E. Ya. Misochko

High resolution EPR spectroscopy of C60F and C70F in solid argon: Reassignment of C70F regioisomers

Free radicals C(60)F and C(70)F were generated in solid argon by means of chemical reaction of photogenerated fluorine atoms with isolated fullerene molecules (C(60) or C(70)). High resolution anisotropic electron paramagnetic resonance (EPR) spectra of C(60)F and C(70)F at low temperature have been obtained for the first time. The spectrum of C(60)F is characterized by an axially symmetric hyperfine interaction on (19)F nucleus. The hyperfine coupling constants A(iso)=202.8 MHz (Fermi contact interaction) and A(dip)=51.8 MHz (electron-nuclear magnetic-dipole interaction) have been measured for C(60)F in solid argon. Quantum chemical calculations using hybrid density-functional models (either PBE0 or B3LYP) with high-quality basis sets give a theoretical estimate of the hyperfine coupling constants in good agreement with the measurements. The electron spin density distribution in C(60)F is theoretically characterized using the Hirshfeld atomic partitioning scheme. Unlike C(60), five isomers of C(70)F can in principle be produced by the attachment of a fluorine atom to one of the five distinct carbon atoms of the C(70) molecule (denoted A, B, C, D, and E, from pole to equator). The measured high resolution EPR spectrum of the C(70)+F reaction products is interpreted to show the presence of only three regioisomers of C(70)F. Based on the comparison of the measured hyperfine constants with those estimated by the quantum chemical calculation, an assignment of the spectra to the isomers (A, C, and D) is made, which differs strongly from the previous one [J. R. Morton, K. F. Preston, and F. Negri, Chem. Phys. Lett. 221, 59 (1994)]. The new assignment would allow the conclusion that the low-temperature attachment of F atom to the asymmetric C=C bonds of C(70) molecule, namely, C(A)[Double Bond]C(B) and C(D)=C(E), shows remarkably high selectivity, producing only one of the two isomers in each case, A and D, respectively. Theoretical investigation of the reaction mechanism is made, and it shows that the attachment reaction should have no barrier in the gas phase. The thermodynamic equilibration of the C(70)F isomers is excluded by the high activation energy ( approximately 30 kcal/mol) for the F atom shifts. The explanation of the high selectivity presents a challenge for theoretical modeling.

Reactions of translationally excited and thermal fluorine atoms with CH4 and CD4 molecules in solid argon

Solid state reactions of fluorine with methane have been studied by ultraviolet laser photolysis of dilute mixtures of the two reagents in solid argon at 13–30 K. Using a combination of EPR and FTIR spectroscopies, three distinct mechanisms of product formation have been identified. At temperatures below 18 K, product formation is dominated by direct photolysis of F2–CH4 heterodimers, resulting in formation of closed-shell complexes HF–CH3F. A small fraction of reaction intermediate, a nonplanar methyl radical trapped in the reaction cage with HF and F, is also formed. This intermediate decays to product on a time scale of 103 s at 13 K after the photolysis period. Above 20 K, photogenerated F atoms undergo thermally activated diffusion in argon, and they react with isolated CH4 molecules to form CH3–HF complexes. The rate constant for this process is ∼10−25 cm3 s−1 at 20 K, and exhibits an activation energy of 1.7 kcal/mol. A third reaction channel involves reaction of F with CH4 to generate isolated pla...

