Intermolecular Exchange Interactions Research Articles

Evaluation of the anionic effect on the formation of biologically active {CuII-phenx; x = 1, 2, 3} fragments - Synthetic and structural variations, antimycobacterial and antiblastoma effects

Two cationic and molecular copper(II) complexes - mixed ligand [{Cu(phen)2benz}{Cu(phen)2Cl}]2+·2Cl-·benz-·H3O+·4·.5H2O (1), [Cu(benz)2phen] (2) (benz- = m-Cl-benzoate anion; phen = 1,10-phenantroline) and [Cu(phen)3]2+·2OTf-·2EtOH (3) (OTf = trifluoromethylsulfonate, triflate anion, CF3SO3-) were synthesized using various synthetic approaches and thoroughly characterized by spectroscopic methods and single crystal X-ray diffraction analysis. The structures of the complexes according to X-ray diffraction data are characterized by a diverse geometric environment of the complexing agent - the cationic form of 1 comprises two structurally nonequivalent moieties: the Cu (1) and Cu (2) complex-forming atoms coordinate, in chelate manner, two phen moieties each and the benz- (1)/Cl- (2) anions, respectively, thus forming different-ligand formula units, a trigonal–bipyramidal {Cu(1)N4Cl} and a pseudo octahedral one {Cu(2)N4O2}. The copper(II) ion in the complex 2 coordinates one phen ligand in a bidentate mode and two benz- anions to produce a distorted planar square environment {CuN2O2}. In complex 3, the copper(II) cation is surrounded by three phenanthroline fragments {Cu(phen)3}2+ and non-coordinated OTf- ions. The results of an EPR study 1 show that in the solid phase, intermolecular exchange interactions that cause exchange narrowing of the spectrum are efficient, whereas in solutions, resolved spectra of copper(II) ion with rhombic symmetry of the g-tensor are observed. An in vitro study of the biological properties of 1–3 toward nonpathogenic mycobacterial strain Mycolicibacterium smegmatis showed an increase in biological effectiveness depending on the structure of complexes (MIC, 1 (2 nmol/disk) > 3 (5 nmol/disk) > 2 (12.5 nmol/disk)). Complex 1 was also tested for antitumor activity against an ovarian adenocarcinoma SKOV3 demonstrated high efficiency: the IC50 value is almost 9 times higher than the activity of cisplatin.

Five-Spin Copper(II) Nitroxide Complex with Apparently Compressed Octahedral Geometry: Design, Synthesis, and Magnetostructural Studies

Multispin Cu(II)-nitroxide compounds have attracted significant interest in the field of molecular magnetism; however, complexes involving more than three spins in a single molecule have rarely been reported. In this work we describe the synthesis and detailed physicochemical study of the solvated six-coordinated five-spin complex [CuL2] (L = 2,2′-(1H-pyrrole-2,5-diyl)bis(4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl)). This complex features a tridentate meridional coordination of the biradical L, resulting in an exchange-coupled cluster of four radical and one Cu(II) spin. The structural and magnetic properties of [CuL2] have been thoroughly investigated using single-crystal X-ray diffraction (XRD), SQUID magnetometry, electron paramagnetic resonance (EPR), Fourier-transform infrared (FTIR) spectroscopy, and quantum chemical calculations (QCC). An apparently compressed octahedral coordination geometry has been found by XRD, which, however, stems from the superposition of two isomeric structures of elongated octahedral CuN2O4 units. Neither X-ray nor FTIR has revealed any structural transitions in [CuL2] at 8–300 K, whereas the magnetic behavior studied by magnetometry and QCC is governed by temperature-dependent populations of high-spin multiplets. EPR spectroscopy revealed a drastic temperature dependence of the line width in [CuL2], which changes by almost 2 orders of magnitude within 4–300 K. A comparison with EPR and QCC data for close-structure four- and three-spin analogues has revealed an interplay of the temperature-dependent population of high-spin multiplets and intermolecular exchange interactions, which govern the line width in such systems.

