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Apologies in advance, I have a background in physics, so this question might seem awfully simple.

Anyway, I am interested in the chiral spin selectivity effect in organic molecules, and have read a paper in which they synthesize helicene molecules with side groups R = n-Pr, i-Pr with absolutely no reference only a slight hint as to what it is an abbreviation of in either the paper or supplementary information. I thus assume that it must be some very common notation, and my guess is that Pr refers to some kind of propane/propene/propanol, and n and i refers to some normal and altered structure, respectively. But I don't know.

I would be grateful if someone could shed some light on it for me.

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Propyl is a simple hydrocarbon with three carbons in it. Propane is $\ce{CH3CH2CH3}$.

Propyl is the unit formed by attaching that chain to something else (which implies replacing one of the carbons with another bond). There are two ways to do this: one attaches the propyl unit using the end carbon on the chain (giving $\ce{X-CH2CH2CH3}$); the other involves attaching the middle carbon to something else (giving $\ce{CH3CHXCH3}$). The first is called (at least in older terminology) normal-propyl, n-propyl or $\ce{n-Pr}$; the second is called iso-propyl, i-propyl or $\ce{i-Pr}$. These reduce the amount of space required to write the structure.

In your compound helicene I'd guess that the propyls hanging off the rings are there to make the compounds far more soluble in organic solvents as flat aromatic structures tend to be fairly insoluble due to the way the pack in crystals.

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