Theoretical study of free and encapsulated carboxylic acid and amide dimers

The homodimers and heterodimers of four carboxylic acids RCOOH and four amides RCONH 2 , where R = H, methyl, phenyl, p ‐ethylphenylene have been calculated via DFT(M06‐2X), MP2, and CCSD(T) methods in conjunction with the 6‐31G(d,p), 6‐311+G(d,p), aug‐cc‐pVTZ, and aug‐cc‐pVQZ basis sets to find out how the substituent R influences their dimerization energy (Δ E ) and their hydrogen bond length. The calculated Δ E values range from 14 to 17 kcal/mol for the different dimers in the gas phase, with the ordering carboxylic homodimers ≥ amide‐carboxylic dimers > amide homodimers. The M06‐2X/6‐311+G(d,p) geometries and Δ E values are in very good agreement with the corresponding MP2/aug‐cc‐pVQZ and CCSD(T)/aug‐cc‐pVTZ values. Coencapsulation of the dimers was studied to examine its effect on the calculated bond lengths and strength of the hydrogen bonds at the M06‐2X/6‐31G(d,p) level of theory. The experimental and calculated % distributions of the encapsulated dimers, taking into account statistical factors are in good agreement.