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Midsized annulenes are known to undergo rapid π-bond shifting. Given that heavy-atom tunneling plays a role in planar bond shifting of cyclobutadiene, we computationally explored the contribution of heavy-atom tunneling to planar π-bond shifting in the major (CTCTCTCT, 5a) and minor (CTCTTCTT, 6a) known isomers of [16]annulene. UM06-2X/cc-pVDZ calculations yield bond-shifting barriers of ca. 10 kcal/mol. The results also reveal extremely narrow barrier widths, suggesting a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using canonical variational transition state theory (CVT) as well as with small curvature tunneling (SCT) contributions, via direct dynamics. For the major isomer 5a, the computed SCT rate constant for bond shifting at 80 K is 0.16 s–1, corresponding to a half-life of 4.3 s, and indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. This contrasts with the CVT rate constant of 8.0 × 10–15 s–1 at 80 K. ...

Surface-enhanced Raman spectroscopy (SERS) underpins a wide range of commercial and fundamental applications. SERS often relies on ligands, usually thiols, bound to a noble metal surface. The difficulty of straightforward thiol synthesis combined with their instability on surfaces highlights the need for alternative ligand design. We present the first example of SERS utilizing N-heterocyclic carbene ligands. A general three step synthesis is presented for functionalized NHC-CO2 adducts. These ligands are deposited on SERS-active gold film-over-nanosphere substrates (AuFONs) in solvent-free and base-free conditions, which prevents fouling. The resulting films are found to be robust and capable of postsynthetic modifications.

Mass spectrally detected products of ligand exchange reactions of the nanoparticle [Au25(SC2H4C6H5)18](1-), (abbrev. Au25(SC2Ph)18), where the dithiol is toluene-3,4-dithiol, CH3C6H3(SH)2, include nanoparticles containing both doubly (bidentate, or chelating) and singly bonded dithiol. The bidentate binding displaces two of the original -SC2Ph ligands, and singly bonded dithiol displaces one -SC2Ph ligand, while maintaining, for mass spectrally detected species, occupancy of 18 ligation sites. Extended exchange reaction times result in an apparent maximum of six chelated dithiolates. In the Au25(SC2Ph)18 nanoparticle, six semi-rings of -S(R)-Au-S(R)-Au-S(R)- act as the protecting ligand shell surrounding a Au13 core; the chelation is suggested to involve binding of dithiolates to adjacent semi-rings, rather than to a single semi-ring. Both high resolution ESI and lower resolution MALDI spectra support the product assignments. A minor extent of bidentate ligand incorporation is sufficient to severely compromise the well-known Au25(SC2Ph)18 UV-vis fine structure and to alter its voltammetric pattern, reflecting either associated semi-ring distortion and/or decay of the exchange product.

New approaches to electrospray ionization mass spectrometry (ESI-MS)-with exact compositional assignments-of small (Au25) nanoparticles with uniform and mixed protecting organothiolate monolayers are described. The results expand the scope of analysis and reveal a rich chemistry of ionization behavior. ESI-MS of solutions of phenylethanethiolate monolayer-protected gold clusters (MPCs), Au25(SC2Ph)18, containing alkali metal acetate salts (MOAc) produce spectra in which, for Na+, K+, Rb+, and Cs+ acetates, the dominant species are MAu25(SC2Ph)182+ and M2Au25(SC2Ph)182+. Li+ acetates caused ligand loss. This method was extended to the analysis of Au25 MPCs with mixed monolayers, where thiophenolate (-SPh), hexanethiolate (-SC6), or biotinylated (-S-PEG-biotin) ligands had been introduced by ligand exchange. In negative-mode ESI-MS, no added reagents were needed in order to observe Au25(SC2Ph)18- and to analyze mixed monolayer Au25 MPCs prepared by ligand exchange with 4-mercaptobenzoic acid, HSPhCOOH, which gave spectra through deprotonation of the carboxylic acids. Adducts of tetraoctylammonium (Oct4N+) with -SPhCOO- sites were also observed. Mass spectrometry is the only method that has demonstrated capacity for measuring the exact distribution of ligand-exchange products. The possible origins of the different Au25 core charges (1-, 0, 1+, 2+) observed during electrospray ionization are discussed.

An efficient stereoselective synthesis of the orally active NK(1) receptor antagonist Aprepitant is described. A direct condensation of N-benzyl ethanolamine with glyoxylic acid yielded a 2-hydroxy-1,4-oxazin-3-one which was activated as the corresponding trifluoroacetate. A Lewis acid mediated coupling with enantiopure (R)-1-(3,5-bis(trifluoromethyl)phenyl)ethan-1-ol afforded a 1:1 mixture of acetal diastereomers which was converted into a single isomer via a novel crystallization-induced asymmetric transformation. The resulting 1,4-oxazin-3-one was converted via a unique and highly stereoselective one-pot process to the desired alpha-(fluorophenyl)morpholine derivative. Interesting and unexpected [1,2]-Wittig and [1,3]-sigmatropic rearrangements were identified during the optimization of these key steps. In the final step, a triazolinone side chain was appended to the morpholine core. The targeted clinical candidate was thus obtained in 55% overall yield over the longest linear sequence.

