Ultrafast dynamics in iron tetracarbonyl olefin complexes investigated with two-dimensional vibrational spectroscopy

Abstract

The dynamics of iron tetracarbonyl olefin complexes has been investigated using two-dimensional infrared (2D-IR) spectroscopy. Cross peaks between all CO-stretching bands show that the CO-stretch modes are coupled, and from the cross-peak anisotropies we can confirm previous assignments of the absorption bands. From the pump-probe delay dependence of the diagonal peaks in the 2D-IR spectrum we obtain a correlation time of ∼3 ps for the spectral fluctuations of the CO-stretch modes. We observe a multi-exponential pump-probe delay dependence of the cross-peak intensities, with rate constants ranging from 0.1 ps(-1) to 0.6 ps(-1). To determine whether this delay dependence originates from fluxionality of the complex or from intramolecular vibrational relaxation (IVR), we modulate the free-energy barrier of fluxional rearrangement by varying the pi-backbonding capacities of the olefin ligand in two iron tetracarbonyl olefin complexes: Fe(CO)(4)(cinnamic acid) and Fe(CO)(4)(dimethyl fumarate). Since the pi-backbonding strongly influences the rate of fluxionality, comparing the dynamics in the two complexes allows us to determine to what extent the observed dynamics is caused by fluxionality. We conclude that on the time scale of our experiments (up to 100 ps) the cross-peak dynamics in the iron complexes is determined by intramolecular vibrational energy relaxation. Hence, in contrast to previously investigated irontricarbonyl and ironpentacarbonyl complexes, iron tetracarbonyl olefin complexes exhibit no fluxionality on the picosecond time scale.

Infrared Dynamics of Iron Carbonyl Diene Complexes

“…The authors 29 suggest that FTIR measurements "show some evidence of coalescence" although compared with Grevels's observations on NBD and COD, the evidence is not very strong. Woutersen and colleagues 35 conclude from 2D-IR studies that in Fe(CO) 4 (cinnamic acid) and…”

“…Woutersen and colleagues 35 conclude from 2D-IR studies that in Fe(CO) 4 (cinnamic acid) and Fe(CO) 4 (dimethyl fumarate), cross-peak dynamics is determined by IVR, so the complexes exhibit no fluxionality on the picosecond time scale.…”

Ultrafast Intermolecular Vibrational Energy Transfer in Hexahydro-1,3,5-trinitro-1,3,5-triazine in Molecular Crystal by 2D IR Spectroscopy

“…These works demonstrated the power of the infrared methods in studying such energetic materials, particularly the 2D IR method, which is essentially a coherent and multidimensional pump–probe IR method possessing both high time resolution and high structural sensitivity and can simultaneously excite and probe many vibrational chromophores with varying frequencies in the mid-IR region . For this reason, in recent years, the 2D IR method has attracted extensive attention in the study of the structures and dynamics of condensed-phase molecular systems, as well as interface and surface molecular systems. − …”

Ultrafast Vibrational Energy Transfer through the Covalent Bond and Intra- and Intermolecular Hydrogen Bonds in a Supramolecular Dimer by Two-Dimensional Infrared Spectroscopy

“…In general, the waiting-time evolution of the amplitude ratio of a cross-peak to its corresponding diagonal peak in the T w -dependent 2D IR spectra can be used to obtain the energy transfer rate between the two given vibrational modes . One can either use the positive and negative extremes of the real part of the absorptive 2D IR peak or use the amplitude of the absolute part of the 2D IR peak. , Here, in this work, the latter approach is used.…”

“…It is well known that a specific chemical group, such as CO, is an infrared-active vibrator that can be used to characterize molecular structures at the chemical bond level through its vibrational frequency and intensity observed using the conventional steady-state infrared absorption method (i.e., FTIR), which is classically known to be sensitive to chemical bonds (i.e., IR chromophores) and their structural and local solvent environment conditions. Further, molecular structural changes and associated structural dynamics, as well as the vibrational energy relaxation pathways, can be revealed by the transient nonlinear infrared method, such as pump-probe and two-dimensional infrared (2D IR) spectroscopies. − Particularly, time-dependent 2D IR spectroscopy has been used to study a number of fundamental structural and chemical phenomena of molecules, including vibrational population and anisotropic relaxations, − intramolecular vibrational energy redistribution (IVR), − intermolecular vibrational energy transfer (VET), ,− anharmonic vibrational coupling, , and equilibrium structural dynamics − including hydrogen-bonding dynamics − and structural fluctuation correlations. − Here, one of the advantages of the 2D IR method in studying these dynamical processes is that multiple IR chromophores can be utilized selectively and simultaneously in a relatively narrow-band fashion (single or double frequency) or in a broadband fashion.…”