Ab initio anharmonic vibrational frequency predictions for linear proton-bound complexes OC–H+–CO and N2–H+–N2

Abstract

Ab initio anharmonic transition frequencies are calculated for strongly coupled (i) asymmetric and (ii) symmetric proton stretching modes in the X–H⁺–X linear ionic hydrogen bonded complexes for OCHCO⁺ and N₂HN₂⁺. The optimized potential surface is calculated in these two coordinates for each molecular ion at CCSD(T)/aug-cc-pVnZ (n = 2–4) levels and extrapolated to the complete-basis-set limit (CBS). Slices through both 2D surfaces reveal a relatively soft potential in the asymmetric proton stretching coordinate at near equilibrium geometries, which rapidly becomes a double minimum potential with increasing symmetric proton acceptor center of mass separation. Eigenvalues are obtained by solution of the 2D Schrödinger equation with potential/kinetic energy coupling explicity taken into account, converged in a distributed Gaussian basis set as a function of grid density. The asymmetric proton stretch fundamental frequency for N₂HN₂⁺ is predicted at 848 cm⁻¹, with strong negative anharmonicity in the progression characteristic of a shallow “particle in a box” potential. The corresponding proton stretch fundamental for OCHCO⁺ is anomalously low at 386 cm⁻¹, but with a strong alternation in the vibrational spacing due to the presence of a shallow D_∞h transition state barrier (Δ = 398 cm⁻¹) between the two equivalent minimum geometries. Calculation of a 2D dipole moment surface and transition matrix elements reveals surprisingly strong combination and difference bands with appreciable intensity throughout the 300–1500 cm⁻¹ region. Corrected for zero point (ΔZPE) and thermal vibrational excitation (ΔE_vib) at 300 K, the single and double dissociation energies in these complexes are in excellent agreement with thermochemical gas phase ion data.