Deuterium substitution, effects

We then investigated the excited-state dynamics by analyzing deuterium substitution effects for hydrogen atoms bonded to Cll and C12 of the retinal chromophore. The substitution effects do not appear significantly in the excited-state dynamics until about 60 fs, but they appear after 60 fs. The mode of isomerization at C11-C12 double bond would couple with the mode of hydrogen-out-of-plane and, therefore, isomerization would start at about 60 fs after photon absorption of rhodopsin. 

Kakitani, X, Akiyama, R Hatano, Y., Imamoto, Y Shichida, Y, Verdegem, P., and Lugtenburg, J., Deuterium substitution effect on the excited-state dynamics of rhodopsin, /. Phys. Chem., 102, 1334, 1998. 

Another circumstance which could change the most commonly observed characteristics of the two-stage process of substitution has already been mentioned it is that in which the step in which the proton is lost is retarded because of a low concentration of base. Such an effect has not been observed in aromatic nitration ( 6.2.2), but it is interesting to note that it occurs in A -nitration. The A -nitration of A -methyl-2,4,6-trinitroaniline does not show a deuterium isotope effect in dilute sulphuric acid but does so in more concentrated solutions (> 60 % sulphuric acid kjj/kjj = 4 8). ... 

Fig. 4.9. DifiBoing zero-point energies ofprotium- and deuterium-substituted molecules as the cause of primary kinetic isotope effects.

The distribution of a-bromoketones formed in the reaction of acetylcyclopentane with bromine was studied as a function of deuterium substitution. On the basis of the data given below, calculate the primaiy kinetic isotope effect for enolization of... 

Challis and Rzepa (1975) observed kinetic deuterium isotope effects in the azo coupling of 2-methyl-4,6-di-tert-butylindole (12.139) and its anion. The origin of this effect must also be attributed to steric hindrance of the proton transfer step in the substitution proper, since 2-deuterated methylindole and unsubstituted indole (Binks and Ridd, 1957) do not give isotope effects. 

THE EFFECT OF RING-DEUTERIUM SUBSTITUTION ON THE RATE AND PRODUCTS OF THE REARRANGEMENT OF HYDRAZOBENZENE IN 90% ETHANOL AT 0 °C... 

The work of Melander and Carter (1964) on 2,2 -dibromo-4,4 -di-carboxybiphenyl-6,6 -d2 (1) has been referred to above in the introductory and theoretical sections, where it was pointed out that the availability of two detailed theoretical computations of the inversion barrier (Westheimer and Mayer, 1946, Westheimer, 1947 Hewlett, 1960) made this system especially attractive for the study of steric isotope efifects. Furthermore, in the preferred initial-state conformation the two bromines are probably in van der Waals contact (cf. Hampsoii and Weissberger, 1936 Bastiansen, 1950), and thus initial-state steric effects are unaffected by deuterium substitution in the 6 and 6 positions. The barrier calculations provided two different theoretical values for the non-bonded H Br distance in the transition state which, together with the corresponding H Br potential function, could be inserted in equation (10) to yield values for A AH. For... 

Deuterium isotope effects have been found even where it is certain that the C—H bond does not break at all in the reaction. Such effects are called secondary isotope effectsf" the term primary isotope effect being reserved for the type discussed previously. Secondary isotope effects can be divided into a and P effects. In a P secondary isotope effect, substitution of deuterium for hydrogen p to the position of bond breaking slows the reaction. An example is solvolysis of isopropyl bromide ... 

Deuterium Substitution. The a and P secondary isotope effects affect the rate in various ways (p. 298). The measurement of a secondary isotope effects provides a means of distinguishing between SnI and Sn2 mechanisms, since for Sn2 reactions the values range from 0.95 to 1.06 per a D, while for S l reactions the values are higher. This method is especially good because it provides the minimum of perturbation of the system under study changing from a H to a D hardly affects the reaction, while other probes, such as changing a substituent or the polarity of the solvent, may have a much more complex effect. 

The rates of hydration of substituted phenylpropiolic acids give a rho of —4.77 when plotted against a, comparable to Ihe acid-catalyzed isomerization of czs-cinnamic acid, with a rho value of —4.3. The solvent deuterium isotope effects are 3.7-S.2 for the isomerization of cinnamic acids at... 

Two reasons may be offered for the enhanced /3-deuterium isotope effect in vinyl cations as compared with carbonium ions (193). As pointed out by Noyce and Schiavelli (21), in the transition state of a vinyl cation, the isotopically substituted C—H bond is ideally suited for overlap with the developing vacant p orbital, as the dihedral angle between the empty p orbital and C—H bonds is zero in the intermediate, as shown in structure 239. Shiner and co-workers (195)... 

