ISSN:
1573-4994
Keywords:
HPLC stationary phases
;
ligand length
;
solid-state NMR spectroscopy
;
time resolved fluorescence anisotropy
Source:
Springer Online Journal Archives 1860-2000
Topics:
Physics
Notes:
Abstract Alkyl chain bonded “reversed” HPLC phases consisting of 6 to 30 carbon atoms are investigated by fluorescence spectroscopy, steady-state and time-resolved fluorescence anisotropy, and solid-state NMR spectroscopy. The structure and dynamics of the interphase formed by alkyl chains and liquid phase penetrating each other are studied as a function of alkyl chain length. Increasing alkyl chain lengths lead to enhanced partitioning of the fluorescent probe diphenylhexatriene (DPH) into the interphase, as monitored by fluorescence decay curves. The concomitant spectral red shift of DPH fluorescence excitation maxima is evidence of increased interphase polarizability. Time-resolved fluorescence anisotropy measurements reveal that the motion of the probe molecule in the interphase is “wobble in cone”-like. Cone angles θ and rotational correlation times τR change from θ = 63° and τR = 0.75 ns in C6 phases to θ = 42° and τR = 1.50 ns in C30 phases, thus indicating decreasing probe mobility with increasing ligand length. This interpretation is supported by 13C CP/MAS NMR spectra, which show reduced contributions of alkyl chain gauche conformations, i.e., enhanced interphase order, in phases with long alkyl chains and high surface coverage. A concomitant increase in the line-widths of 1H MAS NMR peaks indicates reduced mobility of the longer chains. The spectroscopic observations are consistent with the results of HPLC separations, where enhanced shape selectivity is found with increasing ligand length, rod-shaped molecules like DPH showing the greatest increase in retention time.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1023/A:1022534110945
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