Microwave Spectra of 11 Polyyne Carbon Chains M

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 astronomical 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 structure, 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. TABLE 3 EASURED OTATIONAL RANSITIONS OF AND M R T HC15NHC17N

HC15NHC17N

Frequency O[C Frequency O[C J@ ] J (MHz) (kHz) (MHz) (kHz)

38 ] 37...... 5468.202 0 39 ] 38...... 5612.102 0 40 ] 39...... 5756.001 0 41 ] 40...... 5899.901 0 42 ] 41...... 6043.800 0 43 ] 42...... 6187.700 0 44 ] 43...... 6331.599 0 45 ] 44...... 6475.499 0 46 ] 45...... 6619.398 0 48 ] 47...... 6907.196 [1 49 ] 48...... 7051.096 0 50 ] 49...... 7194.995 0 51 ] 50...... 7338.894 0 55 ] 54...... 5577.339 0 59 ] 58...... 5982.960 0 60 ] 59...... 6084.366 0 61 ] 60...... 6185.770 [1 62 ] 61...... 6287.178 1 63 ] 62...... 6388.582 0 64 ] 63...... 9209.578 0 6489.984 [2 65 ] 64...... 9353.477 0 6591.391 [1 66 ] 65...... 9497.375 0 6692.800 2 70 ] 69...... 10072.968 0 71 ] 70...... 10216.866 0

NOTES.ÈEstimated experimental uncertainties are 2 kHz. The di†erence between observed and calculated frequencies (O[C) are with respect to those calculated from the best-Ðt constants in Table 2.

TABLE 4 EASURED OTATIONAL RANSITIONS OF AND M R T HC4NC HC6NC

HC4NC HC6NC

Frequency O[C Frequency O[C J@ ] JF@ ] F (MHz) (kHz) (MHz) (kHz)

3 ] 2 ..... 4] 3 8407.078 0 3 ] 2 8407.090 0 2 ] 1 8407.138 0 4 ] 3 ..... 5] 4 11209.442 0 4 ] 3 11209.450 1 3 ] 2 11209.468 [2 5 ] 4 ..... 6] 5 14011.800 0 5 ] 4 14011.806 0 4 ] 3 14011.819 2 6 ] 5 ..... 7] 6 16814.153 [1 6 ] 5 16814.157 [1 5 ] 4 16814.166 1 7 ] 6 ..... 8] 7 19616.502 0 7 ] 6 19616.504 [1 6 ] 5 19616.510 0 9 ] 8 ..... 10485.350 0 10 ] 9 .... 11650.385 1 ] 11 10... 12815.420 2 FIG. 2.ÈSample FTM spectra. (a) Spectrum of the J \ 45 ] 44 of 12 ] 11... 13980.447 [3 HC15N showing the characteristic double-peaked line shape that results 13 ] 12... 15145.480 0 from the Doppler splitting of the Mach 2 supersonic molecular beam 14 ] 13... 16310.508 [1 relative to the two traveling waves that compose the confocal mode of the ] \ ] 15 14... 17475.538 2 Fabry-Perot cavity. (b) The J 4 3 transition ofHC4NC showing resolved nitrogen quadrupole hyperÐne structure. (c) The J \ 15 ] 14 OTES [ ¹ N .ÈEstimated experimental uncertainties are 1 kHz. O C the transition ofCH3(C C)4H showing the distinctive, tightly spaced K same as in Table 3. doublet rotational pattern of the methylsymmetric top molecules. SPECTRA OF 11 POLYYNE CARBON CHAINS 615

closeÈwithin about a factor of 2Èto the detection limit of the spectrometer, requiring detection integration times of about 1 hr each. Relative to the shorter cyanopolyynes, however,HC17N is surprisingly abundant in our supersonic beamÈan order of magnitude more so than predicted by extrapolation from the shorter cyanopolyynes HC3N throughHC9N. Its measured lines (Table 3) are well repro- duced by the expression in equation (1), but owing to the narrow range of transitions from J \ 55 to 66 (Fig. 3), the centrifugal distortion constant could only be determined to about 30%. 3. ISOCYANOPOLYYNES Although the cyanopolyynes have been the subject of much laboratory and radioastronomical study, relatively little is known about their isomers, the isocyanopolyynes. Until now, only the two shortest, HNC and HCCNC, have been detected in laboratory discharges (Saykally et al. 1976; Kru ger et al. 1991) and in astronomical sources (Snyder & Buhl 1971; Kawaguchi et al. 1992). The presence of these energetic isomers in interstellar clouds is important because it provides a striking example of how far chemical processes there can depart from thermal equilibriumÈand a good illustration as well of the crucial role ion-molecule reactions play in the formation of interstellar molecules (Watson 1974; Herbst 1978). By careful adjustment of the laboratory discharge chemistry for HCCNC, we have been able to detect the next two isocyanopolyynesHC4NC and HC6NC with our FTM spectrometer. Astronomical detection of these polar isomers may help clarify the role of speciÐc ion-molecule reactions in the formation of longer cyanopolyyne and isocyanopolyyne chains. The most intense lines ofHC4NC and HC6NC were produced under nearly the same experimental conditions that yielded the best cyanopolyyne spectra. As shown in Table 1, these isochains were made with three di†erent mixtures of precursor gases, each yielding comparably strong lines. The isospectra at best were more than 100 times weaker than those of the more stable cyanopolyynes of the same size; they are slightly more complicated than those of the longer cyanopolyynes in this paper, owing to their shorter lengths, the comparatively low-J transitions studied, and the resolution of nitrogen hfs. The derived rotational, centrifugal distortion, and hyperÐne constants are given in Table 2.

