Nguyen Tien Trung

Decomposition pathways of the neutral and protonated formamide in some lower-lying excited states.

Unimolecular decompositions of neutral (NH2CHO) and protonated (NH3CHO(+)) formamide, an active precursor of biomolecules in prebiotic chemistry, are investigated in the ground (S0) and first triplet (T1) and singlet (S1) excited states. Different decomposition channels including the homolytic bond dissociations, dehydration, decarbonylation, dehydrogenation, etc., are explored using coupled-cluster theory (CCSD(T)/CBS method) for both S0 and T1 states and RASPT2(18,15)/6-31G(d,p) computations for the S1 state. On S1 and T1 energy surfaces, formamide preferentially follows C-N homolytic bond cleavages forming NH2 + HCO radical pairs. Formation of HCN and HNC from dehydration of neutral and protonated formamide via formimic acid and aminohydroxymethylene isomers has higher energy barriers. A strong stabilization upon triplet excitation of the two latter isomers significantly facilitates the interconversions between isomers, and thus considerably reduces the energy barriers for dehydration pathways. The most probable pathways for HCN and HNC generation are found to be dehydration of formamide in the T1 state. Dehydration pathways from the neutral S1 and protonated T1 forms lead to stable complexes of HCN and HNC with water but are associated with large energy barriers. Overall, in the lower-lying excited states of either neutral or protonated formamide, dehydration is not competitive with homolytic C-N bond cleavages, which finally lead to formation of CO.

Complexes of carbon dioxide with dihalogenated ethylenes: structure, stability and interaction

Interactions of ethylene and its 1,2-dihalogenated derivatives with CO2 induce the formation of twenty four molecular complexes with stabilization energies in the range of 1.1 to 7.5 kJ mol−1 as computed at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level. The stability of the parent C2H4⋯CO2 complex is due to a π⋯π* interaction which has not yet been reported in the complexes of CO2-philic compounds and CO2. The cis-XCHCHX⋯CO2 complexes are found to be slightly more stable than the trans-XCHCHX⋯CO2, with X = F, Cl and Br. Generally, the overall stabilization energy of each complex is determined by the C–H⋯O hydrogen bond and the C–X⋯C Lewis acid–base interaction, in which the latter plays a larger role. Substitution of two H atoms in CH2CH2 by the same halogen atoms stabilizes the complexes XCHCHX⋯CO2, and for the same dihalogenated derivatives, the stability of XCHCHX⋯CO2 tends to increase from X = F via Cl and to Br. The obtained results suggest that the contraction of the C–H bond involved in the C–H⋯O hydrogen bond and the blue-shift of its stretching frequency depend not only on a polarization of the C–H bond in the isolated monomer but also on the geometric shape of the complex formed.

Theoretical analysis of the (HNO) 2 , (HNO HNS), and (HNS) 2 dimers — A case of red and blue shifts of N-H stretching frequency

The hydrogen-bonded interactions in the simple (HNZ)2 dimers, with Z = O and S, were investigated using quantum chemical calculations with the second-order Moller–Plesset perturbation (MP2), couple...

Structure, stability and interactions in the complexes of carbonyls with cyanides

Our theoretical study at the CCSD(T)/aug-cc-pVTZ//MP2/aug-cc-pVDZ level for twenty-two stable structures of the interactions between aldehyde RCHO (R=H, CH3, NH2, F, Cl, Br) and cyanides XCN (X=H, F) found that the strength of these complexes is contributed by the C–H···O(N) and/or N–H···N hydrogen bonds and/or >C=O···C Lewis acid–base interaction. The stability of the RCHO···FCN complexes is mainly determined by the >C=O···C Lewis acid–base interaction, while the Csp–H···O hydrogen bond plays a decisive role in the RCHO···HCN complexes. For each complex of RCHO with either HCN or FCN, the substitution of R by CH3 or NH2 strengthens the complex, while the halogen substitution negligibly affects the complex’s stability, as compared to the non-substituted HCHO···XCN complexes. The decomposition analysis using SAPT2+ approach showed that the attractive electrostatic term plays the predominating role of up to 50% overcoming the induction and dispersion terms in stabilizing these complexes. Remarkably, the obtained results give further support to the observation that the change in C–H bond involved in the hydrogen bond and its stretching vibrational frequency depend both on the polarity of the C–H bond in the proton donor and on the gas-phase basicity of the proton acceptor.

An insight into Csp–H⋯π hydrogen bonds and stability of complexes formed by acetylene and its substituted derivatives with benzene and borazine

Theoretical calculations at the MP2/aug-cc-pVDZ level are used to investigate the Csp–H⋯π interactions of C2HX (X = H, F, Cl, Br, CH3, NH2) with C6H6 and B3N3H6 molecules. Twelve stable complexes with similar structures are observed, in which the (C2HX, C6H6) complex is always found to be more stable than the corresponding (C2HX, B3N3H6) complex. The C2HBr⋯C6H6 complex is the most stable, whereas the weakest one is C2HNH2⋯B3N3H6. When replacing one H atom in C2H2 by different X groups, the stability of the complexes increases in the order of NH2 < CH3 < H ≈ F < Cl < Br and is directly proportional to the polarization of the Csp–H bond in isolated monomers and π electron density of aromatic rings. The Csp–H⋯π hydrogen bonds in all the complexes belong to the red-shifting hydrogen bond, and the magnitude of the Csp–H stretching frequency red shift increases when one H atom of C2H2 is replaced by different X groups, except for the (C2HCH3, C6H6) complex. Remarkably, the SAPT analysis indicates that the contribution of dispersion energy towards the total stabilization energy is more important than the electrostatic interaction and other energy components. Substitution of one H atom in C2H2 by an electron-donor or withdrawing X group negligibly affects the role of the electrostatic and dispersion components in stabilizing (C2HX, C6H6) and (C2HX, B3N3H6) compared to (C2H2, C6H6) and (C2H2, B3N3H6) pairs, respectively.

