Wai-Kee Li

Silicon nanotubes: Why not?

Abstract A diamond nanowire (CNW), a silicon nanowire (SiNW), a carbon nanotube (CNT), and a silicon nanotube (SiNT) were studied using the semiempirical molecular orbital PM3 method, with confirmations by calculations at the HF/3-21G and HF/3-21G(d) levels. It was shown that the systems with a diamond structure generally possess larger band gaps than their tubular counterparts. Carbon nanotubular structure shows efficient sp 2 hybridization and π bonding, thus allowing a high stability of the carbon nanotube structure. In contrast, silicon prefers sp 3 hybridization and favors the tetrahedral diamond-like structures, thereby forming the commonly observed nanowires. This distinction can be traced to the differences in the energetics and overlaps of the valence s and p orbitals of C vs Si. Nevertheless, when the dangling bonds are properly terminated, SiNT can in principle be formed. The resulting energy minimized SiNT, however, adopts a severely puckered structure (with a corrugated surface) with SiSi distances ranging from 1.85 to 2.25 A.

A Gaussian‐2 ab initio study of CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+

Using the Gaussian‐2 (G2) theoretical procedure, we have examined the molecular structures and total energies for CH2SH, CH2S−, CH3S−, CH2SH−, CH3SH−, CH3+, and CH3SH+. Contrary to the relative stabilities of CH3S+(C3v;3A2) and CH2SH+(Cs;1A’), the methylthio radical CH3S(Cs;2A’) and the methylthio anion CH3S−(C3v;1A1) are predicted to be more stable than the mercaptomethyl radical CH2SH(C1;2A) and the mercaptomethyl anion CH2SH−(Cs;1A’) by 9.2 and 38.0 kcal/mol, respectively. The CH2SH−(Cs;1A’) anion may exist in the cis configuration or the less stable trans structure. Combined with the results of previous G2 calculations, this calculation yields predictions for the adiabatic ionization energies (IE) of CH3 (9.79 eV), CH2SH (7.41 eV), and CH3SH (9.55 eV), which are in accord with the experimental IEs of 9.84 eV for CH3, 7.536±0.003 eV for CH2SH, and 9.440 eV for CH3SH. The G2 values for the adiabatic electron affinities (EA) of CH2S, CH2SH to trans‐CH2SH−(Cs;1A’), CH2SH to cis‐CH2SH−(Cs;1A’), and CH3S ar...

An isomeric study of N5+, N5, and N5−: a Gaussian-3 investigation

Abstract An isomeric study for N 5 , N 5 + , and N5− has been carried out using the Gaussian-3 (G3) method. At the MP2(FU)/6-31G(d) level, six N5, five N5+, and four N5− isomers are identified. Of these 15 species, 12 of them have not been reported previously. The most stable N5 isomer is a weak N2⋯N3 complex with C2v symmetry, while the most stable N5− isomer is also a weak N2⋯N3− complex with the same symmetry. The most stable species for N5+ has an open-chain structure with C2v symmetry, which has been synthesized and characterized spectroscopically. Natural bond orbital (NBO) analysis suggests that the stability of this cation is enhanced by hyperconjugation.

Experimental and theoretical studies of isomeric C2H5S and C2H5S

Abstract By combining photoionization and photodissociation measurements with ab initio Gaussian-2 (G2) calculations on the C 2 H 5 S and C 2 H 5 S + system, we have concluded that CH 3 CH 2 S is the dominant primary product formed in the 193 nm photodissociation of (CH 3 CH 2 ) 2 S, while CH 3 CHSH + is the product ion formed at the photoionization onset of C 2 H 5 S + from (CH 3 CH 2 ) 2 S. The G2 predictions for the heats of formation at 0 K (Δ H f0 ) for the isomers ofC 2 H 5 S and C 2 H 5 S + (CH 3 CH 2 S:Δ H f0 (G2)=27.5 kcal/mol;CH 3 SCH 2 :Δ f0 (G2)=37.3kcal/mol; CH 3 CH 2 S + :Δ H f0 (G2)=236.5 kcal/mol;CH 3 CHSH + :Δ H f0 (G2)=192.6 kcal/mol; and CH 3 SCH 2 + : Δ H f0 (G2)=195.3 kcal/mol) are in agreement with available experimental Δ H f0 values (CH 3 CH 2 S: Δ H f0 (exp)=31.4±2 kcal/mol; CH 3 SCH 2 : Δ H f0 /(exp)=34.8±2.5 kcal/mol; CH 3 CH 2 S + : Δ H f0 (exp)=238.3±2 kcal/mol; CH 3 CHSH + :Δ H f0 (exp)=189.6±1.0 kcal/mol;and CH 3 SCH 2 + :Δ H f0 (exp)=195.1 kcal/mol).The G2 calculation also yields a value of 35.3 kcal/mol for Δ H f0 (CH 3 CHSH).

