Fouger̀e, Scott Gregory2024-08-132024-08-1319941994https://hdl.handle.net/1828/17848A series of seven carbonyl isocyanates and three carbonyl isothiocyanates of the general formula R-CO-NCE (where E represents either oxygen in the carbonyl isocyanates or sulfur in the carbonyl isothiocyanates) were synthesized based on reported literature methods. The substituents of the carbonyl isocyanates were CH₃, CH₂Cl, CHCl₂, CCl₃, OCH₃, SCH₃, and NCO. The three carbonyl isothiocyanates studied had fluoro, chloro, and acetyl groups as the substituents (F, Cl, and CH₃ respectively). The vapour phase, and in some cases the liquid phase, infrared spectra and liquid phase Raman spectra were recorded for these families of carbonyl iso(thio)cyanates at room temperature. The molecules' fundamental group frequencies were assigned via a vibrational analysis of each spectrum. Although the characteristic group frequencies diagnostic of the iso(thio)cyanato group appeared relatively insensitive to the nature of the substituent, a slight correlation between increasing electronegativity of the substituent and increasing frequency of the bands was noticeable. In some of the spectra, certain features were observed to possess what appeared to be peak pairs. This offered evidence that more than one conformer was present in the spectra at room temperature. Assignments of these observed pairs were based on the assumption that the two planar conformations of the molecules, a result of rotation of the iso(thio)cyanato group about the C-N bond, are the more stable isomers. Distinction between the cisoid and transoid isomers was made on the basis of force-field calculations, MNDO/AM1 calculations, band contour analyses, and variable­ temperature spectral data. Structural preferences of the isomers are dependent upon the nature of the substituent. The variable-temperature spectral information was utilized to qualitatively analyze the stability of the different conformers in relation to their observed peak positions and relative intensities. Assignment of the bands belonging to the carbonyl isothiocyanate molecules is relatively easy when they are compared to the carbonyl isocyanates. The bands diagnostic of the substituent in the isocyanates and isothiocyanates are more or less unaffected by the sulfur/oxygen exchange in the NCE group. However, the bands associated with the iso(thio)cyanate group are mass sensitive and the isothiocyanate stretching fundamentals appear at lower frequencies than their carbonyl isocyanate counterparts (~250-300 cm⁻¹ lower). The bending motions are also mass­ dependent but do not differ in frequency by as great an amount in carbonyl isothiocyanates compared to carbonyl isocyanates. Distinguishing between the possible conformations in carbonyl isothiocyanates is more difficult than in their isocyanate counterparts. The separation between observed features in the C=O region of the spectra in NCS molecules is only 15-20 cm-1 as compared to 25-35 cm⁻¹ in carbonyl isocyanates indicating a closeness in energy between the isomers. With the data obtained from AM1/MNDO calculations, the relative stabilities of the isothiocyanate conformers were predicted. The cisoid isomer is predicted to be more stable in the fluoro- and acetyl compounds whereas the transoid conformer was predicted to be more stable for the chloro- compound. These observations parallel those of the corresponding carbonyl isocyanates.150 pagesAvailable to the World Wide WebThesis