Ab initio calculations of NMR chemical shifts

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.     

Ab initio MO calculations on 1,2-dithietes and valence isomers

The valence isomerization of the 1,2-dithiete parent compound to the open-chain dithial was studied by CASSCF multiconfiguration methods including the CASPT2 perturbational treatment. The isomerization energy remains small at the highest level of theory. In agreement with Jonas and Frenking, the cyclic structure is only then preferred over the acyclic ones if f-functions on the sulfur atoms are considered. If they are included, the 1,2-dithiete is more stable by 3.8 kcal/mol and the barrier amounts to 24.9 kcal/mol at the CASPT2(8,8)/6-31G(2df)//MP2/6-31G* level of theory. According to MP2/6-31G* geometry optimizations, substitution of H by NH₂ and CH₃ reduces the stability of the 1,2-dithiete ring structures relative to the open-chain dithiocarbonyl structures, whereas the inverse holds for acceptor substitution by CN and CF₃. A higher stability of benzodithiete relative to ortho-dithiobenzoquinone is predicted at all employed levels of theory. This is in good agreement with conclusions drawn from experimental results. Whereas the experimental microwave geometry of 1,2-dithiete is well reproduced theoretically, the experimental electron diffraction geometry of 3,4-bis-(trifluoromethyl)-1,2-dithiete differs from the calculated one. © 1996 John Wiley & Sons, Inc.     

Ab initio SCF MO calculations on the CH3Br molecule

Self-consistent fieldab initio calculations have been performed for CH₃Br. The calculated equilibrium conformation is in good agreement with experiment. Valence and core level ionization potentials, Mulliken population analysis and electronic properties are presented. The ionization potentials are in good agreement with the experimental values, except for one case in which the experimental value may be wrongly assigned. The calculated dipole moment, 2.43 Debye, is 34% or 0.6 Debye larger than the experimental value.Two of us (C.W.B. and G. del C.) wish to acknowledge the hospitality of the IBM San José Research Laboratory and joint study agreements between the IBM and the Lawrence Berkeley Laboratory and the National University of Mexico respectively.     

Ab initio valence-bond calculations of H2O

The first and second bond dissociation energies for H₂O have been calculated in anab initio manner using a multistructure valence-bond scheme. The basis set consisted of a minimal number of non-orthogonal atomic orbitals expressed in terms of gaussian-lobe functions. The valence-bond structures considered properly described the change in the molecular system as the hydrogen atoms were individually removed to infinity. The calculated equilibrium geometry for the H₂O molecule has an O-H bond length of 1.83 Bohrs and an HOH bond angle of 106.5°. With 49 valence-bond structures the energy of H₂O at this geometry was ?76.0202 Hartrees. The calculated equilibrium bond length for the OH radical was 1.86 Bohrs and the energy, using the same basis set, was ?75.3875 Hartrees. After correction for zero point energies the calculated bond dissociation energies are: H₂O → OH + H, D₁=75.38 kcal/mole and OH → O+H, D₂=54.79 kcal/mole.     

17O NMR spectroscopy: Proton–oxygen coupling in simple alcohols at natural abundance

¹⁷O NMR data are reported for 3-pentanol ( 1 ), cyclopentanol ( 2 ), cyclohexanol ( 3 ), and cycloheptanol ( 4 ). The ¹⁷O NMR signals for 1–4 appeared as doublets, shown to arise from proton–oxygen coupling (¹J_OH = 76 ± 3 Hz) by proton decoupling experiments. The effect of concentration, temperature, and solvent was examined in detail for 2 . Proton—oxygen coupling was observed at low concentrations, decreased at lower temperatures, and was sensitive to solvent.     

Ab initio MO calculations on binding energies and stabilities of Na+ with ethylene diamine

Binding energies and stabilities of Na⁺ with bidentate ethylene diamine were studied using ab initio MO calculations with a 6-31G^* basis set. The computed results for bidentate ethylene diamine were compared with those for two (monodentate) CH₃NH₂ molecules.     

