Prediction of the ~(13)C NMR Chemical Shifts of 9,10-Dihydrophenanthrene Analogues by the GIAO Method

After the geometry optimizations at the B3LYP/6-31+G(d,p) level, the NMR calculations of a series of 9,10-dihydrophenanthrene analogues have been carried out by GIAO method at the HF/6-31+G(d) level. The calculated 13C NMR chemical shifts are in agreement with the observed values. By a series of linear correlation equations (δpred = a + bδcal.c) of the 13C chemical shifts, accurate prediction of 13C chemical shifts was achieved for the new 9,10- dihydrophenanthrene compound, for which the predicted 13C NMR chemical shifts are in quite good agreement with the experimental values. The linear correlation between δpred and δexptl is excellent, and the square of correlation coefficient, r2, is up to 0.9973. The maximum absolute difference between δpred and δexptl, Δδ, is 4.5 ppm, and the rms error between δpred and δexptl is 2.55 ppm. In the meantime, according to the theoretical predicted result, we could confirm that the new 9,10-dihydrophenanthrene analogue is erianthridin (2,7-dihydroxy-3,4-dimethoxy-9,10-dihydro-phenanthrene).     

Geometric dependence of the B3LYP-predicted magnetic shieldings and chemical shifts

We perform a systematic investigation of how the B3LYP/6-311+G(2d,p) calculated 13C nuclear magnetic shielding constants depend on the 6-31G(d)-optimized geometries for a set of 18 molecules with various chemical environments. For absolute shieldings, the Hartree-Fock (HF)-optimized geometries lead to a mean absolute deviation (MAD) of 5.65 ppm, while the BLYP- and B3LYP-optimized geometries give MADs of 13.07 and 10.14 ppm, respectively. For chemical shifts, the HF, BLYP and B3LYP geometries lead to MADs of 2.36, 5.80, and 4.43 ppm, respectively. We find that the deshielding tendency of B3LYP can be effectively compensated by using the HF-optimized geometries. When we apply the B3LYP//HF protocol to versicolorin A and 5alpha-androstan-3,17-dione, MADs of 1.86 and 1.41 ppm, respectively, are obtained for chemical shifts, in satisfactory agreement with the experiment.     

Iminopropadienones R–NCCCO and carbon suboxide, C3O2. Theoretical and experimental C NMR spectra

The ¹³C NMR data of five iminopropadienones R–NCCCO as well as carbon suboxide, C₃O₂, have been examined theoretically and experimentally. The best theoretical results were obtained using the GIAO/B3LYP/6-31+G**//MP2/6-31G* level of theory, which reproduces the chemical shifts of the iminopropadienone substituents extremely well while underestimating those of the cumulenic carbons by 5–10 ppm. The computationally faster GIAO/HF/6-31+G**//B3LYP/6-31G* level is also adequate.     

Beta-silyl-substituted silaadamantyl,silabicyclo[2.2.2]octyl,silanorbornyl, and 1-silacyclohexyl cations. A theoretical (DFT and GIAO NMR) study

Parent 1-silaadamant-1-yl (1+) and a series of mono-beta-silyl-substituted- (2-Me+, 2-F+, 2-Cl+, 2-Br+), bis-beta-silyl-substituted- (3-Me+), and tris-beta-silyl-substituted (4-Me+)-1-silaadamant-1-yl cations were studied by the DFT method at the B3LYP/6-31G(d,p) level and by GIAO NMR at the B3LYP/ 6-31G(d,p)//B3LYP/6-31G(d,p) level. The geometries, relative energies, NMR chemical shifts, and charge distribution in the bridgehead silylium ions are discussed and compared. The magnitude of the beta-silyl effect (the Si-C-Si+ hyperconjugation) is gauged as a function of structure. Related model studies on the silabicyclo[2.2.2]octyl (5+, 6+, 5a+, and 6a+), silanorbornyl (7+ and 8+), and silacyclohexyl cations (9+ and 10+) were carried out in which the effect of beta-silyl substitution on geometry, stability, and NMR chemical shifts was probed. The acyclic model Me3Si-CH2-Si+(Me)2 (11+) was used to gauge the influence of the twist angle between the p-orbital at Si+ and the C-Si bond on relative stability and on the changes in the 29Si NMR chemical shifts. Finally, interaction of 1+ with H2O and MeOH and 2-Me+ with H2O was also examined. The resulting optimized structures (12+, 13+, and 14+) and the computed NMR chemical shifts are most compatible with the formation of silaoxonium ions.     

