Abstract
A convenient, broadly applicable and durable wood protection was recently published by Kaufmann and Namyslo. This procedure efficiently allows for esterification of wood hydroxyl groups with (1H-benzotriazolyl)-activated functionalized benzoic acids. The result of such wood-modifying reactions is usually monitored by an increase in mass of the wood material (weight percent gain value, WPG) and by infrared spectroscopy (IR). However, diagnostic IR bands suffer from overlap with naturally occurring ester groups, mainly in the hemicellulose part of unmodified wood. In contrast to known NMR spectroscopy approaches that use the non-commonly available solid state techniques, herein we present solution state NMR proof of the covalent attachment of our organic precursors to wood. The finding is based on a time-efficient, non-uniformly sampled (NUS) solution state 1H,13C-HMBC experiment that only needs a tenth of the regular recording time. The appropriate NMR sample of thoroughly dissolved modified wood was prepared by a mild and non-destructive method. The 2D-HMBC shows a specific cross-signal caused by spin–spin coupling over three bonds from the ester carbonyl carbon atom to the α-protons of the esterified wood hydroxyl groups. This specific coupling pathway requires a covalent bonding as a conditio sine qua non. An even more rapid test to monitor the covalent bonding was achieved with an up-to-date diffusion-ordered spectroscopy sequence (Oneshot—DOSY) based on 1H or 19F as the sensitive nucleus. The control experiment in a series of DOSY spectra gave a by far higher D value of (1.22 ± 0.06)∙10−10 m2∙s−1, which is in accordance with fast diffusion of the “free” and thus rapidly moving small precursor molecule provided as its methyl ester. In the case of a covalent attachment to wood, a significantly smaller D value of (0.12 ± 0.01)∙10−10 m2∙s−1 was obtained.
Highlights
Classic wood treatment methods that involve penetration, but later on allow release of the applied inorganic or appropriately functionalized organic compounds have become more and more obsolete as non-sustainable and environmentally benign procedures
The control experiment in a series of diffusion-ordered spectroscopy (DOSY) spectra gave a by far higher D value of (1.22 ± 0.06)·10−10 m2·s−1, which is in accordance with fast diffusion of the “free” and rapidly moving small precursor molecule provided as its methyl ester
We found that solution state diffusion-ordered spectroscopy (DOSY) [19,20,21,22,23,24,25,26,27,28] is a valuable technique well-known from organic macromolecules such as technical polymers
Summary
Classic wood treatment methods that involve penetration, but later on allow release of the applied inorganic or appropriately functionalized organic compounds have become more and more obsolete as non-sustainable and environmentally benign procedures. In the present case, this 2D NMR experiment was only feasible upon application of non-uniform data sampling (NUS) [16,17,18] with the recording of a 10 percent fraction of data that allowed a measurement time of about four days This experiment resulted in the confirmation of a covalent-type bonding of such benzoylation reagents to wood, but the still high expenditure of time prompted us to aim toward a serviceable day-to-day method, again utilizing commonly available solution state NMR probe heads. It should be an appropriate tool to distinguish between fast diffusing small precursor molecules from slow moving wood, which has been modified with the functionalized organic precursor In this field of wood chemistry and structural analysis, up until now, there have only been a few papers that utilize the DOSY technique, and among these, solely fractionated material (lignin) has been investigated [29,30,31] or esterified cellulose [32]. In order to provide a quasi “attachment quick test”, we performed a highly efficient variant of diffusion-ordered spectroscopy (Oneshot-DOSY) [25]
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