Selection rules for collisional energy transfer in homonuclear diatomics. rotationally inelastic collisions

We have made a precise study of the circular polarisation of rotationally resolved features of laser-excited iodine. The J′ = 19, ν′ = 16 level of ³II⁺_ou was excited using circularly polarised dye-laser fluorescence and a quantitative data on polarisation features representing inelastic transfer of ΔJ′ = 30 was recorded. The experimental circular polarisation ratios were compared to those predicted by two totally conserving models, ΔM = 0 and Δθ = 0. The agreement between experimental points and the predictions based on the former lead to the formation of a new selection due on rotationally inelastic transfer namely, ΔM = 0.     

Determination of the nuclear structure and spin density distribution in the cyano-bridged molecular based magnet K2Mn3(H2O)6[Mo(CN)7]2·6 H2O

This article details the local spin density determination in the cyano-bridged, two-dimensional, molecular based magnet K₂Mn₃(H₂O)₆[Mo(CN)₇]₂·6 H₂O (T_c = 39 K). The crystal structure, determined at room temperature by X-ray diffraction, was redetermined at 50 K using unpolarised neutron diffraction. The importance of intermolecular hydrogen-bonding interactions is clearly demonstrated in this study, previously characterised with X-rays. The local spin density was determined from polarised neutron diffraction data at 4 K with an applied field of 3 T. Positive spin densities were observed on the manganese sites, consistent with high-spin d⁵ ions in octahedral fields, whilst a negative spin density was found on the molybdenum sites, signifying delocalisation onto the cyano ligands. The alternating sign of the spin populations on the metal sites, suggests that the primary Mo^III–Mn^II interactions are antiferromagnetic in nature and the delocalisation onto the cyano-bridges clearly demonstrates the role of the ligand bridges in the magnetic exchange pathway.     

Nuclear spin magnetization transfer by doubly selective irradiation

A procedure for doubly selective irradiation of an AX NMR spin system so as to achieve complete magnetization transfer from X to A is described. In combination with suitably gated decoupling this sequence will give substantial savings of time in determinations of T₁ for low-sensitivity spin-½ nuclei, particularly those with negative magnetogyric ratios.     

[Fe(H2O)5(NO)]2+, the “Brown‐Ring” Chromophore

Although the “brown‐ring” ion, [Fe(H₂O)₅(NO)]²⁺ ( 1 ), has been a research target for more than a century, this poorly stable species had never been isolated. We now report on the synthesis of crystals of a salt of 1 which allowed us to tackle the unique bonding situation on an experimental basis. As a result of the bonding analysis, two stretched, spin‐polarised π‐interactions provide the Fe–NO binding—and challenge the concept of “oxidation state”.     

Charge‐Transfer Phase Transition of a Cyanide‐Bridged FeII/FeIII Coordination Polymer

Heterometallic Prussian blue analogues are known to exhibit thermally induced charge transfer, resulting in switching of optical and magnetic properties. However, charge‐transfer phase transitions have not been reported for the simplest FeFe cyanide‐bridged systems. A mixed‐valence Fe^II/Fe^III cyanide‐bridged coordination polymer, {[Fe(Tp)(CN)₃]₂Fe(bpe)?5 H₂O}_n, which demonstrates a thermally induced charge‐transfer phase transition, is described. As a result of the charge transfer during this phase transition, the high‐spin state of the whole system does not change to a low‐spin state. This result is in contrast to FeCo cyanide‐bridged systems that exhibit charge‐transfer‐induced spin transitions.     

Hydrogen‐Bonding Effect on Spin‐Center Transfer of Tetrathiafulvalene‐Linked 6‐Oxophenalenoxyl Evaluated Using Temperature‐Dependent Cyclic Voltammetry and Theoretical Calculations

The stable tetrathiafulvalene (TTF)‐linked 6‐oxophenalenoxyl neutral radical exhibits a spin‐center transfer with a continuous color change in solution caused by an intramolecular electron transfer, which is dependent on solvent and temperature. Cyclic voltammetry measurements showed that addition of 2,2,2‐trifluoroethanol (TFE) to a benzonitrile solution of the neutral radical induces a redox potential shift that is favorable for the spin‐center transfer. Temperature‐dependent cyclic voltammetry of the neutral radical using a novel low‐temperature electrochemical cell demonstrated that the redox potentials change with decreasing temperature in a 199:1 CH₂Cl₂/TFE mixed solvent. Furthermore, theoretical calculation revealed that the energy levels of the frontier molecular orbitals involved in the spin‐center transfer are lowered by the hydrogen‐bonding interaction of TFE with the neutral radical. These results indicate that the hydrogen‐bonding effect is a key factor for the occurrence of the spin‐center transfer of TTF‐linked 6‐oxophenalenoxyl.     

