J . Phys. Chem. 1989, 93, 5679-5684
5679
Structure of Dicyanoacetylene by Electron Diffraction and Coherent Rotational Raman Spectroscopy Kirk W. Brown, Joseph W. Nibler,* Kenneth Hedberg, and Lise Hedberg Department of Chemistry, Oregon State University, Coruallis, Oregon 97331 (Received: December 20, 1988) The pure rotational Raman loss spectrum and a gas-phase electron-diffraction study of N=C-CeC-C=N are reported. The diffraction data yield r: bond lengths of reN = 1.161 ( 5 ) A, rc4 = 1.367 (3) A, and rcZc = 1.198 (11) 8, for the linear geometry, as well as vibrational amplitudes and shrinkage effect values in good agreement with those calculated from a quadratic force field. Harmonic corrections used with the bond lengths give a calculated value of 0.044891 (86) cm-I for the ground-state rotational constant, Bo. A direct Raman spectroscopic measurement of Bo was not possible due to unresolved contributions from thermally populated levels involving mainly two low-frequency bending modes. An analysis of the band maxima leads to an average value B,, = 0.044 867 (1 9) cm-I which is virtually identical with the electron-diffractionBo value. This result is somewhat surprising since B for a long linear molecule is normally found to increase significantly with bending excitation, an expectation consistent with Raman band-shape simulations for C4N2.Several possible causes of this discrepancy are examined by use of harmonic force field and ab initio calculations.
Introduction Dicyanoacetylene, N=C-C=C-C=N, is an unusual linear molecule first reported by Moureu and Bongrand in 1909.I It is a photoreactive species which readily undergoes polymerization and hence its possible use as a precursor for polyacetylenes is of interest.* The vibrational spectrum of C4N2 has also been reexamined r e ~ e n t l y ,stimulated ~ by the detection of similar nitriles in the thermal emission spectrum of the atmosphere of Titan, one of the moons of S a t ~ r n . This ~ has led to the postulate that a 478-cm-l peak in the spectrum recorded by the Voyager space probe is due to the presence of di~yanoacetylene.~ The first vibrational analysis of C4N2 was reported in 1953 by Miller and Hannans Based on Raman spectra of the liquid phase and infrared spectra of the vapor phase, peak frequency assignments were made for eight of the nine fundamental vibrational modes. The assignments were later revised and the last mode identified by Miller et a1.6 in 1955. In 1987, Khanna et al. reported the Raman and infrared spectra for solid C4N2 as well as the liquid-phase Raman and vapor-phase infrared ~ p e c t r a .Revised ~ assignments were given for the symmetric stretching fundamental frequencies and a new set of valence force constants based on these assignments was offered. The structure of C4N2 is of some interest because of its unusual linear arrangement of three alternate triple bonds and because the molecule is simple enough to permit theoretical calculations of its properties. From X-ray crystallography, the C4N2crystal belongs to the monoclinic space group G h - P 2 a with bond and rce = lengths found to be rCSN = 1.14 A, rc-c = 1.37 1.19 A.’ The structure in the vapor phase has not previously been determined. No high-resolution vibrational-rotational data have been recorded and, due to the centrosymmetric nature of C4N2, no microwave spectrum can be observed. The pure rotational spectrum is accessible by Raman methods, however, and we report here the high-resolution Raman loss spectrum of C4N2. These data have been analyzed to obtain a B,, rotational constant which is the average for several low-lying bending states. In addition, band-shape calculations were done in an effort to define a range for the vibration-rotation-constant (Y for these excited states. An electron-diffraction experiment was carried out in order to obtain thermally averaged interatomic distances (ra,rJ and vibrational amplitudes ( I ) for the molecule. Quantities for the conversion of these distance types to reo values were calculated
d,
( 1 ) Moureu, C.; Bongrand, J . C. Bull. Soc. Chim. 1909, 5, 846. (2) Chien, J. C. W.; Carlini, Carlo J . Polym. Sci., Polym. Chem. Ed. 1985,
23, 1383. ( 3 ) Khanna, R. K.; Perera-Jarmer, M. A,; Ospina, M. J. Spectrochim. Acta 1987, 3, 421. (4) Kunde, V. G.; Aikin, A. C.; Hanel, R. A.; Jennings, D. E.; Maguire, W. E.; Sanuelson, R. E. Nature (London) 1981, 292, 683. ( 5 ) Miller, F. A,; Hannan, R. B. J . Chem. Phys. 1953, 21, 110. (6) Miller, F. A,; Hannan, R. B.; Cousins, L. R. J . Chem. Phys. 1955, 23. (7) Hannan, R. B.; Collin, R. L. Acta. Crystallogr. 1953, 6 , 350.
