The first systematic study of π interactions between non-aromatic rings, based on the authors' own results from an experimental X-ray charge-density analysis assisted by quantum chemical ...calculations, is presented. The landmark (non-aromatic) examples include quinoid rings, planar radicals and metal-chelate rings. The results can be summarized as: (i) non-aromatic planar polyenic rings can be stacked, (ii) interactions are more pronounced between systems or rings with little or no π-electron delocalization (
quinones) than those involving delocalized systems (
aromatics), and (iii) the main component of the interaction is electrostatic/multipolar between closed-shell rings, whereas (iv) interactions between radicals involve a significant covalent contribution (multicentric bonding). Thus, stacking covers a wide range of interactions and energies, ranging from weak dispersion to unlocalized two-electron multicentric covalent bonding ('pancake bonding'), allowing a face-to-face stacking arrangement in some chemical species (quinone anions). The predominant interaction in a particular stacked system modulates the physical properties and defines a strategy for crystal engineering of functional materials.
The present review is aimed to compare crystal packing interactions contributing to stacking arrangements of primarily nonaromatic systems referring only briefly to classical aromatic stacking. The ...classical aromatic stacking is mainly based on weak dispersion interactions (E ≤ 1 kcal mol–1) whereas heteroaromatics reveal more electrostatic (or specifically dipolar) contributions (E = 5–10 kcal mol–1). Based mainly on our charge density studies and DFT calculations, the results show that (i) all planar rings stack, regardless of aromaticity (or delocalization of π electrons) and (ii) stacking interactions cover a wide continuum ranging from weak, mainly dispersion interactions (E < 5 kcal mol–1) to unlocalized two-electron multicentric (2e/mc) covalent bonds (“pancake bonds”, E > 15 kcal mol–1). Our recent studies showed that quinones form face-to-face stacks and the energies of interactions exceed 10 kcal mol–1; ours and other authors’ results indicate that interactions between planar radicals involve a significant contribution of covalent bonding. Thus, π-interactions cover a broad range of energies, ranging from ≤1 to ≥20 kcal mol–1, and the interactions span from weak dispersion to multicentric covalent bonding. Therefore, development of a universal model of stacking is needed. In this respect, stacking can be compared to hydrogen bonding, which also ranges between dispersion (weakest hydrogen bonds, such as C–H···S and C–H···Cl) and two-electron/three-centric covalent bonding (the strongest “symmetrical” hydrogen bonds).
This study demonstrates the reversible structural transformation, single-crystal-to-single-crystal, of the ammonium vanadyl (L-tartrate) complex salt from the hydrate phase to the anhydrous phase. ...The transformation can be initiated by stimuli, such as temperature, humidity, or vacuum conditions. The hydrate and anhydrous phases exhibit a tetragonal structure (P41212), with marked differences in hydrogen bonding due to the presence or absence of one water molecule per asymmetric unit. The intricate relationship between crystal packing and intermolecular interactions in the hydrate phase was investigated by crystallographic charge density analysis revealing, at the molecular level, the reasons for the observed 5 orders of magnitude higher proton conductivity of the hydrate phase compared to that of the anhydrous phase. To gain further insight into the processes occurring at the surfaces of grain boundaries and the proton transfer mechanisms in this system, rehydration of the complex salt was carried out by using D2O instead of H2O and monitored by in situ ATR-FTIR spectroscopy. The results highlight the critical role of interfacial water molecules in driving structural transformations and influencing proton conductivity.
