Pham Cam Nam

Surface enhanced Raman scattering of melamine on silver substrate: An experimental and DFT study.

Surface enhanced Raman spectroscopy (SERS) is a powerful vibrational spectroscopy technique that allows amplifying weak Raman signals by the excitation of localized surface plasmons. In this paper, we used nanoscale roughened silver surface to enhance Raman signals of melamine analyte. Silver nanoparticles were synthesized by wet-chemical method and employed for SERS measurement. Theoretically, optimized geometries and vibrational frequencies of melamine and melamine absorbed on silver-cluster surface were calculated by using the B3LYP/6-31G(d) method. Then, the predicted spectrograms are compared with the experimental Raman spectra. As a result, Raman signals of melamine were dramatically enhanced by using obtained silver nanoparticles as the substrate. Typical peaks of melamine at 676 and 983cm(-1) were enhanced and could be obviously observed in experiments. The calculated vibrational frequencies seem to relatively coincide with the experimental values. SERS effect of melamine on nanoscale silver was also explained by analyzing molecular frontier orbitals (HOMO and LUMO) melamine-silver complexes.

Insight into the antioxidant properties of non-phenolic terpenoids contained in essential oils extracted from the buds of Cleistocalyx operculatus: a DFT study

The antioxidant properties of 21 non-phenolic terpenoids contained in essential oil extracted from the buds of Cleistocalyx operculatus have been investigated using density functional theory (DFT)-based computational methods. The C–H bond dissociation enthalpy (BDE), proton dissociation enthalpy (PDE), proton affinity (PA), electron transfer enthalpy (ETE) and ionization energy (IE) were calculated in the gas phase and in two different solvents (water and ethanol) at the ROB3LYP/6-311++G(2df,2p)//B3LYP/6-311G(d,p) level of theory for the former and with the PM6 method for the latter. Quantum chemical descriptors like chemical potential (μ), chemical hardness (η), and global electrophilicity (ω) were calculated in order to evaluate the reactivity and stability of all studied compounds. The interaction of the HOO˙ radical with α-terpinene, an example molecule, was also studied in detail by establishing a potential energy surface (PES). As a result, a kinetic concurrence between the H-abstraction reaction at the weakest C–H bond and addition reactions at the CC double bonds was clarified. On the basis of this mechanism, the antioxidant capacity may happen via the termination-enhancing process. Among the studied compounds, α-terpinene, γ-terpinene, cembrene and abieta-7,13-diene represent potential antioxidants.

A DFT analysis on the radical scavenging activity of oxygenated terpenoids present in the extract of the buds of Cleistocalyx operculatus

The antioxidant capacity of twenty-one oxygenated monoterpene and oxygenated desquiterpene compounds in the extract from Cleistocalyx operculatus has been computationally evaluated. Calculated by (RO)B3LYP/6-311++G(2df,2p)//B3LYP/6-311G(d,p) model chemistries, the thermochemical parameters, namely BDE, IE, PDE, PA and ETE in the gas phase, water and ethanol were determined. In addition, quantum descriptors, which allow the evaluation of the reactivity and stability of the resulting radicals like chemical potential (µ), hardness (η) and global electrophilicity (ω) were also computed. Potential energy surfaces of the reactions between CH3OO˙ and HO˙ radicals with falcarinol and α-vetivone, two typical and potential antioxidant molecules, were established to give more insight into the antioxidant mechanism. The obtained results underline falcarinol as the most effective antioxidant with the lowest BDE of 66.5 kcal mol−1 and PA of 341.3 kcal mol−1 in the gas phase. Among the reactions on falcarinol, the H-abstraction at C3–H position by both CH3OO˙ and HO˙ radicals are favorable with the energy barriers of −18.7 and −54.7 kcal mol−1, respectively. Moreover, NBO analysis helps to clarify the mechanism of antioxidant action which shows that the third lone pair of electrons on O1-atom of CH3OO˙ radical is donated to an unoccupied σ*-antibonding orbital on C3–H, and on the C4C5, C6C7 triple bonds. Similarly, attack of CH3OO˙ and HO˙ radicals on α-vetivone demonstrates that H-abstraction reactions are also more feasible than the addition ones with ΔH values of −7.3 and −41.9 kcal mol−1, respectively. For all considered reactions, the antioxidant molecules preferentially interact with HO˙ radical.

