Eugenii Ya. Misochko

Matrix-isolation and ab initio study of HNgCCF and HCCNgF molecules (Ng = Ar, Kr, and Xe).

We report three new noble-gas molecules prepared in low-temperature Kr and Xe matrices from the HCCF precursor by UV photolysis and thermal annealing. The identified molecules are two noble-gas hydrides HNgCCF (Ng = Kr and Xe) and a molecule of another type, HCCKrF. These molecules are assigned with the help of ab initio calculations. All strong absorptions predicted by theory are found in experiments with proper deuteration shifts. The experiments and theory suggest a higher stability against dissociation of HNgCCF molecules compared to HNgCCH reported previously. Surprisingly, only very tentative traces of HCCXeF, which is computationally very stable, are found in experiments. No strong evidence of similar argon compounds is found here.

A density functional theory study of the zero-field splitting in high-spin nitrenes

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn-Sham and the Pederson-Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%-12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of D(SS) into n-center (n=1-4) interactions shows that the major contribution to D(SS) results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S=2 and 3, if their molecules are constructed from the first-row atoms.

Molecular Structure and Magnetic Parameters of Septet 2,4,6-Trinitrenotoluene

Septet 2,4,6-trinitrenotoluene is the major paramagnetic product formed during the photolysis of 2,4,6-triazidotoluene in cryogenic matrices. This trinitrene displays different electron paramagnetic resonance (EPR) spectra in solid argon and in 2-methyltetrahydrofuran (2MTHF) glass, corresponding to septet spin states with the zero-field splitting (ZFS) parameters D(S) = -0.0938 cm(-1), E(S) = -0.0040 cm(-1) and D(S) = -0.0934 cm(-1), E(S) = -0.0015 cm(-1), respectively. Analysis of these parameters shows that the molecular and electronic structure of the septet trinitrene derived from the EPR spectrum in argon is in good agreement with the expectations from DFT calculations. The very small parameter E(S) in 2MTHF glass is explained by significant changes of the spin densities on the three nitrene units due to interactions of the nitrogen atom with surrounding 2MTHF molecules.

EPR spectrum of the Y@C82 metallofullerene isolated in solid argon matrix: hyperfine structure from EPR spectroscopy and relativistic DFT calculations

The EPR spectrum of the Y@C(82) molecules isolated in solid argon matrix was recorded for the first time at a temperature of 5 K. The isotropic hyperfine coupling constant (hfcc) A(iso) = 0.12 +/- 0.02 mT on the nucleus (89)Y as derived from the EPR spectrum is found in more than two times greater than that obtained in previous EPR measurements in liquid solutions. Comparison of the measured hfcc on a metal atom with that predicted by density-functional theory calculations (PBE/L22) indicate that relativistic method provides good agreement between experiment in solid argon and theory. Analysis of the DFT calculated dipole-dipole hf-interaction tensor and electron spin distribution in the endometallofullerenes with encaged group 3 metal atoms Sc, Y and La has been performed. It shows that spin density on the scandium atom represents the Sc d(yz) orbital lying in the symmetry plane of the C(2v) fullerene isomer and interacting with two carbon atoms located in the para-position on the fullerene hexagon. In contrast, the configuration of electron spin density on the heavier atoms, Y and La, is associated with the hybridized orbital formed by interaction of the metal d(yz) and p(y) electronic orbitals.

Heavy atom effect on magnetic anisotropy of matrix-isolated monobromine substituted septet trinitrene.

The heavy atom effect on the magnetic anisotropy of septet trinitrenes is reported. Septet 1-bromo-3,5-dichloro-2,4,6-trinitrenobenzene (S-1) was generated in a solid argon matrix by ultraviolet irradiation of 1,3,5-triazido-2-bromo-4,6-dichlorobenzene. This trinitrene displays an electron spin resonance (ESR) spectrum that drastically differs from ESR spectra of all previously studied septet trinitrenes. The zero-field splitting (ZFS) parameters, derived from the experimental spectrum, show the parameter |D| = 0.1237 cm(-1) and the unprecedentedly large ratio of E/D = 0.262 that is close to the rhombic limit E/D = 1/3 for high-spin molecules. The CASCI (based on state-averaged CASSCF) and DFT methods were applied to calculate the ZFS tensor focusing on the heavy (bromine) atom effects on the spin-orbit term. These calculations show that the multiconfigurational ab initio formalism and the CASCI method are the most successful for accurate predictions of the spin-orbit term in the ZFS tensor of high-spin nitrenes containing heavy bromine atoms. Due to the presence of the bromine atom in S-1, the contribution of the spin-orbit term to the total parameter D is dominant and responsible for the unusual orientation of the easy Z-axis lying in the molecular plane perpendicular to the C-Br bond. As a result, the principal values D(XX), D(YY), and D(ZZ) of the total tensor D̂(Tot) have such magnitudes and signs for which the ratio E/D is close to the rhombic limit, and the total parameter D is large in magnitude and positive in sign.

