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.     

Ab initio SCF CL study of the electronic spectrum of nitroso-methane

Configuration interaction studies of ground, n_ → π^*, n₊ → π^*, and π → π^* electronically excited states are reported for nitroso-methane in its eclipsed equilibrium geometry. The first (n_ → π^*) and the second (n⁺ → π^*) singlet states are calculated at 2.17 and 7.14 eV. it is shown that a significant delocalization of the nonbonding orbitals on the nitrogen and oxygen is responsible for the large energy gap between these two states. The two lowest triplet states occur at 1.29 and 5.39 eV and are of n_ → π^* and π → π^* origin.     

Low energy,variable angle electron-impact excitation of 1,3,5-hexatriene

A mixture of cis and trans 1,3,5-hexatriene has been studied by electron impact at incident electron energies of 20 eV, 40 eV, 50 eV, and 70 eV, at scattering angles from 0° to 80°, and with effective energy resolutions in the range from 0.05 eV to 0.15 eV. Singlet → triplet transitions with maximum intensities at 2.61 eV and 4.11 eV are observed. The lowest energy spin-allowed excitation which can be detected is the electric dipole-allowed $\text{X}$¹ A_g → 1 ¹B_u transition (in the notation appropriate for the trans isomer). No evidence has been found for a spin-allowed but symmetry-forbidden $\text{X}$¹ A_g → 2 ¹A_g excitation in the vicinity of 4.4 eV transition energy. Many other spin-allowed excitations are observed in the 6–11 eV energy-loss region, and the correlation between these features and those observed in high resolution ultraviolet absorption spectra and other electron-impact spectra is discussed.     

Triplet states of the amide group. Trapped electron spectra of formamide and related molecules

Trapped electron (TE) spectra are obtained using ion cyclotron resonance detection of scavenged electrons. The lowest singlet-triplet transitions, ³(n→π^*), in formamide (HCONH₂) and N,N-dimethyl formamide (HCONMe₂) are found at vertical energies of 5.30 and 5.00 eV, respectively. An unresolved band containing the ³(π→π^*) and ³(n→3s) states appears at higher energies, centered at 6.60 and 6.00 eV, respectively. The TE spectra of formaldehyde (HCHO), acetaldehyde (MeCHO) and acetone (Me₂CO) are obtained for comparison and are used along with results from ab initio theoretical calculations in establishing assignments. Singlet-triplet transitions dominate the spectra of all of these carbonyl containing molecules, to the exclusion of low lying singlet-singlet transitions. This is in agreement with other TE spectra and the expectation that (dσ/dE) will be higher near threshold for singlet-triplet as compared to singlet-singlet transitions.     

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.     

Stark effects on the low energy electronic states of p-benzoquinone single crystals

The Stark effect on the visible absorption spectra of p-benzoquinone single crystals has been measured using modulation techniques. The measurements of second order Stark effects confirm the presence of at least two close lying electronic nπ^* states in both the singlet and triplet systems. Both the origin of the first singlet-singlet and singlet—triplet transition appear as doublets separated by 3.5 and 18 cm^?1, respectively. For the triplet state the low energy component is Stark induced and has not been observed previously in absorption. The doublet components are mixed in the electric field showing that they are of different parity. They are tentatively assigned as inversion doublets of a symmetric double well potential produced by the vibronic interaction of two different electronic nπ^* states.     

Solvent effects on relaxation of the 1B2u state of benzene

Values of triplet yields for dilute solutions of benzene in methylcylohexane, cyclohexane, methanol, ethanol, acetonitrile, perfluoro-n- hexane, and water, and also fluorescence yields and lifetimes in perfluoro-n-hexane are reported. The calculated rate constants for fluorescence and intersystem crossing are only slightly affected by solvent, except for water. The enhancement of “internal conversion” by increasing solvent polarity is interpreted as a result of an increase in the pre-exponential factor for a possible ¹(π,π^*) → ¹(σ,π^*) 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.     

