Anion-Cation-Anion Ion Triplet Characterization by Computation and Photoelectron Spectroscopy.

Ion triplets of the chloride salts of two commonly used weakly coordinating cations are reported (i.e., Cl·NMeCl () and Cl·PPhCl ()).

π-Bond Dissociation Energies: C-C, C-N, and C-O.

Single, double, and triple bond dissociation energies are useful quantities, but students and practicing chemists commonly do not know representative values or how they are obtained. In this paper, select π-bond energies are provided and general methods for their determination are discussed. Relationships between heats of hydrogenation, bond dissociation energies, and π-bond strengths are also addressed.

Carbon Dioxide Stability and C═O π-Bond Strengths.

π-Bond energies for a series of C═O containing compounds including carbon dioxide are determined using G3 and W1 calculations. These two computational methods give results that are in good accord, and in the latter case span a 37 kcal mol range from 75 (CH═O) to 112 (CO) kcal mol. This sensitivity to substituent effects on the π-bond strength is attributed to Coulombic interactions between the oppositely charged carbon and oxygen atoms in the carbon-oxygen double bond.

Metallocenium incorporated charge-enhanced thiourea catalysts.

Several metallocenium derivatives were prepared including -ferrocenium-'-phenylthiourea tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate (BAr, 4a), -methylferrocenium-'-phenylthiourea BAr (4b) and -cobaltocenium-'-phenylthiourea BAr (5). These compounds are competent catalysts for the Friedel-Crafts alkylation of indoles with -β-nitrostyrenes, and are much more active than their reduced non-charged forms. The iron derivatives are less stable than the cobalt analog and were generated and used whereas the cobalt-containing thiourea was isolated and characterized by X-ray crystallography.

Anion-Activated Bases and Nucleophiles Characterized by Photoelectron Spectroscopy.

Negative ion photoelectron spectra at 20 K along with ab initio [CCSD(T)] and M06-2X density functional theory calculations are reported for a series of six basic and nucleophilic pyridine derivatives with an anionic substituent [i.e., 3- and 4-PyrBX, where X = F, 4--BuCH, 4-MeOCH, and 3,5-(MeO)CH].

How generic is iodide-tagging photoelectron spectroscopy: An extended investigation on the Gly·X- (Gly = glycine, X = Cl or Br) complexes.

We report a joint negative ion photoelectron spectroscopy (NIPES) and quantum chemical computational study on glycine-chloride/bromide complexes (denoted Gly·X-, X = Cl/Br) in close comparison to the previously studied Gly·I- cluster ion. Combining experimental NIPE spectra and theoretical calculations, various Gly·X- complexes were found to adopt the same types of low-lying isomers, albeit with different relative energies. Despite more congested spectral profiles for Gly·Cl- and Gly·Br-, spectral assignments were accomplished with the guidance of the knowledge learned from Gly·I-, where a larger spin-orbit splitting of iodine afforded well-resolved, recognizable spectral peaks.

Electrostatically Enhanced 3- and 4-Pyridyl Borate Salt Nucleophiles and Bases.

A variety of electrostatically enhanced 3- and 4-pyridylborate salt catalysts are reported and show significant improvement over an activated noncharged neutral control compound. Their nucleophilicity in a stoichiometric S2 reaction and catalytic performance in a urethane synthesis are evaluated along with three methods for rapidly evaluating the basicity of these species. That is, qualitative titrations in CHCl and CHCl were carried out, two separate solution-state IR studies in CCl and CDCl are reported, and the proton affinities of the anionic components of the salts were computed.

Structure determination of a bis[4-(di-n-butylamino)phenyl](pyridin-3-yl)borane tetramer highlighting a unique geometric conformation of the core 16-membered ring.

The tetramer of bis(4-di-n-butylaminophenyl)(pyridin-3-yl)borane [systematic name: 2λ,4λ,6λ,8λ-tetrabora-1,3,5,7(1,3)-tetrapyridinacyclooctaphane-1,3,5,7-tetrakis(ylium)], CHBN, was synthesized unexpectedly and crystallized. Its structure contains an unusual 16-membered ring core made up of four (pyridin-3-yl)borane groups. The ring adopts a conformation with pseudo-S symmetry that is very different from the two other reported examples of this ring system.

Liberating the Anion: Evaluating Weakly Coordinating Cations.

