Structure of the Cumulene Carbene Butatrienylidene: H 2 CCCC

Transcription

1 JOURNAL OF MOLECULAR SPECTROSCOPY 180, (1996) ARTICLE NO Structure of the Cumulene Carbene Butatrienylidene: H 2 CCCC M. J. Travers,*, Wei Chen, Stewart E. Novick, J. M. Vrtilek,* C. A. Gottlieb, and P. Thaddeus*, *Harvard Smithsonian Center for Astrophysics, Cambridge, Massachusetts 02138; Division of Applied Sciences, Harvard University, Cambridge, Massachusetts 02138; and Department of Chemistry, Wesleyan University, Middletown, Connecticut Received February 28, 1996; in revised form July 3, 1996 The microwave rotational spectra of the J Å 1 R 0 and 2 R 1 transitions of six isotopic species of the singlet carbene 13 butatrienylidene (H 2 CCCC, H 2 CCCC, H 2 C 13 CCC, H 2 CC 13 CC, H 2 CCC 13 C, and D 2 CCCC) were measured with a Fourier-transform microwave spectrometer and the r 0 and r s structures were determined. Both structures are consistent with a cumulenic carbon-chain backbone. Our empirical structure is r s (HC (1) ) Å { Å, r s (C (1) C (2) ) Å { Å, r s (C (2) C (3) ) Å { Å, r s (C (3) C (4) ) Å { Å, and s HC (1) H Å { 0.4. Except for the CC bond closest to the carbene carbon, r s (C (3) C (4) ), an equilibrium geometry determined in a high-level ab initio calculation (M. Oswald and P. Botschwina, J. Mol. Spectrosc. 169, 181 (1995)) is in very good agreement with the experimental structure Academic Press, Inc. I. INTRODUCTION a standard supersonic nozzle pass through a small electric discharge in the nozzle throat and into a high-q Fabry Butatrienylidene (H 2 CCCC; Fig. 1) 2 ev higher in en- Perot microwave cavity. A saturating pulse of microwave ergy (1) than diacetylene (HC 4 H) is the third member of radiation, timed to coincide with the passage of the gas the cumulene carbene series (H 2 C n ) observed in the labora- pulse, is introduced into the cavity, producing a macroscopic tory. The first member in the series, vinylidene (H 2 CC) polarization when a molecular transition lies within the apan isomer of acetylene was studied via negative ion photo- proximately 1 MHz bandwidth of the cavity. The resulting electron spectroscopy of the corresponding anion by Ervin free induction decay ( FID) is digitized and Fourier transet al. (2); pure rotational transitions of H 2 CCC (Ref. (3)) formed to obtain the frequency profile (i.e., the square root and H 2 CCCC (Ref. (4)) in their singlet electronic ground of the power spectrum). In the experiments described here, states were observed at millimeter wavelengths in a low- the supersonic molecular beam was oriented perpendicular pressure discharge through acetylene and were subsequently to the Fabry Perot axis, resulting in linewidths of approxiobserved in the molecular envelope of the carbon-rich star mately 25 khz ( full width at half intensity). IRC / (Refs. (5, 6)) and the cold dark cloud TMC-1 Several nozzle geometries were explored; that which gave (Refs. (5, 7)). the best results is the same as that used in a recent investiga- The ground state spectrum and structure of H 2 CCC are tion of the CCCCH radical (10). The H 2 CCCC was pronow well studied ( 8), but much less is known about duced in a pulsed dc discharge ( 1000 V) through either H 2 CCCC. Owing to its possible importance as a chemical acetylene or diacetylene (HC 4 H) and argon, with about 25% intermediate in acetylene chemistry, we undertook an experi- stronger signals observed in the diacetylene/ argon dismental investigation of the rotational spectra of its carbon- charge; the optimum gas conditions ( a mixture of 0.25% 13 and deuterium isotopic species. This study, in conjunction acetylene and 0.25% diacetylene in 99.5% argon) resulted with theoretical calculations, yields important spectroscopic in a factor of 2 increase in signal intensity relative to the information which should aid the future observations of ex- acetylene/argon discharge. The FTM spectrum of D 2 CCCC cited vibrational and electronic states needed to understand was observed in a discharge through DCCD in Ar. acetylenic chemistry better. Diacetylene was prepared by the standard method of Armitage et al. (11), with slight modifications suggested II. EXPERIMENT by Brandsma (12). Owing to the risk of explosion, only a small quantity of diacetylene was synthesized and it was Rotational spectra of the isotopic species of H 2 CCCC were immediately diluted with argon to a concentration of about measured with a pulsed-jet Fourier transform microwave 10% for safe handling. The carbon-13 isotopomers of ( FTM) spectrometer ( 9). Briefly, gas pulses produced by H 2 CCCC were observed in a discharge through normal /96 $18.00 All rights of reproduction in any form reserved.

