6,328 research outputs found
Experimental and Theoretical Investigation of Regioselectivity for a Series of Ketoimines with Nuclear Magnetic Resonance Spectroscopy and Density Functional Theory
A series of ketoimines bearing pendant quinolyl substituents were prepared by Schiff base condensation of 1,3-diketones with two different substituents: trifluoromethyl and an alkyl/aryl group (e.g., Me, Et, iPr,tBu, Ph. Synthetic reactions of ketoimines with varying alkyl/aryl substituents altered the distribution of regioisomers as measured in isolated yields and detected by 1H and 19F NMR spectroscopy of crude reaction mixtures. Reaction with the least sterically encumbered diketone (CF3with Me) resulted in mixture of ketoimines with virtually quantitative formation of the ketoimine resulting from quinolyl addition to the carbonyl adjacent to the alkyl substituent. As the steric bulk of the hydrocarbon substituent increased (CF3with Et, iPr, or Ph), a mixture of regioisomers continued, which favored quinolyl addition adjacent to the trifluoromethyl substituent. Only a single ketoimine was isolated or observed with CF3and tBu substituents with quinolyl addition adjacent to CF3. In order to investigate the role of steric and electronic influence of differing alkyl/aryl substitution, Density Functional Theory (DFT) calculation were employed to determine probable transition state structures as well an quantify differences in activation energy between the two regioisomers. Transition state structures were calculated using QST3 calculations verified with IRC calculations at the B3LYP level of theory. Comparisons between DDG‡ determined by DFT agreed well with those calculated experimentally and supported steric driven regioselectivity for the series of ketoimines
Phylogeny and New Intrageneric Classification of Allium (Alliaceae) Based on Nuclear Ribosomal DNA ITS Sequences
The internal transcribed spacer region (ITS) of nuclear ribosomal DNA was sequenced from 195 representative species of Allium, two species of Nothoscordum, and one species each of lpheion, Dichelostemma, and Tulbaghia. Within the Allium species the lengths of the ITS regions were in a range from 612 to 661 base pairs and pairwise genetic distances reached up to 46%. The ITS data supported the inclusion of Nectaroscordum, Caloscordum, and Milula into Allium. Subgenera Rhizirideum and Allium, as well as sects. Reticulatobulbosa and Oreiprason were non-monophyletic taxa. Based on the phylogenetic relations, a new classification of genus Allium consisting of 15 monophyletic subgenera is presented. Sections Microscordum, Anguinum, Porphyroprason, Vvedenskya, Butomissa, Cyathophora, and Reticulatobulbosa are raised to subgeneric rank. Sections Austromontana N. Friesen, Eduardia N. Friesen, Mediasia F. O. Khassanov, S. C. Yengalycheva et N. Friesen, Nigrimontana N. Friesen, Falcatifolia N. Friesen, and Condensatum N. Friesen are newly described. Series Daghestanica, Pallasia, and Scabriscapa, as well as subsects. Eremoprasum, Longivaginata, and Sikkimensia are raised to sectional rank. A taxonomic conspectus of Allium at sectional level is given
Regional Trajectories of Entrepreneurship and Growth
The development of regions is considerably shaped by their history. We review research that finds significant persistence of regional levels of entrepreneurship over longer periods of time. It is argued that the long term persistence of regional entrepreneurship indicates the presence and effect of a culture of entrepreneurship that is conducive to new business formation and regional growth. Hence, regional development is characterized by long term trajectories of entrepreneurship. We derive a number of policy implications and propose avenues for further research.</p
Neutron spectroscopic study of crystal field excitations in Tb2Ti2O7 and Tb2Sn2O7
We present time-of-flight inelastic neutron scattering measurements at low
temperature on powder samples of the magnetic pyrochlore oxides Tb2Ti2O7 and
Tb2Sn2O7. These two materials possess related, but different ground states,
with Tb2Sn2O7 displaying "soft" spin ice order below Tn~0.87 K, while Tb2Ti2O7
enters a hybrid, glassy spin ice state below Tg~0.2 K. Our neutron
measurements, performed at T=1.5 K and 30 K, probe the crystal field states
associated with the J=6 states of Tb3+ within the appropriate Fd\bar{3}m
pyrochlore environment. These crystal field states determine the size and
anisotropy of the Tb3+ magnetic moment in each material's ground state,
information that is an essential starting point for any description of the
low-temperature phase behavior and spin dynamics in Tb2Ti2O7 and Tb2Sn2O7.
While these two materials have much in common, the cubic stanate lattice is
expanded compared to the cubic titanate lattice. As our measurements show, this
translates into a factor of ~2 increase in the crystal field bandwidth of the
2J+1=13 states in Tb2Ti2O7 compared with Tb2Sn2O7. Our results are consistent
with previous measurements on crystal field states in Tb2Sn2O7, wherein the
ground state doublet corresponds primarily to m_J=|\pm 5> and the first excited
state doublet to mJ=|\pm 4>. In contrast, our results on Tb2Ti2O7 differ
markedly from earlier studies, showing that the ground state doublet
corresponds to a significant mixture of mJ=|\pm 5>, |\mp 4>, and |\pm 2>, while
the first excited state doublet corresponds to a mixture of mJ=|\pm 4>, |\mp
5>, and |\pm 1>. We discuss these results in the context of proposed mechanisms
for the failure of Tb2Ti2O7 to develop conventional long-range order down to 50
mK.Comment: 12 pages, 6 figures. Version is the same as the published one, except
for figure placement on page
Spin-liquid signatures in the quantum critical regime of pressurized CePdAl
CePdAl is a prototypical frustrated Kondo lattice with partial long-range
order (LRO) at K. Previous bulk experiments under
hydrostatic pressure found signatures for a quantum critical regime that
extends from GPa, where LRO disappears, up to GPa. We employed extensive muon spin relaxation and rotation (SR)
experiments under pressure. The continuous and complete suppression of LRO at
is confirmed. Above and beyond ,
an additional crossover scale characterizes the change from pure to
stretched exponential relaxation in zero field. Remarkably agrees
with previously determined signatures of entropy accumulation above LRO. This
coincidence microscopically evidences fluctuating frustrated spins at with spin-liquid behavior. Power-law divergences of the temperature and
longitudinal field dependences of the relaxation rate, with time-field scaling,
at pressures between and 1.7 GPa characterize this regime as quantum
critical.Comment: 5 pages, 4 figures and supplemental material (3 pages, 4 figures
- …