The S1 ← S0 0–0 transition energies of polychlorinated dibenzofurans (PCDFs) revisited: CIS(D) and MP2 calculations with correction for correlation energies

This paper reexamines the structures and energies of dibenzofuran and twenty PCDFs in S₁–S₃ states. It was demonstrated that, although the CIS method gives a false relative ordering of excited states, the false ordering can be remedied by the CIS(D) method. Moreover the CIS geometries of typical PCDF molecules reasonably agree with their SAC-CI geometries. It was found that molecules chlorinated at the 1- and 9-positions are twisted in the S₂ state but are planar in other states, except for 1,4,6,9-TeCDF and fully chlorinated dibenzofuran (OCDF). The twisted structure of 1,4,6,9-TeCDF occurs in the S₃ state, but the structure of OCDF is twisted in every state. We partitioned the molecule into the parent structure and four chlorine groups and measured the twist energy with reference to the ground state. Then, the S_i ← S₀ 0–0 transition energies (i = 1, 2) calculated using the CIS(D) and MP2 methods could be expressed as a multiple linear equation with components and twist energy. It was further confirmed that if the multiple linear equation is corrected for residual correlation energies of the parent structure, it can predict the S₁ ← S₀ 0–0 transition energies with high precision.     

An ab initio study of the Cl(P)+C2H6→C2H5+HCl abstraction reaction

Electronic energies, geometries, and harmonic vibration frequencies for the reactants, products, and transition state for the Cl(³P)+C₂H₆→C₂H₅+HCl abstraction reaction were evaluated at the HF and MP2 levels using several correlation consistent polarized-valence basis sets. Single-point calculations at PMP2, MP4, QCISD(T), and CCSD(T) levels were also carried out. The values of the forward activation energies obtained at the MP4/cc-pVTZ, QCISD(T)/cc-pVTZ, and CCSD(T)/cc-pVTZ levels using the MP2/cc-pVTZ structures are equal to −0.1, −0.4, and −0.3 kcal/mol, respectively. The experimental value is equal to 0.3±0.2 kcal/mol. We found that the MP2/aug-cc-pVTZ adiabatic vibration energy for the reaction (−2.4 kcal/mol) agrees well with the experimental value −(2.2–2.6) kcal/mol. Rate constants calculated with the zeroth-order interpolated variational transition state (IVTST-0) method are in good agreement with experiment. In general, the theoretical rate constants differ from experiment by, at most, a factor of 2.6.     

Low-lying excited singlet states and S2→S0 luminescence of dipyrido[3,4-b: 2,3-d]phenazine

The absorption, fluorescence and excitation fluorescence spectra dipyrido[3,4-b:2,3-d]-phenazine (DPPZ1) have been measured in non-polar and polar matrices at room temperature, and were taken into account to explain the origin of the relatively weak emission of this molecule in both type of environment. The electronic structure of DPPZ1 was calculated using a modified INDO CI method. The geometry optimization has been performed using the MNDO method. According to the spectra and the results of calculations, the lowest excited singlet state S₁ of DPPZ1 molecule is of n,π^*-type and the next one, S₂ state, is of π,π^*-type. The energy gap ΔE_calc is equal 4770 cm⁻¹. The low efficiency of the emission observed in the hydroxylic solvent can be interpreted in terms of thermal quenching of the π,π^*-type fluorescence. However, experimental results obtained suggest that in nonpolar solvents the emission of the molecule examined is an anomalous S₂→S₀ fluorescence.     

S2 → S0 fluorescence in an aromatic thioketone, xanthione

In addition to the red phosphorescence (T₁(³ A₂n, π^*) → S₀) xanthione exhibits in solution an emission with a maximum at ≈ 23 000 cm⁻¹ and φ_f(298°) = 5 × 10⁻³. It is shown that this emission is fluorescence from the second excited singlet state (S₂ (¹A₁ π, π^*) → S₀).     

π-facial interactions between Cl and [S4N3]. X-ray crystal structure of [S4N3]Cl

The X-ray structure of [S₄N₃]Cl reveals three independent molecules, which all display π-facial interactions between the Cl⁻ and the pseudo-aromatic [S₄N₃]⁺ rings to produce a structure containing “inverse sandwich” systems.     

