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Microwave Spectra of 11 Polyyne Carbon Chains M THE ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES, 129:611È623, 2000 August ( 2000. The American Astronomical Society. All rights reserved. Printed in U.S.A. MICROWAVE SPECTRA OF 11 POLYYNE CARBON CHAINS M. C. MCCARTHY,W.CHEN,M.J.TRAVERS, AND P. THADDEUS Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138; and Division of Engineering and Applied Sciences, Harvard University, 29 Oxford Street, Cambridge, MA 02138 Received 2000 January 21; accepted 2000 March 9 ABSTRACT A summary is given of the laboratory study of the rotational spectra of 11 recently detected carbon chain molecules. ClassiÐed according to their various end groups, these are the cyanopolyynes HC N ¹ 15 andHC17N; the isocyanopolyynes HC4NC and HC6NC; the methylcyanopolyynes CH3(C C)3CN, CH (C¹C) CN,and CH (C¹C) CN; and the methylpolyynes CH (C¹C) H, CH (C¹C) H, 3 ¹ 4 ¹3 5 3 4 3 5 CH3(C C)6H,and CH3(C C)7H. Measured line frequencies and derived spectroscopic constants are given for each. The microwave laboratory astrophysics of the entire set is now complete in the sense that all the astronomically most interesting rotational transitions, including those with nitrogen quadrupole hyperÐne structure, have now been directly measured or can be calculated from the derived constants to a small fraction of 1 km s~1 in equivalent radial velocity. All 11 carbon chains are candidates for astron- omical discovery since they are closely related in structure and composition to ones that have already been discovered in space. Subject headings: ISM: molecules È line: identiÐcation È molecular data È molecular processes È radio lines: ISM 1. INTRODUCTION where J is the angular momentum quantum number for the upper level of the transition, K is that for the component of Highly unsaturated polyynes with alternating triple and angular momentum along the symmetry axis, and B, D, and single carbon-carbon bonds and cumulenes with successive D are the rotational and two leading centrifugal distor- double carbon-carbon bonds represent the dominant struc- JK tion constants. For the molecules here,DJK is nonzero only tural theme of the nearly 100 polyatomic molecules so far for the symmetric tops with methyl terminations. The identiÐed in space, and many more can probably be found derived spectroscopic constants of each polyyne are sum- once rest frequencies have been measured in the laboratory. marized in Table 2. ForHC4NC and the two shortest In a series of short papers we recently reported the detection methylcyanopolyynes, line centroids can be calculated from and spectroscopic characterization of the 11 new long the rotational and centrifugal distortion constants in Table polyyne chains shown in Figure 1. All are calculated to be 2 and the nitrogen hfs from standard expressions for the highly polar, and all are candidates for astronomical detec- hyperÐne energies and intensities (Townes & Schawlow tion because shorter chains of similar structure have already 1955). been identiÐed in space in sources such as Sgr B2, the ] As in a previous Supplement article on carbon chains molecular shell of the evolved carbon star IRC 10216, or (McCarthy et al. 1997b), nothing is added here with respect the cold molecule-rich source TMC-1 in the Taurus cloud to the identiÐcations of the molecules in question. That was complex. With reÐnements in radio receivers and the avail- a crucial consideration in our original discovery papers, and ability of larger and more powerful telescopes and arrays, no new information has come to light to cause us to ques- many, if not all, may eventually be found. The purpose of tion any of the identiÐcations; the original assignments will this Supplement article is to provide in one place a concise therefore be assumed without further discussion. We have and useful summary of our laboratory results, including also omitted much in the way of experimental details, tabulations of measured line frequencies and derived spec- except for those reÐnements in the Fourier transform troscopic constants not contained in the brief previous microwave (FTM) spectrometer (Appendix A) and the dis- accounts of the present work. Table 1 is a brief overview, charge nozzle source (Appendix B) that were important for giving a summary of the laboratory references, the pro- the present work. We conclude with some general obser- duction methods employed, and the frequency bands that vations about the whole set of newly discovered carbon have been covered in the laboratory. chains. As Figure 1 shows, the 11 polyynes are similar in struc- ture, di†ering only in the end groups that terminate their 2. CYANOPOLYYNES carbon chain backbones; they are (1) cyanopolyynes, (2) CyanopolyynesHC CN are the most