In this research the so-called potential morphing method was used to generate reliable interaction potential energy surfaces for weakly bound complexes. The potential morphing method is based on the optimization of modified computed ab initio potential energy surfaces to give predicted spectroscopic data, in agreement with the experimental values. In the standard potential morphing procedure the computed ab initio potential is adjusted by scaling, shifting, and dilating transformations to reproduce the experimental data. In this research, selected systems have been chosen to be studied based on the availability of varied and accurate sets of experimental data. In the present work, accurate interaction potential energy surfaces are obtained for the weakly bound complexes: Ne:HCl, OC:HX (X = F, Cl, Br, I) and HI:CO2. A comprehensive study on the interaction potential of these systems provides fundamental perspectives on the influence of different intermolecular forces. In addition the ground state isotopic isomerization observed in the OC:HI system may suggest a possible structural change of proteins, and other biological macromolecules, in deuterated solvents. In this dissertation, an alternative approach to morphing the potential energy surfaces of non-covalent interactions is also presented. In this approach the morphed potential is generated as a linear combination of ab initio potentials, that are computed at different levels of theory. This new morphing approach is applied to OC:HCl and is found to be of similar accuracy to that of the previous morphing method. In addition, this new method is also extended from four-dimensions to six-dimensions and is applied to the OC:HF system to obtain a vibrationally-complete six-dimensional morphed potential.

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Work reported in this thesis mainly comprises of the assignments and analysis of the rotational spectra and structures of three weakly bound complexes: C2H4•••H2S, C6H5CCH•••H2O and C6H5CCH•••H2S. All the data have been collected using a home built Pulsed Nozzle Fourier Transform Microwave Spectrometer. Apart from this, the thesis also deals with a criterion of classifying a weakly bound complex to a ‘hydrogen-bonded’ one. First chapter of the thesis gives a brief intermolecular interactions and molecular clusters of π system. It also briefly touches on the structural determination by rotational spectroscopy and the basic information one can gain from the rotational spectrum. Second chapter of the thesis gives a brief introduction to the experimental and theoretical methodology. It also gives a description of the software used in the FTMW spectrometer which was rebuilt using Labview 7.1. Third chapter of the thesis deals with the rotational spectra and structure of eight isotopologoues of C2H4•••H2S complex. The lines are split into four components for the parent isotopologue due to the presence of large amplitude motion. The smaller splitting is 0.14 MHz and the higher splitting is 1.67 MHz in (B+C)/2 for the parent isotopologue. Spectral splitting pattern of the isotopologues confirmed that smaller splitting is due to the rotation of ethylene about its C-C bond axis along with the contraction of S-H bond whereas the larger motion arises due to the interchange of equivalent hydrogens of H2S in the complex. A detailed spectral analysis and ab initio calculation for this system have been described in chapter III. The fourth chapter of the thesis describes the rotational spectroscopic studies of five isotopologues of C6H5CCH•••H2O complex. Rotational spectra unequivocally confirm the structure of the complex to be a one where H2O is donating one of its hydrogen to the acetylenic π cloud forming a O-H••• π bond whereas the ring ortho C-H bond forms C-H•••O bond with the water oxygen. For theparent isotopomer the lines are split into two components due to the rotation of H2O about its C2 symmetric axis. The fifth chapter of thesis describes the rotational spectroscopic and ab initio studies of five isotopologues of C6H5CCH•••H2S complex. Rotational spectra indicate the structure to be the one where H2S is sitting on the top of the phenyl ring and shifted towards the acetylenic group. The sixth chapter of the thesis describes a criterion for calling a complex to be hydrogen bonded based on the dynamic structure rather than the static structure of the complex. The question asked is if the anisotropy of the interaction is strong enough to hold the ‘hydrogen bond’ when one takes dynamics into account. The proposed criterion is that the zero point energy of the motion which takes the hydrogen away from the acceptor should be much less than the barrier height of the respective motion supporting at least one bound level below the barrier. Ab initio calculations have been done on four model systems Ar2•••H2O, Ar2•••H2S, C2H4••• H2O and C2H4••• H2S to emphasize this criterion.

