Sarah L. Hinchley

The molecular structure of tetra-tert-butyldiphosphine: an extremely distorted, sterically crowded molecule

The molecular structure of tetra-tert-butyldiphosphine has been determined in the gas phase by electron diffraction using the new DYNAMITE method and in the crystalline phase by X-ray diffraction. Ab initio methods were employed to gain a greater understanding of the structural preferences of this molecule in the gas phase, and to determine the intrinsic P-P bond energy, using recently described methods. Although the P-P bond is relatively long [GED 226.4(8) pm; X-ray 223.4(1) pm] and the dissociation energy is computed to be correspondingly small (150.6 kJ mol(-1)), the intrinsic energy of this bond (258.2 kJ mol(-1)) is normal for a diphosphine. The gaseous data were refined using the new Edinburgh structure refinement program ed@ed, which is described in detail. The molecular structure of gaseous P(2)Bu(t)(4) is compared to that of the isoelectronic 1,1,2,2-tetra-tert-butyldisilane. The molecules adopt a conformation with C(2) symmetry. The P-P-C angles returned from the gas electron diffraction refinement are 118.8(6) and 98.9(6) degrees, a difference of 20 degrees, whilst the C-P-C angle is 110.3(8) degrees. The corresponding parameters in the crystal are 120.9(1), 99.5(1) and 109.5(1) degrees. There are also large deformations within the tert-butyl groups, making the DYNAMITE analysis for this molecule extremely important.

Planar 1,3λ4δ2,2,4‐Benzodithiadiazine and Its Nonplanar 5,6,7,8‐Tetrafluoro Derivative: Gas‐Phase Structures Studied by Electron Diffraction and Ab Initio Calculations

The gas-phase molecular structures of 1,3lambda4delta2,2,4-benzodithiadiazine and 5,6,7,8-tetrafluoro-1,3lambda4delta2,2,4-benzodithiadiazine have been investigated by ab initio calculations and electron diffraction using the SARACEN method of structural analysis. Important structural parameters (r(h1) structure) for the parent compound were found to be: 1.546(3), r(S-N) 1.697(5), r(C-S) 1.784(5), and r(C-N) 1.393(6) A. For the tetrafluoro derivative, these are (r(h1) structure): 1.552(3), r(S-N) 1.723(8), r(C-S) 1.812(9), and r(C-N) 1.396(7) A. Furthermore, the GED experiment (Gas Electron Diffraction) quite convincingly demonstrates the nonplanarity of the former and the planarity of the latter in agreement with DFT calculations; but the results contradict calculations at the MP2 level. The effect of the fluorine atoms on the conformations of the molecules is discussed.

Persistent phosphinyl radicals from a bulky diphosphine: an example of a molecular jack-in-the-box

The structure of the phosphinyl radical, ṖR2 [R = CH(SiMe3)2], has been determined by gas-phase electron diffraction (GED) together with ab initio molecular orbital calculations, and that of the corresponding diphosphine, (PR2)2, has been established by X-ray crystallography; the diphosphine behaves as an energy storage reservoir.

The molecular structure of [In(P3C2Bu2t)] using gas-phase electron diffraction and ab initio and DFT calculations

The molecular structure of [In(P 3 C 2 Bu t 2 )] has been determined by gas-phase electron diffraction using the SARACEN method. The experimental geometric parameters showed good correlation with those obtained from quantum chemical calculations and from a previous X-ray diffraction study. Calculations were performed using various DFT methods and also MP2 theory to identify the most suitable method for calculating structures of this type. The accuracy of the calculations was gauged by reference to experimentally determined parameters. The use of small-core and large-core pseudopotentials on the indium atom was also tested, showing that the lack of electrons explicitly considered in the calculation when a large-core pseudopotential was used affected the accuracy of the calculation. Similar calculations have been performed for the less symmetrical [In(P 2 C 3 BU t 3 )], but electron diffraction data of adequate quality could not be obtained.

An experimental and theoretical study of the molecular structure and vibrational spectra of iodotrimethylsilane (SiIMe3)

The gas-phase molecular structure of iodotrimethylsilane (ITMS) has been determined from electron diffraction data. Infrared and Raman spectra have been completely assigned. The experimental work is supported by ab initio HF and MP2 calculations for the gas-phase structure determination and DFT(B3LYP) calculations, combined with Pulays SQM method, for the vibrational spectra data.

Molecular structures of the 1,6-disubstituted triptycenes Sb2(C6F4)3 and Bi2(C6F4)3 using gas-phase electron diffraction and ab initio and DFT calculations

The structures of the D(3h)-symmetric molecules dodecafluoro-1,6-distibatriptycene and dodecafluoro-1,6-dibismatriptycene [Z2(C6F4)3 (Z = Sb, Bi)] have been determined in the gas phase by electron diffraction, using the SARACEN method, with restraints obtained from quantum chemical calculations. Several methods of ab initio and density functional theory geometry calculations have been performed and recommendations made as to their relative suitabilities for determining the structures of such species. Calculations using the MP2 method with a small-core pseudopotential (aug-cc-pVQZ-PP) on the Sb and Bi atoms and the 6-311G* basis set on the light atoms were found to give the closest correlation with the experimental results for both molecules. Differences in structure were found depending on whether a large-core or small-core pseudopotential was used on the heavy atoms.

Dynamic interaction of theory and experiment: total determination of the gas-phase molecular structure of tri-tert-butylphosphine oxide (OPBut3).

