Probing the transition state with negative ion photodetachment: the chlorine atom + hydrogen chloride and bromine atom + hydrogen bromide reactions

Probing the activated complex of the F + NH3 reaction via a dipole-bound state

Experimental characterization of the transition state poses a significant challenge due to its fleeting nature. Negative ion photodetachment offers a unique tool for probing transition states and their vicinity. However, this approach is usually limited to Franck-Condon regions. For example, high-lying Feshbach resonances with an excited HF stretching mode (vHF = 2-4) were recently identified in the transition-state region of the F + NH3 → HF + NH2 reaction through photo-detaching FNH3- anions, but the direct photodetachment failed to observe the lower-lying vHF = 0,1 resonances and bound states due apparently to negligible Franck-Condon factors. Indeed, these weak transitions can be resonantly enhanced via a dipole-bound state (DBS) formed between an electron and the polar FNH3 species. In this study, we unveil a series of Feshbach resonances and bound states along the F + NH3 reaction path via a DBS by combining high-resolution photoelectron spectroscopy with high-level quantum dynamical computations. This study presents an approach for probing the activated complex in a reaction by negative ion photodetachment through a DBS.

Observation of resonances in the transition state region of the F + NH3 reaction using anion photoelectron spectroscopy

The transition state of a chemical reaction is a dividing surface on the reaction potential energy surface (PES) between reactants and products and is thus of fundamental interest in understanding chemical reactivity. The transient nature of the transition state presents challenges to its experimental characterization. Transition-state spectroscopy experiments based on negative-ion photodetachment can provide a direct probe of this region of the PES, revealing the detailed vibrational structure associated with the transition state. Here we study the F + NH₃ → HF + NH₂ reaction using slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled FNH₃^- anions. Reduced-dimensionality quantum dynamical simulations performed on a global PES show excellent agreement with the experimental results, enabling the assignment of spectral structure. Our combined experimental-theoretical study reveals a manifold of vibrational Feshbach resonances in the product well of the F + NH₃ PES. At higher energies, the spectra identify features attributed to resonances localized across the transition state and into the reactant complex that may impact the bimolecular reaction dynamics.

Preparation of vibrational quasi-bound states of the transition state complex BrHBr from the bihalide ion BrHBr. Phys Chem Chem Phys 2022;24:21250-21260. [PMID: 36040431 DOI: 10.1039/d2cp03120e]

Efficient strategies that allow the preparation of molecular systems in particular vibrational states are important in the application of quantum control schemes to chemical reactions. In this paper, we propose the preparation of quasi-bound vibrational states of the collinear transition state complex BrHBr, from vibrational states of the bihalide ion BrHBr^-, that favor the bond selective breakage of BrHBr. The results shown complement the investigation that we reported in a previous paper, [A. J. Garzón-Ramírez, J. G. López and C. A. Arango, Int. J. Quantum Chem., 2018, 24, e25784], in which we demonstrated the feasibility of controlling the bond selective decomposition of the collinear BrHBr using linear combinations of reactive resonances. We employed a dipole moment surface, calculated at the QCISD/d-aug-cc-pVTZ level of theory, to simulate the interaction of the BrHBr^- ground vibrational state with heuristically optimized sequences of ultrashort infrared linear chirped laser pulses to achieve a target vibrational state, resulting from expanding a chosen linear combination of reactive resonances of BrHBr in terms of vibrational eigenstates of BrHBr^-. The results of our simulations show final states that capture the most relevant features of the target state with different levels of description depending on the sequence of laser pulses employed. We also discuss ways of improving the description of the target state and possible limitations of our approach.

Mass spectrometry studies of nitrene anions

Nitrene anions are a class of reactive intermediates that provide a means for studying the corresponding neutral molecules via electron photodetachment spectroscopy and photoelectron spectroscopy. The added electron makes it possible for protected nitrene anions to be manipulated by external electric and magnetic fields of a mass spectrometer. Nitrene anions also display their own unique reactivities as reagents, which have been investigated using ion/molecule reactions. Mass spectrometry of negative ions has thereby provided information on the electronic states, reactivities, and thermochemical properties of nitrene intermediates. This review also includes a discussion of condensed-phase nitrene anions.

