NMR investigations of hexamethyldisilane confined in controlled pore glasses: Pore size distribution and molecular dynamics studies

In this work, the melting-point depression and molecular dynamics of hexamethyldisilane confined within five controlled pore glasses, with mean diameters ranging from 7.9 to 23.9 nm, are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substance. The melting-point depression in pores with radiusR follows the simplified Gibbs-Thompson equation ΔT=k _p/(R−s) with ak _p value of 74 K · nm and ans value of 1 nm. To our knowledge, this is the first time thek _p value of hexamethyldisilane is reported. Proton spin-lattice relaxation times (T ₁), spin-spin relaxation times (T ₂), and diffusivities (D) are reported as a function of temperature. The confinement in the pores gives rise to substantial changes in the molecular dynamics and the phase behavior. The line-shape measurements reveal a two-phase system assigned to a relatively mobile component at the pore walls and a crystalline solid at the center of the pores. However, theT ₂ measurements show that the mobile phase also embraces a minor component attributed to nonfrozen liquid in pockets or micropores. The diffusivity of the major narrow-line component is approximately three orders of magnitude larger than that in the plastic bulk phase, reflecting fast diffusion of mobile molecules. Below the melting region,T ₁ of the narrow line is significantly shorter thanT ₁ of the broad line, suggesting that the molecular reorientation is more hindered close to the surface than at the center of the pore.     

H and H NMR studies of cyclohexane nanocrystals in controlled pore glasses

The formation and behaviour of cyclohexane and cyclohexane-d₁₂ nanocrystals in mesoporous solids of well-defined dimensional constraints are studied by ¹H and ²H NMR. The NMR line widths, spin–spin relaxation times (T₂), spin–lattice relaxation times (T₁) and diffusivities (D) were measured as a function of temperature, and the results are discussed with reference to the values obtained for the bulk materials. The confined solids exhibit substantial changes in the phase behaviour and molecular dynamics. Thus, the line-shape measurements reveal a two-phase system consisting of a highly mobile component at the surface of the pore and a plastically crystalline phase in the centre of the pore. The liquid-like surface layer in the mesopores is observable well below the reduced transition temperature of the confined cyclohexane. However, the T₂ and diffusion measurements show that the mobile phase also embraces a minor component attributed to non-frozen liquid in pockets or offshoots.     

KINETIC STUDY OF ALKOXIDE PROMOTED DECOMPOSITION OF ORGANIC PHOSPHONIUM COMPOUNDS Evidence for Hexacovalent Intermediate

Abstract

This study reports rate data of the alkoxide promoted decomposition of triphenyl 3-hydroxypropyl phosphonium chloride, diphenyl di(3-hydroxypropyl)phosphonium chloride, and tetraphenyl phosphonium bromide. Comparison of kinetics, rate constants, and activation parameters of the alkoxide promoted decompositions points to different mechanism as compared to the hydroxyl promoted reaction. The alkoxide reaction is believed to proceed via a hexacovalent intermediate. Reasons for the hexacovalent route are discussed.     

STUDY OF THE ALKALINE CLEAVAGE OF THE P[sbnd]C BOND IN PHOSPHINE OXIDES AND DERIVATIVES OF TRICHLOROMETHANEPHOSPHONATE

Abstract

A study of alkaline decomposition of several aromatic phosphine oxides containing p- and o-nitrobenzyl, and trichloromethyl as leaving groups, is reported. The property of the trichloromethyl group as leaving group, and the CCl^? ₃-group's further decomposition in the hydrolysis of diethyl and disodium trichloromethanephosphonates, have also been investigated.     

Spectral analysis of the high-resolution proton-coupled carbon-13 NMR spectra of 1,3,2-dithiarsolanes

The high-resolution proton-coupled ¹³C NMR spectra of 2-chloro- and 2-phenyl-4-methyl-1,3,2-dithiarsolane and 2-chloro- and 2-phenyl-1,3,2-dithiarsolane have been obtained and completely analysed. The spectral analysis has provided all the ¹³C chemical shifts and ¹³C? ¹H coupling constants, including their relative signs. The results of this work are adequately interpreted in terms of two rapidly equilibrating half-chair conformers with the 4-methyl pseudo-axial or pseudo-equatorial. The cis isomers appear to be conformationally biased towards the form with the 4-methyl pseudo-equatorial. The two one-bond ¹³C? ¹H coupling constants of the methylene group are distinctly different. For both the cis and trans isomers, the largest one-bond coupling always involves the methylene proton situated syn to the substituent at arsenic. The observed variation in the geminal and vicinal ¹³C? ¹H coupling constants are discussed in terms of a stereospecific inductive effect of the sulphur hetero-atoms and conformational factors.     

