4

Infrared spectroscopy is based on the principle that radiation with a wavelength between 2500 nm and 20000 nm is absorbed by the molecule, altering its vibrational movements. There are various ways to alter the molecule vibrations: for example, for water we can have a bond length change.

enter image description here

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage[italian]{babel}
\usepackage[
  a4paper,
  margin=15mm,
  bindingoffset=2mm,
  heightrounded,
]{geometry}

\usepackage{amsmath}
\usepackage{chemfig}
\usepackage[version=3]{mhchem}
% motivated by section 15.6 of the pgfmanual, @percusses comment <!-- m --><a class="postlink" href="https://tex.stackexchange.com/questions/423941/making-the-determination-of-extrema-more-elegant-in-tikz-not-pgfplots#comment1060557_423941">https://tex.stackexchange.com/questions ... 557_423941</a><!-- m -->
% as well as Henri Menkes's answer <!-- m --><a class="postlink" href="https://tex.stackexchange.com/a/423952/121799">https://tex.stackexchange.com/a/423952/121799</a><!-- m -->
\tikzset{mark path bounding box/.style = {
    path picture={
      \coordinate (#1-bl) at (path picture bounding box.south west);
      \coordinate (#1-tr) at (path picture bounding box.north east);
      \coordinate (#1-br) at (path picture bounding box.south east);
      \coordinate (#1-tl) at (path picture bounding box.north west);
    }
  }
}

\begin{document}

\schemestart
\chemname{\chemfig{@{H1}H-[:30]@{O1}O-[:-30]@{H2}H}}{Stretching simmetrico}
\schemestop
\chemmove{\path[mark path bounding box=HO](H1)--(O1);
\draw[blue,-latex]([yshift=2mm,xshift=-1mm]HO-bl)--([yshift=2mm,xshift=-1mm]HO-tr);
\path[mark path bounding box=OH](H2)--(O1);
\draw[blue,-latex]([yshift=2mm,xshift=1mm]OH-br)--([yshift=2mm,xshift=1mm]OH-tl);
}
\hspace{64pt}
\schemestart
\chemname{\chemfig{@{H1}H-[:30]@{O1}O-[:-30]@{H2}H}}{Stretching asimmetrico}
\schemestop
\chemmove{\path[mark path bounding box=HO](H1)--(O1);
\draw[blue,-latex]([yshift=2mm,xshift=-1mm]HO-bl)--([yshift=2mm,xshift=-1mm]HO-tr);
\path[mark path bounding box=HO](O1)--(H2);
\draw[blue,-latex]([yshift=2mm,xshift=1mm]HO-tl)--([yshift=2mm,xshift=1mm]HO-br);
}
\end{document}

On the other hand, rocking and scissoring cause a variation of the bond angle:

enter image description here

In the last pic, I add the blue arrow with Inkscape. Is it possible do the same thing with LaTeX? If so, could you tell me how?

3

enter image description here

\documentclass{article}
\usepackage[T1]{fontenc}
\usepackage{textcomp}
\usepackage[italian]{babel}
\usepackage[
  a4paper,
  margin=15mm,
  bindingoffset=2mm,
  heightrounded,
]{geometry}

\usepackage{amsmath}
\usepackage{chemfig}
\usepackage[version=3]{mhchem}
% motivated by section 15.6 of the pgfmanual, @percusses comment <!-- m --><a class="postlink" href="https://tex.stackexchange.com/questions/423941/making-the-determination-of-extrema-more-elegant-in-tikz-not-pgfplots#comment1060557_423941">https://tex.stackexchange.com/questions ... 557_423941</a><!-- m -->
% as well as Henri Menkes's answer <!-- m --><a class="postlink" href="https://tex.stackexchange.com/a/423952/121799">https://tex.stackexchange.com/a/423952/121799</a><!-- m -->
\tikzset{mark path bounding box/.style = {
    path picture={
      \coordinate (#1-bl) at (path picture bounding box.south west);
      \coordinate (#1-tr) at (path picture bounding box.north east);
      \coordinate (#1-br) at (path picture bounding box.south east);
      \coordinate (#1-tl) at (path picture bounding box.north west);
    }
  }
}

\begin{document}

\schemestart
\chemname{\chemfig{@{H1-1}H-[:30]@{O1-1}O-[:-30]@{H2-1}H}}{Stretching simmetrico}
\schemestop
\chemmove{\path[mark path bounding box=HO](H1-1)--(O1-1);
\draw[blue,-latex]([yshift=2mm,xshift=-1mm]HO-bl)--([yshift=2mm,xshift=-1mm]HO-tr);
\path[mark path bounding box=OH](H2-1)--(O1-1);
\draw[blue,-latex]([yshift=2mm,xshift=1mm]OH-br)--([yshift=2mm,xshift=1mm]OH-tl);
}
\hspace{64pt}
\schemestart
\chemname{\chemfig{@{H1-2}H-[:30]@{O1-2}O-[:-30]@{H2-2}H}}{Stretching asimmetrico}
\schemestop
\chemmove{\path[mark path bounding box=HO](H1-2)--(O1-2);
\draw[blue,-latex]([yshift=2mm,xshift=-1mm]HO-bl)--([yshift=2mm,xshift=-1mm]HO-tr);
\path[mark path bounding box=HO](O1-2)--(H2-2);
\draw[blue,-latex]([yshift=2mm,xshift=1mm]HO-tl)--([yshift=2mm,xshift=1mm]HO-br);
}
\hspace{64pt}
\schemestart
\chemname{\chemfig{@{H1-3}H-[:30]@{O1-3}O-[:-30]@{H2-3}H}}{Scissoring}
\schemestop
\chemmove{
\draw[blue,-latex]([yshift=-1mm,xshift=-2mm]H1-3.south west)--([yshift=-1mm,xshift=2mm]H1-3.south east);
\draw[blue,-latex]([yshift=-1mm,xshift=-2mm]H2-3.south west)--([yshift=-1mm,xshift=2mm]H2-3.south east);}
\end{document}

Note that I gave the nodes different names. In this example, it is not necessary, but might be considered better practice.

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