Vibrational relaxation of the free terminal hydroxyl stretch in methanol oligomers: Indirect pathway to hydrogen bond breaking

Abstract

Vibrational relaxation of methanol-d ͑MeOD͒ in carbon tetrachloride has been investigated via ultrafast infrared pump-probe experiments. Exciting at 2690 cm Ϫ1 , only the free O-D ͑where the D is not H-bonded͒ stretching mode is initially populated. For MeOD mole fractions р0.025, a 2.15 ps single exponential decay is observed. At mole fractions у0.0375, the signal decays ͑2.15 ps decay time͒ below zero ͑increased absorption͒ and then recovers on time scales of 22 ps and ӷ300 ps. The increased absorption indicates the formation of additional free ODs caused by the breaking of H-bonds that are not directly coupled to the initially excited vibration. The two-time scale recovery of this signal arises from geminate and nongeminate recombination. The data are fit with a set of kinetic equations that accurately reproduce the data. The results suggest that vibrational relaxation of the initially excited free OD stretch into intramolecular modes of the methanol leads to H-bond breaking. This contrasts studies that suggest direct relaxation of a vibrationally excited OH stretch into an H-bond stretch is responsible for H-bond breaking.

Vibrational population relaxation of hydrogen-bonded phenol complexes in solution: Investigation by ultrafast infrared pump–probe spectroscopy

“…The intensity of the band at 3350 cm À1 is increased compared to the band at 3480 cm À1 as the concentration of phenol increases. This observation is similar to that for the concentration dependence of methanol in CCl 4 [7,[39][40][41][42].…”

“…Based on the above observations, we proposed that the band at 3350 cm À1 was due to the d-type OH stretching mode, while the band at 3480 cm À1 was the ctype. The b-type OH group, which accepts but does not donate, may have a similar frequency to that of phenol monomer, as shown for methanol oligomers in CCl 4 solution [7,[39][40][41][42].…”

“…In addition to the free OH group of the phenol monomer, there are three types of hydrogen-bonding OH group in phenol oligomers. Similar to the assignment for methanol in CCl 4 , an OH group acting as a donor and accepter of hydrogen bonds (denoted as d) has lower frequencies than the one acting as a donor but not an accepter (denoted as c) [7,[39][40][41][42]. In the gas phase, Ebata et al conducted IR spectroscopic measurements of the OH stretching vibrations of phenol dimer and trimer, as well as phenol monomer, in supersonic jets [43].…”

“…Extensive studies have been performed on hydrogen-bonded liquids using IR pump-probe spectroscopy, by probing the OH or NH stretching modes 0301-0104/$ -see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.chemphys.2007.07.025 [5][6][7][8][9][10][11][12]. Very recently, ultrafast IR vibrational echo spectroscopy has been used to investigate the structural evolution of hydrogen bond networks in water and alcohol solutions [13][14][15][16][17][18][19][20].…”

Vibrational Dynamics of Hydrogen-Bonded Complexes Studied by Infrared Pump-Probe Spectroscopy

“…12) It is found that lifetime of the OH stretch depends on the strength of the hydrogen bond; for isolated methanol, the VER times (T 1 ) are in a picosecond order, 13), 14) and it becomes shorter than 1 ps when it forms hydrogen bond. Figure 7 shows IR absorption spectra of the OH and OD stretch region of methanol in isotopically diluted solutions.…”