Microwave Rotation-Tunnelling Spectroscopic and Theoretical Studies on Weakly Bound Molecular Complexes: Intermolecular Bonding across the Periodic Table
Intermolecular interactions appear to be well understood in a broad sense today; at a deeper molecular level, it is still evolving. Spectroscopy in this isolated state proved to be a first step toward understanding the intermolecular interaction at the molecular level. Microwave spectroscopy offers precise structural information on the near-equilibrium geometry of small dimers and trimers in isolation. Computational studies like the Atoms in Molecules (AIM), non-covalent index plots (NCI), and natural bond orbital analysis (NBO) are used to augment rotational spectroscopic investigations. The Ka = 1 transitions of H2S dimer and several isotopomers were observed in a pulsed nozzle Fourier transform microwave spectrometer. These transitions give unequivocal proof that, at ultra-low temperatures, hydrogen sulfide forms S-H⸳⸳⸳S hydrogen-bonded dimer in the same way as water does, even though ice and solid H2S seem substantially different in bulk. Also, using the AIM theory, we have shown that H2S dimer satisfies the necessary and sufficient criterion proposed by Koch and Popelier to be hydrogen-bonded. Although we recently highlight the arbitrariness in relying on some computational tools to characterize a bond. The weakly bound trimer between two hydrogen sulfide molecules and one water molecule, (H2S)2H2O, was identified from its rotational spectrum. The break with axial molecular symmetry allowed us to investigate (H2S)2H2O at a level of structural detail that has not yet been possible for (H2O)3 and (H2S)3 with rotational spectroscopy owing to their zero-dipole moment. Analysis of experimental results reveals that the three monomers are bound in a triangular arrangement through S-H⸳⸳⸳S, O-H⸳⸳⸳S, and S-H⸳⸳⸳O hydrogen bonds with a fair amount of co-operativity. High-resolution spectroscopic data may be used to validate the correctness of a model intermolecular potential energy hyper-surface. In this regard, 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 106 MHz. However, it could not be measured for the Ar-(H2O)2,as the splitting was expected to be several GHz. With the help of a fourfold periodic potential, we have accurately predicted the fingerprints of donor-acceptor interchange tunnelling transitions and measured the splitting of 4257.41(4) MHz in Ar-(H2O)2. Lastly, we have looked beyond hydrogen bonding and explored other intermolecular bonding across the Periodic Table. The slopes of the binding energy versus electron density at the bond critical point were derived for each main group element. Our results show that intermolecular bonding can be classified into two types: intermolecular bonding (IMB) with a shared shell molecule (IMB-S) and intermolecular bonding (IMB) with a closed shell molecule (IMB-C). The IMB-S includes hydrogen, halogen, chalcogen, pnictogen, tetrel (excluding carbon bonds), and boron bonds (but not triel bonds). IMB-C contains lithium, sodium, beryllium, magnesium, triel (excluding boron bonds) and carbon bonds. The binding energy versus electron density plot of the IMB-S class generally has a low slope, whereas the IMB-C type has a high slope. Carbon bonds are distinct from the other members of the group. Carbon is a hesitant partner in tetrel bonds due to the absence of lower energy d-orbitals. The electron density between the two atoms is extremely low, and the binding energy grows fast with electron density, resulting in a high slope value for the carbon bond. The slopes for the Li, Na, Be, Mg, Ca-bonds were found out to be comparable, whereas the slope for the hydrogen bond remains standout. Several similarities eventually lead us to propose a common name, ‘Alkalene bond,’ for the intermolecular bonding in alkali and alkaline earth metals.