Orbital imaging and assessment of different orbital models for the valenceshell of methanol - Comparison of electron momentum spectroscopy measurements with near-Hartree-Fock limit, MRSD-CI, localized valence bond and density functional theory

The momentum distributions of the valence orbitals of methanol have been st udied by electron momentum spectroscopy (EMS) and Hartree-Fock (HF), multi- reference singles and doubles configuration interaction (MRSD-CI), localize d valence bond (VB) and density functional theory (DFT) calculations. The e xperiment was performed using a multichannel EMS spectrometer at a total en ergy of 1200 eV plus the binding energy. Binding-energy spectra measured in the energy range of 6-47 eV are presented for the azimuthal angles phi = 0 degrees and phi = 8 degrees. Synthetic binding-energy spectra from Green's function and HF calculations for the azimuthal angles phi = 0 degrees and phi = 8 degrees in the 6-47 eV energy region are also compared to experimen t. In the inner valence region strong splitting of the 4a' and 3a' ionizati on is observed due to final-state electron correlation effects. The measure d momentum profiles are compared with HF calculations at the level of the t arget HF approximation using basis sets ranging from simple (STO-3G) to lar ge (110-GTO and Trun-pV5Z). DFT calculations at the level of the target Koh n-Sham approximation employing the local density approximation or hybrid fu nctional methods and the large Trun-pV5Z basis set are also compared to exp eriment. The effects of electron correlation and relaxation are also invest igated in the outer valence region by MRSD-CI calculations of the full ion- neutral overlap amplitude using the 110-G(CI) basis set. The shapes of all momentum profiles are well predicted by higher level theory. Some small dis crepancy still exists between all theoretical treatments and experiment in the low-momentum region for the HOMO 2a " orbital. MRSD-CI or DFT (i.e. cor related) methods are needed to adequately describe the shape of the 7a' and (6a' + 1a ") momentum profiles. The s-type character in the 5a' momentum p rofile is underestimated by HF theory and overemphasized by density functio nal theory (DFT). The 110-G(CI) calculation best predicts the shape for the 5a' momentum profile. The shapes of the experimental momentum profiles for the 4a' and 3a' are well-reproduced by large basis set HF and DET calculat ions. In addition, nearly all the inner valence 4a' pole strength is found in the 20-28 eV region while the high-energy 28-47 eV region contains virtu ally all of the 3a' pole strength. Lastly, the HOMO and NHOMO momentum prof iles are compared to the corresponding localized molecular orbitals (LMO) ( i.e. the 'lone-pair' orbitals of VSEPR or qualitative VB theory), the canon ical molecular orbitals (CMO) of molecular orbital (MO) theory and the Kohn -Sham orbitals (KSO) of DFT used in the present work. The experimental resu lts unequivocally support the delocalized CMO and KSO models and are very d ifferent from the LMO description. Although electron correlation effects ar e also important at low momenta in the 7a' NHOMO this orbital is also basic ally a CMO or KSO in character as shown by the CI and DFT calculations. It is noteworthy that the KSOs of DFT, often described as fictitious mathemati cal concepts, in bet very closely fit the experimental results and are very similar to the CMOs given by HF or CI calculations of the full ion-neutral overlap where necessary. These conclusions have profound implications for computer-aided molecular design, molecular modelling and molecular recognit ion. (C) 1999 Elsevier Science B.V. All rights reserved.