Synthesis and Isolation of a Kekulé Hydrocarbon with a Triplet Ground State.

Design, synthesis, and isolation of a Kekulé hydrocarbon with a triplet ground state is described. Its triplet ground state was unambiguously confirmed by ESR experiments, and the structure and fundamental physical properties were also revealed. The key feature of the molecular design is the decrease in the bonding interaction in the singlet state by aromatic stabilization of benzene rings and the increase of the exchange interaction of unpaired electrons which are favorable for the triplet state. These results contribute to the development of hydrocarbon‐based organic magnetic materials.

Carbenes comprise a well-known class of organometallic compounds each consisting of a neutral, divalent carbon and two unshared electrons. Carbenes can have singlet or triplet ground states, each giving rise to a distinct reactivity. Methylene (CH2), the parent hydride, is well-known to be bent in its triplet ground state. Here, we predict the existence of LiCH, a carbene-like organometallic molecule. Computationally, we treat the electronic structure with parametric and variational two-electron reduced density matrix (2-RDM) methods, which are capable of capturing multireference correlation typically associated with the singlet state of a diradical. Similar to methylene, LiCH is a triplet ground state with a predicted 15.8 kcal/mol singlet-triplet gap. However, unlike methylene, LiCH is linear in both the triplet state and the lowest excited singlet state. Furthermore, the singlet state is found to exhibit strong electron correlation as a diradical. In comparison to dissociation channels Li + CH and Li+ + CH-, the LiCH was found to be stable by approximately 77 kcal/mol.

Spin qubit defects in two-dimensional materials have a number of advantages over those in three-dimensional hosts including simpler technologies for defect creation and control, as well as qubit accessibility. In this work, we select theVBCBdefect in the hexagonal boron nitride (hBN) as a possible optically controllable spin qubit and explain its triplet ground state and neutrality. In this defect a boron vacancy is combined with a carbon dopant substituting the closest boron atom to the vacancy. Our density-functional-theory calculations confirmed that the system has dynamically stable spin triplet and singlet ground states. As revealed from our linear response GW calculations, the spin-sensitive electronic states are localized around the three undercoordinated N atoms and make local peaks in the density of electronic states within the bandgap. Using the triplet and singlet ground state energies, as well as the energies of the optically excited states, obtained from solution to the Bethe-Salpeter equation, we construct the spin-polarization cycle, which is found to be favorable for the spin qubit initialization. The calculated zero-field splitting parameters ensure that the splitting energy between the spin projections in the triplet ground state is comparable to that of the known spin qubits. We thus propose theVBCBdefect in hBN as a promising spin qubit.

The potential energy surface of the reaction [(eta5-C5MenH5-n)2M]2(micro2,eta2,eta2-N2) + H2 --> [(eta5-C5MenH5-n)2M][(eta5-C5MenH5-n)2MH](micro2,eta2,eta2-NNH) at low-lying singlet and triplet electronic states of the reactants was investigated using density functional methods, for n = 0 and 4, and M = Ti, Zr, and Hf. Ground electronic states of the Ti complexes are found to be triplet states, while that for the corresponding Zr and Hf complexes are singlet states. In their singlet state, all these complexes satisfy known necessary conditions (they have a side-on-coordinated N2 molecule and appropriate frontier orbitals) for successful addition of an H2 molecule to the coordinated N2, and consequently, add of an H2 molecule with a reasonable energy barrier. Hf complexes show slightly higher reactivity than corresponding Zr complexes, and in turn, both are more reactive than their singlet-state Ti counterparts. The calculated trend in reactivity of Zr and Hf complexes is consistent with the latest experimental data (see refs 13 and 16). However, Ti complexes have the ground triplet state that lacks in appropriate frontier orbitals. As a result, H2 addition to the Ti complexes at their triplet ground states requires a larger activation barrier than the singlet state and is endothermic (lacks of driven force for reaction). On the basis of these results, we predict that the [(eta5-C5Me4H)2M]2(micro2,eta2,eta2-N2) and [(eta5-C5H5)2M]2(micro2,eta2,eta2-N2) complexes cannot react with an H2 molecule for M = Ti, while those for M = Zr and Hf can. It was shown that the difference in the B3LYP (hybrid) and PBE (nonhybrid) calculated energy gaps between the lowest closed-shell singlet and triplet states of the present complexes reduces via first- > second- > third-row transition metals; both hybrid and nonhybrid density functionals can be safely used to describe reactivity of the low-lying low-spin and high-spin states of second- and third-row transition metal complexes.

