Ab initio study of low-lying triplet states of the lithium dimer

Observation of Bose-Einstein condensation in Li27 initiated the interest in the scattering length of two ground state lithium atoms when they approach each other as a radical pair triplet aSigmau+3 state. But some properties of this state are still unknown. In present work, a number of low-lying triplet states of lithium molecule are calculated by multi-configuration self-consistent field (MCSCF) and response techniques with account of spin-orbit coupling, spin-spin coupling and some other magnetic perturbations. The singlet-triplet transition probabilities to the ground state are also presented. Most results are connected with the weakly bound lowest triplet a3Sigmau+ state, whose radiative lifetime and spin-splitting are unknown so far in spite of its great importance in Bose-Einstein condensation. Calculations indicate that this state has a very small spin-splitting, lambdass=-0.01 cm-1, which is negligible in comparison with the line-width in experimental Fourier transform spectra published so far. Similar splitting is obtained for the upper state of the 1(3)Sigmag+--a3Sigmau+ transition. This is in agreement with experimental rovibronic analysis of the 1(3)Sigmag+--a3Sigmau+ band system in which the triplet structure was not resolved. The radiative lifetime of the a3Sigmau+ state is predicted to exceed 10 h.     

Theoretical study on low-lying electronic states of NiH2

The low-lying electronic states of the NiH2 molecule were investigated by using the MCQDPT2 method. In order to accurately describe the strong correlation derived from the nickel 3d9 super-configuration, a set of diffuse secondary 3d' orbitals were included in the active space, yielding a large active space of 12 electrons in 13 orbitals. It is shown that the absolute minimum energy configuration of NiH2 is bent, in agreement with the experimental observation. The global ground state is 1A1 (or A1 in the spin-orbit coupling case), whereas the lowest linear state is 3Deltag (or 3g). Some other cheaper single-configurational and multi-configurational methods were also used to study both states, and their shortcomings are discussed. Our theoretical results suggest that the arrangement of the experimental frequencies of NiH2 and NiD2 may be incorrect.     

Theoretical study on the low-lying electronic states of NiH and NiAt

The low-lying electronic states of NiH and NiAt are investigated by using multireference second-order perturbation theory with relativistic effects taken into account. The potential energy curves as well as the corresponding spectroscopic constants are reported. The results are grossly in good agreement with the available experimental data and should thus be very useful for guiding future experimental measurements. A cross comparison with other nickel monohalides NiX (X = F, Cl, Br, and I) reveals that the change in the spin-orbit splittings when going from lighter to heavier ligands results more from the state interaction than from the relativistic effects of the ligands.     

Theoretical study of the low-lying electronic states of ZnO and ZnS

Theoretical spectroscopic constants (r_e, D_e) and dipole moments (μ, ∂μ/∂r) are determined for the ¹σ⁺, ^1,3Π and ³σ⁺ states of ZnO and ZnS, using extended Gaussian basis sets and incorporating correlation using both configuration- interaction and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are relatively small. At the CPF level, both ZnO and ZnS have ¹Σ⁺ ground states, with the ³Π state lying 209 and 2075 cm⁻¹ higher, respectively. The ³σ⁺ state lies about 1.5 eV higher in ZnO and 2.1 eV higher in ZnS. The ^1,3Π states are relatively close together since the exchange splitting is small with the σ electron localized on Zn and the π electron on oxygen (or sulfur).     

Molecular aniline clusters. II. The low-lying electronic excited states

The lowest electronically excited states of the aniline dimer and trimer related to the lowest π(?)←π transition of the monomer are investigated by applying time-dependent coupled cluster theory, primarily at the level of the (spin-component-scaled) CC2 model. Minimum energy structures in the vicinity of the Franck-Condon points were determined on the individual potential energy surfaces. For the dimer we find an excimer and a head-to-tail configuration (with the monomers substantially displaced relative to the ground state minimum) for the lowest (dark) and second lowest (bright) states, respectively. The excitation is delocalized on both chromophores for both of these states. For the trimer three distinct minima with quite different hydrogen-bonding arrangements are found for the three lowest states. In strong contrast to the dimer the excitation here is clearly localized on the individual aniline chromophores for each of these three states. One of the three geometries is rather similar to the ground state minimum, while the two others are rather different and thus have presumably quite small Franck-Condon factors. It can be expected that only the electronic origin of the first conformer can eventually be detected in the absorption spectrum of the trimer, provided that it is separated by high-enough barriers from other, energetically lower configurations.     

