The basicities of a large number of organic bases and superbases, including nitrogen basic centers in various chemical environments occurring in phosphazenes, amidines, amines, anilines and pyridines, have been studied in acetonitrile by the isodensity polarized continuum model employing two DFT computational schemes differing in the basis sets for final single-point calculations. It turned out that the B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) method serves the purpose giving good agreement for basicities with experiment for both gas phase and acetonitrile solutions treating widely different bonding situations of basic nitrogen atoms on an equal footing. An attempt is made to correlate the experimental pK a (MeCN) values with the proton affinities (PA) in MeCN. The results are less accurate than those achieved by using basicities in acetonitrile. In particular, the PA(MeCN)s frequently failed in reproducing the pK a (MeCN) values in systems possessing multiple intramolecular hydrogen (IMH) bonds formed via corona effects. In such cases the use of basicities is mandatory instead. A useful corollary of these calculations on systems with multiple IMH bonds is that comparison of the theoretical and experimental pK a values can provide an insight into the structure of the most stable conformations in solutions.

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The pKa values of seven novel guanidine derivatives, six of them possessing heteroalkyl substituents capable of forming intramolecular hydrogen bonds, were determined in acetonitrile (MeCN) by using the UV/Vis spectrophotometric titration method. The obtained pKa values range from 24.7 to 27.2. The most basic among the studied guanidines was found to be by ca. 4 pKa units more basic than thewell‐known superbase N1,N1,N3,N3‐tetramethylguanidine (TMG). The trends in the changes in the measured pKa values were compared with the experimental (determined by the extended kinetic method) and theoretical [B3LYP/6‐311+G(2df,p)//B3LYP/6‐31G(d)] gas‐phase proton affinities. It was shown that basicity ordering of the bases with dimethylaminopropyl substituents in acetonitrile follows the trend encountered in the gas phase. However, this is not the case for the methoxypropyl‐substituted guanidines indicating that in these molecules formation of the intramolecular hydrogen bonds is to large extent hindered due to solvation by acetonitrile.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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