The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH(3) is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH(3)NiOH and HNiOCH(3), locate on the deep well, while CH(3)NiOH is more energetically favorable than HNiOCH(3) on both the triplet and singlet states. The main products shall be syngas once HNiOCH(3) is created on the singlet state, whereas the main products shall be methyl radical if CH(3)NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH(3) formation involving an oxidative addition of NiO molecule into the C-H bond of methane, with an energy barrier of 120.3 kJ mol(-1). The syngas formation would be more effective under higher temperature and photolysis reaction condition.

Hydrocarbons with open-shell singlet and triplet ground states have long been studied. In contrast to studies of Kekulé hydrocarbons with an open-shell singlet ground state, studies of non-Kekulé and Kekulé hydrocarbons with a triplet ground state are quite limited, and no hydrocarbon with a triplet ground state has been isolated as single crystals. In this review, our work on the synthesis, isolation, and properties of m-quinodimethane-based non-Kekulé polycyclic hydrocarbon, a kinetically stabilized triangulene derivative, and Kekulé polycyclic hydrocarbon, a kinetically stabilized bisdibenzo[3,4:5,6]cyclohepta[1,2-a:2,1-d]benzene derivative, are described. Triplet ground states of these hydrocarbons were experimentally confirmed by ESR and magnetic measurements, and these are the first example of polycyclic hydrocarbons with a triplet ground state whose structures were characterized by X-ray crystal structural analysis. These studies will enable the development of various polycyclic hydrocarbon multi-radicals with high spin multiplicity.

The ground-state spins of seven diradicals belonging to the fused ring system have been investigated by ab initio restricted and unrestricted formalisms. The systems under study are (1) 4-oxy-2-naphthalenyl methyl, (2) 1,8-naphthalenediylbis(methyl), (3) 8-imino-1-naphthalenyl methyl, (4) 1,8- naphthalenediylbis(amidogen), (5) 8-methyl-1-naphthyl carbene, (6) 8-methyl-1-naphthalenyl imidogen, and (7) 8-methyl-1-naphthyl diazomethane. Out of the seven molecules, only 1 was theoretically investigated earlier. To our knowledge, for 2−7, this work represents the first ab initio investigation. A variety of basis sets have been employed in these calculations. For each spin state, the molecular geometry has been fully optimized at the unrestricted Hartree−Fock (UHF) level using the STO-3G, 4-31G, 6-311G(d), and 6-311G(d,p) basis sets. The UHF optimized geometries have been used for Møller−Plesset (MP) and coupled cluster (CC) calculations as well as the density functional (UB3LYP) treatment. Results in the unrestricted formalism have been given only at UHF and UB3LYP levels for the 6-311G(d) basis. The UHF calculations yield an unrealistically large singlet−triplet (S−T) splitting. Splittings calculated with different bases disagree seriously. The S−T gap is smaller in the split-valence bases. The basis set truncation error can be considerably overcome by calculations involving electron correlation. For these diradicals, any meaningful result would require larger bases with polarization functions. Apart form this difficulty, the optimized geometry turned out to be highly spin-contaminated. The spin-contamination can be significantly reduced by the density functional UB3LYP treatment. Nevertheless, for most of the diradicals, the UB3LYP method did not yield a systematic trend. To avoid spin contamination completely, we have repeated computations in the restricted (open-shell) Hartree−Fock framework. Geometry optimizations were carried out using STO-3G, 6-311G(d), and 6-311G(d,p) bases at the R(O)HF level and 6-311G(d,p) basis at the R(O)B3LYP level for each spin state. The R(O)B3LYP/6-311G(d,p) optimized geometry yields the best total energy for each spin state and hence the most reliable S−T energy difference. Molecules 1−6 are found as ground-state triplets. The calculated results are in agreement with the available experimental findings. Molecules 3 and 7 have widely different geometries in the singlet and triplet states. The calculations using 6-311G(d) and 6-311G(d,p) basis sets show that in molecule 3 the substituents of naphthalene are −NH2 and −CH in singlet but −NH and −CH2 in triplet. The two optimized geometries are tautomeric forms. Molecule 7 is expected to be either a ground-state triplet with a very little S−T gap or a ground-state singlet. This prediction is borne out by the computed results. The R(O)B3LYP/6-311G(d,p) calculation yields a S−T splitting of −21.9 kcal mol-1. The singlet state becomes stabilized by forming an additional condensed ring. The UHF spin density plots obtained from the 4-31G optimized geometries manifest the phenomenon of spin alternation in the ground state.

