嘌呤类化合物13C与15N NMR化学位移的定量构谱相关研究

利用原子电性作用矢量(AEIV)和原子杂化状态指数(AHSI)对嘌呤类化合物中的N原子与C原子进行结构表征并与其核磁共振碳谱(13C NMR与15N NMR)建立了优良的定量构谱相关(QSSR)模型,并用留一法(Leave-One-Out,LOO)交互校验(Cross-Validation,CV),对构建的模型进行了预测。其中12个嘌呤类化合物的60个C原子建模的计算值、留一法交互校验预测值的复相关系数(R)分别为0.961,0.970;12个嘌呤类化合物48个N原子建模的计算值、留一法交互校验预测值的R分别为0.987,0.989。说明AEIV与AHSI描述子具有普适性,对不同的原子均能进行有效的相关预测。     

丁烷衍生物类木脂素13C NMR化学位移预测

借助原子电性作用矢量(AEIV)和原子杂化状态指数(AHSI),对39种丁烷衍生物类木脂素共计854个等价C原子进行表征,并建立用于模拟该类分子13C NMR化学位移的多元线性回归方程.所得定量结构波谱关系(QSSR)模型及留一法交互检验相关系数分别为r=0.981和q=0.962.进一步用从马尾松松针中分离所得新木脂素中20个13C NMR化学位移对模型进行外部验证,预测结果与实验值较接近.表明所建模型有良好稳定性和泛化力,可对丁烷衍生物类木脂素13C NMR谱学数据准确模拟.     

羰基化合物结构参数与其 13C NMR谱化学位移的关系

通过对62个羰基化合物中羰基^13C NMR谱化学位移与其部分结构参数关系研究,发现羰基^13C NMR谱化学位移与烷基极化效应指数(PEI)、部分电荷(qx)的关系可用下式表示：δ=a b∑PEI c∑qx 该式能比现有文献方法较好地表达羰基^13C NMR谱化学位移值随结构变化的规律,从而对解析^13C NMR谱及预测羰基^13C NMR谱的化学位移值提供了理论依据。     

胺类化合物的离子性指数、极化效应指数与其13C NMR谱化学位移关系

胺类化合物的离子性指数、极化效应指数与其13C NMR谱化学位移关系;脂肪胺化合物;13C NMR谱化学位移;离子性指数;极化效应指数;立体效应     

碱基和核苷及其衍生物的13C核磁共振波谱模拟

应用原子电性作用矢量(VAEI)和原子杂化状态指数(AHSI)及γ效应校正表征碱基和核苷及其衍生物81种分子,以多元线性回归建立13C核磁共振定量结构波谱关系模型,得复相关系数r为0.970。经留一法和留组(分子)法交互检验表明,模型具有较好内部预测能力,r分别为0.969和0.969。将该模型用于三类嘧啶和嘌呤衍生物13C核磁共振波谱模拟,得r分别为0.969、0.921和0.884,所建模型外部预测能力较强。     

多环芳香硫化合物的定量结构-气相色谱保留指数关系

应用分子电性距离矢量(MEDV)对114个多环芳香硫化合物(PASHs)进行结构表征,通过多元线性回归建立了PASHs的气相色谱保留指数与MEDV参数之间的定量结构-保留值关系模型;同时采用逐步回归分析进行变量筛选,继而以留一法交互检验对所得优化模型进行预测能力评价,所建立的模型的相关系数为0.9947,交互检验相关系数为0.9940,表明该优化模型具有良好的稳定性和预测能力。此外,通过将样本集按2:1分成校准集和测试集预测,统计分析结果显示所建的模型具有良好的相关性和稳定性。本文所建的定量结构-保留值关系(QSRR)模型为预测PASHs的气相色谱保留指数提供了一个便捷有效的新方法。     

非对映异构体~（13）C NMR化学位移不等性研究

合成了８对非对映异构体酰胺，测定了它们的~（１３）Ｃ　ＮＭＲ化学位移值和相应的非对映异构体基团的化学位移差。根据实验数据提出一种分子优势构象与~（１３）Ｃ　ＮＭＲ化学位移不等性的相关模型。     

