Transition state theory is central to our understanding of chemical reaction dynamics. We demonstrate a method for extracting transition state energies and properties from a characteristic pattern found in frequency-domain spectra of isomerizing systems. This pattern-a dip in the spacings of certain barrier-proximal vibrational levels-can be understood using the concept of effective frequency, ω(eff). The method is applied to the cis-trans conformational change in the S1 state of C2H2 and the bond-breaking HCN-HNC isomerization. In both cases, the barrier heights derived from spectroscopic data agree extremely well with previous ab initio calculations. We also show that it is possible to distinguish between vibrational modes that are actively involved in the isomerization process and those that are passive bystanders.

https://science.sciencemag.org/content/sci/350/6266/1338.full.pdf

Spectroscopic characterization of isomerization transition states.

Unravelling the composition and characteristics of gas and dust lost by asymptotic giant branch (AGB) stars is important as these stars play a vital role in the chemical life cycle of galaxies. The general hypothesis of their mass-loss mechanism is a combination of stellar pulsations and radiative pressure on dust grains. However, current models simplify dust formation, which starts as a microscopic phase transition called nucleation. Various nucleation theories exist, yet all assume chemical equilibrium, growth restricted by monomers, and commonly use macroscopic properties for a microscopic process. Such simplifications for initial dust formation can have large repercussions on the type, amount, and formation time of dust. By abandoning equilibrium assumptions, discarding growth restrictions, and using quantum mechanical properties, we have constructed and investigated an improved nucleation theory in AGB wind conditions for four dust candidates, TiO2, MgO, SiO, and Al2O3. This paper reports the viability of these candidates as first dust precursors and reveals implications of simplified nucleation theories. Monomer restricted growth underpredicts large clusters at low temperatures and overpredicts formation times. Assuming the candidates are present, Al2O3 is the favoured precursor due to its rapid growth at the highest considered temperatures. However, when considering an initially atomic chemical mixture, only TiO2-clusters form. Still, we believe Al2O3 to be the prime candidate due to substantial physical evidence in presolar grains, observations of dust around AGB stars at high temperatures, and its ability to form at high temperatures and expect the missing link to be insufficient quantitative data of Al-reactions.

/pdf/developing-a-self-consistent-agb-wind-model-ii-non-classical-3amh30ruj7.pdf

Developing a self-consistent AGB wind model - II. Non-classical, non-equilibrium polymer nucleation in a chemical mixture

This article reviews laser-spectroscopic studies of the structure, energetics, and dynamics of processes involving small polyatomic molecules, particularly acetylene (ethyne, C2H2). The linear, centrosymmetric structure of C2H2 is deceptively simple, given that aspects of its optical spectra and dynamics have proved to be unusually complicated. The article focuses on the ground electronic state of C2H2, where rovibrational eigenstates are only approximately described in normal-mode terms, because intramolecular processes (such as anharmonic mixing, l-type resonances, and Coriolis coupling) introduce extensive global and local perturbations. These tend to spoil quantum numbers and symmetries that are well-defined in low-order basis states. Such effects within the rovibrational energy states of C2H2 are systematically characterized, together with dynamical descriptions in terms of polyad models and insight into photochemical or photophysical processes that may occur at high vibrational energies, without dir...

Spectroscopy and energetics of the acetylene molecule: dynamical complexity alongside structural simplicity

Rotational analyses have been carried out for the overtones of the ν4 (torsion) and ν6 (in-plane cis-bend) vibrations of the AAu1 state of C2H2. The v4+v6=2 vibrational polyad was observed in high-sensitivity one-photon laser-induced fluorescence spectra and the v4+v6=3 polyad was observed in IR-UV double resonance spectra via the ground state ν3 (Σ+u) and ν3+ν4 (Πu) vibrational levels. The structures of these polyads are dominated by the effects of vibrational angular momentum: Vibrational levels of different symmetry interact via strong a-and b-axis Coriolis coupling, while levels of the same symmetry interact via Darling–Dennison resonance, where the interaction parameter has the exceptionally large value K4466=−51.68cm−1. The K-structures of the polyads bear almost no resemblance to the normal asymmetric top patterns, and many local avoided crossings occur between close-lying levels with nominal K-values differing by one or more units. Least squares analysis shows that the coupling parameters change ...

