Double oriented fiber spectra of syndiotactic polypropylene were obtained. It was possible to redetermine with greater accuracy the unit cell constants, which are: a = 14.50, b = 5.60, c = 7.40 A. Space group: C2221. It was proved that the chain has s(2/1)2 symmetry, corresponding to a succession of internal rotation angles A2B2A2B2. The agreement between experimental and calculated intensities on the hkl layers with l = 0, 1, 2, 3 up to the lowest observed Bragg distance (about 2 A.) is good for the packing model proposed. The structural results are briefly discussed.

tion of amido and hydroxyl groups in model compounds so that the positions of these groups in the ground state 5 more closely resemble their positions in the transition state for lactonization.Perhaps the hydroxyamide 5 will be more susceptible than la to imidazole-catalyzed lactonization. The parent acid of 5 has been reported to lactonize ~3 X 101 234 times faster than hydroxymethylbenzoic acid.19 (19) D. R. Storm and D. E. Koshland, Jr., Proc. Nat. Acad. Set.

I n previous papers, two postulates fit for defining the most probable chain conformation of a tactic polymer in the crystalline state, were enunciated: that is the postulate of geometric equivalence of the monomeric units of a tactic chain with regard to an axis, and the postulate that the conformation is such that the potential energy of the chain, considered as not subjected to external forces, is the lowest1).It has been possible, starting from these postulates, and in connection with analogous work carried out by A. M. LIQUORIS) on many other polymers such as polyisobutylene and polyvinylidene chloride, to justify in a simple way the chain conformations experimentally found by us for different crystalline polymers.In this communication, we briefly refer on some preliminary calculations effected by us, in order t o foresee from the two above postulates the possible chain conformations of isotactic and syndiotactic polypropylenes and to compare them with the known ones.

No abstract

No abstract

No abstract

The first class of square-planar Pt(II) complexes bearing electron-poor alkenes, i.e., [PtMe(N,N-chelate)(η2-CH2CHCOR)]BF4 (R = H, NMe2, Me, OMe), is described. By using N,N ligands with suitable steric properties, it was possible to inhibit olefin dynamic processes in solution, thus allowing a thorough characterization of the complexes. Insertion of methyl acrylate into the Pt−Me bond provides a rare example of migratory insertion of an alkene into a Pt−alkyl bond.

A series of heterobimetallic complexes of general structure [RhL(2){eta(5)-(2-ferrocenyl)indenyl}] (L(2)=cod, nbd, L=CO; cod=cyclooctadiene; nbd=norbornadiene) has been synthesised with the aim of tuning the metal-metal interaction in their mixed-valence ions generated both by chemical and electrochemical oxidation, and the results are compared with those obtained for [RhL(2){eta(5)-(1-ferrocenyl)indenyl}] isomers. Crystallographic studies and DFT calculations provide a detailed description of the structural and electronic features of these complexes evidencing a significant difference in the extent of planarity of the flexible bridging ligand between the 1- and 2-ferrocenyl isomers. Independent experimental probes, in particular the potential splitting in the cyclic voltammograms and the IT bands in the near-IR spectra, are rationalised in the framework of Marcus-Hush theory and at quantum chemistry level by DFT and TD-DFT methods. These methods allow us to establish a trend based on the magnitude of iron-rhodium electronic coupling H(ab) ranging from valence trapped to almost delocalised ions. The quasi planar bridge and the olefin ancillary ligands make [Rh(nbd){eta(5)-(2-ferrocenyl)indenyl}](+) and [Rh(cod){eta(5)-(2-ferrocenyl)indenyl}](+) rare examples of heterobimetallic systems which can be classified as borderline Class II/Class III species.