I'm working on a project to understand the work of the 2016 Nobel laureates and I've come across a problem of not being able to find the right sources to understand basics of helicity (more specifically right-handed helicity (M) and lefthanded helicity (P)) to understand things like the diagram below (I understand cis, trans, I don't know much about the (P,P) and (M,M)).

enter image description here

I would like some suggestions for books or articles that would help me learn more about these concepts.

Thanks in advance!

  • 1
    $\begingroup$ I asked a similar question a while ago (chemistry.stackexchange.com/questions/58517/…), you may find the comment useful $\endgroup$
    – NotEvans.
    Mar 30, 2017 at 17:02
  • $\begingroup$ This wiki article pretty much explains the basics en.wikipedia.org/wiki/Atropisomer $\endgroup$
    – Anon
    Mar 30, 2017 at 17:38
  • $\begingroup$ @Anon I don't think these are atropisomers. There's no free rotation about the double bond. $\endgroup$
    – Zhe
    Mar 31, 2017 at 13:06
  • $\begingroup$ @Zhe Sure they are not, I linked the page as a reference for the M/P notation, as it includes a clear diagram on how it is assigned. $\endgroup$
    – Anon
    Mar 31, 2017 at 13:12
  • $\begingroup$ @Anon Sadly, I would have preferred that they explicitly choose to use either $\ce{A -> A'}$ or $\ce{B' -> A}$ to determine the designation, instead of both. $\endgroup$
    – Zhe
    Apr 1, 2017 at 19:13

1 Answer 1


Any pretty non-old organic stereochemistry book should work.

M/P notation

You present some organic compounds, so let's see definition from the Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013:

P- The helicity rule: stereodescriptors ‘M’ and ‘P

Helicity is the chiral sense of a helical, propeller, or screw-shaped molecular entity (see ref. 37). The ‘helicity’ rule is expressed by Prelog and Helmchen (see subsection 5.1, ref. 36) as: “Depending on Whether the identified helix is left- or right-handed it is designated ‘minus’ and marked ‘M’ or ‘plus’ and marked ‘P’ ”.

The application of this system to the description of conformations and configurations considers the torsion angle between two specified (fiducial) groups are attached to the atoms linked by that bond. The sign of the smaller torsion angle between the fiducial groups defines the sense of chirality sense of the helix (see torsion angle, P-94).

P- The chirality of hexahelicenes is denoted by the stereodescriptors ‘P’ and ‘M’.

Fig.1 - (P)-hexahelicene            Fig.2 - (M)-hexahelicene
(P)-hexahelicene (PIN)                  (M)-hexahelicene (PIN)


The description follows further, the rule can be applied also for biphenyls, allenes, or similar structures, I'd say where along the axis some two approx. perpendicular groups are in clockwise or anticlockwise rotated positions.

The arrows show the helicity orientation in the direction from the observer; but may be confusing(opposite), when one applies common hand and thumb methods for determining a screw thread handedness, but results in the same correct helicity orientation/descriptor.

Δ/Λ notation

In the question you're mentioning molecular motors. These can be also propeller like structures. If they were from inorganic chemistry world, they could use different notation Δ/Λ; from IUPAC Red Book 2005:

IR- The skew-lines convention

Tris(bidentate) complexes constitute a general family for which a useful, unambiguous convention has been developed based on the orientation of skew lines which define a helix.
Examples 1 and 2 represent the delta (Δ) and lambda (Λ) forms of a complex such as $\ce{[Co(NH2CH2CH2NH2)3]^{3+}}$.   (…)

  1. Fig.3 - 'delta' configuration          2. Fig.3 - 'lambda' configuration
    delta (Δ)                      lambda (Λ)

This one is the Δ isomer:

enter image description here

Δ/Λ likely corresponds to organic's P/M respectively (however, in some structures, there might be doubts which parts of the structure should be interpreted as the propeller blades.. see e.g. Q. Molecular knots absolute configuration)

The structures in your ask, 3,3′-dimethyl-2,2′,3,3′-tetrahydro-1H,1′H-4,4′-biphenanthrylidene, do have a combination of a geometrical isomerism at the connector double bond, and two chirality axes (perpendicular to the double bond connector).


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