Verification of Kinetic Theoretical Prediction of Diffusion-influenced Reversible

양민오,신국재,Yang, Min O,Sin, Guk Jae Korean Chemical Society 2000 Bulletin of the Korean Chemical Society Vol.21 No.1

A diffusion-influenced pseudo-first order reversible reaction A + B ⇔C + B is investigated by the molecular dynamics (MD) simulation method. Theoretical finding that the temporal evolution of reactants [conditional probabilities] in the reversible system can be expressed by the irreversible survival probability with an effective rate parameter is confirmed even in the presence of solvent particles. We carry out molecular dynamics simulations for both the irreversible and the reversible cases to evaluate the survival and the conditional probabilities for each cases. When the resultant irreversible survival probability is inserted into the proposed relation, the conditional probabilities given by the simulation are exactly reproduced.

Reduced Density Matrix Theory for Vibrational Absorption Line Shape in Energy Transfer Systems: Non-Condon Effects in Water

양민오 대한화학회 2011 Bulletin of the Korean Chemical Society Vol.32 No.2

Using the projection operator technique, a reduced density matrix theory for linear absorption spectrum of energy transfer systems is developed for the theoretical absorption line shape of the systems with non-Condon transitions. As an application, we considered a model system of OH vibrations of water. In the present model calculation, the OH vibration modes are coupled to each other via intra-molecular coupling mechanism while their intermolecular couplings are turned off. The time-correlation functions appearing in the formulation are calculated from a mixed quantum/classical mechanics method. The present theory is successful in reproducing the exact absorption line shape. Also the present theory was improved from an existing approximate theory, time-averaged approximation approach.

What Function Is Optimal for CCSD(T) Complete Basis Extrapolation for OH Stretching Potential of Water Molecules?

양민오 대한화학회 2018 Bulletin of the Korean Chemical Society Vol.39 No.1

A procedure to calculate the complete basis set limits of electronic energy of water molecules is developed based on two levels of extrapolation scheme. A three-point fitting scheme with the energies computed at the levels of basis function, aug-cc-pVXZ for X = 4–6, is employed to obtain accurate energies of a monomer and a dimer of water. Three extrapolation functions considered in this article are found to work excellently to give accurate Hartree–Fock (HF) energies of water molecules. However, the three functions give slightly different values up to 1–2 mHartree for the complete basis limit of electronic correlation energy. The best performing extrapolating function for the five-point fitting scheme with a larger set of basis functions for X = 2–6 has been determined aiming for the extrapolation from the lowest two levels of basis sets, double and triple zeta basis functions. For this purpose, an exponential function is suggested for HF energy while an exponential function with a square root exponent is suggested for electronic correlation energy. The optimal exponents of the two functions have been determined.

Memory Equations for Kinetics of Diffusion-Influenced Reactions

양민오 대한화학회 2006 Bulletin of the Korean Chemical Society Vol.27 No.10

A many-body master equation is constructed by incorporating stochastic terms responsible for chemical reactions into the many-body Smoluchowski equation. Two forms of Langevin-type of memory equations describing the time evolution of dynamical variables under the influence of time-independent perturbation with an arbitrary intensity are derived. One form is convenient in obtaining the dynamics approaching the steady-state attained by the perturbation and the other in describing the fluctuation dynamics at the steady-state and consequently in obtaining the linear response of the system at the steady-state to time-dependent perturbation. In both cases, the kinetics of statistical averages of variables is found to be obtained by analyzing the dynamics of time-correlation functions of the variables.

Effects of Coulombic Interaction in Diffusion-influenced Reversible Proton Transfer Kinetics of Photoexcited Acids

양민오 대한화학회 2015 Bulletin of the Korean Chemical Society Vol.36 No.8

A kinetic theory based on the memory equation approach is developed to predict the kinetics of photoexcited acid dissociation and diffusion-influenced recombination reactions between ionic species. It is shown that the simple prescription introducing an effective reaction distance to represent the Coulombic interactions is applicable to the formalism of the kinetic theory. For geminate pairs, approximate analytic solutions for the time-dependent concentrations of excited acids and basic anions are derived, and their long-time asymptotic behavior is discussed. Kinetic behavior depending on the magnitude of the Coulombic interaction between excited anions and protons is also discussed.

Steady-State Fluorescence Intensity of Diffusion-Influenced Reversible Excited Acid–Base Reactions

양민오 대한화학회 2020 Bulletin of the Korean Chemical Society Vol.41 No.1

Effect of diffusion on steady-state fluorescence from excited acids and bases.

