{"id":16601,"date":"2016-07-01T07:47:54","date_gmt":"2016-07-01T06:47:54","guid":{"rendered":"http:\/\/www.ch.imperial.ac.uk\/rzepa\/blog\/?p=16601"},"modified":"2016-07-06T09:08:35","modified_gmt":"2016-07-06T08:08:35","slug":"anomeric-effects-at-boron-silicon-and-phosphorus","status":"publish","type":"post","link":"https:\/\/www.ch.ic.ac.uk\/rzepa\/blog\/?p=16601","title":{"rendered":"Anomeric effects at boron, silicon and phosphorus."},"content":{"rendered":"
\n

\n\tThe anomeric effect occurs at 4-coordinate (sp3<\/sup>) carbon centres<\/a> carrying two oxygen substituents and involves an alignment of a lone electron pair on one oxygen with the adjacent C-O σ*-bond of the other oxygen. Here I explore whether other centres can exhibit the phenomenon. I start with 4-coordinate boron, using the crystal structure search definition below (along with R < 0.1, no disorder, no errors).[1]<\/a><\/span>\"anomeric-bo-sq\"\n<\/p>\n

\n\tThe result shows two prominent clusters, one with both torsion angles being 180°, and another with both being ~60°. This latter is the one that implies that there must be two lone pairs, one on each oxygen, that are anti-periplanar to the adjacent B-O bond. There are two more diffuse clusters where only one antiperiplanar alignment is seen. So yes, 4-coordinate boron can exhibit an anomeric effect!\n<\/p>\n

\n\t\"anomeric-bo\"This compares to the carbon-anomeric plot which is shown here for comparison, where the top right cluster of 180° torsions contains proportionately few hits than with boron.\n<\/p>\n

\n\t\"anomeric-co\"The next centre is at 4-coordinate silicon. Again three significant clusters are seen; one with two antiperiplanar lone pair alignments with Si-O bonds, and two more with just one such alignment. The previous hotspot for which both measured torsions were 180° is largely absent. So here, the anomeric effect is much stronger. Notice also that whereas the torsions in the region of 60° for the carbon centre lie along a ridge coincident with the diagonal  (bottom left to top right), that for the silicon centre show a ridge running orthogonal to the diagonal. An interesting point to follow up perhaps?\"anomeric-sio\"\n<\/p>\n

\n\tSince the off-diagonal clusters are relatively prominent, implying just one anomeric interaction, it is of interest to see if this results in any asymmetry in the two Si-O bond lengths. If its present, the effect is small.\n<\/p>\n

\n\t\"anomeric-sio-distances\"\n<\/p>\n

\n\tFinally 4-coordinate group 15 elements. Most of the hits are in fact for P; there are none for N. This shows four clusters; the two on the diagonal show respectively two and no antiperiplanar interactions. The two off-diagonal clusters show just one such orientation. As with  Si, the ridge in the 60° region run orthogonal to the diagonal.\n<\/p>\n

\n\t\"anomeric-gp15-o\"So this little exploration shows that the anomeric effect, best known for sugars and at a carbon centre, is in fact more general to the adjacent elements.\n<\/p>\n

\n\t <\/p>\n

References<\/h2>\n
  1. H. Rzepa, \"Anomeric effects at boron, silicon and phosphorus.\", 2016. https:\/\/doi.org\/10.14469\/hpc\/696<\/a>\n\n<\/li>\n<\/ol>\n\n<\/div> ","protected":false},"excerpt":{"rendered":"

    The anomeric effect occurs at 4-coordinate (sp3) carbon centres carrying two oxygen substituents and involves an alignment of a lone electron pair on one oxygen with the adjacent C-O σ*-bond of the other oxygen. Here I explore whether other centres can exhibit the phenomenon. 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