An ab initio structural and vibrational analysis of gauche,trans,trans- and gauche,cis,trans-hexa-1,3,5-trienes

Vibrational spectra and ab initio analysis of tert-butyl, trimethylsilyl, trimethylgermyl, trimethylstannyl, and trimethylplumbyl derivatives of 3,3-dimethylcyclopropene X. Some regularities

The changes in the vibrational frequencies of 1-tert-butyl and 1,2-di-tert-butyl derivatives of 3,3-dimethylcyclopropene brought about by substitution of the central carbon atom (X) of the tert-butyl moieties by Si, Ge, Sn, or Pb atoms are examined. The most important decrease in the vibrational frequencies implicating the X(CH(3))(3) moieties is noted for substitution of X=C by Si. The substitutions of Si by Ge or Ge by Sn or Sn by Pb are not accompanied by the pronounced frequency shifts observed for the C-->Si transition. An explanation is given for trends in these vibrational frequencies for the transitions X=C-->Si-->Ge-->Sn-->Pb. It is concluded that there are lower limiting values of the vibrational frequencies of a molecular moiety which are approached when the mass of its isovalent atom is increased. This leads to the formation of cluster regions in the vibrational spectra for the frequencies of the SnC(3) and PbC(3) moieties.

Ultrafast diffraction and structural dynamics: the nature of complex molecules far from equilibrium

Studies of molecular structures at or near their equilibrium configurations have long provided information on their geometry in terms of bond distances and angles. Far-from-equilibrium structures are relatively unknown-especially for complex systems-and generally, neither their dynamics nor their average geometries can be extrapolated from equilibrium values. For such nonequilibrium structures, vibrational amplitudes and bond distances play a central role in phenomena such as energy redistribution and chemical reactivity. Ultrafast electron diffraction, which was developed to study transient molecular structures, provides a direct method for probing the nature of complex molecules far from equilibrium. Here we present our ultrafast electron diffraction observations of transient structures for two cyclic hydrocarbons. At high internal energies of approximately 4 eV, these molecules display markedly different behavior. For 1,3,5-cycloheptatriene, excitation results in the formation of hot ground-state structures with bond distances similar to those of the initial structure, but with nearly three times the average vibrational amplitude. Energy is redistributed within 5 ps, but with a negative temperature characterizing the nonequilibrium population. In contrast, the ring-opening reaction of 1,3-cyclohexadiene is shown to result in hot structures with a CC bond distance of over 1.7 A, which is 0.2 A away from any expected equilibrium value. Even up to 400 ps, energy remains trapped in large-amplitude motions comprised of torsion and asymmetric stretching. These studies promise a new direction for studying structural dynamics in nonequilibrium complex systems.

Direct imaging of transient molecular structures with ultrafast diffraction

Ultrafast electron diffraction (UED) has been developed to study transient structures in complex chemical reactions initiated with femtosecond laser pulses. This direct imaging of reactions was achieved using our third-generation apparatus equipped with an electron pulse (1.07 +/- 0.27 picoseconds) source, a charge-coupled device camera, and a mass spectrometer. Two prototypical gas-phase reactions were studied: the nonconcerted elimination reaction of a haloethane, wherein the structure of the intermediate was determined, and the ring opening of a cyclic hydrocarbon containing no heavy atoms. These results demonstrate the vastly improved sensitivity, resolution, and versatility of UED for studying ultrafast structural dynamics in complex molecular systems.

Study of the Electrocyclization of (Z)-Hexa-1,3,5-triene and Its Heterosubstituted Analogues Based on Ab Initio and DFT Calculations

A comprehensive theoretical study of the electrocyclization of (Z)-hexa-1,3,5-triene and its heterosubstituted analogues (E,Z)-[(2Z)-2,4-pentadienylidene]amine and (2Z)-2,4-pentadienal to 1,3-cyclohexadiene, 1,2-dihydropyridine, and 2H-pyran, respectively, was conducted. The study included a conformational analysis of the reactants, locating transition structures and optimizing the closed products. The energy, geometry, and vibrational frequencies of all structures were calculated at the MP2/6-31G and B3LYP/6-31G levels. Also, electron correlation was improved by single-point calculations using the QCISD(T) and MP4SDTQ methods. The results were consistent with experimental values and accurately reproduced the decrease in activation energy of the heterosubstituted derivatives relative to the parent compound. Complementary natural bond orbital (NBO) computations helped to interpret such a decrease. The involvement of a lone pair of the nitrogen or oxygen atom appears to facilitate the interaction between the terminal atoms that bond to each other to close the cycle. In regard to the enthalpies of the electrocyclization reactions studied, the results reveal a clear-cut trend to instabilization in the closed forms when the terminal methylene groups are replaced with the heteroatoms. Thus, the process is clearly exothermic for hexatriene, much less so for the pentadienimine, and even slightly endothermic for the pentadienal.