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Nuclear singlet relaxation by scalar relaxation of the second kind in the slow-fluctuation regime

Nuclear singlet relaxation by scalar relaxation of the second kind in the slow-fluctuation regime
Nuclear singlet relaxation by scalar relaxation of the second kind in the slow-fluctuation regime

The singlet state of nuclear spin-1/2 pairs is protected against many common relaxation mechanisms. Singlet order, which is defined as the population difference between the nuclear singlet and triplet states, usually decays more slowly than the nuclear magnetization. Nevertheless, some decay mechanisms for nuclear singlet order persist. One such mechanism is called scalar relaxation of the second kind (SR2K) and involves the relaxation of additional nuclei ("third spins") which have scalar couplings to the spin-1/2 pair. This mechanism requires a difference between the couplings of at least one third spin with the two members of the spin-1/2 pair, and depends on the longitudinal relaxation time of the third spin. The SR2K mechanism of nuclear singlet relaxation has previously been examined in the case where the relaxation rate of the additional spins is on the time scale of the nuclear Larmor frequency. In this paper, we consider a different regime, in which the longitudinal relaxation of the third spins is on a similar time scale to the J-coupling between the members of the spin pair. This regime is often encountered when the spin-1/2 pair has scalar couplings to nearby deuterium nuclei. We show that the SR2K mechanism may be suppressed in this regime by applying a radiofrequency field which is resonant either with the members of the spin pair, or with the third spins. These phenomena are analyzed theoretically and by numerical simulations, and demonstrated experimentally on a diester of [13C2,2H2]-labeled fumarate in solution.

0021-9606
Elliott, S.J.
6cc724de-f75f-4c0f-a00a-e712b4e73b12
Bengs, C.
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Brown, L.J.
75aa95fa-5d27-46a7-9dbe-0f465a664f5b
Hill-Cousins, J.T.
615ef358-b4f3-4736-b5cf-3d05355277f2
O'Leary, D.J.
9f2ce688-b1b7-471c-8e75-9a3e0b800ca1
Pileio, G.
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Levitt, M.H.
bcc5a80a-e5c5-4e0e-9a9a-249d036747c3
Elliott, S.J.
6cc724de-f75f-4c0f-a00a-e712b4e73b12
Bengs, C.
6d086f95-d3e8-4adc-86a5-6b255aee4dd1
Brown, L.J.
75aa95fa-5d27-46a7-9dbe-0f465a664f5b
Hill-Cousins, J.T.
615ef358-b4f3-4736-b5cf-3d05355277f2
O'Leary, D.J.
9f2ce688-b1b7-471c-8e75-9a3e0b800ca1
Pileio, G.
13f78e66-0707-4438-b9c9-6dbd3eb7d4e8
Levitt, M.H.
bcc5a80a-e5c5-4e0e-9a9a-249d036747c3

Elliott, S.J., Bengs, C., Brown, L.J., Hill-Cousins, J.T., O'Leary, D.J., Pileio, G. and Levitt, M.H. (2019) Nuclear singlet relaxation by scalar relaxation of the second kind in the slow-fluctuation regime. The Journal of Chemical Physics, 150 (6). (doi:10.1063/1.5074199).

Record type: Article

Abstract

The singlet state of nuclear spin-1/2 pairs is protected against many common relaxation mechanisms. Singlet order, which is defined as the population difference between the nuclear singlet and triplet states, usually decays more slowly than the nuclear magnetization. Nevertheless, some decay mechanisms for nuclear singlet order persist. One such mechanism is called scalar relaxation of the second kind (SR2K) and involves the relaxation of additional nuclei ("third spins") which have scalar couplings to the spin-1/2 pair. This mechanism requires a difference between the couplings of at least one third spin with the two members of the spin-1/2 pair, and depends on the longitudinal relaxation time of the third spin. The SR2K mechanism of nuclear singlet relaxation has previously been examined in the case where the relaxation rate of the additional spins is on the time scale of the nuclear Larmor frequency. In this paper, we consider a different regime, in which the longitudinal relaxation of the third spins is on a similar time scale to the J-coupling between the members of the spin pair. This regime is often encountered when the spin-1/2 pair has scalar couplings to nearby deuterium nuclei. We show that the SR2K mechanism may be suppressed in this regime by applying a radiofrequency field which is resonant either with the members of the spin pair, or with the third spins. These phenomena are analyzed theoretically and by numerical simulations, and demonstrated experimentally on a diester of [13C2,2H2]-labeled fumarate in solution.

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SJE-SR2K-V16 - Accepted Manuscript
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Accepted/In Press date: 19 December 2018
e-pub ahead of print date: 14 February 2019

Identifiers

Local EPrints ID: 428682
URI: https://eprints.soton.ac.uk/id/eprint/428682
ISSN: 0021-9606
PURE UUID: ccc37f47-e792-44bb-a7d5-b1a022ed35ee
ORCID for L.J. Brown: ORCID iD orcid.org/0000-0002-5678-0814
ORCID for G. Pileio: ORCID iD orcid.org/0000-0001-9223-3896
ORCID for M.H. Levitt: ORCID iD orcid.org/0000-0001-9878-1180

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Date deposited: 06 Mar 2019 17:30
Last modified: 14 Mar 2019 01:49

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