Controlling Through‐Space and Through‐Bond Exchange Pathways in Bis‐Cobaltocenes for Molecular Spintronics

Pinching molecules via chemical strain suggests intuitive consequences, such as compression at the pinched site and clothespin‐like opening of other parts of the structure. If this opening affects two spin centers, it should result in reduced communication between them. We show that for naphthalene‐bridged biscobaltocenes with competing through‐space and through‐bond pathways, the consequences of pinching are far less intuitive: despite the known dominance of through‐space interactions, the bridge plays a much larger role for exchange spin coupling than previously assumed. Based on a combination of chemical synthesis, structural, magnetic, and redox characterization, and a newly developed theoretical pathway analysis, we can suggest a comprehensive explanation for this non‐intuitive behavior. These results are of interest for molecular spintronics, as naphthalene‐linked cobaltocenes can form wires on surfaces for potential spin‐only information transfer.     

Intuitive local picture for spin polarization of chemical bonds

The spin polarization of chemical bonds near radical centers is investigated by an analytical spin unrestricted Hartree–Fock model with numerical examples. The centroid analysis of localized molecular orbitals is also introduced to obtain an intuitive local picture for the spin polarization. The alternation of spin alignments in molecules are discussed with orbital symmetries and Hund's rule through chemical bonds. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010     

Improved methods for 1H-3H heteronuclear shift correlation

A better understanding of the structure of complex 3H-labeled molecules can be obtained by complete assignment of their 1H and 3H solution-state NMR spectra. The assignment process is aided by the detection of heteronuclear chemical shift correlations between 1H and 3H nuclei. Heteronuclear correlation (HETCOR) experiments previously applied to this task exhibit several drawbacks caused by the nature of both the pulse sequences and 1H-3H spin systems. The range of J-couplings involved in 1H-3H coupling networks make it challenging to perform correlation experiments using methods that rely on coherences created during free precession periods and interrupted by transfer pulses. Two alternative HETCOR experiments are demonstrated for 1H-3H systems in the present work and are shown to have advantages over earlier methods. The first experiment is known as hetero-TOCSY and correlates heteronuclear chemical shifts using J-cross polarization. This experiment achieves both homonuclear and heteronuclear mixing and connects the chemical shifts of all 1H and 3H nuclei in a coupling network. A second HETCOR experiment uses the heteronuclear Overhauser effect to obtain through-space correlations between nearby nuclei. The 1H-3H HETCOR experiments are phase sensitive and typically contain more correlations than other methods, which is beneficial for assignment purposes, while being sensitive enough to be applicable to routine analytical samples. The experiments were used to analyze 3H incorporation in sub-milligram quantities of 3H-labeled pharmaceutical derivatives with complex labeling schemes.     

"Through-space" nuclear spin-spin J(PP) coupling in tetraphosphine ferrocenyl derivatives: a (31)P NMR and X-ray structure correlation study for coordination complexes

Herein, we report on (31)P(31)P solution-phase "through-space" nuclear spin-spin coupling constants (J(PP)) from a novel family of organometallic tetraphosphine nickel and palladium complexes. These J(PP) constants were accurately determined through NMR iterative simulation based on the second-order spectra obtained for the compounds. The corresponding solid-state X-ray structures of the complexes were determined, and the "through-space" P.P distances are reported. Due to the blocked conformation of the species in solution, a qualitative and semiquantitative experimental correlation is obtained, which links the geometric parameters and the intensity of the corresponding P.P coupling constant. The lone-pair overlap theory developed for (19)F(19)F and (15)N(19)F "through-space" couplings in organic compounds [J. Am. Chem. Soc. 1973, 95, 7747-7752; 2000, 122, 4108-4116] appears to be a reliable foundation on which to account for our results. Based on the reported observations, the lone-pair overlap model is extended to "through-space" (31)P(31)P coupling, and the model is broadened to encompass metal orbital contributions for coordination complexes. Some of the predictions and consequences of the proposed theory are discussed.     

Computational NMR spectroscopy: reversing the information flow

Work on computational NMR recently carried out at our Laboratory in Padova is reviewed. We summarize our results concerning the calculation of NMR properties (chemical shifts and spin–spin coupling constants) in a variety of contexts, from the structure elucidation of complex organic molecules or molecules containing heavy atoms to weakly interacting species, such as those involved in hydrogen bonding or van der Waals CH-π interactions. We also present some original results, viz. the calculated ¹H and ¹³C spectra of the putative natural substance nimbosodione, the first examples of calculated ¹⁸¹Ta chemical shifts, spin–spin couplings in and through-space coupling constants involving ²⁰⁵Tl.     

