Closo versus Hypercloso Metallaboranes: A DFT Study

The structures and electronic relationship of 9-, 10-, 11-, and 12-vertex closo and hypercloso (isocloso) etallaboranes are explored using OFT calculations. The role of the transition metal in stabilizing the hypercloso borane structures is explained using the concept of orbital compatibility. The hypercloso structures, C6H6MBn-1Hn-1 (n = 9-12; M = Fe, Ru, and Os) are taken as model complexes. Calculations on metal free polyhedral borane BnHn suggest that n vertex hypercloso structures need only n skeleton electron pairs (SEPs), but the structure will have one or more six-degree vertices, whereas the corresponding closo structures with n + 1 SEPs have only four- and five-degree vertices. This high-degree vertex of hypercloso structures can be effectively occupied by transition metal fragments with their highly diffused orbitals. Calculations further show that a heavy transition metal with more diffused orbitals prefers over a light transition metal to form hypercloso geometry, This is in accordance with the fact that there are more experimentally characterized hypercloso structures with the heavy transition metals. The size of the exohedral ligands attached to the metal atom also plays a role in deciding the stability of the hypercloso structure. The interaction between the borane and the metal fragments in the hypercloso geometry is analyzed using the fragment molecular orbital approach. The interconversion of the closo and hypercloso structures by the addition and removal of the electrons is also discussed in terms of the correlation diagrams.

Zeeman effect of NQR in 4,6-dichloropyrimidine and 6-chloro 2,4-dimethoxypyrimidine

Zeeman (35Cl) NQR studies in polycrystalline samples of 4,6-dichloropyrimidine and 6 chloro 2,4 dimethoxypyrimidine show that the asymmetry at the four chemically inequivalent chlorine sites in the former is about 10%, while in the latter (one line) the asymmetry is almost zero. Using a valence-bond picture, C-Cl bonds in 4,6-dichloropyrimidine have been characterised, and the results are also compared with those in a corresponding benzene compound using a simple molecular orbital calculation. The axial symmetry of C-Cl bond in the second compound has been attributed to mesomeric effects.

Stuffed fullerenelike boron carbide nanoclusters

Viable stuffed fullerenelike boron carbide nanoclusters, C50B34, C48B36-2, and their isomers based on an icosahedral B-84 fragment of elemental beta-rhombohedral boron have been investigated using density functional theory calculations. The structure and the stability of these clusters are rationalized using the polyhedral skeletal electron counting and ring-cap orbital overlap compatibility rules. The curvature of the fullerene was found to play a vital role in achieving the most stable isomer C50B34(3B). The large highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gaps, three dimensional aromaticity, and electron detachment energies support their high stability. Further, the IR and Raman active modes were recognized.

Stabilization of an All-Metal Antiaromatic Molecule (Al4Li4) Using BH and C as Caps

It has been reported by Pati et al. (J. Am. Chem. Soc. 2005, 127, 3496) that coordination with a transition metal can stabilize the “antiaromatic”, all-metal compound Al4Li4. Here, we report that it can also be stabilized by capping with a main group element like C and its isoelectronic species BH. Our calculations of binding energy, nuclear independent chemical shift, energy decomposition analysis, and molecular orbital analysis support the capping-induced stability, reduction of bond length alternation, and increase of aromaticity of these BH/C-capped Al4Li4 systems. The interaction between px and py orbitals of BH/C and the HOMO and LUMO of Al4Li4 is responsible for the stabilization. Our calculations suggest that capping can introduce fluxionality at room temperature.

Structure and bonding of MCB5H7 and its sandwiched dimer CB5H6M–MCB5H6 (M = Si, Ge, Sn): Isomer stability and preference for slip distorted structure

We find sandwiched metal dimers CB5H6M–MCB5H6 (M = Si, Ge, Sn) which are minima in the potential energy surface with a characteristic M–M single bond. The NBO analysis and the M–M distances (Å) (2.3, 2.44 and 2.81 for M = Si, Ge, Sn) indicate substantial M–M bonding. Formal generation of CB5H6M–MCB5H6 has been studied theoretically. Consecutive substitution of two boron atoms in B7H−27 by M (Si, Ge, Sn) and carbon, respectively followed by dehydrogenation may lead to our desired CB5H6M–MCB5H6. We find that the slip distorted geometry is preferred for MCB5H7 and its dehydrogenated dimer CB5H6M–MCB5H6. The slip-distortion of M–M bond in CB5H6M–MCB5H6 is more than the slip distortion of M–H bond in MCB5H7. Molecular orbital analysis has been done to understand the slip distortion. Larger M–M bending (CB5H6M–MCB5H6) in comparison with M–H bending (MCB5H7) is suspected to be encouraged by stabilization of one of the M–M π bonding MO’s. Preference of M to occupy the apex of pentagonal skeleton of MCB5H7 over its icosahedral analogue MCB10H11 has been observed.

