Chin-Hui Yu

Extended ab initio studies of the vinylidene–acetylene rearrangement

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 [CCSD(T)]; with single, double, and noniterative inclusion of triple and quadruple excitations [CCSD(TQ)]; and with full single, double, and triple excitations (CCSDT) 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 −42.95 kcal/mol and the reaction barrier is 1.5 kcal/mol. Both agree excellently with experimental values.

THE COOPERATIVITY BETWEEN HYDROGEN AND HALOGEN BONDS

The cooperativity between hydrogen bonds and halogen bonds in X–HCN–Y(X: C2H2, H2O, NH3, HCI, HCN, HF; Y: HF, BrF, Br2 is analyzed with MP2/6-311++G(d, p) and DFT/6-311++G(d, p) calculations using the B3LYP and mPW1PW91 hybrid functionals. The results from the quantum chemical calculations are typically clustered in groups according to the Y-ligand. By choosing the X–HCN–HF group as reference it is possible to describe the interaction between the hydrogen and the halogen bond with a two-parameter model. The value of the first parameter of the model describes the contribution of the X-ligand to the interbond cooperativity in the reference cluster. The second parameter of our model quantifies the changes in interbond cooperativity upon varying the Y-ligand. This simple model can be used to predict the cooperativity in X–HCN–Y trimers with reasonable accuracy and thereby to organize the results systematically. It is further shown that the conclusions drawn from this ordering scheme are independent from the computational method and thereby generally applicable.

A Computational Study: Reactivity Difference between Phosphine- and Amine-Catalyzed Cycloadditions of Allenoates and Enones

Allenoates and enones form cyclopentenes via a phosphine-catalyzed [3 + 2] cycloaddition while the amine-catalyzed [2 + 4] cycloaddition yields dihydropyrans or pyrans. The difference between these catalysts is studied with M06-2X/6-31+G* calculations. The addition of the catalyst to the allenoate is the first step in both pathways followed by the reaction with the enone. The formation of the [3 + 2] phosphorus-ylide is exergonic, and hence, the [3 + 2] cycloaddition is kinetically favored over the [2 + 4] addition. Amines do not stabilize [3 + 2] ammonium-ylides. However, electron-withdrawing groups on the enone enable [2 + 4] cycloadditions. The strength of the electron-withdrawing group further controls the α/γ regioselectivity of the [2 + 4] cycloaddition, and the analysis of the HOMO-LUMO interactions explains why only E-dihydropyrans from the direct γ-[2 + 4] cycloaddition have been observed in experiments. The quantum calculations further reveal a new path to the α-[2 + 4] product starting with an intermediate Rauhut-Currier reaction. This new path is kinetically favored over the direct amine-catalyzed α-[2 + 4] cycloaddition.

Isomers of SO3: Infrared absorption of OSOO in solid argon

Sulfur trioxide (SO3) isolated in solid argon at 12 K was irradiated with light at 193 nm from an ArF excimer laser. Recombination of photofragments O and SO2 produces OSOO that absorbs at 1229.6, 1041.3, and 597.6 cm−1. The assignments are based on observed 34S‐ and 18O‐isotopic shifts. Theoretical calculations using the B‐P86 and the B3‐LYP density functional methods were carried out for five isomers of SO3; energies and vibrational wave numbers were predicted for each one. Observed line positions, infrared intensities, and isotopic shifts fit well with those predicted for cis‐OSOO at the B‐P86 level. Further irradiation of the matrix sample with emission at 248 nm from a KrF laser bleached OSOO and enhanced absorption lines of SO2 and, to a lesser extent, of SO3. The mechanism of formation of OSOO in a matrix cage is discussed.

Photoionization spectra and ionization energies of HSCl, HSSSH, SSCl, and HSSCl formed in the reaction system Cl/Cl2/H2S

Photoionization-efficiency (PIE) spectra in the wavelength range 110–140 nm were measured for products of the reaction system Cl/Cl2/H2S in a discharge-flow reactor coupled to a photoionization mass spectrometer employing a synchrotron as source of radiation. According to PIE spectra of HSCl, HSSSH, SSCl, and HSSCl, obtained for the first time, the ionization energies (IE) derived are (9.887±0.016), ⩽9.09, (9.04±0.03), and (9.266±0.014) eV, respectively. Ab initio calculations of these IE with the GAUSSIAN-2 method agree well with experimental results. Other products observed in the system include S2, HSSH, S3, and SCl2. Their PIE spectra and IE were also measured; in some cases discrepancies with previous reports are found. The formation mechanism of the observed products is discussed.

A computational study of unique properties of pillar[n]quinones: Self‐assembly to tubular structures and potential applications as electron acceptors and anion recognizers

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.

Correlated proton motion in hydrogen bonded systems: tuning proton affinities

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.

Production of HCO from propenal photolyzed at 193 nm: Relaxation of excited states and distribution of internal states of fragment HCO

The dynamics of photodissociation of propenal at 193 nm are studied by detecting laser-induced fluorescence of nascent fragment HCO in its transition B 2A′–X 2A′. Rotational states up to N=30 and K=3 of HCO X 2A′ are populated and vibrational states (000), (010), and (001) are detected. The Ka=1 doublet states and the two spin states for all vibrational levels detected are nearly equally populated. Much less rotational excitation is observed than the distributions calculated on a statistical model—phase space theory. This implies that dissociation occurs from the triplet channel with a small exit barrier. Small rotational excitation arises from the repulsive part of the exit barrier and the geometry of the transition state on the triplet surface. Experimental data yield an energy partitioning with translation, rotation, and vibration of HCO at 3.0, 1.3, and 1.5 kcal/mol, respectively, in total accounting for 11.5% of available energy. These results indicate that the other fragment C2H3 has 3.2 kcal/mol...

A study of the vinylidene-acetylene rearrangement using density functional theory

Abstract The vinylidene-acetylene rearrangement was investigated by density-functional theory, including pure and hybrid methods, with cc-pVDZ, cc-pVTZ, and aug-cc-pVTZ basis sets. Several combinations of exchange and correlation functionals were used. All functionals yield geometries which compare well with the high-level ab initio results. Some combinations provide unphysical data. The BLYP, B3P86, B3PW91, B3LYP, and BHandH methods provide reasonable results. The BLYP method performs the best for this reaction providing geometry, harmonic frequencies and reaction energetics, a barrier of 1.53 kcal/mol and a reaction energy of −41.55 kcal/mol, in agreement with experimental data and high-level ab initio results.

Ab initio study of the dissociation of formyl cyanide

Abstract The energetics of the unimolecullar dissociation of formyl cyanide, HCOCN, along three paths were examined by ab initio computations. The energy barriers for unimolecular dissociation, calculated at the QCISD(T)/6–311(3d, 3p)//QCISD/6–311(d, p) level of theory with correction for zero-point energy, are 64.04 and 62.17 kcal/mol for paths 1 and 2 . The barriers for two successive steps of path 3 , involving an intermediate, HCONC, which lies 13.06 kcal/mol above formyl cyanide, are 42.74 and 50.48 kcal/mol for the first and second steps. The rates of reaction along paths 1, 2 and 3 are 3.60 × 10 −34 , 7.61 × 10 −33 and 1.59 × 10 −33 s −1 at 300 K, and 7.11 × 10 −7 , 2.48 × 10 −6 and 2.20 × 10 −6 s −1 at 700 K, respectively. At room temperature formyl cyanide does not decompose via unimolecular decomposition because the rate constants of all three paths are minute.