Formation Of Molecular Orbitals Research Articles

Magnetic circular dichroism study of the2S→2P spectrum of matrix-isolated copper atoms

The magnetic circular dichroism of the 2S→2P transition of copper atoms isolated in Ne, Ar, Kr, Xe, N2 and CH4 matrices has been measured. The spectra are discussed in the light of three current hypotheses concerning the origin of the observed extra splitting of the absorption bands.The m.c.d. spectra provide no support for the “exciplex” model but they are quite compatible with the “crystal-field” and “Jahn–Teller” interpretations of the splitting.The order of the 2S→2P1/2 and 2S→2P3/2 states is reversed in the Xe matrix as compared with all other hosts and this observation is interpreted in terms of molecular orbital formation involving Cu 4p and Xe 5p atomic orbitals.

Definite evidence for the formation of molecular neutron orbits in the scattering of 12C on 13C

Theoretical and experimental evidence for molecular neutron orbits in the 12 C + 13 C system is given. It is shown in a coupled channel calculation that the neutron orbits in the grazing collision of 12C on 13C are of molecular nature. The neutron transfer probability (or formation probability of the molecular orbit) is greatly enhanced due to the mixing of the 1 p 1 2 and 2 s 1 2 , 1 d 5 2 shells ( l-parity mixing). The cross section of the excitation of the 13 C 1 2 + (3.09 MeV) state is dominated by even molecular states (odd states cannot contribute because they have a node between the two centers) and this prediction is confirmed in the experiment.

K-vacancy production in heavy-ion collisions. II. Multiple- and single-collision excitation in the2pσmolecular orbital

The present paper, one of a four-part series, examines $K$-vacancy cross sections in collisions of symmetric and near-symmetric systems produced with 12-60-MeV Br, 200-330-MeV Kr, 15-80-MeV I, 326-470-MeV Xe, and 90-110-MeV Pb beams. Cross sections are discussed from the point of view of molecular-orbital formation during the collision. In that model, the summed $K$-vacancy cross sections of both collision partners represents essentially the total $2p\ensuremath{\sigma}$ vacancy cross section in the outgoing part of the collision. Experimental summed cross sections using solid targets can have appreciable contributions from multiple-collision effects, which are computed in detail. Proposed single-collision mechanisms of $2p\ensuremath{\sigma}$ vacancy formation are also examined, but none can be established with certainty. The sharing of $2p\ensuremath{\sigma}$ vacancies on the outgoing part of the collision between the $K$ shells of two collision partners is found to agree well with a previously derived formula based on the Nikitin-Demkov long-range coupling model.

Optical Absorption Bands of Off-Center Ag+ in CsBr

The optical absorption bands associated with the d 10 ( 1 S )→ d 9 s ( 1 D , 3 D ) transitions of Ag + in CsBr are studied. The area under the absorption curve decreases with increasing temperature from 10 K to room temperature, indicating that the Ag + ion in CsBr is in a deep off-center potential. Ten optical absorption bands are identified and these are ascribed to the transitions from the d 10 multiplets to the d 9 s multiplets split in a C 3ν symmetry. Good agreement between the experimental and calculated transition energies are obtained, although the poor agreement has been reported for alkali halides with NaCl-structure. This results is discussed in terms of the molecular orbital formation between the Ag + ion and neighboring halogens.

ChemInform Abstract: ORBITAL REDUCTION FACTORS IN THE LOWEST EXCITED STATE OF THE PHTHALOCYANINE RING AND THEIR MEASUREMENT BY MAGNETIC CIRCULAR DICHROISM SPECTROSCOPY

Magnetic circular dichroism (m.c.d.) spectroscopy has been used to measure the angular momentum of the lowest excited state of the phthalocyanine ring as a function both of the central metal ion and of the axial ligands bound to the metal. The magnetic moments have been extracted from experiment using a moments analysis thus providing numbers which are independent of any distortions, static or vibronic, within the excited state. It is shown that on changing the metal ion from cobalt(II) to zinc(II) a gradual increase in angular momentum occurs whereas changing the axial ligand on iron(II) phthalocyanine from dimethyl sulphoxide to cyanide produces a large change in magnetic moment from 3.24 to 4.58 Bohr magnetons. A simple model has been proposed to account for these observations. It involves molecular orbital formation between the eg(dxz, yz) metal d-orbitals and the empty 6eg(π*) orbitals of the phthalocyanine ring. It is shown that this necessarily leads to a reduction of the orbital angular momentum. With simple assumptions it is possible to estimate the molecular orbital coefficients for the metal d-orbital participation in the excited orbitals for a series of iron(II) phthalocyanines with different axial ligands.

Orbital reduction factors in the lowest excited state of the phthalocyanine ring and their measurement by magnetic circular dichroism spectroscopy

Magnetic circular dichroism (m.c.d.) spectroscopy has been used to measure the angular momentum of the lowest excited state of the phthalocyanine ring as a function both of the central metal ion and of the axial ligands bound to the metal. The magnetic moments have been extracted from experiment using a moments analysis thus providing numbers which are independent of any distortions, static or vibronic, within the excited state. It is shown that on changing the metal ion from cobalt(II) to zinc(II) a gradual increase in angular momentum occurs whereas changing the axial ligand on iron(II) phthalocyanine from dimethyl sulphoxide to cyanide produces a large change in magnetic moment from 3.24 to 4.58 Bohr magnetons. A simple model has been proposed to account for these observations. It involves molecular orbital formation between the eg(dxz, yz) metal d-orbitals and the empty 6eg(π*) orbitals of the phthalocyanine ring. It is shown that this necessarily leads to a reduction of the orbital angular momentum. With simple assumptions it is possible to estimate the molecular orbital coefficients for the metal d-orbital participation in the excited orbitals for a series of iron(II) phthalocyanines with different axial ligands.

