Structures, Energies, and Spin鈥揝pin Coupling Constants of Methyl-Substituted 1,3-Diborata-2,4-diphosphoniocyclobutanes: Four-member B鈥揚鈥揃鈥揚 Rings B2P2(CH3)n
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- 作者：Janet E. Del Bene ; Ibon Alkorta ; Jose虂 Elguero
- 刊名：Journal of Physical Chemistry A
- 出版年：2011
- 出版时间：September 29, 2011
- 年：2011
- 卷：115
- 期：38
- 页码：10502-10510
- 全文大小：859K
- 年卷期：v.115,no.38(September 29, 2011)
- ISSN：1520-5215
文摘
An ab initio study has been carried out to determine the structures, relative stabilities, and spin鈥搒pin coupling constants of a set of 17 methyl-substituted 1,3-diborata-2,4-diphosphoniocyclobutanes B2P2(CH3)nH8鈥?i>n, for n = 0, 1, 2, 4, with four-member B鈥揚鈥揃鈥揚 rings. The B鈥揚鈥揃鈥揚 rings are puckered in a butterfly conformation, in agreement with experimental data for related molecules. Isomers with the CH3 group bonded to P are more stable than those with CH3 bonded to B. If there is only one methyl group or if two methyl groups are bonded to two different P or B atoms, isomers with equatorial bonds are more stable than those with axial bonds. However, when two methyl groups are present, the gem isomers are the most stable for molecules B2P2(CH3)2H6 with P鈥揅 and B鈥揅 bonds, respectively. Transition structures present barriers to the interconversion of two equilibrium structures or to the interchange of axial and equatorial positions in the same isomer. These barriers are very low for the isomer with two methyl groups bonded to B in axial positions for the isomer with four axial bonds and for the isomer with geminal B鈥揅 bonds at both B atoms. Coupling constants 1J(B鈥揚), 1J(P鈥揅), 1J(B鈥揅), 2J(P鈥揚), and 3J(P鈥揅) are capable of providing structural information. They are sensitive to the number of methyl groups present and can discriminate between axial, equatorial, and geminal bonds, although not all do this to the same extent. The one-bond coupling constants 1J(B鈥揚), 1J(P鈥揅), and 1J(B鈥揅) are similar in equilibrium and transition structures, but 3J(P鈥揅) and 2J(P鈥揚) are not. These coupling constants and those of the corresponding fluoro-derivatives of the 1,3-diborata-2,4-diphosphoniocyclobutanes demonstrate the great sensitivity of phosphorus coupling to structural and electronic effects.