Infrared Study Of Hydrogen Bonded Complexes PDF Download

The nature of hydrogen bonding within molecular complexes is explored through the use of infrared spectroscopy. Vibrational predissociation of strongly hydrogen bonded dimers has been performed using the molecular beam depletion technique. An investigation of the O-H stretches in (CH$sb3$OH)$sb2$ under low resolution (OPO linewidth = 4 cm$sp{-1}$) yielded uniquely different frequency shifts from the monomer absorption ($-$107 cm$sp{-1}$ for the proton donor and +3 cm$sp{-1}$ for the proton acceptor), indicating that the monomer subunits reside in inequivalent environments. The vibrational bands were Lorentzian in shape, thereby permitting determination of the integrated absorption intensities for both transitions. The transition linestrengths for the proton donor and proton acceptor are enhanced upon complexation by a factor of 12 and 1.6, respectively. A more sophisticated approach was employed to measure the absorption intensities in binary complexes involving HF. By saturating rovibrational transitions in (HF)$sb2$ using a high resolution, single mode color center laser, the vibrational transition moments for two F-H stretches have been measured regardless of the internal state distribution of the clusters in the molecular beam. Details of the experimental procedure, referred to as Saturation Predissociation Spectroscopy (SPS), and data analysis are presented. A slight enhancement ($sim$10%) of the transition moment over the noncomplexed monomer value was observed for the proton acceptor. The proton donor transition moment was substantially enhanced by $sim$100% as a result of hydrogen bonding. This represents a linestrength enhancement factor of $sim$4, since the transition linestrength is related to the square of the transition moment. The results are in excellent agreement with predictions from ab initio calculations. Rovibrational transitions of OC-HF, CO$sb2$-HF, and N$sb2$-HF have also been measured using the SPS technique. Analysis of the laser fluence dependence in the saturation limit produced vibrational transition moments of 0.138(6), 0.128(7) and 0.091(5) D, indicating linestrength enhancements of $sim$2, $sim$1.6 and negligible over monomer HF, for the HF subunit in OC-HF, CO$sb2$-HF and N$sb2$-HF, respectively. The values for all the HF related species are then compared with other molecular properties associated with these complexes, namely, the vibrational frequency shifts, the vibrational predissociation linewidths and the zero point dissociation energies, to characterize the role of the hydrogen bond.