In this work, we carry out steady-state, femtosecond pump–probe and two-dimensional (2D) infrared spectroscopic studies on dimeric π-cyclopentadienyliron dicarbonyl [CpFe(CO)2]2 in the CO stretching vibration frequency region in CCl4 and CH2Cl2. The cis and trans isomers, in terms of the position of two terminal CO groups, are found to coexist in the two solvents. A weak asymmetric stretching peak of the cis-isomer is revealed under that of the IR-active trans-isomer by analyzing the 2D infrared cross peak, which is supported by ab initio computations. Furthermore, vibrational population relaxation is found to be both solute and solvent dependent (ranging from 21 ps to 32 ps) – the fastest dynamics is found for the trans-isomer in the polar solvent environment, which is believed to be associated with the availability and the number of efficient energy accepting channels for solvent molecules. The spectral diffusion dynamics of the CO stretching vibrations, occurring on an even faster time scale (1 ps to 3 ps), mainly exhibits solvent dependence – faster dynamics is found in the polar solvent, involving weak and rapidly fluctuating hydrogen bonding interactions between CH2 groups of the solvent and the terminal carbonyls of solutes.

N-Acylglucosamine is an important component in many oligosaccharides in eukaryotes, where it plays a very important biological role. Located between a glucose ring and an alkyl group of such species is an amide unit (−CONH−), which exhibits an infrared absorption band, mainly due to the C═O stretching, in the region of 1600–1700 cm–1, similar to the amide-I band found in polypeptides. In this work, vibrational properties of such an “amide-I mode” in N-propionyl-d-glucosamine (GlcNPr) are examined in three typical solvents (water, methanol, and dimethylsulfoxide) by using steady-state infrared and femtosecond infrared dispersed pump–probe spectroscopies. As a result of solute–solvent interactions, multiple structured GlcNPr–solvent clusters are formed in water and methanol but are unlikely in dimethylsulfoxide. The vibrational relaxation rate of the amide-I mode is slightly frequency-dependent, supporting the presence of multiple solvated structures. Further, the amide-I lifetime is significantly shorter in GlcNPr than that in a well-known monopeptide, N-methylacetamide, which can be attributed to the presence of additional downstream vibrational modes caused by the sugar unit. Ab initio molecular dynamics simulations are used to reveal microscopic details of the first solvation shell of GlcNPr. Our results demonstrate that the amide-I mode in glucosamine exhibits both structural and solvent sensitivities that can be used to characterize the three-dimensional arrangement of sugar residues and their structural dynamics in glycopeptides.

In this work, we carry out steady-state, femtosecond pump–probe and two-dimensional (2D) infrared spectroscopic studies on dimeric π-cyclopentadienyliron dicarbonyl [CpFe(CO)2]2 in the CO stretching vibration frequency region in CCl4 and CH2Cl2. The cis and trans isomers, in terms of the position of two terminal CO groups, are found to coexist in the two solvents. A weak asymmetric stretching peak of the cis-isomer is revealed under that of the IR-active trans-isomer by analyzing the 2D infrared cross peak, which is supported by ab initio computations. Furthermore, vibrational population relaxation is found to be both solute and solvent dependent (ranging from 21 ps to 32 ps) – the fastest dynamics is found for the trans-isomer in the polar solvent environment, which is believed to be associated with the availability and the number of efficient energy accepting channels for solvent molecules. The spectral diffusion dynamics of the CO stretching vibrations, occurring on an even faster time scale (1 ps to 3 ps), mainly exhibits solvent dependence – faster dynamics is found in the polar solvent, involving weak and rapidly fluctuating hydrogen bonding interactions between CH2 groups of the solvent and the terminal carbonyls of solutes.