Ph. Millie

Dipole binding: An experimental test for small cluster structure calculations

An experimental method for the discrimination between nearly degenerate isomers of size selected clusters of closed‐shell polar molecules is presented. It is based upon electron attachment properties of dipole fields since a minimum value (≊2.5 D) of molecular dipoles is required for electron binding. When neutral clusters are created in different configurations only those with large enough resulting dipole moments bind electrons and give birth to stable anions. These dipole‐bound anions have the geometries of their neutral parents which are here calculated within the framework of the exchange perturbation theory as developed by Claverie. Anions are created by Rydberg electron transfer to cold neutral clusters containing up to six molecules of acetonitrile, water, ammonia, or methanol. Structure and dipole moment calculations account well for the ‘‘magic’’ numbers observed in mass spectra distributions. Detailed comparisons between experimental data and calculated geometries lead to informations on the st...

Fluorescence spectra and intramolecular vibrational redistribution in jet-cooled perylene

Abstract The jet-cooled fluorescence spectra of perylene excited to the S 1 state with E vib = 0–1600 cm −1 are recorded and analyzed. For E vib −1 only the resonant fluorescence was detected. Ground- and excited-state frequencies of 14 low-frequency normal modes are determined. A drastic change in frequency of the “butterfly” modes upon electronic excitation shows that perylene slightly deviates from planarity in its ground state and is more rigid in the excited singlet state. For a number of levels in the E vib = 800–1600 cm −1 range, the fluorescence is composed of the resonant emission and of non-resonant (“‘relaxed’”) bands. It is shown that apparently single bands in the fluorescence-excitation spectrum correspond to ovelapping bands pumping different molecular eigenstates resulting from the intrastate coupling. The relative role of the anharmonicity and of the Coriolis interaction are discussed. The data are treated in terms of a selective coupling between doorway and hallway states with the coupling constant rapidly decreasing with the difference in the overall vibrational quantum number between initial and final state.

Intramolecular charge transfer and trans-cis isomerization of the DCM styrene dye in polar solvents. A CS INDO MRCI study

Abstract The solvent-induced changes of trans-cis isomerization efficiency and electronic structure of the excited state of the DCM dye have been considered by means of CS INDO MRCI calculations. The potential energy curves, dipole moments and atomic charge densities as a function of two internal coordinates, namely the rotation angle about the central “double” bond and the twisting of the dimethylamino group, have been obtained for the ground state and the lowest excited states. The structural requirements for the existence of ICT (intramolecular charge transfer) excited states have been investigated by considering internal rotations about three single bonds. The reliability of the potential surfaces and of the solvation models has been discussed with reference to test-calculations on the DMABN molecule. In the first excited singlet state of DCM, the low-energy barrier for the trans-cis isomerization has been found unaffected by the solvent polarity. The only singlet excited state presenting a large ICT character has been found to be the S 2 state for a perpendicularly twisted conformation of the dimethylamino group (TICT state). The assumption of a deactivation of the trans-isomer in the locally excited state through the TICT funnel has been largely discussed with reference to the simplifications of the present theoretical approach.

Vibrational redistribution in jet-cooled perylene intermediate coupling case

Fluorescence excitation spectra (FES) and fluorescence spectra (FS) under single vibronic level excitation of perylene in a supersonic jet are reported. The fluorescence from the lowest vibronic levels of the S1 state is purely resonant while for δEvib > 62; 1600 cm−1 broad-band emission spectra weakly dependent on νexe are explained by vibrational redistribution with a characteristic time 5 <Tred < 100 ps. In the intermediate δEvib range (700–1600 cm−1) complex emission spectra correspond to a strong coupling between the doorway state and a small number of “dark” states. The data are discussed in terms of a model assuming a rapid variation of the anharmonic coupling constant with the overall change of the vibrational quantum number between interacting levels.

The second conformer of the benzene—argon2 van der Waals complex

Abstract From a rotational contour study of UV vibronic bands of the benzene—Ar 2 complex as well as from theoretical structures modelling, conclusive arguments are presented, demonstrating the existence of a second (2, 0) stable conformer of C s symmetry, in addition to the already known symmetric (1, 1) species.

A time-resolved photoelectron study of the double excited-state proton-transfer reaction in 7-azaindole dimer

Abstract A study employing picosecond and subpicosecond excitation in a mass and photoelectron spectrometer is reported for the 7-azaindole (7-AI) dimers, reactive and unreactive. The 7-AI photoelectron spectrum is structured and has a sharp ionisation threshold at 8.17 eV above the neutral ground electronic state. The reactive dimer ionisation threshold was measured as 7.19 eV. The excited-state lifetime of the reactive dimer was measured by a technique that monitors the ionisation signal as a function of pulse duration. 1 + 1 resonance ionisation photoelectron spectra were recorded using 0.8 ps and 5 ps pulses. Our results indicate a lifetime substantially less than a picosecond, however, consistent with recent real time studies. Advantages of the method are discussed.

