A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Abstract:The term "hydrogen bond" has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X-HؒؒؒY hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

The vibrational OH stretch spectra have been measured for size-selected pure water clusters ͑H 2 O͒ n , in the size range n 8 10. Comparison between experiment and calculations suggests that the spectra originate from a small number of "microcrystalline" structures, based on the cubic octamer. The n 8 spectra are caused by two isomers of D 2d and S 4 symmetry. The proposed lowest energy nonamer and decamer structures are derived from the octamer by insertion of one and two two-coordinated molecules, respectively, into the cube edges. [S0031-9007 (98)05604-X] PACS numbers: 36.40.MrWater clusters have been the focus of intense interest during the last several years. One of the objectives is to understand how the properties of ͑H 2 O͒ n evolve towards condensed phase behavior. One may ask, for example, at which size a cold cluster starts displaying attributes of a small crystal. The initial stages of the evolution as a function of n have been largely understood. Recently, an elegant series of far-infrared vibration-rotation-tunneling [1,2] and infrared [3] laser spectroscopic studies demonstrated conclusively a cyclic structure for n 3 5, and a transition towards a three-dimensional cage structure at n 6 [4]. Similar conclusions were drawn from the double resonance ion-dip infrared experiments on water clusters connected to a benzene molecule [5]. At present, the challenge is to understand the evolution in the "cage" regime n $ 7. A number of theoretical studies addressed this problem [6-9]. The only pertinent experimental results focus on the OH stretch spectroscopy of larger clusters without size selection [10,11] and n 7, 8 clusters attached to benzene [5,9].The present Letter reports the first measurement of the infrared spectroscopy of the OH stretch mode of pure water clusters in the size range n 8 10. The OH stretch spectra of hydrogen bonded H 2 O, which are redshifted by hundreds of cm 21 with respect to gaseous H 2 O, are known to be strikingly sensitive to hydrogen bond coordination and to bonding geometry [7,12]. Thus they can be used to probe the cluster structure. Moreover, the spectroscopy of clusters can serve as a benchmark for the calibration of flexible force fields for studies of condensed H 2 O and H 2 O surfaces.The experimental method which we apply is a combination of size selection by momentum transfer in a scattering experiment with atoms, with the infrared depletion technique [13,14]. This technique has been developed in our laboratory in Göttingen and mainly applied to cluster sizes n # 6. In the first step the different clusters are dispersed into different angles according to their masses and detected by a mass spectrometer. Then the OH stretch vibrational mode of the water molecules is excited by infrared laser radiation. The detector records the depletion in the cluster signal caused by the clusters which are dissociated by the absorbed radiation.The experimental setup consists of a crossed molecular beam apparatus with an angular dependent detection of the scattered beam with a resol...

Ionization and dissociation reactions play a fundamental role in aqueous chemistry. A basic and well-understood example is the reaction between hydrogen chloride (HCl) and water to form chloride ions (Cl(-)) and hydrated protons (H(3)O(+) or H(5)O(2)(+)). This acid ionization process also occurs in small water clusters and on ice surfaces, and recent attention has focused on the mechanism of this reaction in confined-water media and the extent of solvation needed for it to proceed. In fact, the transformation of HCl adsorbed on ice surfaces from a predominantly molecular form to ionic species during heating from 50 to 140 K has been observed. But the molecular details of this process remain poorly understood. Here we report infrared transmission spectroscopic signatures of distinct stages in the solvation and ionization of HCl adsorbed on ice nanoparticles kept at progressively higher temperatures. By using Monte Carlo and ab initio simulations to interpret the spectra, we are able to identify slightly stretched HCl molecules, strongly stretched molecules on the verge of ionization, contact ion pairs comprising H(3)O(+) and Cl(-), and an ionic surface phase rich in Zundel ions, H(5)O(2)(+).