Configuration energies (CE) of the d-block elements (Groups 3−11) are electronegativities evaluated from the formula CE = (pes + qed)/(p + q). es and ed are the multiplet-averaged one-electron energies of the s- and d-orbitals of atoms which are in the lowest energy of the configurations sndm and sn-1dm+1, and whose highest known oxidation state is (p + q). The orbital energies are obtained from spectroscopic data. Configuration energies generally increase across a row, with the highest values occurring at nickel, silver, and gold; all are lower than the CE of silicon, the least electronegative nonmetal (except for gold which has a CE equal to that of silicon). Down the groups configuration energies invariably decrease from the first row to the second row; for Groups 7−12, the third-row element has a CE higher than that of the second-row element, due to increasing relativistic stabilization of the 6s orbitals.

We report the first collision-induced dissociation tandem mass spectrometry (CID MS/MS) of a thiolate-protected Au nanoparticle that has a crystallographically determined structure. CID spectra assert that dissociation pathways for the mixed monolayer Na(x)Au(25)(SC(2)H(4)Ph)(18-y)(S(C(2)H(4)O)(5)CH(3))(y) centrally involve the semi-ring Au(2)L(3) coordination (L = some combination of the two thiolate ligands) that constitutes the nanoparticle's protecting structure. The data additionally confirm charge state assignments in the mass spectra. Prominent among the fragments is [Na(2)AuL(2)](1+), one precursor of which is identified as another nanoparticle fragment in the higher m/z region. Another detected fragment, [Na(2)Au(2)L(3)](1+), represents a mass loss equivalent to an entire semi-ring, whereas others suggest involvement (fragmentation/rearrangement) of multiple semi-rings, e.g., [NaAu(3)L(3)](1+) and [NaAu(4)L(4)](1+). The detailed dissociation/rearrangement mechanisms of these species are not established, but they are observed in other mass spectrometry experiments, including those under non-CID conditions, namely, electrospray ionization mass spectrometry (ESI-MS) with both time-of-flight (TOF) and FT-ICR analyzers. The latter, previously unreported results show that even soft ionization sources can result in Au nanoparticle fragmentation, including that yielding Au(4)L(4) in ESI-TOF of a much larger thiolate-protected Au(144) nanoparticle under non-CID conditions.

For both [4n] and [4n + 2]annulenes, cases of configuration change in which the number of trans CC bonds changes by 1 must involve a Mobius π-bond shifting step. The known configuration change in [14]annulene (1, CTCTCTT → 2, CCTCTCT), for which Δtrans = 1, is thus predicted to involve a Mobius antiaromatic transition state for bond shifting. Using unrestricted density-functional theory (DFT), we have located the key transition state for this process, as well as other stationary points involved in the mechanism. CASPT2(14,14)/cc-pVDZ//(U)BH&HLYP/6-311+G** results provide a barrier of 19.3 kcal/mol for the overall process 1 → 2, in good agreement with the known experimental barrier (21.3 kcal/mol). The bond-shifting transition state has substantial singlet diradical character and is strongly antiaromatic on the basis of its large positive NICS value. [14]Annulene thus represents the first example of a concerted reaction for which a Mobius antiaromatic transition state provides the only possible pathway.

Efforts to prepare new organic conducting materials have focused primarily on the preparation of new organic donors or modification of existing donors. Herein, we report the preparation and preliminary characterization of a new intercalation compound, FeOCl(ET){sub 1/4}, where ET = bis(ethylenedithio)tetrathiafluvalene. Structural and electronic studies show that the material contains stacks that are partially oxidized, resulting in high electrical conductivity due to the intercalated electron donors.

Catalytic antibodies (abzymes) have been elicited for a variety of chemical reactions. Abzymes have been developed to catalyze high-energy' and highly disfavored2 reactions through induction of an immunological response using a hapten that structurally resembles the transition-state or high-energy species encountered in a given reaction.' In general, catalytic antibodies exhibit high affinities (

Catalysis of enolizaiion by acetic acid is nnt due to an acetic acid molecule acting on the ketonic oxygen and a water molecule removing the proton from carbon, but instead to hydrogen ion acting on the ketonic oxygen and acetaie ion removing the proton from carbon, in the case of alpha -phenylisocaprophenone (a ketone with one alpha hydrogen) in aqueous solution with 8.4 M dioxane (72% dioxane by volume) at 100 . The magnitude of the protium/tritium isotope effect at the carbcn is diagnostic of the attacking nucleophile, increasing as the basicity and reactivity of the nucleophile increases. For catalysis by acetic acid, it has a value (11.4) which is too high for nucleophilic attack by water, but correct for attack by acetate ion. A relation between tritium ived and compared with experimental data. (auth)