Change angle in the Newman projection

Question

I would like to be able to modify the angular position of the foreground atoms in the Newman projection: in the documentation, I only saw the modification of the angle for the carbon of the second plane.

Does somebody have an idea ?

Answer 1

Well, I gave this question some thought. Actually it is unnecessary to be able to change the angle of the three frontal atoms in a Newman projection. As why that is so, let's first see what a Newman projections actually is.

In a Newman projection you visualize the orientation of substituents on two adjacent tetrahedral carbon atoms. So the frontal substituents are taken as fixed and by rotation of carbon-carbon axis you can see how the substituents on the second backside carbon atom interact with the frontal substituents. In essence you can visualize steric hindrance and explain energy fluctuations in that rotation.

If you want to rotate the substituents on the frontal carbon atom, you simply switch the point of view along the axis, making the frontal one now the backside carbon atom.

When you are dealing with other than tetrahedral atoms a Newman projection is exchanged for a Fischer projection. Where the Newman projection focuses on the spatial stereochemistry, the Fisher projection focuses on the geometric stereochemistry. So where you can tell whether a structure is eclipsed or staggered in a Newman projection, you can tell if a structure has cis or trans orientation in a Fisher projection.

EDIT

As seen in the comments the OP has a specific application of the Newman projection in mind. This small MWE uses the pic option to pass arguments to a drawing. the angles can now arbitrary chosen to draw a Newman projection:

\documentclass[11pt]{article}
\usepackage[a4paper,margin=2cm]{geometry}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.markings,decorations.pathmorphing,backgrounds,calc,shapes.geometric,shapes.misc}
\begin{document}
    \begin{tikzpicture}
        [pics/arms/.style args={#1,#2,#3}{code={\draw  plot[polar comb, mark=*] coordinates { (#1:2) (#2:2) (#3:2)};}}
        ]
    \begin{scope}[on background layer, thick]
        \draw[blue] (0,0) pic {arms={25,45,75}};
        \filldraw[black!10] (0,0) circle[radius=1];
    \end{scope}

    \begin{scope}[thick]    
        \draw[red] (0,0) pic {arms={180,225,315}};
        \filldraw[black] (0,0) circle[radius=0.2];
    \end{scope}
\end{tikzpicture}   


\end{document}

Resulting in:

Adding the option to label the positions with groups using the chemmacros package and its options, the code becomes:

\documentclass[11pt]{article}
\usepackage[a4paper,margin=2cm]{geometry}
\usepackage{tikz}
\usetikzlibrary{decorations,decorations.markings,decorations.pathmorphing,backgrounds,calc,shapes.geometric,shapes.misc}
\usepackage[charter]{mathdesign}
\usepackage[minimal]{chemmacros}
\chemsetup{greek=mathdesign,formula=chemformula}
\chemsetup[chemformula]{format={\sffamily}}
\begin{document}
    \begin{tikzpicture}
        [pics/arms/.style args={#1,#2,#3}{code={\draw  plot[polar comb, mark=*] coordinates { (#1:2) (#2:2) (#3:2)};
            \draw plot coordinates {(#1:2.5)} node(atm1) {};
            \draw plot coordinates {(#2:2.5)} node(atm2) {};
            \draw plot coordinates {(#3:2.5)} node(atm3) {};
        }}
        ]
    \begin{scope}[on background layer, thick]
        \draw[blue] (0,0) pic {arms={25,45,75}};
        \filldraw[black!10] (0,0) circle[radius=1];
        \node at (atm1) {\ch{CH3}};
        \node at (atm2) {\ch{C2H5}};
        \node at (atm3) {\ch{H}};
    \end{scope}

    \begin{scope}[thick]    
        \draw[red] (0,0) pic {arms={180,225,315}};
        \filldraw[black] (0,0) circle[radius=0.2];
        \node at (atm1) {\ch{CH3}};
        \node at (atm2) {\ch{H}};
        \node at (atm3) {\ch{Cl}};
    \end{scope}
\end{tikzpicture}   


\end{document}

which results in: