Characterization of the Conformational Probability of N-Acetyl-Phenylalanyl-NH2 by RHF, DFT, MP2 Computation and AIM Analyses, Confirmed by Jet-Cooled Infrared Data

Computational and experimental determinations were carried out in parallel on the conformational probability of N-Acetyl-Phenylalanine-NH2 (NAPA). Ab initio computations were completed at the BLYP/6-311G(df,p), B3LYP/6-31G(d), B3LYP/6-31G(d,p), and B3LYP/6-31+G(d) levels of theory, labeled L/61fp, B/6, B/6p, and B/6+, respectively. Three experimentally identified conformers were compared with theoretical data, confirming their identities as the Lanti, Lgauche+, and Lgauche- (BACKBONESIDECHAIN) conformers. Evidence comes from matching experimental and theoretical data for all three constituent N-H stretches of NAPA, with a Experimental-Theoretical = ~1-3 cm-1, ~0-5 cm-1, and ~1-6 cm-1, at the L/61fp and B/6+ levels, respectively. Corrected-ZPE relative energies were computed to be 0.14, 0.00, 0.26 and 0.00, 0.67, 0.57 (kcal*mol-1) for the Lanti, Lgauche+, and Lgauche- conformers, respectively, at the L/61fp and B/6+ levels, respectively. The MP2/6-31+G(d) level of theory was subsequently found to give similar relative energies. Characterization of the intramolecular interactions responsible for red and blue shifting of the N-H stretches showed the existence of the following intramolecular interactions: C=O[i]- - -HN[i], (Ar[i])-C- - -HN[i+1], (Ar[i])-C-H- - -O=C[i-1] for Lanti; C=O[i-1]- - -HN[i+1], (Ar[i])-C- - -HN[i+1], (Ar[i])-C-H- - -O=C[i] for Lgauche+; and C=O[i-1]- - -HN[i+1] for Lgauche-. Each of these interactions were further investigated and subsequently characterized by orbital population and Atoms-In-Molecules (AIM) analyses, with the identity of overlap and bond critical points (BCP) serving as 'scoring criteria', respectively. Experimental and theoretical carbonyl stretches were also compared and showed good agreement, adding further strength to the synergy between experiment and theory

Large Shape Staggering in Neutron-Deficient Bi Isotopes

The changes in the mean-square charge radius (relative to Bi-209), magnetic dipole, and electric quadrupole moments of Bi-187,Bi-188,Bi-189,Bi-191 were measured using the in-source resonance-ionization spectroscopy technique at ISOLDE (CERN). A large staggering in radii was found in Bi-187,188,189(g), manifested by a sharp radius increase for the ground state of Bi-188 relative to the neighboring Bi-187,189(g). A large isomer shift was also observed for Bi-188(m). Both effects happen at the same neutron number, N = 10(5), where the shape staggering and a similar isomer shift were observed in the mercury isotopes. Experimental results are reproduced by mean-field calculations where the ground or isomeric states were identified by the blocked quasiparticle configuration compatible with the observed spin, parity, and magnetic moment.Peer reviewe