Non-empirical SCF and Cl Study of the ground and excited states of thioformaldehyde

Ab initio SCF and Cl calculations are reported for ground and various low-lying Rydberg and valence excited states of thioformaldehyde H₂CS. A double-zeta basis of near Hartree-Fock quality is employed in this work and the importance of polarization functions is also assessed. The calculations indicate uniformly larger CX bond lengths in this system than for H₂CO in the corresponding electronic states; they also lind potential minima for H₂CS non-planar nuclear conformations in the (n,π^*) and (π,π^*) excited states but in each case the calculated inversion barriers are seen to be smaller than those encountered in formaldehyde. The vertical transition energies to the various excited states studied are also found to be significantly smaller in H₂CS than in H₂CO but the order of electronic states is concluded to be virtually identical for the two systems. The lowest-lying excited states are the ^3,1(n,π^*) species calculated at 1.84 and 2.17 eV respectively; the first two allowed transitions are indicated to be the Rydberg species (n,s_R) and (n,px_R) at 5.83 and 6.62 eV. These are followed by the two allowed transitions σ → π^* and π → π^* at 7.51 and 7.92 eV respectively, both well below the first ionization limit in H₂CS. The much smaller splitting between the ^3,1(π,π^*) species in H₂CS than in H₂CO is attributed to the relatively diffuse charge distribution of the sulfur atom compared to that of oxygen.     

All-valence-electron Cl calculations on the electronic spectrum of diborane

All-valence-electron Cl calculations have been carried out for diborane B₂H₆ and its positive ion employing a rather large double-zeta AO basis including polarization functions in order to study the electronic spectrum of this system. Transitions from four different valence MOs are found to lead to low-lying electronic transitions of both Rydberg and valence type in each case. Ad mixture of valence character in the otherwise Rydberg-like (nx, 3s), (ny, 3s) and (σ, 3pz) transitions calculated to lie between 11.0 and 11.6 eV is indicated as being primarily responsible for the highly intense shoulder found in this region of the B₂H₆ spectrum. The other strong feature with essentially continuous absorption peaking at 9.3 eV is suggested to result from superposition of several Rydberg-type transitions in the generally broad absorption pattern expected for the ¹(π,π^*) species at significantly higher vertical excitation energy. Quite good agreement is obtained between calculation and experiment for all of the six lowest IPs of diborane and also for the locations of the ¹(n, π^*) and ¹(σ, π^*) transitions previously assigned to the two weak features observed at 6.8 and 8.3 eV in this spectrum.     

Detection of a second triplet state in thiophosgene by electron-impact spectroscopy

The electron-impact energy-loss spectrum of thiophosgene was investigated at incident energies of 25 eV and 40 eV and scattering angles from 0° to 80°. In these spectra we observe a previously unreported triplet state at 3.1 ± 0.1 eV which is tentatively assigned as the 1³A₁ (π, gp^*) state. This state may play a role in intramolecular radiationless transitions in this molecule.     

The absorption spectrum of the anthracene molecule in the vacuum ultraviolet

The optical absorption of anthracene vapour for photon energies from 5 to 8.5 eV was found to differ in finer structure from the spectra reported earlier for parts of this range. Above the strong ¹B_2u long axis polarized πlπ^* transition at 5.24 eV three short axis polarized ¹B_1u ππ^* transitions are assigned on the basis of the oriented gas model in comparison to spectra from anthracene single crystals. A tentative new assignment for most of the additionally observed sharp Rydberg bands leading to the first ionization potential at 7.47 eV is given.     

Molecular orbital studies on the optical activity of chiral nitrosamines and nitramines

The chiroptical properties of the two lowest energy singlet-singlet transitions in a series of N-nitrosopiperidine derivatives are examined on a CNDO/S-CI molecular orbital (MO) model in which rotatory strengths are calculated directly from total molecular electronic wave functions. Similar calculations are carried out for the three lowest energy singlet-singlet transitions in a series of chiral N-nitropiperidine derivatives. The results obtained for the low energy n→π^* transitions in these compounds are compared to those predicted by sector rules proposed for chiral N-nitrosamine and N-nitramine systems.     

