Ming-Bao Huang

Electronic states and radiative transitions in LiAr

Abstract Ab initio MRD-CI calculations have been carried out on the ground and the eight lowest excited electronic states of LiAr, correlating with excited Li atom states up to 3d 2 D. The ground (X 2 ∑ + (2s)) and 2 2 Σ + (2p) electronic states are repulsive while the higher excited states show shallow Rydberg minima. Rates of radiative bound-bound and bound-free transitions have been also calculated.

Two-Way Effects between Hydrogen Bond and Intramolecular Resonance Effect: An Ab Initio Study on Complexes of Formamide and Its Derivatives with Water

Ab initio calculations up to MP2/aug-cc-pVTZ//MP2/cc-pVTZ level, including natural charge population and natural resonance theory analyses, have been carried out to study the two-way effects between hydrogen bond (H-bond) and the intramolecular resonance effect by using the H-bonded complexes of formamide ( FAO) and its derivatives ( FAXs, X represents the heavy atoms in the substituent groups, CH 2, NH, SiH 2, PH, and S) with water as models. Unlike NH 3 and NH 2CH 3 which prefer being H-bond acceptors ( HA) to form H-bond with water, the amino groups in the six monomers, because of the resonance effect, prefer being H-bond donors ( HD) rather HA. Six monomers can all form HD complexes with water, and only two ( FAC and FASi) with the weakest resonance effect are able to form HA complexes with water. The HD H-bond and resonance effect enhance each other (positive two-way effects) whereas the HA H-bond and resonance effect weaken each other (negative two-way effects). The H-bond energies in the six HD complexes are nearly linearly correlated with the weights of the dipolar resonance in Paulings model and the N-C bond lengths; the correlation coefficients are 0.91 and 0.93, respectively. The positive two-way effects also happens in FAO-water complex, in which the FAO CO group serves as HA ( HA co ). Interestingly, when the HD and HA co H-bonds are present in FAO H-bond complex simultaneously, the enhancements are much more significant, and the energies of the two types of H-bonds are much larger than those when only one type of H-bond is present, reflecting the cooperative effects. By using the knowledge to the two-way effects, we computationally designed a molecule ( FAO- BH 3 ) to increase H-bond energy. Because of the oxygen lone pair donation to the empty pi orbital of BH 3, FAO- BH 3 has a much stronger resonance effect than FAO. As a result, the H-bond energy (-5.55 kcal/mol) in HD H 2O ... FAO- BH 3 complex is much greater than the -3.30 kcal/mol in the HD H 2O...FAO complex. The two-way effects can be rationalized as follows: the resonance effect leads to intramolecular charge shifts in the monomers which facilitate or prevent the charge donation or acceptation of their H-bond partners. Therefore, the H-bonds are strengthened or weakened. In reverse, the charge donations or acceptations of their H-bond partners facilitate or prevent the intramolecular charge shifts in the monomer moieties, which enhance or weaken the resonance effect. The understanding to the two-way effects may be helpful in drug design and refinement by modulating the H-bond strength and in building empirical H-bond models to study large biological molecules. The study supports Paulings resonance model.

The 1 2A1, 1 2B2, and 1 2A2 states of the SO2+ ion studied using multiconfiguration second-order perturbation theory