Diradicals have attracted the attention of chemists due to their unique electronic structures and properties originating from unpaired electrons. One of the fundamental motifs of diradicals is quinodimethane; p- and o-quinodimethanes are singlet Kekulé hydrocarbons, while m-quinodimethane is a triplet non-Kekulé hydrocarbon. Most of the hydrocarbon diradicals studied to date have been limited to p- and o-quinodimethane-based non-fused-ring and fused-ring open-shell singlet diradicals and m-quinodimethane-based non-fused-ring triplet diradicals. In this account, studies on m-quinodimethane-based fused-ring diradicals, including an open-shell singlet Kekulé hydrocarbon, an open-shell singlet zwitterion, non-Kekulé hydrocarbon-based triplet diradical and diradical cation, and a triplet Kekulé hydrocarbon, are summarized. They are designed, successfully synthesized, and isolated as crystals, and their fundamental electronic structures and properties have been elucidated by optical, electrochemical, and magnetic measurements, together with DFT calculations. A series of studies showed that controlling the interaction between the two unpaired electrons of m-quinodimethane through an appropriate molecular design would produce polycyclic diradicals with various open-shell singlet diradical characters and the energy differences between singlet and triplet states.

Benzenoid hydrocarbons with two phenalene moieties, diphenalenes,comprising seven benzene rings, form three groups, with the seventhbenzene ring fused to (i) zethrene, (ii) uthrene, or (iii) it linksphenalene subunits. The DFT/cc-pVTZ calculations indicate that fourmolecules in the former group are more stable in the singlet state,four in the next in the triplet, whereas in the latter group, threeare more stable as singlets and two as triplets. The singlet–tripletgap usually exceeds | ± 10| kcal/mol, but for two molecules ofthe (iii) group, it is close to 5 kcal/mol. We show also that thediphenalenes’ Gibbs free energy differences can be presentedas a multivariate function of the number of border Bay, Cove, andFjord motifs. Diphenalenes, including [4]­helicene moiety, are inherentlynonplanar. Although the calculations implied a slight skewness ofother diphenalenes, it appeared apparent as their lowest harmonicfrequencies overcame the barrier between the left and right skewedforms. For the rings at the phenalene junction, the HOMA and INICSaromaticity indices reveal a substantial decrease of aromaticity inthe stable singlet state molecules. All molecules in the triplet state,and those in an excited singlet state, exhibit only a slight decline,if any. Unexpectedly, the HOMA averaged over all rings indicated themore stable singlet state to be less aromatic than the less stabletriplet one, which contradicts the fairly common assumption that thegreater aromaticity, the greater stability. The alternating partialspins pattern in the triplet state diphenalenes differs between theKekuléan and non-Kekuléan ones at the phenalenes junctionbond. It can be utilized to mnemotechnically predict whether an assemblyof two phenalenes will have a singlet or triplet ground state.

Linkages between vinylidene (= C :) and nitrenes (- N :), through methyne and its derivatives (CX), give allylic carbenonitrenes, C =(X) C – N , as a new brand of reactive intermediates, with conceivable singlet, triplet, or quintet ground states ( X=H , 1, X = CH 3, 2, X = COOH , 3, X = F , 4, X = OH , 5, X X= OMe , 6, X X= CF 3, 7, X X= CN , 8, and X X= NH 2, 9). High-spin quintet (5 A ″) ground states are found for 1 and 2 at eight ab initio and DFT levels of theory. At the same levels, triplet (3 A ″) ground states prevail for 3–8. Low-spin singlet (1 A ') input structures of 1, 2, and 4–9 cyclize spontaneously through optimization to their corresponding aromatic X-azacyclopropenylidenes, with multiplicities irrelevant to allylic carbenonitrenes. Researchers may aim for generating 3–8 with triplet states, or even 1 and/or 2 with quintet states, but we do not recommend going for generation of 9 with any multiplicity, and/or formation of 1–8 with singlet states.

