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Philip Coppens

Publications and source records attributed to Philip Coppens.

At least 37 records · Page 2Linked to original sources
PubMed2004

Cu(I)(2,9-bis(trifluoromethyl)-1,10-phenanthroline)2+ complexes: correlation between solid-state structure and photoluminescent properties.

The 90K solid-state structures, room temperature absorption, and room temperature and 17 K emission spectra of seven different salts of [Cu(I)(bfp)(2)](+) (bfp = 2,9-bis(trifluoromethyl)-1,10-phenanthroline) have been determined. To quantify the distortion of the Cu coordination environment, a distortion parameter zeta is defined that is a combined measure of the flattening, rocking, and wagging distortions of the complex cations. In general, the distortion in the (bfp) cations is less than found previously for Cu(I)(dmp)(2) (dmp = 2,9-dimethyl-1,10-phenanthroline) salts, in particular the flattening is reduced because of the bulkier 2,9-substituents. The 17 K lifetimes range up to 1.8 mus in the series of solids examined and, with the marked exception of the BF(4)(-) salt, correlate linearly with the distortion parameter zeta. The emission wavelength red-shifts with decreasing lifetime, which implies that an increased ground-state distortion is associated with a smaller energy gap.

Journal Article↗
PubMed2004

Solid-state spectroscopic properties and the geometry of binuclear rhodium(I) diisocyanoalkane complexes.

A series of crystalline dinuclear rhodium complexes with different bridging diisocyano ligands and different counter ions have been studied by low-temperature crystallographic and solid-state spectroscopic techniques. The Rh-Rh distances vary from 4.5153(3) to 3.0988(7) angstroms, and the twist angles around the Rh-Rh line from 58.3(1) to 0 degree, both depending on the size and conformational rigidity of the bridging ligand. For very long distances as occur in the [Rh(2)(dimen)(4)](2+) salts the absorption is significantly blue-shifted compared to other complexes. For a given cation a shorter Rh-Rh bond gives a red shift of the phosphorescence emission band, indicating a smaller energy gap between the ground and emitting excited states. An exception occurs for the [Rh(2)(1,6-diisocyanohexane)(4)](2+) ion, in which dimer formation in the calixarate salt lengthens the Rh-Rh intramolecular bond length without affecting the emission spectrum.

Journal Article↗
PubMed2004

The interplay between experiment and theory in charge-density analysis.

The comparison of theory and experiment remains a cornerstone of scientific inquiry. Various levels of such comparison applicable to charge-density analysis are discussed, including static and dynamic electron densities, topological properties, d-orbital occupancies and electrostatic moments. The advantages and drawbacks of the pseudoatom multipole are discussed, as are the experimentally constrained wavefunctions introduced by Jayatilaka and co-workers, which combine energy minimization with the requirement to provide a reasonable fit to the X-ray structure factors. The transferability of atomic densities can be exploited through construction of a pseudoatom databank, which may be based on analysis of ab initio molecular electron densities, and can be used to evaluate a host of physical properties. Partitioning of theoretical energies with the Morokuma-Ziegler energy decomposition scheme allows direct comparison with electrostatic interaction energies obtained from electron densities represented by the pseudoatom formalism. Compared with the Buckingham expression for the interaction between non-overlapping densities, the agreement with theory is much improved when a newly developed hybrid EP/MM (exact potential/multipole model) method is employed.

Models, Chemical↗
PubMed2004

Single- and double-linkage isomerism in a six-coordinate iron porphyrin containing nitrosyl and nitro ligands.

Density Functional theoretical calculations confirm the experimental observation that the low-temperature photolysis of (TPP)Fe(NO)(NO2) (as a KBr pellet) results in the generation of linkage isomers involving the axial NO and NO2 groups and suggest the possible formation of the double linkage isomer (TPP)Fe(ON)(ONO). The energy difference between the ground state (porphine)Fe(NO)(NO2) and the double-linkage isomer (porphine)Fe(ON)(ONO) is 1.57 eV, which is comparable to the 1.59 eV calculated previously for the nitrosyl-to-isonitrosyl linkage isomerism in the five-coordinate (porphine)Fe(NO) analogue.

Iron↗
PubMed2004

Geometry changes of a Cu(I) phenanthroline complex on photoexcitation in a confining medium by time-resolved X-ray diffraction.

Using a stroboscopic technique, in which the molecule is repeatedly excited and the structural change is probed more than 5000 times per second immediately after excitation, we performed a 16 K time-resolved single-crystal study of the microsecond lifetime triplet state of the Cu(I)phenanthroline derivative[Cu(I)(dmp)(dppe)][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane). The geometry changes on excitation differ for the two symmetry-independent molecules, but are in the same direction as calculated for an isolated reference molecule, although the flattening distortion in the crystal is significantly smaller, implying that the reorganization energy is greatly affected by the confining medium.

Journal Article↗
PubMed2004

Structure and photoluminescence of a benzil nanocolumn in a C-methylcalix[4]resorcinarene-based framework.

A new framework based on C-methylcalix[4]resorcinarene and the flexible nonconjugated spacer 1,4-bis(imidazol-1yl-methyl)benzene encloses a large one-dimensional channel, containing benzil nanocolumns. Unlike in a previously reported series of benzil-containing supramolecular solids with conjugated linker molecules, benzil luminescence is observed, but the lifetime of 580 ns at 77 K is considerably shorter than the 145 micros reported for neat benzil at room temperature.

