Deuterium fraction

Fig. 7.15 (continued) (c) Superposed experimental and calculated 30.41 MHz (7 T) 15N NMR spectra of 15N enriched DPP at different temperatures and deuterium fractions xd in the mobile sites. The four sharp lines with temperature dependent positions stem from a small quantity of 15N labeled tetramethyltertraaza-[14]annulene reference in a separate capsule (Klein, O., Limbach, H. H., et al. J. Am. Chem. Soc. 126, 11718 (2004))... 

For solutions of the methoxide ion in methanol [17], (p = 0.76 was obtained (More O Ferrall, 1969). This result was later slightly modified to (p = 0.74 (Gold and Grist, 1971) and a value for the solvated proton in methanol [18], (p = 0.625, was also measured. The result for deuterium fractionation was deduced as (p = 0.7 from observations of tritium fractionation (Al-Rawi ei al., 1979) and tp = 0.74 has been obtained more recently (Baltzer and Bergman, 1982). Values for several alkoxide ions in alcohol were used to reach conclusions about the solvation of the alkoxide ions [19] in these solutions (Gold et al., 1982). 

Table 11.13 Deuterium fractionation in the water-hydrogen-methane system, after Bottinga (1969) (wv = water vapor, me = methane, hy = hydrogen).

Fig. 10 Changes in the hydrogen bond between Zn OH and Ser48 during hydride transfer to NAD in the liver alcohol reaction.40 The values are deuterium fractionation factors.

The observed overabundance of deuterated species in molecular clouds and outer disks compared to the measured interstellar D/H ratio of 10 5 is well established. A classical isotopic deuterium fractionation is possible at low temperatures of 10-20 K owing to disbalance between forward and reversed reaction efficiencies H+ + HD 5 H2D+ + H2 + 232K (e.g. Millar et al. 1989 Gerlich et al. 2002). The temperature dependency in an isotope exchange reaction is a consequence of the zero-point vibrational energy difference for the isotopically substituted molecules (Bigeleisen Mayer 1947 Urey 1947). This leads to an elevated ratio ol H2t)+/H compared to HD/H2, which is quickly transferred into other molecules by ion-molecule reactions (see e.g. Roberts Millar 2000 Roberts et al. 2003). For example, the dominant reaction pathway to produce DCO+ is via ion-molecule reactions of CO with H2D+. In disks it results in a DCO+ to HCO+ ratio that increases with radius owing to the outward decrease of temperature (Aikawa Herbst 2001 Willacy 2007 Qi et al. 2008). 

E. Nilsson, et ah. Atmospheric deuterium fractionation HCHO and HCDO yields in the CH2DO -I- O2 reaction, Atmos. Chem. Phys. 7 (2007) 5873-5881. 

Edison, A. E, Markley, J. L., and Weinhold, F., Ab initio calculations of protium/deuterium fractionation factors in clusters, J. Phys. Chem. 99, 8013-8016 (1995). 

The change of the solubility parameter 8eixi maybe assumed to be linear with deuterium fraction e and composition Xj (0[Pg.28]

Roberts, H., Herbst, E. and Millar, T. J. (2003). Enhanced deuterium fractionation in dense interstellar cores resulting from multiply deuterated H7. Astrophysical Journal, 591, L41. 

The kinetics of the HHHH-transfer in the cyclic tetramer of 3,5-diphenyl-4-pyr-azole (DPP) has been evaluated recently [27]. The overall kinetic HHHH/DDDD isotope effects were found to be only around 12. This value indicated absence of a single barrier HHHH process where one would expect a larger overall effect. Instead, the Arrhenius pattern depicted in Fig. 6.36 could be explained in terms of a stepwise HH+HH process according to the profile of Fig. 6.17, where two hydrons are transferred in each step, leading to the expected isotope effects depicted in Fig. 6.19(b) and (c). This means that the rate constants of the HHHD and the HDHD reaction are very similar, and also those of the DDHH, DDHD, DDDD reactions. This leads to a very special dependence of the rate constants observed on the deuterium fraction in the mobile proton sites. The mole fractions of all isotopologs according to a statistical distribution are depicted in Fig. 6.37(a), and the sums of mole fractions of the relevant species exhibiting similar rate constants in Fig. 6.37(b). It is clear, that practically only three different species and rate constants are observed in this case. 

