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)-HaF- and C(N)-HaO interactions much enhanced. The calculated vibrational frequencies and NMR chemical shifts are consistent with experimental trends.