High-Frequency Fe–H and Fe–H₂ Modes in a trans-Fe(η²-H₂)(H) Complex: A Speed Record for Nuclear Resonance Vibrational Spectroscopy

Abstract

Nuclear resonance vibrational spectroscopy (NRVS) and density functional theory (DFT) are complementary tools for studying the vibrational and geometric structures of specific isotopically labeled molecular systems. Here we apply NRVS and DFT to characterize the trans-[57Fe­(η2-H2)­(H)­(dppe)2]­[BPh4] [dppe = 1,2-bis­(diphenylphosphino)­ethane] complex. Heretofore, most NRVS observations have centered on the spectral region below 1000 cm–1, where the 57Fe signal is strongest. In this work, we show that state-of-the-art synchrotron facilities can extend the observable region to 2000 cm–1 and likely beyond, in measurements that require less than 1 day. The 57Fe–H stretch was revealed at 1915 cm–1, along with the asymmetric 57Fe–H2 stretch at 1774 cm–1. For a small fraction of the H2-dissociated product, the 57Fe–H stretch was detected at 1956 cm–1. The unique sensitivity to 57Fe motion and the isolated nature of the Fe–H/H2 stretching modes enabled NRVS to quantitatively analyze the sample composition.

The True Nature of the Energy Calibration for Nuclear Resonant Vibrational Spectroscopy: A Time-Based Conversion

“…This nature leads to a very weak NRVS signal for the Fe−H feature although it is still the best method to observe a Fe−H vibration. For example, several Fe−H and Fe−H 2 stretching modes in trans-[ 57 Fe(η 2 -H 2 )(H)(dppe) 2 ][BPh 4 ] (H 2 FeH for a simple description) were published [48] and are re-illustrated as in Figure 5a, again with the traditional calibration process (1) (blue, corresponding to E* real ). The vibrational modes associated with the NRVS peaks at 1773 and 1915 cm −1 were assigned to the asymmetric Fe-H stretching mode from the Fe(H 2 ) component and the Fe-H stretching from the "local" FeH bond [48].…”

Nuclear Resonance Vibrational Spectroscopy: A Modern Tool to Pinpoint Site-Specific Cooperative Processes

“…People need 36-48 h to move in and set up the HRM and other instruments at the beginning of each beamtime [8]. Nevertheless, researchers are rewarded with amazing NRVS results [7,8,45,47,[65][66][67][68][69].…”

“…A rough recommendation is that NMA should be utilized for smaller/high-symmetry systems where the size of the parameter space can be reasonable and group theory insights are useful. However, in recent years, DFT calculations have been popular as a straightforward approach to interpret vibrational spectra including NRVS [ 7 , 14 , 30 , 32 , 36 , 38 , 44 , 66 , 68 ].…”

“…In addition to the Fe–H bending features, Fe–D or Fe–H stretching features at high-energy regions stand out from all other features and can provide additional and better reference points for DFT calculation vs. proposed structures. Although it is still not possible to directly observe Fe–H/D stretching features in a dilute sample, the features inside various chemical complexes have been observed and published [ 7 , 68 , 102 ]. Figure 15e1 shows the best NiFe hydrogenase model complex so far, which has an Fe–D stretching peak at 1101 cm −1 and doublet Fe–H stretching features at 1476 and 1539 cm −1 [ 7 ].…”

“…The right insert shows the simplified crystal structure for this model. Figure 15e2 illustrated several Fe–H stretching modes and the NRVS for the trans-[ 57 Fe(η 2 H 2 )(H)(dppe) 2 ][BPh 4 ] complex (HFe(H 2 ) for short) [ 68 ]. The left inserts show the vibrational modes for the NRVS peaks at 1770 and 1914 cm −1 ; the former is the asymmetric Fe–H stretching mode from the Fe(H 2 ) structure, while the latter is the Fe–H stretching from the FeH structure.…”