Accurate Prediction of Hyperfine Coupling Constants in Muoniated and Hydrogenated Ethyl Radicals: Ab Initio Path Integral Simulation Study with Density Functional Theory Method
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- 作者：Kenta Yamada ; Yukio Kawashima ; Masanori Tachikawa
- 刊名：Journal of Chemical Theory and Computation
- 出版年：2014
- 出版时间：May 13, 2014
- 年：2014
- 卷：10
- 期：5
- 页码：2005-2015
- 全文大小：469K
- 年卷期：v.10,no.5(May 13, 2014)
- ISSN：1549-9626
文摘
We performed ab initio path integral molecular dynamics (PIMD) simulations with a density functional theory (DFT) method to accurately predict hyperfine coupling constants (HFCCs) in the ethyl radical (C尾H3鈥揅伪H2) and its Mu-substituted (muoniated) compound (C尾H2Mu鈥揅伪H2). The substitution of a Mu atom, an ultralight isotope of the H atom, with larger nuclear quantum effect is expected to strongly affect the nature of the ethyl radical. The static conventional DFT calculations of C尾H3鈥揅伪H2 find that the elongation of one C尾鈥揌 bond causes a change in the shape of potential energy curve along the rotational angle via the imbalance of attractive and repulsive interactions between the methyl and methylene groups. Investigation of the methyl-group behavior including the nuclear quantum and thermal effects shows that an unbalanced C尾H2Mu group with the elongated C尾鈥揗u bond rotates around the C尾鈥揅伪 bond in a muoniated ethyl radical, quite differently from the C尾H3 group with the three equivalent C尾鈥揌 bonds in the ethyl radical. These rotations couple with other molecular motions such as the methylene-group rocking motion (inversion), leading to difficulties in reproducing the corresponding barrier heights. Our PIMD simulations successfully predict the barrier heights to be close to the experimental values and provide a significant improvement in muon and proton HFCCs given by the static conventional DFT method. Further investigation reveals that the C尾鈥揗u/H stretching motion, methyl-group rotation, methylene-group rocking motion, and HFCC values deeply intertwine with each other. Because these motions are different between the radicals, a proper description of the structural fluctuations reflecting the nuclear quantum and thermal effects is vital to evaluate HFCC values in theory to be comparable to the experimental ones. Accordingly, a fundamental difference in HFCC between the radicals arises from their intrinsic molecular motions at a finite temperature, in particular the methyl-group behavior.