Ghanshyam L. Vaghjiani

Fourier Transform Infrared Studies in Hypergolic Ignition of Ionic Liquids

A class of room-temperature ionic liquids (RTILs) that exhibit hypergolic activity toward fuming nitric acid is reported. Fast ignition of dicyanamide ionic liquids when mixed with nitric acid is contrasted with the reactivity of the ionic liquid azides, which show high reactivity with nitric acid, but do not ignite. The reactivity of other potential salt fuels is assessed here. Rapid-scan, Fourier transform infrared (FTIR) spectroscopy of the preignition phase indicates the evolution of N 2O from both the dicyanamide and azide RTILs. Evidence for the evolution of CO 2 and isocyanic acid (HNCO) with similar temporal behavior to N 2O from reaction of the dicyanamide ionic liquids with nitric acid is presented. Evolution of HN 3 is detected from the azides. No evolution of HCN from the dicyanamide reactions was detected. From the FTIR observations, biuret reaction tests, and initial ab initio calculations, a mechanism is proposed for the formation of N 2O, CO 2, and HNCO from the dicyanamide reactions during preignition.

Liquid Azide Salts and Their Reactions with Common Oxidizers IRFNA and N2O4

Several imidazolium-based ionic liquid azides with saturated and unsaturated side chains were prepared, and their physical and structural properties were investigated. The reactivity of these new as well as some previously reported ionic liquid azides with strong oxidizers, N 2O 4 and inhibited, red-fuming-nitric acid (IRFNA), was studied.

Thermal decomposition mechanism of 1-ethyl-3-methylimidazolium bromide ionic liquid.

In order to better understand the volatilization process for ionic liquids, the vapor evolved from heating the ionic liquid 1-ethyl-3-methylimidazolium bromide (EMIM(+)Br(-)) was analyzed via tunable vacuum ultraviolet photoionization time-of-flight mass spectrometry (VUV-PI-TOFMS) and thermogravimetric analysis mass spectrometry (TGA-MS). For this ionic liquid, the experimental results indicate that vaporization takes place via the evolution of alkyl bromides and alkylimidazoles, presumably through alkyl abstraction via an S(N)2 type mechanism, and that vaporization of intact ion pairs or the formation of carbenes is negligible. Activation enthalpies for the formation of the methyl and ethyl bromides were evaluated experimentally, ΔH(‡)(CH(3)Br) = 116.1 ± 6.6 kJ/mol and ΔH(‡)(CH(3)CH(2)Br) = 122.9 ± 7.2 kJ/mol, and the results are found to be in agreement with calculated values for the S(N)2 reactions. Comparisons of product photoionization efficiency (PIE) curves with literature data are in good agreement, and ab initio thermodynamics calculations are presented as further evidence for the proposed thermal decomposition mechanism. Estimates for the enthalpy of vaporization of EMIM(+)Br(-) and, by comparison, 1-butyl-3-methylimidazolium bromide (BMIM(+)Br(-)) from molecular dynamics calculations and their gas phase enthalpies of formation obtained by G4 calculations yield estimates for the ionic liquids enthalpies of formation in the liquid phase: ΔH(vap)(298 K) (EMIM(+)Br(-)) = 168 ± 20 kJ/mol, ΔH(f,xa0gas)(298 K) (EMIM(+)Br(-)) = 38.4 ± 10 kJ/mol, ΔH(f,xa0liq)(298 K) (EMIM(+)Br(-)) = -130 ± 22 kJ/mol, ΔH(f,xa0gas)(298 K) (BMIM(+)Br(-)) = -5.6 ± 10 kJ/mol, and ΔH(f,xa0liq)(298 K) (BMIM(+)Br(-)) = -180 ± 20 kJ/mol.

