Search

In this study, the UV photodissociation of gas phase ion pairs of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [emim]+[tf2n]-, is shown to proceed primarily through radical intermediates. [emim]+[tf2n]- ion pairs have been shown previously to undergo two-photon-dependent dissociation, but the mechanisms of this have not been probed in detail. By employing a two-laser pump probe spectroscopy and time-dependent density functional theory (TD-DFT) calculations, we have illustrated that one of the major UV photodissociation pathways in [emim]+[tf2n]- ion pairs is an intermolecular electron transfer wherein the anion transfers an electron to the cation resulting in two neutral open-shelled products. These products were observable for at least 1.6 μs post photodissociation, the experimental limit, via detection of the [emim]+ cation. This data demonstrates that the likely photoproducts of [emim]+[tf2n]- UV photodissociation are two neutral species that separate spatially, demonstrated through lack of observed relaxation pathways such as electron recombination. TD-DFT and frontier molecular orbital analysis calculations at the MN15/6-311++G(d,p) level are employed to aid in identifying excited state characteristics and support the interpretations of the experimental data. The energetic onset of the intermolecular electron transfer is consistent with previously observed [emim]+[tf2n]- absorption spectra in the bulk and gas phases. The similarities between bulk and gas phase UV spectra imply that this electron-transfer pathway may be a major photodissociation channel in both phases.

Increasing experimental results indicate that optically excited plasmonic metal nanoparticles can drive photochemical reactions at photon flux comparable to that of solar radiation. However, experimental evidence that provides insight into the mechanism of the reactions on plasmonic surfaces has been limited. Here, using plasmon-enhanced N-demethylation (PEND) of methylene blue (MB) as model reaction, we report mechanistic analysis of photochemical reactions on plasmonic gold nanoparticles under different adsorption and atmospheric conditions using surface-enhanced Raman scattering as operando spectroscopy to monitor the reaction as a function of exposure time to the light source. We found that in air and oxygen atmospheres and in the presence of co-adsorbed water molecules, MB undergoes photochemical N-demethylation to yield thionine (complete N-demethylation product) and other partial N-demethylation products that have distinct vibrational signatures. The product signals are negligible when the MB-parti...

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.

The dissociation of cyclopentane has been investigated in a diaphragmless shock tube with laser schlieren densitometry at nominal post-shock pressures, P 2 , of 35, 70, 150, and 300 Torr and temperatures, T 2 , of 1472–2074 K. These are the first experimental data reported in the high temperature fall-off regime for cyclopentane. The experimental density gradients were simulated using a chemical-kinetic model with good agreement between simulations and experiments. Rate coefficients for dissociation of cyclopentane were obtained and mechanistic details were elucidated. An RRKM model was developed to best fit the experimental results, and the rate coefficients calculated are: k 1( 35 Torr) = (7.93 ± 3.96) × 10 84 T − 19.815 exp(−64366/T) s −1 , k 1( 70 Torr ) = (4.79 ± 2.39) × 10 77 T − 17.705 exp( − 62316/T) s −1 , k 1( 150 Torr ) = (2.42 ± 1.21) × 10 69 T − 15.295 exp( − 59862/T) s −1 , k 1 ( 300 Torr ) = (4.66 ± 2.33) × 10 61 T − 13.065 exp( − 57493/T) s −1 , and k 1∞ = (1.69 ± 0.85) × 10 16 T − 0.005 exp( − 42983/T). Results show that cyclopentane dissociation is similar to that of cyclohexane, and involves formation of a biradical intermediate. The high-pressure limit rates for cyclopentane dissociation are also compared with previous results in the literature and show good agreement with a study by Tsang (1978) [21].

