E.S.R. spectra of X-irradiated methylene diphosphonic acid

X-irradiation of single crystals of methylene diphosphonic acid gives rise to E.S.R. spectra which have been assigned to (OH)₂(O)P-?H₂ and

The former species is the more abundant of the two and the methylene group undergoes rapid re-orientation about a two-fold axis at room temperature but is stationary at 77 K. The ¹H and ³¹P coupling constants in this radical are very similar to those in Ph₃P⁺CH₂, and support the previous conclusions [1] that there is little delocalization of the unpaired electron to the phosphorus atom.     

Nematic-phase N.M.R. investigation of rotational isomerism: conformations of 2-furanaldehyde

An unrestricted Hartree-Fock molecular orbital procedure has been used to compute some properties of the lowest-lying spectroscopic states of NH₂, BH₂ and CH₂ ⁺. Computed bond angles for the ²B₁ and ²A₁ states of NH₂ and BH₂ are in good agreement with experimental data; bond lengths are about 6 per cent too great. Among other states for which predictions are made, the ²B₂ state of NH₂ has novel equilibrium geometry, with a predicted angle of 26·57°.

Dissociation products are considered. It is predicted that for the isoelectronic systems CH₂ ⁺ and BH₂ there is a contrast:

Other properties, such as isotropic coupling constants, force constants and dipole moments, are also reported.     

Hyperfine coupling constants and their dependence on charge densities

In paramagnetic aromatic systems the relation between the hyperfine coupling constants a _H for the protons and the spin density ρ on the adjacent carbon atom is usually given by the relation a _H = -Qρ (McConnell) in which Q is a constant characteristic for the C-H bond. In the paramagnetic ions of aromatic molecules the carbon atoms have in general besides a non-zero spin density also a non-zero excess charge density.

If one takes into account both the effect of the spin density ρ and the excess charge density ε on the C-H bond, the following formula is obtained for the proton hyperfine coupling constant:

in which Q and K are predicted to be positive constants of the same order of magnitude.

Comparison with experimental data gives Q = 31·2 gauss; K = 17 gauss.     

Rotational band contour analysis in the 2760 Å system of p-chlorofluorobenzene

The origin band of the 2760 å system of p-chlorofluorobenzene has been shown to be a type B band of a prolate asymmetric top. The electronic assignment of the system is therefore ¹ B ₂-¹ A ₁.

The excited state rotational constants are:

compared with the estimated ground state constants:

The rotational origin of the band is at 36275·1 ± 0·2 cm^-1.     

Estimation of hyperfine coupling constants of 29Si nuclei in the radical anions of trimethylsilyl-substituted π-electron systems

An attempt has been made to relate the coupling constant a _Si of a ²⁹Si nucleus in the radical anions of nine trimethylsilyl derivatives of π-electron systems to the spin populations ρ _Si and of the silicon and the substituted carbon centre , respectively. For the parameter Q _CSi of the relation an absolute value of circa 20 gauss is found, whereas Q _Si appears not to be significantly different from zero. The coupling constants a _Si calculated by means of this relation agree moderately well with the experimental values (standard error circa 0·6 gauss). An HMO model in which the trimethylsilyl substituents are treated as pseudo-π-centres, with the parameters h _Si = -1·5 and k _CSi = 0·55, reproduces satisfactorily the spin distribution in the corresponding radical anions.     

Ab initio potential surfaces for the à and states of HCN+

Ab initio configuration interaction calculations for the à ²Σ⁺ and states of HCN⁺ are presented. Minima occur at r _CH = 2·03 a ₀, r _CN = 2·25 a ₀ (à ²Σ⁺) and r _CH = 2·75 a₀, r _CN = 2·26 a ₀ (). The potential surface for the state has a local maximum as the hydrogen atom is pulled away from CN. The barrier height is calculated to be 0·27 eV.     

On the effective Zeeman Hamiltonian for a linear molecule in an orbitally degenerate state

Values of the dissociation energies and the interaction constants in the long-range potential

for the B ³Π_Ou+ states of Br₂ and Cl₂ have been obtained from least-squares fits to the attractive part of the Rydberg-Klein-Rees potentials. The term in r ^-6 is not negligible even at large internuclear distances corresponding to the last few vibrational levels. The validity of the long-range inverse-power expansion in the internuclear distance r has been investigated. The expansion is valid outside the electron-overlap region given by

in agreement with the result found in the analysis of I₂.     

A re-interpretation of the E.P.R. spectrum of the 1,2,3,6,7,8-hexahydropyrene anion radical

The γ radiolysis of carbon tetrachloride at 77 K results in the formation of free radicals which are stable at 185 K and have been identified as ?Cl₃. These radicals trapped in crystallites of CCl₄ or at ‘boundaries’ give very different E.S.R. spectra. Their a _13C coupling constants are practically identical (117 G et 114 G). The structure of ?Cl₃ radicals has been determined: it is nonplanar and the angle has a value of 116°. The action of O₂ on the ?Cl₃ radicals leads to the CCl_{3 2} peroxide radicals whose a _13C coupling at 200 K is 6·15 G.

Après irradiation γ à 77 K du tétrachlorure de carbone, le seul radical stable à 185 K est le radical ?Cl₃. Le radical ?Cl₃, suivant qu'il est piégé au sein des cristallites de CCl₄ ou aux joints de grains, donne un spectre R.P.E. très différent.