Photodissociation of H2O2 at 193 and 222 nm: Products and quantum yields

The primary quantum yields of OH(Xu20092Π),H(2S), and oxygen atoms [O(1D)+O(3P)] produced in the photodissociation of H2O2 at 193 and 222 nm have been measured at 298 K. At 193 nm, the primary quantum yields were observed to be 1.51±0.18, 0.16±0.04, and <0.02, for Φ(OH), Φ(H), and the sum of Φ(O) and Φ(Ou20091S), respectively. At 222 nm, the OH yield was Φ(OH)=2.02±0.35, the H atom yield was Φ(H)=0.024±0.012, and Φ(O) was <0.002. The errors quoted above are 2σ, precision plus estimated systematic errors. The OH product was directly monitored by pulsed laser‐induced fluorescence, and the atomic species were detected via cw resonance fluorescence. The OH quantum yields reported here were measured relative to known product quantum yields in the dissocation of H2O2 at 248 nm. H(2S) yields were measured relative to those in photolysis of HBr and HCl, (at 193 nm) or CH3SH (at 222 nm), whereas O atoms yields were measured relative to O3 photolysis at both wavelengths. The present results indicate unit dissociation of H2O2 at both 222 and 193 nm with only two major products OH (∼80% at 193 nm, 98% at 222 nm) and H(2S) (∼20% at 193 nm, 2% at 222 nm). Up to 15% of the OH produced in the 193 nm photolysis may be vibrationally excited; however, no evidence for vibrationally excited OH was observed at 222 nm.The primary quantum yields of OH(Xu20092Π),H(2S), and oxygen atoms [O(1D)+O(3P)] produced in the photodissociation of H2O2 at 193 and 222 nm have been measured at 298 K. At 193 nm, the primary quantum yields were observed to be 1.51±0.18, 0.16±0.04, and <0.02, for Φ(OH), Φ(H), and the sum of Φ(O) and Φ(Ou20091S), respectively. At 222 nm, the OH yield was Φ(OH)=2.02±0.35, the H atom yield was Φ(H)=0.024±0.012, and Φ(O) was <0.002. The errors quoted above are 2σ, precision plus estimated systematic errors. The OH product was directly monitored by pulsed laser‐induced fluorescence, and the atomic species were detected via cw resonance fluorescence. The OH quantum yields reported here were measured relative to known product quantum yields in the dissocation of H2O2 at 248 nm. H(2S) yields were measured relative to those in photolysis of HBr and HCl, (at 193 nm) or CH3SH (at 222 nm), whereas O atoms yields were measured relative to O3 photolysis at both wavelengths. The present results indicate unit dissociation of H2...

Heats of Vaporization of Room Temperature Ionic Liquids by Tunable Vacuum Ultraviolet Photoionization

The heats of vaporization of the room temperature ionic liquids (RTILs) N-butyl-N-methylpyrrolidinium bistrifluorosulfonylimide, N-butyl-N-methylpyrrolidinium dicyanamide, and 1-butyl-3-methylimidazolium dicyanamide are determined using a heated effusive vapor source in conjunction with single photon ionization by a tunable vacuum ultraviolet synchrotron source. The relative gas phase ionic liquid vapor densities in the effusive beam are monitored by clearly distinguished dissociative photoionization processes via a time-of-flight mass spectrometer at a tunable vacuum ultraviolet beamline 9.0.2.3 (Chemical Dynamics Beamline) at the Advanced Light Source synchrotron facility. Resulting in relatively few assumptions, through the analysis of both parent cations and fragment cations, the heat of vaporization of N-butyl-N-methylpyrrolidinium bistrifluorosulfonylimide is determined to be DeltaH(vap)(298.15 K) = 195 +/- 19 kJ mol(-1). The observed heats of vaporization of 1-butyl-3-methylimidazolium dicyanamide (DeltaH(vap)(298.15 K) = 174 +/- 12 kJ mol(-1)) and N-butyl-N-methylpyrrolidinium dicyanamide (DeltaH(vap)(298.15 K) = 171 +/- 12 kJ mol(-1)) are consistent with reported experimental values using electron impact ionization. The tunable vacuum ultraviolet source has enabled accurate measurement of photoion appearance energies. These appearance energies are in good agreement with MP2 calculations for dissociative photoionization of the ion pair. These experimental heats of vaporization, photoion appearance energies, and ab initio calculations corroborate vaporization of these RTILs as intact cation-anion pairs.

Photodissociation of bromocarbons at 193, 222, and 248 nm: Quantum yields of Br atom at 298 K

The primary quantum yields, ΦBr, for the formation of Br atom in the photodissociation of CF2Br2 and CH3Br at 248, 222, and 193 nm, and of CF3Br at 222 and 193 nm were measured at 298 K. The bromine atoms were directly detected via resonance fluorescence following pulsed laser photolysis of the molecules of interest. The Br atom quantum yields in CF2Br2 photolysis increased with decreasing wavelengths: 1.01±0.15, 1.63±0.19, and 1.96±0.27 at 248, 222, and 193 nm, respectively. The ΦBr values in CH3Br and CF3Br were close to unity at all the wavelengths: 0.92±0.15 and 1.12±0.16 at 222 and 193 nm, respectively, for CF3Br; 1.01±0.16, 1.10±0.20, and 1.05±0.11 at 248, 222, and 193 nm, respectively, for CH3Br. Quantum yield of H atom formation in the photolysis of CH3Br at 193 nm was measured to be 0.002±0.001. H atom could not be detected in the photolysis at 248 and 222 nm. In all cases the ΦBr values were found to be independent of buffer gas pressure or the photolysis laser fluence. Our results suggest that ...

