G. Gómez

Ni(COD)2: an efficient promoter in the construction of 5-cyclopent-2-enone derivatives from allyl halides, alkynes and carbon monoxide

Abstract Cyclopentenone derivatives are obtained from alkynes, allyl halides and carbon monoxide in a single synthetic operation by the use of Ni(COD) 2 with minimal formation (if any) of Ni(CO) 4 . The initial formation of a π-allyl nickel complex, first intermediate in the so far postulated mechanism, is a valid pathway to the three component cycloaddition. However, care should be taken to minimize the side reactions in the process, the extension of which depends on the nature of both the alkyne and the allyl derivatives. Reliable monitoring of the components throughout the process results in good yields of the cycloadducts.

Optical imaging of L723 : the structure of HH 223

Aims. We imaged the Lynds 723 dark nebula (L723) with the aim of studying the morphology of the Herbig-Haro object HH 223 and other line-emission nebula detected in the region. Methods. We obtained deep narrow-band images in the Hα and [SII] lines and in the continuum nearby Hα of a field of ∼5 � of the L723 dark nebula centered on HH 223. Results. The Hα and [SII] images reveal the detailed morphology of HH 223, unresolved in previous optical images. Both images show a quite complex knotty, wiggling structure embedded in a low-emission nebula. Comparison between the [SII] and Hα fluxes of the knots are indicative of variations in the excitation conditions through HH 223. In addition, several other faint nebula are detected in Hα a few arcmin to the SE and to the NW of HH 223, all of them lying projected onto the east-west pair of lobes of the quadrupolar CO outflow. Comparison between the Hα and the continuum images confirms the HH-like nature of the Vrba object V83, while the Vrba objects V84 and V85 are identified as faint field stars.

Nebular Spectra of the Type Ic Supernovae 1997B and 1997X

New spectra of the Type Ic supernovae 1997B and 1997X at nebular phase are shown. Although one of them, SN 1997X, was studied with some detail around its maximum and its properties compared with other well-known SNe Ib/c, both of these events were later followed within its nebular phase. Our aim is to explore the behavior of these supernovae when they reached the nebular phase. Comparisons with data of other SNe Ic at similar nebular phases, mainly by using spectra from the Canarias Database, are also performed. The results derived from these comparisons reinforce the quite heterogeneous behavior found in SNe Ic nebular spectra. In particular, the widths of the [O I] and [Ca II] emissions found in SN 1997B are wider than the average for Type Ic events, while the [O I] width of SN 1997X is narrower than the average and is nearly comparable to typical [O I] widths found in SNe Ib.

Effect of the curing régime on compressive strength

In a previous issue the Instituto Eduardo Torroja in Spain described the development of lightweight panels made from ordinary Portland cement and rice husk ash (RHA). This paper discusses the crucial effect of the curing regime on compressive strength with temperatures maintained at 30C and relative humidity at 80 percent.