Add time:09/09/2019 Source:sciencedirect.com
Structures have been optimised using ab initio molecular orbital calculations for cyclic ions 1 and β-substituted ions 2 where R1 and R2 are the three possible combinations of H and CH3 and Z is NH2, PH2, AsH2, OH, SH and SeH. The 3–21G basis set was used for ions with Z = NH2, OH, SH, and PH2, and the MINI 1 basis set for ions containing SeH and AsH2. Unsymmetrically substituted rings have C1Z bonds which are longer than C2Z and this difference is overemphasised by the small basis set calculations. All bridged ions are lower in energy than the β substituted ions except for the combination of R1 = R2 = CH3 and Z = OH+. For Group VI elements the ability of substituents to bridge increases with the size of the bridging atom, i.e., OH < SH < SeH. The opposite trend is found for substituents containing elements from Group V where the order is NH2 > PH2 ≈ AsH2.
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