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WALTERF. EDGELL AND MICHAEL 1'. DUNKLE
Some Aspects of the Infrared Spectrum of Liquid Iron Pentacarbonyl and Its n-Heptane Solutions
by Walter F. Edge11 and Michael P. Dunkle Chemistry Department, P u r d u e Cnicersity, Lafayette, I n d i a n a
(Received February 18, 1965)
An attempt was made to verify the vibrational assignment of Fe(C0)5 in the 5 p region by obtaining the frequencies of the infrared bands in this region for the pure liquid for comparison with the Raman yalues. Only a very broad unresolvable band was obtained. Solutions of varying concentrations in n-heptane were studied and revealed a breakdown in Dah selection rules. The study was extended to other regions with similar results. These, together with band contours and their changes, are discussed in terms of molecular motion and other aspects of the condensed phases. Although the original question was not resolved cleanly, the results are consistent with the frequency assignment made earlier.
Introduction The two strong bands a t 2014 and 2034 cm.-' in the infrared spectrum' of Fe(C0)5 have been assigned to the C-0 stretching frequencies V6 (Az") and V ~ O(E') while the Raman lines2 at 1984, 2031, and 2114 cm.-' have been assigned as v2 (Al'), vl0 (E'), and VI AI').]^^ A pillar in this assignment is the coincidence between the infrared band a t 2034 cm.-1 and the Raman line a t 2031 cm.-l. However, the infrared measurements were made on the compound in the vapor state, while the Raman spectrum was obtained for the liquid. Since vapor-liquid frequency shifts may be as large as 20 em.-', these data do not actually settle the matter of which of the two strong infrared bands in question coincide with a Raman line and hence is the E' mode. To do this requires a comparison of spectra obtained for the same phase. I n attempting to resolve this point, we have obtained evidence of an interesting breakdown in Dah selection rules in the pure liquid and in concentrated solutions.
Results Previous reports of the infrared spectrum of liquid Fe(CO)5 show complete absorption a t 5 p.3,4 We obtained the details of this absorption complex by examining the spectrum of the liquid in very thin films. As can be seen in Fig. l(h), one very broad and highly asymmetric band is found with the maximum of absorption a t 1983 ~ m . - l . ~The ~ breadth of this band, T h e Journal of Physical Chemistry
half-width 40 cm. -l, and its unsymmetrical character testify that it is composed of more than one band. However, even the thinnest of films gave no more than a hint of resolution. I n contrast to this result, Fe(C0)5 in CCI, solution was found by Bor and nlarko5 to give two reasonably narrow bands a t 1998 and 2021 cm.-'. Neither of these frequencies is coincident with that of a Raman line. This result and the fact that the position of maximum absorption found above in the liquid band coincides with the 1982 em.-' Raman line of the liquid create a difficulty for the frequency assignment despite internal support for it from the combination bands. Moreover, the question of why the liquid state band is so broad needs to be answered, To resolve these points, we examined the 5 p spectrum of Fe(CO), in successively more concentrated solutions in a solvent (n-heptane) which gives narrow bands. Thus, it was hoped to approach the environment of an Fe(CO)5 molecule in the pure liquid, step by step, and (1) W. F. Edgell and W. E. Wilson, quoted by W. G. Fateley and E. R. Lippincott, Spectrochim. Acta, 10, 8 (1957). (2) H. Stammreich, 0. Sala, and Y. Tavares, J . Chem. Phys., 30,
856 (1959). (3) W. F. Edgell, W. E. Wilson, and R. Summitt, Spectrochim. Acta, 19, 863 (1963). (4) R. Sheline and K. Pitzer, J . Am. Chem. Soc., 72, 1107 (1950) (4a) KOTEADDED I X PRooF.-tvhile this paper was in press I>.H. Jones and R. 9. hIcDowell sent the authors a paper submitted for publication in which they also report this observation. (5) G. Bar and L. Marko, Spectrochim. Acta, 16, 1105 (1962).
