Bruce A. Smart

An ab initio study of some free-radical homolytic substitution reactions at sulfur, selenium and tellurium

Abstract Ab initio molecular orbital calculations using pseudopotential basis sets and electron correlation (MP2, QCISD) predict that homolytic substitution by methyl, silyl, germyl and stannyl radicals at the chalcogen atom in methanethiol, methaneselenol and methanetellurol, with the expulsion of methyl radical, proceeds smoothly. With the exception of dimethyl- λ 4 -tellanyl at the MP2 and QCISD levels of theory, no hypervalent (9-E-3) intermediates were located in any of the reactions in this study. Reactions of germyl and stannyl radicals at methaneselenol and methanetellurol are predicted to be reversible.

On the existence of SH3, SeH3, and TeH3: Discrepancies between all‐electron and pseudopotential calculations

Ab initio calculations using both pseudopotential and double and triple‐ζ all‐electron basis sets, with and without electron correlation (MP2, QCISD), have been performed on the λ4‐sulfanyl (SH3), λ4‐selanyl (SeH3), and λ4‐tellanyl (TeH3) radicals. All‐electron basis sets of double‐ζ quality predict that SH3 and SeH3 correspond to transition states on their respective potential energy surfaces. In contrast, the pseudopotentials of Hay and Wadt predict that SH3 and SeH3 correspond to local minima at the QCISD level of theory while the pseudopotentials of Christiansen and Stevens predict transition states. By comparison, TeH3 proved to be a local minimum at all levels of theory. Interestingly, when a very large (triple‐ζ) all‐electron basis set was used, SH3 proved to be a transition state; however, in this instance the potential energy surface was found to be much flatter than in the case for which a double‐ζ basis set was used, suggesting that further improvements in the basis set may lead to a local minimum. Further improvements in the all‐electron selenium basis also led to a local minimum for SeH3 at the QCISD level of theory.

Eremophilane derivatives with a novel carbon skeleton from Ligularia veitchiana

Abstract Ligulaverin A 1, an eremophilane derivative with a novel carbon skeleton, was isolated from the medicinal plant Ligularia veitchiana, along with four analogues (ligulaverin B 2, ligulaverin C 3, ligulaverin D 4 and ligulaverin E 5). Their structures were elucidated by NMR techniques and X-ray diffraction. The biosynthetic route to this nineteen-carbon skeleton is discussed.

An ab initio study of some free-radical homolytic substitution reactions at halogen

Abstract Ab initio molecular orbital calculations at the HF/3-21G(*) and HF/HUZ-SV(**) levels of theory predict that homolytic substitution by alkyl and other radicals at the selenium atom in alkyl selenides proceeds via a T-shaped transition structure in which the attacking and leaving radicals are colinear, or nearly so. The calculated energy barriers suggest that the nature of the leaving group is important in these reactions, whereas the nature of the attacking alkyl radical is not. HF/HUZ-SV ** calculations put the energy barrier for the attack on methaneselenol by the alkyl radicals in this study at 96–105 kJ mol −1 . MP2 single-point correlation correction reduces most of these barriers to about 71–80 kJ mol −1 , while approximately 68 kJ mol −1 is obtained using the MP4 correction. Formation of methyl radical from methaneselenol by reaction with silyl radical is predicted to be extremely favourable with an MP4/HUZ-SV**//HF/HUZ-SV** calculated barrier of only 12.6 kJ mol(−1), reinforcing the importance of this type of reaction in generating alkyl radicals.

The molecular structure of thiazole, determined by the combined analysis of gas-phase electron diffraction (GED) data and rotational constants and by ab initio calculations

