Wolf Mark E, Vandezande Jonathon E, Schaefer Henry F
Center for Computational Quantum Chemistry, University of Georgia, 140 Cedar Street, Athens, Georgia 30602, USA.
Phys Chem Chem Phys. 2021 Sep 14;23(34):18535-18546. doi: 10.1039/d1cp03302f. Epub 2021 Aug 24.
The reactions between substituted isocyanates (RNCO) and other small molecules (e.g. water, alcohols, and amines) are of significant industrial importance, particularly for the development of novel polyurethanes and other useful polymers. We present very high-level ab initio computations on the HNCO + HO reaction, with results targeting the CCSDT(Q)/CBS//CCSD(T)/cc-pVQZ level of theory. Our results affirm that hydrolysis can occur across both the N[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds of HNCO via concerted mechanisms to form carbamate or imidic acid with ΔH barrier heights of 38.5 and 47.5 kcal mol. A total of 24 substituted RNCO + HO reactions were studied. Geometries obtained with a composite method and refined with CCSD(T)/CBS single point energies determine that substituted RNCO species have a significant influence on these barrier heights, with an extreme case like fluorine lowering both barriers by close to 15 kcal mol and most common alkyl substituents lowering both by approximately 3 kcal mol. Natural Bond Orbital (NBO) analysis provides evidence that the predicted barrier heights are strongly associated with the occupation of the in-plane C-O* orbital of the RNCO reactant. Key autocatalytic mechanisms are considered in the presence of excess water and RNCO species. Additional waters (one or two) are predicted to lower both barriers significantly at the CCSD(T)/aug-cc-pV(T+d)Z level of theory with strongly electron withdrawing RNCO substituents also increasing these effects, similar to the uncatalyzed case. The 298 K Gibbs energies are only marginally lowered by a second catalyst water molecule, indicating that the decreasing ΔH barriers are offset by loss of translational entropy with more than one catalyst water. Two-step 2RNCO + HO mechanisms are characterized for the formation of carbamate and imidic acid. The second step of these two pathways exhibits the largest barrier and presents no clear pattern with respect to substituent choice. Our results indicate that an additional RNCO molecule might catalyze imidic acid formation but have less influence on the efficiency of carbamate formation. We expect that these results lay a firm foundation for the experimental study of substituted isocyanates and their relationship to the energetic pathways of related systems.
取代异氰酸酯（RNCO）与其他小分子（如水、醇和胺）之间的反应具有重要的工业意义，特别是对于新型聚氨酯和其他有用聚合物的开发。我们对HNCO + HO反应进行了非常高水平的从头算计算，结果以CCSDT(Q)/CBS//CCSD(T)/cc-pVQZ理论水平为目标。我们的结果证实，水解可以通过协同机制在HNCO的N=C和C=O键上发生，形成氨基甲酸酯或亚胺酸，其ΔH势垒高度分别为38.5和47.5 kcal/mol。总共研究了24个取代的RNCO + HO反应。用复合方法获得并经CCSD(T)/CBS单点能量优化的几何结构表明，取代的RNCO物种对这些势垒高度有显著影响，极端情况如氟使两个势垒都降低近15 kcal/mol，最常见的烷基取代基使两个势垒都降低约3 kcal/mol。自然键轨道（NBO）分析提供了证据，表明预测的势垒高度与RNCO反应物的面内C-O*轨道的占据密切相关。在存在过量水和RNCO物种的情况下考虑了关键的自催化机制。在CCSD(T)/aug-cc-pV(T+d)Z理论水平下，预测额外的水（一个或两个）会显著降低两个势垒，具有强吸电子性的RNCO取代基也会增强这些效应，类似于未催化的情况。第二个催化剂水分子仅略微降低了298 K时的吉布斯自由能，表明降低的ΔH势垒被多于一个催化剂水分子导致的平动熵损失所抵消。表征了生成氨基甲酸酯和亚胺酸的两步2RNCO + HO机制。这两条途径的第二步呈现出最大的势垒，并且在取代基选择方面没有明显的模式。我们的结果表明，额外的RNCO分子可能催化亚胺酸的形成，但对氨基甲酸酯形成效率的影响较小。我们期望这些结果为取代异氰酸酯的实验研究及其与相关体系能量途径的关系奠定坚实基础。