Heterometallic {Fe18M6} (M = Y, Gd, Dy) Pivalate Wheels Display Solvent-Induced Polymorphism

A new series of heterometallic wheels, isolated as [Fe18M6(piv)12(Htea)18(tea)6(N3)6]·n(solvent) (MIII = Y (1a, 1b), Gd (2a, 2b), and Dy (3a, 3b); Hpiv = pivalic acid, H3tea = triethanolamine), forms by the reaction of trinuclear μ3-oxo-linked or hexanuclear μ-OH-linked Fe(III) pivalate clusters with rare earth nitrates, H3tea, and azide ligands in MeOH/MeCN or EtOH/MeCN media under ultrasonic irradiation. Single-crystal X-ray diffraction showed that wheels 1a–3a prepared from MeOH/MeCN solutions crystallize in the triclinic space group P1̅ and have Ci symmetry, whereas wheels 1b–3b received from EtOH/MeCN solution crystallize in the trigonal space group R3̅ and have C3i symmetry. Magnetic studies reveal medium antiferromagnetic exchange interactions within the Fe3 trimeric unit (with the exchange coupling parameters of JFe3 = −13.1 cm–1 for 1a, JFe3 = −11.6 cm–1 for 1b) and weak intermolecular antiferromagnetic exchange interactions (λmf = −0.366 mol cm–3 and −0.368 mol cm–3 (zJmf = −0.19 cm–1), respectively). This leads to spin ground states of S = 5/2 for each {Fe3} unit. Substitution of diamagnetic Y(III) centers by paramagnetic Ln(III) centers (in 2a/2b by Gd(III) and in 3a/3b by Dy(III) centers) results in ferromagnetic exchange interactions between the Fe(III) and Ln(III) centers in addition to the predominant antiferromagnetic Fe···Fe interactions in 1a/1b.

The contribution of intermolecular spin interactions to the London dispersion forces between chiral molecules.

Dispersion interactions are one of the components of van der Waals (vdW) forces that play a key role in the understanding of intermolecular interactions in many physical, chemical, and biological processes. The theory of dispersion forces was developed by London in the early years of quantum mechanics. However, it was only in the 1960s that it was recognized that for molecules lacking an inversion center, such as chiral and helical molecules, there are chirality-sensitive corrections to the dispersion forces proportional to the rotatory power known from the theory of circular dichroism and with the same distance scaling law R-6 as the London energy. The discovery of the chirality-induced spin selectivity effect in recent years has led to an additional twist in the study of chiral molecular systems, showing a close relation between spin and molecular geometry. Motivated by it, we propose in this investigation to describe the mutual induction of charge and spin-density fluctuations in a pair A-B of chiral molecules by a simple physical model. The model assumes that the same fluctuating electric fields responsible for vdW forces can induce a magnetic response via a Rashba-like term so that a spin-orbit field acting on molecule B is generated by the electric field arising from charge density fluctuations in molecule A (and vice versa). Within a second-order perturbative approach, these contributions manifest as an effective intermolecular exchange interaction. Although expected to be weaker than the standard London forces, these interactions display the same R-6 distance scaling.

Halo‐Substituted Blatter Radicals and Their Role in Modulating Magnetic Interaction in Metal Complexes: A Combined Experimental and Theoretical Study

AbstractHerein, we report the synthesis, structure, and magnetic properties of three new halo‐substituted benzotriazinyl radicals (Rad3a‐3c where a=Cl, b=Br, c=I) and two metal complexes [M(hfac)2(Rad3c)⋅CHCl3](M=Cu(II) (1), Zn(II) (2) and hfac: hexafluoroacetylacetonate) as representative examples. Structural analyses disclose that all the radicals are isostructural. Compared to the reported benzotriazinyl radicals, the N4 atom of pyridine flips opposite to the spin bearing (N1) centre, and it is stabilized by secondary C−H⋅⋅⋅N interaction. The π⋅⋅⋅π stacking interaction between the spin bearing centre and pyridyl ring decreases as the size of the halogen increases from ‐Cl to ‐I. The stacking distance is substantially longer than the other coordinating benzotriazinyl radicals reported so far. The dc magnetic studies reveal the presence of strong intermolecular antiferromagnetic exchange interaction between the radical dimers, which decreases in the order of Rad3a→Rad3b→Rad3c. Complex 1 shows a strong intramolecular through‐bond metal‐radical ferromagnetic interaction, whereas a weak intermolecular through‐space radical⋅⋅⋅radical antiferromagnetic exchange interaction is observed in complex 2. Furthermore, the broken symmetry (BS)‐DFT calculations were performed to get more insight into the exchange interaction for all the radicals and metal complexes.