Theoretical investigations of Fe porphyrins. V. Low-lying electronic states of bisammineporphinatoiron(III)

Ab initio CI calculations are reported on the lowest doublet, quartet, and sextet states of [Fe^III(P)(NH₃)₂]⁺. The low-spin ground state is calculated as (d_xy² (d_π)³ with d_xy(d_π)⁴ higher by 0.15 eV. The near-ir bands at ～1 eV observed in low-spin ferriheme proteins are attributed to (π → d_π) transitions. The lowest high-spin state is ⁶A_1g, and the near-ir transitions of the high-spin ferriheme proteins observed at ～1.2 eV are attributed to higher ⁶[tripsextet] excited states [i.e., ring triplet, metal sextet]. The 30-ps “triplet” transient populated with low quantum yield observed in laser-flash studies on Fe^III(TPP)CI [TPP = tetrapbenylporphyrin] may be an ¹[tripsextet] state.     

Decay Dynamics of N,N-Dimethylthioacetamide in S3(ππ*) State (cited: 1)

The decay dynamics of N,N-dimethylthioacetamide after excitation to the S₃(ππ^*) state was studied by using the resonance Raman spectroscopy and complete active space selfconsistent field method calculations. The UV-absorption and vibrational spectra were assigned. The A-band resonance Raman spectra were obtained in acetonitrile, methanol and water with the laser excitation wavelengths in resonance with the first intense absorption band to probe the Franck-Condon region structural dynamics. The CASSCF calculations were carried out to determine the excitation energies and optimized structures of the lowerlying singlet states and conical intersection point. The A-band structural dynamics and the corresponding decay mechanism were obtained by the analysis of the resonance Raman intensity pattern and the CASSCF calculated structural parameters. The major decay channel of _3,FC(ππ^*)→S₃(ππ^*)/S₁(nπ^*)→_{1(nπ^*) is proposed.}     

A comparative multi-reference configuration interaction study of the low-lying states of two thione isomers of thiophenol

Multi-reference configuration interaction, MR-CI (including extensivity corrections, named +Q), calculations were performed on the S₀–S₃ states of cyclohexa-2,4-diene-1-thione (thione 24 ) and cyclohexa-2,5-diene-1-thione (thione 25 ), which are thione isomers of thiophenol. Several types of uncontracted MR-CIS and MR-CISD wavefunctions were employed, comprising MR-CI expansions as large as ~365 × 10⁶ configuration state functions. The nature of the studied excited states was characterized. Vertical excitation energies (ΔE) and oscillator strengths (f) were computed. The most intense transitions (S₀ → S₂ for 24 and S₀ → S₃ for 25 ) did not change with the wavefunction, although a variation as large as ~1 eV was obtained for the S₃ state of 24 , at the highest (MR-CI+Q) level. On the other hand, ΔE changed at most by ~0.56 eV for 25 as the wavefunction changes, at the same level. The S₁ state of both thiones was found to have nπ* character and is in the visible region. For 24 , S₂ and S₃ are ππ* and nπ* states, respectively, while for 25 the reverse order is obtained. S₂ and S₃ are in the range ~3.5 to 5.2 eV, again at the highest level. It is the first time that the excited states of the title molecules are studied. The computed results agree with the experimental onset of photoreactions of thiones 24 and 25 found by Reva et al (Phys. Chem. Chem. Phys., 2015 , 17, 4888).     

Charge transfer shake-up satellites in X-ray photoelectron spectra of solid compounds of 3d0 ions

Satellite structure has been observed at 8 eV and 12 eV below the main metal 2p peaks for compounds of Sc³⁺ and Ti⁴⁺. These features are attributed to charge transfer “shake-up”: transitions (t_2g → t_2g^* and e_g → e_g^* respectively). The 8 eV satellites are only found in sulphate complexes of Sc³⁺. This is the first observation of satellites which are probably due to t_2g → t_2g^* monopole charge transfer transitions. The probability for this transition is usually low, resulting in the observation of satellites due to the e_g → e_g transition.     