Part A. Generalized valence bond calculations on cyclopropene and vinylmethylene lead to the following conclusions: (1) the allyl-type π-system is strongly distorted by the presence of the unpaired sigma electron leading to a methylene-like triplet, ^3A^, but a 1,3-diradical-like singlet state, ^1A^; (2) the lowest-lying singlet state of vinylmethylene has the form of a singlet methylene ^1A lying 12 kcal/mole above the triplet ground state, while the diradical singlet state lies at 14 kcal/mole. Part B. Generalized valence bond calculations on trimethylenemethane indicate that the ground state is the planar triplet with the planar singlet state 26 kcal/mole higher. The rotational barrier for the triplet state is 18 kcal/mole, while one component of the planar singlet prefers the bisected geometry by 7 kcal/mole. Oscillator strengths for vertical transitions and ionization potentials are also reported. Part C. Generalized valence bond calculations on vinylidene predict that the ground state is a singlet with a methylene-like triplet at 2 eV higher. With extensive CI calculations, we find CC bond energy of D_0(h_2C=C) = 150.1 kcal/mole and a heat of formation of 111.5 kcal/mole at 298°K. The dipole moment for the singlet is calculated to be 2.23D, while the dipole moment for the triplet is 0.55D. Part D. Generalized valence bond calculations on aminonitrene indicate that the ground state is a singlet (^1A_1) with a low-lying triplet state (^3A_2) at 15 kcal/mole. We find the nitrogen-nitrogen bond dissociation energy for the singlet state is 70.4 kcal/mole. The dipole moment is found to be 4.036D for the ^1A_1 and 2.351D for the ^3A_2 state of aminonitrene. The ionization potential is calculated to be 9.4 eV. Part E. The kinetic distribution of isomeric xylenes formed on the thermal aromatization of dl- and meso-1,1'-dimethyl-3,3'-bicylopropenyl and of 3,3'-dimethyl-3,3'-bicyclopropenyl has been determined by extrapolation of the time-dependent xylene percentages to zero percent conversion. The data is most consistent with a mechanism involving initial cleavage of one of the cyclopropene rings, followed by expansion of the other ring, closure to Dewar benzene and finally opening of the Dewar to form aromatic products.

Organic semiconductors with high-spin ground states are fascinating because they could enable fundamental understanding on the spin-related phenomenon in light element and provide opportunities for organic magnetic and quantum materials. Although high-spin ground states have been observed in some quinoidal type small molecules or doped organic semiconductors, semiconducting polymers with high-spin at their neutral ground state are rarely reported. Here we report three high-mobility semiconducting polymers with different spin ground states. We show that polymer building blocks with small singlet-triplet energy gap (ΔES-T) could enable small ΔES-T gap and increase the diradical character in copolymers. We demonstrate that the electronic structure, spin density, and solid-state interchain interactions in the high-spin polymers are crucial for their ground states. Polymers with a triplet ground state (S = 1) could exhibit doublet (S = 1/2) behavior due to different spin distributions and solid-state interchain spin-spin interactions. Besides, these polymers showed outstanding charge transport properties with high hole/electron mobilities and can be both n- and p-doped with superior conductivities. Our results demonstrate a rational approach to obtain high-mobility semiconducting polymers with different spin ground states.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A New Spectrum of CH1 in the Region 60009000 A. Herzberg, A. G. Abstract A year ago a spectrum was found EG. Herzberg and J. Shoosmith, Nature 183, 1801(1959)3 near 1400 A which on the basis of its fine structure could be definitely assigned to CH1. More recently, under conditions very similar to those used for obtaining the 1400 A group (flash photolysis of CH2N2 at partial pressures of about 0.02 mm in the presence of a large excess of inert gas), a new spectrum was found in the region 600O~9000 A which consists of a large number of absorption bands with wide and somewhat complicated fine structure. The spectrum is similar in appearance to the a bands of NH2. A preliminary analysis suggests strongly that the lower state is the expected lowest singlet state 1A2 of bent CH1, while the lower state of the 1400 A group is the ~~,,- state of linear CH2. The existence of two kinds of CH1, a bent and a linear form corresponding to the lowest singlet and triplet state, respectively, is in accordance with what might be expected from theory. From the slight difference in the conditions of production of the two absorption spectra of CH1 it appears likely that the I~~- state is the lowest. Because of the low excitation energy of the first singlet state above the triplet ground state it is likely that the red bands will turn out to be of some astrophysical interest. They might be expected to be observable in emission in the spectra of comets, and in absorption in the spectra of the outer planets and of low temperature carbon stars. In order to assist in possible identifications, a table of the strongest lines of the new CH1 bands will be presented. Publication: The Astronomical Journal Pub Date: 1960 DOI: 10.1086/108059 Bibcode: 1960AJ.....65Q..53H full text sources ADS |

