Ab initio studies of NMR chemical shifts for calix[4]arene and its derivatives

Ab initio calculations are performed for the calix[4]arene (1) and its derivatives (2 and 3), in this study. ¹H and ¹³C NMR measured spectral data given in our previous work are used to elucidate the structures of the prepared calix[4]arenes (1–3). The molecular geometry and chemical shift are calculated by using ab initio calculations based on the Hartree-Fock (HF) and the density functional theory (DFT) in the ground state. The results obtained from both methods are in agreement with the experimental results. The results of molecular geometry and chemical shifts show that DFT approach is closer to the experimental data than HF method.     

Benzooxirene. Ab initio calculations

Ab initio calculations have been performed on benzooxirene, the corresponding oxo carbene (“ketocarbene”), and the transition state linking the two. At the highest level used, QCISD(T)/6-31G^*//MP2(FULL)/6-1G^* with MP2(FULL)/ 6-31G^* zero point energy corrections, the relative energies of the oxirene, the transition state and the carbene are 0, 24.6, and −17.8 kJ mol⁻¹. Correlation energy effects are very important in this system: at the QCISD(T) level the oxirene lies above the carbene, as at the MP4 and HF levels, but at the MP2 level the ordering is reversed. Benzooxirene is probably slightly nonplanar: the HF/6-31G^* geometry is C_2v but the MP2(Fermi contact)/6-31G^* geometry is C_s with a 6-/3-ring coplanarity deviation of about 6.9 °, although in the MP2(FULL)/6-31G^* geometry this is reduced to about 3.1 °.     

Ab initio calculations on urea

Multiconfiguration wave functions constructed from contracted Gaussian-lobe functions have been found for the ground and valence-excited states of urea. ICSCF molecular orbitals of the excited states were used as the parent configurations for the CI calculations except for the ¹A₁(π → π*) state. The ¹A₁(π → π*) state used as its parent configuration an orthogonal linear combination of natural orbitals obtained from the second root of a three-configuration SCF calculation. The lowest excited states are predicted to be the n π → π* and π → π* triplet states. The lowest singlet state is predicted to be the n π → π* state with an energy in good agreement with the one known UV band at 7.2 eV. The π → π* singlet state is predicted to be about 1.9 eV higher, contrary to several previous assignments which assumed the lowest band was a π → π* amide resonance band. The predicted ionization energy of 9.0 eV makes this and higher states autoionizing.     

Ab initio calculations on porphin

Ab initio SCF calculations are reported for the porphin molecule. The positions of the central protons have been optimized, and the equilibrium geometry is found to be a linear NH ? HN arrangement. The NH vibrational frequencies have been computed and are compared to experimentally measured quantities. Several low ionized states have also been studied in separate spin-restricted SCF calculations. The lowest state is found to have B_1u symmetry with an ionization potential of 8.0 eV.     

Ab initio molecular orbital calculations

A series of non-empirical calculations on furan, pyrrole and 1,2,5-oxadiazole are reported in which the effect of polarisation functions added to the minimal 7s 3p basis on each atom is studied. The effect on these planar molecules is largely through the rather than the-system. A comparison with the results of work with scaled functions is reported. Both series are shown to lead to much improved agreement with the electron spectroscopy energy levels. The effect on the dipole moments of these changes in basis is more variable but, with the exception of furan, the agreement with experiment is improved in the present method.

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Zusammenfassung Für die Moleküle Furan, Pyrrol und 1,2,5-Oxadiazol wurde eine Reihe von nichtempirischen Rechnungen durchgeführt, in denen der Einfluß von zusätzlichen Polarisationsfunktionen zur minimalen 7s 3p-Basis an jedem Atom untersucht wird. Die Ergebnisse werden mehr durch die Art der Beschreibung des Systems der-Elektronen als durch diejenige der-Elektronen beeinflußt. Ein Vergleich mit den Ergebnissen bei Verwendung skalierter Funktionen wird durchgeführt. Beide Reihen von Ergebnissen zeigen eine verbesserte Übereinstimmung zu den Energiemeßwerten der Elektronenspektroskopie. Die Änderungen des berechneten Dipolmoments bei derartigen Basisvariationen sind größer als bei früheren Methoden. Die Übereinstimmung mit dem Experiment wird, mit Ausnahme von Furan, jedoch verbessert.

