ZEKE Photoelectron Spectroscopy of <i>p</i>‑Fluorophenol···H₂S/H₂O Complexes and Dissociation Energy Measurement Using the Birge–Sponer Extrapolation Method

In this work we have shown that the Birge–Sponer extrapolation method can be successfully used to determine the dissociation energies (<i>D</i>₀) of noncovalently bound complexes. The O–H···S hydrogen-bonding interaction in the cationic state of the <i>p</i>-fluorophenol···H₂S complex was characterized using zero kinetic energy (ZEKE) photoelectron spectroscopy. This is the first ZEKE report on the O–H···S hydrogen-bonding interaction. The adiabatic ionization energy (AIE) of the complex was determined as 65 542 cm^–1. Various intermolecular and intramolecular vibrational modes of the cation were assigned. A long progression was observed in the intermolecular stretching mode (σ) of the complex with significant anharmonicity along this mode. The anharmonicity information was used to estimate the dissociation energy (<i>D</i>₀) in the cationic state using the Birge–Sponer extrapolation method. The <i>D</i>₀ was estimated as 9.72 ± 1.05 kcal mol^–1. The ZEKE photoelectron spectra of analogous complex FLP···H₂O was also recorded for the sake of comparison. The AIE was determined as 64 082 cm^–1. The intermolecular stretching mode in this system, however, was found to be quite harmonic, unlike that in the H₂S complex. The dissociation energies of both the complexes, along with those of a few benchmark systems, such as phenol···H₂O and indole···benzene complexes, were computed at various levels of theory such as MP2 at the complete basis set limit, ωB97X-D, and CCSD­(T). It was found that only the ωB97X-D level values were in excellent agreement with the experimental results for the benchmark systems for the ground as well as the cationic states. The dissociation energy of the (FLP···H₂S)⁺ complex determined by the Birge–Sponer extrapolation was about ∼18% lower than that computed at the ωB97X-D level

Dissociation Energies of Sulfur-Centered Hydrogen-Bonded Complexes

In this work we have determined dissociation energies of O–H···S hydrogen bond in the H₂S complexes of various phenol derivatives using 2-color-2-photon photofragmentation spectroscopy in combination with zero kinetic energy photoelectron (ZEKE-PE) spectroscopy. This is the first report of direct determination of dissociation energy of O–H···S hydrogen bond. The ZEKE-PE spectra of the complexes revealed a long progression in the intermolecular stretching mode with significant anharmonicity. Using the anharmonicity information and experimentally determined dissociation energy, we also validated Birge–Sponer (B-S) extrapolation method, which is an approximate method to estimate dissociation energy. Experimentally determined dissociation energies were compared with a variety of ab initio calculations. One of the important findings is that ωB97X-D functional, which is a dispersion corrected DFT functional, was able to predict the dissociation energies in both the cationic as well as the ground electronic state very well for almost every case

Acid–Base Formalism Extended to Excited State for O–H···S Hydrogen Bonding Interaction

Hydrogen bond can be regarded as an interaction between a base and a proton covalently bound to another base. In this context the strength of hydrogen bond scales with the proton affinity of the acceptor base and the p<i>K</i>_a of the donor, i.e., it follows the acid–base formalism. This has been amply demonstrated in conventional hydrogen bonds. Is this also true for the unconventional hydrogen bonds involving lesser electronegative elements such as sulfur atom? In our previous work, we had established that the strength of O–H···S hydrogen bonding (HB) interaction scales with the proton affinity (PA) of the acceptor. In this work, we have investigated the other counterpart, i.e., the H-bonding interaction between the photoacids with different p<i>K</i>_a values with a common base such as the H₂O and H₂S. The 1:1 complexes of five <i>para</i> substituted phenols <i>p</i>-aminophenol, <i>p</i>-cresol, <i>p</i>-fluorophenol, <i>p</i>-chlorophenol, and <i>p</i>-cyanophenol with H₂O and H₂S were investigated experimentally and computationally. The investigations were also extended to the excited states. The experimental observations of the spectral shifts in the O–H stretching frequency and the S₁–S₀ band origins were correlated with the p<i>K</i>_a of the donors. Ab initio calculations at the MP2 and various dispersion corrected density functional levels of theory were performed to compute the dissociation energy (<i>D</i>₀) of the complexes. The quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) method, natural bonding orbital (NBO) analysis, and natural decomposition analysis (NEDA) were carried out for further characterization of HB interaction. The O–H stretching frequency red shifts and the dissociation energies were found to be lower for the O–H···S hydrogen bonded systems compared to those for the O–H···O H-bound systems. Despite being dominated by the dispersion interaction the O–H···S interaction in the H₂S complexes also conformed to the acid–base formalism, i.e., the <i>D</i>₀ and the O–H red shift scaled with the p<i>K</i>_a of the donor, similar to that observed in the O–H···O interaction. However, the two classes of H-bonds follow different correlations. In addition we also discuss the nuances associated with the similarity and differences in the hydrogen bonding properties of the two classes in the ground electronic state as well as in the excited state

