1973ApJ. . .184 . .7575 The AstrophysicalJournal,184:757-761,1973September15 the surfacesofdustgrainsinclouds.Thegreatvarietymoleculesfound the densestclouds,particularlyinregionsofgalacticcenterandOrion grains inthemolecularcloudsarediscussed. lene. Theimplicationsofthisfortheeffectivenesshydrogenationreactionsonsurfaces nebula, opensthepossibilityofstudyingchemistryreactionsbywhich and Welch1971forareview).Themoleculesaregenerallybelievedtobeformedon new toolsforthestudyofphysicalconditionsintheseregions(seeRank,Townes, Subject headings:molecules,interstellar—nebulaeradiolines product ofcyanoacetylene)andtheringcompoundspyrimidine,pyridine.Theestimated they mightbeformed. presence ofatomicormolecularhydrogen.Asweshalldiscussin§III,thereissome been discoveredinSgrB2theJ=1-0transitionbyTurner(1971),isaveryun- vation foroursearchacrylonitrilewasthatcyanoacetylene(fig.\a\whichhas upper boundonthecolumndensityofacrylonitrileinSgrB2isafewpercentthatcyanoacety- (fig. \b,c,d)andtheimplicationsofourfailuretodetectthesemolecules.Themoti- pyrimidine, {d)pyridine. saturated moleculeandwouldbeexpectedtohydrogenatedacrylonitrileinthe © 1973.TheAmericanAstronomicalSociety.Allrightsreserved.PrintedinU.S.A. The discoveriesofcomplexmoleculesininterstellarcloudshaveprovidedvaluable © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem We reportheretheresultsofasearchforacrylonitrile,pyrimidine,andpyridine We reporttheresultsofanegativesearchat6mmforacrylonitrile(expectedashydrogenation Fig. 1.—Chemicalstructure of{a)cyanoacetylene,{b)acrylonitrile,(vinylcyanide), (c)1,3- Department ofBiology,BrookhavenNationalLaboratory,Upton,NewYork A SEARCHFORINTERSTELLARACRYLONITRILE, Received 1973January29;revisedApril23 State UniversityofNewYorkatStonyBrook Department ofEarthandSpaceSciences, PYRIMIDINE, ANDPYRIDINE c) a) H-C=C-C =N Martha N.Simon H I I. INTRODUCTION Michal Simon ABSTRACT AND 757 d) b) H-C H C-C =N 'H H 1973ApJ. . .184 . .7575 53 hm 6-3 5-1 1 , hm -1 pyridine shouldbemetinthesesources. temperature is100°K.Observationsofmoleculessuchasformamide(Rubinetal principally collisional.Forexample,forthe5o5-44acrylonitriletransition, permitted; thetransitionwesearchedforobeys“a”selectionrules.Table1lists it iscloselyrelatedtopyrimidine. and RNAbases,thymine,cytosine,uracil.Wealsosearchedforpyridinesince which isquiteabundant(SnyderandBuhl1971),toacrylonitrile.Pyrimidinealso clouds. Pyrimidine,asix-memberedring,couldbeformedbytheadditionofHCN, evidence thathydrogenationandhydrogenexchangereactionsoccurintheinterstellar Acrylonitrile... 5-4o4 758 position switching.Table1listsantenna-temperature upperlimits(expressedas bank of50filters1.2MHzwide.Thesingleside-band systemtemperaturewastypic- conditions requiredforthecollisionalexcitationofacrylonitrile,pyrimidine,and (Barrett, Schwartz,andWaters1971)indicatethepresenceofregionsinthesesources hydrogen densitiesh(H)>3x10cm"arerequiredforexcitationifthegas the rotationalconstantsgiveninMicrowaveSpectralTablesandareexpectedto which wesearchedforthesethreemolecules.Thefrequencieswerecalculatedfrom the transitions,energyofupperleveltransition,andfrequenciesat respectively {MicrowaveSpectralTables1968).Sinceacrylonitrileisasymmetric,its of biochemicalinterestsinceitistheunsubstitutedringanalogforthreeDNA peak-to-peak values)forthethreemoleculesinOrion [observedat«(1950)=53247?0 characterized by«(H)>10cmandgastemperaturesaround100°K.Thus,the nitrile correspondstonegligiblefrequencydifferencesinthesplitcomponents. be accurateto1or2MHz.Theelectricquadrupolesplittingofthelevelsinacrylo- dipole momenthastwocomponents,p=3.68Debyeand1.25{Micro- ally 1500°K.Theobservationsweremadebyboth frequencyswitchingandtelescope Einstein spontaneoustransitionprobabilityis7.1x10“s,sothatmolecular The receiverwascenteredatvelocityoffsetsfrom thelocalstandardofrest10and Radio AstronomyObservatory.The42-50Gcspectral-line receiverwasusedwitha wave SpectralTables1968).Transitionsaccordingtotwosetsofselectionrulesare 8(1950) =-5°2420':0]andSgrB2[at«(1950) 1744l?0 8(1950)=-28°22'30':0]. under contractwiththeNational ScienceFoundation. 