Microwave Spectroscopic and Computational Studies on Hydrogen-Bonded Complexes

Abstract

Hydrogen bonding and van der Waals interactions are key contributors to numerous biological phenomena and have significant implications in synthetic chemistry as well. In supramolecular chemistry and crystal engineering, the orientational preference of more specialised intermolecular interactions, such as hydrogen and halogen bonding, is frequently used to construct macromolecules. Microwave spectroscopy provides valuable insights into the structure and dynamics of molecules. We utilised a Pulse Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer at IISc Bangalore, India and a Chirped-Pulse Fourier Transform Microwave (CP-FTMW) spectrometer at Newcastle University, United Kingdom, to record the rotational spectrum of weakly bonded complexes formed in a supersonic expansion. The computational studies like the Atoms in Molecules (AIM) theory, non-covalent index (NCI) plots, natural bond orbital analysis (NBO), and symmetry-adapted perturbation theory (SAPT) are also used to understand the nature of bonding present in the systems. The phenylacetylene···CH3OH complex, 1-fluoronaphthalene monomer, 1-fluoronaphthalene···(H2O)1-2 complexes, and NH3@18-crown-6 complex will be discussed in the talk. Phenylacetylene (PhAc) is a multifunctional molecule and has been termed a “Hydrogen Bonding Chameleon”. The previous reports on PhAc···CH3OH complex show different results. IR-UV double resonance spectroscopic results found that CH3OH donates the hydrogen bond to the phenyl-π system, whereas the FTIR spectroscopic results show that the CH3OH donates the hydrogen bond to the acetylenic-π system. The rotational spectrum confirms the result of this complex and is consistent with the structure where CH3OH donates the H-bond to the acetylenic π-system, and CH3OH accepts a weak H-bond through the ortho hydrogen of the PhAc. The rotational transitions were split, indicating the internal motion of the CH3 group in CH3OH. The observed global minimum structure has been compared with several CH3OH-containing complexes to understand the internal rotation of the CH3 group and its effect on V3 barrier height. The rotational spectrum of 1-fluoronaphthalene (1FN) monomer is revisited. We accurately determined atomic coordinates using the substitution method, revealing a nearly uniform inertial defect (-0.14 amu Å2) across all isotopologues. The negative inertial defect is attributed to the low out-of-plane bending mode of the 1FN ring. A formula based on empirical data was utilized for the computation of the lowest out-of-plane bending mode, which was then compared with the bending mode estimated using the harmonic and anharmonic frequency calculations. The rotational spectrum of the 1FN···(H2O)1-2 complex was investigated For 1FN···H2O, H2O acts as a proton donor alongside a C-H···O weak interaction where H2O serves as a weak proton acceptor. Experimentally obtained inertial defect (-1.30 amu Å2) of 1FN···H2O complex indicated an effective planar geometry. For 1FN···(H2O)2, the preliminary analysis confirms the structure in which the (H2O)2 is interacting from the top of the 1FN plane by forming O-H···F and O-H···C hydrogen bonds. Finally, the hydrogen bonding, inversion, and tunnelling of NH3 in the complex with 18-crown-6 (CE) were examined. The analysis of the potential energy scan of NH3 inversion revealed that the barriers are significantly influenced by N-H···O hydrogen bonding interactions. The barrier for this inversion was determined to be 12.1 kcal/mol, in contrast to the 5.3 kcal/mol observed for free NH3. Importantly, the configuration of the CE remained unaltered throughout the study. To gain further insight into the tunnelling motion of NH3 through the CE ring, a 1D relaxed scan and a 2D rigid scan were conducted. The investigation yielded a barrier of approximately 11 kcal/mol for this motion.