![Sample our Physical Sciences journals, sign in here to start your FREE access for 14 days]({"type":"image" , "src" : "/sda/110819/TOC-Subject-Sample-2021-Physical-Sciences.jpg"})
Abstract
An investigation has been made of the structures of the title compounds via theoretical calculations, high resolution NMR analysis, and X-ray diffraction analysis. Conformational analyses of these compounds in the gas phase were performed using ab inito methods with GAUSSIAN 94. Single point calculations at the MP4/6-31G level for 3-oxa-7-azabicyclo[3.3.1]nonan-9-one revealed an energy difference of only ΔE = 1.497 kcal/mole between the chair-chair (CC) and boat-chair (BC) conformers. Full geometry optimization of the oxygen-containing ketone at the Hartree-Fock (HF) level using a 6-31G basis set indicated only a negligible variation in interatomic distances in the equilibrium geometry when the phenyl group was included in the calculation versus when the phenyl group was replaced by hydrogen. Full geometry optimization for the sulfur-containing ketone (phenyl replaced by a hydrogen) at the HF level using the 6-31G basis set again demonstrated that the BC form was slightly more stable than the CC form, identical to that found with the oxygen counterpart. Full geometry optimization of the two hydroperchlorates (phenyl replaced by hydrogen) resulted in the CC form being suggested as the more stable with H-bonding between the hydrogen on nitrogen and the respective heteroatom, a situation which likely enhances stability. Proton NMR analyses of both compounds using NOESY techniques strongly supported a predominance of the BC form in solution for the oxygen-containing ketone. Severe signal overlap in the sulfur ketone (even at 600 MHz) prevented in depth analysis of the structure. Using NOESY, DQCOSY, and HMBC techniques, NMR analyses of the corresponding hydroperchlorates implied CC forms for each in solution. The X-ray diffraction analysis of the oxygen-containing ketone revealed a CC in the solid state.
