π Interactions Studied with Electronic Structure Methods: The Ethyne Methyl Isocyanide Complex and Thioanisole

Abstract

Two molecular systems for which previous studies had found qualitative differences in the results from calculations with the B3LYP and MP2 methods are investigated with a range of different electronic structure methods, including meta and double hybrid density functionals and DFT-D (DFT augmented with an empirical dispersion term). The performance of the different methods is assessed by comparison to estimated CCSD(T)/CBS (complete basis set) results. The first molecular system studied is the ethyne methyl isocyanide complex (CH3NC-C2H2), which exhibits π hydrogen bonds involving the C≡C and N≡C triple bonds. Earlier work on this system had shown that B3LYP predicts significantly longer hydrogen-bond distances than MP2. Here, we show that this is likely due to missing dispersion in the B3LYP calculations. On the basis of the CCSD(T) results, the ethyne methyl isocyanide interaction energy is estimated to be 12 ± 1 kJ/mol. B3LYP significantly underestimates the stability of the complex, whereas MP2 slightly overestimates. M05-2X, B3LYP-D, and (CP-corrected) mPW2-PLYP-D give results in close proximity to the CCSD(T) reference values. The second molecule investigated is thioanisole (C6H6SCH3), which can adopt two different conformations (thiomethyl group either planar or perpendicular with respect to the benzene ring). Potential energy curves for rotation around the C(sp(2))-S bond are computed and compared to the estimated CCSD(T)/CBS curve. CCSD(T) predicts the planar conformation to be the global minimum, with a plateau region near the perpendicular conformation (∼4 kJ/mol higher in energy than the planar conformation). The shape of the curve, and location of minima and barriers, is very dependent on the method and basis set employed. MP2, B3LYP, M05-2X, mPW2-PLYP, and mPW2-PLYP-D (employing basis sets of double- or triple-ζ quality) give results in reasonable agreement with the CCSD(T) results, whereas B3LYP-D and M06-L give vastly overestimated barriers at the perpendicular conformation.

Dispersion Interactions with Density-Functional Theory: Benchmarking Semiempirical and Interatomic Pairwise Corrected Density Functionals

“…Thus, the M06 functionals do indeed capture more of the correlation relevant for weak, nonlocal interactions than that captured by the standard semilocal and hybrid functionals. This agrees well with previous reports of their success in describing weakly bound systems. − Furthermore, comparison of the M06-L results with those of the nonempirical TPSS meta-GGA demonstrate unequivocally that the M06 approach owes its success to the empirical construction rather than to the meta-GGA construct. In fact, the TPSS results are markedly worse than those of M06-L and are surprisingly close to those of B3LYP, despite the considerable differences between these two functionals.…”

“…Both strategies have been put to extensive use in the past few years for accurate structural predictions of various properties of dispersively bound systems (see, e.g., refs − ). Given the significant differences between them, it is important to assess the level of accuracy that can be expected from both.…”

Applications and validations of the Minnesota density functionals

“…Bretherick and van Mourik [224] examined the complex of ethyne with methyl isocyanide and the conformations of thioanisole, both of which had earlier shown difference between DFT and WFT. In both cases M05-2X was in good agreement with the best WFT results, although M06-L was found to be inaccurate in the second case.…”

Does hydrohalic acid HX (X = F, Cl) form true N-protonated twisted amide salts? Effects of anions on the ion-pair interactions and on the amide moiety in N-protonated tricyclic twisted amide salts

“…Accurate structural predictions of various properties of dispersive bound systems have been extensively studied. [44][45][46][47][48][49][50][51][52][53][54][55] In the present study, I report for the first time (i) the accurate optimized geometries of N-protonated tricyclic twisted amide salts (I-VI), (ii) the interactions between cation-anion pairs, (iii) the effects of nonbonded interactions between ionpairs on the nature of N-HÁ Á ÁF, N-HÁ Á ÁO, NÁ Á ÁH-F and NÁ Á ÁH-F hydrogen bonding in amide salts (I, X = BF 4 À , II, X = OTs À , III, X = MeSO 3 À , IV, X = HSO 3 À , V, X = F À , VI, X = Cl À ). I have tried to explain that the hydrohalic acid HX (X = F, Cl) does not form true N-protonated twisted amide salts.…”