2,4,6-三甲氧基苯-1-O-D葡萄糖苷的核磁共振谱的理论研究

The 1H and 13C-NMR of 2,4,6-trimethoxyphenol-1-O-D-glucopyranoside(Compound 1) isolated from Celastrus angulatus (Celastraceae) was calculated theoretically at the both levels HF/6-311+G(2d,p)//B3LYP/6-31G(d) and HF/6-311+G(2d,p)//B3LYP/6-31G(d,p) using the GIAO (gauge-independent atomic orbital) method. Statistical error analysis for theoretically predicted δH and δC values versus those experimentally observed for compound 1 was discussed. The results show that the theoretically predicted δH and δC values of β conformer of compound 1 are more close to the experimentally observed values than α conformer, and the β conformer of compound 1 is more stable than α conformer according to molecular energy theoretically calculated. So compound 1 is assigned to be 2,4,6-trimethoxyphenol-1-O-β-D-glucopyranoside, which is in good consistence with the conclusion deduced by the anomeric proton signal (δH=4.80, J=7.3 Hz) experimentally observed.     

Structural elucidation of nitro-substituted five-membered aromatic heterocycles utilizing GIAO DFT calculations

The GIAO (Gauge Including Atomic Orbitals) DFT (Density Functional Theory) method is applied at the B3LYP/6-31+G(d,p)//B3LYP/6-31+G(d), B3LYP/6-311++G(d,p)//B3LYP/6-31+G(d), B3LYP/6-311+G (2d,p)//B3LYP/6-31+G(d) and B3LYP/6-311++G(d,p)//B3LYP/6-311++G(d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants for 25 nitro-substituted five-membered heterocycles. Difference (1D NOE) spectra in combination with long-range gHMBC experiments were used as tools for the structural elucidation of nitro-substituted five-membered heterocycles. The assigned NMR data (chemical shifts and coupling constants) for all compounds were found to be in good agreement with theoretical calculations using the GIAO DFT method. The magnitudes of one-bond (1JCH) and long-range (nJCH, n>1) coupling constants were utilized for unambiguous differentiation between regioisomers of nitro-substituted five-membered heterocycles.     

Substituent effects on the basicity of 3,7-diazabicyclo[3.3.1]nonanes

Basicity constants for a series of 3,7-diazabicyclo[3.3.1]nonane derivatives in acetonitrile with a variation over 13 orders of magnitude have been determined using a spectrophotometric titration technique. An excellent correlation between basicity and calculated proton affinities obtained at PCM-B3LYP/6-31+G(d)//B3LYP/6-31G(d) level was found. The results are discussed in terms of substituent effects and compared to (15)N NMR chemical shifts.     

Predicting 9Be nuclear magnetic resonance chemical shielding tensors utilizing density functional theory

The structures of a series of beryllium containing complexes have been optimized at the B3LYP/6-31G(d) level and their (9)Be magnetic shielding values have been determined using B3LYP/6-311G+g(2d,p) and the gauge-including atomic orbital (GIAO) method. The calculated chemical shifts are in excellent agreement with experimental values. The performance of a variety of NMR methods (SGO, IGAIM, CSGT) were also examined but were found to be inferior to the GIAO method at the chosen level of theory employed. The theoretical method has been utilized to predict the beryllium chemical shifts of structurally characterized complexes for which no measured (9)Be NMR spectrum exists, and to investigate a literature complex with an unusual (9)Be NMR chemical shift. A new standard for beryllium NMR in nonaqueous solvents has been suggested.     

Ab initio study of 2,4-substituted azolidines. II. Amino-imino tautomerism of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one in water solution

