Yao Y.,NRC Steacie Institute for Molecular Sciences |
Hoffmann R.,Cornell University
Journal of the American Chemical Society | Year: 2011
Molecular and crystalline structures of (BH 3) n have been theoretically studied in the pressure regime from 1 atm to 100 GPa. At lower pressures, crystals of the familiar molecular dimer are the structure of choice. At 1 atm, in addition to the well-characterized β diborane structure, we suggest a new polymorph of B 2H 6, fitting the diffraction lines observed in the very first X-ray diffraction investigation of solid diborane, that of Mark and Pohland in 1925. We also find a number of metastable structures for oligomers of BH 3, including cyclic trimers, tetramers, and hexamers. While the higher oligomers as well as one-dimensional infinite chains (bent at the bridging hydrogens) are less stable than the dimer at ambient pressure, they are stabilized, for reasons of molecular compactness, by application of external pressure. Using periodic DFT calculations, we predict that near 4 GPa a molecular crystal constructed from discrete trimers replaces the β diborane structure as the most stable phase and remains as such until 36 GPa. At higher pressures, a crystal of polymeric, one-dimensional chains is preferred, until at least 100 GPa. © 2011 American Chemical Society.
O'Dell L.A.,NRC Steacie Institute for Molecular Sciences
Progress in Nuclear Magnetic Resonance Spectroscopy | Year: 2011
Highlights: Experimental methods for the direct detection of 14N are surveyed. Advantages, disadvantages and practicalities of each technique are discussed. Includes single-crystal, ultra-wideline, MAS and overtone spectroscopy. © 2011 Published by Elsevier B.V. All rights reserved.
Hockett P.,NRC Steacie Institute for Molecular Sciences |
Bisgaard C.Z.,NRC Steacie Institute for Molecular Sciences |
Bisgaard C.Z.,Copenhagen University |
Clarkin O.J.,Queen's University |
And 2 more authors.
Nature Physics | Year: 2011
Chemical reactions are manifestations of the dynamics of molecular valence electrons and their couplings to atomic motions. Emerging methods in attosecond science can probe purely electronic dynamics in atomic and molecular systems 1-6. By contrast, time-resolved structural-dynamics methods such as electron 7-10 or X-ray diffraction 11 and X-ray absorption 12 yield complementary information about the atomic motions. Time-resolved methods that are directly sensitive to both valence-electron dynamics and atomic motions include photoelectron spectroscopy and high-harmonic generation: in both cases, this sensitivity derives from the ionization-matrix element 18,19. Here we demonstrate a time-resolved molecular-frame photoelectron-angular-distribution (TRMFPAD) method for imaging the purely valence-electron dynamics during a chemical reaction. Specifically, the TRMFPADs measured during the non-adiabatic photodissociation of carbon disulphide demonstrate how the purely electronic rearrangements of the valence electrons can be projected from inherently coupled electronic-vibrational dynamics. Combined with ongoing efforts in molecular frame alignment 20 and orientation 21,22, TRMFPADs offer the promise of directly imaging valence-electron dynamics during molecular processes without involving the use of strong, highly perturbing laser fields 23. © 2011 Macmillan Publishers Limited. All rights reserved.
Sussman B.J.,NRC Steacie Institute for Molecular Sciences
American Journal of Physics | Year: 2011
The dynamic Stark effect is the quasistatic shift in energy levels due to the application of optical fields. The effect is in many ways similar to the static Stark effect. However, the dynamic Stark effect can be applied on rapid time scales and with high energies, comparable to those of atoms and molecules themselves. The dynamic Stark effect due to nonresonant laser fields is used in a myriad of contemporary experiments to hold and align molecules, to shape potential energy surfaces, and to make rapid transient birefringence. Five approaches of increasing sophistication are used to describe the dynamic Stark effect. One application, molecular alignment, is summarized and a comparison is made between the dynamic Stark effect and Stokes light generation in a Raman scattering process. © 2011 American Association of Physics Teachers.
