A new analysis of the optical properties of the molecular rotor 1,4-diphenyl-1,3-butadiyne (diphenyl-diacetylene, DPDA) is presented, taking account of the conformational dynamics. The absorption spectra are interpreted in terms of simultaneous contributions from planar as well as non-planar rotamers, characterized by a temperature dependent equilibrium distribution. The investigation is based on IR Linear Dichroism and UV Synchrotron Radiation Linear Dichroism (SRLD) spectroscopy on oriented samples in stretched polyethylene (PE), and on variable temperature UV spectroscopy. The study is supported by the results of detailed quantum chemical Time Dependent Density Functional Theory (TD-DFT) calculations. The resulting analysis has profound implications for the understanding of the optical, photochemical, and photophysical characteristics of this and related chromophores, of importance in a variety of applications.

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Deuterium equilibrium isotope effects (EIEs) for a cage diol and 2,6-dihydroxyacylaromatics and complexes thereof containing intra- or intermolecular hydrogen bonds have been calculated using harmonic and anharmonic vibrational frequencies using Gaussian '03 and the HF, B3LYP, and MP2 levels of theory. The predicted isotope effects have been compared with experimental NMR data, and the origins of the isotope effects have been characterized in terms of zero-point vibrational energy differences and enthalpic and entropic contributions to the free energy difference between labeled species. Reliable free energy predictions based upon harmonic frequencies were found for systems whose isotope effects are governed by bond stretching effects and for systems whose isotope effects are determined by low-frequency vibrational modes. In contrast, thermochemical predictions based upon anharmonic frequencies were found to be far less consistent. Vibrational entropy is predicted to play an important role in modulating and, in some cases, governing isotopic site preferences in hydroxyl-derived intra- and intermolecular hydrogen bonds.

A new class of compounds, the 2-hydroxythioacetophenones, and related compounds have recently been synthesized. The hydrogen-bond system has been characterized by NMR chemical shifts and deuterium isotope effects on these as well as by DFT calculations. Use of solid-state (13)C NMR has enabled measurements of the intrinsic deuterium isotope effects of the most abundant tautomer of beta-thioxoketones. The compounds show very interesting long-range deuterium isotope effects on the thiocarbonyl carbon. The intramolecular hydrogen bonds of o-hydroxythioacetophenones are found to be slightly stronger than those of the corresponding acetophenones. The reactivity and stability of the compounds can be related to hydrogen bonding and to the presence of electron donating substituents.

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