^<13>C Nuclear Magnetic Resonance Spectra of Antipyrine Derivatives and Their Application to Hansch Analysis

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The ¹³C nuclear magnetic resonance spectra of thirty 4-substituted antipyrine derivatives were examined to obtain the following conclusions : (i) the shielding constants of antipyrine were estimated using the CNDO/2 method ; the signal assignment using the shielding constants was consistent with that based on the experiments ; (ii) the ¹³C-²H coupling constants of antipyrine were estimated using the CNDO/2 method ; the calculated values are in good agreement with those observed ; (iii) the additive equation on the chemical shifts of the sp³-hybridized carbon was estimated ; the so-called steric effects were observed as a result of this analysis ; and (iv) the Hansch analyses were carried out regarding C-4 of the antipyrine derivatives as the so-called biological active center ; the agreement between the values estimated by the Hansch equation and those observed was fairly good for the analgesic activities.