Daily duration of vitamin D synthesis in human skin with relation to latitude, total ozone, altitude, ground cover, aerosols and cloud thickness

Vitamin D production in human skin occurs only when incident UV radiation exceeds a certain threshold. From simulations of UV irradiances worldwide and throughout the year, we have studied the dependency of the extent and duration of cutaneous vitamin D production in terms of latitude, time, total ozone, clouds, aerosols, surface reflectivity and altitude. For clear atmospheric conditions, no cutaneous vitamin D production occurs at 51 degrees latitude and higher during some periods of the year. At 70 degrees latitude, vitamin D synthesis can be absent for 5 months. Clouds, aerosols and thick ozone events reduce the duration of vitamin D synthesis considerably, and can suppress vitamin D synthesis completely even at the equator. A web page allowing the computation of the duration of cutaneous vitamin D production worldwide throughout the year, for various atmospheric and surface conditions, is available on the Internet at http://zardoz.nilu.no/~olaeng/fastrt/VitD.html and http://zardoz.nilu.no/~olaeng/fastrt/VitD-ez.html. The computational methodology is outlined here.     

13C?1H coupling constants in 9, 10-dihydroanthracene

The ¹H and ¹³C NMR spectra of 9,10-dihydroanthracene have been obtained at 2.1 and 9.4 T using selective decoupling of the methylene protons. Complete spectral analyses of the experimental spectra have provided the chemical shifts and coupling constants. The ¹³C? ¹H coupling constants in 9, 10-dihydroanthracene and biphenylene have been well accounted for by MNDO theoretical calculations of the molecular geometries and bond orders in these compounds.     

Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

In this work, the molecular dynamics of four organic compounds confined in silica pores of nominal diameter 6 and 20 nm are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substances. By using organic compounds forming soft plastic crystals on freezing as adsorbates, damage to the pore structures can be avoided. NMR lineshapes, spin-lattice relaxation times (T ₁), spin-spin relaxation timesT ₂ and diffusivities are reported as a function of temperature. Since the porous grains are much greater than the distance travelled by the molecules during the experiment, intracrystalline NMR parameters were obtained. However, the shortT ₂ (∼1 ms) encountered in both the bulk and confined samples prohibited measurements ofT ₂ and the diffusivity in the low-temperature ordered phases. The confinement in the pores gives rise to substantial changes in the phase behavior and molecular dynamics. Thus, the¹H lineshape observations of the confined samples clearly reveal a narrow-line component superimposed on a broad resonance at temperatures well below the transition point of the bulk material. In the freezing region, the narrow-line component is attributed to the surface layer and the undercooled liquid in the smaller pores that remains unfrozen. In the two-component, low-temperature region, the narrow component corresponds to the surface layer, while the broad component originates from the crystalline phase at the center of the pores.     

1H and 13C NMR studies on 1,3,2-dioxarsolanes

The ¹H and ¹³C NMR spectra of a series of 1,3,2-dioxarsolanes have been obtained at 2.1 T and some at 9.4 T. The chemical shifts and spin-spin coupling constants have been obtained from complete spectral analyses of the ¹H and proton-coupled ¹³C spectra. The spectral data are interpreted on the basis of two rapidly interconverting half-chair conformers with a pseudoaxial substituent at arsenic. Unique assignment of syn/anti or cis/trans geometries have been made from ¹H or ¹³C NMR spectroscopy alone. The syn and trans isomers of the 4-methyl- and 4,5-dimethyl-1,3,2-dioxarsolanes, respectively, appear to be conformationally biased towards the forms with pseudoequatorial methyl groups. The general trends in the geminal and vicinal ¹H? ¹H and ¹³C? ¹H coupling constants are interpreted in terms of stereospecific, electronegativity and lone-pair effects of the oxygen heteroatoms and conformational factors. The NMR data on the 1,3,2-dioxarsolanes are discussed with reference to related 1,3-dithoxa- and 1,3-dithia- five-membered rings with As, P, S or C at the 2-position.