Using 1H and 13C NMR and DFT calculations, the structure and interactions of the symmetric ethereally fenced hexaarylbenzene receptor 1 with hydronium ions were studied. Both 1 and its equimolecular ...complex 1·H3O+ exhibit C 3v symmetry. According to DFT, two similar optimal structures of the complex exist, the more stable one being 15.4 kJ/mol lower in energy. The equilibrium between 1 and 1·H3O+ complexes is characterized by the stabilization constant K = 1.97 × 106 (i.e., the binding constant η = 6.3) according to both proton and carbon NMR spectra. The exchange dynamics between 1 and the complex measured by the delay-varied CPMG sequence had to be corrected for the internal exchange processes in both 1 (conformation change) and the complex (vacillation between the two minima). After this correction, the correlation time of exchange was found to be 4.76 × 10−5 s. Such relatively fast exchange can be explained only by it being mediated by the excess water molecules present in the system.
Interaction of dibenzo-18-crown-6 (DBC) with H3O+ (HP) in nitrobenzene-d 5 and dichloromethane-d 2 was studied by using 1H and 13C NMR spectra and relaxations, FTIR spectra, and quantum chemical DFT ...calculations. NMR shows that the DBC•HP complex is in a dynamic equilibrium with the reactants, the equilibrium constant K being 0.66 × 103, 1.16 × 104, and 1.03 × 104 L·mol−1 in CD2Cl2, nitrobenzene, and acetonitrile, respectively. The complex appears to have a C 2v symmetry in NMR, but FTIR combined with DFT normal mode calculations suggest that such high symmetry is only apparent and due to exchange averaging of the structure. FTIR spectra as well as energy-optimized DFT calculations show that the most stable state of the complex in solution is that with three linear hydrogen bonds of HP with one CH2−O−CH2 and two Ar−O−Ar oxygen atoms. The structure is similar to that found in solid state but adopts a somewhat different conformation in solution. The dynamics of exchange between bound and free DBC was studied by NMR transverse relaxation. It was found to be too fast to give reproducible results when measured with the ordinary CPMG sequence or its variant DIFTRE removing residual static dipolar interaction, but it could be established by rotating-frame measurements with high intensity of the spin-lock field. The correlation time of exchange was found to be 5.6 × 10−6 and 3.8 × 10−6 s in dichloromethane and nitrobenzene, respectively. Such fast exchange can be explained by cooperative assistance of present water molecules.
Cellulose diacetate (CDA) was acylated with 2-bromoisobutyryl bromide (BriBr) or with dichloroacetyl chloride (ClAcCl) giving polyfunctional macroinitiators for ATRP grafting of styrene (St), MMA and ...butyl acrylate (BuA). Under various reaction conditions, macroinitiators with variable degrees of functionalisation could be prepared. The macroinitiators with 2-bromoisobutyryl (BriB) groups were grafted with St or BuA, those with dichloroacetyl (ClAc) functions were used for graft copolymerization of MMA. Graft copolymers with chemically different grafts as well as tunable lengths and densities of grafts were synthesized in this way. Poly(CDA-
g-St) and poly(CDA-
g-MMA) were further used as macroinitiators of BuA polymerization, giving polyCDA-
g-(St-
b-BuA) and polyCDA-
g-(MMA-
b-BuA) graft copolymers with diblock grafts.
