Communication: Spectroscopic consequences of proton delocalization in OCHCO+

Abstract

Quartic force field-derived vibrational frequencies and spectroscopic constants for the isomeric pair SNO and OSN and isotopologues The Journal of Chemical Physics 143, 084308 (2015) Even though quartic force fields (QFFs) and highly accurate coupled cluster computations describe the OCHCO + cation at equilibrium as a complex between carbon monoxide and the formyl cation, two notable and typical interstellar and atmospheric molecules, the prediction from the present study is that the equilibrium C ∞v structure is less relevant to observables than the saddle-point D ∞h structure. This is the conclusion from diffusion Monte Carlo and vibrational self-consistent field/virtual state configuration interaction calculations utilizing a semi-global potential energy surface. These calculations demonstrate that the proton "rattle" motion (ν 6 ) has centrosymmetric delocalization of the proton over the D ∞h barrier lying only 393.6 cm −1 above the double-well OCHCO + C ∞v minima. As a result, this molecule will likely appear D ∞h , and the rotational spectrum will be significantly dimmer than the computed equilibrium 2.975 D center-of-mass dipole moment indicates. However, the proton transfer fundamental, determined to be at roughly 300 cm −1 , has a very strong intensity. This prediction as well as those of other fundamentals should provide useful guides for laboratory detection of this cation. Finally, it is shown that the two highest energy QFF-determined modes are actually in good agreement with their vibrational configuration interaction counterparts. These high-level quantum chemical methods provide novel insights into this fascinating and potentially common interstellar molecule. C 2015 AIP Publishing LLC. [http://dx

Rovibrational Characterization and Interstellar Implications of the Proton-Bound, Noble Gas Complexes: ArHAr+, NeHNe+, and ArHNe+

“…While QFF data are shown to provide narrow ranges for the position of most fundamental vibrational frequencies, some modes in a selection of these proton-bound complexes are not as reliable. Those modes where the PES is flat or has a double well as present in NNHNN + and OCHCO + , respectively, are troublesome, but nearly all of the other modes are well-described with the QFFs compared to vibrational structure computations with larger PESs. , Furthermore, the molecules where a clearly defined miniumum is present, such as NN–HCO + and CO–HNN + , appear to provide the same, trustworthy QFF behavior and accuracies as the known benchmarks for less exotic systems. , …”

“…One class of molecules whose rovibrational spectral data have been recently characterized through quantum chemical computation is the mass-57 proton-bound complexes: OCHCO + , NNHNN + , NN–HCO + , CO–HNN + , their isotopologues, and some of their third-row analogues. − These molecules are particularly fascinating for astrochemical observation because the “proton rattle” or “proton shuttle” motions are exceptionally bright vibrational modes . The total molecular charge is almost exclusively centered on the proton, yet it contains less than 2% of the total molecular mass.…”

Anharmonic quantum nuclear densities from full dimensional vibrational eigenfunctions with application to protonated glycine

“…This quantity is the nuclear analogue of electron density in Density Functional Theory for electronic structure calculations. The Cartesian coordinate space representation allows for the visualization of probability density isosurfaces in 3D, as for the nuclear ground state distributions obtained from Diffusion Monte Carlo calculations 31,37,[54][55][56][57][58] , and in a similar fashion as routinely done for electron density 59,60 . We are therefore able to represent with nuclear density differences the nuclear motion associated with each peak in vibrational spectra in three-dimensional (3D) space, and to visually spot couplings resulting in specific non-local nuclear density patterns in a quantum mechanical framework.…”

Quantum astrochemical spectroscopy

“…Quantum chemical vibrational treatment requires wave functions composed of 3N26 variables, one for each degree-offreedom. Molecules of five atoms can require thousands of total points in the QFF, [88][89][90] and larger molecules on the scale of even small acenes can require millions. The current, stateof-the-art approaches are attempting to address this issue but with some sacrifice to the accuracy and an increase in the complexity of the methodologies that are employed.…”