NMR parameters of SiC-doped (6,0) zigzag single-walled boron phosphide nanotubes: a density functional study

Abstract  

Nuclear magnetic resonance (NMR) parameters including isotropic and anisotropic chemical shielding parameters and electronic structures were calculated using density functional theory (DFT) for silicon–carbide-doped boron phosphide nanotubes. Geometry optimizations were carried out at the B3LYP/6-31G* level of theory using the Gaussian 03 program suite. The isotropic and anisotropic chemical shielding parameters were calculated for the sites of various ¹³C, ²⁹Si, ¹¹B, and also ³¹P atoms in pristine and SiC-doped (6,0) zigzag boron phosphide nanotube models. The calculations indicated that doping of ¹¹B and ³¹P atoms by C and Si atoms had a more significant influence on the calculated shielding tensors than did doping of the B and P atoms by Si and C atoms. In comparison with the pristine model, Si- and C-doping of P and B sites of the zigzag nanotubes reduces the energy gaps of the nanotubes and increases their electrical conductance.     

Quantum-chemical Investigation of the Hyprevalent Intramolecular Coordination X←N (X = C,Si, Ge) in Quasimonocyclic Models of IVa Group Atranes

The structure and the pentacoordination effect in quasimonocyclic models of IVa group atranes were investigated by ab initio[MP2 (full) /6-311+G **] and the density functional [B3LYP/6-311+G **] quantum chemical calculations. The calculations revealed considerable stabilization of the quasimonocyclic conformations relative to their free-of-strain trans-s-transconformations, which is caused by the formation of secondary (R)XN (X=C, Si, Ge) bonds of the hypervalent type. The strength of the intramolecular (R)XN coordination increases in the order X=C, Si, Ge. The nature of attractive (R)XN coordination is determined by donor-acceptor interaction of the nitrogen lone electron pair and antibonding orbital which primary localize at the X-R bond. Energy of X ··· N (X=Si, Ge) contact is about 3-7 kcal mol^-1.     

Dipole moments,structure, and transannular interactions in silatranes containing planar fragments

The method of dipole moments and theoretical calculations (DFT B3LYP/6-31G^*) were used for structural assessment of silatranes N[CH₂(RMeC₆H₂)O]₃SiR¹ containing planar fragments in the six-membered semirings. They are endo structures with transannular N→Si interaction which involves, along with nitrogen and silicon, oxygen atoms adjacent to silicon.     

The structure of 1-chlorosilatrane: An ab initio molecular orbital and a density functional theory study

The molecular geometries of the 1-chloro-, 1-fluoro-, 1-methyl-, and 1-hydrogenosilatranes were fully optimized by the restricted Hartree-Fock (HF) method supplemented with 3-21G, 3-21G(d), 6-31G(d), and CEP-31G(d) basis sets; by MP2 calculations using 6-31G(d) and CEP-31G(d) basis sets; and by GGA-DFT calculations using 6-31G(d5) basis set with the aim of locating the positions of the local minima on the energy hypersurface. The HF/6-31G(d) calculations predict long (>254 pm) and the MP2/CEP calculations predicted short (∼225 pm) equilibrium Si(SINGLE BOND)N distances. The present GGA-DFT calculations reproduce the available gas phase experimental Si(SINGLE BOND)N distances correctly. The solid phase experimental results predict that the Si(SINGLE BOND)N distance is shorter in 1-chlorosilatrane than in 1-fluorosilatrane. In this respect the HF results show a strong basis set dependence, the MP2/CEP results contradict the experiment, and the GGA-DFT results in electrolytic medium agree with the experiment. The latter calculations predict that 1-chlorosilatrane is more polarizable than 1-fluorosilatrane and also support a general Si(SINGLE BOND)N distance shortening trend for silatranes during the transition from gas phase to polar liquid or solid phase. The calculations predict that the ethoxy links of the silatrane skeleton are flexible. Consequently, it is difficult to measure experimentally the related bond lengths and bond and torsion angles. This is the probable origin of the surprisingly large differences for the experimental structural parameters. On the basis of experimental analogies, ab initio calculations, and density functional theory (DFT) calculations, a gas phase equilibrium (r_e) geometry is predicted for 1-chlorosilatrane. The semiempirical methods predict a so-called exo minimum (at above 310 pm Si(SINGLE BOND)N distance); however, the ab initio and GGA-DFT calculations suggest that this form is nonexistent. The GGA-DFT geometry optima were characterized by frequency analysis. © 1996 by John Wiley & Sons, Inc.     

