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.     

Symmetry-adapted-cluster configuration interaction study of the doublet states of HCl+: potential energy curves, dipole moments, and transition dipole moments

The electronic structure of the HCl(+) molecular ion has been calculated using the general-R symmetry-adapted-cluster configuration interaction (SAC-CI) method. The authors present the potential energy curves, dipole moments, and transition dipole moments for a series of doublet states. The data are compared with the previous CASSCF and MCSCF calculations. The SAC-CI results reproduce quite well the data available in literature and extend the knowledge on the HCl(+) electronic structure for several higher states. The calculated R-dependent behavior of both dipole moments and transition dipole moments for a series of bound and unbound states reveals an intricate dissociation process at intermediate distances (R>R(e)). The pronounced maxima in transition dipole moment (TDM) describing transitions into high electronic states (X (2)Pi-->3 (2)Pi, X (2)Pi-->3 (2)Sigma, 2 (2)Pi-->3 (2)Pi, 3 (2)Pi-->4 (2)Pi) occur at different interatomic separations. Such TDM features are promising for selection of excitation pathways and, consequently, for an optimal control of the dissociation products.     

Ab initio vibrational transition dipole moments and intensities of formaldehyde

Vibrational transition dipole moments and absorption band intensities for the ground state of formaldehyde, including the deuterated isotopic forms, are calculated. The analysis is based on ab initio SCF and CI potential energy and dipole moment surfaces. The formalism derives from second-order perturbation theory and involves the expansion of the dipole moment in terms of normal coordinates, as well as the incorporation of point group symmetry in the selection of the dipole moment components for the allowed transitions. Dipole moment expansion coefficients for the three molecule-fixed Cartesian coordinates of formaldehyde are calculated for internal and normal coordinate representations. Transition dipole moments and absorption band intensities of the fundamental, first overtone, combination, and second overtone transitions are reported. The calculated intensities and dipole moment derivatives are compared to experiment and discussed in the context of molecular orbital and bond polarization theory.     

Efficient calculation of vibrational magnetic dipole transition moments and rotational strengths

A priori prediction of vibrational magnetic dipole transition moments and vibrational strengths requires the calculation of the overlap integral of the derivatives of the electronic wavefunction with respect to nuclear displacement and an external magnetic field. The efficient calculation of this integral, using coupled Hartree-Fock theory, is described.     

Metal—ligand and π singlet—triplet transitions in phthalocyanine films studied by inelastic electron tunneling spectroscopy

Inelastic electron tunneling spectroscopy (IETS) on AlAl₂O₃/PcPb tunneling junctions at 4.2 K has been used to study electronic transitions involving π, π^≠ and metal-ligand orbitals of phthalocyanine (Pc) films (H₂Pc, FePc, CoPc, NiPc, CuPc, ZnPc). The results are compared with calculations for Pc molecules.     

Potential-energy surfaces in singlet and triplet silylene

MC SCF calculations with the 6-31G^** basis set, augmented by second-order configuration interaction, are used to analyze the potential-energy curves in the lowest singlet and triplet states of SiH₂. The adiabatic singlet-triplet splitting is predicted to be 17.7 kcal/mol, and the curves are predicted to cross at ≈ 129°. Thus, bulky substituents may open the angle in the singlet sufficiently to invert the ordering of the two states.     

On the evaluation of electric and magnetic dipole transition moments in the ZDO approximation

The implications of the ZDO approximation for the evaluation of electric and magnetic dipole transition moments are analyzed using an overlap ordered expansion of the type proposed by Fischer-Hjalmars. The relative merits of electric dipole length and electric dipole velocity matrix elements are discussed from the point of view of their use in conjunction with the ZDO approximation.

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Zusammenfassung Die Folgerungen aus der ZDO-Näherung für die Berechnung elektrischer und magnetischer Dipolübergangsmomente werden mit Hilfe einer geordneten Entwicklung der Überlappungsmatrix, wie von Fischer-Hjalmars vorgeschlagen, untersucht. Die relativen Vorzüge der Benutzung von Matrixelementen des Dipolmomentoperators einerseits und des Impulsoperators andererseits werden im Rahmen der ZDO-Näherung diskutiert.

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Résumé Analyse des implications de l'approximation du recouvrement différentiel nul dans l'évaluation des moments de transition dipolaire électrique et magnétique, en utilisant un développement en série selon les puissances du recouvrement du type de celui proposé par Fischer-Hjalmars. Les mérites relatifs des éléments de matrice des opérateurs longueur dipolaire électrique et vitesse dipolaire électrique sont discutés du point de vue de leur utilisation dans le cadre de l'approximation à recouvrement différentiel nul.

     

Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

We observe in glyoxal cooled in a supersonic free jet the fluorescence of individual rotational levels of the S₁ state excited by a cw laser. We use the technique of singlet—triplet magnetic resonance near an anticrossing to measure matrix elements V₃₁ as a function of rotational quantum numbers N_s, N_t, K_s, K_t. The experimental results are compared with theoretical models of singlet—triplet couplings and we show that the spin-vibronic interaction is the dominant singlet—triplet interaction in glyoxal.     

