Isolable p- and m-[((t)Bu2MeSi)2Si]2C6H4: disilaquinodimethane vs triplet bis(silyl radical)

Isomeric p- and m-disilaquinodimethanes 2 and 4 were synthesized by the reductive dehalogenation of the corresponding p- and m-bis(halosilyl)benzenes 1 and 3, respectively, and were isolated and structurally characterized. The X-ray diffraction and solid-state NMR studies of 2 revealed its singlet quinodimethane structure featuring two exocyclic Si═C double bonds with some singlet biradical contribution. In contrast, the X-ray crystallography and EPR measurements of 4 disclosed its biradical nature, described as a triplet ground state bis(silyl radical).     

Syntheses, optical and intramolecular magnetic properties of mono- and di-radicals based on nitronyl-nitroxide and oxoverdazyl groups appended to 2,6-bispyrazolylpyridine cores

This paper presents the synthesis of a series of nitronyl-nitroxide (NN), oxoverdazyl (OVZ) based mono-, and bi-radicals attached to 4-phenyl-2,6-bispyrazolylpyridine coupling unit, their optical, electron spin resonance (ESR) spectroscopic studies and computational analysis. The ESR studies revealed that the axial zero-field splitting (zfs) parameter of the NN biradical (|D/hc| = 0.00719 cm(-1)) is larger than the OVZ biradical (|D/hc| = 0.00601 cm(-1)). Additionally both biradicals displayed forbidden half-field transitions (ΔM(s) = ±2; g(av) ~ 4.01) at 170 K demonstrating their triplet nature. The cryogenic ESR measurements of the two biradicals showed a Curie magnetic behaviour of the ΔM(s) = ±2 signal intensities (χ(EPR)) down to 4.2 K. A detailed comparative analysis of the strength of hyperfine coupling, spin density distribution, zfs and the spin-spin exchange coupling (J) of both NN and OVZ based biradicals showed that the ground state spin multiplicity of both biradicals is probably triplet (S = 1) or it is nearly degenerate singlet-triplet states with J(NN)?J(OVZ).     

Magnetic interactions in supramolecular N-O...H-C[triple bond]C- type hydrogen-bonded nitronylnitroxide radical chains

Two paramagnetic building blocks, 2-(4-ethynyl-1-phenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (3) and 2-(5-ethynyl-2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (4) were synthesized and crystallized. Single crystal X-ray studies of 3 and 4 show the formation of supramolecular head-to-tail one-dimensional H-bonded (N-O...H-C[triple bond]C- type) chain structures with O...C distances of 3.181 and 3.155 A, respectively. High-resolution isotropic liquid state (c相似文献   

A dinuclear Ni(I) system having a diradical Ni2N2 diamond core resting state: synthetic, structural, spectroscopic elucidation, and reductive bond splitting reactions

One-electron reduction of the square-planar nickel precursor (PNP)NiCl ( 1) (PNP (-) = N[2-P(CHMe 2) 2-4-methylphenyl] 2) with KC 8 effects ligand reorganization of the pincer ligand to assemble a Ni(I) dimer, [Ni(mu 2-PNP)] 2 ( 2), containing a Ni 2N 2 core structure, as inferred by its solid-state X-ray structure. Solution magnetization measurements are consistent with a paramagnetic Ni(I) system likely undergoing a monomer <--> dimer equilibrium. The room-temperature and 4 K solid-state X-band electron paramagnetic resonance (EPR) spectra display anisotropic signals. Low-temperature solid-state X-band EPR data at 4 K reveal rhombic values g z = 1.980(4), g x = 2. 380(4), and g y = 2.225(4), as well as a forbidden signal at g = 4.24 for the Delta M S = 2 half field transition, in accord with 2 having two weakly interacting metal centers. Utilizing an S = 1 model, full spin Hamiltonian simulation of the low-temperature EPR spectrum on the solid sample was achieved by applying a nonzero zero-field-splitting parameter ( D = 0.001 cm (-1)), which is consistent with an S = 0 ground state with a very closely lying S = 1 state. Solid-state magnetization data also corroborate well with our solid-state EPR data and reveal weak antiferromagnetic behavior ( J = -1.52(5) cm (-1)) over a 2-300 K temperature range at a field of 1 Tesla. Evidence for 2 being a masked "(PNP)Ni" scaffold originates from its reaction with N 2CPh 2, which traps the Ni(I) monomer in the form of a T-shaped species, Ni(PNPNNCPh 2), a system that has been structurally characterized. The radical nature of complex 2, or its monomer component, is well manifested through the plethora of cooperative H-X-type bond cleavage reactions, providing the nickel(II) hydride (PNP)NiH and the corresponding rare functionalities -OH, -OCH 3, -PHPh, and -B(catechol) integrated into the (PNP)Ni moiety in equal molar amounts. In addition to splitting H 2, compound 2 can also engage in homolytic X-X bond cleavage reactions of PhXXPh to form (PNP)Ni(XPh) (X = S or Se).     

