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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°acid = 389.1 ± 0.8 kcal mol-1, and isotopic labeling established that the α-position of the three-membered ring is the favored deprotonation site. Interestingly, the structure of the base proved to be a key factor in correctly determining the proper ionization site (i.e., secondary amide ions are needed, and primary ones and OH- lead to incorrect conclusions since they scramble the deuterium label). An experimental measurement of the electron affinity of 1-phenylcyclopropyl radical (EA = 17.5 ± 2.8 kcal mol-1) was combined with the ionization energy of hydrogen (313.6 kcal mol-1) to afford BDE = 93.0 ± 2.9 kcal mol-1. This enabled the effect of the phenyl substituent to be evaluated and compared to other situations where it is attached to an sp³- or sp²-hybridized carbon center. M06-2X, CCSD(T), G4, and W1BD computations were also carried out, and a revised C-H BDE for cyclopropane of 108.9 ± 1.0 kcal mol-1 is recommended. [ABSTRACT FROM AUTHOR]

The interactions between five amino acid based anions([AA]−(AA = Gly, Phe, His, Try, and Tyr)) and N7,N9-dimethylguaniniumcation ([dMG]+) have been investigated by the hybrid densityfunctional theory method B3LYP together with the basis set 6-311++G(d,p).The calculated interaction energy was found to decrease in magnitudewith increasing side-chain length in the amino acid anion. The interactionbetween the [dMG]+cation and [AA]−anionin the most stable configurations of ion pairs is a hydrogen bondinginteraction. These hydrogen bonds (H bonds) were analyzed by the quantumtheory of atoms in molecules (QTAIM) and natural bond orbital (NBO) analysis. Finally, several correlations between electron densities inbond critical points of hydrogen bonds and interaction energy as wellas vibrational frequencies in the most stable configurations of ionpairs have been checked. [ABSTRACT FROM AUTHOR]

Bridgehead C-H bond dissociation enthalpies of 105.7 ± 2.0, 102.9 ± 1.7, and 102.4 7plusmn; 1.9 kcal mol-1 for bicyclo[2.2.1]heptane, bicyclo[2.2.2]octane, and adamantane, respectively, were determined in the gas phase by making use of a thermodynamic cycle (i.e., BDE(R-H) = ΔHºacid(H-X) - IE(H) + EA(X)). These results are in good accord with high-level G3 theory calculations and the experimental values along with G3 predictions for bicyclo[1.1.1]pentane, bicyclo[2.2.1]hexane, bicyclo[3.1.1]heptane, and bicyclo[4.2.1]nonane the gas found to correlate with the flexibility of the ring system. Rare examples of alkyl anions in the gas phase are also provided. [ABSTRACT FROM AUTHOR]

Electrospray ionization (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) combined with H/D exchange reactions was utilized to explore the existence of different b5 + and b4 + fragment ion conformers/isomers of hexapeptide WHWLQL in the gas phase. Distinct H/D exchange trends for protonated WHWLQL ([M + H]+) and its b5 + and b4 + fragment ions (with ND3) were observed. Isolated 12Call isotopomers of both b5 + and b4 + fragment ions yielded bimodal distributions of H/D exchanged product ions. The H/D exchange reaction kinetics also confirmed that b5 + and b4 + fragment ions exist as combination of slow-exchanging (“s”) and fast-exchanging (“f”) species. The calculated rate constant for the first labile hydrogen exchange of [M + H]+ (k[M + H]+ = 3.80 ± 0.7 × 10–10 cm3 mol–1 s–1) was ∼30 and ∼5 times greater than those for the “s” and “f” species of b5 +, respectively. Data from H/D exchange of isolated “s” species at longer ND3 reaction times confirmed the existence of different conformers or isomers for b5 + fragment ions. The sustained off-resonance irradiation collision-activated dissociation (SORI-CAD) of WHWLQL combined with the H/D exchange reactions indicate that “s” and “f” species of b5 + and b4 + fragment ions can be produced in the ICR cell as well as the ESI source. The significance of these observations for detailed understanding of protein sequencing and ion fragmentation pathways is discussed. [Copyright &y& Elsevier]

Hydrogen bonds are the dominant motif for organizing the three-dimensional structures of biomolecules such as carbohydrates, nucleic acids, and proteins, and serve as templates for proton transfer reactions. Computations, gas-phase acidity measurements, and pKa determinations in dimethyl sulfoxide on a series of polyols indicate that multiple hydrogen bonds to a single charged center lead to greatly enhanced acidities. A new class of Brønsted acids, consequently, is proposed. [ABSTRACT FROM AUTHOR]

