Han Myoung Lee

Structures, energies, vibrational spectra, and electronic properties of water monomer to decamer

The correlation of various properties of water clusters (H2O)n=1–10 to the cluster size has been investigated using extensive ab initio calculations. Since the transition from two dimensional (2-D) (from the dimer to pentamer) to 3-D structures (for clusters larger than the hexamer) is reflected in the hexamer region, the hexamer can exist in a number of isoenergetic conformers. The wide-ranging zero-point vibrational effects of the water clusters having dangling H atoms on the conformational stability by the O–H flapping or proton tunneling through a small barrier (∼0.5 kcal/mol) between two different orientations of each dangling H atom are not large (∼0.1) kcal/mol). Large dipole moments (>2.5 D) are found in the dimer and decamer, and significant dipole moments (∼2 D) are observed in the monomer, hexamer, and nonamer. The polarization per unit monomer rapidly increases with an increasing size of the cluster. However, this increase tapers down beyond the tetramer. The O–H vibrational frequencies serve ...

Comparative ab initio study of the structures, energetics and spectra of X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] clusters

X−⋅(H2O)n=1–4 [X=F, Cl, Br, I] have been studied using high level ab initio calculations. This extensive work compares the structures of the different halide water clusters and has found that the predicted minimum energy geometries for different cluster are accompanied by several other structures close to these global minima. Hence the most highly populated structures can change depending on temperature due to the entropy effect. As the potential surfaces are flat, the wide-ranging zero point vibrational effects are important at 0 K, and not only a number of low-lying energy conformers but also large amplitude motions can be important in determining structures, energies, and spectra at finite temperatures. The binding energies, ionization potentials, charge-transfer-to-solvent (CTTS) energies, and the O–H stretching frequencies are reported, and compared with the experimental data available. A distinctive difference between F−⋅(H2O)n and X−⋅(H2O)n (X=Cl, Br, I) is noted, as the former tends to favor inter...

Complete basis set limit of Ab initio binding energies and geometrical parameters for various typical types of complexes

Using basis‐set extrapolation schemes for a given data set, we evaluated the binding energies and geometries at the complete basis set (CBS) limit at the levels of the second order Møller–Plesset perturbation theory (MP2) and the coupled cluster theory with singles, doubles, and perturbative triples excitations [CCSD(T)]. The systems include the hydrogen bonding (water dimer), aromatic interaction (benzene dimer), π–H interaction (benzene–water), cation–water, anion–water, π–cation interaction (cation–benzene), and π–anion interaction (anion–triazine). One extrapolation method is to exploit both BSSE‐corrected and BSSE‐uncorrected binding energies for the aug‐cc‐pVNZ (N = 2, 3, 4, …) basis set in consideration that both binding energies give the same CBS limit (CBSB). Another CBS limit (CBSC) is to use the commonly known extrapolation approach to exploit that the electron correlation energy is proportional to N−3. Since both methods are complementary, they are useful for estimating the errors and trend of the asymptotic values. There is no significant difference between both methods. Overall, the values of CBSC are found to be robust because of their consistency. However, for small N (in particular, for N = 2, 3), CBS  NB is found to be slightly better for water–water interactions and cation–water and cation–benzene interactions, whereas CBS  NC is found to be more reliable for bezene–water and anion–water interactions. We also note that the MP2 CBS limit value based on N = 2 and 3 combined with the difference between CCSD(T) and MP2 at N = 2 would be exploited to obtain a CCSD(T)/CBS value for aromatic–aromatic interactions and anion–π interactions, but not for cationic complexes.

STRUCTURES AND ENERGETICS OF THE WATER HEPTAMER : COMPARISON WITH THE WATER HEXAMER AND OCTAMER

In spite of a spate of studies of various water clusters, a few theoretical studies on the water heptamer are available. State-of-the-art ab initio calculations are thus carried out on twelve possible water heptamer structures to explore the conformation as well as spectroscopic properties of this water cluster. Two three-dimensional cagelike structures comprised of seven-membered cyclic rings with three additional hydrogen bondings were found to be the lowest-lying energy heptamer conformers. The global minimum energy structure was found to be 0.5 kcal/mol lower than the other. The zero-point energy uncorrected and corrected binding energies of the global minimum energy structure are 55.2 and 37.9 kcal/mol, respectively. An almost two-dimensional ring conformer, which is only 1 kcal/mol above the global minimum at 0 K, could be more stable above 150 K. The vibrational spectra of different heptamer conformers are discussed and compared with the spectra of the hexamer and octamer water clusters.

Structures, spectra, and electronic properties of halide-water pentamers and hexamers, X-(H2O)5,6 (X=F, Cl, Br, I): Ab initio study

Various structures of halide-water pentamers and hexamers have been investigated using extensive ab initio calculations. Then, we compare the structures, spectra, and electronic properties of the hydrated fluoride, chloride, bromide, and iodide systems. Although some of the fluoride and iodide systems have been investigated earlier, we have carried out more accurate calculations on an enlarged conformational ensemble. The chloride-, bromide-, and iodide-water pentamers and hexamers behave somewhat similarly, but differently from the fluoride-water pentamer and hexamer. Fluoride-water clusters show semisurface (or semi-internal) structures, while chloride-, bromide-, and iodide-water clusters show surface structures. We substantiate our findings by evaluating various electronic properties such as ionization potentials, natural bond orbital charges, dipole moments, and charge-transfer-to-solvent energies, as well as vibrational frequencies of the low-energy halide-water pentamers and hexamers.

