Nicholas J. Head

Flinders University, Tarndarnya, South Australia, Australia

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Publications (16)72.35 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: In order to get insight into transmission mechanisms of the title spin–spin coupling constants, J(C, F1) couplings (n = 4, 5) were calculated at the DFT-B3LYP-6-311-G**/EPR-III level, and couplings were measured in 9 members of the series 1-F,4-X-cubanes, where the α atom of the X group is a carbon atom. Hyperconjugative interactions were evaluated within the NBO approach, using the same level of theory as that employed for calculating spin–spin coupling constants. The unusual J(C4,F1) spin–spin coupling constants known in 1-F,4-X-cubanes are rationalized in this work as originating mainly due to two factors, namely, (i) the strong σ-hyperconjugative interactions involving as donors the cage C–C bonds; (ii) the particular arrangement of cage bonds not involving either the C1 or the C4 carbon atoms, Ci and Cj. For linear X substituents, the direction F1–C1··· C4–X is a three-fold symmetry axis and there are six Ci–Cj equivalent cage bonds, which are involved in (Ci–C j)→(C1–F1)* and (Ci–C j)→(C4–X)* hyperconjugative interactions. This means that coupling pathways involving such interactions are amplified six times, rendering the substrate very efficient for transmitting ‘trans-cage’ coupling constants. The large halogen substituent effects observed for J(C4,F1) spin–spin coupling constants in 1-F,4-X-cubanes (X = halogen atom) are rationalized in terms of interactions between σ-hyperconjugative (involving C–C bonds) and negative hyperconjugative interactions involving the halogen lone-pairs.
    Molecular Physics 02/2006; 104(4):485-492. DOI:10.1080/00268970500276499 · 1.64 Impact Factor
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    ABSTRACT: Photophysical studies with semi-rigid, 1, and flexible, 2, donor-bridge-acceptor (D-b-A2+) molecules with D a porphyrin and A2+ a methyl viologen moiety, were performed in neat polar solvents as well as included in surfactant (DTAB) aqueous and in reverse AOT/n-alkane micelles. The micelles acted as nanoreactors for the photoinduced electron transfer reaction upon laser excitation. In spite of the longer lifetime of the charge separated (CS) state in the semi-rigid tetrad 1(ca. 200 ns vs. ca. 100 ns for the flexible dyad 2), the CS formation quantum yield, for example in acetonitrile, was lower for the former (phi(CS) = 0.13) than for the latter (0.58). Comparison of the time-resolved fluorescence data in neat solvent and in the micelles yielded the phi(CS) values in the dilute micellar solutions. Application of laser-induced optoacoustic spectroscopy at various temperatures to 1 dissolved in a polar organic solvent (benzonitrile, BZN) included in aqueous DTAB nanoreactors afforded structural volume changes for the production in hundreds of ps of the CS state upon excitation of a polar molecule. The contraction during CS formation upon excitation of the collapsed conformer in BZN is attributed to the entering of solvent into the open molecular cavity. The opening upon formation of the CS state due to photoinduced electron transfer in the 1 collapsed conformation arises from the repulsion of the two positively charged ends in this state, as previously calculated. Inclusion of 1 in reverse AOT micelles in various n-alkanes also led to a contraction upon excitation, but the data had much more error due to the limited range of variability of the ratio of thermoelastic parameters. The data obtained with the more flexible "supermolecule" 2 showed the predicted large conformation flexibility of these molecules.
    Physical Chemistry Chemical Physics 01/2006; 7(24):4114-25. DOI:10.1039/b511157a · 4.20 Impact Factor
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    ABSTRACT: The electron-transfer rate constant is measured in two U-shaped donor-bridge-acceptor molecules over a wide range of temperature in acetonitrile and N-methylacetamide (NMA). The electron-transfer rate at high temperature can be well described by a nonadiabatic model of the reaction, but at low temperatures the rate in NMA becomes controlled by the solvent. The results are discussed in terms of theoretical models for the change in reaction mechanism and its dependence on the solute-solvent frictional coupling.