IG 3.1. HC NC F . 3.ÈLower rotational levels ofHC15NHC and17N, showing the 4 transitions measured in the laboratory (vertical bars). The rotational transitions ofHC4NC, one of which is shown in Figure 2, have well-resolved quadrupole hfs over much of the frequency range accessible to our spectrometer; The rotational constant and the leading centrifugal dis- the energy level diagram in Figure 4 shows the Ðve tran- tortion constant were determined by Ðtting the two free sitions measured (Table 4). The quadrupole coupling con- \ parameters in equation (1) to the measured lines (Table 3), stant of HC4NC, eqQ 0.96(2) MHz, is about 4 times yielding a Ðt rms comparable to the measurement uncer- smaller than that of the cyanopolyynes and is of opposite tainty of about 2 kHz. In the frequency range measured, the sign, but it is close to that measured in HCCNC, next term H in the centrifugal expansion is expected to eqQ \ 0.945(1) MHz(Kru ger, Stahl, & Dreizler 1993). make a negligible contribution (of order 1 Hz or less). Only From the spectroscopic constants derived from the labor- at very high rotational transitions, where J approaches atory data (Table 2), the radio spectrum ofHC4NC can be 1000, will neglect of this term result in an error as large as calculated to better than 3 ppm (1 km s~1 in equivalent 1kms~1 in equivalent radial velocities. radial velocity) up to 100 GHz in frequency.

2.2. HC17N 3.2. HC6NC For this cyanopolyyne, nine lines were measured in the The energy level diagram in Figure 4 also shows the frequency range from 5 to 7 GHz; they are systematically seven rotational transitions of the next longer iso- about 3 times weaker than those ofHC15N and are fairly cyanopolyyne,HC6NC, that have been measured from 10 616 MCCARTHY ET AL.

TABLE 5 EASURED OTATIONAL RANSITIONS OF ¹ M R T CH3(C C)2CN Frequency O[C J@ ] JKF@ ] F (MHz) (kHz)

