Electronic structures of intermolecular charge-transfer states in fast electron transfers with tetrathiafulvalene donor. Thermal and photoactivation of [2 + 4] cycloaddition to o-chloranil acceptor.

Tetrathiafulvalene (TTF) spontaneously forms a series of unusual charge-transfer complexes with various quinonoid acceptors such as o-chloranil (CA) that show pronounced near-IR absorption (lambda(CT) = 1100 nm). The successful isolation of the corresponding [1 : 1] donor-acceptor complex from solution and X-ray crystallographic analysis at low temperatures reveal the polarized charge-transfer state: [TTF(q+),CA(q-)] with high degree of charge-transfer (q = 0.6), which is spectrally and crystallographically distinguished from the separate redox (ion-pair) state: [TTF(+*) + CA(-*) ]. The unique interconversion of charge-transfer and electron-transfer states is theoretically well-accommodated by Mulliken theory using semi-empirical valence-bond and molecular-orbital methodologies. Mechanistic implications are discussed of both the thermally activated and the photochemically promoted processes via fast (intracomplex) electron transfer followed by collapse of the adiabatic and the non-adiabatic (vibrationally-excited) ion-pairs, respectively, to the [2 + 4] cycloadduct of tetrathiafulvalene and o-chloranil.

Steric modulations in the reversible dimerizations of phenalenyl radicals via unusually weak carbon-centered pi- and sigma-bonds.

[reaction: see text] Spontaneous self-associations of various tricyclic phenalenyl radicals lead reversibly to either pi- or sigma-dimers, depending on alkyl-substitution patterns at the alpha- and beta-positions. Thus, the sterically encumbered all-beta-substituted tri-tert-butylphenalenyl radical (2*) affords only the long-bonded pi-dimer in dichloromethane solutions, under conditions in which the parent phenalenyl radical (1*) leads to only the sigma-dimer. Further encumbrances of 1* with a pair of alpha, beta- or beta, beta- tert-butyl substituents and additional methyl and ethyl groups (as in sterically hindered phenalenyl radicals 3* - 6*) do not inhibit sigma-dimerization. ESR spectroscopy is successfully employed to monitor the formation of both diamagnetic (2-electron) dimers; and UV-vis spectroscopy specifically identifies the pi-dimer by its intense near-IR band. The different temperature-dependent spectral (ESR and UV-vis) behaviors of these phenalenyl radicals allow the quantitative evaluation of the bond enthalpy of 12 +/- 2 kcal mol(-1) for sigma-dimers, in which the unusually low value has been theoretically accounted for by the large loss of phenalenyl (aromatic) pi-resonance energy attendant upon such bond formation.

Isolation, X-ray structures, and electronic spectra of reactive intermediates in Friedel-Crafts acylations.

[reaction: see text] Reactive intermediates in the Friedel-Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV-vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediates with other cationic electrophiles that have been structurally established via X-ray studies.

Isolation and X-ray structures of labile benzoic- and acetic-acidium carbocations.

New carbocationic salts (via O-protonation of substituted benzoic acids) are prepared for the first time by controlled hydration of the corresponding benzoylium salts and isolated in pure crystalline form. Precise X-ray structural analyses reveal the rather unexpected (electronic) structure of the carboxylic-acidium functionality.

Intervalence (charge-resonance) transitions in organic mixed-valence systems. Through-space versus through-bond electron transfer between bridged aromatic (redox) centers.

