Ionic Liquid Based on α-Amino Acid Anion and N7,N9-Dimethylguaninium Cation ([dMG][AA]): Theoretical Study on the Structure and Electronic Properties

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

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 (<b>1</b>) was experimentally determined using Hess’ law. An equilibrium acidity determination of <b>1</b> afforded Δ<i>H</i>°_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

Effect of Hydrogen Bonds on p<i>K</i>_a Values: Importance of Networking

The p<i>K</i>_a 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 10²¹ times more acidic than <i>tert</i>-butyl alcohol in DMSO and an order of magnitude more acidic than acetic acid (i.e., p<i>K</i>_a = 11.4 vs 12.3). This can be attributed to a 21.9 kcal mol^–1 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 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., p<i>K</i>_a < 0). These hydrogen bond enhanced acids represent an attractive class of Brønsted acid catalysts

Power of a Remote Hydrogen Bond Donor: Anion Recognition and Structural Consequences Revealed by IR Spectroscopy

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 <i>K</i>_CD₃CN(Cl^–) = 1.1 × 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

Interactions of Glutathione Tripeptide with Gold Cluster: Influence of Intramolecular Hydrogen Bond on Complexation Behavior

Understanding the nature of the interaction between metal nanoparticles and biomolecules has been important in the development and design of sensors. In this paper, structural, electronic, and bonding properties of the neutral and anionic forms of glutathione tripeptide (GSH) complexes with a Au₃ cluster were studied using the DFT-B3LYP with 6-31+G**-LANL2DZ mixed basis set. Binding of glutathione with the gold cluster is governed by two different kinds of interactions: Au–X (X = N, O, and S) anchoring bond and Au···H–X nonconventional hydrogen bonding. The influence of the intramolecular hydrogen bonding of glutathione on the interaction of this peptide with the gold cluster has been investigated. To gain insight on the role of intramolecular hydrogen bonding on Au–GSH interaction, we compared interaction energies of Au–GSH complexes with those of cystein and glycine components. Our results demonstrated that, in spite of the ability of cystein to form highly stable metal–sulfide interaction, complexation behavior of glutathione is governed by its intramolecular backbone hydrogen bonding. The quantum theory of atom in molecule (QTAIM) and natural bond orbital analysis (NBO) have also been applied to interpret the nature of interactions in Au–GSH complexes. Finally, conformational flexibility of glutathione during complexation with the Au₃ cluster was investigated by means of monitoring Ramachandran angles

Experimental and Computational Bridgehead C–H Bond Dissociation Enthalpies

Bridgehead C–H bond dissociation enthalpies of 105.7 ± 2.0, 102.9 ± 1.7, and 102.4 ± 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) = Δ<i>H</i>°_acid(H–X) – IE­(H^<b>·</b>) + EA­(X^<b>·</b>)). 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[2.1.1]­hexane, bicyclo[3.1.1]­heptane, and bicyclo[4.2.1]­nonane were found to correlate with the flexibility of the ring system. Rare examples of alkyl anions in the gas phase are also provided