Photoelectron spectroscopy of small IBr[sup −](CO[sub 2])[sub n] (n=0–3) cluster anions

Photoelectron-photofragment coincidence spectroscopy of the mixed trihalides

Photoelectron-photofragment coincidence (PPC) spectroscopy is used to study the photodetachment, photodissociation, and dissociative photodetachment (DPD) of I₂Br^-, IBr₂ ^-, I₂Cl^-, and ICl₂ ^- at 266 nm. The mixed trihalides are asymmetric analogs of the well-studied I₃ ^- anion, with distinguishable dissociation asymptotes and the potential for selective bond breaking. The high beam energy PPC spectrometer used in this study couples an electrospray ionization source, a hexapole accumulation ion trap, and a linear accelerator to produce a 21 keV beam of a particular trihalide. Total, stable, and dissociative photoelectron spectra have been recorded for all the anions, except ICl₂ ^- that does not photodetach at 266 nm. A bound ground state (X) is observed for all the anions, and a dissociative first excited (A) state is also seen for I₂Br^- and I₂Cl^- at low electron kinetic energies (eKE). A 258 nm photoelectron spectrum recorded for I₂Br^- and I₂Cl^- rules out autodetachment of a dipole-bound state as the origin of the low eKE feature. The threshold detachment energy (TDE) of I₂X^- to the X state of the radical is similar to I₃ ^-, whereas the TDE to the radical A state increases with substitution of iodine for a lighter halogen. Two-body DPD is observed for I₂Br^- and I₂Cl^-, resulting in IBr/ICl + I + e^-. For IBr₂ ^- and ICl₂ ^-, the charge symmetric three-body photodissociation of [Br-I-Br]^- and [Cl-I-Cl]^- is seen yielding Br + Br and Br + Br^*, and Cl + Cl and Cl + Cl^* neutral fragments. Evidence for the minimum energy anion structure is observed in all cases, where the iodine atom is located at the center of the trihalide.

Probing the Hydrogen Bonding in Microsolvated Clusters of Au1,2-(Solv)n (Solv = C2H5OH, n-C3H7OH; n = 1-3 for Au-; n =1 for Au2-)

The microsolvation of gold anions in different alcohol solvents is demonstrated by the combination of anion photoelectron spectroscopy and quantum chemical calculations on the Au_1,2^-(Solv)_n (Solv = C₂H₅OH, n-C₃H₇OH; n = 1-3 for Au^-; n = 1 for Au₂^-). The microsolvation structures of these clusters and their corresponding neutrals are assigned by comparing calculations with experiments. In terms of overall regularity, the increasing solvation number (n) and carbon chain extension both can increase the stability of the anion. When n ≥ 2, these clusters have low-energy isomers, where conventional hydrogen bonds (HBs) compete with nonconventional HBs (NHBs). NHBs are dominant when n ≤ 2 and when n is increased, vice versa. Interestingly, a variety of theoretical calculations show that after the hydroxy H atom of the ethanol molecule forms a weak ionic HB with Au^-, there are two lowest conformations of ethanol, trans and gauche, which could be coexisting in the molecular beams. Some theoretical methods also suggest that the gauche isomer is more stable than the trans one, which indicates that Au^- may exist as a gold gauche effect similar to fluorine.

Formation of large clusters of CO2 around anions: DFT study reveals cooperative CO2 adsorption

The cooperative O⋯C secondary interactions compensate for the diminishing effect of primary anion⋯C interactions in anionic clusters of CO₂ molecules.

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 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 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 ΔacidH298 Ko(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 hydroxymethoxide anion, H2C(OH)O. J Chem Phys 2016;145:124317. [PMID: 27782682 DOI: 10.1063/1.4963225]

We report the negative ion photoelectron spectroscopy of the hydroxymethoxide anion, H₂C(OH)O^-. The photoelectron spectra show that 3.49 eV photodetachment produces two distinct electronic states of the neutral hydroxymethoxy radical (H₂C(OH)O^⋅). The H₂C(OH)O^⋅ ground state (X̃ ²A) photoelectron spectrum exhibits a vibrational progression consisting primarily of the OCO symmetric and asymmetric stretches, the OCO bend, as well as combination bands involving these modes with other, lower frequency modes. A high-resolution photoelectron spectrum aids in the assignment of several vibrational frequencies of the neutral H₂C(OH)O^⋅ radical, including an experimental determination of the H₂C(OH)O^⋅ 2ν₁₂ overtone of the H-OCO torsional vibration as 220(10) cm^-1. The electron affinity of H₂C(OH)O^⋅ is determined to be 2.220(2) eV. The low-lying à ²A excited state is also observed, with a spectrum that peaks ∼0.8 eV above the X̃ ²A state origin. The à ²A state photoelectron spectrum is a broad, partially resolved band. Quantum chemical calculations and photoelectron simulations aid in the interpretation of the photoelectron spectra. In addition, the gas phase acidity of methanediol is calculated to be 366(2) kcal mol^-1, which results in an OH bond dissociation energy, D₀(H₂C(OH)O-H), of 104(2) kcal mol^-1, using the experimentally determined electron affinity of the hydroxymethoxy radical.