The molecular structure of thiazole has been determined by the joint analysis of data obtained from gas-phase electron diffraction (GED), microwave (MW) spectroscopy and abinitio molecular orbital calculations. The combined approach, making use of the SARACEN method, has led to a very precise structure in which all independent geometric parameters are well defined, and the quoted uncertainties reflect the true accuracy of the measurements. The ground-state average (≡rα° or rz) structural parameters obtained in the combined study of thiazole were; r[S(1)–C(2)]=172.37(11) pm, r[S(1)–C(5)]=171.38(13) pm, r[C(2)–N(3)]=131.0(2) pm, r[C(4)–C(5)] pm=136.90(19) pm, r[N(3)–C(4)]=137.2(2) pm, r[C(2)–H(6)]=109.8(4) pm, r[C(4)–H(7)]=109.9(4) pm, r[C(5)–H(8)]=109.7(4) pm, C(2)–S(1)–C(5)=89.41(4)°, S(1)–C(2)–N(3)=115.16(6)°, S(1)–C(5)–C(4)=109.52(8)°, C(2)–N(3)–C(4)=109.97(9)°, N(3)–C(4)–C(5)=115.95(11)°, S(1)–C(2)–H(6)=120.7(2)°, C(5)–C(4)–H(7)=125.0(4)°, S(1)–C(5)–H(8)=121.7(2)°.

Infra-red spectra of 13CD3GeH3 and DFT studies of force constants and electrical properties in methylsilane and methylgermane

Abstract New infra-red data are reported for some fundamental bands of 13 CD 3 GeH 3 in the gas phase. Triple-ζ DFT calculations of geometry and force field have been made for CH 4 , SiH 4 , GeH 4 , CH 3 SiH 3 and CH 3 GeH 3 . The DFT force fields for methylsilane and methylgermane are scaled (SDFT) on all available data, including harmonised frequencies and frequency shifts, Coriolis constants and centrifugal distortion coefficients. Comparison of scale factors reveals considerable variation. Empirical force fields (ESDFT) are determined, a number of off-diagonal constants being constrained to SDFT values. Problems remaining in the observed data are identified. Atomic polar tensors from the DFT calculations are used to obtain King effective atomic charges x and bond dipole derivatives d μ d r with respect to terminal bond stretching. Comparison between silanes and germanes reveals common trends in bending and stretching dipole moments together with the inductive effect of a methyl group.

On the stability of trivalent chalcogen radicals—a pseudopotential study of homolytic substitution by a methyl radical at methanethiol, methaneselenol and methanetellurol

Ab initio molecular orbital theory using pseudopotential basis sets and electron correlation predict that homolytic substitution by a methyl radical at sulfur and selenium proceeds via symmetrical transition states with barriers of 87.9 and 63.1 kJ mol –1respectively; the similar reaction at tellurium is predicted to proceed via a symmetrical intermediate which lies 23.5 kJ mol –1 above reactants.

Molecular structure of trimethylamine–gallane, Me3N·GaH3: ab initio calculations, gas-phase electron diffraction and single-crystal X-ray diffraction studies†

The structure of the gallane adduct Me3N·GaH3 has been investigated by ab initio quantum chemical calculations. The results of gas-phase electron-diffraction (GED) measurements, together with earlier microwave measurements, have been reanalysed using the SARACEN method to determine the most reliable structure of the gaseous molecule. Salient structural parameters (rαo structure) were found to be: r(Ga–H) 151.1(13), r(Ga–N) 213.4(4), r(N–C) 147.6(3), r(C–H) 108.4(4) pm; H–Ga–N 99.3(8) and Ga–N–C 108.8(2)°. Unlike the corresponding alane derivative, the adduct is monomeric in the crystalline phase with dimensions very close to those of the gaseous molecule, as confirmed by a redetermination of the structure of a single crystal at 150 K.

Ab initio and analytical potential energy functions of K2Na

Abstract Ab initio variational calculations were performed on K 2 Na + at the MP4 (single, double and quadruple substitutions) level of theory using (20 s , 14 p , 2 d , 1 f ) [13 s , 8 p , 2 d , 1 f ] and (17 s , 11 p , 2 d , 1 f ) [10 s , 6 p , 2 d , 1 f ] basis sets for potassium and sodium respectively. The 1s, 2s and 2p orbitals on potassium, 1s orbital on sodium and their virtual counterparts were not included in the correlation treatment. A 63 discrete potential energy hypersurface was generated using a vibrational t -coordinate system. The minimum energy structure is of C 2v symmetry with a bond length and included bond angle of 3.855 A and 73.9° respectively. From this hypersurface analytical potential functions were obtained using power series expansions, which will be embedded in an Eckart-Watson rovibrational Hamiltonian and from which rovibrational eigenfunctions and eigenergies will be calculated.

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.