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Detection and Characterization of the Cumulene Carbenes H C And and FeIIIFeIII forms of MMOH, based on their Mo¨ ss- 28. Y. Dong et al., ibid., p. 2778. Structure, Mechanism, and Biochemistry (Plenum, bauer-determined percentages in the freeze- 29. S. Mahapatra et al., ibid., p. 8865. New York, 1986). quenched samples. For sample 1, the pre-edge area 30. J. A. Halfen et al., Science 271, 1397 (1996). 39. J. H. Dawson, Science 240, 433 (1988) and refer- of Q 5 {22 units 2 [(27%)(10 units)] 2 [(12%)(14 31. V. K. Yachandra et al., ibid. 260, 675 (1993). ences therein. units)]}/61% 5 29 units. For sample 2, the pre-edge 32. P. J. Riggs-Gelasco, R. Mei, C. F. Yocum, J. E. 40. R. C. Scarrow et al., J. Am. Chem. Soc. 109, 7857 area of Q 5 {18 units 2 [(33%)(10 units)] 2 [(23%)(14 Penner-Hahn, J. Am. Chem. Soc. 118, 2387 (1996). (1987). units)]}/44% 5 26 units. 33. K. R. Reddy, M. V. Rajasekharan, S. Padhye, F. 41. Supported by NSF grant MCB-9405723 (L.Q.), NIH 23. J. E. Penner-Hahn et al., J. Am. Chem. Soc. 108, Dahan, J.-P. Tuchagues, Inorg. Chem. 33, 428 grants GM-40466 (J.D.L.) and GM-22701 (E.M.), a 7819 (1986). (1994). University of Minnesota Graduate School Doctoral 24. D. M. Kurtz Jr., Chem. Rev. 90, 585 (1990). 34. K. E. Liu et al., J. Am. Chem. Soc. 117, 4997 (1995). Dissertation Fellowship (L.S.), and an NIH Predoc- 25. J. M. Berg and R. H. Holm, in Iron-Sulfur Proteins,T. 35. T. Ookubo et al., ibid. 118, 701 (1996). toral Traineeship (J.C.N., GM-08277). Beamline X9B G. Spiro, Ed. (Wiley, New York, 1982), pp. 3–66. 36. Y. Dong, S. Yan, V. G. Young Jr., L. Que Jr., Angew. at the NSLS was supported by NIH grant 26. V. L. Pecoraro, M. J. Baldwin, A. Gelasco, Chem. Chem. Int. Ed. Engl. 35, 618 (1996). RR-001633. We thank M. Chance and E. Sheuring Rev. 94, 807 (1994). 37. K. Kim and S. J. Lippard, J. Am. Chem. Soc. 118, at NSLS Beamline X9B for technical guidance. 27. Y. Zang, Y. Dong, L. Que Jr., K. Kauffmann, E. 4914 (1996). Mu¨ nck, J. Am. Chem. Soc. 117, 1169 (1995). 38. P. R. Ortiz de Montellano, Ed., Cytochrome P-450: 8 October 1996; accepted 27 November 1996 Detection and Characterization of the type R-branch (DJ 5 1, where J is the rotational angular momentum quantum Cumulene Carbenes H2C5 and H2C6 number) transitions of H2C5 and 13 transi- tions of H2C6 were measured in the fre- M. C. McCarthy,* M. J. Travers, A. Kova´ cs, Wei Chen, quency range from 8 to 23 GHz. In both Stewart E. Novick, C. A. Gottlieb, P. Thaddeus molecules, the rotational transitions are grouped into fairly tight triplets with an Two cumulene carbenes, H2C5 and H2C6, were detected in a supersonic molecular beam intensity ratio of approximately 3:2:3 and a by Fourier transform microwave spectroscopy. Their rotational and leading centrifugal splitting of about 60.1% of the frequency distortion constants were determined with high accuracy, such that the entire radio (for example, 617 MHz at 13.4 GHz for spectrum can now be calculated. Like the known carbenes H2C3 and H2C4, both H2C6). The relative intensities are those molecules have singlet electronic ground states and linear carbon-chain backbones. expected at low temperature for a slightly They can be produced in sufficiently high concentrations in the laboratory that their asymmetric top with two equivalent H at- electronic spectra, expected to lie in the visible, should be readily detectable by laser oms, and the splitting is in quantitative spectroscopy. The microwave spectra of other, more exotic isomers may be detectable agreement with that calculated by scaling as well. from the rotational constants of H2C3 (7) and H2C4 (5). Because of ortho-para spin statistics, rotational levels where K (the projection of the rotational angular mo- Carbenes are highly reactive organic mol- establish the geometric and electronic mentum on the C2 symmetry axis) is 61, ecules with two nonbonded electrons local- structure of such C chain carbenes are es- which lie about 14 K above the K 5 0 levels ized on a single C atom. They are important sential if we are to understand their chem- in both carbenes (Fig. 2), are metastable. intermediates in terrestrial chemistry, and ical reactivities (4), but few have been stud- Therefore, they are populated in our molec- several have now been detected in the in- ied spectroscopically because of their high ular beam even though the rotational tem- terstellar gas or in circumstellar shells (1). reactivity. Although we have recently de- perature is only about 3 K. Of particular interest in both combustion tected long hydrocarbon radicals and closed We determined spectroscopic constants