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Single-walled carbon nanotubes (SWCNTs) have been the focus of intense research, and the body of literature continues to grow exponentially, despite more than two decades having passed since the first reports. As well as extensive studies of the fundamental properties, this has seen SWCNTs used in a plethora of applications as far ranging as microelectronics, energy storage, solar cells, and sensors, to cancer treatment, drug delivery, and neuronal interfaces. On the other hand, the properties and applications of double-walled carbon nanotubes (DWCNTs) have remained relatively under-explored. This is despite DWCNTs not only sharing many of the same unique characteristics of their single-walled counterparts, but also possessing an additional suite of potentially advantageous properties arising due to the presence of the second wall and the often complex inter-wall interactions that arise. For example, it is envisaged that the outer wall can be selectively functionalized whilst still leaving the inner wall in its pristine state and available for signal transduction. A similar situation arises in DWCNT field effect transistors (FETs), where the outer wall can provide a convenient degree of chemical shielding of the inner wall from the external environment, allowing the excellent transconductance properties of the pristine nanotubes to be more fully exploited. Additionally, DWCNTs should also offer unique opportunities to further the fundamental understanding of the inter-wall interactions within and between carbon nanotubes. However, the realization of these goals has so far been limited by the same challenge experienced by the SWCNT field until recent years, namely, the inherent heterogeneity of raw, as-produced DWCNT material. As such, there is now an emerging field of research regarding DWCNT processing that focuses on the preparation of material of defined length, diameter and electronic type, and which is rapidly building upon the experience gained by the broader SWCNT community. This review describes the background of the field, summarizing some relevant theory and the available synthesis and purification routes; then provides a thorough synopsis of the current state-of-the-art in DWCNT sorting methodologies, outlines contemporary challenges in the field, and discusses the outlook for various potential applications of the resulting material.

The tridentate title ligand was found to act as an (NeqNaxOeq) = (Namide,Nimine,Ophenolate) ligand as well as an (NaxNeqOax) ligand in pentacoordinate silicon complexes depending on the substituents at the Si atom. Its two notably different coordination modes in (NeqNaxOeq)Si(CH2)3 and (NaxNeqOax)SiPhMe were studied with X-ray structure analysis as well as 29Si CP/MAS NMR spectroscopy together with quantum chemical calculations of geometry and NMR chemical shielding tensors.

Using restricted Hartree–Fock (RHF) and density functional theory (DFT) methods at gauge invariant atomic orbital (GIAO) condition, chemical shielding of 13 C and 15 N nuclei of imidazole and its 4-nitro derivative at different temperatures were calculated. Since we have used the realized structures determined by X-ray and neutron diffraction methods, accuracy of calculated results is improved. By partially optimization of structures of 4-nitroimidazole, determined by X-ray crystallography, the obtained results of these structures are also better than nonoptimized structures. Additionally, we have obtained an optimized geometry for hydrogen bond in structures mentioned above.

A density functional study was carried out to investigate the C-H···O hydrogen bonding effects on the 17O electric field gradient and chemical shielding tensors in crystalline 1-methyluracil. Since the structural coordinates were obtained from a neutron diffraction study, no geometry optimization was needed in performing the calculations. It is demonstrated that C-H···O hydrogen bonding has different influences on O2 and O4 EFG and CS tensors. In the tetramer model of 1-methyluracil, which is much closer to the real crystalline phase, the influence of C-H···O on the O4 EFG and CS tensors is significant, whereas this influence is negligible for O2.

Chemical shielding tensors are calculated for the carbons in a series of 4-aminoquinolines with different substituents at the 7-position. The sigma(11) component is used as a measure of the relative pi-electron density at each carbon. By comparing the pi-electron density at each carbon with the log K of binding to heme (Kaschula et al. J. Med. Chem. 2002, 45, 3531), the drug-heme association is found to increase with increasing pi-electron density at the carbons meta to the substituent and with decreasing pi-electron density at the carbons ortho and para to the substituent. The greatest change in pi-electron density is at the ortho carbons, and log K increases with a decrease in pi-electron density on the ring containing the substituent, which corresponds to an increase in the pi-dipole between the two rings. An examination of the solution structures of the pi-pi complexes formed by amodiaquine and quinine with heme (Leed et al. Biochemistry 2002, 41, 10245. de Dios et al. Inorg. Chem. 2004, 43, 8078) shows that the pi-dipoles in each drug and in the porphyrin ring of heme may be paired. The chloro-substituted compound has an association constant that is an order of magnitude higher than the other compounds in the series, but the pi-electron density at the ring containing the substituent is not correspondingly low. This lack of correlation indicates that the Cl-substituted compound may be binding to heme in a manner that differs from the other compounds in the series.

