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Oral 16
Abstract
The Interplay of MD <strong>and</strong> QM calculations to elucidate singlet state relaxation
Pär Håkansson, Giuseppe Pileio, <strong>and</strong> Malcolm H Levitt
Department of Chemistry, University of Southampton, SO17 1BJ, Southampton, UK
(e-mail: p.hakansson@soton.ac.uk)
Under certain conditions systems of two spin-1/2
support singlet states that are immune to the intra-pair dipolar Molecular model of N 2O in DMSO
relaxation superoperator therefore often showing characteristic
decay constants T S which exceed T 1 by an order of magnitude
or more. These enhanced lifetimes are of particular interest as
polarization (<strong>and</strong> indeed hyperpolarization) can be stored for
longer time intervals with important application in MRI, for
example. Because of the extended time regime, many other
“minor” relaxation mechanism interplay to relax these states.
An exact knowledge of these mechanisms can indicate the
conditions to maximize <strong>and</strong> extend even further singlet
lifetimes. However, even relatively small molecules in solution
impose a number of challenges in the task of modeling T 1 <strong>and</strong>
T S from molecular dynamics simulation (MD) <strong>and</strong> quantum mechanics calculations QM.
Computational cost is one major bottleneck. In addition, in order to accurately model long T S ,
attention is required to the weak <strong>and</strong> rather poorly explored relaxation mechanisms such as spinrotation.
In this work we use novel approaches to address chemical shift anisotropy, spin-rotation <strong>and</strong>
inter-molecular dipole-dipole relaxation mechanisms within the joint framework of MD <strong>and</strong> QM
calculations. We take particular care of molecular flexibility. The CSA contribution, for example, is
computed using a timescale-separated spin Hamiltonian, paying particular attention to internal
molecular degrees of freedom. In the case of spin-rotation, the explicit time dependent spin
Hamiltonian is considered. We demonstrate a good quantitative agreement between experiments <strong>and</strong>
calculations without adjustable parameters. We discuss the field-dependence of T S <strong>and</strong> T 1 .
<strong>8th</strong> <strong>Conference</strong> on Fast Field Cycling NMR Relaxometry, Turin 23-25 May 2013