The third species we study is N,N'-bis(3-aminopropyl)-1,3diaminobutane, also called spermine. It is another tetramine and is known to be a particularly promising candidate for scalable deployment of carbon capture systems . Spermine is very similar to norspermine. The difference is that the middle alkane between the secondary amines is butane instead of propane. This difference alters the length between the primary amines enough that it can span across three unit cell repetitions in the c direction, while retaining its linear character. See Figure .