower the lowest bending frequency will be, other things being equal. (2) DCA contains two nominally single bonds, so the system should be less rigid than the completely cumulated double bond system of C&O,. Instead of the frequency being lower, it is almost twice as high (107 VS. 63 cm-l). This now can be interpreted in a rational manner. For DCA one cannot write reasonable

- resonance structures containing =C- or other non-linear bonds. Consequently the frequency is “normal,” and the intensity is not lowered by such structures.*

Part III. Diacetylene JONES [ll] has made an excellent study of the infrared and Raman spectra of

this molecule, H-C=C--CCC-H. Although he was not able to reach the lowest bending mode, vg of species rU, he had very good evidence from seven sum and difference tones that it is at 220 cm-l. Because we wanted some idea of its intensity, we have observed it directly. It is at 221.5 cm-l in the gas, and is very strong.

+ * One can write N=C-C=C-C=N and its mirror image, but these reduce the total number

of bonds by one and therefore have much higher energy and are unimportant for the ground electronic state.

[9] F. A. MILLER, R. B. HASNAN, Jr. and L. R. COUSINS, J. Chem. Phys. 21, 110 (1953); 23, 2127 (1955).

[IO] F. A. MILLER, G. L. CARLSON and W. B. WHITE, Spectrochirn. Acta 15, 709 (1959). [ll] A. VALLANCE JONES, Proc. Roy. Sot. A211, 255 (1952).


PREPARATION AND MEASUREMENT

The method described by JONES was used with the following slight modifica- tions: The N, stream carrying the product was passed through a reflux condenser, a column of molecular sieves, and two liquid nitrogen traps. About 5 ml of material was caught in the second trap. An infrared survey spectrum showed only a few weak impurity bands, so the sample was used without further purification.

The spectrum was examined with the Beckman IR-11 from 33 to 540 cm-l at 700 torr in a 10 cm cell. It was noted that the compound is unstable. In the liquid state at room temperature it turns noticeably yellow in a minute or so, even under vacuum. Our gas sample at 700 torr deposited yellow material on the two polyethylene cell windows (and not on the sides of the cell). This deposit was thicker on the window toward the quartz mercury arc source. It may therefore have been caused by the U.V. radiation, which is only partially removed by a roughened mirror, or by the heat from the arc. In any event impurity bands appeared at 227 (w) and 462 (a) cm-l. There was no trouble at low pressures, although the sample was not in the instrument as long.

The two observed bands are given in Table 1 and Fig. 1. The 221.5 cm-l band is very strong; the Q branch gives 25% T at 13 torr and 10 cm path for a 1.8 cm-l slit. Thus it fits the usual behavior of bending modes. We can find no explanation for 276 cm-l.

Part IV. Dimethylacetylene Very recently KOPELMAN [12] has published a paper on the far infrared spec-

trum of dimethylacetylene. He reports the lowest bending mode, y12 of species e,,, to have several upper stage bands. Since we have been interested in hot bands for some time, and in low bending modes, we have repeated the work. We found no evidence for the following features which have been reported: 1. The bands at 171 and 177 cm-l. 2. The fine structure in parts of the band with spacings of l-5-2 cm-l. Because our spectral slit width was 1.0 cm- I, this should have been detectable.

We prefer to interpret the band as a conventional one with the Q branch at 194 cm-l, and P and R branches which are poorly defined but are at ~184 and 203 cm-l.

EXPERI~~ENTAL

A Beckman IR-11 double beam spectrophotometer and a one meter cell were used. Pressures were 28, 5, and ~1 torr. Only the region 160-220 cm-l was examined. Results are given in Table 1 and Fig. 1.

[12] R. KOPELMAN, J. Chem. Phys. 41, 1547 (1964).

observation of the lowest bending frequencies of carbon suboxide, dicyanoacetylene, diacetylene and...

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