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Philip Coppens

Publications and source records attributed to Philip Coppens.

At least 19 recordsLinked to original sources
PubMed2003

What can time-resolved diffraction tell us about transient species?: excited-state structure determination at atomic resolution.

The author describes his work for which he coined the word 'photocrystallography', a technique which consists of using a laser to pump, or excite, a molecular crystal while the X-ray diffractometer probes its structure at the atomic level. The technique is being used to study highly reactive excited molecules that exist for just millionths or even billionths of a second using very intense light sources at the National Synchrotron Light Source at Brookhaven National Laboratory and the Advanced Photon Source at Argonne National Laboratory.

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PubMed2003

MLCT state structure and dynamics of a copper(I) diimine complex characterized by pump-probe X-ray and laser spectroscopies and DFT calculations.

The molecular structure and dynamics of the photoexcited metal-to-ligand-charge-transfer (MLCT) state of [Cu(I)(dmp)(2)](+), where dmp is 2,9-dimethyl-1,10-phenanthroline, in acetonitrile have been investigated by time-domain pump-probe X-ray absorption spectroscopy, femtosecond optical transient spectroscopy, and density functional theory (DFT). The time resolution for the excited state structural determination was 100 ps, provided by single X-ray pulses from a third generation synchrotron source. The copper ion in the thermally equilibrated MLCT state has the same oxidation state as the corresponding copper(II) complex in the ground state and was found to be penta-coordinate with an average nearest neighbor Cu-N distance 0.04 A shorter than that of the ground state [Cu(I)(dmp)(2)](+). The results confirm the previously proposed "exciplex" structure of the MLCT state in Lewis basic solvents. The evolution from the photoexcited Franck-Condon MLCT state to the thermally equilibrated MLCT state was followed by femtosecond optical transient spectroscopy, revealing three time constants of 500-700 fs, 10-20 ps, and 1.6-1.7 ns, likely related to the kinetics for the formation of the triplet MLCT state, structural relaxation, and the MLCT excited-state decay to the ground state, respectively. DFT calculations are used to interpret the spectral shift on structural relaxation and to predict the geometries of the ground state, the tetracoordinate excited state, and the exciplex. The DFT calculations also indicate that the amount of charge transferred from copper to the dmp ligand upon photoexcitation is similar to the charge difference at the copper center between the ground-state copper(I) and copper(II) complexes.

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PubMed2003

On the choice of d-orbital coordinate system in charge-density studies of low-symmetry transition-metal complexes.

The distribution of the d electrons over the corresponding orbitals in transition-metal complexes is a central concept in the theory of metal-ligand bonding. The description requires the assignment of an axis of quantization, which is unambiguous in symmetric environments but not clear-cut in the now commonly encountered case of a low-symmetry coordination environment. As the d-electron population can be derived from accurate diffraction data using the methods of charge-density analysis [HollaDay et al. (1983). Acta Cryst. A39, 377-387], the need for an appropriate procedure is relevant in this area of crystallography. Several criteria for the choice of coordinate system based on the resulting orbital populations are discussed. They are tested on a cobalt atom in a trigonal bipyramidal site and applied to transition-metal sites in Cu(II)-alanyl-valine, and an open zirconocene. The population of the d-orbital cross terms for the different coordinate-system orientations is used to judge the results. In the cases examined, the intuitively most reasonable coordinate system corresponds to the one with smaller value of the sum of the populations of the d-orbital cross terms.

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PubMed2003

Investigation of Zr-C, Zr-N, and potential agostic interactions in an organozirconium complex by experimental electron density analysis.

