Host molecules effectively prefer to recognize F ion rather than H2O through the upper rim,
utilizing strong C-H (N-H)...F- interactions. The electropositive field space ("electropositive
field space") at the lower rim of the host molecules can also act as a good binding site for small anions such as F-. The electron withdrawing substituent at the para-position (X) makes the C(N)Ha...F- and C(N)–Ha...O interactions much enhanced. The calculated vibrational frequencies and NMR chemical shifts are consistent with experimental trends.