INFRARED SPECTRUM OF LIQUID IRON PENTACARBONYL
I970
2050
I970
2050
CM-I
Figure 1. The infrared spectrum of Fe(CO), in n-heptane near 5 F : ( a ) 0.5%; ( h ) 5%; ( c ) 10%; (d) 13%; (e) 20%; , ( f ) 30%; (g) SOYGby volume; (h) pure liquid; Raman line positions are indicated by rectangles.
hence observe Ihe gradual formation of the broad ab-. sorption complex of the liquid. I n this way, it was; hoped to obtain the liquid-state frequencies of the two infrared-active fundamentals by extrapolation. The results are shown in Fig. 1. In the most dilute solution, Fig. l(a), the two infrared active fundamentals appear as sharp and strong bands a t 2000.3 and 2022.9 cm.-l, while a weak combination band is apparent a t 1965 cm. -I. Closer examination, however, also shows a very weak: shoulder a t 1987 cm. -I, which has no counterpart in the vapor phase infrared spectrum. Three important developments take place in the spectrum as the mole ratio of Fe(C0)6 to n-heptane is increased. First, both of the dominant bands of the dilute solution become increasingly broad. By the time the solution is 70Ye Fe(C0)6by volume, it is difficult to persuade oneself that these two bands can be distinguished separately in ithe absorption complex. One finds here the reason why the absorption complex of the liquid is not resolvable even in the thinnest films we achieved. The second change is the significant development of
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the 1987 cm.-l band. I n the 0.5% solution, it is only a small rise on the smooth contour of the foot of the 2000 cm.-' band. It grows in intensity, with increasing concentration, until its maximum dominates the absorption complex of the pure liquid. The third development of relevance has to do with band frequencies. An examination of Fig. l ( g and h) shows that it is not possible to measure directly the positions of the several band maxima a t concentrations greater than 50%. To obtain liquid state values requires a long extrapolation from the more dilute solution values and introduces considerable uncertainty regarding exact positions. Severtheless, it is possible to specify an interval for the center of each band which is consistent both with the extrapolation and with the shape of the unresolved band of the pure liquid. The 2023 ern.-' band undergoes no change in the frequency of its maximum as the Fe(C0)6 concentration is increased to 20%. At 30% the maximum has shifted to 2022.3 cm.-l and a t 50% to 2021.4 cm.-l. Both the direction and the extent of the shift could be explained by the assumption that the shift was caused by overlapping by the 2000 cm.-l band and not by an actual change in the band position. If this were true, the extrapolated band position for the pure liquid would be 2023 cm.-l. Actually, the position of the maximum a t each concentration is a lower limit to the frequency of the nonoverlapped band a t that concentration. Extrapolation gives about 2020 cm.-l for the lower limit to the band position in the pure liquid. In this process, it was assumed that the frequency variation would be similar to those found for the bands a t 1965 and 1987 cm.-l which can be observed over the whole concentration range (see below). The shape of the high-frequency portion of the band, see Fig. l(h), suggests an upper limit of about 2028 cm.-l for this band position in the pure liquid. The determination of the liquid-state frequency of the 2000 em.-* band is even more compromised by overlapping than that of the 2023 em.-' band. While the overlapping comes from both the 1987 and 2023 cm.-I bands, it is greater from the former and hence overlap bias would lower the frequency. Again, the frequency changes in the maximum could be a result of the overlapping. Extrapolation, together with the shape of the band complex, leads to 1994-2003 cm.-' ai3 the range for the nonoverlapped band position for the pure liquid. The weak band which appears in the dilute solution a t 1987 cm.-' shifts to about 1983 em.-' as it changes into a strong band of the pure liquid. While it is difi-. cult to locate this band's position accurately a t intermediate concenlxations, it appears that most of the Volume 68, A-umber 9