    Journal of the American Chemical Society 10/2004; 126(34):10778-86. DOI:10.1021/ja049539d · 11.44 Impact Factor
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    ABSTRACT: A systematic determination of electronic coupling matrix elements in U-shaped molecules is demonstrated. The unique architecture of these systems allows for the determination of the electronic coupling through a pendant molecular moiety that resides between the donor and acceptor groups; this moiety quantifies the efficiency of electron tunneling through nonbonded contacts. Experimental electron-transfer rate constants and reaction free energies are used to calibrate a molecular-based model that describes the solvation energy. This approach makes it possible to experimentally determine electronic couplings and compare them with computational values.
    Journal of the American Chemical Society 09/2002; 124(34):10171-81. DOI:10.1021/ja025683s · 11.44 Impact Factor
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    ABSTRACT: A dyad with terminal porphyrin (P) and viologen (MV2+) chromophores covalently linked by a flexible seven-atom hexanoate chain has been synthesized. Ab initio and semi-empirical solvent continuum calculations suggest that the preference for ‘extended’ or ‘folded’ conformations of the dyad depend strongly on solvent. Fluorescence lifetime and transient absorption studies in acetonitrile indicate that approximately 80% of the dyad population are in a suitable conformation to undergo photoinduced electron transfer (PET) to produce P+–MV+ with a rate of 5×108s−1. A long-lived charge-separated state lifetime of 170ns is observed. It is proposed that electrostatic repulsion between the positively charged end groups resulting from PET increases the average donor–acceptor separation thus retarding the charge recombination process.
    Journal of Photochemistry and Photobiology A Chemistry 05/2000; 133(1):105-114. DOI:10.1016/S1010-6030(00)00215-X · 2.29 Impact Factor
  • Journal of the American Chemical Society 05/2000; 122(21):5220-5221. DOI:10.1021/ja000611r · 11.44 Impact Factor
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    ABSTRACT: A solution to the vexing problem of investigating solvent-mediated electron transfer under conditions of frozen solvent motion may be at hand with the synthesis of novel U-shaped systems such as 1 (R=CO(2)Me) and 2 (R=CH(2)OMe), in which the imide-functionalized cleft provides the point of attachment of the "solvent" molecule, either covalently or by hydrogen bonding.
    Angewandte Chemie International Edition 12/1999; 38(21):3219-3222. DOI:10.1002/(SICI)1521-3773(19991102)38:213.0.CO;2-4 · 11.34 Impact Factor
  • Angewandte Chemie 11/1999; 111(21):3408-3411. DOI:10.1002/(SICI)1521-3757(19991102)111:21<3408::AID-ANGE3408>3.0.CO;2-X
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    ABSTRACT: A series of 5-substituted (X) 2,2-dimethoxyadamantanes (4, X = F, Cl, Br, COOCH3, OCH3, CH3, C6H5, p-NO2C6H4, p-BrC6H4, SiMe3, and SnMe3) as well as 2-methylene-5-tert-butyladamantane (6, Y = CH2; X = C(CH3)3) have been synthesized and characterized. Reduction of the ketals 4 under ionic conditions with triethylsilane and phenylsilane provide π-facial diastereoselectivities for hydride trapping of 5-substituted (X) 2-methoxyadamant-2-yl cations (1, R = OCH3 ↔ 5). A comparison of this data with known diastereoselectivities for nucleophilic capture of tertiary and secondary 5-substituted (X) 2-adamantyl cations highlights that stereoselectivity in these systems is a function of electron demand. Diastereoselectivities for the hydrochlorination of 6 (Y = CH2, X = C(CH3)3) in CH2Cl2 and NO2CH3 have been determined and compared with the corresponding data for the silicon analogue (6, Y = CH2; X = SiMe3). Low electron demand coupled with the stereoelectronic requirement of double hyperconjugation appears the most likely explanation, rather than a long-range steric factor, for the total lack of stereoselectivity in the Cl- capture of the 2-methyl-5-(trimethylsilyl)adamant-2-yl cation (1, Y = CH2; X = SiMe3) in NO2CH3 as solvent.