Correlation of the zero-field splittings with the n,π* and π,π* triplet levels of benzaldehydes

The n,π^* and π,π^* triplet state energies of p-chlorobenzaldehyde and p-mathoxybenzaldehyde were determined in several hosts with the aid of phosphorescence and phosphorescence excitation spectra. A linear relationship expected from the theory considering spin-orbit interaction between the closely located n,π^* and π,π^* triplet states was found to be satisfied. The spin—orbit interaction parameter, <vb>G₂>vb>² was found to be 83 cm^?2 for benzaldehydes.     

The Low‐Lying Excited States of 2,2′‐Bithiophene: A Theoretical Analysis

The low‐energy regions of the singlet→singlet, singlet→triplet, and triplet→triplet electronic spectra of 2,2′‐bithiophene are studied using multiconfigurational second‐order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1¹B_u and 2¹B_u, are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2³A_g and 2³B_u, separated about 0.4 eV from the two lowest singlet excited states, can be populated by intersystem crossing from nonplanar singlet states.     

Rotational isomerism in fluoro- and chloro-benzaldehydes

The results of the study of rotational isomerism in the nπ^* electronic spectra of o- and m-substituted fluoro- and chlorobenzaldehydes are reported. It has been found that trans-O is the more stable rotamer in the ground electronic states as well as in the lowest lying singlet and triplet electronic states in all four molecules.     

Excited electronic and ionized states of the nitramide molecule, H2NNO2, studied by the symmetry-adapted-cluster configuration interaction method

Symmetry-adapted-cluster configuration interaction (SAC-CI) wave functions were employed to compute 16 singlet and 13 triplet vertical transitions, and 14 ionized states including relative intensities of the nitramide molecule, H₂NNO₂. This molecule is the simplest neutral closed-shell molecule which has an N–NO₂ bond and is a member of the nitramine family, R,R′N(NO₂), an important class of energetic materials with practical applications. The present nitramide results showed strong similarities with the ones of the N, N-dimethylnitramine molecule, which has also an N–NO₂ bond and was previously studied using the SAC-CI method. Experimental ultraviolet and photoelectron band spectra of the nitramide molecule could be successfully assigned. All the singlet transitions have valence character. The computed singlet and triplet transitions, excepting a singlet one, result from excitation originating in the four highest occupied molecular orbitals, which have close energies. Most of the singlet and triplet transitions involved mixing of singly excited configurations. The strongest computed transition, at 6.8 eV, is a mixture of two nπ_NO2 → π^* configurations corresponding to excitations from the highest occupied molecular orbital (HOMO) to the first two virtual orbitals and has an optical oscillator strength value of 0.2665. The computed ionized states described the whole measured spectrum, have excellent agreement when compared with the measured ionization potentials and revealed an inversion of the ordering of the first states not expected according to Koopmanns’ theorem, thereby showing the limitations of the latter.     

Electronic absorption of carboxylic acids and their anions

Electronic spectra of formic, acetic, mono-, di-, trichloro- and trifluoroacetic, glycolic, cyanoacetic, pivalic, α-methoxyacetic, lactic, oxalic, tartaric and citric acids and betaine and of corresponding anions were recorded. The acid forms of all the carboxylic acids studied show a medium-strong π → π^* and a weak n → π^* absorption band, the latter in the 220–250-nm region. The corresponding anions (or the completely dissociated forms of polybasic acids) show the π → π^* absorption bands, but no indication of a shoulder corresponding to a n → π^* transition. Changes in the absorbance in the wavelength region corresponding to the n → π^* transition with addition of alkali metal hydroxides can be used for end-point detection in titrations. Changes of these absorbances in solutions of buffers or strong acids can be used for pK determinations. A pK value of 0.89 (at μ = 0.5) was found for dichloroacetic acid; approximate pK values were established by means of the H_o acidity scale for trichloroacetic acid (—0.80) and trifluoroacetic acid (—0.92). Finally, absorbances in the 220–250-nm region can be used for determination of carboxylic acids in solutions of strong acids, and some buffers, like phosphate or borate.     

