HC#: The Largest Molecules in the Interstellar Medium A. Arnau, I. Tuiion, and E. Silla Universidad de Valencia, Dr. Moliner 50, 46100-Burjassot, Valencia, Spain J. M. Andres Col. legi Universitari de Castell6, Castello, Spain
t h e substance of the skv constitutes a fifth element. differpnr from that whirh runstitutes the sublunar aorld . . . i t is logic lo eonreivr that suhsrnnrc a* ingeneralnle and
Table 1. Molecules Detected In the interstellar Medlurn (18)' Hydrogen
incurruprihle, inrpt toexprrlmenl anv inrrcment or decrease,and
inept to be altered..
.
("About Sky", Chapters 2 and 3, Aristotle)
. ..the philosophy is written in that very big book open before our eyes, I want to say, the Universe, but it can't be understood without learning the language, the characters in which it has been written. It is written in mathematical language and its characters are trianeles. .. . circlesand other eeometrieal forms. without them it is itnposslhlr to under\rand an; word; wrrhuut r h i m rt is aa to rpin unrmamably intuadark lnhyrinlh.. . ("I1 saggistare", Galileo Galilei) Traditionally the interstellar medium (ISM) has been considered to be a vacuum. In the Middle Ages the scholastic idea of a "perfect" universe was in disagreement with the reality of a material universe. The extraterrestrial world was then conceived as a model of perfection where the moon, the planets and the stars, all of them considered as perfect spheres, were floating into an immaterial space. It was Galileo Galilei (1564-1642) who. usine the first refractor telescope (1609) noted imperfections in some of the extraterrestrial bodies. such as the irregular surface of the moon or the Great ~ e Spot d of ~ u ~ i t e'r . However, the ISM was still conceived as a big vacuum until some decades ago. The telescope alone cannot detect the secrets of that cold region of the cosmos. We had to employ spectroscopic techniques to reveal that the ISM, although it has a very low material density (10-10%m-3), is not a vacuum. About 50 vears aeo. it was discovered that the visible light from stars behind ;me interstellar regions gave ahsorntionlines from s i m ~ l substances e such as Ca. Caf. Na. Fe, T;+, K, CH, CH+, an; CN. From this i t was concl"ded that the ISM was governed by an elemental chemistry of atomic and diatomic species, nothing more. However, when the waves from the ISM in the millimeter range were explored by the first radiotelescopes, something unexpected happened. Two basic species for life, NH3 and H20, were discovered (I) a t the end of the '60's a t 1.26 and 1.35 cm, respectively; the absorption of HzCO a t 6 cm was discovered by Snyder et al. (2-4) in Sgr A and Sgr B; and in 1971 more than 12 molecules had been detected in the interstellar medium (ISM). Now more than 70 interstellar molecules are known (see Table 1). This list ranges from the most simple and abundant molecule (H2) to HCxIN, the largest molecule discovered there. The existence in the ISM of all these molecules and the size of some of them shows that, although the chemical complexity of the ISM is not that of the Earth, a t least space is not a complete vacuum as it was thought. The largest interstellar molecule, HCnN, is the last-discovered (5) member of the cyanopolyynes family (HC,N, n = 3,5,7,9, and 11).All the family has been detected in the
H2
Molecules containing only C and H CH CHt
C=C =H
CSCH HaCCWH
MoleC~leSc~ntaining0 OH Hz0 CO HCO
HCOt HOCt? H&O CH&O
CH3CH0 CHnOH CHsCH20H e C C 0
Moie~uIescontaining N CN
HCN HNC N2H+ NHI
H&N+ NH2CN CHINH CHzNHa CH3CN
CHPHICN H2C=CHCN HC-CN H(W)&N H(CX)&N
Mole~ulescontaining 0 and N NO
HNO?
HNCO
HOCN?
NHICHO
MoleCUieS containing S and SI SO, SN. CS, HIS, SOz. OCS. HCSt, H&S. CHSSH. HNCS. SiO, SiS T h e question marks indicate tentative detections.
ISM. and thev all have a coniueated. unsaturated carbon chaih terminated at one end h i an H atom and at the other by the cyano group (CN); i.e., HC5N: For the smaller cvanonolwnes names like cvanoacetvlene (HC3N) andcyanodiaceiyl&e (HCsN) can be ;xed; however, for lareer molecules of the familv a more formal terminoloev ". is perhsps better (67): HC,N, H-(C--C),-CN, cyanohexatriyne HC,N, H-(C--C)6CN, cyanooctotetrayne HC,,N, H-(C=C)rCN, cyanodecapentayne The first members of the familv can be obtained in the laboratory, and they were discovered in the ISM using their known rotational spectra, cyanoacetylene (HCsN) was discovered by Turner (8)in 1971, cyanodiacetylene (HCsN) by Averv et al. (9) in 1976. and cvanohexatrivne (HC7N) hv rotb bet sl. (10) in 1978. evert he less, thelaiger memhersof the family are unstnhle in the laboratory. and they cannot be obtained on the Earrh. Their discovery has been made possihle bs theoretical predictions of their rotational spectra and the subsequent detection by radiotelescopes. ~y&ooctotetrayne (HC9N)was discovered by Broten et al. (11) in Heile's cloud 2 in 1978, and finally cyanodecapentayne (HCllN) by Bell et al. (5)in the surroundings of the carbon cold star IRC Volume 67
Number 11 November 1990
905
Table 2. STO-30 Optlmlzed Bond Lengths (In A) tor Cyanopolyynes (tic*) Bond H--C N--C, C1-C2 C2--Cs C,--Ca CI*~ c5-2~ ca+7 C 4 a
HCIN
HCSN
HC,N
HC.N
HC,,N
HC,IN
HG6N
1.069 1.159 1.409 1.175
1.068 1.160 1.404 1.182 1.404 1.175
1.067 1.160 1.404 1.183 1.399 1.183 1.403 1.176
1.067 1.160 1.404 1.183 1.398 1.164 1.397 1.183 1.403 1.176
1.067 1.160 1.404 1.183 1.398 1.184 1.396 1.184 1.397 1.163 1.403 1.176
1.067 1.160 1.404 1.183 1.398 1.184 1.396 1.185 1.396 1.184 1.397 1.183 1.402 1.176
1.067 1.160 1.403 1.183 1.398 1.184 1.396 1.185 1.395 1.185 1.396 1.184 1.397 1.183 1.402 1176
c d $ Cs*?o CtbC17 61*12
c?2+13
c11+11
C..--C...
