Experimental and Theoretical Electron Momentum Spectroscopic Study of the Valence Electronic Structure of Tetrahydrofuran under Pseudorotation

Abstract

The most populated structure of tetrahydrofuran (THF) has been investigated in our previous study using electron momentum spectroscopy (EMS). Because of the relatively low impact energy (600 eV) and low energy resolution (DeltaE = 1.20 eV) in the previous experiment, only the highest occupied molecular orbital (HOMO) of THF was investigated. The present study reports the most recent high-resolution EMS of THF in the valence space for the first time. The binding energy spectra of THF are measured at 1200 and 2400 eV plus the binding energies, respectively, for a series of azimuthal angles. The experimentally obtained binding energy spectra and orbital momentum distributions (MDs) are employed to study the orbital responses of the pseudorotation motion of THF. The outer valence Greens function (OVGF), the OVGF/6-311++G** model, and density function theory (DFT)-based SAOP/et-pVQZ model are employed to simulate the binding energy spectra. The orbital momentum distributions (MDs) are produced using the DFT-based B3LYP/aug-cc-pVTZ model, incorporating thermodynamic population analysis. Good agreement between theory and experiment is achieved. Orbital MDs of valence orbitals exhibit only slight differences with respect to the impact energies at 1200 and 2400 eV, indicating validation of the plane wave impulse approximation (PWIA). The present study has further discovered that the orbital MDs of the HOMO in the low-momentum region (p < 0.70 a.u) change significantly with the pseudorotation angle, phi, giving a v-shaped cross section, whereas the innermost valence orbital of THF does not vary with pseudorotation, revealing a very different bonding mechanism from the HOMO. The present study explores an innovative approach to study pseudorotation of sugar puckering, which sheds a light to study other biological systems with low energy barriers among ring-puckering conformations.

A dynamical (e,2e) investigation of the structurally related cyclic ethers tetrahydrofuran, tetrahydropyran, and 1,4-dioxane

“…[32][33][34][35] On the one hand, THF has a relatively flat 5-member ring that possesses puckering out of the planar configuration, with the flatness of this ring structure producing a number of conformations that lie close in energy. Further, low potential energy barriers for pseudo-rotation between its conformations enable essentially free rotation between minima.…”

“…Further, low potential energy barriers for pseudo-rotation between its conformations enable essentially free rotation between minima. 32,33 As such, THF exists in conformers with either C 2 or C s symmetry found along the pseudorotation coordinate. 32,33 On the other hand, it has been well established that both THP and 1,4-dioxane exist in their lowest energy chair conformations.…”

“…32,33 As such, THF exists in conformers with either C 2 or C s symmetry found along the pseudorotation coordinate. 32,33 On the other hand, it has been well established that both THP and 1,4-dioxane exist in their lowest energy chair conformations. 34,35 These, respectively, have C s and C 2h symmetries.…”

“…In the interest of making fruitful comparisons regarding the behavior of these three cyclic ethers, we consider THF through an average over the C 2 to C s conformations; with experimental results having revealed that both conformations exist in near equal proportions at roomlike temperatures. 32,33 As our experiment is performed under room-like temperature conditions, this is a reasonable approximation given the complexity of the calculations. The full details of this averaging approach have been described elsewhere.…”

“…15 Information regarding the ionization of the cyclic ethers has been obtained by ultraviolet photoelectron spectroscopy (PES), [36][37][38] Penning ionization electron spectroscopy (PIES), 36 and electron momentum spectroscopy. 33,39 In all the species under consideration, the HOMO is well separated from the other orbitals. Here the HOMOs are 9b + 12a , 15a , and 8a g for THF, THP, and 1,4-dioxane, respectively.…”