Mass analyzed threshold ionization spectroscopy of p-cyanophenol cation and the CN substitution effect

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Abstract

The adiabatic ionization energy of p-cyanophenol has been determined to be 72 698 ± 5 cm−1 (9.0134 ± 0.0006 eV) on the basis of mass analyzed threshold ionization (MATI) spectrscopy. Analysis of the newly obtained MATI spectra gives the respective frequencies of 399, 517 and 820 cm−1 for the ring deformation 6a, C–CN bending, and breathing vibrations of the p-cyanophenol cation. Comparing these experimental data with those of phenol leads to a better understanding about the influence of the CN substituent on the ionization energy and molecular vibration.

Introduction

p-Cyanophenol has received attention in the studies concerning the changes of structure and acidity upon electronic transition because it consists of an electron-donating OH group and an electron-withdrawing CN group at the para position [1], [2], [3], [4], [5]. It has been investigated by ab initio calculations [1], [2], excitation and dispersed laser-induced fluorescence (LIF) spectroscopy [3], [4], [5], resonantly enhanced multiphoton ionization spectroscopy [5]. These studies reveal many molecular properties of p-cyanophenol in the ground S0 and electronically excited S1 states. The ionization energy (IE) of this molecule has been reported [6]. Nevertheless, detailed spectroscopic data of p-cyanophenol in the cationic ground D0 state are not yet available in the literature.

Since their developments, both zero-kinetic energy (ZEKE) photoelectron [7], [8] and mass-analyzed threshold ionization (MATI) [9], [10] methods have become powerful tools for providing vibrationally resolved cation spectra. In this Letter, we report the MATI spectra of p-cyanophenol recorded by ionizing via three vibronic states. The results give precise adiabatic IE and active vibrations of the cation. Comparing these data with those of phenol derivatives helps us to understand the CN substitution effect resulting from its electronic-withdrawing nature.

Section snippets

Experimental methods

The experiments reported were performed with a laser-based TOF mass spectrometer as described in our previous publication [11]. The solid p-cyanophenol (95% purity, Sigma–Aldrich) sample was heated to about 135 °C, seeded into 1–2 bar of helium and expanded into the vacuum through a pulsed valve with a 0.5 mm diameter orifice. The molecular beam was collimated by a skimmer located 15 mm downstream from the nozzle orifice.

The excitation/ionization source consists of two independent tunable UV laser

Vibronic spectra

Kleinermanns and co-workers [3], [4] reported that the origin of the S1  S0 electronic transition of p-cyanophenol lies at 35547.46 cm−1 on the basis of their LIF experiments. They also pointed out the first transition appeared in the one-color resonant two-photon ionization (1C-R2PI) spectrum is 000+801cm-1. To the best of our knowledge, the 1C-R2PI spectrum of this molecule has never appeared in the literature. Here, we applied both one-color and two-color (2C) R2PI to record the vibronic

Discussion

Ab initio calculations show that substitution of a functional group on an aromatic ring leads to a lowering in the zero point level energy (ZPL) of electronic states. The extent of the lowering reflects the strength of the interaction of the substituent and the ring. If the degree of the lowering of the ZPL of the upper electronic state is greater than that of the lower one, it gives rise to a red shift in the transition energy. Oppositely, it gives a blue shift [17], [18].

Table 2 lists the

Conclusion

We have recorded the vibronic spectra of p-cyanophenol by using the 2C-R2PI technique which involves scanning the frequency of excitation laser while fixing that of ionization laser at 37 368 cm−1. The origin of the S1  S0 electronic excitation is found to be 35 548 ± 2 cm−1, which is in excellent agreement with that reported on the basis of the LIF experiments. In addition, the MATI spectra of p-cyanophenol have been recorded by ionizing via the intermediate states involving the vibrationless and the

Acknowledgment

We gratefully thank the National Science Council of the Republic of China for financial support of this work under Grant Number NSC-93-2113-M-001-023.

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    Visiting scholar from Department of Electronics, Shanxi University, Taiyuan 030006, Shanxi, China.

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