[Winga]

Butanal
http://www.aist.go.jp/RIODB/SDBS/sdbs/owa/sdbs_sea.cre_frame_disp?sdbsno=1925

3-methylbutanal
http://www.aist.go.jp/RIODB/SDBS/sdbs/owa/sdbs_sea.cre_frame_disp?sdbsno=350

Why the peak of aldehydic proton in 3-methylbutanal has no splitting (spin-spin coupling) but present in butanal?

[dexangeles]

I don't see any splitting on both NMR Spectra
If you noticed, the 3-methylbutanal NMR spectra was done under 400 MHz while the butanal was done under a 90 MHz spectra
the splitting you think you see on butanal NMR is because a 90 MHz display is not as fine as a 400 MHz display.
to confirn this, you need to have the splitting data
some sites allow you to zoom into each peak to determine it's splitting (singlet, doublet, multiplet...)

hope this answers your question

[Winga]

For butanal spect, I think the splitting peaks are overlapped with the main peak!

It doesn't matter that both spectra have not splitting on aldehydic proton.
No splitting that means no spin-spin couplings between aldehydic proton and its nearby protons, right?

But why?
I can't explain it. Both compounds are not alcohols, there should be no rapid proton exchange occurs, right?