Structural Evolution of Burmese Amber during Petrifaction Based on a Comparison of the Spectral Characteristics of Amber, Copal, and Rosin

Table 1

Infrared peak attributions of the samples.

Group	Saturated or unsaturated	Infrared peak position (cm^−1)			Evolution of spectral feature
		Burmese ambers	Copal	Rosin
CH2, CH3	Saturated	2925, 2866, 2847, 1458, 1375	2957, 2869, 1464, 1384	1466, 1389	Diminishing absorption intensity from Burmese amber to copal to rosin
ν (CH2−CH3)	Saturated	2731, 2672	2728, 2670	2724, 2667	Decreasing absorption intensity from Burmese amber to copal to rosin. Hydrogen bond cleavage during the formation of amber forms a stable saturated hydrocarbon structure.
C=O	Unsaturated	1719–1724	1720–1726	1710–1722	The peak is split in the spectra of the amber samples.
Unsaturated six-membered ring	Unsaturated	1600	—	—	Weak absorption in spectra of amber samples
Isopropyl	Saturated	1383	1384, 1369	1372, 1386	Diminishing absorption intensity from Burmese amber to copal to rosin
Isopropyl skeleton	Saturated	1162	1164	1161	The absorption intensity of Burmese amber, copal, and rosin is almost the same
Intramolecular hydrogen bonding between carboxylic acids	Saturated	—	—	2651, 2534	No absorption peaks are seen in the spectra of copal and amber.
Trans-structure, hydroxy group	—	975	975	—	—