Ching-Han Hu

Carbonyl–water hydrogen bonding: The H2CO–H2O prototype

The potential energy surface (PES) of the water–formaldehyde complex has been examined using ab initio methods. Three energetically low‐lying stationary points were located on the potential surface corresponding to one minimum and two transition states. All stationary points were examined using a double‐ζ plus polarization (DZP) basis set at the self‐consistent field (SCF), single and double excitation configuration interaction (CISD), and single and double excitation coupled‐cluster (CCSD) levels of theory. In addition, the minimum was more thoroughly investigated through the use of the triple‐ζ plus double polarization (TZ2P) basis set, the TZ2P plus higher angular momentum functions [TZ2P(f,d)] basis, and the TZ2P basis set augmented by a set of diffuse functions (TZ2P+diff) with those same theoretical methods. For each of the stationary points, geometrical parameters, absolute energies, classical binding energies, and zero‐point vibrational energy corrections are reported. Additional information conce...

Toward the infrared spectroscopic observation of SiH+5: the silanium ion

Ab initio quantum mechanical methods, including the self-consistent field, single and double excitation configuration interaction, and single and double excitation coupled cluster, have been applied to six stationary points on the SiH+5 potential energy hypersurface. Equilibrium geometries were determined using analytic energy first derivative techniques. Relative energies of stationary points have been obtained. Harmonic vibrational frequencies of the global minimum were obtained at all levels of theory. Basis sets used include double-zeta plus polarization and triple-zeta plus double polarization. SiH+5 should be regarded as involving weakly bound H2 and SiH+3 subunits, with a dissociation energy of only about 10 kcal/mol. Pseudorotation was found to be unfavorable in the SiH+5 ion.

Is dodecahedral P20 special

SummaryThe laboratory study of phosphorus clusters by laser-based mass spectrometric methods indicates, tentatively, that P+21 may be special. A plausible interpretation might place a P+ ion interior to a dodecahedral P20 molecule.Ab initio quantum mechanical methods have been applied to the P20 molecule using contracted gaussian basis sets as large as (9s 6p 4d 3f) on each phosphorus atom. At the highest level of theory, dodecahedral P20 is predicted to lie 23 kcal/mol above five separated P4 molecules.

Reaction barrier for the methyldiazenyl radical decomposition (CH3N2→CH3+N2)

The reaction of ground state (2A’) CH3N2→CH3+N2 was studied using the ab initio quantum mechanical techniques, including the self‐consistent field (SCF), single and double excitation configuration interaction (CISD), single and double excitation coupled cluster (CCSD), and the single, double, and perturbative triple excitation coupled cluster [CCSD(T)]. The classical barrier for the methyldiazenyl radical decomposition was predicted at the highest level of theory to be 4.5 kcal/mol and was 2.3 kcal/mol when zero‐point vibrational energy corrections are included. This result is pertinent to the apparently conflicting experimental results of the Rice and Berkeley groups. The very small theoretical reaction barrier agrees with the recent experimental observation that lifetime of CH3N2 is very short, at the picosecond range.

Catenanes: A molecular mechanics analysis of the (C13H26)2 Structure 13‐13 D2

Molecular mechanics (MM4) studies have been carried out on the catenane (C13H26)2, specifically 13‐13D2. The structure obtained is in general agreement with second‐order perturbation theory. More importantly, the MM4 structure allows a breakdown of the energy of the molecule into its component classical parts. This allows an understanding of why the structure is so distorted, in terms of CC bonding and nonbonding interactions, van der Waals repulsion, CCC and CCH angle bending, torsional energies, stretch‐bend, torsion‐stretch, and bend–torsion–bend interactions. Clearly, the hole in 113‐membered ring is too small for the other ring to fit through comfortably. There are too many atoms trying to fit into the limited space at the same time, leading to large van der Waals repulsions. The rings distort in such a way as to enlarge this available space, and lower the total energy of the molecule. While the distortions are spread around the rings, one of the nominally tetrahedral CCC bond angles in each ring is opened to 147.9° by MM4 (146.8° by MP2). The stability of the compound is discussed in terms of the strain energy.

Structure and decomposition barrier of the ethyldiazenyl radical

Abstract The structure and dissociation of ground-state ( 2 A ′) C 2 H 5 N 2 , the ethyldiazenyl radical, were studied using ab initio quantum mechanical techniques, including the self-consistent-field (SCF), single and double excitation configuration interaction (CISD), single and double excitation coupled cluster (CCSD) and single, double and perturbative triple excitation coupled cluster (CCSD(T)). The activation barrier for the decomposition of the ethyldiazenyl radical to form N 2 plus C 2 H 5 was predicted to be smaller than that of CH 3 N 2 . The tunneling frequency of C 2 H 5 N 2 estimated within the Wentzel-Kramers-Brillouin approximation gave a tunneling rate corresponding to a lifetime of 0.3 ps for the ground vibrational state. The energetics and rate of C 2 H 5 N 2 decomposition are similar to those of the methyldiazenyl radical CH 3 N 2 , and are very different from those of HN 2 , the lifetime of which is in the nanosecond range.

Accurate prediction of the enthalpies of formation for xanthophylls

This study investigates the applications of computational approaches in the prediction of enthalpies of formation (ΔHf) for C‐, H‐, and O‐containing compounds. Molecular mechanics (MM4) molecular mechanics method, density functional theory (DFT) combined with the atomic equivalent (AE) and group equivalent (GE) schemes, and DFT‐based correlation corrected atomization (CCAZ) were used. We emphasized on the application to xanthophylls, C‐, H‐, and O‐containing carotenoids which consist of ∼ 100 atoms and extended π‐delocaization systems. Within the training set, MM4 predictions are more accurate than those obtained using AE and GE; however a systematic underestimation was observed in the extended systems. ΔHf for the training set molecules predicted by CCAZ combined with DFT are in very good agreement with the G3 results. The average absolute deviations (AADs) of CCAZ combined with B3LYP and MPWB1K are 0.38 and 0.53 kcal/mol compared with the G3 data, and are 0.74 and 0.69 kcal/mol compared with the available experimental data, respectively. Consistency of the CCAZ approach for the selected xanthophylls is revealed by the AAD of 2.68 kcal/mol between B3LYP‐CCAZ and MPWB1K‐CCAZ.