Triplet Spin Multiplicities Research Articles

Global ab initio potential energy surfaces for the O2(Σ3g−)+O2(Σ3g−) interaction

Completely ab initio global potential energy surfaces (PESs) for the singlet and triplet spin multiplicities of rigid O(2)((3)Σ(g)(-))+O(2)((3)Σ(g)(-)) are reported for the first time. They have been obtained by combining an accurate restricted coupled cluster theory with singles, doubles, and perturbative triple excitations [RCCSD(T)] quintet potential [Bartolomei et al., J. Chem. Phys. 128, 214304 (2008)] with complete active space second order perturbation theory (CASPT2) or, alternatively, multireference configuration interaction (MRCI) calculations of the singlet-quintet and triplet-quintet splittings. Spherical harmonic expansions, containing a large number of terms due to the high anisotropy of the interaction, have been built from the ab initio data. The radial coefficients of these expansions are matched at long range distances with analytical functions based on recent ab initio calculations of the electric properties of the monomers [M. Bartolomei, E. Carmona-Novillo, M. I. Hernández, J. Campos-Martínez, and R. Hernández-Lamoneda, J. Comput. Chem. (2010) (in press)]. The singlet and triplet PESs obtained from either RCCSD(T)-CASPT2 or RCCSD(T)-MRCI calculations are quite similar, although quantitative differences appear in specific terms of the expansion. CASPT2 calculations are the ones giving rise to larger splittings and more attractive interactions, particularly in the region of the absolute minima (in the rectangular D(2h) geometry). The new singlet, triplet, and quintet PESs are tested against second virial coefficient B(T) data and, their spherically averaged components, against integral cross sections measured with rotationally hot effusive beams. Both types of multiconfigurational approaches provide quite similar results, which, in turn, are in good agreement with the measurements. It is found that discrepancies with the experiments could be removed if the PESs were slightly more attractive. In this regard, the most attractive RCCSD(T)-CASPT2 PESs perform slightly better than the RCCSD(T)-MRCI counterpart.

MRCI study on spectroscopic parameters and molecular constants of ground and three low-lying excited electronic states of the GeO molecule

The ground and three low-lying excited electronic states of the GeO molecule, including Σ +, Σ − and Π symmetry with singlet and triplet spin multiplicities, have been investigated employing the full valence complete active space self-consistent field (CASSCF) method followed by the highly accurate valence internally contracted multireference configuration interaction (MRCI) approach. The potential energy curves (PECs) of these states have been calculated with the cc-pV5Z basis set. And the effect on the PECs by the relativistic corrections has been taken into considerations. The way to consider the relativistic corrections is to use the second-order Douglas–Kroll Hamiltonian approximation. For the four electronic states, the spectroscopic parameters ( T e , D 0, D e , R e , ω e , ω ex e , α e and B e ) of 10 isotopes ( 70Ge 16O, 72Ge 16O, 73Ge 16O, 74Ge 16O, 76Ge 16O, 70Ge 18O, 72Ge 18O, 73Ge 18O, 74Ge 18O, 76Ge 18O) have been determined. The spectroscopic parameters have been compared with those of previous investigations reported in the literature, and excellent agreement has been found between the present results and the available data. With the PECs determined here, the first 30 vibrational states for each species are computed for the four electronic states when the rotational quantum number J equals zero ( J = 0). The vibrational level G( υ), inertial rotation constant B υ and centrifugal distortion constant D υ are determined when J = 0, which are in excellent agreement with the available experimental data. The ro-vibrational levels have been calculated for these low-lying electronic states of 10 isotopes when J ⩽ 29. And the transition lines are evaluated according to the transition rule. Comparison with the available experimental observations shows that the present ro-vibrational levels are reliable.

