Analysis of the Proton Transfer Bands in the Infrared Spectra of Linear N₂H⁺···OC and N₂D⁺···OC Complexes Using Electric Field-Driven Classical Trajectories

Abstract

In this work, we describe ab initio calculations and assignment of infrared (IR) spectra of hydrogen-bonded ion−molecular complexes that involve a fluxional proton: the linear N 2 H + •••OC and N 2 D + •••OC complexes. Given the challenges of describing fluxional proton dynamics and especially its IR activity, we use electric fielddriven classical trajectories, i.e., the driven molecular dynamics (DMD) method that was developed by us in recent years and for similar applications, in conjunction with high-level electronic structure theory. Namely, we present a modified and a numerically efficient implementation of DMD specifically for direct (or "on the fly") calculations, which we carry out at the MP2-F12/AVDZ level of theory for the potential energy surface (PES) and MP2/AVDZ for the dipole moment surfaces (DMSs). Detailed analysis of the PES, DMS, and the time-dependence of the first derivative of the DMS, referred to as the driving force, for the highly fluxional vibrations involving H + /D + revealed that the strongly non-harmonic PES and non-linear DMS yield remarkably complex vibrational spectra. Interestingly, the classical trajectories reveal a doublet in the proton transfer part of the spectrum with the two peaks at 1800 and 1980 cm −1 . We find that their shared intensity is due to a Fermi-like resonance interaction, within the classical limit, of the H + parallel stretch fundamental and an H + perpendicular bending overtone. This doublet is also observed in the deuterated species at 1360 and 1460 cm −1 .

Intramolecular Proton Transfer in the Hydrogen Oxalate Anion and the Cooperativity Effects of the Low-Frequency Vibrations: A Driven Molecular Dynamics Study

“…The adapted equations of motion accounting for the driven term are where V ( q ) is the molecular interaction potential, m are atomic masses, and p and q are the 3 N Cartesian momenta and coordinates, respectively. We typically vary the electric field strength on the [50, 250] mV/bohr range to calibrate energy absorption. , Present DMD simulations at fundamental frequencies were carried out with the electric field strength 130 mV/bohr. To induce appreciable energy absorption in combinations and overtone bands, that are forbidden in the double harmonic limit, we use the electric field at stronger intensity, 200 mV/bohr.…”

“…In our previous work dealing with shared proton dynamics away from the harmonic limit, we analyzed vibrational spectra of protonated water clusters, − a protonated nitrogen dimer, N 4 H + , and isoelectronic species N 2 H + ···OC, using the dipole-driven MD method. Below, we refer to the dipole-driven MD method simply as DMD.…”

“…Xu and Meuwly also reported MD simulations at B3LYP/6-31G levels of theory that produced satisfactory results compared to the experiment, 3 where it was concluded that a barrier height of 4.2 kcal/mol best reproduced the position and width of the IR absorption associated with the PT. 4 In our previous work dealing with shared proton dynamics away from the harmonic limit, we analyzed vibrational spectra of protonated water clusters, 15−19 a protonated nitrogen dimer, N 4 H + , 20 and isoelectronic species N 2 H + •••OC, 21 using the dipole-driven MD method. Below, we refer to the dipoledriven MD method simply as DMD.…”

Permutationally invariant polynomial representation of polarizability tensor surfaces for linear regression analysis

“…From the above it is clear that the external force is directly proportional to the derivative of the full PTS tensor. The force is added to the equations of motion, and the molecule's internal energy (kinetic and potential) is integrated along the trajectory, as shown by Equation S7, to “measure” the vibrational energy absorption from the field 32–35 . If the driving frequency is close to a fundamental vibration n , that is, , and the polarizability derivative has a sizeable, formally non‐zero, component along the vector of motion, the dynamics will be resonant and the internal energy will increase rapidly.…”

Proton Transfer in Nitromethane–Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations

“…Proton transfer reaction is a basic and significant process in the chemistry and energy system, and its mechanisms play important roles in understanding ion–molecule reactions, radiation chemistry, interstellar chemistry, atmospheric chemistry, and biological systems . Proton transfer and electron transfer are two basic and extensive reactions in the chemical process.…”