Measurement of Donor-Acceptor Interchange Tunnelling in Ar(H2O)2 using Rotational Spectroscopy and a Re-look at Its Structure and Bonding

Abstract

In this work, we have measured the donor-acceptor interchange tunnelling splitting in the ground vibrational state of Ar(H2O)2. In the previous investigations, the donor-acceptor tunnelling splitting in fully deuterated species, Ar(D2O)2, was measured to be about 106 MHz. It could not be measured for Ar(H2O)2, as the splitting was expected to be several GHz. Besides, for (D2O)2, both A1 and B1 states have non-zero statistical weight appearing on both sides of the E1 state, having a semi-rigid rotor type spectrum making their observation easier, compared to that of (H2O)2, which has zero statistical weight for the B1 state. With the help of a fourfold periodic potential, we have accurately predicted the fingerprints of b-dipole A1+↔A1- transitions and observed them using a pulsed nozzle Fourier transform microwave spectrometer. Measurement of these transitions enabled us to determine the donor-acceptor tunnelling splitting of 4257.41(4) MHz in Ar(H2O)2. More detailed structural parameters of Ar(H2O)2 have been evaluated in this work and critically compared with (H2O)2. Atoms in Molecules (AIM), non-covalent interactions (NCI) index and natural bond orbital (NBO) analysis have also been carried out for both (H2O)2 and Ar(H2O)2, and these calculations confirm that the presence of argon (Ar) does not affect the structure and bonding in (H2O)2.