Zgierski M.Z.,NRC Steacie Institute for Molecular Sciences |
Fujiwara T.,University of Akron |
Lim E.C.,University of Akron
Accounts of Chemical Research | Year: 2010
Photosynthesis, which depends on light-driven energy and electron transfer in assemblies of porphyrins, chlorophylls, and carotenoids, is just one example of the many complex natural systems of photobiology. A fuller understanding of the spectroscopy and photophysics of simple aromatic molecules is central to elucidating photochemical processes in the more sophisticated assemblies of photobiology. Moreover, developing a better grasp of the photophysics of simple aromatic molecules will also enhance our ability to create and improve practical applications in photochemical energy conversion, molecular nanophotonics, and molecular electronics. In this Account, we present a concerted experimental and theoretical study of aromatic ethynes, aromatic nitriles, and fluorinated benzenes, illustrating the important roles that the low-lying πσ* state plays in the electronic relaxation of these aromatic compounds. Diphenylacetylene, 4-dialkylaminobenzonitriles, 4-dialkylaminobenzethynes, and fluorinated benzenes exhibit fluorescence that strongly quenches as the excitation energy is increased for gas-phase systems and at elevated temperatures in solution. Much of this interesting photophysical behavior can be attributed to the presence of a dark intermediate state that crosses the fluorescent ππ* state. Our quantum chemistry calculations, as well as time-resolved laser spectroscopies, indicate that this dark intermediate state is the πσ* state that arises from the promotion of an electron from the π orbital of the phenyl ring to the σ* orbital localized in the C - X group (where X is CH and N) or on the C - X group (where X is a halogen). These crossings not only lead to the strong excitation energy and temperature dependence of fluorescence but also induce highly interesting πσ-mediated intramolecular charge transfer in 4- dialkylaminobenzonitriles. Most previous studies on the excited-state dynamics of organic molecules have examined aromatic hydrocarbons, nitrogen heterocycles, aromatic carbonyl compounds, and polyenes, which have low-lying excited states of ππ* character (hydrocarbons and polyenes) or nπ* and ππ* character (carbonyls and N-heterocycles). These studies have revealed important involvement of selection rules (promoting vibrational modes and spin-orbit coupling) and Franck-Condon factors for radiationless transitions, which have important effects on photophysical properties. The recent experimental and time-dependent density functional theory (TDDFT) calculations of aromatic ethynes, nitriles, and perfluorinated benzenes described in this Account demonstrate the importance of the bound excited state of a πσ* configuration in these molecules. © 2010 American Chemical Society.
Lofgreen J.E.,University of Toronto |
Moudrakovski I.L.,NRC Steacie Institute for Molecular Sciences |
Ozin G.A.,University of Toronto
ACS Nano | Year: 2011
We have prepared molecularly imprinted mesoporous organosilica (MIMO) using a semicovalent imprinting technique. A thermally reversible covalent bond was used to link a bisphenol A (BPA) imprint molecule to a functional alkoxysilane monomer at two points to generate a covalently bound imprint precursor. This precursor was incorporated into a cross-linked periodic mesoporous silica matrix via a typical acid-catalyzed, triblock copolymer-templated, sol-gel synthesis. Evidence of imprint sites buried in the pore walls was found through careful characterization of the imprinted material and its comparison to similarly prepared non-imprinted mesoporous organosilica (NIMO) and pure periodic mesoporous silica (PMS). After thermal treatment, the imprinted material (MIMO-ir) removed more than 90% of appropriately sized bisphenol species from water, yet showed significantly lower binding for both smaller and larger molecules containing phenol moieties. Identically treated NIMO-ir showed much poorer retention behavior than MIMO-ir for the same bisphenol species and behaved only slightly better than PMS-ir. © 2011 American Chemical Society.
McKellar A.R.W.,NRC Steacie Institute for Molecular Sciences
Journal of Molecular Spectroscopy | Year: 2010
Most applications of synchrotron radiation lie in the ultraviolet and X-ray region, but it also serves as a valuable continuum source of infrared (IR) light which is much brighter (i.e. more highly directional) than that from normal thermal sources. The synchrotron brightness advantage was originally exploited for high spatial resolution spectroscopy of condensed-phase samples. But it is also valuable for high spectral resolution of gas-phase samples, particularly in the difficult far-IR (terahertz) range (1/λ ≈ 10-1000 cm -1). Essentially, the synchrotron replaces the usual thermal source in a Fourier transform IR spectrometer, giving a increase of up to two (or even more) orders of magnitude in signal at very high-resolution. Following up on pioneering work in Sweden (MAX-lab) and France (LURE), a number of new facilities have recently been constructed for high-resolution gas-phase IR spectroscopy. In the present paper, this new field is reviewed. The advantages and difficulties associated with synchrotron IR spectroscopy are outlined, current and new facilities are described, and past, present, and future spectroscopic results are summarized. Crown Copyright © 2010 Published by Elsevier Inc. All rights reserved.