     

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     

Cluster models and ab initio calculations of (19)F NMR isotropic chemical shifts for inorganic fluorides

(19)F NMR isotropic chemical shift (delta(iso)) calculations are performed in crystallized compounds using the GIAO method with the B3LYP hybrid functional at DFT level. Clusters centered on the studied fluorine atoms mimic the crystalline structures. The 6-311+G(d) basis set is chosen for the central fluorine atom, and the LanL2DZ basis set for the others. The metal atoms are described by the 3-21G(2d) basis set or, when not available, by the CRENBL basis set with the corresponding ECP, and augmented with 2d polarization functions when existing. First, for high-symmetry systems (MF, MF(2), and MF(3) compounds), a systematization of the cluster building up from coordination spheres is proposed, generalized to fluoroperovskites and fluoroaluminates KAlF(4) and RbAlF(4). When applied to rather low symmetry systems such as barium fluorometalates BaMgF(4), BaZnF(4), and Ba(2)ZnF(6), the definition of the coordination spheres is far from easy. Then, for structures built up from a MF(6) octahedron network, we may define different "starting clusters": [FM(2)F(8)] for the shared fluorine atoms, [FMF(4)] for the unshared ones, and [FBa(4)](7+) for the "free" ones. Analogous "starting clusters" are then tested on compounds from the NaF-AlF(3), BaF(2)-AlF(3), and CaF(2)-AlF(3) binary systems and for alpha-BaCaAlF(7) that are also built up from a MF(6) octahedron network. For each of these corresponding fluorine sites, delta(iso) values are calculated with the "starting clusters" and several larger clusters and compared to the experimental delta(iso) values. For the barium-containing clusters, the RMS deviation is equal to 51 ppm. It is suggested that this result may be related to the poor quality of the barium basis sets for which no polarization functions are available for the moment. In total, chemical shifts were calculated for 122 fluorine sites, in a various range of compounds. For the clusters without barium, the ab initio method leads to a RMS equal to 22 ppm, which is a quite nice result keeping in mind that the (19)F chemical shift range is larger than 200 ppm.     

Ab initio quantum-mechanical calculations on trifluoromethylamine

Extensive ab initio molecular-orbital calculations were carried out on trifluoromethylamine (TFM) to elucidate changes in geometry and electronic structure upon fluorination. The calculations show that the decomposition of CF₃NH₂ is slightly endoenergetic, and the heats of atomization of CF₃NH₂ and CH₃NH₂ show decreased stability of the species upon fluorination. Characteristic of CF₃NH₂ is a highly polar, strong, short CN bond. More limited calculations were carried out on CF₃OH and CH₃OH, and the electronic structure of CF₃OH is found to be generally similar to that of CF₃NH₂. The reduced basicity of the fluorinated amine cannot be ascribed to the inductive effect; the enhanced acidity of the fluorinated alcohol reflects the weakening of the OH bond. No evidence leads to a confirmation of the existence of nitrogen–fluorine hyperconjugation in the fluorinated amine.     

Ab initio quantum chemical calculations of a cluster C8H12

Using unrestricted Hartree–Fock–Roothaan approximation and density functional theory on 6‐31G* basis, ab initio calculation cluster C₈H₁₂ has been carried out. It was shown as a result of calculations that a ground state for the conformation given is a septet state. This cluster can be used for investigation of the kinematic mechanism of magnetic exchange and magnetic ordering in polyradicals. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Ab initio molecular modeling of 13C NMR chemical shifts of polymers. 1. Ethylene–norbornene copolymers

Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force‐field computations allowed us to define the main conformational characteristics of ethylene–norbornene (E–N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and ¹³C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio ¹³C chemical shifts computations [gauge‐invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6‐311+G(2d,p) basis set on properly energy‐minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E–N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distortions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows the great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

硫代甲酰胺双聚体的量子化学计算

在MP2/6 31G(d)和MP2(FC)/6 311 G(d,p)水平上,对硫代甲酰胺(HC-SNH2)及其3种构型双聚体进行几何全优化计算,经振动频率分析,确认为势能超曲面上的稳定驻点.然后在MP2/6 311 G(2df,2p)水平上进行单点能计算和基组重叠误差(BSSE)校正以获得相互作用能.并利用自然键轨道(NBO)理论和分子中的原子(AIM)理论探讨HCSNH2之间相互作用的本质.     

