“…We consider first the interaction of an allyl π system with a parallel oxygen lone pair substituted at position 2. Neglecting the small symmetry breaking effect of the hydroxyl proton, only allyl orbitals φ 1 and φ 3 are of the proper symmetry to mix with the oxygen lone pair (Figure ), and the resulting orbitals are those of a heteroatomic trimethylenemethane (TMM) equivalent. − This mixing is weakly stabilizing for heteroatomic 6-electron system and thus favors distortion of 3,5-DDP toward a bicyclic structure (as probably does also inductive stabilization of the allyl anion by the substituting oxygen). 5 Mixing of allyl anion and nonbonding oxygen orbitals to form 2-oxyallyl (left) and 1-oxyallyl (right) MOs.…”
Geometries and singlet-triplet splittings for the 10 geometrical isomers of didehydrophenol are characterized at a variety of levels of electronic structure theory. The influence of the hydroxyl group is primarily to increase/decrease the weight of zwitterionic singlet mesomers that place positive/negative charge adjacent to oxygen in valence bond descriptions of the arynes. For some of the meta isomers, this interaction stabilizes distortion in the direction of a bicyclic geometry. The net effect, relative to the unsubstituted benzynes, is to increase the singlet-triplet splittings in 2,3-, 2,6-, and 3,5-didehydrophenol and to decrease that splitting in 2,4- and 2,5-didehydrophenol (3,4-didehydrophenol is essentially unaffected). As shown for other arynes, the singlet-triplet splittings can also be accurately estimated by correlation with proton hyperfine coupling constants in antecedent monoradicals, these values being accessible from very economical calculations.
“…We consider first the interaction of an allyl π system with a parallel oxygen lone pair substituted at position 2. Neglecting the small symmetry breaking effect of the hydroxyl proton, only allyl orbitals φ 1 and φ 3 are of the proper symmetry to mix with the oxygen lone pair (Figure ), and the resulting orbitals are those of a heteroatomic trimethylenemethane (TMM) equivalent. − This mixing is weakly stabilizing for heteroatomic 6-electron system and thus favors distortion of 3,5-DDP toward a bicyclic structure (as probably does also inductive stabilization of the allyl anion by the substituting oxygen). 5 Mixing of allyl anion and nonbonding oxygen orbitals to form 2-oxyallyl (left) and 1-oxyallyl (right) MOs.…”
Geometries and singlet-triplet splittings for the 10 geometrical isomers of didehydrophenol are characterized at a variety of levels of electronic structure theory. The influence of the hydroxyl group is primarily to increase/decrease the weight of zwitterionic singlet mesomers that place positive/negative charge adjacent to oxygen in valence bond descriptions of the arynes. For some of the meta isomers, this interaction stabilizes distortion in the direction of a bicyclic geometry. The net effect, relative to the unsubstituted benzynes, is to increase the singlet-triplet splittings in 2,3-, 2,6-, and 3,5-didehydrophenol and to decrease that splitting in 2,4- and 2,5-didehydrophenol (3,4-didehydrophenol is essentially unaffected). As shown for other arynes, the singlet-triplet splittings can also be accurately estimated by correlation with proton hyperfine coupling constants in antecedent monoradicals, these values being accessible from very economical calculations.
“…Geometry Optimizations. The NO 3 + structure optimizations performed in this study were guided by previous theoretical work on NO 3 17-28 and NO 3 + . − We initiated exploratory calculations at the Hartree−Fock level of theory from the optimized NO 3 geometries reported by Stanton et al Optimizations of the NO 3 + equilibrium structure begun from the NO 3 ( D 3 h ) structure ( r e = 1.236 Å) yielded D 3 h structures with r e ranging from 1.17 to 1.18 Å. Optimizations of the NO 3 + equilibrium structure begun from the NO 3 2L1S C 2 v structure (two long and one short N−O bond lengths: r L = 1.266 Å, r S = 1.198 Å, θ O*NO = 126.4°) yielded C 2 v ring structures similar to those found for the isoelectronic CO 3 molecule (Figure ). − Optimizations of the NO 3 + equilibrium structure begun from the NO 3 1L2S C 2 v structure (one long and two short N−O bond lengths: r L = 1.351 Å, r S = 1.206 Å, θ O*NO = 114.0°) resulted in dissociation and were not considered further. Note that none of these optimizations produced a NO 3 + ( C 2 v ) structure similar to the one with 120° bond angles but different N−O bond lengths which Monks et al identified .…”
The equilibrium structure and vibrational frequencies of the nitrate cation, NO 3 + , have been investigated with an extensive set of ab initio calculations. Two stationary points were identified on the NO 3 + potential energy surface: a symmetric D 3h structure and a C 2V ring structure similar to that found for the isoelectronic CO 3 molecule. Geometry optimizations executed at the CCSD(T)/aug-cc-pVTZ level of theory yielded the following data. NO 3. Calculations performed at the B3LYP, QCISD, CCSD, and CCSD(T) levels of theory all predict the C 2V structure to be lower in energy than the D 3h structure. Relative energy calculations performed with the Gaussian and complete basis set model chemistry algorithms also predict the C 2V structure to be the most stable NO 3 + conformation. These results are supported by vibrational frequency calculations which suggest that the D 3h structure may correspond to a second-order saddle point rather than a true minimum on the NO 3 + potential energy hypersurface. The symmetry breaking observed in the present NO 3 + calculations is similar to that observed in ab initio studies of the NO 3 equilibrium structure and is used to examine symmetry breaking across the nitrate series NO 3 -, NO 3 , NO 3 + . † Part of the special issue "Harold Johnston Festschrift".
“…Van de Guchte et al 9 show that 'A; (open Y ) of Con is lower than 'A, (closed Y) by 0.33 eV at the CI/DZPI9 level. However, they neglected to consider triple and quadruple excitations, which respectively add 1.90 and -1.40 eV to this energy difference at the AMP4 level for NO;.…”
Section: Comparison To Isoelectronic Counterpartsmentioning
Ab initio calculations have been performed at the self-consistent field (HF') level, and its perturbative extensions up to fourth-order (MPn), for several electronic states of nitroxylium (NO;) as well as for a large number of reference species. Geometries are optimized at the HFiDZ and HF/DZP levels (double zeta and double zeta plus polarization bases). The ground state is found to be the D, 'A; state, with the C,,, 'A, (closed Y ) state higher by 0.94 eV. The relationship between adding electrons or oxygen atoms to NO+ and NO; is explored, especially in relation to fragmentation energies of NOgq (q = 0 or 1). A comparison is drawn between NO; and two isoelectronic species, CO, and C(CH,),, where no surprises are found.
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