A versatile, pulsed anion source utilizing plasma-entrainment: Characterization and applications

A novel pulsed anion source has been developed, using plasma entrainment into a supersonic expansion. A pulsed discharge source perpendicular to the main gas expansion greatly reduces unwanted “heating” of the main expansion, a major setback in many pulsed anion sources in use today. The design principles and construction information are described and several examples demonstrate the range of applicability of this anion source. Large OH−(Ar)n clusters can be generated, with over 40 Ar solvating OH−. The solvation energy of OH−(Ar)n, where n = 1-3, 7, 12, and 18, is derived from photoelectron spectroscopy and shows that by n = 12-18, each Ar is bound by about 10 meV. In addition, cis– and trans– HOCO− are generated through rational anion synthesis (OH− + CO + M → HOCO− + M) and the photoelectron spectra compared with previous results. These results, along with several further proof-of-principle experiments on solvation and transient anion synthesis, demonstrate the ability of this source to efficiently produce cold anions. With modifications to two standard General Valve assemblies and very little maintenance, this anion source provides a versatile and straightforward addition to a wide array of experiments

Photoelectron Spectroscopy of the Thiazate (NSO–) and Thionitrite (SNO–) Isomer Anions

Anion photoelectron spectra of the thiazate (NSO‾) and thionitrite (SNO‾) isomers are reported. The NSO‾ photoelectron spectrum showed several well-resolved vibronic transitions from the anion to the NSO radical neutral. The electron affinity of NSO was determined to be 3.113(1) eV. The fundamental vibrational frequencies of NSO were measured and unambiguously assigned to be 1202(6) cm‾¹ (ν₁, asymmetric stretch), 1010(10) cm‾¹ (ν₂, symmetric stretch), and 300(7) cm‾¹ (ν₃, bend). From the presence of vibrational hot band transitions, the fundamental vibrational frequencies of the NSO‾ anion were also measured: 1280(30) cm‾¹ (ν₁, asymmetric stretch), 990(20) cm‾¹ (ν₂, symmetric stretch), and 480(10) cm‾¹ (ν₃, bend). Combined with the previously measured ΔacidH⁰₂₉₈ K(HNSO), D₀(H‾NSO) was found to be 102(5) kcal/mol. Unlike the results from NSO‾, the SNO‾ photoelectron spectrum was broad with little structure, indicative of a large geometry change between the anion and neutral radical. In addition to the spectrally congested spectrum, there was evidence of a competition between photodetachment from SNO‾ and SNO‾ photodissociation to form S‾ + NO. Quantum chemical calculations were used to aid in the interpretation of the experimental data and agree well with the observed photoelectron spectra, particularly for the NSO‾ isomer

Photoelectron Spectroscopy of the Aminomethoxide Anion, H₂C(NH₂)O⁻

We report the photoelectron spectrum of the aminomethoxide anion (H₂C(NH₂)O⁻). The electron affinity (EA) of the aminomethoxy radical is determined to be 1.944(1) eV. Transitions to the ground (X̃ ²A″) and first excited (Ã ²A′) electronic states of aminomethoxy are observed, with the term energy measured to be T₀(Ã ← X̃) = 0.085(1) eV. A long vibrational progression is observed for the transition to the ground X̃ ²A″ electronic state of aminomethoxy, primarily consisting of OCN bending and HNH wagging vibrations, leading to the assignment of these two fundamental vibrational frequencies of H₂C(NH₂)O· X̃ ²A″. The gas-phase acidity of aminomethanol is calculated at the G4 level of theory to be ΔacidH0Ko = 374.0 kcal mol⁻¹, which, when combined with the experimental EA of aminomethoxy in a thermochemical cycle, provides a determination of the O–H bond dissociation energy, D₀(H₂C(NH₂)O–H) as 106(2) kcal mol⁻¹. Comparisons of the EAs and T₀(Ã ← X̃) for the aminomethoxy, methoxy, ethoxy, and hydroxymethoxy radicals provides insight into how the substituent group affects the electronic structure of singly substituted alkoxy radicals

Photochemistry of (OCS)<inf>n</inf>^- cluster ions

We report the photochemistry of (OCS)n- cluster ions following 395 nm (n=2-28) and 790 nm (n=2-4) excitation. In marked contrast to (CO2)n-, extensive bond breaking and rearrangement is observed. Three types of ionic products are identified: S2-(OCS)k, S-(OCS)k/OCS2-(OCS)k-1, and (OCS)k-. For n<16, 395 nm dissociation is dominated by S2--based fragments, supporting the theoretical prediction of a cluster core with a C2v(OCS)2- dimer structure and covalent C-C and S-S bonds. A shift in the branching ratio in favor of S--based products is observed near n=16, consistent with an opening of the photodissociation pathway of OCS- core-based clusters. These monomer-based cluster ions may coexist with the dimer-based clusters over a range of n, but electron detachment completely dominates photodissociation as long as their vertical electron detachment energy, increasing with addition of each solvent molecule, is less then the photon energy. An (OCS)2- conformer of C2 symmetry with a covalent C-C bond is believed to be responsible for 790 nm dissociation of (OCS)2-, yielding primarily OCS- products. The yield of OCS-, and thus the importance of the C2 form of (OCS)2- cluster core, decreases with increasing n, perhaps due to more favorable solvation of the C2v form of (OCS)2- and/or a solvent-induced increase in the rate of interconversion of conformers. The (OCS)k- products observed in 395 nm photodissociation of the larger (n≥7) clusters are attributed to photofragment caging. Formation and dissociation mechanisms of clusters with different core types are discussed. The photochemical properties of (OCS)n- are compared to those of the isovalent (CO2)n- and (CS2)n- species. © 1998 American Institute of Physics