+
10°216 in 1982; both were found by extrapolation of rotational constanu from the smaller cy&wpoly).nes. Table 2shows thegeometriesofthuse speciesof the family alreadv discovered in the 1S.M (HCA'. n = 3.5.7.9. . . . , and 1 I ) . and~thbse( H C ~ ~and N HC~:N) hhihh are still undiscovered there.althoueh their existence is hwothesized.'l'hese eeomF MO "ab kitio" etries have hien optimized with ~ H SCF method with the STO-3G hasis set, using the MONSTERGAUSS program (12,131. The theoretical geometries are consistent, and they agree very well with the known experimental values (14) for the smallest molecules (HCxN and HC,N). A slight shortening of the simple C-C bonds by r delocaliration is observed as we advance into the series. in agreement with spectroscopic data (15,16).This shortening is. lareer when the bond is in the middle of the molecule. The -~same r delocalization produces a slight shortening in the CH bonds and a slight lengthening of triple bonds C-C, as we go on into the series. The G N bond hardly changes among the different members of the familv. With these geometries the rotational constants for HC,N (see Tahle 3) have been calculated. These constants are very important for finding the undiscovered molecules of the family: HC13N and HClsN, because with their Ba values we can predict their rotational lines and find them with radiotelescopes. A quadratic least-square fit between experimental (Bod and theoretical rotational constants (BoT),gave the equation:
-
~
~
~~
~
~
, B
= -2.7817. ~o-~B,'
+ 1.0065BOT+ 2.8168.
Journal of Chemical Education
Table 4. Known Dipolar Moments tor Cyanopolyynes ( 6 ) (In Debyes) HCaN
HCsN
HCiN
HCsN
HCI,N
3.72
4.33
5.0
5.6
5.0
molecules discovered in the ISM, also are the largest linear molecules found anvwhere. These compounds have relatively intense spectral I& hecause of their linear structure and their very large dipolar moments (see Table 4 1 . This fact makes their detection easier. Although the existence in the ISM of molecules as complex as HC,N can be surprising, it must be expected that, as detection methods advance, more complex and bigger molecules will be discovered. The knowledge of the interstellar molecules, and particularly of those built by carbon chains, help us understand something more about the chemistry of the universe and the origin of life. Literature Clted
A. c.; H.; Thornton. D. c.;Welch, W. J. Phya. Rr". 1701-1705. 2. Snyder.L.E.;Buhl,D.;Zuckerman. B.: Palmer,P.Phvs.R P U L P ~1969,22.679481. L. 3. Snyder, L. E.; Zuekeman, B.: Buhl, D.; Palmer. P.; Aslrophy.$. J. 1969, 166, LL171. Cheung, Rank, D.M.; Tomes. C. Lett. 1968,21,
.
.
7. Handbook ofChemi8Lrv and Physics. 39th od.: W e s t , Ed.;CRC: Florida; p C-1. 8. Turner.B.Astrophys. J. 1971.163. L3LL39. 9. Avery,L. W.:Broten,N.W.;MacLeod.J.M.:Oka,T.:Kroto,H. W.Astrophys.J. 1976.
GHz
This equation let us optimize the obtained results and ~ r e d i cwith t much more security the rotational constants of ;ndiscovered molecules (see able 3). These rotational constants have heen calculated using different basis sets (17); thus, with a 6-31-G basis the ohtained Bo for HClxN was 0.1073 GHz, and with 3-21-G basis for HC15Nmolecule a Bo = 0.0724 GHz was obtained. The HC.N molecules, in addition to being the largest
906
Table 3. Observed, Theoretical STO-30, and Obtalned by Ouadratlc Least-Square Flt (8%) Rotational Constants tor Cyanopolyynes(HCfl) (All In GHz)
12. Peterson. M. R.: Poirier, R. A. Program MONSTERGAUSS, 1985, Univenify of Toronto, ON, Canada. 13. Hehre. W.J.:Lsthan, W.A.;Ditchfield.R.;Newtan.M.D.;Pople.J.A.;Q.C,P.E.236. 14. Alexander, A. J.: Kmto, H. U'.;Walton, D. R. M. J. Molecular Sprctroscopy 1976.62, 175-180. 15. Dsle, J. In Chemistry of Acetylen~s,Viehe, H. G., Ed.; Dekker: New York, 1967: Chapter 1. 16. Co1en.B. F.;Hitchmek, P.B.:Wa1ton.D. R. M.J.Chem. Sor.Doltan T m n s 1975.44% 445. 17. Defrees. D. J.: McLean,A. 0. Chem. Phya. Lett. 1986.158.640-544. 18. Smith, 0.Phil. Trans. R.Soc. L a n d 1987.A323.269-286.