Photoexcitation of water by Lyman-α (121.6 nm) induces a dissociation reaction that produces OH(A2Σ+) + H. Despite this reaction being part of numerous studies, a combined understanding of the product and fluorescence yields is still lacking. Here, the rotational and vibrational distributions of OH(A) are determined from dispersed fluorescence following photoexcitation of both room-temperature and jet-cooled water vapor, for the first time in the same experiment. This work compares new data of state-resolved fluorescence with literature molecular branching ratios and brings previous studies into agreement through careful consideration of OH(A) fluorescent and predissociation lifetimes and confirms a fluorescent quantum yield of 8%. Comparison of the room-temperature and jet-cooled OH(A) populations indicate the temperature of H2O prior to excitation has subtle effects on the OH(A) population distribution, such as altering the rotational distribution in the ν′ = 0 population and affecting the population in...

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, additiona...

The decomposition of ortho-benzyne radicals (o-C6H4), generated from the dissociation of a new precursor, fluorobenzene (C6H5F), has been investigated in a diaphragmless shock tube in a combined laser schlieren densitometry, LS, (P2 = 30 ± 2, 59 ± 3, 121 ± 5 Torr, 2050 < T2 < 2980)/time-of-flight mass spectrometry, TOF-MS (P5 = 1150 ± 200 Torr, 2300 < T5 < 2800 K) study. The LS density gradient profiles were simulated, and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C6H5F → o-C6H4 + HF, o-C6H4 → C4H2 + C2H2, and o-C6H4 → C6H3 + H were obtained. Good agreement with Xu et al. [Proc. Combust. Inst., 31(2007) 231-239] with respect to the o-benzyne dissociation branching ratio was found. However a strong pressure dependence was also observed in o-benzyne dissociation which was not seen by Xu et al. For o-C6H4 → C4H2 + C2H2: k2a,120Torr = (1.2 ± 0.4) × 1063 T−14.27 exp(−52,710/T) k2a,60Torr = (1.4 ± 0.5) × 1060 T−13.595 exp(−50,538/T), k2a,30Torr = (4.0 ± 1.3) × 1057 T−13.015 exp(−48,628/T) s−1. The inclusion of high temperature o-benzyne abstraction reactions is necessary to simulate the reacting system, and we report estimates of the rate of o-C6H4 + C6H5F.

In this study we investigate the effect of deuteration and molecular beam temperature on the hydrogen bond in the ionic liquid [emim][tf2n]. Using IR/UV double resonance spectroscopy, we probe the microscopic structure of the [emim][tf2n] ion pair and its mono-deuterated, [emim-d1][tf2n], analog. Comparisons of the infrared absorption frequencies between these two species show that there are multiple conformers of the ion pair present in the gas phase and trapped through the molecular beam cooling process. Furthermore, each conformer has a characteristic red shift in the frequency of its C2–H group that reveals the variation in strength of a hydrogen bond between the cation and anion.

The thermal decomposition of nitromethane provides a classic example of the competition between roaming mediated isomerization and simple bond fission. A recent theoretical analysis suggests that as the pressure is increased from 2 to 200 Torr the product distribution undergoes a sharp transition from roaming dominated to bond-fission dominated. Laser schlieren densitometry is used to explore the variation in the effect of roaming on the density gradients for CH3NO2 decomposition in a shock tube for pressures of 30, 60, and 120 Torr at temperatures ranging from 1200 to 1860 K. A complementary theoretical analysis provides a novel exploration of the effects of roaming on the thermal decomposition kinetics. The analysis focuses on the roaming dynamics in a reduced dimensional space consisting of the rigid-body motions of the CH3 and NO2 radicals. A high-level reduced-dimensionality potential energy surface is developed from fits to large-scale multireference ab initio calculations. Rigid body trajectory simulations coupled with master equation kinetics calculations provide high-level a priori predictions for the thermal branching between roaming and dissociation. A statistical model provides a qualitative/semiquantitative interpretation of the results. Modeling efforts explore the relation between the predicted roaming branching and the observed gradients. Overall, the experiments are found to be fairly consistent with the theoretically proposed branching ratio, but they are also consistent with a no-roaming scenario and the underlying reasons are discussed. The theoretical predictions are also compared with prior theoretical predictions, with a related statistical model, and with the extant experimental data for the decomposition of CH3NO2, and for the reaction of CH3 with NO2.