Crystallographic and theoretical studies of 4,4′-dimethyl-7,7′-bi([1,2,5]thiadiazolo[3,4-b]pyridylidene)–chloranilic acid (1/1) with intermolecular O–H···N hydrogen bonds and S···O heteroatom interactions

Abstract  The molecular and crystal structure of a 1:1 co-crystal of 4,4′-dimethyl-7,7′-bi([1,2,5]thiadiazolo[3,4-b]pyridylidene)–chloranilic acid, (1), has been determined by X-ray diffraction at the monoclinic space group P2₁/c with cell parameters of a = 8.422(6), b = 7.343(4), c = 16.112(7) ?, β = 104.988(8)°, V = 962.5(10) ?³ and Z = 2. In the crystal structure, two components connect via the intermolecular O–H···N hydrogen bonds [2.804(4) ?] and S···O heteroatom interaction [2.945(3) ?] with R ₂ ²(7) couplings to form a unique and infinite one-dimensional supramolecular tape structure. The calculations of (1) at the HF/6-31G(d), MP2/6-31G(d), and B3LYP/6-31G(d) levels can almost reproduce X-ray geometry. In addition, the distances of the intermolecular O–H···N and S···O interactions by MP2/6-31G(d) and B3LYP/6-31G(d) levels agree well with those in the crystal. The calculated binding energies corrected BSSE and ZPE are −4.487 (HF), −7.473 (MP2), and −5.640 (B3LYP) kcal/mol. The results suggest that the complex (1) is very stable and the dispersion interaction is significantly important for the attractive intermolecular interaction in (1). The NBO analysis has revealed that the n(N) → σ*(O–H) interaction gives the strongest stabilization to the system and the major interaction for the intermolecular S···O contact is n(O) → σ*(S–N). Index Abstract  In the crystal structure of the title compound, the molecules are linked by intermolecular O–H···N hydrogen bonds and short S···O heteroatom interactions with R ₂ ²(7) couplings to construct a unique and infinite one-dimensional supramolecular tape structure.      

Quantum-Chemical Study of the Electronic and Steric Structure of Vinyltetrazoles: II. 2-Vinyltetrazoles

The total Mulliken charges on the C and N atoms, populations of the S-trans-(N¹) conformers, and rotation barriers in the molecules of 2-vinyl-5-R-tetrazoles (R = H, CH₃, CH = CH₂, C₆H₅, CH₂Cl, CF₃) were calculated ab initio (HF/6-31G**, MP2/6-31G**). The results were compared with the ¹H and ¹³C NMR data for these compounds.     

On a proposed radiation-induced polaronic hole in silicon dioxide

Ab-initio self-consistent-field molecular-orbital (SCF MO) Hartree–Fock (HF) calculations using the STO-3G, 6-31G, and 6-31G* basis sets, were performed to model quasi-tetrahedral silicon species in silicon dioxide. Mostly nine-atom clusters, [Si(OH)₄]^qt, with charge number q_t = 0 or + 1, were studied. The positions of the Si and O atoms were varied to achieve minimum total energies, while the protons were held fixed in the O-(neighboring)Si direction to simulate the rigid crystal surroundings. The α-quartz-type local symmetry C₂ was found to be retained for the neutral cluster, but not for the ionic one. The unrestricted HF calculations indicate that the latter paramagnetic centre, (q_t = +1), has its spin population almost entirely on one short-bonded oxygen ion bonded weakly to its neighboring Si, and is quite high in energy (9.55 eV with 6-31G) compared to the diamagnetic centre (q_t = 0). The ionization energy is much higher than the self-trapping potential of the polaronic hole, a fact which may account for the failure so far to observe a [SiO₄]⁺¹ center in quartz by means of continuous-wave electron paramagnetic resonance spectroscopy. Calculations on the [SiO₄]⁺¹ center agree well with ultraviolet spectra, and with the [hole portion of a] proposed radiation-induced exciton in quartz. The hole in [Si(OH)₄]⁺¹ can be shifted from a short-bonded to a long-bonded oxygen to give the excited state [Si(OH)₄]_es⁺¹. Conclusions reached with the nine-atom clusters were confirmed by a series of calculations on the extended model [Si(OSiH₃)₄]^qt. Comparisons with the known isoelectronic species [AlO₄]⁰ were carried out.     

