DOI:10.1002/cphc.201600414 Articles On the Formation of N3H3 Isomers in Irradiated Ammonia Bearing Ices:Triazene (H2NNNH) or Triimide (HNHNNH) MarkoFçrstel,[a, d] Yetsedaw A. Tsegaw,[b] Pavlo Maksyutenko,[a, d] Alexander M. Mebel,[c] Wolfram Sander,[b] and Ralf I. Kaiser*[a, d] The remarkable versatility of triazenesinsynthesis, polymer theoretical studies with our novel detection scheme of photo- chemistry and pharmacology has led to numerousexperimen- ionization-driven reflectron time-of-flight mass spectroscopy tal and theoretical studies.Surprisingly,only very little is we can obtain information on the isomersoftriazene formed known aboutthe most fundamental triazene:the parentmole- in the films. Using isotopically labeled starting material, we can cule with the chemical formula N3H3.Here we observe molecu- additionally gain insightinthe formation pathways of the iso- lar,isolated N3H3 in the gas phase after it sublimes from ener- mers of N3H3 under investigation and identify the isomers getically processed ammonia and nitrogen films. Combining formedastriazene (H2NNNH) andpossibly triimide(HNHNNH). 1. Introduction During the last decades, triazenes—a class of organic mole- life time of at least 1mswas also inferred as an intermediate cules carrying the =N N=N moiety—have received substan- in the radiolysis of an aqueous solution of hydrazine based on À À tial attention both from the theoretical and organic chemistry asingle absorption feature at 230 nm.[6] The cyclic isomer of [1] communities. Derived from cis-and trans-triazene (HN=NNH2 ; triazene, cyclotriazane, was first reported crystallographically in Scheme1), the substituted counterparts have significant appli- zeolite A, where it was stabilized by asilver cation as [1a,c] [1d] + [7] + cations in synthetic chemistry, polymer science, and phar- Ag(N3H3) . Finally,lithium-ion-complexed speciesLi(N3H3) of macology as antitumordrugs such as Dacarbazine, Temozolo- unknown structures were generated in amicrowavedischarge mide, and Mitozolomide[2] with their biological activity attribut- of hydrazine–helium mixtures.[8] ed to their purported capability to alkylate deoxyribonucleic The aforementioned spuriousindication of “triazene” iso- [3] acid (DNA). Althoughtriazenes have been synthesized for mers (N3H3)triggered significant computational efforts span- over 65 years, their stem compound[4] trans-and cis-triazene ning three decades.[9] An early study by Nguyen et al. identified (Scheme 1) could not be isolatedsince triazene undergoes a cis-and trans isomer of triazene (HN=NNH2)with all six [5] facile acid-catalyzed decomposition. An unknownisomer of atoms arranged in the same plane (Cs symmetry);the trans 1 N3H3 has been detected mass spectrometrically via signalat isomer is thermodynamically more stable by 27 to 38 kJmolÀ mass-to-charge m/z=45 as atransient speciesbydischarging compared to the cis structure (Scheme 1).[9b,c] Alater study by hydrazine(N2H4); the ionization energy of the unknown isomer Magers et al. identified two additional isomersoftriazene, trii- [5a] was reported to be 9.6 0.1 eV. Triazene (N3H3)with ahalf- mide (azimine) andcyclo-triazane (triaziridine) with triimide Æ 1 about 54 to 130 kJmolÀ and cyclo-triazane about 170 to 1 [a] Dr.M.Fçrstel, Dr.P.Maksyutenko, Prof. Dr.R.I.Kaiser 190 kJmolÀ less stable than trans-triazene. Therein it was Department of Chemistry pointed out that triazeneis“not exactly planar” and that “low- University of Hawaii, 2545 McCarthy Mall temperature isolation of these species would likely succeed”.[9d] 96822 Honolulu HI (USA) [9a] E-mail:[email protected] These findings were refinedbyPye et al. 1-amino-1,1-dia- [b] Y. A. Tsegaw,Prof. Dr.W.Sander zene was first located on the potential energy surface by Salter [9e] Lehrstuhl fürOrganische Chemie II et al., who suggested that this isomer (called isotriazene in 1 Ruhr UniversitätBochum their paper) is 42 to 50 kJmolÀ lower in energy than cyclo-tria- 44780 Bochum(Germany) zane. Combined, these resultsshow that the stabilities of tria- [c] Prof. Dr.A.M.Mebel zenes decrease in the order trans-triazene, cis-triazene, triimide, Department of Chemistry and Biochemistry Florida InternationalUniversity 1-amino-1,1-diazene and cyclo-triazane (Scheme 1). 