Quantum calculations of the photoelectron spectra of the OH-·NH3 anion: implications for OH + NH3→ H2O + NH2 reaction dynamics

We present the results of quantum dynamics calculations for analyzing the experimentally measured photoelectron spectra of the OH^-·NH₃ anion complex. Detachment of an excess electron of OH^-·NH₃ initially produces a molecular arrangement, which is close to the transition-state structure of the neutral OH + NH₃→ H₂O + NH₂ hydrogen abstraction reaction due to the Franck-Condon principle, and thus finally leads to the OH + NH₃ or H₂O + NH₂ asymptotic channel. We used both the path integral method and the reduced-dimensionality quantum wave packet method to simulate the photoelectron spectra of the OH^-·NH₃ anion. The calculated spectra were found to be in qualitative agreement with the experimental spectra. It was found that the photodetached complex mainly dissociates into the OH + NH₃ channel; however, we found that the hydrogen exchange process also contributes to the photodetachment spectra.

Franck–Condon simulations of transition-state spectra for the OH + H2O and OD + D2O reactions

Quantum wave packet calculations in reduced dimensions were performed to analyze the experimentally measured transition-state spectra of the OH + H₂O and OD + D₂O hydrogen exchange reactions.

Quantum dynamics analysis of transition-state spectrum for the SH + H2S → H2S + SH reaction

Quantum dynamics calculations were performed to understand transition-state spectroscopy of the SH + H₂S hydrogen atom transfer reaction.

Dissociative photodetachment vs. photodissociation of aromatic carboxylates: the benzoate and naphthoate anions

Photodetachment leads to a stable radical and to dissociation. Both processes are characterized by the kinetic energy release of the neutral particles.

Slow Photoelectron Velocity-Map Imaging of Cryogenically Cooled Anions

Slow photoelectron velocity-map imaging spectroscopy of cryogenically cooled anions (cryo-SEVI) is a powerful technique for elucidating the vibrational and electronic structure of neutral radicals, clusters, and reaction transition states. SEVI is a high-resolution variant of anion photoelectron spectroscopy based on photoelectron imaging that yields spectra with energy resolution as high as 1-2 cm^-1. The preparation of cryogenically cold anions largely eliminates hot bands and dramatically narrows the rotational envelopes of spectral features, enabling the acquisition of well-resolved photoelectron spectra for complex and spectroscopically challenging species. We review the basis and history of the SEVI method, including recent experimental developments that have improved its resolution and versatility. We then survey recent SEVI studies to demonstrate the utility of this technique in the spectroscopy of aromatic radicals, metal and metal oxide clusters, nonadiabatic interactions between excited states of small molecules, and transition states of benchmark bimolecular reactions.

Dynamical resonances in chemical reactions

The transition state is a key concept in the field of chemistry and is important in the study of chemical kinetics and reaction dynamics.

Nonadiabatic quantum dynamics calculations of transition state spectroscopy of I + HI and I + DI reactions: the existence of long life vibrational bonding resonances

Nonadiabatic quantum dynamics calculations were performed to understand the transition state spectroscopy of I + HI and I + DI reactions.

Dynamics of transient speciesviaanion photodetachment

Recent experimental and theoretical advances in transient reaction dynamics probed by photodetachment of polyatomic anions are reviewed.

Time-of-flight mass spectrometry: Introduction to the basics

The intention of this tutorial is to introduce into the basic concepts of time-of-flight mass spectrometry, beginning with the most simple single-stage ion source with linear field-free drift region and continuing with two-stage ion sources combined with field-free drift regions and ion reflectors-the so-called reflectrons. Basic formulas are presented and discussed with the focus on understanding the physical relations of geometric and electric parameters, initial distribution of ionic parameters, ion flight times, and ion flight time incertitude. This tutorial is aimed to help the applicant to identify sources of flight time broadening which limit good mass resolution and sources of ion losses which limit sensitivity; it is aimed to stimulate creativity for new experimental approaches by discussing a choice of instrumental options and to encourage those who toy with the idea to build an own time-of-flight mass spectrometer. Large parts of mathematics are shifted into a separate chapter in order not to overburden the text with too many mathematical deviations. Rather, thumb-rule formulas are supplied for first estimations of geometry and potentials when designing a home-built instrument, planning experiments, or searching for sources of flight time broadening. © 2016 Wiley Periodicals, Inc. Mass Spec Rev 36:86-109, 2017.

