Water molecule excitation transition

At its best, the study of solvent kies by the formalism given can be used to learn about proton content and activation in the transition state. For this reason it is known as the proton inventory technique. The kinetics of decay of the lowest-energy electronic excited state of 7-azaindole illustrates the technique.25 Laser flash photolysis techniques (Section 11.6) were used to evaluate the rate constant for this very fast reaction. From the results it was suggested that, in alcohol, a double-proton tautomerism was mediated by a single molecule of solvent such that only two protons are involved in the transition state. In water, on the other hand, the excited state tautomerism is frustrated such that two water molecules may play separate roles. Diagrams for possible transition states that can be suggested from the data are shown, where of course any of the H s might be D s. 

When exposed to electromagnetic radiation of the appropriate energy, typically in the infrared, a molecule can interact with the radiation and absorb it, exciting the molecule into the next higher vibrational energy level. For the ideal harmonic oscillator, the selection rules are Av = +1 that is, the vibrational energy can only change by one quantum at a time. However, for anharmonic oscillators, weaker overtone transitions due to Av = +2, + 3, etc. may also be observed because of their nonideal behavior. For polyatomic molecules with more than one fundamental vibration, e.g., as seen in Fig. 3.1a for the water molecule, both overtones and... 

Excitation spectra have been of considerable use recently in studying both hydration numbers (by lifetime measurements) and inner-sphere complexation by anions (by observing appearance of the characteristic frequencies for e.g. the Eu3+ 5D0-+ 7F0 transition for the different possible species). Thus using a pulsed dye laser source, it was possible to demonstrate the occurrence of inner sphere complexes of Eu3+ with SCN, CI or NO3 in aqueous solution, the K values being 5.96 2, 0.13 0.01 and 1.41 0.2 respectively. The CIO4 ion did not coordinate. Excited state lifetimes suggest the nitrate species is [Eu(N03)(HzO)6,s o.4]2+ the technique here is to compare the lifetimes of the HzO and the corresponding D20 species, where the vibrational deactivation pathway is virtually inoperative.219 The reduction in lifetime is proportional to the number of water molecules complexed.217 218... 

The temperature-dependent Raman spectra are depicted in Fig. 4-27a, b. Figure 4-27a shows the spectra of H2O-I (the water molecules in the inner coordination sphere) from 133-223 K. Figure 4-27b shows the spectra of H2O-II (the water molecules in the outer sphere). The spectra above 223 K are not shown because of the overlap with fluorescence that is observed with the 514.5 nm excitation. Plots of the variations of band frequency with temperature are illustrated in Fig. 4-28a, b for H2O-I and H2O-II. Two discontinuities are observed at 195 5K and 140 5K, indicative of three distinct phases occurring in the temperature range studied, as indicated in Fig. 4-28a. The higher-frequency OH stretch region, as shown in Fig. 4-28b does not show any discontinuities for H2O-I. A plot of full width at half maximum intensity (FWHM) vs. T for H2O-I shows a discontinuity at 140 K (Fig. 4-28c, d). Additional support for these phase transitions was found from the temperature dependences of the UO vibrational mode, lattice vibrations and the NO3 ion vibrations (translations and rotations). 

Table 13-2. Vertical electronic n —> 7T transition energy of acetone in gas phase, Egas, and aqueous solution, Ewat, in units of eV. Excitation energy in aqueous solution was obtained from the combined QM/MM calculations treating acetone molecule at the quantum mechanical level of theory as indicated in the first column and using the polarizable potential for water molecules as a statistical average over 1200 molecular configurations extracted from classical MD simulation. The solvent shift in excitation energy, AE (in cm-1), is evaluated as a difference between excitation energies in water and in vacuum...

The fluorescence polarization excitation spectrum has been measured for thymine in aqueous solution. " The depolarization at the red edge is attributed to the hidden n, ir transition. Ionization of the lowest excited singlet and triplet states have been determined by the effect of pH on the absorption, fluorescence, and phosphorescence spectra of purines and pyrimidines. " Spectral, polarization, and quantum yield studies of cytidylyl-(3, 5 )-adenosine have also been published. Intermediates in the room-temperature flash photolysis of adenine and some of its derivatives have been identified hydrated electron, radical cations and anions, and neutral radicals resulting from their reactions have been assigned. Photoionization occurs via the triplet state. FMN encapsulated in surfactant-entrapped water pools interacts with polar head groups, entrapped water molecules, and outer apolar solvent. ... 

The reaction given in Eq. 3 represents ionization and electronic excitation of water molecules this occurs on the timescale of an electronic transition. The positive radical ion H20 + is known to undergo the ion-molecule reaction (Eq. 4) in the gas phase with a rate constant of 8 x 10 dm mol s [2], which sets the lifetime of the ion at less than 10 s in the liquid. The electronically excited states H2O are known to dissociate in the gas phase, as shown in Eq. 5, and the electron released in the ionization event is known to become thermalized and solvated in less than... 

The situation is quite different for n- r transitions. The lone electron pair is particularly well stabilized by polar and particularly by protic solvents so it becomes energetically more difficult to excite. Figure 2.45 shows the spectrum of N-nitrosodimethylamine in different solvents. Results of calculations indicate that the negative solvatochromism of carbonyl compounds can be explained on the basis of the structural changes due to the formation of hydrogen bonds (Taylor, 1982). Molecular dynamics simulations, however, indicate that the net blue shift is primarily due to electrostatic interactions (Blair et al., 1989). A large number of water molecules around the entire formaldehyde are responsible for the total blue shift the first solvation shell only accounts for one-third of the full shift. 

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