The teaching method of virtual co-teaching helps the mutual sharing of new experiences and methods of pedagogues in teaching in formal education, through digital technologies. It also facilitates the ...involvement of practitioners in teaching, which can be a key mechanism for ensuring quality education. This article focused on comparing the effectiveness of co-teaching in two environments, face-to-face and virtual. In terms of methodology, an experiment was used, which was implemented in three independently selected schools, from which eight groups of thirty students were formed. The phenomenon measured was the level of understanding of the material taught during these lessons of history, mathematics and a foreign language. The test results of the individual groups were subsequently analyzed. From the results, we can confirm the null hypothesis based on the independent samples t-test and the Mann-Whitney U test, which indicates that there is no statistically significant difference between the test results of the two groups. However, it is essential to emphasize the added value in virtual co-teaching, which is primarily in the economic aspect and flexibility. Specifically, it is a reduction in the cost of commuting, reduced time and easy availability of experts. These results form the basis for follow-up research that can focus on improving the quality of teaching using this method.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Using 1H, 13C, and 133Cs NMR spectra, it is shown that calix4arene-bis(t-octylbenzo-18-crown-6) (L) forms complexes with one (L·Cs+) and two (L·2Cs+) Cs+ ions offered by cesium bis(1,2-dicarbollide) ...cobaltate (CsDCC) in nitrobenzene-d 5. The ions interact with all six oxygen atoms in the crown-ether ring and the π electrons of the calixarene aromatic moieties. According to extraction technique, the stability constant of the first complex is log βnb(L·Cs+) = 8.8 ± 0.1. According to 133Cs NMR spectra, the value of the equilibrium constant of the second complex is log K nb (2)(L·2Cs+) = 6.3 ± 0.2, i.e., its stabilization constant is log βnb(L·2Cs+) = 15.1 ± 0.3. Self-diffusion measurements by 1H pulsed-field gradient (PFG) NMR combined with density functional theory (DFT) calculations suggest that one DCC– ion is tightly associated with L·Cs+, decreasing its positive charge and consequently stabilizing the second complex, L·2Cs+. Using a saturation-transfer 133Cs NMR technique, the correlation times τex of chemical exchange between L·Cs+ and L·2Cs+ as well as between L·2Cs+ and free Cs+ ions were determined as 33.6 and 29.2 ms, respectively.
Interaction of trioctylphosphine oxide (TOPO) with fully ionized hydrated protons (HP) was studied in acetonitrile-d 3 and nitrobenzene-d 5 using 1H, 13C, and 31P NMR, PFG NMR, and magnetic ...relaxation, and the experimental results were confronted with high-precision ab initio DFT calculations. Relative chemical shifts of NMR signals of TOPO (0.02 mol/L) under the presence of HP in the molar ratio β = 0−2.0 mol/mol show binding between TOPO and HP. Self-diffusion measurements using 1H PFG NMR demonstrate that larger complexes with higher content of TOPO are generally formed at β < 0.75. Analyzing the dependence of 31P NMR chemical shifts on β by the use of program LETAGROP, we obtained very good fitting for the assumed coexistence of three complexes (TOPO) i ·HP (named C i ), where i = 1, 2, 3. The logarithms of the respective stabilization constants log K i were found to be 3.63, 4.67, and 7.23 in acetonitrile and 3.91, 6.04, and 7.92 in nitrobenzene. The 31P NMR chemical shifts Δδ i corresponding to these complexes are 39.35, 29.51, and 19.72 ppm in acetonitrile and 38.37, 28.47, and 18.63 ppm in nitrobenzene. These values and the calculated values of α i =C i /TOPO0 were utilized in the analysis of the system dynamics. This was done by measuring the transverse 31P NMR relaxation by the CPMG sequence with varying delays t p between the π pulses in the mixtures with β = 0.5, 1.25, and 1.5. Calculating the probabilities of imaginable exchange processes shows that only three of them can have significant influence on relaxation rate R 2, namely C1 ↔ TOPO, C2 ↔ C1, and C3 ↔ C2. Using the slopes of the R 2−t p −1 dependences in the above three mixtures, the following correlation times were obtained: τ10 = 2.5 × 10−6, τ21 = 7.4 × 10−5, τ32 = 11.3 × 10−5 s. The DFT calculations support the hypothesis that complexes C1 to C3 are the main species in the mixtures of TOPO with HP, with the only exception that additional water molecules are bound to the complexes in the case of C1 and C2. Schematically, the compositions of the three stable complexes is 3TOPO·H3O+, 2TOPO·H3O·H2O+, and TOPO·H3O·2H2O+. The relative 31P NMR shifts calculated for the optimized structures of C1, C2, and C3 are in very good agreement with the experimentally observed values.
