4 research outputs found
Introducing Enantioselective Ultrahigh-Pressure Liquid Chromatography (eUHPLC): Theoretical Inspections and Ultrafast Separations on a New Sub-2-μm Whelk-O1 Stationary Phase
A new chiral stationary phase for ultrahigh-pressure
liquid chromatography
(UHPLC) applications was prepared by covalent attachment of the Whelk-O1
selector to spherical, high-surface-area 1.7-μm porous silica
particles. Columns of varying dimensions (lengths of 50, 75, 100,
and 150 mm and internal diameters of 3.0 or 4.6 mm) were packed and
characterized in terms of permeability, efficiency, retention, and
enantioselectivity, using both organic and water-rich mobile phases.
A conventional HPLC Whelk-O1 column based on 5.0-μm porous silica
particles and packed in a 250 mm × 4.6 mm column was used as
a reference. Van Deemter curves, generated with low-molecular-weight
solutes on a 100 mm × 4.6 mm column packed with the 1.7-μm
particles, showed <i>H</i><sub>min</sub> (μm) and
μ<sub>opt</sub> (mm/s) values of 4.10 and 5.22 under normal-phase
and 3.74 and 4.34 under reversed-phase elution conditions. The flat
C term of the van Deemter curves observed with the 1.7-μm particles
allowed the use of higher-than-optimal flow rates without significant
efficiency loss. Kinetic plots constructed from van Deemter data confirmed
the ability of the column packed with the 1.7-μm particles to
afford subminute separations with good efficiency and its superior
performances in the high-speed regime, compared to the column packed
with 5.0-μm particles. Resolutions in the time scale of seconds
were obtained using a 50-mm-long column in the normal phase or polar
organic mode. The intrinsic kinetic performances of 1.7-μm silica
particles are retained in the Whelk-O1 chiral stationary phase, clearly
demonstrating the potentials of enantioselective UHPLC in terms of
high speed, throughput, and resolution
Chiral Peropyrene: Synthesis, Structure, and Properties
Herein we describe the synthesis,
structure, and properties of
chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne
derivatives as precursors, chiral peropyrenes were formed after a
4-fold alkyne cyclization reaction promoted by triflic acid. Due to
the repulsion of the two aryl substituents within the same bay region,
the chiral peropyrene adopts a twisted backbone with an end-to-end
twist angle of 28° that was unambiguously confirmed by X-ray
crystallographic analysis. The chiral peropyrene products absorb and
emit in the green region of the UV–visible spectrum. Circular
dichroism spectroscopy shows strong Cotton effects (Δε
= ±100 M<sup>–1</sup> cm<sup>–1</sup> at 300 nm).
The Raman data shows the expected D-band along with a split G-band
that is due to longitudinal and transversal G modes. This data corresponds
well with the simulated Raman spectra of chiral peropyrenes. The chiral
peropyrene products also display circularly polarized luminescence.
The cyclization reaction mechanism and the enantiomeric composition
of the peropyrene products are explained using DFT calculations. The
inversion barrier for racemization was determined experimentally to
be 29 kcal/mol and is supported by quantum mechanical calculations
Chiral Peropyrene: Synthesis, Structure, and Properties
Herein we describe the synthesis,
structure, and properties of
chiral peropyrenes. Using <i>p</i>-terphenyl-2,2″,6,6″-tetrayne
derivatives as precursors, chiral peropyrenes were formed after a
4-fold alkyne cyclization reaction promoted by triflic acid. Due to
the repulsion of the two aryl substituents within the same bay region,
the chiral peropyrene adopts a twisted backbone with an end-to-end
twist angle of 28° that was unambiguously confirmed by X-ray
crystallographic analysis. The chiral peropyrene products absorb and
emit in the green region of the UV–visible spectrum. Circular
dichroism spectroscopy shows strong Cotton effects (Δε
= ±100 M<sup>–1</sup> cm<sup>–1</sup> at 300 nm).
The Raman data shows the expected D-band along with a split G-band
that is due to longitudinal and transversal G modes. This data corresponds
well with the simulated Raman spectra of chiral peropyrenes. The chiral
peropyrene products also display circularly polarized luminescence.
The cyclization reaction mechanism and the enantiomeric composition
of the peropyrene products are explained using DFT calculations. The
inversion barrier for racemization was determined experimentally to
be 29 kcal/mol and is supported by quantum mechanical calculations
Helical Sense-Responsive and Substituent-Sensitive Features in Vibrational and Electronic Circular Dichroism, in Circularly Polarized Luminescence, and in Raman Spectra of Some Simple Optically Active Hexahelicenes
Four
different hexahelicenes, 5-aza-hexahelicene (<b>1</b>), hexahelicene
(<b>2</b>), 2-methyl-hexahelicene (<b>3</b>), and 2-bromo-hexahelicene
(<b>4</b>), were prepared and their
enantiomers, which are stable at r.t., were separated. Vibrational
circular dichroism (VCD) spectra were measured for compound <b>1</b>; for all the compounds, electronic circular dichroism (ECD)
and circularly polarized luminescence (CPL) spectra were recorded.
Each type of experimental spectrum was compared with the corresponding
theoretical spectrum, determined via Density Functional Theory (DFT).
Following the recent papers by Nakai et al., this comparison allowed
to identify some features related to the helicity and some other features
typical of the substituent groups on the helical backbone. The Raman
spectrum of compound <b>1</b> is also examined from this point
of view