A combined experimental and computational investigation has been performed to investigate the structure and vibrational stretching frequencies of Cl-(NMA)(H2O)0-2Ar2 cluster ions.  The experimental infrared spectra reveal that that the chloride ion is bound to the hydrogen of the amide N-H group, forming an ionic hydrogen bond with the N-H stretching frequency red-shifted by over 600 cm-1 compared to neutral, gas-phase NMA.  The red-shift of this very anharmonic N-H stretch is not adequately captured with standard static ab initio electronic structure calculations.  Instead, ab initio calculations are combined with molecular dynamics techniques in a Born-Oppenheimer Molecular Dynamics (BOMD) approach.  The BOMD simulated spectra of Cl-(NMA)(H2O)0-2 reproduce the experimental spectra very well, capturing all but ~100 cm-1 of the large red-shift in the N-H stretch.  The 100 cm-1 error is due to the lack of zero point energy in the BOMD method.  The impressive agreement of the BOMD calculations with the experimental spectra is achieved without using any scaling factors or adjustable parameters.

IRPD and BOMD study of Cl-(N-methylacetamide)(H2O)0-2Ar2 cluster ions

247th National Meeting (2014)

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