Byung Jin Mhin

Ab initio studies of the water dimer using large basis sets: The structure and thermodynamic energies

Ab initio calculations with various large basis sets have been performed on the water dimer in order to study the structure, energetics, spectra, and electrical properties. As a reference system, the calculations of the water monomer were also performed. The second order Mo/ller‐Plesset perturbation theory (MP2) using a large basis set (O:13s,8p,4d,2f/H:8s,4p,2d) well reproduces various water monomer experimental data except for the somewhat underestimated absolute energy and hyperpolarizability. The monomer energy calculated with the fourth‐order Mo/ller–Plesset perturbation theory (MP4) with the above basis set is −76.407 hartrees, which is only 0.073 hartree above the experimental energy. To compare the theoretical and experimental dimer structures and thermal energies accurately, we summarized the quantum statistical thermodynamic quantities with corrections for anharmonic vibration, rotation, rotation–vibration coupling, and internal rotation. With the correction for the anharmonic binding potential ...

What is the global minimum energy structure of the water hexamer? The importance of nonadditive interactions

The global minimum energy structures of the water hexamer predicted by widely used analytic water potentials are very different from each other, while the cyclic hexamer does not appear to be a low‐lying energy structure. However, high levels of ab initio calculation predict that a number of low‐lying energy conformers including the cyclic conformer are almost all isoenergetic due to the balance of two‐body and nonadditive interactions. For modeling of water potentials, we suggest that the binding energy of the dimer be between −5.0 and −4.7 kcal (mol dimer)−1, while the three‐body corrections be taken into account to a large extent.

Structures, energetics, and spectra of aqua‐sodium(I): Thermodynamic effects and nonadditive interactions

Using extensive ab initio calculations including electron correlation, we have studied structures, thermodynamic quantities, and spectra of hydrated sodium ions [Na(H2O)+n (n=1–6)]. Various configurations were investigated to find the stable structures of the clusters. The vibrational frequency shifts depending on the number of water molecules were investigated along with the frequency characteristics depending on the presence/absence of outer‐shell water molecules. The thermodynamic quantities of the stable structures were compared with experimental data available. Entropy‐driven structures for n=5 and particularly for n=6 are noted in the calculations, which can explain the peculiar experimental thermal energies. On the other hand, the enthalpy effect to maximize the number of hydrogen bonds of the clusters with the surrounding water molecules seems to be the dominant factor to determine the primary hydration number of Na+ in aqueous solution. The nonadditive interactions in the clusters are found to be...

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.

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...

HARMONIC VIBRATIONAL FREQUENCIES OF THE WATER MONOMER AND DIMER : COMPARISON OF VARIOUS LEVELS OF AB INITIO THEORY

Various levels of ab initio theory using various basis sets have been tested for the energy, structure, and harmonic vibrational frequencies of the water monomer. The level of the single, double, and perturbative triple excitation coupled‐cluster method [CCSD(T)] using a large basis set (O:13s,8p,4d,2f/ H:8s,4p,2d) reproduced the experimental harmonic vibrational frequencies of the water monomer within the error of 0.6 cm−1. Other calculational methods seem to have inherent errors in predicting vibrational frequencies. Even with the above large basis set significant differences between symmetric and asymmetric stretching frequencies were found at various levels of calculational method including the commonly used Moller–Plesset 2nd (MP2) and 4th (MP4) order perturbation theories. The harmonic vibrational frequencies and force constants of the water dimer at various levels of ab initio theory have also been studied, and their shifts in the dimer relative to the monomer are discussed. At the Hartree–Fock (HF) level frequency shifts in the dimer relative to the monomer as well as hydrogen bonding strength in the dimer are underestimated, while at the MP2 level these are overestimated. The values at the CCSD(T) level seem to be reliable.

Ab initio studies of the water hexamer: near degenerate structures

Abstract Ab initio HF-SCF calculations have been performed on several structures of the water hexamers which are expected to have low-lying energies in the potential energy hypersurface. The SCF results with the DZP basis sets show that the hexamer with the global minimum energy has the structure of the cyclic oxygens with the S 6 symmetry. However, the energy of the hexamer with open-book-shaped oxygens is almost as low as that of the cyclic hexamer.

Entropy-driven structures of the water octamer

Abstract Ab initio Hartree-Fock and Moller-Plesset second-order perturbation calculations have been performed on several structures of the water octamer which are expected to be in low-lying energy states. The ab initio results calculated with double zeta plus polarization basis sets predict that the global minimum energy octamer has a cubical structure with D 2d symmetry at 0 K, while the cubical structure with S 4 symmetry is more stable than the D 2d structure above ≈40 K. However, above ≈230 K, the cyclic octamers with S 8 and S 4 symmetries are more stable than the cubical species because of the entropy effect.

PHOTOSWITCH AND NONLINEAR OPTICAL SWITCH : THEORETICAL STUDIES ON 1,2-BIS-(3-THIENYL)-ETHENE DERIVATIVES

The 1,2-bis-(3-thienyl)-ethene derivatives are known to be good photoswitches. A large number of experiments have been carried out on different classes of these molecules to find out the most effective photoswitch. We have selected several highly efficient representative model photoswitch molecules of this class and studied their structures, photophysics, and different molecular properties at the ground and vertically excited states using density functional technique together with its time-dependent analog. These analyses are motivated toward the understanding of the effective molecular criteria, which are to be satisfied by a molecule in order to be good photoswitch. The theoretical investigations indicate that the capped ethene derivatives of this class of molecules are more effective photoswitches than the uncapped ones. Our contention has been verified by carrying out similar calculations on a well-known thermally irreversible photoswitch molecule of this class. Since the transition of the open to the...

Intramolecular charge transfer of π-conjugated push–pull systems in terms of polarizability and electronegativity

We have derived a simple expression to evaluate the amount of intramolecular charge transfer (ICT) of π-conjugate push–pull systems from the properties of electronegativity (χ) and polarizability (α) of the corresponding push and pull systems. This simple model is verified from ab initio calculations of disubstituted benzenes, stilbenes, and butadienes (push–pull systems) and their monosubstituted (push or pull) systems with various donors and acceptors. The bond length alternation (BLA) is often used as a good structural parameter to describe the amount of ICT; however, it is not a complete parameter because the amounts of ICT for the same sets of donor/acceptor pairs are different for different bridge systems. Here, we report a parameter composed of polarizability and electronegativity to give a consistent amount of ICT for different bridge systems. In particular, when a highly electropositive donor is used, the polarizability of an acceptor is the most determining factor for ICT. On the basis of this m...