Tetrahydrofuran (THF), a heterocyclic five-member rings, and an analogue of the sugar moiety in the sugar-phosphate backbone of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA), is a prototype of pseudorotation (sugar puckering). The
subtle energy differences caused by the conformations formed as a result of pseudorotation have daunted both experimental and theoretical studies for many years
until recent joint study using electron momentum spectroscopy (EMS) [1]. The diagnostic identification of the most stable conformation of THF in gas-phase has
been based on different symmetries of the highest occupied molecular orbitals (HOMO) of the local minimal structures, Cs and C2. However, due to experimental resolution, little information in valence space for the conformers is revealed, which warrants a theoretical investigation for the pseudorotation of THF. Our study [2] into valence orbital based responses to the pseudorotation of THF using dual space analysis (DSA) [3], is based on a SAOP/et-pQVZ density functional theory (DFT) model. It is found that although the total energies and other properties, such as dipole moments and even the ionization potential energies of the three characteristic configurations of THF, i.e., Cs, C2 and C1, are nearly in distinguishable, their electron spectra (excitations) indicate that they are different species. Orbital based analysis certainly shows their association and difference in valence space, through the uniqueness of pseudorotation.

References
[1] T. C. Yang, G. L. Su, C.G. Ning, J. K. Deng, F. Wang, S. F. Zhang, X.G. Ren and Y.R. Huang, J. Phys. Chem. A, 111, 4297-4933(2007).
[2] F. Wang, J. Phys. Chem. A, 107, 10199-10207(2003).
[3] P. Duffy, J. A. Sordo, F. F. Chen and F. Wang, (to be submitted, 2007)