Vibrational analysis of the electronic spectrum of ethylene based onab initio SCF-CI calculations

Ab initio calculations for CH₂ twisting and CC stretching vibrational wavefunctions and energy levels are reported for various electronic states of ethylene C₂H₄. Electronic transition moments between these states are also obtained to allow a calculation of the oscillator strengths for vibrational transitions involved in various electronic band systems; from this study it is concluded that thevertical electronic energy differenceΔE _e may differ significantly from the energy of the absorption maximumΔE _max with which it is often equated. In particular it is found in the case of theπ→π ^* singlet-singlet excitation of ethylene that theΔE _e value overestimates the most probable vibrational transition energy (7.89 eV) by some 0.4 eV, thereby offering an explanation for the fact that previous attempts to predict the location of theV-N Franck-Condon absorption maximum have consistently obtained substantially higher results than the 7.66 eV value actually observed. Similar calculations for various Rydberg species and for theN-T transition are also found to obtain a quite consistent representation of the electronic spectrum of this system.     

Studies on the circular dichroism of penam and penam sulfoxides

Optical rotatory properties of models of 6-aminopenicillanic acid (6-AP A) as obtained from earlier molecular orbital and rotatory strength calculations are corroborated by new CD data for 6-AP A. The new spectra extend down to 180 nm, revealing a positive Cotton effect near 185 nm due to carbonyl π→π^* transitions. The pH sensitivity of the optical activity of the β-lactam amide π→π^* and the 203-nm n→π^* transitions is explainable in terms of electrostatic effects and orbital interactions present in α-aminoketones and 4-membered rings which tend to flatten out the NH₂ group. These same interactions are responsible for the unusually low pK_a of 4.75 for the amino group; (the carboxyl group of 6-AP A has a normal pK_a of 2.5). The previous calculational procedures are extended to the sulfoxides of 6-AP A, and the predicted CD spectra are compared to new experimental curves and also to available data for penicillin sulfoxides. The MO's associated with the principal bands are illustrated with the aid of electron density maps. The positive Cotton effect observed and calculated near 230 nm is due to two charge transfer transitions within the asymmetric penam nucleus. Similar to the situation with the unoxidized penams, the sulfoxide of 6-AP A displays a very low pK_a for the amino group (3.8) and a change toward a less positive CD band at 202 nm upon protonation of this group.     

The electronic structure of CrO3+ and MoO3+ complexes

The results of ab initio molecular orbital calculations for [CrOf_s]^2? and polarised single crystal electronic spectra of [MoOCl₃(Op(NMe₂)₃)₂] and Ph₄As[MoOCl₄(H₂O)] are presented. These data are consistent with the electronic transitions of the MO³⁺ moieties, O_2pπ → M_dxy and M_dxy → Md_dxy,dyz being the lowest energy transitions in the spectra of their respective complexes, both these transitions being of low intensity.     

Excited electronic states of the fluorobenzenes by variable angle electron impact spectroscopy

Electron-impact excitation spectra of benzene, fluorobenzene, o-difluorobenzene, 1,3,5-trifluorobenzene, 1,2,3,4-tetrafluorobenzene, pentafluorobenzene, and hexafluorobenzene have been measured at impact energies of 50 eV and either 25 eV or 30 eV, and scattering angles from 5° to 80°. Each molecule shows an absorption maximum at about 3.9 eV corresponding to a singlet → triplet, π → π^*, transition. In benzene, fluorobenzene, o-difluorobenzene, and 1,3,5-trifluorobenzene, an additional singlet → triplet transition was detected at about 5.6 eV. Three singlet → singlet transitions analogous to the 4.90, 6.20, and 6.95 eV transitions in benzene are seen in each of the fluorine-substituted molecules. The more highly substituted compounds exhibit an additional singlet → singlet transition that is most clearly observed in the hexafluorobenzene spectrum with a peak at 5.32 eV.     

Electronic band structure of solid CO2 as determined from the hv-dependence of photoelectron emission

