The Structural Arrangement and Relative Abundance of Aliphatic Units May Effect Long-Wave Absorbance of Natural Organic Matter as Revealed by 1 H NMR Spectroscopy

I. V. Perminova, E. A. Shirshin, A. I. Konstantinov, A. Zherebker, V. A. Lebedev, I. V. Dubinenkov, N. A. Kulikova, E. N. Nikolaev, E. Bulygina, R. M. Holmes

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    Abstract

    The objective of this study was to shed light on structural features which underlay intensity of long wave absorbance of natural organic matter (NOM) using 1 H NMR spectroscopy. For this purpose, a set of the NOM samples was assembled from arctic and nonarctic sampling sites (the Kolyma river basin and Moscow region, respectively). It was to ensure a substantial difference in the humification degree of the isolated organic matter - the biogeochemical proxy of the long-wave absorbance of NOM. The assembled NOM set was analyzed using solution-state 1 H NMR spectroscopy. The distribution of both backbone and exchangeable protons was determined using acquisition of spectra in three different solvents. The substantially higher contribution of nonfunctionalized aliphatic moieties CHn (e.g., materials derived from linear terpenoids, MDLT) in the arctic NOM samples was revealed as compared to the nonarctic ones. The latter were characterized with the higher content of CHα protons adjacent to electron-withdrawing groups which belong to carboxyl rich alicyclic moieties (CRAMs) or to aromatic constituents of NOM. We have calculated a ratio of CHn to CHα protons as a structural descriptor which showed significant inverse correlation to intensity of long wave absorbance assessed with a use of E 4 /E 6 ratio and the slope of absorption spectrum. The steric hindrance of aromatic chromophoric groups of the NOM ensemble by bulky nonfunctionalized aliphatic moieties (e.g., MDLT) was set as a hypothesis for explanation of this phenomenon. The bulky aliphatics might increase a distance between the interacting groups resulting in inhibition of electronic (e.g., charge-transfer) interactions in the NOM ensemble. The obtained relationships were further explored using Fourier transform mass spectrometry as complementary technique to 1 H NMR spectroscopy. The data obtained on correlation of molecular composition of NOM with 1 H NMR data and optical properties were very supportive of our hypothesis that capabilities of NOM ensemble of charge transfer interactions can be dependent on structural arrangement and relative abundance of nonabsorbing aliphatic moieties.

    Original languageEnglish
    Pages (from-to)12526-12537
    Number of pages12
    JournalEnvironmental Science and Technology
    Volume52
    Issue number21
    DOIs
    Publication statusPublished - 6 Nov 2018

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