H/D isotope effects on the geometry and infrared spectrum of the protonated ammonia dimer

The two isotopomers, N 2 H 7 + and N 2 D 7 + , have been investigated using a six-dimensional quantum mechanical model. In both cases a structural symmetrization due to zero point vibrational motion is observed. The probability distribution along the proton/deuteron transfer coordinate is found to have a single maximum, which is flattened in the deuterated case due to the energetic proximity of the barrier. Being a strong hydrogen bonding case, the secondary geometric isotope effect leads to a bond compression by 0.004 Å. Most notable changes in the infrared absorption spectrum in the range below 500 cm−1 concern a red-shift of the asymmetric proton stretching fundamental transition by about 47% for the fully deuterated case.