摘要:

IntroductionThe chemical reactivity of a molecule is often interpreted in terms of charge distribution of its atoms. Atomic charges are used to indicate the preferred direction for a reagent to approach a substrate. Although the chemists have an intuitive feeling for the reactivity based on charge distribution, assigning precise reactivity from atomic charge failed in several cases.1,2In recent years, reactivity descriptors3–7such as hardness, softness, Fukui functionetc.have emerged as powerful tools in predicting the reactive sites of molecules. These reactivity descriptors are derived from density functional theory. Global hardness and global softness8–12represent the reactivity of a molecule as a whole. On the other hand, the Fukui function13defines the reactivity of an atom in a molecule and it is a local property. Fukui function and local softness, which is closely related, are suited to describe the relative reactivity of different substrates.Pearson's hard–soft acid–base (HSAB)12,14–16principle suggested that hard–hard and soft–soft interactions are favorable over hard–soft interactions.17–19Again it has been found that soft–soft interactions are preferred in the site of the maximum Fukui function,i.e.frontier control,20–22but on the other hand, hard–hard interactions are preferred in the site of the minimum Fukui function,23,24i.e.charge control.In this paper we have presented the reactivity parameters, the local softnesssk+andsk−and Fukui functionsfk+andfk−of α,β-unsaturated carbonyl compounds, namely acrolein (H2C&z.dbd;CHCHO), methylacrylate (H2C&z.dbd;CHCOOCH3), methylmethacrylate (H2C&z.dbd;C(CH3)COOCH3), cinnamaldehyde (C6H5CH&z.dbd;CHCHO), cinnamoylchloride (C6H5CH&z.dbd;CHCOCl) and acryloylchloride (H2C&z.dbd;CHCOCl), and the most reactive sites of nucleophilic attack were derived. In these compounds the carbonyl carbon is slightly positive, due to mesomeric and resonance effects. The β-carbon, which is conjugated with the carbonyl carbon develops partial positive charge. Experimentally it has been found that a nucleophile attacks at the β-carbon atom of α,β-unsaturated carbonyl compounds.25Here we have investigated the reactive sites of these carbonyl compounds towards a nucleophile using a local hard–soft acid–base approach.

ISSN:1460-2733    

DOI:10.1039/b301675g

出版商:RSC    

年代:2003

数据来源: RSC

摘要:

1.IntroductionStudies of the chemical interactions between metal clusters and organic molecules can provide useful information leading to deeper understanding of some important heterogeneous catalytic processes in the organometallic chemistry field.1–3The structures and properties of metal–aromatic molecular complexes have been studied extensively in recent years.4–15Duncan and coworkers4–6have investigated the formation and dissociation processes of metal–benzene complexes by reflectron time-of-flight mass spectrometry (RTOFMS) and laser photodissociation spectroscopy (LPDS). They have elucidated the dissociative charge-transfer processes of those complexes. Kaya and co-workers7–9have systematically studied the properties of 3d transition metal–benzene complexes ([Mm(C6H6)n] (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni)) in the gas phase, including their structures, photodissociation processes, bonding energies, electron affinities, and ionization energies. They revealed two types of structures for those metal–benzene complexes: (i) multiple-decker sandwich structures; and (ii) metal clusters fully covered with benzene molecules (rice-ball structures). The sandwich structures are characteristic for the early transition metals (Sc–V), whereas the latter structures involve the late transition ones (Fe–Ni). Theoretical efforts in understanding their properties have recently been carried out by Pandeyet al.10by using the density functional theory and generalized gradient approximation (DFT-GGA). All of their calculation results agreed well with the available experimental data. It should be noted that all these research efforts concerned only neutral or cation complexes. Very recently, several negative metal-cluster–benzene complexes were reported,11–14and their photoelectron spectra were studied.On the other hand, the dissociation of benzene molecules on metal clusters is also an important topic because of the interest in the catalytic reaction intermediates. On the surface of many active metal clusters, for example V,11Pt,15Nb and Rh,16etc., benzene can be dehydrogenated or dissociated, wherein the hydrogen atoms are always observed to evolve as H2, since these reactions are thermodynamically preferred. In this article, we report on the generation of a rare kind of complexes, [Mm-phenyl]−(M = Mn–Cu), by the gas phase reaction of laser vaporized metal species with benzene vapor seeded in argon gas. The formation mechanisms, which involve metal cluster anions inducing the selective benzene C–H cleavage, were proposed.

ISSN:1460-2733    

DOI:10.1039/b302761a

出版商:RSC    

年代:2003

数据来源: RSC

摘要:

1IntroductionAs a result of a series of circular dichroism, X-ray diffraction and UV-vis spectrum studies, the manner in which metal ions form complexes with peptides is well understood.1–8The cobalt(iii) tetripeptide, Co(NH3)2(l-ala–gly–gly) (Fig. 1) is a near-planar complex. The UV-vis spectrum shows that the absorption of the molecule tends to be weak in the near infrared region. The crystal structure is triclinic with non-centric space groupP1. These two factors exhibit a possibility that it may be a candidate as a IR nonlinear optical (NLO) material. However, to our knowledge, there are no experimental or theoretical studies concerning the NLO properties on this complex. In this paper we carried out anab initiocalculation to study the electron structure, UV-vis and IR spectra and the second-order polarizabilityβof the cobalt(iii) tetripeptide complex.Molecular structure and orientation of Co(NH3)2(l-ala–gly–gly).

ISSN:1460-2733    

DOI:10.1039/b307832a

出版商:RSC    

年代:2003

数据来源: RSC