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Application of the PM6 method to modeling the solid state.

Stewart JJ - J Mol Model (2008)

Bottom Line: The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated.Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids.The origin of these errors was investigated, and a strategy for improving the method proposed.

View Article: PubMed Central - PubMed

Affiliation: Stewart Computational Chemistry, 15210 Paddington Circle, Colorado Springs, CO 80921, USA. MrMOPAC@OpenMOPAC.net

ABSTRACT
The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated. Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids. The origin of these errors was investigated, and a strategy for improving the method proposed.

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1,8-bis(hexamethyltriaminophosphazenyl)naphthalene
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Fig3: 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene

Mentions: N–H–N In 1-dimethylamino-8-dimethylammonionaphthalene saccharin dihydrate (CSD entry AJOHUC), saccharine donates a proton to the “proton sponge” 1,8-bis(dimethylamino)-naphthalene to form an ionic crystal. The reported structure has that proton asymmetrically positioned between the two nitrogen atoms: N–N′: 2.56, N–H: 1.35, N′–H: 1.26 Å. The optimized PM6 structure gives N–N′: 2.68, N–H: 1.71, N′–H: 1.11. PM6 thus both underestimates the bridging power of the proton and exaggerates the asymmetry of the bond.A closely related species, 4,5-bis(dimethylamino)-1,8-dihydroxynaphthalene, exists as the Zwitterion in the solid (CSD entry RISBIE). In this system, the geometry of the N–H–N′ structure is symmetric, N–H: 1.27 Å, N–N′: 2.57 Å, and the O–H–O structure is unsymmetric, O–H: 1.00 Å, O–O′: 2.45 Å. PM6 predicts both the N–H–N′ (N–H: 1.12 Å, N–N′: 2.68 Å) and the O–H–O′ structures (O–H: 1.09 Å, O–O′: 2.49 Å) to be unsymmetric. In the gas phase, 4,5-bis(dimethylamino)-1,8-dihydroxynaphthalene would most likely exist as the neutral species; a B3LYP/6–31G(d) calculation predicts the energy of the Zwitterionic form to be 0.6 kcal/mol above that of the neutral form; however, PM6 incorrectly predicts that the Zwitterion should be 15.8 kcal/mol more stable than the neutral form.In 2005, an even stronger proton sponge, 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN, was reported [25]. The effect of steric crowding in HMPN arising from the –N = P(N(Me2))3 groups distorts the naphthalene skeleton so that the reported C1–C9–C10–C5 torsion angle, Fig. 3, is 173.9°. The fully optimized PM6 crystal structure predicted this angle to be 170.3°. For the gas-phase structure, B3LYP/6–31G* predicted the torsion angle to be 172.8° [25] while PM6 gave an angle of 165.8°, indicating that the PM6 model was producing either a less rigid naphthalene structure or a greater steric repulsion energy.Fig. 3


Application of the PM6 method to modeling the solid state.

Stewart JJ - J Mol Model (2008)

1,8-bis(hexamethyltriaminophosphazenyl)naphthalene
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2386531&req=5

Fig3: 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene
Mentions: N–H–N In 1-dimethylamino-8-dimethylammonionaphthalene saccharin dihydrate (CSD entry AJOHUC), saccharine donates a proton to the “proton sponge” 1,8-bis(dimethylamino)-naphthalene to form an ionic crystal. The reported structure has that proton asymmetrically positioned between the two nitrogen atoms: N–N′: 2.56, N–H: 1.35, N′–H: 1.26 Å. The optimized PM6 structure gives N–N′: 2.68, N–H: 1.71, N′–H: 1.11. PM6 thus both underestimates the bridging power of the proton and exaggerates the asymmetry of the bond.A closely related species, 4,5-bis(dimethylamino)-1,8-dihydroxynaphthalene, exists as the Zwitterion in the solid (CSD entry RISBIE). In this system, the geometry of the N–H–N′ structure is symmetric, N–H: 1.27 Å, N–N′: 2.57 Å, and the O–H–O structure is unsymmetric, O–H: 1.00 Å, O–O′: 2.45 Å. PM6 predicts both the N–H–N′ (N–H: 1.12 Å, N–N′: 2.68 Å) and the O–H–O′ structures (O–H: 1.09 Å, O–O′: 2.49 Å) to be unsymmetric. In the gas phase, 4,5-bis(dimethylamino)-1,8-dihydroxynaphthalene would most likely exist as the neutral species; a B3LYP/6–31G(d) calculation predicts the energy of the Zwitterionic form to be 0.6 kcal/mol above that of the neutral form; however, PM6 incorrectly predicts that the Zwitterion should be 15.8 kcal/mol more stable than the neutral form.In 2005, an even stronger proton sponge, 1,8-bis(hexamethyltriaminophosphazenyl)naphthalene, HMPN, was reported [25]. The effect of steric crowding in HMPN arising from the –N = P(N(Me2))3 groups distorts the naphthalene skeleton so that the reported C1–C9–C10–C5 torsion angle, Fig. 3, is 173.9°. The fully optimized PM6 crystal structure predicted this angle to be 170.3°. For the gas-phase structure, B3LYP/6–31G* predicted the torsion angle to be 172.8° [25] while PM6 gave an angle of 165.8°, indicating that the PM6 model was producing either a less rigid naphthalene structure or a greater steric repulsion energy.Fig. 3

Bottom Line: The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated.Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids.The origin of these errors was investigated, and a strategy for improving the method proposed.

View Article: PubMed Central - PubMed

Affiliation: Stewart Computational Chemistry, 15210 Paddington Circle, Colorado Springs, CO 80921, USA. MrMOPAC@OpenMOPAC.net

ABSTRACT
The applicability of the recently developed PM6 method for modeling various properties of a wide range of organic and inorganic crystalline solids has been investigated. Although the geometries of most systems examined were reproduced with good accuracy, severe errors were found in the predicted structures of a small number of solids. The origin of these errors was investigated, and a strategy for improving the method proposed.

Show MeSH