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141.
Regioselectivity in isoquinoline alkaloid synthesis is analyzed here. Our experiments have shown that substituents on the aromatic ring of the starting amine are determinant in isoquinoline synthesis. The use of dicyclohexylcarbodiimide in activating carboxylic acids for electrophilic aromatic substitution reactions is presented for the first time. 相似文献
142.
An expression for the Casimir-Polder potential between molecules in a homogeneous dispersive and absorptive dielectric medium is derived. The effect of retardation on the interaction energy is discussed by examining the wave-zone and nonretarded limits of the potential. Unlike Lifshitz theory, the interaction energy is not derived from the potential between macroscopic bodies. In this work, a Green function that explicitly accounts for absorption in the medium is obtained. This function leads to possible dissipation effects and presents a near-zone form that vanishes in the limit of nonabsorptive medium. Employing a two-level model, it is shown that the retarded van der Waals dispersion potential in a medium may become repulsive as a consequence of absorption by the medium. It is suggested that the repulsive dispersion force may delay precipitation of nonpolar molecules from a dielectric solvent or even inhibit chemical reaction between them. 相似文献
143.
Molecular dynamics simulations have been carried out to investigate the structure and dynamics of liquid methanol confined in 3.3 nm diameter cylindrical silica pores. Three cavities differing in the characteristics of the functional groups at their walls have been examined: (i) smooth hydrophobic pores in which dispersive forces prevail, (ii) hydrophilic cavities with surfaces covered by polar silanol groups, and (iii) a much more rugged pore in which 60% of the previous interfacial hydroxyl groups were replaced by the bulkier trimethylsilyl ones. Confinement promotes a considerable structure at the vicinity of the pore walls which is enhanced in the case of hydroxylated surfaces. Moreover, in the presence of the trimethylsilyl groups, the propagation of this interface-induced spatial ordering extends down to the central region of the pore. Concerning the dynamical modes, we observed an overall slowdown in both the translational and rotational motions. An analysis of these mobilities from a local perspective shows that the largest retardations operate at the vicinity of the interfaces. The gross features of the rotational dynamics were analyzed in terms of contributions arising from bulk and surface states. Compared to the bulk dynamical behavior, the characteristic timescales associated with the rotational motions show the most dramatic increments. A dynamical analysis of hydrogen bond formation and breaking processes is also included. 相似文献
144.
Timothy J. Boyle Leigh Anna M. Ottley Todd M. Alam Mark A. Rodriguez Pin Yang Sarah K. Mcintyre 《Polyhedron》2010
The first study into the alcohol solvation of lanthanum halide [LaX3] derivatives as a means to lower the processing temperature for the production of the LaBr3 scintillators was undertaken using methanol (MeOH). Initially the de-hydration of {[La(μ-Br)(H2O)7](Br)2}2 (1) was investigated through the simple room temperature dissolution of 1 in MeOH. The mixed solvate monomeric [La(H2O)7(MeOH)2](Br)3 (2) compound was isolated where the La metal center retains its original 9-coordination through the binding of two additional MeOH solvents but necessitates the transfer of the innersphere Br to the outersphere. In an attempt to in situ dry the reaction mixture of 1 in MeOH over CaH2, crystals of [Ca(MeOH)6](Br)2 (3) were isolated. Compound 1 dissolved in MeOH at reflux temperatures led to the isolation of an unusual arrangement identified as the salt derivative {[LaBr2.75·5.25(MeOH)]+0.25 [LaBr3.25·4.75(MeOH)]−0.25} (4). The fully substituted species was ultimately isolated through the dissolution of dried LaBr3 in MeOH forming the 8-coordinated [LaBr3(MeOH)5] (5) complex. It was determined that the concentration of the crystallization solution directed the structure isolated (4 concentrated; 5 dilute) The other LaX3 derivatives were isolated as [(MeOH)4(Cl)2La(μ-Cl)]2 (6) and [La(MeOH)9](I)3·MeOH (7). Beryllium Dome XRD analysis indicated that the bulk material for 5 appear to have multiple solvated species, 6 is consistent with the single crystal, and 7 was too broad to elucidate structural aspects. Multinuclear NMR (139La) indicated that these compounds do not retain their structure in MeOD. TGA/DTA data revealed that the de-solvation temperatures of the MeOH derivatives 4–6 were slightly higher in comparison to their hydrated counterparts. 相似文献
145.
