共查询到20条相似文献,搜索用时 11 毫秒
1.
A rather neglected family of heterocyclic compounds, the 3-pyrazolines or 2,3-dihydro-1H-pyrazoles, has been studied theoretically at the B3LYP/6–311++G(d,p) level to obtain geometries and energies and at the GIAO/B3LYP/6–311++G(d,p) level for NMR chemical shifts. The calculated barriers of inversion of the N-substituents reproduce adequately the scarce experimental data. The calculated 1H and 13C chemical shifts are consistent with those reported in the literature. A combination of both results, geometries, and 13C chemical shifts, shed light on the conformation of N-phenyl groups. 相似文献
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Wiberg KB Hammer JD Zilm KW Keith TA Cheeseman JR Duchamp JC 《The Journal of organic chemistry》2004,69(4):1086-1096
The MPW1PW91/6-311+G(2d,p) and MP2/6-311+G(2d,p) GIAO nuclear shieldings for a series of monosubstituted acetylenes have been calculated using the MP2/6-311G(2d,p) geometries. Axially symmetric substituents such as fluorine may lead to large changes in the isotropic shielding but have little effect on the tensor component (zz) about the C[triple bond]C bond axis. On the other hand, substituents such as vinyl and aldehyde groups lead to essentially no difference in the isotropic shielding but are calculated to give a large zz paramagnetic shift to the terminal carbon of the acetylene group, without having much effect on the inner carbon. The tensor components of the chemical shifts for trimethylsilylacetylene, methoxyacetylene, and propiolaldehyde have been measured and are in reasonable agreement with the calculations. The downfield shift at the terminal carbon of propiolaldehyde along with a small upfield shift at the adjacent carbon has been found to result from the coupling of the in-plane pi MO of the acetylene with the pi* orbital that has a node near the central carbon. The tensor components for acetonitrile also have been measured, and the shielding of cyano and acetylenic carbons are compared. 相似文献
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The nuclear magnetic resonance chemical shift is one of the most powerful properties available for structure determination at the molecular level. A review of advances made in the ab initio calculation of chemical shielding during the past five years is presented. Specifically, progress in the areas including the effects of an unpaired electron, electron correlation, and relativistic effects into ab initio chemical shielding calculations, the tensor nature of the chemical shift, and intramolecular and intermolecular effects on the chemical shift will be covered. 相似文献
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The 17O NMR spectra of some esters are discussed and are correlated with 13C NMR chemical shift and IR carbonyl streching band data. 相似文献
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I. I. Chervin M. D. Isobaev Yu. I. Él'natanov Sh. M. Shikhaliev L. V. Bystrov R. G. Kostyanovskii 《Russian Chemical Bulletin》1981,30(8):1438-1445
Conclusions A linear correlation was obtained between the barriers to pyramidal inversion of the phosphorus and the31P chemical shifts of acyldiisopropylphosphines. It was shown that it can be applied to various acyl-, diacyl-, triacyl-, and cyanophosphines if the increments of the31P chemical shifts of the substituents are taken into account.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 8, pp. 1769–1776, August, 1981. 相似文献
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Germanium chemical shifts were studied theoretically by the ab initio molecular orbital method. The compounds studied were GeMe4?xClx and GeMe4?xHx(x = 0–4). The calculated values of the germanium chemical shifts agreed well with the available experimental values. The germanium chemical shift is due to the p-electron mechanism that reflects the ligand electronic effect on the p-p* excitation term in the second-order paramagnetic term. For GeMe4?xHx, the chemical shift is almost linear to the number of the ligand, x. On the other hand, a U-shaped dependence is predicted for the chemical shifts of the GeMe4?xClx series and is shown to be caused by the strong and nonadditive electron-withdrawing ability of the Cl ligand. The diamagnetic contribution is relatively small for the chemical shift and is determined solely by a structural factor. © 1994 John Wiley & Sons, Inc. 相似文献
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K. A. Chernyshev V. A. Semenov L. B. Krivdin 《Russian Journal of Organic Chemistry》2013,49(3):379-383
Effect of solvation on the accuracy of DFT quantum-chemical calculations of 15N NMR chemical shifts of pyrrole, N-methylpyrrole, and pyridine was studied. The use of continuum model is sufficient to obtain consistent theoretical σN values for weakly polar aprotic solvents, whereas solvation effects in strongly polar and protic solvents should be taken into account in the explicit form. 相似文献
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In this investigation, semiempirical NMR chemical shift prediction methods are used to evaluate the dynamically averaged values of backbone chemical shifts obtained from unbiased molecular dynamics (MD) simulations of proteins. MD-averaged chemical shift predictions generally improve agreement with experimental values when compared to predictions made from static X-ray structures. Improved chemical shift predictions result from population-weighted sampling of multiple conformational states and from sampling smaller fluctuations within conformational basins. Improved chemical shift predictions also result from discrete changes to conformations observed in X-ray structures, which may result from crystal contacts, and are not always reflective of conformational dynamics in solution. Chemical shifts are sensitive reporters of fluctuations in backbone and side chain torsional angles, and averaged (1)H chemical shifts are particularly sensitive reporters of fluctuations in aromatic ring positions and geometries of hydrogen bonds. In addition, poor predictions of MD-averaged chemical shifts can identify spurious conformations and motions observed in MD simulations that may result from force field deficiencies or insufficient sampling and can also suggest subsets of conformational space that are more consistent with experimental data. These results suggest that the analysis of dynamically averaged NMR chemical shifts from MD simulations can serve as a powerful approach for characterizing protein motions in atomistic detail. 相似文献
