Ground Singlet Electronic States Research Articles

Gas-Phase Reactivity of Quinoline-Based Singlet Oxenium Cations.

Isomeric quinolyloxenium cations were generated in the gas phase in an ion trap mass spectrometer to explore their reactions. The structures of some products were identified via collision-activated dissociation experiments involving model compounds to demonstrate that they have the expected heavy atom connectivity. The lack of radical reactions suggests that the cations have closed-shell singlet electronic ground states. Calculations (CASPT2/CASSCF(16,14)/cc-pVTZ//CASSCF(16,14)/cc-pVTZ) predict that their closed-shell singlet (1A') ground states are lower in energy by ca. 25 kcal mol-1 than their lowest-lying excited states. All cations are reactive toward dimethyl disulfide, dimethyl sulfide, and allyl iodide and most toward water and moderately reactive toward cyclohexane, reflecting their strongly electrophilic nature. They form adducts with nucleophiles in exothermic reactions (ca. 50 kcal mol-1 for dimethyl sulfide) that can fragment or be stabilized via IR emission. Most water adducts spontaneously isomerize to lower-energy tautomers. The nucleophiles preferentially add to those carbon atoms in the benzene ring that have the greatest positive charge (but not the carbonyl carbon). The cations react with cyclohexane via hydride abstraction by the oxygen atom. This is the only reaction that initially involves the oxygen atom and hence reflects the formally positively charged, monovalent oxygen atom in these cations.

Full Dynamical and Ab Initio Investigation of the Electron Transfer-Mediated Decay Mechanism of He+ in the Presence of Heavier Alkali Dimers.

We have studied the electron transfer-mediated decay (ETMD) process for the 1s ionized state of the He atom in the presence of a heavier alkali homonuclear dimer (Na2, K2, and Rb2) as well as heteronuclear dimer (LiNa, NaK, and KRb). In our computation, we have considered all the alkali dimers being in the singlet electronic ground state. The electron transfer from the alkali dimer to He (1s-1) leads to the emission of another electron from the alkali dimer into the continuum. We have investigated the impact of the distance of the He atom from the center of mass of the alkali dimer on the ETMD decay width. We also performed the Born-Oppenheimer molecular dynamics simulation to understand the impact of nuclear dynamics on the ETMD process.

Unusual quadruple bonds featuring collective interaction-type σ bonds between first octal-row atoms in the alkaline-earth compounds AeOLi2 (Ae = Be-Ba).

