Vladimir V. Matveev

Orientational mobility and relaxation spectra of dendrimers: Theory and computer simulation

The developed theory of the orientational mobility of individual segments of a perfectly branched dendrimer is used to calculate the relaxation spectrum of a dendrimer. Frequency dependences of NMR relaxation 1/T(1) and of the nuclear Overhauser effect have been theoretically calculated from the Brownian dynamics simulation data. The dendrimer segmental orientational mobility is governed by three main relaxation processes: (i) the rotation of the dendrimer as a whole, (ii) the rotation of the dendrimers branch originated from a given segment, and (iii) the local reorientation of the segment. The internal orientational mobility of an individual dendrimer segment depends only on the topological distance between this segment and the terminal shell of the dendrimer. Characteristic relaxation times of all processes and their contributions to the segmental mobility have been calculated. The influence of the number of generations and the number of the generation shell on the relaxation times has been studied. The correlation between the characteristic times and the calculated relaxation spectrum of the dendrimer has been established.

NMR Studies of Carbosilane Dendrimer with Terminal Mesogenic Groups

The 4-generation carbosilane dendrimer with terminal cyanobiphenyl mesogenic groups in dilute solution of CDCl(3) was investigated using (1)H NMR technique. The spectrum was obtained and the relaxation time, T(1), was measured in the temperature range 320-225 K. For the first time, the extrema of T(1) values were achieved for majority of the dendrimer functional groups. The values of activation energies of the dendrimer functional groups were obtained. The relaxation data for outer and inner methyl groups show that the dendrimer investigated has dense corona and hollow core. This structure is formed because the mesogenic groups do not allow terminal segments to penetrate into the dendrimer, that is, the backfolding effect is absent. The NMR spectral and relaxation data give evidence for changing conformation of the dendrimer internal segments with decreasing temperature. This reorganization is most likely connected with a change of dendrimer size. We suppose that our experimental results will provide additional information for understanding principles of dendrimer nanocontainer operation. NMR can possibly be a tool for indicating the encapsulation effect as well as the dendrimer effective size.

Average relaxation time of internal spectrum for carbosilane dendrimers: Nuclear magnetic resonance studies

A new theoretical description of the interior mobility of carbosilane dendrimers has been tested. Experiments were conducted using measurements of the (1)H NMR spin-lattice relaxation time, T(1H), of two-, three- and four-generation carbosilane dendrimers with three different types of terminal groups in dilute chloroform solutions. Temperature dependences of the NMR relaxation rate, 1/T(1H), were obtained for the internal CH(2)-groups of the dendrimers in the range of 1/T(1H) maximum, allowing us to directly evaluate the average time of the internal spectrum for each dendrimer. It was found that the temperature of 1/T(1H) maximum is practically independent of the number of generations, G; therefore, the theoretical prediction was confirmed experimentally. In addition, the average time of the internal spectrum of carbosilane dendrimers was found to be near 0.2 ns at room temperature, and this value correlates well with the values previously obtained for other dendrimer structures using other experimental techniques.

Nuclear magnetic resonance in nanocrystalline iron

We have observed for the first time the nuclear magnetic resonance (NMR) of 57Fe in iron powder with an average size of the metal core of particles on the order of 10 mm. The signal relaxation times have been determined, which differ significantly from the corresponding values in the bulk material. Dependences of the spin echo signal amplitude on the magnetic field have been measured. It is suggested that features of the observed NMR signal can be explained assuming a monodomain structure of iron nanoparticles. Using Mössbauer spectroscopy as a supplementary technique, the ratio of oxide and metal phase fractions in the powder has been estimated. It is shown that the NMR and Mössbauer spectroscopy data are consistent with the results of electron-microscopic observations and confirm nanostructural character of the substance studied.

Unexpected Temperature Behavior of Polyethylene Glycol Spacers in Copolymer Dendrimers in Chloroform.

We have studied copolymer dendrimer structure: carbosilane dendrimers with terminal phenylbenzoate mesogenic groups attached by poly(ethylene) glycol (PEG) spacers. In this system PEG spacers are additional tuning to usual copolymer structure: dendrimer with terminal mesogenic groups. The dendrimer macromolecules were investigated in a dilute chloroform solution by 1H NMR methods (spectra and relaxations). It was found that the PEG layer in G = 5 generations dendrimer is “frozen” at high temperatures (above 260 K), but it unexpectedly becomes “unfrozen” at temperatures below 250 K (i.e., melting when cooling). The transition between these two states occurs within a small temperature range (~10 K). Such a behavior is not observed for smaller dendrimer generations (G = 1 and 3). This effect is likely related to the low critical solution temperature (LCST) of PEG and is caused by dendrimer conformations, in which the PEG group concentration in the layer increases with growing G. We suppose that the unusual behavior of PEG fragments in dendrimers will be interesting for practical applications such as nanocontainers or nanoreactors.

