Theoretical study on the effect of intramolecular hydrogen bonding on OH stretching overtone decay lifetime of ethylene glycol, 1,3-propanediol, and 1,4-butanediol

It is well known that the existence of hydrogen bonds causes the vibrational spectra for the donor OH bond to broaden. Most times this broadening is attributed to inhomogeneous contributions, however in this paper, we studied the homogeneous contribution coming from the decay lifetime of the OH stretching excitation. Alkane diol, which possesses an OHb⋯OHf (b and f stand for hydrogen bonded and free, respectively) intramolecular hydrogen bond, provides us with a controllable molecule to systematically study the effect of intramolecular hydrogen bond on the decay rate of the OH vibrational excitation. In the present study we performed local mode vibrational calculation of gas phase ethylene glycol (EG), 1-3 propanediol (PD), and 1-4 butanediol (BD) using the potential energy curves calculated using B3LYP/6-31+G(d,p), MPW1PW91/6-311+G(2d,p), M06-2X/6-311+G(2df,2p), and MP2/6-311++G(3df,3pd) methods. In addition, we studied the dynamics of OH overtone excited state (ΔvOH = 3, 4) for the two most stable conformers of the aforementioned diols using the direct dynamics calculation employing the B3LYP/6-31+G(d,p) method. From the trajectory simulations we showed that the decay lifetime from the donor OHb excitation decreases while that from the acceptor OHf excitation increases as we increase the intramolecular hydrogen bond strength in going from EG, PD, to BD. These results are consistent with the gas phase experimental results of Kjaergaard et al. where the OHb peaks, which are sharp for EG disappear for BD. From the detailed analysis on the trajectories, we observed that the decrease in decay lifetime for the OHb bond excitation originates from the increase in the energy flow to the OHb⋯O torsion/bend motion, while the increase in lifetime for OHf bond is due to the fixing of the OHf geometry by accepting a donor hydrogen. Furthermore, by plotting the calculated red shift versus the decay lifetime we obtained the following relationship Tdecay(fs) = 343176(Δω(cm−1))−1.36.