1971) andmethylcyanide(Solomonetal.inSgrB2alcoholOrion 62 kms,respectively. 0 0 2 2 ab -1 1 The electricdipolemomentsofpyridineandpyrimidineare2.262.42Debye, The observationswerecarriedoutwiththe36-foot (11-m)telescopeoftheNational © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem Fairly highmolecularhydrogendensitiesarerequirediftheexcitationis Molecule Transition(cm) TheNationalRadioAstronomy ObservatoryisoperatedbyAssociatedUniversities, Inc., MARTHA N.SIMONANDMICHAL Upper Level Energy of Observational Parameters 15-2 4.75 47.3550.080.10<2x10cm 6.22 44.3900.090.17 6.48 46.2660.080.13 II. THEOBSERVATIONS TABLE 1 v(GHz) SgrB2OrionComments 0 ATU K) in SgrB2 Vol. 184 1973ApJ. . .184 . .7575 1 152 16-2 162 6 3 J =1-0transitionarenotgivenbytheirdegeneracies.Ifthethermaldistribution expected tobehydrogenatedmoresaturatedformsinahydrogen-richenvironment The HandHDexchangereactionsthathavebeen invokedbySolomonandWoolf from cyanoacetyleneisapossiblemechanismfortheformationofsomecomplex relative abundanceofacrylonitriletocyanoacetyleneislessthanafewpercent.This this workisthat,eveniftheabsolutevaluesofcolumndensitiesareinerror, width athalf-maximumof19kms“observedincyanoacetylenepertainstoacrylo- catalytically tolowertheactivationenergiesof reactions betweenatoms.Extension and Salpeter(1972a,6),complex-moleculeformation cantakeplacereadilyonthe reactions. Indeed,thephysicalconditionsin OrionmolecularsourceorinSgr molecules thathavebeendetected.Likelycandidatesforhydrogenationreactionsare: assumption isincorrect,thesenseofpopulationdepartureslikelytobesamefor et al.(1973)show,forexample,thattheintensitiesofhyperfinetransitions lene energylevelsmaywellbeincorrect.ThecyanoacetyleneobservationsofMcGee an upperboundontheacrylonitrilecolumndensityinSgrB2somewhathigherthan with thesameassumptionswehaveemployed,theirlackofdetectionconvertsinto results. Turner,Gordon,andWrixon(1972)reportasearchforthe3-2transition; molecules aredistributedthermallyovertherotationallevels.Assumingline extensive cyanoacetyleneobservationsofSgrB2inthe7=1-0transition,McGeeetal. (Gezari, Joyce,andSimon 1973),showsthatenoughmolecularhydrogen should surfaces ofgrains,sincethegrainact not onlytocollectatomsbutalso B2 wouldappeartobeveryfavorableforthese reactions.AsdiscussedbyWatson (Jefierts, Penzias,andWilson1973),requiresimilar conditionstothehydrogenation (1973) toexplaintheobservedhighabundance ofthedeuteratedformHCN such asinSgrB2. result issurprising,sincetheveryunsaturatedmoleculecyanoacetylenewouldbe both acrylonitrileandcyanoacetylene.Theobservationalresultthatemergesfrom 2 x10cm". nitrile, T=100°K,andthatthehypotheticalacrylonitrilesourcewouldfillour levels, andthatthecyanoacetylenesourcefilled~3'beamwidthusedin parable. AssumingT=50°K,athermalpopulationofthecyanoacetyleneenergy our value.Theassumptionofthermalpopulationtheacrylonitrileandcyanoacety- (1973) derivedaminimumcolumndensityof6.0x10moleculescmifthe observations, Turner(1971)inferredacolumndensityof2.1x10cm~.From the twomoleculesaresosimilarandconditionsrequiredforexcitationcom- “stick” tothegrainsurfaces suchthatthegrainalwayshassomeHon it. SgrB2, a(H)^10cm" , r^100°K,withtheestimate ~ 30-50°K of thecalculationsWatsonandSalpeter(1972a) totheconditionsassumedfor assumptions usedtoinferthecyanoacetylenecolumndensity.Thisispossible,since acrylonitrile column-densityupperlimitmaybeestimatedbyinvokingthesame upper limitsmustclearlybeveryuncertainforanundetectedline.However,the No. 3,1973 2 032 ex ex ~2!2 beamwidth,weobtainthelimitonacrylonitrilecolumndensity(NL}< 2 2 gas © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem The