摘要:

Dalton made a bold assumption in his atomic hypothesis by stating that atoms retained their mass and their identity in chemical combination. Its vindication had to await Rutherford's nuclear model of the atom. The continuing evolution of chemistry led to the realization that atoms exhibit not only a unique mass but also characteristic additive properties, thereby making it possible to recognize their presence in a molecule and to predict the molecule's static and reactive properties. The theoretical vindication of the model of a functional group as the carrier of chemical information had to await the work of Feynman and Schwinger. Their generalization of physics leads to a unique definition of an atom as an open quantum system and makes possible the renormalization that is required to account for the short-range nature of the forces that enable one to identify a given group in any environment. The lecture will demonstrate that the proper open systems predicted by the quantum action principle define the atom and that this definition accounts for the retention of an atom's chemical identity by enabling one to replace the quantum mechanical observables for force and energy with dressed, real space density distributions whose forms parallel the transferable topology of the electron density distribution.Key words: atom, action principle, atoms in molecules, functional groups.

ISSN:0008-4042    

DOI:10.1139/v98-113

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

O-Alkylation of the flexible macrocycle1with 2-(chloromethyl)pyridine in the presence of Cs2CO3resulted in the preferential formation of partial-cone-2. The cone-to-partial-cone ratio of2increased in the presence of K2CO3, and led to almost equal in the presence of NaH. The template effect of the alkali metal cations plays an important role in thisO-alkylation reaction. The structural characterization of these products is also discussed. The two-phase solvent extraction data indicated that tris[(2-pyridylmethyl)oxy]hexahomotrioxacalix[3]arenes2show moderate extractability forn-butylammonium ion and high extractability for Ag+. The1H NMR titration of cone-2with AgSO3CF3clearly demonstrates that a 1:1 complex is formed, which is stable on the NMR time scale. Thus, the synergism of cyclophane moiety and two or three lower-rim side chains with pyridyl groups play a significant role on the complexation of tris[(2-pyridylmethyl)oxy] derivatives cone-2and partial-cone-2with Ag+ion andn-butylammonium ion.Key words: macrocycles, calixarenes,O-alkylation, ionophores, cation-N interaction.

ISSN:0008-4042    

DOI:10.1139/v98-102

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The selective introduction of one or two nitro groups by direct replacement oftert-butyl groups via theipsoaromatic nitration of the meta-bridged aromatic compounds having two arene rings is described. Theipso-nitration at thetert-butyl groups ofsyn- andanti-di-tert-butyl-dimethoxy[n.2]metacyclophanes1is attributed to the highly activated character of the aryl ring and the increased stabilization of a &sgr;-complex intermediate arising from the dienone-type &sgr;-complex intermediate possible in the case of an internally methoxy substituent. However, only onetert-butyl group isipso-nitrated in the mild reaction conditions such as copper(II) nitrate in an acetic anhydride solution because of deactivation of the second aromatic ring by the introduced nitro group. A first &sgr;-complex intermediate would be stabilized by two types of the through-space electronic interaction, i.e., face-to-face overlapping and intra-annular interaction with the opposing benzene ring, thus accelerating the reaction. The higher yields of mononitrated product are obtained in syn-conformer than in anti-conformer. Therefore, face-to-face overlapping might be much more favourable to stabilize the first &sgr;-complex intermediate than the intra-annular interaction. On the other hand, nitration of1with fuming nitric acid afforded the two-foldipso-nitration product3in quantitative yield. Thus, the number ofipso-nitration at thetert-butyl groups of1was strongly affected by the reactivity of the nitration reagent.Key words: cyclophanes, electrophilic aromatic substitutions,ipso-nitration, &sgr;-complex intermediates, through-space electronic interactions.

