The effect of a mechanical force on quantum reaction rate: quantum Bell formula

The purpose of this note is to derive a quantum-mechanical analog of Bell's formula, which describes the sensitivity of a chemical reaction to a mechanical pulling force. According to this formula, the reaction rate depends exponentially on the force f, i.e., k(f) ~ exp(f/f(c)), where the force scale f(c) is estimated as the thermal energy k(B)T divided by a distance a between the reactant and transition states along the pulling coordinate. Here I use instanton theory to show that, at low temperatures where quantum tunneling is dominant, this force scale becomes f(c) ~ ?ω/a (in the limit where frictional damping is absent) or f(c) ~ ?τ(-1)/a (in the strong damping limit). Here ω is a characteristic vibration frequency along the pulling coordinate and τ is a characteristic relaxation time in the reactant state. That is, unlike the classical case where f(c) is unaffected by dissipation, this force scale becomes friction dependent in the quantum limit. I further derive higher-order corrections in the force dependence of the rate, describe generalizations to many degrees of freedom, and discuss connection to other quantum rate theories.     

Chemical clock reactions: The effect of precursor consumption

During a clock reaction an initial induction period is observed before a significant change in concentration of one of the chemical species occurs. In this study we develop the results of Billingham and Needham (1993) who studied a particular class of inhibited autocatalytic clock reactions. We obtain modified expressions for the length of the induction period and show that characteristic clock reaction behaviour is only observed within certain parameter limits. This revised version was published online in July 2006 with corrections to the Cover Date.     

Natural resonance theory: III. Chemical applications

We describe quantitative numerical applications of the natural resonance theory (NRT) to a variety of chemical bonding types, in order to demonstrate the generality and practicality of the method for a wide range of chemical systems. Illustrative applications are presented for (1) benzene and polycyclic aromatics; (2) CO₂, formate, and related acyclic species; (3) ionic and polar compounds; (4) coordinate covalent compounds and complexes; (5) hypervalent and electron-deficient species; (6) noncovalent H-bonded complex; and (7) a model Diels-Alder chemical reaction surface. The examples exhibit the general harmony of NRT weightings with qualitative resonance-theoretic concepts and illustrate how these concepts can be extended to many new types of chemical phenomena at a quanitative ab initio level. © 1998 John Wiley & Sons, Inc. J Comput Chem 19: 628–646, 1998     

Nonadiabatic transitions in chemical reactions: A quantum mechanical study

This work presents an exact quantum mechanical treatment of a reactive three-atom collinear model system incorporating nonadiabatic couplings. It was assumed that nonadiabatic transitions are induced by the vibrational motion only. The main findings are: (i) The reaction process can create conditions in which weak nonadiabatic couplings terms ( for which the Massey parameter was round 10) may cause large probabilities (～0.5) for transitions from one electronic surface to the other. In other words, the reaction process is able in certain cases to create a near resonance situation which makes the non-adiabatic transition almost independent of the magnitude of the coupling term. For this to happen the two surfaces need not be proximate, nor need they “almost” cross along a certain line (ii) In cases where the main nonadiabatic transitions take place outside the interaction region one may, at least qualitatively, decouple the reaction process from the nonadiabatic one. Thus, under the conditions specified one may first treat the reactive system on the ground state surface without including the excited interacting surface and then treat the nonadiabatic process independently.     

Chemical surface modification of self-assembled monolayers by radical nitroxide exchange reactions

This article describes the application of nitroxide exchange reactions of surface-bound alkoxyamines as a tool for reversible chemical modification of self-assembled monolayers (SAMs). This approach is based on radical chemistry, which allows for introduction of various functional groups and can be used to reversibly introduce functionalities at surfaces. To investigate the scope of this surface chemistry, alkoxyamines with different functionalities were synthesized and were then applied to the immobilization of, for example, dyes, sugars, or biotin. Surface analysis was carried out by contact angle, X-ray photoelectron spectroscopy, and fluorescence microscopy measurements. The results show that this reaction is highly efficient, reversible, and mild and allows for immobilization of various sensitive functional groups. In addition, Langmuir-Blodgett lithography was used to generate structured SAMs. Site-selective immobilization of a fluorescent dye could be achieved by nitroxide exchange reactions.     