This article deals with American travels (mainly to stay in Mexico and Brazil) of Czech traveler Čeněk Paclt (1813-1887). Paclt belongs to the half-forgotten travelers of the nineteenth century, ...although in professional circles, his name is known. Among Czech adventurers and travelers, however, he has an important position, because he holds one primacy. Paclt is also the first Czech, who has provably traveled through all five inhabited continents and has left written evidence about his journeys. He came to the American land in the year 1846. Paclt traveled over Mexico, Brazil and large part of the East Coast of the United States of America. He went through Mexican territory during the Mexican-American War (1846-1848) as an American soldier. He was on the American continent in the years 1846-1857.
According to 1H and 13C NMR spectra, relaxations, and PFG NMR self-diffusion measurements, partially hydrophobic additives methyl–ethyl ketone (MEK), methyl-isopropyl ketone (MIPK), and ...methyl-t-butyl ketone (MTBK) facilitate the conformation change and subsequent self-association of the copolymer Pluronic L64. The correlation time (4–9 ms) and activation energy (43–52 kJ/mol) of transition between its conformation states decrease with the increasing hydrophobicity and bulkiness of the additive. The temperature of the first PPO self-association decreases in the same order (by 4 K for MTBK). The interaction of the additives was indirectly proved by the decrease of their rotational and translational mobility in the presence of L64. The rotational correlation time τc is between 3 and 6 × 10–11 s, whereas that of the same molecules in the absence of L64 is lower than 6 × 10–12 s. The normalized self-diffusion coefficient decreases to about 0.7 of its original value in the presence of L64. The interaction of the additive with the PPO block is transient but effective enough to facilitate its conformational change and self-association. Its mediation by a water molecule bound to PPO as a possible mechanism is suggested.
Controlled radical reversible addition-fragmentation chain transfer (RAFT) polymerisation was used to prepare water-soluble polymer–drug carriers based on copolymers of
...N-(2-hydroxypropyl)methacrylamide (HPMA) with a hydrazide group-containing monomer, showing well-defined structure with narrow molecular weight distribution (approx. 1.1–1.2). The anticancer therapeutic doxorubicin was bound to the polymeric carrier by a hydrazone bond, enabling pH-controlled release under mildly acid conditions that mimics the environment in endosomes/lysosomes of tumour cells. RAFT polymerisation facilitated the synthesis of semitelechelic copolymers, which were used in the synthesis of monoclonal anti-CD20 antibody–polymer–drug conjugate designed for cell-specific tumour targeting. They were also used for producing a biodegradable high-molecular-weight graft polymer–drug conjugate that degrade in the presence of glutathione, which is designed for passive targeting to solid tumours. The conjugates exhibited well-defined structures with narrow molecular weight distributions of approx. 1.3 and pH-controlled drug release.
Using 1H and 13C 1D and 2D NMR spectra, pulsed field-gradient (PFG) diffusion measurements, and 13C relaxations supported by density functional theory (DFT) calculations, the temperature-dependent ...behavior of (EO) m (PO) n (EO) m block copolymers (m/n = 31/14, 31/72, and 17/1) in D2O below and at the critical micellar temperature (CMT) was investigated in order to understand the nature of primary self-association acts and their true driving force. It was shown that a conformation change of the PO block followed by mild and reversible association with other PO blocks and eventually with the inner parts of EO blocks starts at temperatures 10−12 K below the CMT. The primary process is the entropy-driven disintegration of the PPO hydration envelope based on cooperation of hydrophobic hydration and hydrogen bonding. The partial dehydration of PPO is followed by its conformation change. Both processes are cooperative and reversible with a correlation time of the order 0.01 s and an activation energy of 51.3 kJ/mol. The PPO chain in a staggered conformation is prone to self-association starting at temperatures 5−6 K below CMT. In (EO) m (PO) n (EO) m block copolymers, this process is complicated by the stripping of PEO chains of a part of hydrogen-bound water and entwining them with PPO. It is shown that only inner (PPO-near) parts of PEO take part in the process, the end-groups remaining free.
PDF
