Table 6 Wavenumbers (cm⁻¹), intensities and assignments of bands occurring in the IR (KBr, ATR and DFT) spectra of vanillic, isovanillic and o-vanillic acids.

From: Spectroscopic, antioxidant and cytotoxicity studies of vanillic acids

o-VA				VA				IsoVA				Assignment (No.)
IR_KBr	IR_ATR	Theor.		IR_KBr	IR_ATR	Theor.		IR_KBr	IR_ATR	Theor.
[cm⁻¹]	[cm⁻¹]	[cm⁻¹]	Int.	[cm⁻¹]	[cm⁻¹]	[cm⁻¹]	Int.	[cm⁻¹]	[cm⁻¹]	[cm⁻¹]	Int.
		3750	166.1			3756	176.8			3755	173.7
3236 m	3223 w	3642	552.9	3482 s	3479 m	3802	191.9	3415 m	3415 w	3812	150.0	ν(OH)_ar/H2O
3080 w		3210	7.4	3097 w	3093 vw	3215	1.1	3053 w		3210	2.9	νCC (2)
3015 m	3015 w	3196	5.4	3024 sh		3175	10.6	2978 sh		3191	4.2	νCC (20a)
		3138	33.2			3139	30.9			3145	25.5	ν_asCH (CH₃)
2952 m	2952 w	3096	40.6	2957 m	2957 w	3098	40.2	2947 m	2944 w	3109	36.0	ν_asCH (CH₃)
2869 m	2869 w	3024	78.7	2855 w	2843 vw	3025	76.2	2849 w	2836 w	3030	83.6	ν_sCH (CH₃)
2715 − 2521				2794 − 2537				2666 − 2504				ν(OH)
1655 vs.	1650 s	1745	903.6	1679 vs.	1677 vs.	1726	819.5	1688 vs.	1681 s	1733	805.7	ν(C = O)
1619 m	1617 w	1640	154.4			1632	266.8	1616 m	1616 m	1635	107.8	νCC (8b)
1587 m	1586 m	1615	10.3	1599 s	1597 s	1621	18.0	1587 m	1587 m	1616	126.0	νCC (8a)
		1515	86.7	1521 s	1521 s	1536	162.7	1516 m	1515 s	1545	109.7	νCC (19b)
1461 vs.	1453 s	1494	23.4	1470 w	1464 sh	1494	13.6	1451 m	1453 m	1497	20.8	δ_asCH₃
					1456 m			1420 m	1420 m			β(OH)
		1467	132.1	1432 s	1433 s	1447	57.8			1442	110.1	νCC (19a)
1398 m	1396 m	1479	11.8	1382 m	1395 m	1480	15.9	1358 m	1357 m	1479	11.7	δ_asCH₃
1308 s	1308 m	1470	133.2		1338 w	1477	30.6			1469	7.5	δ_sCH₃
1240 vs.	1236 vs.	1313	136.1	1298 vs.	1300 sh	1353	303.9	1306 vs.	1304 vs.	1362	229.1	ν(C-OH)
		1256	744.0	1283 vs.	1281 vs.	1307	528.8	1273 s	1270 s	1359	126.7	νCC (14)
1184 m	1184 m	1249	8.1	1238 s	1238 vs.	1251	276.2			1253	186.5	νCH (13)
				1206 s	1205 s			1225 s	1226 s			β(OH)
		1249	8.1	1184 sh		1251	276.2	1184 w	1185 w	1253	186.5	ν(O-CH₃)
1085 m	1083 w											βCH (18a)
1054 s	1052 s	1178	0.7	1112 m	1112 m	1188	26.0	1133 m	1133 m	1177	194.1	βCH (18b)
		1103	57.9		1058 w	1169	144.7	1092 m	1092 m	1173	395.6	βOH; βCH
		1163	9.0	1028 m	1027 m	1165	18.6	1020 m	1020 m	1165	39.2	δ_asCH₃
894 m	892 m			919 m	916 m			929 m	926 m			γOH
834 w	834 w	847	34.8	884 w	884 m	910	34.6	882 w	881 m	948	40.0	νCH (7b)
813 vw	813 vw	827	4.5			840	20.2	826 w	828 m	855	9.0	γCH (10a)
748 s	746 s	762	106.0	762 m	760 s	776	92.9	765 s	763 vs.	782	66.3	β(C = O)
		696	51.7	724 w	725 w	722	35.0	722 w	728 vw	762	34.1	αCCC (1)
673 m	667 m			636 w	637 m			630 m	630 m			γ(C = O)
613 w	617 w	614	27.9			620	26.4	604 vw		629	79.2	αCCC (6b)
		595	13.9	588 w		587	40.2	552 w		598	26.2	γ(OH); γCH
564 vw		579	18.4	565 vw		526	82.2			539	100.0	ϕCC (16a)
				537 sh		544	40.3			523	27.0	αCCC (6a)
486 m		494	156.4	510 m		492	11.7	484 w		491	18.1	ϕCC (16b)

s – strong; m – medium; w – weak; v – very; sh – shoulder.
The symbol “ν” denotes stretching vibrations, “β” denotes in-plane bending modes, “γ” designates out-of-plane bending modes; “α(CCC)” denotes the aromatic ring out-of-plane bending modes, and “φ(CCC)” designates the aromatic ring in-plane bending modes.

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Table 6 Wavenumbers (cm⁻¹), intensities and assignments of bands occurring in the IR (KBr, ATR and DFT) spectra of vanillic, isovanillic and o-vanillic acids.

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