The relative stabilities of the tautomers of 2-aminothiazolidine-4-one and 4-aminothiazolidine-2-one were calculated at the MP2/6-31+G(d,p) level by considering their mono- and trihydrated complexes. Single-point calculations at the MP4/6-31+G(d,p)//MP2/6-31+G(d,p) level of theory were performed to obtain more accurate energies. The values of proton transfer barriers in the isolated, mono- and trihydrated tautomers of 2-aminothiazolidine-4-one (2AT) and 4-aminothiazolidine-2-one (4AT) were calculated for two different mechanisms of tautomerisation. In the absence of water, the process of proton transfer should not occur. Addition of water molecules decreases the barrier making the process faster, as the participation of two water molecules in a proton transfer reaction is more favorable than the participation of only one water molecule. To estimate the effect of the medium (water) on the relative stabilities of the tautomers of the studied compounds we applied the polarizable continuum model (PCM). (13)C NMR chemical shieldings were calculated using the GIAO approach at MP2/6-31+G(d,p) optimized geometries. HF and the DFT B3LYP functional with 6-31+G(d,p) basis set were employed. The quantum chemical results for the chemical shifts in gas phase and in polar solvents (water and DMSO) were compared with experimental data. TD DFT B3LYP/aug-cc-pVTZ calculations were performed to predict the absorption maxima of tautomers A and B of 2AT and 4AT.     

NMR spectra, GIAO and charge density calculations of five-membered aromatic heterocycles

The B3LYP/6-31+G(d) molecular geometry optimized structures of 17 five-membered heterocycles were employed together with the gauge including atomic orbitals (GIAO) density functional theory (DFT) method at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p) and B3LYP/6-311+G(2d,p) levels of theory for the calculation of proton and carbon chemicals shifts and coupling constants. The method of geometry optimization for pyrrole (1), N-methylpyrrole (2) and thiophene (7) using the larger 6-311++G(d,p) basis sets at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,p) and B3LYP/cc-pVTZ levels of theory gave little difference between calculated and experimental values of coupling constants. In general, the (1)H and 13C chemical shifts for all compounds are in good agreement with theoretical calculations using the smaller 6-31 basis set. The values of nJHH(n=3, 4, 5) and rmnJ(CH)(n=1, 2, 3, 4) were predicted well using the larger 6-31+G(d,p) and 6-311++G(d,p) basis sets and at the B3LYP/6-31+G(d,p), B3LYP/6-311++G(d,p), B3LYP/6-31+G(2d,2p) levels of theory. The computed atomic charges [Mülliken; Natural Bond Orbital Analysis (NBO); Merz-Kollman (MK); CHELP and CHELPG] for the B3LYP/6-311++G(d,p) geometry optimized structures of 1-17 were used to explore correlations with the experimental proton and carbon chemical shifts.     

The prediction of (1)H chemical shifts in amines: a semiempirical and ab initio investigation

Twenty one conformationally fixed amines and their N,N-dimethyl derivatives were obtained commercially or synthesized. These included cis and trans 4-t-butyl cyclohexylamine, 2-exo and 2-endo norbornylamine, 2-adamantylamine, 4-phenylpiperidine, 1-napthylamine and tetrahydro-1-napthylamine. The (1)H NMR spectra of these amines were measured in CDCl(3) solution, assigned and the (1)H chemical shifts given. This data was used to investigate the effect of the amino group on the (1)H chemical shifts in these molecules. These effects were analyzed using the CHARGE model. This calculates the electric field and steric effects of the amino group for protons more than three bonds removed, together with functions for the calculation of two-bond and three-bond effects. The rotational isomerism about the C--N bond of the amino group was investigated by ab initio calculations of the potential energy surface (PES) about this bond at the HF/3-21G level. The resulting conformers were then minimized at the B3LYP/6-311 + + G (d,p) level. These geometries were then used to calculate the (1)H chemical shifts in the above compounds by CHARGE and the ab initio gauge-invariant atomic orbital (GIAO) method at the B3LYP/6-311 + + G(d,p) level and the shifts were compared with those observed. The compounds investigated gave 170 (1)H chemical shifts ranging from 0.60 to 8.2 ppm. The rms errors (obs.-calc.) were ca 0.1 ppm (CHARGE) and ca 0.2 ppm (GIAO). Large deviations of ca 1.0 ppm were observed for the NH protons in the GIAO calculations. The complex spectra of alkyl and aryl amines can thus be successfully predicted by both ab initio and semiempirical methods except for the NH protons, for which the ab initio calculations are not sufficiently accurate.     

