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Title: Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy

Abstract

Using polarization-selective two-dimensional infrared (2D IR) spectroscopy, we measure anharmonic couplings and angles between the transition dipole moments of the four cyanide stretching (ν{sub CN}) vibrations found in [(NH{sub 3}){sub 5}Ru{sup III}NCFe{sup II}(CN){sub 5}]{sup −} (FeRu) dissolved in D{sub 2}O and formamide and [(NC){sub 5}Fe{sup II}CNPt{sup IV}(NH{sub 3}){sub 4}NCFe{sup II}(CN){sub 5}]{sup 4−} (FePtFe) dissolved in D{sub 2}O. These cyanide-bridged transition metal complexes serve as model systems for studying the role of high frequency vibrational modes in ultrafast photoinduced charge transfer reactions. Here, we focus on the spectroscopy of the ν{sub CN} modes in the electronic ground state. The FTIR spectra of the ν{sub CN} modes of the bimetallic and trimetallic systems are strikingly different in terms of frequencies, amplitudes, and lineshapes. The experimental 2D IR spectra of FeRu and FePtFe and their fits reveal a set of weakly coupled anharmonic ν{sub CN} modes. The vibrational mode anharmonicities of the individual ν{sub CN} modes range from 14 to 28 cm{sup −1}. The mixed-mode anharmonicities range from 2 to 14 cm{sup −1}. In general, the bridging ν{sub CN} mode is most weakly coupled to the radial ν{sub CN} mode, which involves the terminal CN ligands. Measurement of the relative transition dipole momentsmore » of the four ν{sub CN} modes reveal that the FeRu molecule is almost linear in solution when dissolved in formamide, but it assumes a bent geometry when dissolved in D{sub 2}O. The ν{sub CN} modes are modelled as bilinearly coupled anharmonic oscillators with an average coupling constant of 6 cm{sup −1}. This study elucidates the role of the solvent in modulating the molecular geometry and the anharmonic vibrational couplings between the ν{sub CN} modes in cyanide-bridged transition metal mixed valence complexes.« less

Authors:
; ; ; ; ; ;  [1]
  1. Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195 (United States)
Publication Date:
OSTI Identifier:
22255056
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Chemical Physics; Journal Volume: 140; Journal Issue: 8; Other Information: (c) 2014 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
37 INORGANIC, ORGANIC, PHYSICAL AND ANALYTICAL CHEMISTRY; ABSORPTION SPECTROSCOPY; AMMONIA; ANHARMONIC OSCILLATORS; COUPLING CONSTANTS; COUPLINGS; CYANIDES; DIPOLE MOMENTS; FORMAMIDE; FOURIER TRANSFORMATION; GROUND STATES; HEAVY WATER; INFRARED SPECTRA; LIGANDS; POLARIZATION; SOLVENTS; TRANSFER REACTIONS; TRANSITION ELEMENTS

