Ultrafast two-dimensional infrared spectroscopy for molecular structures and dynamics with expanding wavelength range and increasing sensitivities: from experimental and computational perspectives

Revealing Local Structure of Angiotensin Receptor-Neprilysin Inhibitor (S086) Drug Cocrystal by Linear and Nonlinear Infrared Spectroscopies

Structurally knowing the active sites of a drug is important for understanding its therapeutic functions. S086 is a novel angiotensin receptor-neprilysin inhibitor that consists of the molecular moieties of EXP3174 (the active metabolite of the angiotensin receptor blocker losartan) and sacubitril (a neprilysin inhibitor prodrug) in a 1:1 molar ratio. There are two forms of cocrystals of S086, namely, ξ-crystal and α-crystal, which were formed both via intermolecular coordination bonding to calcium ions, with the aid of internal water. The binding state of multiple carboxyl anions (COO-) to Ca2+ of EXP3174 and sacubitril was examined in this study using infrared (IR) absorption spectroscopy, in which the asymmetric stretching (as) and symmetric stretching (ss) modes of the COO- groups were used as IR probes. Ultrafast two-dimensional (2D) IR spectroscopy was utilized for spectrally assigning the origin of multiple COO- groups by the presence or absence of interchromophore vibrational coupling. Key structural variation between the two crystal forms was found: in the unit cell of ξ-crystal, the ratio of "bridging" and "bidentate" types of COO- binding to Ca2+ for four EXP3174 molecules is 2:2, while the ratio is predicted to be 3:1 in the case of α-crystal. However, in both crystals, four sacubitril molecules are believed to similarly form a "trident" type of COO- binding to Ca2+. Our study demonstrates that linear and nonlinear IR spectroscopies can be used to characterize local crystal structures of drugs and reveal subtle difference between similar crystal structures.

Chlorine-Modified Soluble Melem-Based Graphitic Carbon Nitrite: Facile Synthesis, Catalytic Property and Ultrafast 2D IR Spectroscopic Characterization

On the basis of thermal etching bulk graphitic carbon nitride (g-C3N4), a mild hydrochloric acid treatment method was used in this work to produce g-C3N4 nano-sheets (CNNS) and further carbon nitride with chloride-modification (CNCl). The latter has thinner layer and smaller particle size and exhibit greatly improved dispersibility and solubility in water, DMSO and other polar solvents. A typical photocatalytic reaction in solution driven by CNCl shows a significantly improved photocatalytic performance over bulk g-C3N4 and CNNS. Steady-state analytical tools including SEM, mass, UV-Vis, and IR spectroscopies, and time-resolved two-dimensional infrared (2D IR) vibrational spectroscopy, were used together in this work. Better solubility, more flexible structure, smaller size, easier generation of free radicals and lower recombination rate of electron-hole pair, are believed to be reasons for the superior photocatalytic performance of CNCl.

Structures and dynamics of 8-oxo-7,8-dihydro-2'-deoxyguanosine in neutral and basic aqueous solutions by spectroscopy

8-oxo-7,8-dihydro-2'-dexyoguanine (8-oxo-dG) can be tautomerized to a 6-enolate,8-keto tautomer through nearby-NH deprotonation at elevated pH. In this work, the N3-protonated 8-oxo-dG tautomers in deuterated pH-buffer solutions were studied using steady-state UV/Vis, FTIR, and ultrafast two-dimensional IR spectroscopies. The presence of 6,8-diketo and C6-anionic tautomers at neutral to basic conditions (pD = 7.4-12.0) was revealed by UV/Vis and FTIR results and was further confirmed by 2D IR signals in both diagonal and off-diagonal regions. However, the C6-enol tautomer, which may be an intermediate during the transition from 6,8-diketo to C6-enolate,C8-keto, was not observed appreciably due to its extreme low population. Furthermore, the neutral-to-anionic tautomeric transition of N3H-8-oxo-dG studied in this work occurs under more basic conditions than the N1H-8-oxo-dG reported previously, showing a higher pKa value for N3H than N1H. Finally, vibrational relaxation of the carbonyl stretching mode was found to be both molecular site dependent and pD dependent for 8oxo-dG. Taken together, this work shows that the ultrafast infrared spectroscopic method is effective for examining tautomers and their dynamics in nucleic acids.

