Vibrational dynamics of the bifluoride ion. III. F–F (ν1) eigenstates and vibrational intensity calculations

Publisher Website
Google Scholar
Abstract
This paper concludes a theoretical study of vibrational dynamics in the bifluoride ion FHF−, which exhibits strongly anharmonic and coupled motions. Two previous papers have described an extended model potential surface for the system, developed a scheme for analysis based on a zero‐order adiabatic separation of the proton bending and stretching motions (ν2,ν3) from the slower F–F symmetric‐stretch motion (ν1), and presented results of accurate calculations of the adiabatic protonic eigenstates. Here the ν1 motion has been treated, in adiabatic approximation and also including nonadiabatic couplings in close‐coupled calculations with up to three protonic states (channels). States of the system involving more than one quantum of protonic excitation (e.g., 2ν2, 2ν3 σg states; 3ν2, ν2+2ν3 πu states; ν3+2ν2, 3ν3 σu states) exhibit strong mixing at avoided crossings of protonic levels, and these effects are discussed in detail. Dipole matrix elements and relative intensities for vibrational transitions have been computed with an electronic dipole moment function based on ab initio calculations for an extended range of geometries. Frequencies, relative IR intensities and other properties of interest are compared with high resolution spectroscopic data for the gas‐phase free ion and with the IR absorption spectra of KHF2(s) and NaHF2(s). Errors in the ab initio potential surface yield fundamental frequencies ν2 and ν3 100–250 cm−1 higher than those observed in either the free ion or the crystalline solids, but these differences are consistent and an unambiguous assignment of essentially all transitions in the IR spectrum of KHF2 is made. Calculated relative intensities for stretching mode (ν3, σu symmetry) transitions agree well with those observed in both KHF2 [e.g., bands (ν3+nν1), (ν3+2ν2), (3ν3), etc.] and the free ion (ν3,ν3+ν1). Calculated intensities for bending mode (ν2, πu symmetry) transitions agree well with experiment for the ν2 fundamental in the free ion and KHF2(s), and for a πu transition in KHF2 which we assign to ν2+2ν3, but are far too small to explain the prominence of progression bands (ν2+nν1) and especially the strong overtone 3ν2 in the spectrum of KHF2(s). Intensity of the progression bands (ν2+nν1) in KHF2 can be explained by hydrogen bonding between adjacent FHF− ions; in NaHF2(s) where such interaction is absent, the band (ν2+ν1) is 50–100 times weaker, in agreement with calculations. The relatively high intensity of the 3ν2 band, which also appears strongly in NaHF2(s), remains the major unexplained feature of the bifluoride spectrum in these solids. Suggestions are made for further experiments on the FHF− and FDF− systems which could test predictions of this dynamical analysis.

This publication has 21 references indexed in Scilit: