Ab initio Molecular Orbital and Valence-Bond methods and the semiempidcal, AM1 method are applied in the studies of: 1. The proton affinity of diacetylene The gas-phase ion-molecule reaction between diacetylene and a proton was studied theoretically at the MP4SDQ/6-311G** level. The geometries, calculated harmonic vibrational frequencies and the proton affinity of the most stable structures are compared. The results from this study are supported by selected ion flow tube measurements and are compared with other calculations 2. The gas-phase reaction of CH₃CN and CH₃⁺. The reaction between CH₃CN and CH₃⁺ was studied at the MP4SDQ/6-31G* level of theory in order to determine the products and establish the multistep dissociation pathway of the reaction. The location and height of the transition states in the process is used as a criteria for the feasibility of the proposed pathway. The result is compared with the expediential and theoretical studies of the same system done by Wincel and coworkers [146]. 3. The geometries and force constants of small-sized organotin compounds The calculations on 12 small-sized organotin compounds were done at the HF/3- 21G* level of theory. The objective of this study was to provide the force constants of SnX and X-Sn-Y types for the use in Molecular Mechanic calculations of organotin compounds. The calculated geometries and harmonic vibrational frequencies of stannane and methyl stannane are compared with experimental results in order to measure the reliability of the calculations. 4. The chemical properties of CnO, CnO⁺, CnHO⁺, CnS, CnS⁺ and CnHS⁺ species in the instellar clouds CnO and CnS when n = 2 and 3 have been reported to be found in some interstellar clouds. These species in such environments are subjected to ionization and protonation processes. Theoretical studies of these species were done at the MP4SDQ/6-311G** level of theory. The calculations suggest that these species are more stable in protonated forms and could be intermediates of some steady state processes. 5. Theoretical study of C₆H₄+ formation in acetylenic flames C₆H₄⁺ has been detected as an intermediate in acetylenic flames. The semiempirical AM1 method was used to determine the most stable products and to establish a chemical mechanism of the reaction between C₄H₂⁺ and C₂H₂. The results from AM1 method were refined by ab initio calculations at the HF/4-31G andMP4SDQ/6-31G* level. From this study, only chemical pathways involved acyclic structure isomers are feasible. 6. A Valence-Bond study of BH2 radical In this study a Valence-Bond program was used on IBM PC/AT microcomputer to study the correlation between the nuclear bond angle and the angle of hybrid orbitals of BH₂. The energies of BH₂ from the Valence-Bond calculations were also compared with the energies from the Molecular Orbital method at the HF, MP4SDQ and CI level with best orbital energy basis sets, 10s6p/2s1p for boron atom and 6s/1s for hydrogen atoms. GAUSSIAN series programs, the MICROMOL package, the GAMESS program, a Valence-Bond program and the MOPAC program were used to perform the calculations.