Triple-Bond Vibrations: Emerging Applications in Energy and Biological Sciences

Triple bonds, such as that formed between two carbon atoms (i.e., C≡C) or that formed between one carbon atom and one nitrogen atom (i.e., C≡N), afford unique chemical bonding and hence vibrational characteristics. As such, they are not only frequently used to construct molecules with tailored chemical and/or physical properties but also employed as vibrational probes to provide site-specific chemical and/or physical information at the molecular level. Herein, we offer our perspective on the emerging applications of various triple-bond vibrations in energy and biological sciences with a focus on C≡C and C≡N triple bonds.

Dissecting amide-I vibrations in histidine dipeptide

The amide-I vibrational characteristics and conformational preferences of the model compound - histidine dipeptide (Ac-His-NHCH₃, HISD) in gas phase and solution have been revealed with the help of ab initio calculations and wavefunction analyses. The Gibbs free energy surfaces (FESs) of solvated HISD were smoothed by solvent effect to exhibit different structural populations concerning various external environments. It was shown that the most stable conformations of HISD in CHCl₃ and gas phase are C7eq, while those in DMSO and water are β and PPII, respectively. Compared with ALAD, the number of accessible conformational states on these FESs was predicted to be reduced due to the steric effect of imidazole group. The two amide-I normal modes of HISD were found to have intrinsically secondary structural dependencies, and be sensitive to surrounding environments. The average amide-I_a frequencies of HISD isomers in these environments were predicted to be almost the same as those of ALAD, while the amide-I_b mean frequencies were estimated to be lower than ALAD due to the intramolecular interactions between the imidazole group and amino-terminal amide unit. The good linear correlations between amide-I frequencies and the atomic electrostatic potentials (ESPs) of amide groups were also found to interpret the solvent-induced amide-I frequency shifts of HISD at the electronic structure level. These results allow us to gain a deep understanding of amide-I vibrations of HISD, and would be helpful for the site-specific conformational monitoring and spectral interpretation of solvated polypeptides.

Probing the local structure and dynamics of nucleotides using vibrationally enhanced alkynyl stretching

Monitoring the site-specific local structure and dynamics of polynucleotides and DNA is important for understanding their biological functions. However, structurally characterizing these biomolecules with high time resolution has been known to be experimentally challenging. In this work, several 5-silylethynyl-2'-deoxynucleosides and 5-substituted phenylethynyl-2'-deoxynucleosides on the basis of deoxycytidine (dC) and deoxythymidine (dT) were synthesized, in which the alkynyl group shows intensified CC stretching vibration with infrared transition dipole moment magnitude close to that of typical CO stretching, and exhibits structural sensitivities in both vibrational frequency and spectral width. In particular, 5-trimethylsilylethynyl-2'-dC (^TMSEdC, molecule 1a) was examined in detail using femtosecond nonlinear IR spectroscopy. The solvent dependent CC stretching frequency of 1a can be reasonably interpreted mainly as the hydrogen-bonding effect between the solvent and cytosine base ring structure. Transient 2D IR and pump-probe IR measurements of 1a carried out comparatively in two aprotic solvents (DMSO and THF) and one protic solvent (MeOH) further reveal solvent dependent ultrafast vibrational properties, including diagonal anharmonicity, spectral diffusion, vibrational relaxation and anisotropy dynamics. These observed sensitivities are rooted in an extended π-conjugation of the base ring structure in which the CC group is actively involved. Our results show that the intensified CC stretching vibration can potentially provide a site-specific IR probe for monitoring the equilibrium and ultrafast structural dynamics of polynucleotides.