Potentiometric and spectrophotometric investigation of nickel-monoethanol-amine complexes

A detailed study of nickel-monoethanolamine complexes has been made employing potentiometric and spectrophotometric methods. The conditions for the formation of mono as well as polynuclear complexes have been investigated by potentiometric method. Evidence is presented for the formation of the following complexes and their stability constants are determined: NiA2+, Ni22+, Ni32+, NiA42+, NiA52+, NiA22+, Ni2A24+ and Ni3A36+. Combining potentiometric data with the spectrophotometric data, absorption spectra of the pure mononuclear complexes NiA2+ to NiA42+ and NiA2+6 have been computed. The absorption spectrum of NiA2+6 has been discussed on the basis of ligand field and molecular orbital theories. The absorption spectra of intermediate complexes have been interpreted on the basis of average ligand field theory. There has been good agreement between the experimental (10,400 cm-1) value of 10 Dq of NiA2+6 and the calculated value of 10 Dq (11,400 cm-1) on the basis of M.O. theory.

An ab-initio study of the proton affinity of conjugated schiff-base and related nitrogen compounds: an analysis of the triggering site of bacteriorhodopsin

The chemical groups which take part in the proton transfer reaction in bacteriorhodopsin have been studied by ab initio quantum chemical methods. The various factors such as conjugation with a linear system, electron delocalization of the guanidine type, cis-trans isomerism, geometry distortion and hydrogen bonding with charged groups can influence the properties of a given chemical group. Several systems are studied at 4-31G and STO-3G levels. Some of the Schiff-base analogues and guanidine type molecules are characterized by their molecular orbital diagrams, energy levels and the nature of charge distribution. Also, the effects of the above-mentioned factors on proton affinity are studied. It is hoped that the values thus obtained can be helpful in evaluating various structural models for proton transfer.

Electronic structure of square planar CuO46− clusters

The results of spin-polarized MSXagr calculations show that the ground state of the CuO 4 6– cluster is essentially non-magnetic in spite of odd number of electrons in the system for short Cu–O distances (1.90 Å) as found in the highT c superconductors. This arises due to the fact that the unpaired electron resides in a molecular orbital with primarily oxygen 3s character. The stability of this molecular orbital is found to be sensitive to the cluster geometry and thus, increase in Cu–O distance (as well as other changes affecting oxygen-oxygen distance) tend to favour a magnetic state. From these calculations we have also estimated the Coulomb correlation strength within the Cu 3d to be about 5.3 eV.

Systematic Organic UV Photoelectron Spectroscopy

Photoelectron spectroscopy (PES) provides valuable information on the ionization energies of atoms and molecules. The ionization energy (IE) is given by the relation.hv = IE + T where hv is t h e energy of the radiation and T i s the kinetic energy of the electron. The IEs are directly related to the orbital energies (Koopmans' theorem). By employing UV radiation (HeI. 21.2 eV. or HeII. 40.8 eV). extensive data on the ionization of valence electrons in organic molecules have been obtained in recent years. These studies of UV photoelectron spectroscopy. originated by Turner, have provided a direct probe into the energy levels of organic molecules. Molecular orbital calculations of various degrees of sophistication are generally employed to make assignments of the PES bands. Analysis of the vibrational structure of PES bands has not only provided structural information on the molecular ions, but has also been of value in band assignments. Dewar and co-workers [1, 2) presented summaries of available PES data on organic molecules in 1969 and 1970. Turner et al. [3] published a handbook of Hel spectra of organic molecules in 1970. Since then, a few books [4-7] discussing the principles and applications of UV photoelectron spectroscopy have appeared of which special mention should be made of the recent article by Heilbronner and Maier [7]. There has, however, been no comprehensive review of the vast amount of data on the UV-PES of organic molecules published in the literature since 1970.

Structure of the rhenium X-ray LIII absorption edge in caesium hexachlororhenate(IV)

The X-ray LIII absorption-edge structure of rhenium in Cs2[ReCl6] has been measured with a bent-crystal X-ray spectrograph. An analysis in terms of molecular-orbital (m.o.) theory has been attempted. The energies of the m.o. levels, crystal-field splitting parameter, effective magnetic moment, magnetic susceptibility, and Landég parameter have been determined from this analysis. An estimate of the Re–Cl bond length has also been made.

