A Fermi resonance and a parallel-proton-transfer overtone in the Raman spectrum of linear centrosymmetric N₄H⁺: A...

We present molecular dynamics (MD), polarizability driven MD (α-DMD), and pump-probe simulations of Raman spectra of the protonated nitrogen dimer N₄H⁺, and some of its isotopologues, using the explicitly correlated coupled-cluster singles and doubles with perturbative triples [CCSD(T)]-F12b/aug-cc-pVTZ based potential energy surface in permutationally invariant polynomials (PIPs) of Yu et al. [J. Phys. Chem. A 119, 11623 (2015)] and a corresponding PIP-derived CCSD(T)/aug-cc-pVTZ-tr (N:spd, H:sp) polarizability tensor surface (PTS), the latter reported here for the first time. To represent the PTS in terms of a PIP basis, we utilize a recently described formulation for computing the polarizability using a many-body expansion in the orders of dipole-dipole interactions while generating a training set using a novel approach based on linear regression for potential energy distributions. The MD/α-DMD simulations reveal (i) a strong Raman activity at 260 and 2400 cm^-1, corresponding to the symmetric N-N⋯H bend and symmetric N-N stretch modes, respectively; (ii) a very broad spectral region in the 500-2000 cm^-1 range, assignable to the parallel N⋯H⁺⋯N proton transfer overtone; and (iii) the presence of a Fermi-like resonance in the Raman spectrum near 2400 cm^-1 between the Σ_g ⁺ N-N stretch fundamental and the Π_u overtone corresponding to perpendicular N⋯H⁺⋯N proton transfer.