William T. Raynes

Nuclear spin–spin coupling in the acetylene isotopomers calculated from ab initio correlated surfaces for 1J(C, H), 1J(C, C), 2J(C, H), and 3J(H, H)

Ab initio calculated coordinate and internal valence coordinate coefficients for each of the four spin–spin coupling surfaces of the acetylene molecule—1J(C, H), 1J(C, C), 2J(C, H), and 3J(H, H) are presented. Calculations were carried out at the SOPPA(CCSD) level using a large basis set. Couplings were calculated at 35 geometries (including equilibrium) giving 35 distinct sites on the 1J(C, C) and 3J(H, H) surfaces and 53 distinct sites on the 1J(C, H) and 2J(C, H) surfaces. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. All couplings are sensitive to geometry with the principal features being (a) an even steeper increase of J(C1, H1) with CC bond stretching than with CH bond stretching—an example of “unexpected differential sensitivity” (or UDS), (b) very opposite variations of 2J(C1, H2) with variations of the CC and C2H2 bond lengths, (c) very opposite variations of 1J(C, C) with a CC stretch and a CH stretch and (d) very oppos...

SCF theory for multiple perturbations

A fully coupled SCF perturbation theory is developed in matrix form for any number of simultaneous perturbations. Explicit expressions are given for all energy terms up to fourth order in the perturbation parameters. In succeeding papers these expressions are used to produce expressions for and values of several molecular properties dependent upon the simultaneous application of electric and magnetic fields.

Density-dependent magnetic shielding in gas phase 13C N.M.R.

Density-dependent 13C nuclear magnetic shielding has been found for each of the pure gases CH4, C2H6, C2H4, CO and CO2, and for several binary mixtures of gases. For methane gas the density dependence is greater at higher temperatures in contrast to expectation and the observed temperature dependence of the shielding at zero density is attributable to nuclear motion. 13C magnetic shielding is considerably higher in the gas phase than in the liquid phase and the difference varies for chemically non-equivalent 13C nuclei by amounts which are well above the level of experimental error.

Unexpected differential sensitivity of nuclear spin–spin-coupling constants to bond stretching in BH4−, NH4+, and SiH4

Correlated ab initio calculations are reported, which show that stretching one of the Si–H bonds in silane from its equilibrium value increases the one-bond coupling J(Si, H) of the other three bonds by more than the increase of that of the stretched bond. This “unexpected differential sensitivity” in silane is considerably greater in terms of reduced coupling constants than the same effect discovered previously in methane. This UDS effect is also found to occur in the BH4− and NH4+ ions and comes overwhelmingly from the Fermi contact term. For the proton–proton coupling UDS due to the stretch of a single bond, also found earlier in methane, occurs in NH4+, just fails to appear in silane and is absent in BH4−. The origins of UDS and the signs of the coupling constant derivatives are discussed.

Nuclear spin-spin coupling in the methane isotopomers

Abstract The coupling constants 1 J (C, H) and 2 J (H, H) of the methane molecule have been calculated as functions of bond-length extension and compression in the vicinity of equilibrium geometry. This has facilitated the prediction of the temperature dependences of these couplings. The calculations were carried out using various polarization propagator methods. There is a very large contribution from electron correlation to both couplings. The bond-length dependence is dominated by the Fermi-contact part of the coupling. 1 J (C, H) is calculated to increase by 0.054 Hz upon increasing the temperature of 13 CH 4 from 200 to 400 K. This result is less than the observed value of 0.083 Hz due to the neglect of higher-order terms, including those involving the angle dependence of the coupling. 2 J (H, D) in 12 CH 3 D is calculated to be virtually temperature independent. The calculated total carbon-proton coupling at 300 K is 126.31 Hz, which is only 1 Hz greater than that experimentally observed. The calculated total proton-proton coupling at 300 K is −14.24 Hz, which is numerically greater by about 2 Hz than that calculated from a recent measurement on 12 CH 3 D.

