Two commercially available reversed phases for high-performance
liquid chromatography (one dimethyl-n-octadecylsilane phase and one
di-isobutyl-n-octadecylsilane phase) and the native silica
substrate were
subjected to deuterium exchange in a mobile phase-like medium of
acetonitrile and deuterium oxide (90/10,
v/v). 29Si cross-polarization
magic-angle-spinning (CP MAS) NMR was used to obtain information on
the
amounts and relaxation behavior of the internal silanols of the silica
substrate, and the residual silanols
of the alkylsilane-derivatized phases. Consequences for the
correlation of silanol 29Si CP MAS NMR
data
and chromatography are discussed. Detection of deuterated silanol
(SiOD) signals in 29Si CP MAS NMR
of deuterium-exchanged phases depends on a transfer of magnetization
(cross-polarization) from silane
protons to silanol silicon atoms. It is concluded that a large
portion of the residual silanol NMR signal
of the phases stems from internal silanols and from silanols that are
spatially very close to an alkylsilane
attachment site. The former are considered chromatographically
irrelevant while the latter are assumed
to be inaccessible for analytes during the chromatographic separation
process for reasons of steric constraints.
One bulky di-isobutyl-n-octadecylsilane is capable of
cross-polarizing proton magnetization to 0.8 residual
surface silanols, whereas this is only 0.3 for the conventional
octadecylsilane with dimethyl side groups.
However, the lower surface concentration of bulky alkylsilanes
leaves more space for free, unhindered
silanols. The most important factor determining the relaxation
behavior of the residual silanol NMR
signal is the mobility of the octadecylsilane side chains, bulky
silanes having the larger mobility quenching
effect on residual silanols.
The synfacial heterodinuclear μ‐Cot complexes (Cot = cyclooctatetraene) [(CpCr) (CpM)]μ‐Cot (Cp = cyclopentadienyl; M Fe, 3; M Co, 4) are formed in a thermal reaction of the mononuclear mixed sandwich compound CpCr(n6‐Cot) and CpMLn [M Fe, Ln = benzene (Bz); M Co, Ln = (C2H4)2]. 3 possesses two unpaired electrons whereas 4 has only one unpaired electron and is ESR active. From the molecular structure of 3 and from the ESR data of 4 it can be deduced that the unpaired electrons are localized at the Cr centers predominantly forcing a close electronical relation between the heterodinuclear compounds 3 and 4 and the mononuclear sandwich complexes chromocene and CpCrBz, respectively.
29-Silicon
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Citation for published version (APA):Scholten, A. B., Haan, de, J. W., Claessens, H. A., Ven, van de, L. J. M., & Cramers, C. A. (1994). 29-Silicon NMR evidence for the improved chromatographic siloxane bond stability of bulky alkylsilane ligands on a silica surface. Journal of Chromatography, A, 688(1-2), 25-29. DOI: 10.1016/0021-9673%2894%2900928-7, 10.1016/0021-9673(94)00928-7
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AbstractA stable bond stationary phase for reversed-phase high-performance liquid chromatography, with a diisobutyl-noctadecylsilane derivatized surface. was studied using "Si cross-polarization magic angle spinning (CP MAS) NMR. Fumed silica surfaces (Aerosil), trimethylsilylated to different extents, were used to illustrate the effect of ligand surface loading on the hydrogen bonding contribution IO the ligand silane CP MAS NMR signal. Spectral comparison of the diisobutyl-n-octadecylsilane derivatized silica with the conventional dimethyl-n-octadecylsilane derivatized silica revealed significantly decreased hydrogen bonding of residual silanols to the ligand siloxane bond in the diisobutyl-n-octadecyl phase. This illustrates the increased steric protection of the ligand siloxane bond by the bulky alkyl substituents, which is assumed to be the reason for the improved hydrolytic stability at low pH of this phase.
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