Proton transfer or hemibonding? The structure and stability of radical cation clusters

The basin hopping search algorithm in conjunction with second-order Moller-Plesset perturbation theory is used to determine the lowest energy structures of the radical cation clusters (NH_3)_n^+, (H_2O)_n^+, (HF)_n^+, (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+, where n=2-4. The energies of the most stable structures are subsequently evaluated using coupled cluster theory in conjunction with the aug-cc-pVTZ basis set. These cationic clusters can adopt two distinct structural types, with some clusters showing an unusual type of bonding, often referred to as hemibonding, while other clusters undergo proton transfer to give an ion and radical. It is found that proton transfer based structures are preferred by the (NH_3)_n+, (H_2O)_n^+, and (HF)_n^+ clusters while hemibonded structures are favoured by (PH_3)_n^+, (H_2S)_n^+ and (HCl)_n^+. These trends can be attributed to the relative strengths of the molecules and molecular cations as Brønsted bases and acids, respectively, and the strength of the interaction between the ion and radical in the ion-radical clusters

Assessment of density functional approximations for the hemibonded structure of water dimer radical cation

Due to the severe self-interaction errors associated with some density functional approximations, conventional density functionals often fail to dissociate the hemibonded structure of water dimer radical cation (H2O)2+ into the correct fragments: H2O and H2O+. Consequently, the binding energy of the hemibonded structure (H2O)2+ is not well-defined. For a comprehensive comparison of different functionals for this system, we propose three criteria: (i) The binding energies, (ii) the relative energies between the conformers of the water dimer radical cation, and (iii) the dissociation curves predicted by different functionals. The long-range corrected (LC) double-hybrid functional, omegaB97X-2(LP) [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2009, 131, 174105.], is shown to perform reasonably well based on these three criteria. Reasons that LC hybrid functionals generally work better than conventional density functionals for hemibonded systems are also explained in this work.Comment: 10 pages, 5 figures, 4 table

Surface Affinity of the Hydronium Ion: The Effective Fragment Potential and Umbrella Sampling

The surface affinity of the hydronium ion in water is investigated with umbrella sampling and classical molecular dynamics simulations, in which the system is described with the effective fragment potential (EFP). The solvated hydronium ion is also explored using second order perturbation theory for the hydronium ion and the empirical TIP5P potential for the waters. Umbrella sampling is used to analyze the surface affinity of the hydronium ion, varying the number of solvent water molecules from 32 to 256. Umbrella sampling with the EFP method predicts the hydronium ion to most probably lie about halfway between the center and edge of the water cluster, independent of the cluster size. Umbrella sampling using MP2 for the hydronium ion and TIP5P for the solvating waters predicts that the solvated proton most probably lies about 0.5–2.0 Å from the edge of the water cluster independent of the cluster size

INFRARED SPECTROSCOPY OF WATER CLUSTER RADICAL CATIONS (H2_2O)n_n+^+ ({\em n} = 3 to 11)

Author Institution: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578,JapanTo obtain structural information about radical cationic hydrogen-bonded water networks, we have measured size-selected infrared spectra of the water cluster cations (H$_2$O)$_n$^+ ({\em n} = 3--11) in the OH stretching region. The spectra of smaller-sized clusters ({\em n} \leq 6) show a free OH band associated with the OH radical. This band indicates that nominal water cluster cations (H$_2$O)$_n$^+$form H$^+$(H$_2$O)$_{n-1}$OH type structures, and that the OH radical lies in the network terminal. For larger-sized clusters, the analyses of the hydrogen-bonded OH stretching bands aided by quantum chemical calculations evidence the existence of the OH radical in the clusters. Detailed cluster structures will be discussed on the basis of the experimental spectra

INFRARED SPECTROSCOPY OF LARGE-SIZED PHENOL-WATER CLUSTERS PhOH-(H2_2O)n_n ( 10 ≤\leq n ≤\leq 50)

Author Institution: Department of Chemistry, Graduate School of Science, Tohoku University, Sendai 980-8578,JapanWe report infrared spectra of moderately size-selected phenol-(H$_2$O)$_{n-1}$ (~10 $\leq$ n $\leq$ ~50), which have essentially the same network structures as (H$_2$O)$_n$. The spectra in the OH stretching region are observed. Detailed analyses of these spectra aided by density functional theory calculations reveal the development process of the hydrogen bond network