2011
INFRARED SPECTRA OF COMPLEXES OF TETRACYANOETHYLENE
The Infrared Spectra of the Anion and Weak Charge=Transfer Complexes of Tetracyanoethylene
by Jack Stanley, Donald Smith, Bobby Lather, and J. P. Devlin Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma
(Received January 14, 1966)
The infrared spectra of oriented crystals of the weak complexes of TCNE with hexa- and pentamethylbenzene have been observed using polarized radiation. The dichroic behavior of the activated modes has been interpreted in terms of Ferguson’s theories of activation and a vibronic contribution to intensities recently proposed by Brown. These theories have also proved useful in assigning the unusual spectrum of KfTCNE- which was measured from thin films deposited by crossing molecular beams of potassium and TCNE. The combined spectroscopic data for the anion and complexes indicate somewhat less than 10% electron transfer in bot,h weak complexes.
Introduction Detailed studies of the vibrational modes of tetracyanoethylene (TCNE) have been reported from a number of laboratories in recent y e a r ~ . l - ~Consequently, the major spectral features from 100 to 3000 cm-l can be assigned with some ~onfidence.~TCNE is well known as a strong r acid and, in fact, forms quite stable highly colored crystals when complexed with such donors as hexamethylbenzene (HMB) and pentamethylbenzene (PMB). These crystals are needlelike and it is reasonable to assume that this form results from the alternate stacking of donor and acceptor molecules such that all molecular planes are nearly perpendicular to the needle axis. For these reasons, TCNE presents an unusual opportunity for study of the effects of complex formation on the position and optical activity of acceptor modes in chargetransfer complexes. Therefore, we have undertaken studies of the weak charge-transfer complexes PMB TCNE and HMB-TCNE. The dichroic behavior of activated acceptor modes was of particular interest. It is generally assumed that significant charge transfer occurs in the solid state of weak charge-transfer complexes such as HMBeTCNE. We hoped to calibrate the complex spectra for per cent transfer by making comparisons with the neutral molecule (0% transfer) and anion radical (lOOyotransfer) spectra. It was also hoped that knowledge of the anion spectrum would simplify interpretation of the complex
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spectra and vice versa. Therefore, a concomitant study of the spectra of the sodium and potassium salts of TCNE. - has been made. This paper reports primarily single-crystal studies of the complexes plus infrared spectral studies of thin polycrystalline films of the anion salts.
Experimental Section Crystals of the charge-transfer complexes have been grown in highly oriented mats by slowly evaporating ether solutions prepared from equimolar quantities of resublimed TCNE and the reagent grade donor compounds. Under an optical microscope the mats appear to be closely packed, parallel, needle-shaped crystals. These crystal mats were mounted in a Beckman micro solid-sample holder and beam condenser and their infrared dichroism was studied in radiation polarized by a AgCl polarizer. This approach has proved vastly more fruitful than initial efforts to use several larger individually grown single crystals. During sampling, the plane of polarization was always parallel to the spectrometer base, and interaction of the (1) F.A. Miller, et al., Spectrochim. Acta, 20, 1233 (1964). (2) A. Rosenberg and J. P. Devlin, ibid., 21, 1613 (1965). (3) T. Takenaka and 5. Hayashi, BuEl. Chem. SOC.Japan, 37, 1216 (1964). (4) Takenaka gives a reverse assignment of the ag and bag cyanide group stretching modes compared to that presented in ref 1 and 2. However, the calculations in ref 2 and our current work with TCNE complexes support Takenaka’s assignment.
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2012
radiation with the crystal needles was observed with the needle axis both parallel and perpendicular to the plane of polarization. The infrared spectra of KBr pellets of the 1: 1 complexes have also been recorded. The anion spectra have been measured from thin films deposited in vacuo by crossing molecular beams of TCNE and the alkali metals at an infrared window in a modification of the standard low-temperature infrared cell. A tendency for dianion formation was avoided by using excess TCNE which was pumped from the window before recording the spectra. The resulting clear but highly colored thin films were quite suitable for infrared studies. This sampling procedure was adopted because of the extreme sensitivity of the anion to moisture and oxygen. Spectral studies have been restricted to the sodium and potassium salts with emphasis on samples prepared a t room temperature. However, deposits prepared a t -160" were found to be glassy and, as a result, provide some interesting information. All spectra have been recorded on a Beckman IR-7 spectrometer equipped with a CsI interchange.