The oxidation by Cr(VI) of aliphatic hydrocarbons containing a tertiary carbon atom has been studied by several groups of workers. Sager and Bradley showed that oxidation of triethylmethane yields triethylcarbinol as the primary product with a primary kinetic isotope effect of about 1.6 (later corrected by Wiberg and Foster to 3.1) for deuterium substitution at the tertiary C-H bond. Oxidations... 

The effect of solvent upon k2 has been reported , and it was concluded that the activated complex is not sufficiently polar to be called ionic . The oxidations of toluene and triphenylmethane exhibit primary kinetic deuterium isotope effects of 2.4 and ca. 4 respectively. No isotopic mixing occurred during formation of the Etard complex from a mixture of normal and deuterated o-nitrotoluene . The chromyl chloride oxidation of a series of substituted diphenylmethanes revealed that electron-withdrawing substituents slow reaction while electronreleasing groups have the opposite effect, the values ofp andp being —2.28 + 0.08 and —2.20 + 0.07 respectively . ... 

NNN Rat Liver microsomes assay for a-hydroxylation and the effect of a-deuterium substitution 33... 

The effects of deuterium substitution on the rates of a-hydroxylation of NNN have been measured. The results obtained in vitro, with rat liver microsomes, showed only a small deuterium isotope effect of 1.2 for 2 -hydroxylation, whereas a significant effect of 2.4-2.7 was observed for 5 -hydroxylation (33). Analogous results were obtained 2n vivo when the urinary metabolites... 

Shifts due to either a- or (3-deuterium substitution are also quite significant, usually leading to well resolved fluorine signals for the deu-terated and undeuterated species, which can be useful in characterization of deuterium labeled fluorinated compounds. An example of the a-effect is shown in Figure 2.11, which depicts the fluorine NMR spectrum of F—CH2CH2CH2CH2CH2CD2—F, a molecule that has... 

For the ds-l-chloro-2-fluoroethylene, an a-deuterium isotope effect (one D) of 0.6 ppm is observed, along with a trans-P-deuterium isotope effect of 0.4 ppm. Looking at the /rau.v-l -chloro-2-fluoroethylcnc system, the a-deuterium isotope effect is 0.5 ppm and the c/s-P-isotope effect is 0.2 ppm. For the 1-chloro-l-fluoroethylene system, trans-P-isotope effect is 0.5 ppm, with the ds-(3-isotope effect being 0.4 ppm. From these results, it appears that trans-deuterium substitution transmits its isotope effect better than ds-deuterium substitution, the same trend as is observed in transmission of coupling constants. 

Taking into consideration the deuterium isotope effect (k Yfy/kxfD) = 2.3), they concluded that fca< k a< /cb and that the rate-determining step was the first substitution. Ligand substitution was thought to proceed by the Ia mechanism, on the basis of the negative AS and the independence of the rate on the concentration of acetylacetone. This feature is compatible with the results with tris(acetylacetonato)metal(III) previously obtained [21]. Furthermore, in the second transition series the kt value decreases in the order... 

The deuterium isotope effects on chemical shift consists of intrinsic isotope effect (direct perturbation of the shielding of X atom) and equilibrium isotope effect (perturbation of the equilibrium caused by the isotopic substitution). The values of deuterium isotope effects are to some extent independent of chemical shifts and allow determination of the mole fraction of the forms in equilibrium. 

Data from microwave spectra on the centrifugal effect of rotational transitions of selenophene and its deuterium-substituted derivatives have been determined experimentally and compared with the calculated theoretical values of the centrifugal stretching constants by means of the force constants determined from the solution of the inverse vibrational problem.26 The two sets of values show good agreement, indicating that the system of force constants obtained for selenophene correctly reflects the characteristic features of the force field of the molecule. 

The effect of deuterium isotope substitution on Tc has been studied for three members of the KL-(BEDT-TTF)2Ag(CF3)4(l,l,2-trihaloethane) family [12, 34-36]. In all cases, the Tc increased upon deuteration with the effect ranging from 0.21 to 0.36 K. These results are similar to the deuterium isotope effect observed in the p"-(BEDT-TTF)2SF5CH2CF2S03 [34, 37] and k-(BEDT-TTF)2Cu(NCS)2 [38] superconductors. 

In a more recent study (105), the intramolecular deuterium isotope effect and the relative rates of benzylic hydroxylation for six para-substituted (OCH3, CH2D, H, Cl, Br,... 

TABLE 4.2 Intramolecular deuterium isotope effects for benzylic hydroxylation of substituted toluenes catalyzed by CYP1A2, CYP1B1, CYP2C9, CYP2E1, and CYP101... 

The role of a-electrons in the stabilization of aromatics has been repeatedly emphasized by Shaik and Hiberty48 and has recently received experimental confirmation after a study of the effects of deuterium substitution of annulenes with internal hydrogens, such as the anti-aromatic [16] annul ene and the aromatic [18]annulene.49 50... 

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