OH bonds in electric fields: electron densities and vibrational frequency shifts

The changes in the electron density distribution caused by varying the OH bond length for HDO and OH− in uniform electric fields are investigated and related to the frequency shifts for the uncoupled O-H stretching vibration. Numerical integration of difference density maps, Δϱ=ϱ(rOH+ ΔrOH) -ϱ(rOH), reproduces the electronic contribution to δμ,∥total/ δrOH, if the integration is carried out to a distance of ≈3.5 Å from the O atom. The frequency shift δν is proportional to −E∥ × [dμ∥permanent(roH) /drOH] - 12E∥,[δμ∥induced(E∥, rOH )/δrOH] and electron density maps corresponding to the electronic part of these terms are presented. Experimentally it has been found that the OH− ion shows a frequency upshift when bound in a moderately strong field, while water molecules show a downshift. The electron density maps show why dμ∥permanent(roH) /drOH is positive for HDO and negative for OH −, resulting in a downshift for bound water an an upshift for bound OH−. For positive fields, δμ∥induced/ δroH is positive for both HDO and OH − and gives a downshift contribution.