Citation Formats

Slenkamp, Karla M., Lynch, Michael S., Van Kuiken, Benjamin E., Brookes, Jennifer F., Bannan, Caitlin C., Daifuku, Stephanie L., and Khalil, Munira, E-mail: mkhalil@chem.washington.edu. Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy. United States: N. p., 2014. Web. doi:10.1063/1.4866294.
Slenkamp, Karla M., Lynch, Michael S., Van Kuiken, Benjamin E., Brookes, Jennifer F., Bannan, Caitlin C., Daifuku, Stephanie L., & Khalil, Munira, E-mail: mkhalil@chem.washington.edu. Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy. United States. doi:10.1063/1.4866294.
Slenkamp, Karla M., Lynch, Michael S., Van Kuiken, Benjamin E., Brookes, Jennifer F., Bannan, Caitlin C., Daifuku, Stephanie L., and Khalil, Munira, E-mail: mkhalil@chem.washington.edu. Fri . "Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy". United States. doi:10.1063/1.4866294.
@article{osti_22255056,
title = {Investigating vibrational anharmonic couplings in cyanide-bridged transition metal mixed valence complexes using two-dimensional infrared spectroscopy},
author = {Slenkamp, Karla M. and Lynch, Michael S. and Van Kuiken, Benjamin E. and Brookes, Jennifer F. and Bannan, Caitlin C. and Daifuku, Stephanie L. and Khalil, Munira, E-mail: mkhalil@chem.washington.edu},
abstractNote = {Using polarization-selective two-dimensional infrared (2D IR) spectroscopy, we measure anharmonic couplings and angles between the transition dipole moments of the four cyanide stretching (ν{sub CN}) vibrations found in [(NH{sub 3}){sub 5}Ru{sup III}NCFe{sup II}(CN){sub 5}]{sup −} (FeRu) dissolved in D{sub 2}O and formamide and [(NC){sub 5}Fe{sup II}CNPt{sup IV}(NH{sub 3}){sub 4}NCFe{sup II}(CN){sub 5}]{sup 4−} (FePtFe) dissolved in D{sub 2}O. These cyanide-bridged transition metal complexes serve as model systems for studying the role of high frequency vibrational modes in ultrafast photoinduced charge transfer reactions. Here, we focus on the spectroscopy of the ν{sub CN} modes in the electronic ground state. The FTIR spectra of the ν{sub CN} modes of the bimetallic and trimetallic systems are strikingly different in terms of frequencies, amplitudes, and lineshapes. The experimental 2D IR spectra of FeRu and FePtFe and their fits reveal a set of weakly coupled anharmonic ν{sub CN} modes. The vibrational mode anharmonicities of the individual ν{sub CN} modes range from 14 to 28 cm{sup −1}. The mixed-mode anharmonicities range from 2 to 14 cm{sup −1}. In general, the bridging ν{sub CN} mode is most weakly coupled to the radial ν{sub CN} mode, which involves the terminal CN ligands. Measurement of the relative transition dipole moments of the four ν{sub CN} modes reveal that the FeRu molecule is almost linear in solution when dissolved in formamide, but it assumes a bent geometry when dissolved in D{sub 2}O. The ν{sub CN} modes are modelled as bilinearly coupled anharmonic oscillators with an average coupling constant of 6 cm{sup −1}. This study elucidates the role of the solvent in modulating the molecular geometry and the anharmonic vibrational couplings between the ν{sub CN} modes in cyanide-bridged transition metal mixed valence complexes.},
doi = {10.1063/1.4866294},
journal = {Journal of Chemical Physics},
number = 8,
volume = 140,
place = {United States},
year = {Fri Feb 28 00:00:00 EST 2014},
month = {Fri Feb 28 00:00:00 EST 2014}
}
  • Picosecond infrared spectroscopy has been used to investigate electron transfer and vibrational excitation and relaxation in the mixed-valence transition-metal dimer [(NC)[sub 5]M[sup II]CNM[sup III](NH[sub 3])[sub 5]][sup [minus]](M = Ru, Os). Optical excitation into the metal-to-metal charge transfer band results in formation of the excited state redox isomer [(NC)[sub 5]M[sup III]CNM[sup II](NH[sub 3])[sub 5]][sup [minus]]. Subsequent back electron transfer results in reformation of the ground state and occurs with [tau] < 0.5 ps. Upon return to the ground electronic state, large amounts of energy (up to 7 quanta, 14 000 cm[sup [minus]1]) are placed into the terminal MC[triple bond]N stretching mode.more » The rate of the subsequent energy relaxation was measured; decay times ranged from <0.5 to 6 ps for the ruthenium analog and from 1.8 to 18 ps for the osmium analog. Results are compared to recent theoretical models of Jortner and Bixon. 36 refs., 10 figs., 5 tabs.« less
  • We study the effects of electron-electron interactions in halogen-bridged mixed-valence transition-metal linear-chain complexes ({ital MX} chains) applying a simple 3/4-filled two-band discrete tight-binding (extended) Peierls-Hubbard model with both on-site and intersite electron-phonon couplings. We employ a variety of methods: perturbation theory, Hartree-Fock approximation, and exact diagonalization in the limit of classical adiabatic phonons, and a variational approach allowing for a finite phonon frequency, i.e., quantum phonons and isotope effect. This variety of methods has proved necessary to obtain a complete picture, due to the structural richness of this model. We investigate the competition between the electron-electron and electron-phonon interactions inmore » a wide range of parameter regimes for both ground and excited states. We focus on values relevant to the {ital MX} chains, probing the experimental variation as {ital X} and {ital M} are varied among {ital X}=Cl, Br, and I and {ital M}=Pt, Pd, and Ni (spanning electron-phonon-interaction-dominated to electron-electron-interaction-dominated materials).« less
