Solvent shifts

Schroeder J and Troe J 1985 Solvent shift and transport contributions in reactions in dense media Chem. Phys. Lett. 116 453... 

Polar solvents shift the keto enol equilibrium toward the enol form (174b). Thus the NMR spectrum in DMSO of 2-phenyl-A-2-thiazoline-4-one is composed of three main signals +10.7 ppm (enolic proton). 7.7 ppm (aromatic protons), and 6.2 ppm (olefinic proton) associated with the enol form and a small signal associated with less than 10% of the keto form. In acetone, equal amounts of keto and enol forms were found (104). In general, a-methylene protons of keto forms appear at approximately 3.5 to 4.3 ppm as an AB spectra or a singlet (386, 419). A coupling constant, Jab - 15.5 Hz, has been reported for 2-[(S-carboxymethyl)thioimidyl]-A-2-thiazoline-4-one 175 (Scheme 92) (419). This high J b value could be of some help in the discussion on the structure of 178 (p. 423). 

Bond alternation A-= SAJV Fm s.-s/ Dilution shift, g Solvent shift is B A Aat"... 

The methyl ester (100, R = CH3), derived from this A-nor acid by treatment with diazomethane, is different from the ester (102) obtained either by Favorskii rearrangement of 2a-bromo-5a-cholestan-3-one (101) or by the action of cyanogen azide on 3-methoxy-5a-cholest-2-ene (103) followed by hydrolysis on alumina. The ketene intermediate involved in photolysis of (99) is expected to be hydrated from the less hindered a-side of the molecule to give the 2j -carboxylic acid. The reactions which afford (102) would be expected to afford the 2a-epimer. These configurational assignments are confirmed by deuteriochloroform-benzene solvent shifts in the NMR spectra of esters (100) and (102). ... 

However, polar solvents shift the equilibrium toward the Z conformer 12a, which has a significantly larger dipole moment. 

The influence of the solvent on the oxidation of film under conformational relaxation control is illustrated in Fig. 47, which shows chronoamperograms obtained by steps from -2000 to 300 mV vs. SCE at room temperature (25°C) over 50 s in 0.1 M LiC104 solutions of different solvents acetonitrile, acetone, propylene carbonate, (PC), dimethyl sulfoxide (DMSO), and sulfolane. Films were reduced over 120 s in the corresponding background solution. Despite the large differences observed in the relative shape of the curves obtained in different solvents, shifts in the times for the current maxima (/max) are not important. This fact points to a low influence of the solvent on the rate at which confor-... 

It is expected, therefore, that the cyclic hexamer also exhibits a characteristic tendency to complex with cations. In fact, the addition of an acetonitrile solution of metal thiocyanates to a solution of the cyclic hexamer in the same solvent shifted the carbonyl absorption to a lower wave number46,52 The shift values depended upon the kind of metal ions present, and the largest shift value of 40 cm-1 was observed for barium thiocyanate (molar ratio of Ba2+/hexamer = 10). In addition to the shift of the carbonyl absorption, the intensities of the C—O-C stretching vibrations around 1200 cm-1 varied appreciably. 

Table 3.7. Solvent Shifts for Various Transitions in p-Substituted BenzophenoneyW4>,°...

State Intensity (log e) Solvent shift, cm-1 Energy range, cm-1 S-T splitting, cm-1... 

They are applicable to compounds in the common NMR solvents - but not in D6-benzene (or D5-pyridine). The substituent effects are additive, but don t place too much reliance on chemical shifts predicted using the table, in compounds where more than two groups are substituted next to each other, as steric interactions between them can cause large deviations from expected values. Note that Table 5.4, like all others, does not cater for solvent shifts, etc ... 

Kawski A (1992) Solvent-shift effect of electronic spectra and excitation state dipole moments. In Rabek JR (ed) Progress in photochemistry and photophysics. CRC, New York, pp 1-47... 

