TUNCHEM

11.23  We successfully finished the TUNCHEM advanced computing exploratory project. However, this is just the beginning of another step of our big research endeavor on exploring tunneling reactivity paradigms to drive chemical transformations.

10.23  Concomitantly with a new experimental research project, we are performing multidimensional quantum tunneling computations (CVT/SCT) to investigate the existence of a pure thermally activated tunneling regime in a target reaction system.  We hope to publish these new results during next year. Moreover, we also applied CVT/SCT computations to investigate H/D primary kinetic isotope effects in some special deep H-tunneling reactions. Our surprising results are now being prepared to be submitted for publication.   

09.23   Rate constants with multidimensional quantum tunneling were computed using CVT/SCT methodology implemented in POLYRATE for one heavy-atom tunneling reaction model. The electrocyclic ring-expansion of 2-formyl-2H-benzazirine to the corresponding cyclic-ketenimine (left picture) occurs by spontaneous heavy-atom tunneling in an argon matrix at 10 K with an half-life of ~220 h (k ~8.8 E-7 s-1) (J. Am. Chem. Soc., 2017, 139, 17649−17659).

A benchmark of the relative energies computed at different DFT levels was performed against the high-quality DLPNO-CCSD(T)/def2-QZVPP//revDSD-PBEP86/6-311+G(2d,p) data (with ZPEV included). The results indicate wB97XD/6-311+G(2d,p) as a resonably good DFT method to computed the PES of the corresponding transformation (Table 2).  

The reaction rate constants computed without (CVT) and with multidimensional quantum tunneling (CVT/SCT) at the wB97XD/6-311+G(2d,p) level are given in the form of Arrhenius plots in Figure 2. The computed half-life at 10 K is ~14.2 h (k ~1.4 E-5 s-1) in reasonable  agreement with the experimental half-life of ~220 h (k ~8.8 E-7 s-1).  

To improve the accuracy, we tested computations using variational transition state theory with interpolated single-point energies (VTST–ISPE; dual-level approach). The fully converged reaction path was calculated at wB97XD/6-311+G(2d,p)  level and single-point energies calculated at DLPNO-CCSD(T)/def2-QZVPP level, on the geometries obtained with the first method.  The simplest implementation was followed, consisting on applying the VTST-ISPE method with higher-level calculations only at the reactants, products, and saddle point. Using this approach, the computed half-life at 10 K is ~770 h (k ~2.2 E-7 s-1) in better agreement with the experimental half-life of ~220 h (k ~8.8 E-7 s-1). 

 The POLYRATE input and output files are available here (4) (5). 

07.23   Rate constants with multidimensional quantum tunneling effects included were successfully computed using CVT/SCT (canonical variational theory and small-curvature tunneling) methodology implemented in the POLYRATE for two H-atom tunneling reaction models. The methylhydroxycarbene rearrangement to acetaldehyde (left picture) is known to occur by H-atom tunneling control in an argon matrix at 10 K with an half-life of ~66 min (Science, 2011, 332, 1300−1303).

We first benchmarked the relative energies computed at different DFT levels using the def2-TZVP basis set against the high-quality AE-CCSD(T)/cc-pCVQZ data (Science, 2011, 332, 1300−1303). The results indicate B3LYP/def2-TZVP as the best DFT method with an excellent accuracy (Table 1). CVT/SCT computations are very demanding because require the calculation of vibrationally adiabatic ground state energy VGa along the minimum energy path (i.e. second derivative hessian matrices need to be calculated for several molecular geometries along the MEP) and therefore we can only afford to use a fast DFT functional/basis set methodology.

The corresponding reaction rate constants computed without (CVT) and with multidimensional quantum tunneling effects (CVT/SCT) at the B3LYP/def2-TZVP level are presented in the form of Arrhenius plots in Figure 1. The results show that up to 200 K the methylhydroxycarbene rearrangement to acetaldehyde occurs only by deep H-atom tunneling. The computed half-life at 20 K is ~58 min (k ~2.0 E-4 s-1) in excellent agreement with the experimental half-life of ~66 min. The POLYRATE input and output files are available here (3).  

05.23  Computational programs for multidimensional tunneling calculations [POLYRATE 17-C] were successfully installed and are ready to run [using the supercomputer Navigator at LCA-UC]. The software uses GAUSSRATE 17-B subroutines to interface POLYRATE 17-C with the electronic structure program GAUSSIAN 16. The software performance was successfully evaluated using some test runs. POLYRATE script files are available here (1) (2).

04.23  Our TUNCHEM project start today!