Intramolecular proton transfer in 2-(2 '-hydroxyphenyl)benzoxazole: the reliability of ab initio calculations on simplified structures

It is common practice in Computational Chemistry to model the behaviour of bulky compounds from simplified structures. This procedure enables the use of higher computational levels, with generally improved results. The most c rucial problem in this methodology is choosing how simple the structures in question should be, in fact, oversimplification in this context can lead t o a highly different behaviour relative to the starting one. In order to ch eck how valid this approach is, in this work we meddled intramolecular prot on transfer in 2-(2'-hydroxyphenyl)benzoxazole in its ground and first exci ted singlet states from structures simplified to a variable extent. Ab init io calculations at the HF/3-21G level in the ground state and the CIS/3-21G level in the excited state included geometric optimization of potential ta utomers and the intervening transition states. The results obtained reveal that the structure of the original compound must be simplified carefully if spurious conclusions are to be avoided. (C) 1999 Elsevier Science B.V. All rights reserved.