EPR and solid-state NMR studies of poly(dicarbon monofluoride) (C2F)n

Poly(dicarbon monofluoride) (C2F)n was studied by electron paramagnetic resonance (EPR) and solid-state nuclear magnetic resonance (NMR). The effects of physisorbed oxygen on the EPR and NMR relaxation were underlined and extrapolated to poly(carbon monofluoride) (CF)n and semi-covalent graphite fluoride prepared at room temperature. Physisorbed oxygen molecules are shown to be an important mechanism of both electronic and nuclear relaxations, resulting in apparent spin-lattice relaxation time and line width during NMR and EPR measurements, respectively. The effect of paramagnetic centers on the 19F spin-lattice relaxation was underlined in accordance with the high electron spin density determined by EPR. 19F magic angle spinning (MAS) NMR, 13C MAS NMR, and 13C MAS NMR with 19F to 13C cross polarization (CP) underline the presence of two types of carbon atoms, both sp3 hybridized: some covalently bonded to fluorine and the others linked exclusively to carbon atoms. Finally, a C-F bond length of 0.138 +/- 0.002 nm has been determined thanks to the re-introduction of dipolar coupling using cross polarization.     

Comparative analysis of hydrogen bonding with participation of the nitrogen, oxygen and sulfur atoms in the 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives based on the 1H, 13C, 15N spectroscopy and DFT calculations

The N-H...X (X = N,O,S) intramolecular hydrogen bond in the series of 2(2'-heteroaryl)pyrroles and their trifluoroacetyl derivatives is examined by the (1)H, (13)C, (15)N spectroscopy and density functional theory (DFT) calculations. The influence of the hydrogen bond on coupling and shielding constants is considered. It is shown that the N-H...N intramolecular hydrogen bond causes a larger increase in the absolute size of the (1)J(N,H) coupling constant and a larger deshielding of the bridge proton than the N-H...O hydrogen bond. The effect of the N-H...S interaction on the (1)J(N,H) coupling constant and the shielding of the bridge proton is small. The NMR parameter changes in the series of the 2(2'-heteroaryl)pyrroles due to N-H...X hydrogen bond and the series of the 1-vinyl-2-(2'-heteroaryl)-pyrroles due to C-H...X hydrogen bond have the same order. The proximity of the nitrogen, oxygen or sulfur lone pair to the F...H hydrogen bridge quenches the trans-hydrogen bond spin-spin couplings (1h)J(F,H-1) and (2h)J(F,N).     