THE CALCULATION AND ANALYSIS OF ISOTOPE EFFECTS ON THE NUCLEAR SPIN-SPIN COUPLING CONSTANTS OF METHANE AT VARIOUS TEMPERATURES

New ab initio symmetry-coordinate carbon–proton and proton–proton spin–spin coupling surfaces for the methane molecule have been computed. Calculations have been performed at the SOPPA(CCSD) level using a large-basis set and a grid of 55 geometries on the two surfaces. The nuclear motion in the isotopomers CH4, CH3D, CH2D2, CHD3 and CD4 has been averaged over these surfaces to give values of J(13C, H) and J(13C, D) for the 13C isotopomers and J(H, D) for the 12C isotopomers at selected temperatures. Calculated isotope effects on J(C, H) and J(C, D) are very close to the values observed experimentally. This validates analysis into the various nuclear motion contributions to show that bond stretching at first order dominates and that angle bending at second order is also significant. However, the bond-stretching terms stem from the ‘other’ three bonds and there is surprisingly very little contribution from the bond containing the coupled nuclei. For the proton–deuteron coupling it is second-order bending wh...

Nuclear spin–spin coupling in silane and its isotopomers: Ab initio calculation and experimental investigation

Ab initio calculated symmetry coordinate and internal valence coordinate coefficients for the two spin–spin coupling surfaces of the silane molecule—1J(Si, H) and 2J(H,H)—are presented. Calculations were carried out at the level of the second-order polarization propagator approximation involving coupled-cluster singles and doubles amplitudes [SOPPA(CCSD)] using a large basis set for a total of 78 different geometries corresponding to 133 distinct points on the 1J(Si, H) surface and 177 distinct points on the 2J(H,H) surface. The results were fitted to fourth order in Taylor series expansions and are presented to second order in the coordinates. Both couplings are sensitive to geometry—more so than found for methane in earlier calculations. The surfaces are averaged over a very accurate, recent ab initio force field to give values for the couplings in silane and its variously deuterated isotopomers over a range of temperatures. For J(Si, H) in 29SiH4 both stretching and bending contribute to the nuclear mo...

Calculations of the force field of the methane molecule

SCF and CAS SCF calculations have been carried out to determine the quadratic and some of the higher force constants of the methane molecule. The calculated SCF value of r e is 1·08258 A which is very close to Gray and Robiettes experimental value of 1·0858 (±0·0010) A. Correlation contributions to most of the quadratic constants are significant and are largely recovered by the CAS SCF calculations. The thirty-three independent quartic symmetry co-ordinate force constants are given explicitly for the first time together with additional, dependent quartic constants and some of them calculated at the SCF level.

The effects of rotation and vibration on the carbon-13 shielding, magnetizabilities and geometrical parameters of some methane isotopomers

An ab initio carbon-13 shielding surface to quadratic terms in displacement coordinates has been used to calculate the effects of temperature on the shielding in 13CH4 and its four deuterated isotopomers. Some of the second order terms are significant. For 3CH4 the total nuclear motion correction at 300 K is −3·695 ppm (−2%). The isotope effects observed by Alei and Wageman along the series 13CHnD4-n, are explained quantitatively to a high degree and the predictions of the temperature dependence of the shielding are in agreement with the meagre experimental data available. A similar calculation is reported for the temperature dependence of the molar magnetizabilities of 12CH4 and 12CD4 using an ab initio magnetizability surface to second order. Again some quadratic terms are important and the nuclear motion correction at 300 K is this time +2%. Modern SQUID magnetometers should be able to measure the variations predicted. Results are also given for the effects of vibration and rotation on the bond lengths...

The 1H2H, 17O1H coupling constants and the 16O/18O induced proton isotope shift in water

Abstract At very low concentration in nitromethane the coupling constants J (H,D) and J( 17 O,H ) of the water molecule at 353 K are found to be 1.091(±0.005) and 80.6(±0.1) Hz, respectively. The high accuracy was achieved by lineshape analysis using the QUADR program for the proton resonance and temperature variation for the 17 O-resonance. Measurements of the proton isotope shift due to 16 O/ 18 O substitution reveal to noticeable shift in contradiction to an early reported detection by others.