Influence of vibration in the reactive scattering of D + MuH: the effect of dynamical bonding

The dynamics of the D + MuH(v = 1) reaction has been investigated using time-independent quantum mechanical calculations. The total reaction cross sections and rate coefficients have been calculated for the two exit channels of the reaction leading, respectively, to DMu + H and DH + Mu. Over the 100-1000 K temperature range investigated the rate coefficients for the DMu + H channel are of the order of 10(-10) cm(3) s(-1) and those for the DH + Mu channel vary between 1 × 10(-12) and 8 × 10(-11) cm(3) s(-1). These results point to a virtually barrierless reaction for the DMu + H channel and to the presence of a comparatively small barrier for the DH + Mu channel and are consistent with the profiles of their respective collinear vibrationally adiabatic potentials (VAPs). The effective barrier in the VAP of the DH + Mu channel is located in the reactant valley and, consequently, translation is found to be more efficient than vibration for the promotion of the reaction over a large energy interval in the post threshold region. Below this barrier, the DH + Mu channel can be accessible through an indirect mechanism implying crossing from the DMu + H pathway. The most salient feature found in the present study is revealed in the total reaction cross section for the DMu + H channel, which shows a sharp resonance caused by the presence of a deep well in the vibrationally adiabatic potential. This well has a dynamical origin, reminiscent of that found recently in the vibrationally bonded BrMuBr complex [Fleming, et al., Angew. Chem., Int. Ed., 2014, 53, 1], and is due to the stabilizing effect of the light Mu atom oscillating between the heavier H and D isotopes and to the bond softening associated with vibrational excitation of MuH.

"Vibrational bonding": a new type of chemical bond is discovered

A long-sought but elusive new type of chemical bond, occurring on a minimum-free, purely repulsive potential energy surface, has recently been convincingly shown to be possible on the basis of high-level quantum-chemical calculations. This type of bond, termed a vibrational bond, forms because the total energy, including the dynamical energy of the nuclei, is lower than the total energy of the dissociated products, including their vibrational zero-point energy. For this to be the case, the ZPE of the product molecule must be very high, which is ensured by replacing a conventional hydrogen atom with its light isotope muonium (Mu, mass = 1/9 u) in the system Br-H-Br, a natural transition state in the reaction between Br and HBr. A paramagnetic species observed in the reaction Mu +Br2 has been proposed as a first experimental sighting of this species, but definitive identification remains challenging.

The hydrogen bond strength of the phenol–phenolate anionic complex: a computational and photoelectron spectroscopic study

The phenol–phenolate anionic complex was studied in vacuo by negative ion photoelectron spectroscopy using 193 nm photons and by density functional theory (DFT) computations at the ωB97XD/6-311+G(2d,p) level.

Strong, low-barrier hydrogen bonds may be available to enzymes

The debate over the possible role of strong, low-barrier hydrogen bonds in stabilizing reaction intermediates at enzyme active sites has taken place in the absence of an awareness of the upper limits to the strengths of low-barrier hydrogen bonds involving amino acid side chains. Hydrogen bonds exhibit their maximal strengths in isolation, i.e., in the gas phase. In this work, we measured the ionic hydrogen bond strengths of three enzymatically relevant model systems in the gas phase using anion photoelectron spectroscopy; we calibrated these against the hydrogen bond strength of HF2(-), measured using the same technique, and we compared our results with other gas-phase experimental data. The model systems studied here, the formate-formic acid, acetate-acetic acid, and imidazolide-imidazole anionic complexes, all exhibit very strong hydrogen bonds, whose strengths compare favorably with that of the hydrogen bifluoride anion, the strongest known hydrogen bond. The hydrogen bond strengths of these gas-phase complexes are stronger than those typically estimated as being required to stabilize enzymatic intermediates. If there were to be enzyme active site environments that can facilitate the retention of a significant fraction of the strengths of these isolated (gas-phase), hydrogen bonded couples, then low-barrier hydrogen bonding interactions might well play important roles in enzymatic catalysis.

THE PARTIAL POTENTIAL ENERGY SURFACE AND SCATTERING RESONANCE STATE OF THE STATE-TO-STATE REACTION Br + HBr(v) → BrH(v′) + Br

In this paper, the partial potential energy surface (PPESs) of the Br + HBr and Br - + HBr systems including the minimum energy path and the vibrational potential curves were constructed at MP2/6-311++G** level, based on the conception and constructing approach of the PPESs previously proposed. These results obtained from the PPESs were compared with those from the high resolved threshold photodetachment spectrum of the BrHBr - anion measured by Neumark et al., J Phys Chem94, 1377–1388, 1990. On the basis of the PPESs, the scattering resonance states of the Br + HBr (v) → BrH (v′) + Br state-to-state reaction were studied and the satisfactory results were obtained. Subsequently, we calculated the width and lifetime of the resonance states in this reaction by the one-dimensional square potential well model, and obtained some results consistent to the experiments.