Intervalence absorption bands appearing in the diagnostic near-IR region are consistently observed in the electronic spectra of mixed-valence systems containing a pair of aromatic redox centers (Ar(*)(+)/Ar) that are connected by two basically different types of molecular bridges. The through-space pathway for intramolecular electron transfer is dictated by an o-xylylene bridge in the mixed-valence cation radical 3(*)(+) with Ar = 2,5-dimethoxy-p-tolyl (T), in which conformational mobility allows the proximal syn disposition of planar T(*)(+)/T redox centers. Four independent experimental probes indicate the large through-space electronic interaction between such cofacial Ar(*)(+)/Ar redox centers from the measurements of (a) sizable potential splitting in the cyclic voltammogram, (b) quinonoidal distortion of T(*)(+)/T centers by X-ray crystallography, (c) "doubling" of the ESR hyperfine splittings, and (d) a pronounced intervalence charge-resonance band. The through (br)-bond pathway for intramolecular electron transfer is enforced in the mixed-valence cation radical 2a(*)(+) by the p-phenylene bridge which provides the structurally inflexible and linear connection between Ar(*)(+)/Ar redox centers. The direct comparison of intramolecular rates of electron transfer (k(ET)) between identical T(*)(+)/T centers in 3(*)(+) and 2a(*)(+)( )()indicates that through-space and through-bond mechanisms are equally effective, despite widely different separations between their redox centers. The same picture obtains for 3(*)(+) and 2a(*)(+)( )()from theoretical computations of the first-order rate constants for intramolecular electron transfer from Marcus-Hush theory using the electronic coupling elements evaluated from the diagnostic intervalence (charge-transfer) transitions. Such a strong coherence between theory and experiment also applies to the mixed-valence cation radical 7(*)(+), in which the aromatic redox S center is sterically encumbered by annulation.

Molecular recognition of NO/NO+ via multicenter (charge-transfer) binding to bridged diarene donors. Effect of structure on the optical transitions and complexation thermodynamics.

Bridged diarenes form very strong [1:1] complexes with nitrosonium/nitric oxide in which the NO moiety is optimally sandwiched in the cleft between a pair of cofacial aromatic rings which act as a molecular "Venus flytrap". The spectral features of these associates are generally similar to those for [1:1] and [2:1] nitrosonium complexes with mononuclear alkyl-substituted benzenes, and they are appropriately described within the LCAO molecular-orbital methodology and the Mulliken (charge-transfer) formulation of donor/acceptor electronic transitions. The thermodynamics study indicates that the efficient binding is determined by (i) the close matching of the donor/acceptor redox potentials and (ii) the ability of bridged diarenes for multicentered interactions with a single NO moiety. The best fit of the electronic and structural parameters is provided by a calixarene host that allows the interacting centers to be arranged in a manner similar to those extant in [2:1] nitrosonium complexes with analogous (nonbridged) aromatic donors; this results in its very strong noncovalent binding with nitrosonium/nitric oxide with the formation constant of K(B) approximately 10(8) M(-)(1) and free-energy change of -DeltaG degrees = 45 kJ mol(-)(1). Such strong, selective, and reversible bindings of nitrosonium/nitric oxide by (cofacial) aromatic centers thus provide the basis for the development of efficient NO sensors/absorbents and also suggest their potential relevance to biochemical systems.

Steric hindrance as a mechanistic probe for olefin reactivity: variability of the hydrogenic canopy over the isomeric adamantylideneadamantane/sesquihomoadamantene pair (a combined experimental and theoretical study).

Access to each C=C face of adamantylideneadamantane (AA) and sesquihomoadamantene (SA) is hindered by the hydrogenic canopy consisting of four beta-hydrogens; otherwise, these olefins have quite normal environments. X-ray crystallography and density functional (DFT) calculations show a 0.5 A larger annular opening in the protective cover of AA than that in SA. This contributes to the remarkable differences in reactivity toward various reagents, not only by limiting access to the olefin site in SA but also by inhibiting reactions which force these hydrogens closer together. Thus, AA is subject to typical olefin-addition reactions with bromine, sulfuryl chloride, m-chloroperbenzoic acid, dioxygen, and so forth, albeit sometimes at attenuated rates. On the other hand, SA is singularly unreactive under identical reaction conditions, except for the notable exceptions that include Brønsted (protonic) acids, a nitrosonium cation, and dichlorine. The exceptions are characterized as three sterically limited (electrophilic) reagents whose unique reactivity patterns are shown to be strongly influenced by steric access to the C=C center. As such, the different degrees of steric encumbrance in the isomeric donors AA and SA shed considerable light on the diverse nature of olefinic reactions. In particular, they evoke mechanistic features in electrophilic addition versus electron transfer, which are otherwise not readily discernible with other less hindered olefinic donors. Transient structures of the olefinic-reaction intermediates such as the protonated carbocations AA-H+ and SA-H+ as well as the cation radicals AA*+ and SA*+ are probed by the combination of X-ray crystallographic analyses and density functional theoretical computations.

