TY - JOUR
T1 - Breakdown of linear response theory under low-power excitation in NMR. II. The case of “long-lived” signals in homogeneously broadened dipolar spin systems
AU - Gong, Zhaoyuan
AU - Walls, Jamie D.
N1 - Funding Information:
We would like to dedicate this work to the memory of Dr. Anatoly Khitrin. We acknowledge support from the National Science Foundation under Nos. CHE-1056846, CHE-1626015, and CHE-1807724. Some of the numerical simulations were conducted using resources of the University of Miami Center for Computational Science.
Publisher Copyright:
© 2018 Author(s).
PY - 2018/12/1
Y1 - 2018/12/1
N2 - In this work, the previous linear response theory developed to describe low-power, radiofrequency (RF) excitation in inhomogeneously broadened spin systems [Z. Gong and J. D. Walls, J. Chem. Phys. 145, 164201 (2016)] is applied to the problem of low-power excitation in homogeneously broadened dipolar spin systems when the strength of the RF pulse, νRF, is much less than the homogeneous linewidth, Δν12. Application of a low-power pulse for a time Tp with a nominal flip-angle of Θ generates a broad signal with a “dip” at the RF transmitter frequency that deepens with increasing Θ. When a delay is placed before signal acquisition, only a negative, “long-lived” signal from the narrow “dip” remains. If a πX-pulse is applied after low-power excitation, a “long-lived” signal lasting a time t ≈ Tp after the πX-pulse is generated where dephasing due to B0 inhomogeneity, anisotropic bulk magnetic susceptibility, and chemical shift anisotropy is refocused while dephasing due to nonzero chemical shift differences is only partially refocused. Contrary to previous observations, experiments in powdered hexamethylbenzene demonstrate that these “long-lived” signals can exist even in the absence of nonzero chemical shift differences. Additional experimental demonstrations in powdered and single-crystalline adamantane and ferrocene samples are also presented.
AB - In this work, the previous linear response theory developed to describe low-power, radiofrequency (RF) excitation in inhomogeneously broadened spin systems [Z. Gong and J. D. Walls, J. Chem. Phys. 145, 164201 (2016)] is applied to the problem of low-power excitation in homogeneously broadened dipolar spin systems when the strength of the RF pulse, νRF, is much less than the homogeneous linewidth, Δν12. Application of a low-power pulse for a time Tp with a nominal flip-angle of Θ generates a broad signal with a “dip” at the RF transmitter frequency that deepens with increasing Θ. When a delay is placed before signal acquisition, only a negative, “long-lived” signal from the narrow “dip” remains. If a πX-pulse is applied after low-power excitation, a “long-lived” signal lasting a time t ≈ Tp after the πX-pulse is generated where dephasing due to B0 inhomogeneity, anisotropic bulk magnetic susceptibility, and chemical shift anisotropy is refocused while dephasing due to nonzero chemical shift differences is only partially refocused. Contrary to previous observations, experiments in powdered hexamethylbenzene demonstrate that these “long-lived” signals can exist even in the absence of nonzero chemical shift differences. Additional experimental demonstrations in powdered and single-crystalline adamantane and ferrocene samples are also presented.
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U2 - 10.1063/1.5036753
DO - 10.1063/1.5036753
M3 - Article
C2 - 30579319
AN - SCOPUS:85058921469
VL - 149
JO - Journal of Chemical Physics
JF - Journal of Chemical Physics
SN - 0021-9606
IS - 23
M1 - 234203
ER -