Photoinduced Processes in Hydrogen Bonded System: Photodissociation of Imidazole Clusters

Excited state hydrogen transfer dynamics in phenol-(NH3)2 studied by picosecond UV-near IR-UV time-resolved spectroscopy

Time-evolutions of excited state hydrogen transfer (ESHT) in phenol (PhOH)-(NH₃)₂ clusters have been measured by three-color picosecond (ps) ultraviolet (UV)-near infrared (NIR)-UV pump-probe ion dip spectroscopy. The formation of a reaction product, ˙NH₄NH₃, is detected by its NIR absorption due to a 3p-3s Rydberg transition. The ESHT reactions from all of the vibronic levels show biexponential time-evolutions, even from the S₁ origin. Based on the biexponential time-evolution, it is suggested that there is a second reaction path via the triplet πσ* state, which gives the slow component. The fast time-evolution of the ESHT reaction from the S₁ origin is measured to be 268 ps, which is 10-times slower than that in PhOH-(NH₃)₃, and a higher barrier between the ππ* and reactive πσ* states is suggested. The size dependence of the ESHT reaction rates is discussed based on a potential distortion due to the proton transferred state in the ππ* potential surface.

Strained hydrogen bonding in imidazole trimer: a combined infrared, Raman, and theory study

This is not how three imidazole molecules prefer to arrange, as a combined IR, Raman and computational analysis unambiguously shows.

Photodissociation of aniline N–H bonds in clusters of different nature

The solvent effects on the photodissociation of aniline in cluster environments have been investigated by H-photofragment velocity map imaging at 243 nm, mass spectrometry after electron ionization, and ab initio calculations.

NMR studies of active-site properties of human carbonic anhydrase II by using (15) N-labeled 4-methylimidazole as a local probe and histidine hydrogen-bond correlations

By using a combination of liquid and solid-state NMR spectroscopy, (15) N-labeled 4-methylimidazole (4-MI) as a local probe of the environment has been studied: 1) in the polar, wet Freon CDF3 /CDF2 Cl down to 130 K, 2) in water at pH 12, and 3) in solid samples of the mutant H64A of human carbonic anhydrase II (HCA II). In the latter, the active-site His64 residue is replaced by alanine; the catalytic activity is, however, rescued by the presence of 4-MI. For the Freon solution, it is demonstrated that addition of water molecules not only catalyzes proton tautomerism but also lifts its quasidegeneracy. The possible hydrogen-bond clusters formed and the mechanism of the tautomerism are discussed. Information about the imidazole hydrogen-bond geometries is obtained by establishing a correlation between published (1) H and (15) N chemical shifts of the imidazole rings of histidines in proteins. This correlation is useful to distinguish histidines embedded in the interior of proteins and those at the surface, embedded in water. Moreover, evidence is obtained that the hydrogen-bond geometries of His64 in the active site of HCA II and of 4-MI in H64A HCA II are similar. Finally, the degeneracy of the rapid tautomerism of the neutral imidazole ring His64 reported by Shimahara et al. (J. Biol. Chem.- 2007, 282, 9646) can be explained with a wet, polar, nonaqueous active-site conformation in the inward conformation, similar to the properties of 4-MI in the Freon solution. The biological implications for the enzyme mechanism are discussed.

Photodissociation of pyrrole-ammonia clusters below 218 nm: quenching of statistical decomposition pathways under clustering conditions

The excited state hydrogen transfer (ESHT) reaction in pyrrole-ammonia clusters (PyH·(NH(3))(n), n = 2-5) at excitation wavelengths below 218 nm down to 199 nm, has been studied using a combination of velocity map imaging and non-resonant detection of the NH(4)(NH(3))(n-1) products. Special care has been taken to avoid evaporation of solvent molecules from the excited clusters by controlling the intensity of both the excitation and probing lasers. The high resolution translational energy distributions obtained are analyzed on the base of an impulsive mechanism for the hydrogen transfer, which mimics the direct N-H bond dissociation of the bare pyrrole. In spite of the low dissociation wavelengths attained (~200 nm) no evidence of hydrogen-loss statistical dynamics has been observed. The effects of clustering of pyrrole with ammonia molecules on the possible statistical decomposition channels of the bare pyrrole are discussed.

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters and their excited state hydrogen transfer dynamics

Hole-burning spectra of m-fluorophenol/ammonia (1:3) clusters are measured by four-color UV-near IR-UV-UV hole-burning spectroscopy. Cis and trans isomers of the cluster are clearly distinguished in the (1:3) cluster. Picosecond time evolutions of the excited state hydrogen transfer (ESHT) reaction in the (1:3) clusters are measured by the ion depletion due to 3p-3s Rydberg transition of reaction products ⋅NH(4)(NH(3))(2) lying in the near infrared region. From the wavelength dependence of the time evolution, we have concluded 1) the initial formation of a metastable ⋅NH(4)-NH(3)-NH(3) radical and 2) successive isomerization to the most stable NH(3)-⋅NH(4) -NH(3) radical in both cis and trans isomers. The reaction lifetimes of ESHT are determined by the rate equation analysis as 32.4 and 31.8 ps for the cis and trans isomer, respectively, and the isomerization and its back-reaction lifetime of both isomers are determined to be 3.3 ps and 11.2 ps. The almost same reaction rates are consistent with the similarity of the hydrogen bond networks in both clusters.