https://ir.nctu.edu.tw:443/bitstream/11536/8473/1/000258336100016.pdf

Darling–Dennison resonance and Coriolis coupling in the bending overtones of the ÃAu1 state of acetylene, C2H2

Isomerization between the cis and trans conformers of the S1 state of acetylene is studied using a reduced dimension discrete variable representation (DVR) calculation. Existing DVR techniques are combined with a high accuracy potential energy surface and a kinetic energy operator derived from FG theory to yield an effective but simple Hamiltonian for treating large amplitude motions. The spectroscopic signatures of the S1 isomerization are discussed, with emphasis on the vibrational aspects. The presence of a low barrier to isomerization causes distortion of the trans vibrational level structure and the appearance of nominally electronically forbidden A1A2←X1Σg+ transitions to vibrational levels of the cis conformer. Both of these effects are modeled in agreement with experimental results, and the underlying mechanisms of tunneling and state mixing are elucidated by use of the calculated vibrational wavefunctions.

Reduced dimension discrete variable representation study of cis-trans isomerization in the S1 state of C2H2.

New laser-induced fluorescence spectra of supersonic jet cooled acetylene (C2H2) in the wavelength region 230–205 nm have led to an improved understanding of the vibrational structure of the A 1Au state. Among the new bands observed are two weak perturbed bands at 46008 cm−1 and 46116 cm−1. Rotational analyses of these bands, together with the corresponding ‘hot’ bands arising from the ground state v4 fundamental, have shown that the upper states have asymmetric top K structure that is unaffected by a axis Coriolis coupling; this means that they do not involve overtones of the low frequency bending vibrations and therefore must be combinations of ag vibrational normal modes. From their positions in the manifold, their vibrational assignments can only be 22 031 0; and 11 0;31 0. These assignments lead to values of x 22, x13, and a revised value for the symmetric CH stretching frequency, ν1 = 2880.5cm−1; this revised value is 160cm−1 lower than the previously accepted value, but consistent with new ab initi...

New vibrational assignments in the Ā1 Au-[Xtilde] 1Σ+ g electronic transition of acetylene, C2H2: the v′1 frequency

Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

The LIF and H-atom action spectra of the A−X transition of acetylene in a supersonic jet were observed in the excitation-energy range of 47 000−50 600 cm-1, where 60 vibrational states were identified in the A state through rotational analyses of respective bands. Most of these states originate from anharmonic couplings of the Franck−Condon-allowed 2m3n states with the 4i6j states where i + j = even. This fact is consistent with the determined rotational constants of respective states whose values are mostly between those of the 2m3n(m ≤ 1, n ≤ 4) and the 41 or 61 state. The line intensities of the H-atom action spectra increase at the higher excitation energies, indicating more efficient predissociation. The lifetimes of respective excited states were evaluated from the observed spectral line widths. In a higher excitation-energy region, the lifetime is shortened from 100 ps at 47 000 cm-1 to 25 ps at 50 600 cm-1, which exceeds the adiabatic dissociation threshold of the A state by 933 cm-1 and is the w...

Nami Yamakita

Papers

Darling–Dennison resonance and Coriolis coupling in the bending overtones of the ÃAu1 state of acetylene, C2H2

New vibrational assignments in the Ā1 Au-[Xtilde] 1Σ+ g electronic transition of acetylene, C2H2: the v′1 frequency

Stretch-bend combination polyads in the Ã1Au state of acetylene, C2H2

Predissociation of Excited Acetylene in the Ã1Au State around the Adiabatic Dissociation Threshold as Studied by LIF and H-Atom Action Spectroscopy

Zeeman quantum beat observed by IR–UV double resonance LIF spectroscopy of acetylene in the Ã1Au3ν3′+ν6′ and 3ν3′+ν4′ungerade vibrational states