Quantum chemical calculations are reported for the complexes of alkaline earth metals AeOLi2 (Ae = Be-Ba) at the BP86-D3(BJ)/def2-QZVPP and CCSD(T)/def2-QZVPPQZVPP levels. The nature of the Ae-OLi2 bond has been analyzed with a variety of methods. The AeOLi2 molecules exhibit an unprecedented σ donor bond Ae→OLi2 where the (n)s2 lone-pair electrons of the Ae atom are donated to vacant O-Li2 antibonding orbitals having the largest coefficient at lithium. This is a covalent bond where the accumulation of the associated electronic charge is located at two positions above and below the Ae-OLi2 axis. The bifurcated component of orbital interactions is structurally related to the recently proposed collective bonding model, but exhibits a completely different type of bonding. The most stable isomer of AeOLi2 has a C 2v geometry and a singlet (1A1) electronic ground state. The bond dissociation energy (BDE) of the Ae-OLi2 bonds exhibits a zig-zag trend from BeOLi2 to BaOLi2, with BeOLi2 having the largest BDE (D e = 73.0 kcal mol-1) and MgOLi2 possessing the lowest BDE (D e = 42.3 kcal mol-1) at the CCSD(T) level. The calculation of the atomic partial charges by the Hirshfeld and Voronoi methods suggests that Be and Mg carry small negative charges in the lighter molecules whereas the heavier atoms Ca-Ba have small positive charges. In contrast, the NBO and QTAIM methods give positive charges for all Ae atoms that are larger for Ca-Ba than that calculated by the Hirshfeld and Voronoi approaches. The molecules AeOLi2 have large dipole moments where the negative end is at the Ae atom with the polarity Ae→OLi2. The largest dipole moments are predicted for the lighter species BeOLi2 and MgOLi2 and the smallest value is calculated for BaOLi2. The calculation of the vibrational spectra shows a significant red-shift toward lower wave numbers for the Ae-OLi2 stretching mode in comparison to diatomic AeO. Besides the Ae→OLi2 σ-donor bonds there are also three dative bonds due to Ae←OLi2 backdonation which consist of one σ bond and two π bonds. The appearance of strong Ae→OLi2 σ donation leads to quadruple bonds AeOLi2 in all systems AeOLi2, even for the lightest species with Ae = Be, Mg. The valence orbitals of Ca, Sr, and Ba, which are involved in the dative interactions, are the (n)s and (n-1)d AOs whereas Be and Mg use their (n)s and (n)p AOs. The EDA-NOCV results are supported by the AdNDP calculations which give four 2c-2e bonding orbitals. Three bonding orbitals have occupation numbers ∼2. One σ orbital has smaller occupation numbers between 1.32 and 1.73 due to the delocalization to the lithium atoms. The analysis of the electronic structure with the ELF method suggests multicenter bonds with mainly trisynaptic and tetrasynaptic basins, which also support the results of the EDA-NOCV calculations.

Understanding Antiferromagnetic and Ligand Field Effects on Spin Crossover in a Triple-Decker Dimeric Cr(II) Complex.

Two possible explanations for the temperature dependence of spin-crossover (SCO) behavior in the dimeric triple-decker Cr(II) complex ([(η5-C5Me5)Cr(μ2:η5-P5)Cr(η5-C5Me5)]+) have been offered. One invokes variations in antiferromagnetic interactions between the two Cr(II) ions, whereas the other posits the development of a strong ligand-field effect favoring the low-spin ground state. We perform multireference electronic structure calculations based on the multiconfiguration pair-density functional theory to resolve these effects. We find quintet, triplet, and singlet electronic ground states, respectively, for the experimental geometries at high, intermediate, and low temperatures. The ground-state transition from quintet to triplet at an intermediate temperature derives from increased antiferromagnetic interactions between the two Cr(II) ions. By contrast, the ground-state transition from triplet to singlet at low temperature can be attributed to increased ligand-field effects, which dominate with continued variations in antiferromagnetic coupling. This study provides quantitative detail for the degree to which these two effects can act in concert for the observed SCO behavior in this complex and others subject to temperature-dependent variations in geometry.

The magnetocaloric effect of the lanthanide fluorides: Using polarized neutron scattering to probe a magnetocaloric suitable for hydrogen liquefaction

This work reports the competitive magnetocaloric effect of some simple lanthanide fluoride materials with cations with high magnetic anisotropy. Of these, HoF3 is particularly promising due to exhibiting a high magnetocaloric entropy change under modest applied fields at higher temperatures, which only decreases modestly with temperature such that it has the potential for cooling for hydrogen liquefaction. Spin-polarized neutron spectroscopy indicates that its promising conventional magnetocaloric effect is likely due to the presence of ferromagnetic fluctuations of highly anisotropic magnetic moments, while its singlet electronic ground state and low-temperature magnetic ordering lead to a decrease in its magnetocaloric performance below 4 K.

An Isolable Bis(Germylene)-Stabilized Plumbylone.

