Mass-spectrometric study of intramolecular coordination Si → O Interaction in gaseous (aroyloxymethyl)trifluorosilanes and methyl(aroyloxymethyl)fluorosilanes

Conclusions Mass-spectrometric criteria were developed for the assessment of the intramolecular coordination Si O interaction in the molecules of (aroyloxymethyl)trifluorosilanes and their analogs, methyl(aroyloxymethyl)difluorosilanes, and dimethyl(aroyloxymethyl)fluorosilanes. The successive replacement of fluorine atoms by methyl groups leads to a decrease in the intramolecular interaction of the silicon and carbonyl oxygen atoms.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1508–1511, July, 1987.     

Solvatochromism of Heteroaromatic Compounds: XX. 4(5)-Nitroimidazole

4(5)-Nitroimidazole in solution is stabilized as the 5-nitro isomer due to formation of hydrogen bond with an aprotic protophilic solvent. Amphiprotic medium favors displacement of the tautomeric equilibrium toward the 4-nitro isomer via formation of a solvate complex where 4-nitroimidazole acts as hydrogen bond acceptor. The observed specific solvatochromic effect in the UV spectrum of 4-nitroimidazole and related heterocyclic nitro compounds is determined by the electronic configuration of the excited ,^*-state.     

Solvatochromism of Heteroaromatic Compounds: X. 2-Phenylpyrrolide Anion

2-Phenylpyrrolide anion was obtained in high-polarity media containing MeONa as an ionizing agent. The bathochromic shift of the long-wave absorption band in going from the neutral to deprotonated form is explained by the difference in the molecular orbital characteristics of the parent molecule and the anion, as well as the by change in the equilibrium conformation on deprotonation. According to ab initio calculations, the conformational rearrangement contributes partly to the observed shift; the greatest contribution is from the destabilization of the highest occupied molecular orbital.     

Solvatochromism of Heteroaromatic Compounds: VIII. 2-(Tricyanovinyl)-5-phenylpyrrole

Rotational isomerism of 2-(tricyanovinyl)-5-phenylpyrrole and formation of complexes with a three-centered (bifurcational) hydrogen bond were studied by UV spectroscopy and quantum-chemical calculations. In the gas phase and in weakly and moderately protophilic media these compounds have mainly the syn-periplanar form stabilized by the intramolecular hydrogen bond involving the orthogonal system of the nitrile group and the NH hydrogen atom. No intermolecular H complex is formed in such solvents. In tetrahydrofuran, cyclohexanone, pyridine, and some other media a complex conformational equilibrium is observed. Interaction with aliphatic alcohols, amides, dimethyl sulfoxide, and triethylamine results in stabilization of the H complex of the anti-periplanar conformer.     

Solvatochromism of Heteroaromatic Compounds: XI. 1-Ethyl-2-formylbenzimidazole

Two regression dependences of the energy of the intramolecular charge-transfer transition in 1-ethyl-2-formylbenzimidazole on the polarity and polarizability of the medium have been established; one relates to the syn-periplanar conformer and the other, to the anti-periplanar conformer. The different solvatochromism of the two molecular forms is associated with the different sensitivity of low-energy electronic transitions of these conformers to nonspecific solvation.     

(Aroyloxymethyl)trifluorosilanes: a new class of pentacoordinate silicon compounds

A new class of organosilicon compounds containing a pentacoordinate silicon atom, (aroyloxymethyl)trifluorosilanes (AFS), has been prepared. The presence of an intramolecular coordinate F₃Si ← OC bond is supported by X-ray diffraction, IR spectroscopy and dipole moment data. Si ← O coordination in AFS has been shown to remain intact in the gaseous and liquid states below 420 K as well as in solution in most organic solvents. The dielectric constant of the medium significantly affects the nature of the coordinate Si ← O bond and that of the SiF, CO, COC bonds. In pyridine the intramolecular coordinate Si ← O interaction is disturbed or greatly weakened.At 420–500 K a reversible reaction takes place: ArC$\text{OOCH}{}_2\text{Si}$F₃ α ArCOOCH₂SiF₃ The changes in enthalpy and entropy are equal in this case: ?△H  8.1 ± 0.7 kcal/mol; ?°_S  15.3 ± 0.8 e.u. _T  450 K An assignment of bands in the IR spectra of AFS resulting from vibrations of atoms in the SiF₃, >CO and COC groups is given. The electron impact-induced primary disintegration of the molecular ion 4-XC₆H₄COOCH₂Si

mainly involves abstraction of a fluorine atom.     