Crystal structures and 77Se NMR spectra of molybdenum(IV) areneselenolates having intramolecular NH...Se hydrogen bonds

Salts of the monooxomolybdenum(IV,V) areneselenolates having intramolecular NH...Se hydrogen bonds, [Mo(IV)O(Se-2-RCONHC6H4)4]2- (R = t-Bu, CH3, CF3) and [Mo(V)O(Se-2-t-BuCONHC6H4)4]-, were synthesized and characterized by 1H nuclear magnetic resonance (NMR), 77Se NMR, electron spin resonance (ESR), UV-visible spectra, X-ray analysis, and electrochemical measurements. 77Se-1H correlated spectroscopy (COSY) indicated a significant correlation between amide 1H and selenolate 77Se atoms through an NH...Se hydrogen bond with 1J(77Se-1H) = 5.4 Hz coupling. The hydrogen bonds contribute to the positive shift in the Mo(V)/Mo(IV) redox potential. In the crystal structure of (PPh4)2[Mo(IV)O(Se-2-CH3CONHC6H4)4], an NH...O=Mo hydrogen bond was found. Ab inito calculations support the presence of intramolecular NH...O=Mo and NH...Se hydrogen bonds.     

ESIPT-regulated Mechanoresponsive Luminescence Process byIntroducing Intramolecular Hydrogen Bond in NaphthalimideDerivatives#br#

Herein, two compounds, 4-2′-hydroxybenzylidenehydrazinyl-N-butyl-1,8-naphthalimide(BN-1) and 4-benzylidenehydra-zinyl-N-butyl-1,8-naphthalimide(BN-2), were synthesized to explore the hydrogen bonding effect on mechanoresponsive luminescent(MRL). The results showed that compound BN-1 exhibited strong emission in solution and solid-state compared with compound BN-2. After grinding, the emission intensity of compound BN-1 sharply decreased by as much as 15 times with an obvious red-shift from 552 nm to 577 nm. The control compound BN-2, by contrast, did not change so much before and after grinding. Single crystal analysis suggests that BN-1 molecule formed strong intramolecular interaction via ―N=N···H―O hydrogen bond with a distance of 0.2632 nm. An excited-state intramolecular proton transfer(ESIPT) based fluorophore featured this intramolecular hydrogen bond. The intramolecular hydrogen bond as well as other intermolecular interactions can rigidify the molecular conformation of compound BN-1 in solid-state, and thus suppress the nonradiative pathways, resulting in strong emission. These intra- and intermolecular interactions were destroyed by mechanical stimuli, accompanied by molecular conformation change that decreases the luminescence and blocks the ESIPT process. The MRL process was also demonstrated by scanning electron microscopy and powder X-ray diffraction. The molecular stacking mode changed from crystalline to a disordered amorphous state after grinding.     

EPR analysis and DFT computations of a series of polynitroxides

Polynitroxides with varying numbers of nitroxide groups (one to four) derived from different aromatic core structures show intramolecular electron spin-spin coupling. The scope of this study is to establish an easy methodology for extracting structural, dynamical, and thermodynamical information from the EPR spectra of these polynitroxides which might find use as spin probes in complex systems, such as biological and host/guest systems, and as polarizing agents in dynamic nuclear polarization (DNP) applications. Density functional theory (DFT) calculations at the B3LYP/6-31G(d) level provided information on the structural details such as bond lengths and angles in the gas phase, which were compared with the single crystal X-ray diffraction data in the solid state. Polarizable continuum model (PCM) calculations were performed to account for solvent influences. The electron paramagnetic resonance (EPR) spectra of the polynitroxides in chloroform were analyzed in detail to extract information such as the percentages of different conformers, hyperfine coupling constants a, and rotational correlation times τ(c). The temperature dependence on the line shape of the EPR spectra gave thermodynamic parameters ΔH and ΔS for the conformational transitions. These parameters were found to depend on the number and relative positions of the nitroxide and other polar groups.     

Theoretical investigation of intramolecular magnetic interaction through an ethylenic coupler

We show that an ethylenic coupler provides a very strong intramolecular magnetic interaction. A recently synthesized nitronyl nitroxide derivative, D-NIT2, is investigated by ab initio quantum chemical methods. The broken symmetry approach yields a coupling constant -541 K that is in good agreement with the observed value in solid state.     