Mechanism of inner-sphere electron transfer via charge-transfer (precursor) complexes. Redox energetics of aromatic donors with the nitrosonium acceptor.

Spontaneous formation of colored (1:1) complexes of various aromatic donors (ArH) with the nitrosonium acceptor (NO+) is accompanied by the appearance of two new (charge-transfer) absorption bands in the UV-vis spectrum. IR spectral and X-ray crystallographic analyses of the [ArH,NO+] complexes reveal their inner-sphere character by the ArH/NO+ separation that is substantially less than the van der Waals contact and by the significant enlargement of the aromatic chromophore. The reversible interchange between such an inner-sphere complex [ArH,NO+] and the redox product (ArH+.+ NO.) is quantitatively assessed for the first time to establish it as the critical intermediate in the overall electron-transfer process. Theoretical formulation of the NO+ binding to ArH is examined by LCAO-MO methodology sufficient to allow the unambiguous assignment of the pair of diagnostic (UV-vis) spectral bands. The MO treatment also provides quantitative insight into the high degree of charge-transfer extant in these inner-sphere complexes as a function of the HOMO-LUMO gap for the donor/acceptor pair. The relative stabilization of [ArH,NO+] is traced directly to the variation in the electronic coupling element H(AB), which is found to be substantially larger than the reorganization energy (lambda/2). In Sutin's development of Marcus-Hush theory, this inequality characterizes a completely delocalized Class III complex (which occupies a single potential well) according to the Robin-Day classification. The mechanistic relevance of such an unusual (precursor) complex to the inner-sphere mechanism for organic electron transfer is discussed.

Charge-transfer forces in the self-assembly of heteromolecular reactive solids: successful design of unique (single-crystal-to-single-crystal) Diels--Alder cycloadditions.

Electron donor/acceptor (EDA) interactions are found to be a versatile methodology for the engineering of reactive heteromolecular crystals. In this way, a series of the charge-transfer pi-complexes between bis(alkylimino)-1,4-dithiin acceptors and anthracene donors are shown to form heteromolecular (1:1) crystalline solids that spontaneously undergo stereoselective [2 + 4] Diels--Alder cycloadditions. The flexible nature of the 1,4-dithiin moiety allows this homogeneous topochemical transformation to proceed with minimal distortion of the crystal lattice. As a result, a unique (single) crystal phase of the Diels--Alder adduct can be produced anti-thermodynamically with a molecular arrangement very different from that in solvent-grown crystals. Such a topochemical reaction between bis(methylimino)-1,4-dithiin and anthracene proceeds thermally and homogeneously up to very high conversions without disintegration of the single crystal. This ideal case of the mono-phase topochemical conversion can be continuously monitored structurally (X-ray crystallography) and kinetically (NMR spectroscopy) throughout the entire range of the crystalline transformation. The resultant "artificial" crystal of the Diels--Alder adduct is surprisingly stable despite its different symmetry and packing mode compared to the naturally grown (thermodynamic) crystal.

Diels-Alder topochemistry via charge-transfer crystals: novel (thermal) single-crystal-to-single-crystal transformations.

