On the influence of semirigid environments on proton transfer along molecular chains

The dynamics of proton transfer along ammonia chains (chemical composition N(x)H(+)(3x+1), x=2, 4, and 6) in a constraining environment is investigated by ab initio molecular dynamics simulations. A carbon nanotube of defined length and diameter is used as an idealized constraining environment such that the ammonia chain is forced to maintain its quasilinear geometry. It is found that, although the energetics of proton transport shows considerable energetic barriers, proton translocation along the wire is possible at finite temperature for all chain lengths studied. The proton transport involves rotational reorientation of the proton-carrying ammonia molecule. High level ab initio calculations (MP2/aug-cc-pVTZ) yield barriers for internal rotation of 9.1 kcal/mol for NH(4) (+)-NH(3) and 11.7 kcal/mol for OH(3) (+)-OH(2), respectively. The infrared spectrum calculated from the dipole-dipole autocorrelation function shows distinct spectral features in the regions (2000-3000 cm(-1)) where the NHN proton transfer mode is expected to absorb. Assigning moderate opposite total charges between 0.002 and 0.2e to the carbon atoms at the end caps of the nanotube leads to a considerable speedup of the proton transfer.