Ladderlike oligomers; intramolecular hydrogen bonding, push-pull character, and electron affinity

Symmetrical 2,5-bis(2-aminophenyl)pyrazines have been synthesized by application of the Stille coupling strategy. These cotrimers feature three important properties, namely strong intramolecular hydrogen bonding, push-pull character, and high electron affinity. The presence of intramolecular hydrogen bonds has been confirmed by 1H NMR, IR spectroscopy, and single-crystal X-ray diffraction. The hydrogen bond strength can be increased by substituting the amino groups with stronger electron-withdrawing functionalities. Despite the anticipated enhanced pi-conjugation through planarization, a hypsochromic shift was observed in the UV/Vis spectra, explained by a decrease in push-pull character. The electron affinity of the cotrimers was deduced from the first reduction potentials measured by cyclic voltammetry and is related to the electron-withdrawing character of the amino substituents. The results obtained have been compared with those of the corresponding 4-aminophenyl analogues and show that intramolecular hydrogen bonds can be used to design polymers with enhanced pi conjugation as well as a high electron affinity.     

Kinetically blocked stable heptazethrene and octazethrene: closed-shell or open-shell in the ground state?

Polycyclic aromatic hydrocarbons with an open-shell singlet biradical ground state are of fundamental interest and have potential applications in materials science. However, the inherent high reactivity makes their synthesis and characterization very challenging. In this work, a convenient synthetic route was developed to synthesize two kinetically blocked heptazethrene (HZ-TIPS) and octazethrene (OZ-TIPS) compounds with good stability. Their ground-state electronic structures were systematically investigated by a combination of different experimental methods, including steady-state and transient absorption spectroscopy, variable temperature NMR, electron spin resonance (ESR), superconducting quantum interfering device (SQUID), FT Raman, and X-ray crystallographic analysis, assisted by unrestricted symmetry-broken density functional theory (DFT) calculations. All these demonstrated that the heptazethrene derivative HZ-TIPS has a closed-shell ground state while its octazethrene analogue OZ-TIPS with a smaller energy gap exists as an open-shell singlet biradical with a large measured biradical character (y = 0.56). Large two-photon absorption (TPA) cross sections (σ((2))) were determined for HZ-TIPS (σ((2))(max) = 920 GM at 1250 nm) and OZ-TIPS (σ((2))(max) = 1200 GM at 1250 nm). In addition, HZ-TIPS and OZ-TIPS show a closely stacked 1D polymer chain in single crystals.     

The para‐didehydropyridine,para‐didehydropyridinium,and related biradicals—a contribution to the chemistry of enediyne antitumor drugs

Structure and stability of seven singlet (S) biradicals formed by Bergman cyclization from enediynes are investigated with unrestricted DFT using B3LYP/6‐31G(d,p) and B3LYP/6‐311+G(3df,3pd). The corresponding triplets (T) are also calculated and compared with their S states utilizing the on‐top pair density and the S‐T difference on‐top pair density. A relationship between the geometry of a S biradical, its stability, and its biradical character is established using the on‐top pair density and calculated S‐T splittings. Through‐bond coupling between the single electrons of the S biradical can be enhanced by the incorporation of a N atom into para‐didehydrobenzene 1 due to lowering of antibonding orbitals, shortening of ring bonds by anomeric effect, and increased overlap between the interacting orbitals. Strong through‐bond interactions lead to a stabilization of the S state and an increase of the S‐T splitting. Because through‐bond interactions also determine the degree of coupling between the single electrons, stabilization of the S biradical, and an increase of the S‐T splitting always means a lowering of the biradical character and the H abstraction ability, which is relevant for the use of N‐containing enediynes and their biradicals in connection with the design of new antitumor drugs. The S para‐didehydropyridine biradical 2 is strongly stabilized and, therefore, has only reduced biradical character. However, the latter can be enhanced by protonation, because this always leads to a lengthening of ring bonds and a reduction of the overlap between interacting orbitals. In the weakly acidic medium of a tumor cell, S biradicals containing an amidine group can be protonated to yield S biradicals with high biradical character (low S‐T splittings, small changes in bond alternation relative to the T state), which will abstract H atoms from the DNA of a tumor cell. © 2000 John Wiley & Sons, Inc. J Comput Chem 22: 216–229, 2001     