FTIR as a Method for Qualitative Assessment of Solid Samples in Geochemical Research: A Review

This study aims to collect information about soil investigation by FTIR. As we know, the FTIR technique is most often used in organic and bioorganic chemistry, while in geochemistry FTIR spectroscopy is not used very often. Therefore, there is a problem with the identification and interpretation of the IR spectra of minerals contained in sediments and soils. The reason for this is a deficiency of data about characteristic wavenumbers for minerals. Therefore, this study reviews and sums up, in one place, published articles that are connected to an investigation of minerals from 2002 to 2021 (based on the Scopus database). Additionally, the present review highlights various analytical techniques (ATR-FTIR, DRIFT, 2D-IR, and SR-FTIR) and discusses some of them for geochemical study. Additionally, the study describes helpful tools in the data pre-processing of IR spectra (normalization, baseline correction, and spectral derivatives).

Ultrafast Two-Dimensional Infrared Spectroscopy Resolved a Structured Lysine 159 on the Cytoplasmic Surface of the Microbial Photoreceptor Bacteriorhodopsin

Bacteriorhodopsin (bR) is a light-driven microbial receptor, and lysine 159 (K159) is a charged residue on the cytoplasmic (CP) side of its E-F loop. However, its conformation and function remain unknown due to fast surface dynamics. By utilizing a ¹³C, ¹⁵N-labeled lysine (K) as an isotope probe, we created a network of site-specific amide-I vibrational signatures (backbone carbonyl stretch) to identify the frequency contribution of the labeled residues to the amide-I excitonic band structure. Thus, the red-shifted amide-I frequency in the ¹³C, ¹⁵N-lysine-labeled bR (uK-bR) to the unlabeled bR (WT-bR) could be differentiated and examined by ultrafast two-dimensional vibrational echo infrared (2D IR) spectroscopy. Our results showed that the backbone carbonyl of K159 is located at a high frequency of ca. 1693 cm^-1 and has a vibrational excited-state relaxation time shorter than the bulk helical amide-I mode at the same frequency, suggesting that K159 may possess a hydrogen-bonded γ-turn structure with E161, one of the carboxylate residues on the CP surface of bR. The 2D solid-state NMR study of uK-bR also revealed conformational dependent lysine residues, from which K159 was found to involve the turn motif. This γ-turn structure maintained by K159 may help to stabilize the E-F loop and support E161 in attracting protons from the bulk during the late stage of the bR photocycle. The combined spectroscopic approach illustrated in this work may be applied to map residue-specific local structures and dynamics of other receptors and large proteins.

Solution Structures and Ultrafast Vibrational Energy Dissipation Dynamics in Cyclotetramethylene Tetranitramine

Steady-state and time-resolved infrared (IR) studies of cyclotetramethylene tetranitramine (HMX) were carried out, using the asymmetric nitro stretch as probe, to investigate its solution structures and vibrational energy transfer processes in pure DMSO and in DMSO/water mixture. Linear IR spectrum in the nitro stretching mode region shows two major bands and one minor band in DMSO but changes to a two-major band mainly picture when adding water as antisolvent of HMX, suggesting a transition from well solvated and less perfect b-conformation to a less solvated and close-to-perfect b-conformation. The latter bears a similar asymmetric nitro stretch vibration profile as the b-polymorph in crystal form. DFT computations of the nitro stretching vibrations suggest HMX in DMSO may be in a NO2 group rotated b-conformation. Two-dimensional IR cross-peak intensity reveals intramolecular energy transfer between the axial and equatorial nitro groups in the β-HMX on the ps time scale, which is slightly faster in the mixed solvent case. The importance of water as an antisolvent in influencing the equilibrium solvation structure, as well as the vibrational and orientational relaxation dynamics of HMX, is discussed.