We report new measurements of the laser induced fluorescence spectra of the HCCl A 1 A″← X 1 A ′ system in the 5150–6150 A region, obtained under jet-cooled conditions using a pulsed discharge source. The pure bending states (0, ν 2 ′ ,0) with ν 2 ′ =5–8 and combination states (0, ν 2 ′ , ν 3 ′ ) with ν 2 ′ =4–6 and ν 3 ′ =1,2 have been measured and the rotational structure fit to a asymmetric top rotational Hamiltonian, which yielded precise values for the band origins and effective rotational constants. Fluorescence lifetimes were measured for rotational levels in each band.

To further investigate the Renner-Teller effect and barriers to linearity and dissociation in the simplest singlet carbene (HCF), we recorded fluorescence excitation spectra of the pure bending transitions 2 n 0 with n = 0-7 and the combination bands 1 1 02 n 0 with n = 1-6 and 2 n 0 3 1 0 with n = 0-3 in the HCF A 1 A" ← X 1 A' system. The spectra were measured under jet-cooled conditions, using a pulsed-discharge source, and rotationally analyzed to yield precise values for the band origins and rotational constants. The derived A-state parameters are in excellent agreement with the predictions of ab initio electronic structure theory. The approach to linearity in the A state is evidenced in a sharp increase in the rotational constant A, as first reported by Kable and co-workers, and a minimum in the vibrational intervals near the 2 7 level. A fit of the vibrational intervals for the pure bending levels yields a barrier to linearity of 6300 ′ 270 cm - 1 above the vibrationless level. Our observation of the K a ' = 1 level of 1 1 2 6 places a lower limit on the A state barrier to dissociation of ∼ 8555 cm - 1 above the vibrationless level.

We report new measurements of fluorescence excitation spectra and single rovibronic level fluorescence lifetimes for (0,υ2′,0)←(0,0,0) bands with υ2′=0–6 in the HCF A 1 A ″← X 1 A ′ system. Due primarily to the Renner–Teller interaction, upper state levels with Ka⩾1 display an obvious lengthening of lifetime with increasing energy which is strongly correlated with the presence of vibronic and rovibronic perturbations, and increases dramatically as the barrier to linearity is approached. This system provides a textbook example of the small molecule limit of radiationless transitions.

The fluorescence excitation spectrum of the NH2 A 2 A 1 ← X 2 B 1 system in the region 2900–4300 A has been measured under jet-cooled conditions using a pulsed discharge source. A total of fourteen bands involving the pure bending states (0,υ2′,0) were observed and analyzed, including 10 bands not previously measured to our knowledge. The spectra are largely free from rotational perturbations at the low N values accessed in this experiment, and the molecular constants were obtained from a least squares fit to the determined spin–rovibronic term values. The derived constants are in good agreement with theoretical predictions incorporating the effects of orbital angular momentum on the spin–rotational fine structure.

The recombination of allyl radicals (C3H5), generated from the dissociation of 1,5-hexadiene or allyl iodide dilute in krypton, has been investigated in a diaphragmless shock tube using laser schlieren densitometry, LS, (900-1700 K, 10 ± 1, 29 ± 3, 57 ± 3, and 120 ± 4 Torr). The LS density gradient profiles were simulated and excellent agreement was found between simulations and experimental profiles. Rate coefficients for C3H5I → C3H5 + I and C3H5 + C3H5 → C6H10 were obtained and showed strong fall-off. Second order rate coefficients for allyl radical recombination were determined as k(1a,124Torr) = (2.6 ± 0.8) × 10(55)T( -12.995) exp(-8426/T), k(1a,57Torr) = (1.7 ± 0.5) × 10(60)T( -14.49) exp(-9344/T), and k(1a,30Torr) = (7.5 ± 2.3) × 10(66)T( -15.935) exp(-10192/T) cm(3) mol(-1)s(-1). The contribution of a disproportionation channel in allyl radical reactions was assessed, and the best agreement was obtained with no more than 5% disproportionation. Notably, because both the forward and back reactions of C6H10 ⇌ C3H5 + C3H5 were measured, utilizing two different precursors, the equilibrium constant of this reaction could be found, suggesting an entropy of formation of 1,5-hexadiene, 87.3 cal mol(-1 )K(-1), which is significantly smaller than that group additivity predicts, but larger than other reference literature values.

