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A time-dependent equation-of-motion coupled-cluster singles and doubles (TD-EOM-CCSD) method is implemented, which uses a reduced basis calculated with the asymmetric band Lanczos algorithm. The approach is used to study weak-field processes in small molecules induced by ultrashort valence pump and core probe pulses. We assess the reliability of the procedure by comparing TD-EOM-CCSD absorption spectra to spectra obtained from the time-dependent coupled-cluster singles and doubles method, and observe that spectral features can be reproduced for several molecules, at much lower computational times. We discuss how multiphoton absorption and symmetry can be handled in the method, and general features of the core-valence separation projection technique. We also model the transient absorption of an attosecond x-ray probe pulse by the glycine molecule.

Among the species discovered in the interstellar medium and planetary atmospheres, a crucial role is played by the so-called “interstellar” complex organic molecules (iCOMs) because they are the signature of the increasing molecular complexity in space. Indeed, they may represent the connection between simple molecules and biochemical species like amino acids and nucleobases. In particular, HCN and the related CN radical are the starting points of rich nitrile chemistry. In this framework, we have undertaken a computational investigation of the gas-phase reaction mechanisms involving different C2N2H5 radicals and their fragments, stemming from the addition of the cyano radical to the nitrogen atom of methylamine. Aiming at exploiting an accurate yet cost-effective protocol, a combination of CCSD­(T)-based composite schemes and density functional theory has been employed. The exploration of the plausible chemical reaction channels has led to the identification of 12 different products, as well as 28 transition states connecting reactants, intermediates, and products. Aminoacetonitrile (H2NCH2CN), proposed as an intermediate in the formation of the smallest amino acid glycine, and the CH2NH2 radical appear as products energetically accessible under astrophysical conditions.

New spectroscopic experiments and state-of-the-art quantumchemical computations of creatinine in different aggregation states unequivocally unveiled a significant tuning of tautomeric equilibrium by the environment: from the exclusive presence of the amine tautomer in the solid state and aqueous solution to a mixture of amine and imine tautomers in the gas phase. Quantum-chemical calculations predict the amine species as the most stable tautomer by about 30 kJ mol À 1 in condensed phases. On the contrary, moving to the isolated forms, both Z and E imine isomers become more stable by about 7 kJ mol À 1 .Since the imine isomers and one amine tautomer are separated by significant energy barriers, all of them should be present in the gas phase. This prediction has indeed been confirmed by high-resolution rotational spectroscopy, which provides the first experimental characterization of the elusive imine tautomer. The interpretation of the complicated hyperfine structure of the rotational spectrum, originated by three 14 N nuclei, makes it possible to use the spectral signatures as a sort of fingerprint for each individual tautomer in the complex sample.