The near ultraviolet photodissociation dynamics of 2- and 3-substituted thiophenols: Geometric vs. electronic structure effects

Bond analysis in meta- and para-substituted thiophenols: overlap descriptors, local mode analysis, and QTAIM

CONTEXT

This study delves into the chemical nuances of thiophenols and their derivatives through a comprehensive computational analysis, moving beyond traditional energetic perspectives such as bond dissociation enthalpy and S-H dissociation dynamics. By employing the overlap model along with its topological descriptors (OP/TOP), quantum theory of atoms in molecules (QTAIM), and local vibrational mode (LVM) theories, the research provides a deeper understanding of the S-H and C-S bonding scenarios in substituted thiophenols. The investigation follows the electron-donating capacity of S-H substituent variation with the nature and positioning of other ring substituents. Energy profile analyses indicate distinct stability differences in the cis and trans conformations of meta- and para-PhSH systems, influenced by the electron-donating strength of these substituents. The study also uncovers significant variations in S-H bond distances and descriptor values, particularly in para-substituted PhSH, reflecting the influence of electron-donating or withdrawing substituents. In contrast, alterations at the meta-position show minimal effects on C-S bond descriptors, while para-substitutions markedly influence C-S bond characteristics, demonstrating a clear correlation with the electron-donating or withdrawing capabilities of the substituents. This research sheds light on the intricate bond dynamics in aromatic systems with diverse substituents, highlighting the complex interaction between electronic effects and molecular conformation.

METHODS

The study employs the ω B97X-D/Def2TZVP level of theory for molecular geometries, ensuring accurate characterization of structures as true minima via analytical harmonic frequency determination. The electronic properties of S-H and C-S bonds in variously substituted thiophenols were analyzed using OP/TOP, QTAIM, and LVM methodologies. Computational processes, including conformational scans, geometry optimizations, and vibrational frequency calculations, were conducted using Gaussian 09, with ultra-fine integration grids and tight convergence criteria for the SCF procedure. Bond descriptors were computed utilizing ChemBOS, Multiwfn, and LModeA software, providing a robust and detailed examination of bond properties.

Non-Born-Oppenheimer effects in molecular photochemistry: an experimental perspective

Non-adiabatic couplings between Born-Oppenheimer (BO)-derived potential energy surfaces are now recognized as pivotal in describing the non-radiative decay of electronically excited molecules following photon absorption. This opinion piece illustrates how non-BO effects provide photostability to many biomolecules when exposed to ultraviolet radiation, yet in many other cases are key to facilitating 'reactive' outcomes like isomerization and bond fission. The examples are presented in order of decreasing molecular complexity, spanning studies of organic sunscreen molecules in solution, through two families of heteroatom containing aromatic molecules and culminating with studies of isolated gas phase H₂O molecules that afford some of the most detailed insights yet available into the cascade of non-adiabatic couplings that enable the evolution from photoexcited molecule to eventual products. This article is part of the theme issue 'Chemistry without the Born-Oppenheimer approximation'.

Substitution-induced Nonplanarity of 3-Fluorothioanisole in the First Electronically Excited State

Vibronic spectra of 3-fluorothioanisole (3FTA) in the first electronic excited state (S₁) and the cationic ground state (D₀) have been obtained by one-color resonant two-photon ionization (1C-R2PI) and mass-analyzed threshold ionization (MATI) spectroscopy. Spectroscopic measurements and theoretical calculations indicate that both cis- and trans-rotamers of the 3FTA molecule are stable and coexist in the S₀ (the electronic ground state) and D₀ states, and the cis-rotamer is shown to be slightly more stable than the trans-rotamer. In the S₁ state, theoretical calculations predict a stable gauche-structure of 3FTA, manifested by the observation of strong activation of the vibrational modes involving the motion of the -SCH₃ group in the low-frequency regions of the 1C-R2PI and MATI spectra. The electronic excitation energy from the S₀ state to the S₁ state (E₁) and the adiabatic ionization energy (IE) are respectively determined to be 34 820 ± 3 and 65 468 ± 5 cm^-1 for cis-3FTA, and those of the trans-rotamer are respectively determined to be 35 047 ± 3 and 65 644 ± 5 cm^-1. The structural properties of the stable rotamers of 3FTA and their comparison with other F- and Cl-substituted thioanisole derivatives are discussed as well.

Photoionization Spectroscopic and Theoretical Study on the Molecular Structures of cis- and trans-3-Chlorothioanisole

Resonance-enhanced two-photon ionization (R2PI) and mass-analyzed threshold ionization (MATI) spectra are measured for the cis- and trans-3-chlorothioanisole (3ClTA). The first electronic excitation energy (E 1) and the adiabatic ionization energy (IE) of the cis-rotamer are determined to be 33 959±3 and 65 326±5 cm-1, respectively, and those of the trans-rotamer are determined to be 34102±3 and 65 471±5 cm-1, respectively. Density functional theory (DFT) calculations confirm that both the cis- and trans-rotamers of 3ClTA are stable and coexist in their respective S0, S1, and D0 states. Both rotamers adopt planar structures with cis- being slightly more stable than trans- in the respective S0, S1, and D0 states. The conformation, substitution, and isotope effects on the molecular structure, active vibrations, and electronic transition and ionization energies of 3ClTA are analyzed.