ISSN:0008-4042    

DOI:10.1139/v98-111

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The electronic structure, conformation, and molecular orbitals of some nickel(II) azides, thiocyanates, and isothiocyanates have been studied. Three different basis sets: split valence (SV), split valence with sixd-Gaussians (SV6D), and double zeta (DZ) sets, were used to find the best ground state for nickel. It has been found that the combination, DZ-3F, gives results closest to the experimental values. The electronic structures of the nickel azides studied were completely different from those of the nickel thiocyanates. On the other hand, the electronic structures of the nickel thiocyanates studied were highly comparable to those of the corresponding nickel isothiocyanates. Molecular orbitals were computed for the complexes studied and the types of electronic transitions expected were identified and discussed.Key words: Ni(II) azides, thiocyanates, and isothiocyanates: ab initio SCF calculations; MO calculations on some Ni(II) complexes; theoretical treatment of some Ni(II) ions and salts; geometry and energetics of some nickel(II) azides, thiocyanates, and isothiocyanates.

ISSN:0008-4042    

DOI:10.1139/v98-109

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Four new bis-macrocyclic bolaamphiphiles were prepared to explore the effects of hydrophobic substitutions on ion transport. In bilayer vesicles the new compounds were remarkably similar to more hydrophilic derivatives prepared previously. Planar bilayer conductance experiments showed the new compounds induced an unique current-time signal consisting of a rapid rise time, followed by a slower decay time. Signal shape was cation dependent and was related to a modest selectivity between cations. Cation-anion selectivity was very high, approaching an ideal cation selectivity. One compound also showed voltage dependence of the signal shape and duration. Qualitative changes in signal shape, duration, and voltage dependence were provoked by variation in the electrolyte pH and by masking the head-group electrostatic interactions with low levels of barium ions. A model for the signal shape is proposed, involving a rapid current rise due to aggregate restructuring, followed by slower decay due to development of the local Donnan potential that results from the high cation-anion selectivity.Key words: ion channel, synthesis, bilayer membrane, bilayer clamp, mechanism.

ISSN:0008-4042    

DOI:10.1139/v98-097

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The hydrogen bond network influence on the carbonic anhydrase II (CAII) zinc binding site has been studied theoretically by using the semiempirical AM1 method. To this aim, quantum mechanical reduced models of wild-type CAII and several CAII variants have been constructed. We have shown that, when a direct metal ligand donates a hydrogen bond to an indirect metal ligand, the first-shell residues enhance their electrostatic interaction with the zinc cation. Thus, the hydrogen-bond network is able to modulate the zinc binding affinity and the zinc-water pKa.Key words: hydrogen bond network, carbonic anhydrase II, Zn2+metalloenzyme ligands.

ISSN:0008-4042    

DOI:10.1139/v98-098

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

The reaction of (NBu4)2[Zn(dsit)2] (dsit = 4,5-diselenolate-1,3-dithiole-2-thione) with AuClL in a 1:4 ratio leads to [Au2(dsit)L2] (L = PPh3(1), PPh2Me (2)), transferring the diselenolate group. Complex2reacts further with equimolar amounts of [Au(OClO3)L], affording the trinuclear complex [Au3(dsit)(PPh2Me)3]ClO4(3). When the reaction of (NBu4)2[Zn(dsit)2] with [Au2Cl2(P-P)] was carried out in a 1:2 ratio, complexes [Au2(dsit)(P-P)]n(P-P = dppe (1,2-bis(diphenylphosphine)ethane),n= 1 (4); P-P = dppm (bis(diphenylphosphine)methane),n= 2 (5)) are obtained. Anionic derivatives such as (NBu4)[Au(dsit)2] (6) could be obtained by reaction of the zinc complex with (NBu4)[AuBr4] in a 1:1 ratio while other gold complexes require a tin complex as ligand transfer. So [Sn(dsit)Me2] (7), obtained from the zinc complex and [SnCl2Me2], reacts with PPN[Au(C6F5)Cl] (PPN = bis(triphenylphosphine)iminium) or [AuBr2(S2CNR2)] affording [Au(dsit)(S2CNR2)] (R = Me (8); R = Bz (9)) or (PPN)2[Au2(dsit)(C6F5)2] (10), respectively. Electrochemical oxidation of6gives (NBu4)0.4[Au(dsit)2] (11) and the reaction with (TTF)3(BF4)2(TTF = tetrathiafulvalene) affords (TTF)[Au(dsit)2] (12). Electrical conductivities of these complexes at room temperature in compacted pellets are 3 × 10-8(11) and 5 × 10-3(12) S cm-1.Key words: diselenolate, dsit, gold complexes, tin complexe