hydrogenationreactionwehavediscussedfortheformationofacrylonitrile Several othertransitionsofacrylonitrilehavebeensearchedforwithnegative Upper limitsforthemolecularcolumndensityfromantenna-temperature SEARCH FORINTERSTELLARMOLECULES HNCO +HHCONH. HCN +H->HCNH,(1) CO +H->COCHgOH, 2 2 2 in. DISCUSSION 759 1973ApJ. . .184 . .7575 This workwassupported inpartbyBrookhavenNationalLaboratory underthe and othermembersofthe N.R.A.O.staffinTucsonforgreathelpwithobservations. generous assistanceininitiatingtheseobservations andE.K.Conklin,B.Rather, have beendetected,andthosethatdetected areoflowabundance;this carbon doublebondshavebeendetected;hence,noringcompounds auspices oftheAtomicEnergy Commission. remains tobeexplainedhowmoleculeswithinthe sameregionandwithpresumably may accountforourfailuretodetectacrylonitrile, pyrimidine,andpyridine.It hydrogenated atall,containingjustenoughhydrogen nottobefreeradicals(e.g., effective whentheydo(e.g.,CHOHfromCO). However,moleculesareoftennot detected indicatenotonlythatthesereactionsoccurbutalsotheyarevery similar historiescanexistinsuchextremesofhydrogenation. HCN andparticularlyHCCN).Fewmoleculesof anintermediatedegreeofsaturation should takeplaceonthesurfaceofgrains.Somemoleculesthathavebeen detected (see,e.g.,deZafraetal.1971). It isalsointerestingtonotethatthepresentlevelsofreceiversensitivity,nocarbon- that partiallysaturatedmoleculesarenotveryabundantrelativetotheirprecursors. to notethat,withintheuncertaintiesdiscussedabove,itappearsthat[HCNH]/[HCN] very unsaturated.ThemoleculesHCO,NH,andCHHOrepresentaninter- The moleculesCHCNandCCHhaveonecarbonfullysaturated,theothers mediate stageofsaturation(acrylonitrilewouldfallinthiscategory).Itisinteresting two distincttypesofobservedmoleculesthatareintermediatetothesecategories. hydrogen). Table2includesonlycomplex(morethanoneheavyatom)molecules: thick, inothers(HCO,CHOH)anomaliesthelevelpopulationsareknowntoexist. molecule formationcouldbedetermined.Reliableabundanceestimatesarenotoriously free radicalshavebeenomitted,butfallintheveryunsaturatedcategory.Thereare saturated (i.e.,moleculeshavingnobondsavailableforthefurtheradditionof (i.e., moleculeshavingdoubleortriplebondsandverylittlehydrogen)completely been detectedseemtofallintotwoclasseswhicharelistedintable2;veryunsaturated as thoseabovewereknown,theeffectivenessofhydrogenationapathwaycomplex « 1(squarebracketsdenoteabundances)andthat[HCO]/[CO]1,suggesting However, certaingeneralizationscanbemade.Thecomplexmoleculesthathave difficult; insomecases(HCN,CO)thisisbecausetheobservedlinesareoptically 3 2 23 3 23 2 760 We thankD.Buhl,L.E.Snyder,andB.Turner forhelpfuldiscussionsand The previousdiscussionhassuggestedthathydrogenationreactionscertainly © American Astronomical Society •Provided bytheNASA Astrophysics DataSystem If therelativeabundancesofdifferentmoleculesparticipatinginreactionssuch are chemicallyunstable. Unsaturated Molecules * Thesemoleculescanbefurtherhydrogenatedbuttheproducts o=c=s Si=0 HN=C=0 HCeeN c=s 0=0 HC=C—C=N Classification ofObservedInterstellarMolecules MARTHA N.SIMONANDMICHAL TABLE 2 Very SaturatedMolecules HCONH* choh HCOOH* 2 3 Vol. 184 1973ApJ. . .184 . .7575 Microwave SpectralTables.1968,NBSPub.No.70(Washington,D.C.:U.S.Government JefFerts, K.B.,Penzias,A.A.,andWilson,R.W.1973,Ap.J.{Letters),inpress. Barrett, A.H.,Schwartz,P.R.,andWaters,J.W.1971,Ap.{Letters),168,L101. .19726,Ap.J.,175,659. Turner, B.E.1971,Ap.J.{Letters),163,L35. Gezari, D.Y.,Joyce,R.R.,andSimon,M.1973,Ap.J.{Letters),179,L67. de Zafra,R.L.,Thaddeus,P.,Kutner,M.,Scoville,N.,Solomon,P.Weaver,H.,andWilliams, Solomon, P.M.,andWoolf,N.J.1973,preprint. Solomon, P.M.,JefFerts,K.B.,Penzias,A.A.,andWilson,R.N.1971,Ap.J.{Letters),168, Snyder, L.E.,andBuhl,D.1971,Ap.J.{Letters),163,L47. Rubin, R.H.,Swenson,G.W.,Benson,C.,Tigelaar,H.L.,andFlygare,W.1971,Ap.J. 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