The title reaction has been investigated in a diaphragmless shock tube by laser schlieren densitometry over the temperature range 1163-1629 K and pressures of 60, 120, and 240 Torr. Methyl radicals were produced by dissociation of 2,3-butanedione in the presence of an excess of dimethyl ether. Rate coefficients for CH(3) + CH(3)OCH(3) were obtained from simulations of the experimental data yielding the following expression which is valid over the range 1100-1700 K: k = (10.19 ± 3.0)T(3.78) exp((-4878/T)) cm(3) mol(-1)s(-1). The experimental results are in good agreement with estimates by Curran and co-workers [Fischer, S. L.; Dryer, F. L.; Curran, H. J. Int. J. Chem. Kinet.2000, 32 (12), 713-740. Curran, H. J.; Fischer, S. L.; Dryer, F. L. Int. J. Chem. Kinet.2000, 32 (12), 741-759] but about a factor of 2.6 lower than those of Zhao et al. [Zhao, Z.; Chaos, M.; Kazakov, A.; Dryer, F. L. Int. J. Chem. Kinet.2008, 40 (1), 1-18].

State-resolved reactions of CH3D molecules containing both C-H and C-D stretching excitation with Cl atoms provide new vibrational spectroscopy and probe the consumption and disposal of vibrational energy in the reactions. The vibrational action spectra have three different components, the combination of the C-H symmetric stretch and the C-D stretch (nu1 + nu2), the combination of the C-D stretch and the C-H antisymmetric stretch (nu2 + nu4), and the combination of the C-D stretch and the first overtone of the CH3 bend (nu2 + 2nu5). The simulation for the previously unanalyzed (nu2 + nu4) state yields a band center of nu0 = 5215.3 cm(-1), rotational constants of A = 5.223 cm(-1) and B = 3.803 cm(-1), and a Coriolis coupling constant of zeta = 0.084. The reaction dynamics largely follow a spectator picture in which the surviving bond retains its initial vibrational excitation. In at least 80% of the reactive encounters of vibrationally excited CH3D with Cl, cleavage of the C-H bond produces CH2D radicals with an excited C-D stretch, and cleavage of the C-D bond produces CH3 radicals with an excited C-H stretch. Deviations from the spectator picture seem to reflect mixing in the initially prepared eigenstates and, possibly, collisional coupling during the reaction.

Experiments explore the influence of different C-H stretching eigenstates of CH3D on the reaction of CH3D with Cl(2P3/2). We prepare the mid |110|0(A1,E), mid |200|0(E), and mid |100|0+nu3 +nu5 eigenstates by direct midinfrared absorption near 6000 cm(-1). The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the |200|0(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to|100|0+nu3 +nu5 excitation, indicating mode-selective chemistry in which the C-H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, |110|0(A1) and |110|0(E), the ratio of reaction cross sections is 1.00 +/- 0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excited local modes corresponding to one quantum of C-H stretch in each of two distinct oscillators react exclusively to form C-H stretch excited CH2D products. Conversely, eigenstates containing stretch excitation in a single C-H oscillator form predominantly ground vibrational state CH2D products. Analyzing the product state yields for reaction of the |110|0(A1) state of CH3D yields an enhancement of 20 +/- 4 over the thermal reaction. A local mode description of the vibrational motion along with a spectator model for the reactivity accounts for all of the observed dynamics.

