VIBRATIONAL OVERTONE EXCITATION OF METHYL HYDROPEROXIDE AND HYDROXYL RADICAL FORMATION

Author Institution: Chemistry Department, Smith College, Northampton, MA 01063Laser photoacoustic spectra of methyl hydroperoxide in the regions of exciting four and five quanta of O-H stretch show features due to excitation of torsional motion about the O-O bond in addition to O-H stretch. Detection of OH radicals by laser-induced fluorescence confirms that states near five quanta of O-H stretch dissociate along the O-O bond. Photoacoustic and action spectra are compared with simulations from a simple model that assumes adiabatic separation of torsion and O-H stretch and that takes into account the dipole moment dependence on dihedral angle

INFRARED SPECTRA OF CHLORIDE-FLUOROBENZENE COMPLEXES: ELECTROSTATICS VS. H BONDING

Author Institution: JILA and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309So far, there has been relatively little experimental information on the vibrational spectroscopy of anions involving aromatic molecules. We investigate the behavior of fluorinated benzene molecules as ligands bound to chloride ions. This constitutes a particularly interesting case, as their properties can be subtly tuned through the choice of the number and positions of F atoms in the molecule. The charge distribution in the aromatic ligand changes drastically in going from benzene with its negatively charged ring ($\pi$ system) and positively charged periphery (H atoms) to perfluorobenzene (positive ring, negative periphery). Consequently, Cl$^-$ binds to C$_6$H$_6$ via bifurcated H-bonds in the plane of the ligand, while it binds to C$_6$F$_6$ above the aromatic ring. We trace the bonding behavior of $\mathrm{Cl^- \cdot C_6F_nH_{6-n}}$ (n = 0 - 5) through the IR spectra of the complexes for all possible numbers and distributions of F atoms

NEGATIVE ION PHOTOELECTRON SPECTRA OF HALOMETHYL ANIONS

Author Institution: JILA, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309; Department of Chemistry, The Ohio State University, Columbus, Ohio 43210; Department of Chemistry and Chemical Physics Program, University of Nevada, Reno, Nevada 89557; JILA, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309Halomethyl anions undergo a significant geometry change upon electron photodetachment, resulting in multiple extended vibrational progressions in the photoelectron spectra. The normal mode analysis that successfully models photoelectron spectra when geometry changes are modest is unable to reproduce the experimental data using physically reasonable parameters. A three-dimensional anharmonic coupled-mode analysis was employed to accurately reproduce the observed vibrational structure. We present the 364 nm negative ion photoelectron spectra of the halomethyl anions CHX$_2^-$ and CDX$_2^-$ (X = Cl, Br, I) and report electron affinities, vibrational frequencies, and geometries

Theoretical Investigation of the Reactivity of Sodium Dicyanamide with Nitric Acid

There is a need to replace current hydrazine fuels with safer propellants, and dicyanamide (DCA^–)-based systems have emerged as promising alternatives because they autoignite when mixed with some oxidizers. Previous studies of the hypergolic reaction mechanism have focused on the reaction between DCA^– and the oxidizer HNO₃; here, we compare the calculated pathway of DCA^– + HNO₃ with the reaction coordinate of the ion pair sodium dicyanamide with nitric acid, Na­[DCA] + HNO₃. Enthalpies and free energies are calculated in the gas phase and in solution using a quantum mechanical continuum solvation model, SMD-GIL. The barriers to the Na­[DCA] + HNO₃ reaction are dramatically lowered relative to those of the reaction with the bare anion, and an exothermic exit channel to produce NaNO₃ and the reactive intermediate HDCA appears. These results suggest that Na­[DCA] may accelerate the ignition reaction

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