DFT-GIAO calculations of 19F NMR chemical shifts for perfluoro compounds

The (19)F NMR shieldings for 53 kinds of perfluoro compounds were calculated by the B3LYP-GIAO method using the 6-31G(d), 6-31+G(d), 6-31G(d,p), 6-31++G(d,p), 6-311G(d,p), 6-311++G(d,p), 6-311G(2d,2p), 6-311++G(2d,2p), 6-311++G(2df,2p), 6-311++G(3d,2p), and 6-311++G(3df,2p) basis sets. The diffuse functions markedly reduce the difference between the calculated and experimental chemical shifts. The calculations using the 6-31++G(d,p) basis set give the chemical shifts within 10 ppm deviations from experimental values except for the fluorine nuclei attached to an oxygen atom, a four- and a six-coordinated sulfur atom, and FC(CF(3))(2) attached to a sulfur atom.     

Ab initio calculations of silicon-halogen-silicon double bridges

Summary Structure and stability of molecular clusters modelling halogen (F, Cl) double bridges between silicon atoms, H₃SiF₂SiH₃ (1), H₃SiF₂SiF₃ (2), H₃SiCl₂SiH₃ (3), and H₃SiCl₂ SiCl₃ (4), have been investigated by an ab initio pseudopotential method. Asymmetrical bridges Si-X...Si with one strong Si-X bond and one weak Si...X bonding interaction (X=F, Cl) result from the geometry optimization using the LP-31 G basis set. Dissociation energy calculations using the MP2/LP-31G*//LP-31G procedure and considering the basis set superposition error provide a decrease of stability of the structures in the order2>4>3>1. The results are discussed with respect to formation and decomposition of halogenated reaction overlayers formed during the etching of silicon by halogen atoms.

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Ab-Initio-Berechnungen von Silizium-Halogen-Silizium-Doppelbrücken

Zusammenfassung Struktur und Stabilität von molekularen Clustern, die Halogen(F, Cl)-Doppelbrücken zwischen Siliziumatomen modellieren, H₃SiF₂SiH₃ (1), H₃SiF₂SiF₃ (2), H₃SiCl₂SiH₃ (3) und H₃SiCl₂SiCl₃ (4), werden mittels eines Ab-Initio-Pseudopotentialverfahrens untersucht. Bei der Geometrieoptimierung unter Verwendung des LP-31 G-Basissatzes ergeben sich asymmetrische Brücken Si-X...Si mit einer starken Si-X-Bindung und einer schwachen bindenden Si...X-Wechselwirkung (X=F, Cl). Dissoziationsenergieberechnungen durch das MP2/LP-31 G*//LP-31 G-Verfahren unter Berücksichtigung des Basissatzüberlagerungsfehlers liefert eine abnehmende Stabilität der Cluster in der Reihenfolge2>4>3>1. Die Resultate werden im Zusammenhang mit der Bildung und dem Zerfall von halogenierten Reaktionsschichen, welche während des reaktiven Ätzens von Silizium mit Halogenatomen gebildet werden, diskutiert.

     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIV. Solvation effects in calculations of chemical shifts in 13C NMR spectra of chlorine-containing compounds

Analysis of precision factors in calculations of ¹³C NMR chemical shifts in the series of saturated and unsaturated organochlorine compounds was performed in the framework of the method of electron density functional theory GIAO-DFT-KT3/pcS-2 in the gas phase and with accounting for solvent effect by the polarized continuum model IEF-PCM. The accounting for solvation effects in calculations of ¹³C NMR chemical shifts within the framework of the IEF-PCM model is not fundamental for organochlorine compounds, yet it considerably improves the precision of calculations up to 2.5 ppm.     

