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排序方式: 共有294条查询结果,搜索用时 46 毫秒
71.
Jeffrey M. McMahon Anne-Isabelle Henry Kristin L. Wustholz Michael J. Natan R. Griffith Freeman Richard P. Van Duyne George C. Schatz 《Analytical and bioanalytical chemistry》2009,394(7):1819-1825
Finite element method calculations were carried out to determine extinction spectra and the electromagnetic (EM) contributions
to surface-enhanced Raman spectroscopy (SERS) for 90-nm Au nanoparticle dimers modeled after experimental nanotags. The calculations
revealed that the EM properties depend significantly on the junction region, specifically the distance between the nanoparticles
for spacings of less than 1 nm. For extinction spectra, spacings below 1 nm lead to maxima that are strongly red-shifted from
the 600-nm plasmon maximum associated with an isolated nanoparticle. This result agrees qualitatively well with experimental
transmission electron microscopy images and localized surface plasmon resonance spectra that are also presented. The calculations
further revealed that spacings below 0.5 nm, and especially a slight fusing of the nanoparticles to give tiny crevices, leads
to EM enhancements of 1010 or greater. Assuming a uniform coating of SERS molecules around both nanoparticles, we determined that regardless of the
separation, the highest EM fields always dominate the SERS signal. In addition, we determined that for small separations less
than 3% of the molecules always contribute to greater than 90% of the signal.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
72.
Bouchoux G Guillemin JC Lemahieu N McMahon TB 《Rapid communications in mass spectrometry : RCM》2006,20(8):1187-1191
The gas-phase basicity (GB) of aminoacetonitrile (NH2CH2CN, 1) has been determined from measurement of proton transfer equilibrium constants in an ion cyclotron resonance mass spectrometer (GB(1) = 789.3 +/- 1.0 kJ x mol(-1)). Molecular orbital calculations up to the G2 level demonstrate that protonation occurs preferentially on the nitrogen atom of the NH2 group, and provide a theoretical proton affinity (PA(1)) of 824.0 kJ x mol(-1). Exact calculation of the entropy associated with hindered rotations and consideration of Boltzman distribution of conformers allow a theoretical estimate of the molar protonation entropy S degrees (1H+) - S degrees (1) = 8.6 J x mol(-1) x K(-1). Combining this value with experimental GB(1) leads to an 'experimental' proton affinity of 819.2 kJ x mol(-1), in close agreement with the G2 expectation. 相似文献
73.
The apparent Michaelis constant, K(M), for glutamate oxidase (GluOx) immobilised on Pt electrodes increased systematically with enzyme loading. The effect was due, at least in part, to electrostatic repulsion between neighbouring oxidase molecules and the anionic substrate, glutamate (Glu). This understanding has allowed us to increase the Glu sensitivity of GluOx-based amperometric biosensors in the linear response region (100+/-11 nA cm(-2)microM(-1) at pH 7.4; SD, n=23) by incorporating a polycation (polyethyleneimine, PEI) to counterbalance the polyanionic protein. Differences in the behaviour of glucose biosensors of a similar configuration highlight a limitation of using glucose oxidase as a model enzyme in biosensor design. 相似文献
74.
Swallen SF Mapes MK Kim YS McMahon RJ Ediger MD Satija S 《The Journal of chemical physics》2006,124(18):184501
The translational dynamics of the low molecular weight glass-former tris(naphthylbenzene) have been studied on the length scale of a few nanometers at the glass transition temperature Tg. Neutron reflectivity was used to measure isotopic interdiffusion of multilayer samples created by physical vapor deposition. Deposition with the substrate held at Tg-6 K allows observation of dynamics characterizing the equilibrium supercooled liquid. The diffusion coefficient measured at q = 0.03 A(-1) was determined to be 1x10(-17) cm2/s at 342 K (Tg). The self-part of the intermediate scattering function I(s)(q,t) decays exponentially. Samples deposited well below Tg show a substantial thermal history effect during subsequent translational motion at Tg. 相似文献
75.
McMahon SA Leonard GA Buchanan LV Giraud MF Naismith JH 《Acta crystallographica. Section D, Biological crystallography》1999,55(5):1108
In the paper by McMahon, Leonard, Buchanan, Giraud & Naismith [Acta Cryst. (1999). D55, 399-402] an author's error has resulted in the fifth sentence of the Abstract being incorrect. The sentence should read 'They are monoclinic, space group P21, with unit-cell dimensions a = 71.12, b = 58.42, c = 96.38 A, beta = 96.38 degrees. 92% (native) and 94% (selenomethionine) complete data sets have been recorded to 2.9 A (Rmerge = 5.0%) and 3.0 A (Rmerge = 6.9%), respectively.' 相似文献
76.
