Efficient Quantum Mechanical Calculations of Mode-Specific Tunneling Splittings upon Fundamental Excitation in the Dimer of Formic Acid

Study on depolymerization kinetics of formic acid dimers in binary mixture

In this study, polarization Raman spectra were collected for binary mixtures of formic acid/methanol and formic acid/acetonitrile with different volume fractions. The broad band of formic acid in the CO vibration region was divided into four vibration peaks, corresponding to CO symmetric and anti-symmetric stretching vibration from cyclic dimer, CO stretching from open dimer, and CO stretching from the free monomer. The experiments showed that as the volume fraction of formic acid in the binary mixture decreased, the cyclic dimer gradually converted to the open dimer, and at a volume fraction of 0.1, fully depolymerized into monomer form (free monomer, solvated monomer, and hydrogen bonding monomer clusters with solvent). The contribution percentage of the total CO stretching intensity of each structure at different concentrations was quantitatively calculated using high resolution infrared spectroscopy, and the results were consistent with the conclusions predicted by polarization Raman spectroscopy. Concentration-triggered 2D-COS synchronous and asynchronous spectra also confirmed the kinetics of formic acid diluted in acetonitrile. This work provides a spectroscopic method for studying the structure of organic compounds in solution and concentration-triggering kinetics in mixtures.

An Ab Initio Neural Network Potential Energy Surface for the Dimer of Formic Acid and Further Quantum Tunneling Dynamics

We construct a full-dimensional ab initio neural network potential energy surface (PES) for the isomerization system of the formic acid dimer (FAD). This is based upon ab initio calculations using the DLPNO-CCSD(T) approach with the aug-cc-pVTZ basis set, performed at over 14000 symmetry-unique geometries. An accurate fit to the obtained energies is generated using a general neural network fitting procedure combined with the fundamental invariant method, and the overall energy-weighted root-mean-square fitting error is about 6.4 cm^-1. Using this PES, we present a multidimensional quantum dynamics study on tunneling splittings with an efficient theoretical scheme developed by our group. The ground-state tunneling splitting of FAD calculated with a four-mode coupled method is in good agreement with the most recent experimental measurements. The PES can be applied for further dynamics studies. The effectiveness of the present scheme for constructing a high-dimensional PES is demonstrated, and this scheme is expected to be feasible for larger molecular systems.

Multidimensional H-atom tunneling in the catecholate monoanion

We present the catecholate monoanion as a new model system for the study of multidimensional tunneling. It has a symmetrical O-H double-well structure, and the H atom motion between the two wells is coupled to both low and high frequency modes with different strengths. With a view to studying mode-specific tunneling in the catecholate monoanion, we have developed a full (33) dimensional potential energy surface in transition state (TS) normal modes using a Distributed Gaussian Empirical Valence Bond (DGEVB) based approach. We have computed eigenstates in different subspaces using both unrelaxed and relaxed potentials based on the DGEVB model. With unrelaxed potentials, we present results up to 7D subspaces that include the imaginary frequency mode and six modes coupled to it. With relaxed potentials, we focus on the two most important coupling modes. The structures of the ground and vibrationally excited eigenstates are discussed for both approaches and mode-specific tunneling splitting and their trends are presented.