HF Formation through Dissociative Electron Attachment-A Combined Experimental and Theoretical Study on Pentafluorothiophenol and 2-Fluorothiophenol

In chemoradiation therapy, dissociative electron attachment (DEA) may play an important role with respect to the efficiency of the radiosensitizers used. The rational tailoring of such radiosensitizers to be more susceptive to DEA may thus offer a path to increase their efficiency. Potentially, this may be achieved by tailoring rearrangement reactions into the DEA process such that these may proceed at low incident electron energies, where DEA is most effective. Favorably altering the orbital structure of the respective molecules through substitution is another path that may be taken to promote dissociation up on electron capture. Here we present a combined experimental and theoretical study on DEA in relation to pentafluorothiophenol (PFTP) and 2-fluorothiophenol (2-FTP). We investigate the thermochemistry and dynamics of neutral HF formation through DEA as means to lower the threshold for dissociation up on electron capture to these compounds, and we explore the influence of perfluorination on their orbital structure. Fragment ion yield curves are presented, and the thermochemical thresholds for the respective DEA processes are computed as well as the minimum energy paths for HF formation up on electron capture and the underlying orbital structure of the respective molecular anions. We show that perfluorination of the aromatic ring in these compounds plays an important role in enabling HF formation by further lowering the threshold for this process and through favorable influence on the orbital structure, such that DEA is promoted. We argue that this approach may offer a path for tailoring new and efficient radiosensitizers.

Real-Time Tunneling Dynamics through Adiabatic Potential Energy Surfaces Shaped by a Conical Intersection

Dynamic shaping of the adiabatic tunneling barrier in the S-H bond extension coordinate of several ortho-substituted thiophenols has been found to be mediated by low-frequency out-of-plane vibrational modes, which are parallel to the coupling vector of the branching plane comprising the conical intersection. The S-H predissociation tunneling rate (k) measured when exciting to the S₁ zero-point level of 2-methoxythiophenol (44 ps)^-1 increases abruptly, to k ≈ (22 ps)^-1, at the energy corresponding to excitation of the 152 cm^-1 out-of-plane vibrational mode and then falls back to k ≈ (40 ps)^-1 when the in-plane mode is excited at 282 cm^-1. Similar resonance-like peaks in plots of S₁ tunneling rate versus internal energy are observed when exciting the corresponding low-frequency out-of-plane modes in the S₁ states of 2-fluorothiophenol and 2-chlorothiophenol. This experiment provides clear-cut evidence for dynamical "shaping" of the lower-lying adiabatic potential energy surfaces by the higher-lying conical intersection seam, which dictates the multidimensional tunneling dynamics.

Insights into the Mechanism of Nonadiabatic Photodissociation from Product Vibrational Distributions. The Remarkable Case of Phenol

The fate of a photoexcited molecule is often strongly influenced by electronic degeneracies, such as conical intersections, which break the Born-Oppenheimer separation of electronic and nuclear motion. Detailed information concerning internal energy redistribution in a nonadiabatic process can be extracted from the product state distribution of a photofragment in photodissociation. Here, we focus on the nonadiabatic photodissociation of phenol and discuss the internal excitation of the phenoxyl fragment using both symmetry analysis and wave packet dynamics. It is shown that unique and general selection rules exist, which can be attributed to the geometric phase in the adiabatic representation. Further, our results provide a reinterpretation of the experimental data, shedding light on the impact of conical intersections on the product state distribution.

Vibronic structure and predissociation dynamics of 2-methoxythiophenol (S1): The effect of intramolecular hydrogen bonding on nonadiabatic dynamics

