RESUMEN
The work in this report describes the syntheses, electrospray ionization mass spectromtery, structures, and experimental and density functional theoretical (DFT) magnetic properties of four tetrametallic stars of composition [M(II)(Cu(II)L)3](ClO4)2 (1, M = Mn; 2, M = Ni; 3, M = Cu; 4, M = Zn) derived from a single-compartment Schiff base ligand, N,N'-bis(salicylidene)-1,4-butanediamine (H2L), which is the [2 + 1] condensation product of salicylaldehyde and 1,4-diaminobutane. The central metal ion (Mn(II), Ni(II), Cu(II), or Zn(II)) is linked with two µ2-phenoxo bridges of each of the three [Cu(II)L] moieties, and thus the central metal ion is encapsulated in between three [Cu(II)L] units. The title compounds are rare or sole examples of stars having these metal-ion combinations. In the cases of 1, 3, and 4, the four metal ions form a centered isosceles triangle, while the four metal ions in 2 form a centered equilateral triangle. Both the variable-temperature magnetic susceptibility and variable-field magnetization (at 2-10 K) of 1-3 have been measured and simulated contemporaneously. While the Mn(II)Cu(II)3 compound 1 exhibits ferromagnetic interaction with J = 1.02 cm(-1), the Ni(II)Cu(II)3 compound 2 and Cu(II)Cu(II)3 compound 3 exhibit antiferromagnetic interaction with J = -3.53 and -35.5 cm(-1), respectively. Variable-temperature magnetic susceptibility data of the Zn(II)Cu(II)3 compound 4 indicate very weak antiferromagnetic interaction of -1.4 cm(-1), as expected. On the basis of known correlations, the magnetic properties of 1-3 are unusual; it seems that ferromagnetic interaction in 1 and weak/moderate antiferromagnetic interaction in 2 and 3 are possibly related to the distorted coordination environment of the peripheral copper(II) centers (intermediate between square-planar and tetrahedral). DFT calculations have been done to elucidate the magnetic properties. The DFT-computed J values are quantitatively (for 1) or qualitatively (for 2 and 3) matched well with the experimental values. Spin densities and magnetic orbitals (natural bond orbitals) correspond well with the trend of observed/computed magnetic exchange interactions.
RESUMEN
In this work, we have demonstrated the use of a fiber Bragg grating (FBG) sensor to measure the pressure profile of blast waves generated inside a vertical shock tube (VST). An FBG pressure sensor probe has been designed and developed that can be incorporated into the wall of the VST. The VST facility is used to generate blast waves with decay times of the order of a few milliseconds to simulate explosive events. Pressure measurement experiments have been carried out at different incident blast wave peak pressures inside the VST. The FBG pressure sensor measurements are validated against a standard piezoelectric pressure transducer at an acquisition rate of 1 MHz. The pressure signals of both sensors are found to match well with similar rise times and decay profiles. The validated FBG pressure sensor is then incorporated into a sand column mounted in the test section of the VST to measure the pressure profile of blast wave-induced stress waves in granular media. The FBG and piezoelectric pressure sensor data are compared using fast Fourier transform analysis and continuous wavelet transform. The feasibility of FBG sensors for blast pressure measurement under harsh conditions imposed inside shock tube environments is established.
RESUMEN
Two µ-phenoxo-µ1,1-azide dinuclear CoIIICoII complexes [CoIII(N3)2L1(µ1,1-N3)CoII(N3)]·MeOH (1) and [CoIII(N3)2L2(µ1,1-N3)CoII(N3)]·MeOH (2) (HL1 and HL2 are two Schiff base ligands having N2O-N2O compartments) both possess one hexacoordinate Co(iii) and one pentacoordinate Co(ii) center. DC magnetic susceptibility and magnetization measurements show an appreciable amount of positive magnetic anisotropy (D/hcâ¼ 40 cm-1) that is also confirmed by ab initio CASSCF calculations. AC susceptibility measurements of 1 reveal that it exhibits a slow magnetic relaxation with two relaxation channels. The external magnetic field supports the low-frequency (LF) channel that escapes on heating more progressively than the high-frequency (HF) branch. The relaxation time is as slow as τ = 255 ms at T = 1.9 K and BDC = 0.6 T, where the LF mole fraction is 69%. The complex 2 also displays similar field-supported slow magnetic relaxation with two relaxation channels.
RESUMEN
The work in this paper presents the syntheses, characterization, catecholase activity, and electrospray ionization mass spectroscopic (ESI-MS positive) study of three mixed-valence dinuclear Co(III)Co(II) complexes of composition [Co(III)Co(II)L(N(3))(3)]·CH(3)CN (1), [Co(III)Co(II)L(OCN)(3)]·CH(3)CN (2), and [Co(III)Co(II)L(µ-CH(3)COO)(2)](ClO(4)) (3), derived from a tetraimino diphenolate macrocyclic ligand H(2)L, obtained on [2 + 2] condensation of 4-ethyl-2,6-diformylphenol and 2,2'-dimethyl-1,3-diaminopropane. While 1 and 2 are diphenoxo-bridged, 3 is a heterobridged bis(µ-phenoxo)bis(µ-acetate) system. Utilizing 3,5-di-tert-butyl catechol (3,5-DTBCH(2)) as the substrate, the catecholase activity of all the three complexes has been checked in methanol/acetonitrile/N,N-dimethyl formamide. While 2 and 3 are inactive, complex 1 shows catecholase activity with turnover numbers of 482.16 h(-1) and 45.38 h(-1) in acetonitrile and methanol, respectively. Electrospray ionization mass (ESI-MS positive) spectra of complexes 1-3 have been recorded in acetonitrile solutions and the positive ions have been well characterized. The ESI-MS positive spectrum of complex 1 in the presence of 3,5-DTBCH(2) has also been recorded and, interestingly, two positive ions [Co(III)Co(II)L(N(3))(2)(3,5-DTBCH(-))H](+) and [Co(II)Co(II)L(µ-3,5-DTBCH(2-))Na](+) have been identified.