Deuterium isotope effects on the carbon-13 chemical shifts of carbocations

Solvolysis of 2-substituted-9-(otrho-substituted phenylmethyl)fluoren-9-yltrimethylammonium ions in various solvents. The effect of steric crowding on alkene formation

“…Secondary P-deuterium isotope effects of approximately 1.3 (for two hydrogens (deuteriums)) found in the present study are in accord with literature values found for other solvolytic reactions leading to carbocation formation (38)(39)(40)(41). It is generally considered (38) that secondary P-deuterium isotope effects arise due to hyperconjugative assistance by the P-hydrogens to the formation of the carbocation.…”

Isotopic Perturbation of Resonance?a Term of Controversy

“…We therefore have to look for suitable model systems which give a reliable estimate. This is very difficult, since it was shown that isotope effects are dependent on the chemical shifts 7,[17][18][19] and for cations intrinsic isotope effects are expected to be larger than for neutral compounds. We chose to compare the values with those in analogously substituted benzenes (4-6) which were partly available to us from earlier work 20,21 and by a new preparation of 6 (see Scheme 3).…”

“…This accounts for the sign change, since it has been shown that the sign of both substituent increments and deuterium isotope effects is dependent on the chemical shift of the carbon atom in question. 7,18,19 Comparing the deuterium isotope effects in the cations 1, 2 and 3 with the neutral reference systems 4, Scheme 2…”

“…Subsequently, this effect was used to distinguish non-classical cations from equilibrating structures. [5][6][7] Other workers [7][8][9] expanded the term to 'perturbation of hyperconjugation' and Forsyth and co-workers even discussed 9 a 'through space perturbation of resonance' and used extensively the concept of isotopic perturbation of hyperconjugation by comparing different carbocations. 10 Similarly, for neutral aromatic compounds, hyperconjugation was used to explain isotope effects by different workers.…”

Factor Analysis of Deuterium Isotope Effects on 13C NMR Chemical Shifts in Schiff Bases

“…Such an effect usually causes an increase in electron density in the neighboring bonds, some shielding of the neighboring nuclei and, in consequence, a decrease in chemical shifts. [1,6,7] The magnitudes of the intrinsic effects are most pronounced at the site of deuteration and generally fall off quite rapidly as a function of bond separation between the substitution site and the observation site. In principle, the intrinsic isotope effects are independent of temperature as long as vibrational excitation and other temperature-dependent changes of the nuclear geometry are negligible.…”