Spectroscopic Identification and Bonding Properties of HNCOCa⁺: a Matrix Isolation and Computational Study

Metal (iso)cyanides dominate the molecular inventory of metal-bearing species in the interstellar medium. Their oxide counterparts, metal (iso)cyanates, have potential as interstellar molecules and have received significant attention. However, cationic complexes HNCOM+ as precursors to metal (iso)cyanates are rarely studied. Herein, we investigated HNCOCa+ by exploiting infrared spectrometry with isotopic substitutions and quantum chemical calculations. For comparison, the light and heavy alkaline earth metal cationic complexes HNCOBe+ and HNCOBa+ were also explored. HNCOCa+ and HNCOBe+ rather than HNCOBa+ can be experimentally generated by the reactions of metal cations with HNCO. The observed antisymmetric and symmetric NCO stretching vibrations in HNCOCa+ (2362.6 and 1330.4 cm−1) are higher than those in free HNCO (2268.5 and 1320.3 cm−1) but lower than those in HNCOBe+ (2426.4 and 1355.2 cm−1). These shifts can be explained by the charge polarization within the NCO fragment in HNCOBe+ and HN-COCa+. Bonding analysis suggests that HNCO–Be+ bond favors covalent character (54%) while HNCO–Ca+ bond has higher electrostatic character (57%). The dominant electrostatic interaction (64%) in HNCO–Ba+ bond results in the low bond energy, which might account for its absence in experiments.