Electronic Structure of Rh(2)(&mgr;-CO)(CO)(2)(H(2)PCH(2)PH(2))(2). An Example of a Non-A-Frame Structure

Calculations based on density functional theory (DFT) and Hartree-Fock configuration interaction (HF-CI) methodology have been carried out to investigate the rhodium-rhodium coupling in Rh(2)(CO)(2)(dppm)(2), 1 (dppm = Ph(2)PCH(2)PPh(2)) and in Rh(2)(&mgr;-CO)(CO)(2)(dppm)(2), 2. DFT geometries, obtained with the Dgauss program, are in good agreement with those determined from X-ray, but HF geometries, calculated using the same basis sets, yield bond distances systematically too long. Calculations indicate that the rhodium atoms in 1 are linked by a single bond. The insertion of a semibridging carbonyl between the two metal atoms leads to a shortening of the rhodium-rhodium distance and also to a noticeable weakening of the metal-metal interaction. Both effects, and also the stabilization of the HOMO of 2, are related to an observed change from square planar to tetrahedral of the ligand environment of the Rh atom proximal to the inserted CO. Both MO analysis and bond characterization from the topology of the charge density confirm the existence of a bonding interaction between the semibridging carbonyl and the distal rhodium atom. The electronic structures of the dicationic complex [Rh(2)(CO)(3)(dppm)(2)](2+) and of the A-frame-like, isoelectronic system Rh(2)Br(2)(&mgr;-CO) (dppm)(2) are also discussed. The electron deformation density is derived from 2 by means of several methodological approaches, namely, HF, HF-CI, DFT, and DFT + gradient corrections. The HF deformation density obtained in the plane containing the metals and the three CO ligands is discussed, as well as the "correlation density" obtained from the difference maps DFT - HF and CI - HF.     

Reactivity of metallic nitride endohedral metallofullerene anions: electrochemical synthesis of a Lu3N@Ih-C80 derivative

Electrochemically generated Lu(3)N@I(h)-C(80) dianions react with the electrophile, PhCHBr(2), to produce a methano derivative of Lu(3)N@I(h)-C(80)(CHC(6)H(5)) (1) with high regioselectivity. The compound was characterized by MALDI-TOF, NMR, and UV-vis-NIR absorption spectroscopy. Electrochemical characterization of this Lu(3)N@I(h)-C(80)(CHC(6)H(5)) derivative showed the typical irreversible reductive behavior of the pristine Lu(3)N@I(h)-C(80), similar to those observed for Bingel adducts of Lu(3)N@I(h)-C(80). Using the same conditions, the reaction between Sc(3)N@I(h)-C(80) dianions and PhCHBr(2) was conducted for comparison. Unexpectedly, no nucleophilic reaction was observed, indicating that Sc(3)N@I(h)-C(80) dianions are not reactive toward the electrophile. Theoretical studies for both dianionic Lu(3)N@I(h)-C(80) and Sc(3)N@I(h)-C(80) showed that the HOMO is more highly localized on the fullerene cage for [Lu(3)N@I(h)-C(80)](2-) and more localized on the inside cluster for [Sc(3)N@I(h)-C(80)](2-), providing an explanation for the drastically different reactivities observed.     

Quantum mechanics: cleaning up metaphysical baggage

The dynamic electronic structure of atoms and molecules can be directly observed by means of the (e, 2e) reaction, which measures the distribution of energies and momenta of two electrons in coincidence after a knockout reaction initiated by an electron beam of known momentum incident on a molecular gas target. The molecular state for each event is identified by the electron separation energy. The recoil momentum for each event is known from the difference of measured initial and final momenta. It has been verified that values of this momentum are equal under suitable conditions to the momentum of the electron in the target immediately before knockout. Thus the spherically-averaged electron momentum distribution for each molecular orbital is measured. This is directly related to molecular orbitals calculated by the methods of quantum chemistry. Properties obtained by this method for different types of molecules are discussed.     