We report herein the synthesis of a stable plumbylone (3) by reduction of a bromodigermylplumbylene (2) with 2.2 equiv of potassium graphite (KC8 ). The molecular structure of 3 was established by a single-crystal X-ray diffraction study and features a two-coordinated Pb center with an acute Ge-Pb-Ge bond angle. Computational studies showed that this complex (3) possesses a singlet electronic ground state with a Pb0 center. Its high thermal stability can be most likely ascribed to the delocalization of π electrons over the Ge-Pb-Ge moiety. A preliminary reactivity study demonstrates that complex 3 can deliver Pb0 atoms to an organic azide producing a tetrameric imido complex [(PbNDipp)4 ] (Dipp=2,6-i Pr-C6 H3 , 4) and perform a metathesis reaction with GeCl2 ⋅dioxane to produce a bis(germylene)-stabilized germylone (5), highlighting the synthetic utility of 3.

An Isolable Bis(Germylene)‐Stabilized Plumbylone

AbstractWe report herein the synthesis of a stable plumbylone (3) by reduction of a bromodigermylplumbylene (2) with 2.2 equiv of potassium graphite (KC8). The molecular structure of3was established by a single‐crystal X‐ray diffraction study and features a two‐coordinated Pb center with an acute Ge−Pb−Ge bond angle. Computational studies showed that this complex (3) possesses a singlet electronic ground state with a Pb0center. Its high thermal stability can be most likely ascribed to the delocalization of π electrons over the Ge−Pb−Ge moiety. A preliminary reactivity study demonstrates that complex3can deliver Pb0atoms to an organic azide producing a tetrameric imido complex [(PbNDipp)4] (Dipp=2,6‐iPr‐C6H3,4) and perform a metathesis reaction with GeCl2⋅dioxane to produce a bis(germylene)‐stabilized germylone (5), highlighting the synthetic utility of3.

Odd-Number Cyclo[n]Carbons Sustaining Alternating Aromaticity.

Cyclo[n]carbons (n = 5, 7, 9, ..., 29) composed from an odd number of carbon atoms are studied computationally at density functional theory (DFT) and ab initio complete active space self-consistent field (CASSCF) levels of theory to get insight into their electronic structure and aromaticity. DFT calculations predict a strongly delocalized carbene structure of the cyclo[n]carbons and an aromatic character for all of them. In contrast, calculations at the CASSCF level yield geometrically bent and electronically localized carbene structures leading to an alternating double aromaticity of the odd-number cyclo[n]carbons. CASSCF calculations yield a singlet electronic ground state for the studied cyclo[n]carbons except for C25, whereas at the DFT level the energy difference between the lowest singlet and triplet states depends on the employed functional. The BHandHLYP functional predicts a triplet ground state of the larger odd-number cyclo[n]carbons starting from n = 13. Current-density calculations at the BHandHLYP level using the CASSCF-optimized molecular structures show that there is a through-space delocalization in the cyclo[n]carbons. The current density avoids the carbene carbon atom, leading to an alternating double aromaticity of the odd-number cyclo[n]carbons satisfying the antiaromatic [4k+1] and aromatic [4k+3] rules. C11, C15, and C19 are aromatic and can be prioritized in future synthesis. We predict a bond-shift phenomenon for the triplet state of the cyclo[n]carbons leading to resonance structures that have different reactivity toward dimerization.

Cationic Carbene Analogues: Donor-Free Phosphenium and Arsenium Ions.

Carbenes and their analogues have constantly enthralled chemists with their intriguing reactivity of ambiphilic character stemming from their electronic structures. Phosphenium and arsenium ions are fiercely reactive cationic species, the stabilization of which has been so far achieved in the condensed phase by dispersing the positive charge through electromeric conjugation with at least one electron‐rich substituent (frequently amido groups). Although observed in the gas phase, the isolation of dicoordinate phosphenium and arsenium ions lacking such stabilizing ligands has eluded chemists for decades. Herein we show that by judicious choice of aromatic substituents, dicoordinate, donor‐free, Lewis‐superacidic phosphenium and arsenium ions can be kinetically stabilized. They feature singlet electronic ground states possessing a vacant p‐orbital and an electron lone pair with predominantly s‐character.