Excited States of SnSi: A Configuration Interaction Study

Electronic structure and spectroscopic properties of the ground and low-lying excited states of SnSi within 4 eV have been investigated by using a multireference singles and doubles configuration interaction (MRDCI) method that includes relativistic effective core potentials. Potential energy curves of a number of Lambda-S states of singlet, triplet, and quintet spin multiplicities are constructed. Spectroscopic parameters (T(e), r(e), omega(e), D(e), and mu(e)) of 27 bound Lambda-S states are reported. The ground state of SnSi belongs to the X(3)Sigma(-) symmetry with an estimated dissociation energy (D(e)) of 2.49 eV. However, with the inclusion of the spin-orbit coupling, D(e) reduces to 2.11 eV. Spectroscopic properties of at least 36 Omega states are determined. Transition probabilities of several singlet-singlet and triplet-triplet transitions are calculated. Partial radiative lifetimes of some of these transitions are estimated. A number of weak Omega-Omega transitions with partial radiative lifetimes of the order of milliseconds or more is also predicted here.

Electronic states and spectroscopic properties of GeSi

Electronic structure and spectroscopy of the GeSi molecule have been investigated by performing ab initio based multireference configuration interaction calculations. Potential energy curves of 29 Λ-S states of singlet, triplet, and quintet spin multiplicities have been constructed. Spectroscopic constants of 24 bound states within 36 000 cm −1 are reported and compared with the available data. The calculated dissociation energy of GeSi in the ground state is 2.80 eV. Effects of the spin–orbit coupling on the spectroscopic properties of the molecule have been found to be small. However, the computed zero-field-splitting of the ground state compares well with the earlier prediction. Transitions such as 2 3Σ −– X 3Σ −, 3 3Σ −– X 3Σ −, 4 3Π– A 3Π, 5 3Π– A 3Π etc. are relatively strong. Radiative lifetimes for several dipole allowed and spin-forbidden transitions are calculated. The estimated lifetimes of the 2 3Σ −, 3 3Σ −, and 5 3Π states are about 109, 33, and 62 ns, respectively. Dipole moments of most of the low-lying states of GeSi are also reported.

Dimetallocene carbonyls: The limits of the 18-electron rule and metal–metal multiple bonding in highly unsaturated molecules of the early transition metals

The structures and energetics of Cp 2M 2(CO) (M = Mn, Cr, and Ti) have been investigated by density functional theory (DFT) for comparison with previously reported studies on Cp 2V 2(CO). For all four metals energetically competitive structures are found with singlet, triplet, quintet, and septet spin multiplicities. However, significant spin contamination was found for the triplet Cp 2Mn 2(CO) and Cp 2Cr 2(CO) structures. The lowest energy structures for Mn and Cr are both septet states, namely Cp 2Mn 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and an Mn Mn distance of 2.523 Å and Cp 2Cr 2(μ-CO) with a two-electron donor bridging carbonyl group and a Cr Cr distance of 2.436 Å. However, for the Ti analogue the lowest energy structure is singlet Cp 2Ti 2(η 2-μ-CO) with a four-electron donor bridging carbonyl group and a Ti Ti distance of 2.364 Å. The higher energy singlet structures of Cp 2M 2(CO) have the very short metal–metal distances of 1.879 Å (M = Mn) and 1.729 Å (M = Cr) suggesting very high formal bond orders of five and six, respectively.

MRSDCI study on potential energy curves and ro-vibrational spectra of the BBr molecule

Four low-lying electronic states of the BBr molecule, including Σ+ symmetry and Π symmetry with singlet and triplet spin multiplicities, have been studied using the complete active space self-consistent-field method followed by multi-reference single and double excitation configuration interaction calculation. For the ground state, four isotopes (11B79Br, 11B81, 10B79Br, 10B81Br) of the BBr molecule have been studied, since their experimental spectroscopic constants and ro-vibrational energy levels are abundant. For the three excited states, just the most natural abundance isotope–11B79Br–is investigated. The spin–orbital coupling effect for the a3Π state is also considered. Finally, the analytical potential energy functions of these states are fitted except for the 3Σ+ state because its potential well is too shallow to be fitted with available functions. These results have been compared in detail with those of the investigations reported in the literature.

AsP分子基态和低激发态的多参考组态相互作用研究

Nine low-lying electronic states of the AsP molecule, including Σ+, Π, and Δ symmetries with singlet, triplet, and quintet spin multiplicities, are studied using multi-reference configuration interaction method. The potential energy curves and the spectroscopic constants of these nine states are determined, and compared with the experimental observed data as well as other theoretical works available at present. Three quintet states are reported for the first time. Furthermore, the analytical potential energy functions of these states are fitted using Murrell-Sorbie function and least square fitting method.

