Simple organometallic molecules, especially those with a single ligand, are the desired model systems to investigate the metal-ligand interactions. For such a molecule, a quantitative relationship between the geometry and the electronic configuration would be instructive to test the existing theories and to access more complicated systems as well. As a matter of fact, microwave spectroscopy could be the best approach to address this issue by measuring the pure rotational spectrum of a metal-containing molecule. By doing so, microwave spectroscopy can provide the most reliable bond lengths and bond angles for the molecule based on the rotational constants of a set of isotopologues. On the other hand, from the fine-structure and hyperfine-structure of the spectrum, microwave spectroscopy can also describe the electronic manifold, charge distribution and bonding nature of the molecule in a quantitative way.Fourier transform microwave spectrometers have been the most popular equipment to measure the pure rotational spectrum for three decades owing to the high resolution and super sensitivity. With the advances in digital electronics and the molecular production techniques, hyperfine structures of metal-containing molecules can be easily resolved even for the rare isotopologues in their nature abundance by this type of spectrometers.In this dissertation, molecules bearing metals in a wide range covering both the main group and transition metals were studied. By taking advantage of both the traditional and newly developed molecular production techniques in the gas phase (for example, metal pin-electrodes and discharged assisted laser ablation spectroscopy), we obtained spectra of molecules containing magnesium, aluminum, arsenic, copper and zinc. Our subjects include metal acetylides (MgCCH, AlCCH and CuCCH), metal dicarbides (CCAs), metal cyanides (CuCN, ZnCN) as well as other metal mono-ligand molecules. For the zinc metal, complexes with two simple ligands were also investigated, such as HZnCl and HZnCN. We strongly believe that researchers in different disciplines would benefit from our laboratory studies: theoretical chemists can use our experimental results for calibration; astrophysicists would interpret their telescope observations by matching our precisely measured frequencies; material scientists could find new functional materials by linking the bulky properties of certain materials with our spectroscopic results of the monomers.

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.