Dynamic spin polarization in organic semiconductors with intermolecular exchange interaction

It is shown that in organic semiconductors where organic magnetoresistance (OMAR) is observed, the exchange interaction between electrons and holes localized at different molecules leads to dynamic spin polarization in the direction of the applied magnetic field. The polarization appears even at room temperature due to the non-equilibrium conditions. The strong spin polarization requires exchange energy to be comparable with Zeeman energy in the external field and be larger or comparable with the energy of hyperfine interaction of electron and nuclear spins. The exchange interaction also modifies the lineshape of OMAR.

A high-frequency EPR study of magnetic anisotropy and intermolecular interactions of Co(II) ions

Changing the anion in a crystal lattice induces distinct structural distortions in the [CoS4]2+ core for a family of complexes with the general molecular formula [Co(L1)4]X2, where L1 = thiourea (NH2CSNH2) and X = I (1), Br (2), and [Co(L1)4](SiF6) (3). The magnetic anisotropy (D) for 1–3 was quantitatively determined with a magnitude of −153(2), −168(5) and < -400 GHz, respectively, by HF-EPR investigation. Also, intermolecular exchange interactions were determined experimentally, whereby an antiferromagnetic exchange for 1 (-5.5 GHz) and 2 (-4.1 GHz), and a ferromagnetic interaction for 3 (+3.5 GHz) are witnessed (based on H = -JS1S2). The exchange interactions, computed using DFT methods on a model complex of 1, disclose the presence of an antiferromagnetic exchange interaction, consistent with the experimental observation. Overall, the present study provides convincing experimental evidence for the sizable influence not only of the first but also of the second coordination sphere on the magnetic anisotropy and exchange interactions of Co(II) ions.

Nickel(II) complexes with 2-Hfur and N-donors: The magnetic effects of the structural variations, thermal properties and antimycobacterial activity against Mycolicibacterium smegmatis

New complexes of nickel(II) with 2-furoic acid (2-Hfur) and different N-donor ligands of the composition [Ni(fur)2(phen)(H2O)2]·H2O (1) (phen = 1,10-phenantroline), [Ni(fur)2(2,2′-bpy)] (2, 2,2′-bpy = 2,2′-bipyridine), [Ni(fur)2(4,4′-bpy)(CH3OH)2]n (3, 4,4′-bpy = 4,4′-bipyridine), [Ni(fur)2(pz)4]·2MeCN (4) (pz = pyrazole) have been synthesized. The structure of the compounds was established by single crystal X-ray diffraction analysis. Complexes 1, 2, and 4 are mononuclear, complex 3 is a 1D coordination polymer. The DC magnetic susceptibility data measured for 2 and 3 are indicative of antiferromagnetic exchange interactions between the metal centers, which proved to be negligibly weak in 2 and more pronounced in 3. A quantum-chemical analysis of intermolecular (for 2) and intra- and interchain (for 3) magnetic exchange pathways confirms these findings, revealing the presence of magnetic chains formed by a network of very weak intermolecular antiferromagnetic exchange interactions in the crystal of 2 and a strengthening of antiferromagnetic exchange coupling in the polymeric chains of 3. Thermal behavior of 1 and 2 was studied by simultaneous thermal analysis (STA). According to PXRD data, the final thermolysis product of complex 2 is metal Ni nanocrystals (d = 14.7(3) nm). Analysis of the results of biological activity of 1–4in vitro against Mycolicibacterium smegmatis showed a low sensitivity of the mycobacterial wall to Ni2+ cations.

