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.     

The intensities of low-energy electronic absorption transitions of some carbonyls and thiocarbonyls

The intensities of low-energy electronic transitions for some carbonyls and thiocarbonyls have been calculated from CNDO wavefunctions.Quite good agreement with experimental results has been obtained, where the latter are available. A satisfactory approximation for calculating intensities employs only one-center integrals. From the calculated trends in oscillator strengths, the absorption of thiophosgene at 4.46 eV can be identified as the π→π* ¹A₁←X?¹A₁ system. Another, very weak, system of thiophosgene at ≈ 3.9 eV is tentatively assigned to an n→π* ¹A₁ ← X?¹ A₁ transition, with the n orbital localized on the chlorine atoms.     

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.     

Conformational dependence of the 1Lb and n → π* rotatory strengths in α-substituted phenylacetic acids

The chiroptical properties of a series of chiral phenylacetic acid derivatives are examined on a theoretical model in which electronic rotatory strengths are calculated directly from molecular wave functions derived from semiempirical molecular orbital calculations. The CNDO/S SCF-MO model is used to calculate ground state wave functions and excited states are constructed in the virtual orbital-configuration interaction approximation. Of special interest are the rotatory strengths associated with the ¹L_b transition of the phenyl chromophore and the n → π^* transition of the carboxyl chromophore. Calculations are carried out on a large number of conformational isomers of the compounds: α-methyl phenylacetie acid (and its methyl ester), α-methylmandelic acid (and its methyl ester), and mandelic acid (and its methyl ester). The dependence of rotatory strength (¹L_b and n → π^*) on conformational variables is examined and discussed, and comparisons between available experimental data and the calculated results are made. Coupling between the phenyl and carboxyl chromophoric moieties is considered and possible spectra-structure relationships are examined.     

Optical rotatory strength calculation by evaluating the gradient matrix through the equation of motion

A straightforward generalization of Linderberg's equation-of-motion-based formula for the matrix elements of the linear momentum operator is proposed. The essential feature of the modification is the abandonment of the zero differential overlap (ZDO) approximation for the electric transition integrals. It is expected that this new formula gives better transition moments and, consequently, better optical rotatory strength values. The results of this modification are analysed numerically for the rotatory strengths of the twisted hydrogen peroxide and for the 1,6-diazaspiro[4,4]-nonane-2,7-dione, C₇H₁₀N₂O₂, molecule, using a CNDO Hamiltonian. For both systems a definitive improvement of calculated rotatory strengths resulted.     

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.     

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.     

Molecular electronic structure of metal phthalocyanines

The molecular–electronic structure of the metal phthalocyanines (Fe, Co, Ni and Cu) has been determined by the molecular orbital treatment. Coulomb integrals of the metal atom occurring in the secular determinants have been approximated equivalent to the valence state ionization energy (VSIE) of a metal orbital for a particular charge configuration. The calculated π-electron charge densities have been found to be higher on the nitrogen atoms as compared to the other atoms in the molecule. This is in agreement with the e.s.r. studies of the metal phthalocyanines. To test the correctness of the molecular orbital calculations, the π-π* transitions (14,000 cm^?1 ? 30000 cm^?1), d-d* transitions (20000 cm^?1 ? 60000 cm^?1) and charge transfer transitions (15000 cm ^?1 ? 30000 cm^?1) have been calculated in the metal phthalocyanine molecules. The calculated frequencies have been compared with the observed ones and found in fair agreement.     

Closed-shell SCF–CIMO treatment of all-reactive electrons in complex heteroorganic molecules

A semiempirical molecular orbital (MO ) method is outlined that is designed for correlation and prediction of the spectra and other properties of complex molecules. Within the ZDO approximation, the reactive (π-bonding and nonbonding) electrons are treated explicitly considering the σ electrons as an unreactive static potential. The electronic repulsion integrals are evaluated using the multipole expansion formulas, which allow the characterization of each orbital by specification of its quadrupole moment, from which all two-center repulsion integrals involving it would be computed. A preliminary application of the all reactive electron self-consistent field (ARE -SCF -CI ) MO method is made to organic carbonyls as well as some selected cyanine dyes. The predicted n → π* and π → π* transition energies and intensities are in good agreement with experimental data. The calculated ionization potentials are 0.2–0.7 eV lower than the observed values. The present method, although it requires further possible refinements, bridges the gap between the simplicity of the PPP method and the required overall interpretation of the electronic properties.     

