In the broad water stretching band (2900−3700 cm-1), frequency-dependent vibrational energy relaxation (VER), and spectral diffusion both occur on the time scale of a few picoseconds. Ultrafast IR−Raman spectroscopy of water is used to study both processes. VER is also studied in solutions of HDO in D2O (HDO/D2O). The OH stretch (νOH) lifetime for water and HDO is ∼1 ps. The OD stretch (νOD) lifetime for D2O is ∼2 ps. Stretch decay generates substantial excitation of the bending modes. The lifetimes of bending vibrations (δ) in H2O, HDO, and D2O can be estimated to be in the 0.6 ps ≤ T 1 ≤ 1.2 ps range. νOH decay in water produces δH 2 O with a quantum yield 1.0 ≤ φ ≤ 2.0. In HDO/D2O solutions, νOH(HDO) decay generates νOD(D2O), δHDO, and δD 2 O. The quantum yield for generating νOD(D2O) is φ ≈ 0.1. The quantum yield for generating both δHDO and δD 2 O is φ ≥ 0.6. Thus, each νOH(HDO) decay generates at minimum 1.2 quanta of bending excitation. After narrow-band pumping, the distribution of excitations within the stretch band of water evolves in time. Pumping on the blue edge instantaneously (within ∼1 ps) generates excitations throughout the band. Pumping on the red edge does not instantaneously generate excitations at the blue edge. Excitations migrate uphill to the blue edge on the 0−2 ps time scale. The fast downhill spectral diffusion is attributed to excitation hopping among water molecules in different structural environments. The slower uphill spectral diffusion is attributed to evolution of the local liquid structure. Shortly after excitations are generated, an overall redshift is observed that is attributed to a dynamic vibrational Stokes shift. This dynamic shift slows down the rate of excitation hopping. Then energy redistribution throughout the band becomes slow enough that the longer VER lifetimes of stretch excitations on the blue edge can lead to a gradual blue shift of population over the next few picoseconds.
In a suspension of reverse micelles, in which the surfactant sodium dioctyl sulfosuccinate (AOT) separates a water nanodroplet from a bulk nonpolar CCl4 phase, ultrafast vibrational spectroscopy was used to study vibrational energy transfer from the nanodroplet through the AOT interfacial monolayer to the surrounding CCl4. Most of the vibrational energy from the nanodroplet was transferred to the polar AOT head group within 1.8 picoseconds and then out to the CCl4 within 10 picoseconds. Vibrational energy pumped directly into the AOT tail resulted in a slower 20- to 40-picosecond transfer of energy to the CCl4.
Ultrafast anti-Stokes Raman scattering after intense mid-IR excitation is used to study vibrational energy relaxation (VER) and vibrational cooling (VC) in neat liquid acetonitrile. The mid-IR pulse (3000 cm-1) excites a combination of C−H stretching fundamentals and C−H bending overtones, which are coupled by Fermi resonance. Vibrational excitation decays from the pumped C−H stretch in 5 ps. Almost no energy is transferred from C−H stretch to C⋮N stretch (2253 cm-1). The C⋮N stretch behaves as a VER “blocking group” which keeps vibrational energy in the CH3−C moiety. A 5 ps buildup, which mirrors the C−H stretch decay, is seen in the C−H bending modes (∼1500 cm-1) at about one-half the C−H stretch energy, and in the lowest energy vibration at 379 cm-1, a C−C⋮N bend. By diluting the acetonitrile with CCl4, it is shown that the buildup of population in the C−C⋮N bend mirrors the buildup of excitation of the bath. Monitoring the C−C⋮N bend allows us to track the instantaneous fraction of energy which has been transferred from acetonitrile vibrations to the bath. The 5 ps buildup of the C−C⋮N bend to about one-half its final value shows C−H stretch decay populates the v = 1 C−H bending vibrations, rather than the v = 2 C−H bending overtones. The decay of daughter C−H bend excitations to C−C stretching vibrations (918 cm-1) and the decay of the C−C stretching excitation are also observed. Combined with the C−C⋮N bending data, a rather complete picture of VER and VC in acetonitrile is obtained. VC in acetonitrile takes about 300 ps. An interesting energy recurrence phenomenon is observed. A C−H bending excitation (v = 2) is initially excited by the laser. That excitation is transferred to C−H stretching vibrations, whose decay repopulates the same bending vibration (v = 1).
Ultrafast anti-Stokes Raman spectroscopy of liquid nitromethane (NM) after mid-IR excitation in the C−H stretching region (∼3000 cm-1) is used to study vibrational energy redistribution with ∼1 ps time resolution. Both vibrational energy relaxation (VER) and vibrational cooling (VC) are discussed. Raman probing of CCl4 mixed with the NM is used to monitor the buildup of excitation in the bath of collective lower frequency excitations (phonons). Combining the intramolecular and bath data, a new and intuitive way of visualizing VC in a polyatomic liquid is presented. In NM, VC occurs in three stages. First, energy deposited in the C−H stretch (and a small amount in first overtones of NO2 stretching and CH bending vibrations) is redistributed to every other vibration in a few picoseconds. Second, the higher energy daughter vibrations of the C−H stretch decay (∼1600−1400 cm-1) relax by populating the lower energy vibrations (∼1100−480 cm-1) in ∼15 ps. Third, the lower energy vibrations excited in the first two stages relax by exciting the bath in 50−100 ps. Although the average vibrational energy decreases with time, this process differs from the usual vibrational cascade description of VC.
Phase grating spectroscopy has been used to follow the optically triggered tertiary structural changes of carboxymyoglobin (MbCO) and carboxyhemoglobin (HbCO). Probe wavelength and temperature dependencies have shown that the grating signal arises from nonthermal density changes induced by the protein structural changes. The material displaced through the protein structural changes leads to the excitation of coherent acoustic modes of the surrounding water. The coupling of the structural changes to the fluid hydrodynamics demonstrates that a global change in the protein structure is occurring in less than 30 ps. The global relaxation is on the same time scale as the local changes in structure in the vicinity of the heme pocket. The observed dynamics for global relaxation and correspondence between the local and global structural changes provides evidence for the involvement of collective modes in the propagation of the initial tertiary conformational changes. The energetics can also be derived from the acoustic signal. For MbCO, the photodissociation process is endothermic by 21 +/- 2 kcal/mol, which corresponds closely to the expected Fe-CO bond enthalpy. In contrast, HbCO dissipates approximately 10 kcal/mol more energy relative to myoglobin during its initial tertiary structural relaxation. The difference in energetics indicates that significantly more energy is stored in the hemoglobin structure and is believed to be related to the quaternary structure of hemoglobin not present in the monomeric form of myoglobin. These findings provide new insight into the biomechanics of conformational changes in proteins and lend support to theoretical models invoking stored strain energy as the driving force for large amplitude correlated motions.
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