Infrared Spectrum and Stability of the H₂O−HO Complex: Experiment and Theory

Abstract

Infrared action spectroscopy is utilized to characterize the gas-phase, hydrogen-bonded H(2)O-HO complex, a primary interaction in the hydration of the hydroxyl radical. The OH radical stretch of the H(2)O-HO complex is identified at 3490 cm(-1), shifted 78 cm(-1) to lower frequency of the OH monomer transition. The stability of the complex, D(0) < or = 5.14 kcal mol(-1), is derived from the highest observed OH product channel in the associated product state distribution. The assignment is supported by high level ab initio calculations of the spectral shift of the binary complex from free OH and its dissociation energy, D(e)(CBS-infinity) = 5.6 kcal mol(-1). A second weaker feature, appearing 15 cm(-1) to lower frequency at 3475 cm(-1), is attributed to a hot band, the OH radical stretch originating from an out-of-plane H(2)O bending state, based on two-dimensional calculations of frequencies and strengths of transitions involving the coupled vibrational modes.

Perspective: Water cluster mediated atmospheric chemistry

“…Accordingly, the binary OH-H 2 O complex has been extensively studied by theoretical and experimental methods. 31,40,88,93,94,96 This complex was observed and investigated by gas phase microwave spectroscopy. 93,96 Infrared spectra were obtained in rare gas matrices leading to agreement as to the global minimum geometry which features a hydrogen bond between the hydrogen of OH and the oxygen in H 2 O.…”

“…93,96 Infrared spectra were obtained in rare gas matrices leading to agreement as to the global minimum geometry which features a hydrogen bond between the hydrogen of OH and the oxygen in H 2 O. 40 The influence of water on the gas phase reactivity of OH and HO 2 with other molecules such as SO 3 , CH 4 and DMS has been investigated with varying conclusions about the potential for water catalysis. 35,101,102 The unstable radical HOOO can form water complexes with longer atmospheric lifetimes than the HOOO radical, allowing for the possibility of reaction with other species.…”

“…4,9,25 Water can form open and closed shell molecular complexes with low binding energies (van der Waals interactions) and more strongly bound complexes (dipolar interactions and hydrogen bonding) with atmospheric species (e.g., O 2 , N 2 , Ar, OH, HO 2 , RO 2 , O 3 , OCS, SO 2 , SO 3 , NO, SH, ClO, NH 3 , HNO 3 , HCl, H 2 SO 4 , organic acids, aldehydes, and ketones, etc.). 19,[26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42] We illustrate results of calculations of the abundance of water complexes with altitude in Figure 1, pointing out that while the temperature minimum at the tropopause enhances the equilibrium constant, it also reduces the partial pressure of water and other monomeric condensable species leading to an overall decrease of the complex abundance with altitude. 4,8,10,12,29 Nevertheless, even low concentrations of weakly bound complexes can have a significant effect on atmospheric chemistry.…”

Tuning the intermolecular proton bond in the H5O2+ ‘Zundel ion’ scaffold

“…For example, at the harmonic level, the shared proton motion strongly couples to other nominally 'backbone' vibrations in larger protonated complexes [25,[38][39][40][41][42]. This has been studied in detail in the case of the protonated dimethyl ether dimer (Me 2 OÁH + ÁOMe 2 ), where strong coupling between motion of the shared proton and CAO stretches is recovered in harmonic vibrational calculations [21].…”

Overtone vibrational spectroscopy in H2-H2O complexes: A combined high level theoretical ab initio, dynamical and experimental study

“…Since formation of dimers represents the first step in aggregation to larger clusters and ultimately condensation, study of such species offers insight into intermolecular interactions relevant from gas-phase to condensed phase regimes. [4][5][6] In particular, bimolecular clusters encompass a variety of noncovalent bonding prototypes, ranging from pure van der Waals interactions 7 in Ar-Ar (D 0 ≈ 84 cm −1 ) to the significantly stronger hydrogen bonding in water dimer (D 0 ≈ 1105(10) cm −1 ). [8][9][10] A striking feature of these weakly bound van der Waals systems is the propensity for each component to retain a significant fraction of monomeric character, for example, as noted in the small shifts of infrared transition frequencies upon complexation.…”