環状メタノール4分子クラスターの配座異性体

Abstract

メタノール4分子の環状クラスター(CH₃OH)₄のOH基とCを同一面内に配置した初期構造から構造最適化計算を行った．HF/STO-3Gでは，初期構造のO-O間距離が2.7 Å以上の場合に最安定構造が高い頻度で得られる．O-O間距離が短い場合には他の安定な配座異性体が2種類得られる．この約2.7 Åの距離はOH基の水素結合力の範囲を示唆する．HF/6-31G (d)では，虚振動を持つ平面構造で収束する．B3LYP/6-31G (d)では，メチル基が環面外の同じ側に向いた構造で収束する．これらの結果は，酸素の非共有電子対の軌道の広がりによる安定化とメチル基の反発力の均衡から説明でき，配座異性体同士の変換傾向を反映する．計算過程で力定数を計算(calcall)すると初期O-O間距離に関わらず最安定構造が得られる．

Translated Abstract

We investigate optimized structures of a cyclic methanol cluster (CH₃OH)₄ by starting different-sized planar symmetric structures, where all the H atoms of the OH groups direct to the next O atom, and all the COH parts are in-plane. At the HF/STO-3G level, initial O-O distances at 2.7 Å and more lead to the most stable structure, whereas those less than 2.7 Å result in two other stable conformers. The border distance indicates the efficient range of hydrogen bonding forces of the OH groups. HF/6-31G (d) results in a planar structure with negative frequencies, whereas B3LYP/6-31G (d) gives a conformer whose methyl groups face the same direction. These structures result from the balance between the stabilization due to the orbital broadening of unshared electron pairs of O and the repulsive force of the methyl groups, reflecting the tendency for transformation among the conformers. To calculate force constants with the calcall option is effective in obtaining the most stable structure regardless of the initial O-O distance.

Figures

Figure 1.

 Typical conformers of (CH₃OH)₄ optimized by HF/STO-3G method. TE denotes total energy.

Figure 2.

 A starting structure of a cyclic methanol cluster, (CH₃OH)₄ (O-O distance = 2.70 Å).

Figure 3.

 Dependency of total energy of the structures optimized by HF/STO-3G on the O-O distance of starting adjacent methanol molecules for (CH₃OH)₄. The three total energy values are specific for type udud (●), uddd (×), and dddd (▲).

Figure 4.

 Some of the optimized structures with highest occupied molecular orbitals (isovalue = 0.005) for (CH₃OH)₄.

Figure 5.

 Dependency of optimized structures on the O-O distance of starting adjacent methanol molecules for (CH₃OH)₄ calculated by different methods and basis sets. The obtained structures are specific for type udud (●), uddd (×), dddd (▲), mmmm (◇), and transition-state dddd (△).

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