Computational determination of the S1(Ã1A″) absorption spectra of HONO and DONO using full-dimensional neural network potential energy surfaces.

The Ã1A″ ← X̃1A' absorption spectra of HONO and DONO were simulated by a full six-dimensional quantum mechanical method based on the newly constructed potential energy surfaces for the ground and excited electronic states, which were represented by the neural network method utilizing over 36 000 ab initio energy points calculated at the multireference configuration interaction level with Davidson correction. The absorption spectrum of HONO/DONO comprises a superposition of the spectra from two isomers, namely, trans- and cis-HONO/DONO, due to their coexistence in the ground X̃1A' state. Our calculated spectra of both HONO and DONO were found to be in fairly good agreement with the experiment, including the energy positions and widths of the peaks. The dominant progression was assigned to the N=O stretch mode (20n) associated with trans-HONO/DONO, which can be attributed to the promotion of an electron to the π* orbital of N=O. Specifically, the resonances with higher vibrational quanta were found to be in the domain of the Feshbach-type resonances. The assignments of the spectra and mode specificity therein are discussed.

Quantum state-to-state nonadiabatic dynamics of the charge transfer reaction H+ + NO(X2Π) → H + NO+(X1Σ+): Influence of ro-vibrational excitation of NO.

Quantum state-to-state nonadiabatic dynamics of the charge transfer reaction H+ + NO(X2Π, vi = 1, 3, ji = 0, 1) → H + NO+(X1Σ+) has been studied based on the recently constructed diabatic potential energy matrix. It was found that the vibrational excitation of reactant NO inhibits the reactivity, while the rotational excitation of reactant NO has little effect on the reaction probability. These attributes were also observed in the semi-classical trajectory calculations employed in the adiabatic representation. Such an inhibitory effect of the vibrational excitation of reactant NO was owing to lower accessibility of the conical intersection and avoided crossing regions, which are located in the wells with respect to the Π diabat, as evidenced by the analysis of the population of the time-independent wave functions. Calculated vibrational state distributions of the product show that the decrease of the reaction mainly leads to the less formation of low vibrational states (vf < 6), and the product vibrational state distributions are more evenly populated for vi = 1 and 3, suggesting a non-statistical behavior. However, the overall shapes of the product rotational distributions remain unchanged, indicating that the redistribution of energy into the rotation of product NO is sufficient in the charge transfer process between H+ and NO. While the reaction is dominated by the forward and backward scattering in differential cross sections (DCSs), consistent with the complex-forming mechanism, a clear forward bias in the DCSs appears, indicating that the occurrence of the reaction is not sufficiently long to undergo the whole phase space of the interaction configurations.

Full-dimensional potential energy surface for the photodissociation of HNCO via its S1 band.

A full-dimensional potential energy surface (PES) for the first excited state S1(1A'') of HNCO has been built up by the neural network method based on more than 36 000 ab initio points, which were calculated at the multireference configuration interaction level with Davidson correction using the augmented correlation consistent polarized valence triple zeta basis set. It was found that two minima, namely, trans and cis isomers of HNCO, and another seven stationary points exist on the S1 PES for the two dissociation pathways: HNCO(S1) → H + NCO/NH + CO. Particularly, a new out-of-plane transition state between the two minima was found in this work, thanks to including all the degree of freedoms for this system. The adiabatic excitation energy of the S1(1A'') ← S0(1A') transition and dissociation energies D0(HNCO → H + NCO) and D0((HNCO →NH(a1Δ) + CO) calculated on the PES are in good agreement with experimental results. In addition, based on the newly constructed S1 PES, the percentage of products H + NCO in the photodissociation of HNCO(S1) was obtained by a quasi-classical trajectory method at the photon wavelengths ranging from 190 to 225 nm, which is in reasonably good agreement with earlier theoretical and experimental results. For the dissociation lifetimes of the trajectories, they were calculated to be less than 5 ps, which is also consistent with experimental observations.

Nonadiabatic quantum dynamics of the charge transfer reaction H+ + NO(X2Π) → H + NO+(X1Σ+).

Nonadiabatic quantum dynamics of the charge transfer (CT) reaction H+ + NO(X2Π) → H + NO+(X1Σ+) is investigated on a new diabatic potential energy matrix (PEM) including the 12A' and 22A' states of HNO+/HON+ at the multireference configuration interaction level with Davidson correction using a large basis set. The diabatization of the two coupled states was achieved by the adiabatic-to-diabatic transformation with a mixing angle and the final diabatic PEM was obtained by fitting each matrix element separately using a three-dimensional cubic spline interpolation including more than 22 000 ab initio points. The reaction was found to be dominated by the resonances supported by the double well associated with HNO+ and HON+ species, manifested by the oscillatory structures in the reaction probabilities and product rotational distributions. The product vibrational states were highly excited due to the large exothermicity of the reaction. Consistent with the complex-forming mechanism, the differential cross sections (DCSs) were found to be dominated by the forward and backward scatterings. A clear forward bias in the vibrational state resolved DCSs suggests that the non-statistical behavior of the reaction mainly comes from the low vibrational states of the product. In addition, the rate constants of the reaction in the temperature range from 50 to 500 K were computed for the first time and found to be in fairly good agreement with the available experimental results at 300 K. In particular, compared to other reactions involving neutral species in this system including N, O, and H atoms, such a CT reaction was found to be much more reactive, which has rate constants more than thirty times larger.

