To investigate other possible catalysts, we considered a set of small three-atomic molecules: ozone (O 3 ), selenium dioxide (SeO 2 ), and nitrous oxide (N 2 O). O 3 and SeO 2 were chosen as direct analogs of the SO 2 .[37-39] SeO 2 is also a ubiquitously used organic catalyst. In addition, sulfur trioxide (SO 3 ) was also tested, as it was shown to produce a similar intermediate in a tetrahydrofuran (THF) medium. The found reactive pathways for the chosen inorganic molecules are shown in Figure . For ozone and nitrous oxide, we did not find any path leading to the desired phenyl isocyanate, as they tend to oxidize the molecule into N-oxobenzamide. SeO 2 and SO 3 follow the same pathway as SO 2 yielding the desired compound. The effective barrier height (Equation ) in benzene with SO 2 as a catalyst is 12 kcal/mol, whereas with SeO 2 it is only 3 kcal/mol, i.e., it should be a more active catalyst than the original. SO 3 in THF gives an effectively barrier-less reaction, which probably indicates the nonphysical nature of assumptions made in the derivation of Equation 2 for this particular pathway. However, the barrier from the intermediate towards the products is lower by 2 kcal/mol, which indicates SO 3 also being a superior catalyst compared to the SO 2 . As an alternative organic catalyst, we first briefly investigated the possibility of replacing the S--O moiety with C--O in the [3+2]-cycloaddition leading to the intermediate formation. The initial screening was done using the r 2 SCAN-3c method in the gas phase. For this, we tested acetone, methyl 3,3,3-trifluoropyruvate, and 3,5-di-tertbutyl-o-benzoquinone (Figure ). All these compounds can provide a similar reaction pathway as the SO 2 through the 1,3-dipolar cycloaddition reaction. The computed effective reaction barrier (Equation ) in the gas phase for the SO 2 at the r 2 SCAN-3c level of theory is 9 kcal/mol, whereas for acetone it is 17 kcal/mol, for methyl 3,3,3trifluoropyruvate it is 11 kcal/mol, and for 3,5-di-tert-butyl-O-benzoquinone it is 10 kcal/mol. Thus, all investigated alternative organic molecules are predicted to be significantly less effective catalysts for the reaction of interest. ) in THF at DLPNO-CCSD(T)/CBS//r 2 SCAN-3c level of theory were the following: for a pathway through Intermediate A is 19 kcal/mol, while for a pathway through Intermediate A is 23 kcal/mol. The colors of the atoms illustrate whether they initially belong to the reactant (blue) or catalyst (red).