MP2 and QCISD(T) study on the convergence of interaction energies of weak O–H⋯F–C, C–H⋯O, and C–H⋯F–C hydrogen bridges

Abstract

The equilibrium structures and the hydrogen bond strengths of the weakly bonded systems: H 2O · CH 3F ( 1), CH 4 · H 2O ( 2), CH 4 · CH 3F ( 3), C 2H 4 · CH 3F ( 4), C 2H 2 · CH 3F ( 5), and cyclo-C 3H 5F · cyclo-C 3H 5F ( 6) were investigated by quantum chemical computations employing a series of correlation-consistent basis sets, cc-pVXZ and aug-cc-pVXZ [X = 2(D), 3(T), 4(Q)], and the MP2 and QCISD(T) methods. The binding energies (BE) and the counterpoise corrected binding energies (CPBE) were extrapolated to the one-particle basis set limit by two-point (3, 4) procedures giving the BE(lim) and CPBE(lim) values. It is shown that the simple average of the total energies BE(lim) and CPBE(lim) [Δ E AV(lim)] calculated at the MP2 level with non-augmented basis set differs by less than 3% ( 1a, 1b, 2, 3), and 3–6% ( 4– 6) from the corresponding values obtained with the aug-cc-pVXZ sets. The same is valid for the Δ E AV(lim) values calculated with the QCISD(T) method for which the analogous differences are less than 2%. The MP2 interaction energies are very close to the QCISD(T) ones. With respect to the MP2 data the QCISD(T) BE(lim) and CPBE(lim) values are systematically shifted to a more negative values [by 0.09–0.35 kJ mol −1 (cc-pVXZ) and 0.03–0.43 kJ mol −1 (aug-cc-pVXZ)]. For the majority of the cases studied the counterpoise corrected CPBE values are only useful as an upper limit estimate of the interaction energies. With respect to the extrapolated, limiting values, the use of the 1/2 CP correction for given X provides considerably improved interaction energies. The best compromise between accuracy and computational costs provide the cc-pVTZ and aug-cc-pVDZ basis sets, for which the interaction energies with 1/2 CP corrections (Δ E AV) differ mostly less than 10% (cc-pVTZ) and 15% (aug-cc-pVDZ) from the extrapolated limiting values. The addition of diffuse functions to the cc-pVXZ basis set for X ⩾ 3 improves the Δ E AV only marginally. It is also shown that the C–H⋯F and C–H⋯O bridges of 2– 4 and 6 share structural features with the so-called blue shifted hydrogen bonds, while the C–H⋯F interaction of 5 and the O–H⋯F interaction of 1 display similarities with conventional red shifted systems.