The structure is shown of benzoic acid, one of the two pertinent reference molecules for the discussion of the properties of PABA, the para-aminobenzoic acid.
The MO eigenvalues of benzoic acid are provided in the file behind the link eigenvalues. Find the eigenvalues of the HOMO and the LUMO and compare the HOMO-LUMO gap to the respective gaps in PABA and aniline.