The word "rationalistical" is pronounced as /ˌræʃənəlɪsˈtɪkəl/. It is derived from the word "rational" and the suffix "-istical" which means "relating to or characterized by". The spelling of this word is based on the phonetic sounds of its syllables. The "a" sound in the first syllable is followed by an "i" which creates the long "a" sound. The stress is on the second syllable, which is pronounced as "lis". Overall, the word means relating to rationality or being based on reason.
Pert. to or resembling rationalism.
Etymological and pronouncing dictionary of the English language. By Stormonth, James, Phelp, P. H. Published 1874.