C-13 NMR Query in 2-Aminobutane: C3 Resonance

[Miffymycat]

In 2-aminobutane, C3 resonates at lower field (ca 33ppm) compared to C1 (ca 23ppm). Is there a simple rationale for this? The extra inductive effect from C4 should increase electron density around C3 and so shift it upfield compared to C2? Any thoughts/answers greatly appreciated.

 

[Bystander]

Steric effect between amine group and C4?