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frequency shift occurs for solutions more concentrated than 20% and is nearly complete a t 7001, Fe(C0)6. The frequency of the weak combination band at 1965 cm.-I is better observed than any of the others in this region and, hence, its frequency behavior was used as a guide in the extrapolations discussed above. A small and gradual shift is observed for dilute solutions, reaching about 1962.7 cm.-l for a 30% Fe(C0)6 solution. Beyond 30y0, the shift is more pronounced until the liquid state value of 1955 cm-' has been nearly reached a t 70%. Thus, it is seen that, not counting the combination band, the liquid state spectrum consists of three strong bands: 2020-2028, 1994-2003, and 1983 cm.-l. I n contrast, the dilute solution shows only two strong bands, 2022.9 and 2000.3 cm.-', and a very weak band a t 1987 cm.-l. We shall interpret this difference in the discussion section as a breakdown in the gas-phase selection rules. The fact that the "extra" band a t 1983 cm.-' for the liquid coincides with a Raman line and does not appear in the infrared spectrum of the gas led us to examine the liquid state infrared spectrum (at 0.05 mm. thickness) a t frequencies corresponding to the other Ram an-ac t ive- inf rared-inactive fundamentals. These occur in the Raman effect of the liquid a t 2114, 753, 414, and 377 ern.-'. We found weak bands a t 2117, 753, and 378 cm.-l. The question now becomes whether these bands, in fact, arise from a breakdown in selection rules or are combination bands. Consequently, we examined the infrared spectrum of a 10% solution a t a thickness of 0.50 mm., an arrangement which puts nearly the same number of Fe(C0)5 molecules in the beam as for the pure liquid spectrum at 0.05 mm. The bands a t 2117 and 378 cm.-l virtually disappeared, while the 750 cm.-' region is "covered" by n-heptane absorption. This result is especially striking for the 2117 em.-' band since it is close to combination bands a t 2110 and 2143 cm.-l, and their intensities are virtually unchanged in the experiment. These results confirm that a breakdown of the Dah selection rules occurs for Fe(C0)6 in the pure liquid state and in concentrated solutions. As a consequence of the above findings, we looked for other evidence of interesting features in the liquid state spectrum. The most striking case was found in the pair of bands between 600 and 650 cm.-I; see Fig. 2. When the spectrum of a dilute solution (6.5y0Fe(C0)6 in nheptane) was examined, the bands were symmetrical and narrow, half-widths about 6.5 cm. -', and appeared a t 643.6 and 616.6 cm.-'. The same result was obtained for an 11.1% solution. At 20Yc, the band maxima shift to 643 and 616.3 em.-' still without significant change in band shape. At 33.37,, the posiThe Journal of Physical Chemistry
WALTERF. EDGELL AND MICHAEL P. DUNKLE
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600
I
I
660
C M-' Figure 2. The infrared spectrum of Fe(COX in n-heptane near 16 p: ( a ) 6.5%; ( b ) 2,07,; (c) 337,; ( d ) 50%; (e) 67% by volume; ( f ) pure liquid.
tions are 640.6 and 615.6 ern.-' and a pronounced asymmetry appears in the 641 cm.-l band. The extent of this is shown by the fact that the high frequency halfwidth is twice the low frequency half-width. At 50% Fe(CO)s, the positions are 639 and 615 cm.-'; the 639 em.-' band is broader and somewhat more asymmetric than for the 33% solution. This effect increased again a t 66.7% Fe(C0)6. For the pure liquid, khe band positions are 638 and 614 cm.-'. The former band is still unsymmetrical, but is less so than for either the 66.7 or 50y0solutions as shown by a smaller "excess" intensity on the high frequency side of the band. I n contrast with the behavior of this band, the 614 crn.-l band undergoes only a very modest increase in breadth with concentration. The slight asymmetry found in this band for the liquid and the concentrated solutions is consistent with the overlap from the 638 cm.-' band which occurs.
