Theoretical Calculation of the OH Vibrational Overtone Spectra of 1,5-Pentanediol and 1,6-Hexanediol

It is well-known that intramolecular hydrogen bonding affects the relative energetics of conformers, as well as the OH stretching peak positions, intensities, and width. In this study we simulated the Delta v(OH) = 3, 4 overtone spectra of 1,5-pentanediol (PeD) and 1,6-hexanediol (HD) using the peak positions, intensities, and width calculated from the B3LYP/6-31+G(d,p) method. Furthermore, room temperature free energy calculations were performed using B3LYP/6-31+G(d,p) MP2/6-31+G(d,p), and MP2/6-311++G-(3df,3pd) to obtain the relative population of the conformers. From the calculation of 109 and 381 distinct conformers for PeD and HD, respectively, we find that for these long chain diols the intramolecular hydrogen bonded conformers are not the most dominant conformation at room temperature. This is in stark contrast with shorter chain diols such as ethylene glycol for which the hydrogen bonded conformer dominates the population at room temperature. On, the other hand, we found that the correlation between the hydrogen bonded OH red shift versus the homogeneous width, Gamma = 0.0155(Delta omega)(136), which was derived for shorter chain diols, is valid even for these longer chain diols. We also showed that the intramolecular hydrogen bonded OH initially decays through the CCOH torsion and COH bending mode no matter how long the alkanediol chain length is for 1,n-alkanediols for n up to 6.