Conformational comparison of urea and thiourea near the CCSD ( T ) complete basis set limit
Conformational comparison of urea and thiourea near the CCSD ( T ) complete basis set limit
复制标题
尿素和硫脲在 CCSD ( T ) 完整基组极限附近的构象比较
DOI：
10.1002/qua.27075
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发表时间：
2022
影响因子：
2.2
通讯作者：
Tschumper, Gregory S.
中科院分区：
文献类型：
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作者：
Barlow, Kayleigh R.;Tschumper, Gregory S.
尿素和硫脲的全局极小值与其他低位固定点一起被表征。用CCSD(T)方法和三重ζ关联一致性基组对每个结构进行优化，然后计算简谐振动频率。用正则和显式相关的CCSD(T)技术在完全基组极限附近评估的相对能量揭示了两个系统的一些微妙但重要的细节。这些计算解决了一个矛盾，证明了无论结构是用MP2，CCSD还是CCSD(T)优化的，尿素的C2v二级鞍点的电子能量都至少比C2全局最小值高1.5kcal −1。此外，尿素有效地有一个极小值而不是两个极小值，因为在CCSD(T)CBS极限下，在CS局部极小值中一个氨基的反转电子势垒消失了。用相同的从头算方法和大的基组对这两个体系进行了表征，最终确定了硫脲中一个或两个NH_2基团的反转电子势垒比尿素中的要小得多。CCSDT(Q)/cc-pVTZ计算表明，高阶电子关联效应对相对能量的影响很小，并且一致地被相反符号和可比量级的核关联效应所抵消。
The global minima of urea and thiourea were characterized along with other low‐lying stationary points. Each structure was optimized with the CCSD(T) method and triple‐ζcorrelation consistent basis sets followed by harmonic vibrational frequency computations. Relative energies evaluated near the complete basis set limit with both canonical and explicitly correlated CCSD(T) techniques reveal several subtle but important details about both systems. These computations resolve a discrepancy by demonstrating that the electronic energy of the C2vsecond‐order saddle point of urea lies at least 1.5 kcal mol−1above the C2global minimum regardless of whether the structures were optimized with MP2, CCSD, or CCSD(T). Additionally, urea effectively has one minimum instead of two because the electronic barrier for inversion at one amino group in the Cslocal minimum vanishes at the CCSD(T) CBS limit. Characterization of both systems with the same ab initio methods and large basis sets conclusively establishes that the electronic barriers to inversion at one or both NH2groups in thiourea are appreciably smaller than in urea. CCSDT(Q)/cc‐pVTZ computations show higher‐order electron correlation effects have little impact on the relative energies and are consistently offset by core correlation effects of opposite sign and comparable magnitude.