Exchange timescale

Another difference arises from the way in which the tracers are introduced into the ocean. CFCs have the simplest boundary conditions, as they dissolve as inert gases, following gas exchange and solubility rules (Warner et ai, 1996 Warner and Weiss, 1985). A typical gas exchange timescale for CFCs is of order 1-2 months, depending on wind speed and mixed-layer depth. Radiocarbon also enters the ocean via gas exchange (as CO2), but its gas exchange timescale is amplified by the... 

The impact of the difference in the boundary conditions between tracers can be seen in Figure 8, which compares the meridional distributions of tritium and CFC-11 in the central Pacific. The common element of both distributions is the tongue of elevated tracer concentrations being subducted in the northern subtropics, advected equatorward, and upwelled in the tropics. This feature was first noted in tritium by Fine et al. (1981, 1983, 1987) and places important constraints on the exchange timescales for subtropical-tropical overturning in the Pacific. This, in turn is potentially an important regulatory element... 

The hydration of more inert ions has been studied by O labelling mass spectrometry. 0-emiched water is used, and an equilibrium between the solvent and the hydration around the central ion is first attained, after which the cation is extracted rapidly and analysed. The method essentially reveals the number of oxygen atoms that exchange slowly on the timescale of the extraction, and has been used to establish the existence of the stable [1 10304] cluster in aqueous solution. 

The timescale is just one sub-classification of chemical exchange. It can be further divided into coupled versus uncoupled systems, mutual or non-mutual exchange, inter- or intra-molecular processes and solids versus liquids. However, all of these can be treated in a consistent and clear fashion. 

These experiments yield T2 which, in the case of fast exchange, gives the ratio (Aoi) /k. However, since the experiments themselves have an implicit timescale, absolute rates can be obtained in favourable circumstances. For the CPMG experiment, the timescale is the repetition time of the refocusing pulse for the Tjp experiment, it is the rate of precession around the effective RF field. If this timescale is fast witli respect to the exchange rate, then the experiment effectively measures T2 in the absence of exchange. If the timescale is slow, the apparent T2 contains the effects of exchange. Therefore, the apparent T2 shows a dispersion as the... 

In presence of molecular motion the NMR line shape will change. A particularly simple situation arises, if the motion is rapid on timescale defined by the inverse width of the spectrum in absence of motion 6 1. In this fast exchange limit, which in 2H NMR is reached for correlation times tc < 1CT7 s, the motion leads to a partially averaged quadrupole coupling and valuable information about the type of motion can directly be obtained from analysis of the resulting line shapes. The NMR frequency is then given by... 

One further point needs to be mentioned when probing the feasibility of a particular experiment. Apart from its dependence on temperature and concentration (for instance of ions, solutes, impurities, isotopes), relaxation times - in particular the longitudinal relaxation time Tj - depend on the field strength. This can be understood from the concept that energy exchange is most efficient if the timescale of molecular motion is equal to the Larmor frequency. Often, molecular motion takes place over a wide range of frequencies, so that the func-... 

If you take a pure sample of ethanol, and run its NMR spectrum in dry CDCI3, the hydroxyl proton will appear as a well-defined triplet, which couples to the adjacent -CH2-, rendering it a multiplet. This is because the hydroxyl proton remains on the oxygen for relatively long periods of time, as there is nothing in the solution to entice it off, i.e., exchange (if any) is said to be very slow on the NMR timescale (less than about 1 s). 

The presence of a trace of acid and water however, causes collapse of the hydroxyl-OH to a singlet (at lower held), the proton can now protonate, and de-protonate the oxygen very rapidly, as the process is catalysed by the acid, i.e., exchange is said to be fast on the NMR timescale (less than about 10-6 s). 

I decays to the ground state by fluorescence emission with a lifetime of 3.3 ns (see above), then I returns to A in less than a nanosecond, a process that is slightly thermally activated, strongly sensitive to deuteration, and apparently involves several intermediates [118, 132, 144]. Because no production of the B state is observed during I deexcitation and return to A, states I and B must exchange on timescales at least slower than the nanosecond. 

As mentioned above the 1,1-organoboration reaction is reversible, and exchange is slow on the NMR timescale. This statement is in agreement with the chemical behavior of equilibrated mixtures of products. Thus, treatment of silicon borahomoadamantane derivative 92 with bis(trimethylstannyl)ethyne leads to the tin-containing compound 93 and liberation of bis(trimethylsilyl)ethyne (Scheme 44). With pyridine, the equilibrium is moved toward 1-boraadamantane completely due to the complexation <2001JOM(620)51>. 

NaB (C6H5 )4/l 8-crown-6/dioxolane systems the exchange was found to be slow on the nmr timescale two 23Na resonances were observed for solutions containing an excess of the sodium salt. 

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