Natural Spin Density Research Articles

Theoretical calculation of the adiabatic electron affinities of the monosubstituted benzaldehyde derivatives

In the current study, several computational models were examined to calculate accurate adiabatic electron affinity (AEA) of twelve m- and p-monosubstituted benzaldehyde derivatives. The examined models are as follows: (i) composite high-level ab initio (G3B3, G4, CBS-Q, and CBS-QB3), (ii) Three hybrid DFT approaches (B3LYP, CAM-B3LYP, and wB97XD) with two basis sets (6–31 + G(d,p) and 6–311++G(2df,2p)) and (iii) single point calculation using fifteen DFT approaches with 6–311++G(2df,2p) at the B3LYP/6–31 + G(d,p) geometry. Several statistical descriptors were computed to validate the calculated AEAs based on the available experimental results. Results revealed that G3B3 and CBS-QB3 are the most accurate result, while G4 and CBS-Q methods yield less accurate ones. Also, the wB97XD and CAM-B3LYP in combination with 6–311++G(2df,2p) able to calculate accurate AEAs. Low CPU time single point calculation strategy by using wb97 and wb97X approaches can compute AEAs value as accurately as G3B3 and CBS-QB3 methods. The AEAs of other several monosubstituted benzaldehydes were also predicted by using the wb97, wb97X, and wb97XD. The effect of the nature and position of substituents on the natural spin density and natural charge is also studied and discussed.

A comparison of the C‒H bond dissociation enthalpies of sulfur-containing fused heterocyclic compounds to the C‒H bond dissociation enthalpies in other heterocycles

The C‒H bond dissociation enthalpies (BDEs) of 26 N, O, S-containing mono-heterocyclic compounds were assessed by the composite high-level ab initio methods G4 and CBS-QB3. In addition, the C‒H BDEs of 32 heterocyclic compounds were evaluated by 13 density functional theory methods. The BMK method showed the lowest root mean square error of 7.2 kJ/mol and the correlation coefficient (R2) was 0.9874 after being compared with the experimental values. Subsequently, we used this method to study the C‒H BDE values of the different positions as well as the substituent effects on benzene and heterocycles in sulfur-containing fused heterocyclic compounds. Both the natural charge distributions of benzo[b]thiophene and benzothiazole and natural spin densities of benzo[b]thiophene derivative radicals were conducted by NBO analysis for further understanding the essence of the C‒H bond.

Computational Study on the Intramolecular Carbene-CO Coupling in M(CH2)(CO)3 Radicals (M = Co, Rh, Ir)

The intramolecular carbene-carbonyl coupling has been investigated for the simple M(CH2)(CO)3 (M = Co, Rh, Ir) radical complexes at the DFT PBEPBE/TZVP level of theory. The coupling is predicted to be very fast for the cobalt-containing system, but it is still feasible for the systems based on the other two metals. The back-way reaction, that is, the conversion of the ketene complex into carbonyl-carbene complex, cannot be excluded from the Ir-containing system in CH2Cl2, and it is even favored in gas phase. The intermolecular ketene formation by the addition of external CO onto the CH2 moiety is the favored pathway for the Ir-complex. The Laplacian distribution, as well as the natural spin density distribution of all the species, being involved in the reaction, gives explanation for the significant difference between the nature of the Co-complex and the Rh- and Ir-systems.

Interpretation of the characteristic fragmentation mechanisms through determining the initial ionization site by natural spin density: A study on the derivatives of tryptophan and tryptamine

The characteristic fragmentation mechanisms of the derivatives of tryptophan and tryptamine are investigated by the density functional theory (DFT) at the B3LYP/6–31+G (d, p) level. The main primary α-cleavage fragmentation has been predicted through determining the initial ionization site by calculated spin density of molecular radical cation and the variation of molecular structures from neutral to cationic form. The results show that the more intensive base peak produced by main primary α-cleavage fragmentation predicted in the present agrees well with those of experimental mass spectra, which is useful for characterizing the fragmentation mechanisms of such indole alkaloids. More importantly, the novel method for determining the most likely initial ionization site proposed in this study is prior to that based on the ionization energy (IE). It will play an important role in unraveling the mechanisms of radical-driven fragmentation not only for small molecules, but also for gas-phase peptides occurring in electron capture dissociation (ECD), electron transfer dissociation (ETD), or low-energy CID of peptide radical cations. However, this study is inappropriate for negative systems.