Microsolvation of the Phenol Cation (Ph⁺) in Nonpolar Environments:  Infrared Spectra of Ph⁺−L_n (L = He, Ne, Ar, N₂, CH₄)

Abstract

Infrared photodissociation spectra of several phenol-L n cation clusters (Ph + -L n ; L ) He, Ne, Ar, N 2 , CH 4 ) are recorded in the vicinity of the O-H stretch vibration (ν 1 ) of bare Ph + . The Ph + -L n complexes are produced in an electron impact (EI) ion source, which generates predominantly the most stable isomer of each cluster ion. The spectra of all dimers (n ) 1) show strong ν 1 transitions (at 3537, 3534, 3464, 3365, 3365 cm -1 for L ) He, Ne, Ar, N 2 , CH 4 ), which are attributed to proton-bound structures based upon the complexationinduced redshifts, ∆ν 1 . A linear correlation between ∆ν 1 and the proton affinity of L is observed. In the case of Ph + -Ar, a weak transition at 3536 cm -1 is assigned to the ν 1 band of the less stable π-bound isomer. The analysis of photofragmentation branching ratios and systematic frequency shifts in the spectra of larger Ph + -L n clusters (n e 2 for CH 4 , n e 5 for Ar, n e 7 for N 2 ) provide information about the microsolvation process of Ph + in nonpolar environments. The ν 1 transitions of the most stable isomers display small incremental blueshifts with respect to the dimer transitions, suggesting that further solvation causes little destabilization of the intermolecular proton bond to the first ligand. In the case of the Ph + -(N 2 ) n complexes, the existence of two isomers is observed in the size range n ) 5-7. For several Ph + -L n clusters, the most stable cation structures produced in the EI source differ considerably from the geometries observed by resonant enhanced multiphoton ionization (REMPI) of the corresponding neutral precursors. The limitations of REMPI techniques (arising from the Franck-Condon principle) for the generation and spectroscopic characterization of cluster cations are discussed.

Weak hydrogen bonding motifs of ethylamino neurotransmitter radical cations in a hydrophobic environment: infrared spectra of tryptamine+–(N2)n clusters (n ≤ 6)

“…As TRA + is expected to form weak H-bonds with nonpolar N 2 molecules, their characteristic features become accessible by direct comparison with the strong conventional H-bond observed in TRA + -H 2 O. The current TRA + -(N 2 ) n study extends our previous characterization of A + -(N 2 ) n clusters with more simple aromatic chromophores (A + ), 32 such as benzene, 33,34 phenols and naphthols, [35][36][37][38][39] anilines, [40][41][42] imidazole, 43 cyclopropenyl, 44,45 and indole (In). 46 These studies revealed that the microsolvation process of acidic aromatic ions in molecular nitrogen is dominated by the competition between two principal binding motifs, namely H-bonding to the acidic functional OH and NH groups and p-stacking to the aromatic ring.…”

“…46 The blue shaded band profile of the intense n NH band of the H-bound isomer, with a large width of B25 cm À1 and a band origin close to the P-branch head, is typical for the excitation of a proton donor stretch vibration and thus confirms the isomer assignment. 32,37,[53][54][55] The computed structures of the relevant TRA + -N 2 isomers are illustrated in Fig. 3, and important parameters are listed in Table 1.…”

Mass analyzed threshold ionization spectra of phenol⋯Ar2: ionization energy and cation intermolecular vibrational frequencies

“…16 MATI, 17 ZEKE, 18 photoionization efficiency (PIE), 19 and IR spectroscopy 20 are consistent with a p-bonded phenol + Á Á ÁAr structure also in the cation ground state. In contrast, the IR photodissociation spectrum of phenol + Á Á ÁAr generated in an electron impact (EI) ion source 21 shows that the most stable isomer in the D 0 state has a H-bound geometry, in line with ab initio calculations. 14,22,23 This has been interpreted by the fact that resonant photoionization of the p-bonded neutral phenol-Ar precursor complex can generate only the p-bonded structure in the cation due to the Franck-Condon principle.…”

Mass analyzed threshold ionization detected infrared spectroscopy: isomerization activity of the phenol–Ar cluster near the ionization threshold

“…The former can be assigned to n H OH , because its frequency matches with n H OH of ionized PhOH + -Ar. [5][6][7][8] The observation of n H OH indicates the occurrence of p -H isomerization, since the Rydberg states are prepared by Franck-Condon restricted photoexcitation from the p-bound S 1 state. The n H OH frequency observed here (B3490 cm À1 ) is somewhat blue-shifted from that observed for cold PhOH + -Ar clusters generated by collisional aggregation of PhOH + and Ar (n H OH = 3468 cm À1 ).…”

“…This interaction switching was derived from IR spectra of ionized clusters, PhOH + -Rg, generated by aggregation of Rg atoms following ionization of PhOH, which leads predominantly to the formation of the most stable isomer. [5][6][7][8][9][10][11][12][13] The IR spectra show broad red-shifted hydrogen-bonded OH stretching vibrations (n H OH ) of structures in which the Rg atoms bind to the OH group. Recent high level ab initio calculations have also supported this switching in the interaction motif upon ionization.…”