Gaston Berthier

Theoretical study of trans-cis photoisomerism in polymethine cyanines

Abstract The trans-cis photoisomerism of polymethine dyes has been interpreted so far using different and rather inconsistent models of the potential energy surfaces. In order to search for a unified electronic model, we tackled the problem again from an intramolecular point of view. Our study consisted in qualitative MO considerations followed by explicit (CS INDO) calculations of the S 0 , T 1 and S 1 potential energy curves for a proper model system: pentamethine cyanine isomerizing around the 2–3 and 3–4 bonds. Torsional energy levels for the calculated potential curves were also obtained. The main conclusions are: (i) the photoreaction proceeds through a “non-spectroscopic” (perp) S 1 minimum which exists also in the isolated molecule, (ii) this twisted excited species has charge transfer character (TICT), as opposed to the biradicaloid character of the “non-spectroscopic” S 1 minimum involved in the trans→cis photoisomerization of olefines (e.g., stilbene). The possible consequences on the dynamics of the excited state relaxation in non-polar solvents are envisaged.

Ab initio study of the electronic structure and hyperfine coupling properties in simple hydrocarbon radicals. II. Short‐range and long‐range interactions in alkyl free radicals

Nonempirical calculations of the ground−state energy, proton and carbon−13 coupling constants of methyl, ethyl, n−propyl, and cyclopropyl radicals have been performed in the frame of spin−restricted LCAO−SCF open−shell and first−order double−perturbation theories. The rotation barrier is negligible for the ethyl radical. The two barriers of the n−propyl radical are, respectively, 0.3 kcal/mole (0.4 exptl) and 4.7 kcal/mole for the rotations about the Ċ−Cα and Cα−Cβ bonds. The cyclopropyl radical is found to be nonplanar with an out−of−plane angle of 41° and an inversion barrier of 3.80 kcal/mole. For the equilibrium conformations, the computed carbon−13 splittings of the radical carbon of methyl (1), ethyl (2), n−propyl (3), and cyclopropyl (4) are +31.04, +37.90, +37.72, and +138.83 G, respectively; the theoretical α −carbon−13 splittings are −18.14 (2), −17.15 (3), and −8.35 G (4); the β −carbon−13 splitting is +11.8 G for the stable conformation of the n−propyl radical. The calculated coupling constant...

Ab initio study of the electronic structure and hyperfine coupling in simple hydrocarbon radicals. I. Test of the calculation method on methyl and vinyl

Nonempirical calculations of the energy, proton, and carbon−13 hyperfine splittings for methyl (ĊH3) and vinyl radicals (H2C=ĊH) in their equilibrium geometry are presented. The spin−restricted SCF method and first−order double−perturbation theory including all spin−adapted monoexcited states with three unpaired electrons have been used. The basis set consists of Gaussian−type orbitals contracted in a double−zeta form. The orbital exponents of the hydrogen functions have been simultaneously varied using a scaling procedure in order to simulate a minimum Slater hydrogenoid orbital. The sensitivity of SCF energies and hyperfine splittings to variations in values of the orbital exponent ζH have been investigated. The lowest total energy is obtained for ζH=1.15. Although calculated hydrogen hyperfine splittings increase with ζH, neither the zeroth−order nor the first−order spin density, computed from canonical MO’s or quasilocalized equivalent MO’s can be correlated to ζ3H. A structural analysis of the contri...

Vibrational properties of polyatomic molecules by quantum-chemical methods

Force constants and dipole moment derivatives have been computed for the molecules CH3 X(X=F, Cl, Br, I) using MO wavefunctions with pseudo-potentials for the interaction between the inner shell and valence electrons. The values obtained at the SCF approximation level from a set of gaussian valence orbitals contracted in double-zeta form and enriched with polarization functions compare well with experimental assignments of force and interaction constants, as well as integrated intensities from infrared data. The transferability of atomic force fields and polar tensors (second derivatives of total energies and first derivatives of dipole moments with respect to atomic displacements in cartesian coordinates) is discussed.

He I photoelectron spectroscopy of four isotopologues of formic acid: HCOOH, HCOOD, DCOOH and DCOOD

Abstract He I photoelectron spectra of four isotopologues of formic acid, HCOOH, HCOOD, DCOOH and DCOOD have been measured, mainly with an electron kinetic energy resolution of 15 meV. Quantum chemical calculations of geometries, vibrational mode frequencies and the potential energy distributions characterising each normal mode were made for the neutral 1 1 A ′ ground state, the cation ground state 1 2 A ′ and first excited ion state 1 2 A ″ of the four isotopologues. The results were used to analyse observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region and this analysis provided values for several vibrational mode frequencies of the ion states. The calculated structure and internal dynamics of the 1 2 A ′ ground state of the ion provide satisfactory agreement with experiment but for the 1 2 A ″ excited state a more refined theoretical treatment, permitting greater structural flexibility, is required. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of the four isotopologues. Analysis is also made of PES features concerning the higher energy states of the ions between 12.2 and 21 eV. The adabatic ionization energies of the 2 2 A ′ , 2 2 A ″ , 3 2 A ′ and 4 2 A ′ states in this energy region were determined. Vibrational frequencies were obtained for the 3 2 A ′ ion state of the isotopologues and new assignments were made concerning the energy and structure of the 4 2 A ′ ion state, whose adiabatic energy was found to lie about 60 meV below the 3 2 A ′ state.

Electronic spectra and trans—cis isomerism of streptopolymethine cyanines. A CS INDO CI study

Abstract The understanding of the role played by cyanine dyes in various fields of application calls for a thorough knowledge of the excited state properties of their parent chromophores, i.e. cationic streptopolymethyne cyanines. For this aim we performed a detailed CS INDO CI investigation on the electronic spectra of tri-, penta- and heptamethine cyanines, both unsubstituted (TC, PC, HC) and terminally substituted by methyl groups (BMTC, BMPC, BMHC). The study comprised S0Sn, and S0Tn transitions of the trans and all mono-cis isomers. CI expansions involved solely π orbitals of the polymethine chains and pseudo-π orbitals of the substituents and included all singly-excited and the most important doubly- and triply-excited configurations. The spectroscopic effects of methyl substitution are fairly well described and are shown to be especially important in the upper excited states. The identification of the photochemically formed stereoisomers is attempted in terms of mono-cis isomers. For BMPC, in particular, we report the absorption spectrum of the phototropic form and show that the photoisomer is identifiable as the 3–4 cis form.

Calcul approché des contributions dipolaires et orbitales au couplage Jcc à l'aide de fonctions d'onde LCAO-SCF non empiriques

AbstractThe contributions of the spin-dipolar and orbital terms to the nuclear spin coupling constants

Topological implications of Y-conjugation for electronic transitions of cyanine dyes

J_{13_C - 13_C }