I have written quite a number of posts on using quantum mechanics computations to predict NMR spectra that can aid in identifying chemical structure. Perhaps the most robust technique is Goodman’s DP4 method (post), which has seen some recent revisions (updated DP4, DP4+). I have also posted on the use of computed coupling constants (posts). Grimblat, Gavín, Daranas and Sarotti have now combined these two approaches, using computed 1H and 13C chemical shifts and 3JHH coupling constants with the DP4 framework to predict chemical structure.1 They describe two different approaches to incorporate ..read more

Can vibrational spectroscopy be used to identify stereoisomers? Medel, Stelbrink, and Suhm have examined the vibrational spectra of (+)- and (-)-α-pinene, (±)-1, in the presence of four different chiral terpenes 2-5.1 They recorded gas phase spectra by thermal expansion of a chiral α-pinene with each chiral terpene. For the complex of 4 with (+)-1 or (-)-1 and 5 with (+)-1 or (-)-1, the OH vibrational frequency is identical for the two different stereoisomers. However, the OH vibrational frequencies differ by 2 cm-1 with 3, and the complex of 3/(+)-1 displays two different OH stretches that d ..read more

A major topic of this blog has been the growing body of studies that demonstrate that dynamic effects can control reaction products (see these posts). Often these examples crop up with valley ridge inflection points. Another cause can be bispericyclic transition states, first discovered by Caramella et al for the dimerization of cyclopentadiene.1 The Houk group now reports on the first trispericyclic transition state.2 Using ωB97X-D/6-31G(d), they examined the reaction of the tropone derivative 1 with dimethylfulvene 2. Three possible products can arrive from different pericyclic reactions: 3 ..read more

For the past twelve years, I have avoided posting on any of my own papers, but I will stoop to some shameless promotion to mention my latest paper,1 since it touches on some themes I have discussed in the past. Back in 2011, Iwamoto, et al. prepared the complex of C601 surrounded by [10]cycloparaphenylene 2 to make the Saturn-like system 3.2 Just last year, Yamamoto, et al prepared the Nano-Saturn 5a as the complex of 1 with the macrocycle 4a.3 The principle idea driving their synthesis was to utilize a ring that is flatter than 2. The structures of 3 and 5b (made with the parent macrocycle 4b ..read more

Selecting the appropriate density functional for one’s molecular system at hand is often a very confounding problem, especially for non-expert or first-time users of computational chemistry. The DFT zoo is vast and confusing, and perhaps what makes the situation worse is that there is no lack of benchmarking studies. For example, I have made more than 30 posts on benchmark studies, and I made no attempt to be comprehensive over the past dozen years! One such benchmark study that I missed was presented by Mardirossian and Head-Gordon in 2017.1 They evaluated 200 density functional using the MGC ..read more

The Pascal group has synthesized dodecaphenyltetracene 1.1 While this paper has little computational work, it is of interest to readers of this blog since I have discussed many aspect of aromaticity. This new tetracene is notable for its large twisting along the tetracene axis: about 97° in the x-ray structure. I have optimized the structure of 1 at B3LYP-D3(BJ)/6-311G(d) and its structure is shown in Figure 1. It is twisted by about 94°. The computed and x-ray structures are quite similar, as seen in Figure 2. Here the x-ray structure is shown with red balls, the computed structure with gray ..read more

Chemists are constantly checking the limits of theories, and the limits of bonding is one that has been subject to many tests of late. I have posted on two recent papers (here, here) that probe just how long a C-C bond can be, and now Li, Miller, and co-workers report a structure that pushes that limit even further out.1 They prepared and obtained the x-ray structure of five derivatives of o-carborane, namely compounds 1, 2a, 3a, 3b and 4. In all of these, the C-C bond in the carborane is stretched well beyond that of a typical C-C bond (see Table 1). The longest case is in 3b where the C-C bo ..read more

Pericyclic reactions remain a fruitful area of research despite the seminal publication of the Woodward-Hoffmann rules decades ago. Here are two related papers of pericyclic reactions that violate the Woodward-Hoffmann rules. First, Solomek, Ravat, Mou, Kertesz, and Jurícek reported on the thermal and photochemical electrocyclization reaction of diphenylcetherene 1a.1 Though they were not able to directly detect the intermediate 2, through careful examination of the photochemical reaction, they were able to infer that the thermal cyclization goes via the formally forbidden conrotatory pathway ..read more

Interesting 18 π-electron systems involving cyclooctadecanonenetriyne rings have been synthesized and examined by computations.1 The mono-, di- and tri-C18 ring compounds 1, 2, and 3 were prepared and the x-ray structure of 2 was obtained. The B3PW91/6-31G(d,p) optimized geometries of 1-3 and of the tetra ring 4 are shown in Figure 1. 1 2 3 4 Figure 1. B3PW91/6-31G(d,p) optimized geometries of 1-4. Since the rings are composed of 18 π-electrons in the π-system perpendicular to the nearly planar ring, the natural question is to wonder if the ring is aromatic. The ..read more

A recent paper by Papov, Shao, Bagdasarian, Benton, Zou, Yang, Houk, and Nelson uncovers a vinyl cation insertion reaction that once again involves dynamic effects.1 They find that vinyl triflates and cyclic vinyl triflates will react with [Ph3C]+[HCB11Cl11]– and triethylsilane to generate vinyl cations that can then be trapped through a C-H insertion reaction. For example, cyclohexenyl triflate 1 reacts in a cyclohexane solvent to give the insertion product 2. The reactions of isomers 3 and 4 give different ratios of the two products 5 and 6. In both cases, the cyclohexyl is trapped predomin ..read more