Dibenzo[a,f]pentalene ([a,f]DBP) is a highly antiaromatic molecule having appreciable open-shell singlet character in its ground state. In this work, DFT calculations at the B3LYP/6-311+G(d,p) level of theory were performed to explore the efficiency of three strategies, that is, BN/CC isosterism, substitution, and (di)benzoannulation of [a,f]DBP, in controlling its electronic state and (anti)aromaticity. To evaluate the type and extent of the latter, the harmonic oscillator model of aromaticity (HOMA) and aromatic fluctuation (FLU) indices were used, along with the nucleus-independent chemical shift NICS-XY-scan procedure. The results suggest that all three strategies could be employed to produce either the closed-shell system or open-shell species, which may be in the singlet or triplet ground state. Triplet states have been characterized as aromatic, which is in accordance with Baird's rule. All the singlet states were found to have weaker global paratropicity than [a,f]DBP. Additional (di)benzo fusion adds local aromatic subunit(s) and mainly retains the topology of the paratropic ring currents of the basic molecule. The substitution of two carbon atoms by the isoelectronic BN pair, or the introduction of substituents, results either in the same type and very similar topology of ring currents as in the parent compound, or leads to (anti)aromatic and nonaromatic subunits. The triplet states of all the examined compounds are also discussed.

Density functional B3LYP/6-311+G(3df )//B3LYP/6-31G* calculations of potential energy surfaces (PES) have been performed for the Ti+CO2→TiO+CO reaction in the triplet, quintet, and singlet electronic states. The results indicate that in the ground triplet state the most favorable reaction mechanism involves insertion of the Ti atom into a CO bond [via a η2-C,O coordinated t-(TiOC)O complex] to produce a triplet t-OTiCO molecule with the energy gain of 43.9 kcal/mol and the latter can further dissociate to TiO(3Δ)+CO with the total reaction exothermicity of ∼30 kcal/mol. The addition mechanism leading to the same TiO(3Δ)+CO products via a metastable η2-O,O complex t-cyc-TiCO2 is also feasible at ambient temperatures since the highest barrier on the reaction pathway is only 4.7 kcal/mol. The reaction mechanisms in excited singlet and quintet electronic states have many similar features with the ground state reaction but also exhibit some differences. In the singlet state, the reaction can follow A″1 and A′1 pathways, of those the insertion via a s-(TiOC)O (1A′) complex leading to s-OTiCO (1A′) and then to TiO(1Δ)+CO does not have an activation barrier. The insertion mechanism on the A″1 PES depicts a low barrier of 1.8 kcal/mol and leads to s-OTiCO (1A″), which dissociates into TiO(1Δ)+CO. The addition pathways via η2-O,O coordinated complexes require to overcome significant barriers, 7.8 and 34.9 kcal/mol for the A″1 and A′1 states, respectively. In the quintet state, the reaction at low and ambient temperatures can proceed only by coordination of Ti(5F) toward CO2 with formation of η2-C,O q-(TiOC)O, η2-O,O q-cyc-TiCO2, and η1-O q-TiOCO bound by 9.7, 6.1, and 4.6 kcal/mol, respectively, relative to the reactants. The η2-C,O and η1-O coordinations occur without barriers, while the η2-O,O coordination has an entrance barrier of 4.2 kcal/mol. The calculated PESs show that the carbon dioxide reforming into CO in the presence of Ti atoms should take place spontaneously.