Archive for July, 2018

Aminomethylene carbene does not rearrange by tunneling

Eckhardt and Schreiner have spectroscopically characterized the aminomethylene carbene 1.1 Their characterization rests on IR spectra, with comparison to the computed AE-CCSD(T)/cc-pCVQZ anharmonic vibrational frequencies, and the UV-Vis spectra, with comparison to the computed B3LYP/6–311++G(2d,2p) transitions.

1 can be converted to 2 by photolysis. Interestingly, 1 does not convert to 2 after 5 days on the matrix in the dark. This is in distinct contrast to hydroxycarbene and related other carbene which undergo quantum mechanical tunneling (see this post and this post). Examination of the potential energy surface for the reaction of 1 to 2 at AE-CCSD(T)/cc-pCVQZ (see Figure 1) identifies that the lowest barrier is 45.8 kcal mol-1, about 15 kcal mol-1 larger than the barrier for the hydroxycarbene rearrangement. Additionally, the barrier width for 12 is 25% larger than for the hydroxycarbenes. Both of these suggest substantially reduced tunneling, and WKB analysis predicts a tunneling half-life of more than a billion years. The stability of 1 is attributed to the strong π-donor ability of nitrogen to the electron-poor carbene. This is reflected in a very short C-N bond (1.27 Å).

Figure 1. Structures and energies of 1 and 2 and the transition states that connect them. The relative energies (kcal mol-1) are computed at AE-CCSD(T)/cc-pCVQZ.


1) Eckhardt, A. K.; Schreiner, P. R., "Spectroscopic Evidence for Aminomethylene (H−C̈−NH2)—The
Simplest Amino Carbene." Angew. Chem. Int. Ed. 2018, 57, 5248-5252, DOI: 10.1002/anie.201800679.


1: InChI=1S/CH3N/c1-2/h1H,2H2

2: InChI=1S/CH3N/c1-2/h2H,1H2

Schreiner &Tunneling Steven Bachrach 23 Jul 2018 1 Comment

Another long C-C bond

This is a third post in a series dealing with very short or very long distances between atoms. Ishigaki, Shimajiri, Takeda, Katoono, and Suzuki have prepared three related analogues of hexaphenylethane that all have long C-C bonds.1 The idea is to create a core by fusing two adjacent phenyls into a naphylene, and then protect the long C-C bond through a shell made up of large aryl groups, 1. Fusing another 5-member ring opposite to the stretched C-C bond (2) creates a scissor effect that should stretch that bond further, even more so in the unsaturated version 3.

Their M062-x/6-31G* computations predict an increasing longer C-C bond (highlighted in blue in the above drawing): 1.730 Å in 1, 1.767 Å in 2, and 1.771 Å in 3. The structure of 3 is shown in Figure 3.

Figure 1. M06-2x/6-31G(d) optimized structure of 3.

These three compounds were synthesized, and characterized by IR and Raman spectroscopy. Their x-ray crystal structures at 200 K and 400K were also determined. The C-C distances are 1.742 Å (1), 1.773 Å (2) and 1.798 Å (3) with distances slightly longer at 400 K. These rank as the longest C-C bonds recorded.


1) Ishigaki, Y.; Shimajiri, T.; Takeda, T.; Katoono, R.; Suzuki, T., "Longest C–C Single Bond among Neutral Hydrocarbons with a Bond Length beyond 1.8 Å." Chem 2018, 4, 795-806, DOI: 10.1016/j.chempr.2018.01.011.


1: InChI=1S/C40H26/c1-5-17-32-27(11-1)23-24-28-12-2-6-18-33(28)39(32)36-21-9-15-31-16-10-22-37(38(31)36)40(39)34-19-7-3-13-29(34)25-26-30-14-4-8-20-35(30)40/h1-26H

2: InChI=1S/C42H28/c1-5-13-33-27(9-1)17-18-28-10-2-6-14-34(28)41(33)37-25-23-31-21-22-32-24-26-38(40(37)39(31)32)42(41)35-15-7-3-11-29(35)19-20-30-12-4-8-16-36(30)42/h1-20,23-26H,21-22H2

3: InChI=1S/C42H26/c1-5-13-33-27(9-1)17-18-28-10-2-6-14-34(28)41(33)37-25-23-31-21-22-32-24-26-38(40(37)39(31)32)42(41)35-15-7-3-11-29(35)19-20-30-12-4-8-16-36(30)42/h1-26H

Uncategorized Steven Bachrach 10 Jul 2018 No Comments