AbstractHyperconjugative, steric, and electrostatic effects were evaluated as possible sources of the helicity in linear perfluorinated alkanes through analysis of natural bond orbitals and classical electrostatics. Contrary to previous rationalizations, which indicate dominating steric or electrostatic effects, this analysis indicates that hyperconjugative stabilization through σCCσ*CF interactions are the underlying driving force for the origin of the observed helicity in perfluoroalkanes.

AbstractHyperconjugative, steric, and electrostatic effects were evaluated as possible sources of the helicity in linear perfluorinated alkanes through analysis of natural bond orbitals and classical electrostatics. Contrary to previous rationalizations, which indicate dominating steric or electrostatic effects, this analysis indicates that hyperconjugative stabilization through σCCσ*CF interactions are the underlying driving force for the origin of the observed helicity in perfluoroalkanes.

Redox potentials are computed for the active form (compound I) of lignin peroxidase (LiP) using a suitable QM/MM methodology (B3LYP/SDD/6-311G**//BP86/SVP:CHARMM). Allowing for dynamic conformational averaging, a potential of 0.67(33) V relative to ferrocenium/ferrocene is obtained for the active form with its oxoiron(IV) core. The computed redox potential is very sensitive to the charge distribution around the active site: protonation of titratable residues close to the metal center increases the redox potential, thereby rationalising the known pH dependence of LiP activity. A simple MM-charge deletion scheme is used to identify residues that are critical for the redox potential. Two mutant proteins are studied through homology modelling, E40Q and D183N, which are predicted to have an increased redox potential by 140 mV and 190 mV, respectively, relative to the wild type. These mutant proteins are thus promising targets for synthesis and further exploration toward a rational design of biocatalytic systems for oxidative degradation of lignin.

We have studied the T. versicolor laccase T1 site redox potential (RP) at the M06/6-311++G**/SDD(Cu) level of theory, employing QM/MM-optimized geometries (RI-BP86/def2-SVP/def2-TZVP(Cu):CHARMM) of the whole protein system with electronic embedding. The oxidation state of the trinuclear cluster was found to affect the T1 site RP by about 0.2–0.3 V, depending on the protein protonation state. The computed laccase RP can be drastically lowered upon introduction of a protonation state corresponding to a neutral environment, by up to −1.37 V, which is likely an overestimation of the effect in vivo. The gradual change of the protonation state by single points without optimization or equilibration results in a change that is even larger, namely up to about −2.6 V. Thus, the preferred protein conformation supports a high redox potential, compensating for the RP-lowering effect of surface charges. The predicted change in RP on going to the F463M mutant, ca. −0.1 V, is consistent with observations for a related laccase. Based on our results, we also propose and test a D206N mutant but find it to be locked in a conformation with slightly lower RP.

The elusive neutral bicarbonate radical and the carbonate radical anion form an acid/conjugate base pair. We now report experimental studies for a model of bicarbonate radical, namely, methyl carbonate (methoxycarbonyloxyl) radical, complemented by DFT computations at the CAM-B3LYP level applied to the bicarbonate radical itself. Methyl carbonate radicals were generated by UV irradiation of oxime carbonate precursors. Kinetic EPR was employed to measure rate constants and Arrhenius parameters for their dissociation to CO2 and methoxyl radicals. With oleate and cholesterol lipid components, methyl carbonate radicals preferentially added to their double bonds; with linoleate and linolenate substrates, abstraction of the bis-allylic H atoms competed with addition. This contrasts with the behavior of ROS such as hydroxyl radicals that selectively abstract allylic and/or bis-allylic H atoms. The thermodynamic and activation parameters for bicarbonate radical dissociation, obtained from DFT computations, predicted it would indeed have substantial lifetime in gas and nonpolar solvents. The acidity of bicarbonate radicals was also examined by DFT methods. A noteworthy linear relationship was discovered between the known pKa’s of strong acids and the computed numbers of microsolvating water molecules needed to bring about their ionization. DFT computations with bicarbonate radicals, solvated with up to eight water molecules, predicted that only five water molecules were needed to bring about its complete ionization. On comparing with the correlation, this indicated a pKa of about −2 units. This marks the bicarbonate radical as the strongest known carboxylic acid.

We present the first high-level ab initio benchmark study of the interaction energy between fluorocyclohexanes and benzene. These compounds form CH⋯π interactions with aromatic solvents which causes notable shielding of the axial cyclohexane protons. For the recently synthesised all-cis 1,2,3,4,5,6-hexafluorocyclohexane the interaction energy with benzene amounts to −7.9 kcal mol−1 and −6.4 kcal mol−1 at the MP2 and SCS-MP2 levels, respectively (extrapolated to the complete basis set limit), which according to dispersion-corrected density functional calculations, is largely due to dispersion.