2 76 TRAVERS ET AL. FIG. 1. The substitution (r s ) structure of H 2 CCCC obtained from the rotational constants of six isotopic species. diacetylene and a statistical mixture of HCCH, H 13 CCH, and H 13 C 13 CH which was produced by the hydrolysis of Li 2 C 2. The 13 C enriched Li 2 C 2 was prepared at the NIH Stable Isotope Resource, Los Alamos National Laboratory. The lines of the four 13 C isotopic species were comparable in strength and about 20 times weaker than the normal species (see Fig. 2 for a sample spectrum). The implication is that insertion of the 13 C occurs more or less randomly in the discharge. Following the initial detection of the normal species (4), the millimeter-wave absorption spectrum of D 2 CCCC was observed in a dc discharge (0.5 A) through DCCD and He (molar fraction 60:1) at a temperature of 160 K and total pressure of 30 mtorr. Typical integration times in the millimeter-wave measurements were 15 s/mhz. The millimeter-wave rotational lines of the carbon-13 isotopomers were not detected in a 13 CO/C 2 H 2 /He discharge; however, recent measurements of CCCCH (Ref. (13)) have revealed that lines of the carbon-13 species are about four times stronger in a discharge through carbon-13 en- FIG. 2. The 2 0,2 R 1 0,1 transition of H 2 CC 13 CC. (a) The free induction riched C 2 H 2 than in the 13 CO/C 2 H 2 /He discharge. decay (FID) and (b) its Fourier transform (calculated at a resolution of 5 These results again demonstrate that a FTM spectromeis khz/pt). The spectrum, an average of 5000 gas pulses (10 Hz repetition rate), ter such as ours is well suited for observing rare isotopic the result of approximately 8 min integration time. The double-peaked lineshape results from the Doppler shift of the supersonic beam relative to the species in enriched as well as in natural abundance (14). two traveling waves that compose the confocal mode of the Fabry Perot In addition, approximately 500 times less sample is con- cavity. The linewidth of the individual peaks (full width at half intensity) is sumed in the FTM than in the millimeter-wave experi- Ç25 khz. ments. TABLE 1 Microwave Transitions of H 2 CCCC Isotopic Species (in MHz)

3 STRUCTURE OF BUTATRIENYLIDENE: H 2 CCCC 77 III. DATA AND ANALYSIS A total of 24 a-type R-branch microwave (J Å 1 R 0 and 2 R 1) transitions, four for each of the six isotopic species (Table 1), and 40 millimeter-wave transitions of D 2 CCCC between 193 and 274 GHz (Table 2) were analyzed with Watson s S-reduced Hamiltonian ( 15), appropriate for a nearly symmetric prolate top molecule such as H 2 CCCC (k Å ). The sign convention for the eighth-order centrifugal distortion constants L JJK and L KKJ in the extended Hamiltonian ( 16), TABLE 2 Millimeter-Wave Rotational Transitions of D 2 CCCC (in MHz) H Å A 0 B / C 2 J 2 a / 0 D JK J 2 J 2 a 0 D K J 4 a / B / C 2 J 2 0 D J J 4 B 0 C 4 / d 1 J 2 1 (J 2 / / J 2 0) / d 2 (J 4 / / J 4 0) / H J J 6 / H JK J 4 J 2 a / H KJ J 2 J 4 a 0 L JJK J 4 J 4 a 0 L KKJ J 2 J 6 a, [1] is consistent with prior investigations of H 2 CCC (Refs. (3, 8)) and H 2 CCCC (Ref. (4)). Three spectroscopic constants (B, C, and D JK ; Table 3) were least-squares fit to the four transitions of the six isotopic species measured in the FTM experiment (see Table 1). For the carbon-13 species, the fourth-order distortion constant D J and the A rotational constant were constrained to those of the normal species (4), because D J is approximately 250 times smaller than D JK and A is essentially the same in the normal and carbon-13 species of structurally similar H 2 CCC ( Ref. ( 8)). The rms of the fits were comparable to the measurement uncertainties (5 10 khz). For D 2 CCCC, the same set of rotational and centrifugal distortion constants, previously determined for the normal species, were determined (Table 4) in a fit to the combined microwave (Table 1) and millimeter-wave ( Table 2) data. IV. DETERMINATION OF H 2 CCCC STRUCTURE (r 0 ) structure. This method, however, does not correct for differing vibrational amplitudes of the various isotopic spe- cies, resulting in fairly large uncertainties in bond lengths Ideally, one would like to determine an equilibrium (r e ) structure from the measured ground state rotational constants; to do that, however, the effects of zero-point vibrational motion, i.e., the vibration rotation coupling constants (a i ), are required. While calculation of a i ab initio is now feasible for linear molecules with four heavy atoms ( see Ref. ( 17) and references therein), the vibration rotation coupling constants have not yet been reported for H 2 CCCC. Consequently, two methods were used to derive the geometry from the ground state rotational constants. Equations for the moments of inertia (18) were leastsquares fit to the observed moments of the normal and isotopically substituted species ( Table 5), yielding a zero-point