processes and astrochemistry are the cu- shell cyanopolyynes (for example, C11H by fitting a theoretical spectrum calculated mulene carbene chains H2C¢(C¢)nC:, and HC13N), the longest cumulene carbene from a standard asymmetric top Hamiltoni- which may be important building blocks in known to date is H2CCCC (5). an (8) to the observed frequencies. Two the syntheses of long hydrocarbons and By using Fourier transform microwave rotational constants (B and C) and two pure C chains implicated in the formation (FTM) spectroscopy (6) of a supersonic mo- centrifugal distortion constants (DJ and of fullerenes (2). Like organic dyes, cu- lecular beam (Fig. 1), we have now detected DJK) were obtained for each carbene (Table mulene carbenes are likely to have intense the next two members of the cumulene 1). Transition frequencies (nJ4J21) for the optical electronic transitions, which may carbene sequence, H2C5 (H2C¢C¢C¢C¢C:, K 5 0 and the two K 561 ladders of each provide a powerful and convenient way to pentatetraenylidene) and H2C6 (H2C¢C¢ can be calculated from the approximate monitor combustion processes, and they are C¢C¢C¢C:, hexapentaenylidene). Nine a- expression promising candidates for carriers of the in- terstellar optical diffuse bands, the identifi- Table 1. Spectroscopic constants of H2C5 and H2C6 (in megahertz). These rotational and centrifugal cation of which constitutes one of the out- distortion constants are from a least squares fit of Watson’s S-reduced Hamiltonian. The root mean standing unsolved problems in astronomical square (3 kHz) of the fits is comparable to the measurement uncertainties. Uncertainties (1s) are in units spectroscopy (3). Gas phase studies that of the last significant digit. M. C. McCarthy, M. J. Travers, P. Thaddeus, Division of H2C5 H2C6 Engineering and Applied Sciences, Harvard University, Constant 29 Oxford Street, Cambridge, MA 02138, USA, and Har- Laboratory Expected* Laboratory Expected* vard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA. A 277,600† 287,600 268,400† 286,100 A. Kova´ cs and C. A. Gottlieb, Division of Engineering and B 2,304.7844(3) 2,304(5) 1,348.0891(1) 1,345(2) Applied Sciences, Harvard University, 29 Oxford Street, C 2,285.8053(3) 2,285(5) 1,341.3519(1) 1,341(2) Cambridge, MA 02138, USA. 3 W. Chen and S. E. Novick, Department of Chemistry, DJ 3 10 0.104(6) 0.088‡ 0.0283(12) 0.0353§ Wesleyan University, Middletown, CT 06459, USA. DJK 0.0464(2) 0.0164(1) *To whom correspondence should be addressed. *From (10). †Derived assuming a planar structure, that is, 1/C 2 1/A 2 1/B 5 0. ‡From (27). §From (28). 518 SCIENCE z VOL. 275 z 24 JANUARY 1997 REPORTS 21 nJ4J21 5 an uncertainty of #1kms . The rotation- two equivalent bosons, each of spin 1 (12). al constants of H2C5 and H2C6 are within Both cumulene carbenes were observed 1 0.25% of those calculated from ab initio in singlet electronic states, the same spin ~B 1 C! 6 ~B 2 C!d 2 2D K2 J F 2 K,1 JK G equilibrium geometries of Maluendes and multiplicity as found for the shorter chains 2 McLean (10) and high level (coupled clus H C (7) and H C (5). The rotational ~B 2 C! - - 2 3 2 4 3 2 4DJ 1 J (1) ter theory) equilibrium geometries of spectra of H2C5 and H2C6 are consistent H c@A 2 ~B 1 C!/2#J Botschwina (11). The distortion constants with linear C backbones, and, although DJ are in good agreement with those of small departures from linearity are difficult where dK,1 is a Kronecker delta and c (a other species with similar structure and size. to detect spectroscopically, in the present numerical constant) is 32 for K 561 and 8 Further evidence for the identifications of case such departures must be small. The for K 5 0. This expression contains only the molecules was obtained in a deuterium- the leading terms in the power series expan- enriched discharge, where the rotational sion of Polo (9). The frequencies agree with spectra of D2C5 (Fig. 3) and D2C6 were those calculated from the truncated asym- observed. Their rotational constants are metric top Hamiltonian to within 0.4 km those predicted by scaling from the normal s21 up to 300 GHz; at that frequency, ne- species, and the components of the rota- glect of the higher order centrifugal distor- tional triplets now have an intensity ratio of tion terms in the Hamiltonian introduces 1:4:1, as expected for a molecule having Fig. 2. Rotational energy levels of H2C5 and H2C6. The arrows indicate measured transitions. The en- ergy scale is given in both frequency (E/h, left) and temperature (E/k, right), where E is energy, h is the Planck constant, and k is the Boltzmann constant.
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