J(F,H), J(C,H) and J(H,H) couplings involving the individual methyl group protons in 1,2,3,4-tetrachloro-5,6,7,8-tetrafluoro-9-methyltriptycene. Evidence of blue-shifting hydrogen bond

1,2,3,4-tetrachloro-5,6,7,8-tetrafluoro-9-methyltriptycene was studied in NMR spectra at low temperatures where the methyl group dynamics is frozen. Values of 5J(19F,1H), 1J(13C,1H), and 2J(1H,1H) for the individual methyl protons were measured. They are in a fair agreement with the corresponding theoretical values calculated at a density functional theory (DFT) level. The 5J(19F,1H) couplings involve the peri-F nucleus and occur via the 'through space' mechanism. Both the natural bond orbital analysis (at a HF level) and the observed pattern of 1J(13C,1H) coupling values corroborate occurrence in this molecule of intramolecular, blue-shifting hydrogen bonds engaging the methyl hydrogens. The 'through space' 5J(19F,1H) couplings may indicate the routes of electron density transfers that escape detection by the natural bond analysis. A consideration of these effects can enrich the chemical intuition involving this specific sort of H-bonds.     

Synthesis, structure and conformation of partially-modified retro- and retro-inverso psi[NHCH(CF3)]Gly peptides

Partially modified retro- (PMR) and retro-inverso (PMRI) psi[NHCH(CF(3))]Gly peptides, a conceptually new class of peptidomimetics, have been synthesized in wide structural diversity and variable length by aza-Michael reaction of enantiomerically pure alpha-amino esters and peptides with enantiomerically and geometrically pure N-4,4,4-trifluorocrotonoyl-oxazolidin-2-ones. The factors underlying the observed moderate to good diastereocontrol have been investigated. The conformations of model PMR-psi[NHCH(CF(3))]Gly tripeptides have been studied in solution by (1)H NMR spectroscopy supported by MD calculations, as well as in the solid-state by X-ray diffraction. Remarkable stability of turn-like conformations, comparable to that of parent malonyl-based retropeptides, was evidenced, as a likely consequence of two main factors: 1) severe torsional restrictions about sp(3) bonds in the [CO-CH(2)-CH(CF(3))-NH-CH(R)-CO] module, which is biased by the stereoelectronically demanding CF(3) group and the R side chain; 2) formation of nine-membered intramolecularly hydrogen-bonded rings, which have been clearly detected both in CHCl(3) solution and in some crystal structures. The former factor seems to be more important, as turn-like conformations were found in the solid-state even in the absence of intramolecular hydrogen bonding. The relative configuration of the -C*H(CF(3))NHC*H(R)- stereogenic centers has a major effect on the stability of the turn-like conformation, which seems to require a syn stereochemistry. X-ray diffraction and ab initio computational studies showed that the [-CH(CF(3))NH-] group can be seen as a sort of hybrid between a peptide bond mimic and a proteolytic transition state analogue, as it combines some of the properties of a peptidyl -CONH- group (low NH basicity, CH(CF(3))-NH-CH backbone angle close to 120 degrees, C-CF(3) bond substantially isopolar with the C=O) with some others of the tetrahedral intermediate [-C(OX)(O(-))NH-] involved in the protease-mediated hydrolysis reaction of a peptide bond (high electron density on the CF(3) group, tetrahedral backbone carbon).     

