Rotational Spectroscopic and Computational Studies on Inter/Intramolecular Bonding

Abstract

The importance of non-covalent interactions cannot be undermined, as it permeates nearly every sphere of chemistry. The hydrogen bond is the most well-known and ubiquitous non-covalent interaction. In recent times similar interactions involving other elements, that include nearly half the periodic table, have been proposed and studied. The tetrel bond (Group 14 elements) is one such interaction which is analogous to the hydrogen bond1,2. The ‘carbon bond’ is an important subset of these interactions1. Our objective is to understand these non-covalent interactions by studying the structures of weakly bound complexes. Rotational spectroscopy is an extremely accurate technique to obtain the geometrical parameters of molecules in the gas phase. We have used a home-built pulsed nozzle Fourier transform microwave (PNFTMW) spectrometer to record the spectra3. Weakly bound complexes are formed via a supersonic expansion of the gases into a vacuum chamber. These complexes interact with the microwave radiation to give a rotational spectrum. Structural information is then extracted from the rotational spectrum. The rotational spectroscopic studies are supplemented by computational studies such as the Atoms in Molecules theory. There is a paucity of experimental data involving tetrel bonded interactions. This Thesis focusses on hydrogen and tetrel bonded complexes. Chapter 1 of this thesis gives a brief introduction to non-covalent interactions and expounds on the hydrogen and tetrel bonding interactions. This chapter also provides pertinent details about rotational spectroscopy. Chapter 2 details the functioning of the microwave spectrometer and other computational methods used in the Thesis. Chapter 3 discusses the rotational spectra of the propargyl alcohol (PA)∙∙∙water complex. PA is multifunctional molecule having a hydroxyl group and an acetylenic moiety. The hydroxyl group present in both alcohol and water can act as either an H-bond donor or as an H-bond acceptor. This offers different binding options leading to a number of different possible hydrogen bonded structures for the PA∙∙∙H2O complex. The two lowest energy structures calculated for the complex differ only in the position of the non-bonded hydrogen atom. The spectrum obtained indicates the presence of the global minimum structure, G-PW-1a. In this structure, PA donates an O-H∙∙∙O H-bond to H2O and accepts an O-H∙∙∙π H-bond from H2O. The spectra for the isotopologues help to determine the position of the non-bonded hydrogen atom of water. This helps to differentiate between the two lowest energy structures. The matrix isolation IR spectroscopic study on the PA∙∙∙H2O complex cannot differentiate between these two structures. The rotational spectrum shows a doubling of the lines caused by the internal rotation of the H2O moiety about its C2 axis. We were also able to generalize the H-bond donor/acceptor capabilities of the hydroxyl groups in an alcohol∙∙∙water complex based on the electron donating/withdrawing abilities of the groups present in the alcohol. The rotational spectra for the acetonitrile∙∙∙carbon dioxide are discussed in Chapter 4. The ab initio calculations for the CH3CN∙∙∙CO2 complex optimized four tetrel bonded structures. Therefore, investigating this complex provides an opportunity to study tetrel bonded structures in the gas phase. We have observed the rotational spectra corresponding to the two lowest energy structures, the π-stacked and the T-shaped. The spectra for the isotopologues were also recorded. The spectra show hyperfine splitting due to the nuclear quadrupole coupling of the N-14 nucleus. The π-stacked structure has CO2 and CH3CN stacked in a parallel manner with the oxygen end of CO2 interacting with the positively charged C atom of the cyano group in CH3CN. The Atoms in Molecules analysis finds that the methyl C-H forms a hydrogen bond with the same oxygen atom leading to a closed network of non-covalent interactions. In the T-shaped structure the nitrogen end of CH3CN donates electron density to the central positively charged C atom of CO2. The spin-less indistinguishable oxygen nuclei in the C2v symmetry of the T-shaped structure dictates that odd |K-m| levels will be missing. We find no K=1 lines for the m=0 state confirming that T-shaped structure has been observed. The structure of CH5+ has been highly debated upon. Chapter 5 discusses the structure of CH5+ using Atom in Molecules (AIM), natural bond orbital (NBO), and normal coordinate analyses. Contrary to the popular perception, we find that the structure of CH5+ cannot be considered as the complex between CH3+ and H2. It has a pentacoordinate carbon center having five C-H bonds4. The congeners of CH5+, SiH5+ and GeH5+ were also explored to see if they form similar structures as CH5+ or not. We find that the structures are different from CH5+. The structures of SiH5+ and GeH5+ form a complex between the TH3+ and H2. The identity of the H2 moiety is retained in these complexes. The H2 moiety donates electron density to the positively charged T atom forming a tetrel bonded complex. This work led us to classify the 3c-2e bonds based on the connectivity patterns of the three nuclei involved in the 3c-2e. We were able to classify them into V, L, Δ, T, and I (linear) types5. Chapter 6 explores the structures and rotational spectra of the weakly bound complexes of methyl fluoride with water, argon, and itself. We intended to observe a tetrel bonded structure for the CH3F∙∙∙H2O complex. However only the global minimum hydrogen bonded structure having a bent O-H∙∙∙F bond was observed. Three structures are possible for the CH3F∙∙∙Ar complex, the T-shaped, tetrel bonded, and linear structures. These three structures are very close in energy. We have evidence for the formation of the T-shaped structure, where the Ar atom is positioned perpendicular to the C-F bond. The large number of unassigned lines could belong to the other two structures. The global minimum structure for the CH3F∙∙∙CH3F dimer is an antiparallel structure where the two CH3F units are bound by two symmetrical C-H∙∙∙F hydrogen bonds. However, it is not possible to observe this structure because it has a net zero electric dipole moment. So the lines observed could possibly belong to the tetrel bonded structures, linear or skewed linear. In both these structures the electron rich F atom donates electron density to the σ-hole formed over the methyl face of CH3F. Chapter 7 summarizes the results and conclusions for the molecular structures investigated in this Thesis.