Intramolecular rearrangement of thioozonides: Sulfine formation without sulfur atom scrambling,a double isotope crossover study

Decomposition of a mixture of ³⁴S and deuterium labeled thioozonides prepared by photooxidation of dimethylthiophenes in CDCl₃ has shown that intramolecular rearrangement to thioketone S-oxide, a stable 1,3-dipole, proceeds cleanly without sulfur atom scrambling in the presence of sulfur allotropes.     

Spectroscopic investigation of H atom transfer in a gas-phase dissociation reaction: McLafferty rearrangement of model gas-phase peptide ions

Wavelength-selective infrared multiple-photon photodissociation (WS-IRMPD) was used to study isotopically-labeled ions generated by McLafferty rearrangement of nicotinyl-glycine-tert-butyl ester and betaine-glycine-tert-butyl ester. The tert-butyl esters were incubated in a mixture of D(2)O and CH(3)OD to induce solution-phase hydrogen-deuterium exchange and then converted to gas-phase ions using electrospray ionization. McLafferty rearrangement was used to generate the free-acid forms of the respective model peptides through transfer of an H atom and elimination of butene. The specific aim was to use vibrational spectra generated by WS-IRMPD to determine whether the H atom remains at the acid group, or migrates to one or more of the other exchangeable sites. Comparison of the IRMPD results in the region from 1200-1900 cm(-1) to theoretical spectra for different isotopically-labeled isomers clearly shows that the H atom is situated at the C-terminal acid group and migration to amide positions is negligible on the time scale of the experiment. The results of this study suggest that use of the McLafferty rearrangement for peptide esters could be an effective approach for generation of H-atom isotope tracers, in situ, for subsequent investigation of intramolecular proton migration during peptide fragmentation studies.     

Role of four-membered rings in C32 fullerene stability and mechanisms of generalized Stone-Wales transformation: a density functional theory investigation

Density functional theory (DFT) methods have been applied to study C(32) fullerenes built from four-, five-, and six-membered rings. The relative energies of pure C(32) fullerenes have been evaluated to locate three most stable structures, 32:D(4d) with two squares, 1:D(3) without square and 5:C(s) with one square. Structural analysis reveals that there is a rearrangement pathway between the lowest energy classical isomer 1:D(3) and the lowest energy non-classical isomer 32:D(4d), and 5:C(s) behaves just as an intermediate between them. The kinetic processes of generalized Stone-Wales transformation (GSWT) with four-membered rings have been explored and two distinct reaction mechanisms are determined by all the transition states and intrinsic reaction coordinates with PBE1PBE/6-31G(d) approach for the first time. One mechanism is the concerted reaction with a rotating dimer closed to the cage surface and another is the stepwise reaction with a carbene-like sp(3) structure, whereas the latter is sorted into two paths based on four-membered ring vanishing before or after the formation of the carbene-like structure. It is indicated that there is no absolute preference for any mechanism, which depends on the adaptability of different reactants on the diverse mechanisms. Furthermore, it's found that the interconversion process with the participation of squares is more reactive than the rearrangement between C(60)_I(h) and C(60)_C(2v), implying some potential importance of non-classical small fullerenes in the fullerene isomerization.     

Hydrogen atom formation from the photodissociation of water ice at 193 nm

The TOF spectra of photofragment hydrogen atoms from the 193 nm photodissociation of amorphous ice at 90-140 K have been measured. The spectra consist of both a fast and a slow components that are characterized by average translational energies of 2k(B)T(trans)=0.39+/-0.04 eV (2300+/-200 K) and 0.02 eV (120+/-20 K), respectively. The incident laser power dependency of the hydrogen atom production suggests one-photon process. The electronic excitation energy of a branched cluster, (H(2)O)(6+1), has been theoretically calculated, where (H(2)O)(6+1) is a (H(2)O)(6) cyclic cluster attached by a water molecule with the hydrogen bond. The photoabsorption of this branched cluster is expected to appear at around 200 nm. The source of the hydrogen atoms is attributed to the photodissociation of the ice surface that is attached by water molecules with the hydrogen bond. Atmospheric implications are estimated for the photodissociation of the ice particles (Noctilucent clouds) at 190-230 nm in the region between 80 and 85 km altitude.     

Tetrahydropyran formation by rearrangement of an epoxy ester: a model for the biosynthesis of marine polyether toxins

Acid-catalyzed rearrangement of the (S)-epoxide derived from 2alpha-allyl cholestanyl acetate resulted in a 1:1 mixture of a steroidal tetrahydrofuran and a steroidal tetrahydropyran. Formation of a six-membered ring supports the hypothesis that epoxy ester-orthester-cyclic ether rearrangement may be involved in the biosynthesis of ladder-type marine polyether toxins. This reaction represents a new biomimetic preparation of medium ring cyclic ethers.     

