Nuclear wave-packet-propagation-based study of the electron-coupled, proton-transfer process in the charge-transfer state of FHCl exhibiting three electronic states in full-dimensional space.

The charge-transfer (CT) excited state of FHCl (F+H-Cl-), generated by the photodetachment of an electron from its precursor anion (FHCl-) by a photon energy of ∼9.5 eV, is a realistic prototype of two bidirectional-coupled reaction pathways, namely the proton-transfer (PT) and electron-transfer (ET) channels, that produce F + HCl and FH + Cl combinations, respectively. The early-time dynamics of the CT was studied via the time-dependent propagations of nuclear wave packets comprising three nonadiabatically coupled electronic states defined within a three-dimensional space. The detailed analyses of the early-time dynamics revealed an interesting phenomenon in which the onset of PT was ∼80 fs earlier than that of ET, indicating that PT dominated ET in this case. A more significant finding was that the proper adjustment of the electronic-charge distribution for the onset of ET was obtained ∼80 fs after the onset of PT; this adjustment was mediated by the initial movement of the H atom, i.e., the F-H vibration mode. To avail experimental observables, the branching ratio, χ = PT/(PT + ET), and absorption spectrum generating the neutral FHCl molecule from its precursor anion were also simulated. The results further demonstrated the dependences of the χs and spectrum on the change in the initial vibration level of the precursor anion, as well as the isotopic substitution of the connecting H atom with deuterium, tritium, and muonium.

Wavepacket propagations for the early time dynamics of proton-coupled electron transfer in the charge-transfer state of NH3Cl complex.

A charge-transfer (CT) excited state of NH3Cl, generated by photo-detachment of an electron from the anionic NH3Cl- precursor, can be represented as H2N+-H-Cl- and proceeds to two chemical reactions: one reaction generating NH2 and HCl resulting from a proton transfer (PT) and the other reaction producing NH3 and a Cl atom resulting from an electron transfer (ET); both are coupled to form a typical proton-coupled electron transfer (PCET) process. The early time dynamics of this CT were studied using time-dependent wavepacket propagation on three nonadiabatically coupled electronic states in a reduced three-dimensional space. The electronic states were treated using the XMS-CASPT2/aug-cc-pVTZ ab initio methodology. The population dynamics of the three coupled electronic states were analyzed in detail to reveal the initial stage of the PCET process up to ∼100 fs, while the branching ratio, χ = PT/(ET+PT), was determined after wavepacket propagations of up to 2000 fs. Another main result is the dependence of χ on the vibration levels of the initial precursor anion and the isotope substitution of the connecting H atom with deuterium and tritium. Our study reveals the detailed microscopic features of the PCET process embedded in the CT state of the NH3Cl complex and certain systematic dependences of the branching ratio χ on the above factors.

Practical approximation of the non-adiabatic coupling terms for same-symmetry interstate crossings by using adiabatic potential energies only.

A very simple equation, FijApp=[(∂2(Via-Vja)/∂Q2)/(Via-Vja)]1/2/2, giving a reliable magnitude of non-adiabatic coupling terms (NACTs, Fij's) based on adiabatic potential energies only (Via and Vja) was discovered, and its reliability was tested for several prototypes of same-symmetry interstate crossings in LiF, C2, NH3Cl, and C6H5SH molecules. Our theoretical derivation starts from the analysis of the relationship between the Lorentzian dependence of NACTs along a diabatization coordinate and the well-established linear vibronic coupling scheme. This analysis results in a very simple equation, α=2κ/Δc, enabling the evaluation of the Lorentz function α parameter in terms of the coupling constant κ and the energy gap Δc (Δc=|Via-Vja|Qc ) between adiabatic states at the crossing point QC. Subsequently, it was shown that QC corresponds to the point where FijApp exhibit maximum values if we set the coupling parameter as κ=[(Via-Vja)⋅(∂2(Via-Vja)/∂Q2)]Qc1/2/2. Finally, we conjectured that this relation could give reasonable values of NACTs not only at the crossing point but also at other geometries near QC. In this final approximation, the pre-defined crossing point QC is not required. The results of our test demonstrate that the approximation works much better than initially expected. The present new method does not depend on the selection of an ab initio method for adiabatic electronic states but is currently limited to local non-adiabatic regions where only two electronic states are dominantly involved within a nuclear degree of freedom.