A series of commonly used weakly coordinating cations (WCCs) including tetraalkylammonium, bis(triphenylphosphine)iminium (PPN), P2 and P5 phosphazenium, and tetraphenylphosphonium ions were investigated along with five additional tetraarylphosphonium ions [Ar = 4-Me, 4--Bu, 3,5-Me, 4-MeN, and 3,5-(MeO) phenyl derivatives]. Dissociation enthalpies of their chloride complexes were computed in the gas phase and in dichloromethane. These results are compared to infrared spectra of these salts in chloroform- which provide free and bound C-D stretches, F NMR spectra of the corresponding tetrafluoroborate salts, ion-pair equilibrium constants, and reaction rates of the chlorides and acetates with 1-iodooctane.

Observation and Exploitation of Spin-Orbit Excited Dipole-Bound States in Ion-Molecule Clusters.

We report an observation of spin-orbit excited dipole-bound states (DBSs) in arginine-iodide complexes (Arg·I) by using temperature-dependent, wavelength-resolved "iodide-tagging" negative ion photoelectron spectroscopy. The observed DBSs are bound to the spin-orbit excited I(P) level of the neutral Arg·I complex in zwitterionic conformations and identified based on the resonant enhancement due to spin-orbit electronic autodetachment from the I(P) DBS to the I(P) neutral ground state. The observed DBS binding energies are correlated to the dipole moments of neutral Arg·I isomers and tautomers.

Observation of Conformational Simplification upon -Methylation on Amino Acid Iodide Clusters.

This Letter reports a counterintuitive observation that methylation of the glycine-iodide cluster leads to fewer conformations and spectroscopic simplicity. Cryogenic "iodide-tagging" negative ion photoelectron spectroscopy (NIPES) is used to probe specific binding sites of three -methylated glycine derivatives, i.e.

Cryogenic "Iodide-Tagging" Photoelectron Spectroscopy: A Sensitive Probe for Specific Binding Sites of Amino Acids.

This work showcases cryogenic and temperature-dependent "iodide-tagging" photoelectron spectroscopy to probe specific binding sites of amino acids using the glycine-iodide complex (Gly·I) as a case study. Multiple Gly·I isomers were generated from ambient electrospray ionization and kinetically isolated in a cryogenic ion trap. These structures were characterized with temperature-dependent "iodide-tagging" negative ion photoelectron spectroscopy (NIPES), where iodide was used as the "messenger" to interpret electronic energetics and structural information of various Gly·I isomers.

-Vinyl and -Aryl Hydroxypyridinium Ions: Charge-Activated Catalysts with Electron-Withdrawing Groups.

Charge-enhanced Brønsted acid organocatalysts with electron-withdrawing substituents were synthesized, and their relative acidities were characterized by computations, 1:1 binding equilibrium constants () with a UV-vis active sensor, P NMR shifts upon coordination with triethylphosphine oxide, and in one case by infrared spectroscopy. Pseudo-first-order rate constants were determined for the Friedel-Crafts alkylations of -methylindole with -β-nitrostyrene and 2,2,2-trifluoroacetophenone and the Diels-Alder reaction of cyclopentadiene with methyl vinyl ketone. These results along with kinetic isotope effect determinations revealed that the rate-determining step in the Friedel-Crafts transformations can shift from carbon-carbon bond formation to proton transfer to the catalyst's conjugate base.

Electrostatically Enhanced Phosphoric Acids and Their Applications in Asymmetric Friedel-Crafts Alkylations.

A series of electrostatically enhanced phosphoric acid catalysts were synthesized and studied. These compounds possess two positively charged -octylpyridinium or triarylphosphonium ion centers at the 3,3'-positions of the ()-BINOL backbone to enhance reactivity and provide needed steric bulk for enantioselective transformations. Catalytic activities for Friedel-Crafts alkylations of indoles with -nitrostyrenes were studied.

Ferrocene Acidity and C-H Bond Dissociation Energy via Experiment and Theory.

The gas-phase acidity of ferrocene (Δ°() = 391.5 ± 1.3 kcal mol) and electron affinity of the ferrocenyl radical (EA() = 1.

Quantification of Catalytic Activity for Electrostatically Enhanced Thioureas via Reaction Kinetics and UV-vis Spectroscopic Measurement.

