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Accurate Gas Phase Basicities of Hydroxyl Radical Modified Purines Estimated by Advanced Quantum Chemistry Methods

Cysewski P., Kozłowska K.

Polish Journal of Chemistry

2008

Vol. 82, nr 12

2255-2268

The gas phase basicities (GPB) of purines and their hydroxyl radical modified analogs were characterized by different methods and diverse basis sets. The macroscopic and microscopic protolytic properties of six derivatives were analyzed in details. Most of studied model purine analogs, namely 8-oxo-adenosine (AB), 8-oxo-guanosine (GA), xanthosine (GB) and fapy-guanosine (GC) have reduced basicities from 2.0 kcal/mol (AB) to 6.4 kcal/mol (GA) compared to non-modified model purine nucleosides. The fapy-adenosine (AC) and 2-OH-adenosine (AA) are characterized by higher basic character compared to non-modified adenosine. Besides, it is presented the detailed analysis of GPB in accuracies estimated by means of HF, MP2, B3LYP, G3MP2 and G3MP2B3 methods. The B3LYP/aug-cc-pvdz approach seems to be the most accurate among studied methods and precise enough for estimation of GPB. However, the microscopic protonation features are much more sensitive to applied method since the difference in energies between some tautomers are often less than 1 kcal/mol with method dependent succession. The correct sequence of neutral and cationic forms may be however obtained using one of the model composite chemistry approaches, e.g. G3MP2B3. In the cases where B3LYP/aug-cc-pvdz and G3MP2B3 methods lead to contradictory predictions of order of neutral or protonated tautomers the latter is suggested to be used in the interpretation of microscopic protonation properties. Nevertheless, if only macroscopic property is necessary the B3LYP/aug-cc-pvdz level is sufficient since it provides GPB values with 1.0 kcal/mol accuracy.

Comparison of hydrogen bonding with gas-phase and broensted basicity of N(1),N(1)-dimethylformamidines. Substituent effects

Raczyńska E.D.

Polish Journal of Chemistry

1998

Vol. 72, nr 7

1215-1227

Substituent effects observed for series of N(1),N(1)-dimethylformamidines in the hydrogen bonding reaction (as delta Delta G-HB with 4-fluorophenol) in non polar solvent (CCl4) have directly been compared with those found in the proton transfer reaction in the gas phase (as delta Delta GB) and polar solvent (as delta Delta G-alc in azeotropic ethanol). Significant differences between the aryl and alkyl subfamilies are observed. For aryl groups almost linear relations are found. There are only slight differences between the transmission of substituent field/inductive and resonance effects to the reaction centre in the hydrogen bonding and proton transfer reactions. For alkyl (simple alkyl, heteroalkyl and arylalkyl) groups no good relations are found. Hydrogen bonding basicity of alkyl groups depends on substituent steric, polarizability and field/inductive effects. Gas-phase basicity depends on substituent polarizability and field/inductive effects and Broensted basicity depends mainly on substituent field/inductive effect.