The electronic spectrum of the SiC radical: A theoretical study

Electronic structure and spectroscopic properties of the low-lying electronic states of the SiC radical have been determined from the ab initio based configuration interaction calculations. Potential energy curves of 32 Λ–S states of singlet, triplet, and quintet spin multiplicities have been constructed. Spectroscopic constants ( r e, T e, and ω e) of 23 states within 6 eV are reported and compared with the existing data. The dipole moments ( μ e) of most of these states at their respective equilibrium bond lengths have been computed. Effects of the spin–orbit coupling on the spectroscopic properties of SiC have been studied. The E 3Π state is found to be an important one which has not been studied before. A transition of the type E 3Π–X 3Π is predicted to take place in the range 25 000–26 000 cm −1. The partial radiative lifetimes for several electric dipole allowed transitions such as A 3Σ +–X 3Π, B 3Σ +–X 3Π, C 3Π–X 3Π, D 3Δ-X 3Π, E 3Π–X 3Π etc. have been reported.

On the [formula omitted] paradigm

A survey of the existing experimental and theoretical data on the trinitrogen dioxide anion N 3 O 2 - that manifests a controversy as to the number of isomers and their chemical structures is presented. To resolve the controversy, new computational studies are performed at the MP2/aug-cc-pVTZ computational level. Two hitherto unknown isomers are predicted, one with singlet and one with triplet spin multiplicity. The singlet isomer, structurally characterized as N 2 · [ONO] −, is the most stable among all known isomers and accounts for fragmentation patterns observed in the recent dissociative photodetachment experiments.

Intrinsic Flexing Abilities of Molecular Muscles Based on Tetrakis(2,3-thienylene): A Quantum Mechanical Study

The conformational and energetic properties of tetrakis(2,3-thienylene), which is the building block of a single-molecule electroactive mechanical actuator, and some substituted derivatives have been characterized using quantum mechanical calculations. More specifically, calculations have been used to investigate the actuation range and the ability to be oxidized of tetrakis(2,3-thienylene), its homodimer, and some derivatives containing charged substituents. The actuation range of tetrakis(2,3-thienylene) upon oxidation to the 1+ state was found to be about 6%, while oxidation to the 2+ state with triplet and singlet spin multiplicities produces dimensional changes of 5% and 11%, respectively. Interestingly, the singlet dicationic state was predicted to be significantly more stable than the triplet one. Similar changes in the molecular length were obtained for the dimer of tetrakis(2,3-thienylene) upon oxidation, in this case the actuation range of the dication being about 11% for the two spin multiplicities. However, it was found that the actuation range increases to about 14% and 19% when the 3+ and 4+ states are considered for the dimer. Finally, incorporation of substituents on the tetrakis(2,3-thienylene) unit led to stabilization of the charged states, even though no relevant improvement in the range of actuation was detected.

Fluorescence from the [formula omitted] dication: Theory and experiment

The recently discovered CS 2 2 + emission has been investigated experimentally and theoretically, leading to the identification of two separate fluorescent dicationic transitions ( X 3 Σ g - ← 1 3 Σ u - and 1 1Δ g ← 1 1Δ u). The vertical energies of the fluorescent electronic states were directly obtained by detecting photons in coincidence with electron pairs. Their assignment was aided by the use of glass filters to isolate the spectral region of each emission and by performing highly correlated MRCI calculations at a range of geometries. The potential energy curves thus generated predict bound potential energy functions for all linear Σ and Δ states of triplet and singlet spin multiplicities, throughout the energy range studied. Dissociation onsets for different fragmentation pathways are re-evaluated by detecting ions in coincidence with electron pairs. The C + + S + onset is revised downwards to 36.8 ± 0.4 eV and C + + S 2 + formation is found to arise from non-statistical dissociation of a limited number of precursor states, which have been identified to be the electronically excited CS 2 2 + formed in bent structures at these internal energies. Assignments have been attempted for all fragment emissions observed by measuring quantum yields as a function of the selected spectral region.