Analytical modeling of the junction evolution in single-molecule break junctions: towards quantitative characterization of the time-dependent process

The conductance through single-molecule junctions characterized by the break junction techniques consists of the through-space tunneling and through-molecule tunneling conductance, and the existence of through-space tunneling between the electrodes makes the quantitative extraction of the intrinsic molecular signals of single-molecule junctions challenging. Here, we established an analytic model to describe the evolution of the conductance of a single molecule in break junction measurements. The experimental data for a series of oligo(aryleneethynylene) derivatives validate the proposed model, which provides a modeling insight into the conductance evolution for the opening process in a "real" break junction experiment. Further modulations revealed that the junction formation probability and rupture distance of the molecular junction, which reflect the junction stability, will significantly influence the amplitude and position of the obtained conductance peak. We further extend our model to a diffusion and a chemical reaction process, for which the simulation results show that the break junction technique offers a quantitative understanding of these time-dependent systems, suggesting the potential of break junction techniques in the quantitative characterization of physical and chemical processes at the single-molecule scale.     

Difficulties in building radiation-generated three-spin systems using spin-labeled luminophores

Aromatic compounds are well-known acceptors of primary radical ions that are formed under high-energy irradiation of nonpolar systems. Thus formed radical ion pairs recombine and produce magnetosensitive fluorescence, which helps study the short-lived radical ions. It was initially suggested that a simple introduction of a spin label into the original arene would allow an easy transition from two-spin to three-spin systems, retaining the experimental techniques available for radical pairs. However, it turned out that spin-labeled arenes often do not produce magnetosensitive fluorescence in the conditions of a conventional radiochemical experiment. To understand the effect of the introduced spin label, we synthesized a series of compounds with the general structure "stable 3-imidazoline radical-two-carbon bridge-naphthalene" as well as their diamagnetic analogues. By use of this set of acceptors, we determined the processes that ruin the observed signal and established their connection with the chemical structure of the compound. We found that the compounds with flexible (saturated) two-carbon bridges between the luminophore and the stable radical moieties exist in solution in folded conformation, which leads to suppression of luminescence from naphthalene due to efficient through-space exchange quenching of the excited state by the radical. Increasing the rigidity of the bridge by introducing the double bond drastically increases the reactivity of the extended pi-system. In these compounds, the energy released upon recombination is spent in radiationless processes of chemical transformations both at the stage of the radical ion and at the stage of the electronically excited molecule.     

Magneto-optical interactions in single-molecule magnets: Low-temperature photon-induced demagnetization

We show that the irradiation of SMM molecules at optical wavelengths can drive an increase or a decrease of the magnetic moment of a SMM, even though the energy of the photons does not correspond to a precise electronic or spin transition, the light pulse triggering a phonon-assisted spin transition. The process is sensitive to the power of the incident light. This result most probably explains why it has been so far impossible to observe the opening of the hysteresis loop on thin films of SMM with the XMCD technique. The consequences of these observations are manifold: they bring a means of controlling molecular magnets, open prospects in the field of quantum computing, and may enable the realization of coherent microwave sources through stimulated superradiance.     

Effect of the orientation of substituents on the chemical shifts of 13C. IV—13C NMR spectra of N,N-diisopropylamides and -thioamides

N,N-diisopropylamides and -thioamides show hindered rotation around the N? CH bonds, and the presence of mixtures of conformational isomers can be demonstrated at temperatures below 273 K in solution. ¹H and ¹³C NMR spectra of these conformers are measured and assigned. The ¹³C data serve to study through-space effects on ¹³C chemical shifts, which strongly depend on the conformations of the isopropyl groups. For amides, a through-space shielding of the N-methine carbons is found to exist only for conformers in which the methine hydrogen atom is spatially close to the oxygen atom. Chemical shift differences between amides and thioamides can be rationalized in terms of through-bond and through-space contributions, and serve for a better understanding of the shift differences in N,N-dialkylamides and -thioamides.     

Spin-state splittings, highest-occupied-molecular-orbital and lowest-unoccupied-molecular-orbital energies, and chemical hardness

It is known that the exact density functional must give ground-state energies that are piecewise linear as a function of electron number. In this work we prove that this is also true for the lowest-energy excited states of different spin or spatial symmetry. This has three important consequences for chemical applications: the ground state of a molecule must correspond to the state with the maximum highest-occupied-molecular-orbital energy, minimum lowest-unoccupied-molecular-orbital energy, and maximum chemical hardness. The beryllium, carbon, and vanadium atoms, as well as the CH(2) and C(3)H(3) molecules are considered as illustrative examples. Our result also directly and rigorously connects the ionization potential and electron affinity to the stability of spin states.     