Ab initio MO LCAO UHF calculations of the stability in dimeric ethylene radical ions

The stability of ethylene dimer ions has been computed by ab initio methods. The positively charged dimer is stable (0.6–0.7 eV) while the neutral and the negatively charged dimers are unstable with respect to decomposition into monomers. The magnetic hyperfine coupling constants have also been evaluated, hyperfine splittings are half those in the monomer in agreement with experimental data.     

水分子垂直电离能的从头计算

分子的芯电子和价电子电离能可用XPS和UPS测得,并可作理论上的近似计算,其中ab initio Hartree-Fock SCF LCAO-MO法是目前广泛采用的一种方法.它所作的三个近似(非相对论近似、Born-Oppenheimer近似和单电子近似)很明确.除了几个基本物理常数外,不再引进任何参数.但是计算中选用的基函数对所得结果的精确度有较大影响.因此,常常需要以扩大的Slater型函数(STF)或Gaussian型函数(GTF)为基函数.由于所需计算的双电子积分的数目正比于基函数数目的四次方,这样就会使计算化费的计算机时间大大增     

Geometrical consequences of resonance-assisted intramolecular hydrogen-bond formation from Ab initio MO calculations on 2-nitroresorcinol

Ab initio orbital calculations on phenol, nitrobenzene, and 2-nitroresorcinol have been performed with the GAUSSIAN 92 series of programs. Initial RHF/6-31G^* and RHF/6-31G^** optimizations were followed by second-order MØller-Plesset MP2(FC)/6-31G^* optimizations. The general geometrical features of these molecules, and, in particular, the characteristic changes as going from phenol to 2-nitroresorcinol and from nitrobenzene to 2-nitroresorcinol are in good agreement with recent gas-phase electron diffraction studies and with the notion of resonance-assisted intramolecular hydrogen-bond formation in 2-nitroresorcinol.     

PH~3...H~2O体系分子间相互作用的从头计算MO研究

在MP2/6-311++G(3d,3p)水平,对PH~3...H~2O体系可能存在的氢键复合物进行了全自由度能量梯度优化,发现PH~3...H~2O体系存在三个能量极小结构A,B和C。其中结构A和B以H~2O作为质子授体,结构C以PH~3作为质子授体,结构A较结构B和C分别稳定6.52kJ/mol和8.18kJ/mol。结构A具有C~s对称性,其结构中P原子和O原子间距离为354.78nm,键角OHP为171.35ⅲ,属于接近于直线的传统型氢键结构。进一步在高级电子相关校正的MP4SDTQ下,用6-311++G(3df,3pd)基加上键函数{3s3p2d1f},通过BSSE校正,精确计算了结构A的结合能为-10.84kJ/mol。     

Organic peracids: a structural puzzle for 17O NMR and ab initio chemical shift calculations

We have applied (17)O NMR spectroscopy to investigate the structure of the organic peracids formed by reaction of acetic acid (AA) or lactic acid (LA) with aqueous hydrogen peroxide (HP), which are used in several "green chemistry" applications. The interpretation of the experimental spectra has been supported by ab initio calculations of the (17)O chemical shifts for several possible species, using a continuum representation of the solvent. The combined use of these tools has also allowed us to discuss the decomposition mechanism of LA/HP solutions. The calculated electric field gradients for water, HP, and CO(2) (a decomposition product of LA) correlate well with the experimental (17)O line widths.     

Structural characterization of poly(diethylsiloxane) in the crystalline, liquid crystalline and isotropic phases by solid-state 17O NMR spectroscopy and ab initio MO calculations

The structure of poly(diethylsiloxane) (PDES) has been characterized using solid-state NMR of (17)O. The sample studied had a weight-average molecular weight of 2.45 x 10(5). The sample was prepared by utilizing the cationic ring-opening polymerization of (17)O-enriched hexacyclotrisiloxane. Solid-state NMR of (17)O-enriched PDES was measured on the low-temperature beta(1) phase, the high-temperature beta(2) phase, the two-phase system consisting of the liquid crystal and isotropic liquid phase and the isotropic phase. From these data, the molecular structure and dynamics of PDES in the various phases were characterized via the chemical shifts of (17)O, and electric field gradient parameters were determined from NMR and ab initio molecular orbital (MO) calculations. In addition to the solid-state NMR of (1)H, (13)C and (29)Si previously reported on these samples, knowledge of the dynamic behavior of PDES as inferred from the NMR of (17)O in the present study was enhanced significantly. Further, the potential of combining the experimental NMR of (17)O with ab initio MO calculations to characterize the dynamics of polymers containing oxygen is demonstrated.     

