Final Report Summary - NITROSO COMPOUNDS (Exploring the borderline of bonding in group IV nitroso compounds by gas phase electron diffraction (GED))
One aim of the project was the synthesis and characterisation of nitroso tetrel compounds, in particular nitroso silanes (R3SiNO). These nitroso silanes possess a NO functionality which is directly bound to the silicon atom and which is responsible for the predicted low singlet-triplet energy gap ES-T. In the first phase of the project, comprehensive quantum chemical calculations (DFT, HF, B3LYP each at different levels of theory) on several model compounds R3SiNO have been performed (R=H, CH3, NH2, OH, F, Cl) to guide the experimental work. Besides information on the geometry around the silicon atom (dSi-N, dNO, wSiNO) also information on spectroscopic properties was derived in dependence of the substituent. In addition, reaction enthalpies of the reaction between the NO radical and different silyl radicals R3Si have been calculated. The main findings can be summarised as follows:
The silicon-nitrogen bond lengths in all calculated nitroso silanes are much longer (1.807 - 1.919 Å depending on substituent, method and basis set) than in silylamines with average Si-N bond lengths of 1.70 - 1.75 Å. This is an indication that the Si-N bonds in nitroso silanes are much weaker than in silylamines. The NO bond lengths vary only slightly for different substituents within a basis set and method. A comparison of dSi-N, dNO, and wSiNO at the 6-311++g level of theory is depicted. For the determination of ES-T the lowest lying triplet states have been calculated. The geometries of these triplet states exhibit two surprising features. The Si-N bond lengths are significantly shorter (1.712 - 1.805 Å), and the SiNO angles are much wider (130.6 - 134.8 degrees) than for the singlet species (114 - 116.9 degrees). The ES-T was calculated 6.2 to 10.3 kcal / mol at the B3LYP 6-311++g level of theory. Finally, reaction energies of NO with R3Si have been calculated. The results indicated an instaneous formation of the nitrososilanes with Hf ranging from -28 to -35 kcal / mol (B3LYP/6-311++g). A reaction barrier cannot be found due to the radical nature of these reactions. These results made us very optimistic for the synthesis of these compounds in the laboratory, however, although several different synthetic routes have been followed, nitroso silanes, germanes and stannanes could not be detected by any analytical method. It seems that these compounds are highly unstable and decompose or rearrange immediately after they have been formed. The second goal of the project deals with the structure determination by means of gas electron diffraction (GED). Originally, it was intended to characterise the nitroso compounds by this method. However, the inaccessibility of these substances forced us to turn our attention to other molecules with unusual bonding situations. The first molecule which was investigated in the course of this project was tert-butylsilanetriol. This compound shows strong intermolecular interactions in the solid as well as in the liquid phase with other silanetriol molecules. Therefore, we became interested whether such interactions do also exist in the gas phase. Quantum chemical calculations predicted such interactions in the gas phase at 0 K. Nevertheless, the determination of the molecular structure in the gas phase unambiguously revealed the presence of free, isolated silanetriol molecules. Additionally the gas phase structures of BiPh3, ((CH3)2P(CH2)2Cu)2, and Cl3CSCN have been determined. While BiPh3 is proposed as a future material in organic light-emitting diode (OLED)-based applications, ((CH3)2P(CH2)2Cu)2, is of interest due to metallophilic interactions of the copper atoms which are observed also in the gas phase. In Cl3CSCN the bonding situation around the thiocyanate moiety is interesting, especially in the gas phase. The radial distribution curves of these compounds are depicted and some relevant geometric parameters are given.
The silicon-nitrogen bond lengths in all calculated nitroso silanes are much longer (1.807 - 1.919 Å depending on substituent, method and basis set) than in silylamines with average Si-N bond lengths of 1.70 - 1.75 Å. This is an indication that the Si-N bonds in nitroso silanes are much weaker than in silylamines. The NO bond lengths vary only slightly for different substituents within a basis set and method. A comparison of dSi-N, dNO, and wSiNO at the 6-311++g level of theory is depicted. For the determination of ES-T the lowest lying triplet states have been calculated. The geometries of these triplet states exhibit two surprising features. The Si-N bond lengths are significantly shorter (1.712 - 1.805 Å), and the SiNO angles are much wider (130.6 - 134.8 degrees) than for the singlet species (114 - 116.9 degrees). The ES-T was calculated 6.2 to 10.3 kcal / mol at the B3LYP 6-311++g level of theory. Finally, reaction energies of NO with R3Si have been calculated. The results indicated an instaneous formation of the nitrososilanes with Hf ranging from -28 to -35 kcal / mol (B3LYP/6-311++g). A reaction barrier cannot be found due to the radical nature of these reactions. These results made us very optimistic for the synthesis of these compounds in the laboratory, however, although several different synthetic routes have been followed, nitroso silanes, germanes and stannanes could not be detected by any analytical method. It seems that these compounds are highly unstable and decompose or rearrange immediately after they have been formed. The second goal of the project deals with the structure determination by means of gas electron diffraction (GED). Originally, it was intended to characterise the nitroso compounds by this method. However, the inaccessibility of these substances forced us to turn our attention to other molecules with unusual bonding situations. The first molecule which was investigated in the course of this project was tert-butylsilanetriol. This compound shows strong intermolecular interactions in the solid as well as in the liquid phase with other silanetriol molecules. Therefore, we became interested whether such interactions do also exist in the gas phase. Quantum chemical calculations predicted such interactions in the gas phase at 0 K. Nevertheless, the determination of the molecular structure in the gas phase unambiguously revealed the presence of free, isolated silanetriol molecules. Additionally the gas phase structures of BiPh3, ((CH3)2P(CH2)2Cu)2, and Cl3CSCN have been determined. While BiPh3 is proposed as a future material in organic light-emitting diode (OLED)-based applications, ((CH3)2P(CH2)2Cu)2, is of interest due to metallophilic interactions of the copper atoms which are observed also in the gas phase. In Cl3CSCN the bonding situation around the thiocyanate moiety is interesting, especially in the gas phase. The radial distribution curves of these compounds are depicted and some relevant geometric parameters are given.
