Structure of isothiocyanic acid dimers. Theoretical and FTIR matrix isolation studies

Experimental FTIR-MI and Theoretical Studies of Isocyanic Acid Aggregates

Homoaggregates of isocyanic acid (HNCO) were studied using FTIR spectroscopy combined with a low-temperature matrix isolation technique and quantum chemical calculations. Computationally, the structures of the HNCO dimers and trimers were optimized at the MP2, B3LYPD3 and B2PLYPD3 levels of theory employing the 6-311++G(3df,3pd) basis set. Topological analysis of the electron density (AIM) was used to identify the type of non-covalent interactions in the studied aggregates. Five stable minima were located on the potential energy surface for (HNCO)₂, and nine were located on the potential energy surface for (HNCO)₃. The most stable dimer (D1) involves a weak, almost linear N-H⋯N hydrogen bond. Other structures are bound by a N-H⋯O hydrogen bond or by O⋯C or N⋯N van der Waals interactions. Similar types of interactions as in (HNCO)₂ were found in the case of HNCO trimers. Among nine stable (HNCO)₃ structures, five represent cyclic forms. The most stable T1 trimer structure is characterized by a six-membered ring formed by three N-H⋯N hydrogen bonds and representing high symmetry (C_3h). The analysis of the HNCO/Ar spectra after deposition indicates that the N-H⋯O hydrogen-bonded dimers are especially prevalent. Upon annealing, HNCO trimers were observed as well. Identification of the experimentally observed species relied on previous experimental data on HNCO complexes as well as computed data on HNCO homoaggregates' vibrational spectra.

Transient species in the ozonolysis of tetramethylethene

The reaction of alkenes with ozone has great effect on atmospheric oxidation, its transient species can produce OH radicals and contribute to the formation of secondary organic aerosols (SOA). In the present study, the reaction of tetramethylethene (TME) with ozone was investigated using self-assembled low temperature matrix isolation system. The TME and ozone were co-deposited on a salt plate at 15 K, and then slowly warmed up the plate. The first transient species primary ozonide (POZ) was detected, indicating that the reaction followed Criegee mechanism. Then POZ began to decompose at 180 K. However, secondary ozonide (SOZ) was not observed according to Criegee mechanism. Probably, Criegee Intermediate (CI) did not react with inert carbonyl of acetone, but with remaining TME formed tetra-methyl epoxide (EPO).

Structural and spectroscopic properties of complexes formed between HNCS and SO2 in low temperature matrices

Matrix isolation FTIR spectroscopy has been combined with quantum chemical calculations in the aim to characterize complexes of isothiocyanic acid HNCS with SO₂. The geometries of the 1:1, 1:2 and 2:1 complexes were optimized at the MP2 and DFT (B3LYPD3) levels of theory with the 6-311++G(3df,3pd) basis set. Five different HNCS⋯SO₂ structures of the 1:1 stoichiometry were optimized. Three of them involve a weak NH⋯O hydrogen bond whereas two other geometries are stabilized by van der Waals interactions of various types. The HNCS/SO₂/Ar spectra analysis evidences that at least one of the three hydrogen bonded structure is present after deposition of the matrices whereas the most stable van der Waals HNCS⋯SO₂ structure as well as complexes of the 1:2 stoichiometry were detected upon annealing.

Characterisation of isothiocyanic acid, HNCS, in the solid state: trapped by hydrogen bonding

The structural characterisation of [Ph₄P][NCS]·HNCS is reported and structurally characterised. DFT calculations and spectroscopy show a strong hydrogen bond (image courtesy of ESO).