E. Liepiņš

A multinuclear NMR spectroscopy study of alkoxysilanes

Abstract 17 O, 29 Si, and 13 C NMR spectra of more than 100 mono-, di-, tri- and tetra-alkoxysilanes R 4− n Si(OR′) n ; R = C n H 2 n +1 , Ph, CH 2 Cl, CH 2 Br; R′ = C n H 2 n +1 , CH 2 Ph, CH 2 CH 2 Cl, CH 2 CHCH 2 , CH 2 CCH, CH 2 CF 3 . (CH 2 ) 3 Cl, (CH 2 ) 3 CN have been studied. Linear relationships between the chemical shifts of 17 O, 29 Si, 13 C in alkoxysilanes and the inductive and steric constants of substituents R and R′ were observed. Different transmission of electronic effects along the SiO bond in various directions was revealed by means of 13 C, 29 Si, 17 O NMR spectroscopy and correlation analysis. The results are discussed in terms of ( pd ) π -bonding between the oxygen and silicon atoms in compounds containing an SiO bond.

13C, 15N and 29Si NMR spectra of triazasilatranes

Abstract 13C, 15N and 29Si chemical shifts and 29Si1H, 29Si13C and 29Si15N coupling constants as well as SiH bond stretching frequencies in the triazasilatranes (I) (2,5,8,9-tetraaza-1-silatricyclo[3.3.3.01,5] undecanes) and model compounds, tris(alkylamino)silanes with RSi = H, Me, CH2CH (Vi) and C6H5 (Ph) were measured. A stronger intramolecular N → Si bonding was revealed in I compared with their oxygen analogues, silatranes (II). This was assumed to be caused by the higher polarity of the equatorial SiX bonds in I (X = NH) in comparison with II (X = O).

Multinuclear NMR study of chlorinated vinylsilanes

Abstract Silylethylenes of the types (CH 3 ) 3- n X n SiCHCHCl (I = trans , II = cis ), (CH 3 ) 3- n X n SiCHCCl 2 (III), (CH 3 ) 3- n X n SiCCl  CCl 2 (IV) and (CH 3 ) 3- n X n SiCHCHSiX n (CH 3 ) 3- n (V) ( n = 0-3, X = Cl, OEt, OMe), were studied by 1 H, 13 C, 29 Si and 17 O NMR methods. A comparison of the 13 C chemical shifts in I–V with those in the corresponding vinylsilanes reveals that the sensitivity of α, β-olefinic carbon resonances to electronic effects of the Si substituents diminishes in the sequence: I > III > II ≈ IV. Various possible coupling constants in I–V were measured. A linear correlation between 1 J (CC) coupling in I–IV and the sum of the electronegativities of the Si substituents was obtained. The different correlations between 1 J ( 29 Si 13 C) and 2 J ( 29 SiC 1 H) involving X = Cl or OR substituents is explained within the framework of the ( p - d ) π hypothesis.

73GE, 17O, 13C NMR Spectra of alkoxygermanes

Abstract 13 C, 17 O and 73 Ge NMR spectra of seven tetraalkoxygermanes have been investigated. 73 Ge resonance signals in these compounds are more sensitive to structure variations than those of 29 Si in isostructural alkoxysilanes. 17 O and 13 C NMR spectra of alkoxygermanes Me 4- n Ge(OR) n (R = Et, Pr, Bu, i-Bu, s-Bu, CH 2 CHCH 2 , CH 2 CH 2 OMe, CH 2 CF 3 ; n = 1–3) have been studied. It has been shown that the magnitude of ( p—d ) τ -bonding in the GeO bond is smaller than in the SiO bond, but variation in the extent of ( p—d ) τ -overlapping in alkozygermanes with different substituents is similar to that in the isostructural alkoxysilanes.

73Ge NMR spectra of 2-thienyl-, 2-furyl- and 2-(4,5-dihydrofuryl)germanes

The 73Ge NMR spectra of 2-thienyl, 2-furyl- and 2-(4,5-dihydrofuryl)germanes have been studied. The comparison of 73Ge chemical shifts with those of 29Si and 119Sn in isostructural silicon and tin derivatives confirms the existence of additional effects in these compounds. The contribution of these effects to chemical shifts of central atom is different for germanium, silicon and tin compounds.

Radical addition of tetrahydrofuran and tetrahydro-2-furanone to alkenylsilanes in the presence of di(t-butyl) peroxide

Abstract β-(Silylethyl)- and γ-(silylpropyl)-tetrahydrofurans and tetrahydro-2-furanones containing alkyl and phenyl groups at the Si atom were obtained by the reactions of tetrahydrofuran (THF) and tetrahydro-2-furanone with vinyl- and allylsilanes in the presence of di(t-butyl) peroxide (DTBP). To reduce the amount of bis-products, the ratio [heterocycle]:[alkenylsilane]:[DTBP] = 80:4:1 was used.

73Ge, 13C and 1H NMR spectra of methylethynylgermanes

The 73Ge, 13C and 1H NMR spectra of methylethynylgermanes have been studied. The chemical shifts were compared with those of the corresponding isostructural tin compounds and a good correlation has been found. The origin of the chemical shifts could not simply be attributed to the electron densities as estimated by MNDO calculations.

Study of polyfunctional stannylallenes using NMR spectroscopy

Abstract A series of stannylallene derivatives Me 3 SnC(X) C C(Y)Z ( 1 ) (X = H, SnMe 3 , SiMe 3 , GeMe 3 , SC 2 H 5 , Br; Y = H, SnMe 3 ; Z = H, SnMe 3 , SiMe 3 , GeMe 3 , SC 2 H 5 ) were studied by means of 13 C, 29 Si, 119 Sn NMR spectroscopy. It is revealed that in 1 the effects of substituents on chemical shift (CS) values and spin spin coupling constants (SSCC) are additive. The set of linear correlations found between the isotope shifts (IS) and SSCC in the molecules of 1 demonstrates the interrelation of these values.

nJ(29Si13C) coupling constants and 13/12C isotope shifts in 29Si NMR spectra of ethynylsilanes

Abstract The coupling constants n J ( 29 Si 13 C) and isotope shifts n Δ 29 Si( 13/12 C) in the 29 Si NMR spectra of the ethynylsilanes (CH 3 ) 3 SiCCX (X = H, Br, I, SCH 3 , SC 2 H 5 , SC 6 F 5 , CH 3 , CH 2 Cl, CH 2 OC 2 H 5 , CH 2 N(C 2 H 5 ) 2 , CH 2 Si(CH 3 ) 3 , CH 2 Ge(CH 3 ) 3 , CH 2 C(CH 3 ) 3 , CF 3 , CN, C 6 H 5 , C 6 F 5 , Si(CH 3 ) 3 , Ge(CH 3 ) 3 , Ge(C 2 H 5 ) 3 , Sn(CH 3 ) 3 , Sn(t-C 4 H 9 ) 3 ) have been examined. There is a linear correlation between the isotope shifts and coupling constants. Compounds with X = MR 3 (M = Si, Ge, Sn; R = Alk) deviate from these correlations, probably because the concurrent hyperconjugation falls in the order M = Sn > Ge > Si. The data are compared with the 119 Sn NMR spectral data of the isostructural ethynylstannanes.

Ultrahigh resolution 29Si N.M.R. spectroscopy: natural abundance studies of oxygen-18/16 isotope effects on 29Si nuclear shielding

18/16 O Isotope effects on 29Si chemical shifts have been measured in various organosilanes for the first time; their magnitude depends on the nature of the Si–O chemical bond, particularly, on the extent of dπ–pπ conjugation.