Connections between the accuracy of rotational constants and equilibrium molecular structures

Abstract

Rotational spectroscopy is the technique of choice for investigating molecular structures in the gas phase. Indeed, rotational constants are strongly connected to the geometry of the molecular system under consideration....

Benchmark Structures and Conformational Landscapes of Amino Acids in the Gas Phase: A Joint Venture of Machine Learning, Quantum Chemistry, and Rotational Spectroscopy

“…Noted is that the evaluation of the α r 's implies anharmonic force field calculations and that the sum appearing in eq 1 (contrary to individual terms) does not involve any resonance issue at the VPT2 level (for details, see, e.g., refs 11, 86, 87). ΔB vib i being a small fraction of the corresponding B e i (typically 0.5%), 88 it can be determined at an affordable level of theory (B3 in the present context) without significantly affecting the accuracy of the resulting vibrational ground-state rotational constant. 11,89 At the same time, inclusion of vibrational corrections is not warranted if the errors on the computed rotational constants are not much lower than 1% (50 MHz for a constant of 5000 MHz).…”

Toward Accurate yet Effective Computations of Rotational Spectroscopy Parameters for Biomolecule Building Blocks

“…As a consequence, its inclusion is warranted only if the error on the computed equilibrium rotational constants is smaller than the expected vibrational contribution. A recent careful analysis of these aspects provides some general hints [ 69 ]. The magnitude of the vibrational contribution B is typically 0.1% to 0.7% that of the corresponding equilibrium rotational constant, with 0.5% being a very robust guess for semi-rigid molecules.…”

“…Based on these considerations, the optimal level of accuracy associated with predicted rotational constants should be close to 0.1% (10 MHz for a constant of 10 GHz). In terms of structural parameters, such an accuracy corresponds to errors smaller than 0.001 Å for typical bond lengths and 0.001 radians (0.05 degrees) for typical valence angles [ 69 ]. This target accuracy can be surely obtained by expensive composite schemes incorporating high excitation orders in the correlation treatment [ 70 ].…”

“…This target accuracy can be surely obtained by expensive composite schemes incorporating high excitation orders in the correlation treatment [ 70 ]. Nevertheless, the models of the ChS family are able to draw closer to such a high accuracy limit [ 51 , 62 , 69 ]. However, for quite large flexible molecules, even this computational level becomes too expensive and one has to rely on density functional theory (DFT).…”

Benchmarking quantum chemical methods for accurate gas-phase structure predictions of carbonyl compounds: the case of ethyl butyrate

“…These challenges target important physical chemical properties, such as van der Waals interactions, hydrogen bonding, and proton transfer, to identify methodological limitations and bottlenecks, and allow to push forward the development of new methodologies and composite schemes. Such projects are crucial to foster collaborations between experimental and theoretical groups and to ensure the cross-talking between different disciplines [9]. Within the studies of small to medium sized molecules (up to 30 heavy atoms), rotational spectroscopy has long manifested itself as the method of choice to characterize the structure and dynamics of isolated systems [10][11][12].…”