Weak intermolecular interactions have very strong impact on the structures and properties of life giving molecules like H2O, DNA, RNA etc. These interactions are responsible for many biological phenomena. The directional preference of some of these interactions is used for designing different synthetic approaches in the supramolecular chemistry. The work reported in this Thesis comprises of investigations of weak intermolecular interactions in gas phase using home-built Pulsed Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer as an experimental tool and ab-initio and Atoms in Molecules (AIM) theory as theoretical tools. The spectrometer which is coupled with a pulsed nozzle is used to record pure rotational spectra of the molecular clusters in a jet cooled molecular beam. In the molecular beam molecules/complexes are free from interactions with other molecules/complexes and thus, spectroscopy in the molecular beams provides information about the 'isolated' molecule/complex under investigation. The rotational spectra of the molecules/complexes in the molecular beam provide their geometry in the ground vibrational states. These experimental geometries can be used to test the performance and accuracy of theoretical models like ab-initio theory, when applied to the weakly bound complexes. Further the AIM theory can be used to gain insights into the nature and strength of the intermolecular interactions present in the system under investigation. Chapter I of this Thesis gives a brief introduction of intermolecular interactions. Other than hydrogen bonding, which is considered as the most important intermolecular interaction, many other intermolecular interactions involving different atoms have been observed in past few decades. The chapter summarizes all these interactions. The chapter also gives a brief introduction to the experimental and theoretical methods used to probe these interactions. In Chapter II, the experimental and theoretical methods used in this work are summarized. Details of our home-built PN-FTMW spectrometer are given in this chapter. The chapter also discusses briefly the theoretical methods like ab-initio, AIM and Natural bond orbital (NBO) analysis. We have made few changes in the mode of control of one of our delay generators which have also been described. Chapter III and Chapter V of this Thesis are dedicated to the propargyl alcohol complexes. Propargyl alcohol (PA) is a molecule of astrophysical interest. It is also important in combustion chemistry since propargyl radical is considered as the precursor in soot formation. Moreover, PA is a multifunctional molecule, having a hydroxyl (-OH) and an acetylenic (-C≡C-H) group. Both of the groups can individually act as hydrogen bond acceptor as well as donor and thus PA provides an exciting possibility of studying many different types of weak interactions. Due to internal motion of -OH group, PA monomer can exist in gauche as well as trans form. However, rotational spectra of PA-monomer show the presence of only gauche conformer. In Chapter III, rotational spectra of Ar•••PA complex are discussed. The pure rotational spectra of the parent Ar•••PA complex and its two deuterated isotopologues, Ar•••PA-D (OD species) and Ar•••PA-D (CD species), could be observed and fitted within experimental uncertainty. The structural fitting confirmed a structure in which PA is present as gauche conformer and argon interacts with both the O-H group and the acetylenic group leading to Ar•••H-O and Ar•••π interactions respectively. Presence of these interactions was further confirmed by AIM theoretical analysis. In all the three isotopologues c-type rotational transitions showed significant splitting. Splitting patterns in the three isotopologues suggest that it originates mainly due to the large amplitude motion of the hydroxyl group and the motion is weakly coupled with the carbon chain bending motion. No evidence for the complex with trans conformer of PA was found. Although, we could not observe Ar•••trans-PA complex experimentally, we decided to perform ab-initio and AIM theoretical calculations on this complex as well. AIM calculations suggested the presence of Ar•••H-O and a unique Ar•••C interaction in this complex which was later found to be present in the Ar•••methanol complex as well. This prompted us to explore different possible interactions in methanol, other than the well known O-H•••O hydrogen bonding interactions, and eventually led us to an interesting interaction which we termed as carbon bond. Chapter IV discusses carbon bonding interaction in different complexes. Electrostatic potential (ESP) calculations show that tetrahedral face of methane is electron-rich and thus can act as hydrogen/halogen bond acceptor. This has already been observed in many complexes, e.g. CH4•••H2O/HF/HCl/ClF etc., both experimentally and theoretically. However, substitution of one of the hydrogens of methane with -OH leads to complete reversal of the properties of the CH3 tetrahedral face and this face in methanol is electron-deficient. We found that CH3 face in methanol interacts with electron rich sites of HnY molecules and leads to the formation of complexes stabilized by Y•••C-X interactions. This interaction was also found to be present in the complexes of many different CH3X (X=OH/F/Cl/Br/NO2/NF2 etc.) molecules. AIM, NBO and C-X frequency shift analyses suggest that this interaction could be termed as "carbon bond". The carbon bonding interactions could be important in understanding hydrophobic interactions and thus could play an important role in biological phenomena like protein folding. The carbon bonding interaction could also play a significant role in the stabilization of the transition state in SN2 reactions. In Chapter V of this Thesis rotational spectra of propargyl alcohol dimer are discussed. Rotational spectra of the parent dimer and its three deuterated (O-D) isotopologues (two mono-substituted and one bi-substituted) could be recorded and fitted within experimental uncertainty. The fitted rotational constants are close to one of the ab-initio predicted structure. In the dimer also propargyl alcohol exists in the gauche form. Atoms in molecules analysis suggests that the experimentally observed dimer is bound by O-H•••O, O-H•••π and C-H•••π interactions. Chapter VI of the thesis explores the 'electrophore concept'. To observe the rotational spectra of any species and determine its rotational constant by microwave spectroscopy, the species should have a permanent dipole moment. Can we obtain rotational constants of a species having no dipole moment via microwave spectroscopy? Electrophore concept can be used for this purpose. An electrophore is an atom or molecule which could combine with another molecule having no dipole moment thereby forming a complex with a dipole moment, e.g. Argon atom is an electrophore in Ar•••C6H6 complex. The microwave spectra of Ar•••13CC5H6 and Ar•••C6H5D complexes were recorded and fitted. The A rotational constant of these complexes was found to be equal to the C rotational constant of 13CC5H6 and C6H5D molecules respectively and thus we could determine the C rotational constant of microwave 'inactive' 13CC5H6. This concept could be used to obtain the rotational spectra of parallel displaced benzene-dimer if it exists. We recently showed that the square pyramidal Fe(CO)5 can act as hydrogen bond acceptor. Appendix I summarizes the extension of this work and discusses interactions of trigonal bipyramidal Fe(CO)5 with HF, HCl, HBr and ClF. Our initial attempts on generating a chirped pulse to be used in a new broadband spectrometer are summarized in Appendix II. Preliminary investigations on the propargyl•••water complex are summarized in Appendix III.