Electronic and Steric Structure of ClAsX<Subscript>2</Subscript> Molecules: Evaluation by <Superscript>35</Superscript>Cl NQR and Quantum-Chemical Calculations

Molecules of the series ClAsX₂ [X = C₂H₅, N(CH₃)₂, OCH₃] were studied by RHF/6-31G(d) and MP2/6-31G(d) calculations. Their ³⁵Cl NQR frequencies were calculated from the populations of the 3p constituents of the chlorine valence p orbitals. The features of interaction of the geminal atoms in the molecules and the effect of this interaction on the electron distribution in them were analyzed.     

Intramolecular O → Si donor-acceptor interaction in R2P(=O)CH2CH2SiF3 molecules: Synthesis of dialkyl [2-(trifluorosilyl)ethyl]phosphonate and bis[(chloromethyl)dimethylsilyl] styrylphosphonate

The reaction of diethyl [2-(triethoxysilyl)ethyl]phosphonate with boron trifluoride etherate was used to synthesize diethyl [2-(trifluorosilyl)ethyl]phosphonate. The reaction of bis(trimethylsilyl) styrylphosphonate with chloro(chloromethyl)dimethylsilane gave bis[(chloromethyl)dimethylsilyl] styrylphosphonate. Multinuclear ¹H, ¹³C, ¹⁹F, ²⁹Si, and ³¹P NMR spectroscopy established the absence of a P=O → Si coordination bond in these compounds and the four-coordinate state of the silicon atom. Evidence for this finding was obtained by B3LYP/6-31G(d) quantum-chemical calculations. However, the same calculations for R₂P(=O)· CH₂CH₂SiF₃ (R = Me, Me₂N) showed the presence in such molecules of an O → Si coordination bond both in the gas phase and in CHCl₃ solution. The distance between the O and Si atoms in this series molecules decreases with R in the order: MeO > Me > Me₂N. The axial Si-F bond length increases in the same order and parallels the order of the Hammet σ ₀ ^m constants of these substituents, relating to their interaction with π-electron systems.     

Ab initio calculations of complexes of tetrachlorides of Group IVA elements: VIII. Structure of SiCl<Subscript>4</Subscript> complexes with hexamethylphosphoric triamide and dynamics of their formation

Quantum-chemical calculations of the systems SiCl₄←OP[N(CH₃)₂]₃ and SiCl₄←2OP[N(CH₃)₂]₃ with complete optimization of their geometry at various Si←O distances were performed by the RHF/6-31G(d) method. The first system was also calculated by the MP2/6-31G(d) method. The calculations of the systems with the complete geometry optimization resulted in trigonal-bipyramidal and trans-octahedral structures, respectively, having energy minima. When the components of the latter system approach each other, first their mutual polarization occurs, and then it is accompanied by electron density transfer from the H and P atoms of the electron-donor molecules to the Cl atoms of the acceptor. The results of the calculation of the trans-octahedral complex agree with the experimental ³⁵Cl NQR data. The electron density of Cl atoms increases upon complex formation, mainly due to an increase in their p _σ electron density.     

Ab initio calculations of group IVA tetrachloride complexes: IV. Dynamics of the formation of the complex of SiCl<Subscript>4</Subscript> with trimethylamine

Quantum-chemical calculations of the system SiCl₄←N(CH₃)₃ were fulfilled by the RHF/6-31G(d) and B3LYP/6-311G(d) methods with full geometry optimization at varied Si←N distances. The experimental electronic and steric structure of the complex were fit not on full geometry optimization but on the geometry optimization with the Si←N distance fixed at the experimental estimate. The calculations showed that the components polarize each other as come closer together. Furthermore, the electron density is transferred from the H atoms of the donor onto the Cl atoms of the acceptor. The C, N, and Si atoms serve only as electron density conductors. Original Russian Text V.P. Feshin, E.V. Feshina, 2007, published in Zhurnal Obshchei Khimii, 2007, Vol. 77, No. 5, pp. 786–791. For communication XX, see [1].     