Vibronic spectroscopy and the S-H bond predissociation dynamics of 2-methoxythiophenol (2-MTP) in the S₁ (ππ^*) state have been investigated for the first time. Resonant two-photon ionization and slow-electron velocity map imaging (SEVI) spectroscopies have revealed that the S₁-S₀ transition of 2-MTP is accompanied with the planar to the pseudoplanar structural change along the out-of-plane ring distortion and the tilt of the methoxy moiety. The S₁ vibronic bands up to their internal energy of ∼1000 cm^-1 are assigned from the SEVI spectra taken via various S₁ vibronic intermediate states with the aid of ab initio calculations. Intriguingly, Fermi resonances have been identified for some vibronic bands. The S-H bond breakage of 2-MTP occurs via tunneling through an adiabatic barrier under the S₁/S₂ conical intersection seam, and it is followed by the bifurcation into either the adiabatic or nonadiabatic channel at the S₀/S₂ conical intersection where the diabatic S₂ state (πσ^*) is unbound with respect to the S-H bond elongation coordinate, giving the excited (Ã) or ground (X̃) state of the 2-methoxythiophenoxy radical, respectively. Surprisingly, the nonadiabatic transition probability at the S₀/S₂ conical intersection, estimated from the velocity map ion images of the nascent D fragment from 2-MTP-d₁ (2-CH₃O-C₆H₄SD) at the S₁ zero-point energy level, is found to be exceptionally high to give the X̃/Ã product branching ratio of 2.03 ± 0.20, which is much higher than the value of ∼0.8 estimated for the bare thiophenol at the S₁ origin. It even increases to 2.33 ± 0.17 at the ν₄₅ ² mode (101 cm^-1) before it rapidly decays to 0.69 ± 0.05 at the S₁ internal energy of about 2200 cm^-1. This suggests that the strong intramolecular hydrogen bonding of S⋯D⋯OCH₃ in 2-MTP at least in the low S₁ internal energy region should play a significant role in localizing the reactive flux onto the conical intersection seam. The minimum energy pathway calculations (second-order coupled-cluster resolution of the identity or time-dependent-density functional theory) of the adiabatic S₁ state suggest that the intimate dynamic interplay between the S-H bond cleavage and intramolecular hydrogen bonding could be crucial in the nonadiabatic surface hopping dynamics taking place at the conical intersection.

Experimental observation of nonadiabatic bifurcation dynamics at resonances in the continuum

The surface crossing of bound and unbound electronic states in multidimensional space often gives rise to resonances in the continuum. This situation happens in the πσ*-mediated photodissociation reaction of 2-fluorothioanisole; optically-bright bound S1 (ππ*) vibrational states of 2-fluorothioanisole are strongly coupled to the optically-dark S2 (πσ*) state, which is repulsive along the S-CH3 elongation coordinate. It is revealed here that the reactive flux prepared at such resonances in the continuum bifurcates into two distinct reaction pathways with totally different dynamics in terms of energy disposal and nonadiabatic transition probability. This indicates that the reactive flux in the Franck-Condon region may either undergo nonadiabatic transition funneling through the conical intersection from the upper adiabat, or follow a low-lying adiabatic path, along which multiple dynamic saddle points may be located. Since 2-fluorothioanisole adopts a nonplanar geometry in the S1 minimum energy, the quasi-degenerate S1/S2 crossing seam in the nonplanar geometry, which lies well below the planar S1/S2 conical intersection, is likely responsible for the efficient vibronic coupling, especially in the low S1 internal energy region. As the excitation energy increases, bound-to-continuum coupling is facilitated with the aid of intramolecular vibrational redistribution, along many degrees of freedom spanning the large structural volume. This leads to the rapid domination of the continuum character of the reactive flux. This work reports direct and robust experimental observations of the nonadiabatic bifurcation dynamics of the reactive flux occurring at resonances in the continuum of polyatomic molecules.

Electronic spectroscopy of methyl vinyl ketone oxide: A four-carbon unsaturated Criegee intermediate from isoprene ozonolysis

Ozonolysis of isoprene, one of the most abundant volatile organic compounds in the atmosphere, proceeds through methyl vinyl ketone oxide (MVK-oxide), methacrolein oxide, and formaldehyde oxide (CH₂OO) Criegee intermediates. The present study focuses on MVK-oxide, a four-carbon unsaturated carbonyl oxide intermediate, using vacuum ultraviolet photoionization at 118 nm and UV-visible induced depletion of the m/z = 86 mass channel to characterize its first π^* ← π electronic transition. The electronic spectrum is broad and unstructured with its peak at 388 nm (3.2 eV). The MVK-oxide spectrum is shifted to a significantly longer wavelength than CH₂OO and alkyl-substituted Criegee intermediates studied previously due to extended conjugation across the vinyl and carbonyl oxide groups. Electronic excitation results in rapid dissociation at λ ≤ 430 nm to methyl vinyl ketone and O ¹D products, the latter detected by 2 + 1 resonance enhanced multiphoton ionization using velocity map imaging. Complementary electronic structure calculations (CASPT2(12,10)/AVDZ) predict two π^* ← π transitions with significant oscillator strength for each of the four conformers of MVK-oxide with vertical excitation energies (and corresponding wavelengths) in the 3.1-3.6 eV (350-400 nm) and 4.5-5.5 eV (220-280 nm) regions. The computed electronic absorption profile of MVK-oxide, based on a Wigner distribution of ground state configurations and summed over the four conformers, is predicted to peak at 397 nm. UV-visible spectroscopy on the first π^* ← π transition is shown by a combination of experiment and theory to provide a sensitive method for detection of the MVK-oxide Criegee intermediate that will enable further studies of its photochemistry and unimolecular and bimolecular reaction dynamics.

Electronic spectrum and characterization of diabatic potential energy surfaces for thiophenol

We present an accurate simulation of the UV spectrum and a diabatization of three singlet potential surfaces along four coordinates.