Role of barriers to conformational changes in ketones undergoing the Norrish type II process

The Norrish type II process is examined in three ketones containing primary, secondary and tertiary C---H bonds in the γ position relative to the carbonyl groups; the MINDO/3 semiempirical self-consistent field (SCF) molecular orbital (MO) method was used with complete geometry optimization in the unrestricted Hartree—Fock frame for the open-shell species. Results show that barriers to conformational change in ketones play an important role in the triplet reaction.

The influence of lone-pair repulsions on C–C bond lengths: a critical evaluation of the experimental and theoretical evidence

X-Ray structural data, as well as semiempirical and ab initio molecular orbital calculations, reveal no systematic and substantial difference between the C–C bond lengths of cis and trans 1,2-diketones. Additional results on various conformations of 1,2-diimines and 1,2-dithiones follow the same pattern. Therefore, lone-pair repulsions cannot be implicated in the observed lengthening of C–C bonds in isatin and several related molecules. Conjugation in these systems occurs peripherally avoiding the participation of the central C–C bond. Negative hyperconjugative interaction between the oxygen lone pairs and the adjacent C–C σ* orbital is suggested to be the principal reason for the relatively long C–C bond in diketones. This effect is found in both the cis and trans conformations.

Approximate VB-MO schemes for excitation gaps in Hubbard models

The excitation gaps in the singlet and triplet manifolds for finite Hubbard models in one, two and three dimensions have been obtained using different approximate configuration interaction (CI) schemes, as a function of the correlation strength, by using valence bond (VB) functions constructed over the molecular orbital (MO) basis. These are compared with numerically exact results and it is found that the scheme in which all particle hole excitations below a given threshold are included is the method of choice. The excitation energies are well reproduced, in trend as well as magnitude, particularly when the threshold equals the bandwidth of the corresponding noninteracting system.

Solvation and axial ligation properties of (2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrinato)zinc(II)

he solvation of (2,3,7,8,12,13,17,18-octabromo-5,10,15,20-tetraphenylporphyrinato)zinc(II)[Zn(obtpp)], in twelve different solvents results in large red shifts of the B and Q bands of the porphyrin accompanied by enhanced absorbance ratios of the Q bands. These observations are ascribed to the destabilisation of the highest occupied molecular orbital a2u of the porphyrin arising from a flow of charge from the axial ligand to the porphyrin ring through the zinc(II) ion. The binding constants of adducts of [Zn(obtpp)] with neutral bases have been found to be an order of magnitude greater than those observed for the corresponding adducts of (5,10,15,20-tetraphenylporphyrinato)-zinc and vary in the order piperidine > imidazole > pyridine > 3-methylpyridine > pyridine-3-carbaldehyde. The enhanced binding constants and large spectral shifts are interpreted in terms of the electrophilicity of [Zn(obtpp)] induced by the electron-withdrawing bromine substituents in the porphyrin core. The structure of [Zn(obtpp)(PrCN)2] has been determined; it reveals six-co-ordinated zinc(II) with two long Zn–N distance [2.51(4), 2.59(3)Å]. The porphyrin is non-planar and displays a saddle-shaped conformation.

Diastereospecific Coupling of Imines by Low-Valent Titanium: An Experimental and Computational Study

The reaction of phenylsilane (PhSiH3) and titanium(IV) isopropoxide Ti(OiPr)(4)] generates low-valent titanium alkoxides that reduce and reductively couple imines. The C-C coupling reaction is diastereospecific, with exclusive formation of the (+/-)-isomer. The yield is dependent on the concentration of titanium(IV) isopropoxide used. Using imines with varying electronic demand revealed that the coupling is most efficient for unsubstituted imines. The involvement of a trimeric titanium biradical species is invoked to explain the observed concentration dependence and diastereospecificity in the reaction. Multilayer ONIOM (''our own n-layered integrated molecular orbital and molecular mechanics'') calculations were carried out on the plausible intermediates involved by using the Guassian 09 suite of programs. Formation of the trimeric biradical species leading to the formation of the (+/-)-isomer is more favored than formation of the trimeric biradical species, which leads to the meso-isomer. Similar calculations with dimeric intermediates do not predict the (+/-)-selectivity. Mass spectral analysis of the reaction mixture indicates the presence of a trimeric titanium alkoxide species.