Coupled-monomers in molecular assemblies: theory and application to the water tetramer, pentamer, and ring hexamer

We present extensions to the local-monomer (LMon) Model, a general quantum method to describe coupled intramolecular vibrational modes of a molecular cluster consisting of a set of monomers [Y. Wang and J. M. Bowman, J. Chem. Phys. 134, 154510 (2011)], to incorporate monomer-monomer coupling. A central aspect of the LMon model is a local normal-mode analysis, done for each monomer, perturbed by all other mononers. Monomer-monomer coupling is described by several approaches based on these normal-mode analyses. Two are Hückel-type models, where coupling constants for each intramolecular mode are determined non-empirically from normal-mode analyses. One model, the simple one, is limited to nearest-neighbor interactions. The second and more general one determines monomer-monomer couplings from the full and local-monomer Hessians, with no further assumptions. The simple approach is applied to the water tetramer, pentamer and ring hexamer. For the tetramer and ring hexamer cases, artificial degeneracies of the intramolecular energies in the LMon model, owing to the high symmetry of the cluster, are correctly lifted. The general approach to obtain coupling constants is illustrated for the ring hexamer, where new fundamental energies are reported. Other, more rigorous approaches are suggested but not implemented.     

3D Photofixation Lithography in Diels–Alder Networks

3D structures are written and developed in a crosslinked polymer initially formed by a Diels–Alder reaction. Unlike conventional liquid resists, small features cannot sediment, as the reversible crosslinks function as a support, and the modulus of the material is in the MPa range at room temperature. The support structure, however, can be easily removed by heating the material, and depolymerizing the polymer into a mixture of low‐viscosity monomers. Complex shapes are written into the polymer network using two‐photon techniques to spatially control the photoinitiation and subsequent thiol–ene reaction to selectively convert the Diels–Alder adducts into irreversible crosslinks.     

Ab initio calculation of the low-lying vibrational states of C2H2(A) in full dimensionality

We report full-dimensional calculations of vibrational energies of trans-C2H2(A) using the code MULTIMODE and with a full-dimensional potential energy surface obtained by fitting singles and doubles coupled-cluster equations-of-motion (EOM-CCSD) energies using a [3s 2p 1d] atomic natural orbital basis. The EOM-CCSD calculations were done with the code "ACES II". We compare the properties of the potential surface to previous calculations at the trans minimum and also compare the vibrational energies to experimental ones.     

The roaming atom pathway in formaldehyde decomposition

We present a detailed experimental and theoretical investigation of formaldehyde photodissociation to H(2) and CO following excitation to the 2(1)4(1) and 2(1)4(3) transitions in S(1). The CO velocity distributions were obtained using dc slice imaging of single CO rotational states (v=0, j(CO)=5-45). These high-resolution measurements reveal the correlated internal state distribution in the H(2) cofragments. The results show that rotationally hot CO (j(CO) approximately 45) is produced in conjunction with vibrationally "cold" H(2) fragments (v=0-5): these products are formed through the well-known skewed transition state and described in detail in the accompanying paper. After excitation of formaldehyde above the threshold for the radical channel (H(2)CO-->H+HCO) we also find formation of rotationally cold CO (j(CO)=5-28) correlated to highly vibrationally excited H(2) (v=6-8). These products are formed through a novel mechanism that involves near dissociation followed by intramolecular H abstraction [D. Townsend et al., Science 306, 1158 (2004)], and that avoids the region of the transition state entirely. The dynamics of this "roaming" mechanism are the focus of this paper. The correlations between the vibrational states of H(2) and rotational states of CO formed following excitation on the 2(1)4(3) transition allow us to determine the relative contribution to molecular products from the roaming atom channel versus the conventional molecular channel.     

Termolecular kinetics for the Mu + CO + M recombination reaction: A unique test of quantum rate theory

The room-temperature termolecular rate constants, k0, for the Mu + CO + M<==>MuCO + M (M = He, N2, Ar) recombination reaction have been measured by the muSR technique, and are reported for moderator gas pressures of up to approximately 200 bar (densities less, similar 0.4 x 10(22) molec cm(-3)). The experimental relaxation rates reveal an unusual signature, in being dominated by the electron spin-rotation interaction in the MuCO radical that is formed in the addition step. In N2 moderator, k0 = 1.2+/-0.1 x 10(-34) cm(6) s(-1), only about 30% higher than found in Ar or He. The experimental results are compared with theoretical calculations carried out on the Werner-Keller-Schinke (WKS) surface [Keller et al., J. Chem. Phys. 105, 4983 (1996)], within the framework of the isolated resonance model (IRM). The positions and lifetimes of resonance states are obtained by solving the complex Hamiltonian for the nonrotating MuCO system, using an L2 method, with an absorbing potential in the asymptotic region. Accurate values of the vibrational bound and resonance states of MuCO reveal unprecedented isotope effects in comparisons with HCO, due to the remarkable effect of replacing H by the very light Mu atom (m(Mu) approximately (1/9)m(H)). Due to its pronounced zero-point energy shift, there are only two (J = 0) bound states in MuCO. Contributions from nonzero J states to the termolecular rate constants are evaluated through the J-shifting approximation, with rotational constants evaluated at the potential minimum. The value of the important A constant (181 cm(-1)) used in this approximation was supported by accurate J = K = 1 calculations, from which A = 180 cm(-1) was obtained by numerical evaluation. The calculations presented here, with a "weak collision factor" beta c = 0.001, indicative of the very sparse density of MuCO states, give a very good account of both the magnitude and pressure dependence of the experimental rates, but only when the fact that the two initially bound (J = 0) states become resonances for J > 0 is taken into account. This is the first time in IRM calculations of atom-molecule recombination reactions where J not equal to 0 states have proven to be so important, thus providing a truly unique test of quantum rate theory.     

Influence of small amounts of addition‐fragmentation capable monomers on polymerization‐induced shrinkage stress

The effects of the addition of small amounts of multifunctional monomers that contain functional groups capable of undergoing addition‐fragmentation during radical polymerizations are investigated. Specifically, up to 6 wt % of phenyl trithiocarbonate (TTC)‐containing diacrylate was added to conventional thiol‐multiacrylate photopolymerizations where its addition led to up to 60% reduction in polymerization‐induced shrinkage stress. The higher levels of TTC achieve the lowest stress though they also significantly depress the polymerization rate. Using up to 0.5 wt % phenyl TTC successfully reduces the stress by nearly 20%, demonstrating the effectiveness of the phenyl TTC, while minimizing the influence that the RAFT activity of the TTC unit has on the polymerization rate. When the polymerization rates of the TTC‐containing resins are increased by changing the incident light intensity, complete acrylate conversion is achieved and the stress remains up to 40% lower in the TTC‐containing resins. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 1315–1321     

Rate mechanisms of a novel thiol-ene photopolymerization reaction

A novel thiol-ene photopolymerization reaction involving copolymerization of tetrathiol monomer with vinyl silazane is experimentally characterized and is modeled successfully. The overall polymerization rate is found to be controlled by the ratio of the propagation to chain transfer kinetic parameters. The polymerization rate of this mixture, in the presence of added photoinitiator, is approximately first order in ene functionality and is independent of thiol functional group concentration. Initiation rates in this system, when cured utilizing a light centered around 365 nm light, and in the presence of no added photoinitiator, are shown to be proportional to the ene monomer concentration. When the mixture is polymerized utilizing light centered at 254 nm light, and without photoinitiator, the initiation rates are proportional to the thiol monomer concentrations. This novel reaction scheme is further utilized to form ultra rapidly polymerizable polymer derived ceramic structures with high aspect ratios.