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Using a combination of two-dimensional infrared (2D IR) and variable temperature Fourier transform infrared (FTIR) spectroscopies the rapid structural isomerization of a five-coordinate ruthenium complex is investigated. In methylene chloride, three exchanging isomers were observed: (1) square pyramidal equatorial, ( 1 ); (2) trigonal bipyramidal, ( 0 ); and (3) square pyramidal apical, ( 2 ). Exchange between 1 and 0 was found to be an endergonic process (Δ H = 0.84 (0.08) kcal mol −1 , Δ S = 0.6 (0.4) eu) with an isomerization time constant of 4.3 (1.5) picoseconds (ps, 10 −12 s). Exchange between 0 and 2 however was found to be exergonic (Δ H = −2.18 (0.06) kcal mol −1 , Δ S = −5.3 (0.3) eu) and rate limiting with an isomerization time constant of 6.3 (1.6) ps. The trigonal bipyramidal complex was found to be an intermediate, with an activation barrier of 2.2 (0.2) kcal mol −1 and 2.4 (0.2) kcal mol −1 relative to the equatorial and apical square pyramidal isomers respectively. This study provides direct validation of the mechanism of Berry pseudorotation – the pairwise exchange of ligands in a five-coordinate complex – a process that was first described over fifty years ago. This study also clearly demonstrates that the rate of pseudorotation approaches the frequency of molecular vibrations.
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This content will become publicly available on November 27, 2026
Diffusion Monte Carlo Study of the Structure and Spectroscopy of H 3 O –
A potential energy surface for H3O– has been developed based on the NN+(MOB-ML) approach we developed for studies of complexes of OH– with two or three water molecules. Unlike those systems, H3O– has two low-energy isomers, H–·H2O and OH–·H2, which differ in energy by less than 2.5 kcal mol–1, and which are separated by a barrier of roughly 4.5 kcal mol–1. We find that by training the NN+(MOB-ML) model using structures based on diffusion Monte Carlo (DMC) simulations initiated in the two potential minima, we are able to obtain a potential surface that describes both isomers. Using these potentials, the structure and spectra of H3O– and its deuterated analogues are investigated using DMC. These calculations show that the ground state wave function for H3O– is mainly localized in the H–·H2O minimum in the potential, with a small amount of the probability amplitude (<5%) in the region of the OH–·H2 minimum. The delocalization of the wave function into the secondary minimum is lowered by deuteration of the water molecule, while replacing H– with D– increases the isomerization due to the shortening of the average distance between the hydride ion and the hydrogen atom in water to which it is bound. Introducing one quantum of excitation in the H–···H2O stretching vibration increases the amount of isomerization, while the isomerization decreases with additional excitation of this mode. Excitation of the free OH stretch in the water molecule also increases the amount of isomerization, while excitation of the out-of-plane bending vibration suppresses the isomerization. The effects of partial deuteration on the frequencies for these vibrations are also explored.
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- Award ID(s):
- 2517458
- PAR ID:
- 10680880
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- The Journal of Physical Chemistry A
- Volume:
- 129
- Issue:
- 47
- ISSN:
- 1089-5639
- Page Range / eLocation ID:
- 10928 to 10939
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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