Gas‐Phase Infrared Spectroscopy and Multidimensional Quantum Calculations of the Protonated Ammonia Dimer N₂H₇⁺

Abstract

Zero‐point energy matters: In strong, low‐barrier hydrogen bonds, quantum effects cause a structural symmetrization (see picture). Together with the pronounced anharmonicity of vibrational motion, this situation gives rise to peculiar infrared (IR) spectral signatures in the region below 2000 cm−1. For the shared proton in N2H7+, the IR spectrum is elucidated by combining two experimental techniques with anharmonic quantum calculations.

Pseudo-Jahn-Teller origin of the low barrier hydrogen bond in N2H7+

“…For this reason, Asada et al 15 only considered an average of the ␣ 2u -␣ 1g modes. On the other hand, Asmis et al 17 performed a joint experimental-theoretical work where they obtained the N 2 H 7 + infrared spectrum and simulated it using a four-dimensional energy surface. In both works the APES was calculated at the MP2 level of theory.…”

“…The kind of coupling between ␣ 1g and ␣ 2u modes described above can also be found in a model that considers mechanisms such as the dependency of the PJT constant with totally symmetric vibrations, different force constants along Q͑␣ 1g ͒ in the ground and excited states, or cubic terms of the Q͑␣ 1g ͒Q͑␣ 2u ͒ 2 type. Despite the well known fact that these mechanisms strongly affect the dynamics of the nuclear frame of any polyatomic system presenting a PJT, 17,23,51 there are few studies devoted to determine when each of these effects may be important in different molecules. Since they affect differently the vibronic matrix, they could lead to different diabatic behaviors in the dynamics.…”

“…In Table V we also show the assignment of the experimental infrared spectra taken from the work of Asmis et al 17 In our dynamical calculations the only infrared-active vibrational states are those that have a 2u symmetry. Then, we can assign bands A, C and F measured by Asmis et al 17 to the transitions ͑all allowed by symmetry͒ from the ground state to ͉1 1͘, ͉1 2͘, and ͉1 3͘ excited states, respectively, in our bidimensional surface.…”

“…These authors concluded that N 2 H 7 + has a symmetric D 3d structure, in agreement with the experimental spectra by Price et al, 11 the contradiction with the minimum energy structure found by geometry optimizations being due to the neglect of quantum effects of the proton in earlier calculations. Recently, Asmis et al 17 provided a joined experimental and theoretical study addressing the consequences of this quantum-mechanical symmetrization through the ZPE. Their four-dimensional anharmonic model agrees well with experimental data, thus providing a first assignment for the main absorption bands.…”

“…Previous studies 18 indicate that proton transfer is not simply related to a single reaction coordinate that takes H + from one ligand to the other but also that the breathing mode that controls the distance between these ligands plays a very important role. Therefore, another goal of this work will be to carry out a detailed study of the mechanisms behind this coupling and to show whether the infrared spectrum 17 can be understood just using these two modes or if the coupling to a larger number of modes is required to adequately account for all the observed bands.…”