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A261336 Number of achiral molecules containing two linear or branched chiral alkyl chains of n carbon atoms each. 1
0, 0, 0, 1, 3, 10, 30, 88, 255, 742, 2157, 6312, 18563, 54932, 163479, 489264, 1471692, 4447896, 13500689, 41140608, 125818217, 386050543, 1188093392, 3666547089, 11344058829, 35180323336, 109339097119, 340508394528 (list; graph; refs; listen; history; text; internal format)



This series is half the value of the series of chiral planted trees of n nodes (A005628). This number is half the total number of chiral alkanes because the two alkyl chains are required to have the same length with opposite chirality at each chiral center: For each chiral alkane, there is only one other with opposite chirality at each center. The two alkyl chains are attached to a scaffold with a mirror plane between them, thus imparting achirality on the whole molecule. Although such a molecule contains chiral centers, the molecule itself is not chiral--its mirror image is superposable on itself. These compounds are called meso compounds in chemistry.


Table of n, a(n) for n=1..28.

Matthew S. MacLennan, On the Number of Phthalates, Physically Chemist Blog, 17 July 2015.


a(n) = A005628(n) / 2.


Equals half A005628.

Sequence in context: A320564 A094306 A257596 * A026109 A026327 A014531

Adjacent sequences:  A261333 A261334 A261335 * A261337 A261338 A261339




Matthew S. MacLennan, Aug 15 2015



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Last modified October 27 15:27 EDT 2020. Contains 338035 sequences. (Running on oeis4.)