4 78 TRAVERS ET AL. TABLE 3 Spectroscopic Constants of H 2 CCCC Isotopic Species (in MHz) and bond angles, especially those involving light atoms such Uncertainties in the r 0 and r s structures of H 2 CCCC (Taas H. Furthermore, the derived structure depends on the ble 6) were estimated by referring to H 2 CCC (Ref. (8)) particular set of isotopic species chosen (i.e., the r 0 structure and H 2 CCO (Ref. (20)) two molecules with similar geis not unique). ometry whose r e structures were derived from experimental In the second method, the coordinates of each atom were rotational constants and vibration rotation coupling condetermined from moment of inertia differences, resulting in stants calculated ab initio. For both H 2 CCC and H 2 CCO, partial cancellation of zero-point vibrational effects (19). the r s structure closely matches that of the r e structure (all Thus, by measuring the spectrum of each singly substituted bonds to within Å and the HCH angle to 0.3 ). Thereisotopic species (doubly substituted in the case of deuterium fore, by analogy with propadienylidene and ketene, the at equivalent positions), the substitution (r s ) structure was uncertainties in bond lengths and angle for H 2 CCCC should determined. There is, however, still some uncertainly in locating the position of the hydrogen atoms owing to their be similar. large amplitude vibrational motion. V. DISCUSSION TABLE 4 Spectroscopic Constants of D 2 CCCC (in MHz) Except for the C C bond closest to the carbene carbon, which is about 0.01 Å too short, the equilibrium geometry determined in a high-level ( coupled cluster theory) calculation (1) is in very good agreement with the experimental (r s ) structure (see Table 6). As shown in Fig. 3, there is an unoccupied p-type orbital perpendicular to the molecular plane in H 2 CCC. Owing to the interaction of the p electrons with the empty molecular orbital, vibrational motion in this coordinate (n 6 : the CCC out-of-plane bend) is predicted to be soft. An ab initio calculation of the force constant (F 66 ), vibrational frequency (n 6 ), and vibration rotation coupling constant confirmed that there are large uncertainties for properties related to this mode ( 8). Similar effects may be present in H 2 CCCC, however, the unoccupied orbital is now in the plane of the molecule (see Fig. 3) and the vibrational coordinate is the C (2) C (3) C (4) in-plane bend. Following our measurements, Botschwina reported largescale ab initio calculations of the out-of-plane bending potentials in H 2 CCC and H 2 CCCC (Ref. (21)); he finds that the CCC out-of-plane bending potential is considerably steeper in H 2 CCCC than in H 2 CCC. The C (2) C (3) C (4) inplane bending potential of H 2 CCCC has not yet been calcu-

5 STRUCTURE OF BUTATRIENYLIDENE: H 2 CCCC 79 TABLE 5 Moments of Inertia and Inertial Defects of H 2 CCCC Isotopic Species (in uå 2 ) lated; if this coordinate is shallow, it might possibly account for the apparent shortening of the terminal C C bond in H 2 CCCC. FTM spectroscopy of supersonic molecular beams may provide the means for obtaining fundamental spectroscopic and structural information of longer members in the cumulene carbene series. Attempts to detect the next members in the series (H 2 CCCCC and H 2 CCCCCC) at millimeter wavelengths have so far proven unsuccessful, leading to speculation as to whether the carbon-chain backbones of longer members in the cumulene carbene series are linear and whether H 2 C 5,H 2 C 6,rrr have triplet, rather than sin- glet, electronic ground states. Even if the carbon-chain backbones are bent, this should not prevent identification of these near-prolate molecules at microwave or millimeter wavelengths. If, on the other hand, H 2 C 5,H 2 C 6,rrr have triplet electronic ground states, the dipole moments would be nearly ten times smaller and each rotational transition would be split by the spin spin interaction into three resolved components; a mixed blessing with respect to identification. In FTM spectroscopy, however, the FID ( see Sect. II) is proportional to the first power (rather than the square) of the dipole moment and reactive species with small dipole moments such as CCN have been observed (22). TABLE 6 H 2 CCCC Structures

