The aldehyde 1 can be unambiguously characterized with the low field NMR data. The aldehyde H-atom, as well as the aliphatic methoxy-group signals can be assigned easily (red and purple asterisks, figure 3). In fact, we do observe second order effects in the aromatic region, with a roofing effect where the peaks tend to point to each other and the intensity is not quite what you’d expect. Although we have an overlap of signal ‘B’ (yellow asterisk, δ = 7.39 (s, 1H, C(CHO)CHC(OCH3)) ppm) and signal ‘A’ (blue asterisk, δ = 7.46 (dd, 3/4JHH = 7.5, 1.8 Hz, 1H, C(CHO)CHCH) ppm), I was still able to collect the scalar coupling constants here. For signal C (green asterisk, δ = 6.95 (dd, 3/4JHH = 7.4, 1.8 Hz, 1H, C(CHO)CHCH) ppm), we also observe a ‘second order-ish’ signal shape, but again, integral and J couplings have the right values.
So, although we’ve encountered second order effects with this substrate, we were able to get every information needed from the low field spectrum. For comparison I acquired high field data of the exact same sample on a 400 MHz instrument (figure 4).