The coordination polymers of Cr 3+ acetylacetonate with diorganophosphinic acids

Polymers based on diphenylthio-, diphenyldithio-, dibutyldiethyl-, diethylthio-, diethyldithio-phosphinic acid with chromium acetylacetonate were synthesized and studied. Their infrared, ultraviolet and paramagnetic electron spectra were recorded. The substitution of the oxygen atoms by those of sulphur reduced the heat resistance of the coordination polymers. A scheme is suggested for the formation of molecular orbitals from the orbitals of the central Cr 3+ ion and the ligands in the polymers.

K-Shell Ionization of Beryllium by Proton and Heavy Ions

Experimental cross sections are presented for the ionization of K-shells of beryllium in collision with heavy ions and proton; C + , N + , O + , Ne + and Ar + for bombarding energy ranging from 0.3 to 30 keV per amu, while that for protons is 10 to 200 keV per amu. Proton data show good agreement with the prediction for Coulomb excitation of K-shell electrons. The date for heavy ions are several orders of magnitude larger than that for proton. These results are understood in terms of pseudomolecule formation and crossings of molecular orbitals during the collisions. Heavy ion cross sections show the trend to converge to the theoretical Coulomb excitation cross sections in high energies above about 10 keV per amu.

On the influence of covalence on paramagnetic spin-lattice relaxation

The paramagnetic spin-lattice relaxation times of an isolated Ir4+ ion inserted in the crystalline lattice (NH4)2PtCl6 are computed. The computation is carried out for a Van Vleck relaxation mechanism taking account of the possibility of the formation of molecular orbits. It is shown that the effect of covalence in this case can be taken into account effectively in terms of the modified quantities ¯r2 and ¯r4, the mean square and mean fourth power of the electron radius. The results of calculations agree better with experiment as compared to estimates obtained without using molecular orbitals.

Anisotropic magnetic susceptibilities for octahedral copper (II) salts with tetragonal distortion

The method of calculating molecular susceptibilities for tetragonally distorted octahedral copper (II) complexes using a model based on molecular orbitals is described. The method involves a number of parameters among which are orbital energy levels in copper, orbital reduction factor caused by formation of molecular orbitals, temperature and the spin orbit coupling parameter. The effect on the anisotropic magnetic moments when these are varied is demonstrated and discussed. The model is fitted to available anisotropic susceptibility data for K 2[Cu(H 2O) 6](SO 4) 2 and the best fit values for the orbital reduction parameters are derived through an error analysis.

Paramagnetic anisotropies and covalency of some tetrahedral copper(II) complexes

The principal and average magnetic moments of six copper(II) complexes with stereochemistries ranging between tetrahedral and square planar are reported. The results are interpreted in terms of a 2D free-ion term perturbed by the crystal field and spin–orbit coupling and also of the covalent bonding in the compounds, by way of the orbital reduction factor. Very large and anisotropic reduction of orbital angular momentum is inferred in these systems and this may be related to both the crystal-field potential and to the degree of molecular orbital formation in these complexes.

Molecular orbitals of dibenzenech romium

1. Group theory is applied to consideration of the formation of the molecular orbitals of dibenzenechromium from the atomic orbitals of Cr and the orbitals of the two benzene molecules. 2. It is found that the following orbitals can interact: the occupied orbitals of the benzene (DB) molecules of symmetry a1g with the 4s orbital of Cr (or a hybridized orbital from 4s and 3d z2), the occupied orbital DBa2u with 4pz, the occupied orbital DBe1g with 3dxz, 3dyz, the occupied orbital DBe1u with 4px, 4py, and the free orbital DBe2g with 3dxy,\(3d_{x^2 - y^2 } \). 3. This corresponds to the formation of three donor-acceptor bonds from occupied orbitals of C6H6 and free d2sp3 Cr orbitals, and of a donor-acceptor bond from the occupied dxy,\(d_{x^2 - y^2 } \)Cr orbitals and free C6H6 orbitals.

The stabilisation of low valent states of the transition metals: Introductory lecture

The ligands most effective in stabilising low valent states are uncharged ligands where carbon is the ligand atom, e. g. CO, PhNC and C 6 H 6, tertiary phosphines, arsines and chelate heterocyclic amines. Anionic ligands, even CN − and RC C −, are much less effective. All the factors which affect the stabilities of complexes must influence the stabilisation of unusual valent states of the transition metals. The most important is the formation of π-type molecular orbitals between the metal and ligand atoms When the ligand orbitals are of low energy and are vacant the bonding π-type molecular orbitals can contain the electrons from the d-orbitals of the metal, and those added during the reduction of the complex until they are all filled. In this way very low valent states of the metal atom can be stabilised. On the other hand, if the ligand orbitals are filled they provide enough electrons to fill the π-type molecular orbitals and the metal electrons are forced into higher energy antibonding orbitals from which some may be readily removed by oxidation, and so higher valent states are stabilised. The electronegativities of the ligand atoms are an important secondary factor, because high electronegativity causes the above processes to be most effective in valency stabilisation. Thus the most electronegative ligand atoms of their types, viz. C and F −, stabilise the lowest and the highest valent states respectively.