How accurate is the determination of equilibrium structures for van der Waals complexes? The dimer N2O⋯CO as an example

Abstract

Plausible methods for accurate determination of equilibrium structures of intermolecular clusters have been assessed for the van der Waals dimer N2O•••CO. In order to assure a large initial dataset of rotational parameters we first measured the microwave spectra of the 15 N2O••• 12 CO and 15 N2O••• 13 CO isotopologues, expanding previous measurements. Then, an anharmonic force field was calculated ab initio and a semiexperimental equilibrium structure was determined. The dimer structure was also calculated at the coupled-cluster level of theory using very large basis sets with diffuse functions and counterpoise correction. It was found that the contributions of the diffuse functions and of the counterpoise correction are not additive and do not compensate each other although have almost the same value but opposite signs. The semiexperimental and ab initio structures were found in fair agreement, the equilibrium distance between the center of mass of both monomers being 3.825(13) Å and the intermolecular bond length r(C … O) = 3.300(9) Å. In this particular case, the mass-dependent method did not permit to determine reliable intermolecular parameters. The combination of experimental rotational constants and results of ab initio calculations thus proves very sensitive to examine the accuracy of structural determinations in intermolecular clusters, offering insight for other aggregates.

A New Ab Initio Potential Energy Surface and Rovibrational Spectra for the CO–N2O Complex

“…The obvious discrepancies between the ab initio calculations and the observed results suggested that a reliable PES of CO−N 2 O is required for the computations of rovibrational energy levels. Recently, Demaison et al 12 evaluated reasonable methods to determine the equilibrium structures of the CO−N 2 O system and found that the combination of experimental observations with theoretical results is sensitive to assess the accuracy of structure determinations. They focused only on plausible methods of equilibrium structure determinations without constructing a thorough PES for CO−N 2 O in this theoretical work.…”

Stacked but not Stuck: Unveiling the Role of π→π* Interactions with the Help of the Benzofuran–Formaldehyde Complex

“…Differently, our approach leads to the determination of an accurate equilibrium structure by rigorously correcting experimental rotational constants for computed vibrational contributions (examples for intermolecular complexes are available in refs. [ 20i , 20l , 23 ]). The latter have been obtained from anharmonic computations that are expensive for a system as large as BZF–FA.…”