Overcoming the Failure of Correlation for Out-of-Plane Motions in a Simple Aromatic: Rovibrational Quantum Chemical Analysis ofc-C₃H₂

Abstract

Truncated, correlated, wave function methods either produce imaginary frequencies (in the extreme case) or nonphysically low frequencies in out-of-plane motions for carbon and adjacent atoms when the carbon atoms engage in π bonding. Cyclopropenylidene is viewed as the simplest aromatic hydrocarbon, and the present as well as previous theoretical studies have shown that this simple molecule exhibits this behavior in the two out-of-plane bends (OPBs). This nonphysical behavior has been treated by removing nearly linear dependent basis functions according to eigenvalues of the overlap matrix, by employing basis sets where the spd space saturatation is balanced with higher angular momentum functions, by including basis set superposition/incompleteness error (BSSE/BSIE) corrections, or by combining standard correlation methods with explicitly correlated methods to produce hybrid potential surfaces. However, this work supports the recently described hypothesis that the OPB problem is both a method and a basis set effect. The correlated wave function's largest higher-order substitution term comes from a π → π* excitation where constructive interference of both orbitals artificially stabilizes the OPB. By employing schema to overcome this issue, the symmetric OPB ν is the predicted to be the second-brightest transition, and it will be observed very close to 775 cm. However, more work from the community is required to formulate better how carbon atoms interact with their adjacent atoms in π-bonded systems. Such bonds are ubiquitous in all of chemistry and beyond.

Fundamental Vibrational Frequencies and Spectroscopic Constants of Substituted Cyclopropenylidene (c-C3HX, X = F, Cl, CN)

“…Because of their larger magnitudes, computed A rotational constants are known to experience larger absolute errors than the B and C constants . This trend is similar to that observed in a previous study on c-C 3 H 2 itself, in which the rotational constants were found to agree very well with experiment, suggesting that F12-TZ QFFs may effectively handle the rotational spectra of c-C 3 H 2 and its related molecules …”

“…Larger step sizes of 0.010 and 0.020 Å or rad are also used in the out-of-plane symmetry internal coordinates in order to alleviate potential problems brought about by known issues with out-of-plane bending (OPB) motions in hydrocarbons, and in c-C 3 H 2 in particular. − Step sizes of 0.010 and 0.020 have been previously shown to mitigate this problem for the related c-(CH)­C 3 H 2 + molecule and for c-C 3 H 2 itself. , The out-of-plane coordinates correspond to the torsional coordinates S 10 , S 11 , and S 12 in c-C 3 HCN and to S 8 and S 9 in c-C 3 HF and c-C 3 HCl, respectively.…”

How do the Hückel and Baird Rules Fade away in Annulenes?

“…Both I ring and MCI are among the less fallible aromaticity indices available in the literature [2,36,45] and, therefore, they have been used in a plethora of cases involving a difficult assessment of aromaticity [46][47][48][49][50][51][52][53][54]. However, these indices present some drawbacks that prevent their use in large rings [55] and, therefore, we have recently designed [55] and tested [40,56] a new electronic aromaticity index, AV1245, based on MCI but free of the shortcomings of this index.…”

The performance of explicitly correlated methods for the computation of anharmonic vibrational frequencies

“…Such has been proposed as a means of computing vibrational frequencies and other spectroscopic values, but results have been mixed. However, recent work has shown that CCSD(T)‐F12b with a triple‐zeta basis set may achieve a nice Pauling point for a cancellation of errors generating fundamental vibrational frequencies that are closely coincident with much more costly CcCR QFF VPT2 results . However, the reliability of such an overlap has not been systematically documented.…”