I did the 400 MHz NMR for several para-disubstituted benzene molecules, and the theoretical intensity values were calculated by chemical shift and coupling constant. Comparing the experimental intensities with the calculated theoretical intensities, I found out that the percent differences are always less than 3% for peaks with lower than 8.2 ppm, and always more than 10% for the peaks with higher than 8.2 ppm. I do not understand why the percent differences are so large for the peaks with higher frequencies (typically higher than 3280 Hz).
Did you do a T1 relaxation study and set the dwell time for the pulse sequence to > 3T1 so the spins have sufficient time to relax back to ground state? This should give you accurate peak intensities.
In addition to increasing the relaxation delay because of the long relaxation time of the aromatic protons I would also increase the number of scans for a better s/n, because even 3% seems a little high.
As already mentioned above, there are two possibilities, may be both.
1. Your relaxation delay (D1) may be too short for the aromatics. Run a quick series T1 experiment and use at least 5 x T1 as your relaxation delay.
2. If your 1H 90 pulse is too long, the frequency range it can excite properly gets diminished. That is, suppose you have a long 90 pulse and the offset (o1p on Bruker) is somewhere close to the aliphatic region, your aromatics may not be fully excited. Like Dr. Anklin said, try moving the offset and see how the intensity changes.
In addition make sure that you are collecting only the signals you need, in other make sure have sweep width that over just the area of you sample so that you wont be collecting too much noise and have a beter instensity and resolution,and by doing so set you aquisation parameter and your op1 correctly.
Goodluck.
Thank you all for answering me in such detail! This is my first time to ask a question here, and I couldn't be more grateful for getting all the good answers in such short time!
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