Radical stabilization energies (RSEs) for a wide variety of nitrogen-centered radicals and their protonated counterparts have been calculated at G3(MP2)-RAD and G3B3 level. The calculated RSE values can be rationalized through the combined effects of resonance delocalization of the unpaired spin, electron donation through adjacent alkyl groups or lone pairs, and through inductive electron donation/electron withdrawal. The influence of ring strain effects as well as the synergistic combination of individual substituent effects (captodatively stabilized N-radicals) have also been explored. In symmetric N-radicals the substituents may also affect the relative ordering of electronic states. In most cases the π-type radical (unpaired spin distribution perpendicular to the plane of the N-radical) is found to be most stable. Closed shell precursors of biological and pharmaceutical relevance, for which neither experimental nor theoretical results on radical stabilities exist, have been included.

The stability of N‐centered radicals and radical cations of potential relevance in C–H amidation reactions has been quantified using highly accurate theoretical methods. Combination with available C–H bond energies for substrate fragments allows for the prediction of reaction enthalpies in 1,5‐hydrogen atom transfer (HAT) steps frequently encountered in reactions such as the Hofmann–Löffler–Freytag (HLF) reaction. Protonation of N‐radicals is found to be essential in classical HLF reactions for thermochemically feasible HAT steps. The stability of neutral N‐radicals depends strongly on the type of N‐substituent. Among the electron‐withdrawing substituents, the trifluoroacetyl (TFA) group is the least and the toluenesulfonyl (tosyl) group the most stabilizing. This implies that TFA‐aminyl radicals have the broadest and tosyl‐aminyl radicals the smallest window of synthetic applicability. In how far the intramolecular C–H amidation reactions compete with hydrogen abstraction from common organic solvents can be judged based on a comparison of reaction thermodynamics.magnified image

Abstract. Kinetics and mechanisms of the oxidation of methoxyurea and N-methylhydroxyurea were studied in neutral and basic aqueous solutions. The obtained pH dependences of the oxidation rates indicate that for both hydroxyureas the reactive species are the deprotonated ones. The second order rate constants, the activation enthalpies and the activation entropies for the reactions of methoxyurea (O-methylhydroxyurea) and N-methylhydroxyurea anions with Fe(CN) 6 3− at 25 o C, I = 2 mol dm −3 (NaClO 4 ) were determined as (5.06 ± 0.01)  10 2 mol −1 dm 3 s −1 , (1.92 ± 0.02)  10 4 mol −1 dm 3 s −1 , 27 ± 1 kJ mol, and 107 ± 4 J mol −1 K −1 , respectively. The pK a value of methoxyurea at 25 o C and 2 mol dm −3 ionic strength was determined kinetically as 12.7 ± 0.1 and the thermodynamic parameters for the deprotonation reaction were determined as Δ a H = 43 ± 1 kJ mol, and. When the kinetic results are compared with the data reported for hydroxyurea, an inverse dependence of the rate constants on the pK a of the hydroxyurea derivatives at 25 o C is observed. Such unexpected behaviour has been explained by the ab initio calculations and NBO analysis of HOMOs for all three hydroxyureates. (doi: 10.5562/cca1799)

The most prominent features responsible for binding of flavonoid aglycones to the IIA region of human serum albumin (HSA) were determined based onin vitrofluorescence measurements and density functional theory calculations.

The inner filter effect (IFE) hinders fluorescence measurements, limiting linear dependence of fluorescence signals to low sample concentrations. Modern microplate readers allow movement of the optical element in the vertical axis, changing the relative position of the focus and thus the sample geometry. The proposed Z -position IFE correction method requires only two fluorescence measurements at different known vertical axis positions ( z -positions) of the optical element for the same sample. Samples of quinine sulfate, both pure and in mixtures with potassium dichromate, showed a linear dependence of corrected fluorescence on fluorophore concentration ( R 2 > 0.999), up to A ex ≈ 2 and A em ≈ 0.5. The correction extended linear fluorescence response over ≈98% of the concentration range with ≈1% deviation of the calibration slope, effectively eliminating the need for sample dilution or separate absorbance measurements to account for IFE. The companion numerical IFE correction method further eliminates the need for any geometric parameters with similar results. Both methods are available online at https://ninfe.science.

