Brian A. Pettitt

13C spin-lattice relaxation in rotationally disordered solids

13C spin-lattice relaxation times and 14N linewidths at selected temperatures in liquid and solid I (plastic) succinonitrile are reported. Effective rotational correlation times are derived from these and the results for solid I are discussed in terms of a model which assumes that the main contributions to the rotational disorder come from gauche↔trans isomerization and the jumping of the central C-C bond of the trans species between the 3-fold axes of the b.c.c. lattice. The general applicability of the 13C technique is emphasized and the results are compared with those of other experiments reported in the literature.

A deuterium NMR study of orientational order in tertiary-butyl bromide

Abstract An analysis of NMR lineshapes and spin-lattice relaxation times for the 2 H nucleus is reported for the liquid and solid phases I, II, and III of (CD 3 ) 3 CBr. The main results are a discontinuous increase of the rotational correlation time on melting (with a concomitant increase of the Arrhenius parameter) and, for phase II, a demonstration of the feasibility of measuring order parameters on the different timescales probed by the absorption profile and the relaxation experiment.

AB initio calculations of 2H and 14N quadrupolar coupling constants in hydrogen bonded dimers

Abstract SCF MO calculations at the 6-31G** level of approximation are reported for 2H and 14N electric field gradients in HCN⋯HCN, HCN⋯HF, and CH3CN⋯HF dimers, with emphasis on the configurational dependence of these quantities in (HCN)2. In comparison with available experimental nuclear quadrupolar coupling constants, the calculated values for the monomers and dimers exhibit an accuracy of ≈ 10%, which is comparable to that of other spectroscopic parameters. The implications of hydrogen bonding for quadrupolar spin-lattice relaxation rates are briefly discussed.

A deuterium NMR study of orientational order in camphor

Abstract This investigation of the 2 H NMR absorption profile of D-camphor-3,3- d 2 demonstrates that the solid 1 (cubic)-solid II (rhombohedral) phase transition, through which complete rotational disorder was thought to be preserved, is actually accompanied by the onset of weakly preferred orientations. It is shown that throughout the temperature range of solid II, the spectra can be simulated by introducing temperature-dependent average order parameters analogous to those of liquid crystals and other partially ordered structures. Relaxation rates and angular correlation times are also presented for solid I.

A carbon-13 NMR study of carbon tetrabromide

Abstract 13C spin—lattice relaxation times and selected linewidths are reported for CBr4 (natural abundance) as determined by conventional techniques in the liquid, sold I (fcc, “plastic crystal”) and solid II (monoclinic, “rigid”) phases. The linewidths are less than 50 Hz and the T1s are between 0.5 and 1.5 s. Comments are made on the complexity of the relaxation of spin 1 2 nuclei by quadrupolar nuclei.

Carbon-13 and nitrogen-14 nuclear magnetic resonance study of triethylenediamine

13 C and 14N n.m.r. relaxation times are reported for the orientationally disordered solid I phase of triethylenediamine (TEDA). Analysis of the associated correlation times within the rotational diffusion model indicates that reorientation of the molecular C3 axis is more hindered than reorientation about this axis, the anisotropy diminishing with increasing temperature. The 13C chemical shift and one-bond scalar coupling constants in the solid I phase are noted as being within experimental error of those measured for TEDA dissolved in dimethyl sulphoxide.

On the Franck−Condon principle in momentum space

Abstract Using 39 K 1 H as an example, we examine the momentum-space characteristics of rovibronic transitions in diatomic molecules. It is shown that only Franck-Condon transitions for which there is no change in the rotational quantum number have a clear interpretation in terms of the overlap of momentum wave functions; in which case, the largest Franck-Condon factors are for states which are maximally coherent over the full range of accessible momentum.

Rovibrational momentum densities of diatomic molecules in the rigid rotor-harmonic oscillator approximation

Abstract The rigid rotor-harmonic oscillator model in the momentum representation is used to examine the rovibrational momentum density of a diatomic molecule. The effects on this function of rotational and vibrational excitation, thermal averaging, and nuclear spin statistics are exhibited in calculations on N 2 and thehydrogen halides, and a brief comparison with electron momentum densities is made.