A new method to aid the determination of structures of sterically crowded molecules in the gas phase by dynamically linking the gas-phase electron diffraction (GED) refinement process with computational methods has been developed. The procedure involves refining the heavy-atom skeleton of the molecule using the GED data while continually updating the light-atom positions during the refinement using computational methods, in this case molecular mechanics. This removes errors associated with the assumption of local symmetry for the light-atom groups, which can affect the final values of the heavy-atom parameters. The refinement of the molecular structure of tri-tert-butyl phosphine oxide has been used to illustrate this new technique, which we call the DYNAMITE (DYNAMic Interaction of Theory and Experiment) method. Re-examination of the structure using this method has resulted in a shorter P-O distance than was found in a less sophisticated anaylsis, and is consistent with the molecule being regarded as O=PBut3, rather than O(-)-P+But3.

Bis(tert-butyl)sulfurdiimide, S(NBut)2, and tris(tert-butyl)sulfurtriimide, S(NBut)3: structures by gas electron diffraction, X-ray crystallography and ab initio calculations

The molecular structures of bis(tert-butyl)sulfurdiimide [S(NBut)2] and tris(tert-butyl)sulfurtriimide [S(NBut)3] have been investigated in the gas phase by electron diffraction and ab initio calculations, and in the solid phase by low-temperature X-ray diffraction. The structures of each were found to be similar in both phases, and the calculated structures agree well with those in the gas phase. Ab initio calculations at levels up to MP2(fc)/cc-pVTZ for S(NBut)2 predict that the E/Z conformer (Cs symmetry) is the preferred arrangement by as much as 36.5 kJ mol−1 over the E/E conformer. Important structural parameters [ab initio (re)/GED (ra)/X-ray] for the E/Z conformer of S(NBut)2 are S(1)–N(2) [152.9/153.8(3)/152.8(3) pm], S(1)–N(3) [155.5/156.5(4)/154.4(3) pm], N(2)–C(4) [147.3/146.2(4)/147.7(5) pm], N(3)–C(5) [147.9/147.0(4)/148.9(4) pm], N(2)–S(1)–N(3) [116.9/117.8(6)/117.4(2)°], S(1)–N(2)–C(4) [125.9/125.9(6)/128.1(2)°] and S(1)–N(3)–C(5) [117.1/116.7(7)/118.2(2)°]. One conformer of S(NBut)3 with C3h symmetry was located at the MP2(fc)/6-31G* level. The gas and solid-phase studies both returned C3 structures, with the butyl groups moved a little out of the SN3 plane. Important structural parameters [ab initio (re)/GED (ra)/X-ray] for S(NBut)3 are SN [152.8/153.5(3)/151.0(2), 151.0(2), 151.1(2) pm], N–C [148.7/147.2(4)/148.3(3), 148.5(3), 148.3(3) pm], C–C [152.8/150.8(2)/152.4(4), 152.6(4), 153.0(3) pm], SN–C [123.2/122.9(4)/126.2(2), 125.5(2), 126.0(2)°], C–C–C (mean) [110.4/108.3/110.0°] and NSN–C (mean) [180.0/173.0(5)/179.4°]. Theoretical predictions at the MP2(fc)/6-31G* level were used to restrain some of the refining parameters for both structures using the SARACEN method. The lowest energy structure of bis(tert-butyl)sulfurdiimide was found to be the E/Z conformer, and the structure of tris(tert-butyl)sulfurtriimide is such that each fragment with two NBut ligands has the E/Z conformation.

1,1,2-Tri-tert-butyldisilane, But2HSiSiH2But: vibrational spectra and molecular structure in the gas phase by electron diffraction and ab initio calculations

The molecular structure of 1,1,2-tri-tert-butyldisilane, But2HSiSiH2But, has been determined in the gas phase by electron diffraction (GED) and ab initio molecular-orbital calculations. Vibrational spectra are consistent with a vapour consisting of one conformer, identified by the structural study as a syn arrangement in which each of the butyl groups eclipses an Si–H bond. Important structural parameters (ra) for the conformer are: Si–Si 236.3(8), Si–C (mean) 191.1(3), C–C 154.5(1), C–H 112.4(1) pm, Si(1)–Si(2)–C(21) 116.0(8), Si(2)–Si(1)–C(11) 111.2(10), Si(2)–Si(1)–C(12) 108.7(9), C(11)–Si(1)–C(12) 121.1(11) and C(21)–Si(2)–Si(1)–H(13) –6.2(11)°, where C(11), C(12) and C(21) are the central carbon atoms of the three tert-butyl groups. These experimental observations are supported by theoretical predictions obtained at the D95*/MP2 level, which also identify two higher-energy conformers.

The molecular structure of using gas-phase electron diffraction and ab initio and DFT calculations

The molecular structure of [In(P 3 C 2 Bu t 2 )] has been determined by gas-phase electron diffraction using the SARACEN method. The experimental geometric parameters showed good correlation with those obtained from quantum chemical calculations and from a previous X-ray diffraction study. Calculations were performed using various DFT methods and also MP2 theory to identify the most suitable method for calculating structures of this type. The accuracy of the calculations was gauged by reference to experimentally determined parameters. The use of small-core and large-core pseudopotentials on the indium atom was also tested, showing that the lack of electrons explicitly considered in the calculation when a large-core pseudopotential was used affected the accuracy of the calculation. Similar calculations have been performed for the less symmetrical [In(P 2 C 3 BU t 3 )], but electron diffraction data of adequate quality could not be obtained.