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Author: search.filters.author.Ngaojampa, Chanisorn

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    Nuclear quantum and H/D isotope effects on three-centered bonding diborane: Path integral molecular dynamics simulations
    (2020-05-15)
    Daengngern, Rathawat
    ;
    Kobayashi, Osamu
    ;
    Kungwan, Nawee
    ;
    Ngaojampa, Chanisorn
    ;
    Tachikawa, Masanori
    Nuclear quantum and H/D isotope effects of bridging and terminal hydrogen atoms of diborane (B<inf>2</inf>H<inf>6</inf>) molecules were systematically studied by classical ab initio molecular dynamics (CLMD) and ab initio path integral molecular dynamics (PIMD) simulations with BHandHLYP/6-31++G** level of theory at room temperature (298.15 K). Calculated results clearly show that H/D isotope effect appears in the distribution of hydrogen (deuterium) of B<inf>2</inf>H<inf>6</inf> (B<inf>2</inf>D<inf>6</inf>). Geometry of B<inf>2</inf>H<inf>6</inf> also plays a significant role in the nuclear quantum effect proved by PIMD simulations, but slightly deviated from its equilibrium structure when simulated via CLMD simulation. The bond lengths between boron atoms R (B1 … B2) and the bridging hydrogen atoms R<inf>HH</inf> (H<inf>B</inf>1 … H<inf>B</inf>2) of the B<inf>2</inf>H<inf>6</inf> molecule obtained from PIMD simulations are slightly longer than those of the deuterated form of the diborane (B<inf>2</inf>D<inf>6</inf>) molecule. The principal component analysis (PCA) was also employed to distinguish the important modes of bridging hydrogen as related to the nuclear quantum and H/D isotope effects. The highest level of contribution obtained from PCA of PIMD simulations is bending, while various mixed vibrations with less contribution were also found. Therefore, the nuclear quantum and H/D isotope effects need to be taken into account for a better understanding of diborane geometry.
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    Heteroatom effect on photophysical properties of 2-(2′-hydroxyphenyl)benzimidazole and its derivatives as fluorescent dyes: A TD-DFT study
    (2017-08-01)
    Manojai, Natthaporn
    ;
    Daengngern, Rathawat
    ;
    Kerdpol, Khanittha
    ;
    Ngaojampa, Chanisorn
    ;
    Kungwan, Nawee
    The effects of hetero nitrogen substitution on the photophysical properties and excited-state intramolecular proton transfer (ESIPT) of HBX derivatives (HBI, HBO and HBT where X=NH, O and S, respectively) were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations at B3LYP/6-311+G(d,p) level, which is the suitable method of choice validated from various density functionals. Geometries and absorption spectra of all derivatives as well as emission spectra of selected derivatives were calculated at the same level of theory. The hetero nitrogen substitution of HBX resulted in the red shift emission spectra, which implies that the lone pair of electron in the substituted nitrogen has the effect on the π-conjugated system. In addition, results of frontier molecular orbital analysis show that vertical S<inf>0</inf>→S<inf>1</inf> transition of these molecules corresponds essentially to the excitation from HOMO (π) to LUMO (π*). The potential energy curves (PECs) of selected derivatives were used to investigate the occurrence of ESIPT and the chance was found in this order: HBT>HBI>HBO. The hetero nitrogen substituted on 1,4 and 3,4 position of HBX derivatives could easily facilitate the ESIPT process, because the nitrogen substitution makes O–H bond of selected HBX derivatives weaker than those of other derivatives and the hydrogen bond in N–H becomes stronger in the excited state, resulting in lower PT barrier. The obtained information of the electronic structure, the photophysical property and the chance of ESIPT of hetero nitrogen substituted HBX molecules is useful for molecular design of fluorescent molecular probes.