11200 SW 8thStreet, Miami, FL 33199 (USA) Here, we exploit anovel experimental approach to synthe- [d] Dr.M.Fçrstel,Dr. P. Maksyutenko, Prof. Dr.R.I.Kaiser size N3H3 in low-temperature matrices via an interaction of ion- W. M. Keck Research Laboratory in Astrochemistry izing radiation with frozenfilms containing ammonia (NH3)and University of Hawaii, 2545 McCarthy Mall nitrogen (N )along with their deuterated (ND )and 15N-labeled 96822HonoluluHI(USA) 2 3 (15N )counterparts. Upon sublimation of the newly formed Supporting Information and the ORCID identification number(s) for the 2 author(s) of this article can be found underhttp://dx.doi.org/10.1002/ molecules, N3H3 is identified for the first time via fragment-free cphc.201600414. single-photon vacuum ultraviolet (VUV) photoionizationcou- ChemPhysChem 2016, 17,2726 –2735 2726 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Articles Scheme1.Structures, ionization energies,and relativeenergies with respect to the most stable isomer of the triazene. Distances are given in ngstrom, angles in degrees. Plain and italic numbersare calculated at the B3LYP/6-311G** and CCSD(T)/6-311G** levels of theory,respectively. Ionization energiesare adiabatic values. Numbersinparenthesis in italics denote values derived with CCSD(T)/6-311G** optimized geometries. The two bottom rows show the struc- tures of the ionic species used for the calculation of the adiabatic ionization energy. pled to areflectrontime-of-flightmassspectrometer(PI- Experimental Section ReTOF-MS)[10] through its parention at mass-to-charge m/z= Ice layers with thicknesses of 600 50 nm were prepared from four 45. This observation is substantiated by the detection of its iso- Æ different gases along with their mixtures. These ices were ammonia topically labeled counterparts N3D3 (m/z=48) along with (NH ), D3-ammonia (ND ), ammonia and nitrogen (NH ,N,1:1.0 15 15 15 3 3 3 2 Æ NN2H3(m/z=46), N2NH3 (m/z= 47), and N3H3 (m/z=48), re- 0.2) and ammonia and 15N-nitrogen (NH , 15N ,1:1 0.2) with puri- 3 2 Æ spectively. ties as follows:NH3 (Matheson;99.999%), ND3 (Isotopes Inc; 15 99+%D), N2 (Matheson;99.9999%) and N2 (Cambridge Isotope ChemPhysChem 2016, 17,2726 –2735 www.chemphyschem.org 2727 2016 Wiley-VCH Verlag GmbH &Co. KGaA, Weinheim Articles Inc. 98+% 15N). The gases and/or their mixtures were introduced into the main chamber via aglass capillary array and condensed Table 1. Data applied to calculate the irradiation doseper molecule:* marks values from CASINO simulations. onto a5.5 0.2 Kcold, rhodium-coated silver wafer.The deposition Æ of each gas took about ten minutes. During the deposition, the initial kinetic energy of the electrons, Einit 5keV pressure in the main vacuum chamber increased from (5 2) 11 8 Æ irradiation current, I 15 2nA 10À torr to (3 1)10À torr.The ice thickness was monitored Æ Æ total number of electrons (3.4 0.3)1014 [11] Æ during the deposition via in situ He–Ne laser interferometry. averagekinetic energyofbackscattered electrons, 3.3 0.9 keV Using Equation (1), alaser wavelength of l=632.8 nm and refrac- Æ Ebs*(NH3 ice) [12] [13] tive indices of NH3 and N2 of 1.35 0.05 and 1.2 0.1 and an averagekinetic energyofbackscattered electrons, 3.3 0.9 keV Æ Æ Æ angle of incidence of q=48,the number of observed interference Ebs*(NH3 :N2)ice) fringes (N )can be related to the thickness (d)ofthe ice: fraction of backscattered electrons, f *(NH ice) 0.34 0.1 f bs 3 Æ fraction of backscattered electrons, f *(NH :N ice) 0.35 0.1 bs 3 2 Æ average kinetic energyoftransmitted electrons, 1.5 0.5 keV Nf l Æ 1 d 1 E *, (NH ice) ð Þ ¼ p 2 2 ð Þ trans 3 2 n sin q average kinetic energyoftransmitted electrons, 1.6 0.5 keV À Æ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Etrans*(NH3:N2 ice) The composition of the ice mixtures was determined by relating fraction of transmitted electrons, f *(NH ice) 0.16 0.05 1 trans 3 the NH absorption features at 1092 cmÀ with avalue of 1.7 Æ 3 fraction of transmitted electrons, ftrans*(NH3 :N2 ice) 0.08 0.05 17 [14] Æ 10À cm to the ice thickness determined by interferometry.Each average penetration depth, l*(NH ice) 365 80 nm 3 Æ sample was then irradiated for 60 min with 5keV electrons at acur- average penetration depth, l*(NH3 :N2 ice) 350 80 nm Æ 3 rent of 15 2nAbyscanning the electron beam over the target density of the NH3 ice, 1 0.66 0.05 gcmÀ Æ 3 Æ 2 density of the NH :N ice, 1 0.75 0.08 gcmÀ surface of 0.9 0.1 cm at an angle of 708 with respect to the sur- 3 2 Æ Æ irradiated area, A 0.9 0.1 cm2 face normal of the substrate. The average deposited dose D per ir- Æ total number of molecules processed (7 3)1017 radiated molecule can be calculated using Equation (2): Æ dose per NH molecule, D (NH ice)1.5 0.4 eV 3 3 Æ dose per NH molecule, D (NH :N ice) 1.3 0.4 eV 3 3 2 Æ Itm dose per N molecule, D (NH :N ice)1.9 0.5 eV 2 D Einit ftransEtrans fbsEbs 2 2 3 2 Æ ð Þ ¼ eNA 1 Alð À À Þ ð Þ where I, t, m, e, NA, 1, A and Einit are the irradiation current, irradia- generated using the frequency doubled (222.6 nm) output of tion time, molecular mass of the molecule, the electron charge, adye laser (445.1 nm, Coumarin 450, Sirah, Cobra-Stretch) which Avogadro’sconstant, the density of the ice, the irradiated area of was pumped by the third harmonic of an Nd:YAG laser (355 nm).