4 ] 3 ...... 1 4] 3 6224.175 1 03] 2 6224.223 [1 4 ] 3 6224.315 [1 5 ] 4 6224.348 0 5 ] 4 ...... 1 5] 4 7780.296 0 04] 3 7780.341 [1 5 ] 4 7780.394 1 6 ] 5 7780.414 [1 6 ] 5 ...... 1 6] 5 9336.385 [1 05] 4 9336.437 0 6 ] 5 9336.468 [1 7 ] 6 9336.485 0 7 ] 6 ...... 1 7] 6 10892.464 0 6 ] 5 10892.476 2 8 ] 7 10892.500 [2 06] 5 10892.522 1 7 ] 6 10892.544 0 8 ] 7 10892.558 2 8 ] 7 ...... 1 8] 7 12448.535 1 7 ] 6 12448.540 1 9 ] 8 12448.560 [2 07] 6 12448.601 0 8 ] 7 12448.617 0 9 ] 8 12448.627 0 9 ] 8 ...... 1 9] 8 14004.601 0 8 ] 7 14004.605 2 10 ] 9 14004.621 0 08] 7 14004.677 1 9 ] 8 14004.686 [3 IG F . 4.ÈLower rotational levels ofHC4NC and HC6NC showing the 10 ] 9 14004.697 1 transitions measured in the laboratory (vertical bars). 10 ] 9 ...... 1 10] 9 15560.665 1 9 ] 8 15560.665 0 11 ] 10 15560.678 [2 to 18 GHz (Table 4). Even at our high resolution, a precise 09] 8 15560.749 1 value for eqQ could not be determined, but the evident 10 ] 9 15560.757 [1 broadening of the rotational lines is consistent with a coup- 11 ] 10 15560.766 1 ling constant eqQ \ 1.0 MHz. The lines ofHC6NC are 11 ] 10...... 1 10] 9 17116.724 [1 about 20 times weaker than those ofHC4NC, a decrement 11 ] 10 17116.724 0 close to that observed for the cyanopolyyne series from 12 ] 11 17116.737 1 010] 9 17116.816 0 HC5NHCto7N. Detection of HC4NC and HC6NC in space may be difficult, but searches should be aided by the 11 ] 10 17116.825 0 ] high polarity of these chains, calculated (Botschwina et al. 12 11 17116.832 0 1998) to be [3.25 and [3.49 D, respectively. NOTES.ÈEstimated experimental uncertainties are 1 kHz. O[C the same as in Table 3. 4. METHYLCYANOPOLYYNES The shorter methylcyanopolyynesCH (C¹C)CN and ¹ 3 \ CH3(C C)2CN are readily observable in the laboratory the ortho levels in our rotationally cold(Trot 2.5 K) molec- and have been found in the astronomical source TMC-1 ular beam. Because very sharp lines only 5 kHz wide are (Matthews & Sears 1983; Broten et al. 1984). Owing to the routinely achieved in our experiments, it has also been pos- lack of laboratory data, longer chains have not yet been sible to resolve the nitrogen hfs in the lowest rotational ¹ ¹ found in space, but those up toCH3(C C)4CN have been transitions ofCH3(C C)2CN (Fig. 5) and determine included in models of interstellar clouds (Herbst & (Table 5) the quadrupole coupling constant eqQ. The Leung 1989), where they are predicted to exist in appre- derived rotational and centrifugal distortion constants are ciable quantities. Methylcyanopolyynes have a distinctive in good agreement with those previously reported by Alex- rotational signature at low rotational temperature, speciÐ- ander et al. (1978). cally, doublets produced by the closely spaced rotational The three new methylcyanopolyynes were produced by a transitions from the K \ 0 and K \ 1 levels (see Fig. 2c). low-current 1300 V discharge synchronized with a 440 ks The ortho-para nuclear spin statistics of these symmetric long gas pulse at a stagnation pressure of 2 atm. The gas tops with three equivalent methyl group protons forbid mixture consisted of dilute (0.5%) methylcyanoacetylene in ¹ radiative transitions between levels of di†erent spin sym- Ne for the chains up toCH3(C C)4CN and a mixture of metry. As a result, the K \ 1 para levels, which lie over 7 K methylcyanoacetylene and diacetylene (0.5% each) in Ne for \ ¹ above the K 0 ortho levels, are comparably populated to CH3(C C)5CN. For all three, somewhat weaker signals TABLE 6 TABLE 6ÈContinued MEASURED ROTATIONAL TRANSITIONS OF CH (C¹C) CN 3 3 Frequency O[C ] ] Frequency O[C J@ JKF@ F (MHz) (kHz) J@ ] JKF@ ] F (MHz) (kHz) 19 ] 18 14239.364 0 ] [ 9 ] 8 ...... 