Infrared Spectra of H⁺(H₂O)_5-8 Clusters:  Evidence for Symmetric Proton Hydration

Abstract

Protonated water clusters, H + (H 2 O) n (n ) 5-8), from a supersonic expansion have been investigated by vibrational predissociation spectroscopy and ab initio calculations. The experimental spectra were obtained at an estimated cluster temperature of 170 ( 20 K. Recorded absorption bands at the frequency range of 2700-3900 cm -1 are attributed to the free-and hydrogen-bonded-OH stretches of the ion core and the surrounding solvent molecules. Ab initio calculations, performed at the B3LYP/6-31+G* level, indicate that geometries of the H + (H 2 O) 5-8 isomers are close in energy, with the excess proton either localized on a single water molecule, yieldingSystematic comparison of the experimental and computed spectra provides compelling evidence for both cases. The unique proton-transfer intermediate H 5 O 2 + (H 2 O) 4 was identified, for the first time, by its characteristic bonded-OH stretching absorptions at 3178 cm -1 . The existence of five-membered-ring isomers at n ) 7 is also evidenced by the distinct bonded-OH stretches at 3500-3600 cm -1 and by the free-OH stretch of threecoordinated H 2 O at 3679 cm -1 . Among these H + (H 2 O) 7 isomers is a newly discovered H 5 O 2 + -containing pentagonal ring, which is computed to be lowest in Gibbs free energy at 170 K. Its spectroscopic signature is the splitting of two equivalent bonded OH stretches into a doublet (3544 and 3555 cm -1 ) by vibrational coupling through ring closure. No profound spectral evidence, however, was found for the formation of four-membered rings although it is predicted to be favorable in terms of total interaction energy for H + (H 2 O) 7 . Six-membered ring and three-dimensional cagelike structures are also stable isomers but they are less strongly bound. The preference of five-membered-ring formation at n ) 7 appears to be the result of a delicate balance between entropy and enthalpy effects at the presently investigated cluster temperature. The correlation of this investigation with other studies of neutral water clusters and of the hydration of biological macromolecules is emphasized.

Investigations of Protonated and Deprotonated Water Clusters Using a Low-Temperature 22-Pole Ion Trap

“…38,39 Notably, all of the spectra show 3 distinct features centered at ν 0 ) 3740, 3715, and 3650 cm -1 , corresponding respectively to the asymmetric, the 2-coordinated, and the symmetric free-OH stretches of the water molecules in either the first solvation shell or the second solvation shell of the H 3 O + or H 5 O 2 + ion core. 18 The full widths at half-maximum (fwhm) of these three absorption bands are all larger than 15 cm -1 , but they decrease significantly when H 2 is introduced into the storage cell. An average reduction factor of 1.7 is found for the spectrum acquired at 200 K compared to that at 77 K. Table 2 lists the spectral assignment, vibrational frequencies, and bandwidths of the individual peaks observed at three different bath temperatures in comparison to those obtained at room temperature without H 2 buffer gas.…”

“…Identification of them has been successful in a coronadischarged supersonic expansion in our previous experiment. 18 Because the source condition used by this new spectrometer is similar to that of the room-temperature 8-pole ion trap apparatus used in ref 18, we expect that these two chainlike isomers exist in the present 22-pole ion trap mass spectrometer as well.…”

“…The structures and energetics of these isomers have been previously reported. 18,23 We depict in Figure 2 only the isomeric structures of H + (H 2 O) 4-8 that have been identified in a corona-discharged supersonic expansion at an estimated cluster temperature of 170 K. [16][17][18] For clusters of n e 6, they are predominantly the chainlike structures (namely, HW4I, HW5I, HW6I, and HW6II), whereas at n g 7, ring structures (HW7III, HW7IV, HW7VI, HW7VIII, HW7X, and HW8II) exist. Such a preference of five-membered ring formation in H 3 O + (H 2 O) n stands as an interesting contrast to its structural analogue NH 4 + (H 2 O) n , 33 of which the formation of symmetric fourmembered rings is strongly favored.…”

First principle study on the structure of H+(H2O)6

“…The structure of protonated water cluster have been the subject of recent experimental studies [1][2][3][4][5][6] . In this work, we have chosen to investigate the structure of H + (H 2 O) 6 by examining the low-lying minima with first principle methods for several reasons.…”

“…Jiang et al examined seven isomers with B3LYP/6-31+G* and concluded that the 402 structure is the most stable form after including ZPE corrected under harmonic oscillator approximation 1 . Their conclusion is supported by the high-level ab initio calculations by Mella et al 14 .…”