“…Indeed, this is the result provided by molecular mechanics and molecular dynamics simulations conducted in vaccuum, where the long-range nonbonded interactions are accounted for by Coulombic, dipolar, and dispersion contributions. The results obtained in the present NMR study for dilute solutions, however, are in plain contradiction with these predictions and raise serious doubts concerning the validity of standard molecular modeling and dynamics simulations of isolated molecules for rationalizing intramolecular complex formation and conformational behavior in systems containing aromatic groups. , …”
Section: Resultscontrasting
confidence: 91%
“…This effect is not possible for 1,3-diacetoxy-2-methylpropane ( M2 ) and is reflected in the much higher relative energies (∼+4 kJ mol -1 ) for the corresponding U-shaped conformers. Similar results have been obtained previously in molecular modeling studies of polymers and model compounds and have been used to rationalize the degree of excimer formation observed in fluorescence experiments. , 2 Schematic representations of the structures of U-shaped conformers of AN .…”
Proton nuclear magnetic resonance spectroscopy has been used to investigate intramolecular interactions between different aromatic groups in a series of diesters. The materials investigated comprise two aromatic groups linked by a 2-methyl-1,3-propanedioxy spacer. This spacer permits U-shaped conformations which place the two terminal aromatic groups close together, parallel in a face-to-face arrangement. For the symmetrical diesters, 1,3-bis(9-anthracenecarbonyloxy)-2-methylpropane (A2) and 1,3-bis[(3,5-dinitrobenzoyl)-oxy]-2-methylpropane (N2), neither the chemical shifts of the aromatic group protons nor the vicinal coupling constants measured in the spacer provide any evidence for a high fraction of U-shaped conformers. In both cases, the conformational distribution of the spacer is similar to that found for 1,3-diacetoxy-2-methylpropane (M2), indicating that the planar aromatic groups in A2 and N2 experience no significant mutual attractive interactions. In contrast, substantial upfield shifts are observed in the resonance frequencies of all aromatic protons in the anthacenyl and 3,5-dinitrophenyl groups of the unsymmetrical diester, 1-(9-anthracenecarbonyloxy)-3-[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (AN), relative to those for the aromatic protons of the respective monoesters and symmetrical diesters. Analysis of the temperature dependence of the vicinal coupling constants indicates highly populated gauche states of the two central C-C bonds of the spacer chain, consistent with a total fraction of U-shaped conformers of about 80% at ambient temperature. Transient nuclear Overhauser effect experiments yield values of between 0.4 and 0.6 nm for average distances between protons on the dinitrophenyl ring and those on the anthracenyl ring, implying a distance of approximately 0.3 nm between the planes of the two aromatic groups. The stabilization of the U-shaped conformers in AN is rationalized in terms of quadrupole interactions between the two aromatic groups. The quadrupole moments associated with the two aromatic groups in AN have opposite sign, resulting in a significant attractive interaction when the groups are oriented face-to-face. For the symmetrical diesters, A2 and N2, the interacting aromatic groups have identical quadrupole moments and the interaction is repulsive in the face-to-face arrangement.
“…Indeed, this is the result provided by molecular mechanics and molecular dynamics simulations conducted in vaccuum, where the long-range nonbonded interactions are accounted for by Coulombic, dipolar, and dispersion contributions. The results obtained in the present NMR study for dilute solutions, however, are in plain contradiction with these predictions and raise serious doubts concerning the validity of standard molecular modeling and dynamics simulations of isolated molecules for rationalizing intramolecular complex formation and conformational behavior in systems containing aromatic groups. , …”
Section: Resultscontrasting
confidence: 91%
“…This effect is not possible for 1,3-diacetoxy-2-methylpropane ( M2 ) and is reflected in the much higher relative energies (∼+4 kJ mol -1 ) for the corresponding U-shaped conformers. Similar results have been obtained previously in molecular modeling studies of polymers and model compounds and have been used to rationalize the degree of excimer formation observed in fluorescence experiments. , 2 Schematic representations of the structures of U-shaped conformers of AN .…”
Proton nuclear magnetic resonance spectroscopy has been used to investigate intramolecular interactions between different aromatic groups in a series of diesters. The materials investigated comprise two aromatic groups linked by a 2-methyl-1,3-propanedioxy spacer. This spacer permits U-shaped conformations which place the two terminal aromatic groups close together, parallel in a face-to-face arrangement. For the symmetrical diesters, 1,3-bis(9-anthracenecarbonyloxy)-2-methylpropane (A2) and 1,3-bis[(3,5-dinitrobenzoyl)-oxy]-2-methylpropane (N2), neither the chemical shifts of the aromatic group protons nor the vicinal coupling constants measured in the spacer provide any evidence for a high fraction of U-shaped conformers. In both cases, the conformational distribution of the spacer is similar to that found for 1,3-diacetoxy-2-methylpropane (M2), indicating that the planar aromatic groups in A2 and N2 experience no significant mutual attractive interactions. In contrast, substantial upfield shifts are observed in the resonance frequencies of all aromatic protons in the anthacenyl and 3,5-dinitrophenyl groups of the unsymmetrical diester, 1-(9-anthracenecarbonyloxy)-3-[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (AN), relative to those for the aromatic protons of the respective monoesters and symmetrical diesters. Analysis of the temperature dependence of the vicinal coupling constants indicates highly populated gauche states of the two central C-C bonds of the spacer chain, consistent with a total fraction of U-shaped conformers of about 80% at ambient temperature. Transient nuclear Overhauser effect experiments yield values of between 0.4 and 0.6 nm for average distances between protons on the dinitrophenyl ring and those on the anthracenyl ring, implying a distance of approximately 0.3 nm between the planes of the two aromatic groups. The stabilization of the U-shaped conformers in AN is rationalized in terms of quadrupole interactions between the two aromatic groups. The quadrupole moments associated with the two aromatic groups in AN have opposite sign, resulting in a significant attractive interaction when the groups are oriented face-to-face. For the symmetrical diesters, A2 and N2, the interacting aromatic groups have identical quadrupole moments and the interaction is repulsive in the face-to-face arrangement.