Ab initio MO calculations and 17O NMR at natural abundance of para-substituted acetophenones

¹⁷O NMR chemical shifts and calculated (ab initio MO theory) electron densities are reported for a series of para-substituted acetophenones, X? C₄H₆? COCH₃, where X = NH₂, OCH₃, F, Cl, CH₃, H, COCH₃, CN, NO₂. The ¹⁷O shifts are very sensitive to the para substituent and cover a range of some 51 ppm. Donors induce upfield shifts and acceptors downfield shifts. The substituent chemical shifts (SCS) correlate precisely with σ_I and σ_R⁺ using the Dual Substituent Parameter (DSP) method. The derived transmission coefficients ρ_I and ρ_R indicate that polar and resonance mechanisms contribute approximately equally to the observed substituent effects. The shifts also correlate well with calculated π-electron densities (slope = 1500 ppm per electron) confirming their electronic origin. λ values are also reported, and the role of the average excitation energy, ΔE, in determining ¹⁷O SCS values is discussed. It is concluded that variations in ΔE are minor and that the local Δ-electron density is the dominant feature controlling ¹⁷O SCS values.     

Ab initio calculations on blocked Pro-Ala dipeptides

The effect of the chirality of the amino acid at position i + 2 on a β-turn was investigated by a grid scan ab initio calculation on the Ac- -Pro- -Ala-NH₂ and Ac- -Pro- -Ala-NH₂ blocked dipeptides. Th6-31G basis set was used to estimate the effect of the alanyl side chain on the conformation of the peptide backbone in a blocked dipeptide as a simple, but complete model for a reverse turn. This study provides a quantum mechanical evaluation of the ability of the NH at the i + 3 residue to form the H-bond that closes the 10 membered ring which stabilizes the turn. The lowest energy of all 64 probed conformations of the -Ala containing peptide corresponded to a good type II β-turn with a hydrogen bond distance between the acetyl oxygen and the amide terminal hydrogen of 2.21 Å. A comparison with the nonblocked dipeptide ab initio study indicates that the presence of the end blocks enhances the propensity of the -Ala-containing dipeptide for a type II β-turn, but does not seem to enhance the propensity of the -Ala-containing dipeptide for a type I β-turn. The energies and geometric parameters for the lowest four optimized conformations identified by the grid scan search for each molecule have been calculated.     

Application of quantum chemical calculations of C NMR chemical shifts to quinoxaline structure determination

Comparison of experimental and theoretical (GIAO DFT) ¹³C NMR chemical shifts allows the reliable assignment of isomeric structures of heteroaromatic compounds. This methodology was applied to establish the structures of isomeric quinoxalines. A modern 1D NOE technique permitted independent proof of the proposed structures.     

Ab initio CI calculations on free-base porphin

Ab initio configuration interaction (CI ) calculations were carried out on low-lying singlet and triplet π–π* states and ionized states of free-base porphin. We take into account single and double excitations from σ and π electrons in the CI calculations. The composite natural orbitals were employed in order to reduce the size of orbital set to be used in the CI . The calculated excitation energies were in good agreement with experimental values. The use of split-valence-type basis and the inclusion of correlation effects of σ electrons were proved to be important to describe the low-lying π–π* states, especially the Soret band.     

Ab initio calculations on bismuth cluster polycations

Ab initio calculations on bismuth polycationic species of the types Bi(n(n-2))+, Bi(n(n-4))+, and Bi(n(n-6))+ (n = 3 - 12) were performed at the Hartree-Fock and density functional theory levels in order to investigate their general properties and the applicability of Wade's rules on bismuth polycations. Some exceptions to Wade's rules were encountered, and, moreover, several predicted and calculated minima show only meta-stable behavior. The bonding in bismuth polycations is characterized by a high degree of electron delocalization and "three-dimensional aromaticity".     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VIII. Solvation effects on 15N NMR chemical shifts of nitrogen-containing heterocycles

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 σ_N values for weakly polar aprotic solvents, whereas solvation effects in strongly polar and protic solvents should be taken into account in the explicit form.