| 1. |
- Brinck, Tore, et al.
(författare)
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Solvation of carbanions in organic solvents : A test of the polarizable continuum model
- 2000
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Ingår i: Journal of Physical Chemistry B. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 104:42, s. 9887-9893
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Tidskriftsartikel (refereegranskat)abstract
- The solvation of carbanions in the solvents N,N-dimethylformamide (DMF) and tetrahydrofuran (THF) has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from present and previously reported electrochemical measurements of reduction potentials of the corresponding radicals. Theoretical solvation energies are obtained from quantum chemical calculations using the polarizable continuum model (PCM). It is found that the solvation energy is relatively independent of molecular size and structure for the saturated carbanions. This indicates that the negative charge is strongly localized to the anionic carbon. The conjugated carbanions have considerably lower absolute solvation energies (\ DeltaG degrees (sol)\) than the saturated carbanions. This is a consequence of the strong delocalization of the negative charge in the former group. The propargyl anion is also found to have a surprisingly low absolute solvation energy. However, high-level quantum chemical calculations show that the electronic structure has large contributions from two different resonance structures, CH=CCH2- and -CH=C=CH2, which results in a significant charge delocalization. There is good agreement between calculated and experimental solvation energies for both the conjugated and nonconjugated primary anions. However, the PCM method consistently underestimates the absolute solvation energies of the secondary and tertiary carbanions. This is attributed to an insufficient treatment of first-layer solvation effects in the method. According to the experimental measurements, the absolute solvation energies are on average 2-3 kcal mol(-1) lower in THF than in DMF. The theoretical data indicate a considerably larger solvent effect, 7-10 kcal mol(-1). The discrepancy between theory and experiment may partly be attributed to the use of a supporting electrolyte in the measurements, but the main cause seems to be that the short-range interaction tendencies of the solvent cannot be Fully characterized by its dielectric constant.
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| 2. |
- Brinck, Tore, et al.
(författare)
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Solvation of sulfur-centered cations and anions in acetonitrile
- 2002
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Ingår i: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 106:37, s. 8827-8833
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Tidskriftsartikel (refereegranskat)abstract
- The solvation of substituted phenylsulfenium ions and thiophenoxides in acetonitrile has been analyzed on the basis of experimental and theoretical data. Experimental solvation energies are obtained from previously reported oxidation and reduction potentials of the corresponding arylthiyl radicals in combination with theoretically calculated ionization potentials and electron affinities at the B3LYP/6-31+G(d) level. These calculations provide a consistent set of values in contrast to the data sets obtained in our previous paper (Larsen et al., J. Am. Chem. Soc. 2001, 123, 1723). The extracted solvation data show the expected substituent dependency for both kinds of ions, i.e., the absolute value of the solvation energy decreases as the charge becomes more delocalized. For the thiophenoxides there is good agreement between the experimental solvation energies and solvation energies computed using the polarizable continuum model (PCM). The solvation of the arylsulfenium ions, is much stronger than predicted by the PCM method. This can be attributed to the formation of a strong covalent bond of the Ritter type between the arylsulfenium ion and one molecule of acetonitrile. When this interaction is included in the solvation energy calculations by means of a combined supermolecule and PCM approach the experimental data are reproduced within a few kcal mol(-1). While the energy difference of the singlet and triplet spin states of the arylsulfenium ions is almost negligible in gas phase, the singlet cation is undoubtedly the dominating species in solution, since the triplet cation lacks the ability to form a covalent bond with acetonitrile.
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| 3. |
- Holm, A. H., et al.
(författare)
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Elucidation of the thermochemical properties of triphenyl- or tributyl-substituted Si-, Ge-, and Sn-centered radicals by means of electrochemical approaches and computations
- 2005
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 127:8, s. 2677-2685
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Tidskriftsartikel (refereegranskat)abstract
- Redox potentials of a number of triphenyl- or tributyl-substituted Si-, Ge-, or Sn-centered radicals, R3M., have been measured in acetonitrile, tetrahydrofuran, or dimethyl sulfoxide by photomodulated voltammetry or through a study of the oxidation process of the corresponding anions in linear sweep voltammetry. For the results pertaining to the Ph3M. series (including literature data for M = C), the order of reduction potentials follows Sn > Ge > C > Si, while for the two oxidation potentials, it is C > Si. The effect of the R group on the redox properties of R3Sn. is pronounced in that the reduction potential is more negative by 490 mV in tetrahydrofuran (390 mV in dimethyl sulfoxide) when R is a butyl rather than a phenyl group. The experimental trends have been substantiated through quantum chemical calculations, and they can be explained qualitatively by considering a combination of effects, such as charge capacity being most pronounced for the heavier elements, resonance stabilization present for the planar Ph3C. and all R3M+, and finally a contribution from solvation. The solvation of R3M- is observed to be relatively strong because of a rather localized negative charge in the pyramidal geometry. However, there is no evidence in the calculations to support the existence of covalent interactions between solvent and anions. The solvation of R3M+ is relatively weak, which may be attributed to the planar geometry around the center atom, leading to more spread out charge than that for a pyramidal geometry. Although the calculated solvation energies based on the polarizable continuum model approach exhibit the expected trends, they are not able to reproduce the experimentally derived values on a detailed level for these types of ions. An evaluation of the general performance of the continuum model is provided on the basis of present and previous studies.
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| 4. |
- Holm, A. H., et al.
