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

Tracking the structural change of the predissociating molecule near the transition state

Despite its profound significance, the molecular structural changes near the transition state, driven by the vibronic coupling, have remained largely unexplored, leaving a crucial aspect of chemical reactions shrouded in uncertainty. Herein, the dynamical behavior of the reactive flux on the verge of chemical bond breakage was revealed through the spectroscopic characterization of a large amplitude vibrational motion. Highly excited internal rotor states of S1 methylamine (CH3ND2) report on the structural change as the molecule approaches the transition state, indicating that the quasi-free internal rotation is strongly coupled to the reaction coordinate as their energies near the maximum of the reaction barrier for the N-D chemical bond predissociation. Energy-dependent behavior of the rate constant perfectly correlates with that of the molecular structural change in the N-D bond length, providing unprecedented crucial information about how vibrational energy flows into the reaction coordinate on the adiabatic potential energy surfaces.

Chlorine Substitution Effect on the S1 Relaxation Dynamics of Chlorobenzene and Chlorophenols

The S1 state relaxation dynamics of chlorobenzene (CB), 3-chlorophenol (3-CP), 3-CP·H2O, and 2-chlorophenol·H2O (2-CP·H2O) have been investigated by means of picosecond time-resolved pump-probe spectroscopy in a state-specific manner. For CB, the S1 state relaxes via the S1-S0 internal conversion in the low internal energy region (<2000 cm-1), whereas the direct C-Cl bond dissociation channel mediated by the upper-lying repulsive πσCCl* state is opened to give the rather sharp increase of the S1 relaxation rate in the high internal energy region (>2000 cm-1). A similar dynamic feature has been observed for 3-CP in terms of the lifetime behavior with an increase in the S1 internal energy, suggesting that the H atom tunneling dissociation reaction from OH might contribute less compared to the internal conversion, although it is not clear at the present time whether or not the sharp increase of the S1 relaxation rate in the high internal energy region of 3-CP (>1500 cm-1) is entirely due to that of the internal conversion. The fact that the internal conversion is facilitated by the Cl substitution implies that the energetic location of the S1/S0 conical intersection should have been strongly influenced by chlorine substitution on the aromatic ring. The approximate energetic location of the saddle point of the S1(ππ*)/πσCCl* conical intersection along the seam coordinate for CB or 3-CP could be inferred from the energy-dependent S1 lifetime measurements. It is discussed in comparison with the dynamic role of the S1(ππ*)/πσCCl* conical intersection, which is strongly influenced by the O-H···Cl intramolecular hydrogen bond in the rather complicated yet ultrafast S1 relaxation dynamics of the cis-2-CP. The S1 lifetimes of 3-CP·H2O and 2-CP·H2O reveal the importance of the conformational structures, especially in terms of the intramolecular hydrogen bonding.

Manipulating Quantum Interference in Two-Color (ω+3ω) Strong Laser Field Photodissociation Dynamics of D2. J Phys Chem A 2024;128:5021-5027. [PMID: 38885171 DOI: 10.1021/acs.jpca.4c03176] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

In a strong field regime, exploring the molecular photodissociation process and revealing the underlying reaction mechanism remain challenging tasks due to the dramatic changes of molecular potentials caused by the applied fields. In this paper, we investigate the strong field photodissociation dynamics of D2+ in a synthesized VUV + IR (266 + 800 nm) two-color laser field by solving the three-dimensional time-dependent Schrödinger equation. We show that the Aharonov-Bohm-like quantum interference in the photofragment angular distribution can be controlled by varying the ellipticity of the VUV light. We demonstrate that the interference phenomenon originates from the geometric phase accumulation of the nuclear wave function when it undergoes cyclic evolution on light-induced potential energy surfaces. This work has implications for the control of chemical reactions of a small molecular system through the geometric phase.

