Given these limitations of the Hammett correlation and the fact that we also had ortho-substituted anilines in our set, we selected to apply a multivariate approach. A robust model with two parameters was identified: the natural population analysis (NPA) charge on the ring carbon connected to the nitrogen and a ring vibration aligned with the para substituent axis (μ and ν respectively, Figure .e.). In this model increased positive charge adjacent to the nitrogen and lower vibration frequencies lead to higher yields. The vibration frequencies are difficult to interpret intuitively because they are influenced by changes to charge and the mass and location of the substituents (see SI Figure for further details). However, the variation in charge confirms the conclusions gained from the Hammett analysisthe ability of substituents to stabilize the iminium transition state, through both resonance and inductive effects, is responsible for the yield of the reaction (See Figure .d-f.).