Rotational Characterization of the Elusive gauche-Isoprene

Conformational equilibria and interaction preference in the complex of isoprene-maleic anhydride

The rotational spectrum of the isoprene-maleic anhydride complex has been investigated by pulsed jet Fourier transform microwave spectroscopy and interpreted with complementary quantum chemical calculations. Theoretical predictions have yielded four plausible isomers, all residing within an energy window of 12 kJ mol-1. However, two distinct isomers characterized by a π-π stacked configuration have been experimentally observed in pulsed jets, which have differed in the orientation of isoprene over maleic anhydride. The relative population ratio of the two detected isomers has been estimated to be NI/NII ≈ 3/1 from rigorous measurements of the relative intensity on a set of μc-type transitions. Remarkably, this study underscores the pivotal role played by the interaction between the CC bonding orbital (π) of isoprene and the CC antibonding orbital (π*) of maleic anhydride in stabilizing the target complex.

Rotationally Resolved Infrared Spectroscopy of Supersonic Jet-Cooled Isoprene

The high-resolution infrared spectrum of isoprene has been observed under supersonic jet-cooled conditions in the region of the ν₂₆ vibrational band near 992 cm^-1. The spectrum was assigned and fit using a standard asymmetric top Hamiltonian, and an acceptable fit was obtained for transitions to excited state energy levels with J ≤ 6, with an error in the fit of 0.002 cm^-1. For excited state energy levels with J > 6, a perturbation was present that prevented fitting using the standard asymmetric top Hamiltonian. Based on previous anharmonic frequency calculations and observed vibrational bands of isoprene, the perturbation is most likely caused by Coriolis coupling between the ν₂₆ and ν₁₇ vibrations or a combination band that lies near the ν₂₆ band. The excited state rotational constants from the fit show reasonable agreement with previous anharmonic calculations performed at the MP2/cc-pVTZ level of theory. The jet-cooled spectrum is compared with previous high-resolution measurements of this band at room temperature and shows that understanding the perturbation will be necessary to accurately model this vibrational band.

Ring-Opening Dynamics of the Cyclopropyl Radical and Cation: the Transition State Nature of the Cyclopropyl Cation

We provide compelling experimental and theoretical evidence for the transition state nature of the cyclopropyl cation. Synchrotron photoionization spectroscopy employing coincidence techniques together with a novel simulation based on high-accuracy ab initio calculations reveal that the cation is unstable via its allowed disrotatory ring-opening path. The ring strains of the cation and the radical are similar, but both ring opening paths for the radical are forbidden when the full electronic symmetries are considered. These findings are discussed in light of the early predictions by Longuet-Higgins alongside Woodward and Hoffman; we also propose a simple phase space explanation for the appearance of the cyclopropyl photoionization spectrum. The results of this work allow the refinement of the cyclopropane C-H bond dissociation energy, in addition to the cyclopropyl radical and cation cyclization energies, via the Active Thermochemical Tables approach.

High-resolution CH stretch spectroscopy of jet-cooled cyclopentyl radical: First insights into equilibrium structure, out-of-plane puckering, and IVR dynamics

