Trigonal symmetry

FlO. 1. Theoretical curves of OC—M—CO bond angle in M(CO)3 groups with approximate trigonal symmetry, with transition metal M enneacoyalent (u = 9) or octacovalent (< = 8), as functions of the bond number n from n = 1 (single bond from metal to carbonyl) to n = 2 (double bond). 

Gas phase experiments show that the reactivity of small clusters varies greatly with cluster size (H), and experimental work with small clusters on supports has shown (12) that adsorbed molecules or atoms affect the metal-metal bond length. In both experiments the adsorption site is not known. Also the reactions proceed to completion so that only the high coverage case has been measured. We have considered the chemisorption of several atoms on Be 13 and AI13 clusters. For both clusters trigonal symmetry is maintained, and only the metal-metal... 

Most of the complexes to be described here have trigonal symmetry. For a trigonal center, the splitting of the spectral bands due to the lifting of orientational degeneracy is described by two parameters, A1 and A2. The parameter A is proportional to the hydrostatic component of the stress and gives rise to a shift in frequency that is independent of the stress direction, whereas A2 gives rise to a shift that depends on the orientation of the center. 

The assignment of the 809 cm-1 band to a doubly degenerate wagging mode implies trigonal symmetry or higher for the complex. Trigonal symmetry is also consistent with the configuration shown in Fig. 8. In Section IV.2, uniaxial stress results will be reviewed that confirm the... 

The orientation dependence of the stress alignment effect is consistent with the trigonal symmetry of the B—H complex. Stress along the [110] direction lifts the orientational degeneracy of the four BC sites about the boron while stress along the [100] direction does not. (A [111] stress also leads to a dichroism of the expected magnitude.) The sites perpendicular to... 

A pronounced structural sensitivity of the oxidation of Pt surfaces is also seen in Fig. 1. The reaction takes place at the most positive potential on Pt(lll). This is probably due to effective blocking of the surface by oxy-anions with the trigonal symmetry, compatible with the (111) orientation. A detailed analysis of this reaction on Au(lll) has been recently performed by Angerstein-Kozlowska et al. (14). No such blocking is possible for the Pt(100) and Pt(110) surfaces with four-fold and two-fold symmetries. Consequently, the oxidation commences at more negative potentials, probably predominantly determined by the surface energy as found with Au (16). 

If, in addition to the cubic crystal field, a component of lower symmetry is present, such as one having tetragonal or trigonal symmetry (as for the Al sites in a-AUOa), further splitting will occur as shown in Fig. 24. Crystal field splittings for other configurations in both the weak and strong field cases are summarized in a review article by Moffitt and Ballhausen... 

Abragam and Pryce 126) have calculated general expressions for the quantities g, D, T, and P of Equation (31). A frequently occurring situation is that of axial symmetry such as when the ion is in a CF with tetragonal or trigonal symmetry. In this case the tensors g and T have two components each, parallel and perpendicular to the symmetry axis. T can be characterized in this case by a single value D. Neglecting the last two terms, Equation (31) becomes in this case as follows ... 

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