黄嘌呤及其互变异构体的密度泛函理论研究

嘌呤碱及其衍生物在生物系统中起重要作用。对人具有兴奋和利尿作用的茶碱和咖啡碱就是黄嘌呤的甲基衍生物。黄嘌呤存在多种互变异构体,从理论计算的角度研究这些互变异构体的几何结构、电子结构及相对稳定性,进一步研究溶剂对其结构和性质的影响是有意义旧。密度泛函理论方法既考虑了电子相关,又较其它CI（组态相互作用）或MPn（n级微扰）方法节省机时。其计算结果较好,被广泛应用于研究各种化合物。本文采用密度泛函B3LYP／6—311G方法对14种黄嘌呤可能的互变异构体（见图1）,分别在气相和水相中进行几何构型全自由度优化和能量计算,讨论了异构体的相对稳定性,水的溶剂化作用对异构体的能量、几何构型、电荷分布和偶极矩的影响,探讨了溶剂极性对异构体的能量和偶极矩的影响。     

Quadrupolar coupling selective cross-polarization in solid state NMR

A new strategy is presented for achieving selective heteronuclear polarization transfers from half-integer quadrupolar spins in magic-angle spinning (MAS) NMR. By combining cross-polarization with a recently introduced RAPT pulse sequence that selectively excites the signal of a half-integer quadrupolar nucleus based on its quadrupolar coupling constant magnitude, we demonstrate that hetero-nuclei in its close proximity may be selectively excited. Selective 23Na --> 1H polarization transfers are demonstrated in Na2MoO4 x 2 H2O, Na2HPO4 x 2 H2O and a mixture of NaHCO3 and Na2HPO4 x 2 H2O.     

Density functional response theory calculations of three-photon absorption

Three-photon absorption probabilities delta(3PA) have been calculated through application of a recently derived method for cubic response functions within density functional theory (DFT). Calculations are compared with Hartree-Fock (HF) and with a coupled cluster hierarchy of models in a benchmarking procedure. Except for cases having intermediate states near resonance, density functional theory is demonstrated to be in sufficient agreement with the highly correlated methods in order to qualify for predictions of delta(3PA). For the larger systems addressed, a set of acceptor A and donor D substituted pi-conjugated systems formed by trans-stilbene and dithienothiophene (DTT), we find noticeable differences in the magnitude of delta(3PA) between HF and DFT, although similar trends are followed. It is shown that the dipolar structures, TS-AD and DTT-AD, have substantially larger delta(3PA) than other types of modifications which is in accordance with observations for two-photon absorption. This is the first application of density functional theory to three-photon absorption beyond the use of few-state models.     

Density functional theory and Hartree–Fock-density functional theory calculations of O, S, and Ge quadrupole coupling constants

The performance of several density functional theory and Hartree–Fock density functional theory methods in conjunction with Pople type bases for the calculation of ¹⁷O, ³³S, and ⁷³Ge quadrupole coupling constants in gaseous state molecules was investigated.Assessment of the several models was made by linear regression analysis of the calculated gradient of the molecular electric field versus the experimental nuclear quadrupole coupling constant (NQCCs). Calculations for oxygen on six molecules with the B3LYP/6-311++G(3df,3p) model yield a residual standard deviation of 0.057 MHz (1.4%); for sulfur on 12 molecules with the B3LYP/6-311G(3df,3p) model, 0.42 MHz (1.8%); and for germanium on nine molecules with the B3P86/6-311G(2d) model, 0.83 MHz (1.0%).In the case of germyl acetylene, our calculations indicate that the experimental NQCC reported some time ago by Thomas and Laurie [J. Chem. Phys. 44 (1966) 2602] was incorrectly assigned with respect to algebraic sign.Predictions are made of the ¹⁷O and ³³S NQCCs in furan, 4H-pyran-4-one, 4H-pyran-4-thione, and 4H-thiapyran-4-thione; and of the ⁷³Ge NQCC in germyl bromide.     