Salts and ionic cocrystals simultaneously comprising N-oxide and carboxylic acid functional groups constitute a very fertile ground for the investigation of various proton transfer phenomena. This is ...because such compounds combine two types of proton transfer: that is, inter- and intramolecular hydrogen bonding in acid–base systems. To this end, a series of novel salts based on pyridine-2,5-dicarboxylic acid N-oxide (H2pydco) as an organic acid and 2,4,6-triamino-1,3,5-triazine (tata), 2-aminopyrimidine (2a-pym), 2-amino-6-methylpyridine (2a-6mpy), 1,10-phenanthroline (phen), and 9-aminoacridine (9a-acr) as organic bases have been synthesized and characterized by elemental analyses, infrared spectroscopy, and single-crystal X-ray diffraction: (Htata)+(Hpydco)− (1), (H2a-pym)+(Hpydco)− (2), (H2a-6mpy)+(Hpydco)− (3A,B), (Hphen)+(Hpydco)−(H2pydco) (4), and (H9a-acr)+(Hpydco)−·EtOH (5). The unit cells of 3A and 3B differ slightly; however, 3A crystallizes in a chiral orthorhombic space group P212121, while 3B crystallizes in the achiral space group P21/n. In both cases, the asymmetric unit comprises one cation and one anion. The influence of different organic cations on the packing of Hpydco– in the crystal lattice is studied. The most important feature of these crystals is the presence of extensive O–H···O, N–H···O, N–H···N, and C–H···O hydrogen bond networks, which form base-dependent supramolecular synthons: 1, 2, and 3A,B comprise an α-aminopyridinium moiety, and all involve the hydrogen-bonded motif R 2 2(8) with the Hpydco– anion. Compounds 4 and 5, which lack the α-aminopyridinium moiety, reveal different hydrogen-bonding patterns. The interaction energies of each individual hydrogen bond have been estimated using the quantum theory of “atoms-in-molecules”, which led us to the identification of the energetically favorable antielectrostatic N–H···N hydrogen bonds (stabilization energy of 4.0 kcal/mol) between positively charged melaminium species in 1. It has been also established that charge-assisted hydrogen bonding does not always offer an energetic advantage over “noncharged” hydrogen bonds. With the use of Hirshfeld surface (HS) analysis we have also explored the influence of the protonation state of pydco species on the composition of contact contributions, as well as established specific properties of their 2D fingerprint plots. Finally, a comment is provided on the applicability of HS analysis for the exploration of polymorphs featuring intramolecular proton transfer.
A novel heterometallic oxalate-based compound of the formula {Co(bpy)3Mn2(C2O4)3·H2O} n (1; bpy = 2,2′-bipyridine) was synthesized and characterized by elemental analysis, IR spectroscopy, ...single-crystal X-ray diffraction (XRD), and magnetization measurement. The molecular structure of 1 is made of a three-dimensional (3D) anionic network, Mn2(C2O4)3 n 2n–, and tris-chelated cations Co(bpy)32+ occupying the vacancies of the framework. Splitting between the zero-field-cooled (ZFC) and field-cooled (FC) branches of susceptibility below the small peak at 13 K indicates magnetic ordering. Compound 1 was used as a single-source precursor for the formation of the mixed-metal oxide CoMn2O4. This conversion via thermal decomposition was explored by thermal analysis (TGA and DTA), IR spectroscopy, powder XRD, and magnetic susceptibility measurement. From refined structural parameters, it could be seen that the spinel obtained by the thermal treatment of 1 at 800 °C is characterized by the inversion parameter δ = 21%, and therefore the structural formula at room temperature can be written as tetCo0.79Mn0.21octCo0.105Mn0.8952O4. The temperature dependence of magnetization for CoMn2O4 points to at least three magnetic phases: the ferrimagnetic state is observed below 83 K, and up to 180 K blocking of the magnetic moments of nanocrystallites of 31 nm appears, transforming to paramagnetic-like behavior above 180 K. Microstructural characterization of the CoMn2O4 sample was carried out by means of XRD line-broadening analysis.
Stereokemija na drugi način Molčanov, Krešimir
Kemija u industriji; časopis kemičara i tehnologa Jugoslavije,
2019, Letnik:
68, Številka:
1-2
Journal Article
Recenzirano
Odprti dostop
U ovome se članku prikazuje sustavan pristup stereokemiji te tumačenju njezina središnjeg pojma, kiralnosti, jednostavnijim načinom koji se može prilagoditi nastavi kroz eksperimente. Polazeći od ...predmetâ iz svakodnevne uporabe (cipele, zrcala, puževi, itd.), dolazi se do općeg opisa kiralnosti, iz kojega se onda izvodi molekulska kiralnost. Tetraedarski se ugljikov atom može smatrati specijalnim slučajem, koji se može izvesti naknadno.
Chirality is the central concept of stereochemistry, but remains neglected at both high school and
university levels. It is typically introduced through description of Pasteur’s historic resolution of
enantiomers of tartaric acid, and teaching is usually centred on correct assignation of configurations
R
and
S
. This “classical” approach rarely proceeds further, thus leaving serious gaps in the
students’ knowledge.
In this paper, new and systematic approach to the introduction of the concept of chirality, is
described. We start with every-day objects (shoes, mirrors, snails,
etc
.), from which a generalised
concept of chirality is introduced. Once this groundwork is laid, molecular chirality is deduced;
its commonly occurring special cases (such as
R
and
S
configurations of the tetrahedral carbon
atom) are discussed later. This method of teaching basic concepts of stereochemistry can be easily
adopted to learning through experiments.