Results A . Weak Complexes. The infrared spectra for the two orientations of the HMB. TCNE and PMBsTCNE are presented in Figures 1 and 2. It is clear from these spectra that the molecular planes of both the donor and acceptor are oriented nearly perpendicularly to the needle axis in each case. The 960- and 1155-cm-' absorptions, which are produced by carbon-carbon single bond bl, and bzu TCNE stretching modes and, therefore, result from transition dipoles in the molecular plane, appear only very weakly when the needles are parallel to the plane of polarization. Conversely, the 882-cm-l band in the PMB complex, produced by the PMB C-H out-of-plane wagging mode, has negligible activity when the crystal needles are perpendicular to the radiation electric vector. Because of the absence of definitive spectral features, the orientation of the H N B molecular plane is assumed to be analogous to that of PMB. Having established the orientation of the molecular planes, the dichroic behavior of activated and enhanced modes was of particular interest because of the nature of existing theories explaining such activity. Consideration of the curves for TCNE, HNIB, and HMB. TCNE in Figure 3 shows that the most definite activation occurs a t 1562 cm-I while the HMB band a t 1380 cm-I is strongly enhanced and shifted to 1390 cm-l. A new band also appears a t 2243 cm-l but is somewhat hidden by the original TCNE activity. Reference to Figure 1 shows that the activity a t 1562 The Journal of Physical Chemistry
J. STANLEY, D. SMITH,B. LATIMER, AND J. DEVLIN
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Figure 1. Infrared spectrum of PMB .TCNE needlelike crystals in polarized radiation: (A) needles perpendicular, and (B) needles parallel to radiation vector.
i !
Figure 2. Infrared spectrum of HMB .TCNE needlelike crystals in polarized radiation: (A) needles perpendicular, and (B) needles parallel to radiation vector.
cm-l is perpendicular to the molecular planes (Le., parallel to the needle axis) while most of the 1390cm-' activity is in-plane. Since the Raman line of the C=C stretching mode is observed a t 1569 cm-' in pure TCNE, the 1562-cm-l activity in the complex is assigned to this mode despite its perpendicular character. The 1380-cm-l absorption in pure HMB results from the el, methyl-group deformation so some planar activity would be expected. Ferguson and Matsen5t6and Ferguson' have arrived a t selection rules for weak complexes that should apply (5) E. E. Ferguson and F. A. Matsen, J . Chem. Phys., 29, 105 (1958). (6) E. E. Ferguson and F. A. Matsen, J . Am. Chem. SOC.,82, 3268 (1960). (7) E. E. Ferguson, J . Chim. Phys., 61, 257 (1964).
INFRARED SPECTRA OF COMPLEXES OF TETRACYANOETHYLENE
ea-'.
Figure 3. Infrared spectra of TCNE, HMB, and HMB .TCNE in KBr pellets.
in this case. They reason that totally symmetric donor or acceptor modes may be activated by virtue of their influence on the vertical ionization potential or electron affinity of the donor or acceptor molecule, respectively. For planar sandwich complexes, the resulting charge oscillation would be directed perpendicularly to the molecular planes. Thus, the perpendicular activity of the TCNE C=C stretch in both complexes is understandable. Further, since HMB has a totally symmetric methyl deformation at 1390 cm-1 as well as the el, deformation at 1380 cm-l, the perpendicular component of the 1390-cm-' band is not unexpected. However, the theory for isolated 1 : 1 complexes does not explain the unusual enhancement of the 1380-cm-I in-plane activity. The enhancement of the in-plane activity of the HMB 1380-cm-l el, methyl deformation mode may be understood in terms of vibronic contributions recently discussed by Brown.* He shows that the A electrons of a conjugated system will migrate as a result of variations in electron-electron repulsive forces during certain vibrations (e.g., the el, modes of a D6h molecule). An effective reduction in the transition moment for a given mode may result. In a donor system, complex formation would reduce this effect by decreasing the A-electron density. The mode would thus gain additional activity as observed for the el, methyl deformation in HMB. A similar strong effect is observed for the corresponding methyl deformation modes of PMB and durene. Further, as this model would predict, the enhancement increases with acceptor strength (ie., TCNE > chloranil > trinitrobenzene). It is interesting that modification of the vibronic contribution shows up most strikingly in an extra-ring mode. Brown notes
2013