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from a harmonic force field and the resultant geometry leads, after harmonic corrections, to a Bo constant that is nearly the same as the B,, value deduced from the Raman data. Such agreement is not to be expected since B should increase for excited bending states. To explore this aspect further, simple ab initio calculations have been done to examine the importance of bond length changes as the molecule is bent.
Experimental Section The dicyanoacetylene used in this work was prepared in a manner similar to the one described by Moureu and Bongrand.8 Acetylenedicarboxamide, sand, and P2Os were thoroughly mixed by grinding and placed in a 500-mL round bottom flask. This was then connected to a vacuum system which was subsequently evacuated to -40 mTorr. The vessel was immersed in a 220 OC oil bath. White crystals of dicyanoacetylene soon formed in a collection tube held at liquid nitrogen temperature. Upon completion of the reaction, the collection tube was warmed to dry ice temperature and pumped to remove the C 0 2 generated during the reaction. The sample was then warmed to room temperature and left in contact with P205for 2 h to remove water vapor. Due to the low reaction efficiency ( 0 levels, R-branch (A/ = +1) rotational transitions are also allowed, although the intensity of these falls very quickly, as 1 / J . For example, even a t the first line seen in our spectra, S( 12), the overlapping R ( 2 6 ) feature contributes only about 1% to the intensity; hence only S-branch lines are considered in our analysis. Although the instrumental resolution is high and the collisional broadening a t 50 Torr is estimated to be only -0.04 cm-' for C,N2, the plethora of thermally populated states results in only partially resolved structure in the rotational spectrum. This is a consequence of the fact that the two lowest frequency bends occur a t 263 cm-l ( u 7 ) and 107 cm-' (14. Table I V shows that the 15 lowest levels, involving mainly overtone/combinations of
-
(14) Lafferty. W . G.; Maki, A. G.; Plyler, E. K. J . Chem. Phys. 1964, 40, 224.
u7 and u9, account for 2 / 3 of the total population. The state degeneracy of each excited level is also given. For example, the transition which is believed to be the source of the Titan spectral peak a t 478 cm-l is v7 2v9 This upper combination level actually consists of II + II @ states, each having two different B values for the f l sublevels. Given such a variety of B values, it is perhaps surprising that the rotational spectrum is as resolved as it is over the large range of J values observed. To extract a rotational constant representing the average over the many vibrational states, each feature in each of several recorded spectra was subjected to an 1 1-point parabolic fitting operation to find the best peak frequency. These frequencies were then fit to eq 6 , with neglect of the small I* term, to yield values of B,,, D,,, and a small constant (-fO.02 cm-I) representing the calibration error in the Raman loss apparatus for that particular day. After shifting each data set by these calibration constants, all of the data were combined and the process repeated to obtain cm-I. The a B,, of 0.044867 (19) cm-I and D,, of 9.3 ( 6 ) X resultant corrected frequencies and differences (observed - calculated) are given in Table V. Raman Band-Shape Analysis. Examination of the expanded Raman loss spectra shows hints of some regular but poorly resolved structure within each J transition, as illustrated in the top spectra of Figure 4. An attempt was made to extract more information from these data using a simple model to simulate the spectrum. This was done by calculating the frequencies and intensities of the rotational transitions arising from the ground state and all states in Table 111 involving the v7 and u9 modes only. The energy levels were assumed to fit the expression F ( J ) = [BO - CY(+ + c 9 ) ] [ J ( J+ 1) - (I7 + &)*I D [ J ( J + 1) - (17 + 19)2]2 ( 7 ) i.e., it was assumed that a single value of the vibration-rotation constant CY applied for all states involving u7 and u9. These transitions were then summed, using the appropriate degeneracies
+
+
Structure of Dicyanoacetylene
The Journal of Physical Chemistry, Vol. 93, No. 15, 1989 5683 TABLE VI: Comparison of Structural Parameters bond lengths r,,.
