Influence of Charge Order on the Ground States of TMTTF Molecular Salts

(TMTTF)2AsF6 and (TMTTF)2SbF6 are both known to undergo a charge ordering phase transition, though their ground states are different. The ground state of the first is Spin-Peierls, and the second is an antiferromagnet. We study the effect of pressure on the ground states and the charge-ordering using 13C NMR spectroscopy. The experiments demonstrate that the the CO and SP order parameters are repulsive, and consequently the AF state is stabilized when the CO order parameter is large, as it is for (TMTTF)2SbF6. An extension of the well-known temperature/pressure phase diagram is proposed.Comment: 5pages, 5 figures, Proceeding of ISCOM2003, to appear in Journal de Physique I

Electron-lattice coupling and the broken symmetries of the molecular salt (TMTTF)2_2SbF6_6

(TMTTF)$_2$SbF$_6$ is known to undergo a charge ordering (CO) phase transition at $T_{CO}\approx156K$ and another transition to an antiferromagnetic (AF) state at $T_N\approx 8K$. Applied pressure $P$ causes a decrease in both $T_{CO}$ and $T_N$. When $P>0.5 GPa$, the CO is largely supressed, and there is no remaining signature of AF order. Instead, the ground state is a singlet. In addition to establishing an expanded, general phase diagram for the physics of TMTTF salts, we establish the role of electron-lattice coupling in determining how the system evolves with pressure.Comment: 4 pages, 5 figure

Competition and coexistence of bond and charge orders in (TMTTF)2AsF6

(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At Tco=103 K a charge-ordering (CO) occurs, and at Tsp(B=9 T)=11 K the material undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from carbon-13 NMR 1/T1 measurements on spin-labeled samples. Above Tsp, up to about 3Tsp, 1/T1 is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, Tsp increases substantially, while Tco is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the 1D extended Peierls-Hubbard model.Comment: 4 pages, 5 figure

Catalytic Transformations of Alkynes via Ruthenium Vinylidene and Allenylidene Intermediates

NOTICE: This is the peer reviewed version of the following book chapter: Varela J. A., González-Rodríguez C., Saá C. (2014). Catalytic Transformations of Alkynes via Ruthenium Vinylidene and Allenylidene Intermediates. In: Dixneuf P., Bruneau C. (eds) Ruthenium in Catalysis. Topics in Organometallic Chemistry, vol 48, pp. 237-287. Springer, Cham. [doi: 10.1007/3418_2014_81]. This article may be used for non-commercial purposes in accordance with Springer Verlag Terms and Conditions for self-archiving.Vinylidenes are high-energy tautomers of terminal alkynes and they can be stabilized by coordination with transition metals. The resulting metal-vinylidene species have interesting chemical properties that make their reactivity different to that of the free and metal π-coordinated alkynes: the carbon α to the metal is electrophilic whereas the β carbon is nucleophilic. Ruthenium is one of the most commonly used transition metals to stabilize vinylidenes and the resulting species can undergo a range of useful transformations. The most remarkable transformations are the regioselective anti-Markovnikov addition of different nucleophiles to catalytic ruthenium vinylidenes and the participation of the π system of catalytic ruthenium vinylidenes in pericyclic reactions. Ruthenium vinylidenes have also been employed as precatalysts in ring closing metathesis (RCM) or ring opening metathesis polymerization (ROMP). Allenylidenes could be considered as divalent radicals derived from allenes. In a similar way to vinylidenes, allenylidenes can be stabilized by coordination with transition metals and again ruthenium is one of the most widely used metals. Metalallenylidene complexes can be easily obtained from terminal propargylic alcohols by dehydration of the initially formed metal-hydroxyvinylidenes, in which the reactivity of these metal complexes is based on the electrophilic nature of Cα and Cγ, while Cβ is nucleophilic. Catalytic processes based on nucleophilic additions and pericyclic reactions involving the π system of ruthenium allenylidenes afford interesting new structures with high selectivity and atom economy

WebSpectra

This site, created by Barry C. Fam and Craig A. Merlic of the University of California - Los Angeles, was established to provide chemistry students with a library of spectroscopy problems. Interpretation of spectra is a technique that requires practice - this site provides 1H NMR and 13C NMR, DEPT, COSY and IR spectra of various compounds for students to interpret. Hopefully, these problems will provide a useful resource to better understand spectroscopy. The page was given a "Top 5% Chemistry Site" award for its usefulness and content. This is a great resource for anyone interested in either biochemistry or chemistry