Modeling amide-I vibrations of alanine dipeptide in solution by using neural network protocol

Infrared spectroscopy is a powerful tool for the understanding of molecular structure and function of polypeptides. Theoretical interpretation of IR spectra relies on ab initio calculations may be very costly in computational resources. Herein, we developed a neural network (NN) modeling protocol to evaluate a model dipeptide's backbone amide-I spectra. DFT calculations were performed for the amide-I vibrational motions and structural parameters of alanine dipeptide (ALAD) conformers in different micro-environments ranging from polar to non-polar ones. The obtained backbone dihedrals, C = O bond lengths and amide-I frequencies of ALAD were gather together for NN architecture. The applications of built NN protocols for the prediction of amide-I frequencies of ALAD in other solvation conditions are quite satisfactory with much less computational cost comparing with electronic structure calculations. The results show that this cost-effective way enables us to decipher the polypeptide's dynamic secondary structures and biological functions with their backbone vibrational probes.

Revealing the Relationship between Electric Fields and the Conformation of Oxytocin Using Quasi-Static Amide-I Two-Dimensional Infrared Spectra

It is reported that the cis/trans conformation change of the peptide hormone oxytocin plays an important role in its receptors and activation and the cis conformation does not lead to antagonistic activity. Motivated by recent experiments and theories, the quasi-static amide-I 2D IR spectra of oxytocin are investigated using DFT/B3LYP (D3)/6-31G (d, p) in combination with the isotope labeling method under different electric fields. The theoretical amide-I IR spectra and bond length of the disulfide bond are consistent with the experimental values, which indicates that the theoretical modes are reasonable. Our theoretical results demonstrate that the oxytocin conformation is transformed from the cis conformation to the trans conformation with the change of the direction of the electric field, which is confirmed by the distance of the backbone carbonyl oxygen of Cys6 and Pro7, the Ramachandran plot of Cys6 and Pro7, the dihedral angle of C_β-S-S-C_β, and the rmsd of the oxytocin backbone. Moreover, the trans conformation as the result of the turn in the vicinity of Pro7 has a tighter secondary spatial structure than the cis conformation, including stronger hydrogen bonds, longer γ-turn geometry involving five amino acids, and a more stable disulfide bond. Our work provides new insights into the relationship between the conformation, the activation of the peptide hormone oxytocin, and the electric fields.

Ultrafast Structure and Vibrational Dynamics of a Cyano-Containing Non-Fullerene Acceptor for Organic Solar Cells Revealed by Two-Dimensional Infrared Spectroscopy

Non-fullerene molecules, such as ITIC (3,9-bis(2-methylene-(3-(1,1-dicyanomethylene) indanone)-5,5,11,11-tetrakis(4-hexylphenyl)dithieno[2,3-d:2',3'-d']-s-indaceno[1,2-b:5,6-b']-dithiophene), are among the most promising non-fullerene acceptors for organic solar cells (OSCs). In this work, using the cyano stretching mode as a vibrational marker, the structural and vibrational energy dynamics of ITIC were examined on an ultrafast time scale with two-dimensional infrared spectroscopy. Two IR-active modes studied here mainly correspond to two anti-symmetric combinations of symmetric and asymmetric stretching vibrations of two C≡N modes originating from two -C(CN)₂ chromophores that are located across the ITIC system, which were found to have significantly larger off-diagonal anharmonicity than their corresponding diagonal anharmonicities. This indicates strong anharmonic vibrational coupling between the two modes, which is supported by ab initio anharmonic frequency computations. Transient IR results indicate picosecond intramolecular vibrational energy transfer between the two C≡N modes upon excitation. The structural basis for these vibrational and energetic features is the conjugating molecular frame that is composed of a network of single/double bonds connecting the two -C(CN)₂ chromophores and may enable efficient vibration delocalization, in addition to its well-known electron delocalization capability. The importance of the results for the OSC applications is discussed.

Structural dynamics and vibrational feature of N-Acetyl-d-glucosamine in aqueous solution

Molecular dynamics simulations and DFT calculations were performed for the demonstration of the structural dynamics and vibrational feature of N-Acetyl-d-glucosamine (NAG) in solution phase. The interactions between NAG and solvent molecules were evaluated through spatial distribution function and radial distribution function, and the preferred conformations of NAG in aqueous solution were revealed by cluster analysis. Results from normal mode analysis show that the solvent induced structural fluctuation of NAG could be reflected in the vibrational feature of specific chromophores, thus we can evaluate the molecular structure with the help of its vibrational signature based on the built correlation between molecular structure and vibrational frequencies of specific groups.