Scaling factors for carbon NMR chemical shifts obtained from DFT B3LYP calculations

A linear scaling of the calculated chemical shifts is used in order to improve the accuracy of the DFT predicted ¹³C NMR chemical shifts. The widely applied method of GIAO B3LYP/6-311+G(2d,p) using the B3LYP/6-31G(d) optimized geometries is chosen, which allows cost-effective calculations of the ¹³C chemical shifts in the molecular systems with 100 and more atoms. A set of 27 ¹³C NMR chemical shifts determined experimentally for 22 simple molecules with various functional groups is used in order to determine scaling factors for reproducing experimentally measured values of ¹³C chemical shifts. The results show that the use of a simple relationship (δ_scalc = 0.95 δ_calc + 0.30, where δ_calc and δ_scalc are the calculated and the linearly scaled values of the ¹³C chemical shifts, respectively) allows to achieve a three-fold improvement in mean absolute deviations for 27 chemical shifts considered. To test the universal applicability of the scaling factors derived, we have used complex organic molecules such as taxol and a steroid to demonstrate the significantly improved accuracy of the DFT predicted chemical shifts. This approach also outperforms the recently recommended usage of the Hartree-Fock optimized geometries for the GIAO B3LYP/6-311+G(2d,p) calculations of the ¹³C chemical shifts.     

Substituent effects on 13C chemical shifts of ketenimines: a GIAO/HF and DFT study

GIAO/HF and DFT methods were utilized to predict the ¹³C chemical shifts of substituted ketenimines. GIAO HF/6–311+G(2d,p) and B3LYP/6–311+G(2d,p) methods were applied on the optimized B3LYP/6–31G(d) geometries and ¹³C chemical shifts of C_α and C_β of substituted ketenimines were correlated with group electronegativities. HF and DFT calculations indicated that increasing substituent group electronegativity leads to increasing chemical shift of C_β of substituted ketenimines, whereas the C_α values decrease. Copyright © 2003 John Wiley & Sons, Ltd.     

The observed and calculated 1H and 13C chemical shifts of tertiary amines and their N-oxides

A series of model tertiary amines were oxidized in situ in an NMR tube to amine N-oxides and their (1)H and (13)C NMR spectra were recorded. Next, the chemical shifts induced by oxidation (Δδ) were calculated using different GIAO methods investigating the influence of the method [Hartree-Fock (HF), Moeller-Plesset perturbation, density functional theory (DFT)], the functional applied in the DFT (B3LYP, BPW, OPBE, OPW91) and the basis set used [6-31G*, 6-311G**, 6-311 + + G** and 6-311 + + G(3df,3pd)]. The best results were obtained with the HF/6-311 + + G** and OPBE/6-311 + + G** methods. The computation/experiment comparison approach was used for the configuration prediction of chiral amine N-oxides-(R) and (S)-agroclavine-6-N-oxide.     

一种天然产物Wangzaozin A的细胞毒活性

从总序香茶菜Isodon racemosa (Hemsl) Hara植物中分离得到一个对人类肿瘤细胞Bel-7402和HD-8910具有毒活性的对映-贝壳衫烷型二萜Wangzaozin A化合物(1). 应用密度泛函理论(DFT)B3LYP方法, 对该分子的几何构型进行优化, 结果表明用B3LYP/6-31G(d)优化的几何参数与它的X射线衍射结构参数基本一致. 在优化的几何构型基础上, 采用规范不变原子轨道(GIAO)法, 在B3LYP理论水平分别用6-31G(d), 6-31G(d,p), 6-31+G(d,p)和6-31++G(d,p)基组进行核磁共振(NMR)化学位移值计算, 预测的1H和13CNMR化学位移值与实验值吻合; 统计误差分析表明, 用B3LYP/6-31G(d)优化的分子构型接近实际的分子构型. 因此, DFT方法适用这一类型化合物的构型和NMR参数进行预测. 在几何优化的基础上, 在B3LYP/6-31G(d)水平上, 对Wangzaozin A分子的静电位(MEP)进行理论计算. MEP三维图表明, 在Wangzaozin A分子中α-亚甲基环戊酮的羰基和羟基附近出现富电子区域(负电位), 起着供电子作用, 与受体的正电子区域结合. 这些结果从理论上支持了α-亚甲基环戊酮结构是一种抗肿瘤活性中心的看法.     