  • We consider a 3/4-filled, two-band discrete tight-binding Peierls-Hubbard model for an isolated chain of a halogen-bridged, mixed-valence, transition-metal linear-chain complex (HMMC or {ital MX} chain). We have employed the adiabatic approximation in which the quantum fluctuations associated with phonons are implicitly treated as an external field for the electrons, and treat electron-electron effects in the Hartree-Fock approximation. We investigate ground states as functions of the model parameters and doping-induced and photoinduced excitations---kinks, polarons, bipolarons, and excitons. Results for several experimental observables, including the lattice distortion, the excess charge and spin densities of defects, and the optical absorption, are compiled. Formore » the ground state, we find that the bond-order-wave (BOW) portion of the one-band phase diagram is eliminated from the two-band phase diagram, in agreement with the lack of real materials in the pure BOW phase. The extent of electron-hole asymmetry and of spatial localization or delocalization of defects is explored. Two separate solitons or polarons are compared with corresponding bipolarons. We demonstrate explicitly the need to employ the two-band model for a realistic modeling of the {ital MX} systems, focusing on three specific systems: (a) highly distorted, valence-localized (strongly charge-disproportionated) PtCl, (b) moderately distorted PtBr, and (c) weakly distorted, valence-delocalized (weak charge-density wave) PtI. The compilation of results reported here constitutes a reference resource against which the rapidly expanding experimental data can be compared.« less
  • Cyanide-bridged mixed-valence complexes are interesting examples of strongly covalently linked redox systems which, nevertheless, exist in valence-localized form. As mixed-valence species, they display fairly intense intervalence (or metal-to-metal) charge-transfer transitions ([epsilon] [approx] 3000 M[sup [minus]1] cm[sup [minus]1]), which tend to be shifted toward the visible region from the near-infrared on account of substantial redox asymmetry. The authors have recently succeeded in obtaining (by femtosecond transient absorbance spectroscopy) a direct measure of the thermal kinetics (k[sub ET]) of the highly exothermic back-electron-transfer reaction which follows intervalence excitation in one of these complexes, (H[sub 3]N)[sub 5]Ru-NC-Fe(CN)[sub 5][sup [minus]]. With the available structural,more » kinetic, and thermodynamic information, the only additional ingredient needed to complete the picture of intramolecular electron transfer is a knowledge of initial-state/final-state electronic coupling. From the work of Burewicz and Haim, one estimate of the size of the coupling element (H[sub if]) is, in fact, already available. They report H[sub if] = 1500 cm[sup [minus]1], on the basis of a Hush-Mulliken analysis of the oscillator strength for intervalence absorption. The purpose of this paper is to report on an alternative electrochemical investigation of electronic coupling as well as donor/acceptor orbital mixing - with particular emphasis on comparisons to the oscillator strength method.« less
  • Patterns in the cyanide stretching frequencies have been examined in several series of monometal- and CN{sup {minus}} bridged transition metal complexes. Metal-to-cyanide back-bonding can be identified as a major factor contributing to red shifts of v{sub CN} in monometal complexes. This effect is complicated in cyanide-bridged complexes in two ways: (a) when both metals can back-bond to cyanide, the net interaction is repulsive and results in a blue shift of v{sub CN}: and (b) when a donor and acceptor are bridged, V{sub CN} undergoes a substantial red shift (sometimes more than 60 cm{sup {minus}1} lower in energy than the parentmore » monometal complex). These effects can be described by simple perturbational models for the electronic interactions. Monometal cyanide complexes and CN{sup {minus}}-bridged backbonding metals can be treated in terms of their perturbations of the CN{sup {minus}} {pi} and {pi}* orbitals by using a simple, Hueckel-like, three-center perturbational treatment of electronic interactions. However, bridged donor-acceptor pairs are best described by a vibronic model in which it is assumed that the extent of electronic delocalization is in equilibrium with variations of some nuclear coordinates. Consistent with this approach, it is found that (a) the oscillator strength of the donor-acceptor charge transfer (DACT) absorption is roughly proportional to the red shift of v{sub CN} and (b) there are strong symmetry constraints on the coupling.« less