Anomalous solvent shift effect and observation of phosphorescence in bilirubin

Bilirubin IX-α has a large extinction coefficient but shows a weak blue-shift in solvents of increasing dielectric constant. A blue-shift is typical of an n → π^* transition, and is here interpreted in terms of the amide group present in the terminal pyrrole rings. Compounds undergoing n → π^* transitions usually form triplet states. With bilirubin, an emission is observed at 77 K; evidence is presented that this may be phosphorescence from an excited triplet state. The energy of this triplet level is 230 kJ mole^?1, thus bilirubin should be capable of sensitizing the formation of ¹Δ_g O₂.     

Relative magnitudes of singlet and triplet state dipole moments: the lowest nπ* excited states of p-methylbenzaldehyde and p-chlorobenzaldehyde

The molecular dipole moment changes upon ¹nπ* and ³nπ* excitation of p-methylbenzaldehyde and p-chlorobenzaldehyde isolated in p-dimethoxybenzene host crystals have been determined from measurements of the Stark splittings in phosphorescence excitation and phosphorescence spectra. Within the experimental uncertainty, the changes are identical for the corresponding triplet and singlet states. This result contrasts with similar determinations for formaldehyde, benzophenone, and 4,4′-dichlorobenzophenone and with predictions based on simple molecular orbital and electron correlation arguments. The result is considered to be a consequence of a relatively large mixing of ³nμ* and ³μμ* states.     

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.     

A non-empirical SCF and CI study of the electronic spectrum of pyrrole

Various electronically excited states of pyrrole have been studied by ab initio SCF and CI calculations including π → π^* and π → Rydberg excitations. Optically allowed valence type transitions are found at energies higher than 6.5 eV whereas all the lower singlet states are of Rydberg type. In addition to the experimentally known triplet states at 4.23 and 5.10 eV, several new triplet transitions with energies from 5.71 to 7.10 eV are predicted. In most cases good agreement with experimental data is found.     

Experimental and theoretical studies of the valence-shell dipole excitation spectrum and absolute photoabsorption cross section of hydrogen fluoride

Absolute cross sectional measurements are reported of the valence-shell dipole excitation spectrum of HF obtained from suitably calibrated high impact energy, small momentum transfer, electron energy-loss scattering intensities. Detailed assignments are provided of all prominent features observed on the basis of concomitant single- and coupled-channel RPAE calculations. The measured spectrum, obtained at an energy resolution of = 0.06 eV (fwhm) in the = 9 to 21 eV interval, includes a dissociative feature centered at = 10.35 eV assigned as X¹Σ⁺ → (1π^?14σ)A¹Π, as well as numerous strong, sharp bands in the = 13 to 16 eV excitation energy region. These bands are attributed on basis of the present calculations to Rydberg (1π^?1npπ)-valence (3σ^?14σ) mixing in X¹Σ⁺ → ¹Σ⁺ excitation symmetry, which gives rise to a long conventional progression, and to strong 1π → nsσ, moderate 1π → ndσ, and weak 1π → npσ Rydberg series in X¹Σ⁺ → ¹Π excitation symmetry. A weaker 1π → ndπ Rydberg series also contributes to the spectrum in X¹Σ⁺ → ¹Σ⁺ symmetry. The calculated and measured excitation energies and f numbers, particularly for the X¹Σ → (1π^?14σ)A¹Π, → (1π^?13pπ)B¹Σ⁺, → (1π^?13sσ)C¹Π, and → (3σ^?14σ)D¹Σ⁺ transitions, are in good quantitative accord, suggesting that the overall nature of the HF spectrum is generally clarified on basis of the present studies. Finally, tentative assignments are provided of weak features observed above the 1π^?1 ionization threshold. As in previously reported joint experimental and theoretical studies of the valence-shell spectrum of F₂, high-resolution optical VUV measurements and calculated potential energy curves aid in the assignment and clarification of the HF spectrum.     

Triplet states of furan,thiophene, and pyrrole

Low-lying triplet electronic states have been detected in furan, thiophene, and pyrrole by the method of variable-angle, electron-impact spectroscopy. Singlet → triplet transitions occur with maximum intensity at 3.99 eV and 5.22 eV in furan, 3.75 eV and 4.62 eV in thiophene, and 4.21 eV in pyrrole. A weak transition at 5.22 eV in pyrrole is assigned as the lowest observed singlet → singlet excitation in that molecule.