本文藉由密度泛函數理論（Density Functional Theory, DFT）和天然鍵性軌域（Natural Bond Orbital, NBO）分析來探討Y•••H-X氫鍵對不正常鹼基酸性的影響。不正常鹼基包含尿嘧啶、黃嘌呤和次黃嘌呤，以及失去氫原子之後所形成的尿嘧啶離子、黃嘌呤離子和次黃嘌呤離子，而每一個鹼基可以透過三個不同的位置與HF、H2O和NH3結合而形成氫鍵。
大多數鹼基錯合物在失去氫原子而形成離子錯合物之後，會變得比較穩定，而由Y•••H間距的縮短及H-X鍵的拉長也可看出所形成的鹼基離子錯合物比較穩定。若不考慮結合位置，氫鍵穩定能大小依序為：HF-鹼基錯合物 > H2O-鹼基錯合物 > NH3-鹼基錯合物；透過決定係數（R2）則可以得知氫鍵穩定能與不正常鹼基氫原子斷鍵能有正相關性。結果顯示不正常鹼基與HF、H2O和NH3所形成的氫鍵越強，則氫原子斷鍵能越小，酸性越大。
透過天然鍵性軌域分析，可以得知分子內電子非定域化的大小（E(2)值）和氫鍵穩定能、Y•••H間距及H-X鍵長之間有不錯的相關性。結果顯示鹼基在形成氫鍵之後的電子非定域化越大，則氫鍵穩定能越好、Y•••H間距越短且H-X鍵越長。NBO分析的其他數據，如X-H鍵中X的s-特徵值、X-H反鍵結軌域中氫的佔有機率，以及兩個軌域之間的Fock矩陣值，皆可以解釋電子非定域化大小改變的現象。

The effects of Y•••H-X hydrogen bonding on the acidity of abnormal bases has been studied through Density Functional Theory (DFT) and Natural Bond Orbital (NBO) analysis. Abnormal bases include uracil, xanthine, and hypoxanthine, as well as uracil, xanthine, and hypoxanthine ions when they lost H-atom, and each base could combine with HF, H2O, and NH3 to form hydrogen bonding through three different positions.
Most base complexes would become stable when they lost H-atom, shorten the Y•••H distance, and stretch the H-X bond-length. Without regard to the combination of positions, stabilization energies of hydrogen bonding are the following trend: HF-base complexes > H2O-base complexes > NH3-base complexes; it could be seen from the coefficient of determination (R-squared values) that stabilization energies of hydrogen bonding have positive correlation with energies of H-atom leaving. The results show that the stronger hydrogen bonding of abnormal bases combined with HF, H2O, and NH3 is, the smaller energies of H-atom leaving of bases are, and the larger the acidity of bases is.
Through NBO analysis, it could be known that electronic delocalization [E(2)] would have good correlation with energies of H-atom leaving, Y•••H distances, and H-X bond-lengths. The results show that, when forming hydrogen bonding of bases, the larger electronic delocalization is, the larger the energy of H-atom leaving is, the shorter the Y•••H distance is, and the more stretching the H-X bond-length is. The other data of NBO analysis, such as s-character of X in X-H bond, the rate of occupation of H-atom in X-H antibonding, and the Fock matrix between two orbitals, could also explain the phenomenon of changes of electronic delocalization.

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