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.     

Free radicals in γ-irradiated olefinic silica compounds

Results are reported from -irradiation of compounds of structure . each of which gives a characteristic ESR spectrum having a hyperfine structure whose resolution is somewhat reduced by the coupling of the radical to the silica. The splittings are 23–25 Oe (typical of alkyl radicals), except for the phenyl compound. Improved resolution is provided by varying the temperature between –196 and 20C. The bond to the surface gives the radical high thermal stability; contact with air at 20C converts the alkyl radicals to peroxyl ones. Hyperfine splitting from the hydrogen H_p 5000 Oe is observed in all compounds except.     

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.     

Anomalous chemical shifts of organic molecules adsorbed on spherical carbon adsorbents

The adsorption of simple organic molecules on synthetic carbon adsorbents of the SCA brand, thermovacuum treated at 2273 K, was studied by the PMR method. It was found that adsorbed molecules appear in the PMR spectra in the form of one or several signals, shifted toward strong magnetic fields in comparison with the proton signal of molecules in the condensed phase. The strongest shielding effect on the part of the surface of the material is experienced by adsorbent molecules in the SCA micropores. It was shown that the chemical shift does not depend on the nature of the adsorbed molecules and is apparently determined by the site of coordination in the interplanar gap of the graphitized portions of the carbon material. It is suggested that the anomalous values of the chemical shift of the protons of the adsorbed molecules are due to the influence of ring currents of the graphitized surface.Translated from Teoreticheskaya Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 80–85, January–February, 1992.     

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.