Journal Article↗
PubMed2004

Calculation of electrostatic interaction energies in molecular dimers from atomic multipole moments obtained by different methods of electron density partitioning.

Accurate and fast evaluation of electrostatic interactions in molecular systems is still one of the most challenging tasks in the rapidly advancing field of macromolecular chemistry, including molecular recognition, protein modeling and drug design. One of the most convenient and accurate approaches is based on a Buckingham-type approximation that uses the multipole moment expansion of molecular/atomic charge distributions. In the mid-1980s it was shown that the pseudoatom model commonly used in experimental X-ray charge density studies can be easily combined with the Buckingham-type approach for calculation of electrostatic interactions, plus atom-atom potentials for evaluation of the total interaction energies in molecular systems. While many such studies have been reported, little attention has been paid to the accuracy of evaluation of the purely electrostatic interactions as errors may be absorbed in the semiempirical atom-atom potentials that have to be used to account for exchange repulsion and dispersion forces. This study is aimed at the evaluation of the accuracy of the calculation of electrostatic interaction energies with the Buckingham approach. To eliminate experimental uncertainties, the atomic moments are based on theoretical single-molecule electron densities calculated at various levels of theory. The electrostatic interaction energies for a total of 11 dimers of alpha-glycine, N-acetylglycine and L-(+)-lactic acid structures calculated according to Buckingham with pseudoatom, stockholder and atoms-in-molecules moments are compared with those evaluated with the Morokuma-Ziegler energy decomposition scheme. For alpha-glycine a comparison with direct "pixel-by-pixel" integration method, recently developed Gavezzotti, is also made. It is found that the theoretical pseudoatom moments combined with the Buckingham model do predict the correct relative electrostatic interactions energies, although the absolute interaction energies are underestimated in some cases. The good agreement between electrostatic interaction energies computed with Morokuma-Ziegler partitioning, Gavezzotti's method, and the Buckingham approach with atoms-in-molecules moments demonstrates that reliable and accurate evaluation of electrostatic interactions in molecular systems of considerable complexity is now feasible.

Algorithms↗
PubMed2003

Solid-state structure dependence of the molecular distortion and spectroscopic properties of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion.

The relation between the geometry and spectroscopic properties of a series of salts of the Cu(I) bis(2,9-dimethyl-1,10-phenanthroline) ion, (Cu((I))(dmp)(2))(+), is explored. The distortions from the idealized D(2)(d)() geometry, which include flattening, rocking of the dmp ligands, and displacement of the Cu atoms out of the dmp planes, show considerable variation, indicating the importance of packing forces in the crystalline environment. The change in the absorption spectra upon flattening of the complex, expressed as the variation of the angle between the dmp planes, which varies from 88 degrees in the BF(4) and tosylate salts to 73 degrees in the picrate, agrees qualitatively with parallel DFT calculations. No correlation is found between ground state geometry and luminescence lifetimes, recorded both at room temperature and at 16 K. The low temperature lifetimes vary by a factor of 8 among the (Cu((I))(dmp)(2))(+) salts examined, the longest lifetime (2.4 micros at 16 K) being observed for the tosylate salt.

Journal Article↗
PubMed2003

Multiple conformations of benzil in resorcinarene-based supramolecular host matrixes.

Six supramolecular complexes incorporating benzil as a guest, CMCR*bipy*benzil (alpha) 1 (CMCR = C-methylcalix[4]resorcinarene), CMCR*bipy*benzil (beta) 2, CMCR*2bpe*benzil*ethanol 3 (bpe = trans-1,4-bis(pyridyl)ethylene), CMCR*2bpe*benzil*2H2O 4, CMCR.2bpeh*benzil*ethanol 5 (bpeh = bis-(1-pyridin-4-yl-ethylidene)-hydrazine), and CECR*2bpe.benzil 6 (CECR = C-ethylcalix[4]resorcinarene), have been synthesized by hydrothermal and conventional methods and characterized by X-ray diffraction. Resorcinarene adopts a boat conformation in 1-4 and a bowl conformation in 5 and 6. Compounds 1-4 show a brick-wall-like framework, in which two benzil molecules are incorporated. For 5, bpeh spacers link CMCR molecules to give a one-dimensional wavelike polymer in which one benzil guest is embedded within the polymer cavity. Complex 6 forms a carcerand-like capsule in which two benzil guests are encapsulated. The O=C-C=O torsion angles vary from 91.8 to 139.3 degrees and correlate with the length of the central C-C bond. The benzil concentration, which is approximately 6.2 mol/L in the neat crystals, varies between 1.01 and 1.51 mol/L in the structures studied, corresponding to a 6-fold dilution. The benzil molecules are disordered in the larger cavities of 4 and 5. The two benzoyl fragments are almost perpendicular in 3, which has the next largest cavity size when solvent volume is excluded, whereas a nearly trans-coplanar conformation occurs for the cavity with the smallest volume in 6.

Journal Article↗
PubMed2003

Formation of pyrene nano-rods within a supramolecular framework.

The combination of trimesic acid and the tri-dentate linker 1,3,5-tri(4-pyridyl)-2,4,6-triazine leads to the assembly of a bilayer structure, which contains four pyrene molecules per cavity; adjacent tetrameric aggregates within the channels formed by the superimposed layers combine to yield pyrene nano-rods.

Journal Article↗
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.

Journal Article↗
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.

Journal Article↗
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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