Exchange pathways

The main reservoirs and exchange pathways of the global P cycle are schematically presented in Fig. 14-7. This representation is primarily taken from Lerman et al. (1975) and modified to include atmospheric transfers. The mass of P in each reservoir and rates of exchange are taken from Mackenzie et al. (1993) and Follmi (1996). 

The rate of exchange was found to be dependent on the hydrogen-ion concentration (up to 5 X 10 M) in a manner which led to these authors suggesting that the exchange pathways... 

Addition of chloride in the range 5 x 10 to 3 x 10 M, at constant acidity and ionic strength, was found to increase the rate of exchange. This was interpreted in terms of an exchange pathway involving the VCl ion, viz. 

From a detailed study of the exchange, at various temperatures (in the range 0 to 20 °C) and acidities at a constant ionic strength of p = 1.0 Af, the kinetic parameters were calculated, k and k 2 k 2 = k2K- have values of 0.48 l.mole". sec and 0.22 sec", respectively, at 0 °C. For the exchange pathway associated with ky, values of the activation enthalpy and entropy of 12.6 kcal.mole" and — 14 cal.deg . mole , respectively, were reported. For the second pathway... 

Masters and Schwartz have made a study of the exchange in perchlorate media and have found two exchange pathways operate. One pathway involves exchange via the process... 

To summarize the exchange and donation mechanisms of Tetralin, pathway 1 in Figure 2 is the predominant pathway of exchange and donation as determined by preferential incorporation of protium into the a-position of Tetralin. Tetralin-di2 loses a deuterium atom from the a-position, and a protium atom is incorporated into the a-position. This equilibrium is the exchange pathway. Continued loss of deuterium from the a-Tetralinyl radical eventually leads to naphthalene, and this reaction is the donation pathway. 

Figure 7 (a) crystal structure and (b) magnetic exchange pathway in 16(5... 

Determination of reaction mechanisms by combining the observed intermediates in a catalytic cycle. To do this, it is often necessary to measure under different conditions - that is, variable temperature NMR. The use of high-pressure NMR cells is crucial in order to measure under the real catalytic conditions. The EXSY experiment helps to unravel exchange pathways, both intra-and intermolecular. 

Haake et al reported on the H/D exchange from the 2-position of 1,3,4-trimethylimidazolium ion (6) and found the predominant exchange pathway to involve 0D as base. The rate is approximately 3 x 10 times slower than that at which the deuteroxide ion abstracts a proton from structurally similar thiazolium ions, in contrast with previous work where a diphenylimidazolium ion was found to exchange at the... 

Scheme 1.11 Exchange pathway for the interchange of diastereoisomers of[Rh(R,R-dipamp)mac]. ...

Figure 1.33 Possible exchange pathways for the inversion at le in complexes of the type [PtIMejL-L], (L-L = MeTe(CH2)3TeMe and PhTe(CH2)3TePh). je EXSY spectrum of [PtlMe3 Mele(CH2)3leMe ] in CDCI3 at 313 K. The circles indicate the position of the exchange cross peaks between the DL and the meso isomers. No exchange between the two meso isomers is observed in this compound.

The rearrangement of 5-bromo-3-methoxy-2-phenylimidazolo[l,2-a] pyridine (70) into the 8-isomer 73 also undoubtedly involves a sequential metalation, transmetalation, and metal-halogen exchange pathway involving intermediates 71-75 and the noncatalytic generation of the 5,8-dibromo derivative 75 (Scheme 20) (83S987). 

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