Pancake bonding in dimers of semiquinoid radical cations in a novel compound, TMPD chloride co-crystal with 2,5-dichlorohydroquinone, is studied by a combination of X-ray charge density and quantum ...chemical computations. The rings in a pancake-bonded dimer are parallel with a transversal offset, the interplanar separation distance is 3.1191(1) Å, and the centroid distance is 3.2022(2) Å. AIM analysis of experimental charge density revealed four symmetry-independent bonding critical points and a cage critical point between the TMPD cations in a dimer, with a maximum electron density of 0.055 e Å–3. A highest occupied molecular orbital extends between both rings of a dimer. The estimated energy of the covalent component is −3.24 kcal mol–1.
Five charge transfer complexes of electron donor N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD or Wurster’s blue) with quinoid electron acceptors tetrafluoro- (F4Q), tetrachloro- (Cl4Q), ...tetrabromoquinone (Br4Q), 7,7,8,8-tetracyanoquinodimethane (TCNQ), and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) were prepared and studied by X-ray crystallography, impedance spectroscopy, and DFT computations. Mixed donor–acceptor π-stacks were observed in all compounds; in TMPD·F4TCNQ, the moieties formed 2D stacked arrays. All moieties had a partial radical character due to charge transfer. The presence of two-electron multicenter bonding (pancake bonding) was confirmed by quantum chemical computation. In TMPD·F4Q, alternating longer and shorter interplanar distances were consistent with formation of pancake-bonded dimers; in complexes of TMPD with Cl4Q, Br4Q, and TCNQ, the stacks were equidistant implying existence of pancake-bonded polymers.
A one-dimensional (1D) ladder-like coordination polymer {NH4{Cu(bpy)}2(C2O4)Fe(C2O4)3·H2O} n (1; bpy = 2,2′-bipyridine) containing Cu(bpy)(μ-C2O4)Cu(bpy)2+ cationic units linked by oxalate ...groups of Fe(C2O4)33– building blocks was investigated as a new type of photoactive solid-state system. It exhibits a photocoloration effect when exposed to direct sunlight or UV/vis irradiation. The photochromic properties and mechanism were studied by powder and single-crystal X-ray diffraction, UV/vis diffuse reflectance, IR and electron paramagnetic resonance spectroscopy, magnetization and impedance measurements, and density functional theory calculations. The process of photochromism involves simultaneous intramolecular electron transfers from the oxalate ligand to Fe(III) and to CuII(bpy)(μ-C2O4)CuII(bpy)2+, leading to the reduction of the metal centers to the electronic states Fe(II) and Cu(I), accompanied by the release of gaseous CO2.
Two heterometallic coordination polymers {NH(CH3)2(C2H5)8Mn4Cl4Cr4(C2O4)12} n (1) and {NH(CH3)-(C2H5)28Mn4Cl4Cr4(C2O4)12} n (2) were obtained by slow evaporation of an aqueous solution containing ...the building block A3Cr(C2O4)3 A = (CH3)2(C2H5)NH+ or (CH3)(C2H5)2NH+ and MnCl2·2H2O. The isostructural compounds comprise irregular two-dimensional (2D) oxalate-bridged anionic layers Mn4Cl4Cr4(C2O4)12 n 8n– with a Shubnikov plane net fes topology designated as (4·82), interleaved by the hydrogen-bonded templating cations (CH3)2(C2H5)NH+ (1) or (CH3)(C2H5)2NH+ (2). They exhibit remarkable humidity-sensing properties and very high proton conductivity at room temperature 1.60 × 10–3 (Ω·cm)−1 at 90% relative humidity (RH) of 1 and 9.6 × 10–4 (Ω·cm)−1 at 94% RH of 2. The layered structure facilitates the uptake of water molecules, which contributes to the enhancement of proton conductivity at high RH. The better proton transport observed in 1 compared to that in 2 can be tentatively attributed to the higher hydrophilicity of the cations (CH3)2(C2H5)NH+, which is closely related to their affinity for water molecules. The original topology of the anionic networks in both compounds leads to the development of interesting magnetic phases upon cooling. The magnetically ordered ground state can be described as the coupling of ferromagnetic spin chains in which Mn2+ and Cr3+ ions are bridged by bis(bidentate) oxalate groups into antiferromagnetic planes through monodentate-bidentate oxalate bridges in the layers, which are triggered to long-range order below temperature 4.45 K via weaker interlayer interactions.
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