Pyrrolydine (PRL) with a nitrogen-containing saturated five-membered ring is usually involved in the biochemistry, medicinal and pharmaceutical chemistry [1-4]. There are axial- and equatorial- conformations of the PRL, due to the different direction of the N-H bond [5, 6]. In general, these two conformers can interconvert to each other with a very low barrier (
Theoretical calculations of neutral PRL molecule have been studied and shown that two conformers (PRL-ax and PRL-eq) with very close energy exist. Many experimental methods, such as gas-phase electron diffraction [9-11], infrared and Raman spectroscopy [5, 12, 13], have also been used to detect the structure preferment and pseudorotation of PRL. However, these methods are performed at the room temperature or in liquid phase. In the early study of microwave spectroscopy study, only the PRL-ax conformer has been observed . Due to the improvement of equipment and the better cooling condition of the supersonic jet, the PRL-eq conformer has been found and considered to be the most stable structure [7, 15]. The above studies are all in the neutral state, and no spectroscopic study of gas-phase PRL molecule in its cationic state has been performed.
To obtain the IR spectroscopy of neutral or cationic molecules in supersonic jet, IR dissociation with vacuum-ultraviolet (VUV) photoionization and mass-selected detection has been proven to be a powerful tool [16-20]. In this technique, the VUV (usually 118 nm) light is utilized as an ionization source. With the VUV light, most of biomolecules can be softly ionized, even though there is no any chromophore group in the molecules. With this method, IR spectra of neutral and cationic tetrahydrofuran (THF), another representative compounds with heterocyclic five-membered ring, have been measured. IR spectra of neutral and cationic THF indicated that the acidity of the CH bond next to the oxygen atom of THF is greatly enhanced after ionization, due to the hyperconjugation interaction . The difference between PRL and THF is the heteroatom incorporated to the five-membered ring. Thus, an interesting question that arises is which H atom is more acidic in the cationic PRL, the C-H or the N-H bond?
To answer the question, IR spectra combined with theoretical calculations of neutral and cationic PRL have been carried out. The C-H bond is proved to be more acidic than that of N-H bond in the cationic PRL. Hyperconjugation induced redshift of the CH bands is indicated by the IR spectral features and DFT calculations.Ⅱ. METHODS
IR spectra of neutral and cationic PRL were recorded by IR predissociation spectroscopy based on the vacuum-ultraviolet photoionization detection. Details of the experimental setups have been reported previously [22, 23], and only a brief description is given here. Gaseous PRL seeded in the He carrier gas was produced on supersonic expansion through a pulsed valve (Parker General Valve series 9) with total pressure of
The VUV ionization light (118 nm) was generated by tripling the third harmonic output (355 nm) of a Nd:YAG laser through a Xe-Ar mixture (1:10) in the gas cell that was directly attached at the vacuum chamber of TOF-MS. The tunable IR light was generated from the OPO/OPA system (Laser Vision). And its pulse energy were 3-5 mJ/pulse at the region of 2600-3600 cm
Geometry optimization, IR spectra, and nature bond orbital (NBO) analyses of neutral and cationic PRL molecule were performed with Gaussian 09 packages . All the calculations were performed with B3LYP/6-311++(d, p). Gas-phase acidities (
FIG. 1 shows the observed and calculated IR spectra of neutral PRL in the CH and NH region of 2600-3600 cm
As we know, the typical CH stretching vibration of alkyl group is at the region of 2850-3000 cm
FIG. 3 presents the observed and calculated IR spectra of cationic PRL in the CH and NH region of 2600-3600 cm
As we have mentioned above, the typical CH stretching vibration of alkyl group is at the region of 2850-3000 cm
As shown in the FIG. 1 and FIG. 3, the PRL-H (
The redshifts of the C
Hyperconjugation induced redshift of the CH bond in the neutral and cationic PRL was investigated by IR-VUV spectroscopy combined with theoretical calculations. With the negative hyperconjugation, the electron in the HOMO mainly delocalizes to
Supplementary materials: Mulliken charge distributions of the products after dehydrogenation processes of cationic PRL predicted with B3LYP/6-311++G(d, p) are available in FIG. S1.Ⅴ. ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation of China (No.U1732146, No.21273083) and the Project under Scientific and Technological Planning Grant (No.201805010002) by Guangzhou City.
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