Circular emission from molecular iodine

The resonance emission from molecular iodine excited by circularly polarised light is circularly polarised. We report the circular emission spectrum of iodine excited by the 5145 A line of Ar⁺. In zero field the prominent rotational triplet and the high energy P,R doublet are strongly circularly polarisation ratios close to theoretical.     

Polarised vibrational studies of the α-glycine single crystal: Part I. Polarised Raman spectra—the Problem of effective local Raman tensors for the glycine zwitterions

Complete (full) set of the polarised Raman spectra for the α-glycine single crystal at room temperature are presented. For the strongest bands arising from the ν_sCH₂, ν_aCH₂, δCH₂, τCH₂, νCOO⁻, δCOO and ν_sCCN vibrations effective local Raman tensors are determined applying the Tsuboi approach. The obtained results are compared to literature data available on this problem. Unfortunately, significant differences are observed between our data and other published results. This is due to the improper single crystal sample preparation for polarised Raman spectra measurements in the past. The obtained results are presented in forms which allow to apply these data for the polarised Raman spectra of other crystals of glycine compounds.     

Intrakonfigurations‐, Trip‐Multiplett‐ sowie anomal polarisierte A1gund A2g‐Übergänge in elektronischen und vibratorischen Resonanz‐Raman‐Spektren von (spinentarteten) trans‐Di(cyano)phthalocyaninatorhenaten

Intraconfigurational, Trip‐Multiplet, and Anomalously Polarised A_1g and A_2g Transitions in Electronic and Vibrational Resonance Raman Spectra of (Spin‐Degenerate) trans ‐Di(cyano)phthalocyaninatorhenates Brown bis(tetra(n‐butyl)ammonium) trans‐di(cyano)phthalocyaninato(2‐)rhenate(II) ( 1 ) is prepared by melting bis(phthalocyaninato(2‐)rhenium(II)) with tetra(n‐butyl)ammonium cyanide. According to electrochemical data, 1 is oxidised by iodine to yield blue tetra(n‐butyl)ammonium trans‐di(cyano)phthalocyaninato(2‐)rhenate(III) ( 2 ), whose cation exchange in the presence of bis(triphenylphosphine)iminium salts has been confirmed by x‐ray structure determination. 1 and 2 dissolve without dissociation of the cyano ligands in conc. sulfuric acid. Dilution with cold water precipitates blue trans‐di(cyano)phthalocyaninato(2‐)rhenium(III) acid. 1 and 2 are oxidised by bromine yielding violet trans‐di(cyano)phthalocyaninato(1‐)rhenium(III). Oxidation of 2 with dibenzoylperoxide and N‐chlorsuccinimide is described. 1 and 2 are characterised by polarised resonance Raman(RR) spectra, FIR/MIR spectra, and UV‐Vis‐NIR spectra. Due to a Kramers degenerate ground electronic state of low‐spin Re^II, a polarisation anomaly of the totally symmetric vibrations a_1g at 598 and 672 cm^–1 with depolarisation ratios ρ_l > 3 is observed in the RR spectra of 1 . Weak bands in the unusual UV‐Vis‐NIR spectrum of 1 , starting at 10200 cm^–1, are attributed to trip‐multiplet (TM) transitions. An electronic RR effect is detected for 2 . The selectively enhanced anomalously polarised line at 1009 cm^–1 with ρ_l ≈ 15 and the (de)polarised lines between 1688 and 2229 cm^–1 are attributed to intraconfigurational transitions A_1g → A_2g > A_1g, B_1g, B_2g, E_g arising from the ³T_1g ground electronic state of low‐spin Re^III split by spin‐orbit coupling and low symmetry (D ). Some of their vibronic bands are detected in the IR spectrum between 1900 and 4000 cm^–1. B and Q transitions of 2 at 16700 and 31900 cm^–1, respectively, as well as eight weak TM transitions are observed between 5050 and 26100 cm^–1.     

