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41.
Villaume S Strich A Daniel C Perera SA Bartlett RJ 《Physical chemistry chemical physics : PCCP》2007,9(46):6115-6122
The transition energies to the low-lying singlet and triplet excited states of Cr(CO)(6) are computed by equation-of-motion coupled cluster singles and doubles (EOM-CCSD) and similarity transformed equation-of-motion coupled cluster singles and doubles (STEOM-CCSD) methods with all-electrons basis sets. Both experimental and optimized geometries are used for the calculations. Calculations with various basis sets, among them one of the largest calculations performed at the EOM-CCSD level, based on atomic natural orbitals with 627 functions, were used to evaluate the basis set influence on computed transition energies. The presence of a shoulder at 3.9 eV in the experimental absorption spectrum, assigned to the (1)A(1g)-->(1)T(2u) transition, which was not reproduced by recent density functional theory (DFT) or multi-state complete active space perturbation theory (MS-CASPT2) is supported by the present STEOM-CCSD calculations with a theoretical value of 3.92 eV. In addition to this weak (1)A(1g)--> a (1)T(2u) absorption, we observe two strong absorptions corresponding to (1)A(1g)--> a (1)T(1u) at 4.37 eV (vs. an experimental value of 4.46 eV) and (1)A(1g)--> b (1)T(1u) at 5.20 eV (vs. an experimental value of 5.53 eV). Both are characterized as metal-to-ligand charge-transfer (MLCT) allowed transitions. The first metal-centered (MC) absorption at 4.37 eV in our best calculation is degenerate with the lowest MLCT absorbing state. The one-dimensional potential energy curves associated to the low-lying singlet MLCT and MC states as a function of the chromium axial carbonyl bond distance q(a) = [Cr-CO(axial)] show that an avoided crossing exists between the a (1)T(1g) (MC) and a (1)T(1u) (MLCT) states near 1.92 A, which is very close to the equilibrium Cr-CO distance. Moreover, the MC state seems to be dissociative for the CO loss. These two important features could explain the ultra-fast dissociation of CO (100 fs) observed in recent low intensity laser probed gas phase experiments. 相似文献
42.
We report here on the successful observation of the unbiased thermodynamic selectivity of ion-selective sensors working in normal pulse chronopotentiometric mode (pulstrodes). In contrast to ion-selective electrodes, the pulstrodes do not require careful counterbalancing of the transmembrane ionic fluxes to achieve unbiased thermodynamic selectivity. The pulstrodes can work under asymmetric conditions, which are often encountered in practice. The composition of the inner filling solution did not affect the sensor response, indicating that the transmembrane flux of primary ions was indeed effectively suppressed in the absence of ion exchanger. For the K-selective sensor considered here, an improvement of Mg discrimination by a factor of 1000 was demonstrated. 相似文献
43.
Carr MJ Perera SD Jelínek T Stíbr B Clegg W Kilner CA Kennedy JD 《Chemical communications (Cambridge, England)》2007,(34):3559-3561
B16H20 and [RhCl2(eta5-C5Me5)]2 with tmnd give [(eta5-C5Me5)2Rh2B11H15], which has an unprecedented thirteen-vertex macropolyhedral cluster core based on a nido ten-vertex {MB9} subcluster and a nido five-vertex {MB4} subcluster fused with their open-face {B2} edges in common. 相似文献
44.
K. Prasanna U. Perera Khalil A. Abboud Dennis W. Smith Mariusz Krawiec 《Acta Crystallographica. Section C, Structural Chemistry》2003,59(3):o107-o110
The molecule of 3,3′,4,4′‐tetrakis(phenylethynyl)biphenyl, C44H26, is approximately planar and is located on a crystallographic inversion center. Bis[3,4‐bis(phenylethynyl)phenyl] ether, C44H26O, has molecules located on twofold symmetry axes, whereas the molecule of 2,2‐bis[3,4‐bis(phenylethynyl)phenyl]propane, C47H32, does not exhibit any molecular symmetry. 相似文献
45.
M. C. Senake Perera 《Journal of Polymer Science.Polymer Physics》1999,37(11):1141-1153
Methyl‐methacrylate‐grafted natural rubber was prepared by free radical polymerization of methyl methacrylate in natural rubber latex, and their structure and dynamics were investigated by dynamic mechanical analysis and solid‐state nuclear magnetic resonance (NMR). Samples were prepared by chemical initiation and high‐energy radiation. The changes of glass transition temperature and tan δ max with different total poly(methyl methacrylate (PMMA) content are reported. The effect of the change in composition in copolymers on tan δ peak width, tan δ max, and area under the tan δ curve are used to understand the miscibility and damping properties. Solid‐state 13C‐NMR measurements were carried out to determine several relaxation time parameters, such as rotating frame and laboratory frame proton and carbon relaxation times. Cross polarization times and carbon relaxation times were interpreted based on the changes in the molecular motion. Proton relaxation times were interpreted based on the heterogeneity of the matrix. Results confirmed phase separation and a presence of an interfacial region. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1141–1153, 1999 相似文献
46.