The crystal structure and electron density (ED) distribution of an imine coupling product with an open zirconocene, Zr(2,4-C(7)H(11))[(i-Pr)NCHPhCH(2)CMe=CHCMe=CH(2)] (C(7)H(11) = dimethylpentadienyl), have been derived from accurate synchrotron X-ray diffraction measurements. The molecular structure reveals asymmetric coordination of Zr by the pentadienyl (2,4-C(7)H(11)) ligand ( = 2.56(6) A), the butadiene fragment ( = 2.43(5) A), and the amide nitrogen atom (Zr-N = 2.0312(5) A) of the second ligand. The study of the ED and its topological analysis affords new insight into the bonding and electronic structure of the title zirconium complex. The interactions between the metal center and the ligands are represented by a Zr-N bond path and one Zr-C bond path with each of the pentadienyl and butadiene moieties, contrary to the usually depicted global metal-ligand bonding. The butadiene and pentadienyl groups exhibit a polarization of the corresponding pi-like ED in the C-Zr directions, indicating that the whole conjugated systems are nonetheless involved. The 4d atomic orbitals of Zr exhibit unusual populations according to ligand field considerations, which reveal a high degree of sigma-donation from the conjugated pi systems of both ligands. As deduced from numerical integration over the topologically defined atomic basins, the Zr to ligand charge transfer is 1.48 e to the C(17)NH(24) ligand and 0.68 e to dimethylpentadienyl. Topological analysis of a short intramolecular Zr.(C,H) contact provides no indication of the presence of agostic interactions, despite a small Zr-N-C angle of 102.87(4) degrees. Thus, no bond path and BCP (bond critical point) of the ED are found in the Zr.(C,H) region, which would have provided evidence for such direct interactions, nor is there any evidence for charge accumulation between the Zr and H atoms, or for lengthening of the C-H bond involved in the putative interaction. These characteristics, similar to those in other distorted situations, may be common for other electron-deficient d(0) complexes.

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PubMed2003

Three-fold interpenetrating three-dimensional networks based on C-methylcalix[4]resorcinarene incorporating benzophenone guest molecules.

A new solid based on C-methylcalix[4]resorcinarene and the linker molecule bis(4-pyridylmethylidyne)hydrazine (bpmh) has been prepared, in which brick-wall sheets are linked in the third dimension to give networks with three intersecting perpendicular channels; the networks interpenetrate three-fold, but nevertheless leave cavities capable of including sizable guest molecules.

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PubMed2003

Theoretical analysis of the triplet excited state of the [Pt2(H2P2O5)4]4- ion and comparison with time-resolved X-ray and spectroscopic results.

A full understanding of the nature of excited states of transition metal complexes is important for understanding their chemical reactivity and role as intermediates in photochemically induced reactions. The ground and excited states of the [Pt(2)(pop)(4)](4-) ion are investigated using density functional theory (DFT). Calculations with different functionals employing quasi-relativistic Pauli and ZORA formalisms all predict a Pt-Pt bond shortening and a slight Pt-P lengthening upon excitation to the lowest triplet state, the latter in apparent contradiction to experimental EXAFS results. The PW86LYP functional with the ZORA relativistic treatment is found to produce good agreement with time-resolved crystallographic and spectroscopic results. A topological bond path between the Pt atoms is found in both the ground and the excited states, though the electron localization function (ELF) indicates weak Pt-Pt covalent bonding for the excited state only. The spin density is mainly localized on the Pt atoms, giving insight into the ability of the triplet excited state to abstract hydrogen and halogen atoms from organic substrates.

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PubMed2003

Light-induced metastable linkage isomers of ruthenium sulfur dioxide complexes.

The irradiation of ruthenium-sulfur dioxide complexes of general formula trans-[Ru(II)(NH(3))(4)(SO(2))X]Y with laser light at low temperature results in linkage isomerization of SO(2), starting with eta(1)-planar S-bound to eta(2)-side S,O-bound SO(2). The solid-state photoreaction proceeds with retention of sample crystallinity. Following work on trans-[Ru(NH(3))(4)Cl(eta(1)-SO(2))]Cl and trans-[Ru(NH(3))(4)(H(2)O)(eta(1)-SO2)](C(6)H(5)SO(3))(2) (Kovalevsky, A. Y.; Bagley, K. A.; Coppens, P. J. Am. Chem. Soc. 2002, 124, 9241-9248), we describe photocrystallographic, IR, DSC, and theoretical studies of trans-[Ru(II)(NH(3))(4)(SO(2))X]Y complexes with (X = Cl(-), H(2)O, or CF(3)COO(-) (TFA(-))) and a number of different counterions (Y = Cl(-), C(6)H(5)SO(3)(-), Tos(-), or TFA(-)). Low temperature IR experiments indicate the frequency of the asymmetric and symmetric stretching vibrations of the Ru-coordinated SO(2) to be downshifted by about 100 and 165 cm(-1), respectively. Variation of the trans-to-SO(2) ligand and the counterion increases the MS2 decay temperature from 230 K (trans-[Ru(II)(NH(3))(4)(SO(2))Cl]Cl) to 276 K (trans-[Ru(II)(NH(3))(4)(SO(2))(H(2)O)](Tos)(2)). The stability of the MS2 state correlates with increasing sigma-donating ability of the trans ligand and the size of the counterion. Quantum chemical DFT calculations indicate the existence of a third eta(1)-O-bound (MS1) isomer, the two metastable states being 0.1-0.6 eV above the energy of the ground-state complex.

