Ab initio valence bond calculations and the spin-paired diradical character of S 4 2+

Quantum chemical calculations of77Se and125Te nuclear magnetic resonance spectral parameters and their structural applications

“…2+ (E = S, Se, Te) at GIAO-DFT(BPW91), [187,188] GIAO-DFT(B3PW91), [168,188] and GIAO-CASSCF [189][190][191][192] levels. The authors found that either pure density functionals (e.g., BPW91) or multiconfigurational ab initio methods are needed to calculate the 77 Se chemical shifts of tetra-atomic chalcogen rings with sufficient accuracy.…”

“…[ 136,147 ] Bayse proposed a solution to that problem in Bayse, [ 152 ] which occurred to be in accordance with the idea suggested by Bühl et al, [ 136 ] consisted in the involvement of multireference methods. Indeed, Tuononen et al [ 153 ] have investigated the δ( 77 Se) of 6π‐electron ring molecules and ions E 2 N 2 and E 4 2+ (E = S, Se, Te) at GIAO‐DFT(BPW91), [ 187,188 ] GIAO‐DFT(B3PW91), [ 168,188 ] and GIAO‐CASSCF [ 189–192 ] levels. The authors found that either pure density functionals (e.g., BPW91) or multiconfigurational ab initio methods are needed to calculate the 77 Se chemical shifts of tetra ‐atomic chalcogen rings with sufficient accuracy.…”

Ab initio valence bond calculations for the ground state of ozone

“…By application of the identity, A -B = AB * AB for a Pauling 'three-electron bond' 1,2958 it may be deduced5c96 that resonance between (1) and (2) is equivalent to resonance between the Lewis octet structures (4)- ( 10). Therefore resonance between (1) and (2) must afford a lower energy than will resonance between the familiar (zwitterionic) Lewis structures (4) and (5) in particular.…”

“…Values of 2 and $ for the parameters k and 1 indicate that the 0-m-electrons of Lewis structures (4) and (5) delocalize more appreciably than do the ii electrons. This reflects the fact that (6) is the primary Lewis component of the increased-valence structures.…”

“…In the above study, the optimum hybridization for the central atom does not correspond to the hybridization that occurs when it is assumed that the bond angle To estimate approximately the extent to which s-p hybridization occurs on the terminal oxygen atoms, we have replaced the 2pu and 2s AOs on these atoms by the hybrid AOs 2 p u + ~2 s and ~2 ~0 -2 4 and calculated the energies for the resonance (1) c* (2) f+ (15) c* (16) (table 3, see later) for selected values of the hybridization parameter K. When K = 0.15, the energy is near a minimum. This value for K has been used in some of the calculations that involve the increased-valence structures.…”