A4 Article in conference proceedings
Limitations in High Resolution NMR Analysis of Humic Acids (2008)


Hänninen, K. (2008). Limitations in High Resolution NMR Analysis of Humic Acids. In I. Perminova, & N. Kulikova (Eds.), From molecular understanding to innovative applications of humic substances. The Proceedings of the 14th Meeting of International Humic Substances Society, Moscow–Saint Petersburg, Russia 14-19, 2008 (pp. 81-82). Venäjä: Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, 119992 Moscow, Russia.


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Publication details

All authors or editors: Hänninen, Kari

Parent publication: From molecular understanding to innovative applications of humic substances. The Proceedings of the 14th Meeting of International Humic Substances Society, Moscow–Saint Petersburg, Russia 14-19, 2008

Parent publication editors: Perminova, Irina; Kulikova, Natalia

Publication year: 2008

Pages range: 81-82

Publisher: Department of Chemistry, Lomonosov Moscow State University, Leninskie Gory 1-3, 119992 Moscow, Russia

Place of Publication: Venäjä

Publication language: English

Open Access: Publication channel is not openly available

Additional information: Limitations in High Resolution NMR Analysis of Humic Acids Kari Hänninen University of Jyväskylä, Department of Biological and Environmental Science, P.O.Box 35, FIN-40014 University of Jyväskylä Keywords: humic acid, C-13 NMR, olefinic and aromatic double bonds 1. INTRODUCTION According to the wet chemical determinations the structure of humic acids (HA) is not very aromatic. Yields of aromatic degradation products are usually less than 3%. On the other hand, according to the C-13-NMR analyses the aromatic structure of HA is claimed to be more than 30%. The aim of this study was to find out why two chemical methods give contradictory rather than expected complementary results when applied to the structural study of HA. 2. MATERIALS AND METHODS This study is made deductively, studying the potentials of modern high resolution NMR spectroscopy to resolve reliably olefinic and aromatic double bond carbon and protons in the structure of HA. 3. RESULTS AND DISCUSSION Olefinic and aromatic protons and carbons in single, well-defined compounds having conjugated carbon-carbon double bonds can be differentiated from each other by high resolution NMR. The number of carbons and protons in the structures of these single compounds are relatively few. So the number of magnetically non-equivalent carbons and protons are also low. NMR spectra contain now only few signals which furthermore are very sharp. In C-13 NMR spectrum the signals are practically sharp lines, the chemical shift values of which can be accurately determined. The spectrum also exhibits a characteristic fine splitting provided by the spin-spin coupling of the adjacent protons or carbons. This method is important in 1-H NMR spectroscopy (C-13 NMR is usually run with the decoupling mode). For structural identification of organic molecules the combined data of chemical shift values and that of spin-spin coupling is required. HA have a very heterogeneous structure. In C-13 NMR or 1-H NMR spectroscopy this means that the number of magnetically non-equivalent protons and carbons are very large. Each of them gives, of course, a sharp peak with low intensity, but with slightly different chemical shift value. Actually, these shifts are so close to each other that the resolution power of the present NMR instruments cannot differentiate them as isolated lines. This is causing broadening of the line widths which is appearing in 1-H or in C-13 NMR spectra as a single broad peak. First of all, this means that spin-spin coupling cannot provide any fine splitting and so reliable structural identification thereof is not possible. The structural identification of HA must be based on the chemical shift values alone. The chemical shift values of olefinic and aromatic protons or carbons are, however, overlapping due to broad peak. This renders the differentiation of olefinic carbons or protons from those of aromatic ones extremely uncertain, practically impossible. The assignment of a certain C-13 NMR or 1-HNMR signal of HA as aromatic in terms of chemical shift values is possible only, when it is otherwise proved that the particular HA in question does not contain olefinic double bonds. So far the non-existence of olefinic structures in HA has not proved. In spite of that, the signals due to the unsaturated carbons of HA are widely assigned solely as aromatic in many HA studies. This assignment is more or less wishful thinking. In terms of aromaticity the spectral determinations comes closer to the values provided by wet chemical analyses when we accept the fact that the carbon-carbon double bonds in HA are more olefinic than aromatic of nature. 4. CONCLUSIONS The 1-H and C-13 NMR spectra of HA are rather reliably telling the amount of carbon-carbon unsaturation, the sum of olefinic and aromatic double bonds. Due to the limitations of the high resolution NMR analysis in the HA research, the interpretation of 1-H and C-13 NMR signals solely as a


Free keywords: humic acid; C-13 NMR; olefinic and aromatic double bonds


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