A CHIRPED PULSE FTMW STUDY OF THE STRUCTURE OF PHENOL DIMER

Abstract

Phenol dimer has been studied extensively and is considered a benchmark molecular complex for \emph{ab initio} theory due to a long range dispersion interaction between the rings as well as an intermolecular hydrogen bond. Previously, the structure had been determined using RCS$^{,}$ and high resolution UV measurements; however, several assumptions were integrated into the structure because a full isotopically substituted structure could not be determined. In this study, the rotational spectrum of the dimer as well as $^{13}$C and $^{18}$O isotopologue spectra that were seen in natural abundance were obtained using chirped pulse Fourier transform microwave spectroscopy (CP-FTMW). The structure was determined using both linear least squares fitting ($r_0$ structure) and the Kraitchman substitution analysis ($r_s$ structure). \emph{Ab initio} calculations were performed for the dimer using MP2/cc-pVTZ cp, B3LYP/6-31G(d,p), M06-2X/6-31G(d,p), and M06-2X/6-311++G(d,p), while CCSD calculations are currently under way. Changing the level of theory and the basis set dramatically changes the structure. The MP2 calculation underestimates the hinge angle (C-O-O-C dihedral angle), while the B3LYP overestimates it. The M06-2X calculations seem to give the best cost-to-benefit ratio when compared to the $r_s$ structure, but they show poorer agreement with increasing basis set size.