Theoretical study of the excited singlet and triplet states of alloxan

The possibility of excited‐state protomeric shifts in the biologically important molecule, alloxan, is investigated. We have focused on the S₁ and T₁ excited states of alloxan and its hydroxy tautomers. Modifications brought in by excitation on the relative stabilities, activation barriers, and optimized geometries, computed at the MNDO, AM1, and PM3 levels of approximation, have been discussed for both excited electronic states. The absorption and fluorescence spectra for the three tautomers are also discussed. Results show significant changes in the geometries on excitation, although the changes are similar for the singlet and triplet excited states. Though the relative stability orders do not change, the 2‐hydroxy tautomer is stabilized, while the 4‐hydroxy tautomer gets destabilized on excitation. The excited states are (n,π*) states, involving the promotion of a nonbonding oxygen lone pair from the CO? CO? CO moiety, which explains why the oxygens of this group become less basic and the 4‐hydroxy tautomer gets destabilized on excitation. However, the activation barriers do not reduce significantly on excitation, and this precludes the possibility of ground‐ or excited‐state proton transfer in the gas phase. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Theoretical characterization of the low-lying electronic states of NbC

We have studied the potential-energy curves and the spectroscopic constants of the ground and low-lying excited states of NbC by employing the complete active space self-consistent field method with relativistic effective core potentials followed by multireference configuration-interaction calculations. We have identified 23 low-lying electronic states of NbC with different spin multiplicities and spatial symmetries within 40,000 cm(-1). At the multireference single and double configuration interaction level of theory the 2sigma+ and 2delta states are nearly degenerated, with the 2delta state located 187 cm(-1) lower than the 2sigma+ state. The estimated spin-orbit splitting for the 2delta state results in a 2delta(3/2) ground state and A 2sigma+ which is placed 650 cm(-1) above the ground state, in reasonable agreement with the experimental result, 831 cm(-1). Our computed spectroscopic constants are in good agreement with experimental values although our results differ from those of a previous density-functional investigation of the excited states of NbC, mainly due to the strong multiconfigurational character of NbC. In the present work we have not only suggested assignments for the observed states but also computed more electronic states that are yet to be observed experimentally.     

Theoretical description of the low-lying excited states of CuCl

The low-lying excited states of CuCl have been investigated theoretically in the Hartree-Fock approximation. Spin-orbit interactions have been included semi-empirically using an atoms-in-molecules technique. All six excited states that were previously characterized experimentally are found to arise from fine structure sublevels of the Cu⁺(3d⁹4s ^1,3D)Cl⁻(3p⁶¹S) configuration.     

Theoretical study of the absorption and nonradiative deactivation of 1-nitronaphthalene in the low-lying singlet and triplet excited states including methanol and ethanol solvent effects

The photophysics of the 1-nitronaphthalene molecular system, after the absorption transition to the first singlet excited state, is theoretically studied for investigating the ultrafast multiplicity change to the triplet manifold. The consecutive transient absorption spectra experimentally observed in this molecular system are also studied. To identify the electronic states involved in the nonradiative decay, the minimum energy path of the first singlet excited state is obtained using the complete active space self-consistent field∕∕configurational second-order perturbation approach. A near degeneracy region was found between the first singlet and the second triplet excited states with large spin-orbit coupling between them. The intersystem crossing rate was also evaluated. To support the proposed deactivation model the transient absorption spectra observed in the experiments were also considered. For this, computer simulations using sequential quantum mechanic-molecular mechanic methodology was used to consider the solvent effect in the ground and excited states for proper comparison with the experimental results. The absorption transitions from the second triplet excited state in the relaxed geometry permit to describe the transient absorption band experimentally observed around 200 fs after the absorption transition. This indicates that the T(2) electronic state is populated through the intersystem crossing presented here. The two transient absorption bands experimentally observed between 2 and 45 ps after the absorption transition are described here as the T(1)→T(3) and T(1)→T(5) transitions, supporting that the intermediate triplet state (T(2)) decays by internal conversion to T(1).     

Theoretical studies on low-lying electronic states of cyanocarbene HCCN and its ionic states

Geometries of 10, 7, and 6 low-lying states of the HCCN neutral radical, its anion and cation, were optimized by using the complete active space self-consistent field (CASSCF) method in conjunction with the aug-cc-pVTZ basis set, respectively. Taking the further correlation effects into account, the second-order perturbations (CASPT2) were carried out for the energetic correction. Vertical excitation energies (T(v)) at the ground state geometry of the HCCN neutral radical were calculated for 11 states. The results of our calculations suggest that the spin-allowed transitions of HCCN at 4.179, 4.395, 4.579, 4.727 and 5.506 eV can be attributed to X(3)A' --> 2(3)A', X(3)A' --> (3)A', X(3)A --> 3(3)A', X(3)A' --> 2(3)A', and X(3)A' --> 3(3)A', respectively. The singlet-triplet splitting gap of HCCN is calculated to be 0.738 eV. The vertical and adiabatic ionization energies were obtained to compare with the PES data. The results we obtained were consistent with the available experiment results.     

