Valence orbital response to pseudorotation of tetrahydrofuran: A snapshot using dual space analysis

Abstract

The pseudorotation of tetrahydrofuran (THF) (C4H8O) has been studied using density functional theory, with respect to the valence orbital responses to the ionization potentials and to orbital electron and momentum distributions. Three conformations of THF, the global minimumstructure Cs, local minimum structure C2, and a transition state structure C1, which arecharacteristic configurations on the potential energy surface, are examined using the SAOP∕et-pVQZ//B3LYP∕6-311++G** models with the aforementioned dual space analysis. It is noted in the ionization energy spectra that the minimum structures Cs and C2 are not directly connected by pseudorotation, but through the transition state structure C1. As a result, some orbitals of the Cs conformer are able to “correlate” to orbitals of the C2 conformer without a strict symmetry constraint, i.e., orbital 7a′ of the Cs conformer is correlated to orbital 5b of the C2 conformer. It is also noted that although the valence orbital ionization potentials are not significantly altered by the pseudorotation of THF, their spectra (mainly due to excitation) are quite different indeed. Detailed orbital analysis based on dual space analysis is given. The valence orbital behavior of the conformations is orbital dependent. It can be approximately divided into three groups: the “signature group” is associated with orbitals experiencing significant changes. The frontier orbitals are in this group. The “nearly identical group” includes orbitals without apparent changes across the conformations. Most of the orbitals showing a certain degree of distortion during the pseudorotation process belong to the third group. The present study demonstrates that a comprehensive understanding of the pseudorotation of THF and its dynamics requires multidimensional information and that the information gained from momentum space is complementary to that from the more familiar coordinate space.

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

“…18 The experimental orbital MDs obtained in the present work support the previous theoretical findings. 18 In Figure 7, a spectroscopic factor of the inner valence orbitals given in Table 1 has been employed to fit the theoretical simulations to the observed MDs.…”

“…18 Part (b) of Figure 5 gives the orbital electron density contours of the HOMOs as a function of the pseudorotation angle φ at 0°(C s ), 15°, 30°, 45°(C 1 ), 60°, 75°, and 90°(C 2 ), which represent half of the angles in part (b) of Figure 5 due to the symmetry at φ ) 90°(part (a) of Figure 5, insert). As indicated previously, 18 the HOMOs of THF at various φ angles are indeed dominated by O(2p) + H(1s) orbitals, resulting in an s x p y -hybridized orbital MDs in momentum space (part (b) of Figure 5 at 0°, i.e., the HOMO density distributions of the C s conformer of THF). In fact, the HOMO of the C s conformer is dominated by the O(2p z ) orbital, with contributions from the 2p z and 2p x orbitals of C(4) and C(5) as well as the 2p y orbitals of C(3) and C(2), which are enhanced by the 1s orbitals of H(8), H(6), H(9), and H(7).…”

“…When the energy moves into the inner valence shell, for example, IP > 18 eV, the OVGF model is not so useful due to its limitations. However, the DFT-SAOP/et-pVQZ model 18 is able to reproduce reliable binding energies in the entire valence space for THF. Table 1 reports the valence orbital ionization energies of THF observed in the EMS experiment, with comparisons to a previous PES experiment and quantum mechanically calculated IPs.…”

“…18 The experimental binding-energy spectrum is recorded in a range of 7-37 eV at the impact energy of 1200 eV plus binding energies, which are obtained through the integration of the electron density map in Figure 2 over all azimuthal angles.…”

“…The innermost valence orbital pair of orbitals 7 and 6, which are formed dominantly by the O2s and C2s orbitals, 18 are invariant with respect to the pseudorotation angle φ, which is in contrary to the HOMO, the outermost valence orbital of THF. It indicates that the pseudorotation mechanism of THF is orbital-dependent.…”