Detection of the Elusive Triazane Molecule (N3H5) in the Gas Phase

DOI:10.1002/cphc.201500560 Communications

Detection of the Elusive Triazane Molecule (N3H5)inthe Gas Phase MarkoFçrstel,[a] PavloMaksyutenko,[a] BrantM.Jones,[a] Bing-Jian Sun,[b] Shih-Hua Chen,[b] Agnes. H.-H. Chang,*[b] and Ralf I. Kaiser*[a]

We report the detection of triazane (N3H5)inthe gas phase. Tri- ful and only potential decompositionproducts of triazane azane is ahigher order nitrogen hydride of ammonia (NH3) (N3H5)—diazane (HNNH) and ammonia (NH3)—could be sam- [1c] [1d] and hydrazine (N2H4)offundamentalimportance for the un- pled. Finally,Fuji et al. made an effort to detect free tria- derstanding of the stability of single-bonded chains of nitro- zane in the microwave discharge of hydrazine, but only the + gen atomsand apotentialkey intermediate in hydrogen–nitro- lithium salt of triazane (LiN3H5 )and the triazane cation + gen chemistry. The experimental resultsalongwith electronic- (N3H5 )could be identified. Therefore, whether free triazane structurecalculations reveal that triazane presentsastable (N3H5)can surviveprompt dissociation in the gas phase has re- molecule with anitrogen–nitrogen bond length that is afew mained an unanswered question. This lack of experimental evi- picometers shorter than that of hydrazineand has alifetime dence on the stability of triazane has triggered substantial exceeding 6 2 msatasublimation temperature of 170 K. Tria- computational efforts.[1a,b] An evaluation of the thermodynamic Æ zane was synthesized throughirradiation of ammonia ice with stability, accounting for hydrogen bonding as well as hyper- energetic electrons and was detected in the gas phase upon conjugation effects, reveals three triazane conformers with C1 sublimation of the ice through soft vacuum ultraviolet (VUV) and Cs symmetries, which range in stability within 24 kJ 1 [1a,b] photoionization coupled with areflectron-time-of-flight mass molÀ . Accounting for potentialdecomposition pathways, spectrometer.Isotopic substitutionexperiments exploiting such as ammonia and diazene elimination, Schlegel and [1b] [D3]-ammonia ice confirmed the identificationthrough the de- Skancke concludedthat “there appears to be no low energy tection of its fully deuterated counterpart[D5]-triazane (N3D5). channels for the unimolecular decomposition of triazane” and that triazane represents a“promising candidate(s) for synthesis.”

The triazane molecule (N3H5)has been long postulated to be Here, we report the experimental detection of the hitherto akey transientspecies in nitrogen–hydrogen chemistry and elusivetriazane molecule (N3H5)inthe gas phase. Our experi- has received considerable interest from the viewpoint of chem- ments are combined with ab initio electronic-structure calcula- ical bonding theory and as apromising high-energymaterial.[1] tions to verify the stability of triazane in the gas phase. The ex- Unlike carbon chemistry,the nitrogen–nitrogen single bond is periments were carriedout in an ultrahigh vacuum (UHV) sur- 11 [3] destabilized by the nonbonding electron pairs of the neighbor- face science machine operated at apressure of a3x10À torr. ing nitrogen atoms, resulting in significantly weaker nitrogen– Amorphous ices of ammonia (NH3)and [D3]-ammonia (ND3) 1 nitrogen single bonds(157–168kJmolÀ )compared to their were prepared in separate experiments with thicknesses of 1 carbon–carbon counterparts (345–355 kJmolÀ ). Consequently, 500 50 nm at 5.5 0.2 K. Nonequilibrium chemistry was in- Æ Æ single-bonded hydrides of nitrogen are extremelyrare and duced by irradiating these ices with energetic electrons (5 keV) only the methane (CH4)and ethane (C2H6)analoguesofnitro- for one hour at an averaged dose of 1.9 0.2 eV per molecule. Æ 1 gen—ammonia (NH3)and hydrazine (N2H4)—have been isolat- After irradiation, the ices were heated at arate of 0.5 KminÀ ed to date. The next higher homologue, triazane, wasreported (temperature-programmed desorption;TPD). During the TPD crystallographically in azeolite, where it was stabilized by com- process, the subliming neutralmolecules were first ionized 2+ [2] plexion to silver ions as Ag2(N3H5)3 . Attempts to isolate tria- through single vacuum ultraviolet (VUV) photon ionization at zane from triazanium sulfite salts, (N3H6)2SO3,were unsuccess- 10.49 eV,then mass resolved in areflectron time-of-flight mass spectrometer (Re-TOF), and eventually identified by their arrival [a] Dr.M.Fçrstel, Dr.P.Maksyutenko, Dr.B.M.Jones, Prof. Dr.R.I.Kaiser times through amultichannel plate (see the Supporting Department of Chemistry Information). University of Hawaii The mass spectra of the subliming molecules released from 2545 McCarthy Mall Honolulu,HI96822-2275 (USA) the irradiated ammonia ices are shown as afunction of tem- E-mail:[email protected] peratureinFigure 1. These spectra are dominated by the am- + [b] Dr.B.-J. Sun, Dr.S.-H. Chen, Prof. Dr.A.H.-H. Chang monia and[D3]-ammonia parentions at m/z=17 [NH3 ;ioniza- Department of Chemistry tion energy (IE)=10.07 0.02 eV] in the irradiated ammonia Æ NationalDong Hwa University ices shiftedby3to m/z=20 (ND +)inthe exposed [D ]-ammo- Shou-Feng,Hualien, 974(Taiwan) 3 3 E-mail:[email protected] nia samples (Figure 2). As the temperature rises, hydrazine Supporting Information for this article is available on the WWW under (N2H4)together with its deuterated counterpart (N2D4)are ob- http://dx.doi.org/10.1002/cphc.201500560. servable through their parent ions at m/z= 32 and 36 (IE=

