P.K. Nandi

Ab initio SCF study of the nature of bonding in Si2H2 and Si2H4

Abstract Ab initio SCF calculations using several basis sets have been performed on a number of structural isomers of Si 2 H 2 and Si 2 H 4 , and their nature of bonding has been studied on the basis of bond index, valency and localized molecular orbitals (LMOs). Excepting the transition states, the stability of the Si 2 H 2 isomers is found to increase with decreasing molecular valency and increasing inert-pair effect in Si. In the case of Si 2 H 4 also, the most stable structure is associated with the least molecular valency. Analysis of the LMOs reveals that Si exhibits rather complicated hybridization.

Theoretical study of multi-centre bonding using a delocalised MO approach

Abstract The nature of the bonding in about thirty electron-deficient molecules has been studied on the basis of two-centre and multi-centre (mainly three-centre) bond indices and atomic valencies calculated from ab initio SCF density matrices using 4-31G (or 6-31G) and 6-31G∗ (or 6-31G∗∗) basis sets. For comparison, the density matrices corresponding to non-orthogonal and symmetrically orthogonalised basis functions have been employed. The nature of the bonding deduced on the basis of bond indices and valencies has been confirmed by localised MO calculations. It has been observed that a three-centre bond is invariably associated with a positive and significantly high value (⩾0.1) of the bond index. For a four-centre bonding interaction the corresponding bond index is generally positive, but rather small.

Ab initio theoretical study of the electronic structure, stability and bonding of dialkali halide cations

Abstract The ground state electronic structures and dissociation energies of the dialkali halide cations M 1 M 2 M + (M 1 , M 2 = Li, Na; X = F, Cl) have been calculated at the HF and post-HF (MP n ) levels using a variety of basis sets. The calculated values are in good agreement with the corresponding experimental data. All the species are predicted to be linear with only one minimum in the corresponding potential energy surface. In the formation of mixed halides, the reaction between M + 1 and M 2 X, where M 1 is lighter than M 2 , is more exothermic than the alternative reaction. A bonding analysis has been made on the basis of Mulliken and natural atomic charges and bond orders.

The role of valence interaction in some cation-molecule complexes

Abstract The role of valence interaction in the 1 : 1 and 1 : 2 complexes between Li + , Na + , Be 2+ , Mg 2+ and CO and N 2 was qualitatively assessed using charge-density-related quantities such as charge transfer, the bond index of the cation-donor atom bond, the valency of interaction and the change in molecular valency calculated at the HF/6–31G ∗ //3–21G level. It has been observed that the relative stability of the complexes can be rationalised on the basis of valence interaction. The molecular hardness parameters have also been calculated. The maximum hardness principle is found to be satisfied in the case of (MCO) n + -(MOC) n+ pairs, and the interaction between (MXY) n + and XY follows the qualitative prediction from the HSAB principle, where M n + is the metal ion and XY is the base.

EFFECT OF SPIN CONTAMINATION IN UHF WAVEFUNCTIONS ON CHARGE DENSITY-BASED LOCAL QUANTITIES

Abstract The effect of spin contamination in UHF wavefunctions on charge density-based local quantities has been investigated for 16 open-shell molecules. These quantities are found to be quite sensitive to spin contamination. When the 〈Ŝ2〉-error in the UHF method is appreciable, the calculated local quantities deviate markedly from the classically expected values. The spin contamination can be totally eliminated either by the ROHF or by the SUHF (spin constrained UHF) method. In the latter the 〈Ŝ2〉-error is constrained using a Lagrangian multiplier, λ. The SUHF solutions gradually converge to the ROHF solutions as λ → ∞. However, most of the spin contamination is removed by using very small positive values of λ. The lowest value of λ for which the 〈Ŝ2〉-error (calculated up to the fourth place of decimal) vanishes is referred to as the threshold λ, or λt. Using λ = λt excellent agreement is obtained between the SUHF and ROHF values for various local quantities.

Bonding in beryllium carbonyls

Abstract The nature bonding in Be(CO) 2 and Be 2 (CO) 4 has been discussed on the basis of bond indices, valencies and localized MOs. Similar calculations have been performed also on HBeBeH in order to elucidate the nature of the BeBe bond in Be 2 (CO) 4 . According to the present calculations the BeBe bond in Be 2 (CO) 4 is a single bond and not a strong double bond as has been claimed earlier.

Ab initio theoretical study of the electronic structure, stability and bonding of MXY− ions (M = H, Li, Na; X, Y= F, C1)

Abstract Equilibrium structural parameters and dissociation energies of the bihalide ions, MXY − (M = H, Li, Na; X, Y = F, Cl) have been calculated at the HF and post-HF (MP n ) levels using 6–31 + G * and 6–311 + G * basis sets. All the species are predicted to be linear, and the calculated quantities are in good agreement with the corresponding experimental data, where available. The nature of bonding in these species has been discussed on the basis of their atomic charges and two- and three-center bond indices. It has been observed that, with the exception of HCl 2 –, none of the species strictly obeys the recently proposed bond index criteria of three-center four-electron bonding.

A closer look into the causes of discrepancy between Löwdin and Mulliken atomic charges

Abstract A relation between Lowdin (L) and Mulliken (M) SCF density matrices ( D ) for closed-shell molecules is established. This is of the form where the residual terms are expressed in terms of P = 2 CC and d = S-I ( I is the identity matrix and S the AO overlap matrix) matrices. It has been observed that the discrepancy between Lowdin and Mulliken atomic charges can be qualitatively explained on the basis of the diagonal terms of the R 2 matrix which is a multiple of the commutator of d and [ P,d ] matrices.

Effect of basis sets and population analysis schemes on the calculation of group electronegativity

Abstract The group electronegativities of several molecules were calculated using different basis sets and population analysis schemes. It was observed that satisfactory results are obtained using the 6-31G∗ basis set and the Mulliken population analysis scheme. The transferability of the group electronegativities was examined by calculating their values for a number of HG and LiG molecules, where G stands for the group.

Natural populations and charge transfer in hydrogen and alkali bihalide ions

Equilibrium structural parameters and dissociation energies of the bihalide ions, (MXY) (M = H, Li or Na; X and Y = F or Cl) have recently been calculated [l] at ab initio HF and post-HF(MPn) levels using 6-31+G* and 6-31 l+G* basis sets. All the species were predicted to have a linear structure with a double-minima potential at R(X . . . Y) > &(X...Y). In the case of heterobihalide ions the structure (X-M . . . Y)), where X is a smaller atom than Y, corresponds to the global minimum in the potential energy surface. For the (HXY)species &(X . . . Y) is shorter than the sum of the van der Waals radii, Rx + RY. Owing to the presence of core electrons in Mf (M = Li or Na) and also its larger size, R, (X . . . Y) is longer than Rx + RY in alkali bihalide ions. These anions are binary complexes of a halide ion with hydrogen or alkali halides. Their nature of bonding was deduced by us [l] on the basis of atomic charges, charge transfer (CT) and bond indices obtained at the HF/6-31 l+G* level using the Mulliken population analysis (MPA) [2]. It was found that the CT or valence interaction plays an almost equal important role both in (HXY)and (MXY)-. This is an unacceptable result. In view of the well-known deficiency [3,4] of MPA, both atomic charges and the amount of CT are highly exaggerated especially for the alkali bihalide ions.