Wenjun Tang Dr. Andrew G. Capacci Xudong Wei Dr. Wenjie Li Dr. Andre White Dr. Nitinchandra D. Patel Jolaine Savoie Joe J. Gao Sonia Rodriguez Dr. Bo Qu Dr. Nizar Haddad Dr. Bruce Z. Lu Dr. Dhileepkumar Krishnamurthy Dr. Nathan K. Yee Dr. Chris H. Senanayake Dr. 《Angewandte Chemie (International ed. in English)》2010,49(34):5879-5883
146.
This study examined the time course of cochlear suppression using a tone-burst suppressor to measure decrement of distortion-product otoacoustic emissions (DPOAEs). Seven normal-hearing subjects with ages ranging from 19 to 28 yr participated in the study. Each subject had audiometric thresholds ≤ 15 dB HL [re ANSI (2004) Specifications for Audiometers] for standard octave and inter-octave frequencies from 0.25 to 8 kHz. DPOAEs were elicited by primary tones with f(2)?= 4.0 kHz and f(1)?= 3.333 kHz (f(2)/f(1)?= 1.2). For the f(2), L(2) combination, suppression was measured for three suppressor frequencies: One suppressor below f(2) (3.834 kHz) and two above f(2) (4.166 and 4.282 kHz) at three levels (55, 60, and 65 dB SPL). DPOAE decrement as a function of L(3) for the tone-burst suppressor was similar to decrements obtained with longer duration suppressors. Onset- and setoff- latencies were ≤ 4 ms, in agreement with previous physiological findings in auditory-nerve fiber studies that suggest suppression results from a nearly instantaneous compression of the waveform. Persistence of suppression was absent for the below-frequency suppressor (f(3)?= 3.834 kHz) and was ≤ 3 ms for the two above-frequency suppressors (f(3)?= 4.166 and 4.282 kHz). 相似文献
147.
Constantin JG Fris AR Appignanesi G Carignano M Szleifer I Corti H 《The European physical journal. E, Soft matter》2011,34(11):126
The structure of water clusters (H2O)n (n = 40 -200) and bulk water were examined by molecular dynamics simulations using the TIP4P-ice water model. The analysis of
the low-temperature structures in terms of the local structure index (LSI) showed a bimodal distribution. This finding supports
the two-state picture derived from the analysis of the inherent dynamics of bulk SPC/E water. The water molecules at the outer
interface of the coldest clusters are more structured than those in the inner core. The geometrical constraint of the interface
forces the surface molecules to lose one neighbor and adopt a local angular distribution of hydrogen bonds resembling that
found in the basal plane of ice Ih. 相似文献
148.
Accordino SR Rodriguez Fris JA Sciortino F Appignanesi GA 《The European physical journal. E, Soft matter》2011,34(5):48-7
Several evidences have helped to establish the two-state nature of liquid water. Thus, within the normal liquid and supercooled
regimes water has been shown to consist of a mixture of well-structured, low-density molecules and unstructured, high-density
ones. However, quantitative analyses have faced the burden of unambiguously determining both the presence and the fraction
of each kind of water “species”. A recent approach by combining a local structure index with potential-energy minimisations
allows us to overcome this difficulty. Thus, in this work we extend such study and employ it to quantitatively determine the
fraction of structured molecules as a function of temperature for different densities. This enables us to validate predictions
of two-state models. 相似文献
149.
150.
The influence of large amplitude pairing fluctuations is investigated in the framework of beyond mean field symmetry conserving configuration mixing calculations. In the numerical application the finite range density dependent Gogny force is used. We investigate the nucleus 54Cr with particle number and angular momentum projected wave functions considering the axial quadrupole deformation and the pairing gap degree of freedom as generator coordinates. We find that the effects of the pairing fluctuations increase with the excitation energy and the angular momentum. The self-consistency in the determination of the basis states plays an important role. 相似文献