Unusual Spin States in Noble Gas Inserted Noble Metal Halocarbenes, FNgCM (Ng = Kr, Xe, Rn; M = Cu, Ag, Au)

Recent experimental detection of noble gas (Ng) insertedfluorocarbenes, viz., FKrCF and FXeCF, which weretheoretically predictedby our group earlier and very recent experimental evidences on gold-halogenanalogy motivated us to explore the possibility of the existence ofnoble gas inserted noble metal fluorocarbene, FNgCM (Ng = Kr, Xe,and Rn; M = Cu, Ag, and Au) molecules. Ab initio quantumchemical calculations have been performed to investigate structure,stability, vibrational frequency, charge distribution and bondinganalysis of FNgCM molecules by employing DFT, MP2, and CCSD(T) methods.For the purpose of comparison FNgCH molecules have also been studied.One of the important outcomes of the study is that the predicted FNgCH,FNgCCu and FNgCAg molecules are more stable in their triplet electronicstates, whereas the FNgCAu molecules are found to be more stable intheir singlet potential energy surface, similar to the recently observedFNgCF (Ng = Kr and Xe) molecules, although the singlet state is thelowest energy state for all the precursor carbene molecules. The goldatom behaves as a better electron donor due to the pronounced relativisticeffect as compared to hydrogen, copper and silver atoms, resultingin stabilization of the singlet carbene molecule indicating halogenlike chemical behavior of gold. These molecules are found to be thermodynamicallystable with respect to all plausible 2-body and 3-body dissociationchannels, except the one that leads to the formation of the globalminimum products. However, metastable nature of the predicted moleculeshas been proved by studying the saddle point corresponding to thetransition from the minima to the global minimum products. Sufficientbarrier heights provide the kinetic stability to the predicted FNgCMmolecules, which prevent them from dissociating into their respectiveglobal minimum products. All the results clearly indicate that theF-Ng bond is mostly ionic in nature with certain amount ofcovalent character while Ng-C bond is found to be covalentin nature. Furthermore, atoms-in-molecule (AIM), energy decompositionanalysis (EDA) and charge distribution analyses suggest that the predictedFNgCM molecules essentially exist in the form of [F](delta-)[NgCM](delta+). The calculated results also indicate thatit may be possible to prepare and characterize the predicted moleculesby suitable experimental technique(s).