“…This effect is not possible for 1,3-diacetoxy-2-methylpropane ( M2 ) and is reflected in the much higher relative energies (∼+4 kJ mol -1 ) for the corresponding U-shaped conformers. Similar results have been obtained previously in molecular modeling studies of polymers and model compounds and have been used to rationalize the degree of excimer formation observed in fluorescence experiments. , 1 Molecular structures of the diesters and monoesters investigated. 2 Schematic representations of the structures of U-shaped conformers of AN .…”
Proton nuclear magnetic resonance spectroscopy has been used to investigate intramolecular
interactions between different aromatic groups in a series of diesters. The materials investigated comprise
two aromatic groups linked by a 2-methyl-1,3-propanedioxy spacer. This spacer permits U-shaped conformations
which place the two terminal aromatic groups close together, parallel in a face-to-face arrangement. For the
symmetrical diesters, 1,3-bis(9-anthracenecarbonyloxy)-2-methylpropane (A2) and 1,3-bis[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (N2), neither the chemical shifts of the aromatic group protons nor the vicinal coupling
constants measured in the spacer provide any evidence for a high fraction of U-shaped conformers. In both
cases, the conformational distribution of the spacer is similar to that found for 1,3-diacetoxy-2-methylpropane
(M2), indicating that the planar aromatic groups in A2 and N2 experience no significant mutual attractive
interactions. In contrast, substantial upfield shifts are observed in the resonance frequencies of all aromatic
protons in the anthacenyl and 3,5-dinitrophenyl groups of the unsymmetrical diester, 1-(9-anthracenecarbonyloxy)-3-[(3,5-dinitrobenzoyl)oxy]-2-methylpropane (AN), relative to those for the aromatic protons of the
respective monoesters and symmetrical diesters. Analysis of the temperature dependence of the vicinal coupling
constants indicates highly populated gauche states of the two central C−C bonds of the spacer chain, consistent
with a total fraction of U-shaped conformers of about 80% at ambient temperature. Transient nuclear Overhauser
effect experiments yield values of between 0.4 and 0.6 nm for average distances between protons on the
dinitrophenyl ring and those on the anthracenyl ring, implying a distance of approximately 0.3 nm between
the planes of the two aromatic groups. The stabilization of the U-shaped conformers in AN is rationalized in
terms of quadrupole interactions between the two aromatic groups. The quadrupole moments associated with
the two aromatic groups in AN have opposite sign, resulting in a significant attractive interaction when the
groups are oriented face-to-face. For the symmetrical diesters, A2 and N2, the interacting aromatic groups
have identical quadrupole moments and the interaction is repulsive in the face-to-face arrangement.
“…David et al 25 concluded therefore that excimer formation must be due, in P(ACE), to the interaction of next‐to‐nearest neighbours, which was supported by later experimental work 26, 27. On the other hand, molecular modelling28 and excited‐state semi‐empirical calculations29 on acenaphthylene diads has suggested the possibility that despite the expected rigidity of the P(ACE) structure, nearest neighbour interactions should not be ignored in describing excimer formation in P(ACE).…”
The rate and efficiency of energy migration in a series of polymers containing the acenaphthylene chromophore were investigated. Both poly(acenaphthylene) homopolymer [P(ACE)] and an acenaphthylene/maleic anhydride copolymer [P(ACE/MA)] were studied by time-resolved fluorescence methods. Magic-angle fluorescence decays and time-resolved fluorescence anisotropy measurements on these polymers were carried out in low-temperature glasses and analysed in terms of rapid energy migration occurring between the ACE chromophores. In the case of P(ACE) the rate of complete fluorescence depolarisation was found to be greater than 2 × 10 10 s −1 , suggesting extremely rapid excitation energy migration. The rate of energy migration was slowed significantly to ∼3.8 × 10 9 s −1 in the case of P(ACE/MA). These findings are supported by analysis of the magic-angle fluorescence decays analysed using the model developed by Fredrickson and Frank.
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