A practical and efficient diabatization that combines Lorentz and Laplace functions to approximate nonadiabatic coupling terms.

A fixed relation of α × ß = 1.397 between the α- and ß-parameters of a Lorentz function and a Laplace function that approximates nonadiabatic coupling terms and maximizes the overlap area between the two functions was found. The mixing angle corresponding to the geometric average between the potential couplings calculated using the individual path-integral of the two functions was then used in the construction of diabatic states and the coupling of the states. Employing the new method, the actual computation of nonadiabatic coupling terms at just a few geometries before and after the guessed conical intersection is enough, and the remaining steps are straightforward and almost automatic. The new method was tested for the one-dimensional LiF system and the two-dimensional space of the collinear case of NH3Cl, and promising results were achieved.

Factors Affecting the Branching Ratio of Photodissociation: Thiophenol Studied through Quantum Wavepacket Dynamics.

The photodissociation dynamics of thiophenol (PhSH) excited to the 1(1) ππ* state was investigated by time-dependent quantum wavepacket propagation within two-dimensional (2D) space consisting of the S-H bond and -SH torsion. We systematically studied the dependence of the branching ratio (Ã/X(~)) between the two electronic states of the phenylthiyl radical (PhS(.) ) on several factors of the 2D potential energy surfaces (PESs). The effect of a reduced initial barrier to the first ππ*/πσ* conical intersection (CI) was found to be marginal, whereas the effects of a reduced torsional barrier of -SH on the excited ππ* state and the mitigated slope of the πσ* PES between the first (ππ*/πσ*) and the second (πσ*/S0 ) CIs were noticeable. The effect of the slope on the branching ratio has never been previously noticed. It was shown that the branching ratio can be sufficiently above unity without pre-excitation of the torsion mode of -SH, which has been assumed so far.

Branching Ratio between Proton Transfer and Electron Transfer Channels of a Bidirectional Proton-Coupled Electron Transfer.

Rigorous quantum dynamical study of concerted proton-coupled electron transfer (PCET) on the time scale of a few femtoseconds (fs) has been rarely reported. Herein, a time-dependent quantum wavepacket propagation method was applied to the dynamics of the charge-transfer excited electronic state of FHCl corresponding to F(+)HCl(-). The dynamics corresponds to a bidirectional PCET with two dissociation channels: the electron transfer (ET, generating FH+Cl) and proton transfer (PT, generating F+HCl) paths. The calculated branching ratio (Cl/F), 0.78, implies a surprising fact: PT prevails over ET. A detailed analysis of the proton movement and electron readjustment suggests that the proton movement starts ∼3 fs earlier than the electron movement, and the electron readjustment is triggered by the initial movement of the proton. The branching ratio drastically inverts to 1.24 because of a reduced nonadiabatic effect in the isotope-substituted system, FDCl.

Quantum wave packet propagation study of the photochemistry of phenol: isotope effects (Ph-OD) and the direct excitation to the 1πσ* state.

An earlier time-dependent quantum wave packet propagation study of the photochemistry of Ph-OH [J. Chem. Phys. 2005, 122, 224315] is extended to investigate isotope effects (for Ph-OD) and the dynamics initiated by direct (vibronically induced) excitation to the (1)πσ* state. The isotope effect is significant only when the initially excited state is (1)ππ*, that is, there are noticeable changes not only in the time scale but also in the branching ratio (Ã/X̃) for the electronic states of the product Ph-O radical. In contrast, the isotope effect on the dynamics initiated by direct excitation to the (1)πσ* state is very small. Our most important observation for the dynamics initiated by direct excitation to the (1)πσ* state is that the initial excitation of the O-H stretch mode does not result in a noticeable enhancement of the product Ph-O radical in the à state, which corresponds to a dissociating H atom with low kinetic energy. The initial excitation of the CCOH torsion mode is the main reason for the enhancement of the product Ph-O radical in the à state that was observed in a vibrationally mediated two-photon experiment [J. Chem. Phys.2008, 128, 104307].