N=C-C-C-C-N H -C=C-C=C-H H-C=C-C=N N=C-C=N H -C-C-H H-C-N
gas,'ED solid: X-ray re: a b initio gas: Raman gas,dMW gas: IR gas,' Raman gas,g IR
1.161 ( 5 ) 1.14 1.141 1.157 (1) 1.154 (6)
1.367 (3) 1.37 1.368 1.376 (2) 1.382 (1) 1.389 (10)
A 1.198 (11) 1.19 1.187 1.205 1.203 ( I ) 1.2086 ( 1 )
1.15313 (2)
"This work. *Reference 6. 'Reference 26. The C=C distance was assumed. dReference 21. 'Reference 28. /Reference 29. CReference 30.
I , ,
, , , ,
6.6
7
I
I
I
7.4
I
I
/
I
I
,
/
I
I
I
l
7.8 0 . 2 8.6
Figure 4. Comparison of experimental and calculated Raman loss spectra of C,N2 for low- and high-J regions. The experimental spectra were taken at 0.007-cm-' data intervals. As discussed in the text, the calculated spectra are based on a simple model involving a single value of the
vibration-rotation interaction constant, a, for the low-lying bending levels.
and Boltzmann weights, the Raman line-strength factors, and a Lorentzian line shape of 0.04 cm-l to account for collisional broadening. For each J transition, this model predicts five significant lines, corresponding to u7 u9 values of 0-4. These lines are evenly spaced by 4 a J , with the maximum intensity occurring a t about u7 + u9 = 2 . Thus one might expect that B,, = Bo - 2cy. The constant cy is expected to be negative for states involving the bending modes since the end-to-end atom distances are less for a bent geometry. Typically cybend/Bis -0.001 to -0.004 for linear molecule^,'^ and such a range was explored for C4N2. Figure 4 shows the contours calculated for low and high J transitions for several values of cy. Because of the assumption that Bo = E,, + 2a, the center of each transition set is well matched to the experimental peak average but it is clear that the peak modulation depth is changed greatly as cy varies. A t high Jvalues, the five-line multiplets spread and start to overlap in a constructive or destructive fashion, producing a higher or lower apparent frequency modulation in the calculated spectrum. Such interference may account for the reduced modulation seen for Raman shifts greater than 16 cm-' where J > 90. In view of the simplified nature of the model, it is not surprising that it was not possible to produce a very satisfactory match simultaneously for low and high J values. However, it does appear that an average negative cy value of about -0.0002 (1) gives the best overall fit to the spectrum and hence Bo is about 0.0445 (1) cm-I. Because this value is not very well determined, it was not used as a constraint in a combined electron diffraction-spectroscopy least-squares structural determination, one of our original intents.
+
Discussion Bond Lengths. The rao bond lengths obtained from the refinement of the electron-diffraction data are listed in Table VI along with some ro distances for similar compounds. Comparison of the bond lengths of C4N, in the gas and solid phases suggests that condensation produces a small contraction of the triple bonds and an expansion of the C-C single bonds. This is somewhat surprising since packing forces in the solid should preferentially compress the weaker C-C bonds, while any long-range delocalization of the *-electrons would be expected to result in longer triple bonds in the solid. It may be, however, that these small differences are spurious since no uncertainties were given in the X-ray studies. As can be seen in the table, molecules with alternate single and triple bonds have a C-C distance of 1.37 A, a remarkably short
-
( I 5) Herzberg, Gerhard Molecular Spectra and Molecular Structure III: Electronic Spectra and Electronic Structure of Polyatomic Molecules; Van Nostrand: Princeton, NJ. 1966.
