Alessandra Degli Esposti

Ab initio calculation of the OH (X 2Π, A 2Σ+)+Ar potential energy surfaces and quantum scattering studies of rotational energy transfer in the OH (A 2Σ+) state

The potential energy surfaces of OH+Ar, which correlate asymptotically with OH(X 2Π)+Ar(1S) and OH(A 2Σ+)+Ar(1S), have been calculated using the coupled electron pair approximation (CEPA) and a very large basis set. The OH–Ar van der Waals complex is found to be bound by about 100 cm−1 in the electronic ground state. In agreement with several recent experimental studies the first excited state is found to be much more stable. The A state potential energy surface has two minima at collinear geometries which correspond to isomeric OH–Ar and Ar–OH structures. The dissociation energies De are calculated to be 1100 and 1000 cm−1, respectively; both forms are separated by a barrier of about 1000 cm−1. The equilibrium distances for OH–Ar and Ar–OH are calculated to be 2.9 and 2.2 A, respectively, relative to the center of mass of OH. In order to investigate the nature of the strong binding in the A state, we have calculated accurate dipole and quadrupole moments as well as dipole and quadrupole polarizabilities ...

Photophysical characterization of free-base corroles, promising chromophores for light energy conversion and singlet oxygen generation

A detailed photophysical characterization of a series of six free-base corroles with different substitution patterns at meso-positions is presented. In air-free toluene at 295 K, the luminescence quantum yields range from 0.13 to 0.22 and lifetimes vary from 4.1 ns to 6.3 ns whereas in air-saturated toluene the lifetimes range from 3.8 ns to 5.6 ns. In glassy toluene at 77 K the lifetimes have values in the range 5.2 ns to 7.9 ns. Transient absorption spectra of the singlet excited states have positive bands art λ 680–700 nm. The transient absorption spectra of the lowest triplet display a band around 460–470 nm and bleaching features in the Q bands region, no absorption above 700 nm. The triplet lifetimes in air-free toluene are in the interval 50–150 μs. The lowest excited states react with oxygen and sensitized singlet oxygen (1Δg) yields of 0.51–0.77 are determined via its luminescence. Electron and energy transfer from the excited states to molecular oxygen is discussed. Effect of polar solvents on the luminescence parameters indicate that charge transfer (CT) excited states play a very minor role. Thermal and photo stability is excellent for most of the examined compounds in toluene and dichloromethane.

Quantum scattering studies of the Λ doublet resolved rotational energy transfer of OH(X 2Π) in collisions with He and Ar

Three dimensional potential energy surfaces for the collision systems OH(X 2Π)+He and OH(X 2Π)+Ar have been calculated using the coupled electron pair approximation (CEPA) and large basis sets. The asymptotically degenerate 2Πx and 2Πy states split into two states of 2A′ and 2A″ symmetry, respectively, when the C∞v symmetry is lifted by the approach of the noble gas atom. The average and half difference of the calculated points on the A″ and A′ potential energy surfaces were fitted to analytical functions, which were then vibrationally averaged. These potential energy surfaces have been used in quantum scattering calculations of cross sections for collision induced rotationally inelastic transitions. Test calculations showed that the cross sections obtained from exact close‐coupling calculations (CC) and within the coupled states approximation (CS) are in close agreement for these systems, and therefore the CS approximation has been used in all further calculations. Rotational transitions with Λ doublet r...

Quantum scattering study of rotational energy transfer in OH(A 2Σ+, v’=0) in collisions with He(1S)

A quantum mechanical study of rotational energy transfer (RET) in OH(A 2Σ+, v’=0) in thermal collisions with He(1S) has been performed. The interaction potential of OH(A)+He was computed using the coupled electron pair approximation (CEPA) and a very large basis set. An analytical fit of the resulting OH–He potential was employed in close‐coupling (CC) and coupled states (CS) calculations of integral RET cross sections for collision energies up to 5000 cm−1. The cross sections were integrated over a Boltzmann energy distribution to yield thermally averaged rate coefficients. State‐to‐state RET coefficients for the lowest 11 fine structure levels of OH(A, v’=0) were calculated as a function of the temperature. The agreement between the theoretical and recently measured values at 300 K is very good. The data for the OH(A)+He system are compared to the results of a previous theoretical study of the OH(A)+Ar system [A. Degli Esposti and H.‐J. Werner, J. Chem. Phys. 93, 3351 (1990)]. The theoretical findings f...

THE INTRAMOLECULAR VIBRATIONS OF PROTOTYPICAL POLYTHIOPHENES

Inelastic neutron scattering experiments are combined with infrared and Raman data to obtain a uniquely defined description of the intramolecular vibrations of three oligomers of polythiophene. Through refinement of ab initio force fields, the three vibrational spectra of each oligomer are simulated with remarkable accuracy. Two different basis sets of atomic orbitals are used: the first, is 6‐31G* and is used to optimize the geometries and calculate the relevant force fields of α‐2T and α‐4T, the second is 3‐21G* and is used for the same purpose for α‐4T and α‐6T. To improve agreement with the experiment, the force fields are scaled. In this way, one set of scaling parameters is generated for the 6‐31G* basis and another for the 3‐21G* basis. The parameters are common to both molecules calculated with either basis sets and are believed to be transferable to higher isomers. The fitting procedure is applied in steps: first, the calculated vibrational frequencies are assigned on the basis of the experimenta...

