Simulation of proton transfer along ammonia wires: An “ ab initio ” and semiempirical density functional comparison of potentials and classical molecular dynamics

• Published in: The Journal of chemical physics 2002-02, Vol.116 (6), p.2572-2585
• Main Authors: Meuwly, Markus, Karplus, Martin
• Format: Article
• Language: English
• ISSN: 0021-9606; 1089-7690
• DOI: 10.1063/1.1431285

Protonated ammonia clusters of the composition (NxH3x+1)+ with x=2,3,4 are investigated by using the gradient corrected, three-parameter functional by Becke based on the functional by Lee, Yang, and Parr (B3LYP/6-31G**) and self-consistent charges density functional tight-binding (SCC–DFTB) methods for calculating the potential energy surface and forces in the Born–Oppenheimer approximation. They are used for classical molecular dynamics simulations at temperatures ranging from 5 K to 600 K. Results from the two methods are compared for proton transfer in N2H7+. The number of proton transfer events as a function of temperature is similar, although at low temperatures, SCC–DFTB cuts off more rapidly than B3LYP/6-31G**. Calculated vibrational spectra agree well for the intermolecular N–N and intramolecular N–H stretch excitations. Both approaches lead to broad, relatively unstructured bands extending over about 1500 cm−1 for the proton transfer coordinate. Simulations at the SCC–DFTB/MD level for larger (NxH3x+1)+ (x⩽4) clusters are presented and discussed. They show significant structural reorganization within the cluster. Consecutive proton hops within a few tenths of a fs are observed. A N2H7+ cluster immersed in a water shell containing 25 water molecules was studied by the mixed quantum mechanical/molecular mechanics (QM/MM) method with SCC–DFTB for the QM part. The presence of water appears to impede proton transfer. Including corrections for basis set superposition error in the MP2/aug-cc-pVTZ and B3LYP/6-31G** calculations has a small effect. It increases the barrier heights from 0.78 kcal/mol to 1.28 kcal/mol (MP2) and from 0.10 kcal/mol to 0.27 kcal/mol (B3LYP), respectively.