Tunnel-exchange states of square-planar bielectron transfer clusters

The tunnel states of square-planar bielectron transfer dⁿ-dⁿ-dⁿ⁺¹-dⁿ⁺¹ (n=0–4) clusters are considered. The nature of the ground spin state is revealed in the limiting case of a strong double exchange. It is shown that the magnetic properties of these systems radically differ from those of clusters with one migrating electron (hole) and tetrameric tetrahedral bielectron transfer clusters. Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 826–833, September–October, 1996. Translated by I. Izvekova     

Magnetic properties of tetrameric tetrahedral clusters with two-electron transfer

Tunnel-exchange states of tetrahedral tetrameric clusters dⁿ-dⁿ-dⁿ⁺¹-dⁿ⁺¹ with double transfer are considered to calculate the energy levels and wave functions. A method based on the second quantization technique and group-theoretical approach is developed. The nature of the ground spin state of the above systems is determined. It is shown that the structures have magnetic properties that radically differ from the properties of clusters with one migrating electron (hole). Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 3, pp. 458–470, May–June, 1996.     

Double exchange in polynuclear mixed-valence clusters. 2. Iron-sulfur proteins [Fe4S4]+ and [Fe4S4]3+

The energy states of tetrameric iron clusters with d⁶-d⁶-d⁶-d⁵ and d⁵-d⁵-d⁵-d⁶ electronic configurations, which form the metal frameworks of the [Fe₄S₄]⁺ and [Fe₄S₄]³⁺ iron-sulfur proteins, are calculated using a general approach. It is revealed that the structural distortions of these systems significantly affect the composition of the magnetic states. The effect of the isotropic Heisenberg exchange is considered. Valencia University, Spain. Bordeaux University, France. Moldova State University. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 816–825, September–October, 1996. Translated by I. Izvekova     

Electronic and vibronic states of an asymmetric trimeric mixed-valence cluster

Electronic and adiabatic vibronic states of d¹-d¹-d² trimeric clusters are considered in terms of the many-electron theory of mixed-valence states. The initial model includes two d-orbitals on each center and allows for Heisenberg and double exchange, asymmetry of the system, and vibronic coupling with local shifts of ligands. Influence of distortions in the trimers on the structure of tunnel-exchange states, on ferro-or antiferromagnetism of the clusters, and on the form of the adiabatic potential is examined. State University, Moldova Republic. Valencia University, Spain. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 35, No. 4, pp. 35–45, July–August, 1994. Translated by L. Smolina     

Magnetic properties and tunnel exchange states of dimeric mixed-valence clusters with two-electron transfer

Tunnel states of dimeric mixed-valence clusters with dⁿ-dⁿ⁺² type two-electron transfer are considered. General expressions for the matrix elements of the double exchange operator are obtained taking into account the contributions of the excited states. The energy spectrum of the clusters is calculated and the magnetic properties are studied. Allowance for the dⁿ⁺¹-dⁿ⁺¹ excited states considerably affects the energy spectrum and the magnetic properties of the systems. Translated fromZhurnal Strukturnoi Khimii, Vol. 38, No. 6, pp. 1038–1045, November–December, 1997.     

Vibronic reduction of Heisenberg and double exchange in multielectron mixed-valency clusters

Vibronic reduction is considered for Coulomb interaction between centers for Heisenberg and double exchange in multielectron mixed-valency d²-d³ clusters. A simplified multimode vibration model indicates how effects arise via vibrations from the correlation interaction between electron shells. Vibronic reduction in double exchange can alter the magnetic parameters for the cluster's ground state.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 2, pp. 114–121, March–April, 1992.     

Vibronic coupling effects in trimeric mixed-valence clusters

The spectrum of a trimeric mired-valence cluster d₁–d₁–d₂ is calculated in the Piepha-Krausz-Schatz vibronic model. Based on the vibronic spectra obtained magnetic properties of the d₁–d₁–d₂ cluster are investigated It is shown that spin of the system's ground state may change not only as a result of double exchange reduction by vibronic coupling, but also due to a diflerence in the eneqy dependence of the lowest vibronic states on the vibronic coupling constant for diflerent total spin values. Unlike previous studies of vibronic interactions in the d₁–d₁–d₂ cluster, in this study the diagonalizations of the vibronic-coupling and double-exchange matrices are fuljilled simultaneously, using Lanczos' method. Due to this, all types 4 vibronic state mixing by double exchange could be taken into account properly.Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 20–32, September–October, 1993.Translated by L. Smolina     