An ab initio computational study of the properties of 10 dihydrogen-bonded complexes of HnMH (M = Be, Al, Ga, Si, Ge) with the rare gas derivatives HArF and HKrF has been carried out at the MP2(full)/6-311++G(2d,2p) level of theory. Red shifts of H-Rg and Rg-F along with blue shifts of M-H vibrational stretching frequency were predicted. Variations of the 1H chemical shielding of the HRgF molecules versus the H⋯H distance of the complexes were also studied. © 2005 Elsevier B.V. All rights reserved.

Ab initio and density functional studies of the properties of 11 linear dihydrogen-bonded complexes pairing XeH2 with different proton donor molecules was undertaken at the MP2/6-311++G(2d,2p)/LJ18, MP2/DGDZVP, and B3LYP/DGDZVP computational levels. Red shifts of H-X along with blue shifts of Xe-H vibrational stretching frequencies were predicted. A linear correlation was established between interaction energies versus dipole moment enhancements of neutral complexes (R2 = 0.99). It is shown that there are linear correlations between absolute chemical shielding of 129Xe and 1H versus the charge on these atoms in XeH2 studied complexes (R2 = 0.99 and 0.96, respectively). © 2006 Elsevier B.V. All rights reserved.

Calculations of the phosphorus NMR chemical shielding in 5-coordinate phosphorus compounds have been carried out using the gauge-including-atomic-orbital (GIAO) 6-311+G(nd,p) basis set at both scaled density functional theory (sDFT) and estimated infinite order Moller–Plesset (EMPI) approaches. Results are generally in accord with previous studies on 3-coordinate phosphorus compounds but fail badly for compounds containing multiple chlorine atoms and indicate a need for a relativistic treatment of these species. We observe that some compounds with reported experimental 31P NMR chemical shifts far downfield of the calculated values are in fact in the range known from experiment and calculation to be in that expected for phosphonium ions; the reported structures need to be reconsidered.

We report the results of a quantum chemical investigation of the 31P nuclear magnetic resonance (NMR) isotropic chemical shieldings (σiso) and the 31P NMR chemical shielding tensor principal values...

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.

The absolute orientations of the three 13 C chemical shielding tensors in the phosphoenolpyruvate (PEP) moiety in a PEP-model compound with known crystal structure are reported. The study uses a fully 13 C-enriched polycrystalline sample of triammonium phosphoenolpyruvate monohydrate, (NH 4 ) 3 (PEP)⋅H 2 O, and 13 C MAS NMR experiments fulfilling various different 13 C rotational-resonance conditions. The absolute 13 C chemical shielding tensor orientations are derived by iterative fitting, employing numerically exact simulations, of various rotational-resonance 13 C MAS NMR lineshapes of the three- 13 C-spin system in fully 13 C-enriched (NH 4 ) 3 (PEP)⋅H 2 O. The implications of the results of this study for future, biochemically oriented solid-state NMR studies on the PEP moiety are outlined.

NMR chemical shielding calculations have been carried out on a series of selenium compounds employing our scaled density functional (DFT) approach as well as our EMPI method which combines a particular mixture of Hartree–Fock and Moller–Plesset calculations. The EMPI calculations provide results equivalent to coupled cluster methods while the scaled DFT approach provides considerable improvement over other DFT and ab initio methods.

