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排序方式: 共有693条查询结果,搜索用时 31 毫秒
1.
We introduce a statistical ensemble for a single vortex filament of a three dimensional incompressible fluid. The core of
the vortex is modeled by a quite generic stochastic process. We prove the existence of the partition function for both positive
and a limited range of negative temperatures.
Received: 31 July 2000 / Revised version: 6 March 2001 / Published online: 20 December 2001 相似文献
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4.
Volonterio A Bellosta S Bravin F Bellucci MC Bruché L Colombo G Malpezzi L Mazzini S Meille SV Meli M Ramírez De Arellano C Zanda M 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4510-4522
Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon). 相似文献
5.
Massimiliano Aschi Marina Attina Fulvio Cacace 《Research on Chemical Intermediates》1996,22(7):645-658
Experiments based on FT-ICR mass spectrometry provide conclusive evidence for the operation of an alternative mechanism of electrophilic aromatic substitution occurring within the complex formed upon addition of gaseous arenium ions to a variety of proelectrophiles, including alkyl halides, alcohols, epoxides, esters and diazoalkanes. The factors affecting the efficiency of the unconventional substitution are examined in the light of the kinetic results of the FT-ICR experiments. 相似文献
6.
Aschi M Lucente G Mazza F Mollica A Morera E Nalli M Paglialunga Paradisi M 《Organic & biomolecular chemistry》2003,1(11):1980-1988
The conformational study of a new group of synthetic peptides containing 4-amino-1,2-dithiolane-4-carboxylic acid (Adt), a cysteine-related achiral residue, has been carried out through a joint application of computational and experimental methodologies. Molecular Dynamics simulations clearly suggest the tendency of this molecule to adopt a gamma-turn conformation in vacuum and help in analyzing the complex and crucial conformational behaviour of the dithiolane ring which appears to preferentially adopt a C(S)-like structure. Electronic structure calculations carried out in solution using the Density Functional Theory also indicate the preservation of the gamma-like folding in apolar solvents and the helix-like one in more polar solvents. A comparison with the achiral 1-aminocycloalkane-1-carboxylic acid (Ac5c) has been carried out using the same computational tools. NMR and IR data on dipeptide derivatives containing the Adt or Ac5c residue show that in chloroform solution all the models prefer a gamma-turn structure, centered at the cyclic residue, stabilized by an intramolecular H-bond, whereas in a more polar solvent, i.e. dimethyl sulfoxide, this folding is not maintained. The experimental conformational studies, extended to N-Boc protected tripeptides, clearly indicate the remarkable tendency of both the five-membered C(alpha)-tetrasubstituted cyclic amino acids Adt and Ac5c to induce the gamma-turn structure also in models able to adopt the beta-bend conformation. 相似文献
7.
Cordaro M Di Donna L Grassi G Maiuolo L Mazzotti F Perri E Sindona G Tagarelli A 《European journal of mass spectrometry (Chichester, England)》2004,10(5):691-697
An evaluation of the gas-phase ion chemistry of rotenone (1) by electrospray ionisation (ESI) mass spectrometry (MS) and tandem mass spectrometry (MS2) is presented, aiming at providing tools for its determination in natural and biological matrices. The behaviour of its cycloadducts with benzonitrile-N-oxide (2) and 2,4,6-trimethylbenzonitrile-N-oxide (3) was also evaluated and the MS data thus obtained have provided evidence into the mechanism of formation of the key product ion at m/z 192 which can be considered a marker in the MS and MS2 spectra of rotenone and its derivatives. 相似文献
8.
Bonini BF Comes-Franchini M Mazzanti G Ricci A Sala M 《The Journal of organic chemistry》1996,61(21):7242-7243
9.
Cimas A Rayón VM Aschi M Barrientos C Sordo JA Largo A 《The Journal of chemical physics》2005,123(11):114312
The singlet potential-energy surface for the N(2D)+CH2F(2A') reaction has been studied employing both second-order M?ller-Plesset and density-functional theories. The energies of the involved species have been refined using the Gaussian-2, complete basis set, and coupled-cluster singles and doubles (triples) methods. The reaction proceeds through the formation of an initial intermediate, which does not involve any activation barrier. Based on the energy profile for the singlet potential-energy surface, the preferred product should be the most exothermic one, namely, HCN+HF, followed by HNC+HF and FCN+H2. This result seems in contradiction with a computational study of the kinetics of the title reaction in terms of the statistical theories, which leads to the prediction that the production of HNC+HF should be the dominant channel. Consequently, a limited molecular-dynamics study has been carried out, concluding that in fact the system behaves in a nonstatistical way. According to the molecular-dynamics study, the most exothermic channel, HCN+HF, should be the dominant one. An analysis of the possible role of the singlet surface in the reaction of N(4S) with CH2F(2A') has also been carried out. The computational study shows that the microcanonical coefficients for the nonadiabatic channels are much smaller than the competing adiabatic ones. Therefore, the reaction of N(4S) with CH2F(2A') should proceed on the triplet surface without spin change. 相似文献
10.
Amadei A Marinelli F D'Abramo M D'Alessandro M Anselmi M Di Nola A Aschi M 《The Journal of chemical physics》2005,122(12):124506
In this paper we extend the perturbed matrix method by explicitly including the nuclear degrees of freedom, in order to treat quantum vibrational states in a perturbed molecule. In a previous paper we showed how to include, in a simple way, nuclear degrees of freedom for the calculation of molecular polarizability. In the present work we extend and generalize this approach to model vibroelectronic transitions, requiring a more sophisticated treatment. 相似文献