Dissociation chemistry of hydrogen halides in water

To understand the mechanism of aqueous acid dissociation, which plays a fundamental role in aqueous chemistry, the ionic dissociation of HX acids (X=F, Cl, Br, and I) in water clusters up to hexamer is examined using density functional theory and Møller-Plesset second-order perturbation methods (MP2). Further accurate analysis based on the coupled clusters theory with singles and doubles excitations agrees with the MP2 results. The equilibrium structures, binding energies, electronic properties, stretching frequencies, and rotational constants of HX(H(2)O)(n) and X(-)(H(3)O)(+)(H(2)O)(n-1) are calculated. The dissociated structures of HF and HCl can be formed for n>/=4, while those of HBr and HI can be formed for n>/=3. Among these, the dissociated structures of HX (X=Cl, Br, and I) are more stable than the undissociated ones for n>/=4, while such cases for HF would require much more than six water molecules, in agreement with previous reports. The IR spectra of stable clusters including anharmonic frequencies are predicted to facilitate IR experimental studies. Undissociated systems have X-H stretching modes which are highly redshifted by hydration. Dissociated hydrogen halides show three characteristic OH stretching modes of hydronium moiety, which are redshifted from the OH stretching modes of water molecules.

Hydrogen-Release Mechanisms in Lithium Amidoboranes

Alkali-metal amidoboranes have been recently highlighted as materials that satisfy many of the criteria required to make hydrogen-storage media. It is, therefore, crucial for us to understand the dehydrogenation mechanism of these materials for further development towards making successful hydrogen-storage media. In the present study, we attempt to shed light on the mechanisms involved in the loss of one molar equivalent of H(2) from solid lithium amidoboranes by using high-level ab initio calculations of monomeric and dimeric compounds in the gas phase. In the lithium amidoborane dimer, H(2) is released by the formation of LiH, which is followed by a redox reaction of the dihydrogen bond formed between the strongly basic H(-) in LiH and H(delta+) bonded to N. In the dehydrogenation process, the Li cation catalyzes the intermolecular N-B bond formation; this could lead to new pathways for N-B polymerization. After the release of the first molecule of H(2), a Li cation binds to a nitrogen atom, resulting in a lowering of the energy barrier for the second dehydrogenation process per dimer. These results will be useful for the design of future hydrogen-storage media.

Structures and spectra of iodide–water clusters I−(H2O)n=1–6: An ab initio study

To investigate the structures of I−(H2O)n=1–6, extensive ab initio calculations have been carried out. Owing to very flexible potential surfaces of the system (in particular for n=5 and 6), the lowest energy structures are characterized from various possible low-lying energy conformers. In contrast to some previously reported structures, we find a new lowest energy structure (followed by a few low-lying energy conformers) for n=5 and four nearly isoenergetic conformers for n=6. These conformers have surface and near-surface structures with the coordination number of 4. The present results provide the information of possible structures in recent profuse experiments of infrared spectra of I−(H2O)n=1–6 and charge transfer from the excited iodide ion to water molecules. Our predicted ionization potentials and OH stretching frequencies are in good agreement with the experimental data available, while only the cases of the OH frequencies for n=4 and the ionization potential for n=5 need consideration of conform...

Highly stable CO2/N2 and CO2/CH4 selectivity in hyper-cross-linked heterocyclic porous polymers.

The largest obstacles for landfill/flue gas separation using microporous materials are small adsorption values and low selectivity ratios. This study demonstrates that these adsorption and selectivity challenges can be overcome by utilizing a series of hyper-cross-linked heterocyclic polymer networks. These microporous organic polymers (MOPs) were synthesized in a single step by inexpensive Friedel-Crafts-catalyzed reactions using dimethoxymethane as an external linker. The amorphous networks show moderate Brunauer-Emmett-Teller surface areas up to 1022 m(2) g(-1), a narrow pore size distribution in the range from 6 to 8 Å, and high physicochemical stability. Owing to the presence of the heteroatomic pore surfaces in the networks, they exhibit maximum storage capacities for CO2 of 11.4 wt % at 273 K and 1 atm. Additionally, remarkable selectivity ratios for CO2 adsorption over N2 (100) and CH4 (15) at 273 K were obtained. More importantly, as compared with any other porous materials, much higher selectivity for CO2/N2 (80) and CO2/CH4 (15) was observed at 298 K, showing that these selectivity ratios remain high at elevated temperature. The very high CO2/N2 selectivity values are ascribed to the binding affinity of abundantly available electron-rich basic heteroatoms, high CO2 isoteric heats of adsorption (49-38 kJ mol(-1)), and the predominantly microporous nature of the MOPs. Binding energies calculated using the high level of ab initio theory showed that the selectivity is indeed attributed to the heteroatom-CO2 interactions. By employing an easy and economical synthesis procedure these MOPs with high thermochemical stability are believed to be a promising candidate for selective CO2 capture.

Aqua–potassium(I) complexes: Ab initio study

A number of conformers of aqua-K+ complexes, K+(H2O)n (n=1–10) have been investigated using high level ab initio calculations, to elucidate the structures and thermodynamic energies of the hydrated potassium ions. Since the coordination number of K+ is around six in the bulk water, the focus of the present study has been the n=5 and 6 clusters. In contrast to previous studies which have used only the enthalpies to compare against the experimental numbers, the present study also employs free energies. As a result, the predictions of a number of hitherto unknown conformers are in excellent agreement with the experimental results. The maximum coordination number for K+ in ligands containing O atoms is evaluated to be around eight from the energetics of structures possessing only the first hydration shell of water molecules around the K+ ion. It is of interest to note that the hydration of the K+ ion is less structured than that of the Na+ ion, since the water–water interaction becomes more important in the a...