It is well known that the relative stabilities of the host guest complexes in solution are due to a combination of steric, electronic and solvent effects [1-2]. In this work the formation of p-tertbutylcalix[ 4]tube (1) and p-H-calix[4]tube (3) complexes with alkali metal cations Na+, K+, Rb+ and Cs+ are studied theoretically. At first, the formation of corresponding complexes in two kinds of complexation mode for a calix[4]tube with an alkali metal cation i.e. poly ether tube (p.e.t.) and benzene rings pocket (b.r.p.) were investigated. The result of calculations in the gas phase showed that in all cases interaction and formation energy values for p.e.t. forms are significantly larger than those for b.r.p. and among the complexes in p.e.t. forms the sodium complexes have maximum amounts of interaction and formation energies. Whereas K+ selectivity is observed for calix[4]tubes in aqueous environments [3], calculations show that calix[4]tubes bind Na+ most strongly in the gas phase. Thus, the K+ selectivity of calix[4]tubes in solution is not an intrinsic property of the calix[4]tube itself. The selectivity is reversed for Na+ and K+ because the K+ leaves much easier the solvents molecules and has a greater chance to go into the calix[4]tubes.
