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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Gas-Phase Basicity of Glycine: A Comprehensive ab Initio Study
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Gas-Phase Basicity of Glycine: A Comprehensive ab Initio Study

机译：甘氨酸的气相碱度：全面的从头开始研究

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摘要

Geometry-optimized structures for the most stable conformers of glycine and protonated glycine were obtained using the Hartree-Fock and second-order Moller-Plesset perturbation (MP2) methods with the 3-21G~*, 6-31G~*, 6-31G~(**), 6-31+G~(**), 6-311G~(**), 6-311+G~(**), and 6-311++G~(**) basis sets. Analyses of results indicate that the MP2/6-31G~*, MP2/6-31+G~(**), and, MP2/6-311+G~(**) levels of theory are more suited for protonation studies. Considerations were given to potential applications of single-point calculations using higher correlation methods such as MP4, QCISD(T), and CCSD(T) and larger basis sets including 6-311+G(3df,2p) and aug-cc-pVTZ. An ideal gas basicity of 203.5 kcal/mol at 298.15 K, which was calculated at the MP4/6-31+G(2d,2p) composite level for electronic properties and at the MP2/6-31G~* level for thermodynamic properties with corrections of basis-set superposition error and conformational equilibrium effect, is shown to be sufficiently accurate by systematic deductions. This theoretical value is in good agreement with the lower of the two mass spectrometric values, 202.5 and 207.0 kcal/mol, assigned as the gas-phase basicity (GB) of glycine based on two different basicity scales. comparisons with GB calculations on ammonia and methylamine reveal that certain protonation properties remain fairly constant among molecules undergoing amino N-protonations. Several findings from this study help formulate practical strategies for calculating the GBs of larger molecules, including the use of density functional theory.
机译：使用Hartree-Fock和二阶Moller-Plesset扰动（MP2）方法并使用3-21G〜*，6-31G〜*，6-31G获得了最稳定的甘氨酸和质子化甘氨酸构象的几何优化结构〜（**），6-31 + G〜（**），6-311G〜（**），6-311 + G〜（**）和6-311 ++ G〜（**）基础套。结果分析表明MP2 / 6-31G〜*，MP2 / 6-31 + G〜（**）和MP2 / 6-311 + G〜（**）的理论水平更适合于质子化研究。考虑了使用更高相关性方法（例如MP4，QCISD（T）和CCSD（T））以及更大的基础集（包括6-311 + G（3df，2p）和aug-cc-pVTZ）进行单点计算的潜在应用。理想的气体碱性在298.15 K时为203.5 kcal / mol，这是根据电子性能的MP4 / 6-31 + G（2d，2p）复合能级和热力学性能的MP2 / 6-31G〜*能级计算得出的。系统推论表明，基集叠加误差和构象平衡效应的校正是足够准确的。该理论值与两个基于两个不同碱度标度的甘氨酸的气相碱度（GB）的两个质谱值中的较低值202.5和207.0 kcal / mol较低。与关于氨和甲胺的GB计算的比较表明，在经历氨基N质子化的分子中，某些质子化性质保持相当恒定。这项研究的一些发现有助于制定实用的策略来计算较大分子的GB，包括使用密度泛函理论。

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