A novel paramagnetic coordination polymer, fabricated from Co(NCS)2 and 2-pyridinecarbaldehyde isonicotinoylhydrazone

We report on a mixed cobalt(II)/cobalt(III) heteroleptic coordination polymer of the salt-like structure {[(CoIIIL2)2CoII(NCS)2][CoII(NCS)4]}n∙a(solvent) (1∙a(solvent)), obtained by a reaction of Co(NCS)2 with 2-pyridinecarbaldehyde isonicotinoylhydrazone (HL). Complex 1∙a(solvent) contains 15.52% of solvent molecules, which fits to six molecules of EtOH per one [(CoIIIL2)2CoII(NCS)2][CoII(NCS)4] species, and the structure of the crystals can be assigned to the {[(CoIIIL2)2CoII(NCS)2][CoII(NCS)4]}n∙6EtOH composition. 1∙a(solvent) was found to be stable after drying at 80 °C for several days, yielding a solvent-free complex 1. The FTIR data of both structures supports the complete loss of solvents after drying, while the general framework remains intact. As evident from the comparison of the experimental X-ray powder diffraction data of 1 with the calculated powder pattern generated from the single crystal X-ray data of 1∙a(solvent), the former complex partially loses its crystallinity; however, the experimental powder pattern resembles the calculated one. Thus, the overall structure of 1 is similar to that of 1∙a(solvent). The crystal structure of 1∙a(solvent) is constructed from the V-shaped (CoIIIL2)+ cations, where the metal ion is N4S2-coordinated by two L. These (CoIIIL2)+ cations are interlinked through CoII(NCS)2 molecules by coordination via free 4-pyridyl nitrogen atoms, yielding a 1D polymeric cationic chain {[(CoIIIL2)2CoII(NCS)2]2+}n, which are interlinked into a 3D framework through a bunch of bifurcated (N)C–S⋯π(2-Py) and linear (NN)C–H⋯O non-covalent interactions. Due to such 3D aggregation, a half of rhombohedral cells are filled by the interlayered [CoII(NCS)4]2– anions, while the other half of cells is free from non-volatile species and filled with volatile solvent molecules, yielding 1D tubular voids, which occupy about 17% of unit cell volume. Direct current variable-temperature magnetic susceptibility measurements on a polycrystalline sample of 1∙a(solvent) were carried out in the temperature range 1.8–300 K. The magnetic behaviour of 1∙a(solvent) indicates the presence of magnetic anisotropy of the CoII ions and/or very weak intermolecular exchange interactions. The surface and pore properties of 1 and HL were calculated by N2 adsorption at 77 K. In addition, CO2 adsorption properties of 1 and HL at different temperatures were determined. It was found that 1 increased both textural and CO2 adsorption properties.

Single crystal growth, structural characterization and magnetic properties study of an antiferromagnetic trinuclear iron(III) acetate complex with uncoordinated hexamine

Structural characterization of novel Fe(III) basic acetate with 1,3,5,7-tetraazaadamantane (hexamine – HEX), [Fe3(μ3-O)(OAc)6(H2O)3)](NO3)(HEX)2(H2O)5] (1), revealed a molecular structure typical for Fe(III) basic carboxylates, with a triangular array formed by three iron atoms being equilateral and with an uncoordinated hexamine molecule and nitrate anion present in the crystal structure. Despite the hexamine molecule being uncoordinated, it plays a crucial role in the formation of a supramolecular network. The magnetic properties of the compound were characterized by magnetic susceptibility, Mössbauer and EPR spectroscopy. These studies indicate the presence of high-spin iron ions in +3 oxidation state with non-Curie-like behaviour. A small room temperature value of the effective magnetic moment and a decrease of the product χT with decreasing temperature both speak in favour of antiferromagnetic interactions between the three individual Fe(III) ions of complex 1 with a total ground state spin of St = 1/2 per trinuclear cluster. EPR spectroscopy confirmed the formation of a St = 1/2 ground state, but also suggested the presence of weak intermolecular exchange interactions between two (or more) clusters of neighbouring molecules. The temperature-dependent Mössbauer spectra show a slowing-down of the fast spin-lattice relaxation rates of the paramagnetic Fe(III) ion below about 60 K, however, yet no static spin regime is reached down to 3.6 K.