Deciphering the structural preference encoded in amide-I vibrations of lysine dipeptide in gas phase and in aqueous solution

Protein's biological function is critically associated with its structural feature, which is encoded in its amino acid sequence. For evaluation of conformational fluctuation and folding mechanism, DFT calculations were performed on the model compound - lysine dipeptide (LYSD) in gas phase to demonstrate the correlation between amide-I vibrations and secondary structure. Molecular dynamics simulations were carried out for the structural dynamics of LYSD in aqueous solution. The results show that LYSD tends form C_7eq, C₅, β, PP_II and α conformations in the gas phase and primarily presented PP_II and α conformations in aqueous solution. The obtained amide-I vibrational frequencies of LYSD conformers were assigned, thus build the correlations between amide-I probes and secondary structure of LYSD. These results provide theoretical insights into the structural feature of LYSD through amide-I vibrations, and would shed light on site specific structural prediction of polypeptides.

Quasi-Static Two-Dimensional Infrared Spectra of the Carboxyhemoglobin Subsystem under Electric Fields: A Theoretical Study

There is no doubt that electric fields of a specific frequency and intensity could excite certain vibrational modes of a macromolecule, which alters its mode coupling and conformation. Motivated by recent experiments and theories, we study the mode coupling between the Fe-CO mode and CO-stretch mode and vibration energy transfer among the active site and proteins in carboxyhemoglobin (HbCO) under different electric fields using the quasi-static two-dimensional infrared spectra. This study uses iron-porphyrin-imidazole-CO and two distal histidines in HbCO as the subsystem. The potential energy and dipole moment surfaces of the subsystem are calculated using an all-electron ab initio (B3LYP-D3(BJ)) method with the basis set Lanl2dz for the Fe atom and 6-31G(d,p) for C, H, O, and N atoms. Although the subsystem is reduced dimensionally, the anharmonic frequency and anharmonicity of the CO-stretch mode show excellent agreement with experimental values. We use the revealing noncovalent interaction method to confirm the hydrogen bond between the H_ε atom of the His63 and the CO molecule. Our study confirms that the mode coupling between the Fe-CO mode and CO-stretch mode does not exist when the subsystem is free of electric field perturbation, which is coupled when the electric field is -0.5142 V/nm. In addition, with the increases of distance between the active site and the His92, there is no vibrational energy transfer between them when the electric field is 1.028 V/nm. We believe that our work could provide new ideas for increasing the dissociation efficiency of the Fe-CO bond and theoretical references for experimental research.

Application of a transparent window vibrational probe (azido probe) to the structural dynamics of model dipeptides and amyloid β-peptide

The azido asymmetric stretching motion is widely used for the elucidation of the intrinsic conformational preference and folding mechanism of protein since it has strong vibrational absorbance in the spectral transparent windows. However, the possible secondary structural disturbance induced by the insertion of azido group in the side chain of polypeptides should be carefully evaluated. Here, DFT calculation and enhanced sampling method were employed for model dipeptides with or without azido substitution, and the outcome results show that the lower potential energy basins of isolated model dipeptides are consistent with the preferred structural distributions of model dipeptides in aqueous solution. The azido asymmetric stretching frequency shows its sensitivity to the backbone configurations just like amide-I vibration does, and the azido vibration exhibits great potential as a structural reporter in the transparent window. For the evaluation of the application of azido group in biologically related system, the structural dynamics of Aβ_37-42 and N₃-Aβ_37-42 fragments and the self-assemble process of their protofiliments in aqueous solution were demonstrated. The outcome results show that the structural fluctuations of Aβ_37-42 and its protofilament in aqueous solution are quite similar with or without azido substitution, and the dewetting transitions of Aβ_37-42 and N₃-Aβ_37-42 β-sheet layers are both complete within 30 ns and assemble into stable protofilaments. Therefore, the azido asymmetric vibrational motion is a minimally invasive structural probe and would not introduce much disturbance to the structural dynamics of polypeptides.

