Research Seminar Abstract
The ν(OH) peaks in solid-state FTIR spectra of compounds with OH groups or H2O molecules undergo significant changes when the OH group(s) participate in hydrogen bonding. Typical changes are (i) red shifts of the peak position(s) by up to hundreds of cm−1, (ii) broadening of the peak(s) by up to hundreds of cm−1, and (iii) a ten-fold or greater increase in the integrated intensity of the peak(s). In typical metal salt hydrates, the νasym(OH) and νsym(OH) peaks are usually so broad at room temperature that they overlap, and the ν(OH) region is usually not discussed in detail (e.g., Cu(H2O)4(SO4)·H2O and Mg(H2O)6(BF4), see below; the red arrows indicate the positions of the ν(OH) peaks for H2O(g)). In this presentation I will show that this region can contain a wealth of information about the relative strengths of hydrogen bonds in metal salt hydrates when the water molecules are only weakly hydrogen bonded to the anion. Salt hydrates of very weakly-coordinating anions such as PF6−, B12F122−, and Al(OC(CF3)3)4−, but not BF4−, have very sharp ν(OH) peaks (FWHM = 5‒30 cm−1) with small red shifts. This allows comparisons of the relative hydrogen bond strengths of coordinated H2O molecules as a function of metal cation, as a function of weakly-coordinating fluoroanion, or as a function of different O(H)···F distances in the same compound. These comparisons rely on an analysis of FTIR spectra of isotopically-dilute deuterated samples that contain only one O–D oscillator (e.g., Co(HOD)(H2O)(B12F12)) per formula unit.