Spin-free quantum chemistry. XVI. Spin correlation

We discuss the calculation of quasielastic critical neutron scattering for systems defined by a spin-free Hamiltonian. The system dependent property which determines the critical scattering is the thermal average of a spin-free operator called the general unpaired electron correlation operator. For localized models, we obtain the scattering as the Fourier transform of an unpaired site correlation function. The algebraic relationship between this correlation function and the usual spin correlation function is discussed as are its relationships to thermodynamic properties. Correlations in the infinite interaction range model, linear spin wave model, and infinite chain Heisenberg model are given. In the last case, an estimate of the low temperature correlation length is made.     

Spin-free quantum chemistry. XIII. Spin waves

We have obtained the Bloch spin wave dispersion formula using the methods of spin-free quantum chemistry. The spin-free eigenvectors are waves in spin-free space. This development makes the point that Bloch spin waves are dynamically spin-free. The neutron diffraction transition moment for spin waves is calculated employing the antisymmetrized projections of vector products of spin-free eigenkets and spin kets and is found to be agreement with results of Moorhouse.     

并五苯分子电荷注入导致自旋极化的密度泛函理论研究

用密度泛函理论研究了不带自旋的空穴注入并五苯后体系的自旋相关特性. 电荷注入后并五苯分子中存在自发自旋极化行为. 当注入电荷量达一定程度,分子磁矩随注入电荷量的增加呈线性增长,最大磁矩可达1μB. 注入电荷和并五苯分子的相互作用导致分子体系结构发生变化,同时电荷密度分布及自旋密度分布也发生了变化. 注入电荷先填充自旋劈裂的碳原子pz轨道.     

Spin-free quantum chemistry. XII. Coarse structure magnetic theory

The coarse structure magnetic theory presented here is a reformulation of conventional magnetic theory which emphasizes the spin-free (i.e., the coulombic) nature of the electron-electron interaction. We show that the magnetic properties of a system for which fine and hyperfine structure can be neglected depend only on the energy spectrum of a spin-free Hamiltonian. As an example, we treat the Heisenberg linear chain.     

The spin distribution in low-spin iron porphyrins

In many low-spin (S = 1/2) iron porphyrin derivatives, electron spin resonance (ESR) spectra indicate that one of the d(pi) orbitals of iron, either a d(xz) or d(yz), depending on the axial ligands of the porphyrin complex as well as their orientation, is essentially singly occupied; the unpaired electron is almost completely located at the metal. In contrast, nuclear magnetic resonance (NMR) and electron nuclear double resonance (ENDOR) spectroscopy convincingly show that a significant share of the unpaired electron is delocalized. This apparent contradiction is explained by the present density-functional-theory (DFT) calculations performed on a heme a model as well as on bis-imidazole-ligated iron porphyrin without substituents. The calculations show that the integrated spin density at the iron atom is nearly one, in agreement with the ESR measurements. However, significant areas with opposite (beta) spin are found along the Fe-N bond axes, thus evoking a need for additional alpha-spin density to be present in the porphyrin ring, ring substituents, and the axial ligands to keep the net amount of unpaired spin exactly one. The gross spin density, that is, the sum of unpaired alpha and beta spins, amounts to about 1.3 electrons. It seems that the degree to which alpha and beta spin dominate in different regions of the heme structure, as evidenced in these calculations, has not been previously observed.     

Synthesis, physical properties, multitemperature crystal structure, and 20 K synchrotron X-ray charge density of a magnetic metal organic framework structure, Mn3(C8O4H4)3(C5H11ON)2

A new magnetic metal organic framework material has been synthesized, Mn3(C8O4H4)3(C5H11ON)2, 1. Magnetic susceptibility measurements from 2 to 400 K reveal anti-ferromagnetic ordering at approximately 4 K and a total magnetic moment of 6.0 micro(B). The magnetic phase transition is confirmed by heat capacity data (2-300 K). The crystal structure is studied by conventional single-crystal X-ray diffraction data at 300, 275, 250, 225, 200, 175, 150, 125, and 100 K, and synchrotron data at 20 K. There is a phase transition between 100 and 20 K due to ordering of the diethylformamide molecules. The X-ray charge density is determined based on multipole modeling of a second 20 K single-crystal synchrotron radiation data set. The electron distributions around the two unique Mn centers are different, and both have substantial anisotropy. Orbital population analysis reveals large electron donation (1.7 e) to each Mn atom and the maximum possible number of unpaired electrons is 3.2 for both Mn sites. Thus, there is a considerable orbital component to the magnetic moment. Bader topological analysis shows an absence of Mn-Mn bonding, and the magnetic ordering is via super-exchange through the oxygen bridges. Formal electron counting suggests Mn2+ sites, but this is not supported by the Bader atomic charges, Mn1 = +0.11 e, Mn2 = +0.17 e. The topological measures show the dominant metal-ligand interactions to be electrostatic, and a simple exponential correlation is derived between Mn-O bond lengths and the values of nabla2rho at the bond critical points.     