First, high-resolution sub-Doppler infrared spectroscopic results for cyclopentyl radical (C5H9) are reported on the α-CH stretch fundamental with suppression of spectral congestion achieved by adiabatic cooling to Trot ≈ 19(4) K in a slit jet expansion. Surprisingly, cyclopentyl radical exhibits a rotationally assignable infrared spectrum, despite 3N − 6 = 36 vibrational modes and an upper vibrational state density (ρ ≈ 40–90 #/cm−1) in the critical regime (ρ ≈ 100 #/cm−1) necessary for onset of intramolecular vibrational relaxation (IVR) dynamics. Such high-resolution data for cyclopentyl radical permit detailed fits to a rigid-rotor asymmetric top Hamiltonian, initial structural information for ground and vibrationally excited states, and opportunities for detailed comparison with theoretical predictions. Specifically, high level ab initio calculations at the coupled-cluster singles, doubles, and perturbative triples (CCSD(T))/ANO0, 1 level are used to calculate an out-of-plane bending potential, which reveals a C2 symmetry double minimum 1D energy surface over a C2v transition state. The inversion barrier [Vbarrier ≈ 3.7(1) kcal/mol] is much larger than the effective moment of inertia for out-of-plane bending, resulting in localization of the cyclopentyl wavefunction near its C2 symmetry equilibrium geometry and tunneling splittings for the ground state too small (<1 MHz) to be resolved under sub-Doppler slit jet conditions. The persistence of fully resolved high-resolution infrared spectroscopy for such large cyclic polyatomic radicals at high vibrational state densities suggests a “deceleration” of IVR for a cycloalkane ring topology, much as low frequency torsion/methyl rotation degrees of freedom have demonstrated a corresponding “acceleration” of IVR processes in linear hydrocarbons.

Infrared Comb Spectroscopy of Buffer-Gas-Cooled Molecules: Toward Absolute Frequency Metrology of Cold Acetylene

We review the recent developments in precision ro-vibrational spectroscopy of buffer-gas-cooled neutral molecules, obtained using infrared frequency combs either as direct probe sources or as ultra-accurate optical rulers. In particular, we show how coherent broadband spectroscopy of complex molecules especially benefits from drastic simplification of the spectra brought about by cooling of internal temperatures. Moreover, cooling the translational motion allows longer light-molecule interaction times and hence reduced transit-time broadening effects, crucial for high-precision spectroscopy on simple molecules. In this respect, we report on the progress of absolute frequency metrology experiments with buffer-gas-cooled molecules, focusing on the advanced technologies that led to record measurements with acetylene. Finally, we briefly discuss the prospects for further improving the ultimate accuracy of the spectroscopic frequency measurement.

Binding Site Switch by Dispersion Interactions: Rotational Signatures of Fenchone-Phenol and Fenchone-Benzene Complexes

Non-covalent interactions between molecules determine molecular recognition and the outcome of chemical and biological processes. Characterising how non-covalent interactions influence binding preferences is of crucial importance in advancing our understanding of these events. Here, we analyse the interactions involved in smell and specifically the effect of changing the balance between hydrogen-bonding and dispersion interactions by examining the complexes of the common odorant fenchone with phenol and benzene, mimics of tyrosine and phenylalanine residues, respectively. Using rotational spectroscopy and quantum chemistry, two isomers of each complex have been identified. Our results show that the increased weight of dispersion interactions in these complexes changes the preferred binding site in fenchone and sets the basis for a better understanding of the effect of different residues in molecular recognition and binding events.

Decomposition of the simplest ketohydroperoxide in the ozonolysis of ethylene

Hydroperoxides from the ozonolysis of alkenes, in addition to Criegee intermediates, have been proposed as an atmospheric source of OH radicals in the absence of sunlight, but have remained largely elusive due to their reactivity. A weak peroxide bond enables facile OH elimination, and subsequent β-scission can lead to a variety of decomposition products depending on the nature of the peroxide. In this paper we explore this process theoretically for the simplest ketohydroperoxide, hydroperoxyacetaldehyde (HPA), which is believed to be formed in the ozonolysis of ethylene. Despite it being the most stable C₂H₄O₃ species in this reaction scheme, lower in energy than the starting materials by around 100 kcal mol^-1, HPA has only been directly observed once in the ozonolysis of ethylene by photoionization mass spectrometry appearance energy. Here we report predictions of the rotational spectrum of HPA conducted in support of microwave spectroscopy experiments. We suggest a new dissociation path from HPA to glyoxal [HOOCH₂CHO → HCOCH₂O + OH → CHOCHO + H], supported by thermochemical calculations. We encourage the search for glyoxal using complementary experimental methods, and suggest possible future experimental directions. Evidence of glyoxal formation from ethylene ozonolysis might provide evidence of this underappreciated path in an important and long studied reaction system.