The 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) electronic states of the SO(2) (+) ion have been studied using multiconfiguration second-order perturbation theory (CASPT2) and two contracted atomic natural orbital basis sets, S[6s4p3d1f]/O[5s3p2d1f] (ANO-L) and S[4s3p2d]/O[3s2p1d] (ANO-S), and the three states were considered to correspond to the observed X, B, and A states, respectively, in the previous experimental and theoretical studies. Based on the CASPT2/ANO-L adiabatic excitation energy calculations, the X, A, and B states of SO(2) (+) are assigned to 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2), respectively, and our assignments of the A and B states are contrary to the previous assignments (A to (2)A(2) and B to (2)B(2)). The CASPT2/ANO-L energetic calculations also indicate that the 1 (2)A(1), 1 (2)B(2), and 1 (2)A(2) states are, respectively, the ground, first excited, and second excited states at the ground-state (1 (2)A(1)) geometry of the ion and at the geometry of the ground-state SO(2) molecule. Based on the CASPT2/ANO-L results for the geometries, we realize that the experimental geometries (determined by assuming the bond lengths to be the same as the neutral ground state of SO(2)) were not accurate. The CASPT2/ANO-S calculations for the potential energy curves as functions of the OSO angle confirm that the 1 (2)B(2) and 1 (2)A(2) states are the results of the Renner-Teller effect in the degenerate (2)Pi(g) state at the linear geometry, and it is clearly shown that the 1 (2)B(2) curve, as the lower component of the Renner splitting, lies below the 1 (2)A(2) curve. The UB3LYP/cc-pVTZ adiabatic excitation energy calculations support the assignments (A to (2)B(2) and B to (2)A(2)) based on the CASPT2/ANO-L calculations.

Hyperfine structure in HCS and related radicals: a theoretical study

Abstract The hyperfine structure of the HCS radical and the isovalent HCO, HSiS and HSiO radicals was studied using the B3LYP and MRSDCI methods. The B3LYP calculations predict an isotropic proton hyperfine coupling constant a (H) of 127.4–129.1 MHz for HCS, in excellent agreement with the experiment. The calculations confirm the experimental fact that the a (H) value of HCS is much smaller than the a (H) values of HCO, HSiS and HSiO, for which we present a simple explanation on the basis of the analysis of the spin densities on the heavy atoms.

THEORETICAL STUDY ON INFRARED VIBRATIONAL SPECTRA OF BORIC-ACID IN GAS-PHASE USING DENSITY FUNCTIONAL METHODS

Abstract Density functional theory (DFT) methods with various exchange-correlation functionals such as SVWN, BVWN, BVWN5, BLYP, B1LYP, B3LYP, B3PW91, and BH and H are employed in a theoretical study of molecular boric-acid in gas-phase. In the calculations, the split valence 6-311++G** and 6-31G* basis sets were used. The geometry, zero-point vibrational energies (ZPVEs), and harmonic infrared vibrational (IR) frequencies are predicted. The calculated C 3h -symmetry geometrical parameters are compared with Hartree–Fock (HF) calculation results and experimental data. IR frequencies predicted by the BLYP, B3LYP, and B3PW91 calculations are in good agreement with experimental data. The frequency calculations presented here also suggest that the C 3h -symmetrical structure corresponds to a minimum in the potential energy surface, but neither is D 3h - or C 3 -symmetrical structure.

INSERTIONS OF METHYLIDYNE AND SILYLIDYNE INTO METHANE AND SILANE

Ab initio molecular orbital calculations have been carried out to study the insertion reactions of methylidyne and silylidyne into methane and silane. The calculations included: (i) optimization calculations to locate the stationary points along the insertion paths at the second-order Moller–Plesset perturbation theory level (MP2) with 6-31G(d,p), 6-311G(d,p) and 6-311++G(d,p) basis sets; (ii) MP2/6-31G(d,p) frequency calculations to characterize the stationary points; (iii) MP2/6-31G(d,p) intrinsic reaction coordinate (IRC) calculations to trace the insertion paths; and (iv) MP4/6-311++G(2d,p) single point calculations plus the corrections with the zero point energies to check the stability of energetic results. In the reaction path of the silylidyne insertion into SiH4 an intermediate complex separating the reactants from the transition state exists and it is stabilized owing to the interaction between the empty silicon p-orbital of silylidyne and the Si–H σ-bonding molecular orbital of silane. The energetic results indicate that the insertion channel for the SiH+SiH4 reaction is feasible, and the predicted insertion reaction path of the SiH+SiH4 reaction confirms kinetic experimental observations. The reaction path of the silylidyne insertion into methane is predicted to be somewhat similar to that of the silylidyne insertion into silane, but the energy well is very shallow and the barrier is significantly high. The insertions of methylidyne into SiH4 and CH4 are predicted to occur with no energy barriers.

