Anomalous chemical shifts of benzene on spherical carbon adsorbents

Benzene adsorbed on spherical carbon adsorbents (SCA) heat-treated at 1273 K and above displayed in PMR spectra two signals with anomalous chemical shifts:=0 and=–10 ppm. The spectral characteristics of benzene adsorbed from the vapor phase at pressures p/p_s of 0.95, 0.666, and 0.106 and temperature 293 K were compared with those of benzene acquired on freezing of the liquid phase. The signal at =–10 ppm was found to be due to benzene molecules in micropores, while the signal at =0 was due to surface-bound benzene in larger-diameter pores.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 77–79, January–February, 1992.     

Anomalous chemical shifts of water and organic molecules adsorbed on graphitized surfaces of various types

¹H NMR spectroscopy methods have been used to study adsorption of water and organic molecules on graphitized carbon materials. As follows from the results obtained, the adsorbed molecules have anomalously low chemical shifts.     

Adsorption and mobility of water molecules and organic substances in carbon adsorbents 5. Self-diffusion of adsorbed molecules and fractal properties of carbon adsorbents

The self-diffusion coefficients of molecules of C₆H₆ and C₆H₁₄ adsorbed in two active carbons (AC) are not a function of the observation time in the time range from 1 to 600 msec. These AC have extensive microporous zones, and the AC studied do not exhibit fractal properties in the range of movements of 2–40 m for one and 1–100 m for the other.For previous communication, see [1].Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 10, pp. 2189–2192, October, 1989.     

NMR chemical shifts of molecules encapsulated in single walled carbon nanotubes

We present density functional theory calculations of the nuclear magnetic resonance spectroscopy of molecules encapsulated within single walled carbon nanotubes. Ring currents in the nanotube induce shifts in the chemical shift of the nuclei comprising the encapsulated molecule. These changes in the chemical shifts are shown to have characteristic dependence on the chirality of the surrounding nanotubes.     

Adsorption and mobility of molecules of water and organic compounds in carbon adsorbents

The mobility of molecules of water and fluorine-containing organic compounds adsorbed in a mixture on active carbons has been studied by the NMR pulsed field gradient technique (¹H and¹⁹F resonances). Samples with various H₂O/C₆F₆ and H₂O/C₂Cl₃F₃ ratios have been examined. The mobility of components at total fill factors > 0.8 has been shown to decrease in comparison with the adsorption of pure substances while the diffusion activation energy increases. The results are interpreted on the basis of adsorption mechanisms of water and organic compounds on active carbons.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 56–61, January, 1993.In conclusion the authors would like to express their thanks to the German Research Society for the financial support of this work.     

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.     

Quantum-chemical calculation of NMR chemical shifts of organic molecules: III. Intramolecular coordination effects on the 31P NMR chemical shifts of phosphorylated N-vinylazoles

Theoretical and experimental studies on magnetic shielding of the phosphorus nucleus in trichloro-[2-(1H-pyrazol-1-yl)ethenyl]phosphonium hexachlorophosphate(V) and 1,1,1,1-tetrachloro-1H-1λ⁶-pyrazolo-[1,2-a][1,2,3]diazaphosphol-8-ium-1-ide showed that intramolecular coordination of the phosphorus atom in the chlorophosphonium group to the nitrogen atom in the pyrazole ring leads to upfield shift of the phosphorus signal (to δ_P 170 ppm) and that the contribution of the spin-orbital contribution to the ³¹P chemical shift reaches 15%. Relativistic effects and effects of the medium are determining in the theoretical calculation of ³¹P NMR chemical shifts.     

Quantum chemical calculations of NMR chemical shifts of organic molecules: XI. Conformational and relativistic effects on the 31P and 77Se chemical shifts of phosphine selenides

Conformational and relativistic effects on the ³¹P and ⁷⁷Se chemical shifts of phosphine selenides were analyzed in terms of the ZORA-GIAO-B1PW91/TZP approach. The effect of conformation of phosphine selenides related to internal rotation about the single P-C bonds was found to be insignificant, while the contribution of relativistic spin-orbit interaction to the calculated values of ⁷⁷Se chemical shifts did not exceed 10 ppm. On the other hand, relativistic effects arising from magnetic shielding of the phosphorus nucleus in the P=Se fragment by selenium are fairly strong (25–30 ppm), which indicates the necessity of including the contribution of relativistic spin-orbit interaction in the calculation of ³¹P chemical shifts in phosphine selenides.     

Prediction of the 13C NMR chemical shifts of organic species adsorbed on H-ZSM-5 zeolite by the ONIOM-GIAO method

The ONIOM-GIAO method has been used to accurately predict 13C NMR chemical shifts for a series of organic species adsorbed on H-ZSM-5 zeolite. This is useful for the spectroscopic identification of complicated catalytic systems.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: VI. Accuracy of DFT calculations of 29Si chemical shifts of four-coordinate silicon compounds

Systematic analysis of factors affecting the accuracy of DFT calculations of ²⁹Si NMR chemical shifts in four-coordinate silicon compounds showed that the best agreement with the experimental values is attained using B1PW91 and PBE0 functionals in combination with the TZP basis set. In calculations of ²⁹Si chemical shifts by quantum-chemical methods particular attention should be given to the contribution of relativistic spin-orbit interaction and conformational equilibrium.     

