Biomedical subjects

Chantal Daniel

Publications and source records attributed to Chantal Daniel.

7 recordsLinked to original sources

PubMed2006

Photochemistry of CH3Mn(CO)5: a multiconfigurational ab initio study.

The electronic spectroscopy of CH3Mn(CO)5 has been investigated by means of ab initio multiconfigurational MS-CASPT2/CASSCF calculations. The absorption spectrum is characterized by a series of Metal-Centered (MC) excited states in the UV energy domain (below 290 nm) that could be responsible for the observed photoreactivity starting at 308 nm. The upper part of the spectrum is overcrowded between 264 and 206 nm and dominated by a high density of Metal-to-Ligand-Charge-Transfer (MLCT) states corresponding mainly to 3d(Mn) --> pi*(CO) excitations. A non-negligible contribution of Metal-to-sigma-Bond-Charge-Transfer (MSBCT) states corresponding to 3d(Mn) --> sigma*(Mn-CH3) excitations is also present in the theoretical spectrum of CH3Mn(CO)5. However, in contrast to other transition metal hydrides and methyl substituted (HMn(CO)5, HCo(CO)4, and CH3Co(CO)4) these MSBCT transitions do not participate to the lowest bands of the spectrum as main contributions. The photochemistry of CH3Mn(CO)5, namely the loss of a CO ligand vs. the metal-methyl bond homolysis, is investigated by means of MS-CASPT2 states correlation diagrams. This study illustrates the complexity of the photodissociation mechanism of this class of molecules, which involves a large number of nearly degenerate electronic states with several channels for fragmentation.

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PubMed2006

trans-cis Photoisomerization of the styrylpyridine Ligand in [Re(CO)3(2,2'-bipyridine)(t-4-styrylpyridine)]+: role of the metal-to-ligand charge-transfer excited states.

The trans-cis isomerization of the styrylpyridine carbon-carbon double bond induced by visible light irradiation in fac-[Re(CO)(3)(bpy)(stpy)](+) (bpy = 2,2'-bipyridine; stpy = t-4-styrylpyridine) has been investigated by means of quantum-chemical methods. The structures of the various cis and trans conformers of [Re(CO)(3)(bpy)(stpy)](+) have been optimized at the density functional theory (DFT) level. Three rotational conformers for the most stable trans isomer lie within 2.3 kJ mol(-1) each other. The energy difference between the cis and trans isomers is 27.0 kJ mol(-1). The electronic spectroscopy of the most stable conformers has been investigated by time-dependent DFT (TD-DFT) and complete active space self-consistent field/CAS second order perturbation theory (CASSCF/CASPT2) calculations. The lowest absorption bands are dominated by metal-to-ligand charge-transfer (MLCT, d(Re)-->pi*(bpy)) transitions calculated at about 25,000 cm(-1) and by a strong intraligand (1)IL (pi(stpy)-->pi*(stpy)) transition in the near UV region. On the basis of CASSCF potential energy curves (PECs) calculated as a function of the torsion angle of the C=C bond of the styrylpyridine ligand, it is shown that the role of the low-lying MLCT states is important in the photoisomerization mechanism. In contrast to the free organic ligand, in which the singlet mechanism is operational via the (1)IL (S(1)) and electronic ground (S(0)) states, coordination to the rhenium steers the isomerization to the triplet PEC corresponding to the (3)IL state. From the (3)IL(t) (t = trans) the system evolves to the perpendicular intermediate (3)IL(p) (p = perpendicular) following a 90 degrees rotation around the styrylpyridine C=C bond. The metal center acts as a photosensitizer because of the presence of photoactive MLCT states under visible irradiation. The position of the crossing between the (3)IL and electronic ground state PEC determines the quantum yield of the isomerization process.

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PubMed2005

Quantum chemical study of the electronic structure of NiCH2 + in its ground state and low-lying electronic excited states.

The electronic structure of NiCH(2) (+), representative of transition metal carbene ions, is investigated by means of several methods of quantum chemistry. The relative stabilities of the four low-lying doublet electronic states ((2)A(1), (2)A(2), (2)B(1), and (2)B(2)) are determined at the coupled cluster singles and doubles level (CCSD) and triples level [CCSD(T) and CCSDT-3] with both a Hartree-Fock and density functional theory (Kohn-Sham) reference. The equation-of-motion coupled cluster for treatment of excited states in singles and doubles approximation (EOM-CCSD) is used to characterize the transition energies from the (2)A(1) electronic ground state to the low-lying doublet excited states. The (2)A(2) and (2)B(1) states are nearly degenerate, found to be separated by 940 cm(-1) at the EOM-CCSD level, in agreement with the CASSCF energy ordering. The (2)B(2) state is calculated to be higher in energy by more than 1.0 eV. The spin purity of the low-lying doublet and quadruplet states described by CCSD calculations based on the unrestricted open-shell Hartree-Fock reference is discussed.

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PubMed2003

Deciphering the reaction dynamics underlying optimal control laser fields.

Femtosecond high-resolution pump-probe experiments have been used together with theoretical ab initio quantum calculations and wave packet dynamics simulations to decode an optimal femtosecond pulse that is generated from adaptive learning algorithms. This pulse is designed to maximize the yield of the organometallic ion CpMn(CO)3 while hindering the competing fragmentation. The sequential excitation and ionization of the target ion are accomplished by an optimized field consisting of two dominant subpulses with optimal frequencies and time delays.