Density functional calculations of NMR chemical shifts and ESR g-tensors

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     

Solid state NMR studies and density functional theory (DFT) calculations of conformers of quercetin

This work presents the first comparative analysis of two crystallographic modifications of quercetin (3,3',4',5,7-penta-hydroxyflavone). The existence of dihydrate and unhydrated forms of quercetin in the solid state is confirmed by several experimental techniques e.g. X-ray diffraction of powders, DSC, TGA, and NMR. Our studies allow an understanding of the complexity of quercetin samples obtained from different sources. A PASS-2D experiment is employed to establish principal values of 13C chemical shift tensors for both modifications. Solid state NMR spectroscopy and DFT GIAO calculations provide unique information about NMR shielding and electron density distribution for different conformers. It has been concluded that changes of conformation and hydrogen bonding pattern have great influence on bond order parameters of quercetin. Theoretical calculations and experimental data do not exclude the existence of the syn conformer of quercetin, which so far was not considered in the condensed phase.     

Density functional theory predictions of isotropic hyperfine coupling constants

The reliability of density functional theory (DFT) in the determination of the isotropic hyperfine coupling constants (hfccs) of the ground electronic states of organic and inorganic radicals is examined. Predictions using several DFT methods and 6-31G, TZVP, EPR-III and cc-pVQZ basis sets are made and compared to experimental values. The set of 75 radicals here studied was selected using a wide range of criteria. The systems studied are neutral, cationic, anionic; doublet, triplet, quartet; localized, and conjugated radicals, containing 1H, 9Be, 11B, 13C, 14N, 17O, 19F, 23Na, 25Mg, 27Al, 29Si, 31P, 33S, and 35Cl nuclei. The considered radicals provide 241 theoretical hfcc values, which are compared with 174 available experimental ones. The geometries of the studied systems are obtained by theoretical optimization using the same functional and basis set with which the hfccs were calculated. Regression analysis is used as a basic and appropriate methodology for this kind of comparative study. From this analysis, we conclude that DFT predictions of the hfccs are reliable for B3LYP/TZVP and B3LYP/EPR-III combinations. Both functional/basis set scheme are the more useful theoretical tools for predicting hfccs if compared to other much more expensive methods.     

Density functional theory calculations for two-dimensional silicene with halogen functionalization

The electronic structures and band gaps of silicene (the Si analogue of graphene) adsorbed with halogen elements are studied using the density functional theory based screened exchange local density approximation method. It is found that the band gaps of silicene adsorbed with F, Cl, Br and I have a nonmonotonic change as the periodic number of the halogen elements increases. This is attributed to the transfer of contributions to band gaps from Si-Si bonding to Si-halogen bonding.     

Structure assignment in the solid state by the coupling of quantum chemical calculations with NMR experiments: a columnar hexabenzocoronene derivative.

We present a quantum chemical ab initio study which demonstrates a new combined experimental and theoretical approach, whereby a comparison of calculated and experimental (1)H NMR chemical shifts allows the elucidation of structural arrangements in solid-state molecular ensembles, taking advantage of the marked sensitivity of the (1)H chemical shift to intermolecular interactions. Recently, Brown et al. have shown that, under fast magic-angle spinning (MAS) at 35 kHz, the resolution in a (1)H NMR spectrum of the solid phase of an alkyl-substituted hexabenzocoronene (HBC) derivative is sufficient to observe the hitherto unexpected resolution of three distinct aromatic resonances ( J. Am. Chem. Soc. 1999, 121, 6712). Exploiting the additional information about proton proximities provided by (1)H double-quantum (DQ) MAS NMR spectroscopy, it was shown that the results are qualitatively consistent with the aromatic cores packing in a manner similar to that in unsubstituted HBC. Using the HBC-C(12) molecule as an example, we show here that the new combined experimental and theoretical approach allows the observed (1)H chemical shifts to be related in a quantitative manner to the intermolecular structure. In the quantum chemical calculations, a series of model systems of stacked HBC oligomers are used. On account of the marked dependence of the (1)H chemical shift to ring currents arising from nearby aromatic rings, the calculated (1)H chemical shifts are found to be very sensitive to the stacking arrangement of the HBC molecules. Moreover, the ring current effect is found to be particularly long range, with a considerable influence of the second neighbor, at a distance of 700 pm, being observed.     