Generation of Melamine Polymer Condensates upon Hypergolic Ignition of Dicyanamide Ionic Liquids

Abstract : Fuels that can be ignited chemically under ambient conditions upon contact with an oxidizing agent are referred to as hypergols.[1] Engines powered by hypergols do not require electric ignition, making them simple, robust and reliable alternatives to conventional fossil fuels. Commonly used hypergolic fuels include hydrazine and its methylated derivatives, which are extremely toxic, corrosive, and have high vapor pressure. Intense research is underway to develop alternative environmentally friendly liquid propellants with lower toxicity to reduce operational costs and safety requirements associated with handling hydrazine.[2] Ionic liquids (ILs)[3] have recently received considerable attention as energetic materials for propellant applications due to lower vapor pressures, higher densities and, often, an enhanced thermal stability compared to their nonionic analogues.[4] Since 2008, a number of ILs have been reported to be hypergolic when reacted with common oxidizers, such as HNO3.[5-7] Of particular practical interest are hypergolic ILs comprising fuel-rich dicyanamide (DCA) anions.[5] DCA ILs have some of the lowest viscosities among known ILs,[8] which is a very important figure of merit for the efficient fuel supply in bipropellant engines. In this study, using electrospray ionization mass spectrometry (ESI-MS), we discovered that the reaction between DCA ILs and HNO3 yields a precipitate that is composed of cyclic triazines, including melamine and its polymers. The concurrent formation of precipitate siphons materials from the hypergolic reaction pathway,[6] limiting the energy capacity of a fuel. Furthermore, the generation of stable solidstate species during the ignition indeed represents a serious problem for the safe operation of bipropellant engines. We propose a mechanism for the formation of the major polymers via thermal decomposition of DCA ILs, mediated by nitric acid.

Helium Nanodroplet Isolation and Infrared Spectroscopy of the Isolated Ion-Pair 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

The ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide was vaporized at 420 K, and the ion-pair constituents were entrained in a beam of liquid He nanodroplets and cooled to 0.4 K. The vapor pressure was optimized such that each He droplet picked up a single ion-pair from the gas phase. Infrared spectroscopy in the CH stretch region reveals bands that are assigned to intact ion-pairs on the basis of comparisons to ab initio harmonic frequency computations of 23 low energy isomers. The He droplet spectrum is consistent with a weighted sum of the computed harmonic spectra, in which the weights are determined from ab initio computations of the relative free energies at 420 K. Anharmonic resonance polyads in the CH stretch region are treated explicitly, which improves the agreement between the experiment and computed spectra for ion-pairs. For isomers having a strong cation···anion hydrogen bonding interaction, the imidazolium C(2)-H stretch fundamental is shifted to lower energy and into resonance with the overtones and combination bands of the imidazolium ring stretching modes, resulting in a spectral complexity in the CH stretch region that is fully resolved in the He droplet spectrum. The assignment of the infrared spectrum to ion-pairs is confirmed through polarization spectroscopy measurements that reveal the permanent electric dipole moment of the He-solvated species to be 11 ± 2 D. The computed permanent electric dipole moments for the low energy isomers of the [emim(+)][Tf2N(-)] ion-pairs fall in the range 9-13xa0D, whereas the computed dipole moments of decomposition products of the ionic liquid are less than 4.3 D.

Tunable wavelength soft photoionization of ionic liquid vapors

Combined data of photoelectron spectra and photoionization efficiency curves in the near threshold ionization region of isolated ion pairs from [emim][Tf(2)N], [emim][Pf(2)N], and [dmpim][Tf(2)N] ionic liquid vapors reveal small shifts in the ionization energies of ion-pair systems due to cation and anion substitutions. Shifts toward higher binding energy following anion substitution are attributed to increased electronegativity of the anion itself, whereas shifts toward lower binding energies following cation substitution are attributed to an increase in the cation-anion distance that causes a lower Coulombic binding potential. The predominant ionization mechanism in the near threshold photon energy region is identified as dissociative ionization, involving the dissociation of the ion pair and the production of intact cations as the positively charged products.

Thermal Decomposition Mechanisms of Alkylimidazolium Ionic Liquids with Cyano-Functionalized Anions

Because of the unusually high heats of vaporization of room-temperature ionic liquids (RTILs), volatilization of RTILs through thermal decomposition and vaporization of the decomposition products can be significant. Upon heating of cyano-functionalized anionic RTILs in vacuum, their gaseous products were detected experimentally via tunable vacuum ultraviolet photoionization mass spectrometry performed at the Chemical Dynamics Beamline 9.0.2 at the Advanced Light Source. Experimental evidence for di- and trialkylimidazolium cations and cyano-functionalized anionic RTILs confirms thermal decomposition occurs primarily through two pathways: deprotonation of the cation by the anion and dealkylation of the imidazolium cation by the anion. Secondary reactions include possible cyclization of the cation and C2 substitution on the imidazolium, and their proposed reaction mechanisms are introduced here. Additional evidence supporting these mechanisms was obtained using thermal gravimetric analysis-mass spectrometry, gas chromatography-mass spectrometry, and temperature-jump infrared spectroscopy. In order to predict the overall thermal stability in these ionic liquids, the ability to accurately calculate both the basicity of the anions and their nucleophilicity in the ionic liquid is critical. Both gas phase and condensed phase (generic ionic liquid (GIL) model) density functional theory calculations support the decomposition mechanisms, and the GIL model could provide a highly accurate means to determine thermal stabilities for ionic liquids in general.