Temperature Dependence of the Electronic Absorption Spectrum of NO2.

The nitrogen dioxide (NO2) radical is composed of the two most abundant elements in the atmosphere, where it can be formed in a variety of ways including combustion, detonation of energetic materials, and lightning. Relevant also to smog and ozone cycles, together these processes span a wide range of temperatures. Remarkably, high-resolution NO2 electronic absorption spectra have only been reported in a narrow range below about 300 K. Previously, we reported [ J. Phys. Chem. A 2021, 125, 5519-5533] the construction of quasi-diabatic potential energy surfaces (PESs) for the lowest four electronic states (X̃, Ã, B̃, and C̃) of NO2. In addition to three-dimensional PESs based on explicitly correlated MRCI(Q)-F12/VTZ-F12 ab initio data, the geometry dependence of each component of the dipoles and transition dipoles was also mapped into fitted surfaces. The multiconfigurational time-dependent Hartree (MCTDH) method was then used to compute the 0 K electronic absorption spectrum (from the ground rovibrational initial state) employing those energy and transition dipole surfaces. Here, in an extension of that work, we report an investigation into the effects of elevated temperature on the spectrum, considering the effects of the population of rotationally and vibrationally excited initial states. The calculations are complemented by new experimental measurements. Spectral contributions from hundreds of rotational states up to N = 20 and from 200 individually-characterized vibrational states were computed. A spectral simulation tool was developed that enables modeling the spectrum at various temperatures─by weighting individual spectral contributions via the partition function, or for pure excited initial states, which can be probed via transient absorption spectroscopy. We validate these results against experimental absorption spectroscopy data at high temperatures, as well as via a new measurement from the (1,0,1) initial vibrational state.

Photoelectron Spectroscopy of Biacetyl and Its Cluster Anions.

Photoelectron spectroscopy of the biacetyl (dimethylglyoxal) anion reveals the properties of the ground singlet and lowest triplet electronic states of the neutral biacetyl (BA) molecule. Due to the broad and congested nature of the singlet transition, which peaks at a vertical detachment energy VDE = 1.12(5) eV, only an upper bound of the adiabatic electron affinity of BA could be determined: EA(BA) < 0.7 eV. A narrower and more structured triplet band peaking at VDE = 3.17(2) eV reveals the adiabatic electron binding energy of the triplet to be 3.05(2) eV. These results are in good agreement with ab initio (coupled-cluster) calculations. The lowest-energy structures of the anion, singlet, and triplet states of biacetyl are characterized by different orientations of the methyl groups within the molecular frame. In the ground singlet state of neutral BA, the methyl torsion is offset by ∼60° compared to that of the anion, while in the triplet the methyl orientation is similar to that of the anion. Photoelectron spectra of the cluster anions reveal that the intermolecular interactions in the homogeneously solvated (BA) n- clusters are significantly stronger than the interactions of BA- with N2O or even of BA- with H2O. To account for these observations, π-π bonded structures of the dimer and trimer anions of biacetyl are proposed based on density-functional theory calculations. The analysis of the proposed structures indicates that the negative charge in the (BA) n- cluster anions, at least in the dimer and the trimer, is significantly delocalized between all BA moieties present and there is a significant degree of covalent bonding within the cluster.

Aromatic Stabilization and Hybridization Trends in Photoelectron Imaging of Heterocyclic Radicals and Anions.

We examine the photoelectron spectra and laboratory-frame angular distributions in the photodetachment of furanide (C4H3O(-)), thiophenide (C4H3S(-)), and thiazolide (C3H2NS(-)) and compare the results to the previously reported studies of pyridinide (C5H4N(-)) and oxazolide (C3H2NO(-)). Using the mixed s-p model for the angular distributions, the results are interpreted in terms of the effective fractional p character of the highest-occupied molecular orbitals of these heterocyclic anions, revealing trends related to the aromaticity. We conclude that aromatic stabilization across a series of systems may be tracked using the photoelectron angular distributions. In addition, we report an improved (higher-precision) electron affinity (EA) for the thiophenyl radical, EA((•)C4H3S) = 2.089(8) eV. The EA of thiazolyl falls within the 2.5(1) eV range, but it is not clear if this determination corresponds to the 2- or 5-cyclic species or the 2-ring-open isomer. These results are analyzed in conjunction with the properties of other heterocyclic radicals (pyridinyl, furanyl, and oxazolyl) and interpreted in terms of the C-H bond dissociation energies (BDEs) of the corresponding closed-shell molecules. The BDEs of all five-membered-ring heterocyclics studied fall within the 116-120 kcal/mol range, contrasting the lower BDE = 110.4(2.0) kcal/mol of the more aromatic six-membered-ring pyridine. The observed aromaticity trends are consistent with the findings derived from the anion photoelectron angular distributions.

Photoelectron angular distributions for states of any mixed character: an experiment-friendly model for atomic, molecular, and cluster anions.

We present a model for laboratory-frame photoelectron angular distributions in direct photodetachment from (in principle) any molecular orbital using linearly polarized light. A transparent mathematical approach is used to generalize the Cooper-Zare central-potential model to anionic states of any mixed character. In the limit of atomic-anion photodetachment, the model reproduces the Cooper-Zare formula. In the case of an initial orbital described as a superposition of s and p-type functions, the model yields the previously obtained s-p mixing formula. The formalism is further advanced using the Hanstorp approximation, whereas the relative scaling of the partial-wave cross-sections is assumed to follow the Wigner threshold law. The resulting model describes the energy dependence of photoelectron anisotropy for any atomic, molecular, or cluster anions, usually without requiring a direct calculation of the transition dipole matrix elements. As a benchmark case, we apply the p-d variant of the model to the experimental results for NO(-) photodetachment and show that the observed anisotropy trend is described well using physically meaningful values of the model parameters. Overall, the presented formalism delivers insight into the photodetachment process and affords a new quantitative strategy for analyzing the photoelectron angular distributions and characterizing mixed-character molecular orbitals using photoelectron imaging spectroscopy of negative ions.

Spectroscopy of the breaking bond: the diradical intermediate of the ring opening in oxazole.

Bond breaking is a challenging problem in both experimental and theoretical chemistry, due to the transient nature and multi-configurational electronic structure of dissociating molecules. We use anion photodetachment to probe the diradical interactions in the ring-opening reaction of oxazole and obtain a self-consistent picture of the breaking bond. Starting from the closed-shell cyclic molecule, the reaction is launched on the anion potential, as an attached electron cleaves a carbon-oxygen bond. In the photodetachment, two neutral potential regions are sampled. One corresponds to a completely dissociated bond, while the other - to the bond fragments separated by approximately 3 Å. At this chemically relevant distance, signatures of lingering through-space interactions between the radical centers are observed.

Selective deprotonation of oxazole and photoelectron imaging of the oxazolide anion.

A photoelectron imaging study of the oxazolide anion obtained by selective deprotonation of oxazole at the C2 position is reported. The photodetachment transitions observed at 355, 392, and 532 nm are assigned to the ground state of the neutral oxazolyl σ radical. A Franck-Condon analysis of this transition aids in the first determination of the adiabatic electron affinity of oxazolyl, EA = 2.21 ± 0.02 eV. A vibrational progression with a frequency of 890 ± 80 cm(-1) is observed, corresponding to an in-plane ring distortion mode. The photoelectron angular distributions are analyzed using the mixed s-p model, shedding light on the hybrid character of the anion HOMO.