1 9] 8 6744.944 0 20 19 14239.364 2 ] ] 8 ] 7 6744.946 [1 20 19...... 1 19 18 14988.744 1 ] 10 ] 9 6744.965 0 20 19 14988.744 0 ] [ 08] 7 6744.966 0 21 20 14988.744 3 ] 9 ] 8 6744.978 0 01918 14988.799 2 ] 10 ] 9 6744.986 0 20 19 14988.799 0 ] [ 10 ] 9 ...... 1 10] 9 7494.388 3 21 20 14988.799 2 ] ] 9 ] 8 7494.389 4 21 20...... 1 20 19 15738.175 1 ] 11 ] 10 7494.403 2 21 20 15738.175 0 ] [ 09] 8 7494.410 1 22 21 15738.175 2 ] 10 ] 9 7494.420 1 02019 15738.234 2 ] 11 ] 10 7494.424 [1 21 20 15738.234 0 ] [ 11 ] 10...... 1 10] 9 8243.823 [1 22 21 15738.234 2 ] ] 11 ] 10 8243.824 0 22 21...... 1 21 20 16487.606 1 ] 12 ] 11 8243.837 1 22 21 16487.606 0 ] [ 010] 9 8243.851 0 23 22 16487.606 3 ] 11 ] 10 8243.859 [1 02120 16487.668 2 ] 12 ] 11 8243.865 0 22 21 16487.668 0 ] [ 12 ] 11...... 1 11] 10 8993.259 [3 23 22 16487.668 1 ] ] 12 ] 11 8993.261 [1 23 22...... 1 22 21 17237.036 1 ] 13 ] 12 8993.273 0 23 22 17237.036 0 ] [ 011] 10 8993.293 1 24 23 17237.036 2 ] 12 ] 11 8993.298 [1 02221 17237.100 2 ] 13 ] 12 8993.304 0 23 22 17237.100 0 ] [ 13 ] 12...... 1 12] 11 9742.697 [2 24 23 17237.100 1 ] ] 13 ] 12 9742.699 [1 24 23...... 1 23 22 17986.465 1 ] 14 ] 13 9742.708 0 24 23 17986.465 0 ] [ 012] 11 9742.734 1 25 24 17986.465 2 ] 13 ] 12 9742.737 [2 02322 17986.532 2 ] 14 ] 13 9742.743 0 24 23 17986.532 1 ] [ 14 ] 13...... 1 13] 12 10492.135 [1 25 24 17986.532 1 ] ] 14 ] 13 10492.137 0 25 24...... 1 24 23 18735.893 1 ] 15 ] 14 10492.143 0 25 24 18735.893 0 ] [ 013] 12 10492.174 1 26 25 18735.893 2 ] 14 ] 13 10492.176 [1 02423 18735.964 2 ] 15 ] 14 10492.182 1 25 24 18735.964 1 ] 15 ] 14...... 1 14] 13 11241.573 1 26 25 18735.964 0 ] ] 15 ] 14 11241.573 0 26 25...... 1 25 24 19485.321 1 ] 16 ] 15 11241.578 [1 26 25 19485.321 0 ] [ 014] 13 11241.612 [1 27 26 19485.321 2 ] 15 ] 14 11241.619 2 02524 19485.394 2 ] 16 ] 15 11241.619 [1 26 25 19485.394 2 ] 16 ] 15...... 1 15] 14 11991.008 1 27 26 19485.394 0 ] ] 16 ] 15 11991.008 0 27 26...... 1 26 25 20234.748 2 ] 17 ] 16 11991.012 [1 27 26 20234.748 1 ] 015] 14 11991.051 0 28 27 20234.748 0 ] 16 ] 15 11991.056 2 02625 20234.822 2 ] 17 ] 16 11991.056 [1 27 26 20234.822 0 ] 17 ] 16...... 1 16] 15 12740.443 1 28 27 20234.822 0 ] ] 17 ] 16 12740.443 0 28 27...... 1 27 26 20984.174 3 ] 18 ] 17 12740.446 [1 28 27 20984.174 2 ] 016] 15 12740.489 1 29 28 20984.174 1 ] 17 ] 16 12740.493 2 02726 20984.251 2 ] 18 ] 17 12740.493 [1 28 27 20984.251 1 ] 18 ] 17...... 1 17] 16 13489.877 1 29 28 20984.251 0 ] ] 18 ] 17 13489.877 0 29 28...... 1 28 27 21733.596 0 ] 19 ] 18 13489.877 [4 29 28 21733.596 0 ] [ 017] 16 13489.928 2 30 29 21733.596 2 ] 18 ] 17 13489.928 0 02827 21733.678 2 ] 19 ] 18 13489.928 [2 29 28 21733.678 2 ] 19 ] 18...... 1 18] 17 14239.311 1 30 29 21733.678 1 19 ] 18 14239.311 0 NOTES.ÈEstimated experimental uncertainties are 1 kHz. ] [ 20 19 14239.311 3 O[C the same as in Table 3. 018] 17 14239.364 2 618 MCCARTHY ET AL. Vol. 129

