Radical cations and triplet states of 1,2-disubstituted cyclopropanes: comparison of potential surfaces.

Radical ion pairs generated by photo-induced electron transfer from 1,2-disubstituted cyclopropanes to various acceptors undergo return electron transfer in pairs of singlet and triplet multiplicity. The pair energies relative to the reactant ground states and to accessible triplet states, respectively, determine the competition between the recombination pathways. The potential surfaces of the radical cations and triplet states of 1,2-diphenyl-, 1, and 1,2-dimethylcyclopropane, 2, have been examined by density functional theory calculations.

Innermolecular reactions of fluorophenylcarbene inside a hemicarcerand.

Photolysis of fluorophenyldiazirine, incarcerated in hemicarcerand 2, affords fluorophenylcarbene, which attacks an aryl unit of the host, leading (after rearrangement) to a fluoromethoxy/phenyltropone derivative of the hemicarcerand. The incarcerated carbene is probably unstable at temperatures above 100 K.

Properties of the Nucleic-acid Bases in Free and Watson-Crick Hydrogen-bonded States: Computational Insights into the Sequence-dependent Features of Double-helical DNA.

The nucleic-acid bases carry structural and energetic signatures that contribute to the unique features of genetic sequences. Here we review the connection between the chemical structure of the constituent nucleotides and the polymeric properties of DNA. The sequence-dependent accumulation of charge on the major- and minor-groove edges of the Watson-Crick base pairs, obtained from ab initio calculations, presents unique motifs for direct sequence recognition.

Competing gas-phase substitution and elimination reactions of gemini surfactants with anionic counterions by mass spectrometry. Density functional theory correlations with their bolaform halide salt models.

Understanding ion specific effects on the solution properties of association colloids is a major unsolved problem, and we are studying the chemistry of gemini surfactants in the gas-phase by mass spectrometry and density functional theory (DFT) to probe ion specific effects in the absence of water. Products from gas-phase fragmentation chemistry of dication-monoanion pairs, M2+X(-), of C16H33(CH3)2N+-(CH2)(n-) +N(CH3)2C16H33.2X(-) gemini surfactants were determined by using sequential collision induced dissociation mass spectrometry.

Structural, infrared, and density functional theory studies of N,N,N',N'-Tetramethylimidazolidinium dichloride: a model for cation-anion association of headgroups and counterions in the interfacial regions of gemini micelles.

N,N,N',N'-Tetramethylimidazolidinium dichloride (1-Im-1 2Cl) has been studied as a model system for cation-anion interactions in the interfacial regions of gemini micelles by X-ray crystallography, density functional theory (DFT) calculations, and infrared spectroscopy. Single crystals of 1-Im-1 2Cl contain 1-Im-1 dications, whose five-membered rings adopt a distorted envelope conformation. Eight chloride anions surround each dication, two of which are cradled above and below the five-membered ring (apical) and six of which are dispersed about the periphery of the ring (equatorial).

Chlorofluorocarbene: first UV observation of a dihalocarbene in solution.

Chlorofluorocarbene (ClCF), generated by laser flash photolysis of chlorofluorodiazirine, absorbs at 368 nm in pentane. Absolute rate constants are reported for ClCF additions to several alkenes and pyridine. ClCF is less reactive toward alkenes than CCl(2), and it does not react rapidly with oxygen.

Tracking "invisible" alkylchlorocarbenes by their sigma --> p absorptions: dynamics and solvent interactions.

Contrary to implications in the literature, the sigma --> p absorptions of alkylchlorocarbenes (RCCl) are readily acquired by laser flash photolysis with UV-vis detection in solution at ambient temperature. Examples include RCCl with R = methyl, benzyl, t-butyl, 1-adamantyl, and cyclopropyl. These absorptions permit direct monitoring of carbene reactions and the formation of carbene-solvent complexes.

The synthesis of dichlorodiazirine and the generation of dichlorocarbene: spectroscopy and structure of dichlorocarbene ylides.

Reaction of 2,4-dinitrophenoxychlorodiazirine (13) with chloride ions affords dichlorodiazirine (4). Photolysis of 4 generates dichlorocarbene. In laser flash photolysis (LFP) experiments, CCl2 forms chromophoric ylides or oxides with pyridine, 2-picoline, thioanisole, and oxygen.

Diffuse functions in natural bond orbital analysis.

We show that diffuse function augmentation of Pople basis sets at the 6-311G RHF and B3LYP levels strongly impact conclusions drawn from natural bond orbital (NBO) analysis. The large spatial extent of high quantum number Rydberg orbitals introduced by augmentation contribute importantly to the valence space of neighboring atoms due to the likely inadequacy of the 311 valence functions. In contrast, lesser anomalies are found for augmentation of double zeta type 6-31G.

Conformational product control in the low-temperature photochemistry of cyclopropylcarbenes.

[reaction: see text] Different conformers of two fused-ring cyclopropylchlorocarbenes are shown to undergo distinct, wavelength-selectable, photochemistry in low-temperature matrixes. Conformers with H and Cl syn give ring expansion predominantly, whereas those with H and Cl anti mainly fragment.

Incorporation of 2-arylhexa-1,5-diene into pentasil zeolite: A distorted 1-arylcyclohexane-1,4-diyl radical cation at room temperature.

