Qiu Wenqi1, Wei Zhiliang1, Ye Qimiao1, Chen Youhe2, Lin Yulan1, and Chen Zhong1
1Department of Electronic Engineering, Xiamen University, Xiamen, China, People's Republic of, 2Department of Mechanical and Electrical Engineering, Xiamen University, Xiamen, China, People's Republic of
Synopsis
High-resolution multi-dimensional nuclear magnetic
resonance (NMR) spectroscopy serves as an irreplaceable and versatile tool in
various chemical investigations. In this study, a method based on the concept
of partial homogeneity is developed to offer two-dimensional (2D)
high-resolution NMR spectra under inhomogeneous fields. Oscillating gradients
are exerted to encode the high-resolution information, and a
field-inhomogeneity correction algorithm based on pattern recognition is designed
to recover high-resolution spectra. The proposed method improves performances
of 2D NMR spectroscopy under inhomogeneous fields without increasing the
experimental duration or significant loss in sensitivity, and thus may open
important perspectives for studies of inhomogeneous chemical systems.Introduction
Multi-dimensional
(mD) nuclear magnetic resonance (NMR)
spectroscopy is considered to be a versatile and powerful method for
characterizing the structures and dynamics of molecules
[1-3]. However, in some
occasions, sufficiently homogeneous magnetic fields are simply unavailable even
with advanced shimming techniques. Under this circumstance, adjacent peaks in
most kinds of mD NMR spectra take the
risk of overlap, thus deteriorating spectral readability and losing
information. Recently, the partial homogeneity assisted inhomogeneity
correction spectroscopy (PHASICS) has been presented to produce 1D spectra with
enhanced resolutions under inhomogeneous magnetic fields
[4]. In this study,
the PHASICS is equipped with different mixing periods to offer different types
of 2D NMR information in unison with a designed 2D field-inhomogeneity correction
algorithm to yield high-resolution 2D NMR spectra.
Methods
The proposed
pulse sequence is shown in Fig. 1. Replacing the step-wise phase encodings in
the reported PHASICS [4] with synchronous encoding (denoted as SE-PHASICS),
which contains three orthogonal gradients, shortens the acquisition time.
Defining Ginh-x, Ginh-y, and Ginh-z as the
first-order field inhomogeneity along the X, Y and Z axes, we set the encoding
gradients (Gx, Gy, and Gz) according to k=Ginh-x/Gx=Ginh-x/Gx =Ginh-x/Gx, where
k is defined as the synchronous coefficient by virtue of a pre-acquired field
map. In this way, position-independent high-resolution information can be
recorded, and the high-resolution spectrum can be obtained with the aid of a 2D
inhomogeneity correction algorithm based on pattern recognition. If the
inhomogeneity is mainly along one dimension, SE-PHASICS sequence can be
simplified by retaining only single-axial gradient along the same orientation
where field inhomogeneity distributes. In this case, high-order terms of
inhomogeneous field can be conquered.
The encoded high-resolution information
can be recovered by virtue of pattern recognition. As presented in Figure 2,
the 2D inhomogeneity correction algorithm is implemented as follows: (1) reconstruct
the 3D data D(t1, n, ta), perform 3D FT to obtain the frequency-domain
information S(ω1, ω2, ω3) and break it into a series of 2D data; (2) binarize
the 2D spectra, apply dilation operator, and then extract the contours of peaks
to yield central coordinates of different peaks; (3) compare central
coordinates to provide correction information; (4) shifting S(ω1, ω2 and ω3)
along both ω1 and ω2 dimensions according to the correction information, the
high-resolution information can be retrieved by projecting S(ω1, ω2 and ω3) on
to the ω1ω2 plane.
Results and Discussion
A mixture
solution comprising of ethyl 3-bromopropionate and 2-butanone in identical
volume is used to demonstrate the flexibility and usability of Z-PHASICS. The
shimming coils of X1, Y1 and Z1 were detuned to introduce a 3D inhomogeneous
field. This inhomogeneity is sufficiently large that diagonal peaks are
broadened to overlap each other and cross peaks appear with broad line width in
the conventional COSY (Fig. 3c). According to the acquired field map, the
magnetic field inhomogeneity was 0.034, 0.038 and 0.097 G/cm along the X, Y and
Z axes, respectively. The module of t1/2~90o~t1/2 was used to form
the SE-PHASICS-COSY. With the SE-PHASICS, high-resolution information can be
restored. Then, the 2D inhomogeneity correction algorithm unravel the
high-resolution information. The projection spectra convey high-resolution 2D
SECSY, and the COSY spectrum can be recovered from spin echo correlated
spectrum via a simple and direct mathematical conversion [5]. The performance
of SE-PHASICS suffers from certain deterioration when high-order inhomogeneity
terms occur, but is satisfactory when high-order terms are insignificant.
A 200 mM quinine in DMSO-d6 is used to investigate the
performance of Z-PHASICS in analyzing more complicated chemical systems. The
shimming coils from Z1 to Z7 are detuned simultaneously. Modules of t1/2~90o~t1/2
and t1/2~180o~t1/2 are integrated with
Z-PHASICS to construct Z-PHASICS-SECSY and Z-PHASICS-J spectra. Fig. 4b exhibits correlation information clearly
and Fig. 4c presents delicate scalar-coupling splitting pattern, providing
useful information of chemical systems even under severe inhomogeneity (Fig.
4d).
With the enhanced signal-to-noise ratio and resolution
performances under inhomogeneous magnetic fields, the PHASICS based
high-resolution 2D spectroscopy constitutes an alternative for conventional
techniques. The proposed methods may open important perspectives for studies of
inhomogeneous chemical systems.
Acknowledgements
This work was partially supported by the National
Natural Science Foundation of China under Grants 11375147 and
11204256.References
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