Solvatochromism of Heteroaromatic Compounds: XIX. Factors Affecting Quantitative Characteristics of Solvatochromism in Aprotic Inert Media

The effects of aprotic inert media on the electronic absorption spectra of aromatic nitro compounds p-NO₂C₆H₄R were used as evidence for the linear correlation between the slope s _a of the solvatochromism equations _max = ₀ + s _a^* and the dipole moments of the molecules in their ground electronic state _g. A linear correlation was established between 0 and the first ionization potential of subunits C₆H₅R. A new approach to estimating the dipole moment of electronically excited molecules (_e) for molecules like p-NO₂C₆H₄R on the basis of the correlation _e = r_g was proposed.     

Solvatochromism of Heteroaromatic Compounds: XXI. Ion Pairs of 4-Nitropyrazolide Anion

The solvatochromism of the long-wave UV absorption band of the contact ion pairs of the 4-nitropyrazolide anion with alkali metal cations in aprotic protophilic solvents with 24 was studied and quantitatively described using the Kamlet-Taft empirical solvent parameters. Of two possible types of tight ion pairs differing in the murtual arrangement of the counterions, the solvent stabilizes the pair in which the cation is located in the plane of the anion in the vicinity of its nitrogen atoms. Among the factors affecting the quantitative characterisitcs of the solvatochromism of such ion pairs, the major factors are the polarity/polarizability of the solvent and the capability of its molecules for coordination with cations. The peripheral specific solvation of the cation in an ion pair stabilizes its Franck-Condon excited state relative to the ground electronic state. In the quantitative respect, this effect is similar to that characteristic of solvation H complexes.     

Solvatochromism of Heteroaromatic Compounds: XV. 3-Nitro-1,2,4-triazol-5-ones

Effect of the medium on the parameters of the long-wave absorption band in the electronic spectra of H complexes formed by 3-nitro-1,2,4-triazol-5-one and its N-methyl derivatives with aprotic protophilic solvents was described on a quantitative level using Kamlet-Taft solvatochromic parameters. A weak nonadditivity was revealed in the influence of two kinds of intermolecular hydrogen bonds.     

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.     

Quantum chemical study of silicon pentacoordination in (Aroyloxymethyl)trifluorosilanes and their analogs

Pentacoordinated silicon compounds of the series 4-XC₆H₄C(O)O(CH₂)_mSi(CH₃)_3-nF_n (m = 1, 2; n= 1,2,3) with an intramolecular 0→Si bond are studied by ab initia and semiempirical (AMI) quantum chemical methods. The results are compared with published experimental data. The C₆H₅C(O)OCH₂SiF₃ molecule is calculated in an RHF approximation using the 6–31G*basis set. The total energy of the molecule for its geometry optimization is calculated by the MP2 method including electron correlation. This leads to considerably improved agreement between the calculated coordination energy (25.3 kJ/mole) and the experimental value (28.5 kJ/mole). The geometry and the dipole moment calculated by both ab initio (HF/6-31G*//HF/6-31G*, MP2/6-31G*//MP2/6-31G*) methods and by the AMI method are in satisfactory agreement with the experimental data.     

Solvatochromism of Heteroaromatic Compounds: IX. Anions of 3-Nitro-1,2,4-triazol-5-ones

Solvatochromism of the long-wave band in the electronic absorption spectra of 3-nitro-1,2,4-triazol-5-one anions was studied and quantitatively described with the Kamlet-Taft solvent parameters. Azole anions are specifically solvated in amphiprotic solvents. The strength of the hydrogen bond in H complexes with amphiprotic solvents decreases in going to the Franck-Condon excited state. The UV and IR specrta show that 3-nitro-1,2,4-triazol-5-one anions, in contrast to nitroazole anions, are specifically solvated with participation of the carbonyl group. In highly polar organic solvents the 3-nitro-1,2,4-triazol-5-one anion mainly exists in the form of the 1H tautomer.     

Solvatochromism of Heteroaromatic Compounds: XIV. 5-Amino-1-methyl-4-nitropyrazole and Its Analogs

The solvatochromism of H complexes of 5-amino-1-methyl-4-nitropyrazole and 2-nitroaniline derivatives in aprotic protophilic media was described on a quantitative level with the aid of Kamlet-Taft empirical parameters. Specific solvation affects only one of the two long-wave bands, namely that corresponding to an electronic transition involving orbital electron density transfer from the H-bound nitrogen atom. When such transfer does not occur, which is typical of the second transition, specific solvatochromic effect is either weak or absent.     

