Occasionally, someone comments about an old post here, asking a question. Such was the case here, when a question about the dipole moment of cyclopropenylidene arose. It turned out to be 3.5D, but this question sparked a thought about the related molecule below.
Two record breakers for the anomeric effect; one real, the other not.
July 1st, 2021The classic anomeric effect operates across a carbon atom attached to oxygens. One (of the two) lone pairs on the oxygen can donate into the σ* orbital of the C-O of the other oxygen (e.g. the red arrows) tending to weaken that bond whilst strengthening the donor oxygen C-O bond. Vice versa means e.g. the blue arrows weakening the other C-O bond. This effect tends to increase charge separation and the question then arises: how large can this effect get? To try to find out, we are going to do some crystal structure mining in this post! 
A closer look at that fourth bond in C2.
June 2nd, 2021From the last few posts here, you might have noticed much discussion about how the element carbon might sustain a quadruple bond. The original post on this topic from some years ago showed the molecular orbitals of the species CN+, which included two bonding π-types and a low lying nodeless bonding σ-orbital, all with double occupancies and adding up to a triple bond. Discussing now C2 itself, there are two remaining orbitals for consideration which we will for the purpose here call the highest occupied σ-MO or HOσMO (Σu) and the lowest unoccupied σ-MO or LUσMO (Σg) and which are more mysterious.
A reality-based suggestion for a molecule with a metal M⩸N quadruple bond.
May 13th, 2021I noted in an earlier post the hypothesized example of (CO)3Fe⩸C[1] as exhibiting a carbon to iron quadruple bond and which might have precedent in known five-coordinate metal complexes where one of the ligands is a “carbide” or C ligand. I had previously mooted that the Fe⩸C combination might be replaceable by an isoelectronic Mn⩸N pair which could contain a quadruple bond to the nitrogen. An isoelectronic alternative to FeC could also be FeN+. Here I explore the possibility of realistic candidates for such bonded nitrogen.
References
- A.J. Kalita, S.S. Rohman, C. Kashyap, S.S. Ullah, and A.K. Guha, "Transition metal carbon quadruple bond: viability through single electron transmutation", Physical Chemistry Chemical Physics, vol. 22, pp. 24178-24180, 2020. https://doi.org/10.1039/d0cp03436c
A suggestion for a molecule with a M⩸C quadruple bond with trigonal metal coordination.
May 13th, 2021The proposed identification of molecules with potential metal to carbon quadruple bonds, in which the metal exhibits trigonal bipyramidal coordination rather than the tetrahedral modes which have been proposed in the literature[1],[2],[3] leads on to asking whether simple trigonal coordination at the metal can also sustain this theme?
References
- A.J. Kalita, S.S. Rohman, C. Kashyap, S.S. Ullah, and A.K. Guha, "Transition metal carbon quadruple bond: viability through single electron transmutation", Physical Chemistry Chemical Physics, vol. 22, pp. 24178-24180, 2020. https://doi.org/10.1039/d0cp03436c
- A.J. Kalita, S.S. Rohman, C. Kashyap, S.S. Ullah, I. Baruah, L.J. Mazumder, P.P. Sahu, and A.K. Guha, "Is a transition metal–silicon quadruple bond viable?", Physical Chemistry Chemical Physics, vol. 23, pp. 9660-9662, 2021. https://doi.org/10.1039/d1cp00598g
- L.F. Cheung, T. Chen, G.S. Kocheril, W. Chen, J. Czekner, and L. Wang, "Observation of Four-Fold Boron–Metal Bonds in RhB(BO<sup>–</sup>) and RhB", The Journal of Physical Chemistry Letters, vol. 11, pp. 659-663, 2020. https://doi.org/10.1021/acs.jpclett.9b03484
What does a double σ-bond along a bond axis look like?