Density functional theory calculations and vibrational circular dichroism of aromatic foldamers

Ab initio calculations together with vibrational circular dichroism (VCD) have been used for studying the conformations of a quinoline-derived oligoamide bearing a terminal chiral residue. Three helically folded conformers of the dimer, trimer, and tetramer forms of the oligomer were optimized at the density functional theory (DFT) level using the B3LYP functional and the 6-31G* basis set. For each form, the three conformers differ in their helical handedness and in the conformation of the chiral end group. The calculated structures of the tetramer and also the proportions predicted between them based on their calculated Gibbs free energies differences match remarkably well with experimental data collected on an octamer. Specifically, a R-phenethyl terminal group gives rise to a 91:9 ratio between left handed and right handed helices. The predicted VCD spectrum calculated from the Boltzmann population of the individual conformer reproduces very well the experimental VCD spectrum of the tetramer in CDCl3 solution. The DFT calculations performed for the trimer also allow one to assess the preferred handedness of the helix and the conformation of the chiral end group, but the calculated relative populations differ slightly from experimental data. Finally, this study shows that the dimer fragment is not sufficient to obtain valuable information on the conformation of this aromatic oligoamide foldamer.     

Density functional theory calculations of pressure effects on the vibrational structure of alpha-RDX

Pressure effects on the vibrational structure of alpha-RDX were examined using density functional theory (DFT) up to 4 GPa. The calculated vibrational frequencies at ambient conditions are in better agreement with experimental data than are previous single molecule calculations. The calculations showed the following pressure-induced changes: (i) larger shifts for lattice modes and for internal modes associated with the CH(2) and NO(2) groups as compared to the pressure shifts for modes associated with the triazine ring, (ii) enhancement of mixing between different vibrations, for example, between NN stretching and CH(2) scissor, wagging, twisting vibrations, and (iii) increase in mixing between translational lattice vibrations and the NO(2) wagging vibrations, reducing the distinction between internal and lattice modes. The calculated volume and lattice constants at ambient pressure are larger than the experimental values, due to the inability of the present density functional approach to correctly account for van der Waals forces. Consequently, the pressure-induced frequency shifts of many modes deviate substantially from experimental data for pressures below 1 GPa. With increasing pressure, both the lattice constants and the frequency shifts agree more closely with experimental values.     

黄芪异黄酮类化合物抗氧化活性的密度泛函理论研究

本文采用密度泛函理论中的B3LYP方法在6-311G(d,p)水平上对黄芪中的四种异黄酮类化合物毛蕊异黄酮、毛蕊异黄酮苷、芒柄花素、芒柄花苷进行了优化计算,从四个分子的几何结构、酚羟基氢原子上的NBO电荷、酚羟基解离能、HOMO和LUMO能级以及其能级差△E(LUMO-HOMO)等方面分析了四种黄芪异黄酮类化合物清除自由基的活性.C3.位的酚羟基为毛蕊异黄酮苷元及其苷分子的最大可能活性位点,C7位酚羟基也具有一定的活性,可以增加分子本身的抗氧化活性,C7位酚羟基为芒柄花素分子的活性位点.C3,位或C7位上酚羟基氢原子带正电荷越大、酚羟基的解离能越小、△E(LUMO-HOMO)越小、HOMO能级相对越高分子的抗氧化活性越高.糖苷取代C7位酚羟基上的H原子,可以提高HOMO、LUMO的轨道能级,但是分子失去了7位酚羟基,从而降低了毛蕊异黄酮苷分子的抗氧化活性.结果表明,四种黄芪异黄酮类化合物的抗氧化能力大小为:毛蕊异黄酮＞毛蕊异黄酮苷＞芒柄花素＞＞芒柄花苷.对芒柄花素和羟基自由基反应的过渡态进行了计算研究.     