Dynamic Role of the Intramolecular Hydrogen Bonding in the S1 State Relaxation Dynamics Revealed by the Direct Measurement of the Mode-Dependent Internal Conversion Rate of 2-Chlorophenol and 2-Chlorothiophenol

The dynamic role of the intramolecular hydrogen bond in the S1 relaxation of cis-2-chlorophenol (2-CP) or cis-2-chlorothiophenol (2-CTP) has been investigated in a state-specific manner. Whereas ultrafast internal conversion is dominant for 2-CP, the H-tunneling competes with internal conversion for 2-CTP even at the S1 origin. The S0-S1 internal conversion rate of 2-CTP could be directly measured from the S1 lifetimes of 2-CTP-d1 (Cl-C6H4-SD) as the D-tunneling is kinetically blocked, allowing distinct estimations of tunneling and internal conversion rates with increasing the energy. The internal conversion rate of 2-CTP increases by two times at the out-of-plane torsional mode excitation, suggesting that the internal conversion is facilitated at the nonplanar geometry. It then sharply increases at ∼600 cm-1, indicating that the S1/S0 conical intersection is readily accessible at the extended C-Cl bond length. The strength of the intramolecular hydrogen bond should be responsible for the distinct dynamic behaviors of 2-CP and 2-CTP.

Electronic Excitation of ortho-Fluorothiophenol

ortho-Fluorothiophenol (o-FTP) photodissociates through the well-known πσ* process. The fluorine atom of o-FTP introduces a feature in the photodissociation of o-FTP that does not occur in most other πσ* processes because the fluorine atom can form a hydrogen bond with the hydrogen atom of the SH group. Theoretical computations can serve as a good way to study these reactions because they usually proceed very quickly, and the current spectroscopies cannot probe the details of the processes as thoroughly as theory can. Here we use completely renormalized equation-of-motion coupled cluster theory with single and double excitations and a quasiperturbative treatment of connected triple excitations (CR-EOM-CCSD(T)) and quasidegenerate perturbation theory, in particular extended multistate complete-active-space second-order perturbation theory (XMS- CASPT2), to calculate the four lowest singlet states of o-FTP and hybrid density functional theory to optimize the geometries of the two lowest singlet states. We find that ten active electrons in nine active orbitals are sufficient to provide a good reference function for all four states. We find that the ground electronic state and the first excited singlet state both exhibit strongly bent hydrogen bonds. We also use density functional theory with the Tamm-Dancoff approximation and the SMD solvation model to successfully simulate the electronic spectrum of o-FTP in n-hexane solvent.

πσ*-Mediated Nonadiabatic Tunneling Dynamics of Thiophenols in S1: The Semiclassical Approaches

The S-H bond tunneling predissociation dynamics of thiophenol and its ortho-substituted derivatives (2-fluorothiophenol, 2-methoxythiophenol, and 2-chlorothiphenol) in S₁ (ππ*) where the H atom tunneling is mediated by the nearby S₂ (πσ*) state (which is repulsive along the S-H bond extension coordinate) have been investigated in a state-specific way using the picosecond time-resolved pump-probe spectroscopy for the jet-cooled molecules. The effects of the specific vibrational mode excitations and the SH/SD substitutions on the S-H(D) bond rupture tunneling dynamics have been interrogated, giving deep insights into the multidimensional aspects of the S₁/S₂ conical intersection, which also shapes the underlying adiabatic tunneling potential energy surfaces (PESs). The semiclassical tunneling rate calculations based on the Wentzel-Kramers-Brillouin (WKB) approximation or Zhu-Nakamura (ZN) theory have been carried out based on the ab initio PESs calculated in the (one, two, or three) reduced dimensions to be compared with the experiment. Though the quantitative experimental results could not be reproduced satisfactorily by the present calculations, the qualitative trends among different molecules in terms of the behavior of the tunneling rate versus the (adiabatic) barrier height or the number of PES dimensions could be rationalized. Most interestingly, the H/D kinetic isotope effect observed in the tunneling rate could be much better explained by the ZN theory compared to the WKB approximation, indicating that the nonadiabatic coupling matrix elements should be invoked for understanding the tunneling dynamics taking place in the proximity of the conical intersection.

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'.