High-resolution electronic spectra and luminescence properties of some benzonaphthyridines at 77 K. absorption and fluorescence spectra of 1,5-, 1,6- a

Absorption and fluorescence spectra of 1,5-, 1,6- and 4,6-benzo[h]naphthyridines (BN) were examined in Shpolskii matrices and in n-butanol at 77 K. Vibrational analysis of quasilinear spectra of 1,6- and 4,6-BN in n-hexane matrices was performed. Calcula- tions of the electronic structure of the isomers examined were done using a modified INDO CI method. The results of the experiments and calculations prove the π, π^* state to be the lowest excited singlet state of 1,6- and 4,6-BN molecules; in 1,5-BN molecule the S₁ (n, π^*) state is strongly perturbed by the nearby s₂(π π^*) state.     

Magnetic circular dichroism and molecular orbital studies on bicyclo[6,2,0] decapentaene

The magnetic circular dichroism (MCD) spectrum of bicyclo[6,2,0] decapentaene has revealed four skeletal π → π^* electronic transitions in the visible and ultraviolet region. The four MCD bands are assigned to the B₂ ← A₁, A₁ ← A₁. B₂ ← A₁ and A₁ ← A₁ electronic transitions in increasing order of energy.     

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.     

Intercombination lines of Mg I-, Al I- and Si I-like ions in the beam foil spectra of Ti,Fe, Ni and Cu

In delayed spectra of foil-excited beams of Ti, Fe, Ni and Cu ions lines have been observed which are identified with intercombination transitions 3s ^{2 1} S ₀ — 3s 3p ³ P ₁ ⁰ , 3s ² 3p ² P ⁰ — 3s 3p ^{2 4} P and 3s ² 3p ^{2 3} P — 3s 3p ^{3 5} S ₂ ⁰ in magnesiumlike, aluminiumlike and siliconlike spectra, resp. Wavelengths and decay properties have been determined. The results are compared to recent theoretical predictions.     

Broken orbital symmetry study of low-lying excited and N15 ionized states of pyrazine

Ab initio ΔE_SCF calculations of the low-lying n-π^* and π-π^* transitions in pyrazines are reported with D_2h and C_2v symmetry adapted molecular orbitals. The use of broken orbital symmetry is essential for interpreting the emission properties of pyrazine at the computational level used. The energy of the N_is orbital is calculated using these two symmetry constraints with C_2v orbitals leading to results in better agreement with experiment.     

M1 transition probabilities between fine structure components2L J (L ≥ 1)

Spontaneous emission transition probabilities of the magnetic dipole transitions between states of ground state configurations consisting of onenl electron (or a hole) outside a closed shell have been calculated by using relativistic terms of order α² Z ² and using hydrogenic orbitals to calculate the required overlap integrals. The line strengths calculated for the Boron and Fluorine isoelectronic sequences are in excellent agreement with the calculations involving Dirac wavefunctions for all ions uptoZ=60. The maximum difference at the highest value ofZ=92 is about 6%. Our calculated lifetimes for the state 2p ^{5 2} P _1/2 for Fluorine-like Mg IV and Fe XVIII are 5.03 s and 51.7 µs respectively which are in excellent accord with corresponding values 5.00 s and 51.0 µs calculated by using sophisticated configuration interaction wavefunctions within the Breit-Pauli approximation. Our calculated value of lifetime for Thalium-like Pb II is 40.0 ms which is in good accord with the experimental value of 41.2 ms. New results are presented for the highly ionized ions in the Al-like and Cl-like isoelectronic sequences. The present analysis can be exploited for all the ions in the isoelectronic sequences of elements of groups III A, III B VII A of the periodic table.     

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.     

MO-calculations of the energy transfer-activities of organic π-structures in the photo-fries rearrangement—III : A theoretical index for the energy transfer efficiency of organic π-systems in the photo-fries reaction

The energy transfer process between the excited singlet state of N-phenylurethane, NPU^*, and any other π-system, M, will be described theoretically by introducing the integral U, which symbolizes the interaction of the states NPU^*-M and NPU-M* following a suggestion of Labhart et al.¹ The formula for U based on the LCAO-MO-SCP-LCI approximation in the Pariser-Parr-Pople-version enables us to systematise the energy tranfer tendency of any π-system to quench or sensitize the photo-Fries reaction of NPU^* depending on the aggregate geometry NPU-M. The criteria of the complex geometry and the electronic structure of M providing a maximal value of U are collected and discussed. Together with the differences in transition energies of the fluorescence and the absorption spectra of NPU and M, respectively, the maximal values of U yield a theoretical index ?, providing a sequence for different M to inhibit the photo-Fries reaction by energy acception.