May 10th, 2021Introductory chemistry will tell us that a triple bond between say two carbon atoms comprises just one bond of σ-axial symmetry and two of π-symmetry. Increasingly mentioned nowadays is the possibility of a quadruple bond between carbon and either itself or a transition metal, as discussed in the previous post. Such a bond comprises TWO bonds of σ-axial symmetry. Since most people are unfamiliar with such double bonds and in particular with how that second σ-bond sits with the first, I thought it would be interesting to show such an orbital. This one is a localised orbital 41, selected from the previous post for the molecule (PH3)2(CN)2Mo⩸C. Read the rest of this entry »
Two new reality-based suggestions for molecules with a metal M⩸C quadruple bond.
May 8th, 2021Following from much discussion over the last decade about the nature of C2, a diatomic molecule which some have suggested sustains a quadruple bond between the two carbon atoms, new ideas are now appearing for molecules in which such a bond may also exist between carbon and a transition metal atom. A suggested, albeit hypothetical example was C⩸Fe(CO)3[1]. Iron has a [Ar].3d6.4s2 electronic configuration and if we ionise to balance a C4- ligand, the iron becomes formally FeVI or [Ar].3d4. By adding 14 electrons deriving from the seven “bonds” to the 3d4, including a quadruple count from carbon, the Fe formally completes its 18-electron valence shell, as also found in e.g. Ferrocene.
References
- A.J. Kalita, S.S. Rohman, C. Kashyap, S.S. Ullah, and A.K. Guha, "Transition metal carbon quadruple bond: viability through single electron transmutation", Physical Chemistry Chemical Physics, vol. 22, pp. 24178-24180, 2020. https://doi.org/10.1039/d0cp03436c
Dimethyl ketal hydrolysis catalysed by hydroxide and hydronium ions
April 7th, 2021In the preceding post, I looked at a computed mechanism for the hydrolysis of a ketal by water. Of course, pure water consists of three potential catalysts, water itself or [H2O], and the products of autoionisation, [OH–] and [H3O+]. The latter are in much smaller concentration, equivalent to a penalty of ~11.9 kcal/mol on any free energy barrier. Here I take a look at these ion-catalysed routes to see if that penalty can be overcome.
A computational mechanism for the aqueous hydrolysis of a ketal to a ketone and alcohol.
April 1st, 2021The previous post was about an insecticide and made a point that the persistence of both insecticides and herbicides is an important aspect of their environmental properties. Water hydrolysis will degrade them, a typical residency time being in the order of a few days. I noted in passing a dioxepin-based herbicide[1] which contains a ketal motif and which in water can hydrolise to a ketone and alcohol. The reverse (acid catalysed) formation of a ketal is a staple of the taught organic chemistry curriculum. Here as a prelude to looking at the hydrolysis of that dioxepin, I take a look at a possible computational mechanism for the hydrolysis of 2,2-dimethoxypropane using pure water, without the help of acid or base.
References
- P. Camilleri, D. Munro, K. Weaver, D.J. Williams, H.S. Rzepa, and A.M.Z. Slawin, "Isoxazolinyldioxepins. Part 1. Structure–reactivity studies of the hydrolysis of oxazolinyldioxepin derivatives", J. Chem. Soc., Perkin Trans. 2, pp. 1265-1269, 1989. https://doi.org/10.1039/p29890001265
Deltamethrin – a polymorphed insecticide.
March 24th, 2021Deltamethin is a pyrethroid insecticide for control of malaria which has been used for a little while. Perhaps inevitably, mosquitoes are developing resistance to it. So what could be done about countering this? Well, perhaps surprisingly, form a polymorph![1] These crystal structure isomers are often highly undesirable; thus Ritonavir, which changed its polymorphic form during manufacture to become far less active (due it has to be said to insolubility). Now a polymorph of Deltamethin has been discovered, which when applied as a powder, increases its effectiveness more than 10 times against Anopheles mosquitoes and provides a potentially new affordable malaria control solution for countries that are loosing protection.
References
- J. Yang, B. Erriah, C.T. Hu, E. Reiter, X. Zhu, V. López-Mejías, I.P. Carmona-Sepúlveda, M.D. Ward, and B. Kahr, "A deltamethrin crystal polymorph for more effective malaria control", Proceedings of the National Academy of Sciences, vol. 117, pp. 26633-26638, 2020. https://doi.org/10.1073/pnas.2013390117