S1-State Decay Dynamics of Benzenediols (Catechol, Resorcinol, and Hydroquinone) and Their 1:1 Water Clusters

The S₁-state decaying rates of the three different benzenediols, catechol, resorcinol, and hydroquinone, and their 1:1 water clusters have been state-specifically measured using the picosecond time-resolved parent ion transients obtained by the pump (excitation) and probe (ionization) scheme. The S₁ lifetime of catechol is found to be short, giving τ ∼ 5.9 ps at the zero-point level. This is ascribed to the H-atom detachment from the free OH moiety of the molecule. Consistent with a previous report (J. Phys. Chem. Lett. 2013, 4, 3819-3823), the S₁ lifetime gets lengthened with low-frequency vibrational mode excitations, giving τ ∼ 9.0 ps for the 116 cm^-1 band. The S₁ lifetimes at the additional vibronic modes of catechol are newly measured, showing the nonnegligible mode-dependent fluctuations of the tunneling rate. When catechol is complexed with water, the S₁ lifetime is enormously increased to τ ∼ 1.80 ns at the zero-point level while it shows an unusual dip at the intermolecular stretching mode excitation (τ ∼ 1.03 ns at 146 cm^-1). Otherwise, it is shortened monotonically with increasing the internal energy, giving τ ∼ 0.67 ns for the 856 cm^-1 band. Two different asymmetric or symmetric conformers of resorcinol give the respective S₁ lifetimes of 4.5 or 6.3 ns at their zero-point levels according to the estimation from our transients taken within the temporal window of 0-2.7 ns. When resorcinol is 1:1 complexed with H₂O, the S₁ decaying rate is slightly accelerated for both conformers. The S₁ lifetimes of trans and cis forms of hydroquinone are measured to be more or less same, giving τ ∼ 2.8 ns at the zero-point level. When H₂O is complexed with hydroquinone, the S₁ decaying process is facilitated for both conformers, slightly more efficiently for the cis conformer.

Femtosecond Wavepacket Dynamics Reveals the Molecular Structures in the Excited (S1) and Cationic (D0) States

Molecular structures in the electronically excited (S₁) and cationic (D₀) states of 2-fluorothioanisole (2-FTA) have been precisely refined from the real-time dynamics of the femtosecond (fs) wavepacket prepared by the coherent excitation of the Franck-Condon active out-of-plane torsional modes in the S₁ ← S₀ transition at 285 nm. The simulation to reproduce the experiment in terms of the beating frequencies gives the nonplanar geometry of 2-FTA in S₁, where the out-of-plane dihedral angle (φ) of the S-CH₃ moiety is 51° with respect to the molecular plane. The behavior of the fs wavepacket in terms of the amplitudes and phases with the change of the probe (ionization) wavelength (λ_probe = 300-330 nm) provides the otherwise veiled structure of the cationic D₀ state. While the 2-FTA cation adopts the planar geometry (φ = 0°) at the global minimum, it is found to have a vertical minimum at φ ≈ 135° from the perspective of the D₀ ← S₁ vertical transition. Ab initio calculations support the experiment quite well although the simulation using the model potentials could improve the match with the experiment, giving the new interpretation for the previously disputed photoelectron spectroscopic results.

Conformer-Specific Tunneling Dynamics Dictated by the Seam Coordinate of the Conical Intersection

The dynamic role of the conical intersection "seam" coordinate has been first revealed in the H fragmentation reaction of ortho(o)-cresol conformers. One of the (3N - 8) dimensional seam coordinates of the S₁(ππ*)/S₂(πσ*) conical intersection has been identified as the CH₃ torsional potential function. The tunneling dynamics of the reactive flux is dictated by its nuclear layout with respect to the CH₃ torsional angle, as the multidimensional tunneling barrier is dynamically shaped along the conical intersection seam. The effective tunneling-barrier weight-averaged over the quantum-mechanical probability along the CH₃ torsional angle perfectly explains the experimental finding: the sharp variation of the tunneling rate ((700-400) ps^-1) with the CH₃ torsional mode excitations within the narrow (0-100 cm^-1) energetic window. The much longer S₁ lifetime of cis compared to trans is ascribed to the higher-lying S₁/S₂ conical intersection of the former. With the use of distinct lifetimes, vibronic bands of each conformer could be completely separated.