Chris M. Ramsey

Cobalt-catalyzed homocoupling of terminal alkynes: synthesis of 1,3-diynes

Abstract Homocoupling of terminal alkynes proceeds using Co 2 (CO) 8 pretreated with phenanthroline to give good yields of 1,3-diynes under mild conditions.

Heat capacity and electron spin echo evidence for low frequency vibrational modes and lattice disorder in L-alanine at cryogenic temperatures

With the view of understanding the low frequency (40-50 cm(-1)) motional processes in L-alanine around 4 K, we have carried out heat capacity (CP) and electron spin echo (ESE) measurements on L-alanine and L-alanine-d7. The obtained CP data show the so-called boson peak (seen as a maximum in CP/T3 versus T plots) in the low temperature region (1.8-20 K). The phase memory time, T(M), and spin lattice relaxation time, T1, of the spin probe, the so-called first stable alanine radical (SAR1), *CHCH3COOH, have been measured between 4 and 105 K. The obtained relaxation rate 1/T1 shows an anomalous increase which coincides with the emergence of a boson peak in the low temperature region (4-20 K). Together, the ESE and the CP data confirm the existence of a thermally activated dynamic orientational disorder in the lattices of both compounds below 20 K. The results help explain the discrepancy between the CP data from powders and single crystals of alanine, as well as the proanomalous relaxation mechanisms for SAR1 in these lattices, and they also provide a mechanism for the spin-lattice relaxation process for SAR1 at cryogenic temperatures.

Phonon-bottleneck enhanced magnetic hysteresis in a molecular paddle wheel complex of Ru25+

The ruthenium based molecular magnet [Ru2(D(3,5‐Cl2Ph)F)4Cl(0.5H2O)∙C6H14] (hereafter Ru2) behaves as a two-level system at sufficiently low temperatures. The authors performed spin detection by means of single-crystal measurements and obtained magnetic hysteresis loops around zero bias as a function of field sweeping rate. Compared to other molecular systems, Ru2 presents an enhanced irreversibility as shown by “valleys” of negative differential susceptibility in the hysteresis curves. Simulations based on phonon-bottleneck model are in good qualitative agreement and suggest an abrupt spin reversal combined with insufficient thermal coupling between sample and cryostat phonon bath.

Crystalline and water soluble CR(4+) and CR(5+:) model compounds for chromium toxicity studies

We discuss two complexes of Cr(4+) for their possible utility as models for Cr toxicity studies. They are Cr(dien)(O2)2·H2O (dien = diethylenetriamine) and Cr(NH3)3(O2)2, which have been recently characterized by x-ray diffraction and magnetic techniques. We present their optical and infrared absorption spectra as quick aids in their identification procedure. We also summarize the general properties of some well-characterized Cr(5+) compounds. All of these compounds are water soluble with the Cr(5+) samples being fairly stable in basic solutions. They can all be prepared as pure crystals with a shelf life of over 2 years when stored in a refrigerator.

EPR probing of bonding and spin localization of the doublet-quartet states in a spin-frustrated equilateral triangular lattice: Cu3(O2C16H23)6·1.2 C6H12

We present high-frequency (34 and 95 GHz) EPR spectroscopic measurements of the magnetic parameters of both the ground state (spin-doublet) and the excited state (spin-quartet) of the model frustrated-spin triangular lattice of Cu3(O2C16H23)6·1.2 C6H12, containing the Cu36+ core. From 295 down to about 100 K, the EPR spectra from single crystals consist of a well-resolved triplet, but with the central component being overlapped by a single peak. At 4 K, the triplet is replaced by a singlet. The triplet is shown to arise from the quartet state, located at 324 K above the ground state. Its magnetic parameters are: D = –535 G, E = 0, g// = 2.209, g⊥ = 2.057 with the parallel direction being the three-fold axis of the Cu36+ core. The singlet is assigned to the S = 1/2 ground state, with gxx = 2.005, gyy = 2.050, and gzz = 2.282. Its hyperfine structure was that from a single Cu nucleus, with Azz = 157 G, and Axx = Ayy < 60 G, demonstrating that in the doublet state the unpaired electron is localized on only one of the three Cu2+ ions. We ascribe this localization to an antiferromagnetic exchange interaction between Cu36+ cores, with zJ′ = –0.15 K. These results serve as a basis for detailed theoretical calculations of spin dynamics and electronic bonding in a frustrated triangular magnetic lattice. To cite this article: B. Cage et al., C. R. Chimie 6 (2003) 000–000.