The nature of the excited states of p-nitro-N,N-dimethylaniline.

Contrary to numerous derivatives of p-substituted N,N-dimethyl-aniline, p-nitro-N,N-dimethylaniline, does not form the TICT state. This behaviour is predicted by INDO/S calculations. The calculations also reveal an important role of the Coulomb interaction term in the evolution of intramolecular charge-transfer excited-state energy upon twisting of the dimethylamino group

A Circular Dichroism Study of Molecular Association of Cinchona Alkaloids and Carboxylic Acids

Circular dichroism (CD) spectra of cinchona alkaloids and their salts with carboxylic acids have been investigated. A eontributionof the exciton type Cotton effect to the 230 nm CD band of free bases in non-polar solvents is suggested to result from the association of alkaloid molecules. Strong association is observed in the case of dihydrocupreidine - an alkaloid with free phenolic group. Comparison of the CD spectra of salts of alkaloids with mono- and dicarboxylic acids demonstrates that in the latter case the p-band Cotton effects originate from the exciton type interactions between the associated molecules. The unusual rotations of salts of cinchona alkaloids with biphenyl 1,1\u27-dicarboxylic acids have been found to originate from the strong intermolecular exciton coupling, rather than from the »first-order asymmetric transformation«. The existence of the two low energy staggered conformations around the C(8)-C(9) bond in the alkaloid molecule has been confirmed by the MMP2 calculation on a model compound

Low-energy unphysical saddle in polynomial molecular potentials

Vibrational spectra of polyatomic molecules are often obtained from a polynomial expansion of the adiabatic potential around a minimum. For several molecules, we show that such an approximation displays an unphysical saddle point of comparatively small energy, leading to a region where the potential is negative and unbounded. This poses an upper limit for a reliable evaluation of vibrational levels. We argue that the presence of such saddle points is general.Comment: The preprint version of the published Mol. Phys. paper, 19 pages, 3 figure

A Circular Dichroism Study of Molecular Association of Cinchona Alkaloids and Carboxylic Acids

Circular dichroism (CD) spectra of cinchona alkaloids and their salts with carboxylic acids have been investigated. A eontributionof the exciton type Cotton effect to the 230 nm CD band of free bases in non-polar solvents is suggested to result from the association of alkaloid molecules. Strong association is observed in the case of dihydrocupreidine - an alkaloid with free phenolic group. Comparison of the CD spectra of salts of alkaloids with mono- and dicarboxylic acids demonstrates that in the latter case the p-band Cotton effects originate from the exciton type interactions between the associated molecules. The unusual rotations of salts of cinchona alkaloids with biphenyl 1,1\u27-dicarboxylic acids have been found to originate from the strong intermolecular exciton coupling, rather than from the »first-order asymmetric transformation«. The existence of the two low energy staggered conformations around the C(8)-C(9) bond in the alkaloid molecule has been confirmed by the MMP2 calculation on a model compound

Network dynamics and field evolution: The growth of interorganizational collaboration in the life sciences

A recursive analysis of network and institutional evolution is offered to account for the decentralized structure of the commercial field of the life sciences. Four alternative logics of attachment - accumulative advantage, homophily, follow-the-trend, and multiconnectivity-are tested to explain the structure and dynamics of interorganizational collaboration in biotechnology. Using multiple novel methods, the authors demonstrate how different rules for affiliation shape network evolution. Commercialization strategies pursued by early corporate entrants are supplanted by universities, research institutes, venture capital, and small firms. As organizations increase their collaborative activities and diversify their ties to others, cohesive subnetworks form, characterized by multiple, independent pathways. These structural components, in turn, condition the choices and opportunities available to members of a field, thereby reinforcing an attachment logic based on differential connections to diverse partners. © 2005 by The University of Chicago. All rights reserved

Novel Chiral Pyromellitdiimide (1,2,4,5-Benzenetetracarboxydiimide) Dimers and Trimers: Exploring Their Structure, Electronic Transitions and Exciton Coupling

The chiral but highly symmetrical acyclic and cyclic pyromellitic diimide dimers and trimers 2-5 have been obtained and characterized for the first time. The pyromellitdiimide chromophores in these molecules are linked by a rigid diequatorially 1,2-disubstituted cyclohexane skeleton. The structures of the compounds have been determined in detail by molecular modeling and, in the case of cyclic dimer 4 and trimer 5, by means of X-ray diffraction analysis. The electronically excited states of the pyromellitdiimide chromophore (1a) have been studied in these and other model compounds by means of linear dichroism (LD), magnetic circular dichroism (MCD), and circular dichroism (CD) spectroscopy. CD spectra of the rigid cyclic trimer 5 have provided the most detailed information on the excited states of the pyromellitdiimide chromophore. The low-energy tail (340-360 nm) of the absorption envelope can be assigned to out-of-plane polarized n-pi* transitions (I, II). The higher energy bands are due to contributions from up to six pi-pi* transitions, these being polarized either along the long (IV-VI, VIII) or short axis (III, VII). The results of ab initio CIS/cc-pVDZ and semiempirical INDO/S-CI calculations have been compared with the experimental data. CD Cotton effects in the region 200260 nm, which result from exciton interactions between electric dipole allowed transitions of two pyromellitdiimide chromophores in compounds 2-5, provide reliable and useful information concerning the conformation and absolute configuration of these molecules, which may be extrapolated to other oligoimide systems