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P. Pyykkö 《Chemical physics》1983,74(1):1-7
A relativistic analogue to Ramsey's theory of NMR chemical shifts is formulated. Four-component spinor one-electron wavefunctions and relativistic magnetic hamiltonians are used. In contrast to the third-order Pauli approximation theory of Nakagawa et al., the main relativistic effects are then included in the usual second-order theory. 相似文献
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Pinjari RV Dhumal NR Gejji SP 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2007,67(3-4):1144-1149
Successive substitution of CH groups of cubane (CH)(8), by isoelectronic nitrogen atoms leads to a class of energy-rich azacubanes (CH)(8-alpha)N(alpha) (with alpha=1-8). In the present work, we systematically investigate how substitution of nitrogen in a cubanoid influence deshielding of carbon and manifest in the chemical shift in NMR spectra calculated using the second order M?ller-Plesset (MP2) perturbation level of theory. PMR spectra predict a large downshift of delta(H)=7.9 ppm in heptaazacubane owing to the more number of nitrogen and the stronger C-H...N interactions. These chemical shifts are explained by the net atomic charges derived from the population analysis based on Hirshfeld partitioning scheme. 相似文献
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Bühl M 《Magnetic resonance in chemistry : MRC》2006,44(7):661-668
53Cr chemical shifts of CrO4(2-), Cr2O7(2-), CrO3X-, CrO2X2(X = F, Cl), and Cr(CO)5L (L = CO, PF3, CHNH2, CMeNMe2) are computed, using geometries optimized with the gradient-corrected BP86 density functional, at the gauge-including atomic orbitals (GIAO)-, BPW91-, and B3LYP levels. For this set of compounds, substituent effects on delta(53Cr) are better described with the pure BPW91 functional than with B3LYP, in contrast to most other transition-metal chemical shifts studied so far. For selected cases, 53Cr NMR line widths can be rationalized in terms of electric field gradients (EFGs) computed with the BPW91 functional, but in general other factors such as molecular correlation times appear to be dominating. 53Cr chemical shifts and EFGs are predicted for CrO3, Cr(C6H6)2, Cr(C6H6)CO3, and, with reduced reliability, for Cr2(mu2-O2CH)4. 相似文献
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An overview is given on recent advances of density functional theory (DFT) as applied to the calculation of nuclear magnetic
resonance (NMR) chemical shifts and electron spin resonance (ESR) g-tensors. This is a new research area that has seen tremendous progress and success recently; we try to present some of these
developments. DFT accounts for correlation effects efficiently. Therefore, it is the only first-principle method that can
handle NMR calculations on large systems like transition-metal complexes. Relativistic effects become important for heavier
element compounds; here we show how they can be accounted for. The ESR g-tensor is related conceptually to the NMR shielding, and results of g-tensor calculations are presented. DFT has been very successful in its application to magnetic properties, for metal complexes
in particular. However, there are still certain shortcomings and limitations, e.g., in the exchange-correlation functional,
that are discussed as well.
Received: 24 October 1997 / Accepted: 19 December 1997 相似文献
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Pilar Cabildo Rosa Maria Claramunt Jose Elguero 《Magnetic resonance in chemistry : MRC》1984,22(9):603-607
13C shielding data for 100 derivatives of pyrazole are reported. These include methyl, ethyl, n-propyl, tert-butyl, phenyl, hydroxymethyl, carboxyl, ethoxycarbonyl, cyano, amino, hydrazino, nitro, azido, chloro, bromo and iodo groups as substituents on the ring carbon atoms. 相似文献
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K. A. Chernyshev L. I. Larina E. A. Chirkina V. G. Rozinov L. B. Krivdin 《Russian Journal of Organic Chemistry》2011,47(12):1859-1864
Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ6-pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δP 170 ppm) and that the contribution of the spin-orbital contribution to the 31P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of 31P NMR chemical shifts. 相似文献
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15N chemical shifts in an extensive series of para (15) and meta (15) as well as ortho (8) substituted benzonitriles, X-C6H4-CN, were measured in deuteriochloroform solutions, using three different methods of referencing. The standard error of the average chemical shift was less than 0.03 ppm in most cases. The results are discussed for both empirical correlations with substituent parameters and quantum chemical calculations. The 15N chemical shifts calculated at the GIAO/B3LYP/6-31 + G*//B3LYP/6-31 + G* level reproduce the experimental values well, and include nitrogen atoms in the substituent groups (range of 300 ppm with slope 0.98 and R = 0.998, n = 43). The 15N shifts in hydroxybenzonitriles are affected by interaction with the OH group. Therefore, these derivatives are excluded from the correlation analysis. The resultant 15N chemical shift correlates well with substituent constants, both in the simple Hammett or DSP relationships and the 13C substituent-induced chemical shifts of the CN carbon. 相似文献
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49Ti chemical shifts of TiX4 (X = Cl, Br, F), TiClnMe(4-n) (n = 0-3), Ti(C5H5)2X2 (X = F, Cl, Br) and Ti(CO)6(2-) were computed, using geometries optimized with the gradient-corrected BP86 density functional, at the GIAO (gauge-including atomic orbitals)-Hartree-Fock, BPW91, and B3LYP levels. For this set of compounds, substituent effects on delta(49Ti) are reasonably well described with all methods considered; judged from mean absolute deviations from experiment, B3LYP performs best. Zero-point corrections to the delta(49Ti) values, evaluated from a perturbational approach based on vibrationally averaged effective geometries, turn out to be fairly small. Electric field gradients computed with the B3LYP functional do not correlate with trends in 49Ti NMR linewidths. Attempts are reported to correlate the delta(49Ti) values of Ti[YC(O)CHC(O)Y]2Cl2 (Y = H, Me, CF3, CN, F, Cl and Br) with the rate-limiting propagation barrier for ethylene polymerization using catalysts derived from these precursors. 相似文献