that this also seems to be true for the liquid-solid phase transition of benzene (ie., vCHelu is most affected). Ferguson' has emphasized that charge oscillation between donor and acceptor can accompany vibrational modes which affect the overlap of the donor and acceptor orbitals. In an isolated 1 : l molecular complex such as C6H6.12, this requires that the vibration have the character of a nonplanar rotation or be nonplanar and of the same symmetry species as a translation. In the latter case, the effect is usually masked by the infrared activity allowed under the selection rules for the isolated molecule since the normal vibrational dipole and the new charge oscillation are parallel to each other. An example of the first case has not been identified in our spectra either. However, a likely example of charge oscillation due to a change in overlap has been detected. Figures 1 and 2 show that a moderately intense band at 580 cm-' has nearly equal intensity for both orientations. This band can be firmly assigned to a bl, TCNE mode which is best described as a planar cyanide bend, so that, from the selection rules for the isolated molecule, only the parallel (in-plane) component was expected. This assignment has been checked by observations on HMB and the HMB-chloranil complex as well as through a normal coordinate treatment of the in-plane and out-of-plane modes of TCNE. Neither HMB nor the chloranil-complexed HhIB absorb significantly at this frequency. Further, only a single out8-ofplane TCNE mode, observed at 542 em-' in the complex, is predicted from the normal coordinate analysis. Our explanation of the unexpected 580-em-' perpendicular component calls on Ferguson's overlap concept. It is reasonable that, although nearly parallel, the TCNE plane is slightly offset with respect to the donor molecular plane as has been observed in the HMB.chlorarii1 complex. As a result, two g e m cyano groups may protrude more closely to a given neighbor HMB (PMB) molecule than do the other cyano pair. The pair of cyano groups closest to the particular HMB (PMB) molecule are naturally more involved in overlap with that particular donor molecule. As a result, the bl, cyanide bending mode would produce an alternating increase and decrease in overlap of the orbitals involved in charge transfer. A perpendicular oscillation of charge, accountable for the perpendicular component of the 580-cm-I band, would result. The interpretation of the dichroism in the infrared spectra of the complexes has required the assumption of some charge transfer to the TCNE. The effect of this transfer on the position of the planar TCNE modes (8) T. L. Brown, J . Chem. Phys., 43, 2780 (1965).
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J. STANLEY, D. SMITH,B. LATIMER, AND J. DEVLIN
is not particularly striking. In each complex the bl, and bzu C=N stretching modes were red-shifted 9 cm-I each as was the a, C=C mode. Conversely, the blu and bzU carbon-carbon single bond stretching frequencies were invariant. The significance of these data will be considered in the next section following discussion of the spectrum of the TCNE anion. B. The TCNE Anion Salts. The radical anion of TCNE has been known since 1960.9 Since solution epr studies have shown that the exchange reaction between TCNQ (tetracyanoquinodimethane) and TCNE - yields neutral TCNE, the anion is apparently formed without severe structural modification. This discussion is, therefore, based on the assumption that the TCNE.- structure resembles the DZh TCNE configuration and that D z h selection rules loosely apply in the solid state. However, one glance at the spectra of the anion salts in Figure 4 confirms that they are highly dissimilar to the spectrum of TCNE. The most interesting new features in the K+TCNE.- spectrum are the very intense band a t 1371 cm-I and the marked absence of absorption in the 600-1300-~m-~region where the strongest TCNE fundamental absorptions occur. The intense doublet in the cyanide stretching region (2200 cm-1) as well as the activity in the 500-600-cm-' range must also be considered in interpreting the spectra. A Huckel-type calculation for TCNE predicts that the radical anion electron enters a molecular orbital of B1 symmetry such that the C=N and C=C bonds lose considerable strength while the C-C bond order is slightly increased. Consequently, one would expect the C=N and C=C stretching modes to experience a significant red shift while the single bond stretching frequencies increase. Thus we have designated the C-C modes, which absorb strongly at 1155 and 960 cm-l in TCNE, as the source of the very weak bands at 1187 and 970 cm-1 in K+TCNE -. No other infrared-active TCNE fundamental modes occur between 600 and 2000 cm-l so there is no apparent source for the intense 1371cm-1 band. The assignment of this band is the key to understanding the unusual TCNE . - spectrum. Anderson'O has reported that unusual effects in the reflection spectrum of CSZ(TCNQ)~are best interpreted by recognizing the existence of strong charge-transfer interaction between neighboring TCNQ (tetracyanoquinodimethane) species. The Ferguson-Matsen theory of activation then applies and certain intense bands have been assigned to infrared-forbidden, totally symmetric modes. Using tohisapproach with K(TCNE) the strong band at 1371 cm-l is assignable as the totally symmetric C=C stretch which is observed at
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The Journal of Physical Chemistry
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I
1
1
I
I
I
1
1
1
1
1~
1
1
2300 2200 2100 Zoo0 Is00 1600 1400 I300 1200 1100 IWO 900 800 700 600
w.
