Multispectral multidimensional spectrometer spanning the ultraviolet to the mid-infrared.

Multidimensional spectroscopy is the optical analog to nuclear magnetic resonance, probing dynamical processes with ultrafast time resolution. At optical frequencies, the technical challenges of multidimensional spectroscopy have hindered its progress until recently, where advances in laser sources and pulse-shaping have removed many obstacles to its implementation. Multidimensional spectroscopy in the visible and infrared (IR) regimes has already enabled respective advances in our understanding of photosynthesis and the structural rearrangements of liquid water. A frontier of ultrafast spectroscopy is to extend and combine multidimensional techniques and frequency ranges, which have been largely restricted to operating in the distinct visible or IR regimes. By employing two independent amplifiers seeded by a single oscillator, it is straightforward to span a wide range of time scales (femtoseconds to seconds), all of which are often relevant to the most important energy conversion and catalysis problems in chemistry, physics, and materials science. Complex condensed phase systems have optical transitions spanning the ultraviolet (UV) to the IR and exhibit dynamics relevant to function on time scales of femtoseconds to seconds and beyond. We describe the development of the Multispectral Multidimensional Nonlinear Spectrometer (MMDS) to enable studies of dynamical processes in atomic, molecular, and material systems spanning femtoseconds to seconds, from the UV to the IR regimes. The MMDS employs pulse-shaping methods to provide an easy-to-use instrument with an unprecedented spectral range that enables unique combination spectroscopies. We demonstrate the multispectral capabilities of the MMDS on several model systems.

Interfacial Hydration Dynamics in Cationic Micelles Using 2D-IR and NMR.

Using the thiocyanate anion as a vibrational probe chromophore in conjunction with infrared and NMR spectroscopy, we find that SCN- strongly associates with the cationic head group of dodecyltrimethylammonium bromide (DTAB) micelles, both in normal-phase and reverse micelles. In competition with chloride and iodide ions, we find no evidence for displacement of thiocyanate, in accord with the chaotropicity of the Hofmeister ordering, while lending support to a direct interaction picture of its origin. Ultrafast 2D-IR spectroscopy of the SCN- probe in a range of DTAB micelle sizes (w0 = 4 to w0 = 12) shows little if any size dependence on the time scale for spectral diffusion, which is found to be ∼3.5 times slower than in bulk water (both D2O and H2O). Normal-phase micelles studied with 2D-IR exhibit essentially the same spectral dynamics as do reverse micelles, indicating a lack of sensitivity to interfacial curvature. Combined with 1H NMR chemical shift perturbations, we conclude that the SCN- ions tightly associate with the head groups and are partially buried. The 3-4-fold slowdown in spectral diffusion is consistent with the excluded volume model for interfacial perturbation to hydrogen bond reorientation dynamics. On the basis of these observations and comparisons to previous studies of zwitterionic interfaces probed with phosphate transitions, we conclude that the SCN- spectral dynamics in both reverse- and normal-phase micelles is largely dominated by hydration contributions, and offers a promising probe of interfacial hydration at cationic interfaces. Addition of competitive anions alters neither the IR spectra nor the ultrafast dynamics, indicating that SCN- is robustly associated with the head groups.

Acid-Base Formalism Extended to Excited State for O-H···S Hydrogen Bonding Interaction.

Hydrogen bond can be regarded as an interaction between a base and a proton covalently bound to another base. In this context the strength of hydrogen bond scales with the proton affinity of the acceptor base and the pKa of the donor, i.e., it follows the acid-base formalism. This has been amply demonstrated in conventional hydrogen bonds. Is this also true for the unconventional hydrogen bonds involving lesser electronegative elements such as sulfur atom? In our previous work, we had established that the strength of O-H···S hydrogen bonding (HB) interaction scales with the proton affinity (PA) of the acceptor. In this work, we have investigated the other counterpart, i.e., the H-bonding interaction between the photoacids with different pKa values with a common base such as the H2O and H2S. The 1:1 complexes of five para substituted phenols p-aminophenol, p-cresol, p-fluorophenol, p-chlorophenol, and p-cyanophenol with H2O and H2S were investigated experimentally and computationally. The investigations were also extended to the excited states. The experimental observations of the spectral shifts in the O-H stretching frequency and the S1-S0 band origins were correlated with the pKa of the donors. Ab initio calculations at the MP2 and various dispersion corrected density functional levels of theory were performed to compute the dissociation energy (D0) of the complexes. The quantum theory of atoms in molecules (QTAIM), noncovalent interaction (NCI) method, natural bonding orbital (NBO) analysis, and natural decomposition analysis (NEDA) were carried out for further characterization of HB interaction. The O-H stretching frequency red shifts and the dissociation energies were found to be lower for the O-H···S hydrogen bonded systems compared to those for the O-H···O H-bound systems. Despite being dominated by the dispersion interaction the O-H···S interaction in the H2S complexes also conformed to the acid-base formalism, i.e., the D0 and the O-H red shift scaled with the pKa of the donor, similar to that observed in the O-H···O interaction. However, the two classes of H-bonds follow different correlations. In addition we also discuss the nuances associated with the similarity and differences in the hydrogen bonding properties of the two classes in the ground electronic state as well as in the excited state.