4
$\begingroup$

According to VSEPR theory, for $\mathrm{sp^2}$ hybridisation the bond angle is approximately $120^\circ$. In $\ce{H3CNCS}$, the nitrogen has one lone pair. As lone-pair-bond-pair repulsion is greater than bond-pair-bond-pair repulsion, the bond angle of $\ce{CNC}$ should be less than $120^\circ$, but it is actually greater. How can this be rationalised?

$\endgroup$
5
  • 3
    $\begingroup$ I took a liberty to correct formatting, notations and added compound's name. But where did you find a C−N−C in $\ce{\ce{CH3CNS}}?$ What angle are you talking about, exactly? Have you seen doi.org/10.1071/CH10303, especially its SI available for free? $\endgroup$
    – andselisk
    Jul 5, 2020 at 11:06
  • 1
    $\begingroup$ Actually a typing mistake. It would be CH3NCS. Sorry for the inconvenience. $\endgroup$
    – Shiva
    Jul 5, 2020 at 14:04
  • $\begingroup$ No prob, I corrected the name and bond order accordingly. I'm pretty sure answers from Bond angle in CH3NCO would apply here as well: partial delocalization of the lone pair on N into a π orbital on C. This effect is even more pronounced in silyl isothiocyanate, which is almost linear. $\endgroup$
    – andselisk
    Jul 5, 2020 at 14:05
  • 2
    $\begingroup$ @andselisk the answers aren't really good though. To all who are tempted, I would let this question open for the time being. $\endgroup$ Jul 5, 2020 at 18:46
  • $\begingroup$ Related: What is the Si–N–C bond angle in silyl isothiocyanate? $\endgroup$ Jul 8, 2020 at 16:53

1 Answer 1

8
$\begingroup$

I have just answered What is the CNC bond angle in methyl isocyanate? Pretty much everything written there also applies to methyl isothiocyanate, just a bit more extreme.

Here is the structure calculated on the RI-BP86/def2-SVP level of theory:

molecular structure methyl isothiocyanate

The bond angle is $\angle(\ce{CNC}) = 156^\circ$, so we expect a much larger contribution of the right side in the following resonance description: $$\ce{H3C-N=C=S <-> H3C-N^+\bond{3}C-S^-}$$

Reasons for this are not trivial, but that would exceed the scope of this answer.

TL;DR From VSEPR we would expect a trigonal coordination and a $120^\circ$ angle. Refining with resonance, we'll expect a widening of the bond angle. Everything beyond that needs much more involved models.

$\endgroup$
1
  • $\begingroup$ Similar to an other comment, not about methyl isothiocyanates, but $\ce{R-N=C=S}$ there are at least these 15 entries in the COD with coordinates to compare with: crystallography.net/cod/7116329.html and (just substitute the numbers), COD7116328, 7116327, 7116326, 7116325, 7103098, 4117347, 4108942, 4108356, 4075734, 4024672, 4024671, 2234166, 2233483, and 2214501. $\endgroup$
    – Buttonwood
    Jul 8, 2020 at 21:01

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.