PARTIAL POTENTIAL ENERGY SURFACE AND ITS APPLICATIONS IN REACTIVE RESONANCES

The conception of partial potential energy surface (PPES) is presented in this paper. PPES can be abstracted from complete potential energy surface (CPES), therefore, it can be constructed with ab initio quantum chemical method. For the systems of H + H 2→ H 2+ H , I + HI → IH + I and I -+ HI → IH + I -, the construction and applications of PPES are proposed as typical examples. It can be seen that the applications of PPES demonstrate remarkable virtues in the analysis of reaction mechanism and the formation of scattering resonance states.

Photodetachment-photoelectron spectroscopy of HS- x H2S and DS- x D2S: the transition states of the SH + H2S and SD + D2S reactions

The transition state region for neutral hydrogen transfer reactions can be accessed by photodetachment of a stable negative ion with a geometry similar to that of the neutral transition state. In this work the SH + H(2)S and SD + D(2)S reactions are investigated by photodetachment-photoelectron spectroscopy of HS(-) x H(2)S and DS(-) x D(2)S. The spectra exhibit vibrational structure which is attributed to the antisymmetric stretching mode (H-atom motion) of the neutral transitions state for H-atom transfer. The spectra are compared to one-dimensional simulations performed using a wave packet propagation scheme. Electronic structure calculations of the anionic, neutral and transition-state geometries and calculations of the vertical detachment energy at different levels of theory are used to support the analysis of the spectra. A vertical detachment energy VDE of 3.06 eV has been determined.

Slow electron velocity-map imaging of negative ions: applications to spectroscopy and dynamics

Anion photoelectron spectroscopy (PES) has become one of the most versatile techniques in chemical physics. This article briefly reviews the history of anion PES and some of its applications. It describes efforts to improve the resolution of this technique, including anion zero electron kinetic energy (ZEKE) and the recently developed method of slow electron velocity-map imaging (SEVI). Applications of SEVI to studies of vibronic coupling in open-shell systems and the spectroscopy of prereactive van der Waals complexes are then discussed.

Photoelectron imaging of I2− at 5.826eV

We report the anion photoelectron spectrum of I2- taken at 5.826 eV detachment energy using velocity mapped imaging. The photoelectron spectrum exhibits bands resulting from transitions to the bound regions of the X 1Sigmag+(0g+), A' 3Piu(2u), A 3Piu(1u), and B 3Piu(0u+) electronic states as well as bands resulting from transitions to the repulsive regions of several I2 electronic states: the B' 3Piu(0u-), B" 1Piu(1u), 3Pig(2g), a 3Pig(1g), 3Pig(0g-), and C 3Sigmau+(1u) states. We simulate the photoelectron spectrum using literature parameters for the I2- and I2 ground and excited states. The photoelectron spectrum includes bands resulting from transitions to several high-lying excited states of I2 that have not been seen experimentally: 3Pig(0g-), 1Pig3(1g), 1 3Sigmag-3(0g+), and the 1Sigmag-3(0u-) states of I2. Finally, the photoelectron spectrum at 5.826 eV allows for the correction of a previous misassignment for the vertical detachment energy of the I2 B 3Piu(0u+) state.

Probing chemical dynamics with negative ions

Experiments are reviewed in which key problems in chemical dynamics are probed by experiments based on photodetachment and/or photoexcitation of negative ions. Examples include transition state spectroscopy of biomolecular reactions, spectroscopy of open shell van der Waals complexes, photodissociation of free radicals, and time-resolved dynamics in clusters. The experimental methods used in these investigations are described along with representative systems that have been studied.

Spectroscopic investigation of Al2N and its anion via negative ion photoelectron spectroscopy

Negative ion photoelectron spectroscopy was used to elucidate the electronic and geometric structure of the gaseous Al2N/Al2N- molecules, using photodetachment wavelengths of 416 nm (2.977 eV), 355 nm (3.493 eV), and 266 nm (4.661 eV). Three electronic bands are observed and assigned to the X2Sigma(u)+ <-- X1Sigma(g)+, A2Pi(u) <-- X1Sigma(g)+, and B2Sigma(g)+ <-- X1Sigma(g)+ electronic transitions, with the caveat that one or both excited states may be slightly bent. With the aid of density functional theory calculations and Franck-Condon spectral simulations, we determine the adiabatic electron affinity of Al2N, 2.571 +/- 0.008 eV, along with geometry changes upon photodetachment, vibrational frequencies, and excited-state term energies. Observation of excitation of the odd vibrational levels of the antisymmetric stretch (nu3) suggests a breakdown of the Franck-Condon approximation, caused by the vibronic coupling between the X2Sigma(u)+ and B2Sigma(g)+ electronic states through the nu3 mode.