The 3-21G(N*) basis set: An economical polarized basis set for ab initio studies on nitrogen-containing molecules

The 3-21G basis set shares with its older cousin, the 4-31G basis set, a tendency to overestimate valence angles at nitrogen atoms and to underestimate seriously barriers to inversion at such atoms. The 6-31G* basis set generally yields greatly improved results in these respects. It is here shown that, for a variety of molecules, supplementation of the 3-21G basis set at three- or two-coordinate nitrogen atoms with a set of six d-functions having exponent 1.0 leads to optimized geometries and inversion barriers at such nitrogen centers in good agreement with results obtained with the 6-31G* basis set. This supplemented basis set, designated as 3-21G(N*), also leads to calculated vibrational frequencies in good agreement with those calculated with the 6-31G* basis set. The 3-21G(N*) basis set offers an economical alternative to the 6-31G* basis set, particularly for molecules containing several first-row atoms other than nitrogen.     

Molecular structures of β-amino-α-bromovinyl trifluoromethyl ketones

The molecular structures of β-amino-α-bromovinyl trifluoromethyl ketones was studied by UV, IR, ¹H, ¹³C, and ¹⁵N NMR spectroscopy and using the density functional (B3LYP/6-31G(d,p) and PBE/QZ3P) and ab initio riMP2/cc-pVTZm quantum-chemical calculations. Factors affecting stabilization of the EZE-conformation of the molecules, which is atypical of analogous unsubstituted amino enones, are considered.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 102–106, January, 2005.     

ab initio calculations of complexes of group IVA tetrachlorides: I. Dynamics of complex formation of SiCl4 with pyridine

Quantum-chemical calculations of a trans-octahedral complex of SiCl₄ with pyridine involving complete optimization of its geometry and variation of the coordinate of the complex-formation reaction were fulfilled by the RHF/6-31G(d) method. The calculated electron distributions of chlorine atoms in the complex were confirmed by experimental ³⁵Cl NQR data. As the components of the system SiCl₄←2NC₅H₅ come closer together, the electron density on N atoms increases and on the Si atom decreases as a result of polarization of these components. On further mutual approach of the components, the electron density is transferred from the ligand to chlorine atoms (mainly on their p orbitals).     

Reaction of diphenyl[2-(triethoxysilyl)ethyl]phosphine oxide with boron trifluoride etherate

Diphenyl[2-(trifluorosilyl)ethyl]phosphine oxide was synthesized by the reaction of diphenyl[2-(triethoxysilyl)ethyl]phosphine oxide with boron trifluoride etherate. As shown by the ¹H, ¹³C, ¹⁹F, ³¹P, ²⁹Si multinuclear NMR spectroscopy data, the silicon atom in the molecule is tetracoordinate. The absence of P=O→Si interaction in diphenyl[2-(trifluorosilyl)ethyl]phosphine oxide, as follows from the comparison of the calculated [GIAO B3LYP/6-311++G(2d,p)] and experimental δ(²⁹Si) and δ(³¹P) values, is due to the formation of complex with BF₃ by the phosphoryl oxygen.     

Ab initio calculations of complexes of group IVA element tetrachlorides: II. Dynamics of complex formation of GeCl4 with trimethylamine

RHF/6-31G(d) calculations of the system GeCl₄←N(CH₃)₃ were performed with full geometry optimization and at varied Ge←N distance. Mutual approach of the system components is accompanied by their mutual polarization followed by electron density transfer from the H atoms of the donor to the Cl atoms of the acceptor. The C, N, and Ge atoms act merely as conductors of this electron density. The total energy of the system decreases until the Ge←N distance of 3.412 Å is attained; at this distance, however, the complex is not yet formed. The complex formation involves an increase in the energy by 0.213 eV. The applicability of the RHF/6-31G(d) method to studying the trigonal-bipyramidal complex was assessed.     