Photoelectron energy distribution curves from solid CO₂ have been determined for excitation energies from hv = 14 up to 40 eV using synchrotron radiation. A 1:1 correspondence to the gas-phase photoelectron spectrum is observed for the occupied molecular orbitals. The vertical binding energies E_B^v (E_VAC = 0) and widths (fwhm) of the valence bands of solid CO₂ are determined to be 13.0 and 0.95 eV (1π_g); 16.7 and 1.1 eV (1π_u); 17.6 and 0.85 eV (3σ_u) and 18.8 and 0.8 eV (4σ_g) for the individual bands respectively. The partial photoemission cross sections differ importantly from those of the gas phase in exhibiting pronounced maxima at 5.2 eV (1π_g), 4.4–5.3 eV (1π_u + 3σ_u) and 4.2 eV (4σ_g) above the vacuum level, which is attributed to effects of high density of final (conduction-band) states. Further weaker maxima are observed at higher photon energies. Contrary to the case for the gas phase, the resonances are unperturbed in the solid by degenerate autoionizing molecular Rydberg states. The molecular origin of the resonances in the continuum is discussed and related to X-ray absorption spectra, electron-scattering data and to theoretical cross-section calculations. It is shown that the same set of resonances is observed in the different experiments. The resonances occur however at different energies due to different Coulomb interactions. The photoemission results presented provide also a key to the hitherto unexplained optical spectrum of solid CO₂ in the VUV range, making possible an assignment of the structures observed to Frenkel-type excitons (hv ≤ 15 eV) and interband transitions (hv ? 15 eV).     

Triplet states of fluorocarbonyl compounds determined by trapped electron spectroscopy

Ion cyclotron resonance trapped electron spectra for carbonylfluoride, trifluoroacetyl fluoride, hexafluoroacetone, acetyl fluoride and I,I.I-trifluoroacetone are reported. Comparisons of data obtained for ³(π → π^*) vertical excitation energies are used in conjunction with photoelectron spectroscopic data to deduce the effects of fluorine and trifluoromethyl groups on the energies of σ, π and π^* orbitals associated with the carbonyl group.     

S1 (n,π*), T1 (n,π*) and S2(n,π*) emissions in 3,6-diphenyl-s-tetrazine

Simultaneous emissions from S₁(n,π^*) and S₂(n,π^*) states in 3,6-diphenyl-s-tetrazinc (DPT) have been observed along with weak luminescence from T₁ (n,π^*). The occurrence of the S₂(n,π^*) fluorescence has been justified on the basis of the slow S₂ XXX S₁ internal conversion resulting from the large energy gap between the two states. This is the first case of dual fluorescence where both the emitting states are of (n,π^*) nature.     

The long-wavelength MCD of some quinones and its interpretation by semi-empirical MO methods

Long-wavelength MCD spectra of several quinones are measured in solution at a magnetic field strength of 5.26 T. The B terms for the ππ^* transitions are computed with wavefunctions of the PPP type. The results enable one to deduce the polarization of many of the ππ^* bands. The signals due to nπ^* transitions are identified with the help of CNDO calculations. A very weak long-wavelength A term appearing in the spectra of some of the compounds is identified as due to the S_OT₁ (³nπ^*) absorption.     

Angular overlap parameters for ruthenium(II) complexes

Approximate Angular Overlap Model e_π parameters have been obtained for a number of ligands L by comparison of the t_2g(Ru) → π^*(bpy) transition energies in [Ru(bpy)₂L₂] complexes. The filled t_2g subshell of Ru(II) limits the effects of otherwise strongly π-donating ligands.     

Preferential energy transfer from N2(A) to specific energy levels of Cu atoms

Emission spectra resulting from reaction of “clean” N₂(A³ Σ_u⁺) with copper atoms were studied using a flowing afterglow apparatus. The population distribution of the Cu states was calculated from the spectrum; it indicates that Cu atoms are excited by nearly resonant energy transfer processes. N₂(A,v') + Cu(²S_{$\text{1}\text{2}$}) → N₂(X, v) + Cu^* , and that the transfer is most efficient for N₂(A,v') → N₂(X,v) transitions with large Franck-Condon factors. The preferential energy transfer results in population inversion between some of the Cu states.     

The T3 (π, π) State of acridine

The 00 band maximum of the transition T₃(π, π^*) ← T₁ (π, π^*) of acridine occurs at ≈ 10200 ± 20 cm^?1 in inert (n-hexane, benzene, CCl₄), at 10220 ± 20 cm^?1 in polar (acetonitrile) and at 10170 ± 50 cm^?1 in hydrogen-bonding (methanol, 2-propanol and alkaline water) solvents. Based on the solvent-independent energy of T₁ (π, π^*), the T₃(π, π^*) state of acridine is estimated at 26050 ± 50 cm^?1 in all the solvents.