3.4 Computations for gas phase rearrangements of nitro diaryliodonium cations
To shed insight to the reaction mechanisms of nitro diaryliodonium cations, density functional theory (DFT) calculations were performed with the Gaussian 16 package.43 The geometries were fully optimized at the M062x/Def2-TZVP level in gas phase,44,45 and harmonic vibration frequency calculations confirmed that the optimized structures were either minima (no imaginary vibrations) or transition states (only one imaginary vibration). The Gibbs free energies were given in kcal/mol and geometrical coordinates could be referred to the supporting information.
The schematic potential energy surface and the optimized structures of reactive species in ESI-MS/MS of [5 –OTf]+ resulting from the theoretical calculations were shown in Figure 5 . Two paths of [5 –OTf]+ were considered, includingpath (a ): the Smiles rearrangement via a six-membered ring transition state (IAr2-o-TS1 ); and path(b ): the cleavage of the C–I bond to form another transition state (IAr2-o-TS3 ). The distance between C(ipso) and I, C(ipso) and O atom of nitro group in IAr2-o-TS1 were 3.130 and 2.436 Å, which were both smaller than that of IAr2-o-TS3 (4.367 and 4.589 Å, respectively). These results suggested that Smiles rearrangement of [5 –OTf]+ could facilely occur via a six-membered ring transition state (IAr2-o-TS1 ), leading to the formation of the intermediate IAr2-o-INT1 .
Figure 5 Schematic of potential energy surface and the optimized structures of reactive species in ESI-MS/MS of [5 –OTf]+ (IAr2-o-R ). The Gibbs free energies were given in kcal/mol.
The energy barrier for the Smiles rearrangement of path(a ) via a six-membered ring transition state (IAr2-o-TS1 ) was only 22.7 kcal/mol and 9.5 kcal/mol less than the energy required for the C–I bond cleavage energy barrier of IAr2-o-TS3 (36.0 kcal/mol). Furthermore, the Smiles rearrangement from the hypervalent iodine(III) complexIAr2-o-R to the iodane(I) intermediateIAr2-o-INT1 (-14.9 kcal/mol) was exothermic, but the C–I bond cleavage to product complex IAr2-o-INT3 (30.0 kcal/mol, which might give [Mes]+ at m/z119) was endothermic. Thus, according to Figure 5 , the Smiles rearrangement process of path (a ) formed [Mes-I+-(o -NO2- C6H4)] (IAr2-o-R ) to the intermediate [Ar1-O-(o -NO-C6H4I)]+(IAr2-o-INT1 ) via IAr2-o-TS1 was a favorable reaction pathway both kinetically and thermodynamically. Then, the N–O bond cleavage of intermediate [Ar1-O-(o -NO-C6H4I)]+(IAr2-o-INT1 ) gave [MesO]+ atm /z 135 by loss of neutral [o -NO-C6H4I] (IAr2-o-INT2 ) via transition stateIAr2-o-TS2 (energy barrier of only 12.4 kcal/mol). Such calculation results supported our hypothesis proposed for path(a ) IAr2-o-R to intermediateIAr2-o-INT1 via Smiles rearrangement and finally produced [MesO]+ at m /z 135 with N–O bond cleavage of intermediate IAr2-o-INT1 .
Meanwhile, the C–I bond cleavage to product complexIAr2-o-INT3 (30.0 kcal/mol) might produce the intermediate IAr2-o-INT4 (-10.2 kcal/mol) in path(b ) and such process was exothermic. The N–O bond cleavage of intermediate IAr2-o-INT4 might also give [MesO]+ at m /z 135 and neutral [o -NO-C6H4I] (IAr2-o-INT5 ) via transition stateIAr2-o-TS4 (energy barrier of only 14.4 kcal/mol). The structures of the active species involved in the steps fromIAr2-o-INT3 to IAr2-o-INT5 were same as those in the process from IAr2-o-INT1 toIAr2-o-INT2 , but the species with same structures had minor differences in relative energies due to the different spatial configurations in the finally optimized structures.
Figure 6 Schematic of potential energy surface and the optimized structures of reactive species in ESI-MS/MS of [6 –OTf]+ (IAr2-m-R ). The Gibbs free energies were given in kcal/mol.
The path (b ) of C–I bond cleavage process proposed for [5 –OTf]+ was unfavorable than Smiles rearrangement of path (a ) (Figure 5 ). However, the Smiles rearrangement via O-attacking was not accessible for [6 –OTf]+ and [7 –OTf]+ due to their distances between C(ipso) and O atom of nitro group. Therefore, the stepwise processes involved ion-neutral complexes were considered for the explanation of the unexpected fragment ion [MesO]+ at m/z 135 in ESI-MS/MS of [6 –OTf]+ and [7 –OTf]+. The schematic potential energy surface and the optimized structures of reactive species in MS/MS of [6 –OTf]+ and [7 –OTf]+ resulting from the theoretical calculations were shown in Figure 6 and 7 . Theoretical calculations showed that [6 –OTf]+ and [7 –OTf]+ dissociated via similar mechanism as the path (b ) of C–I bond cleavage process proposed for [5 –OTf]+, which was unfavorable than Smiles rearrangement of path (a ) (Figure 5 ). However, the Smiles rearrangement via O-attacking was not accessible for [6 –OTf]+ and [7 –OTf]+ , due to the fact that the distances between C(ipso) and O atom of nitro group inIAr2-m-R and IAr2-p-R were 4.783 and 7.265 Å, respectively, which were both much longer than that ofIAr2-o-R (3.007 Å).
Figure 7 Schematic of potential energy surface and the optimized structures of reactive species in ESI-MS/MS of [7 –OTf]+ (IAr2-p-R ). The Gibbs free energies were given in kcal/mol.
Therefore, the stepwise processes involved ion-neutral complexes were considered for explanation of the unexpected fragment ion [MesO]+ at m/z 135 in ESI-MS/MS of [6 –OTf]+ and [7 –OTf]+. The energy barriers for the formation of IAr2-m-TS1 and IAr2-p-TS1were 33.0 and 33.4 kcal/mol, respectively, which were both much higher than that of IAr2-o-TS1 (22.7 kcal/mol). These results proved that the Smiles rearrangement process inIAr2-o-TS1 was the most favorable path to give [MesO]+ at m /z 135. Therefore, such calculation results explained why the fragmentation ion [MesO]+ at m /z 135 dominated the MS/MS of [Mes-I+-(o -NO2- C6H4)] from diaryliodonium salts (5 and13 ~16 ) but only with tiny amount of [Mes]+ at m/z 119.
To test the generality of the fragmentation patterns we proposed in this article, we further performed MS/MS experiments on cations from salts17 ~54 (structures shown inScheme S1 ), and their tandem mass spectra and MS/MS data were summarized in Figure S17 ~S54 andTable S1 ~S4 . At the same time, pseudo-MS3 experiments were performed in order to assign the origin of some ions (Figure S55~S57 ). These results showed the fragmentation ion [MesO]+ from [M –OTf]+ (M =17 ~22 ) completely dominated the spectra (Figure S17 ~S22 ) and diaryliodonium cations from salts17 ~36 also conformed well to the fragmentation patterns proposed in Scheme 3 and 4 . The fragmentation patterns of cyclic diaryliodonium cations from salts37 ~54 were also characterized by loss of an iodine atom (Scheme 6 , Figure S37 ~S54 ).
Scheme 6 Proposed fragmentation patterns of [M –OTf]+ from salts 37 ,41 , 43 and 46 .