Accurate structural and spectroscopic characterization of prebiotic molecules: The neutral and cationic acetyl cyanide and their related species

Insights into the Molecular Structure and Spectroscopic Properties of HONCO: An Accurate Ab Initio Study

In an effort to provide the first accurate structural and spectroscopic characterization of the quasi-linear chain HONCO in its electronic ground state, state-of-the-art computational approaches mainly based on coupled-cluster (CC) theory have been employed. Equilibrium geometries have been calculated by means of a composite scheme based on CC calculations that incorporates up to the quadruple excitations and accounts for the extrapolation to the complete basis set limit and core correlation effects. This approach is proven to provide molecular structures with an accuracy better than 0.001 Å and 0.05° for bond lengths and angles, respectively. Incorporation of vibrational effects permits this level of theory to predict rotational constants with an estimated accuracy of 0.1% or better. Vibrational fundamental bands have been evaluated by means of a hybrid scheme based on harmonic frequencies computed using the CC singles, doubles, and a perturbative treatment of the triples method (CCSD(T)) in conjunction with a quadruple-ζ basis set, with all electrons being correlated, and anharmonic corrections from CCSD(T) calculations using a triple-ζ basis set, within the frozen-core approximation. Such a hybrid approach allowed us to obtain fundamental frequencies with a mean absolute error of about 1%. To complete the spectroscopic characterization, vertical electronic excitation energies have been calculated for the lowest singlet and triplet states using the internally contracted multireference configuration interaction (MRCI) method. Computations show that HONCO dissociates into OH + NCO upon the absorption of UV-vis light. In conclusion, we are confident that the highly accurate spectroscopic data provided herein can be useful for guiding future experimental investigations and supporting the characterization of this molecule in atmospheric and astrophysical media, as well as in combustion.

Valence-shell ionization of acetyl cyanide: simulation of the photoelectron and infra-red spectra

The vibrational envelopes of the first and second lines of the acetyl cyanide valence photoelectron spectrum [Katsumata et al., J. Electron Spectrosc. Relat. Phenom., 2000, 49, 113] in the gas phase have been simulated considering the Taylor expansion of the dipole moment from zero up to the second order as well as the changes of geometries/frequencies/normal modes between the initial neutral electronic ground state and the final (15a'^(-1), 3a''^(-1)) cationic states. It is shown that the vibrational profile of the first band (A') extending over 3500 cm^-1 with a vibrational spacing of 500 cm^-1 is not due solely to the overtones (v = 0 → v' = 1, 2, 3,…) of the C-CO bending mode as previously suggested but results from a collection of (v = 0 → v' = 1) transitions with frequencies multiple of 500 cm^-1 associated with the CO stretching at 1550 cm^-1, C-C stretching at 1045 cm^-1 and C-CO, C-CN bending modes at 370/500 cm^-1 completed by combination bands. Our calculations also reveal that the structureless and asymmetric shape of the second band (A'') is due to the activation of the torsion mode at low-frequency (ω ≈ 150 cm^-1) induced by the rotation (60 degrees) of the methyl group blurring the main vibrational progression (ω ≈ 1115 cm^-1) corresponding to the cooperative motions of the methyl CH bending and C-CO bending/CO stretching. Infra-red spectra of the fundamental and both the 15a'^(-1) and 3a''^(-1) cationic states were finally simulated. In contrast to the photoemission spectra, the infrared intensity of the CO stretching motion is very weak. The spectra are mainly dominated by the v = 0 → v = 1 transition of the CN stretching and the CH symmetric bending/stretching modes, providing complementary information between photoemission and infra-red spectroscopies to capture the nature of the cationic states in acetyl-cyanide.

Advances in spectroscopy and dynamics of small and medium sized molecules and clusters

Investigations of the spectroscopy and dynamics of small- and medium-sized molecules and clusters represent a hot topic in atmospheric chemistry, biology, physics, atto- and femto-chemistry and astrophysics.

Structure and spectroscopic properties of low-lying states of the HOC(O)O radical

The HOC(O)O radical is a product of the reaction of HOCO radicals with oxygen atoms. The present study provides theoretical prediction of critical spectroscopic features of this radical that should aid in its experimental characterization. Energies, structures, rotational constants, and harmonic frequencies are presented for the ground and two low-lying excited electronic states of HOC(O)O. The energies for the Ã(2)A″←X̃(2)A' and B̃(2)A'←X̃(2)A' electronic transitions are reported. The band origin of the B̃←X̃ transition of HOC(O)O is predicted to occur in the near infrared region of the spectrum at around 1.5 eV and it is suggested to be the most promising one for observing this radical spectroscopically. The structural and spectroscopic similarities between HOC(O)O and the isoelectronic radical FC(O)O are discussed. The abundance of experimental data on the FC(O)O radical should guide the spectroscopic characterization of HOC(O)O and serve as a benchmark for the structural and spectroscopic parameters obtained from theory.