A Comparison of NH52+ and CH5+ Ions and Deuterated Variants of NHxD(5− x)2+: Real or Artefactual Rotation?

Abstract

Protonated NH₅²⁺ has unusual vibrational and rotational behavior because its three nonequivalent equilibrium structures have nearly identical energies and its five protons scramble freely, while in the CH₅⁺ structure, five hydrogen atoms are bonded to the carbon atom by sharing eight valence electrons. Although a few theoretical papers have been published on quantum mechanics of those systems. Better understanding requires spectral and conformational analyses. CASSCF (8, 9) calculations with the correlation consistent polarized valence double and triple zeta basis sets are accomplished for estimating the vibrational data and zero-point energies of those two ions. The present results indicate that normal modes agree qualitatively with NH_xD_(5-
x)²⁺ and one of the normal modes indicates that NH₅²⁺ is highly fluxional and has a complex spectrum while the broken N-H bonds are reformed all the time. The spectrum of mode 10 is highly complex with red and some blue shifts. In particular, modes 6 and 12 are attributed to the rapid coupling of the N-H stretching normal mode to motions more closely related to isomerization, i.e., bending or rocking. There have been long debates whether NH₅²⁺ has a structure at all or not and if the rotation is real or artefactual.