Double exchange in polynuclear mixed-valence clusters. 1. General solution of the electronic problem

The problem of calculating the electronic energy spectra of mixed-valence clusters with one “extra” electron (dⁿ-dⁿ...dⁿ⁺¹) or hole (dⁿ⁺¹-dⁿ⁺¹...dⁿ) delocalized in the paramagnetic cores of transition metals is solved. Unlike the available particular solutions, which are restricted to small numbers of ions and electrons, the solution proposed in this work is general and is suitable for many-electron systems of arbitrary numbers of nuclei and arbitrary symmetries. The new microscopic approach to the double exchange problem is based on the combination of the sequential (“chain”) scheme of spin coupling and angular momentum method. In terms of this approach, an analytical dependence of the matrix elements of the double exchange, Heisenberg exchange, and vibronic interaction on all spin quantum numbers is obtained. The final equations contain only the 6j symbols and are free of the higher-order nj symbols, which obstructed the solution of the double exchange problem in previous works. Valencia University, Spain. Bordeaux University, France. Moldova State University. Institute of Chemistry, Academy of Sciences, Moldova Republic. Translated fromZhurnal Strukturnoi Khimii, Vol. 37, No. 5, pp. 805–815, September–October, 1996. Translated by I. Izvekova     

Investigation of trifluoroacetolysis of some 2-bromoethanol arylsulfonates

Conclusions By PMR spectroscopy the kinetics of trifluoroacetolysis of 2-bromoethyl-2,2-d₂-toluene and 2-bromoethyl-2,2-d₂-anisole sulfonates, the dependence of rate constant on the presence of lithium perchlorate and on salts with a common ion were investigated. The trifluoroacetolysis of 2-bromoethyl-2,2-d₂-toluene-d₇-sulfonate in the presence of the lithium salt of nondeuterated toluenesulfonic acid was investigated. It was shown that reaction is accompanied by isomerization of the initial sulfonate (1,2-migration of bromine) and occurs by an ion pair mechanism.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 371–375, February, 1987.     

Structure distortion effects in the spectrum of tunnel-exchange states of tetrahedral mixed-valence tetramers

Tunnel-exchange states of a tetrameric mixed-valence (MV) d₁–d₁–d₁–d₂ cluster are considered. Energy levels of distorted pseudotetrahedral conjiprations with rhombic and trigonal symmetry are calculated. It is demonstrated that the correlation diagrams of planar systems are symmetric with respect to the sign of the double exchange parameter. A strong double exchange always results in the ferromagnetic ground state of planar systems. The following types of spectrum defamations of the d₁–d₁–d₁–d₂ tetramer from tetrahedral to planar systems have been found: a) with a weak positive double exchange, deformation of a tetrahedral system does not lead to any change of spin of the antiferromagnetic ground state of the cluster; b) with a strong positive double exchange, the defonnation alters the antiferromagnetic ground (S = 1/2) state of the tetrahedral system to the ferromagnetic (S = 5/2) state of the planar system, or to an intermediate (S = 3/2) state; c) with a negative double exchange, irrespective of its absolute value, the tetrahedral system deformation does not alter the spin of the ground state. With a weak double exchange, this ground state remains antiferromagnetic, and with a strong double exchange, it is ferromagnetic or intermediate.Moldova State University. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 5, pp. 33–46, September–October, 1993.Translated by L. Chernomorskaya     

2H-imidazole-N-oxides as spin traps

The capacity of 2H-imidazole-1-oxides, which contain an aldonitrone group, for spin capture was investigated. The parameters of spin adducts with^.OH,^.CH₃, and^.CH₂OH radicals are reported.Institute of Chemical Kinetics and Combustion, Siberian Branch, Russian Academy of Sciences. Institute of Organic Chemistry, Siberian Branch, Russian Academy of Sciences, 630090 Novosibirsk. Translated from Izvestiya Akademii Nauk, Seriya Khimicheskaya, No. 5, pp. 1064–1068, May, 1992.     