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PubMed2003

Introductory lecture: Time-resolved chemistry at atomic resolution.

Though time-resolved studies are still at an early stage, the field is rapidly being developed and applied to an increasingly broad spectrum of problems with timescales varying from seconds or more down to femtoseconds. In this overview a number of different techniques are discussed, with emphasis on chemical applications in which information is obtained at the atomic level. The need to correlate with theory, both for calibration of theoretical methods and to obtain related information not accessible experimentally, is stressed.

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PubMed2002

Application of charge density methods to a protein model compound: calculation of Coulombic intermolecular interaction energies from the experimental charge density.

A combined experimental and theoretical charge density study of the pentapeptide Boc-Gln-d-Iva-Hyp-Ala-Phol (Boc, butoxycarbonyl; Gln, glutamine; Iva, isovaline; Hyp, hydroxyproline; Ala, ethylalanine; Phol, phenylalaninol) is described. The experimental analysis, based on synchrotron x-ray data collected at 20 K, is combined with ab initio theoretical calculations. The topologies of the experimental and theoretical densities are analyzed in terms of the atoms in molecules quantum theory. Topological parameters, including atomic charges and higher moments integrated over the atomic basins, have been evaluated with the program topxd and are used to calculate the electrostatic interactions between the molecules in the crystal. The interaction energies obtained after adding dispersive and repulsive van der Waals contributions agree quite well with those based on M-B3LYP/6-31G** dimer calculations for two of the three dimers in the crystal, whereas for the third a larger stabilization is obtained than predicted by the calculation. The agreement with theory is significantly better than that obtained with multipole moments derived directly from the aspherical atom refinement. The convergence of the interaction as a function of addition of successively higher moments up to and including hexadecapoles (l = 4) is found to be within 2-3 kJ/mol. Although shortcomings of both the theoretical and experimental procedures are pointed out, the agreement obtained supports the potential of the experimental method for the evaluation of interactions in larger biologically relevant molecules.

Biophysical Phenomena↗
PubMed2002

Aspherical-atom scattering factors from molecular wave functions. 1. Transferability and conformation dependence of atomic electron densities of peptides within the multipole formalism.

In this study, the feasibility of building a database of theoretical atomic deformation density parameters applicable to the construction of the densities of biomacromolecules and to the interpretation of their X-ray diffraction data is discussed. The procedure described involves generation of valence-only structure factors of tripeptides calculated from theoretical densities at the B3LYP level and the refinement of multipole parameters against these simulated data. Our results so far indicate that the backbone pseudoatoms extracted in such a way are highly transferable and fairly invariant with respect to rotations around single bonds in the peptide framework. The ultimate goal is to use the aspherical-atom database for improved macromolecular refinements that are based on high-resolution data and for prediction of electrostatic properties of larger molecules.

Algorithms↗
PubMed2002

The first photocrystallographic evidence for light-induced metastable linkage isomers of ruthenium sulfur dioxide complexes.

Light-induced metastable linkage isomers of trans-[Ru(NH(3))(4)Cl(SO(2))]Cl and trans-[Ru(NH(3))(4)(H(2)O)(SO(2))](C(6)H(5)SO(3))(2) have been identified for the first time using photocrystallographic methods. In both linkage isomers the SO(2) ligand is side bound, but the Ru-O and Ru-S distances are considerably longer and almost equal in the trans-H(2)O isomer. DFT calculations confirm that both isomers correspond to minima on the ground-state potential energy surface and also predict the existence of a second oxygen-bound isomer for both compounds. The decay of the light-induced species has been studied by both DSC and IR. Activation energies for the thermal back-reaction, as derived from the temperature-dependent disappearance of light-induced IR bands, are 50.0 and 58.4 kJ/mol for the two isomers, which is larger than the corresponding numbers for photoinduced side-bound nitrosyl linkage isomers.

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PubMed2002

Synthesis and structure of multicomponent crystals of fullerenes and metal tetraarylporphyrins.