Ring current and anisotropy effects on OH chemical shifts in resonance-assisted intramolecular H-bonds

Ring current effects on resonance-assisted and intramolecularly bridged hydrogen bond protons for 10-hydroxybenzo[h]quinoline 1 and a number of related compounds were calculated and the through-space NMR shieldings (TSNMRS) obtained hereby visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. These calculations revealed that this through-space effect is comparably large (up to 2?ppm) dependent on the position of the intramolecularly bridged OH proton, and therefore, contribute considerably to the chemical shift of the latter making it questionable to use δ(OH)/ppm in the estimation of intramolecular hydrogen bond strength without taking this into account. Furthermore, the anisotropy effects of additional groups on the aromatic moiety (e.g. the carbonyl group in salicylaldehyde or in o-hydroxyacetophenone of ca. 0.6?ppm deshielding) should also be considered. These through-space effects need to be taken into account when using OH chemical shifts to estimate hydrogen bond strength.     

A polarization propagator analysis of the geminal 31P-31P coupling in bis(difluorophosphino)amine⋆

Inner projections of the polarization propagator (IPPP) are used to decompose in through-space and through-bond contributions the two-bond P-P coupling in PF₂-NH-PF₂. This study is carried out using a ground state INDO wavefunction. Results for a phosphorus sp and spd atomic basis sets are compared. Several experimental trends are correctly reproduced using either of them. It is concluded that the overlap of the lone pair of both P atoms constitutes a very efficient pathway for transmitting through-space the spin information associated to the Fermi contact term.Part of a PhD thesis (H.O.G.) to be presented to the University of Buenos AiresFellow of the Argentine National Research Council (CONICET)Member of the Argentine National Research Council (CONICET)     

The structure of poly(carbonsuboxide) on the atomic scale: a solid-state NMR study

In this contribution we present a study of the structure of amorphous poly(carbonsuboxide) (C3O2)x by 13C solid-state NMR spectroscopy supported by infrared spectroscopy and chemical analysis. Poly(carbonsuboxide) was obtained by polymerization of carbonsuboxide C3O2, which in turn was synthesized from malonic acid bis(trimethylsilylester). Two different 13C labeling schemes were applied to probe inter- and intramonomeric bonds in the polymer by dipolar solid-state NMR methods and also to allow quantitative 13C MAS NMR spectra. Four types of carbon environments can be distinguished in the NMR spectra. Double-quantum and triple-quantum 2D correlation experiments were used to assign the observed peaks using the through-space and through-bond dipolar coupling. In order to obtain distance constraints for the intermonomeric bonds, double-quantum constant-time experiments were performed. In these experiments an additional filter step was applied to suppress contributions from not directly bonded 13C,13C spin pairs. The 13C NMR intensities, chemical shifts, connectivities and distances gave constraints for both the polymerization mechanism and the short-range order of the polymer. The experimental results were complemented by bond lengths predicted by density functional theory methods for several previously suggested models. Based on the presented evidence we can unambiguously exclude models based on gamma-pyronic units and support models based on alpha-pyronic units. The possibility of planar ladder- and bracelet-like alpha-pyronic structures is discussed.     

CIDEP effects of intramolecular quenching of singlet and triplet excited states by nitroxide radicals in oligopeptides: a potentially useful new method for investigating peptide secondary structures in solution

Two hexapeptides, each bearing one photoactive alpha-amino acid (Bin or Bpa) and one nitroxide-containing TOAC residue, have been synthesized and fully characterized. FT-IR absorption measurements indicate that a 3(10)-helical conformation is adopted by these peptides in solution. As two amino acid units separate the photoactive residue from TOAC in the peptide sequences, the two moieties face each other at a distance of about 6 A after one complete turn of the ternary helix. Irradiation by a light pulse from an excimer laser populates the excited states localized on the chromophores. An intramolecular interaction between the singlet (Bin) or triplet (Bin and Bpa) excited states and the doublet state of the TOAC nitroxide makes a spin-selective decay pathway possible, that produces transient spin polarization. In addition, in order to determine whether the intramolecular exchange interaction occurs through-bond or through-space, we have prepared linear and cyclic TOAC-Bin dipeptide units. A CIDEP study revealed that a through-space intramolecular interaction is operative. The observation of spin polarization makes the two helical hexapeptides suitable models to test the possibility of application of this novel technique to conformational studies of peptides in solution.     

