Cooksy Andrew L, King Harry F, Richardson William H
Department of Chemistry, San Diego State University, San Diego, California 92182-1030, USA.
J Org Chem. 2003 Nov 28;68(24):9441-52. doi: 10.1021/jo035085b.
Detailed molecular orbital calculations were directed to the cyclopropylcarbinyl radical (1), the cyclopropoxy radical (2), and the cyclopropylaminium radical cation (3) as well as their ring-opened products. Since a considerable amount of data are published about cyclopropylcarbinyl radicals, calculations were made for this species and related ring-opened products as a reference for 2 and 3 and their reactions. Radicals 1-3 have practical utility as "radical clocks" that can be used to time other radical reactions. Radical 3 is of further interest in photoelectron-transfer processes where the back-electron-transfer process may be suppressed by rapid ring opening. Calculations have been carried out at the UHF/6-31G*, MP4//MP2/6-31G*, DFT B3LYP/6-31G*, and CCSD(T)/cc-pVTZ//QCISD/cc-pVDZ levels. Energies are corrected to 298 K, and the barriers between species are reported in terms of Arrhenius E(a) and log A values along with differences in enthalpies, free energies, and entropies. The CCSD(T)-calculated energy barrier for ring opening of 1 is E(a) = 9.70, DeltaG* = 8.49 kcal/mol, which compares favorably to the previously calculated value of E(a) = 9.53 kcal/mol by the G2 method, but is higher than an experimental value of 7.05 kcal/mol. Our CCSD(T)-calculated E(a) value is also higher by 1.8 kcal/mol than a previously reported CBS-RAD//B3LYP/6-31G* calculation. The cyclopropoxy radical has a very small barrier to ring opening (CCSD(T), E(a) = 0.64 kcal/mol) and should be a very sensitive time clock. Of the three series studied, the cyclopropylaminium radical cation is most complex. In agreement with experimental data, bisected cyclopropylaminium radical cation is not found, but instead a ring-opened species is found. A perpendicular cyclopropylaminium radical cation (4) was found as a transition-state structure. Rotation of the 2p orbital in 4 to the bisected array results in ring opening. The minimum onset energy of photoionization of cyclopropylamine was calculated to be 201.5 kcal/mol (CCSD(T)) compared to experimental values of between about 201 and 204 kcal/mol. Calculations were made on the closely related cyclopropylcarbinyl and bicyclobutonium cations. Stabilization of the bisected cyclopropylcarbinyl conformer relative to the perpendicular species is much greater for the cations (29.1 kcal/ mol, QCISD) compared to the radicals (3.10 kcal/mol, QCISD). A search was made for analogues to the bicyclobutonium cation in the radical series 1 and 2 and the radical cation series 3. No comparable species were found. A rationale was made for some conflicting calculations involving the cyclopropylcarbinyl and bicyclobutonium cations. The order of stability of the cyclopropyl-X radicals was calculated to be X = CH2 >> X = O > X = NH2+, where the latter species has no barrier for ring opening. The relative rate of ring opening for cyclopropyl-X radicals X = CH2 to X = O was calculated to be 3.1 x 10(6) s(-1) at 298 K (QCISD).
详细的分子轨道计算针对环丙基甲基自由基(1)、环丙氧基自由基(2)和环丙基铵自由基阳离子(3)及其开环产物。由于已发表了大量关于环丙基甲基自由基的数据,因此对该物种及其相关开环产物进行了计算,作为2和3及其反应的参考。自由基1 - 3作为“自由基时钟”具有实际用途,可用于确定其他自由基反应的时间。自由基3在光电子转移过程中更受关注,其中反向电子转移过程可能会因快速开环而受到抑制。计算是在UHF/6 - 31G*、MP4//MP2/6 - 31G*、DFT B3LYP/6 - 31G和CCSD(T)/cc - pVTZ//QCISD/cc - pVDZ水平上进行的。能量校正到298 K,并根据阿伦尼乌斯E(a)和log A值以及焓、自由能和熵的差异报告物种之间的势垒。CCSD(T)计算得出的1开环的能量势垒为E(a) = 9.70,ΔG = 8.49 kcal/mol,与之前用G2方法计算的E(a) = 9.53 kcal/mol的值相比有利,但高于实验值7.05 kcal/mol。我们CCSD(T)计算的E(a)值也比之前报道的CBS - RAD//B3LYP/6 - 31G*计算结果高1.8 kcal/mol。环丙氧基自由基的开环势垒非常小(CCSD(T),E(a) = 0.64 kcal/mol),应该是一个非常灵敏的时钟。在研究的三个系列中,环丙基铵自由基阳离子最为复杂。与实验数据一致,未发现平分的环丙基铵自由基阳离子,而是发现了一个开环物种。发现了一个垂直的环丙基铵自由基阳离子(4)作为过渡态结构。4中2p轨道旋转到平分排列会导致开环。计算得出环丙胺光离子化的最小起始能量为201.5 kcal/mol(CCSD(T)),而实验值在约201至204 kcal/mol之间。对密切相关的环丙基甲基阳离子和双环丁鎓阳离子进行了计算。与自由基(3.10 kcal/mol,QCISD)相比,阳离子(29.1 kcal/mol,QCISD)中平分的环丙基甲基构象相对于垂直物种的稳定性要大得多。在自由基系列1和2以及自由基阳离子系列3中搜索双环丁鎓阳离子的类似物。未发现可比物种。对一些涉及环丙基甲基阳离子和双环丁鎓阳离子的相互矛盾的计算进行了合理说明。计算得出环丙基 - X自由基的稳定性顺序为X = CH2 >> X = O > X = NH2+,其中后一种物种没有开环势垒。在298 K(QCISD)下,环丙基 - X自由基X = CH2到X = O的相对开环速率计算为3.1×10(6) s(-1)。