value approaching the C=C distance of 1.339 8, in ethylene rather than the C-C value of 1.536 8, in ethane. This suggests that ionic resonance structures such as -N=C=C=C=C=N+ may be important in the bonding description. The increased charge separation possible in N=C-C=C-C=N compared to N E C-C=N should favor such forms and this could account for the slightly smaller C-C distance in the former molecule. The difficulty with this simple picture, however, is that it predicts a corresponding increase in the CEN and C==C distances relative to those in H C N and C2H2whereas the experimental values are essentially constant. The simple valence bond description is thus not a good model for these compounds and a molecular orbital approach is to be preferred. W e have carried out an a b initio calculation (GAUSSIAN86 a t the HF/6-3lG* basis level16) which gives an energy level pattern and an orbital description close to that predicted by a simple Huckel model. The net charge distribution corresponds to -*N=Cf6-C=C-+6C=N-6 with 6 = 0.45 electrons and the central C=C atoms essentially neutral as in C2Hz. The predicted re bond lengths, shown in the table, are sensibly shorter than the experimental ro values. This offers some encouragement that other physical properties such as vibrational frequencies and transition intensities may also be reliably estimated from these calculations. In particular, the three calculated Zg+ symmetric stretching vibrational frequencies (not "scaled") and Raman activities (in brackets) can be compared with the assignments of Khanna et al.3 and Miller et a1.6 mode vl(C=N) Y~(CEC) 4C-C)
calcd
Khanna
Miller
2706 [1300] 2455 [1.3] 640 [ I ]
2271 (VS) 2327 (vs) 640 (vw)
2290 (VS) 2119 (m) 692 (m)
Close absolute agreement is not to be expected but it may be meaningful that the calculated frequency ordering and relative intensity patterns are more consistent with Miller's assignment. This ordering of the C=N and C=C stretching frequencies is also in line with the bond length ordering found experimentally. These two stretching modes are undoubtedly heavily mixed, however, and further study using isotopic substitution will probably be necessary to provide a definitive choice between the two assignments. Rotational Constants. The Bo rotational constant derived from the electron-diffraction bond lengths is 0.044 89 (9) cm-l, nearly identical with the B,, value of 0.04487 (2) deduced from the Raman data. This is somewhat surprising since, as discussed earlier, the contribution of excited bending states should be such as to make B,, > Bo by perhaps 0.0002 cm-l. It is, of course, possible that the harmonic corrections used to obtain Bo from the r, values of the diffraction analysis are insufficient. In the case of C 3 0 2 ,a molecule with an exceptionally low bending mode a t 17 cm-I, an analogous discrepancy between electron diffractionspectroscopic rotational constants was resolved only by explicit inclusion of anharmonic terms involving this mode.I7 This (16) Frisch, M . J.; Binkley, J. S.; Schlegel, H. B.; Raghavachari, K.; Melius, C. F.; Martin, R. L.; Stewart, J. J . P.; Bobrowicz, F. W.; Rohlfing, C. M.; Kahn, L. R.; Defrees, D. J.; Seeger, R.; Whiteside, R. A,; Fox, D.J.; Fleuder, E. M.; Pople, J. A. GAUSSIAN 86; Carnegie-Mellon Quantum Chemistry Publishing Unit: Pittsburgh, PA, 1984.
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J . Phys. Chem. 1989. 93, 5684-5692
“quasi-linear” molecule is quite unusual in that it is thought to have a barrier of 27 ( 16) cm-’ at the linear geometry, the minimum occurring for a central bend angle of 20 (2)’. A clear indication of the inadequacy of the harmonic corrections for “linear” C 3 0 2 is given by the poor agreement between calculated and observed shrinkage effects in Table I l l . I n contrast, the agreement for C4N2 is comparable to that seen for other linear molecules. Similarly, the comparison of calculated and observed mean-square amplitudes shown in Table I indicates that the harmonic force field is acceptable for C4N2. The equality of B,, and Bo implies a distribution of excited-state B values centered about Bo. Smaller B values could come from excited states involving axial stretching modes but these have a much lower thermal population than the bending levels. If, however, the bond lengths should increase appreciably as the molecule is bent, a net decrease in the B value might result. In fact, such a stretch-bend interaction has been observed for C 3 0 2 , the C=C bond lengths extending slightly as the molecule bends.” A similar, but smaller, effect has also been noted for other linear geometries such a5 CO,,’* CS2,I9 and CH,=C=CH,.Zo ( 1 7 ) Ohshima. Y.; Yamamoto, S.; Kuchitsu, K. Acta Chem. Scand. A 1988, 42, 307.
( I 8) Pariseau, M. A,; Suzuki, H.; Overend, J. J . Chem. Phys. 1965, 42, 2335. (19) Smith, D. F. Jr.; Overend, J. J . Chem. Phys. 1971, 54, 3632. (20) Ohshima, Y.; Yamamoto, S.; Nakata, M.; Kuchitsu, K. J . Phys. Chem. 1987, 91, 4696. (21) Tanimoto, M.; Kuchitsu, K.; Morino, Y. Bull. Chem. Soc. Jpn. 1971, 44, 386. (22) Morino, Y . ;Kuchitsu, K.; Hori, Y.; Tanimoto, M. Bull. Chem. Soc. Jpn. 1968, 41, 2349. (23) Cvvin, S. J.; Meisingseth, E. Acra Chem. Scand. 1961, 1289 (24) Smith, W. H.; Lero; G. E. J . Chem. Phys. 1966, 45, 1767. (25) Fusina, L.; Mills, I . M. J . Mol. Spectrosc. 1980, 123. (26) Callomon, J. H.; Stoicheff, B. P. Can. J . Phys. 1957, 3 5 , 373. (27) Westenberg, A. A . ; Wilson, E. B. Jr. J . Am. Chem. Sac. 1950, 7 2 , 199. (28) Maki, A. G . J . Chem. Phys. 1965, 43, 3193. Edwards, H. G. M.; Mansour, H . R. J . Mol. Strucr. 1987, 160, 209.
