The discovery of the hydrogen bond from p-Nitrothiophenol by Raman spectroscopy: Guideline for the thioalcohol molecule recognition tool

Inter- and intra- molecular hydrogen bonding plays important role in determining molecular structure, physical and chemical properties, which may be easily ignored for molecules with a non-typical hydrogen bonding structure. We demonstrated in this paper that the hydrogen bonding is responsible for the different Raman spectra in solid and solution states of p-Nitrothiophenol (PNTP). The consistence of the theoretical calculation and experiment reveals that the intermolecular hydrogen bonding yields an octatomic ring structure (8) of PNTP in the solid state, confirmed by the characteristic S-H---O stretching vibration mode at 2550 cm−1; when it comes to the solution state, the breakage of hydrogen bond of S-H---O induced the S-H stretching vibration at 2590 cm−1. Our findings may provide a simple and fast method for identifying the intermolecular hydrogen bonding.

structure, therefore, the inconsistence between calculation and experiment might be ignited from the strong intermolecular interaction between two neighbour PNTP molecules in solid state, considering the disulfide bonding 36 between the two S-H or hydrogen-bonding 37 between S-H and N-O groups, respectively. By so far, there is no report related to the PNTP crystal structure.
In order to figure out the origin of the two peaks of ca. 2550 cm −1 and 2590 cm −1 observed in the S-H stretching vibration region, we simulated the Raman spectra of 4-Nitrophenyl disulphide (NPDS), the disulfate structure of PNTP (as shown in Fig. 2a), and R 2 2 (8) structure 3 , the hydrogen bonding dimer of two PNTP molecules using density functional theory calculations (seeing in Fig. 2b).
No surprisingly, we can't observe these two peaks of ca. 2550 cm −1 and 2590 cm −1 in the simulated Raman spectrum of NPDS, due to the disappearance of S-H bond via the formation of disulfate. Instead, the peak at ca. 1084 cm −1 of PNTP is split to two peaks at ca. 1059 and 1099 cm −1 of NPDS, which is confirmed by the experimental Raman spectrum of NPDS (seeing Supplementary in Fig. S2).
When it comes to the case of the hydrogen-bonding dimer system as shown in Fig. 2b, the two peaks at 2560 cm −1 and 2593 cm −1 were clearly displayed, very similar to the two peaks observed experimentally. Furthermore, the relative Raman intensity of the 2560 cm −1 peak to the 2593 cm −1 is 5-6 times, a value almost identical to the experimental result shown in Fig. 1a. Whereas hydrogen bonding has a negligible effect on the other main characteristic Raman peaks of PNTP, such as 1084 cm −1 (ν C-S), 1336 cm −1 (ν NO 2 ) and 1593 cm −1 (ν C-C).
Taking a look at the molecular structure of this dimer shown in Fig. 2b inset, we find that an octatomic ring R 2 2 (8) is formed with the hydrogen bonding between S-H and N-O groups of the two neighbour PNTP molecules. Compared to the S-H free of hydrogen bonding (d 2 , 1.348 Å), the interaction between S and H atoms for the d 3 bond (1.351 Å) is weaker due to the formation of the S-H---O structure, which will induce the redshift of S-H vibration from 2593 cm −1 to 2560 cm −1 . Meanwhile, the formation of the hydrogen-bonding octatomic ring greatly increases the Raman scattering cross section of the d 3 bond, which results in a much stronger Raman intensity of d 3 than d 2 (More detailed explanation in supplementary Fig. S3). These phenomenons may be explained with proton transfer mechanism [38][39][40][41] . Therefore, the Raman spectrum obtained from the solid PNTP is not from the PNTP itself, but the dimer structure shown in the insert of Fig. 2b.
Considering the hydrogen bonding is very sensitive to the environment and could be easily broken or formed with the solvation effect 42,43 , we dissolved the PNTP in tetrachloromethane (20 mM) to obtain the normal Raman spectrum in Fig. 3b. One Raman band at 2591 cm −1 is clearly displayed, which can be assigned to the S-H stretching vibration of the free PNTP molecule and is in good agreement with the calculated value of 2594 cm −1 (shown in Fig. 3e). The appearance of 2550 cm −1 band (shown in Fig. 3a) is originated from the association of molecules through the hydrogen bonding. PNTP dissolved in the CCl 4 induced the blue shift of the three main characteristic Raman peaks at 1103 cm −1 (ν C-S), 1344 cm −1 (ν NO 2 ) and 1583 cm −1 (ν C-C) comparing with that of the PNTP solid one (seeing Supplementary in Fig. S4). Besides, Fig. 3c,d also show the infrared spectra of PNTP in the solid state and in CCl 4 solution, respectively. One could clearly observe that the disappearance of the ν S-H--O peak at the 2544 cm −1 (More experimental and theoretical spectra seeing Supplementary in Figs S5 and S6) after the PNTP molecule being

Conclusion
In summary, hydrogen bonding between two PNTP molecules dimer was investigated by Raman spectroscopy and DFT calculations. The Raman spectra sensitively captured partial structure changes of the molecule. Structures and vibrational modes for pure PNTP and hydrogen-bonded R 2 2 (8) complex with the stoichiometric  ratio 1:1 were calculated. Both calculations and experiments show that 2550 cm −1 can be assigned to the S-H stretching vibration of the S-H---O structure, and 2590 cm −1 is assigned to the S-H stretching vibration free of hydrogen bonding. The observation not only indicates that conclusions stemmed from S-H stretching band of the PNTP molecule in the literatures might need to be reinterpreted, but also may provide new insight for the widely investigated interfacial chemical reaction of PNTP [26][27][28][29][30][31][32][33] under external field by Surface-enhanced Raman spectroscopy (SERS) or Tip-enhanced Raman spectroscopy (TERS). Besides, the current work provides a guideline for the thioalcohol molecules recognition tool, a simple and fast method for identifying the intermolecular hydrogen bonding.

Methods
PNTP was purchased from Aldrich. NPDS was purchased from Aladdin. CH 2 Cl 2 was purchased from Sinopharm Chemical Reagent Co. Ltd. CCl 4 was purchased from Tianjin ZhiYuan Reagent Co. Ltd. KBr was purchased from Tianjin FuChen Chemical Reagent Co. Ltd. All reagents were used without further purification. The IR spectra of 20 mM PNTP in CCl 4 was recorded at room temperature with an ATR-FTIR spectrometer (Nicolet iS50). The IR spectra of PNTP solid with KBr was obtained at room temperature using an FT-IR spectrometer (Nicolet 6700).
All the Raman signals were obtained at room temperature using a confocal Raman microscopy system (inVia Renishaw, UK) with a 532-nm laser for excitation. The Raman spectral resolution was 1 cm −1 . The diameter of the light spot was ~1.5 μ m and the laser excitation power was 0.15 mW for measuring the Raman spectrum of solid state of PNTP and NPDS. The diameter of the light spot was ~6.5 μ m and the laser excitation power was 10.5 mW for measuring the Raman spectrum of 20 mM PNTP in CCl 4 and CH 2 Cl 2 . All spectra were recorded using an accumulation time of 10 s.
The theoretical calculations of the molecular Raman spectra and IR spectra and their vibrational modes were performed with the Gaussian 09 software using density functional theory, the B3LYP functional 44,45 , and the 6-311+ G(p, d) level of theory.