σ_IValues for heterocycles

Role of Arginine Guanidinium Moiety in Nitric-oxide Synthase Mechanism of Oxygen Activation

“…We found that arylguanidines exhibit pK a values ranging from 9.3 to 11 (see Fig. 1 and supplemental Table S1), matching those reported in the literature for similar compounds (50,63). As expected, the values obtained for alkylguanidines were found between 11.8 and 12.6, close to the pK a value of L-Arg (12.48) (supplemental Table S1).…”

“…The pK a value of each L-Arg analogue was measured using two different methods. The pK a determination of substituted guanidines in water has been already described for phenyl-substituted tetramethylguanidines (62) and monosubstituted guanidines, such as Ph-Gua (50) and NO 2 Ph-Gua (63). Accordingly, the pK a values of our set of arylguanidines were measured in water by NaOH titration via the simultaneous monitoring of the pH and the electrical conductivity of the guanidinium salt solution (see "Experimental Procedures").…”

Kinetics of the gas-phase elimination of ?-Bromophenylacetic acid under maximum inhibition

“…However, the -system of an aromatic ring is less polar than an olefinic double bond [10] and a decrease in reactivity may be expected. This means that the double bond of the benzene ring is less nucleophilic …”

Gas-phase pyrolytic reactions. Part 6. [1] Behavior of ethyl (hetero)arylcarboxylate esters in thermal elimination reactions

“…of the thermal elimination reactions of these esters is semiconcerted with evident concomitant and commensurate negative and positive charge residues being developed, respectively, at the relevant carbon atoms of the carbonyl moiety and the alkyl group of the ester function (Schemes I & II) [4 -6]. Besides, it is instructive to note the observation that the most reactive esters also possess the most polar transition states [6], and that the converse should also be expected to hold. Further, we have used this feature of charge development to provide the first set of consistent gas-phase Hammett replacement 0 substituent constants of the title heteroaryl ring systems that were found to be in agreement with the corresponding values computed from solution reactions [2,7].…”