ISSN:0008-4042    

DOI:10.1139/v98-101

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Several new dibenzopyrans as well as novel thiadiazaacenaphthenes are obtained via 4 + 2 addition of the thienocoumarins (1) and thienopyridazines (2) to electron-poor olefins.Key words: pyridazines, thienopyridazines, phthalazines, coumarines, thiadiazaacenaphthenes.

ISSN:0008-4042    

DOI:10.1139/v98-095

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Ab initio computations (B3LYP/6-31G(D), coupled with isodesmic reactions) were used to predict&agr;C→H bond dissociation energies (BDEs) for proline as a residue in a model peptide, intended to mimic the environment in proteins. The environment was further constrained to mimic common proline positions in turns of different types. The BDEs were found to be very dependent on the structural constraints imposed by the turn type, implying different structure-mediated susceptibilities to free radical damage to proline residues. Unnatural repair of proline (inversion of chirality) was found to be thermodynamically unfavourable. The predicted BDEs for the proline&agr;C→H bond, in kJ mol-1, to an estimated accuracy of ±10 kJ mol-1are as follows: fully optimizedtransrotamer, 368.6; fully optimizedcisrotamer, 357.7; ß turn type I, 380.7; ß turn type II, 397.8; ß turn type II', 385.4; ß turn type VIa, 374.0; ß turn type VIb, 355.0.Key words: proline, ß -turns, free radical, bond dissociation energy, molecular structure, oxidative dama

ISSN:0008-4042    

DOI:10.1139/v98-107

出版商:NRC Research Press    

年代:1998

数据来源: NRC

摘要:

Pentafluorophenyl-bis(pentafluorophenylthio)sulfonium hexafluoroarsenate, [(C6F5S)2SC6F5][AsF6], was prepared by the reaction of (C6F5)2S2and AsF5in liquid SO2at room temperature. The compound has been characterized by single-crystal X-ray and powder diffraction, and IR and FT-Raman spectroscopy. Crystals of [(C6F5S)2SC6F5][AsF6] are orthorhombic, space groupP212121(cell dimensions measured at room temperature in brackets):a= 13.132(3) [13.157(2)] Å,b= 21.477(4) [21.575(3)] Å,c= 8.770(2) [8.775(1)] Å, &agr; = ß = &ggr; = 90.00°;V= 2473.3(9) Å3,Z= 4, temperature = 181 ± 1 K. The structure consists of [AsF6]-anions and [(C6F5S)2SC6F5]+cations that have a different structure than the related [X2MMMX]+cations (M = S, Se; X = Cl, Br), which contain a trichalcogen chain with a pronounced chalcogen-chalcogen bond alternation. Ab initio calculations suggest the observed [(C6F5S)2SC6F5]+geometry is due to steric and solid state effects. Comparison of the Raman spectra of [(C6F5)S2SC6F5][AsF6], [Cl2SSSCl][AsF6], and [(CH3)2ClS3]+implies that [(CH3)2ClS3]+has a structure with equal sulfur-sulfur distances, and therefore the structure is likely [(CH3S)2SCl]+and probably not the previously reported [(CH3)2SSSCl]+.Key words:[R3X3]+cations, X-ray crystal structure, FT-Raman spectroscopy, [(C6F5S)2SC6F5]+, pentafluorophenyl-bis(pentafluorophenylthio)sulfonium cation, hexafluoroar

ISSN:0008-4042    

DOI:10.1139/v98-100

出版商:NRC Research Press    

年代:1998

数据来源: NRC