Ultrafast Vibrational Energy Transfer through the Covalent Bond and Intra- and Intermolecular Hydrogen Bonds in a Supramolecular Dimer by Two-Dimensional Infrared Spectroscopy

In this work, the structural fluctuations and vibrational energy transfer dynamics in a supramolecular homodimer model, which is composed of 2-(9-anthracene)ureido-6-(1-undecyl)-4[1H]-pyrimidinone (UPAn) with quadruple intermolecular and single intramolecular hydrogen bonds (HBs), have been examined using ultrafast two-dimensional infrared (2D IR) and steady-state IR spectroscopies. A less structurally fluctuating intermolecular HB is found between the pyrimidinone C═O and ureido N-H groups. However, a larger structurally fluctuating intramolecular HB is suggested by the equilibrium and dynamical line-shape measurements of the ureido C═O stretch. Further, dynamical time-dependent 2D IR diagonal and off-diagonal signals show that intra- and intermolecular vibrational energy transfer processes occur on the picosecond timescale, where the latter is more efficient due to intermolecular hydrogen bonding interaction and through-space interaction.

Mapping the amide-I vibrations of model dipeptides with secondary structure sensitivity and amino acid residue specificity, and its application to amyloid β-peptide in aqueous solution

Vibrational spectroscopy has been known as particularly well-suited for deciphering the polypeptide's structure. To decode structural information encoded in IR spectra, we developed amide-I frequency maps on the basis of model dipeptides to correlate the amide-I frequency of interest to the combination of the calculated secondary structure dependent amide-I frequency by using DFT method and the electrostatic potentials that projected onto the amide unit from the micro-environment within molecular mechanics force field. The constructed maps were applied to model dipeptides and amyloid β-peptide fragment (Aβ_25-35). The dipeptide specified map (DS map) and the hybrid map (HYB map) predicted amide-I bands of Aβ_25-35 in solution satisfactorily reproduce experimental observation, and indicate the preference of forming β-sheet and random coil structure for Aβ_25-35 in D₂O just as the results of cluster analysis suggested. These maps with secondary structural sensitivity and amino acid residue specificity open up a way for the interpretation of amide-I vibrations and show their potentials in the understanding of molecular structure of polypeptides in solution.

Ultrafast intramolecular vibrational energy transfer in carbon nitride hydrocolloid examined by femtosecond two-dimensional infrared spectroscopy

In this work, ultrafast vibrational and structural processes in a graphitic carbon nitride hydrocolloid system were studied using a combination of linear infrared and nonlinear two-dimensional infrared (2D IR) spectroscopies. The experimentally observed three IR line shapes in the C=N stretching vibration frequency region were analyzed and attributed to the rigid and conjugated molecular frame of the prepared g-CN molecular species, which is believed to be a dimeric tris-s-triazine, as well as attributed to insignificant solvent influence on the delocalized C=N vibrations. Vibrational transition density cubes were also computed for the proposed g-CN dimer, confirming the heterocyclic C=N stretching nature of the three IR absorption peaks. Intramolecular vibrational energy transfer dynamics and spectral diffusion of the g-CN system were characterized by examining a series of time-dependent 2D IR spectra. A picosecond intramolecular vibrational energy redistribution process was found to occur among these delocalized C=N stretching modes, acting as an efficient vibrational energy transfer channel. This work reasonably connects the experimentally observed IR signature to a specific g-CN structure and also provides the first report on the ultrafast intramolecular processes of such carbon nitride systems. The obtained results are fundamentally important for understanding the molecular mechanisms of such carbon-nitride based functional materials.