Ion-molecule reactions of primary radical cations in the liquid-phase radiolysis of <Emphasis Type="Italic">n</Emphasis>-alkanes: An EPR study

Radiation-chemical yields the liquid-phase radiolysis of C₅–C₁₂ n-alkanes were measured using the spin trap technique. The yields of n-alkyl radicals depended only slightly on the chain length in C₅–C₉ alkanes and amounted up to 30% of the total yield of trapped radicals; they were inhibited by the addition of charge scavengers. An analysis of the experimental results together with data on radicals in irradiated crystalline alkanes and radical cations in freon matrices showed that n-alkyl radicals results from the ion-molecule reactions of primary radical cations, whereas the protonated ions RH₂⁺ as products of these reactions are a source of sec-alkyl radicals. At least 60% of primary radical cations are consumed via these reaction pathways. A part of sec-alkyl radicals is due to gauche-conformers. The relative amount of primary alkyl radicals formed in the degradation of excited states and the subsequent charge neutralization processes should be insignificant.Translated from Khimiya Vysokikh Energii, Vol. 39, No. 1, 2005, pp. 5–14.Original Russian Text Copyright © 2005 by Belevskii, Belopushkin.     

S1←S0 Vibronic spectra and the structure of acetyl fluoride-h3 and acetyl fluoride-d3 molecules

UV absorption spectra of acetyl fluoride-h₃ and -d₃ (CH₃COF and CD₃COF) molecules in the region of S₁S₀ electronic transitions are investigated. The origins (0⁰ ₀ or 14^0± ₀) of these transitions are observed at 39912 and 39904 cm^–1, respectively, and some of the fundamental frequencies of these molecules in the s₀ and S₁ states are determined. In particular, the systems of torsion and inversion (out-of-plane C=O vibrations) energy levels are studied. The geometric parameters of an acetyl fluoride-h₃ molecule in the S₁ state are estimated by the theoretical simulation of the rotational contours of the 0⁰ ₀ (14^0± ₀) band. These data are used to evaluate the potential barriers to internal rotation in the S₀ and S₁ states, which were found to be 360 and 560 cm^–1 for acetyl fluoride-h₃ and 380 and 770 cm^–1 for acetyl fluoride-d₃, respectively, as well as the potential barriers to inversion in the S₁ states, which were found to be 2090 and 2370 cm^–1 for acetyl fluoride-h₃ and acetyl fluoride-d₃, respectively.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1957–1964, November, 1994.In conclusion, the authors would like to acknowledge T. S. Kuznetsova for the synthesis and purification of the AF samples.The research presented here was supported in part by the International Science Foundation under grant No. MJ 1000.     

Influence of magnetic dilution on the spin transition in the complex of iron(II) nitrate with 4-amino-1,2,4-triazole

Influence of magnetic dilution with Zn^II ions on the spin transition in the iron nitrate complex of 4-amino-1,2,4-triazole (AT) was studied by magnetochemistry, Mössbauer spectroscopy, and IR spectroscopy. In studies of the properties of solid phases of Fe _x Zn _1–x (AT)₃(NO₃)₂ (0.01x0.8), it was demonstrated that magnetic dilution results in a lowered spin transition temperature and an increased share of the high-spin form of the iron(II) complex.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 34, No. 6, pp. 145–151, November–December, 1993.Translated by L. Chernomorskaya     

Theoretical study of bimolecular elimination (E2) reactions. Possibility ofsyn E2 elimination in the series of 2-R-2-R'-1-halocyclopropanes

Reactions of the methoxide ion with substituted halocyclopropanes, which result in E2 elimination, have been studied by the semiempirical quantum-chemical AM1 method. The transition states corresponding totrans andcis routes have been localized. The energetic predominance of thetrans route over thecis route is reduced by 2.6 kcal mol^–1 on going from 1-chloropropane to chlorocyclopropane because of the features of cyclopropane geometry. It has been demonstrated that, in the gas phase,cis elimination may predominate overtrans elimination for a particular stereoisomer of 2-cyano-2-methyl-1-halocyclopropanes due to weakening of orbital interactions and Coulomb repulsion between the cyano group and the MeO^– anion in thetrans E2 transition state.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 620–623, April, 1995.     

Comparison effects of different doping on spin-Peierls transition in CuGeO(3)

The comparison between non-magnetic spin-Peierls (SP) and magnetic Néel ground states have been investigated in CuGeO₃ doped with Zn²⁺, Ni²⁺ and Mn²⁺ ions by using the electron spin resonance (ESR) techniques in the temperature range of 3–300 K. It was concluded that the one-dimensional (1D) antiferromagnetic (AF) spin chain formed of spin-1/2 (Cu²⁺) ions is broken by spin-0 (Zn²⁺), spin-1 (Ni²⁺), and spin-5/2 (Mn²⁺) ions, giving uncoupled spins at the end of the chains that give extra contribution to the spectra at lower temperature. An almost linearly dependence of frequency of resonance field has been showed for X-, K- and Q-band spectra. By the analysis of resonance field–frequency relations, the effects of the internal field is refined and thus the spectroscopic g-factor and internal field were calculated to be g = 1.9386 and H_i = 148 G, respectively.     

