James S. Wright

Predicting the antioxidant activity of curcumin and curcuminoids

Abstract The curcuminoids, which include curcumin and related molecules, are effective antioxidants with demonstrated medicinal effects. The curcumin structure contains a variety of functional groups including the β-diketo group, carbon–carbon double bonds and phenyl rings containing varying amounts of hydroxyl and methoxy substituents. The literature on the site of activity and the reaction mechanism(s) responsible for the antioxidant effects is controversial, with some authors claiming that the activity is due to the hydroxyl moiety while others invoke carbon-centered radicals or involvement of the carbonyl groups. We present a systematic theoretical study of these molecules, aimed at clarifying the active sites, and discuss the validity of proposed mechanisms based on our calculations. We also discuss more general aspects of free radical chemistry, especially the selectivity arising from choice of the free radical/antioxidant system.

Stability of carbon-centered radicals: Effect of functional groups on the energetics of addition of molecular oxygen

In this paper we examine a series of hydrocarbons with structural features which cause a weakening of the CH bond. We use theoretical calculations to explore whether the carbon‐centered radicals R• which are created after breaking the bond can be stabilized enough so that they resist the addition of molecular oxygen, i.e. where the reaction R• + O2 → ROO• becomes energetically unfavorable. Calculations using a B3LYP‐based method provide accurate bond dissociation enthalpies (BDEs) for RH and ROO• bonds, as well as Gibbs free energy changes for the addition reaction. The data show strong correlations between ROO• and RH BDEs for a wide variety of structures. They also show an equally strong correlation between the ROO• BDE and the unpaired spin density at the site of addition. Using these data we examine the major functional group categories proposed in several experimental studies, and assess their relative importance. Finally, we combine effects to try to optimize resistance to the addition of molecular oxygen, an important factor in designing carbon‐based antioxidants.

Strongly bound multiply excited states of B+2 and B2

The potential curves, transition energies (Te), and spectroscopic constants (Re, ωe) of several low‐lying electronic states of B+2 and selected doubly excited states of B2 are given. The data have been obtained by using a multireference single‐ and double‐excitation (MRD) configuration interaction (CI) approach and a triple‐zeta plus polarization AO basis set. The B+2 ground state, which is found here to be X 2Σ+g, shows a rather shallow potential curve (Re =4.015 bohr, ωe =423 cm−1) when compared with that of X 3Σ−g of B2(Re =3.00 bohr, ωe =1051 cm−1, exptl.). The first excited state of B+2, namely 1 2Πu, lies at Te =0.30 eV. Moreover, double excitations relative to X 2Σ+g are essential for the description of a large number of excited states, such as σuσg→π2u(1 4Σ−u, 1 2Σ−u, 1 2Δu, 1 2Σ+u) and σ2u→π2u(1 4Σ−g,1 2Σ−g, 1 2Δg, 1 2Σ+g). Similarly, 2 2Πu arises from the triple excitation σ2uσg→π3u. In the same order as given above, such multiple excitations lead to a significant gain in bond strength (i.e., sh...

Abstraction vs insertion in O(1D)+H2→OH+H

It is shown that in the reaction O(1D)+H2→OH+H, the total reaction cross sections are almost entirely due to insertion and the contribution to the total cross section from abstraction is negligible. (AIP)

Rational Design, Synthesis, and Optical Properties of Film-Forming, Near- Infrared Absorbing, and Fluorescent Chromophores with Multidonors and Large Heterocyclic Acceptors

A new series of film-forming, low-bandgap chromophores (1 a,b and 2 a,b) were rationally designed with aid of a computational study, and then synthesized and characterized. To realize absorption and emission above the 1000 nm wavelength, the molecular design focuses on lowering the LUMO level by fusing common heterocyclic units into a large conjugated core that acts an electron acceptor and increasing the charge transfer by attaching the multiple electron-donating groups at the appropriate positions of the acceptor core. The chromophores have bandgap levels of 1.27-0.71 eV, and accordingly absorb at 746-1003 nm and emit at 1035-1290 nm in solution. By design, the relatively high molecular weight (up to 2400 g mol(-1)) and non-coplanar structure allow these near-infrared (NIR) chromophores to be readily spin-coated as uniform thin films and doped with other organic semiconductors for potential device applications. Doping with [6,6]-phenyl-C(61) butyric acid methyl ester leads to a red shift in the absorption only for 1 a and 2 a. An interesting NIR electrochromism was found for 2 a, with absorption being turned on at 1034 nm when electrochemically switched (at 1000 mV) from its neutral state to a radical cation state. Furthermore, a large Stokes shift (256-318 nm) is also unique for this multidonor-acceptor type of chromophore, indicating a significant structural difference between the ground state and the excited state. Photoluminescence of the film of 2 a was further probed at variable temperatures and the results strongly suggest that the restriction of bond rotations certainly helps to diminish non-radiative decay and thus enhance the luminescence of these large chromophores.

