Jens Dreyer

Coherent low-frequency motions of hydrogen bonded acetic acid dimers in the liquid phase

Ultrafast vibrational dynamics of cyclic hydrogen bonded dimers and the underlying microscopic interactions are studied in temporally and spectrally resolved pump-probe experiments with 100 fs time resolution. Femtosecond excitation of the O-H and/or O-D stretching mode gives rise to pronounced changes of the O-H/O-D stretching absorption displaying both rate-like kinetic and oscillatory components. A lifetime of 200 fs is measured for the v=1 state of the O-H stretching oscillator. The strong oscillatory absorption changes are due to impulsively driven coherent wave packet motions along several low-frequency modes of the dimer between 50 and 170 cm(-1). Such wave packets generated via coherent excitation of the high-frequency O-H/O-D stretching oscillators represent a clear manifestation of the anharmonic coupling of low- and high-frequency modes. The underdamped low-frequency motions dephase on a time scale of 1-2 ps. Calculations of the vibrational potential energy surface based on density functional theory give the frequencies, anharmonic couplings, and microscopic elongations of the low-frequency modes, among them intermolecular hydrogen bond vibrations. Oscillations due to the excitonic coupling between the two O-H or O-D stretching oscillators are absent as is independently confirmed by experiments on mixed dimers with uncoupled O-H and O-D stretching oscillators.

Hydrogen-bonded acetic acid dimers: anharmonic coupling and linear infrared spectra studied with density-functional theory.

Anharmonic vibrational force field calculations provide a quantitative understanding of the width and substructure of the linear IR-absorption spectrum of the O-H stretching mode in acetic acid dimers (CH3-COOH)2 and (CD3-COOH)2. Anharmonic coupling of the high-frequency upsilon(OH) mode to fingerprint and low-frequency modes is included resulting in 11- and 9-dimensional vibrational Hamiltonians. A sixth-order force field covering up to three-body interactions is used. Force constants are calculated by fitting one-dimensional potential-energy surfaces and a finite difference procedure applying density-functional theory [Becke 3 Lee-Yang-Parr 6-311+G(d,p)]. It is demonstrated that both anharmonic coupling to low-frequency modes as well as Fermi resonance coupling with fingerprint modes are important mechanisms explaining the line shape of the O-H stretching IR-absorption band in acetic acid dimers.

Ultrafast vibrational relaxation processes induced by intramolecular excited state hydrogen transfer

Abstract Vibrational spectra between 1000 and 1700 cm−1 are studied after ultrafast transfer of a hydrogen atom in the excited state of 2-(2′-hydroxyphenyl)benzothiazole. Femtosecond pump–probe experiments reveal new vibrational bands of the keto-S1 state, including the carbonyl stretching band formed by hydrogen transfer. Such bands display a negligible spectral reshaping but blue-shift by up to 7 cm−1 following biexponential kinetics with time constants of 700 fs and 15 ps. The blue-shift is attributed to the anharmonic coupling of the fingerprint vibrations to Raman-active low-frequency modes that are excited upon electronic excitation and depopulated by intramolecular redistribution and cooling to the solvent.

Excited state structure of 4-(dimethylamino)benzonitrile studied by femtosecond mid-infrared spectroscopy and ab initio calculations

Abstract Combining femtosecond transient vibrational spectroscopy and high-level calculations is a powerful tool in the determination of excited-state structures. Striking differences in the experimental vibrational pattern of the locally excited states of 4-(dimethylamino)benzonitrile (DMABN) and 4-aminobenzonitrile (ABN) are explained on the basis of molecular structures obtained from ab initio complete-active-space self-consistent-field (CASSCF) calculations, giving evidence for a strong sensitivity of the molecular structure on modest changes in the substituents. The 4.0 ps charge-transfer time for DMABN in acetonitrile is resolved for the first time by tracking the downshifted CN stretching mode.

Unraveling the structure of hydrogen bond stretching mode infrared absorption bands: An anharmonic density functional theory study on 7-azaindole dimers

The structure of the linear infrared absorption spectrum of the N-H stretching mode in 7-azaindole dimers is analyzed by quartic anharmonic vibrational force field calculations based on density functional theory. It is demonstrated that a multiple Fermi resonance model including contributions from 12 fingerprint vibrational modes, most of them containing considerable contributions of N-H bending motions, combined with a single low-frequency mode satisfactorily explains the complex line shape of N-H stretching mode absorption band.

