An official website of the United States government
Nuclear spin relaxation mechanisms are often difficult to isolate and identify, especially in molecules with internal flexibility. Here we combine experimental work with computation in order to determine the major mechanisms responsible for 31 P spin–lattice and singlet order (SO) relaxation in pyrophosphate, a physiologically relevant molecule. Using field-shuttling relaxation measurements (from 2 μT to 9.4 T) and rates calculated from molecular dynamics (MD) trajectories, we identified chemical shift anisotropy (CSA) and spin–rotation as the major mechanisms, with minor contributions from intra- and intermolecular coupling. The significant spin–rotation interaction is a consequence of the relatively rapid rotation of the –PO 3 2− entities around the bridging P–O bonds, and is treated by a combination of MD simulations and quantum chemistry calculations. Spin–lattice relaxation was predicted well without adjustable parameters, and for SO relaxation one parameter was extracted from the comparison between experiment and computation (a correlation coefficient between the rotational motion of the groups).
more »
« less
Weak nuclear spin singlet relaxation mechanisms revealed by experiment and computation
Nuclear spin singlet states are often found to allow long-lived storage of nuclear magnetization, which can form the basis of novel applications in spectroscopy, imaging, and in studies of dynamic processes. Precisely how long such polarization remains intact, and which factors affect its lifetime is often difficult to determine and predict. We present a combined experimental/computational study to demonstrate that molecular dynamics simulations and ab initio calculations can be used to fully account for the experimentally observed proton singlet lifetimes in ethyl-d 5 -propyl-d 7 -maleate in deuterated chloroform as solvent. The correspondence between experiment and simulations is achieved without adjustable parameters. These studies highlight the importance of considering unusual and difficult-to-control mechanisms, such as dipolar couplings to low-gamma solvent nuclei, and to residual paramagnetic species, which often can represent lifetime limiting factors. These results also point to the power of molecular dynamics simulations to provide insights into little-known NMR relaxation mechanisms.
more »
« less
- Award ID(s):
- 2108205
- PAR ID:
- 10337264
- Date Published:
- Journal Name:
- Physical Chemistry Chemical Physics
- Volume:
- 24
- Issue:
- 12
- ISSN:
- 1463-9076
- Page Range / eLocation ID:
- 7531 to 7538
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Thiocarbonyls exhibit unique photophysical properties, characterized by rapid intersystem crossing (ISC) due to favorable singlet−triplet energetics and enhanced spin−orbit coupling. However, the role of hydrogen bonding in modulating the ISC remains underexplored. This study investigates the effect of solvent−solute hydrogen bonding on the ISC dynamics of 7-(diethylamino)-4- methyl-2-sulfanylidene-2H-chromen-2-one (thiocoumarin 1, TC1) using steadystate and time-resolved spectroscopy, complemented by theoretical calculations. Experimental data reveal that in methanol, hydrogen bonding leads to increased fluorescence quantum yield, prolonged singlet-state lifetime, and reduced triplet yield compared to aprotic acetonitrile. Time-resolved spectroscopy identifies an additional long-lived emissive singlet state in methanol, attributed to a hydrogen-bonded state, which slows ISC. Theoretical calculations demonstrate that hydrogen bonding alters the electronic structure and constrains ISC along key nuclear coordinates, including the C S bond vibration and dihedral angles, leading to decreased triplet formation. These findings provide mechanistic insights into hydrogen-bonding-mediated control of ISC in thiocoumarins, with implications for designing functional materials with tunable photophysical properties.more » « less
-
The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T 1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet–triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet–triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors.more » « less
-
null (Ed.)Abstract The removal of electrons located in the core shells of molecules creates transient states that live between a few femtoseconds to attoseconds. Owing to these short lifetimes, time-resolved studies of these states are challenging and complex molecular dynamics driven solely by electronic correlation are difficult to observe. Here, we obtain few-femtosecond core-excited state lifetimes of iodine monochloride by using attosecond transient absorption on iodine 4 d −1 6 p transitions around 55 eV. Core-level ligand field splitting allows direct access of excited states aligned along and perpendicular to the ICl molecular axis. Lifetimes of 3.5 ± 0.4 fs and 4.3 ± 0.4 fs are obtained for core-hole states parallel to the bond and 6.5 ± 0.6 fs and 6.9 ± 0.6 fs for perpendicular states, while nuclear motion is essentially frozen on this timescale. Theory shows that the dramatic decrease of lifetime for core-vacancies parallel to the covalent bond is a manifestation of non-local interactions with the neighboring Cl atom of ICl.more » « less
-
31 P NMR spectroscopy and the study of nuclear spin singlet relaxation phenomena are of interest in particular due to the importance of phosphorus-containing compounds in physiology. We report the generation and measurement of relaxation of 31 P singlet order in a chemically equivalent but magnetically inequivalent case. Nuclear magnetic resonance singlet state lifetimes of 31 P pairs have heretofore not been reported. Couplings between 1 H and 31 P nuclei lead to magnetic inequivalence and serve as a mechanism of singlet state population conversion within this molecule. We show that in this molecule singlet relaxation occurs at a rate significantly faster than spin–lattice relaxation, and that anticorrelated chemical shift anisotropy can account for this observation. Calculations of this mechanism, with the help of molecular dynamics simulations and ab initio calculations, provide excellent agreement with the experimental findings. This study could provide guidance for the study of 31 P singlets within other compounds, including biomolecules.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1CP05537B
