Roger Grinter

The d→p spectra of matrix‐isolated copper atoms: An MCD and synchrotron radiation absorption study

The electronic absorption spectra of copper atoms isolated in argon, krypton, and xenon matrices have been measured in the range 3.6–8.6 eV using a synchrotron radiation source. By correlating the results with gas‐phase data, and with the help of magnetic circular dichroism measurements and calculations of energies and intensities, a detailed assignment of the 3d→4p region of the spectra has been proposed. A band at 6.357 eV in the xenon matrix is assigned to the Rydberg 4s→5p transition with a fair degree of confidence, and two high‐energy bands in the krypton matrix are tentatively assigned to Rydberg 3d→5p transitions. The matrix shifts confirm the view that the principal influence of the matrix is upon the upper orbital involved in the transition. Attention is drawn to the fact that matrix perturbations may increase the number of observable spectral bands, either by lifting degeneracies or by mixing allowed character into previously forbidden transitions.

π-Electron contributions to proton-proton spin-spin coupling

Abstract Values of π-electron contributions to (H,H) coupling are calculated for a variety of unsaturated hydrocarbons using several different methods. The choice of parameters for the calculations is discussed in some detail. The π systems are treated using parameters determined by the method of Baird and Dewar (1); the value of β0 is varied so as to fit the estimated π contribution to 5J in butadiene. The results for the various methods of calculation are compared with one another and with experimental coupling constants. Predictions are made for several cases where no experimental values have been reported.

Calculations of the nuclear spin-spin coupling constants of second-row elements. ii. one-bond couplings of silicon and phosphorus to carbon

Abstract Finite perturbation theory calculations at the INDO level of approximation are reported for 1J(SiC) and 1J(PC). Three different sets of molecular orbital parameters have been used, and orbital and spin--dipolar contributions to the coupling have been calculated. The results for 1J(SiC) are in good agreement with experiment and clearly demonstrate the dominance of the Fermi-contact mechanism in this coupling. The results for 1J(PC) are very poor, although they do suggest that orbital and spin-dipolar terms must be included in the calculation if good agreement with experiment is to be obtained, particularly in the case of PIII derivatives. Comparison with other recent calculations of 1J(PC) indicates that, although phosphorus 3d orbitals may be important, inclusion of them is unlikely in itself to be sufficient to ensure good agreement between theory and experiment for couplings to phosphorus. It is tentatively suggested that there may be some basic inadequacy in the INDO/CNDO method when it is applied to the calculation of molecular properties which are very sensitive to the presence of lone-pair electrons.

The magnetic circular dichroism spectrum of the 1 B 2u ←1 A 1g transition of benzene in the vapour phase

The magnetic circular dichroism (MCD) spectrum of the 1 B 2u ←1 A 1g transition of benzene has been measured in the vapour phase. Many of the bands due to transitions between single vibronic levels display A terms. It has been shown that the angular momentum arises by vibronic mixing both of the 1 E 1u state with the 1 B 1u state and of the 1 E 2g states with the 1 A 1g ground state by e 2g vibrations. The magnitudes and signs of the experimental and calculated ratios, A/D, for the A 0 0 vibronic origin are in excellent agreement. Two strong MCD progressions of opposite sign with B-term dispersion have been observed in regions of low absorption. These are identified with vibronic origins due to the v 8 and v 9 e 2g modes. By contrast the MCD spectrum of hexadeuterobenzene vapour has a much lower magneto-rotational strength and displays none of the striking features of the benzene MCD spectrum.

Exchange interactions in trinuclear basic chromium(III) clusters : direct observation of the magnetic spectrum by inelastic neutron scattering

Incoherent inelastic neutron scattering spectra are reported for salts of the complex [Cr3O(OOCCH3)6(OH2)3]+. The data are consistent with predominantly antiferromagnetic coupling between pairs of chromium ions. The complete spectrum of transitions between successive states with total spin S=1/2, 3/2, 5/2, 7/2, and 9/2 has been observed for the first time. Splittings of the ground state S=1/2 have been observed directly and attributed to lowering of symmetry of the triangular cluster. For the chloride salt the data confirm that two sets of complex cations with different degrees of symmetry lowering are present in the crystal, at least at the lowest temperatures used (T=1.4 to 50 K). In principle, the relationship of J values for the symmetry‐lowered case could be described as ‘‘isosceles,’’ with Jab≳Jbc=Jac or Jab<Jbc=Jac; or as ‘‘scalene,’’ with all three J values different. We find that, for at least one of the two sets of metal ion clusters, the scalene case applies, with J values of −11.5±0.2, −10.5±0...

Calculation of the nuclear spin-spin coupling constants of second-row elements. I. Couplings to hydrogen

Abstract Finite perturbation theory calculations at the INDO level of approximation are reported for 1 J( PH ), 1 J( SiH ), 2 J( PCH ) , and 2 J( SiCH ) . Three different sets of molecular orbital parameters have been used. The results of the calculations are generally rather satisfactory, although some important problems arise. In particular the calculated value of 1J(PH) in PH3 is found to be much too small and the effect of strongly electronegative substituents on the couplings does not appear to be completely accounted for. The relative merits of the three parameter sets are discussed, and also the possible effects of the inclusion of 3d orbitals. It is concluded that, at present, there is no compelling evidence for including 3dorbitals.

Application of magnetic circular dichroism spectroscopy to the identification of small, matrix-isolated metal clusters and the assignment of their electronic spectra

Those principles of magnetic circular dichroism which are particularly relevant to the assignment of the spectra of matrix-isolated metal atoms and small molecules formed from such atoms are briefly reviewed. These principles are illustrated through a discussion of the m.c.d. spectra of Agn and Cun isolated in argon and methane matrices.The analysis of the spectra of the silver species presents few problems and for Ag2 and Ag3 the results strongly confirm the assignments of species and electronic states which have been made hitherto. For the copper spectra the situation is much less satisfactory and further work is indicated.

The electronic spectra of gas-phase and matrix-isolated Ag2 below 5.5 eV

Abstract The gas- and matrix-phase electronic spectra of the silver dimer in the energy region below 5.5 eV have been assigned. It is found that a satisfactory interpretation of these spectra, particularly that in the matrix, can only be obtained if the mixing of Σ and Π states by second-order spin—orbit coupling is explicitly included in the analysis. Electron repulsion effects are found to play an important, but less obvious, role.

Magnetic circular dichroism study of the 2S→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.

The magnetic circular dichroism spectra of matrix-isolated benzene

The magnetic circular dichroism (MCD) spectra of the 1 B 2u ←1 A 1g transition of benzene in nitrogen and argon matrices at 20 K have been measured in order to obtain a spectrum devoid of hot bands and complicating rotational structure. The spectrum is dominated by three progressions of B terms, two of positive sign and one of negative. One positive progression and the negative one can be definitely assigned to the e2g modes v 6 and v 9 respectively, whilst the other positive progression may be the e 2g mode v 8. This is in marked contrast with the absorption spectra of the same matrices which reveal only a single progression built upon v 6. The MCD spectra are nicely accounted for in terms of a magnetic mixing between the 1 B 1u and 1 B 2u states, the electric dipole intensity arising from the mixing in of a 1 E 1u state via e 2g vibrational modes.