Vibrational Förster transfer to hydrated protons

Abstract

We have studied the influence of excess protons on the vibrational energy relaxation of the O-H and O-D stretching modes in water using femtosecond pump-probe spectroscopy. Without excess protons, we observe exponential decays with time constants of 1.7 and 4.3 ps for the bulk and anion bound O-D stretch vibrations. The addition of protons introduces a new energy relaxation pathway, which leads to an increasingly nonexponential decay of the O-D stretch vibration. This new pathway is attributed to a distance-dependent long range dipole-dipole ͑Förster͒ interaction between the O-D stretching vibration and modes associated with dissolved protons. The high efficiency of hydrated protons as receptors of vibrational energy follows from the very large absorption cross section and broad bandwidth of protons in water. For a proton concentration of 1M we find that Förster energy transfer occurs over an average distance of 4.5 Å, which corresponds to a separation of about two water molecules.

Vibrational and orientational dynamics of water in aqueous hydroxide solutions

“…This type of Förster energy transfer has been observed for the proton in water. 12 In this latter study the O-D stretch vibration of HDO was observed to show an additional distinctly non-exponential decay that became faster with increasing proton concentration. This additional decay finds its origin in vibrational Förster energy transfer to the background of water molecules hydrating the proton.…”

“…47 For the proton in water a similar observation was made: water molecules outside the first hydration shell of the proton showed the same reorientation time constant as the molecules in bulk liquid water. 12 Interestingly, the vibrational lifetime of the O-D and O-H vibrations is observed to decrease with increasing concentration NaOH/NaOD (Figs. 4 and 5(b)).…”

“…This additional decay finds its origin in vibrational Förster energy transfer to the background of water molecules hydrating the proton. 12 , proton transfer process (1) implies that the O-D group shifts from the second hydration shell to the third hydration shell. This transition will also lead to fluctuations of the vibrational frequencies of the HDO molecule to which O-D belongs.…”

“…The latter is apparent from the fact that for the O-H stretch vibration outside the first hydration shell of D + no acceleration was observed. 12 This difference in the effect of proton/deuteron transfer on the relaxation can be explained from the specific hydration structures of the proton and the hydroxide ion. For the proton, the water molecules in the first hydration shell are forming Zundel (H 5 O 2 + ) or Eigen (H 9 O 4 + ) cations.…”

“…35 It is noteworthy that the contribution of the O-H · · · OH − oscillators to the absorption is significantly weaker than that of water molecules hydrating the proton (H 2 O · · · H + ), as observed in acidic solutions. 12 The linear absorption spectrum of an acidic solution (in absence of deuterium) shows an approximately two times higher total absorbance at ∼ 2500 cm −1 than an aqueous solution of NaOH of the same concentration.…”