[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.

[OrgXemProf]

C-13 chemical shifts depend upon steric environment as well as upon the inductive effect of neighboring substituents.

The apparent "anomaly" that you noted most likely results from steric compression, i. e., the "γ-gauche effect".

For additional information the following reference may offer a helpful starting point:

http://voyager.dvc.edu/~lborowski/chem121index/spectroscopy/13C_NMR_Examples.htm

[OrgXemProf]

The following reference may tell you more than you want to know about the effects of steric interactions on ¹³C chemical shifts in aliphatic systems.

http://www.chem.wisc.edu/areas/reich/nmr/06-cmr-03-shift-effects.htm

[Miffymycat]

Thank you - much appreciated!  There's not much in the way of explanation of gamma gauche effects so one assumes it is electron density shifts eg from C4 C-H bonding orbitals to C-N antibonding, which results in deshielding