The dynamics of the internal phonons tris(quinolin-8-olato) aluminum(III) in crystalline β-phase

A new approach to the analysis of the internal phonons of tris(quinolin-8-olato) aluminum(III) is presented, which enlightens the role played by the ligands in determining the vibrational properties of the organometallic compound and evidences the importance of the contributions arising from the coupling terms among the three quinolinato fragments. An accurate exam of the normal modes of the meridianal isomer evidences the role of the interactions among the fragments in the vibrational dynamics of the ground state. Due to the special attention paid to the quinolinato fragments, a preliminary investigation on the vibrational properties of 8-hydroxyquinoline, taken as a model fragment, was also performed. The vibrational properties of the polymorph species β of the organometallic molecule were obtained refining the calculated frequencies, the dipole moment derivative matrix, and the polarizability derivative tensor derived by the hybrid density functional B3LYP/6-31G* comparing with the frequencies and inte...

The polarized infrared and Raman spectra of α-T6 single crystal: An experimental and theoretical study

The polarized infrared absorption and Raman spectra on the bc plane of a single crystal of α-sexithienyl (α-T6) have been recorded in the range from 50 up to 3500 cm−1. The intermolecular interactions give rise to the Davydov splitting of the ungerade phonon energy levels originated by the molecular pyramidalization along with the CCH bending modes. A μ-Raman investigation on the crystal surface allows identification of the perturbed local packing of the molecules, which are spectroscopically crystal defects. The selective resonant enhancement of the scattering intensity of some Raman modes, obtained by exciting at λex=632.8 nm, indicates that these defects are associated with the lowering of the electronic excited states due to the increased π-orbital overlapping. Moreover, the resonant enhancement allows detection of some of the gerade intramolecular phonons not observed in the bulk. The comparison between the far-infrared absorption spectra recorded at room temperature and at 80 K evidences three latti...

Organometallic quinonoid linkers: a versatile tether for the design of panchromatic ruthenium(II) heteroleptic complexes.

The synthesis, X-ray structure determination, and photophysical investigation of a novel series of heteroleptic ruthenium(II) polypyridine complexes with organometallic linkers are reported. The displayed panchromatic absorption features are assigned by means of time-dependent density functional theory studies.

Analytical energy gradients for multiconfiguration self‐consistent field wave functions with frozen core orbitals

A method to calculate analytical energy gradients for multiconfiguration self‐consistent field (MCSCF) wave functions with frozen core orbitals is presented. Since the core orbitals, which are taken from a closed shell SCF calculation, are not variationally optimized in the MCSCF procedure, it is necessary to determine their derivatives by solving a set of coupled perturbed Hartree–Fock (CPHF) equations. The technique is similar to the calculation of energy gradients for CI wave functions, but is complicated by the fact that the SCF and MCSCF orbitals are different. This makes it necessary to perform a transformation between the two orbital basis sets at an intermediate stage. The CPHF equations are solved by an iterative method, in which optionally part of the Hessian matrix can be constructed and inverted explicitly. Some applications of the method are presented. For the molecule P2S, optimized geometries for two isomers and a saddle point are compared for MCSCF wave functions with frozen and fully opti...

Microwave spectrum and ab initio computations for ethylene carbonate. Part 1.—Conformation and ring inversion

The conformation and ring inversion in ethylene carbonate have been investigated using microwave spectroscopy and ab initio computations. The vibrational satellite spectra of the normal isotopic species of ethylene carbonate and the rotational constants of Backvall et al.(Tetrahedron Lett. 1980, 21, 4985) for cis- and trans-[1,2-2H2]ethylene carbonate show the molecule to have a twisted C2 equilibrium conformation. The vibrational satellite spectra have been analysed in terms of independent ring-bending and twisting vibration. A potential function for the twisting vibration derived from the variation of the rotational constants with vibrational state, and vibrational energy separations obtained from relative intensities and Coriolis perturbations give an equilibrium twist angle of 19° and a barrier to ring inversion of 2.8 kJ mol–1. The twist angle has also been obtained from inertial data, and a value of 15° is obtained using two methods of calculation. Ab initio computations of the ring-puckering potential-energy surface have been made using STO-3G and STO-3-21 G orbitals and complete geometry optimization. The STO-3G computations predict a planar C2ν equilibrium geometry, but with the twisting vibration being lower in frequency and more anharmonic than the bending vibration. The STO-3-21G computations predict a C2 conformation with an equilibrium twist angle of 14° and barrier to inversion through the planar ring conformation of 1.1 kJ mol–1. The computed barrier to ring inversion by pseudorotation is 14.3 kJ mol–1.