Titanium hydrogen shift

As shown in Scheme 23, a low-valence chiral an.sa-bis(indenyl)titanium complex catalyzes 1,3-hydrogen shift of trans-4-tert-buxy -1 -vinylcy-clohexene, an unfunctionalized meso olefin, to give an axially asym-... 

While it is not the purpose here to review all Cp-Ti compounds, there is obviously a very close relationship between CpTi moieties and their rearrangement product (CsH TiH via ring-to-titanium a-hydrogen shift, as has been alluded to above. The nature of the Ti-H species involved in reactions between low-valent titanium compounds and dinitrogen is not always clear. There seems little doubt that some form of bis(cyclopentadienyl) titanium is an active intermediate in dinitrogen coordination reactions (31, 37-39), but there is as yet no conclusive evidence that a Ti-H species is involved in the initial reaction (11), despite some claims to the contrary (40). The existence of different forms of Cp2Ti is referred to again below. 

Interestingly, a similar process is observed for the thermal decomposition of dimethylbis(pentamethylcyclopentadienyl)titanium(IV) (65). The decomposition takes place at elevated temperatures around about 110 °C and results in the formation of fulvene complex 68 and methane. Extensive labeling experiments and kinetic measurements resulted in two reaction pathways compatible with the experimental observations. Either one involves a titanium methylene species 66 as the key intermediate. The next step is an intramolecular C,H-activation leading to fulvene complex 67, similar to the formation of 63. Einally, a hydrogen shift accounts for the final product, fulvene complex 68 (Scheme 10.23) [71]. 

Nagel and Perkins (6) studied carefully the effect of added vanadium on the 7 — 5 transition in titanium hydrides via x-ray analysis. They observed a shift in (H/M)crit (the hydrogen-to-metal ratio at the 7 — 6 transition) to lower... 

In determining the hydrogen content in solid probes of titanium, copper and magnesium alloys and in steels, two express-methods are widely used, namely, the spectral method and method of melting in the flux of inert gas carrier. These fast methods require regular (by one shift of even one hour) calibration on the hydrogen content standards - State Standard Probes (SSP). In the SSP passport, the certified concentration of hydrogen in the probe and the allowed deviation with 95% confidence is indicated. For aluminum alloys, the relative value of the allowed deviation varies from 5 to 30%. 

The first equation is realized at the LKB while the second one is carried out at the LPTF. A first titanium-sapphire laser excites the hydrogen transition. A laser diode (power of 50 mW) is injected by the LD/Rb standard and frequency doubled in a LiBsOs (LBO) crystal placed in a ring cavity. The generated UV beam is frequency compared to the frequency sum (made also in a LBO crystal) of the 750 and 809 nm radiations produced by a second titanium-sapphire laser and a laser diode. A part of the 809 nm source is sent via one fiber to the LPTF. There, a 809 nm local laser diode is phase locked to the one at LKB. A frequency sum of this 809 nm laser diode and of an intermediate CO2 laser in an AgGaS2 crystal produces a wave at 750 nm. This wave is used to phase lock, with a frequency shift S, a laser diode at 750 nm which is sent back to the LKB by the second optical fiber. This 750 nm laser diode is frequency shifted by lyfCOo) + S with respect to the one at 809 nm. In such a way, the two equations are simultaneously satisfied and all the frequency countings are performed in the LKB. Finally, the residual difference between the two titanium-sapphire lasers is measured with a fast photodiode or a Schottky diode. 

In this report we present results of a study of a titanium - ethylene complex formed in an ultra high vacuum chamber and collected on a quartz sensor for mass analysis. The technique we use for mass analysis is unique. It is based on the application of a frequency modulation method to unambiguously track the resonant frequency of a quartz sensor to better than 0.1 ppm. A downward shift in the resonant frequency of the sensor, either due to the initial deposition of the complex or due to the absorption of hydrogen gas, indicates a mass accumulation on its face. This shift can be measured with a 0.1 ppm resolution. 

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