IG ¹ ¹ F . 5.ÈLower rotational levels ofCH3(C C)2CN and CH3(C C)3CN showing the transitions measured in the laboratory (vertical bars) were also observed with a mixture of diacetylene and cyano- sitions. The characteristic K-type doublets, however, were acetylene (1% each) in Ar or Ne. well resolved in all 10 transitions of the former and the 11 transitions of the latter. The measured laboratory fre- ¹ 4.1. CH3(C C)3CN quencies of both molecules between 6 and 14 GHz are given ¹ As Figure 5 shows, 21 transitions of CH3(C C)3CN in Table 7. Precise values for the rotational and centrifugal were measured between 6 and 22 GHz; the measured fre- distortion constants (Table 2) were derived from the labor- quencies are given in Table 6. Most rotational transitions of atory data, and the entire rotational spectra of both mol- ¹ CH3(C C)3CN are a complex blend of lines because the ecules can be predicted again to better than 3 ppm over the splittings from the nitrogen hfs, the K doublets, and the range of interest to radio astronomers. We are unaware of instrumental Doppler doublets are all comparable. At high any ab initio calculations of the dipole moments for the J, however, the nitrogen hfs has largely collapsed and the longer chains, but both should be fairly polar molecules; spectrum has simpliÐed sufficiently to allow the K-type extrapolation from the shorter ones (Bester et al. 1984; doublets to be assigned without ambiguity. For the lower J Arnau et al. 1990) suggests the dipole moments lie in the lines, where assignments and line positions were more diffi- range 5È7D. cult to determine a priori, precise frequencies for the With the addition of successiveC2 units to the carbon hyperÐne-split transitions were obtained by directly Ðtting chain backbone, the line strengths of the longer methyl- the power spectrum in the frequency domain (Haekel & cyanopolyynes typically decrease by a factor of about 7È10. Ma der 1988) using the spectroscopic constants determined A similar decrement in line intensity is observed on from the high-frequency data and an initial estimate of the ascending the series of shorter cyanopolyynes to HC9N nitrogen eqQ of [4.25 MHzÈthat found for (McCarthy et al. 1997b). Under the best conditions, lines of CH (C¹C) CN. A good Ðt with an rms of only 1 kHz was CH (C¹C) CN were fairly easy to detect, but those of 3 2 3 ¹ 4 achieved by this procedure, allowing determination of CH3(C C)5CN were close to the limit of sensitivity, the four spectroscopic constants (B,DJ, DJK, and eqQ)in requiring cooling of the spectrometer and integrations of Table 2. about 1 hr to achieve a signal-to-noise ratio of about 5. ¹ ¹ METHYLPOLYYNES 4.2. CH3(C C)4CNand CH3(C C)5CN 5. Nitrogen hfs was not resolved forCH (C¹C) CN and Much less polar than the carbon chains here with termin- ¹ 3 4 CH3(C C)5CN owing to the high J of the observed tran- al CN groups, the methylpolyynes are nonetheless of No. 2, 2000 SPECTRA OF 11 POLYYNE CARBON CHAINS 619