Electronic spectrum of a photochromic diarylethene derivative in a supersonic free jet. Internal conversion from S2(1B) to S1(2A)

The fluorescence excitation and dispersed fluorescence spectra of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene have been measured in a supersonic free jet. No vibronic structure has been observed in the excitation spectrum. The intensity of fluorescence gradually increases with the excitation energy in the 25,500–28,700 cm⁻¹ region, indicating that the geometry of the molecule substantially changes upon photoexcitation. The dispersed fluorescence spectrum is anomaly Stokes-shifted with respect to the excitation energy, suggesting that the S₂(1B) state is initially excited followed by rapid internal conversion from the S₂(1B) to S₁(2A) state. The fluorescence is due to the S₁(2A)–S₀(1A) transition. Density functional theory calculations at the B3LYP/6-31G^** level have been carried out to investigate stable conformations responsible for the observed spectra.     

Structures of 1,2,3-trihydroxybenzene in the S0 and S1 states

In this paper we report on the structure and vibrations of gaseous pyrogallol (1,2,3-trihydroxybenzene) in the electronic ground state (S₀) and its first electronically excited state (S₁). Both ab initio CASSCF/CASMP2 calculations as well as R2PI spectroscopy have been performed. From the ab initio calculations three minimum energy structures are obtained and the vibrations of two structures are observed in the R2PI spectra. The minimum energy structures differ by their OH torsional angles. The full three-dimensional potential energy surface of the coupled torsional motions is investigated and the three-dimensional eigenvalues are calculated. The most stable structure of pyrogallol contains two intramolecular hydrogen bonds and turns out to be planar in the S₀ state. In the S₁ state the free OH group is rotated out of the plane of the aromatic ring by about 40°. The strong change in geometry of this structure is predicted by the CASSCF calculations and confirmed by the R2PI spectra of pyrogallol and its deuterated species. The low frequency region of the R2PI spectra can be explained by a torsional motion and the out of plane vibration 17b.     

Relative quantum yield of the S2 → S1 fluorescence from azulene

The quantum yield ratio r = φ_{2 → 0}/φ_{2 → 1} of the S₂ → S₀ and S₂ → S₁ fluorescences from azulene has been redetermined. With azulene in isopentane at 190 K, r = 455 ± 100. This value agrees with the lower limit, given by Huppert, Jortner and Rentzepis, but is an order of magnitude lower than that given by Gillispie and Lim.     

Quantum efficiency of S0 and P0,1 levels of Pr doped YF3

We present a detailed investigation of the quantum efficiency of the ¹S₀ and ³P_0,1 levels in Pr³⁺ doped YF₃ under VUV excitation. The quantum efficiency of the ³P_0,1 level was determined by the measurement and comparison of the photon fluxes of the ¹S₀ → ¹I₆ transition around 400 nm with the visible emissions occurring from the ³P_0,1 levels. It was found that already at concentrations as low as 0.01% the ³P_0,1 emission quantum efficiency is only about 61%. The quenching process is most probable caused by an energy transfer cross relaxation process involving the ³P_0,1 levels and the ³H₄ ground state. For concentrations of 0.1%, 0.5%, 1% and 10%, the quantum efficiency was determined to be about 42%, 29%, 17%, and 0.4%, respectively. The total quantum efficiency of the visible emission (380–750 nm) under excitation at 190 nm, determined by measurements of the total photon flux, has a maximum for a Pr³⁺ concentration of 1%. At this value, the absorption efficiency of the 4f5d absorption bands is very high and the ³P_0,1 quenching is only moderate.     

Quantum-chemical analysis of the propeller-shaped molecule [4,4,4]-propellahexaene. A study in heavy-atom tunneling

AM1, CNDO/S + CIS and QCFF/PI quantum-chemical calculations of the electronic states and nuclear configuration of the newly synthesized molecule [4,4,4 ]-propellahexaene are presented. The height and shape of the energy barrier between the two enantiomeric D₃ configurations in the S₀ state are calculated together with the tunneling rate constants for different vibrational levels of the pitch-reversing mode. These results are compared with Eckart barrier type calculations. The Franck-Condon activity of the A₁ modes in the S₀ → S₁ transition is evaluated for the propeller configuration and a predicted isomer.     

