Evidence for Slow Rotation Around the sec‐Alkyl‐to‐Nitrogen Bond in Some N ,N‐Di‐sec‐alkyl Amides

Abstract

The methine proton signals $$\left(\begin{array}{cc}\backslash & \\ –& \mathrm{CH}\\ /& \end{array}\right)$$ of certain N,N‐di‐2‐propyl amides were shown to broaden selectively as the temperature is lowered. For N,N‐di‐2‐propyl isobutyramide, the high‐field methine signal broadened around room temperature, but sharpened again between −50° and −60°C. For other related amides, the low‐field methine signals broke up into subsets of signals at low temperature. This broadening and multiplicity is interpreted as evidence for slow rotation around the 2‐propyl‐to‐nitrogen bonds. This sort of behavior is viewed as potentially a serious complication in determining rotation rates by PMR‐signal‐shape analysis for rotors to which the 2‐propyl and related groups are attached.