readily observed isocyanopolyynes, (3) methylcyanopolyynes, and (4) methyl- 2n and the most numerous class of carbon chains in space. polyynes. All are closed-shell molecules with fairly simple Those up toHC N have been detected in at least one rotational spectra characterized by transitions that are 11 astronomical source (Bell et al. 1997), and rest frequencies separated by harmonic intervals. In the absence of nitrogen for the next longer one,HC N, are available (Travers et al. quadrupole hyperÐne structure (hfs), the radio spectra of all 13 1996). With improvements in production efficiency and can be calculated to high precision from the standard detection sensitivity, we have now detected the rotational expression for the rotational transitions of a symmetric top spectra of the next two in the series,HC NHC and N. molecule: 15 17 The main reÐnements we have made in our production \ [ 3[ 2 lJ?J~1 2BJ 4DJ 2DJK JK , (1) scheme are the use of diacetylene(HC4H) as a precursor gas 611 612 MCCARTHY ET AL. Vol. 129 FIG. 1.ÈMolecular geometries of the present carbon chains, showing the characteristic alternating triple and single carbon-carbon bonds of the polyynes. and Ne as a bu†er gas. When cyanoacetylene(HC3N) is for optimal production ofHC15NHC and17N are similar used in combination with diacetylene rather than acetylene, to those now used forHC13N and shorter cyanopolyynes: a line intensities ofHC11NHC and13N increase by a factor of mixture of 0.5% of cyanoacetylene and 0.5% diacetylene in about 4, and when Ne is used instead of Ar as the bu†er gas Ne, a discharge in the throat of the supersonic nozzle of they increase by another factor of about 2. The conditions about 1900 V and 50 mA, a gas pulse of 300 ks length at a No. 2, 2000 SPECTRA OF 11 POLYYNE CARBON CHAINS 613 TABLE 1 SUMMARY OF PRESENT POLYYNE DETECTIONS Frequency Band Molecule Precursor Gasesa (GHz) Reference Cyanopolyynes: HC15N................. HC3N/HC4H5È11 1 HC17N................. HC3N/HC4H5È71 Isocyanopolyynes: CH3C2CN/HC4H, HC3N, HC3N/HC4H, HC4NC................ or (CN)2/HC4H8È20 2 CH3C2CN/HC4H, HC3N, HC3N/HC4H, HC6NC................ or (CN)2/HC4H10È18 2 Methylcyanopolyynes: ¹ b CH3(C C)2CN ...... CH3C2CN 6È18 3 ¹ CH3(C C)3CN....... CH3C2CN 6È22 3 ¹ CH3(C C)4CN....... CH3C2CN 8È14 3 CH3C2CN/HC4H ¹ CH3(C C)5CN....... orHC3N/HC4H6È12 3 Methylpolyynes: ¹ CH3(C C)4H......... CH3C2H/HC4H9È16 4 ¹ CH3(C C)5H......... CH3C2H/HC4H8È11 4 ¹ CH3(C C)6H......... CH3C4H/HC4H5È11 5 ¹ CH3(C C)7H......... CH3C4H/HC4H5È85 a All precursor gases diluted in Ne. b Previously studied by Alexander et al. 1978. REFERENCES.È(1) McCarthy et al. 1998a; (2) Botschwina et al. 1998; (3) Chen et al. 1998; (4) Travers et al. 1998; (5) Chen et al. 1999. repetition rate of 6 Hz, and a total gas pressure behind the the energy level diagram in Figure 3, the measured lines nozzle of 2.5 atm. cover a fairly wide range of rotational levels, with J ] J [ 1 from J \ 38 to 71. There is no evidence for quadrupole hfs 2.1. HC15N from the 14N nucleus in ourHC N spectra, and none is 15 2 Eighteen lines ofHC15N were measured to an accuracy expected: hyperÐne splittings are of order eqQ/4J and are of 0.1 km s~1 between 5 and 11 GHz (Table 3). Although therefore less than 1 kHz for the lowest observed J tran- [ HC15NHCis somewhat less abundant than13N in our sitions on the assumption that eqQ B 4.3 MHz, the value molecular beam (by a factor of about 3), integrations of only for the shorter cyanopolyynes,HC3N (La†erty & Lovas 10È20 minutes yielded lines with high signal-to-noise ratio, 1978),HC5N (Winnewisser, Creswell, & Winnewisser 1978), as the sample line in Figure 2a demonstrates. As shown in andHC7N (McCarthy et al. 2000). TABLE 2 SPECTROSCOPIC CONSTANTS (IN MHz) ] 6 ] 3 Molecule BD10 DJK 10 eqQ Cyanopolyynes: HC15N ..................... 71.950133(6) 0.0369(9) HC17N ..................... 50.70323(6) 0.025(7) Isocyanopolyynes: HC4NC .................... 1401.18227(7) 34.3(9) 0.96(2) HC6NC .................... 582.5203(1) 5.4(3) \1.0 Methylcyanopolyynes:a ...... ¹ b [ CH3(C C)2CN .......... 778.03974(4) 9.2(2) 4.37(2) 4.25(3) ¹ [ CH3(C C)3CN ........... 374.72127(1) 1.61(2) 1.38(1) 4.2(1) ¹ CH3(C C)4CN ........... 208.73699(2) 0.422(9) 0.543(8) ¹ CH3(C C)5CN ........... 128.0723(2) 0.1665(6) 0.21(1) Methylpolyynes: a ............ ¹ CH3(C C)4H ............. 376.71252(2) 1.55(2) 1.382(9) ¹ CH3(C C)5H ............. 210.23883(3) 0.46(3) 0.566(8) ¹ CH3(C C)6H ............. 129.07609(2) 0.134(6) 0.25(1) ¹ c CH3(C C)7H ............. 84.86220(3) 0.05 NOTE.ÈUncertainties (in parentheses) are 1 p in the last signiÐcant digit. a Constants derived on the assumption that the A rotational constant is 157 GHz. b Previously studied by Alexander et al. 1978. c ¹ Scaled from CH3(C C)6H.
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