We obtained the chemical shielding constants of 29 Si in Li 2 S–SiS 2 glass system on the basis of molecular orbital calculations. The relative chemical shieldings calculated at the HF/6-31G* level is −16.1 and −23.8 ppm for E (1) and E (0), respectively. These calculations are in good agreement with the previous NMR study: σ ( E (2))> σ ( E (1))> σ ( E (0)). It is found theoretically that incorporation of Li 2 S into SiS 2 does not influence the 29 Si chemical shift, while incorporation of Li 2 O into SiO 2 does to a significant extent. It is also found that the existence of oxygen in Li 2 S–SiS 2 glasses increases the chemical shielding of 29 Si by about 25 ppm, which is in good agreement with the experimental result.

A systematic investigation of the dependencies of the isotropic chemical shielding and the chemical shielding anisotropy upon the vertical-stacking distance for the respective 15N, 1H, and 13C nuclei present in simple models representing the aromatic and nonaromatic stacking (i.e., in benzene and formamide dimers accordingly) has been carried out. Although the approaches employed differ in the way they treat the effects of electron correlation and hence their ability to account for the energetics of stacking varies significantly, the qualitative characteristics of the stacking-induced variation in the chemical shielding parameters remain unchanged. This substantiates the use of the computationally cheap, DFT-based methods (the SOS-DFPT-IGLO strategy in particular) in modeling the secondary structural effects on chemical shielding parameters of biomolecular systems.

In this study we present an alternative approach to separating contributions to the NMR shift originating from the Knight shift and chemical shielding by a combination of experimental solid-state NMR results and ab initio calculations. The chemical and Knight shifts are normally distinguished through detailed studies of the resonance frequency as function of temperature and carrier concentration, followed by extrapolation of the shift to zero carrier concentration. This approach is time-consuming and requires studies of multiple samples. Here, we analyzed $^{207}$Pb and $^{125}$Te NMR spin-lattice relaxation rates and NMR shifts for bulk and nanoscale PbTe. The shifts are compared with calculations of the $^{207}$Pb and $^{125}$Te chemical shift resonances to determine the chemical shift at zero charge carrier concentration. The results are in good agreement with literature values from carrier concentration-dependent studies. The measurements are also compared to literature reports of the $^{207}$Pb and $^{125}$Te Knight shifts of $n$- and $p$-type PbTe semiconductors. The literature data have been converted to the currently accepted shift scale. We also provide possible evidence for the "self-cleaning effect" property of PbTe nanocrystals whereby defects are removed from the core of the particles, while preserving the crystal structure., Comment: 34 pages, 9 figures

19 F NMR chemical shielding constants in ( E )-1,2-difluoroethenyllithium ( 1 ) and trifluoroethenyllithium ( 2 ) have been calculated by the SOS-DFPT-IGLO 1 method and employed for an assignment of individual signals in experimental 19 F NMR spectra of polyfluoro-1-lithioethenes. The role of solvation on equilibrium geometries and consequently on the shielding constants is discussed. In comparison to HF 2 or MP2 methods, DFT calculated absolute shielding values for fluoroalkenes differ from the experimental values by an opposite offset error. All methods gave acceptable relative shielding values.

We report on 13C and 19F nuclear magnetic resonance study of acceptor doped fullerenes showing p-type semiconductor behavior. Room temperature 19F spectra are characteristic for rotating (MF6−) complexes, while the 13C spectra show that fullerene molecules are not mobile at temperatures up to 365 K. Reduction of the 13C chemical shielding anisotropy of (MF6)2C60 in comparison to solid C60 is assigned to the charge transfer between C60 and intercalated species.

We report the theoretical 19F NMR shielding tensor magnitudes and orientations for a series of fluoroaromatic species, together with a comparison with experimental results. We discuss results for Hartree−Fock (HF) and second-order Moeller−Plesset theory (MP2) geometry optimized structures and HF-gauge including atomic orbitals (HF-GIAO), sum over states-density functional theory-independent gauges for localized orbitals (SOS-DFT-IGLO) and MP2-GIAO shielding calculations, for several basis set arrangements. In general, MP2 and DFT methods show few improvements over HF methods, at the expense of time (MP2) and accuracy (MP2 and DFT). Pure density functionals overestimate the tensor breadths (spans), an effect that is only partially offset by use of hybrid exchange correlation functionals. HF-GIAO methods in general give good overall predictions of 19F shielding tensor elements. In the case of potassium tetrafluorophthalate, we also demonstrate that use of the charge field perturbation-IGLO technique provide...