Direct vs complex reaction dynamics for F+OH→HF+O

The exerogicity of the reaction F+H2O→HF+OH is sufficient to give HF(v′?1); however, arrested relaxation infrared chemiluminescence experiments on this system show emission from HF(v′?3). The higher vibrational levels are populated by the secondary reaction F(2P)+OH(2Π)→HF(1Σ+)+O(3P). By a combination of SCF–CI calculations and a rotated Morse curve fitting procedure, it is shown that barrier heights on triplet surfaces which correlate reactants and products of the secondary reaction are too high to provide a reaction path. Instead, the reaction proceeds on a singlet surface to produce an HOF complex, followed by rearrangement and a nonadiabatic transition to the triplet surface. An exit‐channel barrier results from the surface crossing. The chemiluminescence data are shown to be in accord with this reaction mechanism.

Theoretical calculation of gas-phase ionization potentials for mono- and polysubstituted benzenes

Abstract This Letter provides an accurate and efficient method to obtain gas-phase ionization potentials for monosubstituted benzene molecules, using density functional theory with the B3LYP functional and geometry/frequencies obtained by the AM1 method. The results show an excellent correlation between calculated and experimental IPs for 20 different monosubstituted benzene rings. The study is extended to include polymethylated benzenes, up to hexamethylbenzene, whose IPs are also shown to be in good agreement with experimental data, making this a promising approach for large, polysubstituted benzene systems.

Theoretical calculations on electronic transitions for H3, including Rydberg and transition state spectra

MRD‐CI calculations have been carried out on the ground and excited electronic states of H3 for D3h, D∞h, C∞v, and C2v geometries. Dipole transition moments between the various electronic states have been also obtained at the different geometries calculated. The present work provides accurate theoretical information relevant to the transition state spectroscopy of H+H2 along a collinear path and also along a perpendicular path. In addition, the present work is the first all‐electron configuration interaction treatment of the Rydberg states of H3, and the results are in excellent agreement with the observed spectra.

Rotated Morse curve–spline potential function for A+BC reaction dynamics: Application to (Cl, HBr), (F,H2), and (H+,H2)

Methods for the fitting of potential energy functions to discrete potential energy data for triatomic molecules are discussed. Several new criteria for successful fitting functions are proposed, and existing criteria are reviewed. Different semiempirical and empirical fitting functions are analyzed in terms of these criteria. The rotated Morse curve–spline (RMCS) method is developed for general triatomic molecules, and applied to the systems Cl+HBr, F+H2, and H++H2. The detailed surface features are well reproduced for Cl+HBr and F+H2, particularly near the minimum energy path. Overall standard deviations between reference functions and RMCS functions are 0.98 and 0.35 kcal/mole for Cl+HBr and F+H2, respectively. Averaged classical trajectories on the F+H2 surface show good agreement between LEPS and RMCS surfaces over the range 0–10 eV, although there is no point‐for‐point matching of individual trajectories. Relative CPU times for trajectories on the RMCS and LEPS surfaces are 3.3:1. A H+3 potential sur...

The effect of bond functions on molecular dissociation energies

Abstract Multi-reference Cl calculations are reported for the ground states of HCl and N 2 at their equilibrium distances, and for their separated atoms. Basis sets of double-zeta and double-zeta plus polarization quality are systematically augmented by additional sets of functions located at the bond centers. It is shown that use of bond functions can lead to either an underestimate or an overestimate of the the bond energy. Optimum basis sets for each molecule were obtained, giving D e values of 4.59 eV for HCl (expt. 4.62 eV) and 9.96 eV for N 2 (expt. 9.905 eV) at the estimated full Cl level. The quality of the potential curves obtained with these basis sets is discussed.