Comparison between PBE-D3, B3LYP, B3LYP-D3 and MP2 Methods for quantum mechanical calculations of polarizability and IR-NMR spectra in C24 isomers, including a novel isomer with D2d symmetry

Abstract

We perform a comprehensive theoretical analysis of C24 isomers in the gaseous phase using the PBE-D3-cc-pVTZ,B3LYP-cc-pVTZ,B3LYP-D3-cc-pVTZ , MP2-6-31G methods.We consider the basis set cc-pVTZ for the MP2 method , carry out optimized (single point) calculations in three isomers (remaining two isomers when convergence was too time consuming) in order to reveal the potentiality of the method.We covered many properties,including geometry optimizations,chemical stability,polarizabilities,nuclear screening constants,Fermi(FE),gap(GE) , atomization energies(AE),thermodynamic analysis,reactivity index,IR, NMR.Calculations were performed for ring (D12h), sheet (D6h) , two cage (D6d,Oh) configurations. We proposed a new structure, the bracelet (D2d) arrangement, stable according to the PBE, B3LYP,B3LYPD3 methods, but a transition state by the MP2 method. Results consistently indicated the D6h isomer is the most stable one among the C24 isomers studied, while the D2d isomer was found to be the least stable.For the gap energy(GE), the B3LYP,B3LYP-D3 methods yielded higher values compared to the PBEs,with an average DFT (PBE, B3LYP) GE of 1.89eV,whereas the MP2 method showed a substantially higher GE value of 7.6 eV, representing an increase of approximately 0.75.The polarizabilities of the C24 isomers were found to be overestimated by the PBE,B3LYP,B3LYP-D3 methods when compared to the corresponding MP2 values.The PBE-D3 method produced higher polarizabilities for the C24 isomers in comparison to the B3LYP, B3LYP-D3, MP2 methods.The investigation confirms that the Oh (D12h) isomer has the smallest (largest) polarizability.The polarizability of D12h is notably affected by the selected DFT method, while that of Oh displays lower sensitivity but shares similarities with D6d.