The 19 F shielding tensors of HF, CH 3 F, CHF 3 , NF 3 , SiF 4 , POF 3 , F 2 , OF 2 , SiF 2 and SF 2 were calculated by DFT–GIAO methods. Acceptable basis set convergence and agreement with experimental data were found for the first six compounds. The performances of the BILYP, B3LYP, B3PW91 and MPW1PW91 hybrid functionals are equally satisfactory, while the G96LYP functional produces poor results in most cases. The optimum DFT level for 19 F shielding tensor calculations is found to be MPW1PW91/tzv2d1f. For the molecules F 2 , OF 2 , SiF 2 and SF 2 , the results are less satisfactory and sizeable deviations from experimental data are observed. It appears that DFT calculations of 19 F shielding tensors are less suited for these compounds at the present level of sophistication.

This paper presents a new method to calculate solid-state effects on NMR chemical shifts. Using full crystal potentials, this new method (CPPCh) eliminates the need to arbitrarily select the point charges that are included in the calculations of the NMR chemical shieldings to take into account intermolecular effects. By eliminating the arbitrary selection of the point charges, the method provides a mechanism to systematically improve the simulation of intermolecular effects on chemical shielding calculations. This new method has been applied to the calculation of the 31P NMR chemical shifts tensors in P4S3. The shielding calculations were done using the DFT approach with the BLYP gradient corrected exchange correlation functional. This method was selected to calculate the 31P chemical shifts because it includes electron correlation effects at a reasonable cost.

The hybrid density functional theoretical method has been successfully applied to the computation of the 59Co NMR chemical shifts and shift tensor components in a hexacoordinated Co(III) porphyrin ...

We report the observation of a new decoupling-induced recoupling phenomenon in multiple-quantum magic-angle-spinning (MQMAS) NMR spectra of half-integer quadrupolar nuclei. The origin of the effect is identical to the second-order recoupling between dipolar and anisotropic chemical shielding interactions under continuous-wave (CW) spin decoupling, which is first observed by Ernst et al. in the solid-state NMR spectra of spin-1/2 nuclei [J. Chem. Phys. 105 (1996) 3387].

We report a solid-state 17O NMR study of the 17O electric field gradient (EFG) and chemical shielding (CS) tensors for the oxonium ion, H3O+, in p-toluenesulfonic acid monohydrate (TAM). Both the 1...

The interrelation between the sum of all 13 C chemical shifts (Σδ 13 C) and the relative stabilities of diastereoisomeric compounds, estimated from computational methods, has been studied. A correlation exists between the degree of 13 C overall shielding and the computed relative stabilities in the cases where differences in the stereochemistry are due to different γ or e interactions. The δ- syn axial interactions cause substantial deshielding, although shielding is expected. The computational methods (molecular mechanics and semiempirical molecular orbital methods) are consistent when given the same most stable isomer within a set of isomers.

Local structure of phospholipid/amine polyion complexes in lyophilized powders of dipalmitoylphosphatidic acid(DPPA)/poly-l-lysine(pLYS) and DPPA/poly-l-arginine(pARG) is investigated by solid-stat...

NMR chemical shielding anisotropy tensors have been computed, employing several basis sets and the GIAO-RHF and GIAO-MP2 formalisms of electronic structure theory, for all the atoms of the five and nine typical backbone conformers of For-Gly-NH2 and For-L-Ala-NH2, respectively. Multidimensional chemical shift plots, as a function of the respective backbone fold, have been generated for both peptide models. On the 2D 1 H NH - 15 N NH and 15 N NH - 13 C plots the most notable feature is that at all levels of theory studied the backbone conformers cluster in different regions. Computed chemical shifts, as well as their averages, have been compared to relevant experimental values taken from the BioMagnetic Resonance Bank (BMRB). At the highest levels of theory, for all nuclei but the amide protons, deviations between statistically averaged theoretical and experimental shifts are as low as 1-3%. These results indicate that chemical shift information from selected multiple-pulse NMR experiments (e.g., 2D-HSQC and 3D-HNCA) could directly be employed to extract folding information for polypeptides and proteins. c 2000 John Wiley & Sons, Inc. J Comput Chem 21: 882-900, 2000

Numerous pathways by which nuclear spin polarization is transferred, rather than dissipated, through the spin relaxation process can be identified and exploited. In this study, application of relaxation-induced polarization transfer has been implemented to obtain the 13C chemical shielding anisotropy (−23 ± 6 ppm) of the methyl group in methylcobalamin dissolved in D2O (300 K).