Influence of the Increasing Number of Organic Radicals on the Structural, Magnetic, and Electrochemical Properties of the Copper(II)–Dioxothiadiazole Family of Complexes

The preparation, structures, and electrochemical and magnetic properties supported by density functional theory (DFT) calculations of three new copper(II) compounds with [1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 1,1-dioxide (td) and its radical anion (td·–) are reported: {[CuIICl(td)](μ-Cl)2[CuIICl(td)]} (1), which incorporates only neutral td ligands; [CuIICl(td·–)(td)]·2MeCN (2), which comprises one neutral td and one radical td·–; and PPN[CuIICl(td·–)2]·2DMA (3), where CuII ions are coordinated by two radical anions td·– (DMA, dimethylacetamide; PPN+, the bis(triphenylphosphine)iminium cation). All three compounds show interesting paramagnetic behavior with low-temperature features indicating significant antiferromagnetic coupling. The magnetic properties of 1 are dominated by CuII···CuII interactions (JCuCu) mediated through the Cl– bridges, while the magnetic properties of 2 and 3 are governed mainly by the td·–···td·– (Jtdtd) and CuII–td·– (JCutd) exchange interactions. The structure of 2 features only two major magnetic coupling pathways enabling the fitting of experimental data with Jtdtd = −36.0(5) cm–1 and JCutd = −12.6(2) cm–1 only. Compound 3 exhibits a complex network of magnetic contacts. Attempt to approximate its magnetic behavior using only a local magnetic contacts model resulted in Jtdtd = −5.6(1) cm–1 and two JCutd constants, −12.4(2) and −22.6(4) cm–1. The experimental fitting is critically compared with the results of broken symmetry density functional theory (BS DFT) calculations for inter- and intramolecular contacts. More consistent results were obtained with the M06 functional as opposed to popular B3LYP, which encountered problems reproducing some of the experimental intermolecular exchange interactions. Electrochemical measurements of 2 and 3 in MeCN showed three reversible nearly overlapping redox peaks appearing in a narrow potential range of −600 to −100 mV vs Fc/Fc+. Small differences between the redox events suggest that such compounds may be good candidates for new switchable materials, where the electron transfer between the metal and the ligand center is triggered by temperature, pressure, or light (valence tautomerism).

Synthesis of Nitroxide Diradical Using a New Approach.

A new synthetic pathway to diradical organic systems is proposed. The effectiveness of this approach was exemplified by the synthesis of a new nitroxide diradical. An interaction of perfluorobiphenyl with lithium tert-butylamide, followed by oxidation of the thusly formed N4,N4′-di-tert-butyl-2,2′,3,3′,5,5′,6,6′-octafluorobiphenyl-4,4′-diamine with meta-chloroperoxybenzoic acid, led to the polyfluorinated nitroxide diradical, N,N′-(perfluorobiphenyl-4,4′-diyl)bis(N-tert-butyl(oxyl)amine), with a good total yield. The polyfluorinated diradical is stable and can be isolated in free form and completely characterized. The structure of the nitroxide diradical was proved by single-crystal X-ray diffraction analysis. According to the X-ray diffraction data, the diradical is considerably twisted: dihedral angles between the planes of the nitroxide groups and aromatic cycles are 65.1° and 69.5°, and between aromatic cycles 52.6°. Quantum chemical calculations predict well-balanced size of both intramolecular and intermolecular exchange interactions with J from −2.65 to −1.14 cm−1.

A Weakly Antiferromagnetically Coupled Biradical Combining Verdazyl with Nitronylnitroxide Units.

