Measurements of the rotational spectra of phenol and 2-pyrone and computational studies of the H-bonded phenol-pyrone dimer

Rotational spectra for the a-type transitions of phenol and a-type and b-type transitions of 2-pyrone in the ground vibrational states were measured using pulsed beam Fourier transform (PBFT) microwave spectroscopy. From the observed a-type spectrum of phenol, which exhibited no complicated tunneling doublet splittings, we obtained the following rotational constants: A ₀ = 5650.494(26), B₀ = 2619.2323(7), C₀ = 1789.8520(7) MHz. For 2-pyrone, the following rotational constants were obtained: A₀ = 5677.6356(10), B₀ = 2882.2458(11), C ₀ = 1912.13275(94) MHz. The centrifugal distortion constant, Δj, for these molecules is less than 0.2 kHz, in good agreement with our predicted, theoretical Δj values. Combined spectral fits using data from this work and previous data provided accurate information on the rotational and centrifugal distortion constants of these molecules. From the measured rotational constants we obtained the following inertial defects (Δ): Δ(2-pyrone) = -0.053 and Δ(phenol) = -0.031 amu Ų. The observed negative inertial defect for these planar molecules (normally a small positive value for planar molecules) suggests that the out-of-plane vibrational potential due to the attached OH and O is highly anharmonic. From the measured inertial defect, we calculated the low frequency out-of-plane vibration to be approximately 110 cm^-1. Quantum chemical calculations were performed in combination with the experiments to determine the molecular and spectroscopic properties of phenol, 2-pyrone and the H - bonded, phenol-pyrone dimer. A well-defined theoretical structure was obtained for the phenol-pyrone dimer from the calculations with electron correlation. Structure optimization calculations using Møller-Plesset perturbation theory predicted a stable bent dimer structure with relatively strong interaction energy in the 28-32 kJ mol^-1 range. This novel, phenol-pyrone dimer forms a single O'-HO hydrogen bond with length about 1.87-1.93 Å, and is further stabilized by π-π and CH - π interactions. Density functional theory (DFT) calculations predicted that a planar nontransition state structure would be stable, but failed to predict a stable bent structure. Experimental searches for the rotational spectrum of phenol-pyrone stable were conducted in the 4-8 GHz range, but no transitions were detected in this study. A number of microwave transitions for the phenol-phenol dimer were detected in this study and used to estimate rotational constants.