The conformational behaviour of Ac-Gly-NHMe in nonpolar, polar and polar protic solutions was systematically studied in this work by theoretical calculations and experimental infrared and 1H NMR spectroscopies. Ac-Gly-NHMe prefers a gauche conformer with a strong seven-membered intramolecular hydrogen bond for the isolated compound and in nonpolar solvents, but such preference changes in polar and polar protic solvents. Elucidation of Ac-Gly-NHMe preferences was also supported by studying the conformers of its CF3-C(O)-Gly-NHMe and Ac-Gly-N(Me)2 derivatives in solution.

No CF⋯HO intramolecular hydrogen bonds (IHBs) in 2-fluoroethanol, 3-fluoropropanol and 4-fluorobutanol can be detected experimentally in solution by NMR and infrared spectroscopies. According to ab initio (MP2/aug-cc-pVDZ) and DFT calculations (B3LYP), a CF⋯HO IHB has no influence on the conformational behavior of 2-fluoroethanol, while it stabilises the global minima of 3-fluoropropanol and 4-fluorobutanol for the isolated molecules. Entropy and bulk solvation effects, even in nonpolar media, such as CCl4, cyclohexane and dichloromethane, are indicated to diminish the population of these global minima, apparently below the detection limit.

Intramolecular CF···FC interactions in selected organofluorine compounds (all-syn-1,2,3,4- and all-syn-1,2,4,5-tetrafluorocyclohexane, 1,8-difluoronaphthalene, 4,5-difluorophenanthrene, 2,2′,5,5′-tetrafluorobiphenyl) were studied at the MP2/aug-cc-pVDZ level using the recently developed noncovalent interaction (NCI) method. For the optimized minima, all CF···FC interactions that are identified by this method are classified as attractive, even in those cases where suitable isodesmic reaction energies fail to provide evidence for an energetic stabilization. Possible relations between these interactions and the observable JFF spin–spin coupling constant values are discussed.

103Rh NMR chemical shifts have been computed at the GIAO-B3LYP level of density functional theory (DFT) for a number of [Rh(COD)(P∩P)]+ complexes [COD = 1,5-cyclooctadiene, P∩P = chelating bis(phosphine) including bis(dimethylphosphino)ethane (dmpe), bis(diphenylphosphino)ethane (dmpe), MeDUPHOS, DIOP, BINAP, and others]. Structures have been optimized using PBE0 and M06 functionals in the gas phase, in a continuum modeling the solvent, and with [PF6]− counteranion included explicitly. Observed trends in δ(103Rh) are well reproduced for pristine PBE0-optimized cations in the gas phase or for ion pairs optimized in a continuum with M06. While there is no overall trend between computed δ(103Rh) values and complex stabilities (evaluated through isodesmic ligand exchange reactions), there is a linear relationship between the 103Rh chemical shifts and the mean Rh–P bond distances. This relationship appears to be remarkably general, encompassing various chelating ring sizes and substituents at P, including remote electron-donating and -withdrawing substituents that are characterized through their Hammett constants. The combination of 103Rh NMR and DFT computations emerges as a useful tool for structure elucidation of Rh–phosphine complexes.

Redox potentials are computed for the active form (compound I) of lignin peroxidase (LiP) using a suitable QM/MM methodology (B3LYP/SDD/6-311G**//BP86/SVP:CHARMM). Allowing for dynamic conformational averaging, a potential of 0.67(33) V relative to ferrocenium/ferrocene is obtained for the active form with its oxoiron(IV) core. The computed redox potential is very sensitive to the charge distribution around the active site: protonation of titratable residues close to the metal center increases the redox potential, thereby rationalising the known pH dependence of LiP activity. A simple MM-charge deletion scheme is used to identify residues that are critical for the redox potential. Two mutant proteins are studied through homology modelling, E40Q and D183N, which are predicted to have an increased redox potential by 140 mV and 190 mV, respectively, relative to the wild type. These mutant proteins are thus promising targets for synthesis and further exploration toward a rational design of biocatalytic systems for oxidative degradation of lignin.

No CF⋯HO intramolecular hydrogen bonds (IHBs) in 2-fluoroethanol, 3-fluoropropanol and 4-fluorobutanol can be detected experimentally in solution by NMR and infrared spectroscopies. According to ab initio (MP2/aug-cc-pVDZ) and DFT calculations (B3LYP), a CF⋯HO IHB has no influence on the conformational behavior of 2-fluoroethanol, while it stabilises the global minima of 3-fluoropropanol and 4-fluorobutanol for the isolated molecules. Entropy and bulk solvation effects, even in nonpolar media, such as CCl4, cyclohexane and dichloromethane, are indicated to diminish the population of these global minima, apparently below the detection limit.

We present the first high-level ab initio benchmark study of the interaction energy between fluorocyclohexanes and benzene. These compounds form CH⋯π interactions with aromatic solvents which causes notable shielding of the axial cyclohexane protons. For the recently synthesised all-cis 1,2,3,4,5,6-hexafluorocyclohexane the interaction energy with benzene amounts to −7.9 kcal mol−1 and −6.4 kcal mol−1 at the MP2 and SCS-MP2 levels, respectively (extrapolated to the complete basis set limit), which according to dispersion-corrected density functional calculations, is largely due to dispersion.