An ab initio molecular orbital study of the structures and energies of neutral and charged bimolecular complexes of NH3 with the hydrides AHn (A = N,O, F,P, S,and Cl)

Hartree-Fock 6-31G(d) structures for the neutral, positive ion, and negative ion bimolecular complexes of NH₃ with the first- and second-row hydrides AH_n (AH_n = NH₃, OH₂, FH, PH₃, SH₂, and ClH) have been determined. All of the stable neutral complexes except (NH₃)₂, the positive ion complexes with NH₃ as the proton acceptor, and the negative ion complexes containing first-row anions exhibit conventional hydrogen bonded structures with essentially linear hydrogen bonds and directed lone pairs of electrons. The positive ion complex NH₄⁺ …? OH₂ has the dipole moment vector of H₂O instead of a lone pair directed along the intermolecular line, while the complexes of NH₄⁺ with SH₂, FH, and ClH have structures intermediate between the lone-pair directed and dipole directed forms. The negative ion complexes containing second-row anions have nonlinear hydrogen bonds. The addition of diffuse functions on nonhydrogen atoms to the valence double-split plus polarization 6-31G(d,p) basis set usually decreases the computed stabilization energies of these complexes. Splitting d polarization functions usually destabilizes these complexes, whereas splitting p polarization functions either has no effect or leads to stabilization. The overall effect of augmenting the 6-31G(d,p) basis set with diffuse functions on nonhydrogen atoms and two sets of polarization functions is to lower computed stabilization energies. Electron correlation stabilizes all of these complexes. The second-order Møller–Plesset correlation term is the largest term and always has a stabilizing effect, whereas the third and fourth-order terms are smaller and often of opposite sign. The recommended level of theory for computing the stabilization energies of these complexes is MP2/6-31+G(2d,2p), although MP2/6-31+G(d,p) is appropriate for the negative ion complexes.     

Quantum-Chemical Study of Electronic and Steric Structure of Vinyltetrazoles: I. 2-Substituted 5-Vinyltetrazoles

The total Mulliken charges on the carbon atoms of the vinyl group, populations of S-trans-(N¹)conformers, and internal rotation energies were calculated ab initio (HF/6-31G**, MP2/6-31G**, and MP2/6-31G**//AM1) for a series of 2R-5-vinyltetrazoles (R = CH₃, C₂H₅, i-C₃H₇, t-C₄H₉, C₆H₅). The calculation results were compared to the available experimental data.     

Theoretical Investigation on the Effect of Different Nitrogen Donors on Intramolecular Se⋅⋅⋅N Interactions

The effect of different donor nitrogen atoms on the strength and nature of intramolecular Se ??? N interactions is evaluated for organoselenium compounds having N,N‐dimethylaminomethyl (dime), oxazoline (oxa) and pyridyl (py) substituents. Quantum chemical calculations on three series of compounds [2‐(dime)C₆H₄SeX ( 1 a – g ), 2‐(oxa)C₆H₄SeX ( 2 a – g ), 2‐(py)C₆H₄SeX ( 3 a – g ); X=Cl, Br, OH, CN, SPh, SePh, CH₃] at the B3LYP/6‐31G(d) level show that the stability of different conformers depends on the strength of intramolecular nonbonded Se ??? N interactions. Natural bond orbital (NBO), NBO deletion and atoms in molecules (AIM) analyses suggest that the nature of the Se ??? N interaction is predominantly covalent and involves nN→σ*_{Se? X} orbital interaction. In the three series of compounds, the strength of the Se ??? N interaction decreases in the order 3 > 2 > 1 for a particular X, and it decreases in the order Cl>Br>OH>SPh≈CN≈SePh>CH₃ for all the three series 1 – 3 . However, further analyses suggest that the differences in strength of Se ??? N interaction in 1 – 3 is predominantly determined by the distance between the Se and N atoms, which in turn is an outcome of specific structures of 1 , 2 and 3 , and the nature of the donor nitrogen atoms involved has very little effect on the strength of Se ??? N interaction. It is also observed that Se ??? N interaction becomes stronger in polar solvents such as CHCl₃, as indicated by the shorter r_{Se ??? N} and higher E_{Se ??? N} values in CHCl₃ compared to those observed in the gas phase.