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PubMed2002

Photodissociation and electronic spectroscopy of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene): quantum wavepacket dynamics based on ab initio potentials.

The photodissociation dynamics of [Re(H)(CO)(3)(H-dab)] (H-dab=1,4-diaza-1,3-butadiene) were studied by means of wavepacket propagations on CASSCF/MR-CCI potentials calculated for the electronic ground state and low-lying excited states as a function of two coordinates, q(a) and q(b), that correspond to the Re-H bond homolysis and to the axial CO loss, respectively. The theoretical absorption spectrum is characterized by two bands, one intense peak centered at lambda=500 nm (21,000 cm(-1)) and one broad band centered at 310 nm (32,500 cm(-1)). The visible band was assigned to the low-lying metal-to-ligand charge-transfer (MLCT) states with a main contribution of the a(1)A'-->c(1)A' transition corresponding to the 3d(xz)-->pi*(dab) excitation. The second band calculated in the UV energy domain was assigned to the d(1)A' (sigma(Mn-H)-->pi*(dab)) state corresponding to a sigma-bond-to-ligand charge-transfer (SBLCT) state. The photodissociation dynamics of the low-lying (1)MLCT and (3)SBLCT states following irradiation in the visible energy domain was simulated by wavepacket propagation on the two-dimensional diabatic potentials V(q(a), q(b)) coupled by the spin-orbit. In contrast to what was found for the manganese analogue, the (1)MLCT state is nonreactive and a rather slow (beyond the ps time scale), nontotal and indirect homolysis of the Re-H bond occurs through (1)MLCT-->(3)SBLCT intersystem crossing.

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PubMed1998

The Spectroscopy of HMn(CO)(5): A CASSCF/MRCI and CASPT2 ab Initio Study.

The electronic structure of the states of HMn(CO)(5) in the near-UV region was investigated by CASSCF/MRCI and CASPT2 ab initio methods with a basis set between double- and triple-zeta quality including very diffuse functions. On the basis of calculated vertical excitation energies and oscillator strengths, the following absorption band assignments could be made: (1) A(1)E (3d(pi) --> 3d(x)()()2(-)(y)()()2) and B(1)E (3d(pi) --> sigma(Mn-H)) transitions underlie the low-intensity shoulder at 229 nm; (2) the high-intensity B(1)A(1) (3d(pi) --> pi(CO,12e) +3d(xy)() --> pi(CO,3b)2) and weak C(1)E (3d(pi) --> pi(CO,3b)2) transitions underlie the central band at 214 nm; and (3) the very high intensity C(1)A(1) (sigma(Mn)(-)(H) --> sigma(Mn)(-)(H)) underlies mainly the band at 193 nm. This identifies the most likely photoinitiating states for the photochemistry of HMn(CO)(5). The a(3)A(1) was shown to have valence character, contrary to previous suggestions.

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PubMed1997

Electronic Structure of the Lowest Excited States of Cr(CO)(4)(2,2'-bipyridine): A CASSCF/CASPT2 Analysis.

The visible and UV absorption spectra of Cr(CO)(4)(bpy) are interpreted according to CASPT2 calculations based on CASSCF reference wave functions using atomic natural orbital (ANO) basis sets. The excitation energies of the lowest singlet MLCT (metal-to-ligand-charge-transfer) states range between 12 630 and 17 580 cm(-)(1). They originate in the excitation of chromium d electrons to the lowest pi(bpy) orbital of b(2) local symmetry. Dipole transition moments calculated for individual MLCT transitions show that the only transition expected to contribute significantly to the intense absorption band in the visible spectral region is the a(1)A(1) --> b(1)A(1) transition calculated at 17 580 cm(-)(1). This result agrees very well with the experimental spectra recorded in weakly solvating C(2)Cl(4), characterized by a band at 17 700 cm(-)(1). The low-energy emission band at 12 850 cm(-)(1) has been attributed to the lowest a(3)B(2) state calculated at 12 560 cm(-)(1). The next set of excited states corresponding to 3d --> pi(bpy,a)()2 excitations range between 25 370 and 26 200 cm(-)(1) for the singlets and between 24 630 and 25 750 cm(-)(1) for the triplets. The singlet excited states corresponding to d --> d excitations are calculated between 29 650 and 37 360 cm(-)(1). These results show that the intense absorption in the near-UV spectral region originates in strongly overlapping absorption bands due to closely spaced transitions into MLCT (a(2)) and dd excited states, respectively. The c(1)B(2) excited state corresponding to the 3d(xz)() --> 3d(z)()()2 excitation, proposed as photoactive in the mechanism of the efficient CO loss under irradiation at 27 630 cm(-)(1), is calculated at 36 320 cm(-)(1) and is far too high to be directly populated in these photochemical studies. Its mixing with one or several low-lying (singlet) MLCT states at the early stage of the reaction path could be responsible for the observed primary reaction. A comparison between the excitation energies of the lowest singlet states of Cr(CO)(4)(bpy) and Cr(CO)(4)(dab) is reported. The most significant feature is the lowering of the MLCT excited states on going from the bpy-containing molecule to its dab (1,4-diaza-1,3-butadiene) analog.

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