Incorporation into a redox-active pentasil zeolite [(Na,H)-ZSM-5] converted 2-arylhexa-1,5-dienes (9a-c; aryl = phenyl, tolyl, anisyl) into 1-arylcyclohexane-1,4-diyl radical cations, 10a-c*+. The ESR spectra of 10a-c*+ (six lines, g = 2.0026; a = 9.

Mechanism of thio acid/azide amidation.

A combined experimental and computational mechanistic study of amide formation from thio acids and azides is described. The data support two distinct mechanistic pathways dependent on the electronic character of the azide component. Relatively electron-rich azides undergo bimolecular coupling with thiocarboxylates via an anion-accelerated [3+2] cycloaddition to give a thiatriazoline.

1-Fluoropropane. Torsional Potential Surface.

The systematic deletion of orbital interactions, using natural bond orbital (NBO) theory at the B3LYP/ 6-311++G(3df,2p) level, provides validation for the anti-C-H/C-F* hyperconjugative interaction providing the backbone for the gauche preference of 1-fluoropropane (FP). The FCCC torsional coordinate taking trans FP to gauche FP is predicted to be strongly contaminated by CCC bending with the result that a large part of the trans → gauche stabilization energy stems from mode coupling. The anti-C-H/C-F* hyperconjugative interaction is also found to play a major, if not determining, role in the coupling.

Dichlorodiazirine: a nitrogenous precursor for dichlorocarbene.

The reaction of 3-p-nitrophenoxy-3-chlorodiazirine with a 1.1:1.0:1.

Endo entry to the nortricyclyl-norbornenyl cation system: stereochemistry in the fragmentation of endo-5-norbornenyl-2-oxychlorocarbene.

Fragmentation of (S)-endo-5-norbornenyl-2-oxychlorocarbene [(S)-8] in cyclohexane-d12 gives approximately 20% (S)-endo-2-chloro-5-norbornene [(S)-7] with approximately 50% ee, 65-70% (R)-exo-2-chloro-5-norbornene [(R)-4] with >95% ee, and approximately 12% (R)-3-nortricyclyl chloride [(R)-5] with approximately 22% ee. (Analogous stereochemical results were also obtained starting with the enantiomeric carbene (R)-8.) The (S)-8 to (S)-7 and (S)-8 to (R)-4 conversions are ascribed mainly to retention and inversion S(N)i transition states, respectively.

Kinetic studies of the cyclization of singlet vinylchlorocarbenes.

[reaction: see text] Vinylchlorocarbenes (R = Me or Cl) cyclize to cyclopropenes with k = (2.4-5.1) x 10(7) s(-1) or (3.

Homolytic fragmentation of allyloxychlorocarbene.

The decomposition of allyloxychlorocarbene in hydrocarbon solvents leads via homolysis to allyl and COCl radicals, which recombine to 3-butenoyl chloride or (after scission of (*)COCl to CO and (*)Cl) to allyl chloride. Labeling experiments show that both products are formed in part with allylic rearrangement.

Unprecedented chemistry of an aryloxychlorodiazirine: generation of a dihalodiazirine and diazirinone.

The reaction of p-nitrophenoxychlorodiazirine with tetrabutylammonium fluoride follows three channels: (1) approximately 17% of p-nitrophenoxide/fluoride exchange to chlorofluorodiazirine and p-nitrophenol, (2) approximately 28% of Cl/F exchange to p-nitrophenoxyfluorodiazirine, and (3) approximately 55% of ipso fluoride attack, affording p-nitrofluorobenzene and the previously unknown diazirinone (diazacyclopropenone).

Solvent-equilibrated homoadamantyl chloride ion pairs from chloroformate or oxachlorocarbene fragmentations.

Fragmentations of 3-homoadamantyl chloroformate and 3-homoadamantyloxychlorocarbene produce identical ion pairs as product-determining intermediates.

Solvent-equilibrated ion pairs from carbene fragmentation reactions.

[R(+) OC Cl(-)] ion pairs were generated in methanol/dichloroethane solutions, with R(+) as the 1-bicyclo[2.2.2]octyl, 1-adamantyl, or 3-homoadamantyl cation.

Carbon tunneling in the ring expansion of noradamantylchlorocarbene.

Irradiation of 3-(3-noradamantyl)-3-chlorodiazirine produced the corresponding noradamantylchlorocarbene, which could be detected in solution with laser flash photolysis via its pyridinium ylide, and in N2 or Ar matrixes at 9 K with IR and UV/vis spectroscopy. At ambient temperatures, or on irradiation in the cryogenic matrixes, the carbene rearranged to 2-chloro-1-adamantene, which could be trapped in solution and which was characterized by IR and UV/vis at low temperatures. In the dark at 9 K, the carbene also slowly ring-expanded to the chloroadamantene, at a rate ca.

Rearrangements concerted with fragmentation of cyclopropylmethoxychlorocarbene and cyclobutoxychlorocarbene in hydrocarbon solvents and Ar matrices.

Fragmentations of cyclopropylmethoxychlorocarbene (6) and cyclobutoxychlorocarbene (10) lead to rearrangments that afford mixtures of cyclopropylmethyl chloride (7), cyclobutyl chloride (8), and 3-butenyl chloride (9). Isotopic substitution studies show that these rearrangments are accompanied by partial exchange of the methylene groups within 6 and 10. Surprisingly, these processes that are typical of carbocations persist in hydrocarbon solvents such as pentane and cyclohexane-d(12).