Effect of Solvent Polarity on the Vibrational Dephasing Dynamics of the Nitrosyl Stretch in an Fe super(II) Complex Revealed by 2D IR Spectroscopy

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2013-07, Vol.117 (29), p.6234-6243-6234-6243
• Main Authors: Brookes, Jennifer F, Slenkamp, Karla M, Lynch, Michael S, Khalil, Munira
• Format: Article
• Language: English
• Subjects: Dynamics; Infrared spectroscopy; Ligands; Nitrosyls; Polarity; Solvents; Spectroscopy; Two dimensional
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp4005345

The vibrational dephasing dynamics of the nitrosyl stretching vibration ( nu sub(NO)) in sodium nitroprusside (SNP, Na sub(2)[Fe(CN) sub(5)NO].2 H sub(2)O) are investigated using two-dimensional infrared (2D IR) spectroscopy. The nu sub(NO) in SNP acts as a model system for the nitrosyl ligand found in metalloproteins which play an important role in the transportation and detection of nitric oxide (NO) in biological systems. We perform a 2D IR line shape study of the nu sub(NO) in the following solvents: water, deuterium oxide, methanol, ethanol, ethylene glycol, formamide, and dimethyl sulfoxide. The frequency of the nu sub(NO) exhibits a large vibrational solvatochromic shift of 52 cm super(-1), ranging from 1884 cm super(-1) in dimethyl sulfoxide to 1936 cm super(-1) in water. The vibrational anharmonicity of the nu sub(NO) varies from 21 to 28 cm super(-1) in the solvents used in this study. The frequency-frequency correlation functions (FFCFs) of the nu sub(NO) in SNP in each of the seven solvents are obtained by fitting the experimentally obtained 2D IR spectra using nonlinear response theory. The fits to the 2D IR line shape reveal that the spectral diffusion time scale of the nu sub(NO) in SNP varies from 0.8 to 4 ps and is negatively correlated with the empirical solvent polarity scales. We compare our results with the experimentally determined FFCFs of other charged vibrational probes in polar solvents and in the active sites of heme proteins. Our results suggest that the vibrational dephasing dynamics of the nu sub(NO) in SNP reflect the fluctuations of the nonhomogeneous electric field created by the polar solvents around the nitrosyl and cyanide ligands. The solute solvent interactions occurring at the trans-CN ligand are sensed through the pi -back-bonding network along the Fe-NO bond in SNP.