Thermal rearrangement of alkylaryldihydrosemibullvalene derivatives: hydrogen atom transfer processes

Alkylaryldihydrosemibullvalene derivatives, besides exhibiting vinylcyclopropane type rearrangements typical of tetraarylated analogues, also undergo H· atom transfer from methyl groups, most easily understood in terms of biradical intermediates resulting from alternative modes of cyclopropane ring scission.     

Photobleaching of radiation induced trapped electrons in neutral and alkaline glasses. Hydrogen atom formation

Photomobilization of trapped electrons in 7 M NaClO₄- and 9 M NaOH-glass gives rise to trapped hydrogen atoms. This is probably due to the reaction e^?_m + H₂O → H + OH^?. Experiments with an electron scavenger indicate that electrons are not precursors to radiolytically produced hydrogen atoms. It seems that the mobile electrons produced during photobleaching are not slowed down to thermal energy before they react to produce hydrogen atoms, since the yield of the latter species is strongly dependent on the wavelength of the bleaching light.     

Hydrogen exchange in a six-membered ring transition state. Evidence for a stepwise McLafferty rearrangement

Mass spectra of 2-methyl-5-chloro-3-N-ethyl-benzothiazolium iodide and labeled counter-parts are discussed with particular emphasis on the loss of ethylene from the moiety produced by thermal elimination of HI from this salt. This process demonstrates the stepwise nature of a McLafferty rearrangement involving a hydrogen transfer to an olefinic carbon.     

Tin atom formation in a graphite furnace atomizer

Absorbance measurements resolved in time and space are presented for tin vaporized under various conditions in a graphite furnace. Mass spectroscopie studies coupled with temporal and spatial absorbance measurements indicate that oxygen entrained in the inert sheath gas significantly attenuates the free atomic tin population through the rapid formation of SnO. Addition of 10% hydrogen to the sheath gas and enclosure of the atomizer, substantially reduce the effect of entrained oxygen. Similar release mechanisms are postulated with and without hydrogen, but it is suggested that the concentration of elemental tin on the surface at the time of vaporization is greater when hydrogen is included. The effects of added sulfate salts were also studied and it was concluded that the formation and prevolatilization of SnS is responsible for the extreme depressions reported in the literature.     

Determination of active sites for H atom rearrangement in dissociative ionization of ethanol

In an electron impact dissociative ionization experiment on C(2)H(5)OH, the formation of molecular ions requiring rearrangement of H atoms has been studied using a momentum spectrometer. H(3) (+), H(2) (+), HOH(+), and H(2)OH(+) observed in the experiment are molecular ions of this type. By comparing the mass spectrum of C(2)H(5)OH with that of its isotopomer C(2)H(5)OD, we determine the proportions of H-bond rearrangements involving carbon and oxygen sites. We find that the formation of H(3) (+) due to the breaking of the O-H bond and rearrangement of the H atoms on the CH(2) site is about 2.5 times as likely as its formation involving atoms from the CH(3) site alone. No such difference is seen in case of the H(2) (+) ion. The role of the O-H bond in formation of all observed ions has been assessed. Kinetic energy distributions of the molecular ions suggest that two or three electronically excited states contribute to their formation.     

Beyond the Roger Brown rearrangement: long-range atom topomerization in conjugated polyynes

Long-range carbon atom topomerization in a 1,3-diyne has been demonstrated for the first time. 1-Phenyl-4-p-tolyl-1,3-butadiyne, (13)C-enriched at C-1, was synthesized and subjected to flash vacuum pyrolysis. At 800 degrees C and 0.01 Torr, this resulted in nearly complete (13)C label equilibration between C-1 and C-2, as seen by NMR analysis. Pyrolysis at 900 degrees C further led to ca. 35% of the label migrating about equally to C-3 and C-4. These results demonstrate that both intrabond and interbond atom exchange processes are operative, with the former having a lower activation barrier. DFT and Moller-Plesset calculations support a mechanism that passes through Brown rearrangement (1,2-shift), closure to trialene (bicyclo[1.1.0]-1,3-butadiene), bond-shift isomerization to exchange C-2 and C-3, and ring opening. The resulting vinylidene can rearrange to a butadiyne with the isotopic label at C-3 or C-4. Consistent with earlier calculations, trialene is predicted to have alternating peripheral bonds, with a weak central sigma bond and significant diradical character. Trialene is predicted [(B3LYP/6-311+G(2d,p)] to lie 64.6 kcal/mol above butadiyne, with barriers of 2.2 and 4.4 kcal/mol, respectively, for ring opening or bond-shift isomerization. Other potential rearrangement mechanisms which pass through tetrahedrene (E(rel) = 167.2 kcal/mol) or 1,2,3-cyclobutatriene (E(rel) = 161.1 kcal/mol) lie at much higher energies.     

