In this work, we have studied the structural changes in soot precursor particles sampled from an isooctane/air rich premixed flame using Fourier transform infra red spectroscopy. Samples are extracted from different heights of the flame and dispersed in dichloromethane for ex-situ measurements in an infrared spectrometer. An absorption spectrum of mature soot collected from the same flame is also taken in the spectrometer for comparison. It has been found that the mature soot structure is much complex with many absorption peaks at different wavenumbers. On the other hand, fewer numbers of peaks are observed in the absorption spectra of soot precursor particles. Baseline method of quantitative analysis is used to compare the concentrations of different functional groups in the structures of the particles. Significant aliphatic and carbonyl functionalities are found to occur in the soot precursor particles in comparison to the aromatic functionalities present in them at lower elevations of the flame. The ratio of concentrations of aliphatic C–H to aromatic C=C functional groups in the nascent soot is found to peak at a certain height from the burner top and then decreases at further height. On the other hand, the ratio of carbonyl to aromatic functional groups monotonically reduces along the flame height. The variations in the non-aromatic structures influence the surface growth and coagulation pattern of soot along the flame.