Molecular mobility of counterion functional groups in ionic liquid 1-ethyl-3-methylimidazolium acetate according to 1H and 13C NMR relaxation data

Abstract1-Ethyl-3-methylimidazolium acetate was studied by NMR relaxation. The temperature dependences of the spin-lattice relaxation rates (1/T1) for 1H and 13C were obtained. The curves with maxima were observed for the majority of the temperature dependences 1/T1, which provided a reliable temperature dependence of the correlation times (τc). In the low-temperature range, the proton relaxation rates tend to an asymptotic value, which is related, most likely, to spin diffusion manifested in the studied samples. The values of correlation times τc calculated for 1H and 13C of the same functional group almost coincide at high temperatures, which confirms that the used approach is adequate for the determination of characteristic times of rotational reorientation of counterions in the studied ionic liquid.

Nuclear magnetic resonance investigation of photomagnetic phenomena in FeBO3

For the first time the NMR technique has been applied to the investigation of photomagnetic phenomena in a magnetic material. In the transparent easy-plane weak ferromagnet FeBO3 the 57Fe NMR spectra were measured with and without light illumination. It was shown that the exposure to light leads to (i) a total NMR frequency shift, as a result of electronic magnetization change, and (ii) a NMR signal increase, which is spectrally inhomogeneous at small external magnetic fields, where NMR spectrum splitting exists. The suggested mechanism for signal enhancement was connected to the light induced decrease of the basic-plane magnetic anisotropy.

Investigation of Melts of Polybutylcarbosilane Dendrimers by 1 H NMR Spectroscopy

Melts of polybutylcarbosilane (PBC) dendrimers from third (G3) up to sixth (G6) generations are investigated by 1H NMR spectroscopy in a wide temperature range up to 493 K. At room temperature, NMR spectra of G3-G5 dendrimers exhibit resolved, solution-like spectra (“liquid” phase). In contrast, the spectrum of the G6 dendrimer is characterized by a single unresolved broad line at whole temperature range, which supports the presence of an anomalous phase state of G6 at temperatures higher than glass transition temperature. For the first time, an unexpected transition of G5 dendrimer from a molecular liquid state to an anomalous state/phase upon temperature increase has been detected using NMR data. Specifically, an additional wide background line appears in the G5 spectrum above 473 K, and this line corresponds to a G5 state characterized by restricted molecular mobility, i.e., a state similar to the “anomalous” phase of G6 melt. The fraction of the G5 dendrimers in “anomalous” phase at 493 K is approximately 40%. Analysis of the spectral shapes suggests that changes in the G5 dendrimers are reversible with temperature.

Molecular dynamics simulation of poly(butyl)carbosilane dendrimer melts at 600 K

The structural properties of melts of poly(butyl)carbosilane (PBC) dendrimers of the third (G3), fifth (G5), and sixth (G6) generations were studied by molecular dynamics simulation at 600 K. A substantial difference was found between the density of the melt of the G6 generation dendrimer and the densities of the melts of the G3 and G5 generation dendrimers. The obtained computer simulation results do not confirm the hypothesis that these differences are caused by physical entanglements between the branches of the neighboring dendrimers (which take place for G6 to a higher extent) and indicate, most likely, the minimization of the interdendrimer free volume due to a more regular packing.

Observation of low-temperature magnetic ordering in mixed-phase Sr-Ca-Cu-O superconductors substituted by iron

The series of unsubstituted Sr2CaCu2O6 and iron-substituted Sr2CaCu2−x Fe x O6 () materials were produced by a high-pressure synthesis route and characterized using X-ray diffraction and SQUID magnetometry. The change of contained phase ratio was found at different substituent concentrations in the samples synthesized under the same conditions: unexpected growth of 0201 and CuO phase with decrease of iron content was observed. Synthesis temperature-dependence of obtained phase fractions was found for the samples with . The superconductivity transition was found for all obtained samples and the highest critical temperature was around 103 K. From the SQUID measurements distortions of the magnetic susceptibility curves were found; these were explained by the magnetic ordering arising at low temperatures. This magnetic ordering was associated with the magnetic moment appearing in iron-doped 0201 phase.