The assignment of these absorptions has been controversial2,164, 165) the low-energy transition near 1640 cm-1 has been assigned either to the C=0 stretching mode or at least considered to have predominatly C=0 character on the basis of solvent shifts. 

Interestingly, the UV maxima of the above dialkyl triafulvenes are only slightly influenced by the polarity and hydrogen-bonding capability of the solvent. In the cyclic triafulvene 195, countercurrent solvent shifts for the two main absorptions are observed. 

Figure 7. Frequency dependence of fix for PNA. Key +, experimental fif data points and—, theoretical fix curve accounting for solvent shift effect. Measured and calculated values agree even better than in Reference 12 due to thefurther refinements described in...

The direct spectroscopic characterization of la in an LFP experiment was achieved by using a different precursor. LFP of p-chlorophenol in aqueous solution produces, at the end of the laser pulse, a transient with absorption maxima at 250, 370, and 384 nm.26 These absorptions are in reasonable agreement with the strong and medium intensity absorptions of la in argon at 10 K (290 and 379 nm), especially if a solvent shift of the highly polar la in water compared to argon is taken into account. The assignment of the transient... 

Applications of TDDFT within the QM/M M Framework -Solvent Shift of the So/St Transition of Acetone in Water... 

As a first test system chosen to probe the capabilities of a QM/ MM approach in reproducing environment-induced shifts in excitation spectra, we have calculated the solvent shift of the first optical excitation (n -> n ) of an acetone molecule in the gas phase and in water (Fig. 1.5) [16]. 

Using the ROKS method a blue shift of 0.23 eV (experimental value 0.21 eV [94]) is calculated for the case in which only acetone itself is included in the QM region. Addition of the first solvation shell has only a tiny effect (a shift of 0.03-0.04 eV) indicating that the solvent shift is basically converged with respect to... 

A fundamental question concerns the state of the adsorbed gas, namely whether it is closer to the gaseous or the liquid state. At 301 K, the solvent shift is mainly observed on the terminal carbon atoms which are more exposed to intermolecular interactions (22). The carbon Cj and C4 of 1-butene experience a small low field shift with respect to the gas, the carbon a small high field shift, while the methinic C2 carbon atom is much more influenced than the other carbon atoms (low field shift) suggesting a specific interaction at this site of the molecule. 

Equations (7.6) and (7.7) provide a means of determining excited dipole moments together with dipole vector angles, but they are valid only if (i) the dipole moments in the FC and relaxed states are identical, (ii) the cavity radius remains unchanged upon excitation, (iii) the solvent shifts are measured in solvents of the same refractive index but of different dielectric constants. 

Diehl, P., Freeman, R. The Influence of Molecular Shape on Solvent Shifts in the Proton Magnetic Resonance Spectra of Polar Solutes. Mol. Phys. 4, 39 (1961). 

Evans, D. F. Solvent Shifts of Nuclear Spin Coupling Constants due to Hydrogen Bonding. J. Chem. Soc. 1963, 5575,... 

G. van der Zwan and J. T. Hynes, Time-dependent fluorescence solvent shifts, dielectric friction and nonequilibrium solvation in polar solvents, J. Phys. Chem. 89, 418M188 (1985). 

This expression shows that the avaage occupancy of the ion s orbital takes on values between 0 and 1, depending on the solvent shift of the ion s energy relative to the Fermi level of the metal. For example, if A 0, we get (/ /> = 0, and the electron is on the metal, as we find in the case of the two-state model. 

Table 1 Main solvent shifts (Aj ) and fluorescence lifetimes (r) of [FM] complexes (F = (/ )-2-naphthyl-1-ethanol F5 = (5)-2-naphthyl-1-ethanol)...

No reaction at all is observed upon irradiation oil 4, R =4-NH2 in methanol or 74, R=4- (or 5-) -OH in aqueous alkaline solution ). From solvent shifts in the absorption spectra and lack of reactivity a lowest (7r,n )- or charge transfer excited state is implied. 

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