Theoretical and Experimental Insights into the Dissociation of 2‑Hydroxyethylhydrazinium Nitrate Clusters Formed via Electrospray

Ionic liquids are used for myriad applications, including as catalysts, solvents, and propellants. Specifically, 2-hydroxyethylhydrazinium nitrate (HEHN) has been developed as a chemical propellant for space applications. The gas-phase behavior of HEHN ions and clusters is important in understanding its potential as an electrospray thruster propellant. Here, the unimolecular dissociation pathways of two clusters are experimentally observed, and theoretical modeling of hydrogen bonding and dissociation pathways is used to help rationalize those observations. The cation/deprotonated cation cluster [HEH₂ – H]⁺, which is observed from electrospray ionization, is calculated to be considerably more stable than the complementary cation/protonated anion adduct, [HEH + HNO₃]⁺, which is not observed experimentally. Upon collisional activation, a larger cluster [(HEHN)₂HEH]⁺ undergoes dissociation via loss of nitric acid at lower collision energies, as predicted theoretically. At higher collision energies, additional primary and secondary loss pathways open, including deprotonated cation loss, ion-pair loss, and double-nitric-acid loss. Taken together, these experimental and theoretical results contribute to a foundational understanding of the dissociation of protic ionic liquid clusters in the gas phase

Photoelectron Spectroscopy of Anilinide and Acidity of Aniline

The photoelectron spectrum of the anilinide ion has been measured. The spectrum exhibits a vibrational progression of the CCC in-plane bending mode of the anilino radical in its electronic ground state. The observed fundamental frequency is 524 ± 10 cm^–1. The electron affinity (EA) of the radical is determined to be 1.607 ± 0.004 eV. The EA value is combined with the N–H bond dissociation energy of aniline in a negative ion thermochemical cycle to derive the deprotonation enthalpy of aniline at 0 K; Δ_acid<i>H</i>₀(PhHN–H) = 1535.4 ± 0.7 kJ mol^–1. Temperature corrections are made to obtain the corresponding value at 298 K and the gas-phase acidity; Δ_acid<i>H</i>₂₉₈(PhHN–H) = 1540.8 ± 1.0 kJ mol^–1 and Δ_acid<i>G</i>₂₉₈(PhHN–H) = 1509.2 ± 1.5 kJ mol^–1, respectively. The compatibility of this value in the acidity scale that is currently available is examined by utilizing the acidity of acetaldehyde as a reference

C–H Bond Strengths and Acidities in Aromatic Systems: Effects of Nitrogen Incorporation in Mono-, Di-, and Triazines

The negative ion chemistry of five azine molecules has been investigated using the combined experimental techniques of negative ion photoelectron spectroscopy to obtain electron affinities (EA) and tandem flowing afterglow-selected ion tube (FA-SIFT) mass spectrometry to obtain deprotonation enthalpies (Δ_acid<i>H</i>₂₉₈). The measured Δ_acid<i>H</i>₂₉₈ for the most acidic site of each azine species is combined with the EA of the corresponding radical in a thermochemical cycle to determine the corresponding C–H bond dissociation energy (BDE). The site-specific C–H BDE values of pyridine, 1,2-diazine, 1,3-diazine, 1,4-diazine, and 1,3,5-triazine are 110.4 ± 2.0, 111.3 ± 0.7, 113.4 ± 0.7, 107.5 ± 0.4, and 107.8 ± 0.7 kcal mol^–1, respectively. The application of complementary experimental methods, along with quantum chemical calculations, to a series of nitrogen-substituted azines sheds light on the influence of nitrogen atom substitution on the strength of C–H bonds in six-membered rings

Electronic States of the Quasilinear Molecule Propargylene (HCCCH) from Negative Ion Photoelectron Spectroscopy

We use gas-phase negative ion photoelectron spectroscopy to study the quasilinear carbene propargylene, HCCCH, and its isotopologue DCCCD. Photodetachment from HCCCH^– affords the <i>X̃</i>(³B) ground state of HCCCH and its <i>ã</i>(¹A), <i>b̃</i> (¹B), <i>d̃</i>(¹A₂), and <i>B̃</i>(³A₂) excited states. Extended, negatively anharmonic vibrational progressions in the <i>X̃</i>(³B) ground state and the open-shell singlet <i>b̃</i> (¹B) state arise from the change in geometry between the anion and the neutral states and complicate the assignment of the origin peak. The geometry change arising from electron photodetachment results in excitation of the ν₄ symmetric CCH bending mode, with a measured fundamental frequency of 363 ± 57 cm^–1 in the <i>X̃</i>(³B) state. Our calculated harmonic frequency for this mode is 359 cm^–1. The Franck–Condon envelope of this progression cannot be reproduced within the harmonic approximation. The spectra of the <i>ã</i>(¹A), <i>d̃</i>(¹A₂), and <i>B̃</i>(³A₂) states are each characterized by a short vibrational progression and a prominent origin peak, establishing that the geometries of the anion and these neutral states are similar. Through comparison of the HCCCH^– and DCCCD^– photoelectron spectra, we measure the electron affinity of HCCCH to be 1.156 ± _0.095^0.010 eV, with a singlet–triplet splitting between the <i>X̃</i>(³B) and the <i>ã</i>(¹A) states of Δ<i>E</i>_ST = 0.500 ± _0.01^0.10 eV (11.5 ± _0.2^2.3 kcal/mol). Experimental term energies of the higher excited states are <i>T</i>₀ [<i>b̃</i>(¹B)] = 0.94 ± _0.20^0.22eV, <i>T</i>₀ [<i>d̃</i>(¹A₂)] = 3.30 ± _0.02^0.10eV, <i>T</i>₀ [<i>B̃</i>(³A₂)] = 3.58 ± _0.02^0.10eV. The photoelectron angular distributions show significant π character in all the frontier molecular orbitals, with additional σ character in orbitals that create the <i>X̃</i>(³B) and <i>b̃</i>(¹B) states upon electron detachment. These results are consistent with a quasilinear, nonplanar, doubly allylic structure of <i>X̃</i>(³B) HCCCH with both diradical and carbene character