Pour les deux types de radicaux, les constantes de couplage a _13C sont voisines (117 G et 114 G). On peut ainsi en déduire que le radical CCl₃ est non plan avec une valeur de 116° pour l'angle . Par action de l'oxygène sur le radical ?Cl₃, on a observé à 200 K le radical CCl_{3 2} dont la constante de couplage a _13C est 6,15 G.     

Line positions and intensities in the υ2 band of H216O

The constants involved in the rotational expansion of the transformed transition moment operator of the v ₂ band of H₂ ¹⁶O have been determined through a fit of about 110 measured line intensities. A comparison between theoretical and experimental values of these constants is given. The coefficient ²μ _x of the expansion of the dipole moment with respect to normal coordinates is deduced to be

Moreover, a knowledge of the transformed transition moment operator has been used to compute the whole spectrum of the v ₂ band.     

Thermodynamic parameters for gaseous sulphate ion

Electrostatic energies and Madelung parameters for M ₂SO₄ (M = Li, Na, K, Rb and Cs) and for MSO₄ (M = Ca) are reported. An assessment of the charge distribution within the sulphate ion (q ₀ = -0·80) is made and values:

and

are assigned to the enthalpies of formation and solvation of the gaseous sulphate ion. The total lattice potential energies are found to be

A value of -1267 kJ mol^-1 is assigned for the oxide ion affinity of sulphur trioxide.     

Molecular motions in plastic succinonitrile: correlation times from incoherent neutron scattering

In the plastic phase (233 K < T < 331 K), the succinonitrile molecule N≡C-(CH₂)₂-C≡N undergoes motions of isomerization between two gauche and a trans conformations and, when in its transform, rotational motions of reorientation from a diagonal of the b.c.c. unit cell to another. After an evaluation of the multiple scattering correction using both a Monte Carlo simulation technique and a semianalytical calculation, the experimental quasi-elastic neutron spectra measured at 302 K were corrected and a comparison is made to a model taking into account the two coupled molecular motions.

The correlation times found by fitting this model to the corrected data are: for the rotational motion, and for the reaction of isomerization gauche?trans.     

The gas-phase electron paramagnetic resonance spectrum of vibrationally excited SO radicals

Gas-phase E.P.R. spectra from the first six vibrational levels in X ³Σ^- SO have been detected and analysed using a Hund's case (b) formalism. The spectra can be fitted using the following molecular constants:

The first and second derivatives of with respect to the normalized internuclear distance ξ were calculated from these constants. A simple analytic formalism is described which enables the variation of with internuclear distance to be separated into contributions from the matrix elements and from the change in energy spacing between interacting states. Both effects are important in the derivatives of , but it is suggested that in SO most of the variation of with rotational and vibrational state is due to the increase in the second-order part of the parameter as the separation between the ground and the dominant perturbing state decreases. This explanation implies that the expression proposed recently by Veseth and Lofthus for the rotational variation of is not accurate, and enables the conflict between the results of their analysis and ab initio calculations to be resolved.     

Tris(dimethylamino)-cyclopropenium radical dication

Tris(dimethylamino)-cyclopropenium cation (I^⊕) undergoes a facile oxidation to the radical dication (I^2⊕). The E.S.R. spectrum of I^2⊕, which covers a range of almost 200 gauss, has been analysed in terms of the coupling constants a _N =7·51 (three ¹⁴N nuclei) and a _H ^CH₃ =8·16 gauss (eighteen protons). The line-width can be expressed as

where M _I(N) is the magnetic quantum number for the set of three equivalent ¹⁴N nuclei. A simple molecular orbital model of the trisamino-cyclopropenium system satisfactorily accounts for some physico-chemical properties of I^⊕. The E.S.R. results for I^2⊕ are compared with those for the iso-π-electronic hexamethyl[3]radialene radical anion (II^?).     

On the Taylor expansion of the expression for the transition probability for normal absorption processes

Molecular reorientation of 2-chloropyrimidine dissolved in CS₂ (0·1 M) has been investigated by means of ¹³C and proton relaxation. Although weakly coupled, the proton system subjected to non-selective 180-τ-90 pulse sequences allows the determination of one autocorrelation and one cross-correlation dipolar spectral density. The proton and carbon-13 relaxation data allow the complete determination of the rotational diffusional tensor:

and

It is shown that scalar relaxation due to nitrogen-14, has no effect on proton longitudinal relaxation time, because of a cross term due to the symmetry of the molecule, although this mechanism could, a priori, have been thought to be important. Finally, the nitrogen relaxation time recalculated with D_xx , D_yy , D_zz and the quadrupolar coupling tensor is in agreement with the observed linewidth.     

Statistical mechanics of solid and liquid mixtures of ortho- and para-hydrogen

It is known from experimental measurements that the configurational free energy of solid and liquid mixtures of p-H₂ and o-H₂ is approximately of the form

where is the mole fraction of o-H₂. Assuming that this dependence of F _conf on is due solely to orientational forces a quantum-mechanical calculation of F _conf is developed which is valid for moderately low temperatures. A simplified statistical model is used consisting of a rigid lattice. The theoretical free energy obtained is however much smaller than the experimental one probably on account of the crudeness of the model. Various refinements are discussed.