Chemical transformations of 4:7,10:13-diepoxy[15]annulenone by carbonyl transposition reactions

The titled annulenone 1 could be converted into new annulenones 3 and 8 by carbonyl transposition reactions. Compound 8 can be regarded as the first 14π hydroxy-enone as a higher homolog of tropolone (6π) and hydroxy[11]annulenones (10π), and was proved to be aromatic.     

Chemical reactions and lasers: Elementary steps and complex systems

In recent years, various methods have been developed to observe and to influence the course of chemical reactions using laser radiation. By selectively increasing the translational, rotational, and vibrational energies of the reaction partners with high intensity visible and UV lasers, direct insight can be gained into the molecular course of the breaking and re-forming of chemical bonds.The production of free radicals by laser radiation can be used in chemical synthesis for the production of monomers. As example the kinetics of the UV-laser induced dehydrochlorination of CH₃CC1F₂ is described.The application of linear and non-linear laser spectroscopic methods allows a non-intrusive observation of the interaction of transport processes with chemical reactions used with high temporal, spectral and spatial resolution. As a simple test system the ignition of O₂–O₂ mixtures by irradiation with a CO₂ laser along the axis of a cylindrical vessel is considered. Mathematical simulation of the ignition process is simulated methematically by solving the corresponding system of conservation equations. Experimental data are presented for velocity components of the flame front from IR-UV double resonance experiments and for the temperature history from infrared absorption measurements using tunable diode lasers.     

The diagrammatic perturbation theory for the interaction of two molecular systems treated by the extended Hückel theory

The general diagrammatic perturbation theory for the interaction of two molecular systems treated by the extended Hückel method is elaborated.     

Microwave-assisted multicomponent reactions: rapid and regioselective formation of new extended angular fused aza-heterocycles

A new multicomponent domino reaction for rapid and regioselective synthesis of highly functionalized benzo[h]naphtho[2,3-a]acridine-15,16(5H,14H)-diones has been established. The reaction can be conducted by using readily available and inexpensive substrates under microwave irradiation. The procedures are facile, avoiding timeconsuming and costly syntheses, tedious work-up and purifications of precursors as well as protection/deprotection of functional groups. This method is much more efficient due to short reaction times and easy work up. The resulting naphthoacridines have been readily converted into benzoquinoxaline-fused benzoquinoline analogues by treating with benzene-1,2-diamine under microwave irradiation. The structural assignment has been ambiguously confirmed by X-ray analysis. A new mechanism has been proposed for this new multicomponent domino process.     

The extended Koopmans' theorem: vertical ionization potentials from natural orbital functional theory

The Piris natural orbital functional, PNOF5, has been used to predict vertical ionization potentials of a selected set of 30 organic and inorganic spin-compensated molecules by means of the extended Koopmans' theorem. Electron affinities of 10 selected radicals have also been estimated as the inverse of the ionization potentials of the anionic species, calculated at the experimental geometries of the neutral radicals. The basis set limit effects have been assessed by inspecting the data obtained for the Dunning's basis set series cc-pVXZ and aug-cc-pVXZ (X = D, T, Q, 5). The performance of the PNOF5 is established by carrying out a statistical analysis of the mean absolute errors (MAEs) with respect to the experiment values. The calculated PNOF5 ionization potentials and electron affinities agree satisfactorily with the corresponding experimental data, with MAEs smaller than 0.5 eV.     

Chemical reactions in microdroplets by electrostatic manipulation of droplets in liquid media

A microchemical reaction method involving microdroplets is proposed. Microdroplets are formed in a chemically stable medium on electric panel devices. These devices are substrates which have electrode arrays or electrode dots, and its surfaces are coated by an insulating film (such as Teflon or polypropylene) to prevent discharge and electrolysis of solutions. Microdroplets can be separately manipulated by a traveling electric field, which arises on applying a sequential voltage to the electrodes. Droplets moved smoothly at 1 Hz and voltage 400 V(0-p). Reagents were then put in droplets that were collided and coalesced, resulting in chemical reactions that included alkalization of phenolphthalein and the luciferin-luciferase reaction.     