An antiferromagnetically (AFM) coupled biradical based on oxoverdazyl and nitronylnitroxide was synthesized in 46 % yield using Sonogashira coupling. The obtained heterobiradical evidenced distinct properties of both radical entities in solution. Depending on the solvent, the prepared biradical crystallized in two different forms. SQUID magnetization measurements on Form II showed coupling constants JintraII /kB =-2.1 K and zJinterII /kB =-11.5 K. Consequently, total intermolecular exchange interactions are five times larger than the intramolecular ones. Further, DFT calculations explained this phenomenon and indicated the advantage of Form I for further in-depth investigations.

Tuning the Magnetic Exchange Interactions in Organic Biradical Networks

An overview of organic biradicals directed towards triplon excitations in higher ordered crystalline states is provided. The topic started from the idea to control the intramolecular spin exchange interaction in a sizable but not too large extent, in order to provide also decent intermolecular spin interactions in a crystalline lattice. The predictability of exchange couplings through DFT calculations of optimized geometries turns out to be limited, such that the molecules need to be synthesized and crystallized for further in‐depth characterizations. For crystal structure geometries, most often then a good agreement of calculated intra‐ and intermolecular exchange interactions with those later determined experimentally is found. Finally, it is demonstrated that by choosing well balanced size of both interactions a field‐induced higher ordered magnetic state becomes accessible.

Controlled Molecular Magnetism of Bi- and Polynuclear Transition Metal Complexes Based on Hydrazones, Azomethines, and Their Analogs

The possibilities of the magnetochemical method for the description of structures and properties of bi- and tetranuclear metallochelates and supramolecular architectures based on coordination compounds with restricted types of ligand systems, mainly hydrazones, azomethines, and their analogs, are reviewed. The known published data for the bi- and polynuclear complexes, whose paramagnetic centers are bound by both intra- and intermolecular exchange interactions, are systematized. A relationship between specific features of the electronic and geometric structures of the complexes and the character of the exchange effects is considered. Magnetostructural relations in the discussed compounds are systematized. The compounds discussed are important model objects for the development of the strategy for the targeted design of one-, two-, and three-dimensional magnetically ordered structures.

Probing through-space and through-bond magnetic exchange couplings in a new benzotriazinyl radical and its metal complexes.

The synthesis and characterization of a new chelating benzotriazinyl radical (Rad2) are described. Crystallographic studies coupled with SQUID magnetometry on Rad2 reveal the presence of discrete radical pairs which are antiferromagnetically coupled. The reaction of Rad2 with the 3d transition metal complexes M(hfac)2·xH2O (hfac- = hexafluoroacetylacetonate) led to mononuclear metal complexes of general formula M(hfac)2(Rad2) [M = Zn(ii) (1); Ni(ii) (2) and Co(ii) (3)] whose structures have been determined by single crystal X-ray diffraction. Compounds 1-3 are isostructural and crystallize in the monoclinic space group P21/n with two molecules in the asymmetric unit. In the case of the Zn(ii) complex (1) through-space intermolecular radicalradical antiferromagnetic exchange interactions viaπ*π* contacts are observed, whereas strong intramolecular through-bond metal-radical ferromagnetic interactions [J = +59.3(9) cm-1] are observed for the Ni(ii) complex (2). For the Co(ii) complex (3), computational and magnetic studies reveal substantial zero field splitting and ferromagnetic metal-radical interactions.

Crystallographic Insights into the Synthesis and Magnetic Properties of Oxoverdazyl Radicals Functionalized by Benzoic Acid

