核自旋偶合常数的计算 Ⅴ.取代苯偶合常数的计算

我们曾研究烃类分子以及1,2-双取代乙烯分子偶合常数的计算.本文提出取代苯偶合常数的计算方法. 苯分子中的氢原子被某些基团取代后,分子中碳原子间的电子密度将发生变化,当然,分子的几何构型和整个分子的电子结构也会发生变化.如果我们只考虑取代基对碳原子间电子密度的影响,据前文的公式,可以得到取代苯偶合常数的计算公式:     

核自旋偶合常数的计算 Ⅶ.二取代萘偶合常数的计算

前文讨论了双取代乙烯和取代苯中质子偶合常数的计算问题,本文把计算原理和方法推广应用于取代萘偶合常数的计算. 与取代苯相似,当萘分子中的氢原子被某些基团取代后,与取代基连接的碳原子及其邻近碳原子之间的电子密度会发生变化,当然,分子的几何构型也可能改变,但一般变化不大.如果我们只考虑取代基对碳原子间电子密度的影响,那么,取代苯偶合常数的计算公式同样适用于取代萘,即:     

核磁共振偶合常数1JCH的计算

借助非正交定域分子轨道(简称为LMO)及其伴基所构成的单位算子, 得到以LMO为基计算NMR偶合常数的二级微扰理论公式, 并计算了CH_4、C_2H_4、C_2H_2分子碳原子和氢原子核上的电荷密度及NMR偶合常数~1J_(CH)。计算结果表明, ~1J_(CH)主要由定域于碳原子和氢原子化学键区域的成键与反键LMO所决定, 并与LMO在碳原子和氢原子核上的电荷密度乘积成比例。     

核自旋偶合常数的计算 Ⅵ.取代苯质子偶合常数的加和性

取代基对苯环~(13)C和环上质子化学位移的影响具有加和性,基于这一性质而进行的化学位移计算已广泛应用于分子结构的测定.我们发现,取代基对苯环质子偶合常数的影响也具有近似的加和性,并提出简单的计算公式.偶合常数计算与化学位移计算结合起来,可更有效地用于分子结构测定. 苯环上质子间偶合常数的计算公式可表示如下:     

核自旋偶合常数2JC-C-F的量子化学计算

用密度泛函B3LYP方法,在6-311G基组上对36个化合物进行构型优化、频率分析,计算了C-C-F的核自旋偶合常数^2JC-C-F。由此拟合出一个基于该方法和相同水平上计算^2JC-C-F的公式,选取四种化合物进行验证,计算值与文献值吻合。     

核自旋偶合常数的计算 Ⅳ.1,2-双取代乙烯偶合常数~3J_(H-H)的计算

NMR偶合常数在判别1,2-双取代乙烯分子构型方面十分有用,但目前尚无简单有效的计算方法.我们在以往计算烃类分子偶合常数~3J_(H-H)的基础上,提出计算1,2-双取代乙烯偶合常数~3J_(H-H)的简单方法,并可以作为确定乙烯型分子构型的判据. 乙烯分子中的氢原子被某些基团取代后,与取代基相连接的碳原子的电子密度ρc会发生改变,分子的几何构型以及电子云分布也可能受到影响,从而影响烯质子的偶合常数~3J_(H-H).如果我们只考虑取代基对ρc的影响,并略去数值很小的空间偶合项~3J_(H-H)~(2),根据前文提出的公式不难得到:     

核磁共振偶合常数~1J_(CH)的计算

借助非正交定域分子轨道(简称为LMO)及其伴基所构成的单位算子,得到以LMO为基计算NMR偶合常数的二级微扰理论公式,并计算了CH_4、C_2H_4、C_2H_2分子碳原子和氢原子核上的电荷密度及NMR偶合常数~1J_(CH)。计算结果表明,~1J_(CH)主要由定域于碳原子和氢原子化学键区域的成键与反键LMO所决定,并与LMO在碳原子和氢原子核上的电荷密度乘积成比例。     

Proton spin coupling constants in methylcyclopentadienes

A detailed procedure for analysing the ¹H n.m.r. spectra of two isomeric methylcyclopentadienes, two dimethylcyclopentadienes and 3-methylindene is reported. Finite perturbation INDO calculations are used to discuss the conformational features of the long range proton coupling constants.     

Proton spin coupling constants in cycloalkenes

The proton-proton coupling constants in cyclopentadiene, cyclohexadiene, cycloheptatriene, cyclooctatetraene, cyclopentene and cyclohexene have been calculated using the finite perturbation INDO method. The carbon geometry was taken from the corresponding electron diffraction studies. The theory gives satisfactory agreement with experiment. In terms of the many factor structural model, distance and angle distortions have been studied in ethylene, cis-1,3-butadiene and cis-2-butene. It has been established that these distortions produce noticeable changes only for those protons whose co-ordinates are changed. The coupling constants across three, four and five bonds are discussed separately in connection with the structural factors and the σ–π contribution. Finally, variations in the proton co-ordinates in cyclopentadiene are used to optimize the proton couplings.     