Theoretical Investigation on Antioxidant Activity of Phenolic Compounds Extracted from Artocarpus Altilis

Theoretical calculations have been performed to predict antioxidant property for phenolic compounds extracted from Artocarpus Altilis. The chosen ONIOM model contains only two atoms of the breaking bond as the core zone and is able to provide reliable evaluation for BDE(O–H) for phenolic compunds. Important characteristics of antioxidants such as the homolytic O-H bond dissociation enthalpy (BDE) and the adiabatic ionization energy (IE) were determined both in gas phase and in solvents. The BDE(O–H) values of compounds 1, 2, 3 and 4 are predicted to be 80.8, 80.3, 79.3, and 77.3 kcal/mol, respectively.

Functionalization of fullerene via the Bingel reaction with α-chlorocarbanions: an ONIOM approach

Two-layer ONIOM method at the ONIOM(B3LYP/6-31G(d):PM6) level of theory was applied to study the cycloaddition reaction of α-chlorocarbanions (CR2Cl─, where R is H, Cl, CH3 CN, and NO2) and fullerene. The results show that the reaction pathways depend on the electron withdrawing functional groups or the electron donating functional groups contained in the α-chlorocarbanions. The energy profile analysis reveals that functionalization of fullerene by CCl3─, C(CH3)2Cl─, and CH2Cl─ is more favorable than by C(CN)2Cl─ and C(NO2)2Cl─ in terms of the thermodynamic point of view.

Density functional theory study of the role of benzylic hydrogen atoms in the antioxidant properties of lignans

Antioxidants are a diverse group of chemicals with proven health benefits and thus potential preventive medicine and therapeutic applications. While most of these compounds are natural products, determining their mechanism of radical scavenging and common motifs that contribute to antioxidant activity would allow the rational design of novel antioxidants. Here the origins of the antioxidant properties of ten natural products of the lignan family were studied in silico by calculating their thermochemical properties by using ROB3LYP/6-311++G(2df,2p)//B3LYP/6-311G(d,p) model chemistry. Three conditions were modelled: gas phase, ethanol and water solvents. The results allowed assigning the antioxidant activity to specific moieties and structural features of these compounds. It was found that the benzylic hydrogen atoms are the most likely to be abstracted to form radicals and hence define antioxidant properties in most of the studied compounds. The results also suggested that the most likely mechanism of HOO• radical scavenging differs by the key moiety: it is hydrogen atom transfer in case the benzylic C-H bonds, however it is proton coupled electron transfer in case of the compounds where O-H bonds are responsible for radical scavenging.

Synthesis of aromatic and indole alpha-glucosinolates

Aromatic and indole glucosinolates are important members of the glucosinolate family of compounds du to their potential medicinal properties. They are known to exert antioxidant and anti-carcinogenic activity either by the natural products themselves, or their metabolic products including indole-3-carbinol and isothiocyanates. Natural glucosinolates are all β-glucosinolates; however, α-glucosinolates are also promising compounds for medicinal applications and hence have to be produced synthetically for any bio-activity studies. Here we report on the successful synthesis of a series of α-glucosinolates: α-neoglucobrassicin, α-4-methoxyglucobrassicin, 2,3-dichlorophenyl-α-glucosinolate for the first time. Testing for anti-inflammatory properties of these synthetic GLs, however, did not yield the expected activity.

Is Vitamin A an Antioxidant or a Pro-oxidant?

Antioxidant efficiency of all-trans-retinol has been studied on the basis of characteristic thermochemical properties using density functional theory. The influence of the solvent polarity has also been evaluated. It is found that retinol may act in parallel as an effective antioxidant via H atom donating as well as a pro-oxidant in yielding reactive hydroxyl radical. In fact, the lowest values of bond dissociation enthalpy were found at the C18-H and C18-OH positions. Retinol was also determined to be a good electron donor but bad acceptor in the single electron transfer (ET) reaction with hydroperoxyl (HOO•) radical. In addition, potential energy surfaces of H atom transfer (HAT) and radical adduct formation (RAF) reactions between retinol and HOO• radical were also investigated in the gas phase and in the solvent. The results demonstrated that the RAF mechanism was generally more predominant than ET and HAT ones. The most favored radical addition position was found at the C2═C3 double bond in the cyclohexenyl ring. Moreover, the radical scavenging reactivity via RAF reactions was strongly exergonic and thermodynamically feasible while the ET one was endergonic. Natural bond orbital analysis showed that the lone pairs of electrons on the oxygen atom of the HOO• radical were donated to the unoccupied antibonding orbital of transferred H atom in HAT reactions. In contrast, in the case of RAF reactions, strong interactions between 2p orbitals on oxygen atoms of the radical and the π orbital of the double bond on the retinol molecule were recognized. The results obtained in this work were in agreement with previous experimental observations.