ChemPhysChem 2015, 16,3139 –3142 3139 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communications

ated, closed-shell nitrogen hydride. The molecularions at + + m/z=47 (N3H5 )and 52 (N3D5 )are stable in the ReTOF depicting–based on the time-of-flight—life times of at least 30 Æ 1 ms; the stabilityofthe molecular ion is consistent with the study of Fuji et al. that identified the triazane cation in the mi- crowavedischargeofhydrazine. Finally,wewould like to out- line that the second ion peak and the shoulder of the TPD at + + m/z=48 (N3D3 )and 45 (N3H3 ), respectively,emerging at

about 160 Kmight origin from N3D3/N3H3 isomers triazene

(H2NNNH)and/or cyclotriazane (c-N3H3)togetherwith their deuterated counterparts. It is important to stress that we also conducted control ex- Figure 1. Reflectron time-of-flight (ReTOF) mass spectra as afunction of the periments, that is, experiments under identical conditions, but temperature of the newly formed products subliming into the gas phase from the irradiated ammonia ice measured at 10.49 eV.The signalofthe withoutirradiating the ices with energetic electrons. In these most abundant species at 17 (ammonia) was truncated to allow for abetter control studies, only ion signals derived from ammonia and representation of the data. [D3]-ammonia were observable.This demonstrates that the abovementioned products and the triazane molecule in partic- ular are aresult of the radiolysis of the ammonia ices, and not an artifact from the photolysis and/or ion–molecule reactions of the subliming ammonia. Therefore, our data provide com- pelling evidence that amolecule with the molecular formulae

N3H5 (ammonia ice) and N3D5 ([D3]-ammonia ice) have been formed in the ices upon exposure to energetic electrons;these specieswere released into the gasphase upon sublimation and then ionizedbysingle-photon ionization. The calculated adiabatic IE of triazane was computed to be 7.7 0.1 eV,which Æ is well below the energy of the vacuum ultraviolet photon of 10.49 eV exploited to ionize the subliming molecules (see the Supporting Information).Considering the root-mean-square 1 Figure 2. Temperature desorption profiles of key species subliming from the velocityof310 msÀ of atriazane molecule subliming at 170 K irradiated ammonia (left) and [D3]-ammonia ices (right). Statistical error bars and the distance betweenthe ice and the photoionization are included in Panels (A)and (B) for the newly identified triazanemolecule. laser of 2mm, we determined alifetimeofthe subliming tria- Traces in blue show the signal from the irradiated sample and traces in red zane and [D5]-triazane molecules of at least 6 2and 7 2 ms, are those measured for the nonirradiated sample. Æ Æ respectively,and possibly much longer.

