2-amino-1-propanol versus 1-amino-2-propanol: Valence band and C 1s core-level photoelectron spectra

Abstract

Valence band and C 1s core-level photoelectron spectra of S-(+)-2-amino-1-propanol (alaninol) and S-(+)-1-amino-2-propanol (isopropanolamine) have been studied by means of synchrotron radiation photoelectron spectroscopy in gas phase. The alaninol, the reduced derivative of the alanine, is a good test system of amino acid-like structures. The isopropanolamine, presenting the inversion of the two functional groups of the alaninol at the chiral carbon, offers the opportunity to study the effect of -OH and -NH(2) structural position on the photoelectron spectra. The influence of the conformational contribution on the electronic structure and the photoelectron spectra has been interpreted using density functional and ab initio theoretical calculations. Agreement has been achieved by taking into account the presence, in gas phase, of two conformers with different population ratios in both chiral systems. The C 1s core-level spectra of alaninol and isopropanolamine are reported and the peak positions of the three carbon atoms of the molecules are assigned.

Conformational Effects in Photoelectron Circular Dichroism of Alaninol

“…In the following discussion we enumerate the experimental peaks as Roman numerals and the DFT-calculated HOMO-(nÀ1) for the nth level. Taking the molecular orbital IEs for conformers A1 and A2 from the OVGF calculation as guideline [16] (marked as the grey short vertical lines in the upper panel of Figure 2), it is possible to guess that HOMO, HOMO-1 and HOMO-2 of the two conformers are not distinguishable in the valence photoelectron spectrum shown, the energy difference being of the order of some tens of meV (the overall photoelectron resolution is about 250 meV). The separation of the conformer energy state is significant for HOMO-3 (DE = 110 meV) but less than the resolution limit.…”

“…We have already carried out a study on the effect of conformer population on the photoelectron spectra of alaninol, which provided some important information on the energetics of the conformers. [16] Here, we will show that the analysis of the CD in photoelectron spectroscopy directly yields information about the structural and electronic properties that is not accessible from photoelectron spectroscopy in linear or unpolarized light. We underline that alaninol absorption is in the VUV wavelength range, where it is difficult to measure the NCD in absorption because of the lack of lock-in techniques.…”

“…where D B for the Ith experimental deconvoluted contribution is a sum of the D parameter weighted by the specific conformer molecular orbital isotropic cross section and the Boltzmann www.chemphyschem.org population coefficient (DE is the calculated energy difference between the two conformers) [16] for every molecular orbital (HOMO-n, HOMO-m: highest occupied molecular orbital) that contributes to the deconvoluted peak. The normalization term contains the cross section and the g term defined in Equation (3).…”

“…The information about the ground-state electronic properties was taken from Catone et al; [16] in the following we summarize the method. Alaninol has a hydrogen bond between the oxydrilic and amino groups that locks the near-free rotation of the molecule, thus limiting the number of conformers.…”

“…[16] The cross sections and the D parameters were obtained by a continuum calculation with the multicentric B-spline function approach, as detailed by Stener et al [24] The ground-state density obtained from a self-consistent-field calculation employing the LB94 exchange-correlation potential with the same DZP basis set defines the KS Hamiltonian employed in continuum calculations. Further details specific to the alaninol molecule can be found in the report by Catone et al [16] Every experimental D value is compared with the theoretical D values of A1 and A2 and the Boltzmann average, with the ground-state IEs as a guideline in composing the sum. In this way it is possible to recognize whether the experimental peak has a definite conformer character or is a superposition of conformer states.…”