Solid state to solution: crystal structure and molecular dynamics simulations of a polyammonium nitrate host

Abstract

The crystal structure of a hexaprotonated octaaza macrocycle, 1,4,7,10,13,16,19,21-octaazacyclotetracosane, has been obtained as the nitrate complex, H₈1·8NO₃. The complex crystallizes in the monoclinic space group P2₁/c with unit cell parameters a=14.43(2), b=8.36(1), c=15.9(1) Å, β=110.8(2)° and U=1793(12) ų. The macrocycle crystallizes in a relatively flat elliptical conformation with dimensions of 11.9 and 7.2 Å, and a depth of 1.8 Å. The crystallographic coordinates formed the basis for molecular dynamics simulations over a 400 ps timeframe using the all-atom CHARMM version 25 package. The major change in the conformational structure of the macrocycle during the simulations was the shifting of all NCCN dihedral angles to a trans conformation. Also, each conformational transition in CNCC and CCNC angles flanking a central CC bond was found to be correlated. The macrocycle remained relatively flat throughout the simulation, resulting in essentially complete hydration within the first 100 ps of the simulation. Extensive water relay networks could be identified throughout the simulation, which are attributed to maintaining and stabilizing anion/cation interactions in solution.