Fourier transform ion cyclotron resonance (FT-ICR) ligand exchange equilibrium experiments have been used to establish a relative scale of sodium binding free energies of about fifty organic molecules. Ab initio calculations yield accurate enthalpies and entropies of complexation for a new set of 30 molecules. These calculations establish an absolute basis for the relative experimental free energy scale. In addition, they provide structural information for the complexes which permits considerable insight into the nature of sodium ion binding. We found that when the binding site is a first row atom, the sodium ion aligns with the molecular dipole axis in order to maximize charge-dipole electrostatic interactions. Strong deviations from this behavior occur when the ion is attached to a heavier atom such as sulfur, chlorine or bromine. For flexible molecules such as the isomers of butyl chloride, there are several isomers of low energy, and differences exist between the enthalpy and free energy orders of stability. Finally, sodium ion affinities have been obtained for several aromatic molecules which lend support to the importance of charge-quadrupole interactions in such cation-pi complexes. 相似文献
77.
Stracener LL Halter RJ McMahon RJ Castro C Karney WL 《The Journal of organic chemistry》2000,65(1):199-204
Ab initio (MP2, CCSD(T)) and density functional theory (BLYP, B3LYP) calculations provide insight concerning novel aspects of structure and bonding in cyclobutylidene (1). Singlet cyclobutylidene ((1)1) adopts a bicyclobutane-like structure (C(s) symmetry) that includes a weak, transannular bonding interaction between the carbene carbon and the opposing CH(2) group. Conformational ring inversion in (1)1 occurs through a transition state of C(2)(v)() symmetry (TS(1)1) with an enthalpy barrier of approximately 3 kcal/mol. Stabilization afforded the singlet state by the transannular interaction appears to be largely offset by a loss of hyperconjugative stabilization from the adjacent C-H bonds. Triplet cyclobutylidene ((3)1) exhibits a C(2)(v)() structure and conventional bonding. The triplet state lies 5.9 kcal/mol above the singlet ground state at the CCSD(T)/TZP//CCSD(T)/DZP level of theory. The singlet-triplet energy gap of cyclobutylidene (-5.9 kcal/mol) lies between that of an acyclic analogue, dimethylcarbene (-1.6 kcal/mol), and a highly strained analogue, cyclopropylidene (-13.8 kcal/mol). The magnitude of the energy gap suggests that triplet cyclobutylidene ((3)1) will be thermally accessible under a variety of experimental conditions. 相似文献
78.
The crystal and molecular structure of (tBu)3AlP(nPr)3 has been determined. The Al(1)?P(1) bond distance [2.594(3) Å] is slightly longer than other aluminum-phosphine complexes; however, the geometry about aluminum is similar to that of the [AlCl(tBu)3]? anion, suggesting that the geometry about the aluminum in tri-tert-butylaluminum complexes is defined by the size of thetert-butyl ligands and not as a consequence of the steric bulk of the Lewis base. Crystal data: Monoclinic,P2t,a=8.932(2),b=16.832(3),c=9.328(2), Å, β=114.36(3)°,V=1277.6(6) Å3,Z=4,R.=0.055,R w =0.053. 相似文献
79.
Herman Gershon Donald D. Clarke John J. McMahon Muriel Gershon 《Monatshefte für Chemie / Chemical Monthly》2001,132(7):833-838
Summary. 2,5-, 3,5-, 3,6-, 3,7-, 4,5-, 5,6-, 5,7-, and 6,7-dibromo-8-quinolinols were prepared and tested against six fungi (Aspergillus niger, Aspergillus oryzae, Myrothecium verrucaria, Trichoderma viride, Mucor cirinelloides, and Trichophyton mentagrophytes) in Sabouraud dextrose broth. Most of the compounds were strongly antifungal, inhibiting five of the fungi below 1μg/cm3, but were somewhat less fungitoxic than the corresponding chloro analogues. M. cirinelloides was not inhibited by any of the compounds except 4,5-dibromo-8-quinolinol. This high level of fungitoxicity is attributed
to intramolecular synergism.
Received December 28, 2000. Accepted (revised) February 3, 2001 相似文献
80.
I.R. Gabitov A.O. Korotkevich A.I. Maimistov J.B. McMahon 《Applied Physics A: Materials Science & Processing》2007,89(2):277-281
Light propagation in a Bragg periodic structure containing thin films with metallic nanoparticles is studied. The plasmonic
resonance frequency, Bragg frequency, and the light carrier frequency are assumed to be close. Exact solutions describing
solitary gap-waves are found, and a light arrest phenomenon due to the nonlinearity of plasmonic oscillations is studied.
PACS 42.55.Tv; 42.65.-k; 42.70.Qs; 81.07.Bc 相似文献