Through‐space 19F–19F spin–spin coupling in ortho‐fluoro Z‐azobenzene

Rastogi, Shiva K.; Rogers, Robert A.; Shi, Justin; Brown, Christopher T.; Salinas, Cindy; Martin, Katherine M.; Armitage, Jacob; Dorsey, Christopher L.; Gao, Chun; Rinaldi, Peter L.; Brittain, William J.

doi:10.1002/mrc.4327

Cited by 13 publications

(9 citation statements)

References 46 publications

(75 reference statements)

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“…There is only one literature report on the experimental determination of phenyl rotation barrier in E -AB and none for Z -AB. We have previously reported through-space 19 F– 19 F coupling for o -fluorosubstituted Z - AB . In this study, we combine theory and variable-temperature NMR to determine the phenyl rotation barrier in Z -AB systems.…”

Section: Introductioncontrasting

confidence: 59%

“…Six compounds are the subject of the present study (Scheme ). Synthesis and characterization of these compounds has been previously reported …”

Section: Resultsmentioning

confidence: 88%

“…In all cases, the conformations placing a fluorine over the adjacent aromatic ring were more stable by 0.2–0.4 kcal/mol . Corroboration of this preferred conformation was obtained by X-ray crystallography of Z 2 reported previously and Z 6 (Supporting Information).…”

Section: Resultsmentioning

confidence: 99%

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Conformational Dynamics of o-Fluoro-Substituted Z-Azobenzene

et al. 2015

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A conformational analysis of o-fluoro Z-azobenzene reveals a slight preference for aromatic C-F/π interaction. Density functional theory (DFT) indicates that the conformation with a C-F/π interaction is preferred by approximately 0.3-0.5 kcal/mol. Ground-state conformations were corroborated with X-ray crystallography. (Z)-Azobenzene (Z-AB) with at least one o-fluoro per ring displays (19)F-(19)F through-space (TS) coupling. 2D J-resolved NMR was used to distinguish through-bond from TS coupling ((TS)JFF). (TS)JFF decreases as the temperature is lowered and the multiplets coalesce into broad singlets. We hypothesize that the coalescence temperature (Tc) corresponds to the barrier for phenyl rotation. The experimentally determined barrier of 8-10 kcal/mol has been qualitatively verified by DFT where transition states with a bisected geometry were identified with zero-point energies of 6-9 kcal/mol relative to ground state. These values are significantly higher that values estimated from previous theoretical studies but lie within a reasonable range for phenyl rotation in hydrocarbon systems.

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Section: Introductioncontrasting

confidence: 59%

“…Six compounds are the subject of the present study (Scheme ). Synthesis and characterization of these compounds has been previously reported …”

Section: Resultsmentioning

confidence: 88%

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Conformational Dynamics of o-Fluoro-Substituted Z-Azobenzene

et al. 2015

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“…The most cogent advice I received during this correspondence was “fluorine NMR can be really tricky.” For this special issue of the Journal of Physical Organic Chemistry , I have chosen to summarize this project, which J.D.R., once again, made me solve myself (a hallmark of his training). Portions of this study have been published previously . The succinct summary of this project is that we measured the barrier to phenyl rotation in a metastable Z ‐azobenzene using the temperature dependence of through‐space (TS) 19 F‐ 19 F spin‐spin coupling.…”

Section: Roberts' Mentorshipmentioning

confidence: 99%

Phenyl rotation barrier in Z‐azobenzene: A physical organic problem in honor of J.D.R.

Brittain

Rastogi

Rogers

et al. 2018

J of Physical Organic Chem

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The Z isomer of azobenzene exhibits nonbonded interactions between ortho substituents on adjacent phenyl groups. For o,o′‐difluoroazobenzene derivatives, through‐space 19F‐19F spin‐spin coupling is observed. The coupling constant decreases as the nuclear magnetic resonance probe temperature is decreased culminating in collapse to a single peak. We have used density functional theory and X‐ray crystallography to argue that phenyl group rotation is responsible for this loss of through‐space coupling. We experimentally measured phenyl rotation barriers in the range of 37 to 44 kJ/mol for o,o′‐difluoroazobenzene derivatives. These rotation barriers are qualitatively similar to literature experimental values for fluxional systems but higher than calculated values for Z‐azobenzene. This is the only project in 35 years that I discussed with J.D.R. This mechanistic interpretation of an unusual nuclear magnetic resonance phenomenon is a classic physical organic investigation inspired by my training with Roberts. I tell this scientific story interwoven with a few observations about working with Roberts.

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“…Compounds used in this study are shown in Scheme 1. The synthesis of 4-6 has been reported [12], 1, 2, 8, 10 are commercially available and 3 was prepared in a conventional manner by the Mills reaction [5]. Compounds 10 and 8 are reagents in a collagenase assay [13].…”

Section: Solubilitymentioning

confidence: 99%

Differential azobenzene solubility increases equilibrium cis/trans ratio in water

Brown

Rastogi

Barrett

et al. 2017

Journal of Photochemistry and Photobiology A: Chemistry

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We have determined aqueous solubility (S w) for purified cis and trans isomers of azobenzene (AB) and AB n derivatives. S w for trans-AB n is 0.05-0.15 mM and the cis isomer is 6-40x more soluble. The effect of differential solubility is important for aqueous sample concentration > S w (trans) where cis/trans ratios >1 were observed via 1 H and 19 F NMR. This phenomenon is general for AB n (aq) that included semifluorinated AB n and peptide conjugates. The van't-Hoff-Dimroth relationship predicts the equilibrium cis/trans ratio is proportional to the ratio of S w values. Comparing NMR data to S w values confirms the trend observed in the cis/trans ratios, but the magnitude of the ratio is lower. This phenomenon is observed in μmol concentration and may be a factor in photopharmacology. We have also studied the thermal cis-to-trans isomerization as a function of temperature for eight AB n compounds. E a values AB n (aq) are ~ 25 kJ mol-1 lower compared to E a measured in nonpolar solvents and ΔS ‡ values in water are more negative so that aqueous rates are 2-5 times greater in water. This class of AB n compounds generally show little solvent effect and this change in E a is small compared to the behavior of donor-acceptor ABs. The inversion mechanism for this isomerization is supported by density functional theory.

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Through‐space ¹⁹F–¹⁹F spin–spin coupling in ortho‐fluoro Z‐azobenzene

Cited by 13 publications

References 46 publications

Conformational Dynamics of o-Fluoro-Substituted Z-Azobenzene

Conformational Dynamics of o-Fluoro-Substituted Z-Azobenzene

Phenyl rotation barrier in Z‐azobenzene: A physical organic problem in honor of J.D.R.

Differential azobenzene solubility increases equilibrium cis/trans ratio in water

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