Spectroscopy of H3+ based on a new high-accuracy global potential energy surface

Three states global fittings with improved long range: singlet and triplet states of H

Full dimensional analytical fits of the coupled potential energy surfaces for the three lower singlet and triplet adiabatic states of H are developed, providing analytic derivatives and non-adiabatic coupling matrix elements. The fits are highly accurate and include an improved description of the long range interactions, including new terms for the description of the long range in the diatomic fits and the atom-diatom dissociation channels. The fits are based on the DIM formalism including three body terms in Hamiltonian matrix elements, each of them obeying S₂ permutational symmetry, where the positive charge is placed in either of the three hydrogen atoms, but the full system obeys S₃ permutational symmetry, invariant under all permutations of the nuclei. The ab initio points used in the fitting are obtained from a complete basis set extrapolation, made for all electronic states. Total root mean square errors of the fits are 27 and 12 cm^-1, for the singlet and triplet states, respectively. The errors in the channels are lower than 2 cm^-1 and 6 cm^-1 for the H + H and H⁺ + H₂ channels respectively. The new fits have been used to calculate the rovibrational bound states of the lowest singlet and lowest triplet states showing very good agreement with previous calculations in the literature.

Near-resonant effects in the quantum dynamics of the H + H2 + → H2 + H+ charge transfer reaction and isotopic variants

The non-adiabatic quantum dynamics of the H + H₂ ⁺ → H₂ + H⁺ charge transfer reactions, and some isotopic variants, is studied with an accurate wave packet method. A recently developed 3 × 3 diabatic potential model is used, which is based on very accurate ab initio calculations and includes the long-range interactions for ground and excited states. It is found that for initial H₂ ⁺(v = 0), the quasi-degenerate H₂(v' = 4) non-reactive charge transfer product is enhanced, producing an increase in the reaction probability and cross section. It becomes the dominant channel from collision energies above 0.2 eV, producing a ratio between v' = 4 and the rest of v's, which that increase up to 1 eV. The H + H₂ ⁺ → H₂ ⁺ + H exchange reaction channel is nearly negligible, while the reactive and non-reactive charge transfer reaction channels are of the same order, except that corresponding to H₂(v' = 4), and the two charge transfer processes compete below 0.2 eV. This enhancement is expected to play an important vibrational and isotopic effect that needs to be evaluated. For the three proton case, the problem of the permutation symmetry is discussed when using reactant Jacobi coordinates.

Identifying Molecular Structural Aromaticity for Hydrocarbon Classification

Determination of aromaticity in hydrocarbons may be as simple as determining the average bond length for the molecule of interest. This would greatly assist in classifying the nature of hydrocarbon chemistry, especially for large molecules such as polycyclic aromatic hydrocarbons (PAHs) where today's aromatic classification methods are prohibitively expensive. The average C-C bond lengths for a test set of known aromatic, antiaromatic, and aliphatic cyclic hydrocarbons are computed here, and they show strong delineating patterns for the structural discernment of these aromaticity classifications. Aromatic molecules have average C-C bond lengths of 1.41 Å or less with the largest molecules, PAHs, having the longest average C-C bond lengths; aliphatic species have such lengths of 1.50 Å or more; and antiaromatic species fall between the two. Consequently, a first-order guess as to the aromaticity of a system may simply arise from its geometry. Although this prediction will likely have exceptions, such simple screening can easily classify most cases, and more advanced techniques can be brought to bear on the cases that lie in the boundaries. Benchmarks for hydrocarbons are provided here, but other classes of molecular structural aromaticity likely will have to be defined on an ad hoc basis.

High Accuracy ab Initio Calculations of Rotational-Vibrational Levels of the HCN/HNC System

Highly accurate ab initio calculations of vibrational and rotational-vibrational energy levels of the HCN/HNC (hydrogen cyanide/hydrogen isocyanide) isomerising system are presented for several isotopologues. All-electron multireference configuration interaction (MRCI) electronic structure calculations were performed using basis sets up to aug-cc-pCV6Z on a grid of 1541 geometries. The ab initio energies were used to produce an analytical potential energy surface (PES) describing the two minima simultaneously. An adiabatic Born-Oppenheimer diagonal correction (BODC) correction surface as well as a relativistic correction surface were also calculated. These surfaces were used to compute vibrational and rotational-vibrational energy levels up to 25 000 cm^-1 which reproduce the extensive set of experimentally known HCN/HNC levels with a root-mean-square deviation σ = 1.5 cm^-1. We studied the effect of nonadiabatic effects by introducing opportune radial and angular corrections to the nuclear kinetic energy operator. Empirical determination of two nonadiabatic parameters results in observed energies up to 7000 cm^-1 for four HCN isotopologues (HCN, DCN, H¹³CN, and HC¹⁵N) being reproduced with σ = 0.37 cm^-1. The height of the isomerization barrier, the isomerization energy and the dissociation energy were computed using a number of models; our best results are 16 809.4, 5312.8, and 43 729 cm^-1, respectively.

