AB initio and configuration interaction calculations for some σ → π* superexcited states of the trans-1,3-butadiene molecule

Some σ → π¹ superexcited states of the trans-1,3-butadiene molcule have been calculated in order to establish them as possible candidates for the 9.52 eV and 11.04 eV transitions observed in the electron impact spectra of this molecule. Four states have been solved self-consistently ( 7a_g→ 2a_u2a_gand 2b_g and 6b_u→ 2a_u, 2bg) and on the basis of extensive CI calculations of transition energies and oscillator strengths, we assign the 11.04 eV transition to the ¹B_g (6b_u→ 2a_u) state. The transition observed at 9.52 eV is more likely to be either a π (la_u) → π1 transition or the first member of a Rydberg series converging to the second ionization potential.     

Electronic absorption spectra of M2L6 compounds containing metal-metal triple bonds of σ2π4 configuration

The electronic absorption spectra of compounds containing metal-metal triple bonds of σ²π⁴ valence electronic configuration are presented and discussed. The lowest-energy transition of M₂L₆ compounds (M = Mo or W, L = CH₂Bu^t or OBu^t) is expected to be the dipole-allowed π → π* (e_u → e_g) transition. This appears to be the case for M₂(CH₂Bu^t)₆ and M₂(OBu^t)₆ compounds, in which the lowest energy absorption bands occur between 26,000 and 28,000 cm⁻¹ (ε = 1.1 x 10³-1.8 x 10³ M⁻¹ cm⁻¹). For M₂(NMe₂)₆ compounds, the lowest energy absorption is not the π → π* transition but is assigned instead to a LMCT transition originating from nitrogen lone-pair orbitals, N_1p → π*, observed at 30,800 cm⁻¹ (ε = 1.4 x 10⁴-1.9 x 10⁴ M⁻¹ cm⁻¹). The π → π* transition is not observed in these compounds, but is presumably masked by the more intense LMCT. These assignments are derived from Xα-SW calculations performed and described by other authors (Bursten et al., J. Am. Chem. Soc. 1980, 102, 4579).     

Ab initio study of 4-monosubstituted pyrimidines in ground and excited n → π* states

Ab initio SCF and SCF -CI calculations have been performed to investigate substituent effects on ground- and excited-state properties of 4-R-pyrimidines, and to compare these with substituent effects in 2- and 4-R-pyridines, with R including the π donating and σ withdrawing groups CH₃, NH₂, OH, F, and C₂H₃ and the σ and π electron-withdrawing groups CHO and CN. Substitution leads to significant changes in the internal angles of the pyrimidine ring, which are independent of the nature of the substituent. The geometry of the pyrimidine ring is more sensitive to substitution in the 4 position than the pyridine ring geometry is to substitution in either the 2 or the 4 position. The isodesmic reaction energies for substituent transfer from the 4 position of pyrimidine to the 2 or 4 position of pyridine indicate that all R groups except CN have a relative stabilizing effect in pyrimidine. The presence of a π donating group leads to an increase in the n→π* transition energy of 4-R-pyrimidines, while the π withdrawing group CN leads to a decrease in the transition energy relative to pyrimidine. Orbital energy differences and virtual excitation energies tend to correlate with n→π* transition energies of 4-R-pyrimidines with saturated R groups, but such correlations are masked by π conjugation, n orbital interaction, and configurational mixing when the unsaturated groups C₂H₃, CHO, and CN are present. The electronic effects of a π donating group are stronger when the group is bonded to pyrimidine than to pyridine, but those of a π withdrawing group are weaker when the group is bonded to pyrimidine.     

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.     

Near ultraviolet spectroscopic studies of 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) and 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil)

The near u.v. spectra of 2,4-diamino-6-piperidinopyrimidine (desoxyminoxidil) and 2,4-diamino-6-piperidinopyrimidine-3-oxide (minoxidil) can be viewed as perturbed pyrimidine spectra. The u.v. properties of pyrimidine and a series of aminopyrimidines, specifically 2,4,6-triaminopyrimidine, are examined to obtain u.v. spectral assignments for desoxyminoxidil and minoxidil. Minoxidil and its desoxy counterpart have C_s symmetry, and all π → π* absorptions are allowed ¹A′ ← ¹A′ transitions. The two lowest energy π →- π* absorptions observed in minoxidil (262 nm, 292 nm) are tentatively assigned as very mild oxygen → pyrimidine ring charge-transfer transitions. Intensity decreases in protic solvents, and the results of simple Hückel molecular orbital calculations indicate that the 292 nm transition has more charge-transfer character than the 262 nm absorption. The protonated species of desoxyminoxidil and minoxidil have very similar u.v. spectra. This is due to the lack of oxygen-related charge transfer in protonated minoxidil, and the high probability that the positive charge resides in similar environments in the minoxidil and desoxyminoxidil molecular frameworks.     