Detection and use of small J couplings in solid state NMR experiments

Over the years, solid-state nuclear magnetic resonance (NMR) spectroscopy has become an important tool for materials science, with its local point of view that is highly complementary to the structural information provided by diffraction techniques, electron microscopy, and molecular modeling, for example. As compared to other interactions that determine the spectral expression of the local structure of the observed nuclei in solid-state NMR experiments, the J coupling, characteristic of the chemical bonds, has received far less attention because of its being generally so small that it is masked in the line-widths. Nevertheless, the scalar or isotropic part of J couplings, which is not averaged by magic angle spinning (MAS), can be evidenced in many systems, and exploited to unequivocally characterize the extended coordination sphere. In a first step we describe the different experiments that permit the observation and the measurement of J couplings, even when dealing with quadrupolar nuclei. We then present new and recently-published results that illustrate the state of the art of NMR methodologies based on or intended for measuring J couplings in solids and the novel perspectives that they open towards better understanding of ordered and disordered materials at the subnanometric scale, a length scale that is otherwise difficult to access.     

Structure determination of beta-Pb2ZnF6 by coupling multinuclear solid state NMR, powder XRD and ab initio calculations

The results from one-dimensional multinuclear (19F, 207Pb and 67Zn) magic-angle spinning nuclear magnetic resonance experiments combined with the use of the ISODISPLACE program allow for the space group determination of beta-Pb2ZnF6 (no. 138 P4(2)/ncm). The structure was refined from X-ray powder diffraction data (a = 5.633 (1) A and c = 16.247 (1) A, Z = 4). beta-Pb2ZnF6 has one six-fold coordinated Zn, one eleven-fold coordinated Pb and five F non-equivalent crystallographic sites and is built from alternated layers parallel to the (a, b) plane; tilted ZnF4(2-) layers of corner sharing ZnF6(4-) octahedra and FPb+ layers of edge sharing FPb4(7+) tetrahedra. The structure of beta-Pb2ZnF6 was then optimized using the ab initio code WIEN2k and the calculated 67Zn EFG is in agreement with the NMR results. 19F-19F proximities and 19F-207Pb connectivities were evidenced using through-space and through-bond NMR correlation experiments, respectively, and support the proposed structure. 19F-207Pb J-coupling was also used to select fluorine resonances depending on the number of neighbouring lead ions, leading to an unambiguous assignment of the different 19F resonances.     

Density functional theory calculations and vibrational spectra of 6-methyl 1,2,3,4-tetrahyroquinoline

The solid phase FTIR and Raman spectra of 6-methyl 1,2,3,4-tetrahydroquinoline (MTHQ) have been recorded in the regions 4000-100 and 3500-100 cm(-1), respectively. The spectra were interpreted with the aid of normal coordinate analysis following a full structure optimization and force field calculations based on the density functional theory (DFT) using the standard B3LYP/6-311+G(**) basis set combination. A close agreement was achieved between the observed and calculated frequencies by refinement of the scale factors.     

Quantum mechanical calculations of NMR J coupling values in the determination of relative configuration in organic compounds

[structure: see text] An approach relying on quantum mechanical calculations of proton-proton and proton-carbon J coupling values is proposed as a tool for assigning the relative configuration on chiral organic compounds. The method is suitable for carbon frameworks containing several adjacent stereogenic centers and may allow significant advances in the extensive use of spin-spin couplings in structural elucidation.     

Density functional calculations on cyclodextrins

Conformations of α-, β-, and γ-CDs under anhydrous conditions in the gas phase were investigated by a density functional method, B3LYP/6-31G(d,p). These calculations resulted in several symmetric conformations with different energies. The lowest energy conformations contain two rings of homodromic hydrogen bonds and are referred to “one-gate-closed” conformations. Different orientations of hydrogen bonds lead to four minima. Other conformational minima were found for “open” conformations which correspond to some extent to experimentally determined structures.