A theoretical study of nuclear spin coupling constants and hyperfine coupling constants of se-heterocyclics

MO calculations based on the finite perturbation theory in the INDO approximation have been carried out on selenophene, eighteen of its monosubstituted derivatives and benzo (b)selenophene. The calculated nuclear spin coupling constants satisfactorily reproduce signs, magnitudes, internal orders and some trends of the experimental values. Comparison of different ZDO calculations provides information on the relative importance of σ and π pathways for the various coupling constants in selenophene and benzo(b)selenophene. Unrestricted Hartree-Fock calculations at the INDO level have been performed on the radical anions of dibenzoselenophene and 2,1,3-benzoselenadiazole, the phenoselenazine radical cation, the phenoselenazine neutral radical and the phenoselenazine nitroxide. The isotropic hyperfine coupling constants have been found to be generally in satisfactory agreement with experiment.     

Semi-empirical MO calculations of nuclear spin coupling constants

The results obtained using different semi-empirical approaches, namely EHMO, IEHMO, CNDO/2 and INDO, in the calculation of spin–spin coupling constants within the framework of the one-electron MO approximation are systematically compared in the case of several classes of organic molecules. While, at a semi-empirical level SCF methods normally provide satisfactory wave function, the use of the simple EHMO seems better able to satisfy the problem connected with the calculation of spin–spin coupling constants, expecially when an appropriate set of AO's is chosen, in order to avoid parametrisation of Dirac monocentric integrals. Charge iteration (IEHMO) seems to improve the results slightly only when heteroatoms are present, but the complexity introduced into the calculations and the greatly increased computer time do not justify the slight improvement achieved, particularly as the method is applied to large molecules and organometallic compounds.     

The spin polarization model for hyperfine coupling constants

Summary The simple spin polarization model for calculation of the spin densities that determine hyperfine coupling constants in free radicals is examined. Spin-unrestricted approaches, both without and with removal of spin contamination, are discussed and compared with spin-restricted treatments. Basis set design and electron correlation effects are also considered. Calculations on small pi radicals are presented to illustrate the arguments. Explanations are advanced for why some of the simpler treatments seem to work better than might be expected.     

Fluorine nuclear spin coupling constants in fluoro-substituted furans

In order to assist experimental assignments in olefinic and aromatic fluorine compounds, fluorine nuclear coupling constants have been obtained from the first-order self-consistent perturbation theory equation with the INDO molecular orbital approximation in a series of fluoro-substituted furans. There is a clear distinction between the magnitudes and signs for the four fluorine coupling positions; the importance of the orbital and spin-dipolar contributions is very clear. Tentative signs are suggested for all the coupling constants.     

BIRD-J-resolved HMBC and BIRD-high-resolution HMBC pulse sequences for measuring heteronuclear long-range coupling constants and proton-proton spin coupling constants in complicated spin systems

Efficient pulse sequences for measuring long-range C-H coupling constants (J(C-H)) and proton-proton spin coupling constants (J(H-H)), named BIRD-J-resolved HMBC and BIRD-high-resolution HMBC, respectively, have been developed. In spin systems possessing a secondary methyl group positioned between protonated carbons (e.g. -CH(2)-CH(CH(3))-CH(2)-), the methine proton splits in a complicated fashion, resulting in difficulty in the determination of its spin coupling constants. For easy and accurate measurements of the long-range J(C-H) and J(H-H) in such a spin system, the BIRD pulse [90°x(H)-180°x (H/C)- 90° (-x)(H)] or [90°x(H)-180°x(H/C)-90° (-x)(H)180°x(C)] is incorporated into the J-resolved portion of the pulse sequence. As a result, the above secondary methyl group can be selectively decoupled, providing simplified cross-peak patterns, which are suitable for the accurate measurements of the long-range J(C-H) and J(H-H).     

反式-PtHL2X配合物中Pt-H核自旋偶合常数的量子化学计算

在计算两个核之间的自旋偶合常数时, 一般只考虑两个核与各自的s价电子的Fermi接触相互作用.这种近似处理不能解释反式-PtHL2X配合物的Pt-H偶合常数.本文考察到铂原子的6s价轨道在氢核处有较大的数值, 除考虑核与各自的s价电子的Fermi接触互作用之外, 引入铂的6s价电子与氢核的Fermi接触相互作用, 称为Fermi交换接触相互作用. 在有限微扰理论和EHMO近似下, 计算了上述配合物的铂氢偶合常数, 得到与实验值一致的结果.