From: Robert H. <ha...@st...> - 2017-05-15 22:48:15
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I'm interested in two things. First, feedback on a proposed amendment to CIP Rule 1b. Second, suggestions for how to officially propose this. Current Rule 1: *(1a) higher atomic number precedes lower;* *(1b) a duplicate atom node whose corresponding nonduplicated atom node is the root or closer to the root ranks higher than a duplicate atom node whose corresponding nonduplicated atom node is farther from the root. *Said differently but with the same meaning: *(1a) higher atomic number precedes lower;**(1b) in comparing two duplicate nodes, lower root distance precedes higher root distance, where "root distance" for a duplicate node is defined as* * the distance from its corresponding nonduplicated atom node to the root node.* Proposed amended rule: *(1a) higher atomic number precedes lower;* *(1b) in comparing two duplicate nodes, lower root distance preceded higher root distance, where "root distance" is defined: (i) in the case of **a duplicate atom for which the atomic number is averaged over two or more atoms in applying Rule 1a, * *the distance from the duplicate node itself to the root node; and (ii) in all other cases, the distance of its corresponding nonduplicated atom node to the root node.* If that means nothing to you, ignore this. But it is a critically important addition for any algorithm if it is to correctly assign the stereochemistry even for very simple compounds based on CIP rules 1-5. For example, without that modification, an algorithm following the rules in IUPAC BB 2013 will arrive at "S" for the descriptor for in this compound: [image: Inline image 1] C1=CC=CC(O)=C1[C@H](C2=CC=CC=C2O)O My second question is, having said that, how do I go about officially stating this? Publish? Where? Bob -- Robert M. Hanson Larson-Anderson Professor of Chemistry St. Olaf College Northfield, MN http://www.stolaf.edu/people/hansonr If nature does not answer first what we want, it is better to take what answer we get. -- Josiah Willard Gibbs, Lecture XXX, Monday, February 5, 1900 |