Influence of polyoxometalate ligands on the nature of high-valent transition metal nitrido species. A theoretical analysis of experimentally known and unprecedented compounds

The electronic structure of group 6-8 transition metal (TM) nitrido derivatives [PW(11)O(39){TM(VI)N}](4-) is studied computationally and the potential reactivity of the polyoxoanions is discussed. The observed electrophilic reactivity for the Ru(VI) nitrido derivative is rationalized from frontier molecular orbital analysis. When we move to the left or down in the periodic table (TM = Os, Tc, Re, Mo and W) the electrophilic character of the polyoxometalate decreases or the cluster should be better regarded as a nucleophile. The DFT analysis of the redox properties suggests that the still unknown high-valent Mn(VI)N and Fe(VI)N units could be stabilized by the porphyrin-like ligand [PW(11)O(39)](7-) and their electronic structure indicates that these anions should have a high potential reactivity towards nucleophiles.     

Identification of a set of genes showing regionally enriched expression in the mouse brain

Background

We have recorded responses from single neurons in murine visual cortex to determine the effectiveness of the input from the two murine cone photoreceptor mechanisms and whether there is any unique selectivity for cone inputs at this higher region of the visual system that would support the possibility of colour vision in mice. Each eye was stimulated by diffuse light, either 370 (strong stimulus for the ultra-violet (UV) cone opsin) or 505 nm (exclusively stimulating the middle wavelength sensitive (M) cone opsin), obtained from light emitting diodes (LEDs) in the presence of a strong adapting light that suppressed the responses of rods.

Results

Single cells responded to these diffuse stimuli in all areas of striate cortex. Two types of responsive cells were encountered. One type (135/323 – 42%) had little to no spontaneous activity and responded at either the on and/or the off phase of the light stimulus with a few impulses often of relatively large amplitude. A second type (166/323 – 51%) had spontaneous activity and responded tonically to light stimuli with impulses often of small amplitude. Most of the cells responded similarly to both spectral stimuli. A few (18/323 – 6%) responded strongly or exclusively to one or the other spectral stimulus and rarely in a spectrally opponent manner.

Conclusion

Most cells in murine striate cortex receive excitatory inputs from both UV- and M-cones. A small fraction shows either strong selectivity for one or the other cone mechanism and occasionally cone opponent responses. Cells that could underlie chromatic contrast detection are present but extremely rare in murine striate cortex.     

Ammonolysis of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives

Ammonolyses of mono(pentamethylcyclopentadienyl) titanium(IV) derivatives [Ti(eta5-C5Me5)X3] (X = NMe2, Me, Cl) have been carried out in solution to give polynuclear nitrido complexes. Reaction of the tris(dimethylamido) derivative [Ti(eta5-C5Me5)(NMe2)3] with excess of ammonia at 80-100 degrees C gives the cubane complex [[Ti(eta5-C5Me5)]4(mu3-N)4] (1). Treatment of the trimethyl derivative [Ti(eta5-C5Me5)Me3] with NH3 at room temperature leads to the trinuclear imido-nitrido complex [[Ti(eta/5-CsMes)(mu-NH)]3(mu3-N)] (2) via the intermediate [[Ti(eta5-C5Me5)Me]2(mu-NH)2] (3). The analogous reaction of [Ti(eta5-C5Me5)Me3] with 2,4,6-trimethylaniline (ArNH2) gives the dinuclear imido complex [[Ti(eta5-C5Me5)Me])2(mu-NAr)2] (4) which reacts with ammonia to afford [[Ti(eta5-C5Me5)(NH2)]2(mu-NAr)2] (5). Complex 2 has been used, by treatments with the tris(dimethylamido) derivatives [Ti(eta5-C5H5-nRn)(NMe2)3], as precursor of the cubane nitrido systems [[Ti4(eta5-C5Me5)3(eta5-C5H5-nRn)](mu3-N)4] [R = Me n = 5 (1), R = H n = 0 (6), R = SiMe3 n = 1 (7), R = Me n = 1 (8)] via dimethylamine elimination. Reaction of [Ti(eta5-C5Me5)Cl3] or [Ti(eta5-C5Me5)(NMe2)Cl2] with excess of ammonia at room temperature gives the dinuclear complex [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) where an intramolecular hydrogen bonding and a nonlineal nitrido ligand bridge the "Ti(eta5-C5Me5)Cl(NH3)" and "Ti(eta5-C5Me5)Cl2" moieties. The molecular structures of [[Ti(eta5-C5Me5)Me]2 (mu-NAr)2] (4) and [[Ti2(eta5-C5Me5)2Cl3(NH3)](mu-N)] (9) have been determined by X-ray crystallographic studies. Density functional theory calculations also have been conducted on complex 9 to confirm the existence of an intramolecular N-H...Cl hydrogen bond and to evaluate different aspects of its molecular disposition.