Structural and electronic properties and chemical fate of free radicals generated from hydroxyurea (HU) and its methylated analogues N-methylhydroxyurea (NMHU) and O-methylhydroxyurea (OMHU) are of utmost importance for their biological and pharmacological effects. In this work the cis/trans conformational processes, tautomerizations, and intramolecular hydrogen and methyl migrations in hydroxyurea-derived radicals have been considered. Potential energy profiles for these reactions have been calculated using two DFT functionals (BP86 and B3LYP) and two composite models (G3(MP2)RAD and G3B3). Solvation effects have been included both implicitly (CPCM) and explicitly. It has been shown that calculated energy barriers for free radical rearrangements are significantly reduced when a single water molecule is included in calculations. In the case of HU-derived open-shell species, a number of oxygen-, nitrogen-, and carbon-centered radicals have been located, but only the O-centered radicals (e1 and z1) fit to experimental isomeric hyperfine coupling constants (hfccs) from EPR spectra. The reduction of NMHU and OMHU produces O-centered and N-centered radicals, respectively, with the former being more stable by ca. 60 kJ mol -1 . The NMHU-derived radical e4 undergoes rearrangements, which can result in formation of several conceivable products. The calculated hfccs have been successfully used to interpret the experimental EPR spectra of the most probable rearranged product 10. Reduction potentials of hydroxyureas, radical stabilization energy (RSE) and bond disocciation energy (BDE) values have been calculated to compare stabilities and reactivities of different subclasses of free radicals. It has been concluded, in agreement with experiment, that reductions of biologically relevant tyrosyl radicals by HU and NMHU are thermochemically favorable processes, and that the order of reactivity of hydroxyureas follows the experimentally observed trend NMHU > HU > OMHU.

The synthesis of 1,1′‐bis(thymine)ferrocene nucleoside is reported. This nucleoside was obtained in a two‐step synthetic methodology including a Michael addition reaction of 1,1′‐bis(3‐chloropropionyl)ferrocene with thymine to afford the bis(thymine) adduct in 44 % yield. In the second step, the two prochiral carbonyl functionalities in the Michael adduct were reduced to hydroxyl groups with sodium borohydride. This apparently straightforward reaction proceeds in a highly stereoselective fashion to yield the title ferrocenyl nucleoside as a racemic mixture that consists of the R,R and the S,S isomers. The absolute configuration of the chiral carbon atoms in the nucleoside was assigned on the basis of single‐crystal X‐ray diffraction analysis of the methyl derivative. Furthermore, the mechanism of reduction of the bis(thymine) adduct was investigated by using DFT calculations. The two critical minima, pre‐reactive complex, and semi‐reduced intermediate, as well as two corresponding transition states were located to support the observed stereoselectivity. The redox properties of 1,1′‐bis(thymine)ferrocene nucleoside, its precursor, and congeners were investigated using cyclic voltammetry. For the title compound a reversible redox process was found at a low potential of −30 mV versus FcH/FcH+ (FcH=Fe(η5‐C5H5)2) as the reference redox couple.

Chlorination of amides is of utmost importance in biochemistry and environmental chemistry. Despite the huge body of data, the mechanism of reaction between amides and hypochlorous acid in aqueous environment remains unclear. In this work, the three different reaction pathways for chlorination of N-methylacetamide by HOCl have been considered: the one-step N-chlorination of the amide, the chlorination via O-chlorinated intermediate, and the N-chlorination of the iminol intermediate. The high-level quantum chemical G3B3 composite procedure, double-hybrid B2-PLYPD, B2K-PLYP methods, and global hybrid M06-2X and BMK methods have been employed. The calculated energy barriers have been compared to the experimental value of ΔG(#)298 ≈ 87 kJ/mol, which corresponds to reaction rate constant k(r) ≈ 0.0036 M(-1) s(-1). Only the mechanism in which the iminol form of N-methylacetamide reacts with HOCl is consistent (ΔG(#)298 = 87.3 kJ/mol at G3B3 level) with experimental results. The analogous reaction mechanism has been calculated as the most favorable pathway in the chlorination of small-sized amides and amide-containing pharmaceuticals: carbamazepine, acetaminophen, and phenytoin. We conclude that the formation of the iminol intermediate followed by its reaction with HOCl is the general mechanism of N-chlorination for a vast array of amides.