Photoelectron spectroscopy and thermochemistry of o-, m-, and p-methylenephenoxide anions

The anionic products following (H + H⁺) abstraction from o-, m-, and p-methylphenol (cresol) are investigated using flowing afterglow-selected ion flow tube (FA-SIFT) mass spectrometry and anion photoelectron spectroscopy (PES). The PES of the multiple anion isomers formed in this reaction are reported, including those for the most abundant isomers, o-, m- and p-methylenephenoxide distonic radical anions. The electron affinity (EA) of the ground triplet electronic state of neutral m-methylenephenoxyl diradical was measured to be 2.227 ± 0.008 eV. However, the ground singlet electronic states of o- and p-methylenephenoxyl were found to be significantly stabilized by their resonance forms as a substituted cyclohexadienone, resulting in measured EAs of 1.217 ± 0.012 and 1.096 ± 0.007 eV, respectively. Upon electron photodetachment, the resulting neutral molecules were shown to have Franck–Condon active ring distortion vibrational modes with measured frequencies of 570 ± 180 and 450 ± 80 cm⁻¹ for the ortho and para isomers, respectively. Photodetachment to excited electronic states was also investigated for all isomers, where similar vibrational modes were found to be Franck–Condon active, and singlet–triplet splittings are reported. The thermochemistry of these molecules was investigated using FA-SIFT combined with the acid bracketing technique to yield ΔacidH°298K values of 341.4 ± 4.3, 349.1 ± 3.0, and 341.4 ± 4.3 kcal mol⁻¹ for the o-, m-, and p-methylenephenol radicals, respectively. Construction of a thermodynamic cycle allowed for an experimental determination of the bond dissociation energy of the O–H bond of m-methylenephenol radical to be 86 ± 4 kcal mol⁻¹, while this bond is significantly weaker for the ortho and para isomers at 55 ± 5 and 52 ± 5 kcal mol⁻¹, respectively. Additional EAs and vibrational frequencies are reported for several methylphenyloxyl diradical isomers, the negative ions of which are also formed by the reaction of cresol with O⁻

Photoelectron Spectroscopy of the Methide Anion: Electron Affinities of •CH₃ and •CD₃ and Inversion Splittings of CH₃‾ and CD₃‾

We report high-resolution photoelectron spectra of the simplest carbanions, CH₃– and CD₃–. The vibrationally resolved spectra are dominated by a long progression in the umbrella mode (ν₂) of •CH₃ and •CD₃, indicating a transition from a pyramidal C₃v anion to the planar D₃h methyl radical. Analysis of the spectra provides electron affinities of •CH₃ (0.093(3) eV) and •CD₃(0.082(4) eV). These results enable improved determination of the corresponding gas-phase acidities: ΔacidH0K°(CH₄) = 414.79(6) kcal/mol and ΔacidH0K°(CD₄) = 417.58(8) kcal/mol. On the basis of the photoelectron anisotropy distribution, the electron is photodetached from an orbital with predominant p-character, consistent with the sp3-hybridized orbital picture of the pyramidal anion. The double-well potential energy surface along the umbrella inversion coordinate leads to a splitting of the vibrational energy levels of the umbrella mode. The inversion splittings of CH₃– and CD₃– are 21(5) and 6(4) cm‾¹, respectively, and the corresponding anion umbrella vibrational frequencies are 444(13) and 373(12) cm‾¹, respectively. Quantum mechanical calculations reported herein show good agreement with the experimental data and provide insight regarding the electronic potential energy surface of CH₃–

Properties of tetramethyleneethane (TME) as revealed by ion chemistry and ion photoelectron spectroscopy

The negative ion chemistry and photoelectron spectra of[CH2=C(CH3)-C(CH2)2]- and [(CH2)2C-C(CH2)2]- have been studied. The negative ion photoelectron spectra reveal the tetramethyleneethane diradical, TME, to have two low-lying electronic states, X̃ and ã. The ground X̃ state is assigned as [TME] 1A and the excited ã state as [TME] 3B1. The energy separation between these states is about 2 kcal mol-1; ΔE[ã 3B1 ← X̃1A] ≅ 0.1 eV. The experimental electron affinities of the neutrals are: Eea[CH2=C(CH3)-C(CH2)2] = 0.654 ± 0.010 eV and Eea[(CH2)2C-C(CH2)2] = 0.855 ± 0.010 eV. The experimental gas phase acidities are: ΔacidH298[CH2=C(CH3)-C(CH 2)CH2-H] = 388 ± 3 kcal mol-1 and ΔacidH298[(CH2)2C-C(CH 2)-CH2-H] = 388 ± 4 kcal mol-1. These findings can be used to establish the bond energies and heats of formation: DH298[CH2=C(CH3)-C(CH2)CH 2-H] = 90 ± 3 kcal mol-1 and ΔfH298[(CH2)2C-C(CH 3)=CH2] = 48 ± 3 kcal mol-1; DH298[(CH2)2C-C(CH2)CH 2-H] = 94 ± 4 kcal mol-1 and ΔfH298[(CH2)2C-C(CH 2)2] = 90 ± 5 kcal mol-1