Velocity dependence of polarised rotational transfer rates: close-coupled calculations on model systems

Expressions for the laboratory-frame rate constants for polarised rotational energy transfer, as a function of molecular velocity, have been derived for atom-diatom systems. These equations require the kinetic energy dependence of the collision-frame cross sections, which have been obtained by close-coupled calculations using four different N₂-rare gas potentials over the temperature range 0 to 400 K. The energy dependence of multipolar cross sections is presented as are results for spherical and cylindrical velocity distributions. These model calculations indicate strongly that information on intermolecular potentials may be obtained from polarisation ratio velocity dependence measurements.     

Graphitic Silicon Nitride: A Metal‐Free Ferromagnet with Charge and Spin Current Rectification

As a first example, herein we show that g‐Si₄N₃ is expected to act as a metal‐free ferromagnet featuring both charge and spin current rectification simultaneously. Such rectification is crucial for envisioning devices that contain both logic and memory functionality on a single chip. The spin coherent quantum‐transport calculations on g‐Si₄N₃ reveal that the chosen system is a unique molecular spin filter, the current‐voltage characteristics of which is asymmetric in nature, which can create a perfect background for synchronous charge and spin current rectification. To shed light on this highly unusual in‐silico observation, we have meticulously inspected the bias‐dependent modulation of the spin‐polarized eigenstates. The results indicate that, whereas only the localized 2p orbitals of the outer‐ring (OR) Si atoms participate in the transmission process in the positive bias, both OR Si and N atoms contribute in the reverse bias. Furthermore, we have evaluated the spin‐polarized electron‐transfer rate in the tunneling regime, and the results demonstrate that the transfer rates are unequal in the positive and negative bias range, leading to the possible realization of a simultaneous logic–memory device.     

Relaxations Studies of 2IZSZ Spin Order with Signal Enhancement by Coherence Transfer

The pulse sequence for generating coherence transfer (or polarization transfer) is invoked to enhance the signal of heteronuclear two spin order (e.g. 2I_ZS_Z) in a spin system with CH moiety. This allows the observation of selective conversion of Zeeman order, in a sample with natural abundant ¹³C nuclei, into two spin order for measuring cross-correlation of chemical shift anisotropy and dipole-dipole interactions. The molecular reorientational correlation time and the orientation of the C–H axis with respect to the principal axes of carboxyl CSA tensor may be determined simultaneously in the relaxation profile of two spin order.     

Spin-density distribution of the high Tcp-O2N·C6F4·CNSSN free radical studied by polarised neutron diffraction

Knowledge of the spin-density distribution in the dithiadiazolyl radical ring (DTDA) constitutes a major step towards the understanding of the magnetic and electronic properties of the rich magnetism of DTDA derivatives. The p-O₂N·C₆F₄·CNSSN radical was chosen as the most favourable CNSSN derivative to study the spin distribution in this kind of free radical by polarised neutron diffraction. Spin-density maps obtained for the p-O₂N·C₆F₄·CNSSN radical show that almost all the spin density is localised on the sulphur and nitrogen atoms of the CNSSN ring. A small negative spin density on the carbon atom of the CNSSN ring and a negligible spin density over the rest of the radical are observed, in good agreement with ab initio calculations.     