Matrix isolation ESR spectroscopy and magnetic anisotropy of D3h symmetric septet trinitrenes

The fine-structure (FS) parameters D of a series of D3h symmetric septet trinitrenes were analyzed theoretically using density functional theory (DFT) calculations and compared with the experimental D values derived from ESR spectra. ESR studies show that D3h symmetric septet 1,3,5-trichloro-2,4,6-trinitrenobenzene with D = -0.0957 cm(-1) and E = 0 cm(-1) is the major paramagnetic product of the photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in solid argon matrices at 15 K. Trinitrenes of this type display in the powder X-band ESR spectra intense Z1-transition at very low magnetic fields, the position of which allows one to precisely calculate the parameter D of such molecules. Thus, our revision of the FS parameters of well-known 1,3,5-tricyano-2,4,6-trinitrenobenzene [E. Wasserman, K. Schueller, and W. A. Yager, Chem. Phys. Lett. 2, 259 (1968)] shows that this trinitrene has [line]D[line] = 0.092 cm(-1) and E = 0 cm(-1). DFT calculations reveal that, unlike C2v symmetric septet trinitrenes, D3h symmetric trinitrenes have the same orientations of the spin-spin coupling tensor D[^]SS and the spin-orbit coupling tensor D[^]SOC and, as a result, have negative signs for both the DSS and DSOC values. The negative magnetic anisotropy of septet 2,4,6-trinitrenobenzenes is considerably strengthened on introduction of heavy atoms in the molecules, owing to an increase in contributions of various excitation states to the DSOC term.

High-resolution electron spin resonance spectroscopy of XeF• in solid argon. The hyperfine structure constants as a probe of relativistic effects in the chemical bonding properties of a heavy noble gas atom

Xenon fluoride radicals were generated by solid-state chemical reactions of mobile fluorine atoms with xenon atoms trapped in Ar matrix. Highly resolved electron spin resonance spectra of XeF* were obtained in the temperature range of 5-25 K and the anisotropic hyperfine parameters were determined for magnetic nuclei 19F, 129Xe, and 131Xe using naturally occurring and isotopically enriched xenon. Signs of parallel and perpendicular hyperfine components were established from analysis of temperature changes in the spectra and from numerical solutions of the spin Hamiltonian for two nonequivalent magnetic nuclei. Thus, the complete set of components of hyperfine- and g-factor tensors of XeF* were obtained: 19F (Aiso=435, Adip=1249 MHz) and 129Xe (Aiso=-1340, Adip=-485 MHz); g(parallel)=1.9822 and g(perpendicular)=2.0570. Comparison of the measured hyperfine parameters with those predicted by density-functional theory (DFT) calculations indicates, that relativistic DFT gives true electron spin distribution in the 2Sigma+ ground-state, whereas nonrelativistic theory underestimates dramatically the electron-nuclear contact Fermi interaction (Aiso) on the Xe atom. Analysis of the obtained magnetic-dipole interaction constants (Adip) shows that fluorine 2p and xenon 5p atomic orbitals make a major contribution to the spin density distribution in XeF*. Both relativistic and nonrelativistic calculations give close magnetic-dipole interaction constants, which are in agreement with the measured values. The other relativistic feature is considerable anisotropy of g-tensor, which results from spin-orbit interaction. The orbital contribution appears due to mixing of the ionic 2Pi states with the 2Sigma+ ground state, and the spin-orbit interaction plays a significant role in the chemical bonding of XeF*.