无环醇~(13)C NMR化学位移与其结构参数的定量关系

用新颖的原子拓扑矢量Y_C、原子平衡电负性q_e、结构信息参数[N_H~i(i=α,β)]和γ校正参数对63个无环饱和脂肪醇的局部化学微环境进行了结构表征,并对化合物~(13)C NMR化学位移进行了QSSR研究.采用偏最小二乘回归得到模型的复相关系数R和标准偏差S分别为0.9915和2.4827;对353个碳原子~(13)C NMR化学位移的实验值与计算值的平均绝对误差仅为2.01×10~(-6).同时,采用留分法(Leave-molecule-out)和外检验方法测试模型的内部稳定性和外部预测能力.与文献结果比较,本研究所用参数少,且计算简便.     

几种苄基哌嗪衍生物13C NMR的计算研究

本文分别采用量子化学从头算Hatree-Fock方法和密度泛函B3LYP方法在6-311G++基组水平下对几种苄基哌嚷衍生物的13C NMR作了计算研究.结果表明两种方法计算得到的各苄基哌嗪衍生物中C原子化学位移的计算值与实验值之间均近似存在线性关系,其中采用考虑了电子相关作用的密度泛函方法计算时,各化合物中碳原子的化...     

离子性指数、极化效应指数烷烃^13C NMR化学位移的 关系研究

定义了烷烃分子中碳原子的离子性指数(INI)，用离子性指数(INI)、极化效应指数(PEI)及N^i~H(i=αβΥ)结构信息参数研究了烷烃的^13CNMR化学位移模型，结果表明，烷烃^13CNMR化学位移(CS)可用下式来定量描述：CS=194.6156-37.7394(INI)+98.6505(ΣPEI)+27.1630(INI/ΣPEI)-652.9106(ΣPEI/INI)+0.7735N^α~H+2.2468N^β~H-0.1742N^γ~H。用上式估算了304个碳原子的化学位移，平均绝对误差仅为0.77δ，标准差0.9860δ，预测值与实验值非常吻合。     

Counterintuitive deshielding on the 13C NMR chemical shift for the trifluoromethyl anion

The trifluoromethyl anion (CF₃⁻) displays ¹³C NMR chemical shift (175.0 ppm) surprisingly larger than neutral (CHF₃, 122.2 ppm) and cation (CF₃⁺, 150.7 ppm) compounds. This unexpected deshielding effect for a carbanion is investigated by density functional theory calculations and decomposition analyses of the ¹³C shielding tensor into localized molecular orbital contributions. The present work determines the shielding mechanisms involved in the observed behaviour of the fluorinated anion species, shedding light on the experimental NMR data and demystify the classical correlation between electron density and NMR chemical shift. The presence of fluorine atoms induces the carbon lone pair to create a paramagnetic shielding on the carbon nucleus.     

The application of empirical methods of 13C NMR chemical shift prediction as a filter for determining possible relative stereochemistry

The reliable determination of stereocenters contained within chemical structures usually requires utilization of NMR data, chemical derivatization, molecular modeling, quantum‐mechanical (QM) calculations and, if available, X‐ray analysis. In this article, we show that the number of stereoisomers which need to be thoroughly verified, can be significantly reduced by the application of NMR chemical shift calculation to the full stereoisomer set of possibilities using a fragmental approach based on HOSE codes. The applicability of this suggested method is illustrated using experimental data published for a series of complex chemical structures. Copyright © 2009 John Wiley & Sons, Ltd.     

13C NMR chemical shifts of carbonyl groups in substituted benzaldehydes and acetophenones: substituent chemical shift increments

13C NMR Substituent chemical shift (SCS) increments have been determined for the carbonyl carbon of a variety of substituted benzaldehydes and acetophenones. The 13C NMR chemical shift of the carbonyl carbon can be predicted for many di- and trisubstituted benzaldehydes and acetophenones through simple additivity of the SCS increments. The magnitude and sign of the SCS increments have been explored using Hartree-Fock 6-31G* calculations to determine the natural atomic charges of the carbonyl carbon. When a substituent capable of intermolecular hydrogen bonding is present, deviations from additivity on the order of 2 ppm are observed in dilution experiments; deviations of up to 6 ppm can result from intramolecular hydrogen bonding.     