Toward a statistical mechanical theory for water: analytical theory for a short-ranged reference system

Starting from a realistic Hamiltonian and making use of recent findings that the properties of associating fluids are determined primarily by short-ranged interactions, this methodology has been implemented using statistical mechanical approaches and thermodynamic perturbation theory for the TIP4P model of water. We focus on the short-range reference system for which an analytic expression for the Helmholtz free energy is derived. It is found that the model (reference system) exhibits, in addition to a faithful representation of the structure of water, the same features that are characteristic for real water, namely, (i) the temperature of the density maximum and its pressure dependence, including the inflection point at high pressures and (ii) the temperature minima of the constant pressure heat capacity and the coefficient of isothermal compressibility.     

Single-molecule force spectroscopy of bimolecular reactions: system homology in the mechanical activation of ligand substitution reactions

The mechanochemistry of the bimolecular nucleophilic substitution of DMSO for substituted pyridines at a square-planar pincer Pd(II) center was investigated using single-molecule force spectroscopy (SMFS). The SMFS data are interpreted in terms of the Bell-Evans model, which gives thermal off-rates for two reactions that agree well with previous, stress-free measurements. The characteristic force dependency of the rupture rate, fbeta, is effectively constant for the two reactions examined (22 +/- 2 and 24 +/- 2 pN), and the system homology in the mechanical response is consistent with expected similarities in the reaction potential energy surfaces.     

Chemical ionization mass spectrometry: Oligomerization reactions in tight ion sources

Association of carbonyl compounds with protons to give cluster ions can be observed in conventional chemical ionization mass spectrometry. The variation of the relative ion currents of proton bound ‘dimers’ formed with acetone and methylethyl ketone have been examined as a function of ion source partial pressures. Multiple ion/molecule reactions with N-methylol derivatives of amides and carbamates and repetitive losses of water give protonated oligomeric species. Ions having m/z values as large as 625 [M₆H? 5H₂O]⁺ have been detected in the chemical ionization spectrum of N-hydroxymethylurethane (M).     

Communication: extended multi-state complete active space second-order perturbation theory: energy and nuclear gradients

The extended multireference quasi-degenerate perturbation theory, proposed by Granovsky [J. Chem. Phys. 134, 214113 (2011)], is combined with internally contracted multi-state complete active space second-order perturbation theory (XMS-CASPT2). The first-order wavefunction is expanded in terms of the union of internally contracted basis functions generated from all the reference functions, which guarantees invariance of the theory with respect to unitary rotations of the reference functions. The method yields improved potentials in the vicinity of avoided crossings and conical intersections. The theory for computing nuclear energy gradients for MS-CASPT2 and XMS-CASPT2 is also presented and the first implementation of these gradient methods is reported. A number of illustrative applications of the new methods are presented.     

Crystallization under stress: A theory of fibrillar crystallization

A theory of crystallization in stretched polymer networks is developed. In it, four principal features are incorporated: (i) crosslinks are displaced by growing crystallites, (ii) network chains are constrained to positions compatible with fixed sample shape and volume, (iii) some network chains remain amorphous, and (iv) the relative direction of a chain through a crystallite may not be the same for all chains. The derived network force exhibits a V or U shape with changing temperature in the crystallization zone that is a close replica of the behavior of gutta percha networks. Postulates of fibrillar-lamellar transitions are not introduced into the calculations.     

Gas phase protonolysis reactions : Chemical ionization mass spectrometry of N-nitrosamines

Gas phase protonolysis reactions of a wide variety of N-nitrosamines have been studied utilizing methane chemical ionization mass spectrometry (CIMS). N-Nitrosamines are protonated at either the N or O atoms of the N-nitroso triad (NNO) leading to aminium radicals, nitrenium ions, ammonium ions, carbenium ions and various elimination reaction. A structure-CIMS reactivity relationship of N-nitrosamines is presented.