Nargenicin model studiess: dehydrative rearrangement of a dihydroxyoctahydronaphthalene

Cyclic ether formation by dehydration of a cis-fused 5-acetyl-1,5-dihydroxyoctahydronaphthalene occurs with rearrangement to give a 12-oxatricyclo[5.4.1^1,7.O^3,8]dodecene rather than the 11-oxatricyclo[4.4.1^1,6.O^2,7]undecene system of the nargenicin antibiotics.     

The formation of a stable o-semidine rearrangement intermediate

Treatment of 5,6,11,12-tetrahydrodibenzo-[b, f][1,2] diazocine with dilute acids affords a spiro-1,2,3,4-tetrahydroquinoline derivative which represents the first isolated stable o-semidine rearrangement intermediate.     

Mobility of a single alkali metal atom on fullerene C60: first principles molecular dynamical study

The dynamical behavior of a single K or Na alkali metal atom on the surface of a C(60) molecule is investigated via Car-Parrinello molecular dynamics simulations in a temperature range up to 300 K. These provide direct evidence for the heteroatom motion, postulated earlier in pioneering experiments, and show that an alkali metal atom can move both on the surface and radially outward from the surface, resulting in a dynamics ranging from diffusive to free orbital motion, on time-scales of up to a few picoseconds.     

Design of stapled DNA-minor-groove-binding molecules with a mutable atom simulated annealing method

We report the design of optimal linker geometries for the synthesis of stapledDNA-minor-groove-binding molecules. Netropsin, distamycin, and lexitropsinsbind side-by-side to mixed-sequence DNA and offer an opportunity for thedesign of sequence-reading molecules. Stapled molecules, with two moleculescovalently linked side-by-side, provide entropic gains and restrain theposition of one molecule relative to its neighbor. Using a free-atom simulatedannealing technique combined with a discrete mutable atom definition, optimallengths and atomic composition for covalent linkages are determined, and anovel hydrogen bond zipper is proposed to phase two molecules accuratelyside-by-side.     

4d-->4f dipole resonance of the metal atom encapsulated in a fullerene cage: Ce@C82

The yield curves for photoions from Ce@C(82) are measured by using synchrotron radiation in the photon energy range from 90 to 160 eV. Parent Ce@C(82) (z+) and fragment ions C(60) (z+) and C(70) (z+) are observed in a mass spectrum (z=1 and 2). The yield curves for doubly charged ionic species exhibit broad resonance in the photon energy region of from 120 to 140 eV which is ascribed to the 4d-->4f giant dipole resonance of the encapsulated Ce atom. The total photoabsorption cross section of Ce@C(82) was determined from partial photoionization cross sections for formation of the parent and fragment ions to be 5.3(-1.1) (+1.8) and 19.6(-3.9) (+6.5) Mb at photon energies of 110 and 130 eV, respectively.     

Novel formation of a fluorinated aziridino[60]fullerene

Reaction of MEMN₃ (methoxyethoxymethylazide) with C₆₀F₁₈ results in replacement of two fluorines by the >NCH₂OCH₂CH₂OMe function giving a C_s symmetrical aziridinofluorofullerene, C₆₀F₁₆NCH₂OCH₂CH₂OMe.     

A highly selective rearrangement of a housane-derived cation radical: an electrochemically mediated transformation

To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14*+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained sigma framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.     

Molecular photoelectric switch using a mixed SAM of organic [60]fullerene and [70]fullerene doped with a single iron atom

We describe a photoswitch fabricated on indium tin oxide (ITO) as a self-assembled monolayer (SAM) of two fullerene molecules, a purely organic [60]fullerene that generates an anodic current and a [70]fullerene doped with a single iron atom. This device generates a bidirectional photocurrent upon irradiation at 340 and 490 nm. The new [70]fullerene iron complex bearing three rigid carboxylic acid legs, Fe[C(70)(C(6)H(4)C(6)H(4)COOH)(3)]Cp, generates only a cathodic current upon photoexcitation between 350 and 700 nm, whereas the organic [60]fullerene absorbs at wavelengths shorter than 500 nm. The quantum efficiency of the photocurrent generation by the mixed SAM is comparable to that of a single-component SAM, indicating that the individual diode molecules on ITO generate photocurrents independently with little cross talk.