High resolution electron paramagnetic resonance spectroscopy of septet pyridyl-2,4,6-trinitrene in solid argon: Fine-structure parameters of six electron-spin cluster

The high resolution 9 GHz electron paramagnetic resonance (EPR) spectrum of septet pyridyl-2,4,6-trinitrene was recorded after the photolysis of 2,4,6-triazido-3,5-dichloropyridine in solid argon matrix at 15 K. Owing to the high resolution of the experimental EPR spectrum, the zero-field splitting parameters of the septet trinitrene were determined with a high accuracy: D(s)=-0.1019+/-0.0004 cm(-1) and E(s)=0.003 25+/-0.000 15 cm(-1). All EPR transitions of the septet trinitrene were, for the first, unambiguously assigned based on the eigenfield calculations of the Zeeman energy levels. The spectrum of the septet trinitrene represents a new type of EPR spectra of septet spin states with nonzero zero-field splitting parameter E(s). The nonvanishing parameter E(s) of the septet trinitrene arises due to magnetic nonequivalence of three triplet centers in the molecule and is manifested in the appearance in the spectrum of separate x and y transitions. The septet spin states of this type display at very low magnetic fields two intense z transitions since the mid R:3D(s)mid R: energy gap between zero-field energy levels W(+/-1) and W(+/-2) fits the quantum of microwave irradiation of a 9 GHz EPR spectrometer. Analysis of the magnetic parameters shows that semiempirical description of the fine-structure tensor for six electron-spin cluster in the septet trinitrene is appropriate for precise estimations of the parameter D(s) but it is too crude to estimate small value of the parameter E(s).

High resolution electron paramagnetic resonance spectroscopy of quintet pyridyl-2,6-dinitrene in solid argon: Magnetic properties and molecular structure

The high resolution X-band electron para magnetic resonance (EPR) spectrum of quintet pyridyl-2,6-dinitrene was recorded after the photolysis of 4-amino-2,6-diazido-3,5-dichloropyridine in solid argon matrix at 15 K. This spectrum represents a new type of powder EPR spectra that are characteristic for quintet spin states with zero-field splitting parameters |E(q)/D(q)| approximately 1/4. All EPR lines of the quintet dinitrene were unambiguously assigned based on the eigenfield calculations of the Zeeman energy levels and angular dependencies of resonance magnetic fields. Owing to the high resolution of the experimental EPR spectrum, zero-field splitting parameters of the quintet dinitrene were determined with a high accuracy: D(q)=0.2100+/-0.0005 cm(-1) and E(q)=-0.0560+/-0.0002 cm(-1). These parameters provide correct information regarding the molecular angle Theta and distance r between two triplet sites in the molecule of quintet dinitrene. The measured molecular angle Theta=114.2 degrees+/-0.2 degrees is in excellent agreement with results of the density functional theory calculations. The analysis of the magnetic parameters shows that the spin population on the nitrene units in the quintet dinitrene is greater than that on the nitrene unit in the triplet nitrene.

Formation of the HHF complex in the reaction of thermal fluorine atoms with hydrogen molecules in solid Ar

Abstract The reaction of F atoms with H 2 in an Ar matrix was studied with EPR spectroscopy. F 2 photolysis produces two well-known doublets of the H atom trapped in different crystal sites. Heating gives rise to an EPR spectrum assigned to the H 2 F complex ( A H = 50.8, A F = 1.85, A H ⩽ 0.15 mT), the formation of which is attributed to the reaction of diffusing thermal F t atoms with H 2 . The shape and width of the spectrum of the complex changes drastically with temperature due to the transitions between two stable configurations: A ( a F = 3.2 mT, a H ⩽ 3.2 mT) and B ( a F = 0.5 mT, a H = 0.4 mT) with an Arrhenius-like dependence of rate ( E a = 80 cal/mol, ω 0 ∼ 3 × 10 10 s −1 ).

Magnetic anisotropy parameters of matrix-isolated septet 1,3,5-trinitreno-2,4,6-trichlorobenzene

An ESR spectrum of high-symmetry septet 1,3,5-trinitreno-2,4,6-trichlorobenzene generated under photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in solid argon at 15 K was recorded. Computer simulation revealed that the spectrum corresponds to the septet spin state with the fine structure parameters DS = −0.0957±0.0006 cm−1 and ES = 0±0.0004 cm−1. These values of the magnetic anisotropy parameters DS and ES are in good agreement with the results of UDFT calculations. The spin-spin (DSS) and spin-orbit (DSO) coupling parameters of septet molecules with D3h symmetry are negative and mutually enhance the magnetic anisotropy of these molecules. The contribution of the spin-orbit coupling to the magnetic anisotropy of 1,3,5-trinitreno-2,4,6-trichlorobenzene is higher than 11% due to the presence of three chlorine atoms in the molecule. This suggests the possibility of further strengthening the magnetic properties of septet 1,3,5-trinitrenobenzenes by introducing bromine and iodine atoms into positions 2, 4, and 6 of their benzene rings.