Theoretical studies on Ru(fppz)2(CO)L (L = N-heterocyclic ligand): Electronic structure, absorption, phosphorescence, and solvatochromism

A series of ruthenium(II) complexes Ru(fppz)₂(CO)L [fppz = 3-trifluoromethyl-5(2-pyridyl)pyrazole; L = pyridine (1), 4-dimethylaminopyridine (2), 4-cyanopyridine (3)] were designed and investigated theoretically to explore their electronic structures, absorption, and emissions as well as the solvatochromism. The singlet ground state and triplet excited state geometries were fully optimized at the B3LYP/LANL2DZ and CIS/LANL2DZ level, respectively. The HOMO of 1–3 is composed of d_yz(Ru) atom and π(fppz). The LUMO of 1 and 2 is dominantly contributed by π*(fppz) orbital, but that of 3 is contribute by π*(L). Absorption and phosphorescence in vacuo, C₆H₁₂, and CH₃CN media were calculated using the TD-DFT level of theory with the PCM model based on the optimized ground and excited state geometries, respectively. The lowest-lying absorption of 1 and 2 at 387 and 391 nm is attributed to {[d_yz(Ru) + π(fppz)] → [π*(fppz)]} transition, but that of 3 at 479 nm is assigned to {[d_yz(Ru) + π(fppz)] → [π*(L)]} transition. The phosphorescence of 1 and 2 at 436 and 438 nm originates from ³{[d_yz(Ru) + π(fppz)] [π*(fppz)]} excited state, while that of 3 at 606 nm is from ³{[d_yz(Ru) + π(fppz)] [π*(L)]} excited state. The calculation results showed that the absorption and emission transition character can be changed from MLCT/ILCT to MLCT/LLCT transition by altering the substituent on the L ligand. The phosphorescence of 1 and 2 does not have solvatochromism, but that of 3 at 606 nm (vacuo), 584 nm (C₆H₁₂), and 541 nm (CH₃CN) is strongly dependent on the solvent polarity, so introducing electron-withdrawing group on ligand L will induce remarkable solvatochromism.     

A molecular orbital study of lithium ion association with bases. The excited carbonyl bases R2CO

SCF CI calculations have been performed to investigate Li^XXX association with excited bases R₂CO. Although association leads to large increases in n → π¹ transition energies, the complexes R₂COLi^XXX remain bound in the n → π¹ state, but are destabilized relative to the ground state. In the Li^XXX-urea complex, the n → π¹ A₂, state lies slightly above a charge-transfer π → σ^* A₂ state.     

Electronic structure of the azide ion using the SCF Xα SW method

The electronic structure of the azide ion is investigated using the SCF Xα scattered wave method. Calculated ionization energies are compared with values determined by electron spectroscopy. Transition state calculations for π_g → π*_u, σ_u → π*_u and σ_g → π*_u single electron transitions yield excitation energies near 5.7, 11.0, and 12.0 eV respectively.     

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.     

The far ultraviolet spectrum of thioformaldehyde

The electronic absorption spectrum of thioformaldehyde has been recorded from 2200 to 1800 Å. Four electronic transitions have been identified in the spectrum and have been assigned to the π → π*, n → 4s, n → 4p_y and n → 4p_z electron promotions.     

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.     

A theoretical interpretation of 13C screening data in unsaturated molecules

Carbon-13 screening constants are calculated within the INDO/S level of approximation to Pople's model. Satisfactory agreement is obtained in most cases between the calculated and observed screening results. An analysis of the contributions of the π → σ*, σ → π* and σ → σ* transitions to the paramagnetic term shows that a linear relationship between ¹³C chemical shifts and the lowest energy transition is not present. The average excitation energies are found to vary appreciably among the molecules studied.     

Monomere und dimere Chrom(III)-Phthalocyanine: Synthese und Eigenschaften von Hydroxopyridinophthalocyaninatochrom(III) und μ-Oxodi(pyridinophthalocyaninatochrom(III))

Monomeric and Dimeric Chromium(III) Phthalocyanines: Synthesis and Properties of Hydroxopyridinophthalocyaninatochromium(III) and μ-Oxodi(pyridinophthalocyaninatochromium(III)) Heating of ?[Cr(OH)Pc^2?]”? in pyridine (Py) gives the paramagnetic (T = 273 K) complexes [Cr(OH)(Py)Pc^2?] (μ_Cr = 3.84 μ_B) and [(Cr(Py)Pc^2?)₂O] (μ_Cr = 1.24 μ_B) by consecutive substitution and condensation reactions. The UV-VIS spectra are characterized by the typical B, Q, and N regions of the Pc^2? ligand being shifted hypsochromically for the dimer with respect to the monomer due to excitonic coupling (1.5 kK). Regions of weak absorbance between 8 and 13 resp. 19 kK are assigned to trip-quartet transitions for both complexes. A weak band at 870 cm^?1 in the FIR/MIR spectra is assigned to v_as(Cr? O? Cr). In the resonance Raman(RR) spectra v(Cr? O) at 514 cm^?1 resp. v_s(Cr? O? Cr) at 426 cm^?1 is selectively enhanced. Further strong RR-lines of the μ-Oxo dimer at 110 and 631 cm^?1 are assigned to a (Py? Cr? O)- resp. internal pyridine deformation of a_1g symmetry. An assignment as 2v_as(Cr? O? Cr) is proposed for the remarkable RR line at 1740 cm^?1.     

