Articles you may be interested inA multifrequency high-field pulsed electron paramagnetic resonance/electron-nuclear double resonance spectrometer Rev. Sci. Instrum. 79, 064703 (2008); 10.1063/1.2937630 17O hyperfine and quadrupole interactions for water ligands in frozen solutions of high spin Mn2+
Published results on transition in a Stokes layer indicate a wide range of transition Reynolds numbers. As thermal effects in a resonance tube (Merkli & Thomann 1975) depend on the state of the boundary layer, the transition Reynolds number was determined, and a critical Reynolds numberAc≈ 400 was found. The observations were made with hot wires and with flow visualization by means of smoke, and provide new details on turbulence in a Stokes layer. With this knowledge an explanation of the large discrepancies between some stability theories and the experiments is suggested. The main point is that turbulence occurs in the form of periodic bursts which are followed byrelaminarimtionin the same cycle and donotlead to turbulent flow during the whole cycle.A further, unexpected result of the present investigation is the discovery of vortex patterns superimposed on the normal laminar acoustic motion.
The mechanisms of
the reaction of CO2 with hindered
alkanolamines are described based on direct reaction monitoring with in situ
13C NMR spectroscopy. Four amines of
increasing steric hindrance were studied and compared with unhindered
primary, secondary, and tertiary alkanolamines. The number and location
of additional methyl (or other) groups on the nitrogen or adjacent
carbon atoms create different degrees of steric hindrance. As steric
hindrance increases, the Lewis basicity (nucleophilicity), as an affinity
of the amine nitrogen to directly attack the electrophilic carbon
of CO2 and form a carbamate, decreases. At the same time,
methyl (or other) groups present on nitrogen or adjacent carbon atoms
do not change the Brønsted basicity of hindered amines. Coupled
with lower Lewis basicity, the CO2–amine–water
reaction equilibrium favors formation of bicarbonate, with higher
CO2 loading capacity and lower thermal stability, both
of which are favorable properties for cyclic CO2 capture.
Due to lower stability of the hindered N–C bond of the carbamate,
hindered amines AMP and MAP in aqueous solution show a phenomenon
of steric acceleration: the more rapid transfer of CO2 from
the nitrogen of the amine (e.g., carbamate) to form the bicarbonate
anion, maintaining relatively high reaction rates. A strong nucleophilic
base such as piperazine present in the amine solution at low concentration
maintains the high reaction rate by attacking free CO2 and
transferring it to the amine as bicarbonate. This described mechanism
helps to improve CO2 capture rates with severely hindered
amines such as in the secondary amine MAMP, which without a promoter
acts as a tertiary amine slowly reacting with CO2. The
mechanism of chemical reaction between CO2 and amines dissolved
in methanol is similar to that in watermethanol attacks CO2 and forms an O–C bond and methylbicarbonate anion
(analogue of bicarbonate) with CO2, which is stabilized
by a protonated amine. The effects of amine concentration, CO2 partial pressure, and temperature are discussed.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.