A set of reference linear correlations is presented to convert ab initio quantum mechanically computed absolute chemical shielding tensors into the more common empirical δ(TMS) values. Large deviat...

The interpretation of nuclear spin relaxation data of biomolecules often requires the accurate knowledge of chemical shielding anisotropy (CSA) tensors, which significantly depend on the environmen...

Ab initio calculations of 31 P nuclear shielding were performed as a function of the number of carbon atoms in twelve alkyl phosphorus compounds: PH 3 , PH 2 (CH 3 ), PH(CH 3 ) 2 , P(CH 3 ) 3 , P(C 2 H 5 )(CH 3 ) 2 , P(C 2 H 5 ) 2 (CH 3 ), P(C 2 H 5 ) 2 (CHC 2 H 6 ), P(C 2 H 5 )(CHC 2 H 6 ) 2 , P(CHC 2 H 6 ) 3 , P(CHC 2 H 6 ) 2 (C(CH 3 ) 3 ), P(CHC 2 H 6 )(C(CH 3 ) 3 ) 2 and P(C(CH 3 ) 3 ) 3 . The calculation of the 31 P shielding tensors employed the GAUSSIAN 98 implementation of the gauge-including atomic orbital (GIAO) and continuous set of gauge transformations (CSGT) by using 6-311++G(2d,2p) basis set methods at both the Hartree–Fock (HF) and density functional levels of theories (DFT). A good linear correlation between the calculated chemical shielding at gas-phase and experimental shift values was obtained.

The influence of carbon doping (C-doping) on the electrostatic properties of the (10,0) zigzag aluminum nitride nanotube (AlNNT) is studied by density functional (DFT) calculations of chemical shielding (CS) tensors at the sites of Al-27 and N-15 nuclei in the optimized undoped and C-doped models. The calculated parameters indicate that the Al and N atoms located at the mouths of nanotube have the smallest and the largest chemical shielding isotropic (CSI) values among other Al and N nuclei, respectively. In the C-doped model, the NMR parameters of those nuclei directly bonded to C atoms undergo significant changes while those of other nuclei undergo minor changes.

[M 6 F] 5+ clusters were chosen to calculate the 19 F NMR chemical shielding for solid state alkali-metal fluorides MF (M=Li, Na, K, Rb) using gauge-independent atomic orbitals (GIAO) at Hartree–Fock (HF) and density functional theory (DFT) levels, respectively. The results agree with those measured experimentally, in particular when the relative deviations within the same series are considered. Our results reveal that the variation of chemical shielding from LiF to RbF correlates well with a combined effect from the lattice factor R −2 and the valency factor Q + Q − .

Theoretical calculations of the 13 C chemical shielding tensor were completed on different fragments of the crystal structure of cadmium acetate dihydrate using the GIAO method. The calculations show the need for including intermolecular interactions in order to reproduce the experimental 13 C chemical shift tensors of charged species. When a large enough fragment of the crystal is included in the calculation, the RMS difference between the experimental and the theoretical chemical shift tensor is 7 ppm, in comparison to differences of 64–20 ppm for calculations on isolated acetate anions and on cadmium acetate with and without waters of hydration. This RMS difference of 7 ppm is comparable to that obtained by calculations on neutral organic molecules, completed using similar computational approaches but without the inclusion of any intermolecular interactions.

The orientation dependence of aluminum chemical shielding is shown to vary from a few ppm in distorted octahedral complexes to more than 400 ppm in linear molecules. The analysis of solid-state 27A...

