Rolf Linder

Tautomers and electronic states of jet-cooled 2-aminopurine investigated by double resonance spectroscopy and theory

We present resonant two-photon ionization (R2PI), IR-UV, and UV-UV double resonance spectra of jet-cooled 2-aminopurine (2AP) as well as Fourier transform infrared (FTIR) gas phase spectra. 2AP is a fluorescing isomer of the nucleobase adenine. The results show that there is only one tautomer of 2AP which absorbs in the wavelength range 32,300-34,500 cm(-1). The comparison with the calculated IR spectra of 9H- and 7H-2AP points to 9H-2AP as the dominating tautomer in the gas phase but the spectra are too similar to allow an unambiguous assignment to the respective tautomer. Hence, we determined vertical and adiabatic excitation energies of both tautomers employing combined density functional theory and multi-reference configuration interaction techniques. For the 0-0 band of the first 1pipi* transition of 9H-2AP we obtain a theoretical value of 32,328 cm(-1), in excellent agreement with the band origin of our R2PI spectrum at 32,371 cm(-1). The first singlet pipi* transition of the less stable 7H-2AP tautomer is predicted to be red-shifted by about 1700 cm(-1) with respect to the corresponding transition in 9H-2AP. From the absence of experimental bands in the energy region between 30,300 and 32,350 cm(-1) we conclude that 7H-2AP is not present to an appreciable extent in the molecular beam. Our calculations yield nearly equal energies for the 1npi* and 1pipi* minima of isolated 2AP, similar to the situation in adenine. The hitherto existing argument that the energetic order of states is responsible for the different spectroscopic properties of these isomers therefore does not hold. Rather, vibronic levels close to the origin of the 1pipi* transition cannot access the conical intersection between the 1pipi* and S(0) states along a puckering coordinate of the six-membered ring, in contrast to the situation in electronically excited 9H-adenine. As a consequence, a rich vibrational structure can be observed in the R2PI spectrum of 2AP whereas the spectrum of 9H-adenine breaks off at low energies.

The vibronic structure of mixed ligand Os(IV) complexes II. Low lying ligand field levels of trans-[OsCl4I2]2−, fac- and mer-[OsCl3I3]2−

Abstract The title compounds were optically investigated by recording the 10 K absorption spectra of the Cs and n-tetrabutylammonium salts pressed in KBr pellets and the 10–20 K emission spectra of the complex ions doped into Cs 2 SnCl 6 . For d-d transitions the spectral region is limited to the range from about 4000–12000 cm −1 . The measured peaks are assigned to zero-phenon transitions (electronic origins) between states belonging to the (t 2g ) 4 subshell configuration and to corresponding vibronic transitions. For the better resolved spectra an assignment to symmetries of respective point groups can be obtained from selection rules using relative peak intensities. These results must be, however, considered as still tentative.

The vibronic structure of mixed-ligand osmium(iv) complexes

Mixed-ligand osmium(iv) complexes, OsCl5X2- and cis-OsCl4X2- 2, with X = Br or I, are optically investigated by recording the 10 K absorption spectra of the caesium and tetrabutylammonium salts in KBr pellets and the 10–20 K emission spectra of complexes doped in Cs2SnCl6, both in the energy region between 5000 and 17 000 cm-1. The measured peaks (origins and vibronics) are due to zero-phonon and vibronic transitions respectively within the (t2g)4 subshell configuration, which gives rise to a maximum of 15 energy levels at low symmetry. From the well-resolved absorption and emission spectra the d-level energies can be obtained for many of the split states that are expected in the region considered. For an assignment to symmetry types, selection rules for electric dipole transitions are applied, comparing peak intensities in the spectra. Ligand-field calculations using angular overlap parameters support these assignments, which must, however, be considered as partially tentative.

Metal pair spectra and exchange coupling parameters of di-μ-hydroxo-bis [tetraamine chromium (III)] bromide

Abstract The absorption, luminescence, and far i.r. spectra of the title compound have been recorded at various temperatures. A computer-assisted assignment of all peaks measured in the vibronic spectra has been carried out using an energy level scheme for the exchange coupled 4 A 2 g 4 A 2 g and 4 A 2 g 2 E g manifolds, calculated from a model considering only bilinear coupling terms in the excited states. Optimal agreement with the experiment is obtained if the energy levels are calculated from the exchange coupling parameters (in cm −1 ) J = 30, j = −0.3 for the ground state, J 11 = 0, J 12 = 125, J 13 = −48, J 33 = 186 for the excited state manifold, from the low symmetry ligand field parameter V = 46, and an energy gap of Δ E = 14 445. The importance of vibronically induced transitions in the assignment of band peaks is emphasized: frequencies due to this transition mechanism are obtained by comparison with the i.r. spectrum and with the electronic transition energies calculated from the model.

Hochaufgelöste vibronische Spektren von Al(acac)3 : Cr3+/Highly Resolved Vibronic Spectra of Al(acac)3: Cr3+

Abstract Highly resolved emission spectra of Cr3+ doped Trisacetylacetonatoaluminum(III) have been measured at various temperatures. By comparison with absorption spectra of dilute crystals the three most intense peaks have been assigned to the same molecular zero phonon transition according to three inequivalent Cr3+ sites in the low temperature phase of the host crystal. The usually unobserved ground state splitting has been resolved (1.0 cm-1) in emission spectra at T = 1.9 K. Band assignments are confirmed by ligand Field calculations using complete trigonal perturbation matrices including spin-orbit coupling. From the obtained parameters K = 725 cm-1 and K′ = - 600 cm-1 the trigonal distortion of the chromium complex in the Al-crystal is determined. This indicates that the phase transition of the host crystal leads to rotations of rigid molecules while the molecular geometry remains.