DFT study of the EPR spectral pattern of propagating methacrylic radicals

The EPR spectral pattern observed in the bulk polymerization of methacrylic monomers was theoretically investigated by DFT methodology. The conformational analysis of the propagating radical by the rotation around the C–C_β bond, was performed using the B3LYP/6-31G^* computational scheme. To obtain accurate values of the isotropic hyperfine coupling constants (hfccs) a higher level protocol, B3LYP/TZVP//B3LYP/6-31G^*, was applied. The experimental 13-line spectrum registered at the first stage of the polymerization was assumed to correspond to a free rotating radical in a fluid medium and it was simulated just considering the most stable conformation. The 9-line spectrum registered at high conversions was interpreted in terms of highly hindered rotational conformers frozen in the very viscous medium. This spectrum was well reproduced by a model which considers the sum of the individual spectra of the conformations spread around the most probable. Each of these contributing spectra was obtained on the basis of the computed hfccs for the considered conformations and weighted by his relative Boltzmann population according to the DFT analysis. Besides, the calculated hfccs showed an excellent agreement with those predicted by the Heller–McConnell relationship, which confirms the coherence of the DFT methodology for this kind of calculations.     

DFT calculations of 31P spin-spin coupling constants and chemical shift in dioxaphosphorinanes

One-bond heteronuclear spin-spin coupling constants (1)J(PX) (X=H, O, S, Se, C and N) between the phosphorus atom and axial and equatorial substituents in dioxaphosphorinanes are computed using density functional theory (DFT). The experimental values of these coupling constants for a variety of substituents can be applied to identify different diastereoisomers. The DFT calculations confirm the systematic trend observed in experiment, and indicate that the computed (1)J(PX) coupling constants are related to the length of the axial and equatorial bonds. A similar relation between the phosphorus chemical shift and the R(PX) bond length appears to be valid, with the exception of selenium substituents.     

Calculations of the isotropic hyperfine coupling constants in free radicals

For a number of free radicals the results of non-empirical (ab initio) and semi-empirical (INDO, DEPAC, CNDO/SP) calculations of the isotropic hyperfine coupling constants are compared.     

DFT calculations as a powerful tool for ESR spin trapping experiments

A combined electron spin resonance (ESR) and density functional theory study has been carried out to obtain deeper insight in the reaction mechanism of the photorearrangement of 3‐hydroxyindolic nitrones. Also, in this case, the combination of these techniques constitutes a powerful tool when discriminating between different reaction pathways. In particular, density functional theory calculations played an important role for unraveling the overlapping of signals coming from different species in the ESR spin trapping experiments. In the present study, the computed energies and the corresponding ESR parameters of each possible isomer hypothesized have been considered, taking also into account their different possibilities of intramolecular H‐bond formation. However, the results obtained indicate that the possibility of intramolecular H‐bonding did not play a determinant role in this case. Copyright © 2016 John Wiley & Sons, Ltd.     

Computational modeling of isotropic electron paramagnetic resonance spectra of doublet state main group radicals

The combined use of theoretical and mathematical methods in the analysis of electron paramagnetic resonance data has greatly increased the ability to interpret even the most complex spectra reported for doublet state inorganic main group radicals. This personal account summarizes the theoretical basis of such an approach and provides an in-depth discussion of some recent illustrative examples of the utilization of this methodology in practical applications. The emphasis is on displaying the enormous potential embodied within the approach.     

Benzylic coupling constants in toluene derivatives by J doubling in the frequency domain and DFT calculations

The recently modified J doubling in the frequency domain method (MJDFDM) allowed the determination of ^{4, 5, 6}J in toluene, a series of 4‐mono‐ and 3,5‐disubstituted toluene derivatives, as well as in 4‐picoline. The methyl and aromatic signals were subjected to successive deconvolution processes, which at the end led to singlet signals and afforded the corresponding coupling constant values with a high degree of accuracy. Density functional theory calculation of benzylic coupling constants by addition of the Fermi contact, the spin‐dipole, the diamagnetic spin‐orbit, and the paramagnetic spin‐orbit terms revealed good agreement between predicted and measured values when the B3LYP/aug‐cc‐pVTZ level of theory was used. Evaluation of the substituent effect over the coupling constant was made for all studied compounds and some limitations of the methodology were evidenced. Copyright © 2009 John Wiley & Sons, Ltd.     

