De Vleeschouwer Freija, De Proft Frank, Ergün Özge, Herrebout Wouter, Geerlings Paul
Research Group of General Chemistry (ALGC), Department of Chemistry, Vrije Universiteit Brussel (VUB), Pleinlaan 2, B-1050 Brussels, Belgium.
Molecular Spectroscopy Research Group, Department of Chemistry, University of Antwerp (UA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium.
Molecules. 2021 Nov 9;26(22):6767. doi: 10.3390/molecules26226767.
Linear triatomic molecules (CO, NO, and OCS) are scrutinized for their propensity to form perpendicular tetrel (CO and OCS) or pnictogen (NO) bonds with Lewis bases (dimethyl ether and trimethyl amine) as compared with their tendency to form end-on chalcogen bonds. Comparison of the IR spectra of the complexes with the corresponding monomers in cryogenic solutions in liquid argon enables to determine the stoichiometry and the nature of the complexes. In the present cases, perpendicular tetrel and pnictogen 1:1 complexes are identified mainly on the basis of the lifting of the degenerate ν 2 bending mode with the appearance of both a blue and a red shift. Van 't Hoff plots of equilibrium constants as a function of temperature lead to complexation enthalpies that, when converted to complexation energies, form the first series of experimental complexation energies on sp tetrel bonds in the literature, directly comparable to quantum-chemically obtained values. Their order of magnitude corresponds with what can be expected on the basis of experimental work on halogen and chalcogen bonds and previous computational work on tetrel bonds. Both the order of magnitude and sequence are in fair agreement with both CCSD(T) and DFA calculations, certainly when taking into account the small differences in complexation energies of the different complexes (often not more than a few kJ mol) and the experimental error. It should, however, be noted that the OCS chalcogen complexes are not identified experimentally, most probably owing to entropic effects. For a given Lewis base, the stability sequence of the complexes is first successfully interpreted via a classical electrostatic quadrupole-dipole moment model, highlighting the importance of the magnitude and sign of the quadrupole moment of the Lewis acid. This approach is validated by a subsequent analysis of the molecular electrostatic potential, scrutinizing the σ and π holes, as well as the evolution in preference for chalcogen versus tetrel bonds when passing to "higher" chalcogens in agreement with the evolution of the quadrupole moment. The energy decomposition analysis gives further support to the importance/dominance of electrostatic effects, as it turns out to be the largest attractive term in all cases considered, followed by the orbital interaction and the dispersion term. The natural orbitals for chemical valence highlight the sequence of charge transfer in the orbital interaction term, which is dominated by an electron-donating effect of the N or O lone-pair(s) of the base to the central atom of the triatomics, with its value being lower than in the case of comparable halogen bonding situations. The effect is appreciably larger for TMA, in line with its much higher basicity than DME, explaining the comparable complexation energies for DME and TMA despite the much larger dipole moment for DME.
与线性三原子分子（一氧化碳、一氧化氮和氧硫化碳）形成端对端硫族键的倾向相比，研究了它们与路易斯碱（二甲醚和三甲胺）形成垂直四价元素（一氧化碳和氧硫化碳）或五价元素（一氧化氮）键的倾向。通过比较液态氩低温溶液中配合物与相应单体的红外光谱，可以确定配合物的化学计量比和性质。在目前的情况下，垂直四价元素和五价元素1:1配合物的识别主要基于简并ν2弯曲模式的解除，同时出现蓝移和红移。以平衡常数作为温度函数的范特霍夫图得出络合焓，将其转换为络合能后，形成了文献中关于sp四价元素键的第一系列实验络合能，可直接与量子化学计算值进行比较。它们的量级与基于卤素和硫族键的实验工作以及先前关于四价元素键的计算工作所预期的量级相符。量级和顺序与CCSD(T)和DFA计算结果都相当吻合，当然要考虑到不同配合物络合能的微小差异（通常不超过几kJ/mol）和实验误差。然而，应该注意的是，氧硫化碳硫族配合物在实验中未被识别，很可能是由于熵效应。对于给定的路易斯碱，首先通过经典的静电四极矩 - 偶极矩模型成功解释了配合物的稳定性顺序，突出了路易斯酸四极矩大小和符号的重要性。随后对分子静电势的分析验证了这种方法，该分析仔细研究了σ和π空穴，以及在向“更高”硫族元素转变时硫族键与四价元素键偏好的演变，这与四极矩的演变一致。能量分解分析进一步支持了静电效应的重要性/主导地位，因为在所有考虑的情况下，静电效应都是最大的吸引项，其次是轨道相互作用和色散项。化学价自然轨道突出了轨道相互作用项中电荷转移的顺序，该项主要由碱的N或O孤对电子向三原子中心原子的给电子效应主导，其值低于可比卤素键合情况。对于三甲胺，这种效应明显更大，这与其比二甲醚高得多的碱性一致，解释了尽管二甲醚的偶极矩大得多，但二甲醚和三甲胺的络合能相当。
