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<p>The vibrational coupled cluster method (VCCM) is applied to simulate the in situ IR spectra of protonated pyridine and pyridine radical cation. The results show that for the protonated pyridine, the in situ IR spectrum is mostly due to transitions in the conformer where the proton is attached to the N atom, which is the most stable conformer. However, for the pyridine radical cation, the VCCM spectrum of p-Py<InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(^{+{\textbf {.}}}\)</EquationSource> <EquationSource Format="MATHML"><math> <mmultiscripts> <mrow /> <mrow /> <mrow> <mo>+</mo> <mo>.</mo> </mrow> </mmultiscripts> </math></EquationSource> </InlineEquation> conformer mimics the experimental spectrum, though it is not the most stable conformer. The transition energies are in good agreement with the experimental values, and the results are found to be more accurate than the second-order perturbation theory (VPT2) calculations. Moreover, the ambiguity of the assignment of an intense transition at the low-frequency region of pyridine radical cation with VPT2 calculation is resolved with the VCCM method. An extensive analysis of the position of protonation and deprotonation on the transitions of different frequency range of the two systems is carried out in this study.</p>

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Ab initio investigation of IR spectrum of cationic pyridine and protonated pyridine: a vibrational coupled cluster study

  • 期刊论文

  • 发表日期：

    2026年4月7日

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  • Sreeshakthi Varadharajan,
  • Subrata Banik

摘要

The vibrational coupled cluster method (VCCM) is applied to simulate the in situ IR spectra of protonated pyridine and pyridine radical cation. The results show that for the protonated pyridine, the in situ IR spectrum is mostly due to transitions in the conformer where the proton is attached to the N atom, which is the most stable conformer. However, for the pyridine radical cation, the VCCM spectrum of p-Py \(^{+{\textbf {.}}}\) + . conformer mimics the experimental spectrum, though it is not the most stable conformer. The transition energies are in good agreement with the experimental values, and the results are found to be more accurate than the second-order perturbation theory (VPT2) calculations. Moreover, the ambiguity of the assignment of an intense transition at the low-frequency region of pyridine radical cation with VPT2 calculation is resolved with the VCCM method. An extensive analysis of the position of protonation and deprotonation on the transitions of different frequency range of the two systems is carried out in this study.