6 80 TRAVERS ET AL. REFERENCES FIG. 3. Generalized valence bond (GVB) representation (see Ref. 1. M. Oswald and P. Botschwina, J. Mol. Spectrosc. 169, 181 (1995). 2. K. M. Ervin, J. Ho, and W. C. Lineberger, J. Chem. Phys. 91, 5974 (1989). 3. J. M. Vrtilek, C. A. Gottlieb, E. W. Gottlieb, T. C. Killian, and P. Thaddeus, Astrophys. J. Lett. 364, L53 (1990). 4. T. C. Killian, J. M. Vrtilek, C. A. Gottlieb, E. W. Gottlieb, and P. Thaddeus, Astrophys. J. Lett. 365, L89 (1990). 5. J. Cernicharo, C. A. Gottlieb, M. Guélin, T. C. Killian, G. Paubert, P. Thaddeus, and J. M. Vrtilek, Astrophys. J. Lett. 368, L39 (1991). 6. J. Cernicharo, C. A. Gottlieb, M. Guélin, T. C. Killian, P. Thaddeus, and J. M. Vrtilek, Astrophys. J. Lett. 368, L43 (1991). 7. K. Kawaguchi, N. Kaifu, M. Ohishi, S. Ishikawa, Y. Hirahara, S. Yamamoto, S. Saito, S. Takano, A. Murakami, J. M. Vrtilek, C. A. Gottlieb, P. Thaddeus, and W. M. Irvine, Publ. Astron. Soc. Jpn. 43, 607 (1991). 8. C. A. Gottlieb, T. C. Killian, P. Thaddeus, P. Botschwina, J. Flügge, and M. Oswald, J. Chem. Phys. 98, 4478 (1993). 9. A. R. Hight Walker, W. Chen, S. E. Novick, B. D. Bean, and M. D. Marshall, J. Chem. Phys. 102, 7298 (1995). 10. W. Chen, S. E. Novick, M. C. McCarthy, C. A. Gottlieb, and P. Thaddeus, J. Chem. Phys. 103, 7828 (1995). (23)) of H 2 CCC and H 2 CCCC. 11. J. B. Armitage, E. R. H. Jones, and M. C. Whiting, J. Chem. Soc., 44 (1951). 12. L. Brandsma, Preparative Acetylenic Chemistry, pp , Elsevier, Amsterdam, ACKNOWLEDGMENTS 13. M. C. McCarthy, C. A. Gottlieb, P. Thaddeus, M. Horn, and P. Botschwina, J. Chem. Phys. 103, 7820 (1995). 14. K. K. Lehmann, F. J. Lovas, and R. D. Suenram, J. Mol. Spectrosc. We thank M. C. McCarthy, W. Klemperer, G. B. Ellison, and P. Botsch- 160, 58 (1993). wina for valuable counsel during the course of these experiments; E. W. 15. J. K. G. Watson, in Vibrational Spectra and Structure, (J. R. Durig, Gottlieb for computer assistance; and T. Gant, T. Zwier, R. Frost, C. Ramos, Ed.), Vol. 6, Chap. 1. Elsevier, New York, and C. Arrington for advice on the synthesis of diacetylene. We also grate- 16. K. M. T. Yamada and S. Klee, J. Mol. Spectrosc. 166, 395 (1994). fully acknowledge L. A. Silks III of the NIH Stable Isotope Resource at 17. P. Botschwina, M. Horn, S. Seeger, and J. Flügge, Mol. Phys. 78, 191 The Los Alamos National Laboratory for assistance in the production of (1993). carbon-13 enriched lithium carbide. The Los Alamos Stable Isotope Re- 18. J. Kraitchman, Am. J. Phys. 21, 17 (1953). source was supported by U.S.P.H.S. Grant RR02231 and by the U.S. Depart- 19. C. C. Costain, J. Chem. Phys. 29, 864 (1958). ment of Energy. SN thanks the National Science Foundation for partial 20. A. L. L. East, W. D. Allen, and S. J. Klippenstein, J. Chem. Phys. support of this research through Grant CHE , 8506 (1995). Note added in proof. Very recently we have observed the lowest rotational 21. P. Botschwina, J. Mol. Spectrosc. 179, 343 (1996). transitions of H 2 C 5 and H 2 C 6 in a FTM molecular beam spectrometer 22. Y. Ohshima and Y. Endo, J. Mol. Spectrosc. 172, 225 (1995). (McCarthy et al., in preparation); both carbenes have linear carbon-chain backbones and singlet electronic ground states. 23. W. A. Goddard III and L. B. Harding, Annu. Rev. Phys. Chem. 29, 363 (1978).