IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

Yan, Juchao; Wilson, Reid W.; Buck, Jason T.; Grills, David C.; Reinheimer, Eric W.; Mani, Tomoyasu

doi:10.1039/d1sc03455c

Cited by 9 publications

(13 citation statements)

References 75 publications

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“…While studying nitriles introduced as probes of electric fields in simple solvents and proteins, we observed a substantial variation of the nitrile intensity, an effect noted before but not analyzed quantitatively for nitriles or carbonyls. ,− Intriguingly, we found that the integrated intensity correlates with the solvent field. As shown in the following, this new VSE circumvents the complication of the blueshift in H-bonding environments.…”

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confidence: 89%

Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

et al. 2022

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Nitriles are widely used vibrational probes; however, the interpretation of their IR frequencies is complicated by hydrogen bonding (H-bonding) in protic environments. We report a new vibrational Stark effect (VSE) that correlates the electric field projected on the −CN bond to the transition dipole moment and, by extension, the nitrile peak area or integrated intensity. This linear VSE applies to both H-bonding and non-H-bonding interactions. It can therefore be generally applied to determine electric fields in all environments. Additionally, it allows for semiempirical extraction of the H-bonding contribution to the blueshift of the nitrile frequency. Nitriles were incorporated at H-bonding and non-H-bonding protein sites using amber suppression, and each nitrile variant was structurally characterized at high resolution. We exploited the combined information available from variations in frequency and integrated intensity and demonstrate that nitriles are a generally useful probe for electric fields.

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confidence: 89%

Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

et al. 2022

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“…For instance, placing an electron-donating group (EDG), such as methoxy, at the para position of benzonitrile can lead to a significant increase in ε CN . − This enhancement effect can be understood from the point of view that increasing the electronic density in the phenyl ring leads to weakening of the CN bond (or that the EDG stabilizes a charge-separated resonance structure in which the CN bond becomes a double bond), hence an increase in the transition dipole moment. Recently, Mani and co-workers studied the CN stretching vibrational bands of a series of nitrile-functionalized oligophenylenes, in either neutral or radical anion form, and found that the ε CN of the charged species can be an order of magnitude larger than that of the neutral molecule and that the ε CN of one of the radical anions [i.e., (L3P) 2 CN] reaches ∼10 4 M –1 cm –1 .…”

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confidence: 99%

Photoenhancement of the C≡N Stretching Vibration Intensity of Aromatic Nitriles

Liu

Feng

Zhou

et al. 2022

J. Phys. Chem. Lett.

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The CN stretching vibration is a versatile infrared (IR) reporter that is useful for a wide range of applications. Aiming to further expand its spectroscopic utility, herein, we show that, using 4-cyanoindole and 4-cyano-7-azaindole as examples, photoexcitation can significantly shift the frequency (νCN) and enhance the molar extinction coefficient (εCN) of this vibrational mode of aromatic nitriles and that, for these indole derivatives, the enhancement factor can reach 13. Moreover, we find that while solvent relaxation at the excited electronic state(s) always leads to an increase in εCN, its effect on νCN depends on the solute and the solvent. Taken together, these results demonstrate that solvent relaxation can differently affect the local environment of the nitrile group and its conjugation with the indole ring and, more importantly, that the CN stretching vibration can serve as a sensitive IR probe of charge and electron transfer processes in which an aromatic nitrile is involved.

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“…Furthermore, to avoid ambiguity, all of those chosen compounds exhibit a single and well-defined CN stretching vibrational band under the corresponding experimental conditions. Following previous studies, ,,,− for each compound, we used density functional theory (DFT) calculations to determine the net charge ( q CN ) on its CN group, using either the ChelpG or RESP fitting method.…”

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confidence: 99%

C≡N Stretching Frequency as a Convenient Reporter of Charge Separation in Molecular Systems

et al. 2023

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Previously, several studies have shown that, for a set of structurally related nitrile compounds, there could be a linear relationship between the total charge on the nitrile group (q CN) and its stretching frequency (νCN). However, it is unclear whether the corresponding frequency and charge properties of structurally different nitrile compounds can be described by a single linear νCN–q CN relationship. Herein, we compute the q CN magnitudes of a large number of nitrile-containing molecules whose νCN values cover a spectral range of ca. 200 cm–1 and are measured under different experimental conditions. Our results reveal that νCN indeed exhibits a linear dependence on q CN, with a slope of 637 ± 30 cm–1/charge. Because the nitrile moiety is a commonly used building block in electronic donor–acceptor (D–A) molecular systems, we believe that this linear relationship will find utility in a wide range of applications where such D–A constructs are used, such as in organic photovoltaic assemblies. In addition, we apply this linear relationship to characterize the degree of charge transfer upon photoexcitation of two indole derivatives, 5-cyanoindole and 6-cyanoindole, and are able to show that in both cases, the fluorescence emission arises from a charge-transfer or La state.

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IR linewidth and intensity amplifications of nitrile vibrations report nuclear-electronic couplings and associated structural heterogeneity in radical anions

Abstract: Quantification of the intensity and linewidth of the ν(CN) IR band in a series of neutral and anionic nitrile-functionalized oligophenylenes reveals that the CN vibration is coupled to nuclear and electronic structural changes.

Cited by 9 publications

References 75 publications

Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

Nitrile Infrared Intensities Characterize Electric Fields and Hydrogen Bonding in Protic, Aprotic, and Protein Environments

Photoenhancement of the C≡N Stretching Vibration Intensity of Aromatic Nitriles

C≡N Stretching Frequency as a Convenient Reporter of Charge Separation in Molecular Systems

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