The ground state vinylidene-acetylene isomerization was investigated by ab initio molecular electronic structure theory. The coupled-cluster method with single, double, and noniterative inclusion of triple excitations ͓CCSD͑T͔͒; with single, double, and noniterative inclusion of triple and quadruple excitations ͓CCSD͑TQ͔͒; and with full single, double, and triple excitations ͑CCSDT͒ were used to treat the effect of electron correlation. Several correlation-consistent polarized valence basis sets, cc-pVXZ, were employed. Theoretical limiting values of the energetics of the reaction were then deduced from the series of computations. With zero-point energy correction, the energy of reaction is Ϫ42.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values.

Bergman cyclization of the enediynes (Z)-3-hexene-1,5-diyne (1), (Z)-3-heptene-1,5-diyne (2), (Z)-4-octene-2,6-diyne (3), (Z)-1-cyclononene-3,8-diyne (4), (Z)-1-cyclodecene-3,9-diyne (5), and (Z)-1-cycloundecene-3,10-diyne (6) has been studied by density functional methods. The reaction of 1 was first studied using the BP86, BLYP, BPW91, and B3LYP functionals with the 6-311G** basis set and the large ANO basis set for the latter two functionals. The BPW91/6-311G** calculations yielded results comparing well with those of high-level ab initio computations. Thus, BPW91/6-311G** was employed to study the reactions of 2−6. Geometry optimizations and harmonic frequency calculations were applied for every reactant, transition structure, and product; frequency calculations were also carried out for other optimized stationary points. The optimized structure of the conformer of 6 with the lowest energy agrees excellently with the X-ray diffraction crystallographic structure. IRC (intrinsic reaction coordinate) calculations were carried out for the transition structures of 4, 5, and 6 to establish the reaction path. The zero-point energy corrected reaction barriers for 1−6 are 25.16, 27.93, 32.25, 12.09, 20.87, and 26.42 kcal/mol, respectively. Thermodynamic data, ΔH, ΔS, ΔG, ΔH a, ΔS a, and ΔG a, have been evaluated at several temperatures. The temperature effect on the free energy is insignificant. The critical distance, which is the distance between the two carbon atoms forming a new bond, in the transition states of all six reactions is approximately 2.0 Å. The IRC analysis shows that the reaction coordinate is close to the critical distance, and the reactant with a larger critical distance is relatively more stable and has a higher barrier. Therefore, a smaller ring, possessing a larger strain energy and a shorter critical distance, has a lower barrier.

The theorem of matching proton affinities (PA) has been widely used in the analysis of hydrogen bonds. However, most experimental and theoretical investigations have to cope with the problem that the variation of the PA of one partner in the hydrogen bond severely affects the properties of the interface between both molecules. The B3LYP/d95+(d,p) analysis of two hydrogen bonds coupled by a 5-methyl-1H-imidazole molecule showed that it is possible to change the PA of one partner of the hydrogen bond while maintaining the properties of the interface. This technique allowed us to correlate various properties of the hydrogen bond directly with the difference in the PAs between both partners: it is possible to tune the potential energy surface of the bonding hydrogen atom from that of an ordinary hydrogen bond (localized hydrogen atom) to that of a low barrier hydrogen bond (LBHB, delocalized hydrogen atom) just by varying the proton affinity of one partner. This correlation shows clearly that matching PAs are of lesser importance for the formation of a LBHB than the relative energy difference between the two tautomers of the hydrogen bond.

Articles you may be interested inThermochemistry of radicals formed by hydrogen abstraction from 1-butanol, 2-methyl-1-propanol, and butanal A compilation is presented of published experimental and computational reports ͑191 references͒ on the structures, vibrational frequencies, molar enthalpies of formation and standard entropies for 26 gas phase boranes for the temperature range from 0 to 1500 K. The thermochemical properties have been collated via standard programs and are listed in a convenient tabular format. Levels of uncertainties in the thermodynamic functions have not been assessed, because of the limited experimental and computational data. The tabulated values were fitted to standard seven-parameter ͑NASA͒ polynomials to facilitate the computation of enthalpies of formation, entropies, and heat capacities for modeling purposes. Within the context of intrinsic uncertainties, the equilibrium compositions of the gas phase were calculated, constrained to constant temperature and volume, for several boron-hydrogen ͑B/H͒ ratios, at various temperatures and pressures. The ͑unex-pected͒ results indicate that in none of the reported gas-phase kinetics studies was thermodynamic equilibrium attained, even though the measured concentration profiles appear to extrapolate to steady state product distributions.

Density functional theory has been used to calculate the thermodynamic properties and molecular orbitals of pillar[n]quinones. Pillar[n]quinones are expected to be effective electron acceptors and the ability to accept more than one electron increases with the size of the interior cavity. Pillar[5]quinone and pillar[7]quinone show a great intramolecular charge transfer upon the electron excitation from highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) as indicated by a large difference of electron distributions between their HOMO and LUMO and a notable dipole moment difference between the ground and first triplet excited state. The aggregation of pillar[n]quinones leads to tubular dimeric structures joined by 2n CH···O nonclassical hydrogen bonds (HBs) with binding energies about 2 kcal/mol per HB. The longitudinal extension of the supramolecular self-assembly of pillar[n]quinone may be adjustable through forming and breaking their HBs by controlling the surrounding environment. The tunability of the diameter of the tubular structures can be achieved by changing the number of quinone units in the pillar[n]quinone. The electrostatic potential maps of pillar[n]quinones indicate that the positive charge in the interior cavity decreases as the number of quinone units increases. Chloride and bromide anions are chosen to examine the noncovalent anion-π interactions between pillar[n]quinones and captured anions. The calculations show that the better compatibility of the effective radius of the anions with the interior dimension of pillar[n]quinone leads to larger stabilization energy. The selectivity of spatial matching and specific interaction of pillar[n]quinone is believed to possibly serve as a candidate for ionic and molecular recognition.