Discussion The salient experimental feature of the 5 p region is the very broad unresolved band for the liquid which breaks up into three strong bands for concentrated
INFRARED SPECTRUM OF LIQUIDIRON PENTACARBOKYL
solutions in n-heptane. In contrast, the dilute solutions show only two strong bands. An Fe(C0)5 molecule of trigonal bipyramid structure has D a h symmetry and hence two fundamentals are infrared active in this region. One concludes, therefore, that the infrared spectrum of the liquid Fe(CO), does not satisfy the n 3 h selection rules. On the other hand, the 5 p spectrum for the dilute solutions agrees in the essentials with the selection rules. Kow, an Fe(C0)6 molecule in a condensed phase is surrounded by several near-neighbor molecules. This might produce a distortion of the molecular conformation. Also, forces occur between the atoms of the molecule and the surroundings as the molecule vibrates (site forces). Moreover, the dipole moment change during vibration has components whose origins lie in the presence of the surrounding molecules. There is nothing which requires these latter two quantities to transform under the Dah covering operations of an Fe(CO), molecule even if it had this symmetry. Any one of those effects could produce a breakdown in the D3h selection rules and lead to the above results. Sow the “extra” band a t 1983 cm.-l for the liquid coincides with a Raman line but no band was found at this place in the infrared spectrum of the gas. This leads to the postulate that it corresponds to a Ramanactive-infrared-inactive fundamental under DBh selection rules, whose intensity in the liquid is due to a nonzero dipole moment change arising from one or more of the above condensed-phase effects. This interpretation is consistent with the fundamental assignment of the Raman line made on other bases.’t3 The presence of the Raman-active-infrared-inactive frequencies 2117, 753, and 378 ern.-’ as weak infrared bands in the spectrum of the liquid which disappear for the dilute solutions plus the fact that the 640 cm. - I band is symmetrical in the latter concentration range confirm the conclusion that D,lh selection rules are essentially valid for the dilute n-heptane solutions and break down for the liquid and concentrated solutions. In the other region of intense absorption, the 640 cm. band is asymmetric and broad for the pure liquid and for the concentrated solutions but is symmetric and narrow for the dilute solutions. Xote that this asymmetry cannot be the result of overlapping from the 615 em.-’ band. The fact that these latter measurements were made with a rather wide spectral slit width of ca. 4 em.-’ suggests that the asymmetry may be due to a splitting of the band into two components, one of which falls off in intensity as the solution becomes more concentrated. Since the 638 em.-’ band has been assigned as an E’ mode, such a splitting could be explained as a moderate removal of the twofold degener-
455
acy caused by one or both of the first two possible condensed phase effects listed above. On the other hand, there is a possibility that these effects may result from changes in the character of the motion on passing from dilute solution to pure liquid. What might be expected? An Fe(CO)Smolecule, of course, rotates freely in the gas phase and, as a result, each vibrational band has P, Q, and R branches. Let us examine what can happen to these branches for a condensed phase band. The molecule will still undergo rotary motion. However, this is no longer a “free” rotation but can be more a torsional oscillation. It is illhstrative to consider a limiting case in which the fundamental vibration which gives rise to the band and the rotary oscillation of the molecule are both considered as simple harmonic motions. The transitions corresponding to the P and R branches of the freely rotating molecule are now the sum and difference transitions v f v , and V Y - v r where v f is the frequency of the fundamental and vr is an appropriate rotary oscillation frequency. These transitions are forbidden in the harmonic oscillator approximation when the electric moment expansion is terminated after the terms first order in the vibratory or the rotary oscillatory displacements. In this case, the ‘Y’ and “R” wings vanish. In place of the Q branch transitions of the freely rotating molecule, one has the “hot band” type transitions v f v , - v r . These are permitted in the infrared spectrum and fall on top of each other in the harmonic oscillator approximation. Thus, in this model, the condensed phase fundamental band consists only of a symmetrical, Q-like branch whose half-width would be determined by the statistical distribution of lifetimes of the states involved in the transitions. Anharmonic cross terms involving both the vibratory and the rotary-oscillatory displacements can occur in the potential energy and, in some cases, may be relatively large. Their presence removes the frequency coincidence of the v f v , - vr transitions and causes successive transitions to be displaced successively more to one side of the most intense, v r = 0 trailsition. Then the intensity of the “Q” branch will fall off more slowly and extend further on one side of the maximum than on the other. This is the characteristic of the 640 em.-’ band in the pure liquid and in concentrated solution and hence thc existence of such terms forms a possible explanation for it. In general, one expects the rotary oscillations of a molecule in a liquid or in solution to have comparatively large amplitudes of motion and to be anharmonic. Both of these effects give intensity to the ‘T” and “R” wiiigs. Because of the large moments of inertia of Fe(CO)S,these wings will tend to be crowded touard thc band center and need not mask the asymmetry of the
