Department of Chemistry, University of Coimbra, 3004-535 Coimbra, Portugal.
J Phys Chem A. 2010 Feb 25;114(7):2506-17. doi: 10.1021/jp911118v.
Monomeric pyrrole-2-carbaldehyde (P2C) was isolated in low-temperature argon and xenon matrices, and its UV-induced photochemistry was studied. The structures of the reagent as well as the reaction photoproducts were characterized by FTIR spectroscopy. Interpretation of the experimental results was assisted by theoretical calculations carried out at the MP2 and DFT (B3LYP) levels with the 6-311++G(d,p) basis set. The compound can assume two conformations, cis and trans, regarding the orientation of the N-C-C=O dihedral angle. The cis form is the conformational ground state, being more stable than the trans by ca. 15 kJ mol(-1). The relative stability of the two conformers was analyzed based on the comparison of their structures and using the natural bond orbital method. In agreement with the calculations, only the signature of the cis conformer was found in the experimental FTIR spectra of matrix-isolated P2C monomers. UV irradiation (lambda > 235 nm) readily converts the cis-P2C into the trans-P2C form, and a photostationary equilibrium is established where the [cis]/[trans] ratio is ca. 3.3:1 in both Ar and Xe. Upon prolonged irradiation, P2C slowly undergoes photolysis to [pyrrole + CO]. In matrices, pyrrole and CO form associates of different geometry, which could be characterized based on their vibrational signatures.
单体吡咯-2-甲醛（P2C）在低温氩气和氙气基质中被分离出来，并研究了其在紫外光照射下的光化学反应。通过傅里叶变换红外光谱法对试剂和反应光产物的结构进行了表征。通过在 MP2 和 DFT（B3LYP）水平上使用 6-311++G（d，p）基组进行的理论计算，辅助了实验结果的解释。该化合物可以假设两种构象，顺式和反式，关于 N-C-C=O 二面角的取向。顺式构象是构象基态，比反式构象稳定约 15 kJ mol(-1)。根据结构比较和自然键轨道方法，分析了两种构象体的相对稳定性。与计算结果一致，在 P2C 单体的实验傅里叶变换红外光谱中仅发现了顺式构象体的特征。紫外光照射（lambda > 235 nm）可轻易将顺式-P2C 转化为反式-P2C 形式，并建立光稳定平衡，其中在 Ar 和 Xe 中 [cis]/[trans] 比值约为 3.3:1。在长时间照射后，P2C 会缓慢发生光解，生成 [吡咯 + CO]。在基质中，吡咯和 CO 形成不同几何形状的缔合物，可以根据它们的振动特征进行表征。