(författare)
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Thermochemistry of arylselanyl radicals and the pertinent ions in acetonitrile
- 2003
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 125:8, s. 2148-2157
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Tidskriftsartikel (refereegranskat)abstract
- Reduction and oxidation potentials of a series of parasubstituted phenylselanyl radicals, XC6H4Se*, have been measured using photomodulated voltammetry in acetonitrile. The thermodynamic significance of these data was substantiated through a study of the oxidation process of the pertinent selenolates in linear sweep voltammetry. Both the reduction and the oxidation potentials correlate linearly with the Hammett substituent coefficients sigma and sigma(+) leading in the latter case to slopes, rho(+), of 2.5 and 3.8, respectively. Through comparison of these slopes with those published previously for the O- and S-centered analogues, it is revealed that the pi-interaction becomes progressively smaller as the size of the radical center increases in the order O, S, and Se. Solvation energies of the pertinent selenolates and selanylium ions have been extracted from thermochemical cycles incorporating the measured electrode potentials for XC6H4Se* as well as electron affinities and ionization potentials obtained from theoretical calculations at the B3LYP/6-31+G(d) level. The extracted data show the expected overall substituent dependency for both kinds of ions; that is, the absolute value of the solvation energy decreases as the charge becomes more delocalized. The data have also been compared with solvation energies computed using the polarizable continuum model (PCM). Interestingly, we find that, while the model seems to work well for selenolates, it underestimates the solvation of selanylium ions in acetonitrile by as much as 25 kcal mol(-1). These large deviations are ascribed to the fact that the PCM method does not take specific solvent effects into account as it treats the solvent as a continuum described solely by its dielectric constant. Gas-phase calculations show that the arylselanylium ions can coordinate covalently to one or two molecules of acetonitrile in strong Ritter-type adducts. When this strong interaction is included in the solvation energy calculations by means of a combined supermolecule and PCM approach, the experimental data are reproduced within a few kcal mol-1. Although the energy difference of the singlet and triplet spin states of the arylselanylium ions is small for the gas-phase structures, the singlet cation is undoubtedly the dominating species in solution because the triplet cation lacks the ability to form covalent bonds.
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| 5. |
- Lund, T., et al.
(författare)
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Oxidation potentials of alpha-hydroxyalkyl radicals in acetonitrile obtained by photomodulated voltammetry
- 2001
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 123:50, s. 12590-12595
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Tidskriftsartikel (refereegranskat)abstract
- Oxidation potentials, E-1/2(ox) of alpha -hydroxyalkyl radicals of the type (RRCOH)-R-1-C-2-O-. (denoted (KH)-H-1 .) have been obtained in acetonitrile by photomodulated voltammetry. The values of E-1/2(ox) increase as the R-1 and R-2 groups are changed from alkyl to aryl and, in particular, strong electron-withdrawing functionalities such as CN and CF3. Using rate data available in the literature for the pinacol photoexchange reaction K + (KH.)-H-1 --> KH. + K-1, it is found that as the difference in the standard potential of the ketone K, E-K(o), and the oxidation potential of (KH)-H-1 . (,) E-1/2(ox) increases there is a modest increase in the exchange rate constant, k(ex). This indicates that even if some charge transfer may occur between the hydroxyalkyl radical and the ketone in the transition state, it is certainly not to the extent of a complete electron transfer. If the exchange reaction is treated as a simple hydrogen atom transfer process within the Marcus model, the intrinsic barrier is found to be 8- 13 kcal mol(-1) due to the changes occurring in bonds, hybridizations, and bond an-les. Finally, acid dissociation constants for (KH.)-H-1 are provided by means of a thermochemical cycle.
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| 6. |
- Vigh-Larsen, J. F., et al.
(författare)
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Fitness Characteristics of Elite and Subelite Male Ice Hockey Players: A Cross-Sectional Study
- 2019
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Ingår i: Journal of Strength and Conditioning Research. - 1533-4287. ; 33:9, s. 2352-2360
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Tidskriftsartikel (refereegranskat)abstract
- The purpose was to evaluate fitness profiles in elite (age 23.5 ± 4.4 years) and subelite (age 19.4 ± 3.1 years) male ice hockey players. Twenty teams from the best (n = 164) and second-best (n = 132) Danish ice hockey division were assessed in-season using a field-test battery consisting of off-ice measurements of countermovement jump (CMJ) performance and body composition, as well as performance tests on the ice. These included the submaximal and maximal Yo-Yo intermittent recovery ice hockey tests, level 1 (Yo-Yo IR1-IHSUB and Yo-Yo IR1-IHMAX), the 5-10-5 pro-agility test, and a straight-line sprint test. Elite players were heavier (85.7 ± 8.1 vs. 80.8 ± 10.0 kg, p ≤ 0.05) and had a higher skeletal muscle mass (41.9 ± 3.9 vs. 38.8 ± 4.7 kg, p ≤ 0.05) than subelite players. Moreover, elite players elicited a superior CMJ (50.1 ± 6.1 vs. 44.9 ± 5.4 cm, p ≤ 0.05), agility (4.76 ± 0.17 vs. 4.96 ± 0.22 seconds, p ≤ 0.05), and sprint (4.49 ± 0.16 vs. 4.71 ± 0.19 seconds, p ≤ 0.05) performance. Finally, elite players outperformed subelite players in Yo-Yo IR1-IHSUB (79.7 ± 6.8 vs. 88.0 ± 5.4% HRmax, p ≤ 0.05) and Yo-Yo IR1-IHMAX tests (2,434 ± 414 vs. 1,850 ± 499 m, p ≤ 0.05). Top elite teams performed. 1.1 and 7% better than bottom elite teams on the agility and CMJ test (p ≤ 0.05), whereas differences approached significance for sprint (p = 0.08) and Yo-Yo IR1-IHMAX (p = 0.08) performance in favor of top-tier teams. No differences were observed between forwards and defensemen. In conclusion, elite-level ice hockey requires a high level of fitness in terms of muscle mass and explosive strength, as well as a well-developed high-intensity intermittent exercise capacity. In addition, these demands seem to apply for both forwards and defensemen.
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