The preparation of fullerene complexes with metal tetraarylporphyrins in the presence of excess ferrocene (Cp(2)Fe) results in the formation of new solvent-free and multicomponent molecular crystals. New isomorphous complexes of C(60) with PyZnTPP (ZnTPP identical with zinc 5,10,15,20-tetraphenyl-21H,23H-porphyrinate) and PyCoTPP (CoTPP identical with cobalt(II) 5,10,15,20-tetraphenyl-21H,23H-porphyrinate) containing Cp(2)Fe and the isostructural C(70) complex with PyZnTPP have been prepared. The crystal structures of the new layered C(60) complexes CoTMPP x C(60) (obtained in the presence of Cp(2)Fe) and CoTMPP x 2C(60) x 3C(7)H(8) (obtained in the absence of Cp(2)Fe) have been described (CoTMPP identical with cobalt(II) 5,10,15,20-tetrakis(p-methoxyphenyl)-21H,23H-porphyrinate). Cobalt atoms of the PyCoTPP and CoTMPP molecules are weakly coordinated to C(60) with Co...C(C(60)) distances in the 2.64-2.82 A range, whereas zinc atoms of PyZnTPP, as well as cobalt atoms of the CoTMPP molecules in the solvent-free phase, form only van der Waals contacts with fullerenes. Different packing arrangements in the crystals of fullerene-porphyrin complexes have been discussed.

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PubMed2002

Excited-state structure by time-resolved X-ray diffraction.

X-ray crystallography has traditionally been limited to the study of the ground-state structure of molecules and solids. Recent technical advances are removing this limitation as demonstrated here by a time-resolved stroboscopic study of the photo-induced 50 micros lifetime excited triplet state of the [Pt(2)(pop)(4)](4-)ion [pop = pyrophosphate, (H(2)P(2)O(5))(2-)], performed at helium temperatures with synchrotron radiation. The shortening of the Pt-Pt bond by 0.28(9)A upon excitation is compatible with the proposed mechanism involving promotion of a Pt-Pt antibonding dsigma* electron to a weakly bonding p orbital. The contraction is accompanied by a 3 degree molecular rotation. The time-resolved diffraction technique described here is applicable to reversible light-driven processes in the crystalline solid state.

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PubMed1998

Multitemperature Resonance-Diffraction and Structural Study of the Mixed-Valence Complex [Fe(3)O(OOCC(CH(3))(3))(6)(C(5)H(5)N)(3)].

The Fe-O and Fe-N bond lengths at two iron sites of the mixed-valence complex [Fe(3)O(OOCC(CH(3))(3))(6)(C(5)H(5)N)(3)] show a pronounced temperature dependence; the bonds from two of the Fe atoms to the central oxygen atoms vary by more than 0.10 Å on cooling to 10 K whereas the bond from the third iron atom is essentially invariant. The variation is such that the longest Fe-O bonds at ambient temperature are the shorter ones at 10 K, with the crossover occurring at about 90 K. The bonds to the axial pyridine ligand show the opposite dependence. The variation is attributed to an equilibrium between different configurations, which interconvert through vibronic coupling, a process that involves electron transfer between the metal atoms. The position of the absorption edge for each of the iron atoms has been determined by resonance-diffraction experiments at the Fe K edge, performed at four different temperatures. At each temperature, the order of the absorption edges corresponds to that of the experimentally determined bond lengths. The crossover near 90 K is confirmed by the resonance experiments. The absorption-edge positions are related to the formal oxidation state by calibration with reference complexes of known oxidation state. The experiments demonstrate the close relation between the changes in coordination geometry and the oxidation states of the iron atoms.

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PubMed1996

X-ray Diffraction Analysis of Geometry Changes upon Excitation: The Ground-State and Metastable-State Structures of K(2)[Ru(NO(2))(4)(OH)(NO)].

The structure of the laser-light-induced metastable state MS(1) of the [Ru(NO(2))(4)(OH)(NO)](2)(-) anion in K(2)[Ru(NO(2))(4)(OH)(NO)] was determined by X-ray analysis at 50 K of a crystal with a 16% excited-state population. Results of an independent determination of the ground-state structure were used in the analysis. The most pronounced geometrical change upon excitation was an increase of the Ru-(NO) distance by 0.097(11) Å, significantly larger than the change of the corresponding distance in sodium nitroprusside (Pressprich, M. R.; White, M. A.; Vekhter, Y.; Coppens, P. J. Am. Chem. Soc. 1994, 116, 5233-5238). A decrease in the angleRu-(N-O) angle from 174.0(2) to 169(1) degrees was observed. The diffraction results provide evidence that the photoinduced state MS(1) of the transition metal nitrosyl complexes is a linkage isomer in which the NO group is attached to the metal atom through the oxygen, instead of through the nitrogen atom, rather than an electronic excited state as reported previously.

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