Singlet-triplet Energy Splitting Research Articles

Electronic structure of the ground state and calculated spectrum of low-lying excited states of Mo 3X 4(aq) 4+9 (X=O, S) cluster ions

The electronic structure of the ground state and electronic transition energies of low-lying states have been calculated using the INDO/S-CI method for Mo 3X 4(aq) 4+ 9 cluster ions(X=O, S). It has been shown that the calculated values are in accordance with those observed experimentally, allowing us to assign the bands. The lowest absorption band results from the dipole-forbidden but spin-allowed electron transition 1 1A 1→1 1A 2 state, and it has been assigned as arising from a metal-based bonding orbital to a metal-based antibonding orbital. The singlet-triplet energy splitting of the two lowest excited states, A 2, E, have been estimated to be about 2400 and 3350 cm −1, respectively, and a red-shift of the absorption bands has been explained by comparing X=S with X=O for the Mo 3X 4(aq) 4+ 9 ions.

Singlet—triplet gap in HCl halocarbene

The singlet—triplet energy separation, geometrical structure, harmonic vibrational frequencies, adiabatic ionization potential and electron affinity, dissociation energies and dipole moment for HCl have been determined by using the linear combination of Gaussian-type orbital-local spin density method (LCGTO-LSD) and employing both the local and nonlocal exchange and correlation potentials. Computations show that HCI has a singlet ground state ( 1A 1) in agreement with a previous theoretical result but in disagreement with recent experimental data. The singlet—triplet energy splitting is found to be 3.7 kcal/mol at the nonlocal level of computation. The calculated spectroscopic parameters agree with experimental ones where available.

Triplet Exciton States in a-Si:H and Its Alloys as Elucidated by Optically Detected Magnetic Resonance Measurements

Triplet exciton states in a-Si:H, a-Si,C:H and a-Si, N:H were investigated by the steady-state and time-resolved optically detected magnetic resonance techniques. The existence of triplet exciton states in a-Si:H and a-Si, N:H materials was confirmed, for the first time from these measurements. The extent of the electron orbitals in the triplet exciton state and its averaged lifetime in these materials were estimated to be ∼10 A and ∼1 ms, respectively. A singlet-triplet exciton splitting energy in a-Si:H was also estimated to be ∼0.15 eV. An impurity-related center and a self-trapped exciton center are considered as the origin for the triplet exciton centers in these materials.

ChemInform Abstract: Structural and Magnetic Properties of Bis(ethanolammonium)hexahalodicuprate(II) Salts: Copper(II) Halide Dimers with a Square‐Pyramidal Type Distortion

Abstract The compounds (C 4 H 16 N 2 O 2 )Cu 2 Cl 6 and (C 4 H 16 N 2 O 2 )Cu 2 Br 6 have been synthesized and their crystal structures determined. EPR and magnetic susceptibility measurements were made. The crystals of both compounds are triclinic, space group P 1 with a = 6.077(1), b =8.068(2), c =8.455(2) A, α=67.99(2), β=79.90(2), γ=78.77(2)°, Z =2, and D calc = 2.08 g/cm 3 for the chloride salt, and a =6.418(2), b =8.262(2), c =8.764(2) A, α=68.17(2), β=80.12(2), γ=78.47(2)°, Z =2, and D calc =2.88 for the bromide salt. The structures, solved by direct methods and refined by least-squares calculations to R =0.036 (chloride) and R =0.049 (bromide), consist of nearly planar Cu 2 X 6 2− dimers stacked to form infinite chains. The coordination of the copper atoms may be described as distorted square pyramidal with an ethanolammonium oxygen semi- coordinated to the sixth coordination site for each copper atom. Both compounds exhibit predominantly antiferromagnetic exchange interactions. The bromide salt exhibits a intradimer singlet-triplet energy splitting of J / k =−64 K and a interdimer splitting of J ′/ k =−21 K, while for the chloride J / k =−6 K and J ′/ k =−6 K is deduced. A comparison with the magnetic behavior of other Cu 2 - X 6 2− dimers is given.

