Interpretation of Hund's rule for first-row hydrides AH (A = lithium, boron, nitrogen, fluorine)

Abstract

Configuration interaction calculations have been carried out for low-lying singlet and triplet states of ('•3 +), BH(UII), (1 11 ,3 "), and FH(UII), with the aim of evaluating singlet-triplet energy component differences. These calculations represent the first systematic study of the quantum mechanical interpretation of Hund's rule in a series of molecules. While early molecular work on Hund's rule suggested that the electron-nuclear attraction may always be deeper in the triplet than in the singlet, exceptions are found in the present work. Also, in certain cases, where configuration mixing and molecular charge separation play a key role, the singlet electron-electron repulsion exceeds that of the triplet. It is also observed that changes in electron-electron repulsion tend to be in the same direction as changes in kinetic energy, whereas changes in electron-nuclear attraction are of the opposite sign, for a given geometry.

Ab Initio spectroscopic characterization of borane, BH, in its electronic state

“…The potential energy functions for various electronic states of BH were determined using the valence full configuration interaction approach by Gagliardi et al [24] and using the multireference configuration interaction approach with large correlation-consistent basis sets by Petsalakis and Theodorakopoulos [33] and by Miliordos and Mavridis. [34] In the latter work, the values of D e , r e , and m were predicted for the ground electronic state of BH to be 29 660 cm 21 , 1.230 Å , and 2261 cm 21 , respectively. The higher-order electron correlation effects on properties of BH were investigated by Halkier et al, [28] Larsen et al, [29] Abrams and Sherrill, [30] and Temelso et al [32] Previous theoretical predictions of the spectroscopic constants for the X 1 R 1 state of BH were recently reviewed by Sinha Mahapatra et al [36] The accurate binding energy D e for the X 1 R 1 state of BH is still not known experimentally.…”

“…In the calculation at the Hartree-Fock level of theory by Cade and Huo, [11] the potential energy function of BH in its X 1 R 1 electronic state was determined. The binding energy D e , the equilibrium internuclear distance r e , and the vibrational fundamental wavenumber m were calculated to be 22 500 cm 21 , 1.220 Å , and 2401 cm 21 , respectively. The electron correlation effects of BH were then discussed by several authors.…”

“…The electron correlation effects of BH were then discussed by several authors. In particular, Pearson et al [12] determined the values of D e , r e , and m using the configuration interaction approach with a modest double-zeta spd basis set to be 26 400 cm 21 , 1.276 Å , and 2041 cm 21 , respectively. The most extensive calculations on the spectroscopic constants for the X 1 R 1 state of BH, using the coupled-cluster approach with large correlation-consistent basis sets, were reported by Martin, [25] Pawłowski et al, [31] and Temelso et al [32] The equilibrium internuclear distance r e was predicted in these studies to be 1.22955, 1.2293, and 1.22917 Å , respectively, whereas the fundamental wavenumber m was predicted to be 2270.6, 2272.3, and 2269.4 cm 21 , respectively.…”

Interpretation of Hund’s multiplicity rule for the carbon atom

“…[11,12,13] The origin of Hund's multiplicity rule for heavier atoms has mostly been examined at the HF level. It is interesting to study the origin of Hund's multiplicity rule in heavier atoms beyond the HF level.…”

Correct Interpretation of Hund’s Rule and Chemical Bonding Based on the Virial Theorem

“…A number of authors have studied Hund's rule for atoms [8][9][10][11][12][13][14][15][16][17][18][19] and light molecules [20][21][22][23][24][25][26][27][28] by Hartree-Fock (HF) and other variational calculations. They have found that the stabilization of the highest multiplicity state relative to the lower multiplicity states is ascribed to a lowering in V en that is gained at the cost of increasing V ee as well as T. Davidson 8,9) has first pointed out that V ee is larger for the triplet than for the singlet by HF calculations for low-lying excited states of the helium atom.…”