O–H···S Hydrogen Bonds Conform to the Acid–Base Formalism

Hydrogen bonding interaction between the ROH hydrogen bond donor and sulfur atom as an acceptor has not been as well characterized as the O–H···O interaction. The strength of O–H···O interactions for a given donor has been well documented to scale linearly with the proton affinity (PA) of the H-bond acceptor. In this regard, O–H···O interactions conform to the acid–base formalism. The importance of such correlation is to be able to estimate molecular property of the complex from the known thermodynamic data of its constituents. In this work, we investigate the properties of O–H···S interaction in the complexes of the H-bond donor and sulfur containing acceptors of varying proton affinity. The hydrogen bonded complexes of <i>p</i>-Fluorophenol (FP) with four different sulfur containing acceptors and their oxygen analogues, namely H₂O/H₂S, MeOH/MeSH, Me₂O/Me₂S and tetrahydrofuran (THF)/tetrahydrothiophene (THT) were characterized in regard to its S₁–S₀ excitation spectra and the IR spectra. Two-color resonantly enhanced multiphoton ionization (2c-R2PI), resonant ion-dip infrared (RIDIR) spectroscopy, and IR-UV hole burning spectroscopic techniques were used to probe the hydrogen bonds in the aforementioned complexes. The spectroscopic data along with the ab initio calculations were used to deduce the strength of the O–H···S hydrogen bonding interactions in these system relative to that in the O–H···O interactions. It was found that, despite being dominated by the dispersion interaction, the O–H···S interactions conform to the acid–base formalism as in the case of more conventional O–H···O interactions. The dissociation energies and the red shifts in the O–H stretching frequencies correlated very well with the proton affinity of the acceptors. However, the O–H···S interaction did not follow the same correlation as that in the O–H···O H-bond. The energy decomposition analysis showed that the dissociation energies and the red shifts in the O–H stretching frequencies follow a unified correlation if these two parameters were correlated with the sum of the charge transfer and the exchange component of the total binding energy

Ultrafast Roaming Mechanisms in Ethanol Probed by Intense Extreme Ultraviolet Free-Electron Laser Radiation: Electron Transfer versus Proton Transfer

Ultrafast H$_2$$^+$ and H$_3$$^+$ formation from ethanol is studied using pump-probe spectroscopy with an extreme ultraviolet (XUV) free-electron laser. The first pulse creates a dication, triggering H2 roaming that leads to H$_2$$^+$ and H$_3$$^+$ formation, which is disruptively probed by a second pulse. At photon energies of 28 and 32 eV, the ratio of H$_2$$^+$ to H$_3$$^+$ increases with time delay, while it is flat at a photon energy of 70 eV. The delay-dependent effect is ascribed to a competition between electron and proton transfer. High-level quantum chemistry calculations show a flat potential energy surface for H$_2$ formation, indicating that the intermediate state may have a long lifetime. The ab initio molecular dynamics simulation confirms that, in addition to the direct emission, a small portion of H$_2$ undergoes a roaming mechanism that leads to two competing pathways: electron transfer from H2 to C$_2$H$_4$O$_2$$^+$ and proton transfer from C$_2$H$_4$O$_2$$^+$ to H$_2$

