The development of structural models for predicting the external morphology of crystalline materials is presented and discussed in terms of their applications to molecular crystals. The predicted crystal morphologies of a number of molecular materials including alpha -sulphur, naphthalene, benzoic acid and hexamine are presented using the Bravais-Friedel-Donnay-Harker, attachment energy and Ising models. The results of the various models are compared both against each other and against the experimentally observed morphologies.
A combined modeling and experimental strategy has been
applied to the problem of stabilisation of a
metastable conformational polymorph. For the first time additives
have been successfully selected which by virtue
of their conformation are able to selectively inhibit the appearance of
the stable β polymorph of l-glutamic acid
and
hence stabilize the metastable α structure.
XwHÉ É Ép (phenyl) is the interaction of a hydrogen atom with the p-system of an aromatic ring in either an intermolecular or an intramolecular fashion. Although the existence of these types of interaction has gained recent attention in the literature, the geometry is poorly understood. This paper attempts to investigate the preferred geometry and interaction strength for OwH, NwH, NwH`, SwH, sp2 CwH and sp1 CwH interactions with phenyl rings. This has been done through searches of the Cambridge Structural Database, combined with semi-empirical and ab initio molecular orbital calculations. It is found that the classical image of T-shaped geometry is rarely adopted and that the preferred geometry involves direct interaction of XwH with the carbon atoms of the phenyl ring. The binding energy associated with the interaction decreases with the electronegativity of the donor atom X.
The spectroscopic properties of solvent soluble derivatives of Brooker's simple merocyanine 4-[(1-methyl-4(1H)-pyridinylidene)ethylidene]-2,5-cyclohexadien-1-one, which can in principle exist in two distinct canonical forms, have been assessed both experimentally and theoretically using molecular orbital methods. 1 H and 13 C NMR evidence in a range of solvents suggests that the merocyanine exists as a resonance hybrid which is weighted toward the zwitterion even in solvents of low dielectric constants. In protic solvents, the large hypsochromic shift observed for the merocyanine in the visible region arises from both a dielectric effect and a hydrogen bonding effect. Theoretically, the PM3/COSMO method gives a reasonable account of the structure and spectroscopic shifts of the merocyanine in aprotic solvents. The large shifts observed arise because solvents with large dielectric constants have a much greater stabilizing effect on the more polar ground state of the merocyanine than they do on the first excited state. While the same method predicts stable hydrogen-bonded structures for a dihydrate and hexahydrate, it is unable to reproduce the known hypsochromic shift for these solvated species. In contrast, a version of the CNDO/S method does predict the correct trends on hydration though the magnitude of the effect is less than that found experimentally.Recent studies have strongly suggested that one of the main factors which contribute to the large solvatochromic shift is a
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.