A Bipodal Dicyano Anchor Unit for Single‐Molecule Spintronic Devices

The conductance through single 7,7,8,8‐tetracyanoquinodimethane (TCNQ) connected to gold electrodes is studied with the nonequilibrium Green’s function method combined with density functional theory. The aim of the study is to derive the effect of a dicyano anchor group, ?C(CN)₂, on energy level alignment between the electrode Fermi level and a molecular energy level. The strong electron‐withdrawing nature of the dicyano anchor group lowers the LUMO level of TCNQ, resulting in an extremely small energy barrier for electron injection. At zero bias, electron transfer from electrodes easily occurs and, as a consequence, the anion radical state of TCNQ with a magnetic moment is formed. The unpaired electron in the TCNQ anion radical causes an exchange splitting between the spin‐α and spin‐β transmission spectra, allowing the single TCNQ junction to act as a spin‐filtering device.     

Two-component relativistic methods for the heaviest elements

Different generalized Douglas-Kroll transformed Hamiltonians (DKn, n=1, 2,...,5) proposed recently by Hess et al. are investigated with respect to their performance in calculations of the spin-orbit splittings. The results are compared with those obtained in the exact infinite-order two-component (IOTC) formalism which is fully equivalent to the four-component Dirac approach. This is a comprehensive investigation of the ability of approximate DKn methods to correctly predict the spin-orbit splittings. On comparing the DKn results with the IOTC (Dirac) data one finds that the calculated spin-orbit splittings are systematically improved with the increasing order of the DK approximation. However, even the highest-order approximate two-component DK5 scheme shows certain deficiencies with respect to the treatment of the spin-orbit coupling terms in very heavy systems. The meaning of the removal of the spin-dependent terms in the so-called spin-free (scalar) relativistic methods for many-electron systems is discussed and a computational investigation of the performance of the spin-free DKn and IOTC methods for many-electron Hamiltonians is carried out. It is argued that the spin-free IOTC rather than the Dirac-Coulomb results give the appropriate reference for other spin-free schemes which are based on approximate two-component Hamiltonians. This is illustrated by calculations of spin-free DKn and IOTC total energies, r(-1) expectation values, ionization potentials, and electron affinities of heavy atomic systems.     

Building symmetric two-dimensional two-photon materials

Two-dimensional multi-annulenic carbon networks are important molecules with possible applications in optoelectronic devices and nonlinear optics. Investigations of two-photon absorption (TPA) cross sections have been carried out in a series of annulenes with a basic building block approach and variable symmetries. Enhancement of the TPA cross section has been observed with an increase in number of building blocks and order of symmetry. Evaluations of the ground-state transition dipole moment and chromophore density are not sufficient to explain the observed enhancement. Estimates of excited-state transition dipole moments made by femtosecond transient absorption measurements are able to successfully predict the observed trend in TPA cross section. It has been observed that the symmetry of the molecule plays a vital role in enhancing the TPA cross section by virtue of increasing the excited-state transition dipole moment.     

Excitation of electronic states in tetrahydrofuran by electron impact

We report on differential and integral cross section measurements for the electron impact excitation of the three lowest lying Rydberg bands of electronic states in tetrahydrofuran. The energy range of the present experiments was 15-50 eV with the angular range of the differential cross section measurements being 15°-90°. The important effects of the long-range target dipole moment and the target dipole polarizability, on the scattering dynamics of this system, are evident from the present results. To the best of our knowledge, there are no other theoretical or experimental data against which we can compare the cross section results from this study.     

Electronic structure of the organic half-metallic magnet: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl

An accurate full-potential density-functional method is used to study the magnetic and half-metallic properties in the pure organic materials: 2-(5-pyrimidinyl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-3-oxoimidazol-1-oxyl. The total and partial density of states and atomic spin magnetic moments are calculated and discussed. It is found that the unpaired electrons in this compound are localized in a molecular orbital constituted primarily of π^*(NO) orbital, and the main contribution of the spin magnetic moment comes from the NO free radicals. It is predicted that this compound is half-metallic magnet. It is also found that there exists ferromagnetic intermolecular interaction in the compound.     