Solid-State Structures and Magnetic Properties of Halide-Bridged, Face-to-Face Bis-Nickel(II)-Macrocyclic Ligand Complexes: Ligand-Mediated Interchanges of Electronic Configuration

Dramatic differences are found between the ambient and 100 K X-ray structures of [L(2)Ni2Br2](ClO4)2 (L(2) = alpha,alpha'-bis{(5,7-dimethyl-1,4,8,11-tetraazacyclotetradeca-6-yl)-o-xylene), in which the bromide-bridged, bimetallic, macrocyclic ligand complexes of nickel(II) are held face-to-face and in which each bimetallic complex has a net triplet spin multiplicity. The ambient structure of this complex consists of very highly ordered, infinite chains of alternating R and S isomers in which the identical Ni(II) coordination spheres are near to the average expected for the high- and low-spin Ni(II) coordination sites, and there is appreciable stereochemical strain in the linkage of the macrocyclic ligands to the phenyl ring. In contrast, every other dinickel complex of the 100 K structure is displaced about 40 pm along the infinite chains to form tetrameric repeat units (pairs of dinickel complexes), in which each dinickel complex has well-defined high-spin and low-spin Ni(II) coordination sites; the high-spin sites are adjacent in the tetramers, and the stereochemical strain in the linkage to the phenyl spacer is relaxed. The molecular magnetic moments and structural contrasts are similar for the 100 K structure and the previously reported ambient structure of [L(2)Ni2Br3](ClO4) complex for which the molecular magnetic moments also correspond to a single triplet state per complex. The halide-bridged, monochloro- and monobromo dinickel complexes also have triplet spin multiplicity, and they crystallize with a coordinated perchlorate completing the axial coordination of the high-spin Ni(II) site, while the other Ni(II) site of these halide-bridged complexes has equatorial Ni-N bond lengths typical of low-spin Ni(II) coordination. The bridging halide is sandwiched between the face-to-face macrocyclic ligand Ni(II) moieties and slightly off the Ni-Ni axis in all of the complexes. The temperature dependence of the magnetic moments of the series of complexes indicates that their singlet-triplet energy gaps are small, with zero point energy differences that are generally less than 10(3) cm(-1). The very weak metal-metal electronic coupling, the triplet state spin multiplicity of each dinickel complex, and the averaged high-spin/low-spin coordination environments of the ambient structure implicate a vibronic mechanism for the electronic configurational exchange in the dibromo and tribromo complexes. The single molecular vibrational mode that correlates with the configurational exchange in these complexes includes the concerted motion of the bridging bromide between the Ni(II) centers. Activation of this vibrational mode is sufficient to effect the configurational exchange. These complexes present especially clear examples of the effects of the coupling of nuclear vibrational motions to the interchange of electronic configuration between two different centers.

Spectroscopy and dynamics of hydride radical van der Waals complexes

Spectroscopic and theoretical studies of hydride radical–rare gas atom complexes (Rg–HX) are reviewed. This family of van der Waals molecules is of interest as they can be used to explore the characteristics of the long-range forces associated with open-shell species. Orbitally degenerate states of HX radicals have an electronic anisotropy that results in van der Waals interactions that are qualitatively different from those exhibited by the corresponding closed-shell systems. Rg–HX complexes, where X is a first- or second-row p-block element, reveal systematic trends where the anisotropic components of the physical interactions are determined by the electronic orbital configuration. Radicals in Σ, Π and Δ states with singlet, doublet and triplet spin multiplicities have been examined. When Rg = He, Ne or Ar the interaction potential energy surfaces can be predicted using high-level ab initio methods. Theoretical studies have established the methods and basis sets that are capable of providing an accurate description of the long-range forces for open-shell molecules. Clusters consisting of an HX molecule with multiple rare gas atoms are model systems for studies of solvated radicals. Potential energy surfaces derived from the binary clusters are being used to construct approximate potentials for Rg n –HX clusters. The equilibrium structures and vibrational dynamics predicted for these systems show that solvated radicals exhibit unique properties. Contents PAGE 1. Introduction 376 2. Experimental methods 377 3. Theoretical considerations 378 4. Binary complexes 384 4.1. BH–Ar and AlH–Ar 384 4.2. CH–Rg complexes (Rg = He, Ne and Ar) 386 4.3. NH–Rg complexes (Rg = He, Ne and Ar) 397 4.4. OH–Rg and SH–Rg complexes 407 5. Complexes of HX radicals with multiple Rg atoms 413 6. Conclusions and future directions 417 Acknowledgements 418 References 418

Quantum chemical density-functional theory calculations of the structures of defect C60 with four vacancies.

