Dynamic isotope effect, water

Fig. 5.17 The dynamic isotope effect for water as measured by (x) spin-lattice relaxation, 1/t2 ( ) self-diffusion, D, and ( + viscosity, t. ...

Investigation of water motion in AOT reverse micelles determining the solvent correlation function, C i), was first reported by Sarkar et al. [29]. They obtained time-resolved fluorescence measurements of C480 in an AOT reverse micellar solution with time resolution of > 50 ps and observed solvent relaxation rates with time constants ranging from 1.7 to 12 ns. They also attributed these dynamical changes to relaxation processes of water molecules in various environments of the water pool. In a similar study investigating the deuterium isotope effect on solvent motion in AOT reverse micelles. Das et al. [37] reported that the solvation dynamics of D2O is 1.5 times slower than H2O motion. 

The second explanation for the solvent isotope effect arises from the dynamic medium effect . At 25 °C the rotational and translational diffusion of DjO molecules in D20 is some 20% slower than H20 molecules in H20 (Albery, 1975a) the viscosity of D20 is also 20% greater than H20. Hence any reaction which is diffusion controlled will be 20% slower in D20 than in H20. This effect would certainly apply to transition state D in Fig. 3 where in the transition state the leaving group is diffusing away. A similar effect may also apply to the classical SN1 and SN2 transition states, if the rotational diffusion of water molecules to form the solvation shell is part of the motion along the reaction co-ordinate in the transition state. Robertson (Laughton and Robertson, 1959 Heppolette and Robertson, 1961) has indeed correlated solvent isotope effects for both SN1 and SN2 reactions with the relative fluidities of H20 and D20. However, while the correlation shows that this is a possible explanation, it may also be that the temperature variation of the solvent isotope effect and of the relative fluidities just happen to be very similar (see below). 

N. Nandi, S. Roy and B. Bagchi, Ultrafast solvation dynamics in water isotope effects and comparison with experimental results, J. Chem. Phys., 102 (1995) 1390-7. 

Two unusual features can be observed in these plots (and, at least for the self-diffusion coefficient, this behaviour is common to all hydrogen-bonded liquids). This ratio is a function of temperature. At constant temperature and pressure, rotation and translation reveal the same isotope effect. From simple sphere dynamics one would expect the rotation to scale as the square root of the ratio of the moments of inertia (=1.38) while for translational mobility the square root of the ratio of the molecular masses ( = 1.05) should be found. This is clearly not the case, indicating that the dynamics of liquid water are really the dynamics of the hydrogen-bond network. The hydrogen bonds in D2O are stronger than those in H2O and thus the mobility in the D2O network decreases more rapidly as the temperature decreases. 

Figure 4. Comparative analysis of H-D isotope effects on elementary charge transfer, including electron photodetachment and localization, and electron-pro-tonated radical couplings in pure water at 294 K The dotted line represents the characteristic limit for which the electronic dynamics are independent of H-D...

Ice samples are put into pre-cooled extraction vessels for evacuation. The ice is then melted and a gas extraction and purification similar to that used for water samples is performed. Because dynamic analysis requires far more sample than the static mode, ultrapure N2 is added to increase bulk pressure by a factor of 10. The mass spectrometric analysis follows conventional procedures of dynamic isotope ratio mass spectrometry, with modifications designed to avoid any fractionating effects, such as thermal diffusion... 

If equilibrium solvation is the only cause of the solvent effect then the Mu reaction should also be a factor 35 faster in aqueous solution compared to the gas phase. This was not observed, the increase of its rate constant in water for addition to benzene amounts to only a factor of 3-5 (Figure 8), and it is not limited by diffusion. The difference was ascribed to a dynamic solvent effect and taken as evidence of Kramers solvent friction which increases with frequency and is thus obviously far more important for the reaction of Mu, the lighter isotope [33]. 

Figure 5 The effect of different marine N cycle processes on nitrate <5 N and concentration, assuming an initial nitrate <5 N of 5%o. The trajectories are for reasonable estimates of the isotope effects, and they depend on the initial nitrate <5 N as well as the relative amplitude of the changes in nitrate concentration (30% for each process in this figure). A solid arrow denotes a process that adds or removes fixed N from the ocean, while a dashed line denotes a component of the internal cycling of oceanic fixed N. The effects of these two types of processes can be distinguished in many cases by their effect on the concentration ratio of nitrate to phosphate in seawater. The actual impact of the different processes on the N isotopes varies with environment. For instance, if phytoplankton completely consume the available nitrate in a given environment, the isotope effect of nitrate uptake plays no major role in the <5 N of the various N pools and fluxes the effect of nitrate generation by organic matter degradation and nitrification, not shown here, will depend on this dynamic. Similarly, the lack of a large isotope effect for sedimentary denitrification is due to the fact that nitrate consumption by this process can approach completion within sedimentary pore waters.

Decoherence in condensed phase typically slows down chemical reactions as has been exemplified by the non-radiative relaxation of solvated electrons [3,18,67]. In the case of an electron in water the difference in the rates of quantum decoherence induced in the electron subsystem by water and deuterated water explains the absence of a solvent isotope effect on the relaxation rate [18,67]. In rare instances, decoherence can enhance chemical reactivity. The SMF approach has been used to provide evidence for acceleration of a chemical reaction in a condensed phase due to the quantum anti-Zeno effect [55]. The mechanism indicates that the anti-Zeno effect involves both delocalization of the quantum dynamics and a feedback loop by coupling to the solvent. Believed to be the first example of the quantum anti-Zeno effect in chemistry, the observed phenomenon suggests the possibility of quantum control of chemical reactivity by choice of solvent. 

Sasmal et al. [104] reported deuterium isotope effect on the solvation dynamics and the anisotropy decay of coumarin 480 (C480) in a microemulsion system with l-pentyl-3-methylimidazolium tetrafluoroborate [pmim][BFJ in TX-lOO/benzene by employing femtosecond upconversion. Retarded solvation dynamics were foimd with replacement of H O by D O in the microemnlsion identifying water molecnles as the main species responsible for solvation. 

We have seen that the effect of a full or partial deuteration of the cation not only leads to line shifts but also significantly changes the intensities and modifies the assignment of the infrared signatures of the different isotopologues. This is due to the soft, anharmonic, and coupled potential of the Zundel cation, where the dynamics and spectroscopy are strongly dominated by Fermi resonances between various coupled zeroth-order vibrations. The discussed quantum dynamical calculations represent an important milestone in our understanding of the spectroscopy and dynamics of protonated water clusters and on their dramatic isotope effects [41], and could only be achieved after a full-dimensional quantum dynamical treatment of the clusters. 

VendreU O, Gatti F, Meyer H-D (2009) Full dimensional (15D) quantum-dynamical simulation ofthe protonated water dimer IV isotope effects in the infrared spectraof D(D20), H(D20) andD(H20) isotopologues. J Chem Phys 131 034308... 

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