Theoretical investigation on multinuclear NMR chemical shifts of some tris(trifluoromethyl)boron complexes

Tris(trifluoromethyl)boron complexes have unusual properties and may find applications in many fields of chemistry, biology, and physics. To gain insight into their NMR properties, the isotropic ¹¹B, ¹³C, and ¹⁹F NMR chemical shifts of a series of tris(trifluoromethyl)boron complexes were systematically studied using the gauge‐included atomic orbitals (GIAO) method at the levels of B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G* and B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p). Solvent effects were taken into account by polarizable continuum models (PCM). The calculated results were compared with the experimental values. The reason that the structurally inequivalent fluorine atoms in a specific species give a same chemical shift in experimental measurements is attributed to the fast rotation of CF₃ group around the B? C(F₃) bond because of the low energy barrier. The calculated ¹¹B, ¹³C(F₃), and ¹⁹F chemical shifts are in good agreement with the experimental measurements, while the deviations of calculated ¹³C(X, X = O, N) chemical shifts are slightly large. For the latter, the average absolute deviations of the results from B3LYP/6‐311 + G(d,p)//B3LYP/6‐311 + G(d,p) are smaller than those from B3LYP/6‐31 + G(d,p)//B3LYP/6‐31G*, and the inclusion of PCM reduces the deviation values. The calculated ¹⁹F and ¹¹B chemical shieldings of (CF₃)₃BCO are greatly dependent on the optimized structures, while the influence of structural parameters on the calculated ¹³C chemical shieldings is minor. Copyright © 2009 John Wiley & Sons, Ltd.     

Comparison of Some Computational Methods for Geometry Optimisation of Phosphorus Acid Derivatives

Several economical methods for geometry optimization, that should be applicable to larger molecules, have been evaluated for 19 phosphorus acid derivatives. MP2/cc-pVDZ geometry optimizations are used as reference points and the geometries obtained from the other methods are evaluated with respect to deviations in bond lengths and angles, from the reference geometries. The geometry optimization methods are also compared to the much used B3LYP/6-31G(d) method. Single point energies obtained by subsequent EDF1/6-31+G(d) or B3LYP/6-31+G(d,p) calculations on the respective equilibrium geometries are also reported relative to the energies obtained from the reference geometries. The geometries from HF/MIDI! optimizations were closer to those of the references than the geometries of the HF/3-21G(d), HF/6-31G(d), and B3LYP/MIDI! optimizations. The EDF1/6-31+G(d) or B3LYP/6-31+G(d,p) single point energies obtained from the HF/3-21G(d), HF/6-31G(d), and B3LYP/MIDI! geometries gave a mean absolute deviation (MAD) from that of the reference geometries of 1.4-3.9 kcal mol m 1 . The HF/MIDI! geometries, however, gave EDF1/6-31+G(d) and B3LYP/6-31+G(d,p) energies with a MAD of only about 0.5 and 0.55 kcal mol m 1 respectively from the energies obtained with the reference geometries. Thus, use of HF/MIDI! for geometry optimization of phosphorus acids is a method that gives geometries of near-MP2 quality, resulting in a fair accuracy of energies in subsequent single point calculations, at a much lower computational cost other methods that give similar accuracies.     

FT-IR and NMR investigation of 1-phenylpiperazine: a combined experimental and theoretical study

FT-IR and (1)H, (13)C, DEPT, COSY, NOESY, HETCOR, INADEQUATE NMR spectra of 1-phenylpiperazine (pp) have been reported for the first time except for its (1)H NMR spectrum. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of pp (C(10)H(14)N(2)) have been calculated by means of the Hartree-Fock (HF) and Becke-Lee-Yang-Parr (BLYP) or Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31G(d) and 6-31G(d,p) basis sets, respectively. Comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for predicting vibrational frequencies and NMR properties.     

FT-IR and NMR investigation of 2-(1-cyclohexenyl)ethylamine: a combined experimental and theoretical study

FT-IR and (1)H, (13)C, DEPT, HETCOR, COSY, and NOESY NMR spectra of 2-(1-cyclohexenyl)ethylamine (CyHEA) have been reported for the first time. The vibrational frequencies and (1)H, (13)C NMR chemical shifts of CyHEA (C(8)H(15)N) have been calculated by means of the Hartree-Fock (HF), Becke-Lee-Yang-Parr (BLYP) and Becke-3-Lee-Yang-Parr (B3LYP) density functional methods with 6-31 G(d) and 6-31 G(d,p) basis sets, respectively. The comparison between the experimental and the theoretical results indicates that density functional B3LYP method is superior to the scaled HF and BLYP approach for vibrational frequencies and predicting NMR properties.