    The Journal of Organic Chemistry 09/1997; 62(18):6177-6182. DOI:10.1021/jo970600p · 4.64 Impact Factor
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    ABSTRACT: Methods are described for the synthesis of 9-hydroxy- and 9-bromopentacyclo[,5),0(3),(8).0(4,7)] nonane (homocubane derivatives) and for the same derivatives of pentacyclo[,5).0(3,8).0(4,7)]decane (basketane). The 9-homocubyl and 9-basketyl radicals generated from these precursors were observed by EPR spectroscopy. In spite of their very large strain energies, both radicals rearranged extremely slowly, and unrearranged products were obtained from homolytic reactions in solution at temperatures below 150 degrees C. At higher temperatures the 9-basketyl radical rearranged by a cascade of three beta-scissions, the ultimate product being 1-(4-cyclobut-2-enyl)cyclohexa-2,4-diene. The Arrhenius parameters for the arrangement were found to be log(A(r)/s(-1)) = 13.6, E(r) = 13.5 kcal mol(-1). The 9-homocubyl radical did not rearrange even at 220 degrees C. An explanation as to why these cage radicals rearrange at least 6 orders of magnitude more slowly than the related cubylcarbinyl radical is presented, and semiempirical SCF-MO calculations are reported.
    ChemInform 04/1994; 116(7). DOI:10.1021/ja00086a009
  • ChemInform 12/1992; 114(27). DOI:10.1021/ja00053a008
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    ABSTRACT: The ESR spectra of cubyl and 4-fluorocubyl radicals were observed in solution; they indicated that hyperconjugation was slight, but that significant spin density reaches C(3) and C(4).
    Journal of the Chemical Society Chemical Communications 11/1990; DOI:10.1039/c39900001589
  • Ernest W. Della, Nicholas J. Head
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    ABSTRACT: The substituent chemical shifts and the carbon-fluorine coupling constants of a range of 4-substituted cubyl fluorides in solvents of varying polarity are presented. Least squares regressional analysis of the data indicates that the fluorine probe, while being somewhat less sensitive to field effects as a result of the low polarizability of the cubane C-F bond, nevertheless responds in the ''reverse'' manner in agreement with precedents established in other alicyclic systems. The dependence of (1)J(CF) upon electronegativity has been interpreted as evidence for the occurrence of sigma-induction over four bonds, the longest reported to date. Significantly, resonance parameters are found to be very important in describing the effect of substituents on both the fluorine chemical shifts and the magnitude of the one-bond and four-bond C-13-F-19 coupling constants. This represents one of the rare occasions that resonance effects have been observed in a fully saturated system in the ground state. A possible mechanism by which these resonance effects may be transmitted is presented.
    The Journal of Organic Chemistry 60(16). DOI:10.1021/jo00121a059 · 4.64 Impact Factor
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    ABSTRACT: The product ratios observed upon fluorodeiodination of a series of caged cyclobutane-containing iodides are explained on the basis of the relative energies of the intermediate cations involved. The relative energies of these cations have been evaluated by ab initio calculations with the inclusion of electron correlation (MP2/6-31G*//RHF/3-21G; MP2/6-31G**), the results of which lend support to the view that hyperconjugative involvement of the cationic center with the alpha,beta and beta,gamma C-C bonds in each cyclobutyl moiety is the critical factor responsible for the stability of the cation in each case. The degree of stabilization of the cations is a reflection of the number and relative importance of several resonance contributors (corresponding to the involvement of the carbon sigma-framework) to their overall structure and is strongly dependent on the geometry of the rigid carbon framework in each case.
    The Journal of Organic Chemistry 58(27). DOI:10.1021/jo00079a037 · 4.64 Impact Factor
  • Ernest W. Della, Nicholas J. Head
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    ABSTRACT: Fluorodeiodination is found to be an attractive procedure for the synthesis of bridgehead fluorides. Thus, treatment of the corresponding iodide with xenon difluoride in dichloromethane at ambient temperature generally leads to high yields of the fluoride. Evidence suggests the intermediacy of the bridgehead cation in this reaction, and accordingly the substrates which are unfavorably disposed to fluorodeiodination are the bicyclo[n.1.1]alkyl iodides. In this context the isolation of a small quantity of methyl 4-fluorobicyclo[2.1.1]hexane-1-carboxylate (46, R = COOMe) is significant because it represents the first occasion on which the elusive 1-bicyclo[2.1.1]hexyl cation has been trapped. We have also demonstrated that synthesis of the iodides themselves can be accomplished efficiently both by Barton halodecarboxylation and by treatment of the carboxylic acid with lead tetraacetate and iodine.
    The Journal of Organic Chemistry 57(10). DOI:10.1021/jo00036a018 · 4.64 Impact Factor