Noninnocence of the ligand atoms in iron‐porphine: Chemical consequences of the delocalized electron spin

It is well recognized that the electronic spin density in transition metal complexes in high‐spin states, tends to delocalize from the metal ion itself to the donor atoms of the ligand. In square planar iron‐porphine [PFe]⁺ the delocalization occurs even further and spin corresponding to roughly one electron is delocalized over a large part of the ligand. In this article, density functional theory is applied to explore the chemical consequences of the delocalized spin in four‐coordinate iron‐porphine. It is shown that the porphine ligand has a moderate affinity for radicals, and that covalent bonds can form through spin‐pairing of the unpaired delocalized electron on the porphine ligand and the unpaired electron of another radical species. The hydrogen atom is used as a probe to evaluate the radical affinity of the different nitrogen and carbon atoms that constitute the porphine ligand. It is computationally predicted that the porphine ligand of four‐coordinate iron‐porphine is kinetically capable of activating weak C? H bonds of, for example, unsaturated organic compounds. Hydrogen atom transfer becomes spontaneous via subsequent homo‐coupling of the organic radical created. Whether or not the radical affinity of the porphine ligand has any mechanistic implications for heme‐containing enzymes is left as an open question.     

2D—HOHAHA和ROESY方法归属复杂天然有机化合物的NMR谱线

用同核化学位移相关谱及旋转坐标系中的同核化学位移相关谱和旋转坐标系中的同核NOE实验等新的二维核磁共振方法对首次从西洋参叶中分离出的Ocotillol型皂甙(OTS)的~1H化学位移进行了完全归属,为OTS分子溶液中的三维空间结构研究提供了可靠的结构参数.     

Recent advances of folded tetraphenylethene derivatives featuring through-space conjugation

Through-space conjugated molecules are interesting building blocks for the construction of functional materials that allow multi-dimensional transport of carrier and energy. However, the well explored through-space conjugated molecules are quite limited, which defers their structure-property correlation establishment and wide-scale application. In this review, we introduce a kind of newly-emerging folded tetraphenylethene derivatives featuring through-space conjugation. Their synthesis, crystal and electronic structures, and optical properties are described, and their representative applications as bipolar charge-transporting materials in organic light-emitting diodes and as single-molecule wires in molecular devices are presented, which are anticipated to provide guidance for the further expansion of through-space conjugated systems.     

Relations between 77Se NMR chemical shifts of (phenylseleno)-benzenes and their molecular structures derived from nine X-ray crystal structures

An extensive library of 77Se chemical shifts have been generated from the NMR measurements on substituted (phenylseleno)benzenes, including 33 new compounds. The variation in chemical shifts cover 265 ppm ranging from 446 to 181 ppm. Crystal structures have been determined for nine selected representatives of the substituted (phenylseleno)-benzenes. The analysis of the crystal structures supported that through-space interactions between selenium and the ortho-substituent observed in the crystal structures also are likely to be present in solution. The variation in the 77Se NMR chemical shifts can be rationalised from the intramolecular interactions with the substituent in the ortho-position. Furthermore it appears that these ortho-effects are roughly additive, and that it is the actual interactions and not the resulting conformational constraints that are responsible for the variations in the 77Se NMR chemical shifts.     

Exchange interaction mechanisms in 1,3,5,7-tetramethyl-2,6-diazaadamantane <Emphasis Type="Italic">N,N’</Emphasis>-dioxyl biradical

The exchange interaction parameters were calculated and the spin density distribution over the organic skeleton of the 1,3,5,7-tetramethyl-2,6-diazaadamantane N,N’-dioxyl biradical was studied based on the results of quantum chemical modeling of the biradical structure by the DFT method using various hybrid functionals (UB3LYP, LC-wPBE, UCAM-B3LYP, UHSEH1PBE) with the 6-311++G(2d,2p) basis set and by the UHF method with the same basis set. The characteristics of the direct orbital overlap between the N atoms of the two nitroxide groups were determined. The values of the J constant, obtained using different calculation methods, were found to be similar to each other. It was established that there is ferromagnetic exchange interaction between the radical sites in the system in question, which occurs predominantly according to the spin polarization mechanism in the 2,6-diazaadamantane core, and the various spin density transfer pathways through the C atoms of the organic skeleton were found to be nonequivalent. The direct overlap of the upper singly-occupied МОs with localization on the nitroxide groups led to a noticeable additional contribution of the antiferromagnetic exchange interaction. Despite the latter factor, the total contribution of these two mechanisms (spin polarization and direct through-space exchange) resulted in the triplet ground state in the biradical studied.