  An increasing number of large molecules have been positively identified in
space. Many of these molecules are of biological interest and thus provide
insight into prebiotic organic chemistry in the protoplanetary nebula. Among
these molecules, acetic acid is of particular importance due to its structural
proximity to glycine, the simplest amino acid. We compute electronic and
vibrational properties of acetic acid and its isomers, methyl formate and
glycolaldehyde, using density functional theory. From computed photo-absorption
cross-sections, we obtain the corresponding photo-absorption rates for solar
radiation at 1 AU and find them in good agreement with previous estimates. We
also discuss glycolaldehyde diffuse emission in Sgr B2(N), as opposite to
emissions from methyl formate and acetic acid that appear to be concentrate in
the compact region Sgr B2(N-LMH).