Ab initio study of low-lying electronic states of the NF2 radical

The equilibrium geometries, excitation energies, vibrational frequencies for the low-lying states X2B1, 2A1, 2B2, 22A1, and 2A2 of the NF2 radical have been calculated at the HF/6-31G* and MP2/6-31G* levels. Our MP2/6-31G* results for the X2B1 and 2A1 states are in good agreement with available experimental data obtained recently via UV spectroscopy and photolysis. Our calculations indicate that inclusion of polarization functions in the basis set and incorporation of correlation energy are both important in ab initio calculations on the NF2 radical.

The Bergman cyclizations of the enediyne and its N-substituted analogs using multiconfigurational second-order perturbation theory.

The Bergman cyclizations of the enediyne and its four N‐substituted analogs [(Z)‐pent‐2‐en‐4‐ynenitrile, 3‐azahex‐3‐en‐1,5‐diyne, malenotrile, and 3,4‐azahex‐3‐en‐1,5‐diyne] have been studied using the complete active space self‐consistent field and multiconfigurational second‐order perturbation theory methods in conjunction with the atomic natural orbital basis sets. The geometries and energies of the reactants, transition states, and products along both the S0 (the ground state) and T1 (the lowest‐lying triplet state) potential energy surfaces (PESs) were calculated. The calculated geometries are in good agreement with the available experimental data. The distance between two terminal carbons in enediyne, which was considered as an important parameter governing the Bergman cyclization, was predicted to be 4.319 Å, in agreement with the experimental value of 4.321 Å. Our calculations indicate that the replacements of the terminal C atom(s) or the middle C atom(s) in the CC bond by the N atom(s) increase or decrease the energy barrier values, respectively. There exist stable ring biradical products on the T1 PESs for the five reactions. However, on the S0 PESs the ring biradical products exist only for the reactions of enediyne, (Z)‐pent‐2‐en‐4‐ynenitrile, and 3‐azahex‐3‐en‐1,5‐diyne.

The o-, m-, and p-benzyne radical cations: a theoretical study

On the basis of the CASPT2 (multiconfigurational second-order perturbation theory) geometry optimization calculations, the ground states of the o-C6H4+ (C2v), m-C6H4+ (C2v), and p-C6H4+ (D2h) radical cations were determined to be 1 2B1, 1 2A2, and 1 2B1u, respectively. For o-C6H4+ and m-C6H4+, the first excited states (1 2A2 and 1 2A1, respectively) lie very close to the respective ground states. The small distance value of 1.419 A between the two dehydrocarbons in the ground-state geometry of m-C6H4+ indicates that there is a real chemical bond between the two dehydrocarbons (the distance in the 1 2A1 geometry of m-C6H4+ is very long as in the m-C6H4 molecule). The (U)B3LYP isotropic proton hfcc (hyperfine coupling constant) calculation results imply that the ground and first excited states of o-C6H4+ will have similar ESR spectrum patterns while the ground and first excited states of m-C6H4+ will have completely different ESR spectrum patterns.

Ab initio calculations of the electronic states of acetyl radical

Abstract Four excited states, A 2 A″, B 2 A′, C 2 A′, and D 2 A″ of the acetyl radical CH 3 CO, which is the intermediate of the Norrish type I photolysis reaction of acetone, were found by MR(SD)CI calculations. Their vertical excitation energy from the ground state X 2 A′ is 60.0, 113.0, 154.5 and 161.4 kcal/mol, respectively. The A 2 A″ state is a bound state while the X 2 A′ state has a low dissociative energy barrier to form CH 3 and CO. Based on these calculations, a state-specific reaction path of one photon and two photon photodissociation of acetone is described.