Surface self-diffusion of organic molecules adsorbed in porous silicon

The pulsed field gradient nuclear magnetic resonance method has been employed to probe self-diffusion of organic guest molecules adsorbed in porous silicon with a 3.6 nm pore size. The molecular self-diffusion coefficient and intrapore adsorption were simultaneously measured as a function of the external vapor pressure. The latter was varied in a broad range to provide pore loading from less than monolayer surface coverage to full pore saturation. The measured diffusivities are found to be well-correlated with the adsorption isotherms. At low molecular concentrations in the pores, corresponding to surface coverages of less than one monolayer, the self-diffusion coefficient strongly increases with increasing concentration. This observation is attributed to the occurrence of activated diffusion on a heterogeneous surface. Additional experiments in a broad temperature range and using binary mixtures confirm this hypothesis.     

Frozen-liquid PMR spectra and interactions of water with spherical carbon adsorbents

The PMR spectrum from a spherical carbon adsorbent shows separate signals from the water in the pores ( 2–0 ppm) and outside the granules ( 5 ppm) at temperatures above 273 K because of the short spin relaxation times in the presence of numerous paramagnetic centers. Below 273 K, the water bound to the carbon after heat treatment above 1273 K may be in two states: A ( _A 2–0 ppm) and B ( _B –12 to –13 ppm). The enthalpy of (H₂O) _A (H₂O) _B is –50 kJ/mole. State B is associated with the graphitization occurring above 1273 K. It is suggested that the anomalous chemical shifts in states A and B are due to axial coordination in relation to the condensed benzene ring systems at the negatively charged surfaces.Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. 1, pp. 111–115, January–February, 1990.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XII. Calculation of the 13C NMR chemical shifts of fluoromethanes at the DFT level

Calculation was carried out of chemical shifts in ¹³C NMR spectra for a series of fluoromethanes CH _n F_4?n (n = 0–4) by the methods of the electron density functional theory GIAO-DFT taking in consideration the solvent effect in the framework of the polarizable continuum model Tomasi IEF-PCM. The best results were obtained at the use of Keal-Tozer KT3 functional combined with Pople standard basis sets 6-311G(d,p) and 6-311++G(d,p), and also with Jensen special set pcS-2 containing tight p-functions. The optimum reference in the calculation of chemical shifts in ¹³C NMR spectra for the fluoromethanes series is TMS.     

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.     

Carbon-13 chemical shifts of inverted carbon atoms

The inverted carbons in 2,4-methano-2,4-dehydroadamantane are shielded by 14.3 ppm relative to the corresponding carbons in 2,4-methanoadamantane. This was explained by the balance of the inverted carbon hybridization deshielding, the shielding effect of the cyclopropane rings formed, and the change in the influence of the neighbouring atoms.     

Adsorption and mobility of water and organic molecules in carbonaceous adsorbents. 4. Self-diffusion coefficients of water molecules and organic substances in active carbon having relatively large micropores

The self-diffusion coefficients of water in active carbon (AC) having relatively wide micropores increase in direct proportion to the degree of filling. The self-diffusion coefficients D of adsorbed C₆H₆, C₆F₆, C₆H₁₂, and H₂O molecules at 298 K do not depend on the size of the molecules and are larger, the greater the mobility of the molecules in the free liquids. The self-diffusion activation energy of the adsorbed molecules varies in the order: H₂O>C₆F₆> C₆H₁₂ C₆H₁₄. C₆H₁₄. For H₂O and C₆H₆ the activation energy increases as the characteristic size of the micropores of the AC increases.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1949–1951, September, 1989.     

Relationship of the critical parameters of adsorbed gases with the initial heats of adsorption on carbon adsorbents

Conclusions The initial heats of adsorption on microporous carbon adsorbents (Saran and S charcoals) are proportional to the critical parameters of the adsorbates. Doubling of the initial heat of adsorption in the slit micropore of S-band charcoal relative to the initial heat of absorption on the basis graphite face was established experimentally.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2803–2805, December, 1985.     

Quantum-chemical calculations of NMR chemical shifts of organic molecules: XIII. Accuracy of the calculation of 15N NMR chemical shifts of azines with account taken of solvation effects

Effects of solvation on the accuracy of the calculation of ¹⁵N chemical shifts in the azine series have been analyzed at the DFT/GIAO level of theory. The best results are obtained with the use of the Keal-Tozer KT2 functional in combination with the Dunning aug-cc-pVTZ basis set with inclusion of solvent effects according to the Tomasi polarizable continuum model (PCM). If specific solvation is strong, additional consideration of solvent effects in the supermolecule approximation is necessary with explicit inclusion of solvent molecules.