Car-Parrinello molecular dynamics simulations and EPR property calculations on aqueous ubisemiquinone radical anion

Car-Parrinello molecular dynamics (CPMD) simulations have been performed on ubisemiquinone radical anion in aqueous solution. The different types of hydrogen bonding formed between the semiquinone and the solvent were studied in terms of frequency and directionality, in comparison with the parent benzosemiquinone radical anion. The EPR parameters (g-tensors and hyperfine coupling constants) were obtained from quantum chemical property calculations performed on snapshots along the MD trajectory, and the contributions of different solvation effects to the EPR parameters have been evaluated. The influence of the anion’s conformational behaviour was examined, including the orientation-dependent effects of hyperconjugation on side-chain hyperfine coupling.     

Coupled cluster and DFT calculations of 14N nuclear quadrupole coupling constants

The dependence of ¹⁴N quadrupole coupling constants calculated using coupled cluster theory on the level of approximation is examined for a series of small molecules. For HCN, HNC, CH₃CN, and CH₃NC, we use the coupled cluster singles‐and‐doubles with a noniterative perturbative triples correction—CCSD(T)—approach, and we analyze the basis set dependence of the results. For aziridine, diazirine, and cyclopropyl cyanide, we use the CCSD(T) approach, but smaller basis sets, and for the largest studied molecules—quinuclidine and hexamine—we present CCSD results. The differences between computed and experimental values for the best basis sets used are ≈ 5% at the CCSD level and decrease noticeably at the CCSD(T) level. The ‐ N≡C bonds are an exception—in this case the quadrupole coupling constants are very small, hence the differences between theory and experiment become larger (up to 9%). We also consider the performance of density functional theory, comparing the results for different density functionals with the coupled cluster values of the same constants. Most of the functionals provide results systematically improved with respect to the Hartree–Fock values, with ¹⁴N coupling constants in ‐ N≡C bonds being again an exception. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem, 2011     

Capture of Gaseous Iodine in Isoreticular Zirconium-Based UiO-n Metal-Organic Frameworks: Influence of Amino Functionalization,DFT Calculations,Raman and EPR Spectroscopic Investigation

A series of Zr-based UiO-n MOF materials (n=66, 67, 68) have been studied for iodine capture. Gaseous iodine adsorption was collected kinetically from a home-made set-up allowing the continuous measurement of iodine content trapped within UiO-n compounds, with organic functionalities (−H, −CH₃, −Cl, −Br, −(OH)₂, −NO₂, −NH₂, (−NH₂)₂, −CH₂ NH₂) by in-situ UV-Vis spectroscopy. This study emphasizes the role of the amino groups attached to the aromatic rings of the ligands connecting the {Zr₆O₄(OH)₄} brick. In particular, the preferential interaction of iodine with lone-pair groups, such as amino functions, has been experimentally observed and is also based on DFT calculations. Indeed, higher iodine contents were systematically measured for amino-functionalized UiO-66 or UiO-67, compared to the pristine material (up to 1211 mg/g for UiO-67-(NH₂)₂). However, DFT calculations revealed the highest computed interaction energies for alkylamine groups (−CH₂NH₂) in UiO-67 (−128.5 kJ/mol for the octahedral cavity), and pointed out the influence of this specific functionality compared with that of an aromatic amine. The encapsulation of iodine within the pore system of UiO-n materials and their amino-derivatives has been analyzed by UV-Vis and Raman spectroscopy. We showed that a systematic conversion of molecular iodine (I₂) species into anionic I⁻ ones, stabilized as I⁻⋅⋅⋅I₂ or I₃⁻ complexes within the MOF cavities, occurs when I₂@UiO-n samples are left in ambient light.     

Raman and photoacoustic infrared spectra of fluorene derivatives: Experiment and calculations

Raman and photoacoustic (PA) infrared spectra of fluorene and four derivatives (2,3-benzofluorene, 2-methylfluorene, 2-ethylfluorene and 1,8-dimethylfluorene) were recorded and analyzed in this investigation. Mid- and far-infrared PA spectra were examined from about 2000 to 100 cm⁻¹. The Raman spectra spanned the same wavenumber range. Observed bands in both PA and Raman spectra were compared with DFT (harmonic) and variational (anharmonic) calculations, and with published spectra. The DFT calculations provided single-molecule frequencies, whereas the variational method yielded results for both monomeric and dimeric species. Many previously unknown bands, including numerous features due to combination and overtone transitions, were identified and assigned in this work.     

DFT calculations of hyperfine coupling constants of organic π radicals and comparison with empirical equations and experiment

Density functional theory calculations (UB3LYP/EPR‐III) for a series of radicals and radical ions were performed to check the internal consistency of the method and its implications to the theoretical concepts of electron paramagnetic resonance such as π–σ spin polarization, hyperconjugation and phenyl hyperconjugation. In the second part, experimental data for seven radicals (43 hyperfine coupling constants) are compared with calculations, yielding a correlation of r² = 0.97. Copyright © 2016 John Wiley & Sons, Ltd.     