Structure and stabilities of the isomers of SiB2H4

Ab initio quantum-mechanical methods at the HF/6–31G*, MP2/6–31G* and MP2/6–31G* levels are used to study the relative stabilities of the isomers of SiB₂H₄. Isomers obtained using the analogy between trivalent boron and divalent silicon are calculated to be more stable compared to isomers where carbon is replaced by the isovalent silicon. 2π aromaticity and the preference of silicon for lower valency control the relative stabilities of SiB₂H₄ isomers.     

Conformational effects in UV absorption spectra of tetrasilanes

UV absorption of cyclic carbosilanes (SiMe₂)₄(CH₂)_n, N = 1–4 (1–4), and Si₄Me₁₀ (5) provides an experimental counterpart to the singlet transition energy and intensity correlation diagrams for the syn-anti conformational transformation in tetrasilane. A new third transition is found between the two previously known singlet transitions. Transition energies are nearly independent of the dihedral angle, while intensities vary widely. All trends agree with CIS/3–21G^*//HF/3–21G^* calculations. The Sandorfy C and ladder C models of σ conjugation fail to describe electronically excited states of tetrasilane, since they do not consider the lateral bonds to substituents.     

Description of the S1(ππ*) state and the reinterpretation of the (1+1) REMPI spectra of N-methylpyrrole as based on the ab intio calculation: a computational spectroscopy

The ab intio calculation was performed to establish the assignment of the title spectra by such as searching for stationary points belonging to lower excited states. The lowest excited state was confirmed to be of ππ* type with an A″ symmetry of a molecular point group C_s (against the previous assumption of πΣ* type) trapped in deep potential minima at the nonplanar staggered conformation (also against the current belief on the involvement of internal rotation). Thus, lower ‘vibrational’ levels in the S₁ state were shown to be tunnel-split levels with various symmetry species for a molecular symmetry group G₁₂. Based on this finding, the spectral data as reported by Philis [Chem. Phys. Lett. 353 (2002) 84] were reassigned while applying the formalism as will be presented in Appendix A.     

Role of homogeneous solvents on photophysics of 3-pyrazolyl-2-pyrazoline derivative

S₀ → S₁ and S₀ → S₂ electronic transitions have been observed in UV–Visible absorption spectroscopy of 3-pyrazolyl-2-pyrazoline (PZ) in different homogeneous solvents. Radiative emissions and relaxation processes from S₁ and S₂ states of PZ have been resolved in water, ethylene glycol and glycerol whereas in polar aprotic and protic solvents the radiative transitions have been observed from S₁ state. The S₂–S₁ electronic energy spacing has been calculated from the absorption maxima of the S₀ → S₂ transitions and fluorescence maxima of the S₁ → S₀ transitions. Solute–solvent interactions have been established to rationalize the photophysical modification of PZ in H-bonding solvents.     

Ab initio study of some peroxides. HOOH, CH3OOH and, CH3OOCH3

The geometries of HOOH, CH₃OOH, and CH₃OOCH₃, were optimized with different basis sets (3-21G, 6-31G^*(*) and D95^**) at different levels of theory (HF, MP2, MP4, and CI). HF/3-21G optimizations result in planar trans conformations for all three peroxides. HF/6-31G^** calculations predict skew conformations for HOOH and CH₃OOH, but a planar trans struture for CH₃OOCH₃. For the larger basis set the calculated bond lengths, especially the O-O bonds, are too short. Optimizations for HOOH including electron correlation at the MP2, MP3, MP4, CI, and CCD level improve the agreement for bond lengths and the OOH angle, but result in dihedral angles Which are too large by 3– 8°. In the case of CH₃OOCH₃, similar calculations at the MP2 and CI level predict planar trans structures instead of the experimentally observed skew conformation. On the other hand, MP4 single point calculations at MP2 optimized parameters result in a correct skew structure. For all three peroxides a computationally “economic” method, i.e., single point calculations at MP2 or MP4 level with HF/3-21G optimized parameters, result in close agreement between calculated and experimental structures.     