Magnetic anisotropy parameters of matrix-isolated septet 2,4,6-tribromo-1,3,5-trinitrenobenzene

An ESR spectrum of highly symmetric septet 2,4,6-tribromo-1,3,5-trinitrenobenzene formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in solid argon at 15 K was recorded. The spectrum corresponds to a septet spin state with the fine structure parameters |D| = 0.202 cm−1 and E = 0. The obtained magnetic anisotropy parameter |D| agrees well with the DFT calculated value that predicts the record high negative magnetic anisotropy D = 0.24 cm−1 in a series of organic septet trinitrenes. Due to the presence of three heavy bromine atoms and high symmetry of the molecule (D3h), the contribution of spin—orbit interactions to the magnetic anisotropy of the obtained trinitrene reaches ∼62% and parameter D in the studied trinitrene twofold exceeds the typical experimental values of |D| for the known septet polynitrenes.

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon

Isolated radicals.NH2 and radical-molecule complexes.NH2−HF, which are products of the reactions of mobile fluorine atoms with NH3 molecules in solid argon, were identified by EPR spectroscopy. The isotropic HFC constants of the complex (aN=1.20,aH=2.40, andaF=0.70 mT) were determined experimentally. The constant of isotropic HFC with the nucleus of hydrogen atom of the HF molecule is less than 0.1 mT. This assignment was confirmed in the experiments on isotope substitution of atoms (H→D),14N→15N) in the NH3 molecule. According to quantum-chemical calculations, the free complex.NH2−HF has a planar structure withC2, summetry and a binding energy of 12 kcal mol−1. Optimization of the arrangement of the complex in the crystal showed that its structure is only slightly distorted in the Ar lattice so that the equilibrium configuration is close to that obtained from gas-phase calculations. Different ratios of relative intensities of the proton triplet lines in the EPR spectra of isolated.NH2 radicals and.NH2−HF complexes were qualitatively explained by different heights of the barriers to rotation of the NH2 fragment in the Ar lattice.

Biopolymer composites based on modified lignin and nanocellulose

New composite lignocellulose materials were prepared using modified lignin and nanocellulose. The materials were studied by elemental and thermal gravimetric analysis, and their tensile tests were performed. The results obtained show that addition of nanocellulose to modified lignin makes possible its use as filler in paper production.

Reactions of photogenerated fluorine atoms with molecules trapped in solid argon. 4. Spectroscopic characteristics of β-C2H2F· radicals generated in reactions of mobile F atoms with C2H2 molecules trapped in solid argon

Reactions of mobile fluorine atoms with C2H2, C2D2, and C2HD molecules in solid argon were studied by ESR and IR spectroscopic techniques. Highly resolved ESR spectra of the stabilized radicals CHF=·CH, CDF=·CD, CHF=·CD, and CDF=·CH were obtained for the first time. Isotropic hyperfine constants on fluorine and proton nuclei were measured. It was found that the radicals formed in the reaction F + C2H2 correspond to the cis-β-C2H2F· isomer. A comparison of the measured HFC constants with the values calculated by modern quantum-chemical methods allows the identification of the isomeric form of the radical, whereas vibrational analysis of the IR absorption spectra gives unreliable results. The calculation of the energy of the radical isomers predicts that cis-β-C2H2F· is more stable than trans-β-C2H2F· by ∼1.0 kJ mol–1.