Magneto-structural correlations in chains of bifolded Cu 2Cl 62− dimers

The magnetic susceptibility of two ACuCl 3 salts containing chains of bibridged Cu 2Cl 6 2− dimers has been measured. Each copper ion bridges to a chloride in an adjacent dimer, thus assuming a 4 + 1 coordination geometry. Structurally, each dimer is characterized by a bridging CuClCu angle, ø, and a fold angle, σ, at each copper center. The high temperature susceptibility data for (4-benzylpiperidinium)CuCl 3 (ø=95.3°, σ=28.9°) obey a Curie-Weiss law which has a positive intercept (θ=13 K), indicating a predominant ferromagnetic interaction. For the low temperature data, the Curie-Weiss plot has a negative intercept (θ′=−4 K), showing that a net antiferromagnetic coupling exists. The data is interpreted quantitatively in terms of a system of ground state triplet dimers, with singlet-triplet energy splitting Δ E(=2 J) of Δ E/k=60 K with interdimer interactions, accounted for with a mean field approximation of J′/ k=−3 K. The salt (paraquat)Cu 2Cl 6 (ø=97.5°, σ=31.7°) behaves as an antiferromagnetic alternating chain with J/k=19 K and J′/ k=2 K. The magnetic properties of two other salts containing structurally similar chains, (CH 3) 2NH 2CuCl 3 and (CH 3) 2CHNH 3CuCl 3, have been reinvestigated using pulsed high field magnetization techniques. Comparison of these systems shows that, with ø constant, there is an approximate linear relationship between Δ E and the fold angle, σ. At ø=95.5°±0.3°, the interaction is ferromagnetic for σ>22° and antiferromagnetic for smaller values of σ. These conclusions are confirmed by extended Hückel MO calculations for the intradimer interactions within the Hoffmann framework for exchange coupling.

Structural and magnetic properties of bis(ethanolammonium)hexahalodicuprate(II) salts: Copper(II) halide dimers with a square-pyramidal type distortion

The compounds (C 4H 16N 2O 2)Cu 2Cl 6 and (C 4H 16N 2O 2)Cu 2Br 6 have been synthesized and their crystal structures determined. EPR and magnetic susceptibility measurements were made. The crystals of both compounds are triclinic, space group P 1 with a= 6.077(1), b=8.068(2), c=8.455(2) Å, α=67.99(2), β=79.90(2), γ=78.77(2)°, Z=2, and D calc= 2.08 g/cm 3 for the chloride salt, and a=6.418(2), b=8.262(2), c=8.764(2) Å, α=68.17(2), β=80.12(2), γ=78.47(2)°, Z=2, and D calc=2.88 for the bromide salt. The structures, solved by direct methods and refined by least-squares calculations to R=0.036 (chloride) and R=0.049 (bromide), consist of nearly planar Cu 2X 6 2− dimers stacked to form infinite chains. The coordination of the copper atoms may be described as distorted square pyramidal with an ethanolammonium oxygen semi- coordinated to the sixth coordination site for each copper atom. Both compounds exhibit predominantly antiferromagnetic exchange interactions. The bromide salt exhibits a intradimer singlet-triplet energy splitting of J/ k=−64 K and a interdimer splitting of J′/ k=−21 K, while for the chloride J/ k=−6 K and J′/ k=−6 K is deduced. A comparison with the magnetic behavior of other Cu 2- X 6 2− dimers is given.

UV laser photochemistry: Evidence for “through-bond coupling” effects in the lifetimes of 2,2-dimethyl-1,3-cyclopentadiyl triplet diradicals.

The 2,2-dimethyl substituted 1,3-diradicals 5a-c possess significantly shorter triplet lifetimes (τ t) than the parent 1,3-cyclopentadiyl 3 , which is being rationalized on the basis of changes in the “through-bond” coupling and thus singlet-triplet energy splitting between the radical sites.

Direct theoretical ab initio calculations in exchange coupled copper(II) dimers: Influence of the choice of the atomic basis set on the singlet–triplet splitting in modeled and real copper dimers

The singlet–triplet splitting (2J=EST) is calculated in bis-[bromo-(N,N-diethylaminoethanolato)copper(II)], belonging to the group of alkoxo-bridged copper(II) dimers, by a recently proposed ab initio method. The singlet–triplet splitting energy is obtained by a perturbation expansion closely related to Anderson’s formalism. The role of all valence electrons is taken into account and it is shown to be very important in order to obtain a satisfying agreement with the experimental result. Two important facts, from a computational point of view, are tested: influence of the atomic basis set size, and influence of modifying of the ligands. Changes in the basis set of the bridging atoms can induce variations in the results from 10% to 20%. As a final result, the theoretical value of the real complex is calculated to 2J(calc)=−1095 cm−1 and agrees with the experimental one 2J(exp)=−817 cm−1.

