Anharmonic vibrations of N–H in Cl−(N-methylacetamide)1(H2O)0–2Ar2 cluster ions. Combined IRPD experiments and BOMD simulations

Abstract

Infrared Predissociation (IRPD) spectra of Cl⁻(NMA)₁(H₂O)_0–2Ar₂ combined with Born–Oppenheimer Molecular Dynamics (BOMD) IR spectra have been acquired, providing the structure and dynamics of these systems. We show that the chloride ion is bound to the hydrogen of the amide N–H group, forming a strong ionic hydrogen bond, weakening the N–H stretch, and shifting it to lower frequency. The presence of water molecules enhances the ionic hydrogen bond by binding to the amide carbonyl oxygen of NMA and shifts the N–H stretch further to lower frequency. The BOMD IR spectra can recapture all, but about 100 cm⁻¹, of the 600 to 700 cm⁻¹ shifts due to the strong N–H stretch anharmonicities observed in experiments. This residual error was found to be due to the lack of zero point energy in the classical treatment of motion in the BOMD method.