1
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Figure 4. Spectra of TCNE and TCNE. - derivatives.
1569 cm-I in the Raman of TCNE. The 198-cm-l red shift' agrees, at least qualitatively, with predictions from the Huckel calculation. Although this is our assignment for the 1371-cm-l band, the mechanism of activation is believed to differ somewhat from that proposed for Cs2(TCNQ)+ The intensity of the 1371-cm-l band would require an unacceptably strong charge-transfer interaction between TCNE. - species. An alternate activation model considers the fact that the TCNE - electron cloud is highly polarizable and, therefore, distorted under the influence of the point-charge cation. The extent of this distortion determines the magnitude of the dipole directed from cation to anion. Further, the intense Raman effects at 1569 and 2247 cm-l in TCNE indicate that the scalar polarizability of TCNE, and thus most likely TCNE -, varies considerably with the symmetric C z N and C=C stretching modes. As the magnitude of the scalar polarizability oscillates with the normal vibration, the distortion of the anion charge cloud by the cation varies such that the dipole directed between cation and anion oscillates. Thus the mode is activated. It is quite likely that this process actually involves fluctuations in the charge-transfer interaction between the anion (donor) and the cation (acceptor). If this is the correct explanation of the 1371-cm-l absorption, it is necessary that the C=N symmetric stretch be similarly activated. A very intense doublet was observed at 2180-2201 cm-l, which, since the splitting vanishes in the glassy phase (see Figure 4), seems to result from a single mode. In line with the assignment of the 1371-cm-1 absorption and the results from the Huckel calculation, this strong doublet has been assigned to the symmetric C&N stretch. (9) W. D. Phillips, J. C. Rowell, and S. I. Weissman, J . Chem. Phys., 3 3 , 626 (1960).
(10) G. R. Anderson, Paper R5,Symposium on Molecular Structure and Spectroscopy, The Ohio State University, 1963.
INFRARED SPECTRAOF COMPLEXES OF TETRACYANOETHYLENE
It is unfortunate that, because of low-frequency oscillations of the donor (anion) and acceptor (cation) molecules against each other, this activation model does not lead to conclusions regarding crystal structure. For example, it might seem at first thought that such activation could not occur in a structure composed of independent stacks represented as --D +A-D +Ad--, where a given cation is equidistant between two anions. However, the low-frequency lattice mode wherein all cations move in a sense opposite to that of the anions would distort a given stack, thus permitting higher frequency intramolecular vibrations to be activated by the proposed mechanism. This might be represented as
where the phase of the high-frequency mode is such that the anion polarizability is increasing. Similar considerations apply to weak complexes discussed in section A." Brown's vibronic contribution theory8 must be invoked to understand the other unusual feature of the TCNEe- spectrum-the absence of activity in the bl, and b2, carbon-carbon single bond stretching modes which produce strong absorptions in TCNE. The prerequisites for a vibronic contribution are present in both TCNE and TCNE.-, but the additional mobile radical electron in the anion is apparently critical. The vibronic contribution should increase as the separation of ground and electronic excited states decreases or as the density and mobility of the T electrons increase. These effects are produced by reducing TCNE to the anion. Thus, following Brown, one could propose that in TCNE.- the vibronic contribution to the activity of the bl, and bzu modes is nearly equal in magnitude but opposite in sign to that of the carbon skeletal distortion. It is noteworthy that both this proposed vibronic washout of the bl, and bZucarbon-carbon stretching modes and the activation of the a, modes result from the high polarizability of the T-electron system of the anion. The net result is a spectrum in which all strong absorption from 700 to 4000 cm-l is produced by transitions which would be symmetry forbidden in the isolated D 2 h anion. Most likely this behavior is not a rare one, however, as preliminary results indicate that identical arguments are necessary to explain the infrared spectrum of disodium hexacyanobutadienediide, 1,1,2,3,3-pentacyanopropenideanion, and an2-dicyanomethylene-1 ,1,3,3-tetracyanopropanediide io n.