Quantum chemical density-functional theory (DFT) calculations have been carried out for the six isomers obtained by removing four adjacent atoms from C60. The most stable isomer consists of twelve 5-member and eighteen 6-member rings, indicating that the removal of some atoms from C60, which contains twelve 5-member rings and twenty 6-member rings, does not always generate larger holes. Each of the other five isomers contains at least one 4-member ring and one larger ring (7-, 8-, 9-, or 10-member ring) besides the 5- and 6-member rings. All isomers have similar structures for singlet and triplet spin multiplicities but with different stabilities. The ground states for two of the isomers are triplets, whereas the ground states for the other isomers are singlets. Furthermore, a comparison between the various isomers allowed one to examine the effect of the structure on the stability of fullerene cages.

Contribution of Diffusion to the Quenching by Oxygen of the Lowest Electronically Excited Singlet and Triplet States of Aromatic Molecules in Liquid Solution under High Pressure

The quenching by oxygen of the lowest electronically excited singlet (S1) and triplet (T1) states of five aromatic molecules in methylcyclohexane (MCH) at pressures up to 400 MPa was investigated. The apparent activation volume for the S1 state, Δ , at 0.1 MPa fell in the range from 14 to 16 cm3/mol, which is significantly smaller than Δ (25 cm3/mol) determined from the pressure dependence of the solvent viscosity, η, whereas that for the T1 state, Δ , at 0.1 MPa changed from +6.1 (anthracene) to −15.2 cm3/mol (triphenylene) and correlated approximately linearly with the triplet energy, ET. However, Δ at 350 MPa was positive and almost independent of ET (3−6 cm3/mol). The pressure dependence of was interpreted in the framework reported previously by us, and that of was attributed to the contribution of diffusion to the quenching in which the encounter complex pair with singlet, triplet, and quintet spin multiplicities formed between the oxygen and T1 state molecules is involved.

Quenching by Oxygen of the Lowest Singlet and Triplet States of Pyrene and the Efficiency of the Formation of Singlet Oxygen in Liquid Solution under High Pressure

Quenching by oxygen of the lowest singlet (S1) and triplet (T1) states of pyrene at pressures up to 400 MPa in liquid solution was investigated. The rate constant of the S1 state, kqS, decreased significantly with increasing pressure, while that of the T1 state, kqT, was nearly independent of pressure at the lower pressure region (<100 MPa) and decreased monotonically with further increase in pressure. The activation volume for kqS, ΔVqS⧧, at 0.1 MPa fell in the range of 12−16 cm3/mol, depending on the solvents examined, whereas that for kqT, ΔVqT⧧, was nearly zero. It was found that both the activation volumes, ΔVqS⧧ and ΔVqT⧧, are significantly smaller than those determined from the pressure dependence of the solvent viscosity, η, ΔVη⧧ (22−25 cm3/mol). For the quenching of the S1 state, the large difference between ΔVqS⧧ and ΔVη⧧ was interpreted in terms of the competition of the quenching with diffusion, and kqS was separated into the contributions of the rate constants for diffusion, kdiff, and for the bimolecular quenching in the solvent cage, kS,bim. For the quenching of the T1 state, it was found that kqT/kdiff increases over 1/9 and approaches 4/9 with increasing pressure. The oxygen concentration dependence on the quantum yield for the formation of singlet oxygen, ΦΔ, in methylcylohexane (MCH) was measured at pressures up to 400 MPa in order to separate ΦΔ into the contributions of the S1 and T1 states. From the results, together with those of the pressure dependence of the quantum yield of T1 state, the branching ratio for the formation of singlet oxygen in the T1 state, fΔT, was found to decrease with increasing pressure. The oxygen quenching of the T1 state from these results was discussed by using the mechanism that involves the encounter complex pairs with singlet, triplet, and quintet spin multiplicities.