Synthesis, spectroscopy, and magnetic characterization of copper(II) and cobalt(II) complexes with 2-amino-5-bromopyridine as ligand

The synthesis, spectroscopic, and magnetic characterization of two new copper(II) and cobalt(II) complexes are described. Both two compounds have the general formula [M(L)₂(Cl)₂] (M = Cu (I), Co (II); L = 2-amino-5-bromopyridine). These complexes were prepared in one-step synthesis and characterized by elemental analysis, FT-IR, UV-Vis, and EPR spectroscopy. Moreover, the single crystal structure of complex I was studied by the X-ray diffraction method. This compound consists of mononuclear units consisting of two ligands linked to metal via the nitrogen of pyridine ring. The UV-Vis spectra of copper(II) and cobalt(II) complexes show three and five absorption bands, respectively, attributed to the d-d transition of the metal ion, ligand → metal charge transfer and π → π* or n → π* transitions of the ligand. The FT-IR spectra show MN₂Cl₂ vibrations at 500–300 cm^?1. The complexes show room temperature magnetic moments of 1.78 and 4.12 μ_B for Cu(II) and Co(II), respectively. The X-band electron spin resonance (ESR) spectra of Cu(II) complex in DMF or DMSO frozen at liquid nitrogen temperature show the typical ΔM_S = ±1 transition.     

Tayloring standard TDDFT approaches for computing UV/Vis transitions in thiocarbonyl chromophores

We report the development of an accurate computational procedure for the calculation of the n → π* (λ_max?1) and π → π* (λ_max?2) transitions of a set of thiocarbonyl derivatives. To ensure converged results, all calculations are carried out using the 6‐311+G(2df,p) basis set for time‐dependent calculations, and the 6‐311G(2df,p) for the ground‐state geometrical optimization. Starting with two hybrids, PBE0 and B3LYP, the Hartree–Fock exchange percentage (α) used is optimized in order to reach excitation energies that fit experimental data. It turns out that BLYP(α) is the more adequate functional for calibration. For the n → π* excitation, the optimal α value lies in the 0.10–0.20 interval, whereas for the π → π* process setting α equal to 0.10 provides the most accurate results. The corresponding mean absolute errors (MAE) are limited to 17 nm for λ_max?1, and to 10 nm for λ_max?2, allowing a consistent and accurate prediction of both transitions. We also assess the merits of the ZINDO//AM1 scheme and it turns out that the semi‐empirical method only provides a poor prediction of the λ_max of thiocarbonyl derivatives, especially for the n → π* transition. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

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.     

Bonding in the ground state and excited states of copper-alkene complexes

The ground state and ¹B₂ excited state of Cu(C₂H₄)⁺ and of CuX(C₂H₄) (X  F, Cl) have been investigated by the Hartree-Fock-Slater (HFS) method. The main metal-ligand interactions in the ground state are ethene π → Cu 4s donation and Cu 3d_π → ethene π^* backdonation, which have comparable contributions to the metal-ligand bond strength. The excitation of CuX(C₂H₄) does not involve an alkene π → metal charge transfer (LMCT), but instead is metal 3d → alkene π^* charge transfer (MLCT) in character. The implications for the photochemistry of olefin-copper(I) complexes are discussed.     

Circulardichroism—LXXV: Cottonogenic derivatives of chiral bidentate ligands with the complex [Mo2 (O2CCH3)4]

[Mo₂(O₂CCH₃)₄] forms optically active complexes in DMSO after addition ofchiral ligands. Bidentate ligands (carboxylic acids, diols, and amino alcohols) but also a primary amine (α-phenylethylamine) were shown to form these complexes. For molar ratios of less than appr. 1.5:1 (ligand to complex) up to 5 Cotton effects (A to E from 600 to 270 nm) can be observed. The signs of those between 300 and 400 nm can be used for empirical determination of absolute configuration of complexing ligands. Bands A-D are assigned to the following transitions: A: mainly π_Mo-Mo→ δ^* (A_1g→ E_g); B: δ →ρ^*_Mo-O (A_1g → A_2g C: mainly δ → π^*_Mo-Mo (A_1g → E_g ) D: δ → δ^* (A_1g → A_2u ). Sector rules for the pair of CD-bands A/C (hexadecant rule) and for CD-band B (hexadecant rule with two additional nodal planes through the MoO_4- planes) are derived from qualitative MO theory.     

Magnetic circular dichroism of some cyano-substituted cyclopentadienide anions

MCD spectra of pentacyanocyclopentadienide anion, tetracyanocyclopentadienide anion and diazotetracyanocyclopentadiene were measured. They all exhibited the Faraday A term or B terms which are caused by π → π^* electonic transitions of the compounds. Transition energies and symmetries of the excited states were calculated by use of the Pariser-Parr-Pople method. The results of the calculations gave a qualitative interpretation of the observed MCD spectra.