Toward extension of the gas-phase basicity scale by novel pyridine containing guanidines

Abstract

The gas-phase basicities (GB) and proton affinities (PA) of six novel guanidine derivatives were measured by the entropy corrected kinetic method. The proton affinities of the dimethylaminopropyl and methoxypropyl guanidine derivatives 8–12, as described earlier in Ref. [5], were refined employing a new set of reference bases and were found to be in good agreement with previously published values. Their GB values were also evaluated. In this way a useful set of potential reference bases with GBs in the range of 1037–1079kJmol−1 was developed allowing their use in further measurements of more basic compounds. The experimentally determined gas-phase basicities correlate well with the pKHB values of the hydrogen bond accepting functional groups within the heteroalkyl side-chain(s). Trends in the changes in the measured GB and PA values for the set of 25 nitrogen bases including the investigated ones were compared with the computed data arrived with the following methods: B3LYP/6-311+G(2df,p)//B3LYP/6-31+G(d,p), MP2/6-311+G(d,p)//B3LYP/6-31+G(d,p), BMK/6-311+G(d,p)//B3LYP/6-31+G(d,p) and M06-2X/6-311+G(d,p)//B3LYP/6-31+G(d,p). It is shown that the B3LYP and BMK functionals slightly overestimate the experimental values for 2-(2-pyridyl)ethyl substituted bases, while the M06-2X, and in particular the MP2 results, are in much better agreement with the absolute GB values.