Synthesis, structure, and unexpected magnetic properties of La3Re2O10

The compound La(3)Re(2)O(10) has been synthesized by solid-state reaction and characterized by powder neutron diffraction, SQUID magnetometry, and heat capacity measurements. Its structure consists of isolated [Re(2)O(10)](9-) dimer units of two edge-shared ReO(6) octahedra, separated by La(3+) within the lattice. The Re-Re distance within the dimer units is 2.488 A, which is indicative of metal-metal bonding with a bond order of 1.5. The average oxidation state of the Re atom is +5.5, leaving one unpaired electron per dimer unit (S = 1/2). Although the closest interdimer distance is 5.561 A, the magnetic susceptibility data and heat capacity measurements indicate this compound exhibits both short- and long-range magnetic order at surprisingly high temperatures. The zero field cooled (ZFC) magnetic susceptibility data show two broad features at 55 and 105 K, indicating short-range order, and a sharper cusp at 18 K, which signifies long-range antiferromagnetic order. The heat capacity of La(3)Re(2)O(10) shows a lambda-type anomaly at 18 K, which is characteristic of long-range magnetic order. DFT calculations determined that the unpaired electron resides in a pi-bonding orbital and that the unpaired electron density is widely delocalized over the atoms within the dimer, with high values at the bridging oxygens. Extended Hückel spin dimer calculations suggest possible interaction pathways between these dimer units within the crystal lattice. Results from the calculations and fits to the susceptibility data indicate that the short-range magnetic ordering may consist of 1-D antiferromagnetic linear chains of coupled S = 1/2 dimers. The magnetic structure of the antiferromagnetic ground state could not be determined by unpolarized neutron powder diffraction.     

Rate constants of atomic hydrogen formation in H3O+(H2O) n + e → H + (H2O) n gas-phase processes

Using the Maxwellian electron velocity distribution and the Breit-Wigner approximation of the reaction cross section, the kinetic parameters of the hydrogen atom formation upon the electron capture by positively charged hydronium-water clusters are estimated. Calculations of the cross sections and rate constants are based on the data of quantum chemical studies of H₃O⁺(H₂O) _n and H₃O(H₂O) _n clusters, particularly on the detailed analysis of the spacing of high-lying states of the radicals and the character of the unpaired electron density distribution, as well as on the general trend in the electron affinity change of the cations depending on the number of water molecules. The lifetimes of the radicals before the dissociation are taken from the classical nonempirical molecular dynamics runs. The results are compared to available experimental data. The article is published in the original.     

On the measure of electron correlation and entanglement in quantum chemistry based on the cumulant of the second-order reduced density matrix

In this paper we propose a functional of the many-body cumulant of the second-order reduced density matrix within the spin-free formalism of quantum chemistry which quantifies the idea of electron correlation and allows one to detect spin entanglement. Its properties are rigorously stated and discussed for spin-adapted pure states. Numerical determinations are performed for both equilibrium conformations and dissociation processes in molecular systems.     

Electron-density topology in molecular systems: paired and unpaired densities

This work studies the partitioning of the electron density into two contributions which are interpreted as the paired and the effectively unpaired electron densities. The topological features of each density field as well as of the total density are described localizing the corresponding critical points in simple selected molecules (local formalism). The results show that unpaired electron-density concentrations occur out of the topological bonding regions whereas the paired electron densities present accumulations inside those regions. A comparison of these results with those arising from population analysis techniques (nonlocal or integrated formalisms) is reported.     

O hyperfine structure in ESR spectra of Cu(II) hydroxyquinolinate: phthalimide

ESR spectra of ¹⁷O isotopic modifications of the Cu(II) hydroxyquinolinate complex substituted in phthalimide single crystals are reported and used to derive principal values and orientation of the hyperfine interaction tensors of the unpaired electron with the ¹⁴N and ¹⁷O nuclear magnetic moment.     

极化中子衍射及在电子自旋密度研究中的应用

极化中子衍射方法常用于研究含未配对电子化合物中电子自旋密度的分布．分子中电子自旋密度分布从一个独特的角度反映化合物的磁性质．本文介绍极化中子衍射方法的背景知识和基本原理．包括中子源、中子和X射线衍射、极化中子衍射,以及一些常用的实验数据处理方法．选用几个实例总结了用极化中子衍射方法得到的电子自旋密度分布在无机和有机化学中的应用．通过单分子磁体[Fe8O2（OH）12（tacn）6]^8＋和氰基桥联化合物K2[Mn（H2O）2]3[Mo（CN）7]2·6H2O,说明如何用该方法研究金属原子间的磁相互作用;并通过Ru（acac）3这个只含一个未配对电子的化合物来说明如何获得化合物中金属和配体上小的自旋密度;最后介绍了该方法在nitronylnitroxide自由基研究中的应用．     

Effects of conjugation length, electron donor and acceptor strengths on two-photon absorption cross sections of asymmetric zinc-porphyrin derivatives

Exceptionally large two-photon absorption cross sections at the infrared region have been revealed by time-dependent density functional theory calculations for asymmetric charge-transfer conjugated zinc-porphyrin derivatives. The largest two-photon cross section is found to be more than one order of magnitude larger than for the conventional two-photon active organic molecules. The calculations show that the formation of strong charge-transfer states depends on the length of the conjugation bridge between the zinc-porphyrin core and the electron donor/acceptor. The two-photon absorption cross section can be greatly enhanced by increasing the strengths of the electron donor/acceptor.