Ground and low-lying excited state potential energy surfaces of diiodomethane in four dimensions

Abstract

We report a set of adiabatic potential energy surfaces (PESs) for diiodomethane, including the ground electronic state and all excited states accessible via single-photon absorption near 260 nm. Although constrained to four dimensions, these PESs capture the essential photochemical processes following photoexcitation--namely, bond breaking and rearrangement among the methyl radical and the two iodine atoms. Constructed using an accurate and efficient spline interpolation algorithm, the PESs reproduce local features with high fidelity and exhibit overall smooth first-order derivatives, making them suitable for molecular dynamics simulations. We identify key stationary points on the ground-state PES and on three excited-state PESs, and map reaction pathways leading to $\text{CH}_2\text{I}+\text{I}$ dissociation via the intermediate formation of a $\text{CH}_2\text{I-I}$ isomer. These PESs provide a valuable resource for molecular dynamics studies, enabling detailed exploration of photochemistry in diiodomethane.

Figures (7)

• Figure 1: Molecular diagram of $\text{CH}_2\text{I}_2$ showing the degrees of freedom and the geometric constraints. The two C-I distances and the two angles between the C-I vectors and the $C_2$ axis ($R_1$, $\alpha_1$, $R_2$, $\alpha_2$) are the coordinates used in constructing the potential energy surfaces. All other degrees of freedom are fixed at the values in the equilibrium geometry. These geometric constraints keep the molecule in $C_s$ symmetry.
• Figure 2: Ab initio potential energy curves of $\text{CH}_2\text{I}_2$ molecule by stretching one C-I bond length $R_1$. All other degrees of freedom are frozen at their values in the equilibrium geometry. 17 potential curves corresponding to the ground and low-lying excited states are grouped by dissociation asymptotes and by symmetry. The blue, red and green curves correspond to $\text{CH}_2\text{I}+\text{I}$, $\text{CH}_2\text{I}+\text{I}^*$, and $\text{CH}_2\text{I}^*+\text{I}$ dissociation channels, respectively. The solid and dashed curves correspond to electronic states with $\Gamma_1$ and $\Gamma_2$ spinor symmetries, respectively. The lower panel shows a zoom-in view of the coupling region.
• Figure 3: Gaussian fitted absorption spectrum (dashed black) of $\text{CH}_2\text{I}_2$ molecule compared with the experimental UV absorption spectrum (solid black, reproduced from Ref. xu2002) for photon energy from 3.4 to 5.3 eV. The theoretical fit is conducted using the sum of the Gaussian components of those $f>0.0005$ excited states in Table \ref{['table1']}. The vertical lines indicate the energy levels of those excited states with the relative heights matching the corresponding oscillator strengths. The different colors and strokes indicate the thresholds and symmetries of the excited states in the same manner as Fig. \ref{['fig2']}. The experimental data are reproduced with permission from J. Chem. Phys. 117, 5722 (2002). Copyright 2002 American Institute of Physics.
• Figure 4: Potential energies and the corresponding first-order derivatives as a function of $R_1$(a), $R_2$(b), $\alpha_1$(c), and $\alpha_2$(d) for $1\Gamma_1$(black), $4\Gamma_1$(blue), $5\Gamma_1$(red), $6\Gamma_1$(gray, panel (b) only), and $7\Gamma_1$(green) electronic states, respectively. For the potential curves along each direction, the remaining DOF are fixed at the values indicated in each panel. The solid lines represent the interpolated potential energies and the corresponding derivatives. The open markers denote the ab initio energy points.
• Figure 5: Error distributions for the interpolated PESs of $1\Gamma_1$ (a), $4\Gamma_1$ (b), $5\Gamma_1$ (c), and $7\Gamma_1$ (d) states, respectively, based on 3000 testing data points.