The effects of small changes in the molecular structure can be observed in the case of the related diarylamidines which are the nitrogen analogs of formic acid and which represent models for nucleic acids. In tetrahydrofuran, for N,N -di-(p-F-phenyl)amidine (DFFA) three forms were observed by NMR, a solvated s-cis-form and a solvated s-trans-form which is in fast equilibrium with a cyclic dimer in which a HH-transfer takes place [24] as illustrated in Fig. 6.43. Fortunately, at low temperatures, the s-cis- and the s-trans-forms were in slow exchange. The rate constants of the HH, HD and DD reactions were determined by dynamic H and i F NMR as a function of concentration, deuterium fraction in the mobile proton sites and of temperature. The dependence of the observed rate constants of the s-trans-form on concentration is depicted in Fig. 6.44. The solid lines were calculated using Eq. (6.39) from which the rate constants in the dimer as well as the equilibrium constants of the dimer formation could be obtained. The Arrhenius... 

Intramolecular H/D exchange gives essentially a statistical mixture of isotopomers, but not always exactly statistical because deuterium usually prefers to be in the HD or DD site (see Chapter 6). Isotopomers can be detected by solution NMR or by IR in low-temperature matrices. Separate resonances for H2 and hydride site isotopes are observed in the spectra of complexes when no intramolecular exchange occurs, but in cases where the reaction in Eq. (9.13) is a rapid one, only averaged chemical shifts and JHD are observed (see Chapter 5). In the fast-exchange H NMR spectra of isotopomers of nonclassical polyhydrides, a phenomenon called isotopic perturbation of resonance (IPR) occurs. For example, in a partially deuterated MH(H2) complex each isotopomer (H3, DH2, and HD2) shows a separate hydride resonance for the species provided the M-H and M(H2) sites have significantly different chemical shifts and there is sizable deuterium fractionation between the sites (see Section 5.4). 

Semilog-plot of deuterium fractionation factor (agas) vs. the reciprocal of temperature for some compounds of hydrogen (Bigeleisen 1969)... 

CHEMICAL PATHWAYS FOR DEUTERIUM FRACTIONATION IN INTERSTELLAR MOLECULES... 

The deuterium fractionation observed for these species must be measured against the benchmark of the cosmic D/H ratio and this is iilustrated for three examples below ... 

Of course, this has only taken the typing out of preparing the chemistry the real science lies in choosing appropriate deuterium fractionation reactions, which are added to the automatically produced reaction list. We now have a reaction scheme, and may use DELOAD, and construct our model. 

Using dynamic solid-state CP MAS NMR spectroscopy, the kinetics of the degenerate intermolecular double and quadruple proton and deuteron transfers in the cyclic dimer of N labelled polycrystalline 3,5-diphenyl-4-bromopyrazole (DPBrP) and in the cyclic tetramer of N labelled polycrystalline 3,5-diphenylpyrazole (DPP) have been studied in a wide temperature range at different deuterium fractions in the mobile proton sites. Rate constants were measured on a millisecond time scale by line shape analysis of the doubly N labelled eompounds and by magnetisation transfer experiments on a second timescale of the singly N labelled compounds in order to minimise the effeets of proton-driven N spin diffusion. The Arhenius curves of all processes were found to be nonlinear and indicated tunneling processes at low temperatures. In a preliminary analysis, they were modelled in terms of the Bell-Limbach tuimeling model. 

The influence of some of these desorption processes on cold core abundances has been investigated by Willacy Williams, Shalabeia Greenberg, and Willacy Millar. The latter also included deuterium fractionation, a process known to be efficient at low temperatures and which provides additional observational constraints. Willacy Millar compared predictions with observations of some 25 species in the cold core TMC-1 and with the D/H abundance ratios of a further 8 species, finding best agreement (to within a factor of 5) in f9 abundances and 5 ratios for the case of H2 driven desorption. The global effect of these desorption processes is to cause a quasi-steady state to develop at times in excess of a million years due to the balance between accretion and desorption. Such a quasisteady state can also occur in models with surface chemistry, as discussed in Section 1.6.3. 

Roberts H, Herbst E, Millar TJ. (2003) Enhanced Deuterium Fractionation in Dense Interstellar Cores Resulting Prom Multiply Deuterated Hg. Astrophys. J. 591 L41-L44. 

Osamura Y, Roberts H, Herbst E. (2005) The Gas-Phase Deuterium Fractionation of Formaldehyde. Astrophys. J. 621 348-358. 

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