Both C-H bond dissociation energies for cyclobutene were measured in the gas phase (BDE = 91.2 ± 2.3 (allyl) and 112.5 ± 2.5 (vinyl) kcal mol-1) via a thermodynamic cycle by carrying out proton affinity and electron-binding energy measurements on 1- and 3-cyclobutenyl anions. The results were compared to those for an acyclic model compound, cis-2-butene, and provide the needed information to experimentally establish the heat of formation of cyclobutadiene. Chemically accurate G3 and W1 calculations also were carried out on cycloalkanes, cycloalkenes, and selected reference compounds. It appears that commonly cited bond energies for cyclopropane, cyclobutane, and cyclohexane are 3 to 4 kcal mol-1 too small and their π bond strengths, as given by BDE1 - BDE2, are in error by up to 8 kcal mol-1. [ABSTRACT FROM AUTHOR]

The heat of hydrogenation of phenylcyclobutadiefle (ΔHohyd = 57.4 ± 4.9 kcal mol-1) was determined via a thermodynamic cycle by carrying out gas-phase measurements on 1 -phenylcyclobuten-3-yl cation. This leads to an antiaromatic destabilization energy of 27 ± 5 kcal mol-1, a difference of 9.6 ± 4.9 kcal mol-1 for the first and second C—H bond dissociation energies of 1 -phenylcyclobutene, and an estimate of 96 ± 5 kcal mol-1 for the heat of formation of cyclobutadiene. These results are compared to G3, G3(MP2), and B3LYP computations and represent the first experimental measurements of the energy of a monocyclic cyclobutadiene. [ABSTRACT FROM AUTHOR]

Radical anions of O-, m-, and p-benzoquinone were produced in a Fourier transform mass spectrometer by low energy electron attachment or collision-induced dissociation and were differentiated. Classical derivatization experiments also were carried out to authenticate the ortho and meta anions. Gas- phase techniques were used to measure the proton affinities of all three radical anions and the electron affinities of o- and m-benzoquinone. By combining these results in thermodynamic cycles, we derived heats of hydrogenation of o-, m-, and p-benzoquinone (AhydHo(1O, 1m, and 1p) = 42.8 ± 4.1, 74.8 ± 4.1, and 38.5 ± 3.0 kcal mol-1, respectively) and their heats of formation (Ad-P(1o, 1 m, and Ip) = -23.1 ± 4.1, 6.8 ± 4.1, and -27.7 ± 3.0 kcal mol-1, respectively). Good accord with the literature value for the para derivative was obtained. Combustion calorimetry and heats of sublimation also were measured for benzil and 3,5- di-tert-butyl-o-benzoquinone. The former heat of formation agreed with previous determinations, while the latter result (AfI-f(g) = -73.09 ± 0.87 kcal mol-1) was transformed to A1H°(lo) = -18.9 ± 2.2 kcal mol-1 by removing the effect of the tert-butyl groups via isodesmic reactions. This led to a final value of AH°(10) = -21.0 ± 3.1 kcal mol-1. Additivity was found to work well for m-benzoquinone, but BDE1 and BDE2 for 1,2- and I ,4-dihydroxybenzene differed by a remarkably small 14.1 ± 4.2 and 23.5 ± 3.7 kcal mol1, respectively, indicating that o- and p-benzoquinone should be excellent radical traps. [ABSTRACT FROM AUTHOR]

Bond energies, acidities, and electron affinities are related in a thermodynamic cycle, and by measuring two of these quantities one can derive the third. This procedure is most often used to determine bond dissociation energies. In this work, we show that ΔHOacid(HX), EA(X), and BDE(HX) are all linearly related for specific classes of compounds (e.g., alcohols and carboxylic acids), and therefore, only one of these quantities is needed to determine the others. This should simplify thermodynamic determinations and enable hundreds of new values to be obtained from already existing data in the literature. The intercepts obtained from plots of ΔHOacid(HX) vs EA(XO provide bond energies which correspond to BDE(HX) when EA(X) = 0. The results for eight different types of compounds are provided and some of these values are discussed. Sensitivity factors are obtained from the slopes of these plots, and their sign (+ vs -) can be predicted by considering physical effects such as hybridization and resonance. [ABSTRACT FROM AUTHOR]

Measures the gas-phase acidity of 3,3-dimethylcyclopropene by bracketing and equilibrium techniques. Evidence showing that the derived value is indistinguishable from that of methylacetylene; Electron affinity of 3,3-dimethylcyclopropenyl radical.