Photoelectron Spectroscopy of cis-Nitrous Acid Anion (cis-HONO(-))

We report photoelectron spectra of cis-HONO(-) formed from an association reaction of OH(-) and NO in a pulsed, plasma-entrainment ion source. The experimental data are assigned to the cis-HONO(-) isomer, which is predicted to be the global minimum on the anion potential energy surface. We do not find evidence for a significant contribution from trans-HONO(-). Electron photodetachment of cis-HONO(-) with 1613, 1064, 532, 355, and 301 nm photons accesses the ground X̃ (1)A' (S0) and excited ã (3)A″ (T1) states of neutral HONO. The photoelectron spectrum resulting from detachment forming cis-HONO (S0) exhibits a long vibrational progression, dominated by overtones and combination bands involving the central O-N stretching and ONO bending vibrations. This indicates that there is a significant change in the central O-N bond length between cis-HONO(-) and cis-HONO (S0). The electron affinity (EA) of cis-HONO is determined to be 0.356(8) eV. We also report the dissociation energy (D0) of cis-HONO(-), forming OH(-) + NO, as 0.594(9) eV, which is a factor of 4 decrease in the central O-N bond strength compared to neutral cis-HONO. The T1 state of cis-HONO is shown to be ∼2.3 eV higher in energy than cis-HONO (S0). Electron photodetachment to form cis-HONO (T1) accesses a transition state along the HO-NO bond dissociation coordinate. The resulting photoelectron spectrum exhibits broad peaks spaced by the terminal N═O stretching frequency. Electronic structure calculations and photoelectron spectrum simulations reported here show very good agreement with the experimental data.

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.

New view of the ICN A continuum using photoelectron spectroscopy of ICN-

Negative-ion photoelectron spectroscopy of ICN(-) (X̃ (2)Σ(+)) reveals transitions to the ground electronic state (X̃ (1)Σ(+)) of ICN as well as the first five excited states ((3)Π(2), (3)Π(1), Π(0(-) ) (3), Π(0(+) ) (3), and (1)Π(1)) that make up the ICN A continuum. By starting from the equilibrium geometry of the anion, photoelectron spectroscopy characterizes the electronic structure of ICN at an elongated I-C bond length of 2.65 Å. Because of this bond elongation, the lowest three excited states of ICN ((3)Π(2), (3)Π(1), and Π(0(-) ) (3)) are resolved for the first time in the photoelectron spectrum. In addition, the spectrum has a structured peak that arises from the frequently studied conical intersection between the Π(0(+) ) (3) and (1)Π(1) states. The assignment of the spectrum is aided by MR-SO-CISD calculations of the potential energy surfaces for the anion and neutral ICN electronic states, along with calculations of the vibrational levels supported by these states. Through thermochemical cycles involving spectrally narrow transitions to the excited states of ICN, we determine the electron affinity, EA(ICN), to be 1.34(5) (+0.04∕-0.02) eV and the anion dissociation energy, D(0)(X̃ (2)Σ(+) I-CN(-)), to be 0.83 (+0.04/-0.02) eV.

Solvent-mediated charge redistribution in photodissociation of IBr(-) and IBr(-)(CO2)

A combined experimental and theoretical investigation of photodissociation dynamics of IBr(-) and IBr(-)(CO(2)) on the B ((2)Σ(1/2)(+)) excited electronic state is presented. Time-resolved photoelectron spectroscopy reveals that in bare IBr(-) prompt dissociation forms exclusively I∗ + Br(-). Compared to earlier dissociation studies of IBr(-) excited to the A' ((2)Π(1∕2)) state, the signal rise is delayed by 200 ± 20 fs. In the case of IBr(-)(CO(2)), the product distribution shows the existence of a second major (∼40%) dissociation pathway, Br∗ + I(-). In contrast to the primary product channel, the signal rise associated with this pathway shows only a 50 ± 20 fs delay. The altered product branching ratio indicates that the presence of one solvent-like CO(2) molecule dramatically affects the electronic structure of the dissociating IBr(-). We explore the origins of this phenomenon with classical trajectories, quantum wave packet studies, and MR-SO-CISD calculations of the six lowest-energy electronic states of IBr(-) and 36 lowest-energy states of IBr. We find that the CO(2) molecule provides sufficient solvation energy to bring the initially excited state close in energy to a lower-lying state. The splitting between these states and the time at which the crossing takes place depend on the location of the solvating CO(2) molecule.

Theoretical investigations of the time-resolved photodissociation dynamics of IBr(-)

The role of laser pulse width as well as other quantum mechanical effects in the interpretation of the observed time-resolved photoelectron spectra (TRPES) of IBr(-) are investigated using conditions that are chosen to reproduce those used for the experimental study of Mabbs et al. [ J. Chem. Phys. 2005 , 122 , 174305 ]. In that study, it was shown that one could correlate shifts in the frequency of the maximum in signal as a function of time to differences between the potential energies of the electronic states that are accessed by the pump and probe lasers. While this classical picture is attractive, it is based on a single trajectory with an initial I-Br separation that is ∼0.3 Å longer than the equilibrium value. In addition, it does not include the role of the pulse widths and other possible quantum effects. In the present work, the six lowest energy electronic states of IBr(-) were calculated at the MR-SO-CISD/aug-cc-pVDZ level of theory/basis set as a function of the I-Br distance. The TRPES of IBr(-) were calculated in three pulse regimes: an infinitesimally short pulse, an intermediate pulse that has a temporal full width at half-maximum (fwhm) of 300 fs, which was chosen to match the experimental value, and one that is 3 times longer than the experimental value. The resulting spectra are qualitatively different, and the sources of these differences are discussed. The intermediate pulse provides very good agreement with experiment with the introduction of no adjustable parameters. The origins of the features of the experimental signal are discussed in terms of this fully quantum mechanical picture.