TABLE 7 TABLE 8 EASURED OTATIONAL RANSITIONS OF¹ AND EASURED OTATIONAL RANSITIONS OF¹ AND M R T CH3(C C)4CN M R T CH3(C C)4H ¹ ¹ CH3(C C)5CN CH3(C C)5H ¹ ¹ ¹ ¹ CH3(C C)4CN CH3(C C)5CN CH3(C C)4HCH3(C C)5H

Frequency O[C Frequency O[C Frequency O[C Frequency O[C J@ ] JK(MHz) (kHz) (MHz) (kHz) J@ ] JK(MHz) (kHz) (MHz) (kHz)

21 ] 20...... 1 8766.915 0 12 ] 11...... 1 9041.056 [1 0 8766.938 0 0 9041.089 [1 22 ] 21...... 1 9184.387 2 13 ] 12...... 1 9794.475 [1 0 9184.408 [1 0 9794.513 1 23 ] 22...... 1 9601.855 [1 14 ] 13...... 1 10547.895 0 0 9601.880 [1 0 10547.933 0 24 ] 23...... 1 10019.327 1 6147.447 [3 15 ] 14...... 1 11301.313 0 0 10019.353 1 6147.460 0 0 11301.355 0 25 ] 24...... 1 10436.795 [1 6403.592 [1 16 ] 15...... 1 12054.731 0 0 10436.823 0 6403.603 [1 0 12054.776 1 26 ] 25...... 1 6659.733 [3 19 ] 18...... 1 14314.981 0 0 6659.747 0 0 14315.032 [1 29 ] 28...... 1 12106.671 [1 20 ] 19...... 1 15068.396 0 0 12106.706 2 0 15068.451 0 30 ] 29...... 1 12524.140 [1 21 ] 20...... 1 15821.810 0 8829.990 0 0 12524.172 [2 0 15821.868 0 8830.015 1 31 ] 30...... 1 12941.609 0 22 ] 21...... 1 9250.464 0 0 12941.644 1 0 9250.488 [1 32 ] 31...... 1 13359.078 1 8196.587 [3 23 ] 22...... 1 9670.938 0 0 13359.111 [1 8196.606 2 0 9670.963 [1 33 ] 32...... 1 13776.545 1 8452.732 0 24 ] 23...... 1 10091.411 0 0 13776.580 0 8452.747 1 0 10091.438 0 34 ] 33...... 1 8708.878 4 25 ] 24...... 1 10511.883 [1 0 8708.889 0 0 10511.913 0 35 ] 34...... 1 8965.017 1 26 ] 25...... 1 10932.358 1 0 8965.033 2 0 10932.387 0 36 ] 35...... 1 9221.159 1 OTES [ 0 9221.174 1 N .ÈEstimated experimental uncertainties are 1 kHz. O C the 44 ] 43...... 1 11270.289 4 same as in Table 3. 0 11270.306 2 methylcyanopolyynes, all four methylpolyynes here have ] [ 45 44...... 1 11526.424 1 readily identiÐable microwave spectra owing to the clearly 0 11526.442 [2 \ \ ] resolved, tightly spaced doublets from the K 0 and K 1 46 45...... 1 11782.565 0 ¹ 0 11782.581 [4 ladders. The lines ofCH3(C C)6H are observed with a signal-to-noise ratio of about 25 in only 10 minutes of inte- OTES [ ¹ N .ÈEstimated experimental uncertainties are 1 kHz. O C the gration, but those ofCH3(C C)7H are much fainterÈby a same as in Table 3. factor of about 10, the same decrement found for other symmetric top polyynes in our spectrometer when suc- astronomical interest because the Ðrst two, methylacetylene cessiveC units are added to the molecular backbone. [CH (C¹C)H]and methyldiacetylene [CH (C¹C) H], 2 3 3 2 Detection of the fainter lines required liquid nitrogen have been observed in several astronomical sources (Irvine cooling. et al. 1981; MacLeod, Avery, & Broten 1984; Walmsley et ¹ ¹ al. 1984). Precise rest frequencies are available for those up 5.1. CH3(C C)4HCHand 3(C C)5H ¹ toCH3(C C)3H (Alexander et al. 1978). Methylpolyynes A total of eight a-type rotational transitions of larger thanCH (C¹C) H have been included in chemical ¹ ¹ 3 3 CH3(C C)4HCHand six of3(C C)5H (Table 8) were models of interstellar clouds (Herbst & Leung 1989), and detected and analyzed in terms of the standard expression astronomical detection of some of these is presumably only for a symmetric top given in equation (1). In each Ðt the A a matter of sensitivity. rotational constant was Ðxed at 157 GHz; the rms of the Ðts As Table 1 shows, the rotational spectra of four methyl- is comparable to the measurement uncertainties of about ¹ \ polyynes CH (C C) H, n 4È7, have now been detected. 1 kHz. For both molecules the rotational and two leading 3\ n \ Those with n 4 and n 5 were detected Ðrst using a dis- quartic centrifugal distortion constants were determined to charge with a dilute mixture of methylacetylene and high accuracy. The derived constants, tabulated in Table 2, diacetylene in Ne (Travers et al. 1998). Typically, a dis- are in excellent agreement with those predicted by extrapo- charge potential of 1300 V in the throat of the nozzle and a lation from the shorter members of the series. gas pulse 200È350 ks long at a stagnation pressure behind ¹ ¹ the pulsed valve of 2 atm produced the strongest lines. 5.2. CH3(C C)6HCHand 3(C C)7H ¹ \ When methyldiacetylene was used instead of methyl- A total of 10 transitions ofCH3(C C)6H from J 22 to acetylene as a precursor, it was possible to extend this series 39 in the two lowest K ladders, and eight transitions of \ ¹ \ \ even further, detecting the next two chains with n 6 and CH3(C C)7H from J 32 to 44 in the K 0 ladder n \ 7 (Chen et al. 1999), shown in Figure 1. Like the long (Table 9), were detected between 5 and 11 GHz. Line fre- 620 MCCARTHY ET AL. Vol. 129

TABLE 9 1. For the molecules here (and by extension for carbon EASURED OTATIONAL RANSITIONS OF¹ AND M R T CH3(C C)6H chains generally) the laboratory astrophysics is well ahead ¹ CH3(C C)7H of the radio astronomy, allowing new astronomical mol- ¹ ¹ ecules to be found without searches in frequency that with CH3(C C)6HCH3(C C)7H large telescopes can be prohibitive in time and cost. As the Frequency O[C Frequency O[C astronomical detection ofHC11N demonstrates, any J@ ] JK(MHz) (kHz) (MHz) (kHz) carbon chain that can be detected in space can probably now be detected with our present microwave instrumen- 22 ] 21...... 1 5679.329 [2 tation, or a slight reÐnement of it. 0 5679.343 1 2. The rotational constants of the polyynes here are ] [ 23 12...... 1 5937.481 1 remarkably well predicted by a simple extrapolation in 0 5937.495 1 which a third-order polynomial in chain length is Ðtted to 24 ] 23...... 1 6195.631 [2 0 6195.645 0 the moments of inertia of the shorter chains in the sequence. 25 ] 24...... 1 6453.784 0 The predicted constants typically agree with those mea- 0 6453.798 2 sured to within a few parts in 105, so that at 5È8 GHz, where 26 ] 25...... 1 6711.934 0 most of these long chains have their most intense rotational \ 0 6711.948 1 transitions in our cold(Trot 2.5 K) molecular beam, a fre- 27 ] 26...... 1 6970.084 [1 quency search of a few MHz or less was required for detec- 0 6970.100 2 tion. The close agreement of extrapolation with experiment 32 ] 31...... 1 8260.838 2 suggests that the moment of inertia of long carbon chains is 0 8260.850 [2 5431.170 [4 ] quite insensitive to structural details such as the rotation- 33 32...... 1 8518.987 1 vibration interactions important in small molecules. Rota- 0 8519.001 [2 5600.902 4 34 ] 33...... 0 5770.620 [2 tional constants of still longer polyynes can probably be 35 ] 34...... 0 5940.341 [4 predicted with very high accuracy from extrapolation; some 36 ] 37...... 0 6110.072 3 predicted constants are given in Table 10. 38 ] 37...... 1 9809.735 1 3. The centrifugal distortion of the polyynes is well 0 9809.753 0 6449.522 6 described by a simple semiclassical theory (Thaddeus et al. 39 ] 38...... 1 10067.884 0 1998), which treats a carbon chain as a thin classical rod 0 10067.903 0 6619.235 [5 with a YoungÏs modulus that is independent of chain length. 44 ] 43...... 0 7467.858 2 As Figure 6 shows, D/B for the three types of polyyne chains NOTES.ÈEstimated experimental uncertainties are 1 kHz. O[C the here is closely proportional to the inverse fourth power of same as in Table 3. the chain length L , the same relation as that found for the acetylenic free radicalsCnH and the cumulene carbenes H2Cn. All carbon chains so far studied apparently distort quencies were again measured to an accuracy of 0.3 ppm or under rotation in this same simple way, regardless of the ¹ better for both molecules. ForCH3(C C)6H, the three spectroscopic constants B,D , andD were determined, ¹ J JK but forCH3(C C)7H only the rotational constant B (Table 2) could be extracted from the data because of the narrow range of transitions measured.

