Solvatochromism of heteroaromatic compounds: XXXI. Energetics of hydrogen bonding between <Emphasis Type="Italic">N</Emphasis>-methyltrifluoromethanesulfonamide and ethers

For a series of H-complexes of ethers with N-methyltrifluoromethanesulfonamide CF₃SO₂NHMe, a proportionality between the spectral shift (Δv_NH) and the enthalpy of complex formation (ΔH) was found. Comparison with hydroxyl-containing H-bond donors revealed the following contradiction: ΔH increases in the order CF₃SO₂NHMe < MeOH < PhOH, whereas Δv_XH increases in the order MeOH < CF₃SO₂NHMe ～ PhOH. The absence of a single Badger-Bauer relationship Δv_XH ～ f(ΔH) is due to the contribution of universal interactions into the enthalpy of H-complex formation. The account for these interactions allowed calculation of the energy of an intermolecular hydrogen bond, which varies in parallel with the spectral shift: MeOH < CF₃SO₂NHMe ～ PhOH.     

Solvatochromism of Heteroaromatic Compounds: XIII. Methyl(aroyloxymethyl)fluorosilanes and (Aroyloxymethyl)trifluorosilanes

The UV spectra of heptane solutions of methyl(aroyloxymethyl)fluorosilanes and (aroyloxymethyl)trifluorosilanes of the general formula 4-RC₆H₄COOCH₂Si(Me)_{3 - 4n} F _n (n = 1-3; R = H, F, Cl, Me, MeO, O2N) were studied. The intramolecular coordination bond Oarr;Si causes a bathochromic shift of the second -* transition, because the energy gap between the ground electronic and Franck-Condon excited states decreases owing to additional stabilization of the latter. The spectral shifts correspond to stabilization of the five-coordinate state of the silicon atom in (aroyloxymethyl)trifluorosilanes by 5-6 kcal mol^{- 1}.     

Solvatochromism of Heteroaromatic Compounds: XXVII. Configuration Isomerism of H Complexes of Methanol with Carbonyl Compounds

The energy minimum of H complexes of methanol with carbonyl compounds corresponds to the nonlinear structure in which the bridging hydrogen atom deviates from the axis of the C=O bond. The stretching vibration bands of the OH bonds in these H complexes, observed in the IR spectra of solutions (CCl₄), have a complex shape or are asymmetrical, which is due to the existence of two configuration isomers differing in the direction of the H bond. Difference in the orientation of the subunits of the complex may cause significant differences in the OH stretching frequencies.     

Solvatochromism of Heteroaromatic Compounds: XXIV. Solvation Complexes of ( = O→Si)(4-Nitrobenzoyloxymethyl)trifluorosilane in Protophilic Solvents

(4-Nitrobenzoyloxymethyl)trifluorosilane containing a five-coordinate silicon atom ( = OSi) is specifically solvated in a protophilic medium to form a complex in which the solvent molecule donates an electron pair and the coordination number of the silicon atom increases to six. Formation of such solvation complexes gives rise to a pronounced positive specific solvatochromic effect in the UV spectra. According to dielectrometric, ¹⁹F NMR, and IR data, and also to ab initio calculations (HF/6-31G*), the solvent molecule occupies an equatorial position.     

UV Spectra and Structure of 2-Diethylamino-3-phenylpropenal: Enamine or Enal?

According to the data of UV, IR, ¹H and ¹³NMR spectra and also quantum-chemical calculations (B3LYP/631G*), Z- and E-isomers of 2-diethylamino-3-phenylpropenal in inert environment exist as antiperiplanar conformers. The energy of electron transitions therein depends on the orientation both f the benzene ring and the unshared electron pair of the nitrogen with respect to the C = C bond. 2-Diethylamino-3-phenylpropenal is an enamine, but the extent of the p^* interaction in its prevailing Z-isomer is small.     

Molecular Structure of Complexes with Bifurcated Hydrogen Bond: I. N-(4-Methyl-2-nitrophenyl)acetamide

According to the results of HF/6-31G* and B3LYP/6-31G* nonempirical calculations, N-(4-methyl- 2-nitrophenyl)acetamide in the synperiplanar and antiperiplanar conformations gives stable complexes with protophilic solvents, which are stabilized by bifurcated (three-center) hydrogen bond. The complexes are characterized by (1) opposite variations of the interatomic distances and angles corresponding to intra- and intermolecular components of the bifurcated hydrogen bond, (2) extension of the intramolecular component and a more pronounced shortening of the intermolecular component with increase in the strength of the three-center H-complex, and (3) nonlinear and nonmonotonic relation between the NH stretching vibration frequency and the energy of complex formation.