8.10 0.15 eV), respectively,asthey are released into the gas Æ phase with ion counts peakingat160 K. With increasing tem- + perature, we also observedsignal at m/z=30 (N2H2 ), 45 + + (N3H3 ), and 47 (N3H5 )along with their deuteratedcounter- + + + parts at m/z=32 (N2D2 ), 48 (N3D3 ), and 52 (N3D5 )peaking at about 170 K. Signals at m/z= 30 and45originate from ionized N H (IE=9.8 0.1 eV) and N H (IE=9.6 0.1 eV), respec- 2 2 Æ 3 3 Æ tively.Nomasses higher than 47 and 52 were observed for the irradiated ammonia and [D3]-ammonia ices, respectively.Itis important to stress that ionsignals depicting identical TPD profiles indicate that fragmentation of the molecules with the + highest mass may occur.Here, signals at m/z=17 (NH3 ), 30 + + (N2H2 ), and 45 (N3H3 ), the TPD profiles of which peak at + about 170 K, match the TPD profileofm/z= 47 (N3H5 ), thus indicating that the lower masses are fragments from amolecule that contains nitrogen andhydrogen atoms with amolecular weightof47, that is, triazane(N3H5). In case of [D3]-ammonia + ices, the fragment masses are shiftedtom/z=20 (ND3 ), 32 + + (N2D2 ), and 48 (N3D3 ), which, in turn, are fragments from [D ]-triazane (N D )upon photoionization. Note that m/z= 47 Figure 3. Structures of three conformersoftriazane, I–III,together with their 5 3 5 computed decomposition pathways of triazane. All energies are given in + + 1 (N3H5 )and 52 (N3D5 )cannot originate from higher masses, kJ molÀ .The molecular structures of the reactants,intermediates, products, as N3H5/N3D5 resembles the heaviest allowable fully hydrogen- and transitionstates are shown in Figure 4.

ChemPhysChem 2015, 16,3139 –3142 www.chemphyschem.org 3140 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communications

Figure 4. Structures of reactant conformers (I–III), the intermediate (INT1), products, and transition states (TS1–TS4); bond lengths and angles are definedin pm and degrees, respectively.Also shown is the structure of the triazane ion. Calculation details and the corresponding energies are given in Table S2 in the Supporting Information.

1 Having provided experimental evidence on the formation, decomposition pathway is endoergic by 428 kJmolÀ .Note 3 identification, and stabilityofthe previously elusive gas-phase that nitrenehas a ÆÀ electronic ground state, which lies 1 triazane (N3H5)molecule together with its deuterated counter- 188 kJmolÀ below the excited singlet state. Even if intersys- part, we turned our attentiontothe underlying N3H5 potential tem crossing occurs, the overall reaction, whichyields ground- 1 energy surface so that we could merge the experimentalfind- state nitrene plus hydrazine, is still endoergicby240 kJmolÀ . 1 ings with theory (see the Supporting Information). Our compu- The computed singlet–triplet splitting of 188 kJmolÀ agrees 1 1 tations identified three triazaneConformers I (0 kJmolÀ ), II well within 12 kJmolÀ with the experimentally determined 1 1 1.[4] (4 kJmolÀ ), and III (23 kJmolÀ )with the relative energetics value of 176 kJmolÀ Second, the triazane II conformer(N3H5) 1 given in parentheses (Figures 3and 4). This stabilitysequence can isomerize through atransition state located 258 kJmolÀ agrees well with previouscomputations,[1a,b] and predict Con- above the reactantthrough a[1,2]-hydrogen shift to INT1 1 former II to be 3–4 kJmolÀ higherinenergy than Conformer I; (Figure 4);the latter represents ahighly unstableintermediate 1 with Conformer III determined to present the highest energy that is 159 kJmolÀ less favorable than the triazane isomer and 1 structure rangingfrom 21 to 22 kJmolÀ above Conformer I. fragments into singlet nitreneplus hydrazine (N2H4)inanover- 1 The computed bond lengths and bond anglesagree, within all endoergic reaction (+428 kJmolÀ ). Third, triazane (N3H5) the error limits,with the ones found crystallographically.[2] Fur- was predicted to undergo unimolecular decomposition form- thermore, our computations identified six potentialdecompo- ing ammonia (NH3)plus cis-diazene(HNNH) in an overall exoer- 1 sition pathways of the triazane molecule (N3H5)inthe gas gic reaction ( 28 kJmolÀ )after passing atransitionstate locat- À1 phase. First, triazane (N3H5)can undergo unimolecular decom- ed 232 kJmolÀ above triazane. Fourth, INT1 was predicted to positionthrough elimination of electronically excited nitrene, decompose through ammonia (NH3)loss to form trans-diazene 1 1 NH(a D); the reversed reaction resembles an insertion of ni- (HNNH) in an overall exoergic reaction ( 48 kJmolÀ ). Fifth, tri- À trene into anitrogen–hydrogen bond of hydrazine (N2H4); the azane (N3H5)was found to eliminate ammonia (NH3)along