Ab initio calculations on CICN and ONCI

Ab initio calculations are presented for the molecules CICN and ONCI with optimization of all geometric parameters. Calculated equilibrium geometries for CICN are in good agreement with microwave data; however, the calculated N-Cl distance in ONCI is about 0.1 Å shorter than obtained by electron diffraction. Orbital energies are calculated by means of Koopmans' theorem and also by ΔSCF calculations. The importance of relaxation energy is shown by comparing the calculated orbital energies with experimental data from photoelectron spectra of the valence levels.     

Ab initio versus CNDO potential surface calculations for Li2O and Al2O

Potential surfaces for Li₂O and Al₂O have been calculated by an ab initio SCF-LCAO-MO method and by the semiempirical CNDO method. For both molecules the semiempirical methods incorrectly imply unreasonable structures with very acute apex angles and very long bond distances — rather more like diatomic Li₂ or Al₂ molecules with O-atoms attached to their bonds. Our ab initio treatment does correctly predict a symmetrical linear configuration for Li₂O with bond distances in excellent agreement with experiment. This method also predicts a linear symmetrical structure for Al₂O, in agreement with experimental gas phase measurements but in disagreement with matrix-isolation studies.     

17O nuclear magnetic resonance studies of some amino acids at natural abundance

¹⁷O chemical shifts of four amino acids at natural abundance have been obtained. Sensitivity was enhanced by accumulating a large number of scans or employing high concentrations at elevated temperature. Routine memory overflow was avoided by using a combination of solvent suppression and phase alternating pulse sequences.     

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.

     

Ab initio calculations on Pro-Ala and Pro-Gly dipeptides

The geometries of the dipeptides -Pro- -Ala, -Pro- -Ala and -Pro-Gly were investigated by a grid scan ab initio calculation. The 6-31G basis set was used to estimate the effect of the alanyl side-chain on the conformation of the peptide backbone and to provide a computational basis for the interpretation of known physical-chemical properties of larger peptides that contain these dipeptides. These calculations furnish a direct quantum mechanical assessment of the energetic consequences of a methyl side-chain in the i + 2 position of a turn. The results of the calculation support the current view that the presence of a -Ala residue in the i + 2 position favors a type II β-turn over a type I β-turn conformation, while -Ala has the opposite effect. Total and relative energies for all the optimized conformations identified by the grid search are given and geometric parameters (bond lengths, bond angles and dihedral angles) and net atomic charges have been calculated.     

Natural abundance 17O NMR spectroscopy of heteroaromatic Nitro compounds

The ¹⁷O chemical shift data, at natural abundance, for selected nitroquinolines, nitroindoles, nitroindazoles and nitrothiophenes are reported. In the absence of a peri or a lone-pair repulsion effect, the nitroquinolines' chemical shifts differ little from those of their carbocyclic analogs. However, the signal for 5-nitroquinoline, 2 , is deshielded by 25 ppm compared to 6-nitroquinoline, 1 , and the ¹⁷O nucleus in 8-nitroquinoline, 3 , is deshielded by 49 ppm compared to that in 1 . Both these shifts are attributed to rotation of the nitro group from the plane of the heteroaromatic ring arising from peri hydrogen interaction and lone pair repulsion, respectively. The signals for nitro groups on electron excessive ring systems (e.g., indoles and thiophenes) are shielded relative to corresponding ones in electron deficient heterocyclic ring system analogs. The chemical shifts for the π-excessive systems are interpreted in terms of electronic effects.