Ground State Proton Transfer in Phenol–(NH₃)_n (n ≤ 11) Clusters Studied by Mid-IR Spectroscopy in 3–10 μm Range

Abstract

The infrared (IR) spectra of size-selected phenol-ammonia clusters, PhOH-(NH(3))(n) (n ≤ 11) in the 3-10 μm wavelength region were measured to investigate the critical number of solvent molecules necessary to promote the ground state proton transfer (GSPT) reaction. While the N-H stretching vibrations did not provide clear information, characteristic changes that are assigned to the GSPT reaction were observed in the skeletal vibrational region. The production of phenolate anion (PhO(-)), which is a product of the GSPT reaction, was established from the appearance of characteristic bands assignable to C-C stretching and C-H bending vibrations of PhO(-) and from the corresponding disappearance of C-O-H bending vibration of PhOH at n = 9. The mid-IR spectroscopy directly proves the structural change induced by the deprotonation from the O-H bond and thus establishes the GSPT reaction as complete at n = 9. No such absorptions were observed for n ≤ 5 in line with a previous report. For n = 6-8, both the proton transferred and the nontransferred signatures were observed in the spectra, showing coexistence of both species for the first time.

Revealing the role of excited state proton transfer (ESPT) in excited state hydrogen transfer (ESHT): systematic study in phenol–(NH3)n clusters

“…However, the size-selected IR spectra of PhOH-(NH 3 ) n (n ¼ 0-11) measured by IR-UV dip spectroscopy raised a doubt about the threshold size of n ¼ 6. 62 The IR spectra showed the signature of PhOH vibrations (such as C-O stretching) even for clusters of n ¼ 6 and the expected marker band (C]O stretching) of PhO À was not found. The disappearance of the C-O stretching band was not clear, even in larger sized clusters, whereas the C-O-H bending vibration, another vibrational signature of PhOH, disappeared at n $ 9.…”

Ultrafast Proton Transfer Reaction in Phenol–(Ammonia)n Clusters: An Ab-Initio Molecular Dynamics Investigation.

“…[2][3][4][5][6] In this regard, the role of ammonia as a solvent in the proton transfer process in the phenol-(ammonia)n clusters have been the subject of intense experimental and theoretical investigations by several research groups over the past three decades to understand the proton transfer reaction in the ground state and hydrogen transfer process in the excited electronic state. [7][8][9][10][11][12][13][14][15][16] Additionally, it has been shown that the excited proton transfer reaction of phenol becomes ultrafast at the air-water interface. 17 The proton transfer reaction in phenol-(ammonia)n clusters were investigated using the infrared spectroscopic method, which revealed that at least six ammonia molecules are required to observe the features corresponding to the proton transferred form.…”

“…17 The proton transfer reaction in phenol-(ammonia)n clusters were investigated using the infrared spectroscopic method, which revealed that at least six ammonia molecules are required to observe the features corresponding to the proton transferred form. 14 On the other hand, electronic structure calculations have shown that five ammonia molecules are adequate to extract the proton from phenol, albeit being energetically unfavourable, and the proton transfer process in the phenol-(ammonia)n clusters becomes increasingly favourable with the increase in the number of ammonia molecules. 16 In a recent work our group, with the aid of electric field calculations along the donor O-H of phenol in the phenol-(ammonia)n clusters, using at the M06-2X/cc-pVTZ level of theory, has shown that a critical electric field of a critical field of 236 MV cm -1 is essential to transfer of a proton from phenol to the surrounding ammonia cluster.…”

Time-Resolved Study of Light-Induced Ground State Proton Transfer from Acid Medium to 4-Nitrophenolate

“…18,19 Techniques using flash photolysis with scanning in the UV-Vis or infrared spectrum also make it possible to follow the formation of transient species. [13][14][15] Experimental characterization of pKa changes or solvation medium, imply kinetic and thermodynamic changes for this proton transfer reaction in ground and excited states. Reactions involving nitrophenol degradation were also performed on suspensions, or colloidal dispersions supported on surfaces of oxides of metals such as titanium and iron.…”