The nature of interactions within a molecule, i.e. chemical bonding, is well understood today. However, our understanding about intermolecular interactions, which has great relevance in nature, is still evolving. Historically there are two types of intermolecular interactions, van der Waals interaction and hydrogen bonding. However, there has been an upsurge of interest in the halogen bonding and lithium bonding during the last decade. The main emphasis of our research is to understand these interactions in detail, in particular non-conventional hydrogen bond acceptors. In this work, weakly bound complexes are studied using Pulsed Nozzle Fourier Transform Microwave Spectrometer, which has been fabricated in our laboratory and various theoretical methods. FTMW spectroscopy in the supersonic beam provides accurate structural information about the near-equilibrium geometry of small dimers and trimers in isolation. The home-built Pulsed Nozzle Microwave spectrometer, having a spectral range of 2-26.5 GHz has been used to record the microwave spectrum of these complexes. The spectrometer consists of a Fabry-Perot cavity, electronic circuit and pumps. Fabry-Perot cavity is the interaction zone of the molecules and radiation. The electronic circuit is used for the polarization and detection of the signal. Mechanical and diffusion pumps are used to maintain the vacuum inside the cavity. The gas molecules of interest are then mixed with a carrier gas and pulsed supersonically inside the cavity through a nozzle of 0.8 mm diameter. The emission from the complexes formed during the expansion is detected by super-heterodyne detection technique and then Fourier transformed. The first chapter of the thesis gives a brief introduction to intermolecular interactions, hydrogen bonding, halogen bonding, lithium bonding and molecular 2 of clusters. The chapter also includes a brief introduction to rotational spectroscopy. The second chapter of the thesis discusses the experimental and theoretical methods. It includes a detailed discussion of the mechanical and electrical parts of the spectrometer and the software used, which is developed in Labview 7.1. The various theoretical methods (ab initio and DFT) and the basis sets are discussed along with Atoms In Molecules Theory and the criteria used to characterize hydrogen bond. In the third chapter, to understand the ability of saturated hydrocarbons to act as hydrogen bond donor and acceptor, interaction of CH4 with H2S is studied using rotational spectroscopy as well as theoretical methods such as ab initio and Atoms In Molecules theory. Three progressions were obtained for the CH4•••H2S complex using microwave spectroscopy. The progressions were independently fitted to a linear top Hamiltonian. Absence of J10 transition in Progression II confirms the presence of higher internal angular momentum state, m=1. This also confirms the internal rotation of the monomers in the complex. Progressions II and III have negative centrifugal distortion constants. Hence both the states are from some excited internal rotation/torsional motion with strong vibrational-rotational coupling. The moment of inertia obtained from the experimental rotational constant confirms the structure in which sulphur of H2S is close to CH4. This also supports the structure in which CH4 is the hydrogen bond donor, if such an interaction is present. AIM analysis and the potential energy barrier for internal rotation show orientational preference and hence hydrogen bonding. The ab initio results show that CH4•••HSH interaction is more favorable than CH3H•••SH2. Ab initio and AIM studies also gave a structure where there is direct interaction between C and S. This is interesting since the electronegativities of C and S are comparable. Experimentally obtained negative distortion constants for the other two states, confirm excited state rotational-vibrational coupling. The experimental data give a floppy structure having internal rotation. In the fourth chapter the complex chosen for investigation is benzene-ethylene. Experiments in condensed phase and theoretical works show evidence of - stacking in benzene dimer, but there is no gas phase spectroscopic evidence available for the same. The lack of permanent dipole moment in the -stacked geometry of benzene dimer is the hindrance in the experimental observation of the same using microwave spectroscopy. Substitution of one of the benzene with ethylene in the -stacked structure will result in a complex having permanent dipole moment. C6H6 C2H4 complex can have, in addition to -stacking, C-H/interaction. There could be a competition between C6H6 and C2H4, either of which can act as H-bond donor. Experiments show the evidence of C-H/interaction, where C2H4 is the hydrogen bond donor. To ascertain hydrogen bond interaction AIM analysis has been carried out. The results show C-H/interaction, where one of the C2H4 hydrogen interacts with the benzene. Even though the aim was to get the -stacked geometry, it could not be obtained. However theory and AIM supports the formation of -stacked complex. In the fifth chapter using theoretical methods the ability of radicals as acceptor of hydrogen, lithium and chlorine bonds are examined with CF3 radical as the model system. As hydrogen bonds are highly sensitive to the environment, the effect of substitution of hydrogen by fluorine is also analyzed. It is found that, even though CH3 and CF3 radicals are topologically different, they interact in a similar fashion. AIM analysis of CF3HY satisfies all the eight criteria proposed by Koch and Popelier for hydrogen bonding. Here the hydrogen bond formed is charge transfer assisted. The interaction energies of the complexes are inversely proportional to the dipole moment of hydrogen bond donors and are proportional to the charge transfer occurring in the complex. Interaction energies from ab initio calculations confirm complexation of CF3 radical with LiY(Y=F, Cl, Br) and ClF. AIM analysis of CF3LiY and CF3ClF complexes show a bond critical point between Li/Cl and the C of CF3 and the condition of mutual penetration is also met. In CF3LiY complexes the interaction energies and charge transferred are directly proportional to the dipole moment of the Li bond donor. In the sixth chapter in order to extend the concept of non-conventional hydrogen bond acceptors to transition metals, complexes of Fe (Fe(CO)5) with HX (X=F,Cl,Br) have been studied theoretically. DFT calculations show that the structure in which the hydrogen of HX interacting with Fe through the sixth co-ordination site is a stable geometry. AIM analysis shows the presence of a bond critical point between the iron and the hydrogen of HX and hence bond formation. Q obtained from NBO analysis shows that there is charge transfer from the organometallic system to the hydrogen bond donor. However the interaction energies of the complexes are proportional to the dipole moment of hydrogen bond donors and are inversely proportional to the charge transfer for these complexes. H-bonding leads to the stabilization of square pyramidal geometry. ‘Hydrogen bond radius’ of iron has also been defined. Studies on the interaction of Fe(CO)5 with ClF and ClH showed that Fe can also act as a chlorine bond acceptor. Seventh chapter provides the overall conclusion and also discusses future direction.