Protonation of serine: conformers, proton affinities and gas-phase basicities at the "gold standard" and beyond

The potential energy surfaces (PESs) of serine and its protonated counterparts are investigated to determine the structures of the minima. A total of 95 neutral serine, 15 N-(amino-) and 46 O-(carbonyl-)protonated serine conformers are found. Their relative energies, geometries and harmonic vibrational frequencies are determined at the MP2/aug-cc-pVDZ level of theory. To obtain highly accurate thermodynamic values, further computations are performed: the ten conformers with the lowest relative energies from each molecule type (neutral, N- and O-protonated) are further optimized using the explicitly correlated CCSD(T)-F12a/cc-pVDZ-F12 method (for neutral serine, harmonic vibrational frequencies were also computed). In addition, auxiliary corrections were determined: basis-set effects up to CCSD(T)-F12b/cc-pVQZ-F12, electron correlation effects up to CCSDT(Q), core correlation and second-order Douglas-Kroll relativistic effects along with zero-point energy contributions. Two important thermodynamic parameters (at 298.15 K), proton affinity (PA)/gas-phase basicity (GB) are calculated considering the two different protonation sites: 218.05 ± 0.2/209.86 ± 0.6 kcal mol^-1 and 205.87 ± 0.2/196.36 ± 0.3 kcal mol^-1 for the amino and carbonyl sites, respectively. The uncertainty of the determined values was approximated based on various sources including auxiliary corrections, basis-set effects, harmonic vibrational frequencies.

C2H5NO Isomers: From Acetamide to 1,2-Oxazetidine and Beyond

This work documents the properties of a number of isomers of molecular formula C₂H₅NO from the most stable, acetamide, through 1,2-oxazetidine and including even higher energy species largely of a dipolar nature. Only two of the isomers have been detected in emissions from the interstellar medium (ISM); possible further candidates are identified, and the likelihood of their being detectable is considered. In general, hardly any of these compounds have been discussed in the existing chemical literature, so this work represents an important contribution extending the canon of chemical bonding which can contribute to machine learning, providing a more exacting test of AI applications. The presence in the ISM of acetamide, CH₃C(O)NH₂, is the subject of current debate with no clear and obvious paths to its formation; it is shown that a 1,3-[H]-transfer from (E,Z)-ethanimidic acid, CH₃C(OH)=NH, is feasible in spite of an energy barrier of 130 kJ mol^–1. It is speculated that imidic acid can itself be formed from abundant precursors, H₂O and CH₃C≡N, in an acid-induced, water addition, autocatalytic reaction on water–ice grains. H₃CC≡NH₃CC≡NH⁺ + H₂OH₃CC(O⁺H₂)=NHH₃CC(OH)=NH + H₃O⁺