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首页> 外文期刊>The journal of physical chemistry, A. Molecules, spectroscopy, kinetics, environment, & general theory >Intrinsic Conformational Preferences of Substituted Cyclohexanes and Tetrahydropyrans Evaluated at the CCSD(T) Complete Basis Set Limit:Implications for the Anomeric Effect
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Intrinsic Conformational Preferences of Substituted Cyclohexanes and Tetrahydropyrans Evaluated at the CCSD(T) Complete Basis Set Limit:Implications for the Anomeric Effect

机译：在CCSD（T）完整基集限制下评估取代的环己烷和四氢吡喃的固有构象偏好：端粒效应的含义

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A series of MP2 and CCSD(T) computations have been carried out with correlation consistent basis sets as large as aug-cc-pV5Z to determine the intrinsic equatorial-axial conformational preference of CH_3-,F-,OCH_3-,and OH-substituted cyclohexane and tetrahydropyran rings.The high-accuracy relative electronic energies reported here shed new light on the intrinsic energetics of these cyclic prototypes for the anomeric effect.At the CCSD(T) complete basis set (CBS) limit,the energy of the equatorial conformation relative to the axial position (DELTA E_((CCSD(T))~(CBS)) is -1.75,-0.20,-0.21,and -0.56 kcal mol~(-1) in methyl-,fluoro-,methoxy-,and hydroxycyclohexane,respectively,while DELTA E_((CCSD)(T))~(CBS) is -2.83,+2.45,+1.27,and +0.86 kcal mol~(-1) for 2-methyl-,2-fluoro-,2-methoxy-,and 2-hydroxytetrahydropyran,respectively.Note that the equatorial and axial conformers are nearly electronically isoenergetic in both fluoro- and methoxycyclohexane.For all eight cyclic species,a zero-point vibrational energy correction decreases DELTA E_((CCSD)(T))~(CBS) by a few tenths of a kilocalorie per mole.Relative energies obtained with popular methods and basis sets are unreliable,including Hartree- Fock theory,the B3LYP density functional,and the 6-31G and 6-311G families of split-valence basis sets.Even with the massive pentuple-zeta basis sets,the HF and B3LYP methods substantially overestimate the stability of the equatorial conformers (by as much as 0.99 and 0.73 kcal mol~(-1),respectively,for 2-methoxytetrahydropyran).Only because of a consistent cancellation of errors do these popular approaches sometimes provide reasonable estimates of the anomeric effect.
机译：已经使用与aug-cc-pV5Z一样大的相关一致基础集进行了一系列MP2和CCSD（T）计算，以确定CH_3-，F-，OCH_3-和OH取代的内在赤道轴向构象偏好此处报道的高精度相对电子能量为这些环状原型的内在能量学提供了新的信息，从而产生了异头作用。在CCSD（T）完全基集（CBS）极限下，赤道构象的能量相对于轴向位置（DELTA E _（（CCSD（T））〜（CBS））在甲基-，氟-，甲氧基-中为-1.75，-0.20，-0.21和-0.56 kcal mol〜（-1），和羟基环己烷，而2-甲基-，2-氟-的DELTA E _（（CCSD）（T））〜（CBS）分别为-2.83，+ 2.45，+ 1.27和+0.86 kcal mol〜（-1）。分别是2-，2-甲氧基和2-羟基四氢吡喃。请注意，赤道和轴向构象异构体在氟代和甲氧基环己烷中几乎都是电子等能量的。对于所有八个环状物种，零穴t振动能量校正使DELTA E _（（CCSD）（T））〜（CBS）每摩尔减少十分之一千卡。通过常用方法和基集获得的相对能量不可靠，包括Hartree-Fock理论，B3LYP密度功能和6-31G和6-311G系列分裂价基集。即使使用庞大的五元组zeta基集，HF和B3LYP方法也大大高估了赤道构象异构体的稳定性（分别高达0.99和0.73）对于2-甲氧基四氢吡喃，它们分别对应于kcal mol〜（-1）。仅由于一致地消除了误差，这些流行的方法有时提供了对端基异构作用的合理估计。

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