Kramers mechanism of indirect spin-spin couplings of nuclei in crystals

Nuclear spin exchange in the ionic clusters [Tl₄Cl]³⁺, situated in the cavities of cation-substituted chlorosodalite, was investigated by the²⁰³Tl and²⁰⁵Tl NMR method. It was shown that nuclear spin exchange in the ionic crystals is realized by analogy with the Kramers mechanism of indirect exchange of electron spins using the electron shells of the bridging atoms.Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated from Zhurnal Strukturnoi Khimii, Vol. 34, No. 2, pp. 74–78, March–April, 1993.     

Ab initio calculations of vibronic activity in phosphorescence microwave double resonance spectra of p-dichlorobenzene

The phosphorescence spectrum of p-dichlorobenzene has been calculated using multiconfiguration self-consistent-field wave functions and the quadratic response technique. Attention has been paid to the intensity distribution of the singlet–triplet (³B_1u¹A_g) transition through a number of vibronic subbands. The second order spin–orbit coupling (SOC) contribution to the spin splitting of the ³B_1u (^3*) state is found to be almost negligible, and the calculations therefore provide a good estimate for the zero-field splitting (ZFS) parameters based only on the electron spin–spin coupling expectation values. Nuclear quadrupole resonance constants for the different Cl isotopes are also calculated to accomplish the ZFS assignment. The electric dipole activity of the spin sublevels in the triplet–singlet transitions to the ground-state vibrational levels is estimated by calculations of derivatives using distorted geometries which are shifted from the equilibrium position along different vibrational modes. A vibrational analysis of the phosphorescence spectrum, based on the SOC-induced mixing of the singlet and triplet states calculated along different vibrational modes, provides reasonable agreement with experimental data.Acknowledgment O. R.-P. would like to thank the European MOLPROP network for support. The authors thank Alexander Baev for fruitful discussions. This work was supported by the Swedish Royal Academy of Science (KVA).     

Inelastic neutron scattering on a d1-d1-2 mixed-valence trimer cluster

We have carried out the calculations for inelastic neutron scattering spectra for thermal neutrons scattering on amixed-valence trimer d¹-d¹-d² cluster, and we have investigated the selection rules. The forbidden transitions include the A₁ A₂ transitions, independently of spin, as well as the¹A₂ ¹E transition. The rest of the transitions with S = 0, 1 are allowed, and in turn are subdivided according to intensity into strong and weak transitions. The scattering cross sections of the latter include the differences in the form factors of the mixedvalence ions (the indicated form factors are close) and are small in magnitude. The scattering cross sections and the line positions in the spectrum depend considerably on the tunneling and exchange parameters of the cluster.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 25, No. 3, pp. 329–332, May–June, 1989.     

Vibrational spectra and harmonic force fields of pyrrolidine derivatives: comparison between HF, MP2 and DFT force fields

Infrared and Raman spectra are reported for the isotopic species of pyrrolidine-d₀ (PY) and -d₁ and for N-methylpyrrolidine-d₀ (NMP), -d₂, -d₃ and -d₈. A complete assignment of the experimentally observed bands to normal modes is presented and discussed in particular in the CH/CD stretching region. The molecular structures and harmonic force fields were calculated ab initio at the Hartree–Fock (HF), the second order Møller–Plesset (MP2) and the density functional theory (DFT) level with the 6-31G* basis set. The force fields were fitted by use of 7 (PY) and 4 (NMP) independent scale factors. The spectra calculated with the DFT force fields are in better agreement with the experiment than those calculated by the MP2 and HF force fields. Though some scaled fundamental frequencies show larger deviations from the experimental ones, the mean percentage deviations of calculated frequencies from experimental fundamentals are less than 2.6% for all isotopic species of PY and NMP under study. The results indicate that density functional theory is a reliable tool to get a deeper insight in the assignment of vibrational spectra and the nature of normal modes of pyrrolidine derivatives.