Molecular crystals crystal field effects

Gatti, C., Saunders, V.R. and Roetti, C. (1994) Crystal field effects on the topological properties of the electron density in molecular crystals the case of urea, J. Chem. Phys., 101, 10686-10696. 

ESR detects a splitting of the Lande factor below 140 K, as is represented in Fig. 6 at T=40 K, consistent with a static JTD [13]. The largest value of the 0-tensor (gzz) was not found parallel to the tetragonal c-axis,suggesting that the direction of elongation is dictated by molecular symmetry and not crystal field effects. The two other values of the 0-tensor are similar, as expected for axial symmetry, but not strictly equal, leading Bietsch et al. [13] to conclude in favor of a D2h distortion. 

Within this framework we consider the bonding electrons as the main source of the EFG. For the crystal field, effects A) and B) are most important. Lattice defects seem to play a minor role in molecular crystals. The influence of the external parameters on NQR is quite important since, with external fields, the crystal field may be influenced experimentally. However, very little work has been done in this field. 

The simplest and most easily recognized crystal field effect is the influence of the symmetry of the crystal on the NQR spectrum. Starting from the molecular field, the NQR spectrum is determined by the isolated molecules. The crystal field then produces a fine structure of the spectrum which can be used to explore the site symmetry of the nuclei considered. 

The determination of the asymmetry parameters and the direction cosines, together with x-ray investigations of the crystal structure, will shed some light on the crystal field effect in solids. NQR powder spectra can only permit very rough and qualitative conclusions on the intermolecular forces. The results on the Mentschukin complexes with AsCl3 show that the metal-chlorine bond is virtually unaffected by the formation of molecular compounds. From singlecrystal NQR spectroscopy, particularly on the As nucleus, some geometrical information about the molecular compounds can be expected. 

Table V.5 Crystal field effect in molecular compounds of trichloroacetic acid with ethers, aldehydes etc.and of some salts of the trichloroacetic acid. A( SC1) <0.7 MHz in the compounds TCA-X A (3SC1) K 1.6 MHz in the salts...

In the alkali metal pseudohalides the contribution of cationic wave functions to the valence band structure can be neglected. The optical absorption spectra can therefore be correlated to transitions involving excited states of the anions. However, one can see solid state effects like the superposition of vibronic structure on the molecular symmetry forbidden transition at 5.39 eV in the crystal spectra of the alkali metal azides (76). In the more complex heavy metal and divalent azides, a whole range of optical transitions can occur both due to crystal field effects and the enhanced contributions from cationic states to the valence band. Detailed spectral measurements on a-PbNe (80), TIN3 (57), AgNs (52), Hg(CNO)2 (72) and AgCNO (72) have been made but the level assignments can at best be described as tentative since band structure calculations on these materials are not available at present. 

The application of the plane wave basis set implies that the periodicity is taken into account and the method is therefore well suited for the simulation of the crystal field effects. For a recent review of Car-Parrinello methodology see Ref. [31]. With Car-Parrinello method the proton motion in PANO including the effects of crystal environment was simulated. The molecular dynamics simulation was carried out at a constant volume, i.e. the unit cell parameters were fixed during the simulation. Fictitious orbital mass was set to 150 a.u. (note that 1 a.u. corresponds to the electron mass) and the propagation time step was set to 2 a.u. 

Both through-bond and pseudocontact contributions can be easily factorized into a series of products of two terms, each term depending either on the nucleus i (topologic and geometric location) or from the lanthanide j (electronic structure and crystal-field effects). For axial complexes, that is, possessing at least a three-fold axis as found in triple-stranded helicates, the molecular magnetic susceptibility tensor written in the principal magnetic axes system is symmetrical xx = mag-... 

Reis et al. report theoretical studies of the urea250 and benzene251 crystals. Their calculations start from MP2 ab initio data for the frequency-dependent molecular response functions and include crystal internal field effects via a rigorous local-field theory. The permanent dipolar fields of the interacting molecules are also taken into account using an SCF procedure. The experimental linear susceptibility of urea is accurately reproduced while differences between theory and experiment remain for /2). Hydrogen bonding effects, which prove to be small, have been estimated from a calculation of the response functions of a linear dimer of urea. Various optoelectronic response functions have been calculated. For benzene the experimental first order susceptibility is accurately reproduced and results for third order effects are predicted. Overall results and their comparison with studies of liquid benzene show that for compact nonpolar molecules environmental effects on the susceptibilities are small. 

Gatti, C., Saunders, V.R. and Roetti, G. (1994) Crystal field effects on the topological properties of the electron density in molecular crystals the case of urea, /. Chem. Phys., 101, 10686-10696. Tsirelson, V.G., Zou, P.F. and Bader, R.F.W. (1995) Topological definition of crystal structure determination of the bonded interactions in solid molecular chlorine. Cryst., A51, 143-153. Platts, J.A. and Howard, S.T. (1996) Periodic Hartree-Fock calculations on crystalline HCN,/. Chem. Phys., 105,4668-4674,... 

Chen, F.P., Hansom, D.M., Fox, D. Origin of Stark shifts and splittings in molecular crystal spectra 1. Effective molecular polarizability and local electric field. Durene and Naphthalene. J. Chem. Phys. 63, 3878-3885 (1975)... 

The shifts of NQR frequencies caused by crystal field effects in molecular crystals reach a maximum of 1.5-2%. However within a series of similar compounds this effect, as a rule, does not exceed 0.3%. 

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