The solid-state [4+2] cycloaddition of anthracene to bis(N-ethylimino)-1,4-dithiin occurs via a unique single-phase topochemical reaction in the intermolecular (1:1) charge-transfer crystal. The thermal heteromolecular solid-state condensation involves the entire crystal, and this rare crystalline event follows topochemical control during the entire cycloaddition. As a result, a new crystalline modification of the Diels-Alder product is formed with a crystal-packing similar to that of the starting charge-transfer crystal but very different from that of the (thermodynamically favored) product modification obtained from solution-phase crystallization. Such a single-phase transformation is readily monitored by X-ray crystallography at various conversion stages, and the temporal changes in crystallographic parameters are correlated with temperature-dependent (solid-state) kinetic data that are obtained by 1H NMR spectroscopy at various reaction times. Thus, an acceleration of the solid-state reaction over time is found which results from a progressive lowering of the activation barrier for cycloaddition in a single crystal as it slowly and homogeneously converts from the reactant to the product lattice.

Web-based distance continuing education: a new way of thinking for students and instructors.

As people have more difficulty taking time away from work to attend conferences and workshops, the idea of offering courses via the Web has become more desirable. Addressing a need voiced by Medical Library Association membership, the authors developed a Web-based continuing-education course on the subject of the librarian's role in evidence-based medicine. The aim of the course was to provide medical librarians with a well-constructed, content-rich learning experience available to them at their convenience via the Web. This paper includes a discussion of the considerations that need to be taken into account when developing Web-based courses, the issues that arise when the information delivery changes from face-to-face to online, the changing role of the instructor, and the pros and cons of offering Web-based versus traditional courses. The results of the beta test and future plans for the course are also discussed.

Silver(I) complexation of (poly)aromatic ligands. Structural criteria for depth penetration into cis-stilbenoid cavities.

Silver(I) complexes with aromatic donors are thoroughly analyzed (with aid of the Cambridge Crystallographic Database) to identify the basic structural factors inherent to the bonding of an arene ligand. Most strikingly, the distance parameter d (which simply measures the normal separation of Ag from the mean aromatic plane) is singularly invariant at d = 2.41 +/- 0.05 A for all silver/arene complexes, independent of the hapticity (eta 1 or eta 2), hybridization, or multiple coordination. As such, a systematic series of stilbenoid ligands has been successfully designed to precisely modulate the penetration of silver(I) into the ligand cleft, and a multicentered poly(arene) ligand (X) designed to form a one-dimensional assembly of Ag/arene units. Simply stated, the depth penetration of silver(I) into the aromatic cavities of various cis-stilbenoid donors can be precisely predicted with a single parameter gamma that measures the separation of the two cofacial aryl groups comprising the cleft. This simple geometric consideration must be taken into account in any successful design of novel (poly)aromatic ligands for silver(I) complexation to constitute new molecular architectures.

Stereoselective Photodimerization of (E)-Stilbenes in Crystalline gamma-Cyclodextrin Inclusion Complexes.

Solid-state irradiation of the crystalline inclusion complex of (E)-stilbene in gamma-cyclodextrin (gamma-CD) yields a single isomer of syn-tetraphenylcyclobutane stereoselectively in high yield. In contrast, the photodimerization of stilbene in solution is very inefficient and unselective, and no photodimer is observed even upon prolonged irradiation of pure crystals. The monosubstituted stilbenes form a pair of photodimers stereoselectively, viz. the syn head-to-head and syn head-to-tail isomers, in comparable yields. The photodimer yields of about 70% and the biphasic decay kinetics of the excited stilbene (as established by picosecond time-resolved diffuse-reflectance spectroscopy) indicate that the stilbene guests are located in at least two distinct sites in the gamma-CD crystal lattice, i.e., a dimerization site where excited stilbene is in close reach of another stilbene guest molecule and an isomerization site where excited stilbene does not find a close neighbor for dimerization and thus undergoes trans --> cis isomerization only.

Photoinduced Coupling of Acetylenes and Quinone in the Solid State as Preorganized Donor-Acceptor Pairs.