The combined effect from the 51V quadrupole coupling and chemical shielding anisotropy (CSA) has been characterized for five orthovanadates (Mg3(VO4)2, Zn3(VO4)2, BiVO4, TaVO5, NbVO5) and two metavanadates (RbVO3 and CsVO3) employing 51V magic-angle spinning (MAS) NMR of the central and satellite transitions. Furthermore, five metavanadates studied earlier (MVO3 where M = Li, Na, NH4, Tl, K) have been reinvestigated using rotor-stabilized MAS NMR, which improves the reliability of the spinning sideband (ssb) intensities. The complete manifolds of ssbs from all seven single-quantum transitions, observed in the MAS NMR spectra, have been analyzed using least-squares fitting and numerical error analysis. The optimized data demonstrate that for the metavanadates the magnitudes of the quadrupole and shielding anisotropy tensors (i.e., CQ, ηQ, δσ, ησ) and of the Euler angle, which relates the principal element of the two tensors, can be determined with high precision. Somewhat larger error limits are observed f...

An ab initio calculation of the principal components of the C-13 chemical shielding tensors of 4,4'-dimethoxybiphenyl are reported by employing the ab initio GIAO approach. The geometric parameters were optimized by using the Gaussian-92 program. The observed values of the C-13 NMR chemical shifts of a powdered sample were determined by using the recently introduced magic angle turning (MAT) experiment. Theoretical calculations successfully reproduced the MAT experimental resonance peaks. The results presented in this paper demonstrate the importance of using tensor information in assigning the experimental resonances. (C) 1998 Elsevier Science B.V.

Made available in DSpace on 2015-05-15T13:30:25Z (GMT). No. of bitstreams: 0 Previous issue date: 2014Bitstream added on 2015-05-15T13:56:35Z : No. of bitstreams: 1 ISSN1424-8220-2014-14-10-19260-19274.pdf: 3483504 bytes, checksum: 72d8f9bc57d4506749e176d816aadb45 (MD5) The ability to integrate multiple materials into miniaturized fiber structures enables the realization of novel biomedical textile devices with higher-level functionalities and minimally-invasive attributes. In this work, we present novel textile fabrics integrating unobtrusive multi-material fibers that communicate through 2.4 GHz wireless networks with excellent signal quality. The conductor elements of the textiles are embedded within the fibers themselves, providing electrical and chemical shielding against the environment, while preserving the mechanical and cosmetic properties of the garments. These multi-material fibers combine insulating and conducting materials into a well-defined geometry, and represent a cost-effective and minimally-invasive approach to sensor fabrics and bio-sensing textiles connected in real time to mobile communications infrastructures, suitable for a variety of health and life science applications. Universidade Estadual Paulista Júlio de Mesquita Filho, Departamento de Química Geral e Inorgânica, Instituto de Química de Araraquara, Araraquara, R. Professor Francisco Degni, S/N, Quitandinha, CEP 14800-900, SP, Brasil Universidade Estadual Paulista Júlio de Mesquita Filho, Departamento de Química Geral e Inorgânica, Instituto de Química de Araraquara

Three approximations to the isotropic chemical shielding functional are used to calculate isotropic hydrogen chemical shieldings in small molecules. The approximations, at most, require the electron density and its angular derivative at the hydrogen nucleus. Results are compared to a variety of theoretical calculations and/or experiments. Deviations from these calculations are at worst off by 40%, and at best off by a few percent.

Accurate measurements of the magnitudes and relative orientations of chemical shielding, J coupling and dipolar coupling tensors for isolated homonuclear 31P−31P spin pairs, using standard 31P MAS ...

We report the first successful density functional calculation of the 59Co chemical shift span as well as the chemical shift skew of [Co(NH3)6]Cl3 = (243 ppm, 0.57), [Co(NH3)4CO3]Br = (1562 ppm, 0.30), and Co(acac)3 = (1115 ppm, −0.42) which agree satisfactorily with the experimental values; viz. [Co(NH3)6]Cl3 = (308 ppm, 0.56), [Co(NH3)4CO3]Br = (1500 ppm, 0.33), and Co(acac)3 = (1230 ppm, −0.51). We have conclusively demonstrated that hybrid HF-DFT XC functionals B3PW91 and B3LYP are more suitable for 59Co shielding calculation compared to the pure density functional BLYP. Furthermore, the results of this study show that the performance of the B3PW91 functional is slightly superior to the B3LYP functional. It is suggested that B3PW91/6-311+G* or B3LYP/6-311+G* is promising for 59Co shielding calculations.

