Table 1.
Overview of the IR band assignments of G‐lignin. Note that Hergert studied milled wood lignins and Faix studied Western hemlock native lignin. Both authors gave assignments for lignin in general, whereas our assignment work is based on G‐DHP. Φ = Ring mode in Wilson/Varsanyi notation. Two charts depicting all ring modes for asym‐trisubstituted and asym‐tetrasubstituted rings can be found in the Supporting Information
| Wavenumber (G‐DHP) | Hergert64 | Faix65 | This work |
|---|---|---|---|
| ~3,400 | O–H stretching (H‐bonded) | O–H stretch | O–H stretch (H‐bonded) |
| 3,086 | C–H stretch of aromatic ring | ||
| 3,065 | C–H stretch of aromatic ring Φ2 | ||
| 3,035 | C–H stretch of aromatic ring | ||
| 3,002 | C–H stretch in methyl and methylene groups | C–H stretch of OCH3 | |
| 2,960 | C–H stretch in methyl and methylene groups | Asymmetric C–H stretch of OCH3 | |
| 2,937 | C–H stretching (methoxyl groups and side‐chain CH) (Assigned to 2,920 in original publication) | C–H stretch in methyl and methylene groups |
Symmetric C–H stretch of OCH3 Antisymmetric stretch of CH2OH |
| 2,875 | C–H stretching (methoxyl groups and side‐chain CH) | C–H stretch in methyl and methylene groups | Symmetric C–H stretch of CH2OH |
| 2,844 | C–H stretch in methyl and methylene groups | Symmetric C–H stretch of OCH3 | |
| 1,720 | C=O stretching of aliphatic ketone | C=O stretch in unconjugated ketone, carbonyl, and in ester groups (frequently of carbohydrate origin); conjugated aldehydes and carboxylic acids absorb around and below 1,700 cm−1 | C=O stretch of unconjugated carbonyls |
| 1,660 | C=O stretching of p‐substituted aryl ketone | C=O stretch; in conjugated p‐subst. aryl ketones; strong electronegative substituents lower the wavenumber |
C=O stretch of conjugated carbonyls (i.e., coniferyl aldehyde, carbonyls in α‐position, carbonyls of quinone methide) C=C stretching of coniferyl alcohol |
| 1,599 | C=C skeletal vibrations (aromatic ring) | Aromatic skeletal vibrations plus C=O stretch; S > G; G condensed > G etherified | Ring stretch Φ8b of G‐rings and Φ8a of S‐ringsa |
| 1,507 | C=C skeletal vibrations (aromatic ring) | Aromatic skeletal vibrations; G > S | Ring stretch Φ19b of G‐rings and Φ19a of S‐ringsa |
| 1,463 | C–H deformation (asymmetric) | C–H deformations; asym. In –CH3 and –CH2– | C–H bending of OCH3 and CH2 |
| 1,455 | C–H deformations; asym. In –CH3 and –CH2– | C–H bending of OCH3 and CH2 | |
| 1,423 | Aromatic skeletal vibrations combined with C–H in‐plane deform. | Ring stretch Φ19a of G‐rings and Φ19b of S‐ringsa | |
| 1,369 | C–H deformation (symmetric) | Aliphatic C–H stretch in CH3; not in OMe; phen. OH |
Ring stretch Φ14 of 4‐OH‐G‐rings and S‐ringsa Ring stretch Φ20a of 5‐5′ structures |
| 1,330 | S‐ring plus G‐ring condensed (i.e., G‐ring substituted in pos. 5) | Ring stretch Φ20a of asymmetric‐tetrasubstituted rings (C–X; X=C, O) | |
| 1,268 | C–O stretching aromatic (methoxyl) | G‐ring plus C=O stretch; G condensed > G etherified | Ring bend Φ7a of G‐rings |
| 1,213 | C–O stretching aromatic (phenol) | C–C plus C–O plus C=O stretch; G condensed > G etherified (authors give range from 1,221 to 1,230) | Ring bend Φ13 of G‐rings |
| 1,190 | Unassigned (methoxyl group) | C–H rocking of methoxy groups | |
| 1,141 | Aromatic C–H in‐plane deformation; typical for G‐units; whereby G condensed > etherified (typical for S‐units): plus secondary alcohols plus C=O stretch |
C–C stretch of coniferyl aldehyde C–H bend Φ18b, Φ15 of G‐rings CH bend Φ18a of 5‐5′ structures |
|
| 1,087 | C–O deformation (aliphatic ether or secondary hydroxyl) | C–O deformation in secondary alcohols and aliphatic ethers |
C–C stretch of Cß and Cγ C–O stretch of methoxy groups of 5‐5′ structures |
| 1,031 | C–O deformation (methoxyl group) | Aromatic C–H in‐plane deformation, G > S; plus C–O deform. In primary alcohols; plus C=O stretch (unconj.) | C–O stretch of methoxy groups of G‐rings |
| 967 | =CH out‐of‐plane deformation (trans) | –HC=CH– out‐of‐plane deform (trans) |
C–H wagging of C=C of coniferyl aldehyde and alcohol In‐phase ring bend Φ7a of 5‐5′ structures |
| 930 | Unassigned (possibly OH out‐of‐plane deformation) | C–H out‐of‐plane; aromatic (authors give range from 915 to 925) | Ring bend Φ7b of G‐rings |
| 855 | C–H out‐of‐plane deformation (one H, aromatic ring) | C–H out‐of‐plane in positions 2, 5, and 6 of G‐units | C–H out‐of‐plane bend Φ10a of G‐rings |
| 820 | C–H out‐of‐plane deformation (two H, aromatic ring) | C–H out‐of‐plane in positions 2, 5, and 6 of G‐units | C–H out‐of‐plane bend Φ11 of G‐rings |
| 775 | Ring bend Φ12 of G‐rings |
Includes G‐units with substituents on ring position 5.