10 research outputs found
Unconventional O–H···C Hydrogen Bonding and Effects of Conformational Changes on Infrared Spectroscopy of <i>o</i>‑Cresol in Solutions
The unconventional
O–H···C intramolecular
hydrogen bonding and the effect of conformational changes on IR spectra
of <i>o</i>-cresol in aqueous solutions were investigated
by using molecular dynamics (MD) simulations, density functional theory
(DFT), and experiments. A facial rotational isomerization between
global minimum with <i>trans</i> conformation and the <i>cis</i> isomer is predicted to take place in gas phase with
a low barrier of about 3.7 kcal/mol through a <i>vertical</i>-like transition state. Upon solvation in aqueous solution, the contents
of energetically high–lying <i>vertical</i> and <i>cis</i> conformations of neutral <i>o</i>-cresol are
increased to 19% and 57%, respectively, in comparison with those (<i>vertical</i>, 0%; <i>cis</i>, 27%) in vacuum. The
IR spectra of aqueous solution are closely related to the relative
population of the different conformations, especially for the <i>cis</i> conformation with hydroxyl group facing to alkyl group.
The appearance of <i>cis</i> conformations and unconventional
O–H···C intramolecular hydrogen bond (HB) caused
the low-frequency shift in OH stretching vibration of the IR spectra,
which was also correlated with cation-like charge distribution and
the decrease in <i>s</i>-component of oxygen hybridization
orbital. However, the intermolecular HB between the <i>o</i>-cresol and surrounding water (<i>o</i>-cresol) molecules
gave rise to more evident shifts in IR spectra than that caused by
the intramolecular HB contributions in <i>cis</i> isomer.
Further identification of intramolecular HB effect is performed through
the comparison of spectrum changes that occurs on passing from aqueous
solution to less interacting (carbon tetrachloride and cyclohexane)
solvents. The absence of the intermolecular HB interaction between <i>o</i>-cresol and carbon tetrachloride (and cyclohexane) solvents
leads to the weaker intensity and narrower width of OH stretching
vibration region (around 3407 cm<sup>–1</sup>) in the IR spectra
than that in aqueous solution
Visible-Light-Promoted Radical C–H Trifluoromethylation of Free Anilines
The trifluoromethyl-substituted
anilines are biologically active
compounds and useful building blocks. In this communication, we have
developed the first visible-light-induced radical trifluoromethylation
of free anilines with the commercially available and easily handled
Togni reagent at room temperature. The resulting products were successfully
transformed into a variety of valuable fluorine-containing molecules
and heterocyclic compounds. This protocol provides an economical and
powerful route to trifluoromethylated free anilines
Role of Planar Conformations in Aggregation Induced Spectral Shifts of Supermolecular Oligofluorenols in Solutions and Films: A Combined Experimental and MD/TD-DFT Study
The supramolecular approach of fluorenol
polymers brings about
excellent self-assembly behavior to fabricate organogels and superstructured
thin films through highly directional noncovalent interactions. To
understand the aggregation effects on electronic structures, the packing
structures and the UV/vis absorption spectra of oligofluorenols (PFOH<i>n</i>, <i>n</i> = 1/3–8), with and without
OC<sub>8</sub>H<sub>17</sub> side chains, were studied experimentally
and theoretically in crystal, amorphous solids, and solutions, respectively.
For the ground state in vacuum the steric repulsion between two adjacent
fluorenol units renders the PFOH oligomers twisted in a helix conformation,
while the molecular aggregation favors the appearance of planar π-conjugated
structures. In comparison with the crystal packing, the content of
planar conformation (with the torsion angle less than 20°) is
increased in amorphous solids. The hydroxyl groups in oligofluorenols
facilitate the formation of hydrogen bonding networks. The red shift
in absorption spectra was observed in a systematic experimental study
of unsubstituted and substituted oligofluorenols with the increasing
concentration both in toluene and chloroform solutions. The subsitituted
oligofluorenol R-PFOH1 with only one OC<sub>8</sub>H<sub>17</sub> side
chain exhibited a shoulder peak at 430–440 nm, which is different
from PFOH1 without side chain and 3R-PFO1 with three OC<sub>8</sub>H<sub>17</sub> side chain. Time-dependent density functional theory
(TDDFT) calculations, which were carried out on conformation ensembles
taken from a series of molecular dynamics (MD) simulations, revealed
that the increase in the content of planar π-conjugated conformations
is correlated to the red shift in the absorption spectra upon increasing
the solution concentrations. The aggregation-induced red-shift in
absorption spectra of oligofluorenols, as well as the blue-shift for
oligothiophenes, was rationalized in a unified way from the increased
(and reduced) content of planar conformations in molecular aggregates