Strong-field induced fragmentation and isomerization of toluene probed by ultrafast femtosecond electron diffraction and mass spectrometry

We investigate the fragmentation and isomerization of toluene molecules induced by strong-field ionization with a femtosecond near-infrared laser pulse. Momentum-resolved coincidence time-of-flight ion mass spectrometry is used to determine the relative yield of different ionic products and fragmentation channels as a function of laser intensity. Ultrafast electron diffraction is used to capture the structure of the ions formed on a picosecond time scale by comparing the diffraction signal with theoretical predictions. Through the combination of the two measurements and theory, we are able to determine the main fragmentation channels and to distinguish between ions with identical mass but different structures. In addition, our diffraction measurements show that the independent atom model, which is widely used to analyze electron diffraction patterns, is not a good approximation for diffraction from ions. We show that the diffraction data is in very good agreement with ab initio scattering calculations

Hydrogen migration in inner-shell ionized halogenated cyclic hydrocarbons

Abstract We have studied the fragmentation of the brominated cyclic hydrocarbons bromocyclo-propane, bromocyclo-butane, and bromocyclo-pentane upon Br(3d) and C(1s) inner-shell ionization using coincidence ion momentum imaging. We observe a substantial yield of CH3+ fragments, whose formation requires intramolecular hydrogen (or proton) migration, that increases with molecular size, which contrasts with prior observations of hydrogen migration in linear hydrocarbon molecules. Furthermore, by inspecting the fragment ion momentum correlations of three-body fragmentation channels, we conclude that CHx⁺ fragments (with x = 0, …, 3) with an increasing number of hydrogens are more likely to be produced via sequential fragmentation pathways. Overall trends in the molecular-size-dependence of the experimentally observed kinetic energy releases and fragment kinetic energies are explained with the help of classical Coulomb explosion simulations

Valence shell electronically excited states of norbornadiene and quadricyclane

The absolute photoabsorption cross sections of norbornadiene (NBD) and quadricyclane (QC), two isomers with chemical formula C7H8 that are attracting much interest for solar energy storage applications, have been measured from threshold up to 10.8 eV using the Fourier transform spectrometer at the SOLEIL synchrotron radiation facility. The absorption spectrum of NBD exhibits some sharp structure associated with transitions into Rydberg states, superimposed on several broad bands attributable to valence excitations. Sharp structure, although less pronounced, also appears in the absorption spectrum of QC. Assignments have been proposed for some of the absorption bands using calculated vertical transition energies and oscillator strengths for the electronically excited states of NBD and QC. Natural transition orbitals indicate that some of the electronically excited states in NBD have a mixed Rydberg/valence character, whereas the first ten excited singlet states in QC are all predominantly Rydberg in the vertical region. In NBD, a comparison between the vibrational structure observed in the experimental 11B1–11A1 (3sa1 ← 5b1) band and that predicted by Franck–Condon and Herzberg–Teller modeling has necessitated a revision of the band origin and of the vibrational assignments proposed previously. Similar comparisons have encouraged a revision of the adiabatic first ionization energy of NBD. Simulations of the vibrational structure due to excitation from the 5b2 orbital in QC into 3p and 3d Rydberg states have allowed tentative assignments to be proposed for the complex structure observed in the absorption bands between ∼5.4 and 7.0 eV

Monitoring the evolution of relative product populations at early times during a photochemical reaction

Identifying multiple rival reaction products and transient species formed during ultrafast photochemical reactions and determining their time-evolving relative populations are key steps toward understanding and predicting photochemical outcomes. Yet, most contemporary ultrafast studies struggle with clearly identifying and quantifying competing molecular structures/species among the emerging reaction products. Here, we show that mega-electronvolt ultrafast electron diffraction in combination with ab initio molecular dynamics calculations offer a powerful route to determining time-resolved populations of the various isomeric products formed after UV (266 nm) excitation of the five-membered heterocyclic molecule 2(5H)-thiophenone. This strategy provides experimental validation of the predicted high (∼50%) yield of an episulfide isomer containing a strained three-membered ring within ∼1 ps of photoexcitation and highlights the rapidity of interconversion between the rival highly vibrationally excited photoproducts in their ground electronic state