THEORETICAL EVALUATION OF THE pKa VALUES OF 5-SUBSTITUED URACIL DERIVATIVES

Density functional theory (DFT) calculations using numerical basis sets were employed to predict the solvation energies, Gibbs free energies and pK a values of a series of 5-substituted uracil derivatives. Obtained results show that solvation energies are not significantly different between DFT methods using the numerical (DNP) and Gaussian basis set (aug-cc-pVTZ). It is noteworthy that the independent and suitable solvation energy of proton of -258.6 kcal/mol has been proposed for the evaluation of pK a values in conjunction with the numerical basis set. In addition, the calculated pK a values suggest that the anti-conformation of 5-formyluracil is the most stable form in the aqueous solution.

Interactions of formaldehyde and its substituted derivatives with HCN: Structure, stability and interaction

Twelve stable structures of the interactions of HCN with RCHO (R = H, F, Cl, Br, NH 2 , CH 3 ) are located on the potential energy surface at the MP2/aug-cc-pVDZ level. Interaction energies including both ZPE and BSSE corrections range from -5.80 to -21.07 kJ.mol -1 . The result of SAPT analysis shows that the electrostatic component has mainly contributed to the stability of the complexes. It is remarkable that the most stable complex of HCHO∙∙∙HCN is P1-H b which has not been reported in the literature. The red-shifting hydrogen bonds of the C-H∙∙∙O and N-H∙∙∙N types are observed in the P1-H b , P1-CH 3 , P1-NH 2 and P2-NH 2 complexes. On the other hand, the C-H∙∙∙N(O) blue-shifting hydrogen bonds are observed in the rest of complexes. The contraction of C-H bond and the blue shift of its stretching vibrational frequency are inversely proportional with its polarity in the isolated monomer. Keywords. Hydrogen bond, interaction energy, RCHO, HCN, QTAIM, SAPT.

A highly sensitive fluorescent chemosensor for simultaneous determination of Ag(I), Hg(II), and Cu(II) ions

Simultaneous determination of the heavy metal ions at the µg/L level in an aqueous solution has always been attractive to scientists. Here, we present a simple, rapid and highly sensitive fluorescent chemosensor for simultaneous determination of mercury, copper, and silver ions in an aqueous solution in the presence of various competitive metal ions, including Na + , K + , Ca 2+ , Ba 2+ , Mg 2+ , Pb 2+ , Cd 2+ , Co 2+ , Zn 2+ , Fe 2+ , Ni 2+ , Al 3+ , and Cr 3+ ions. The detection limits of the sensor for Hg 2+ , Cu 2+ , and Ag + ions are 2.8, 0.8, and 1.0µg/L, respectively. This sensor is potentially suitable for detecting and monitoring these metal ions in drinking water samples according to the regulations of the World Health Organization.

A fluorescent sensor based on dansyl-diethylenetriamine-thiourea conjugate: a through theoretical investigation

A new dansyl-diethylenetriamine-thiourea conjugate (DT) for detection of Hg 2+ ions in aqueous solution has been theoretically designed and compared to our previously published results. The synthetic path, the optimized geometric structure and the characteristics of the DT were found by the theoretical calculations at the B3LYP/LanL2DZ level. Accordingly, the DT can react with Hg 2+ ion to form a product with quenched fluorescence. It is remarkable that the experimental results are in an excellent agreement with the theoretically evaluated data. Keywords. Fluorescence, chemodosimeter, dansyl, diethylenetriamine, thiourea, Mercury, TD-DFT, PET.

Synthesis and characterisation of nanostructured TiO 2/SBA-15 and Ag-TiO 2/SBA-15 mesoporous composites

TiO2/SBA–15 composite photocatalysts with different TiO2 : SiO2 mass ratios of 25 : 75, 40 : 60, 50 : 50 and 75 : 25 have been successfully synthesised using hydrothermal method. Characterisation techniques included X–ray diffraction, transmission electron microscopy, X–ray photoelectron spectroscopy, and nitrogen adsorption–desorption experiments. Remarkably, the obtained products have highly ordered hexagonal mesoporous structure with uniform pore sizes and large specific surface areas. The TiO2 : SiO2 mass ratio of 50 : 50 as compared to the remaining ones is the most suitable for enhancing photocatalytic activity for the TiO2/SBA–15 material. It is noteworthy that in all the Ag–doped samples of TiO2/SBA–15 material (TiO2/SiO2=1) the 4 wt % Ag–TiO2/SBA–15 has the highest photocatalytic ability under sunlight.