T o examine this possibility for C,N2, our a b initio calculations at the 6-3 IC* level were extended by optimization of the structure for fixed and bent conformations. Bond angles of 174.7’ (C= were chosen since these correspond C-C) and 177.8’ (C-C=N) to mean bending amplitudes (thermally averaged) obtained from the force field analysis for the 107-cm-’ bending mode. The effect of bending was quite small: an increase of 0.0004 8, in the CEC distance and a decrease of 0.0001 8, in the C=N and C-C distances. Such changes are insufficient to account for the discrepancies in the Bo rotational constants. Calculations with a larger basis set and electron correlation might change this conclusion of course. One final possible cause of low B values for excited bending states is intriguing, albeit highly speculative: that C4N2,like C,O,, is quasi-linear with an appreciable barrier at the linear geometry. Bending states above the barrier would then be linear and thus have B values smaller than Bo. It should be said that the electron diffraction shrinkage data do not support such an unusual structure, however, and it should also be noted that our theoretical calculations converged a t the linear geometry. Thus a resolution of the small difference between the electron diffraction and spectroscopic Bo values does not seem possible with the data in hand. Further high-resolution Raman and infrared examination of the bending modes and their overtones would be desirable for C 4 N 2 since these could better establish the bending potential function and give more accurate B values of the bending states. Acknowledgment. Research support by the National Science Foundation (CHE88-10070 and CHE88-40436) and the Air Force Office of Scientific Research is gratefully acknowledged. W e also thank Rainer Beck for assistance in obtaining the Raman Loss spectra. Registry No. N=CC=CC=N,
1071-98-3.
(29) Baducci, A.; Ghersetti, S.; Hurlock, S. C.; Rao, K. N. J . Mol. Spectrosc. 1976, 59, 116. (30) Rank, D.H.; Skorinko, G.; Eastman, D. P.; Wiggins, T. A. J . Opt. Sac. Am. 1960, 421
Infrared Spectra of Amide-HF Complexes in Solid Argon Robert B. Bohn and Lester Andrews* Department of Chemistry, University of Virginia, Charlottesville, Virginia 22901 (Received: December 27, 1988)
Hydrogen-bonded complexes of amides and hydrogen fluoride have been prepared in solid argon. FTIR spectra of these complexes compared to spectra of similar carbonyl and amine complexes show that HF is hydrogen bonded to the carbonyl oxygen in the primary intermolecular interaction, which is complemented by a secondary amido hydrogen-fluorine interaction giving a cyclic structure. The amide-HF interaction increases with methyl substitution.
Introduction Amides, R-CONH,, are fundamental molecules in organic chemistry and especially biochemistry.’,2 It is known that all proteins and polypeptides are created by end-on-end addition of amino acids, which displace water, to form oligomers through amide bonds. In this sense, these macromolecules can be envisioned as polymeric chains of amide subunits. Hydrogen bonding plays a crucial role in the intra- and intermolecular interactions within proteins. It necessarily influences the overall 3D structure ( I ) Sigel, H.; Martin, R. B. Chem. Rec. 1982, 82, 385. (2) Homer, R. B.; Johnson, C. D. In The Chemistry ofAmides; Zabicky, J.. Ed.: Interscience-Wiley: London, 1970.
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of the protein, and in turn its chemistry. The hydrogen bonds that are typically found in amides occur between the carbonyl oxygen atoms and the amido hydrogens; those found in the a-helix and P-pleated sheet have lengths of approximately 2.9-3.4 A.3 The hydrogen-bonding properties of amides are direct consequences of its electronic structure. The amide group itself has a large extent of n-conjugation throughout the O C N framework; consequently, two resonance forms, 1 and 2, exist. Form 2 is a zwitterion characterized by planarity of the C O N H 2 subunit as well as C O and C N bond lengths intermediate between single and (3) Walton, A . G .Polypeptides and Protein Structure; Elsevier North Holland: N e w York, 198I .
0 1989 American Chemical Society