Intensified C≡C Stretching Vibrator and Its Potential Role in Monitoring Ultrafast Energy Transfer in 2D Carbon Material by Nonlinear Vibrational Spectroscopy

In this work, an intensity-enhanced C≡C stretching infrared (IR) absorption is observed in hexakis[(trimethylsilyl)ethynyl]benzene (HTEB), whose IR transition dipole magnitude becomes comparable to that of a typical C═O stretch, and the enhancement is believed to be due to a joint effect of π-π conjugation and hyperconjugation associated with a terminal trimethylsilyl group. Using dynamical time-dependent two-dimensional infrared (2D IR) spectroscopy, a picosecond intramolecular energy redistribution process is observed between two nondegenerate C≡C stretching modes, whose symmetry breaking is attributed to a noncovalent halogen-bonding interaction between HTEB and solvent CH₂Cl₂. The rigid structure of HTEB and limited structural dynamics are also inferred from the insignificant initial spectral diffusion value extracted from the 2D IR spectra. This work provides the first nonlinear infrared investigation of the conventionally weak C≡C stretch. The methods outlined are particularly important for detailed understanding of the structure-related processes such as vibrational energy transfer in novel C≡C species containing materials such as graphdiyne.

Linear and Nonlinear Infrared Spectroscopies Reveal Detailed Solute-Solvent Dynamic Interactions of a Nitrosyl Ruthenium Complex in Solution

In this work, the solvation of a nitrosyl ruthenium complex, [(CH₃)₄N][RuCl₃(qn)(NO)] (with qn = deprotonated 8-hydroxyquinoline), which is a potential NO-releasing molecule in the bio-environment, was studied in two bio-friendly solvents, namely deuterated dimethyl sulfoxide (dDMSO) and water (D₂O). A blue-shifted NO stretching frequency was observed in water with respect to that in dDMSO, which was believed to be due to ligand-solvent hydrogen-bonding interactions, one N═O···D and particularly three Ru-Cl···D, that show competing effects on the NO bond length. The dynamic differences of the NO stretch in these two solvents were further revealed by transient pump-probe IR and two-dimensional IR results: faster vibrational relaxation and faster spectral diffusion (SD) were observed in D₂O, confirming stronger solvent-solute interaction and also faster solvent structural dynamics in D₂O than in DMSO. Further, a significant non-decaying residual in the SD dynamics was observed in D₂O but not in DMSO, suggesting the formation of a stable solvation shell in water due to strong multi-site ligand-solvent hydrogen-bonding interactions, which is in agreement with the observed blue-shifted NO stretching frequency. This work demonstrates that small solvent molecules such as water can form a relatively rigid solvation shell for certain transition metal complexes due to cooperative ligand-solvent interactions and show slower dynamics.

Solvent-dependent structural dynamics of an azido-platinum complex revealed by linear and nonlinear infrared spectroscopy

The vibrational and anisotropic relaxation dynamics and structural dynamics of a potential anticancer prodrug, trans,trans,trans-[Pt(N3)2(OH)2(py)2], were investigated using time-resolved infrared pump-probe spectroscopy and ultrafast two-dimensional infrared (2D IR) spectroscopy. Herein, two representative bio-friendly solvents, H2O and DMSO, were used, in which the local structural and dynamical variations were monitored using the antisymmetric linear combination of the two N3 stretching vibrational modes as an infrared probe. It was found that the vibrational relaxation process of the N3 antisymmetric stretching (as) mode in H2O is two to three times faster than that in DMSO. The anisotropic relaxation process of the anticancer prodrug was observed to be hindered in DMSO; this indicated a tighter solvent environment around the sample molecule in this solvent. The vibrational frequency time correlation of the N3 antisymmetric stretching mode in H2O decays with a time constant of 1.94 ps, in agreement with the hydrogen bond formation and breaking times of water. In DMSO, the frequency time correlation of the N3 as mode decays on a much longer time scale; this further indicates its sensitivity to the out-layer DMSO structural dynamics, which are relatively static in the experimental time window.

