Yi Liu
Dept. of Chemistry
New York University, New York, NY 10003
Mark E. Tuckerman
Dept. of Chemistry and Courant Institute of Mathematical Sciences
New York University, New York, NY 10003
March 26, 2001
The structural, dynamical and electronic properties of ionic defects
in liquid ammonia at 260 K created by the addition or removal of a proton
have been studied using the method of ab initio molecular dynamics.
These protonic defects correspond to the
ammonium (NH 
) and amide (NH 
) ions in the liquid and
are the analogs of the H 
O 
and OH 
ions in
water. For this reason, direct comparison between the protonic defects
in ammonia and those in water can be made. In particular, it is found that
the NH exhibits a characteristic cationic solvation pattern, in which
it donates four hydrogen bonds to neighboring ammonia molecules, giving it
a coordination number of 4. The NH ion is found to have a coordination number between
7 and 8 in liquid ammonia, a number higher than would be expected based on the
number of hydrogen bonds it can accept and donate. It is found that
about 40% of this is due to hydrogen bonding but that these hydrogen bonds
are all accepted by the amide nitrogen. Moreover, the hydrogen bonds
form are often arranged in a planar configuration (perpendicular to the C 
axis of the amide), a solvation pattern also exhibited by OH in water.
The rationale for the high coordination of NH is found to differ
markedly from that which emerges from interpretation of spectral data.
Unlike H O and OH in water, no proton transfer is exhibited in either the
NH or the NH systems. The results presented here lead to a possible explanation
for the lack of structural diffusion. Nevertheless, the solvation structures
formed by the NH and NH ions in ammonia and their associated electronic
properties possess many similarities with the water ions in water, and from the studies
performed here, a number of important patterns begin to emerge that may
be applicable to protonic defects in other hydrogen-bonded liquids.
in liquid ammonia
in liquid ammonia
O and OH solvation in water