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The effect of benzene moiety fused with macrocyclic ring on the proton affinity of crown ethers studied in MS/MS experiment

Frański R., Gierczyk B.

Ars Separatoria Acta

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2008

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No. 6

61-68

EN

In this paper we address the question how the presence of an aromatic moiety affects the proton affinity of crown ethers. In order to compare the proton affinities of crown ethers studied (M), we compared the abundances of protonated crown ethers ([M + H]+ ions) with the abundance of the ion [NH2-B15C5 + H]+ (formally also protonated crown ether). Both [M + H]+ and [NH2-B15C5 + H]+ were formed as a result of decomposition of [NH2-B15C5 + H + M]+. The presence of a benzene moiety fused with a macrocyclic ring strongly decreases the ratio [M + H]+/[NH2-B15C5 + H]+. Thus, the higher ring strain caused by the benzene moiety leads to a substantial lowering of the proton affinity of crown ethers. It is also suggested that for protonated benzocrown ethers the ring strain is partly compensated by the proton-đ interaction. The presence of an NO2 group decreases the electron density on the aromatic ring and, consequently, the proton-đ interaction is suppressed. As expected, the proton affinity of benzo-crown ethers increases with increasing size of their cavity since the ring strain is lower for larger molecules. An unexpectedly high proton affinity of dicyclohexano-18-crown-6 (DC18C6) has been observed.

2

Comparison of the Proton (H+) and Alkali Metal Ion (Li+, Na+ and K+) Binding Affinities of Pyruvate and Oxamate Anions in the Gas Phase. Quantum-Chemical Studies

Duczmal K., Hallmann M., Raczynska E., Gal F., Maria P.

Polish Journal of Chemistry

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2007

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Vol. 81, nr 5-6

1011-1020

EN

Proton affinity (PA), lithium, sodium and potassium cation affinities (CA) in the gas phase were estimated by Gaussian-2 (G2), Möller-Plesset (MP2) methods and/or hybrid density functional theory calculations (B3LYP) for pyruvate and oxamate anions. Comparison of these affinities shows that the COO– groups of both anions have similar basicities. Differences between their calculated PAs as well as between their calculated CAs are not larger than 3 kcal mol–1. Somewhat larger differences are evidenced for the formations of the alkali metal complexes with the cation located between the oxygen atoms of the alfaC=O and COO– groups for which the lithium, sodium and potassium cation affinities are larger for oxamate than for pyruvate (by 40.5 kcal mol–1 at theG2 level and by 60.5 kcal mol–1 at the DFT(B3LYP)/6-31++G** level). This is a consequence of the chelate effect on the cation binding seen on the calculatedmost stable structures of the adducts.

3

Proton affinities of rare gases

Kłobukowski M.

Polish Journal of Chemistry

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1998

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Vol. 72, nr 7S

1472-1478

EN

Density functional theory and large Gaussian basis sets were employed in the studies of the molecular ions HeH(+), NeH(+), and ArH(+). The calculated proton affinities of 42, 49-51, and 90-92 kcal/mol for the three rare gases He, Ne, and Ar agree reasonably well with the experimantal values of 42, 49, and 88 kcal/mol. Spectroscopic constants for the ions were evaluated and compared with other accurate values.

4

Aqueous basicity and proton affinity of flexible carboxybetaines, N+(CH2)nCOO-

Barczyński P., Dega-Szafran Z., Szafran M.

Polish Journal of Chemistry

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1998

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Vol. 72, nr 2

277-283

EN

pKa Values of 22 carboxybetaines in water were determined by potentiometric titration of their hydrohalides with KOH and their proton affinities (PA) in the gas phase were calculated using the AM1 method. The linear dependence of the pKa with 1/n implies that the distance between the two charged centers increases monotonically with the number of the methylene group (n). The calculated Pa value for betaine is close to the experimental value. The acceptable results obtained for the correlation between pKa and PA, gives confidence to predict PA for the unknown betaines.