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 HNCOCa⁺. 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.