ChemPhysChem 2015, 16,3139 –3142 www.chemphyschem.org 3141 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Communications

with diimide (H2NN) in an overall endoergic reaction from electron-irradiatedammonia ices might, in future experi- 1 (+55 kJmolÀ ). Finally,aunimolecular decomposition of tria- mental studies, even lead to the synthesis of more-complex ni- zane through asimple nitrogen–nitrogenbond rupture pro- trogen–hydrogen compounds;for example, isovalent mole- cess leads to the amino radical (NH2)together with the hydra- cules of the well-known higher order phosphanes, which have [5] zinyl radical(N2H3)with the overall reaction being endoergic been identified up to octaphosphane (P8H10), thus eventually 1 by 202 kJmolÀ .Based on the overall reaction energies and the transferring Langmuir’s concept of isovalency to main group V location of the transition states involved, our electronic-struc- hydrides nearly acenturyafter its foundation.[6] ture calculations predict that although triazaneisthermody- namically less stable compared to ammonia (NH3)plus cis-/ trans-diazene(HNNH), triazane is kinetically stable, as the asso- Acknowledgements ciated transition state located 258 (trans-diazene) and 1 The authors would like to thank the W. M. Keck Foundation and 232 kJmolÀ (cis-diazene)above triazane cannot be overcome the U. S. Army Research Office (W911NF-14-1-0167) for support. under our experimental conditions. Considering atemperature M.F.acknowledges funding for aPostdoctoral Fellowshipfrom of the subliming triazane molecules of 170 KholdingaMax- the Deutsche Forschungsgemeinschaft (FO 941/1). Computer re- well–Boltzmanndistribution, less than 1part per billion of tria- sourcesatthe National Centerfor High-performance Computer zane is expected to decompose. Consequently,our electronic- of Taiwan were utilized in the calculations. The authors also ex- structurecalculations confirm that gas-phase triazane(NH )is 3 5 press gratitude to Prof. Karl Seff for reviewing and improving the stable under our experimentalconditions. article prior to submission. In conclusion, our combined experimentaland computational study,exploiting soft vacuum ultraviolet (VUV) photoionization coupled with areflectron-time-of-flightmass spectroscopy, Keywords: matrix isolation · nitrogen · nitrogen chain providesconvincingevidence of gas-phase triazane (N3H5)to- molecules · single-bonded nitrogen · triazane gether with its deuterated counterpart(N3D5). Triazaneis [1] a) R. M. Richard, D. W. Ball, J. Mol. Model. 2008, 14,29–37;b)H.B.Schle- ahigher order nitrogen hydrideofammonia (NH3)and hydra- gel, A. Skancke, J. Am. Chem. Soc. 1993, 115,7465 –7471;c)E.Wiberg, N. zine (N H )offundamental importance for the understanding 2 4 Wiberg, A. F. Hollemann, Inorganic chemistry,Academic Press, De Gruyter, of the stabilityofsingle-bonded chains of nitrogen atoms and New York, 2001;d)T.Fujii, C. P. Selvin, M. Sablier,K.Iwase, J. Phys. Chem. apotential key intermediate in hydrogen–nitrogen chemistry. A 2002, 106,3102 –3105. With anitrogen–nitrogenbond length that is slightly shorter [2] Y. Kim, J. W. Gilje, K. Seff, J. Am. Chem. Soc. 1977, 99,7057 –7059. [3] a) S. Maity,R.I.Kaiser, B. M. Jones, Faraday Discuss. 2014, 168,485; (by afew picometers) than that in hydrazine, free triazane b) B. M. Jones,R.I.Kaiser, J. Phys. Chem. Lett. 2013, 4,1965 –1971;c)R.I. presentsanastonishingly stable molecule with alifetime ex- Kaiser,S.Maity,B.M.Jones, Phys. Chem. Chem. Phys. 2014, 16,3399 – ceeding 6 2 msat170 K. Although triazane was predicted 3424. Æ computationally to be thermodynamically unstable with re- [4] J. L. Rinnenthal, K.-H. Gericke, J. Mol. Spectrosc. 1999, 198,115–122. [5] M. Baudler,K.Glinka, Chem. Rev. 1994, 94,1273 –1297. spect to unimolecular decomposition to ammonia (NH )and 3 [6] I. Langmuir, J. Am. Chem. Soc. 1919, 41,868–934. cis-/trans-diazene (HNNH), the energetically unfavorable transition state cannot be overcome, thus making triazane kinetically Manuscript received:July 15, 2015 stable toward fragmentation. The identification of triazane Final Article published:September 2, 2015

ChemPhysChem 2015, 16,3139 –3142 www.chemphyschem.org 3142 2015 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim