A Proposed Hew Convention for Graphic Presentation of Molecular Geometry and Topography Hubert Maehr Roche Research Center, Hoffmann-La Roche Inc., Nutley, New Jersey 071 10
9 Bk+
H Although the configuration of a molecule is unambiguously and clearly representable by a molecular model, this tbreedimensional mode ( I ) is not always practical. One-dimenHzOH H.,d sional molecular representations (e.g., Wisswesser line notaCHs CH, CH20H H tion) (2,3) are limited by the absence of conventions for stereocbemical identifications, but even if such conventions exIla Ilb lllc isted, the resulting stereochemical information would be merely equivalent to a verbal description (4). The ahility to express the molecular architecture in two dimensions is therefore of profound importance to the scientist and is , achieved by a number of conventions ( 5 , 6 ) . The common graphic representations of the cubic dimensionality fall into two categories, namely perspective and OH OH OH projective drawings. Perspective representations are easiest to understand. The most elaborate of these are the cornIVa IVb IVC V puter-generated stereoscopic drawings (7) permitting three-dimensional perception of the object nothing short of More popular are the stylized drawings of molecular avisual inspection of amolecular modelasviewed fromafixed models such a s those of Dreiding, which are exemplified by point. The drawing of [lR,3R,4S,6S,7S,8S,lOR]-4-(dim- IIa (sawhorse notation) and further simplified as in IIIa ethoxymethy1)-1, 7-dimethyl-8-methoxy-2,5,9-trioxatricy(trans-decaline) and IVa (methyl 8-D-glucopyranoside). clo[4.2.2.03J0]decane (Ia) serves as an example (8). Hydrogen substituents unessential for clarity are usually omitted. In general, lower horizontal bonds represent closer proximity to the viewer than upper ones, so that the bond C2-C3 in IVa can be regarded as above the paper plane and C5-05 as lying below. The illusion of depth can be enhanced by the interruption of bonds as in bicyclo[2.2.1]hept-2-ene (V), addition of bolder lines (IVh) or isometric drawing of rings. The cumulative effect of all three possibilities is shown in IVc. The perspective nature of
S;
la
114
Journal of Chemical Education
y
'kt / \
Vla
/k\ Vlb
/ \ Vlc
/C\
Vlla
'..J / \ Vllla
Vllb
/ \ Vld
'\#
\\/
/C\
:'[
/ \ Vllc
/c", /Y!\ /Y\ Vlllb
IXa
IXb
a drawine is maintained and its an~licabilitv extended hv the .. use of th'e so-callrd flying wedge notation:^^ illustratGd in Ilh and Vla, the solid wedec extends toward the viewer. the broken wedge extends below the plane of the paper, and the solid lines indicate bonds in the plane. The solid wedge symbol is often drawn as a bold line for simplicity (VIIa-VIIc). Today, interatomic bonds to he visualized as pointing in the direction away from the viewer are generally drawn hy some type of "broken" symhol as in VIa-VId, arranged in order of increasing simplicity. Thus, wedge outlines, once proposed for the same purpose and exemplified in IXa never became popular, and their use has heen virtually discontinued. We maintain that wedge outlines rather intrinsically represent honds directed toward the viewer, much in the same sense of a solid wedge, rendering VIIIa and VIIIb self-explanatory symbols1. Turning to the projective molecular representations we find three basic cateeories. Frontal oroiections alone the axes of certain bonds hake gained wideap&ation in co~formational analysis. Structure Xa, for example, is a projection along the ('6-('7 axis of s t r ~ ~ c t ula. r e The scereoperceptit~nis enhanced by showing carbon and hydrogen atoms which are closer to the viewer (C10,H6) in larger sizes than more remote ones (C8,H7). In a special notation (Newman projection) ( l o ) ,the substituents are projected onto two planes perpendicular to the projection axis. More specifically, the substituents of the frontal atom are projected onto the frontal plane indicated hy a circle; the second projection plane lies behind the first, and the distance between the two planes, although not visible in the drawing, is equal to the interatomic distance of the two atoms defining the projection axis. Xb and IIc show the
Xa
Xb
Newman notations of Xa and IIa, respectively. Side projections are especially useful for the representation of alicyclic systems and are convenientlv accom~lishedhv the . oroiective " "se of the tlying-wedgr nutatiun. The major molecular skeleton is ~roiectedinto the vlone of the naoer and all hvdroeen , substituenk are omitted. klcohol IIa appears now as i ~ dand D-glucose as XIa. Top projections must be further differentiated according to existing conventions. We distinguish therefore between projections of models and Fischer projections. The former are popular notations for rings and especially for fused polycyclic compounds. Again, substituents are advantageously indicated by the symbols used in VIa or VIIa as exemplified in IIIh. A simpler version uses dots to indicate hydrogen atoms above the plane (IIIc), but lacks the obvious universality of the former convention since this notation is limited to hydrogen substituents. The Fischer projection (11), especially useful for comparative purposes of alicyclic systems with
Ilc
F u a recent example in which wedge outlines are used in ihe same sense as solid wedges, see ref. (9).
Ild
Ile
lllb CHO
OH OH
lllc CHO
H+OH
Xla
CH20H
CH20H
Xlb
Xlc
several chiral centers as in sugars, is a top projection of the flying-wedge representation consisting exclusively of solid lines, as in XIc. with the understandine that vertical bonds are in the plane and horkuntal ones are ;hove the plane of the naner.'rhus, the honds 1tf the molecular backbone have to he io&ted to expose all substituents above the plane of projection as shown in XIb. Current usage of the flying-wedge notation requires an additional comment. There is no controversy about the "wedge notation" for substituents above the plane of the drawing surface as the solid isosceles representing the wedges lend perspectivity to the dlagrnm where Increasing narrowing of the aolrd triangle portrays incrrnsing remoreness from the viewer. The current practice of drawing substituents below the plane, unfortunately, is less consistent. In contrast to representation VIa, it has become popular to draw the broken triangles designating honds to more distant substituents in such a way that the triangle widens with increasing distance from the viewer as shown in IXh. T o be sure, interchanges of symbol combinations VIa and IXb hardly cause problems within detailed perspective drawings but are potentially problematic in certain projections. The symbol combination IXb seems to appear unequivocal to the user in view of the clear meaning of the solid isosceles triangle and the assumption that the triangles originate always at the molecular hackbone located in the plane of the drawing surface and extend toward substituents above and below the plane. As a result, IId is recognized to he identical with IIe even in the absence of solid isosceles triangles at the C2 positions. Broken wedges, unaccompanied by solid ones and without a convention governing their meaning in terms of either VIa or IXb, become ambiguous as the borderline between "backbone" and "suhstituent"vanishes. Imaeine the renlacement of the bromint: subsrituents in hoth li;l and Ile with H at C:l and tetrahvdrofuran-2-vl at CZ where the ring is drawn in too orojection and in the same way for both. ?he resulting t& diVolume 62 Number 2
February 1965
115
agrams for 2-(tetrahydrofuran-2-y1)-1-butanolcould now he addressed as enantiomers. Preserving the principle of perspective illustration and thus avoiding the necessity of a new convention we therefore advocate the use of the broken isosceles in the sense of VIa, where increasing distance from the viewer is concurrent with increasing narrowing of either solid or broken wedges. Problems of Molecular Geometry and Topography2 As chira13 molecules are encountered in practice, we differentiate primarily between enantiomerically pure compounds and racemates, and among the former we distinguish between those with known and unknown absolute configurations. Compounds with unknown absolute configuration and containing more than onr rlement of chiraliry,
