I'm reasonably unfamiliar with chemical notation, and recently I've come across something I'm not sure how to interpret. In this article, the authors refer to the "surface groups" denoted by $\ce{#Ti-F}$ and $\ce{#Ti-OH}$.

Similarly, this article refers to surface functional groups $\ce{#Si-O-}$ and $\ce{#Si-OH}$.

To my (limited) knowledge, "$\ce{#}$" denotes a triple bond. If this is the case, what am I to take from this? Are they referring to a (triple) dangling bond, or is this some notation I'm not familiar with to denote surface species? The first page of the first article does state that "$\ce{#Ti-X}$" refers to surface species, but I'm finding it difficult to believe the notation was chosen arbitrarily.

I appreciate any explanation. As I've said, I am a novice with chemistry and chemical notation.

  • 1
    $\begingroup$ Better representation would be e.g. ⚞Si−OH, but I did not see any official material discussing this topic. $\endgroup$
    – mykhal
    Jun 7, 2020 at 14:39

1 Answer 1


The notation used does not necessarily represent a triple bond - for example, using one of the examples that you have provided:


Which is for the functional group silanol, that is defined by the $\ce{Si-O-H}$ connectivity, it's chemical structure is

enter image description here

Image source

Using this example, the hydrogens represented by the ≡ symbol can be replaced by other chemicals forming other members of the silanol functional group, for example, Trimethylsilanol:

enter image description here

  • 2
    $\begingroup$ To further clarify, that $\ce{#}$ indicates that the $\ce{Si}$ atom has three bonds sub-surface things: it may be three single bonds to three individual atoms or groups as indicated here. It may be one single and one double bond to two atoms or groups. It may be a triple bond to a single atom or group. The $\ce{#Si-OH}$ notation indicates that since we are only interested in surface groups, we don't care about the sub-surface structure and connectivity. $\endgroup$
    – Ben Norris
    Feb 28, 2014 at 10:58

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