Theoretical calculation of the low-lying sextet electronic states of CrF molecule

The potential energy curves have been investigated for the 12 lowest sextet electronic states in the ^2s+1Λ^(±)${}^{2s+1}{\mathrm{\Lambda }}^{(\pm )}$ representation below 53,000 cm⁻¹ of the molecule CrF via CASSCF and MRCI (single and double excitation with Davidson correction) calculations. Seven electronic states have been studied theoretically for the first time. The harmonic frequency ω_e, the internuclear distance R_e, the rotational constant B_e, the electronic energy with respect to the ground state T_e, and the permanent dipole moment μ have been calculated. By using the canonical functions approach, the eigenvalues E_v, the rotational constant B_v and the abscissas of the turning points R_min and R_max have been calculated for the considered electronic states up to the vibrational level v = 39. The comparison of these values to the theoretical and experimental results available in the literature shows a good agreement.     

Theoretical study of low-lying excited states of molecular aggregates. I. Development of linear-scaling TD-DFT

The project aims to develop an integrated linear-scaling time-dependent density functional theory (TD-DFT) for studying low-lying excited states of luminescent molecular materials, especially those fluorescence and phosphorescence co-emitting systems. The central idea will be "from fragments to molecule" (FF2M). That is, the fragmental information will be employed to synthesize the molecular wave function, such that the locality (transferability) of the fragments (functional groups) is directly built into the algorithms. Both relativistic and spin-adapted open-shell TD-DFT will be considered. Use of the renormalized exciton method will also be made to further enhance the efficiency and accuracy of TD-DFT. Solvent effects are to be targeted with the fragment-based solvent model. It is expected that the integrated TD-DFT and program will be of great value in rational design of luminescent molecular materials.     

Effect of low-lying excited triplet states on the physical and chemical properties of aryllithium compounds

Conclusions It is suggested that the physical and chemical behavior of the aryllithium compounds results from the low value of the singlet-triplet state separation EST since this entails that a considerable proportion of the molecules be in the lower excited triplet state.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 2, pp. 307–310, February, 1979.     

Theoretical study of low-lying electronic states and emission spectra of the excimer ions NaHe and NaNe

Ab initio CASSCF calculations using the Gaussian basis set aug_cc_pVTZ developed by Dunning et al. specifically for calculations at the post-HF level was carried out for excimer NaHe⁺ and NaNe⁺ ions in order to determine spectroscopic characteristics and radiation lifetimes of their low-lying excited electronic states. The computed emission spectra of these were shown to be in good agreement with the experimental ones observed recently by Hammer et al. (Hyperfine Interactions 88 (1994) 151). Contrary to the earlier popular opinion, all excited electronic states of these ions, correlating with the limit Rg⁺+Na, and not just the radiating singlet states turn out to be bound. It was shown that under the conditions of an experiment similar to the one performed by Hammer et al., the excited triplet NaHe⁺(1³Σ⁺) states can decay as a result of their interactions with a high-energy Ar⁺ beam. This decay has to be accompanied with the formation of the ions He⁺, which in turn can interact with sodium atoms to yield the radiating NaHe⁺(2¹Σ⁺) states again and, thus to maintain the emission observed in the experiment.     

Theoretical study of photoinduced singlet and triplet excited states of 4-dimethylaminobenzonitrile and its derivatives

Singlet and triplet low-lying states of the 4-dimethylaminobenzonitrile and its derivatives have been studied by the density functional theory and ab initio methodologies. Calculations reveal that the existence of the methyl groups in the phenyl ring and the amino twisting significantly modify properties of their excited states. A twisted singlet intramolecular charge-transfer state can be accessed through decay of the second planar singlet excited state with charge-transfer character along the amino twisting coordinate or by an intramolecular charge-transfer reaction involved with a locally first excited singlet state. Plausible charge-transfer triplet states and intersystem crossing processes among singlet and triplet states have been explored by spin-orbit coupling calculations. The intersystem crossing process was predicted to be the dominant deactivation channel of the photoexcited 4-dimethylaminobenzonitrile.     

Ab-initio study on low-lying states of the TiSi molecule

 The electronic structure of the TiSi molecule was examined using two types of multireference single and double excitation configuration interactions with highly extended basis sets, one including valence correlation and the other including valence and core–valence correlation. A multireference coupled-pair approximation (MRCPA) was further applied to the latter. The calculations suggest a ⁵Δ ground state, and the lowest excited state is ³Π and is only slightly (0.12 eV as estimated by MRCPA) above the ground state. The spectroscopic constants of the low-lying ¹Δ, ³Δ, ¹Π, ⁵Π, and ⁷Σ⁺ states as well as the ⁵Δ ground state and the ³Π excited states were evaluated, and we found that the molecule has only a weak σ bond and that six of the eight valence electrons essentially do not contribute to the bonding. The bonding nature of TiSi in these states is discussed in comparison with the TiC molecule. Received: 7 October 2000 / Accepted: 8 January 2001 / Published online: 3 May 2001