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“Q” branch. Increasing the resistance to the rotary motion will increase v r and broaden the band by extending these wings. We are nom in a position to attempt a general explanation of the observed spectrographic features. A comparison of the vapor spectrum with that of the dilute solution in n-heptane shows that the vapor bands have lost their P and R branches where they were resolved and have become symmetrical (and somewhat more narrow) &-like bands. This suggests that the rotation of the Fe(C0)6 molecules is hindered in the dilute solution. Since the D,h-forbidden bands do not appear or are so weak (e.g., 1987 cm.-’) a t low concentrations, one may conclude that the molecule retains its DBhconformation and that the cavity for the Fe(CO), molecule has this same or higher symmetry as far as the effective site forces and site electric moment terms are concerned. The appearance of the DBh-forbidden bands in the pure liquid or concentrated solution indicates that the cavity no longer has Dlh symmetry as far as the effective site forces and/or site electric moment terms are concerned but does not necessarily require a distortion of the molecular geometry. Certainly, any pronounced distortion of the D3h molecular configuration can be eliminated. The appearance of broader bands with some asymmetry in band contour as one increases the mole fraction of Fe(C0)5 suggests increased anharmonicity in the rotary-oscillatory motions and, when cbupled with g, general trend to lower frequencies, suggests increased site forces which produce increased resistance to the rotary motions. These may be understood if the forces between Fe(C0)5 molecules are greater than those between Fe(C0)5 and a n-heptane molecule. While the features which stem from a lack of Dah symmetry in a molecular cavity would be readily explained by appropriate lack of symmetry in the geometry of the packing, it might be more consistent witl? the above if the forces between Fe(C0j6 molecules were strong enough to tend to produce pairs or similar clusters. But what of the original question regarding the assignment, i.e., which 5 p band arises from the E‘ mode? As can be seen in Fig. l(li), both the Raman lines a t 1984 and 2031 cni.-l fall within the absorption area of infrared band complex. W e have interpreted the presence of the infrared band a t 1983 cm.-* as due to a breakdown in selection rules. The Raman line a t 2031 em.-’ almost falls within the estimated range, 20202028 crn.-’, for the position of the high frequency band in the pure liquid. Thus, the results may be said to be consistent with the assignment of Edge11 and eo-
The Journal of Phvsical Chemistry
WALTERF. EDGELL AND MICHAEL P. DUNKLE
workers. l r 3 However, no clean-cut coincidence between an infrared band and Raman line for the same phase has been established yet in this frequency region. The results of this study show that a demonstration of band coincidence between the infrared and Raman spectra near 2000 cm.-’ cannot be made for the liquid state; such a comparison of condensed phase values should be made for a dilute solution in a solvent like nheptane to be valid. To this end, we are examining the Raman spectrum of Fe(CO)&in solution.
Experimental The iron pentacarbonyl was obtained from Antara Chemicals. Its spectrum was identical with that of a sample reported as 99.87% pure based on a cooling curve determination.6 The n-heptane obtained from Phillips Petroleum Co. was specified as 99 mole % pure. The spectra near 5 and 16 p were recorded under “high resolution” conditions by a Perkin-Elmer Model 421 spectrophotometer equipped with a grating interchange. The gain was set higher than customary for the conditions employed in order to maintain a very “live” pen so there would be no lag in following the band contours. Two successive runs were made, one on top of the other, in order to average out the somewhat larger amount of noise which accompanies the higher gain. The frequency calibrations were made with gaseous samples of deuterium chloride and carbon d i o ~ i d e . ~The relative frequency values in a particular region are accurate to 0.2 em.-’, but absolute values may be in error as much as 1 cm.-I. The spectra near 26 p were recorded with a Reckman IR5A spectrophotometer equipped with a CsBr prism. Demountable cells with NaC1, KBr, and CsBr plates were used, the cell thickness being adjusted to produce curves in the desirable percent transmission range. All the measurements a t 5 p , except those for the most dilute solution, were made in a cell without a spacer whose windows were repolished before this use. Fine adjustments for the thickness of the film of solution or liquid were made by adjusting the pressure on t,he salt plates. The volume per cent was calculated assuming additivity of volumes. Acknowledgment. Thanks are due to U. 6. Atomic Energy Commission for support of this work. (6) R. Summitt, Ph.D. Thesis, Purdue University, 1961. (7) “Tables of Wavenumbers for the Calibration of Infrared Spectrometers,” I.U.P.A.C. Edition, Butterwortlls, London, 1961, p. 583.
