MATRIX ISOLATION INFRARED SPECTROSCOPY OF A SERIES OF 1:1 PHENOL-WATER COMPLEXES

We report here the FTIR spectra of 1:1 complexes of eight fluorophenol derivatives with water measured under matrix isolation condition. In all the complexes, oxygen of water is the hydrogen bond acceptor and phenolic O-H the hydrogen bond donor. The attributes of the O-H...O linkage in the complexes are tuned remotely by fluorine substitutions at different aromatic sites of phenol. The goal of the study is to find the intermolecular interactions that correlate best with the sequence of spectral shifts of the donor O-H stretching (nub{OH}) frequencies. Measurements reveal that the probe phenolic nub{OH} shifts vary by nearly 90% from unsubstituted phenol to pentafluorophenol. Interestingly, this large variation correlates poorly with the predicted binding energies of the complexes. Secondly, although electrostatic interaction is considered to dominate the overall stabilization of such classical hydrogen bonds, we see that the shifts do not display any correlation with this interaction at the hydrogen bonding sites. On the other hand, the purely quantum mechanical charge-transfer interaction energies, as obtained from Natural Bond Orbital analysis,are found to display very good correspondence with the spectral shifts. Thus, we propose that such local charge-transfer type interactions are better descriptors of weakening of the hydrogen bond donor than electrostatic energy parameters