We report on the vibrational mode dependence of the 19F and 1H hyperfine interaction constants in the A1A" state of HCF, determined using polarization quantum beat spectroscopy. The nuclear spin/overall rotation coupling constants display a pronounced energy dependence and mode selectivity which can be traced to variations in both the A rotational constant and nuclear spin/electron orbital coupling constant a. In particular, modes containing C-F stretching excitation display significantly larger 19F spin-rotation constants, which is explained in terms of a decrease in back donation of electron density into the C 2p(pi) orbitals.

The pyrolysis of methyl acetate, 2% and 4% dilute in krypton, was investigated in a diaphragmless shock tube (DFST) using laser schlieren densitometry (LS). Experiments were performed at 122 ± 3 and 63 ± 2 Torr over the temperature range of 1492-2266 K. Master equation models for the four main dissociation paths of methyl acetate based on a prior study by Peukert et al. [S. Peukert, R. Sivaramakrishnan, M. Su and J. Michael, Combust. Flame, 2012, 159, 2312-2323] were refined and formed the basis for simulating the LS experiments. The density gradient profiles from the LS experiments indicate that the initial dissociation proceeds predominantly by breakage of the C-O bond leading ultimately to two methyl radicals and CO2, accounting for 83-88% of the methyl acetate loss over this temperature range. Rate coefficients for dissociation of methyl acetate were satisfactorily simulated with a master equation model, with modelled rate coefficients of k120 Torr = 9.06 × 10(81) × T(-19.07) × exp(-61 600K/T) s(-1), k60 Torr = 3.71 × 10(82) × T(-19.34) × exp(-61 200K/T) s(-1), and of k∞ = 1.97 × 10(30) × T(-3.80) × exp(-47 900K/T) s(-1) for the major channel, based on fitting to 120 Torr and 60 Torr data taken in this study. The model also captures the pressure dependency of methyl acetate dissociation and resolves an earlier discrepancy concerning the mechanism of dissociation of methyl acetate.

The reaction of Cl atoms with CH3D proceeds either by abstraction of hydrogen to produce HCl + CH2D or by abstraction of deuterium to produce DCl + CH3. Using Cl atoms with different amounts of translational energy, produced by photolysis of Cl2 with 309, 355, or 416 nm light, reveals the influence of translational energy on the relative reaction probability for the two channels. These measurements give an estimate of the energy barrier for the reaction for comparison to theory and indicate that tunneling is the dominant reaction mechanism at low collision energies. Adding two quanta of C-H stretching vibration causes the reaction to proceed readily at all collision energies. Detecting the vibrational state of the CH2D product shows that vibrational energy initially in the surviving C-H bond appears as vibrational excitation of the product, an example of spectator behavior in the reaction. The reaction produces both stretch and stretch-bend excited products except at the lowest collision energy. A subtle variation in the reaction probability of the lowest energy rotational states with translational energy may reflect the presence of a van der Waals well in the entrance channel.

Experiments explore the influence of different C–H stretching eigenstates of CH3D on the reaction of CH3D with Cl(P3∕22). We prepare the ∣110⟩∣0⟩(A1,E), ∣200⟩∣0⟩(E), and ∣100⟩∣0⟩+ν3+ν5 eigenstates by direct midinfrared absorption near 6000cm−1. The vibrationally excited molecules react with photolytic Cl atoms, and we monitor the vibrational states of the CH2D or CH3 radical products by 2+1 resonance enhanced multiphoton ionization. Initial excitation of the ∣200⟩∣0⟩(E) state leads to a twofold increase in CH2D products in the vibrational ground state compared to ∣100⟩∣0⟩+ν3+ν5 excitation, indicating mode-selective chemistry in which the C–H stretch motion couples more effectively to the H-atom abstraction coordinate than bend motion. For two eigenstates that differ only in the symmetry of the vibrational wave function, ∣110⟩∣0⟩(A1) and ∣110⟩∣0⟩(E), the ratio of reaction cross sections is 1.00±0.05, showing that there is no difference in enhancement of the H-atom abstraction reaction. Molecules with excit...