6. DISCUSSION Several general comments are suggested by the present work:

TABLE 10 PREDICTED CONSTANTS FOR LONGER POLYYNES (IN MHz)

Molecule B

Cyanopolyynes: HC19N ...... 37.069(2) HC21N ...... 27.919(3) HC23N ...... 21.550(4) Methylcyanopolyynes:a ...... ¹ CH3(C C)6CN ...... 84.193(4) ¹ CH3(C C)7CN ...... 58.293(4) Methylpolyynes:a ...... ¹ CH3(C C)8H ...... 58.747(4) ¹ CH3(C C)9H ...... 42.339(4) FIG. 6.ÈLog-log plot of D/B as a function of chain length L for the OTE N .ÈEstimated 1 p uncertainties are cyanopolyynesHC2n`1N (solid triangles), the methylcyanopolyynes given in parentheses. CH3C2nCN(open circles), and the methylpolyynes CH3C2nCH (solid a Constants derived on the assumption squares). The errors of log (D/B) are generally smaller than the size in the that the A rotational constant is 157 GHz. data points. No. 2, 2000 SPECTRA OF 11 POLYYNE CARBON CHAINS 621 end groups that terminate the carbon chain backbone, the type of carbon-carbon bonding, or the low-lying vibrational structure of the individual molecules. 4. As shown in Figure 7, the abundance of the long carbon chainsHC15NHC and17N in our laboratory discharge is only slightly less than that of the shorter members of the sequence, implying that still longer chains are being produced at similar concentrations in our discharge source and might be detected with slight improvements in instru- mentation. In the interstellar gas, as in our molecular beam, long chains may be more abundant than extrapolation from present radio observations suggest for two reasons: (a) the synthesis of carbon chains in space may be somewhat similar to that in the laboratory and (b) long chains as large as those detected here may behave in the interstellar space more like grains than more familiar smaller molecules and may be fairly stable against processes such as dissociative recombination and photodissociation which rapidly destroy small molecules in the di†use interstellar gas.

FIG. 7.ÈRelative intensity of the strongest rotational lines of the cyanopolyynes (solid circles) and their relative abundance (open circles)in We are indebted to E. S. Palmer for technical assistance the FTM spectrometer, as a function of the number of carbon atoms in the in the construction and maintenance of the FTM spectro- chain. The error bars are estimated 2 p uncertainties. Relative abundances are obtained from line intensities by taking into account the dependence meter, particularly with the microwave electronics, and we on the chain length of the rotational partition functions and dipole thank those colleagues who made partial contributions to moments. The limit of the detection sensitivity is approximately that some of the original papers: P. Botschwina, C. A. Gottlieb, achieved in a total observation time of 3 hr. A.Heyl, S. E. Novick, J.-U. Grabow, and M. R. Munrow.