Yu K.,NRC Steacie Institute for Molecular Sciences |
Ouyang J.,NRC Steacie Institute for Molecular Sciences |
Leek D.M.,NRC Steacie Institute for Molecular Sciences
Small | Year: 2011
In-situ observation of the temporal evolution of the absorption of PbSe nanocrystals (NCs) via a low-temperature noninjection approach is presented. Based on a model reaction of lead oleate (Pb(OA) 2) and n-trioctylphosphine selenide (TOPSe) in 1-octadecene at 35-80 °C, the use of commercially available TOP (90 or 97%) in affecting the formation of the NCs is explored. TOPSe solutions made from TOP 90% exhibited higher reactivity than those made from TOP 97%. 31P NMR spectroscopy detected no dioctylphosphine selenide (DOPSe) but some DOP in ≈ 1.0 M TOPSe/TOP solution (made from TOP 90%), as well as no diphenylphosphine selenide (DPPSe) when DPP was added to the ≈1.0 M solution. Hence, it is proposed that, for the formation of PbSe monomers, an indirect pathway dominates with the formation of a Pb-P complex/intermediate, which results from the activation of Pb(OA) 2 by a phosphine compound (such as DPP, DOP, or TOP) and in turn reacts with TOPSe. With the use of TOP 90% and the addition of secondary phosphine DPP, the formation of PbSe magic-sized nanoclusters (MSNCs) and regular NCs (RNCs) is investigated. With proper tuning of the synthesis conditions, the formation of various PbSe MSNCs versus RNCs is monitored in situ with versus without the addition of DPP, or at different reaction temperatures but otherwise identical synthetic formulation and reaction parameters. Accordingly, the degree of supersaturation (DS) of the PbSe monomer affecting the development of these PbSe MSNCs versus RNCs is proposed; the higher the DS, the more the MSNCs are favored. Also, surface-determined cluster-cluster aggregation is proposed to be the growth mechanism for both the RNCs and MSNCs. For the former, quantized growth is followed by continuous growth. For the latter, the sizes of the magic-sized families are calculated. An indirect reaction pathway is proposed to be dominant for the formation of PbSe monomers, which involves the reaction between lead oleate and diphenyl/dioctyl/ trioctylphosphine (TOP) leading to a Pb-P complex/intermediate, which then reacts with TOPSe. The degree of supersaturation of the PbSe monomer plays a critical role in the formation of PbSe regular nanocrystals (RNCs) and/or magic-sized nanocrystals (MSNCs). Surface-determined cluster-cluster aggregation is responsible for the quantized growth of the RNCs at the initial growth stage and for the MSNCs. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
De Haan H.W.,University of Ottawa |
Paquet C.,NRC Steacie Institute for Molecular Sciences
Magnetic Resonance in Medicine | Year: 2011
The effects of including a hydrophilic coating around the particles are studied across a wide range of particle sizes by performing Monte Carlo simulations of protons diffusing through a system of magnetic particles. A physically realistic methodology of implementing the coating by cross boundary jump scaling and transition probabilities at the coating surface is developed. Using this formulation, the coating has three distinct impacts on the relaxation rate: an enhancement at small particle sizes, a degradation at intermediate particle sizes, and no effect at large particles sizes. These varied effects are reconciled with the underlying dephasing mechanisms by using the concept of a full dephasing zone to present a physical picture of the dephasing process with and without the coating for all sizes. The enhancement at small particle sizes is studied systemically to demonstrate the existence of an optimal ratio of diffusion coefficients inside/outside the coating to achieve maximal increase in the relaxation rate. Copyright © 2011 Wiley Periodicals, Inc.
Yu K.,NRC Steacie Institute for Molecular Sciences
Advanced Materials | Year: 2012
Colloidal semiconductor quantum dots (QDs) have been well appreciated for their potential in nanophotonics with an unprecedented impact in various areas, including light emitting diodes (LEDs) and solar cells. There is an outstanding demand on the control of size and size distribution for the various applications, with rational design supported by fundamental understanding of nucleation and growth. This Research News introduces recent advances in the synthesis of colloidal CdSe magic-sized nuclei (MSN) exhibiting sharp bandgap emission, with a model proposed to illustrate the nature of monomers and their degree of supersaturation (DS) affecting the formation of various CdSe MSN, magic-sized nanoclusters (MSCs), and regular nanocrystals (RNCs). Also, this model addresses tuning the CdSe RNCs into the CdSe MSN with the presence of cadmium acetate (Cd(OAc) 2) affecting the nature of the monomers. The nature and degree of supersaturation (DS) of CdSe monomers plays an essential role in the formation of CdSe magic-sized nuclei (MSN), magic-sized nanoclusters (MSCs), and regular nanocrystals (RNCs) in 1-octadecene (ODE) from reactions with cadmium acetate Cd(OAc) 2 as a Cd source. With low and high myristic acid to Cd(OAc) 2 feed molar ratios such as 1-to-2 and 3-to-1, the resulting Cd precursors are OAc-Cd-MA and MA-Cd-MA, respectively, which play an important role in the formation of the various CdSe NCs. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.