Solvatochromism of Heteroaromatic Compounds: XII. Solvatochromism and Tautomerism of 4-Methyl-3-nitro-1,2,4-triazol-5-one in Aprotic Solvents

The long-wave absorption band in the UV spectrum of 4-methyl-3-nitro-1,2,4-triazol-5-one is highly sensitive to specific solvation by hydrogen bonding with a protophilic solvent. The UV and IR spectra suggest that the compound mainly exists in the form of the N¹H tautomer. The qualitative and quantitative relationships characterizing the solvent effect on the spectral characteristics of the major tautomer were elucidated.     

Solvatochromism of Heteroaromatic Compounds: VII. Bifurcate Hydrogen Bond in Solvatochromic Complexes of 2-(1,2,2-Tritricyanovinyl)pyrrole

IR and UV spectrosocopy and quantum chemistry were used to reveal a bifurcate (three-center) hydrogen bond in the complexes of 2-(1,2,2-tricyanovinyl)pyrrole with hydrogen-bond acceptors. In the gas phase and aprotic inert solvents this compound exists predominantly as the sp conformer stabilized by intra- molecular hydrogen bond involving an orthogonal system of the nitrile group and the NH hydrogen. The formation of the three-center hydrogen bond with an aprotic protophilic solvent shifts the conformational equilibrium to the ap conformer. As the strength of the intermolecular hydrogen bond increases in the series nitromethane < acetonitrile < dioxane, the fraction of the ap conformer increases and becomes prevailing already in the moderately protophilic THF. Solvatochromism of the long-wave absorption band in the elec- tronic spectra of the sp and ap conformers was studied in detail.     

Solvatochromism of Heteroaromatic Compounds: VI. Comparison of the Empirical and Theoretical Approaches to Description of Solvatochromism in Nonspecific Solvation of Nitropyrazoles

The effect of nonspecific solvation on the long-wave absorption band in the UV spectra of isomeric nitropyrazoles was studied. The aprotic inert and aprotic protophilic solvents exert different spectroscopic effects. In the former solvents, the solvatochromism is mainly governed by orientation interactions, and in the latter solvents, along with the orientation interactions, the UV spectrum is appreciably influenced by the induction, polarization, and dispersion interactions, which is due to the presence of readily polarizable nonbonding electron pairs and/or a pi-electron system in molecules of such solvents.     

Geometry of intramolecular silicon complexes: an ab initio estimation of the sensitivity to the effect of medium. (Aroyloxymethyl)trifluorosilanes

The effect of medium on the geometry of the (benzoyloxymethyl)trifluorosilane (1) molecule was studied by the HF and MP2 methods with the 6-31G(d), 6-311G(d), and 6-311+G(d,p) basis sets, as well as using the Onsager SCRF model, the PCM approach, and the data of X-ray diffraction study. Molecule 1 has a low complexation energy (5.4 kcal mol^–1 according to MP2(Full)/6-31G(d)+ZPE calculations and 6.8 kcal mol^–1 according to IR spectroscopy data), while its geometric parameters are the least sensitive to the effects of medium among all hypervalent silicon compounds studied to date. Nevertheless, the results obtained revealed a pronounced deformation of the SiO coordination bond in 1 on going from the gas phase to the polar solution and crystal. This serves as a theoretical substantiation of the hypothesis that substantial changes in the IR and NMR spectral characteristics of the (O—Si)-chelate (aroyloxymethyl)trifluorosilanes upon variation of external factors are due to geometric reasons. Analysis of the electron density distribution according to Bader indicates that the SiO bond in molecule 1 can be treated as a covalent bond of high polarity.     

Solvatochromism of Heteroaromatic Compounds: XXIII. Reversible Specific Solvatochromic Effect in the IR Spectra of 2-Nitrophenol and 2,4,6-Trinitrophenol

2-Nitrophenol and 2,4,6-trinitrophenol form with protophilic solvents complexes with a bifurcate (three-center) hydrogen bond. Their O-H stretching vibration frequencies nonlinearly and unsteadily vary with the H-bond acceptor ability of the medium, which is a spectroscopic evidence for three-center H-complex formation.