HFCN open-shell isomers in solid argon. I. Spectroscopy of the ground and excited states of HFC=N radical

Fluoromethylene amidogen radicals, HFC=N•, were generated in solid argon by solid-state chemical reactions of mobile F atoms with hydrogen cyanide. Highly resolved infrared and electroparamagnetic resonance spectra of HFC=N• were obtained in the temperature range 15–30 K. All six vibrational frequencies and the complete set of isotropic hyperfine coupling constants on magnetic nuclei 1H, 19F, and 14N were determined experimentally. Calculated spectroscopic characteristics are in excellent agreement with experiments, showing that HFC=N• radical has a planar structure in the ground state. Two electronic absorption transitions were observed in the near-ultraviolet and visible spectral region. The first excited 2A″ state of HFC=N• radical is calculated to have a planar structure very similar to the ground state, and lies 20 726 cm−1 above the ground state [at the CCSD(T)/cc-pVTZ level of theory], in good agreement with the experimental value, 20 430 cm−1. The observed Franck–Condon envelope in the laser-induc...

Infrared spectroscopic observation of the radical •XeF3 generated in solid argon

Xenon trifluoride radicals were generated by the solid-state chemical reaction of mobile fluorine atoms with XeF(2) molecules isolated in a solid argon matrix. On the basis of spectroscopic and kinetic FTIR measurements and performed quantum chemical calculations, two infrared absorption bands at 568 (strong) and 523 (very weak) cm(-1) have been assigned to asymmetric and symmetric Xe-F stretching vibrational modes of radical (*)XeF(3), respectively. Chemical reaction of fluorine atom with XeF(2) in a solid argon cage obeys specific kinetic behavior indicating the formation of a long-lived intermediate complex under the condition that the diffusing fluorine atom is attached to isolated XeF(2) at temperatures 20 K < T < 27 K. Subsequent thermally activated conversion in the complex is the main source of novel xenon-containing radical species (*)XeF(3). The rate constant and energy barrier are estimated for the reaction in an argon cage, [XeF(2)-F] --> (K(r)) [XeF(3)], as K(r) approximately 7 x 10(-5) c(-1) at 27 K and E approximately 1.2 kcal/mol, respectively. Quantum chemistry calculations reveal that radical (*)XeF(3) has a planar C(2v) structure. DFT calculations show that formation of the third Xe-F bond in the (*)XeF(3) radical is exothermic, and the binding energy of the third Xe-F bond is 8-20 kcal/mol.

Molecular conformations and magnetic parameters of the compact trimethylenemethane-type triplet diradical.

The ESR spectrum of compact nitroxide (NO)-substituted nitronyl nitroxide (NN) triplet diradical N-tert-butyl-N-oxidanyl-2-amino-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl (1) was recorded in solid argon matrix at 15 K. The zero-field splitting (ZFS) parameters of 1 were derived from the recorded ESR spectrum: |D| = 0.0248 cm(-1) and E = 0.0025 cm(-1). Quantum chemical calculations have been performed using DFT and multiconfigurational ab initio (CAS) methods in order to establish equilibrium geometries of the conformational isomers resulting from twisted conformations of NO and NN moieties. The ZFS parameters of 1 were calculated at these levels of theory to test validity of the calculated structures. The calculation results were analyzed using the measured ZFS parameters and magnetic and structural data from the previous studies (Suzuki, S.; et al. J. Am. Chem. Soc. 2010, 132, 15908; Tretyakov, E. V.; et al. Russ. Chem. Bull. 2011, 60, 2608). It was found that the ab initio method is most successful for accurate predictions of molecular and magnetic parameters. Diradical 1 has only one stable enantiomeric pair in pseudoeclipsed conformations. The two chiral isomers exist in racemic crystals 1 and in solid matrices with molecular parameters close to those attributed to a free molecule. The analysis of the spin density distribution suggests that one unpaired electron occupies NO group at the equilibrium geometry, whereas the torsion of NO group governs the spin density distribution of the second unpaired electron on a conjugated fragment in NN group. The increase in planarity by torsion of NO group enhances the trimethylenemethane-type properties and, therefore, gives rise to larger ferromagnetic exchange interaction. More planar equilibrium geometry and greater (three times) exchange interaction constant J were predicted for hypothetical diradical 1a, where bulky tert-butyl group is replaced by a methyl group in the nitroxide fragment.