1-Cyclohepta-2,4,6-trienyl-selanes--a 77Se NMR study: indirect nuclear 77Se--13C spin-spin coupling constants and application of density functional theory (DFT) calculations

1-Cyclohepta-2,4,6-trienyl-selanes Se(C(7)H(7))(2) (2c), R--Se--C(7)H(7) with R = Bu, (t)Bu, Ph, 4-F--C(6)H(4) (12a,b,c,d) were prepared by the reaction of the corresponding silanes, Si(SeMe(3))(2) and R--Se--SiMe(3), respectively, with tropylium bromide C(7)H(7)Br. In spite of the low stability of the selanes even in dilute solutions and at low temperature, they could be characterised by their (1)H, (13)C and (77)Se NMR parameters. Coupling constants (1)J((77)Se,(13)C) were measured and calculated by DFT methods at the B3LYP/6-311+G(d,p) level of theory. The comparison of experimental and calculated coupling constants (1)J((77)Se,(13)C) included numerous selenium carbon compounds with largely different Se--C bonds, revealing a satisfactory agreement. Both the spin-dipole (SD) and the paramagnetic spin-orbital (PSO) terms contributed significantly to the spin-spin coupling interaction, in addition to the Fermi contact (FC) term.     

EPR of free radicals formed from 3-hydroxyesculetin and related derivatives

EPR spectroscopy of 3-hydroxyesculetin (1, solid and in solution) and of the radicals formed during the aerobic oxidation of alkaline solutions of 1 and related compounds was investigated. 1 in the solid state was studied by pulsed EPR experiments and showed a radical character. The aerobic oxidation of alkaline solutions of 1 was also followed by EPR spectroscopy. A ring contraction occurred leading to a 5,6-dihydroxybenzofuran-2-carboxylate radical. The autoxidation of an alkaline solution of (Z)-3-(3,4-dihydroxyphenyl)-2-hydroxypropenoic acid allowed the observation of a spectrum attributable to 5,6-dihydroxybenzofuran-3-one-2-carboxylate radical. The formation mechanisms of these radicals are discussed.     

1H-1H scalar coupling across two stacked aromatic rings: DFT calculations and experimental proof

1H-1H scalar coupling across two stacked (parallel and eclipsed) aromatic rings has been revealed through the 1D and 2D 1H NMR analysis of a [2,2]paracyclophane and rationalized by means of density functional theory (DFT) calculation of the J values.     

Ab initio and DFT calculations for the structure and vibrational spectra of cyclopentene and its isotopomers

Ab initio calculations using the MP2/cc-pVTZ basis set do an excellent job of predicting the inversion barrier (247 vs. 232 cm⁻¹) and dihedral angle (26°) of cyclopentene. DFT calculations also do an excellent job of predicting the vibrational frequencies of the d₀, d₁, d₄, and d₈ isotopomers. They have also allowed the reassignments of several of the vibrational frequencies.     

Complexes of dihydroxybenzenes with carbon monoxide by DFT calculations and FT-IR matrix spectroscopy

Weakly bound complexes of dihydroxybenzenes and nitrophenols with carbon monoxide were the objects of theoretical studies based on the Becke–Lee–Parr density functional. Stability and vibrational spectra of all stable bimolecular complexes were determined and the theoretical results were integrated by matrix isolation FT-IR spectroscopy measurements.     

Effect of temperature on the EPR properties of a rhamnose alkoxy radical: a DFT molecular dynamics study

It has been shown previously that two distinctive variants (called RHop and RO4) exist of the radiation-induced rhamnose alkoxy radical. Density functional theory (DFT) calculations of the electron paramagnetic resonance (EPR) properties were found to be consistent with two separate measurements at different temperatures [E. Pauwels, R. Declerck, V. Van Speybroeck, M. Waroquier, Radiat. Res., in press]. However, the agreement between theory and experiment was only of a qualitative nature, especially for the latter radical. In the present work, it is examined whether this residual difference between theoretical and experimental spectroscopic properties can be explained by explicitly accounting for temperature in DFT calculations. With the aid of ab-initio molecular dynamics, a temperature simulation was conducted of the RO4 variant of the rhamnose alkoxy radical. At several points along the MD trajectory, g and hyperfine tensors were calculated, yielding time (and temperature) dependent mean spectroscopic properties. The effect of including temperature is evaluated but found to be within computational error.     

Satellite lines in the EPR spectra of nitroxide radicals: experimental data and DFT calculations

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