Central-metal effect on intramolecular vibrational energy transfer of M(CO)5Br (M = Mn, Re) probed by two-dimensional infrared spectroscopy

Vibrational energy transfer in transition metal complexes with flexible structures in condensed phases is of central importance to catalytical chemistry processes. In this work, two molecules with different metal atoms, M(CO)₅Br (where M = Mn, Re), were used as model systems, and their axial and radial carbonyl stretching modes as infrared probes. The central-metal effect on intramolecular vibrational energy redistribution (IVR) in M(CO)₅Br was investigated in polar and nonpolar solvents. The linear infrared (IR) peak splitting between carbonyl vibrations increases as the metal atom changes from Mn to Re. The waiting-time dependent two-dimensional infrared diagonal- and off-diagonal peak amplitudes reveal a faster IVR process in Re(CO)₅Br than in Mn(CO)₅Br. With the aid of density functional theory (DFT) calculations, the central-metal effect on IVR time linearly correlates with the vibrational coupling strength between the two involved modes. In addition, the polar solvent is found to accelerate the IVR process by affecting the anharmonic vibrational potentials of a solute vibration mode.

Controlling Photochemistry via Isotopomers and IR Pre-excitation

It is a photochemist's dream to be able to photoinduce a reaction of a specific molecular species in an ensemble of similar but not identical ones. The problem is that similar molecules often exhibit nearly identical UV-Vis absorption spectra, making them difficult or impossible to distinguish or to select spectroscopically. The ultrafast VIPER (VIbrationally Promoted Electronic Resonance) pulse sequence allows to pick a single species for electronic excitation based on its infrared spectrum. The latter usually shows more features, allowing the discrimination between species than the UV-Vis spectrum. Here, we show that it is possible to induce and monitor species-selective photochemistry even for molecules with virtually identical UV-Vis spectra, which is the case for isotopomers. Next to isotope-selective photochemistry in solution, applications to orthogonal photo-uncaging and species-selective spectroscopy and photochemistry in mixtures are within reach.

Micellar and bicontinuous microemulsion structures show different solute–solvent interactions: a case study using ultrafast nonlinear infrared spectroscopy

Using aqueous and organic probes to simultaneously explore the structural dynamics of reverse micellar and bicontinuous microemulsion structures.

Ultrafast structural molecular dynamics investigated with 2D infrared spectroscopy methods

Ultrafast, multi-dimensional infrared (IR) spectroscopy has been advanced in recent years to a versatile analytical tool with a broad range of applications to elucidate molecular structure on ultrafast timescales, and it can be used for samples in a many different environments. Following a short and general introduction on the benefits of 2D IR spectroscopy, the first part of this chapter contains a brief discussion on basic descriptions and conceptual considerations of 2D IR spectroscopy. Outstanding classical applications of 2D IR are used afterwards to highlight the strengths and basic applicability of the method. This includes the identification of vibrational coupling in molecules, characterization of spectral diffusion dynamics, chemical exchange of chemical bond formation and breaking, as well as dynamics of intra- and intermolecular energy transfer for molecules in bulk solution and thin films. In the second part, several important, recently developed variants and new applications of 2D IR spectroscopy are introduced. These methods focus on (i) applications to molecules under two- and three-dimensional confinement, (ii) the combination of 2D IR with electrochemistry, (iii) ultrafast 2D IR in conjunction with diffraction-limited microscopy, (iv) several variants of non-equilibrium 2D IR spectroscopy such as transient 2D IR and 3D IR, and (v) extensions of the pump and probe spectral regions for multi-dimensional vibrational spectroscopy towards mixed vibrational-electronic spectroscopies. In light of these examples, the important open scientific and conceptual questions with regard to intra- and intermolecular dynamics are highlighted. Such questions can be tackled with the existing arsenal of experimental variants of 2D IR spectroscopy to promote the understanding of fundamentally new aspects in chemistry, biology and materials science. The final part of the chapter introduces several concepts of currently performed technical developments, which aim at exploiting 2D IR spectroscopy as an analytical tool. Such developments embrace the combination of 2D IR spectroscopy and plasmonic spectroscopy for ultrasensitive analytics, merging 2D IR spectroscopy with ultra-high-resolution microscopy (nanoscopy), future variants of transient 2D IR methods, or 2D IR in conjunction with microfluidics. It is expected that these techniques will allow for groundbreaking research in many new areas of natural sciences.