Magnetically Oriented Microcrystal Array: Complete Characterization of Chemical Shift
Anisotropy from Microcrystalline Powder
Ryosuke Kusumi, Fumiko Kimura, and Tsunehisa Kimura
Division of Forest and Biomaterials Science, Kyoto University, Kyoto 606-8502, Japan
E-mail: [email protected]
Solid-state NMR spectroscopy is one of the most widely used methods for investigating crystal
structures, along with the X-ray and neutron diffraction methods. The electronic structure around a
nucleus in a crystal can be determined by characterization of the chemical shift anisotropy (CSA) of
the nucleus. The single-crystal rotation method, in which changes in chemical shifts are measured
as a function of the rotating angle of a single crystal, is a powerful and direct means for complete
characterization of CSA, i.e., the chemical shift tensor described by the three principal values and
directions of the principal axes with respect to a crystal coordinate system. However, this
traditional method requires a relatively large single crystal (several mm in each dimension), which
hinders the use of this powerful method.
We recently proposed the use of a magnetically
oriented microcrystal array (MOMA) as an alternative to
a single crystal.1,2) A MOMA can provide a great
potential to extract the CSA information from a
microcrystalline powder. Previously, we reported that
the 13C chemical shift tensors of the carboxyl and methyl
carbons of the L-alanine crystal can be determined by
application of the standard procedure in the
single-crystal rotation method to an L-alanine MOMA
sample.3)
In the present study, we demonstrate that the
combination of MOMA with the single-crystal rotation
method is applicable to the CSA characterization of
other nuclei such as 31P and 15N. Fig. 1 shows the 31P
solid-state CP NMR spectra of the MOMA sample
prepared from a microcrystalline powder of
phenylphosphonic acid (PPA).
While the 31P
resonance peaks were considerably broad for the
measurement without magic angle spinning (MAS)
shown in Fig. 1(b) compared to that with MAS (Fig.
1(a)), the PPA-MOMA produced sharp and narrow
resonance peaks without resolution enhancement by
MAS. The PPA-MOMA spectra showed that the
positions of the 31P resonance peaks varied
systematically as a function of the angle ψ that is the
sample-rotation angle about the axis inclined by the
magic angle. The similar results were also obtained for
Fig. 1 31P Solid-state CP NMR
15
15
the N solid-state CP NMR spectra of N-enriched
spectra of PPA obtained for (a) a
L-alanine MOMA. On the basis of the above results,
powder sample with MAS at 15 kHz,
the determination of 31P and 15N chemical shift tensors
(b) a powder sample without MAS,
including directions of the principal axes is currently
and (c) a MOMA sample without
underway for the two MOMA samples of PPA and
MAS at different angles ψs.
15
N-enriched L-alanine, respectively.
References
1) T. Kimura, F. Kimura, and M. Yoshino: Langmuir 22 (2006) 3464.
2) T. Kimura, C. Chang, F. Kimura, and M. Maeyama: J. Appl. Crystallogr. 42 (2009) 535.
3) R. Kusumi, G. Song, F. Kimura, and T. Kimura: J. Magn. Reson. 223 (2012) 68.