Crystalline electron donor-acceptor (EDA) complexes of various diarylacetylenes (DA) and dichlorobenzoquinone (DB) are isolated and structurally characterized by X-ray crystallography. Deliberate excitation of either the DB acceptor at lambda(DB) = 355 nm or the 1:2 [DA, 2DB] complex at lambda(CT) = 532 nm in the solid state leads to [2 + 2] cycloaddition and identical (isomeric) mixtures of the quinone methide products. Time-resolved (ps) diffuse reflectance spectroscopy identifies the ion-radical pair [DA(*+), DB(*-)] as the reactive intermediate derived by photoinduced electron transfer in both photochemical procedures. The effects of crystal-lattice control on the subsequent ion-radical pair dynamics are discussed in comparison with the same photocouplings of acetylenes and quinone previously carried out in solution.

Preparation and Structures of Crystalline Aromatic Cation-Radical Salts. Triethyloxonium Hexachloroantimonate as a Novel (One-Electron) Oxidant.

Triethyloxonium hexachloroantimonate [Et(3)O(+)SbCl(6)(-)] is a selective oxidant of aromatic donors (ArH), and it allows the facile preparation and isolation of crystalline paramagnetic salts [ArH(+)(*), SbCl(6)(-)] for the X-ray structure determination of various aromatic cation radicals. The mechanistic relationship between the Meerwein salt [Et(3)O(+)SbCl(6)(-)] and the pure Lewis acid oxidant SbCl(5) is based on a prior ethyl transfer from oxygen to chlorine within the ion pair.

Providing consumer health information through institutional collaboration.

In the past several years, The Claude Moore Health Sciences Library of the University of Virginia Health Sciences Center (HSC) has noted a growing demand for consumer health information. However, because the primary role of the library is to provide information services to health professionals at the HSC, questions have been raised as to the amount of time, energy, and money that should be expended to provide health care information to consumers. The library staff, because it can provide special expertise regarding the availability and utilization of consumer health materials, has felt the responsibility to participate in HSC initiatives that reach a broad audience. Library efforts in that regard include assisting with inventory and management of patient education materials, participating in a community health promotion task force, collaborating with hospital departments in planning a consumer health information center, establishing a consumer health information reference section in the library, and obtaining a grant to offer a networked health information system to local public and community college libraries. Consumers of health information benefit from the enhanced services that result from combining the expertise of health professionals and patient educators with the information management skills of library staff.

Mechanisms of organic oxidation and reduction by metal complexes.

The mechanism of many organic oxidation and reduction reactions can be described in terms of the formation and reaction of free radicals with metal complexes. Redox (trace-metal) catalysis also involves the oxidation and reduction of radical intermediates with a metal species which oscillates between several oxidation states (4). The oxidation and reduction of free radicals with metal complexes follow two general mechanisms, electron transfer and ligand transfer. Direct analogy exists with wholly inorganic descriptions of outer-sphere and innersphere processes. In an electron transfer or outersphere mechanism the redox process is derived largely by transfer of an electron from reductant to oxidant, with only indirect contributions from the solvent and ligand. Carbonium ion intermediates and transition states are important considerations, and the scission of the beta-hydrogen bond is minor during oxidation of alkyl radicals to alkenes. In contrast, ligand transfer or inner-sphere mechanism demands maximum involvement of the ligand in the transition state. Free-radical character prevails; cationic contributions from the organic moiety are minimal. Oxidation and reduction are conjugate processes. In an electron transfer mechanism the oxidation of alkyl radicals to carbonium ions is conceptually represented by a microscopic reverse reaction in which a carbonium ion is reduced to an alkyl radical. A similar duality exists in the interconversion of carbanions and free radicals by metal complexes. The reversibility of the ligand transfer process is easier to observe. For example, the chlorine-transfer oxidation of alkyl radicals is represented by a microscopic reverse reduction of alkyl chlorides to alkyl radicals by cuprous chlorides. A ligand transfer counterpart of the reduction of radicals R*+Cu(II)Cl(n)<---->R-Cl+Cu(I)Cl(n-1) can also be described. Hopefully, these simple redox mechanisms will be utilized in rationalizing complex reactions and formulating new syntheses. The limited number of examples cited in this short review represent only an introduction to the vast area of chemical research to be tapped in the study of the mechanisms and the synthetic utility of oxidation-reduction reactions and catalysis.