Simulating the HeI photoelectron spectrum of Cl2O with new full-dimensional adiabatic potential energy surfaces of Cl2O(X̃1A1), Cl2O+(X̃2B1), and Cl2O+(C̃2A2) and a three-state diabatic potential energy matrix of Cl2O+(Ã2B2, B̃2A1, and 22A1): a quantum mechanical study.

To interpret the HeI photoelectron spectrum of Cl2O (involving four lowest electronic states of Cl2O+), in this work we first constructed the associated adiabatic full-dimensional potential energy surfaces (PESs) of Cl2O(X̃1A1), Cl2O+(X̃2B1), and Cl2O+(C̃2A2) and a diabatic potential energy matrix (PEM) of Cl2O+(Ã2B2, B̃2A1, and 22A1) using the explicitly correlated internally contracted multi-reference configurational interaction with Davidson correction (MRCI-F12+Q) and neural network methods. Particularly for the Ã2B2, B̃2A1, and 22A1 states of Cl2O+ coupled in terms of conical intersection, their diabatization is achieved by the neural network approach based merely on the associated adiabatic energies. With the help of newly constructed adiabatic PESs and the diabatic PEM, the HeI photoelectron spectrum of Cl2O is further computed quantum mechanically. The calculated photoelectron spectrum is found to be in good accord with experiment. The mode specificity in the HeI photoelectron bands of Cl2O is analyzed in detail.

Nonadiabatic heavy atom tunneling in 1nσ*-mediated photodissociation of thioanisole.

The 1nσ*-mediated photodissociation dynamics of thioanisole is investigated quantum mechanically using a three-dimensional model based on a newly constructed diabatic potential energy matrix. The lifetimes of the low-lying S1(1ππ*) resonances are determined and found to accord well with available experimental data. Specifically, our theoretical results demonstrate that the photodissociation of thioanisole at the low-lying S1(1ππ*) levels takes place via the heavy atom tunneling due to the higher S1/S2 conical intersection and two equivalent out-of-plane saddle points appearing on the dissociation path. The isotopic effect on the lifetimes is found to be pronounced, manifesting the nature of the tunneling process. Moreover, the geometric phase effect around the S1/S2 conical intersection is found to slightly impact the lifetimes due to the weak destructive or constructive interferences in this heavy atom tunneling, which differs significantly from the scenario in the nonadiabatic hydrogen atom tunneling. Importantly, the quantum mechanical treatment is essentially required to accurately describe the 1nσ*-mediated photodissociation dynamics of thioanisole owing to involving quantum tunneling and geometric phase effects near the conical intersection.

Constructing Diabatic Potential Energy Matrices with Neural Networks Based on Adiabatic Energies and Physical Considerations: Toward Quantum Dynamic Accuracy.

A permutation invariant polynomial-neural network (PIP-NN) approach for constructing the global diabatic potential energy matrices (PEMs) of the coupled states of molecules is proposed. Specifically, the diabatization scheme is based merely on the adiabatic energy data of the system, which is ideally a most convenient way due to not requiring additional ab initio calculations for the data of the derivative coupling or any other physical properties of the molecule. Considering the permutation and coupling characteristics of the system, particularly in the presence of conical intersections, some vital treatments for the off-diagonal terms in diabatic PEM are essentially needed. Taking the photodissociation of H2O(X~/B~)/NH3(X~/A~) and nonadiabatic reaction Na(3p) + H2 → NaH(Σ+) + H for example, this PIP-NN method is shown to build up the global diabatic PEMs effectively and accurately. The root-mean-square errors of the adiabatic potential energies in the fitting for three different systems are all small (<10 meV). Further quantum dynamic calculations show that the absorption spectra and product branching ratios in both H2O(X~/B~) and NH3(X~/A~) nonadiabatic photodissociation are well reproduced on the new diabatic PEMs, and the nonadiabatic reaction probability of Na(3p) + H2 → NaH(Σ+) + H obtained on the new diabatic PEMs of the 12A1 and 12B2 states is in reasonably good agreement with previous theoretical result as well, validating this new PIP-NN method.