Your clear and straightforward comments are highly appreciated.

Saeed

As long as the studied reaction involves change of weak interaction, one should pay attention to BSSE problem. If the basis set is sufficiently large such as def2-QZVP, or 3-zeta basis set with rich diffuse functions such as aug-cc-pVTZ, then correction for BSSE effect is not quite needed. In the present situations, counterpoise correction evidently complicates the study, so I recommend simply using def2-QZVP or aug-cc-pVTZ and then simply ignored the annoying BSSE problem.

Dear Tian,
Too many thanks for your highly valuable and informative comments. Please, also, let me explain another issue. Indeed, if these exchange reaction is considered as follows:

Acroleine-BF3 complex...nitrone---->nitrone-BF3 complex...Acroleine

Then, BSSE correction is unavoidable. In other words, all reactants and products should be considered as whole complex very similar to what happen during an SN2 reaction. I think the provided exchange reaction as portrayed in the first question does not need BSSE correction whether a large or a small basis set is employed.

Sincerely,
Saeed

If def2-QZVP or aug-cc-pVTZ is used, BSSE problem is small enough to be safely ignored.

Dear Tian,
Please assume BF3 interacts with acroleine as a dipolarophile to form acroleine-BF3 complex. Then, a 1,3dipole such as simplest nitrone is added to this complex so a 1,3DC reaction takes place. Evidently, in addition to the 1,3DC reaction, an exchange reaction can compete in which BF3 is separated from acroleine and bound to nitrone as:

Acroleine-BF3 complex+nitrone--->nitrone-BF3 complex+Acroleine

The challenging issue is that this exchange reaction really needs BSSE correction?

Sincerely,
Saeed