A scientific paper basically comes down to this:
(1) Tell people what the problem is, how you plan to solve it, and what you expect to find. Introduction.
(2) Tell people specifically what you did. Methods.
(3) Describe what you observed. Results
(4) Provide an explanation for the observation. Discussion.
(5) Sum it all up and note some limitations of the study and some future directions for further exploration. Conclusions.
So you're going to want to describe what you observed, particularly focusing on what the differences between the spectra are. That's your Results. And then provide some explanation for why the differences exist. That's your Discussion.
For example, if you see additional peaks in the commercial aspirin versus salicylic acid, why do you suspect that might be the case? Is this consistent with what your predictions were? Can you take some guesses (based on knowledge of NMR peaks or literature search) about what those peaks could be representing? This is the kind of stuff that should be in your discussion.
By the way, if you think about what you should find before you do the experiment, this can make writing a discussion much easier.