Infrared action spectroscopy of doubly charged PAHs and their contribution to the aromatic infrared bands

The so-called aromatic infrared bands (AIBs) are attributed to emission of polycyclic aromatic hydrocarbons (PAHs). The observed variations toward different regions in space are believed to be caused by contributions of different classes of PAH molecules, that is to say with respect to their size, structure, and charge state. Laboratory spectra of members of these classes are needed to compare them to observations and to benchmark quantum-chemically computed spectra of these species. In this paper we present the experimental infrared (IR) spectra of three different PAH dications, naphthalene2+, anthracene2+, and phenanthrene2+, in the vibrational fingerprint region 500–1700 cm−1. The dications were produced by electron impact ionization of the vapours with 70 eV electrons, and they remained stable against dissociation and Coulomb explosion. The vibrational spectra were obtained by IR predissociation of the PAH2+ complexed with neon in a 22-pole cryogenic ion trap setup coupled to a free-electron infrared laser at the Free-Electron Lasers for Infrared eXperiments (FELIX) Laboratory. We performed anharmonic density-functional theory calculations for both singly and doubly charged states of the three molecules. The experimental band positions showed excellent agreement with the calculated band positions of the singlet electronic ground state for all three doubly charged species, indicating its higher stability over the triplet state. The presence of several strong combination bands and additional weaker features in the recorded spectra, especially in the 10–15 μm region of the mid-IR spectrum, required anharmonic calculations to understand their effects on the total integrated intensity for the different charge states. These measurements, in tandem with theoretical calculations, will help in the identification of this specific class of doubly-charged PAHs as carriers of AIBs.

Signature of the neighbor's quantum nuclear dynamics in the electron transfer mediated decay spectra.

We computed fully quantum nuclear dynamics, which accompanies electron transfer mediated decay (ETMD) in weakly bound polyatomic clusters. We considered two HeLi2 clusters – with Li2 being either in the singlet electronic ground state or in the triplet first excited state – in which ETMD takes place after ionization of He. The electron transfer from Li2 to He+ leads to the emission of another electron from Li2 into the continuum. Due to the weak binding of He to Li2 in the initial states of both clusters, the involved nuclear wavepackets are very extended. This makes both the calculation of their evolution and the interpretation of the results difficult. We showed that despite the highly delocalized nature of the wavepackets the nuclear dynamics in the decaying state is imprinted on the ETMD electron spectra. The analysis of the latter helps understanding the effect which electronic structure and binding strength in the cluster produce on the quantum motion of the nuclei in the decaying state. The results produce a detailed picture of this important charge transfer process in polyatomic systems.

Ligand-Mediated Spin-State Changes in a Cobalt-Dipyrrin-Bisphenol Complex.

The influence of a redox-active ligand on spin-changing events induced by the coordination of exogenous donors is investigated within the cobalt complex [CoII(DPP·2–)], bearing a redox-active DPP2– ligand (DPP = dipyrrin-bis(o,p-di-tert-butylphenolato) with a pentafluorophenyl moiety on the meso-position. This square-planar complex was subjected to the coordination of tetrahydrofuran (THF), pyridine, tBuNH2, and AdNH2 (Ad = 1-adamantyl), and the resulting complexes were analyzed with a variety of experimental (X-ray diffraction, NMR, UV–visible, high-resolution mass spectrometry, superconducting quantum interference device, Evans’ method) and computational (density functional theory, NEVPT2-CASSCF) techniques to elucidate the respective structures, spin states, and orbital compositions of the corresponding octahedral bis-donor adducts, relative to [CoII(DPP·2–)]. This starting species is best described as an open-shell singlet complex containing a DPP·2– ligand radical that is antiferromagnetically coupled to a low-spin (S = 1/2) cobalt(II) center. The redox-active DPPn– ligand plays a crucial role in stabilizing this complex and in its facile conversion to the triplet THF adduct [CoII(DPP·2–)(THF)2] and closed-shell singlet pyridine and amine adducts [CoIII(DPP3–)(L)2] (L = py, tBuNH2, or AdNH2). Coordination of the weak donor THF to [CoII(DPP·2–)] changes the orbital overlap between the DPP·2– ligand radical π-orbitals and the cobalt(II) metalloradical d-orbitals, which results in a spin-flip to the triplet ground state without changing the oxidation states of the metal or DPP·2– ligand. In contrast, coordination of the stronger donors pyridine, tBuNH2, or AdNH2 induces metal-to-ligand single-electron transfer, resulting in the formation of low-spin (S = 0) cobalt(III) complexes [CoIII(DPP3–)(L)2] containing a fully reduced DPP3– ligand, thus explaining their closed-shell singlet electronic ground states.

Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide.

The reactions of laser‐ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end‐on bonded NNBeCO and side‐on bonded (η2‐N2)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end‐on bonded complex has a triplet ground state while the side‐on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge‐ and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s02p2 excited configuration and that the metal‐ligand bonds can be described in terms of N2→Be←CO σ donation and concomitant N2←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N−N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.

Beryllium Atom Mediated Dinitrogen Activation via Coupling with Carbon Monoxide

AbstractThe reactions of laser‐ablated beryllium atoms with dinitrogen and carbon monoxide mixtures form the end‐on bonded NNBeCO and side‐on bonded (η2‐N2)BeCO isomers in solid argon, which are predicted by quantum chemical calculations to be almost isoenergetic. The end‐on bonded complex has a triplet ground state while the side‐on bonded isomer has a singlet electronic ground state. The complexes rearrange to the energetically lowest lying NBeNCO isomer upon visible light excitation, which is characterized to be an isocyanate complex of a nitrene derivative with a triplet electronic ground state. A bonding analysis using a charge‐ and energy decomposition procedure reveals that the electronic reference state of Be in the NNBeCO isomers has an 2s02p2 excited configuration and that the metal‐ligand bonds can be described in terms of N2→Be←CO σ donation and concomitant N2←Be→CO π backdonation. The results demonstrate that the activation of N2 with the N−N bond being completely cleaved can be achieved via coupling with carbon monoxide mediated by a main group atom.

Multiple Bonding Between Group 3 Metals and Fe(CO)3−

AbstractHeteronuclear Group 3 metal/iron carbonyl anion complexes ScFe(CO)3−, YFe(CO)3−, and LaFe(CO)3− are prepared in the gas phase and studied by mass‐selective infrared (IR) photodissociation spectroscopy as well as quantum‐chemical calculations. All three anion complexes are characterized to have a metal–metal‐bonded C3v equilibrium geometry with all three carbonyl ligands bonded to the iron center and a closed‐shell singlet electronic ground state. Bonding analyses reveal that there are multiple bonding interactions between the bare group‐3 elements and the Fe(CO)3− fragment. Besides one covalent electron‐sharing metal–metal σ bond and two dative π bonds from Fe to the Group 3 metal, there is additional multicenter covalent bonding with the Group 3 atom bonded to Fe and the carbon atoms.

Multiple Bonding Between Group 3 Metals and Fe(CO)3.

Heteronuclear Group 3 metal/iron carbonyl anion complexes ScFe(CO)3 - , YFe(CO)3 - , and LaFe(CO)3 - are prepared in the gas phase and studied by mass-selective infrared (IR) photodissociation spectroscopy as well as quantum-chemical calculations. All three anion complexes are characterized to have a metal-metal-bonded C3v equilibrium geometry with all three carbonyl ligands bonded to the iron center and a closed-shell singlet electronic ground state. Bonding analyses reveal that there are multiple bonding interactions between the bare group-3 elements and the Fe(CO)3 - fragment. Besides one covalent electron-sharing metal-metal σ bond and two dative π bonds from Fe to the Group 3 metal, there is additional multicenter covalent bonding with the Group 3 atom bonded to Fe and the carbon atoms.