Magnetic and Spectral Properties of 3,5-Dichloropyridine Adducts of Dimeric Copper(II) Acetate, Chloroacetate, and Dichloroacetate

Abstract Three complexes with the empirical formula of Cu(RCOO)2·L, where R=CH3, CH2Cl, or CHCl2 and L=3,5-dichloropyridine, were prepared and studied. These complexes display the antiferromagnetic behavior typical of dimeric copper(II) carboxylates. The singlet-triplet splitting energy (−2J) is 349 cm−1 for the acetate, 340 cm−1 for the monochloroacetate and 312 cm−1 for the dichloroacetate. Comparison of their magnetic and electronic spectral properties with those of the corresponding pyridine adducts indicates that both the −2J and \ ildeνmax(absorption maximum of Band I) values of the respective 3,5-dichloropyridine adducts are higher than those of the pyridine adducts. The influence of the pyridines on both the magnetic and spectral properties was discussed.

Synthesis and Maganetic Susceptibility of Dimeric Copper(II) Acetate Adducts with Triphenylphosphine or Triphenylphosphine Oxide

Abstract Two compounds of stoichiometry Cu(CH3COO)2·L (L=triphenylphosphine or triphenylphosphine oxide) were prepared. These compounds display the antiferromagnetic behavior typical of dimeric copper(II) carboxylates. The singlet-triplet splitting energy is 371 cm−1 for the phosphine adduct and 319 cm−1 for the phosphine oxide adduct.

Methylene singlet–triplet energy splitting by molecular beam photodissociation of ketene

The singlet–triplet splitting in methylene has been determined from the measurements of fragment velocities from ketene photodissociation at 351 and 308 nm in a molecular beam. The splitting is found to be 8.5±0.8 kcal/mol. This agrees with many experimental results, but not with the value of 19.5 kcal/mol derived from recent photodetachment experiments on CH−2.

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in π2 configurations of diatomic molecules

The CEPA-PNO method is used for calculating the energy difference ΔEST between the3∑− and the1Δ states of diatomic molecules in electronic π2 configurations. An analysis of the contribution of electron correlation to ΔEST is performed in terms of physically understandable effects such as direct correlation, dynamic spin polarization, semiinternal and internal excitations. It is shown that these effects are of completely different importance for the molecules treated in this study: For C2 the direct correlation between the two singly occupied π-orbitals is the dominant correlation contribution to ΔEST; for O2, S2, SO the internal excitation πu2 → πg2 is predominant, whereas for NH and PH there is a close competition between the direct correlation and the spin polarization of the underlying σ-orbitals. The basis set dependence of these effects is investigated, in particular for NH. Our final results reproduce experimental values of ΔEST within 0.05–0.10 eV.

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in π2 configurations of diatomic molecules

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in π2 configurations of diatomic molecules

CEPA calculations on open-shell molecules. IV. Electron correlation effects in B1 Rydberg states of H2O

We have calculated the vertical electronic excitation energies of ten 1b1→a1 excited states of H2O belonging to different Rydberg series. The calculations have been carried out with the FRC, SCF, PNO–CI, and CEPA approximations in order to investigate the electron correlation of Rydberg states. We report on correlation contributions to the electron binding energies, to the quantum defect of Rydberg series, and to the singlet–triplet splitting energies. In addition we make predictions for the dipole moments of the Rydberg states based on SCF calculations. Comparison of our results with experiment allows the assignment of transitions belonging to a np series and two nd series and it suggests an analysis for the 1b1→ns1B1 Rydberg series.

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in ?2 configurations of diatomic molecules

CEPA calculations on open-shell molecules. II. Singlet-triplet energy splitting in ?2 configurations of diatomic molecules

The low lying states of ammonia; generalized valence bond and configuration interaction studies

The nine lower singlet and triplet states of ammonia were studied using ab initio generalized valence bond (GVB) and configuration interaction (CI) wavefunctions. The eight lowest (vertical) excited states were found to be Rydberg in nature. Vertical excitation energies were determined to be (in eV): 6.14 and 6.37 for n → 3s; 7.86 and 7.88 for n → 3p x,y , and 7.87 and 8.15 for n → 3p z (in each case the triplet energy is first). These results are in excellent agreement (0.06 eV average error) with the observed electron impact excitation energies (in eV), 6.39 (A 1A″ 2), 7.93 (B 1E″), and 8.26 C 1A′ 1). The small singlet triplet energy splittings are consistent with those expected for Rydberg-like excited states. We find no vertical transition consistent with the small peak at 4.4 eV apparently observed in threshold electron impact experiments.

Phosphorescence study of nitronaphthalenes

The phosphorescence yields and lifetimes of five nitronaphthalenes have been measured at 77°K in EPA in order to provide a better understanding of their lowest triplet which is π, π ∗ . 2-Nitronaphthalene, which is presumed to be planar, exhibits a fivefold decrease in triplet deactivation relative to 1-nitronaphthalene, which is bent with an angle of 49°. The results Suggest that the rate constant for radiationless deactivation of the triplet is affected to a greater extent by substitution than is the rate constant for the radiative process. The highest phosphorescence yield measured was 0.79 for 1,8-dinitronaphthalene. The higher triplet yield of 2-nitronaphthalene relative to 1-nitronaphthalene may be due to the smaller singlet-triplet energy splitting in the former compound.

Energy levels of Tm 168

Fifty-one levels in Tm 168 have been observed up to an excitation energy of 1514 keV utilizing 12-MeV deuterons and the reaction Tm 169(d, t)Tm 168. The Q value of the lowest observed state was determined to be −1776 ± 15 keV. The spectrum below 700 keV has been interpreted in terms of the coupling of the [411↓] Nilsson proton orbital with the neutron orbitals prominent in the low-energy (d, t) spectrum of Er 167. This interpretation has resulted in the determination of relative energies due to the residual neutron-proton interaction for six different configurations. The observed singlet-triplet splitting energies for the [633↑], [521↓], and [512↑] neutron orbitals coupled to the [411↓] proton orbital were measured to be −157, +191, and −234 keV, respectively. Theoretical calculations of these energies made for a zero range spin-dependent central potential gave values of −87, +154, and −255 keV, respectively. The good agreement indicates that the spin-spin interaction can account for most of the singlet-triplet splitting energy.

Two-Quasiparticle States inTb158

Thirty-four levels in ${\mathrm{Tb}}^{158}$ have been observed up to an excitation energy of 1166 keV utilizing 12-MeV deuterons and the reaction ${\mathrm{Tb}}^{159}(d,t){\mathrm{Tb}}^{158}$. The ground-state $Q$ value was determined to be -1870 \ifmmode\pm\else\textpm\fi{} 15 keV. The spectrum has been interpreted in terms of the coupling of the [$411\ensuremath{\uparrow}$] Nilsson proton orbital with the neutron orbitals prominent in the ($d,t$) spectrum of ${\mathrm{Gd}}^{157}$. This interpretation has resulted in the determination of relative energies due to the residual neutron-proton interaction for eight different configurations. The observed singlet-triplet splitting energies for the [$521\ensuremath{\uparrow}$], [$402\ensuremath{\downarrow}$], and [$400\ensuremath{\uparrow}$] neutron orbitals coupled to the [$411\ensuremath{\uparrow}$] proton orbital were measured to be +132, -111, and +136 keV, respectively. Theoretical calculations of these energies made for a zero-range spin-dependent central potential gave values of +174, -186, and +136 keV, respectively.

Two-quasiparticle states in 164Ho

Thirty-one levels in 164Ho have been observed up to an excitation energy of 994 keV utilizing 12 MeV deuterons and the reaction 165Ho(d, t) 164Ho. The ground state Q-value was determined to be −1730±15 keV. The spectrum has been interpreted in terms of the coupling of the [523↑] Nilsson proton orbital with the neutron orbitals prominent in the (d, t) spectrum of 163Dy. Based on this interpretation, the singlet-triplet splitting energies for the [523↓], [642↑], [521↑], [402↓] and [400↑] neutron orbitals coupled to the [523↑] proton orbital were determined to be −144, +67, +171, −85 and +102 keV, respectively. Theoretical calculations of these energies made for a zero-range spin-dependent central potential gave values of −248, +74, +160, −82 and +73 keV, respectively.