Chemical Approach Towards Broadband Spintronics on Nanoscale Pyrene Films

The injection of pure spin current into the non-magnetic layer plays a crucial role in transmitting, processing, and storing data information in the realm of spintronics. To understand broadband molecular spintronics, pyrene oligomer film (≈20 nm thickness) was prepared using an electrochemical method forming indium tin oxide (ITO) electrode/pyrene covalent interfaces. Permalloy (Ni₈₀Fe₂₀) films with different nanoscale thicknesses were used as top contact over ITO/pyrene layers to estimate the spin pumping efficiency across the interfaces using broadband ferromagnetic resonance spectra. The spintronic devices are composed of permalloy/pyrene/ITO orthogonal configuration, showing remarkable spin pumping from permalloy to pyrene film. The large spin pumping is evident from the linewidth broadening of 5.4 mT at 9 GHz, which is direct proof of spin angular momentum transfer across the interface. A striking observation is made with the high spin-mixing conductance of ≈1.02×10¹⁸ m⁻², a value comparable to the conventional heavy metals. Large spin angular moment transfer was observed at the permalloy-pyrene interfaces, especially at the lower thickness of permalloy, indicating a strong spinterface effect. Pure spin current injection from ferromagnetic into electrochemically grown pyrene films ensures efficient broadband spin transport, which opens a new area in molecular broadband spintronics.     

The effect of nuclear spin on the polarisation ratio of resonance fluorescence

The nuclear quadrupole interaction causes the rotational angular momentum of a molecule with nuclear spin to precess about the total angular momentum whose projection along a space fixed axis is conserved. This effects the degree of orientation in the excited state of molecules optically pumped with circularly polarised light and reduces the polarisation ratio of fluorescence from molecules with large nuclear spin. Calculations are presented for the circular polarisation ratio of the forward scattered fluorescence when the exciting radiation is sufficiently broad-banded that it excites the full manifold of quadrupole split components of a given J″ → J′ transition. Results are also presented for the variation of the polarisation ratio and intensity as a narrow excitation source is tuned across the absorption.     

Spin coherence studies of a quasi-one-dimensional crystal: Trap-to-trap energy migration and stimulated echo decay

Optically detected spin coherence experiments including spin locking, two-pulse Hahn echoes and three-pulse stimulated echoes were performed on h₂ traps in isotopically mixed 1,2,4,5-tetrachlorobenzene. Values for the rate constants for trap-to-trap triplet energy migration are obtained for crystals of several different trap concentrations, and the relative importance of various energy transfer pathways is estimated. The results suggest direct superexchange as an important mechanism for triplet energy migration in these crystals. We also find that some out-of-chain energy transfer must occur to account for our results.     

The non-equilibrium ortho/para spin state ratio for molecular H2 formed in the hydrolysis of lithium aluminum hydride

The ortho/para ratio for H₂ formed by reaction of LiAlH₄ with H₂O is <- 2.3 at room temperature, indicating that H₂ is formed in non-equilibrium spin ratios. Presumably only a few states of H₂ are initially formed in the reaction.     

Mechanistic aspects associated with the oxidation of -ascorbic acid at the 1,2-dichloroethane water interface

The biologically relevant oxidation of -ascorbic acid (AH₂) was studied at an externally polarised 1,2-dichloroethane water interface by a combination of electrochemical and spectroscopic techniques. In the presence of an electron acceptor (chloranil) in 1,2-dichloroethane, the basic features of the voltammetric signal are dependent on the concentration ratio of the redox species. In the presence of an excess of chloranil, a reversible signal is readily observed. A similar voltammetric response is developed when the semiquinone radical anion of chloranil (Q√⁻) is generated by homogeneous reduction in the presence of decamethylferrocene (DMFc) in the organic phase. These results suggest that Q√⁻ is formed homogeneously rather than by heterogeneous electron transfer from AH₂ to Q under these conditions. However, in the presence of an excess of ascorbate, an irreversible voltammetric response is the dominant feature. Replacing chloroanil by tetracyanoquinodimethane also provides irreversible features indicating that heterogeneous electron transfer can also take place. The occurrence of both homogeneous and heterogeneous electron transfer pathways is also suggested by potential modulated reflectance (PMR) spectroscopy.     

1H,2H,19F and 31P NMR and ESR investigations of aryl-t-butyl nitroxides

Aspects of translocation of spin to fluorine and phosphorus are discussed. (2p–3d)_π overlap in phosphorus is insignificant. Spin polarization and (2p–3p)_π overlap are opposing spin transfer mechanisms for phosphorus. The³¹P NMR spectrum of the nitroxide from 7a shows some features which are not fully understood. Some gain in resolution of the NMR spectra can be achieved by deuteration, but it is considerably lower than predicted.