# Linear and Non-Linear Middle Infrared Spectra of Penicillin G in the CO Stretching Mode Region

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## Abstract

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## 1. Introduction

## 2. Experimental Section

#### 2.1. Materials

#### 2.2. Instrumental Setup

#### 2.3. Signal Acquisition, Data Treatment and Calculations

## 3. Results and Discussion

#### 3.1. FT-IR Spectra

#### 3.2. Broadband Pump/Probe Experiments

#### 3.3. Vibrational Relaxation

#### 3.4. Evaluation of the Coupling Constants

## 4. Conclusions

## Author Contributions

## Funding

## Data Availability Statement

## Acknowledgments

## Conflicts of Interest

## References

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**Figure 1.**Penicillin G molecule (A) and its IR spectra (B) in ${D}_{2}O$ (blue line) and in $DMSO-d6$ (black line). Assignments are given: (1) $\beta -lactam$$CO$ stretching; (2) $AmideI$; (3) asymmetric stretching of carboxylate group. A large red shift of $AmideII$ (A2), out of the frequency window, is observed upon proton exchange in ${D}_{2}O$.

**Figure 4.**Transient broadband parallel spectra in ${D}_{2}O$ (on the

**left**) and in $DMSO-d6$ (on the

**right**). The black dot lines represent simulations of the transient spectra obtained by summing the linear IR spectrum with its reversed one, shifted by the averaged value of anharmonicity (27 cm${}^{-1}$ for ${D}_{2}O$ and 20 cm${}^{-1}$ for $DMSO-d6$).

**Figure 5.**Bi-exponential decay of excited state absorption and ground state recovery of $\beta -lactam$ CO in ${D}_{2}O$. Fitting is done starting from 200 fs. In the first 200 fs cross phase modulation phenomena due to temporal overlap between pump and probe occur.

**Figure 6.**Narrowband experiment in ${D}_{2}O$: the dot line show the scaled intensities to the most intense signal recorded when the $\beta -lactam$ at 1762 cm${}^{-1}$ is excited. In order to highlight cross peaks (black arrows) the spectrum has been 20 times enlarged. The presence of the cross peak due to the coupling between the $\beta -lactam$ and the carboxylate is clearly visible while the cross peak with the amide I mode is hidden in the noise.

**Figure 7.**Cross peaks in the CO stretching region of Penicillin G in DMSO-d6. Excitation frequencies are centred at 1767 cm${}^{-1}$, 1678 cm${}^{-1}$ and 1622 $cm-1$. Black arrows indicate the position of cross peaks (four times enlarged).

**Table 1.**Experimental and calculated FT-IR frequencies (cm${}^{-1}$) of the three investigated vibrational modes and their assignment.

${\mathit{D}}_{2}\mathit{O}$ | $\mathit{DMSO}-\mathit{d}6$ | $\mathit{Mode}$ | ||
---|---|---|---|---|

$\mathit{\nu}$$\mathit{Exp}$ | $\mathit{\nu}$$\mathit{Calc}$ | $\mathit{\nu}$$\mathit{Exp}$ | $\mathit{\nu}$$\mathit{Calc}$ | |

1762 | 1762 | 1767 | 1767 | $\beta -lactam$ carbonyl stretching |

1640 | 1655 | 1674 | 1674 | amide I |

1601 | 1580 | 1615 | 1588 | carboxylate asymmetric stretching |

**Table 2.**Excited state absorption (ESA) and ground state recovery (GSR) lifetimes in ${D}_{2}O$ and $DMSO-d6$. A thermal contribution of 0.05/0.1 m optical density (OD) is measured in $DMSO-d6$. Time values of the exponentials are in $fs$.

${\mathit{D}}_{2}\mathit{O}$ | $\mathit{DMSO}-\mathit{d}6$ | |||
---|---|---|---|---|

$\mathit{\beta}-\mathit{lactam}$ | $\mathit{\beta}-\mathit{lactam}$ | Amide I | Carboxylate | |

ESA | 1.1 exp(-t/550) | 0.2 exp(-t/324) | 0.3 exp(-t/490) | 0.9 exp(-t/136) |

0.4 exp(-t/1750) | 0.3 exp(-t/2040) | 0.3 exp(-t/2125) | 1.2 exp(-t/2195) | |

$\u2329\tau \u232a$ = 0.9 ± 0.1 ps | $\u2329\tau \u232a$ = 1.4 ± 0.1 ps | $\u2329\tau \u232a$ = 1.3 ± 0.1 ps | $\u2329\tau \u232a$ = 1.3 ± 0.1 ps | |

GSR | 1.5 exp(-t/550) | 0.4 exp(-t/512) | 0.5 exp(-t/580) | 0.5 exp(-t/154) |

0.5 exp(-t/1850) | 0.4 exp(-t/2940) | 0.5 exp(-t/2100) | 1.3 exp(-t/2200) | |

$\u2329\tau \u232a$ = 0.9 ± 0.1 ps | $\u2329\tau \u232a$ = 1.7 ± 0.1 ps | $\u2329\tau \u232a$ = 1.3 ± 0.1 ps | $\u2329\tau \u232a$ = 1.6 ± 0.1 ps |

**Table 3.**Experimental values for coupling constant $\beta $ (in cm${}^{-1}$) evaluated in the weak coupled anharmonic mode model (see text).

${\mathit{D}}_{2}\mathit{O}$ | $\mathit{DMSO}-\mathit{d}6$ | |
---|---|---|

$\beta -lactam$/Amide I | 6.7 | |

$\beta -lactam$/carboxylate | 9.2 | 19 |

Amide I/carboxylate | 8.3 |

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**MDPI and ACS Style**

Ragnoni, E.; Catalini, S.; Becucci, M.; Lapini, A.; Foggi, P.
Linear and Non-Linear Middle Infrared Spectra of Penicillin G in the CO Stretching Mode Region. *Symmetry* **2021**, *13*, 106.
https://doi.org/10.3390/sym13010106

**AMA Style**

Ragnoni E, Catalini S, Becucci M, Lapini A, Foggi P.
Linear and Non-Linear Middle Infrared Spectra of Penicillin G in the CO Stretching Mode Region. *Symmetry*. 2021; 13(1):106.
https://doi.org/10.3390/sym13010106

**Chicago/Turabian Style**

Ragnoni, Elena, Sara Catalini, Maurizio Becucci, Andrea Lapini, and Paolo Foggi.
2021. "Linear and Non-Linear Middle Infrared Spectra of Penicillin G in the CO Stretching Mode Region" *Symmetry* 13, no. 1: 106.
https://doi.org/10.3390/sym13010106