Intermolecular interactions: basis set and intramolecular correlation effects on semiempirical methods. Application to (C2H2)2, (C2H2)3 and (C2H4)2

A detailed study of the intrinsic consistency of the semiempirical method of P. Claverie namely, the effects of the basis set and intramolecular correlation on the multipole distributions of molecular subunits and the influence of the electronic population of each atom in the molecular subunit on its van der Waals radius, is performed on some van der Waals dimers. The validity, limits of this model and the appropriate way to use it is established. In particular, the dependence of the geometry and the interaction energy on the basis set chosen and the intramolecular correlation shows that the multipole distribution involved in the calculation of the electrostatic and polarization terms must be derived from a correlated wave function within an extended basis set. Associated to non local methods for finding stationary points, the method of P. Claverie reproduce reliably the intermolecular geometrical parameters observed for the equilibrium structures and the transition states of the dimer and trimer of acetylene. In addition, a study of the equilibrium structures of the ethylene dimer is presented, the aim of being to clarify the considerable uncertainty in their number and their geometry.

Bond formation between positively charged species. Non-adiabatic analysis and valence-bond model in the CO2+ case

Abstract Ab initio MCSCF calculations of the potential energy curves of the first low-lying states of the CO 2+ dication have been performed. A localization procedure of MCSCF orbitals allows us to discuss the bond formation in the framework of valence-bond theory. The non-adiabatic curves corresponding to C + + O + , C 2+ + O and C + O 2+ are described using both orthogonal and non-orthogonal quasi-atomic orbitals, allowing us to re-examine the currently proposed models of bond formation between positively charged species.

Laser induced fluorescence of jet-cooled non-conjugated bichromophores: bis-phenoxymethane and bis-2,6-dimethylphenoxymethane

Abstract The electronic spectroscopy of bichromophoric molecules linked by a OCH 2 O chain, such as bis-phenoxymethane (1), and bis-2,6-dimethylphenoxymethane (3), has been studied in a supersonic free jet by laser induced fluorescence. The experimental results have been compared to calculations resting on the perturbative CIPSI method, in which the di- and tri-excited configurations involving the π as well the v orbitals have been taken into account. The bis-phenoxymethane (1) molecule shows a single 0—0 transition which is blue-shifted relative to anisole by more than 400 cm −1 . This blue-shift has been theoretically related to the conformation of the bichromophore which displays an out-of-plane distorsion of the ether chain relative to anisole. The calculations clearly show that the observed blue-shift of the transition is related to this distorsion, and not to any electronic coupling between both cycles which is very weak. The single transition experimentally observed corresponds to the most stable structure of (1) which is in a gauche-gauche conformation relative to the CO bonds of the chain. In this structure the cycles are equivalent. The study of van der Waals complexes of (1) with usual solvents confirms this interpretation and shows that the equivalence of the cycles is removed by complexation. This contrasts with the bichromophore (3) whose 0—0 transition is blue-shifted by only 25 cm −1 relative to the 2,6-dimethylanisole subunit. Calculations have shown that in this case, the monomeric subunit as well as the bichromophore are in an out-of-plane conformation due to the steric hindrance introduced by the methyl groups. Moreover the excited state of (3) behaves as a weakly fluorescent exciplex whereas the emission resulting from the excitation of (1) is resonant. For the sake of comparison, the fluorescence excitation spectrum of the van der Waals dimer of anisole has also been studied and exhibits a red-shift with respect to bare anisole. The equilibrium geometry and the exciton coupling have also been calculated for the anisole van der Waals dimer, by means of the exchange perturbation theory.

Infrared spectra of the C2H2–HCl complexes: An experimental and ab initio study

By means of a pulsed slit jet and an infrared tunable diode laser spectrometer, the vibration–rotation absorption spectra of the complexes C2H2–H35Cl and C2H2–H37Cl have been observed for the first time in the 3.6 μm region of the ν1 band correlated with the HCl stretch. All the lines of the spectrum have been assigned for J=0 to 18 and Ka=0, 1, 2, 3. To determine the band origin and the rotational and centrifugal constants, the observed line frequencies have been fitted to those determined by the Watson Hamiltonian in the A reduction. A force constant model has been used to derive the binding energy De of the complex and the intermolecular stretching harmonic frequency from the experimental spectroscopic constants. The available experimental results concerning these complexes and other isotopic forms D35Cl and D37Cl were compared with ab initio calculations performed at the coupled-cluster single double triple [CCSD(T)] level of theory. The comparison turned out to be very good for all the properties con...