Increments for 1H and 13C NMR chemical shifts in areneboronic acids

A series of areneboronic acids were studied by NMR spectroscopy. Increments for the ¹H and ¹³C chemical shifts caused by the boronic acid substituent B(OH)₂ in areneboronic acids were determined. Copyright © 2003 John Wiley & Sons, Ltd.     

Use of an internal reference in 13C chemical shift measurements

The use of the nominal value of the chemical shift of the solvent to calibrate the spectrometer in ¹³C NMR spectroscopy was found to introduce errors due to the effect which the solute has on the solvent. In addition, hexamethyldisiloxane (HM) is proposed as an internal standard; owing to its high boiling point it is easier to manipulate than tetramethylsilane (TMS) and it is therefore possible to prepare solutions of known concentrations. In order to convert the data obtained with this standard to the TMS scale, the chemical shift of HM was determined in 16 solvents using cyclohexane as external standard in a spherical cell (5% w/w concentration of HM) as a function of the solvent factor g². Comparing these results with a similar plot obtained previously for TMS by other workers, it is possible to convert one type of data to the other by a simple linear expression.     

Density functional theory study of 13C NMR chemical shift of chlorinated compounds

The use of the standard density functional theory (DFT) leads to an overestimation of the paramagnetic contribution and underestimation of the shielding constants, especially for chlorinated carbon nuclei. For that reason, the predictions of chlorinated compounds often yield too high chemical shift values. In this study, the WC04 functional is shown to be capable of reducing the overestimation of the chemical shift of Cl‐bonded carbons in standard DFT functionals and to show a good performance in the prediction of ¹³C NMR chemical shifts of chlorinated organic compounds. The capability is attributed to the minimization of the contributions that intensively increase the chemical shift in the WC04. Extensive computations and analyses were performed to search for the optimal procedure for WC04. The B3LYP and mPW1PW91 standard functionals were also used to evaluate the performance. Through detailed comparisons between the basis set effects and the solvent effects on the results, the gas‐phase GIAO/WC04/6‐311+G(2d,p)//B3LYP/6‐31+G(d,p) was found to be specifically suitable for the prediction of ¹³C NMR chemical shifts of chlorides in both chlorinated and non‐chlorinated carbons. Further tests with eight molecules in the probe set sufficiently confirmed that WC04 was undoubtedly effective for accurately predicting ¹³C NMR chemical shifts of chlorinated organic compounds. Copyright © 2012 John Wiley & Sons, Ltd.     

QSAR studies of phenylhydrazine-substituted tetronic acid derivatives based on the 1H NMR and 13C NMR chemical shifts

The quantitative structure–activity relationship models of 40 phenylhydrazine-substituted tetronic acid derivatives were established between the ¹H nuclear magnetic resonance (NMR) and ¹³C NMR chemical shifts and the antifungal activity against Fusarium graminearum, Botrytis cinerea, Rhizoctonia cerealis, and Colletotrichum capsici. The models were validated by R, R², R_A², variance inflation factor, F, and P values testing and residual analysis. It was concluded from the models that the ¹³C NMR chemical shifts of C8, C10, C7, and the ¹H NMR chemical shifts of Ha contributed positively to the activity against Fusarium graminearum, Botrytis cinerea, Colletotrichum capsici, and Rhizoctonia cerealis, respectively. The models indicated that decreasing the election cloud density of specific nucleuses in compounds, for example, by the substituting of electron withdrawing groups, would improve the antifungal activity. These models demonstrated the practical application meaning of chemical shifts in the quantitative structure–activity relationship study. Furthermore, a practical guide was provided for further structural optimization of the antifungal phenylhydrazine-substituted tetronic acid derivatives based on the ¹H NMR and ¹³C NMR chemical shifts.