APPENDIX A

THE COOLED FOURIER TRANSFORM MICROWAVE SPECTROMETER

The present Fourier transform microwave spectrometer shown in Figure 8 is an improved version of the instrument previously described by McCarthy et al. (1997a, 1997b). The sensitivity has been improved by more than an order of magnitude, cooling of the Fabry-Perot cavity and reÐnements to the microwave receiver (Grabow et al. 2000, in preparation) accounting for at least a factor of 5 of this increase. Improvements in production efficiency with the use of larger organic precursors such as cyanoacetylene, diacetylene, methylcyanoacetylene, and methyldiacetylene, and additional optimization of the geometry and electrical characteristics of the discharge nozzle (Appendix B), account for the remainder. The better sensitivity of the microwave receiver results from (1) the use of separate antennas for excitation and detection to eliminate lossy microwave components (i.e., circulators) and unnecessary cables in the receiver front end, (2) improvement in the coupling between the Ðrst-stage ampliÐer and the receiver antenna by mounting the ampliÐer directly behind the antenna inside the vacuum chamber, (3) better coupling of the receiver antenna to the microwave cavity to obtain a higher quality factor Q of the cavity over much of the centimeter-wave band, and (4) cooling the mirrors of the Fabry-Perot and Ðrst-stage ampliÐer to 77 K by liquid nitrogen cooling. Good coupling of the receiver antenna to the Fabry-Perot cavity is important because line intensities are sensitive to the cavity Q. The highest unloaded Q now achieved is 2 ] 105, which is within a factor of 2 of the theoretical limit of about 4 ] 105 set by the ohmic losses in the aluminum mirrors. When the cavity is critically coupled, the loadedQ is one-half the 5 L unloadedQ0, i.e., about 10 ÈsigniÐcantly higher than the loaded Q achieved before the present work. The higher cavity Q produces rotational lines stronger by a factor of 5 or more and was crucial in the detection of most of the polyynes here. Operation with liquid nitrogen reduces the system noise temperature by about a factor of 4, from 800 to 190 K. Above 10 GHz di†ractive losses in the open resonator are negligible, and roughly two-thirds (110 K) of the receiver noise is from the cold ampliÐer and one-third from the 77 K mirrors. Below 10 GHz di†raction from the open resonator contributes signiÐ- cantly to the cavity Q, and the system temperature rises to about 400 K; the modest twofold decrease in the noise level on cooling then results largely from the lower noise Ðgure of the cold ampliÐer. Cooling of the Fabry-Perot cavity and the Ðrst ampliÐer of the receiver was done fairly cheaply, without extensive redesign of the spectrometer. Each mirror is cooled separately by continuously Ñowing liquid nitrogen through a copper coil soldered to a copper disk making good thermal contact with the mirrorÏs back surface. Thin-walled stainless steel tubes connect the two copper coils to liquid nitrogen vacuum feedthroughs. These are coiled inside the vacuum chamber to be sufficiently 622 MCCARTHY ET AL. Vol. 129

Ñexible at 77 K to allow the cavity to be tuned by adjusting the separation of the mirrors. Thermal isolation of the mirrors is achieved by suspension on epoxy strips. The pulsed discharge nozzle is kept close to room temperature by (1) mounting the nozzle assembly on TeÑon stando†s that minimize contact with the cold mirror and (2) surrounding the nozzle with a copper insulating jacket heat sunk with copper braid to the warm walls of the vacuum chamber. Condensation of gas from the supersonic jet does not appreciably degrade the reÑectivity of the cold mirrors.

FIG. 8.ÈSimpliÐed schematic of the 77 K FTM spectrometer showing the Fabry-Perot cavity and an expanded view of the discharge nozzle source

FIG. 9.ÈPulsed discharge nozzle source showing the supersonic molecular beam, the reaction zone, and the TeÑon housing No. 2, 2000 SPECTRA OF 11 POLYYNE CARBON CHAINS 623

APPENDIX B

DISCHARGE NOZZLE FOR THE PRODUCTION OF LONG CARBON CHAINS ReÐnements to the pulsed discharge source contributed signiÐcantly to the detection of long polyynes. As Figure 9 shows, the present nozzle geometry consists of a short TeÑon spacer directly after the nozzle, two oxygen-free high-conductivity copper electrodes separated by a 10 mm spacer of the same dielectric, and a third 20 mm TeÑon spacer following the cathode. The crucial di†erence between this nozzle and the one used on our spectrometer to detect long-chain free radicals and carbenes is the addition of the third spacer. This reÐnement conÐnes the discharge plasma before the free expansion and produces much stronger lines of the longer polyynesÈpresumably because a greater number of collisions prior to expansion allows time for long chains to assemble. ConÐning the discharge plasma also appears to quench production of open-shell molecules, collisions apparently driving the hydrocarbon chemistry to the more stable closed-shell polyynes. The polarity of the discharge plates, as shown in Figure 9, is normally chosen so that the cathode is the second (outer) electrode; the strength of rotational lines usually decreases by a factor of 2È4 when the polarity is reversed.

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