Research topics

1. Ab initio simulation of coherent three-pulse spectroscopies

2. Ab initio simulation of linear and nonlinear spectra of acetic acid dimers

1. Ab initio simulation of coherent three-pulse spectroscopies

The molecular structure forms the basis for understanding and predicting chemical, physical and biological properties. Multidimensional femtosecond vibrational spectroscopy is a novel tool with great promise for determining the dynamics of molecular structures such as proteins or hydrogen-bonded systems on ultrafast time scales that are hardly accessed by NMR or X-ray spectroscopy (see Multidimensional Femtosecond Correlation Spectroscopies of Electronic and Vibrational Excitations S. Mukamel Ann. Rev. Phys. Chem. 2000, 51, 691-729).

Multidimensional infrared spectroscopy constitutes a three-pulse (four-wave mixing) third-order nonlinear technique that is based on the manipulation and control of multiple quantum coherences to probe coupled vibrational modes. Multidimensional (nD) correlation plots result by displaying the signal with respect to n variables. These may include time intervals between pulses, carrier frequencies, or other pulse parameters like envelopes, polarizations, durations, and even phases.

Advantages of multidimensional spectroscopy are:

• Congested ordinary one-dimensional spectra may be disentangled by spreading the signal into additional dimensions, allowing the time scales of environmental bath interactions to be discerned by lineshape analysis.
• Newly appearing cross peaks in two-dimensional correlation plots contain direct signatures of intra- or intermolecular interactions that can be converted into structural and dynamical information.
• Intense tunable 50-100 fs infrared pulses make it possible to monitor structural changes and ultrafast processes such as vibrational energy relaxation and redistribution, charge transfer, conformational fluctuations, and chemical reactions.
• Properties of potential energy surfaces like diagonal and off-diagonal anharmonicities are measured directly.

Four distinct signal contributions that can be measured independently. 

k_I = - k₁ + k₂ + k₃

k_II = + k₁ - k₂ + k₃

k_III = + k₁ + k₂ - k₃

k_IV = + k₁ + k₂ + k₃

We simulated one- and two-color 2D vibrational spectra, either with all three pulses in resonance with the symmetric and antisymmetric C=O stretching bands (see Figure for a typical photon echo 2D spectrum) or with two resonant with the C=O bands and one tuned to the symmetric and antisymmetric N-H stretching transitions. The one-color experiments reveal information on the coupling of amide I modes, which can be used to monitor the secondary structure of peptides and proteins and follow its fluctuations on ultrafast time scales. The two-color experiments yield the coupling of peptide C=O and N--H bonds, which provides additional insight into structural and dynamical properties.

2. Ab initio simulation of linear and nonlinear spectra of acetic acid dimers

Carboxylic acids are important model systems displaying key features of hydrogen-bonded systems in nature. They represent a structural motif of cooperative hydrogen bonds in proteins containing aspartic or glutamic acid and mimic multiple hydrogen bonds in acid base pairs. Acetic acid predominantly forms cyclic C2h-symmetric dimers with two hydrogen bonds in the gas phase as well as in nonpolar solution.

Vibrational spectra provide information about local properties of hydrogen bonded groups. The lineshapes of the high-frequency O-H stretching mode of OH groups involved in the hydrogen bonds are often very complex and highly congested reflecting the complicated underlying microscopic interactions, and consequently, the dynamics and line broadening mechanisms of hydrogen bonds.

Only recently high-resolution infrared (IR) spectra of acetic acid dimer became available from supersonic jet Fourier transform spectroscopy measured in the group of Martin Suhm. These cold gas phase spectra uncover much of the vibrational fine structure and can thus serve as a reference for zero temperature gas phase calculations to analyze the O-H stretching mode IR spectra.

To reach a quantitative understanding of the spectra we performed density functional theory calculations of the linear O-H stretching mode IR spectra taking the following spectral broadening mechanisms into account: (1) anharmonic coupling between the high-frequency O-H stretching vibration and low-frequency modes, (2) Fermi resonance coupling between the v=1 state of the O-H stretching mode and combination tones of fingerprint modes, and (3) a combination of (1) and (2).

From a comparison of calculated and experimental spectra one can conclude that only the combined mechanism accounts for the full spectral width and substructure. We identify 2 Raman active low-frequency modes, the in-plane bending and stretching inter-dimer hydrogen bond modes, and combination states of the C-O stretching, out-of-plane methyl group deformation, in-plane O-H deformation and the C=O stretching IR and Raman active modes to explain the lineshape of the O-H stretching IR absorption spectram in (CH3-COOH)2. The same conclusion holds for the spectrum of (CD3-COOH)2, except that combination bands of the methyl group deformation mode are missing. Only a few peaks of minor intensity observed in the gas phase IR spectrum are not accounted for by the present calculations. Thus, it is demonstrated that both anharmonic coupling to low-frequency modes as well as Fermi resonance coupling with fingerprint modes are essential mechanisms underlying the lineshape of the O-H stretching IR absorption band in acetic acid dimers.

These studies were extended to simulations of the 2D IR spectra to investigate the multidimensional signatures of the respective coupling mechanisms. Coherent 2D IR spectra were predicted for all 3 coupling mechanisms (homogeneous line width = 1 cm-1, vibrational temperature 0 K). Spectra including the R3 diagram only resemble conditions, where the excited vibrational state has decayed already.

a. Anharmonic coupling

b. Fermi resonance coupling to fingerprint combination modes

c. Combined mechanism

In going from anharmonic coupling of the O-H stretching mode to low-frequency modes (3 modes) to Fermi resonance coupling with fingerprint mode combination tones (9 modes) to a combined mechanism (11 modes) the number of individual 2D peaks grows considerably and the complexity of the vibrational signatures increases substantially.

Introduction of homogeneous broadening (36 cm-1) causes many individual peaks to overlap to a single vibrational feature. This spectrum, calculated for ground state absorption only (R3 feynman pathways), may be compared to the experimental 2D IR spectrum measured at a population relaxation time of T=400 fs, at which the excited vibration state of the O-H stretching mode has decayed already. Thus, the spectra contain only diagonal and cross peaks. The most striking diagonal and cross peaks are observed for the most intense bands in the linear spectrum. From the good agreement with experimental results it is concluded that Fermi resonances dominate the T=400 fs experimental 2D spectrum. Low-frequency progressions make a minor contribution to the 2D spectra because of their substantially smaller transition dipoles.

Publications on ab initio simulations of linear and nonlinear spectra of acetic acid dimers

Publications on ab initio simulations of nonlinear response

Publications on excited-state structures and intramolecular charge transfer