Towards completing the cyclopropenylidene cycle: rovibrational analysis of cyclic N3+, CNN, HCNN+, and CNC. Phys Chem Chem Phys 2017;19:22860-22869. [PMID: 28812071 DOI: 10.1039/c7cp04257d]

The simple aromatic hydrocarbon, cyclopropenylidene (c-C₃H₂), is a known, naturally-occurring molecule. The question remains as to whether its isoelectronic, cyclic, fellow aromatics of c-N₃⁺, c-CNN, HCNN⁺, and c-CNC^- are as well. Each of these are exciting objects for observation of Titan, and the rotational constants and vibrational frequencies produced here will allow for remote sensing of Titan's atmosphere or other astrophysical or terrestrial sources. None of these four aromatic species are vibrationally strong absorbers/emitters, but the two ions, HCNN⁺ and c-CNC^-, have dipole moments of greater than 3 D and 1 D, respectively, making them good targets for rotational spectroscopic observation. Each of these molecules is shown here to exhibit its own, unique vibrational properties, but the general trends put the vibrational behavior for corresponding fundamental modes within close ranges of one another, even producing nearly the same heavy atom, symmetric stretching frequencies for HCNN⁺ and c-C₃H₂ at 1600 cm^-1. The c-N₃⁺ cation is confirmed to be fairly unstable and has almost no intensity in its ν₂ fundamental. Hence, it will likely remain difficult to characterize experimentally.

Limited rotational and rovibrational line lists computed with highly accurate quartic force fields and ab initio dipole surfaces

In this work, computational procedures are employed to compute the rotational and rovibrational spectra and line lists for H2O, CO2, and SO2. Building on the established use of quartic force fields, MP2 and CCSD(T) Dipole Moment Surfaces (DMSs) are computed for each system of study in order to produce line intensities as well as the transition energies. The computed results exhibit a clear correlation to reference data available in the HITRAN database. Additionally, even though CCSD(T) DMSs produce more accurate intensities as compared to experiment, the use of MP2 DMSs results in reliable line lists that are still comparable to experiment. The use of the less computationally costly MP2 method is beneficial in the study of larger systems where use of CCSD(T) would be more costly.

Full-dimensional quantum calculations of the dissociation energy, zero-point, and 10 K properties of H7+/D7+ clusters using an ab initio potential energy surface

Diffusion Monte Carlo (DMC) and path-integral Monte Carlo computations of the vibrational ground state and 10 K equilibrium state properties of the H7 (+)/D7 (+) cations are presented, using an ab initio full-dimensional potential energy surface. The DMC zero-point energies of dissociated fragments H5 (+)(D5 (+))+H2(D2) are also calculated and from these results and the electronic dissociation energy, dissociation energies, D0, of 752 ± 15 and 980 ± 14 cm(-1) are reported for H7 (+) and D7 (+), respectively. Due to the known error in the electronic dissociation energy of the potential surface, these quantities are underestimated by roughly 65 cm(-1). These values are rigorously determined for first time, and compared with previous theoretical estimates from electronic structure calculations using standard harmonic analysis, and available experimental measurements. Probability density distributions are also computed for the ground vibrational and 10 K state of H7 (+) and D7 (+). These are qualitatively described as a central H3 (+)/D3 (+) core surrounded by "solvent" H2/D2 molecules that nearly freely rotate.

State specific stabilization of H+ + H2(j) collision complexes

Stabilization of H3(+) collision complexes has been studied at nominal temperatures between 11 and 33 K using a 22-pole radio frequency (rf) ion trap. Apparent binary rate coefficients, k(*) = kr + k3[H2], have been measured for para- and normal-hydrogen at number densities between some 10(11) and 10(14) cm(-3). The state specific rate coefficients extracted for radiative stabilization, kr(T;j), are all below 2 × 10(-16) cm(3) s(-1). There is a slight tendency to decrease with increasing temperature. In contrast to simple expectations, kr(11 K;j) is for j = 0 a factor of 2 smaller than for j = 1. The ternary rate coefficients for p-H2 show a rather steep T-dependence; however, they are increasing with temperature. The state specific ternary rate coefficients, k3(T;j), measured for j = 0 and derived for j = 1 from measurements with n-H2, differ by an order of magnitude. Most of these surprising observations are in disagreement with predictions from standard association models, which are based on statistical assumptions and the separation of complex formation and competition between stabilization and decay. Most probably, the unexpected collision dynamics are due to the fact that, at the low translational energies of the present experiment, only a small number of partial waves participate. This should make exact quantum mechanical calculations of kr feasible. More complex is three-body stabilization, because it occurs on the H5(+) potential energy surface.

Chemistry, astronomy and physics of H3+

The great developments in the chemistry, astronomy and physics of H(3)(+) since 2006, which have led to this Royal Society Theo Murphy Meeting, are reviewed.