Quantitative aspects of all-trans-retinol singlet and triplet quenching by oxygen

In benzene and methanol, the oxygen quenching of all-trans-retinol singlet results in intersystem crossing (enhanced) and energy transfer (to form ¹O₂^*, ¹Δ_g), both with quantum efficiencies considerably less than unity (0.3–0.5). ¹O₂^* formation from retinol triplet quenching by ³O₂ is closer to quantitative (≈ 0.8).     

Ab initio study on NH+: Transition dipole moments,transition probabilities,and radiative lifetimes

The transition electric dipole moments between low-lying valence states of NH⁺ are calculated by an ab initio effective valence-shell Hamiltonian (H^v) method. The H^v calculated transition moments are found to be in good agreement with those by other accurate ab initio methods. The spontaneous emission probabilities for the A²− → X²Π, B²Δ → X²Π, and C²−⁺ → X²Π transitions of NH⁺ are computed. Also, radiative lifetimes for A²∑⁻, B²Δ, and C²∑⁺ states are all theoretically determined using the potential energy functions by H^v. Also, the H^v results are well compared with those computed using the Morse potentials and the rkr potentials which are obtained from experimental data. © 1996 John Wiley & Sons, Inc.     

Study of singlet and triplet 2,6-difluorophenylnitrene by time-resolved infrared spectroscopy

The solution-phase photochemistry of 2,6-difluorophenyl azide was studied by time-resolved infrared (TRIR) spectroscopy. A vibrational band of singlet 2,6-difluorophenyl nitrene (1N) was observed at 1404 cm(-1) between 243 and 283 K. At ambient temperature, it was not possible to detect this intermediate. At 298 K, only the decay products of the singlet nitrene, the isomerized products ketenimine (K) and triplet-2,6-difluorophenyl nitrene (3N), were observed at 1576 and 1444 cm(-1), respectively. The assignments are consistent with density functional theory calculations and previous studies of this system by laser flash photolysis techniques with UV-visible detection.     

Calculated photoelectron spectra of singlet and triplet methylene

Vertical ionization potentials for singlet and triplet methylene are calculated by a CI perturbation method based on ab initio SCF molecular orbitals (6–31 G^** basis). The shape and vibrational fine structure of the first photoelectron band are investigated using the MINDO/3 method. The computed singlet-triplet splitting for methylene is 16.4 kcal/mol, in reasonable agreement with the experimental value of 19.5 kcal/mol.     

Low-lying singlet and triplet electronic states of RhB

The low-lying XSigma+, a3Delta, A1Delta, b3Sigma+, B1Pi, c3Pi, C1Phi, D1Sigma+, E1Pi, d3Phi, and e3Pi electronic states of RhB have been investigated at the ab initio level, using the multistate multiconfigurational second-order perturbation (MS-CASPT2) theory, with extended atomic basis sets and inclusion of scalar relativistic effects. Among the eleven electronic states included in this work, only three (the X1Sigma+, D1Sigma+, and E1Pi states) have been investigated experimentally. Potential energy curves, spectroscopic constants, dipole moments, binding energies, and chemical bonding aspects are presented for all electronic states.     

Conformation control of triplet state diffusion in platinum containing polyfluorene copolymers

The spectral diffusion of singlet and triplet excitons in 9,9-dioctylfluorene-based conjugated copolymers is investigated using photoluminescence spectroscopy at both low (5 K) and room temperature (300 K). Inclusion of a N,N-diphenyl-4-(pyridin-2-yl)aniline moiety into the polymer backbone allows subsequent cyclometalation with platinum acetylacetonate to increase the spin-orbit coupling and yield radiative decay from the triplet state. For suitably low fractions (≤5%) of bulky ligand inclusion, cyclometalated or not, the resulting longer sequences of fluorene units are able to adopt the chain-extended β-phase conformation. Comparison between the phosphorescence spectral diffusion in glassy- and β-phase Pt-copolymer samples provide insight into the triplet exciton transfer in more- or less-disordered conjugated polymer films. It is found in the glassy-phase samples with shorter conjugation lengths that the triplet exciton relaxation becomes frustrated at low temperature due to a freezing out of the thermally activated hops required to move from one conjugated segment to another. In contrast, for films containing β-phase chain segments, with increased conjugation lengths, this frustration is lifted as more hopping sites remain accessible through intra-segment motion. This work demonstrates controlled use of changes in molecular conformation to optimize triplet diffusion properties in a member of the widely deployed fluorene-based conjugated copolymers.     

Vibrational spectra,dipole moments,and conformations of acylic sulfoxides

Conclusions Acyclic aliphatic and aliphatic-aromatic sulfoxides exist in the form of equilibrium mixtures of conformations. In the case of bromomethyl methyl sulfoxide, a conformational equilibrium of three forms is observed, and an equilibrium between two forms basically exists for chloromethyl methyl sulfoxide and halomethyl aryl sulfoxides.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 12, pp. 2758–2764, December, 1980.We thank S. A. Katsyub for supplying the programs for the calculations of the frequencies and the normal modes of vibration.