Charged thiourea derivatives containing one and two methylated or octylated pyridinium ion centers and a tetraarylborate or triflate counteranion are reported. These novel catalysts are much more active in the Friedel-Crafts reactions of trans-β-nitroalkenes with N-methylindoles than the privileged N, N'-bis(3,5-bis(trifluoromethyl)phenyl)thiourea (i.e.

Synthesis of Cyclic Organic Carbonates Using Atmospheric Pressure CO and Charge-Containing Thiourea Catalysts.

Cycloadditions of epoxides with CO to synthesize cyclic five-membered ring organic carbonates are of broad interest from a synthetic, environmental, and green chemistry perspective, and the development of effective catalysts for these transformations is an ongoing challenge. A series of eight charge-containing thiourea salts that catalyze these reactions under mild conditions (i.e.

Asymmetric Arylation of 2,2,2-Trifluoroacetophenones Catalyzed by Chiral Electrostatically-Enhanced Phosphoric Acids.

A series of highly reactive metal-free chiral phosphoric acids possessing positively charged phosphonium ion substituents are reported and have been applied to Friedel-Crafts alkylations of indoles and 2,2,2-trifluoromethyl aryl ketones. These catalysts are orders-of-magnitude more active and have similar or better enantioselectivities than their noncharged analogues. High tolerance to a range of substrates with electron-withdrawing and electron-donating substituents was also observed.

Enantioselective Friedel-Crafts Alkylation between Nitroalkenes and Indoles Catalyzed by Charge Activated Thiourea Organocatalysts.

A series of methylated and octylated pyridinium and quinolinium containing thiourea salts with a chiral 2-indanol substituent are reported. These organocatalysts are positively charged analogues of privileged bis(3,5-trifluoromethyl)phenyl substituted thioureas, and are found to be much more active catalysts despite the absence of an additional hydrogen bond donor or acceptor site (i.e.

Electrostatically Enhanced Phosphoric Acids: A Tool in Brønsted Acid Catalysis.

A novel type of phosphoric acid catalyst with enhanced reactivity is reported. These compounds possess one or two positively charged centers which electrostatically activate them. This is illustrated in several bond-forming transformations including Friedel-Crafts and Diels-Alder reactions as well as a ring-opening polymerization.

Anion Binding of One-, Two-, and Three-Armed Thiourea Receptors Examined via Photoelectron Spectroscopy and Quantum Computations.

Benzene rings substituted with 1-3 thiourea containing arms (1-3) were examined by photoelectron spectroscopy and density functional theory computations. Their conjugate bases and chloride, acetate, and dihydrogen phosphate anion clusters are reported. The resulting vertical and adiabatic detachment energies span 3.

Phenylcyclopropane Energetics and Characterization of Its Conjugate Base: Phenyl Substituent Effects and the C-H Bond Dissociation Energy of Cyclopropane.

The α-C-H bond dissociation energy (BDE) of phenylcyclopropane (1) was experimentally determined using Hess' law. An equilibrium acidity determination of 1 afforded ΔH° = 389.1 ± 0.

Flexible Acyclic Polyol-Chloride Anion Complexes and Their Characterization by Photoelectron Spectroscopy and Variable Temperature Binding Constant Determinations.

Flexible acyclic alcohols with one to five hydroxyl groups were bound to a chloride anion and these complexes were interrogated by negative ion photoelectron spectroscopy and companion density functional theory computations. The resulting vertical detachment energies are reproduced on average to 0.10 eV by M06-2X/aug-cc-pVTZ predictions and range from 4.

Electrostatically Enhanced Thioureas.

A new class of readily prepared thiourea catalysts with one or more positively charged centers and no new hydrogen-bonding sites are exploited in several bond-forming reactions and are orders of magnitude more reactive than Schreiner's thiourea. These findings provide the basis for a new strategy for activating hydrogen-bond catalysts.

Selective binding and extraction of aqueous dihydrogen phosphate solutions via three-armed thiourea receptors.

A series of neutral anion receptors with one to three thiourea arms were synthesized and their binding to tetrabutylammonium chloride, acetate, and dihydrogen phosphate salts in aqueous DMSO mixtures was examined. The three-armed thiourea host was found to strongly and selectively bind H2PO4(-) even in DMSO solutions containing up to 30% water. This enabled the dihydrogen phosphate salt to be extracted from water into chloroform in its dibasic form despite the high heat of the hydration of HPO4(2-).