Dhruthiman R. Mantheni M. P. K. Maheswaram Hany F. Sobhi Naullage Indika Perera Alan T. Riga M. Ellen Matthews K. Alexander 《Journal of Thermal Analysis and Calorimetry》2012,108(1):227-233
Novel dielectric behavior of a linear increase in ionic conductivity prior to melt temperature was observed for active pharmaceutical
ingredients (APIs), organic chemicals, amino acids, and carbohydrates. Though, there are solids like polyolefins and long
chain organic compounds (tetracosane, pentacosane) which do not exhibit this premelt behavior (i.e., the temperature where
the onset of increase in ionic conductivity to melt temperature). We have discovered novel electrical conductivity properties
and other physical analytical variations which can lead to unique synthetic routes of certain chemical entities. The above-mentioned
unique variations are not related to solid–solid transitions which are quite often observed in pharmaceutical crystalline
solids. These new properties are related to amorphous crystalline behavior of a solid. We have also studied the effect of
various experimental variables: such as amount of mass tested, applied frequency at a given electric field and heating rate,
which results in varying the onset temperature of the increase in ionic conductivity. Melting of the solids was correlated
using differential scanning calorimetry (DSC). Activation energies for all the solids were measured in the premelt region
using an Arrhenius plot at a specific frequency since we observed changes in the conductivity with frequency. This study focused
on frequencies 0.1 to 10 Hz, since the conductivity at these frequencies related to surface analysis. This new physical properties
are leading to new electro synthetic procedures to modify or prepare chemicals. 相似文献
47.
Libby Yoerg M. Ellen Matthews Lakshmi Kaza Naullage Indika Perera David W. Ball John Moran Alan T. Riga 《Journal of Thermal Analysis and Calorimetry》2012,108(1):19-24
Three aldohexose monosaccharides, d-glucose, d-mannose, and d-galactose, were examined by scanning temperature dielectric analysis (DEA) from ambient temperatures through their melts.
Phase transitions, including glass transition (T
g) and melting temperature (T
m), were evaluated by differential scanning calorimetry (DSC). The monosaccharides were found to exhibit thermally-induced
dielectric loss spectra in their amorphous-solid phase before melting. Activation energies for electrical charging of each
of the monosaccharides were calculated from an Arrhenius plot of the tan delta (e″/e′, dielectric loss factor/relative permittivity) peak frequency versus reciprocal temperature in Kelvin. The DEA profiles
were also correlated with the DSC phase diagrams, showing the changes in electrical behavior associated with solid–solid and
solid–liquid transitions. 相似文献
48.
Lakshmi Kaza Hany F. Sobhi Jeffrey A. Fruscella Chris Kaul Shravan Thakur Naullage I. Perera Kenneth Alexander Alan T. Riga 《Journal of Thermal Analysis and Calorimetry》2012,109(3):1365-1371
A standard protocol was developed to determine the water content by thermal analysis of milk of magnesia (MoM). Differential scanning calorimetry (DSC) and thermogravimetry (TG) were used in a novel manner for examining the physical characteristics of the commercial pharmaceutical suspensions. Moisture analyzer and oven-dry methods validate the proposed protocol. MoM consists primarily of water and magnesium hydroxide [Mg(OH)2]. Experimental design of the thermal analysis parameters were considered including sample size, flowing atmosphere, sample pan, and heating rate for both DSC and TG. The results established the optimum conditions for minimizing heat and mass transfer effect. Sample sizes used were: (5?C15?mg) for DSC and (30?C50?mg) for TG. DSC analysis used crimped crucibles with a pinhole, which allowed maximum resolution and gave well-defined mass (water) loss. TG analysis used a heating rate of 10?°C/min?1 in an atmosphere of nitrogen. The heat of crystallization, heat of fusion, and heat of vaporization of unbound water are 334, 334, and 2,257?Jg?1, respectively (Mitra et al. Proc NATAS Annu Conf Therm Anal Appl 30:203?C208, 2002). The DSC average water content of (MoM) was 80?wt% for name brand and 89.5?wt% for generic brand, based on the relative crystallization, melting and vaporization heats/Jg?1 of distilled water in the recently purchased (2011) MoM samples. The TG showed a two-step process, losing water at 80?C135?°C for unbound water and bound water (MgO·H2O) at 376?C404?°C, yielding a total average water loss of 91.9?% for name brand and 90.7?% for generic brand by mass. The difference between the high-temperature TG and the lower-temperature DSC can be attributed for the decomposition of magnesium hydroxide or MgO·H2O. Therefore in performing this new approach to water analysis by heating to a high temperature decomposed the magnesium hydroxide residue. It was determined that the TG method was the most accurate for determining bound and unbound water. 相似文献
49.
K. Lawniczak-Jablonska J.J. Jia L. Lin M.M. Grush T.A. Callcott A. Asfaw J.A. Carlisle L.J. Terminello F.J. Himpsel D.L. Ederer J.H. Underwood R.C.C. Perera 《Applied Physics A: Materials Science & Processing》1997,65(2):173-177
1-x MnxS (where 0.05<x<1) were measured as the energy of the exciting radiation was tuned across the S and Mn-L2,3 absorption edges of these compounds. Strong resonance peaks in Mn-L emission spectra and the systematic appearance of new
spectral features in S-L emission spectra were observed. Partial substitution of Zn by a magnetic Mn ion results in strong
hybridization of the Mn 3d orbitals with the sp band of the host semiconductor. A detailed study of resonant inelastic scattering
in the vicinity of the S and Mn-L2,3 absorption edges of these DMS is presented.
Received: 6 March 1997 相似文献
50.
We have implemented a model of I 2 ? (CO2) n (2 ≤n ≤ 17) clusters and present an analysis of the minimum energy structures obtained from a quenching procedure. A discussion of the importance of various potential contributions to the energetics of the clusters is also presented. Given the current state of understanding of structural control of caging and the time scales of recombination and evaporation, this model has important implications for understanding the picosecond dynamics observed by the Lineberger group and for rigorous studies of evaporation rates. 相似文献