Natural and synthetic anion receptors are extensively employed, but the structures of their bound complexes are difficult to determine in the liquid phase. Infrared spectroscopy is used in this work to characterize the solution structures of bound anion receptors for the first time, and surprisingly only two of three hydroxyl groups of the neutral aliphatic triols are found to directly interact with Cl-. The binding constants of these triols with zero to three CF3 groups were measured in a polar environment, and ...(Cl-) = 1.1 x 106 M-1 for the tris(trifluoromethyl) derivative. This is a remarkably large value, and high selectivity with respect to interfering anions such as, Br-, NO3- and NCS- is also displayed. The effects of the third "noninteracting" hydroxyl groups on the structures and binding constants were also explored, and surprisingly they are as large or larger than the OH substituents that hydrogen bond to Cl-. That is, a remote hydroxyl group can play a larger role in binding than two OH substituents that directly interact with an anionic center. [ABSTRACT FROM AUTHOR]

Like-chargeion pairing is commonly observed in protein structuresand plays a significant role in biochemical processes. Density functionalcalculations combined with the conductor-like polarizable continuummodel were employed to study the formation possibilities of doublycharged noncovalently linked complexes of a series of model compoundsand amino acids in the gas phase and in solution. Hydrogen bond interactionswere found to offset the Coulombic repulsion such that cation–cationclusters are minima on the potential energy surfaces and neither counterionsnor solvent molecules are needed to hold them together. In the gasphase the dissociation energies are exothermic, and the separationbarriers span from 1.7 to 15.6 kcal mol–1. Liquid-phasecomputations indicate that the separation enthalpies of the cation–cationcomplexes become endothermic in water and nonpolar solvents with dielectricconstants of ≥7 (i.e., the value for THF). These results revealthat electrostatically defying noncovalent complexes of like-chargedions can overcome their Coulombic repulsion even in low-polarity environments. [ABSTRACT FROM AUTHOR]

The pK, of an acyclic aliphatic heptaol ((HOCH2CH2CH(OH)CH2)3COH) was measured in DMSO, and its gas-phase acidity is reported as well. This tertiary alcohol was found to be 1O~ times more acidic than tert.butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., plC2 = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol' stabilization of the charged oxygen center in the conjugate base by three hydrogen bonds and another 6.3 kcal mol-1 stabilization resulting from an additional three hydrogen bonds between the uncharged I primary and secondary hydroxyl groups. Charge delocalization by both the first and second solvation shells may be used to facilitate enzymatic reactions. Acidity constants of a series of polyols were also computed, and the combination of hydrogen-bonding and electron-withdrawing substituents was found to afford acids that are predicted to be extremely acidic in DMSO (i.e., pK2< 0). These hydrogen bond enhanced acids represent an attractive class of Bronsted acid catalysts. [ABSTRACT FROM AUTHOR]

We have regiospecifically generated the α,2-, α,3-, and α,4-dehydrophenoxide anions by collisional activation of o-, m-, and p-nitrobenzoate. The α,2 and α,4 isomers also were synthesized by reacting o-benzyne radical anion with carbon dioxide and electron ionization of p-diazophenol. All three dehydrophenol radical anions were differentiated from each other and identified by probing their chemical reactivity with several reagents. Each isomer was converted to phenoxide and its corresponding quinone as well. Thermochemical measurements were carried out on all three radical anions and their hydrogenatom affinities, proton affinities, and electron binding energies are reported. These measured quantities are combined in thermodynamic cycles to derive the heats of formation of each of the radical anions and their corresponding carbenes (i.e., α,2-, α,3-, and α,4-dehydrophenol). These results are compared to MCQDPT2, G3, G2+(MP2), and B3LYP calculations and experimental data for appropriate reference compounds. [ABSTRACT FROM AUTHOR]

Understanding the nature of the interaction between metalnanoparticlesand biomolecules has been important in the development and designof sensors. In this paper, structural, electronic, and bonding propertiesof the neutral and anionic forms of glutathione tripeptide (GSH) complexeswith a Au3cluster were studied using the DFT-B3LYP with6-31**-LANL2DZ mixed basis set. Binding of glutathione with thegold cluster is governed by two different kinds of interactions: Au–X(X = N, O, and S) anchoring bond and Au···H–Xnonconventional hydrogen bonding. The influence of the intramolecularhydrogen bonding of glutathione on the interaction of this peptidewith the gold cluster has been investigated. To gain insight on therole of intramolecular hydrogen bonding on Au–GSH interaction,we compared interaction energies of Au–GSH complexes with thoseof cystein and glycine components. Our results demonstrated that,in spite of the ability of cystein to form highly stable metal–sulfideinteraction, complexation behavior of glutathione is governed by itsintramolecular backbone hydrogen bonding. The quantum theory of atomin molecule (QTAIM) and natural bond orbital analysis (NBO) have alsobeen applied to interpret the nature of interactions in Au–GSHcomplexes. Finally, conformational flexibility of glutathione duringcomplexation with the Au3cluster was investigated by meansof monitoring Ramachandran angles. [ABSTRACT FROM AUTHOR]