Abstract
The 1H chemical shielding anisotropy (CSA) is an NMR parameter that is exquisitely sensitive to the local environment of protons in crystalline systems, but it is difficult to obtain it experimentally due to the need to concomitantly suppress other anisotropic interactions in the solid-state NMR (SSNMR) pulse sequences. The SSNMR measurements of the 1H CSA are particularly challenging if the fast magic-angle-spinning (MAS) is applied. It is thus important to confront the results of both the single-crystal (SC) and fast-MAS experiments with their theoretical counterparts. Here the plane-waves (PW) DFT calculations have been carried out using two functionals in order to precisely characterize the structures and the 1H NMR chemical shielding tensors (CSTs) of the solid forms of maleic, malonic, and citric acids, and of L-histidine hydrochloride monohydrate. The level of agreement between the PW DFT and either SC or fast-MAS SSNMR 1H CSA data has been critically compared. It has been found that for the eigenvalues of the 1H CSTs provided by the fast-MAS measurements, an accuracy limit of current PW DFT predictions is about two ppm in terms of the standard deviation of the linear regression model, and sources of this error have been thoroughly discussed.
Highlights
The 1 H chemical shielding anisotropy (CSA) is expected to be one of the solid-state NMR (SSNMR) parameters that should be useful in describing local structural and dynamical effects experienced by the investigated protons [1]
Relatively recently an important alternative to tedious and obviously limited SC experiments appeared, when it became possible to apply the magic-angle spinning (MAS) SSNMR technique with fast rotation rates to powder samples and reliably assess the 1 H CSA parameters
The 1 H CSA data gleaned from this type of the MAS SSNMR measurements during the 2007–ca. mid-2017 period were summarized in Reference [4]
Summary
The 1 H chemical shielding anisotropy (CSA) is expected to be one of the SSNMR parameters that should be useful in describing local structural and dynamical effects experienced by the investigated protons [1]. Most of the 1 H CSA data was obtained by analyzing the rotation patterns from the single-crystal (SC) measurements performed with the decoupling of 1 H–1 H dipolar interactions [2]. Relatively recently an important alternative to tedious and obviously limited SC experiments appeared, when it became possible to apply the magic-angle spinning (MAS) SSNMR technique with fast rotation rates to powder samples and reliably assess the 1 H CSA parameters (several other options are surveyed in Reference [3], which mentions the solution NMR studies of the 1 H CSA). The 1 H CSA data gleaned from this type of the MAS SSNMR measurements during the 2007–ca. It should be noted that above-mentioned SC measurements directly characterize the full 1 H-NMR chemical shielding tensor,
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