Title

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

Date of Completion

January 2004

Keywords

Chemistry, Physical

Degree

Ph.D.

Abstract

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. ^