Effect of isotropic collisions with neutral hydrogen on the polarization of the CN solar molecule

ABSTRACT In this work, we study the solar molecule CN, which presents conspicuous profiles of scattering polarization. We start by calculating accurate potential energy surfaces for the singlet and triplet electronic ground states in order to characterize the collisions between the CN molecule in its X 2Σ state and the hydrogen in its ground state 2S. The potential energy surfaces are included in the Schrödinger equation to obtain the scattering matrix and the probabilities of collisions. Depolarizing collisional rate coefficients are computed in the framework of the infinite order sudden approximation for temperatures ranging from T = 2000 K to T= 15 000 K. We give an interpretation of the results and compare the singlet and triplet collisional rate coefficients. We show that, for typical photospheric hydrogen density (nH = 1015−1016 cm−3), the X 2Σ state of CN is partially or completely depolarized by isotropic collisions.

Transition‐Metal Chemistry of Alkaline‐Earth Elements: The Trisbenzene Complexes M(Bz)3(M=Sr, Ba)

AbstractWe report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3(M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by aD3symmetric structure involving three equivalent η6‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n−1)d AOs of M and strong backdonation from the occupied (n−1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

Transition-Metal Chemistry of Alkaline-Earth Elements: The Trisbenzene Complexes M(Bz)3 (M=Sr, Ba).

We report the synthesis and spectroscopic identification of the trisbenzene complexes of strontium and barium M(Bz)3 (M=Sr, Ba) in low‐temperature Ne matrix. Both complexes are characterized by a D 3 symmetric structure involving three equivalent η6‐bound benzene ligands and a closed‐shell singlet electronic ground state. The analysis of the electronic structure shows that the complexes exhibit metal–ligand bonds that are typical for transition metal compounds. The chemical bonds can be explained in terms of weak donation from the π MOs of benzene ligands into the vacant (n−1)d AOs of M and strong backdonation from the occupied (n−1)d AO of M into vacant π* MOs of benzene ligands. The metals in these 20‐electron complexes have 18 effective valence electrons, and, thus, fulfill the 18‐electron rule if only the metal–ligand bonding electrons are counted. The results suggest that the heavier alkaline earth atoms exhibit the full bonding scenario of transition metals.

Pentadiynylidene and Its Methyl-Substituted Derivates: Threshold Photoelectron Spectroscopy of R1-C5-R2 Triplet Carbon Chains.

Mass-selective threshold photoelectron spectroscopy in the gas phase was employed to characterize the dialkynyl triplet carbenes pentadiynylidene (HC5H), methylpentadiynylidene (MeC5H), and dimethylpentadiynylidene (MeC5Me). Diazo compounds were employed as precursors to generate the carbenes by flash pyrolysis. The R1-C5-R2 carbon chains were photoionized by vacuum ultraviolet (VUV) synchrotron radiation in photoelectron photoion coincidence (PEPICO) experiments. High-level ab initio computations were carried out to support the interpretation of the experiments. For the unsubstituted pentadiynylidene (R1 = R2 = H) the recorded spectrum yields an adiabatic ionization energy (IEad) of 8.36 ± 0.03 eV. In addition, a second carbene isomer, 3-(didehydrovinylidene)cyclopropene, with a singlet electronic ground state, was identified in the spectrum based on the IEad of 8.60 ± 0.03 eV and Franck-Condon simulations. We found that multireference computations are required to reliably calculate the IEad for this molecule. CASPT2 computations predicted an IEad = 8.55 eV, while coupled-cluster computations significantly overestimate the IE. The cyclic isomer is most likely formed from another isomer of the precursor present in the sample. Stepwise methyl-substitution of the carbene leads to a reduction of the IE to 7.77 ± 0.04 eV for methylpentadiynylidene and 7.27 ± 0.06 eV for dimethylpentadiynylidene. The photoionization and dissociative photoionization of the precursors is investigated as well.