- 期刊
羰基化合物之兩個鍵結研究：α位斷鍵能量及羰基π電子之氫鍵受體能力
Two Bonding Studies on Carbonyl Compounds: α-Cleavage Bond Dissociation Energies and Hydrogen Bond Acceptor Capability of Carbonyl π-Electrons
摘要
我們使用的B3LYP/6-311G**方法研究一系列羰基分子(H_2AB, A=C, Si, Ge; B=O, S, Se)，與鹵素取代化合物之α位斷鍵能量(BDE)。我們顯示第二個BDE比第一個來的弱的原因在於AB分子的電子組態由雙自由基態轉化為基態的舒解效應，兩鏈能差值(∆BDE)只與AB有關，幾乎與取代基種類無關，另外，XAB自由基中A-X鍵雖弱，但鍵距正常可用位能面的avoided-crossing來解釋。第二個主題則是有關於羰基π電子的討論。由於尿素二聚體可呈現兩單體共平面(θ=0°)或是垂直(θ=90°)之結構，故尿素分子之羰基π電子也可做為氫鍵受體。兩種構形是分別利用氧原子平面上之非鍵結電子以及羰基上的π電子來做為雙重氫鍵之受體。我們也發現π與體取代基可有效地增強羰基π電子做為氫鍵受體之能力。尿素二聚體之θ=0°以及θ=90°之兩構形具有幾乎相同之鍵能，而對於R_2CO．尿素系列(R=H、F、CI、NH_2、HNHC)而言，此兩種構形之能量差可以歸因於HOMO-LUMO形式之非庫倫作用力。
並列摘要
We made B3LYP/6-311G** theoretical study on the bond dissociation energy (BDE) for the carbonyl compounds (H_2AB, A = C, Si, Ge, B = O, S, Se), monohalogen and dihalogen compounds. We demonstrate that the second BDE in much weaker than the first one due to the electronic relaxation effect on the AB fragment. The difference in BDE (∆BDE) appears to be a property of AB alone, nearly independent of the substituents involved. The normal bond length of the second bond, the weak A-X bond in XAB radical can be interpreted as a result of potential surface avoided-crossing. For our second topics, it appears that the carbonyl π-electrons of urea molecule is a good hydrogen bond acceptor since the bound urea dimer have two monomers either coplanar (θ = 0°) or perpendicular (θ = 90°). They use oxygen in-plane nonbonding electrons and carbonyl π-electrons, respectively, as a bi-dentate hydrogen bond acceptor. We find that the π-donor substituents can enhance effectively the hydrogen bond acceptor capability for the carbonyl π-electrons. The urea dimer has nearly equal bonding energies for the two conformers, θ= 0° and θ= 90°. For the dimer series R_2CO．urea (R = H, F, CI, NH_2, HNHC), the energy difference for the two conformers can be accounted for by the HOMO-LUMO type noncoulombic interaction.