Locally dense basis sets employ large sets of functions on a particular atom or small molecular region, and smaller or attenuated sets elsewhere. This approach has been shown to be very useful in calculating NMR chemical shieldings at the Hartree-Fock level on systems otherwise computationally expensive. In the present work it is shown that such mixed basis sets can also be successfully employed at the second order many-body perturbation theory level (MBPT[2], or MP2). Shielding calculations have been carried out on a variety of carbon-, nitrogen-, oxygen-, and phosphorus-containing molecules and the locally dense shielding results analyzed at Hartree-Fock and MP2 levels, and with our recently introduced estimated infinite order Moller-Plesset (EMPI) approach.

The 59Co chemical shieldings of several hexacoordinated Co(III) complexes have been calculated by the sum-over-states density-functional-perturbation-theoretical method using individual gauges localized orbital (SOS-DFPT-IGLO). The Co TZVP basis set employed in the shielding calculation is tested by calculating the molecular properties of CoH and CoO. The results obtained indicate that the SOS-DFT-IGLO method shows promise in the calculation of 59Co shielding tensors.

The effects of including correlation in the calculation of phosphorus nuclear magnetic resonance (NMR) chemical shielding has been investigated for a variety of molecules in the Hartree-Fock, second-order Moller-Plesset (MP2), and estimated infinite-order Moller-Plesset theory ab initio approaches in the gauge including atomic orbital (GIAO) scheme. The inclusion of correlation in the shielding calculations often leads to significant changes from the Hartree-Fock theory and provides results that are in improved agreement with experiment. © 1996 John Wiley & Sons, Inc.

Ab initio MP2/aug′-cc-pVTZ searches of the potential surfaces of (H2C═PX)2 complexes, with X = F, Cl, OH, CN, NC, CCH, H, CH3, and BH2, have been carried out to identify and characterize the properties of complexes with P···P pnicogen bonds. All (H2C═PX)2 form equilibrium conformation A dimers with C2h symmetry in which A–P···P–A approaches a linear alignment, with A the atom of X directly bonded to P. Conformation A dimers containing the more electronegative substituents are stabilized by a P···P pnicogen bond, have shorter P–P distances, and have binding energies which correlate with the P–P distance. Dimers stabilized by a P···P pnicogen bond and two P···Hb interactions consist of those with the more electropositive substituents, have shorter P–Hb distances, and have binding energies which are too high for their P–P distances. Conformation A complexes with P···Hb interactions in addition to the P···P bond are more stable than the corresponding (PH2X)2 complexes, while with only one exception, complexes stabilized by only a P···P bond are less stable than the corresponding (PH2X)2 complexes. In the region of the potential surfaces with C–P···P–C approaching linearity (conformation B), the only planar equilibrium complex is (H2C═POH)2, which is stabilized primarily by two O–H···P hydrogen bonds. The remaining (H2C═PX)2 complexes are not stabilized by pnicogen bonds, but by π interactions between the two H2C═PX monomers which are in parallel planes. When A–P···P–C approaches linearity, two types of equilibrium structures with P···P bonds exist. Of the conformation C dimers, (H2C═POH)2 is planar and the most stable, with a P···P pnicogen bond and an O–H···P hydrogen bond. (H2C═PH)2 and (H2C═PCH3)2 are also planar, and stabilized by a P···P pnicogen bond and a P···Hb interaction. The absence of a P···Hb interaction results in nonplanar C′ conformations with structures in which the monomers essentially retain their symmetry plane, but the plane of one molecule is rotated about the P···P bond relative to the other. C and C′ dimers are less stable than the corresponding A dimers, except for (H2C═PCH3)2. 31P chemical shielding patterns are consistent with the changing nature of the interactions which stabilize (H2C═PX)2 complexes. EOM-CCSD 31P–31P spin–spin coupling constants increase quadratically as the P–P distance decreases.