The direct detection of a hydrogen bond in the solid state by NMR through the observation of a hydrogen-bond mediated (15)N [bond] (15)N J coupling

A new method for detecting hydrogen bonds in the solid state is presented. Using two-dimensional NMR correlation experiments, it is shown that a hydrogen-bond mediated J coupling can be observed in a powder under magic-angle spinning conditions, even though the J coupling is 2 orders of magnitude smaller than the dominant anisotropic interactions encountered in solid-state NMR. Specifically, the observation of a pair of peaks in a two-dimensional 15N-15N solid-state INADEQUATE experiment due to two nitrogens that have no covalent connectivity is attributed to the presence of a J coupling across a linking hydrogen bond.     

Singlet-triplet gap in triplet ground-state biradicals is modulated by substituent effects

Three S = 1 bis(semiquinone) complexes have been prepared. To ensure ferromagnetic intramolecular exchange coupling, the two semiquinones are attached 1,3 to a 5-substituted phenylene ring. The biradical complexes differ in their meta-substituents: 1-NMe(2)(), X = N,N-dimethylamino; 1-t-Bu, X = tert-butyl; 1-NO(2)(), X = nitro. All three structures have been determined by X-ray crystallography. Results of structural studies indicate that the biradical ligands of all three complexes have nearly identical conformations with average semiquinone ring torsions of 32 degrees +/- 2 degrees relative to the 5-substituted phenylene ring. The exchange parameter, J (Eta = -2JS(1).S(2)), ranges from +31.0 +/- 0.6 cm(-)(1) for 1-NO(2)() to +59.3 +/- 1.2 cm(-)(1) for 1-t-Bu, with J = +34.9 +/- 0.7 cm(-)(1) for 1-NMe(2)(). Since the conformations are nearly identical, the differences in exchange coupling parameter J are due to substituent effects. The experimental results are supported by Hückel theory arguments and previous computational work.     

Reaction mechanism in crystalline solids: kinetics and conformational dynamics of the Norrish type II biradicals from α-adamantyl-p-methoxyacetophenone

In an effort to determine the details of the solid-state reaction mechanism and diastereoselectivity in the Norrish type II and Yang cyclization of crystalline α-adamantyl-p-methoxyacetophenone, we determined its solid-state quantum yields and transient kinetics using nanocrystalline suspensions. The transient spectroscopy measurements were complemented with solid-state NMR spectroscopy spin-lattice relaxation experiments using isotopically labeled samples and with the analysis of variable-temperature anisotropic displacement parameters from single-crystal X-ray diffraction to determine the rate of interconversion of biradical conformers by rotation of the globular adamantyl group. Our experimental findings include a solid-state quantum yield for reaction that is 3 times greater than that in solution, a Norrish type II hydrogen-transfer reaction that is about 8 times faster in crystals than in solution, and a biradical decay that occurs on the same time scale as conformational exchange, which helps to explain the diastereoselectivity observed in the solid state.     

On the solid state structure of 4-iodobenzoic acid

The solid-state structure of 4-iodobenzoic acid has been confirmed by variable temperature X-ray diffraction, variable temperature solid-state NMR and differential scanning calorimetry. 4-iodobenzoic acid crystallizes in the space group P2(1)/n, and dimerizes in the solid state about a center of inversion. Using extensive X-ray crystallographic data collections, the placement of the carboxylate H atoms from the residual electron density in difference Fourier maps was determined. The position of the electron density associated with the proton is found to vary with temperature in that the population of the disordered sites changes with varying temperature. Determination of the crystal structure between the temperatures of 248 and 198 K was not possible due to a phase transition, an endothermic event occurring at 230.77 K. The phase transition is also indicated by a change in the relaxation time of the ring carbon atoms in the solid-state NMR data. Though the dominating force in the dimeric unit in the solid state is the presence of strong hydrogen bonds, there are also van der Waals forces present between the iodine atoms. In the layered structure, the iodine-iodine distance is within the van der Waals contact radii, an interaction which causes a deformation in the electron density of the iodine atoms.