The synthesis and crystallization of two verdazyl radicals, 1,5‐dimethyl‐3‐(4′‐carboxyphenyl)‐6‐oxoverdazyl HIMe and 1,5‐diisopropyl‐3‐(4′‐carboxyphenyl)‐6‐oxoverdazyl HIiPr, are described. The electrochemical studies reveal that the oxidation of the two radicals is reversible, whereas their reduction is irreversible. The EPR spectrum of both radicals essentially exhibits a nine‐line pattern related to the mean hyperfine interaction of the unpaired electron with the nitrogen atoms of the verdazyl cycle. The single‐crystal X‐ray diffraction of the intermediates towards HIiPr allows a fine description of the cyanoborane adduct, which is the key intermediate of this synthesis. The verdazyl radicals themselves are obtained as single crystals. In the case of HIiPr, depending on the solvent, two polymorphs are crystallized. The structure resolution reveals that, in the solid state, the organization of the verdazyl radicals is governed by both H‐bonding and π–π interactions and is reminiscent of the H‐bonded structures that can be present in solution. Within the 1D π stacks observed in the three compounds, the verdazyl–verdazyl distance varies from 4.88 Å in HIMe to 7.90 Å in HaIiPr. This modulation of the distance strongly influences the antiferromagnetic intermolecular exchange interaction between π‐stacked radicals, which goes from J = –90 cm–1 (H = –JΣSiSi+1) for HIMe to –12.96(3) cm–1 for HbIiPr and –0.92 cm–1 for HaIiPr.

Line-shape theory of the X3Σg-→a1Δg,b1Σg+ transitions in O2-O2 collision-induced absorption.

We derive the theory of collision-induced absorption for electronic transitions in the approximation of an isotropic interaction potential. We apply this theory to the spin-forbidden X3Σg-→a1Δg and X3Σg-→b1Σg+ transitions in O2-O2, which are relevant for calibration in atmospheric studies. We consider two mechanisms for breaking the spin symmetry, either by the intermolecular exchange interaction between paramagnetic collision partners or by the intramolecular spin-orbit coupling. The calculations for the exchange-based mechanism employ the diabatic potential energy surfaces and transition dipole moment surfaces reported in Paper I [T. Karman et al., J. Chem. Phys. 147, 084306 (2017)]. We show that the line shape of the theoretical absorption spectra is insensitive to the large uncertainty in the electronic transition dipole moment surfaces. We also perform calculations using a simple model of the alternative mechanism involving intramolecular spin-orbit coupling, which leads to absorption intensities which are well below the experimental results. The relative intensity of this spin-orbit-based mechanism may impact the relative contribution to the absorption by collisions with diamagnetic collision partners, such as the atmospherically relevant N2 molecule. We furthermore show that both the line shape and temperature dependence are signatures of the underlying transition mechanism.

Salicylic aldehyde and 2-N-tosylaminobenzaldehyde tetrazolyl hydrazones and their complexes

Tetrazolyl hydrazones of salicylic aldehyde and 2-N-tosylaminobenzaldehyde as well as zinc(II) and copper(II) complexes on their base have been prepared. Quantum-chemical simulation of tautomerism of the hydrazones in vacuum and in the solutions has been performed. IR and NMR spectroscopy and magnetochemistry studies have revealed that the complexes are mononuclear; however, weak intermolecular exchange interaction of the antiferromagnetic type has been detected in the copper(II) complexes.

Direct visualization of surface phase of oxygen molecules physisorbed on Ag(111) surface: A two-dimensional quantum spin system

We report on the real-space observation of the two-dimensional distorted triangular lattice of physisorbed oxygen $({\mathrm{O}}_{2})$ molecules on an Ag(111) surface by low-temperature scanning tunneling microscopy. The physisorbed state of the ${\mathrm{O}}_{2}$ monolayers was confirmed by measuring their thermal stability, which showed good agreement with previous thermal desorption spectroscopy. The distortion of the observed lattice was reproduced quantitatively by considering the intermolecular exchange interaction in Monte Carlo calculations, indicating a critical role of antiferromagnetic ordering of ${\mathrm{O}}_{2}$ spins. In tunneling spectra, the Kondo resonance was not observed on the ${\mathrm{O}}_{2}$ layer at 4.7 K unlike the case of physisorbed ${\mathrm{O}}_{2}$ on Ag(110). These results indicate that an intrinsic $S=1$ spin of the ${\mathrm{O}}_{2}$ molecules was preserved to form a two-dimensional antiferromagnetic quantum spin system on the surface.