spin-spin splitting

The less deshielded triplet corresponds to the terminal methyl hydrogen atoms that experience spin-spin splitting from the adjacent methylene hydrogen atoms.

The simple rules for the spin-spin splitting of NMR signals described above apply only if the chemical shifts of the coupling partners are substantially larger than the coupling constant between them.

Matrix Mechanics Spin States and Operators, Selection Rules, Transition Frequencies, Chemical Shifts, Spin-spin Splittings, Coupled-Spin Selection Rules, The AB System, Exchange Effects, Quadrupoles, Born-Oppenheimer Approx.