Red‐ and Blue‐shifting Hydrogen Bonds of Cyclic Ketones of Varying Ring Sizes with HF and CHF₃: Comparison of the Results of B3LYP, O3LYP, and M06‐2X Functionals

The density functionals B3LYP, O3LYP, and M06-2X are employed to characterize and compare the properties of hydrogen bonds (HBs) between cyclic ketones of increasing ring size (cyclopropanone to cyclohexanone) and HF and CHF₃. Herein, HF forms the classical red-shifting HBs whereas CHF₃ forms the unconventional blue-shifting HBs with the cyclic ketones. The red-shifting FH···O HBs are found to be characterized by predominantly shared-shell interaction as contrary to the closed-shell (electrostatic) interaction governing the blue-shifting F₃CH···O HBs. This is seen to be consistent with the dominance of hyperconjugative ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:semantics><mml:mrow><mml:mi>n</mml:mi> <mml:mrow><mml:mo>(</mml:mo> <mml:mi>O</mml:mi> <mml:mo>)</mml:mo></mml:mrow> <mml:mo>→</mml:mo> <mml:msup><mml:mi>σ</mml:mi> <mml:mo>*</mml:mo></mml:msup> <mml:mrow><mml:mo>(</mml:mo> <mml:mrow><mml:mtext>HF</mml:mtext></mml:mrow> <mml:mo>)</mml:mo></mml:mrow> </mml:mrow> <mml:annotation>$n \left(\right. \text{O} \left.\right) \rightarrow \left(\sigma\right)^{\star} \left(\right. \text{HF} \left.\right)$</mml:annotation></mml:semantics> </mml:math> charge transfer) effect over the rehybridization effect underlying the redshifting FH···O HBs, while the reverse situation prevails for blueshifting F₃CH···O HBs. The geometry and electronic structure parameters of the cyclic ketones (HB acceptors) show that the polarity of the carbonyl (CO) moiety can be correlated with the ring size to rationalize the electrostatic stabilization of the HBs. The performance of the three density functionals is compared for characterizing the HBs in combination with the triple-ζ basis set 6-311++g(d,p).