The hypothesis that nuclear motion can be described classically has been tested for several critical systems. We investigate the inversion of ammonia and the heat capacities of water and hydrogen. We use conventional ab initio molecular dynamics, which describes nuclear motion classically and the electron cloud using density functional theory. Ammonia inversion is described perfectly by the tunneling of the p orbital through the molecular plane. Nuclear tunneling is not needed to describe this phenomenon. While the investigation of heat capacities is hampered by the brief simulation times and limited system sizes, we can nevertheless make some qualitative statements. Indeed, the heat capacity can be frozen out in molecular dynamics simulations of solids, and hence, a quantized description is not required.
AbstractHigh-resolution rotational spectroscopy was used to investigate the conformational landscape of methyl-3,3,3-trifluoropyruvate, a small, partially-fluorinated molecule, which is of interest because of its chemical properties and reactivity in contrast to the unfluorinated species. Methyl 3,3,3-trifluoropyruvate is also subject to two possible large amplitude motions of the methyl and trifluoromethyl group. However, only the methyl rotor gives rise to the tunneling splitting specific to individual conformers. In the rotational spectrum measured in the frequency region from 6 to 27 GHz, the identified conformers, s-cis and s-trans, were fitted to experimental accuracy, resulting in the accurate determination of the vibrational ground state rotational constants ${A}_{0}=2185.05827\left(36\right)\text{\hspace{0.17em}MHz}$, ${B}_{0}=1023.30031\left(17\right)\text{\hspace{0.17em}MHz}$, and ${C}_{0}=803.520287\left(95\right)\text{\hspace{0.17em}MHz}$ for the s-cis conformer, and ${A}_{0}=2706.9024\left(49\right)\text{\hspace{0.17em}MHz}$, ${B}_{0}=864.889539\left(81\right)\text{\hspace{0.17em}MHz}$, and ${C}_{0}=746.532896\left(71\right)\text{\hspace{0.17em}MHz}$ for the s-trans conformer. Additionally the barrier heights of the methyl rotor ${V}_{3}\left({\text{CH}}_{3}\right)=363.116\left(94\right){\text{\hspace{0.17em}cm}}^{-1}$ and ${V}_{3}\left({\text{CH}}_{3}\right)=389.290\left(80\right){\text{\hspace{0.17em}cm}}^{-1}$ were obtained for the s-cis and s-trans conformer, respectively.
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