全始算研究：CH3N游離能計算及CH3N化學吸附於銅(110)表面之理論分析

Abstract

我們使用量子化學的方法計算游離能。根據游離能計算的策略，我們定義了游離能為，將基態的分子游離的能量；而非穩定的基態與穩定的游離態的能量差。 應用此計算策略，我們以MP2、MP4、QCISD和G2+的方法，計算了三重態CH3N的游離能。與實驗觀測值比較，G2+方法可以得到最好的計算結果，進而可以判斷光電光譜上，每一個能量訊號所代表之離子態。離子態可以分子軌域表示，更可因此確認CH3N的電子組態。 電子組態可以輔助我們對CH3N的化學性質作假設，特別是CH3N在銅表面上的化學吸附。在台大凝態中心表面科學實驗室已進行一系列CH3N吸附在Cu(110)表面的實驗。為了解釋實驗觀測到原子級尺度的結果，我們以密度泛函理論，對CH3N吸附在Cu(110)表面做理論計算。我們以銅原子組成的團簇來模擬Cu(110)的表面，並以LANL2DZ的基底函數詮釋銅；同時以6-31G**的基底函數詮釋CH3N。將計算的結果與實驗結果作比較，我們討論CH3N吸附在Cu(110)表面的機制，是以共價鍵的方式鍵結。而離子性質影響了氫與銅間的作用力，會使得C-H的震動頻率分裂。

The calculation of ionization energies has been performed by using quantum chemistry calculation methods. As a calculation strategy of ionization energies, we have defined that the ionization energies can be considered as how large energies can ionize the molecule, that is, this is not the simple energy difference between stable ground state and stable ionic states. Using the strategy, we have calculated the ionization energies of triplet methylnitrene by applying MP2, MP4, QCISD, and G2+ methods; the G2+ method has shown a good agreement with the experimentally observed ionization energies and we have assigned the origin of the cation states appeared in a photoelectron spectrum [18]. The ionic states can be represented by molecular orbitals (MOs) and we have confirmed the electronic configuration of methylnitrene [1, 3]. The electronic configuration can allow us to assume the chemical properties of methylnitrene, especially for methylnitrene adsorbed on copper surface. There have been a series of experiments about CH3N chemical adsorbing on Cu(110) surface in surface science research group at National Taiwan University [23- 25]. In order to interpret the experimentally observed results in the atomic scale, we have performed the calculation of CH3N on Cu(110) using density functional theory (DFT). We have simulated the Cu(110) surface for various models of Cu cluster by adopting effective core potentials (ECPs), LANL2DZ; while the CH3N is treated with 6-31G** basis set. The calculated results of the cluster models are compared with the experimental results. Based on the comparison, we have discussed that the adsorption mechanism of CH3N on Cu(110) can be a formation of covalence bond between N and Cu, and the ionic properties can affect the H-Cu interaction that makes the degenerate C-H stretching vibrational frequencies split.