Using the power of C-13 NMR to interpret infrared spectra of soil organic matter: A two-dimensional correlation spectroscopy approach

Soil organic matter (SOM) is a complex mixture containing a variety of organic molecular structures. As a consequence, interpretation of the infrared (IR) spectra of SOM is difficult and ambiguous and there is a necessity to establish more reliable IR spectral band assignments. We investigated a novel approach to identify IR spectral bands based on correlation with more easily interpreted nuclear magnetic resonance (NMR) spectra. The IR spectra of HF-treated soils were obtained in both near- and mid-infrared (NIR and MIR) regions and generalized two-dimensional (2D) correlation spectroscopy was employed as the computational correlation tool. 2D NMR/NIR and 2D NMR/MIR heterospectral correlation analyses were performed, separately. We found that NIR spectroscopy could identify aliphatic carbon in SOM as a broad peak occupying the entire NIR region. On the other hand, the MIR spectra contained stronger and more distinct signals than NIR from most of the major carbon types. Bands due to aromatic carbon and carboxyl groups were identified in the regions 4000–3500 cm−1 and 850–500 cm−1, respectively, and bands due to aliphatic carbon appeared around 3500–2600 cm−1 and 2000–1000 cm−1. Most (but not all) of these assignments are consistent with assignments based on MIR spectra of model compounds. These findings will assist in developing new IR spectroscopy tools for characterizing the chemistry of SOM more accurately and, perhaps, for monitoring its changes more sensitively.