Dynamics of Resonant Energy Transfer in OH Vibrations of Liquid Water

양민오 대한화학회 2012 Bulletin of the Korean Chemical Society Vol.33 No.3

Energy transfer dynamics of excited vibrational energy of OH stretching bonds in liquid water is theoretically studied. With time-dependent vibrational Hamiltonian obtained from a mixed quantum/classical calculation, we construct a master equation describing the energy transfer dynamics. Survival probability predicted by the master equation is compared with numerically exact one and we found that incoherent picture of energy transfer is reasonably valid for long-time population dynamics. Within the incoherent picture, we assess the validity of independent pair approximation (IPA) often introduced in the theoretical models utilized in the analysis of experimental data. Our results support that the IPA is almost perfectly valid as applied for the vibrational energy transfer in liquid water. However, proper incorporation of radial and orientational correlations between two OH bonds is found to be critical for a theory to be quantitatively valid. Consequently, it is suggested that the Fster model should be generalized by including the effects of the pair correlations in order to be applied for vibrational energy transfer in liquid water.

수소 결합한 물 분자에서 OH 신축 진동의 국소모드와 정규모드

권세은,양민오 대한화학회 2020 대한화학회지 Vol.64 No.6

The validity of the calculation method based on the local mode in hydrogen-bonded water molecules was investigated by comparing the frequencies of the local and normal modes of OH stretching vibration in water molecules. By calculating a monomer, dimer, and trimer of water molecules using a quantum chemical ab initio theory, we examined how the frequencies of the local and normal modes and the anharmonicity of local modes vary with molecular cluster size. It was shown that, as the number of molecules increases from monomer to trimer, the anharmonicity of OH bonds increases and the difference between local and normal mode frequencies decreases. This confirms that local-mode-based calculations that can easily handle the anharmonicity can be appropriate for the calculation of the OH stretching frequency of water molecules in the condensed phase. 물 분자내의 OH 신축진동(stretching vibration) 운동을 나타내는 정규모드(normal mode)와 국소모드(local mode) 진동수 들을 비교하여 수소결합한 물 분자에 대한 국소모드에 기반한 계산의 타당성을 조사하였다. 물 분자의 단량체, 이합체, 삼량체 에 대한 계산을 수행하여 분자 클러스터 크기가 커짐에 따라 국소모드 진동수, 국소모드의 비조화성, 그리고 국소모드와 정규모 드 진동수들의 유사성이 어떤 경향성을 보이는지 순이론적 양자화학 계산 방법으로 연구하였다. 단량체에서 삼량체로 분자의 갯수가 증가할수록 OH 결합의 비조화성은 증가하며 국소모드와 정규모드 진동수 간의 차이는 줄어드는 것으로 나타났다. 따라서, 응축상에 존재하는 물 분자들의 OH 신축 진동수의 이론적 계산은 비조화성을 쉽게 다룰 수 있는 국소모드에 기반한 방식이 적 절할 수 있음을 확인하였다.

용액상 색소분자의 흡수스펙트럼에 대한 이론적 연구

우정문,양민오,Woo, Jung-Moon,Yang, Min-O 대한화학회 2008 대한화학회지 Vol.52 No.1

분자들의 전자 흡수스펙트럼에 영향을 주는 분자 핵 운동은 이론적으로 양자동역학적 시간상관함수로 표현된다. 본 연구에서는 분자동역학 전산모사와 양자화학적 계산등의 순 이론적 계산을 이용해 고전역학적 시간상관함수를 구하고 두 가지의 준고전역학적 근사방법으로 양자동역학적 시간상관함수를 얻었다. 또한 이차 축적전개 근사식을 이용하여 액체상에 있는 nile blue 색소분자의 전자전이 흡수스펙트럼을 얻었다. 계산 결과는 실험에서 얻은 스펙트럼과 비교적 잘 일치하였으며, 실험 결과와의 비교를 통해, 본 계의 용매화 동역학의 시간 척도는 1ps 보다 길며 색소분자에 인접한 용매분자들이 용매화에 영향을 주는 주 성분임을 확인하였다. Molecular motion influencing the absorption spectrum of a chromophore in liquid is theoretically described by a quantum mechanical time correlation function. In the present paper, we developed a theoretical method to calculate such a quantum mechanical time-correlation function from a classical time-correlation function using semi-classical approximations. The calculated time-correlation function was combined with the second order cumulant expansion method to calculate the absorption spectrum of nile blue in acetonitrile. Reasonably good agreement with experimental spectrum was obtained. From the comparison with experimental spectrum, we concluded that the time scale of solvation dynamics of the system should be longer then 1ps and the first shell of solvent is the major contribution to the solvation dynamics.

Density Functional Theory for Calculating the OH Stretching Frequency of Water Molecules

전기영,양민오 대한화학회 2016 대한화학회지 Vol.60 No.6

The anharmonic frequency of a local OH stretching mode of a water monomer and dimer was calculated using various levels of density functional theory. The quantum chemical potential energy curves as a function of the OH bond distance were calculated, and they were fitted with the Morse potential function to analytically obtain the fundamental transition frequency. By comparing those values with the frequencies similarly calculated using an ab initio quantum chemical method, the coupled cluster theory including both single and double excitations with the perturbative inclusion of triple excitation in the complete basis limit, the accuracy of various density functional methods in the calculation of anharmonic vibration frequency of water molecules was assessed. For a water monomer, X3LYP and B3LYP methods give the best accuracy, whereas for a water dimer, B972, LCBLYP, ωB97X, ωB97 methods show the best performance.