Structure of pyridazine in the S1 state: experiment and theory.

The molecular structure of pyridazine in the first electronically excited state (S(1)) is deduced from the combined use of resonance-enhanced two-photon ionization and mass-analyzed threshold ionization spectroscopic methods. The equation-of-motion coupled-cluster single and double (EOM-CCSD) calculation gives the distorted planar geometry for the most stable structure of the S(1) pyridazine. The symmetry constraint of C(2v) is relaxed to that of C(s), and consequently many in-plane vibrational modes are found to be optically active in both S(1)-S(0) and D(0)-S(1) excitation spectra, being appropriately assigned from the comparison of their frequencies with ab initio values. This indicates that the S(1)-S(0) excitation is partially localized, and provides an alternative explanation for the long-standing spectroscopic puzzle in S(1) pyridazine.

State-selective predissociation dynamics of methylamines: the vibronic and HD effects on the conical intersection dynamics.

The photodissociation dynamics of methylamines (CH(3)NH(2) and CD(3)ND(2)) on the first electronically excited state has been investigated using the velocity map ion imaging technique probing the H or D fragment. Two distinct velocity components are found in the H(D) translational energy distribution, implying the existence of two different reaction pathways for the bond dissociation. The high H(D) velocity component with the small internal energy of the radical fragment is ascribed to the N-H(D) fragmentation via the coupling of S(1) to the upper-lying S(2) repulsive potential energy surface along the N-H(D) bond elongation axis. Dissociation on the ground S(0) state prepared via the nonadiabatic dynamics at the conical intersection should be responsible for the slow H(D) fragment. Several S(1) vibronic states of methylamines including the zero-point level and nnu(9) states (n=1, 2, or 3) are exclusively chosen in order to explore the effect of the initial quantum content on the chemical reaction dynamics. The branching ratio of the fast and slow components is found to be sensitive to the initial vibronic state for the N-H bond dissociation of CH(3)NH(2), whereas it is little affected in the N-D dissociation event of CD(3)ND(2). The fast component is found to be more dominant in the translational distribution of D from CD(3)ND(2) than it is in that of H from CH(3)NH(2). The experimental result is discussed with a plausible mechanism of the conical intersection dynamics.

A theoretical study of the low-lying excited electronic states of thiocarbonyl chlorofluoride and their dissociation pathways.

The spectroscopic constants for the ground (X (1)A(1)) and low-lying triplet and singlet excited states (a (3)A("),A (1)A("),B (1)A(')) of thiocarbonyl chlorofluoride (ClFCS) were obtained using the equation-of-motion coupled-cluster singles and doubles method. The calculated vibrational frequencies of the electronic states were within 4% of the experimental values for 21 of the frequencies, but four calculated frequencies were 20%-40% away from the corresponding experimentally reported values, suggesting the need to reexamine previous experimental spectra. The spectroscopic properties of the radical fragments (FCS, ClCS, and CClF) were also studied, and the correlation diagram between the excited electronic states of ClFCS and possible combinations of dissociation fragments were obtained. The potential energy surfaces (PESs) of the excited electronic states of ClFCS along possible dissociation pathways were also studied. The main qualitative dynamical features of the S(1)(A (1)A("))<--S(2)(B (1)A(')) fluorescence of ClFCS, which may occur in spite of the small barrier (8 kcalmol) on the S(2) PES to the dissociation of C-Cl bond, are discussed.