Linear and nonlinear infrared signatures of local α- and 310-helical structures in alanine polypeptides

Vibrational exciton Hamiltonians for the amide I and amide A modes of both the α- and 310-helical conformations of a fifteen unit polyalanine oligomer CH3–CO(Ala)15–NHCH3 are constructed using density-functional calculations for smaller model peptides. Energy levels as well as the transition dipoles of all singly and doubly excited-state manifolds are calculated. A variety of C13-substituted isotopic derivatives are examined with respect to their ability to reveal differences in local secondary structures in two-dimensional infrared spectra in the amide I region. Amide mode anharmonicities are predicted to be valid indicators of secondary helical structures.

The hydrogen-bonded 2-pyridone dimer model system. 1. Combined NMR and FT-IR spectroscopy study.

2-Pyridone (PD), converting to 2-hydroxypyridine (HP) through a lactam-lactim isomerization mechanism, can form three different cyclic dimers by hydrogen bond formation: (PD)(2), (PD-HP), and (HP)(2). We investigate the complexation chemistry of pyridone in dichloromethane-d(2) using a combined NMR and Fourier transform infrared (FT-IR) approach. Temperature-dependent (1)H NMR spectra indicate that at low temperatures (<200 K) pyridone in solution predominantly exists as a cyclic (PD)(2) dimer, in exchange with PD monomers. At higher temperatures a proton exchange mechanism sets in, leading to a collapse of the doublet of (15)N labeled 2-pyridone. Linear FT-IR spectra indicate the existence of several pyridone species, where, however, a straightforward interpretation is hampered by extensive spectral overlap of many vibrational transitions in both the fingerprint and the NH/OH stretching regions. Two-dimensional IR correlation spectroscopy applied on concentration-dependent and temperature-dependent data sets reveals the existence of the (PD)(2) cyclic dimer, of PD-CD(2)Cl(2) solute-solvent complexes, and of PD-PD chainlike dimers. Regarding the difference in effective time scales of the NMR and FT-IR experiments, milliseconds vs (sub)picoseconds, the cyclic dimers (PD-HP) and (HP)(2), and the chainlike conformations HP-PD, may function as intermediates in reaction pathways through which the protons exchange between PD units in cyclic (PD)(2).

Vibrational excitation and energy redistribution after ultrafast internal conversion in 4-nitroaniline

Nonequilibrium vibrational excitations of para-nitroaniline (PNA, 4-nitroaniline) occurring after internal conversion from the photoexcited charge transfer state are studied by picosecond anti-Stokes Raman scattering. Vibrational excess populations with distinctly different picosecond rise and decay times are found for a number of modes with frequencies between 860 and 1510 cm−1, including the overtone of a non-Raman active mode. A nonthermal distribution of vibrational populations exists up to about 6 ps after photoexcitation. The time-resolved experiments are complemented by steady-state infrared and Raman measurements as well as calculations based on density functional theory, providing a detailed analysis of the steady-state vibrational spectra of PNA and two of its isotopomers. A weakly Raman active vibration at about 1510 cm−1 displays the fastest rise time and a pronounced excess population and—thus—represents the main accepting mode. We suggest that an out-of-plane mode giving rise to the overtone...

Real-Time Observation of the Formation of Excited Radical Ions in Bimolecular Photoinduced Charge Separation: Absence of the Marcus Inverted Region Explained

Unambiguous evidence for the formation of excited ions upon ultrafast bimolecular photoinduced charge separation is found using a combination of femtosecond time-resolved fluorescence up-conversion, infrared and visible transient absorption spectroscopy. The reaction pathways are tracked by monitoring the vibrational energy redistribution in the product after charge separation and subsequent charge recombination. For moderately exergonic reactions, both donor and acceptor are found to be vibrationally hot, pointing to an even redistribution of the energy dissipated upon charge separation and recombination in both reaction partners. For highly exergonic reactions, the donor is very hot, whereas the acceptor is mostly cold. The asymmetric energy redistribution is due to the formation of the donor cation in an electronic excited state upon charge separation, confirming one of the hypotheses for the absence of the Marcus inverted region in photoinduced bimolecular charge separation processes.

Ab initio simulation of the two-dimensional vibrational spectrum of dicarbonylacetylacetonato rhodium(I)

The complete anharmonic cubic and quartic force field of the two carbonyl stretching vibrations of a rhodium di-carbonyl complex is calculated at the density functional level and used to simulate the third-order vibrational response function. The infrared photon echo spectrum calculated using the diagonalized resulting exciton Hamiltonian is in qualitative agreement with measured values. Quartic terms in the potential are critical for reproducing the experimental transition energies and transition dipoles.