Theoretical studies of electronic absorption and emission spectra of Pt(saloph)

The performance of ab initio calculations for the ground and excited states of the Pt(saloph) complex is examined in detail. The S₀–S_i and T₁–T_i absorption spectra are calculated, and the transition between the ground S₀ state and the excited S₁ state involves the HOMO-2, HOMO-1, HOMO and LUMO. Moreover, calculations show that the emissive singlet is of mixed MLCT/LLCT characteristic. On the other hand, the molecular geometry of the complex is nearly planar in the ground state while the geometry is obviously nonplanar in the excited state of S₁(π, π^*) in the gas phase.     

A comparative study on the nature and strength of O–O, S–S, and Se–Se bond

A computational study on dichalcogenide molecules (R₂X₂; X = O, S, Se; R = H, CH₃, NH₂) has been carried out employing B3LYP and MP2 levels using 6-31+G*, 6-311+G*, 6-311++G**, and PVDZ basis sets. The relative energies have been evaluated at G2MP2 also. The rotational barriers and bond dissociation energies indicate that S–S bond is stronger than Se–Se and O–O bond. NBO analysis at MP2/6-31+G* suggest the presence of partial π character between X–X bond that decreases in the order S–S > Se–Se > O–O. Fuki functions for nucleophilic and electrophilic attack fail to distinguish the reactivity of S and Se. The proton affinities of the O₂H₂, S₂H₂, Se₂H₂ decrease in the order Se > S > O.     

Reaction of NS with N3

Thionitrosyl hexafluoroarsenate, [NS][AsF₆], reacted with caesium azide, [Cs][N₃], to form S₂N₂ which polymerized to give (SN)_x. The structures of the following likely intermediate species were calculated at correlated MP2/6–31G(d,p) level of theory (relative energies in kcal mol⁻¹): thionitrosyl azide, N₃---N=S (37.8), thiazyl azide, N₃---SN (39.2) and cyclic N₄S (0.0).^‡     

Role of structural flexibility in the fluorescence and photochromism of salicylideneaniline: the general scheme of the phototransformations

The mechanism of light-induced transformation in the salicylideneaniline molecule was studied by semiempirical PM3 calculations. The structures and energies of the minima and saddle points (transition states) on the S₀, S₁ and T₁ potential energy hypersurfaces (PESs) were obtained, together with the gradient lines on the PESs. The structure-energy scheme was compared with the experimental findings. According to the results obtained, the following principle processes are observed: fast S₁ excited state intramolecular proton transfer (ESIPT), followed by typical ESIPT fluorescence; the formation of two S₁ twisted intramolecular charge transfer (TICT) structures which quench the ESIPT fluorescence; the diabatic formation of two ground state metastable coloured “post-TICT” structures responsible for photochromism.     

Theoretical study on the structure and UV–visible spectrum of pyridine–chloranil charge-transfer complex

The ground-state structure of the charge-transfer complex formed by pyridine (Py) as electron donor and chloranil (CA) as acceptor has been studied by full geometry optimization at the MP2 and DFT levels of theory. Binding energies were calculated and counterpoise corrections were used to correct the BSSE. Both MP2 and DFT indicate that the pyridine binds with chloranil to form an inclined T-shape structure, with the pyridine plane perpendicular to the chloranil. The CP and ZPE corrected binding energies were calculated to be 14.21 kJ/mol by PBEPBE/6-31G(d) and 23.21 kJ/mol by MP2/6-31G(d). The charge distribution of the ground state Py–CA complex was evaluated with the natural population analysis, showing a net charge transfer from Py to CA. Analysis of the frontier molecular orbitals reveals a σ–π interaction between CA and Py, and the binding is reinforced by the attraction of the O₇ atom of CA with the H₂₃ atom of Py. TD-DFT calculations have been performed to analyze the UV–visible spectrum of Py–CA complex, revealing both the charge transfer transitions and the weak symmetry-relieved chloranil π–π^* transition in the UV–visible region.