Facile synthesis, crystal structures, and high-spin cationic states of all-para-brominated oligo(N-phenyl-m-aniline)s.

Syntheses of both the dimer (3) and the trimer (4) of all-para-brominated poly(N-phenyl-m-aniline)s (2c) were achieved in a one-pot procedure from the parent nonbrominated oligomers and benzyltrimethylammonium tribromide [(BTMA)Br(3)]. An X-ray crystallographic analysis revealed that 4 has a U-shaped structure, suggesting that 2c easily adopts helical structures. Furthermore, the redox properties were investigated by the UV-vis and EPR measurements. It was confirmed that the both 3 and 4 can be oxidized into the dications 3(2+) and 4(2+) with triplet spin-multiplicity.

Effect of Pressure on the Intersystem Crossing between the Encounter Complex Pairs Involved in Triplet−Triplet Annihilation of 2-Acetonaphthone in Liquid Solution

The bimolecular rate constant, kTT, for triplet−triplet annihilation (TTA) of 2-acetonaphthone (ACN) in methylcyclohexane (MCH) and n-hexane at pressures up to 400 MPa at 25 °C has been measured. It was found that kTT decreases significantly with increasing pressure, and the activation volumes for kTT, ΔVTT⧧, were determined to be 18 and 11 cm3/mol, whereas those for the solvent viscosity, ΔVη⧧, were 24 and 23 cm3/mol in MCH and n-hexane, respectively. The significant difference between ΔVTT⧧ and ΔVη⧧ was attributed to the competition of the quenching with diffusion. From the analysis on the basis of the pressure dependence of the solvent viscosity, η, on the quenching, the observed rate constant, kTT, was separated into the contributions of the rate constant for diffusion, kdiff, and the bimolecular rate constant for quenching, kbim, in the solvent cage. By using the values of kdiff thus determined, 2kTT/kdiff was found to be about 4/9 at 0.1 MPa and to approach unity with increasing pressure. The results were interpreted by a kinetic model that involves the encounter complex pairs, i(M*M*)en (i = 1, 3, 5), with singlet, triplet, and quintet spin multiplicities. It was concluded that the rate constant for the dissociation, k-diff, decreases more significantly than that for the intersystem crossing, kenisc(i), between the encounter complex pairs with increasing pressure (Scheme 4).

Electronic states of CF +

Ab initio multireference configuration interaction calculations have been carried out on the ground and excited electronic states of the cation CF + of singlet and triplet spin multiplicities. The results on the ground and the first excited state, a 3Π, are in good agreement with experimental and previous theoretical results. The potential energy curve of a 3Π has a deep minimum of 2.27 eV at 1.227 Å. The potential of the 1 1Π state has a very shallow local minimum (0.10 eV) at 1.307 Å. The potential energy curves of the higher excited states show mostly shallow minima at large internuclear distance, but they are repulsive at the r e of the ground state.

Laser photolysis studies of electron transfer between triplet naphthoquinones and amines

Electron transfer between triplet naphthoquinones ( 3Nap ∗; 1,4-naphthoquinone, 2-methyl-1,4-naphthoquinone and 2,3-dimethyl-1,4-naphthoquinone in the triplet state) and N, N-dimethylaniline (DMA) or triethylamine (TEA) in acetonitrile (MeCN) and a mixture of MeCN and H 2O (4:1, v/v) was studied by laser photolysis techniques at 295 K. By analysis of the transient absorption spectrum observed after the completion of electron transfer, the absorption spectra and coefficients of the naphthoquinone anion radicals (Nap .−) were determined on the basis of those of the DMA cation radical (DMA .+) in MeCN and MeCNH 2O (4:1, v/v). The values of the rate constants ( k et) for electron transfer between 3Nap ∗ and DMA were found to be close to those for diffusion-controlled processes as predicted by the Rehm-Weller theory on electron transfer. Using the determined molar absorption coefficients of Nap .−, the efficiencies ( Γ rad) for the information of Nap .− and DMA .+ were shown to be unity since the radical ion pair produced ( 3[Nap .− + DMA .+]) readily dissociates into Nap .− and DMA .+ due to the triplet spin multiplicity.