Water transition dipoles

Our method has evolved during many studies over the last two decades. These include studies on the effect of strong internal electric fields in crystals on optical transition dipole directions of nucleic acid bases [2, 3], QM-MM predictions of time-dependent solvatochromism on 3-methylindole (3MI) in water [4], and on tryptophan in several proteins [5-8]. More recently, the same techniques have been... 

First, as the molecule on which the chromophore sits rotates, this projection will change. Second, the magnitude of the transition dipole may depend on bath coordinates, which in analogy with gas-phase spectroscopy is called a non-Condon effect For water, as we will see, this latter dependence is very important [13, 14]. In principle there are off-diagonal terms in the Hamiltonian in this truncated two-state Hilbert space, which depend on the bath coordinates and which lead to vibrational energy relaxation [4]. In practice it is usually too difficult to treat both the spectral diffusion and vibrational relaxation problems at the same time, and so one usually adds the effects of this relaxation phenomenologically, and the lifetime 7j can either be calculated separately or determined from experiment. Within this approach the line shape can be written as [92 94]... 

Within the mixed quantum/classical approach, at each time step in a classical molecular dynamics simulation (that is, for each configuration of the bath coordinates), for each chromophore one needs the transition frequency and the transition dipole or polarizability, and if there are multiple chromophores, one needs the coupling frequencies between each pair. For water a number of different possible approaches have been used to obtain these quantities in this section we begin with brief discussions of each approach to determine transition frequencies. For definiteness we consider the case of a single OH stretch chromophore on an HOD molecule in liquid D2O. 

One way to include these local quantum chemical effects is to perform ab initio calculations on an HOD molecule in a cluster of water molecules, possibly in the field of the point charges of the water molecules surrounding the cluster. In 1991 Hermansson generated such clusters from a Monte Carlo simulation of the liquid, and for each one she determined the relevant Bom Oppenheimer potential and the vibrational frequencies. The transition-dipole-weigh ted histogram of frequencies was in rough agreement with the experimental IR spectrum for H0D/D20 [130],... 

We have described our most recent efforts to calculate vibrational line shapes for liquid water and its isotopic variants under ambient conditions, as well as to calculate ultrafast observables capable of shedding light on spectral diffusion dynamics, and we have endeavored to interpret line shapes and spectral diffusion in terms of hydrogen bonding in the liquid. Our approach uses conventional classical effective two-body simulation potentials, coupled with more sophisticated quantum chemistry-based techniques for obtaining transition frequencies, transition dipoles and polarizabilities, and intramolecular and intermolecular couplings. In addition, we have used the recently developed time-averaging approximation to calculate Raman and IR line shapes for H20 (which involves... 

In-plane and out-of-plane rotational dynamics of CigRB at the toluene/water interface was evaluated using time-resolved TIR fluorescence spectroscopy [27]. The known transition dipole moment for the absorption of rhodamine B(RB) at about 530 nm (So Si) is almost parallel to that for the emission at about 570 nm (Si -> So) [28]. Time-resolved in-plane fluorescence anisotropy (r[Pg.213]

Fig. 4. The anatomy of a p-like state . Two isodensity contour maps (+ 0.01 and + 0.03 a.u." ) of the same LUMO orbital are shown side by side. Unlike the p-Uke orbitals in one-electron models, LUMO states in MQC MD-DFT and CIS models have the lobes pushed outwards between the first and the second solvation shells, with < 20% of the spin density residing inside the cavity. This results in considerable firagmentation of the diffuse part of the wavefunction. The O 2p orbitals are strongly polarized, with opposite signs of the orbitals attained by water molecules on the opposite sides of the cavity in the direction of transition dipole moment.

Interestingly, the multielectron model also accounts for the failure of the PTHB experiment since part of the transition dipole moment is carried out by O 2p orbitals in water molecules, their rapid... 

This deformation can be followed by UV-vis spectroscopy directly at the air/water interface. Upon compression of the monolayer, the absorbance changes not very much around the point where the first increase in surface pressure is recorded (see Figure 6.17), This is caused by the disappearance of the voids between the domains. Above the kink in the isotherm, howevei the K-n band (-323 nm) decreases while the band at 248 nm increases. This increase is due to the increase in chromophore concentration upon compression of the monolayer. The decrease of the 7t-ic band is caused by a preferential orientation of the chromophorra perpendicular to the surface. Chromophores oriented perpendicularly are not detected because the transition dipole of the n-n band is parallel to the probing light. The band at 248 nm, however is not sensitive to the orientation of the chromophore, because the transition dipole moment has a component perpendicular to the long axis of the molecule (see Figure 6.17). 

Fig. 4. Time dipole correlation functions C(t) of water in critical state (left top), in bulk liquid water at 30°C (left center), in a monolayer on fluorophlogopite mica (left bottom), in LTA bonded to the first 4 Na+ ions (right top), in SB A-15 heated to 300°C for 2 hrs (right center), and in fully hydrated SBA-15 (right bottom). The normalized total correlation functions, obtained according to Eq. (9) involve vibrations of the transition dipole of the (v+5) band displayed as rapid oscillations. Rotational correlations including angular perturbations appear as envelopes of the vibrational correlation functions. The inertial rotational motion about the least rotational axis of the water molecule is indicated as a quadratic decay C(t) - (kT/I) t2 at times 0 - 0.05 psec in each C(t) vs. t graph. The graphs on the left are reproduced from ref. 18.

The graph below shows the absorption spectrum of a molecule in water (refractive index = 1.33). The spectrum is simplified for ease of integration. Calculate the dipole strength of the absorption band and the transition dipole moment. Specify the units of both quantities. Save your results for use in the exercises for Chaps. 5 and 6. 

A number of metal ions, especially ions of transition metal elements (such as Cu, Zn, Fe, Co and Ni) exist in the form of relatively stable aqua complexes (hydrates) in aqueous solutions at the natural pH values of food materials. Aqua complexes of metal ions are often part of more complicated complexes of mineral compounds with various organic ligands. The binding of water is subject to electrostatic interactions between positive ions and coordinated water molecule dipoles. Generally, the molecules attached to the central ion are called ligands (7-1). 

---