Microwave spectroscopy and quantum chemical calculations of triple bonded molecules and benzyl compounds

Date of Completion

January 2004


Chemistry, Physical




Triple bonded hydrocarbons. The rotational spectrum of 5-hexynenitrile and n-butyl acetylene, respectively, was measured with high-resolution microwave spectroscopy. Rotational constants and other spectroscopic constants were measured for the conformations belonging to each molecule. Nuclear quadrupole splittings were observed and characterized for each assigned conformer of 5-hexynenitrile. The spectra of the singly substituted 13C isotopomers were assigned for two of the four observed conformers of n-butyl acetylene. Also, a Kraitchman analysis was done to characterize the substitution structures of the parent isotopomers for these conformations. Ab initio calculations were carried out on the aforementioned compounds and n-butyl cyanide in order to assess the relative energy ordering of each compound's conformations. ^ Benzyl-X compounds. We have observed the microwave spectrum of benzyl alcohol and its OD isotopomers at high resolution in a pulsed-jet Fourier transform microwave spectrometer. The spectrum is consistent an asymmetric stable conformation characterized by a C–C–C–O dihedral angle of approximately 60°. Tunneling interactions strongly perturb the spectrum. Tunneling interactions between two equivalent conformational minima is manifested by transitions split into doublets. The observed splittings diminish upon deuterium substitution. ^ Previous studies on benzyl alcohol have suggested that a weak attraction between the π electrons of the phenyl ring and the substituent –OH group as the reason for the observed stable conformation. A theoretical analysis of the atomic charges in benzyl alcohol suggests another possible explanation for the observed structure. Atomic charges, generated by fits to the electrostatic potential, indicate a relatively strong dipole-dipole coupling between the –CH group in the methylene side chain and the closest –CH group in the phenyl ring, which results in a nearly planar orientation of the –CH group in the methylene side chain with the phenyl ring. ^