Computational study on the molecular inclusion of andrographolide by cyclodextrin.

Due to the poor water solubility of andrographolide (andro), an inclusion technique has been developed to modify its physical and chemical properties so as to improve its bioavailability. In contrast with the immense experimental studies on the inclusion complexes of andro:cyclodextrin, no computational study has so far been carried out on this system. In this work, preliminary docking experiments with AutoDock were performed. Density Functional Theory (DFT) and Austin Model 1 (AM1) calculations upon the docking instances were applied to investigate the two possible modes of molecular inclusions between andro and x-cyclodextrin (xCD, where x is α, β or γ). Atoms-in-Molecules (AIM) analysis based on the B3LYP/cc-pVDZ wavefunction was applied to verify the existence of the intermolecular hydrogen bonds. It was found that the most stable complex among the six possible inclusion complexes was the one formed between andro and βCD with andro’s decalin ring moiety wrapped by CD at a ratio of 1:1. The hydrogen bonds between andro and CD were responsible for the stability of the inclusion complexes. The calculated data were found to be consistent with the experimental results. Thus, the results of this study can aid new drug design processes.

A Gaussian‐2 ab initio study of van der Waals dimers R1R2 and their cations R1R+2 (R1, R2=He, Ne, Ar, and Kr)

Predictions for the ionization energies (I.E.s) of van der Waals dimers R1R2 and the bond dissociation energies (D0’s) of the dimer ions R1R2+ (R1, R2=He, Ne, Ar, and Kr) have been calculated using the ab initio Gaussian‐2 theoretical procedure. Despite only fair agreements observed between theoretical and experimental geometries, the theoretical I.E.s and D0’s are found to be in excellent agreement with available experimental findings.

Adiabatic ionization energy of CH3SSCH3

The ionization energy (IE) for CH3SSCH3 has been measured by the pulsed molecular beam photoionization mass spectrometric method. The experimental IE of 8.18±0.03 eV is in excellent agreement with the theoretical prediction of 8.15 eV calculated using the ab initio Gaussian‐2 procedure, indicating that the experimental ionization onset can be assigned as the adiabatic IE for CH3SSCH3. The observation of the adiabatic IE(CH3SSCH3) is attributed to the low potential energy barrier for rotation about the S–S bond, which allows CH3SSCH3 to exist dynamically in a wide range of CSSC dihedral angles.

Theoretical investigation of four conformations of HNIW by B3LYP method

Abstract The density function theory method has been used to study the geometry and electronic structure of hexanitrohexaazaisowurtzitane (HNIW). Four conformations of HNIW were obtained. Full geometry optimization, calculation of vibrational frequency, and NBO analyses for the four conformations of HNIW were performed at the B3LYP/6-31G ∗∗ level. The geometry and electronic structures of four conformations were analyzed. The C–C bond linking two five-membered rings and the boat-shaped six-membered ring are important factors in stabilizing the molecular skeleton. The N–N bonds in N-nitro group are possibly more easily broken than the other bonds. The spatial orientation of NO 2 has some influence on the molecular structure and energy.

Theoretical Studies on the Nonlinear Optical Properties of Octupolar Tri-s-triazines.

The first-, second-, and third-order static and frequency-dependent polarizabilities of a series of octupolar tri-s-triazines have been investigated by using the ab initio coupled perturbed Hartree-Fock (CPHF) method. Effects of substitution have also been considered. The results show that α, β, and γ values for octupolar tri-s-triazines are much larger than those for s-triazine in both static and frequency-dependent cases. Attaching groups containing π systems such as azide and ethenyl to the tri-s-triazine molecule results in a significant increase of first-, second-, and third-order polarizabilities. Our calculations suggest that the octupolar tri-s-triazines may be prospective candidates for nonlinear optical materials.

Experimental and theoretical studies of isomeric CH3S2 and CH3S+2

By combining molecular beam photodissociation and photoionization measurements with ab initio Gaussian‐2 (G2) calculations on the CH3S2 and CH3S+2 systems, we have shown that CH3SS is the dominant isomer formed in the photodissociation process, CH3SSCH3+hν(193 nm) →CH3S2+CH3. The experimental ionization energy for CH3SS (8.97±0.02 eV) and the heat of formation at 0 K for CH3SS+ (217.7±1.2 kcal/mol) are in excellent agreement with the G2 results. The photoionization efficiency spectrum observed for CH3SS is also consistent with the theoretical prediction that the Franck–Condon factor for the photoionization process, CH3SS+hν →CH3SS++e−, is not favorable. Based on the statistical modeling of experimental rates obtained previously for HS loss in the unimolecular decomposition of CH3SSCH+3 and the comparison with G2 ab initio predictions, we conclude that CH2SSH+ is most likely the isomer structure formed near the experimental appearance energy (11.07 eV) observed for the photodissociative ionization process,...