2015
The relatively rich TCNEa- spectrum in the range 500-700 cm-l is consistent with the above interpretation. Two a, modes are expected in this region. These modes are known to produce relatively small fluctuations in scalar polarizability but, nevertheless, should be somewhat activated. Further, a strong band at 554 cm-l in TCNE is produced by an out-of-plane mode. No vibronic contribution to the activity of such a mode would be expected. Thus, it should retain activity in the anion ( i e . , classically, .rr-electron polarizability perpendicular to the molecular plane is small). It is noteworthy that our data for Na(TCNE) are consistent with the proposed assignment. Because the sodium cation has a greater electron affinity than does the potassium cation, the electron transfer to TCNE should be greater in K(TCSE) than Na(TCNE). Thus the smaller red shifts observed for the C=N and C=C stretching frequencies in Na(TCNE) were expected.
Conclusions and Summary The current theories of activation of vibrational modes in charge-transfer complexes have been found adequate to explain the observed dichroic behavior of the activated modes in single crystals of aromatic TCNE complexes. Spectroscopic studies of the anion of TCNE indicate that the electron transfer in the weak complexes is roughly 510%. This number is derived from comparison of the 9-cm-' average red shift of the C=X and C=C modes of TCNE in the complex with the 130-cm-l average shift found for the anion (-100% transfer). The highly unusual anion spectrum has been analyzed, and the major features were assigned by invoking two theories which emphasize the unusual polarizability of the anion. All available data seem consistent with a pseudo-charge-transfer activation of the ag modes accompanied by deactivation of the bl, and b2, stretching modes by a vibronic contribution. The Huckeltype calculation has added credence to the assignment. Measurement of the Raman frequency of the C=C mode in the anion would constitute a critical check on the proposed assignment. However, the character of the anion salts, highly colored reactive solids, and the inaccessibility of a suitable Raman instrument have delayed this measurement. Acknowledgments. This work has been made possible by support from the National Science Foundation (11) H. B. Friedrichs and (1966).
W-.B. Person, J . Chem. Phys.,
Volume 70, Number 6
44, 2161
June 1966
H. L. FRISCH, J. L. KATZ,E. PRAESTGAARD, AND J. L. LEBOWITZ
2016
under Grant GP-3878. Valuable discussions with Professor Lionel RaiT of our department are gratefully acknowledged. Professor Willis Person has also made
helpful suggestions. Valuable assistance has been provided by the Oklahoma State University Research Foundation.
High-Temperature Equation of State-Argon
by H. L. Frisch, Bell Telephone Laboratories, Murray H a , New Jeraey 07971
J. L. Katz, North American Aviation Science Center, Thousand Oaka, California 91960
E. Praestgaard, Chemical Laboratory III, H . C. &st&
Institute, Copenhugen @, Denmark
and J. L. Lebowitz Department of Physics, Belfer Craduate School of Science, Yeehiva Univeraitu, New York, N e ~ rY w k (Received January 17, 1966)
We have studied the first two terms of the high-temperature equation of state, p / p k T = a(p) b(p)/kT c/(kT)a .,of a simple spherical fluid whose intermolecular potential is the sum of a hard core and soft (mostly attractive) contribution. a(p) and b ( p ) are known functionals of the pressure and radial distribution function of a fluid whose potential is solely composed of the hard-core contribution. Approximate expressions for a(p) are obtained by using the hard-sphere equation of state and radial distribution function of the approximate Percus-Yevick theory. We show that b(p) can be expressed directly as a quadrature of the Laplace transform of the approximate radial distribution function (which is explicitly known); no inversion of the transform is necessary. Choosing for the soft potential a truncated LennardJones potential, we compare the resulting first two terms of the series with experimental data for argon for densities between 40 and 600 amagats and temperatures from 0 to 150’. The intercept a(p) is in good agreement with that theoretically computed. The theory can reproduce the b ( p ) , found from experiment, if the parameters of the truncated Lennard-Jones potential are varied by about 5%.
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+ ..
1. Introduction At sufficiently high temperatures the behavior of even dense, simple (supercritical) fluids will be largely determined by the “effective” hard cores of the molecules. The “soft” mostly attractive part of the interThe Journal of Phyaieal Chemistfy
molecular potential may then be treated as a small perturbation. It then becomes convenient to approximate the actual i n ~ ~ ~ o l e c uPotential, lar u(r),which is not perfectly known anyway as a function of the intermolecular distance, T , by a sum of two terms in eq 1.1.
