Quantum theory of chemical reactions in the presence of electromagnetic fields

We present a theory for rigorous quantum scattering calculations of probabilities for chemical reactions of atoms with diatomic molecules in the presence of an external electric field. The approach is based on the fully uncoupled basis set representation of the total wave function in the space-fixed coordinate frame, the Fock-Delves hyperspherical coordinates, and the adiabatic partitioning of the total Hamiltonian of the reactive system. The adiabatic channel wave functions are expanded in basis sets of hyperangular functions corresponding to different reaction arrangements, and the interactions with external fields are included in each chemical arrangement separately. We apply the theory to examine the effects of electric fields on the chemical reactions of LiF molecules with H atoms and HF molecules with Li atoms at low temperatures and show that electric fields may enhance the probability of chemical reactions and modify reactive scattering resonances by coupling the rotational states of the reactants. Our preliminary results suggest that chemical reactions of polar molecules at temperatures below 1 K can be selectively manipulated with dc electric fields and microwave laser radiation.     

Quantum control of molecular orientation by two-color laser fields

We demonstrate molecular orientation by using phase-controlled two-color omega+2omega laser pulses with an intensity of 1.0x10(12) W/cm(2) and a pulse duration of 130 fs. The orientation of three iodine-containing molecules (IBr, CH(3)I, and C(3)H(5)I) was monitored by the directional asymmetries of the photofragment angular distribution in dissociative ionization. In all three molecules, the directional asymmetry showed an oscillating behavior dependent on the relative phase difference between omega and 2omega pulses. The phase dependence of the directional asymmetry observed in iodine ions and counterpart ions were out of phase with each other. This result shows that a phase-controlled omega+2omega optical field discriminates between parallel and antiparallel configurations of aligned molecules that have a permanent dipole. This method performed well because (1) molecular orientation can be achieved by all-optical fields; (2) the direction of orientation is easily switched by changing the sign of the quantum interference; and (3) this method is free from any resonance constraint and thus can be applied to any molecule.     

Effect of absolute laser phase on reaction paths in laser-induced chemical reactions

Potential surfaces, dipole moments, and polarizabilities are calculated by ab initio methods [unrestricted MP2(full)/6-311++G(2d,2p)] along the reaction paths of the F+CH4 and Cl+CH4 reaction systems. It is found that in general dipole moments and polarizabilities exhibit peaks near the transition state. In the case of X=F these peaks are on the products side and in the case of X=Cl they are on the reactants side indicating an early transition state in the case of fluorine and a late transition state in the case of chlorine. An analysis of the geometric changes along the reaction paths reveals a one-to-one correspondence between the peaks in the electric properties and peaks in the rate of change of certain internal geometric coordinates along the reaction path. Interaction with short infrared intense laser fields pulses leads to the possibility of interferences between the dipole and polarizability laser-molecule interactions as a function of laser phase. The larger dipole moment in the Cl+CH4 reaction can lead to the creation of deep wells (instead of energy barriers) and new strongly bound states in the transition state region. This suggests possible coherent control of the reaction path as a function of the absolute phase of the incident field, by significant modification of the potential surfaces along the reaction path and, in particular, in the transition state region.     

Importance of cytochromes in cyclization reactions: Quantum chemical study on a model reaction of proguanil to cycloguanil

Proguanil, an anti‐malarial prodrug, undergoes cytochrome P450 catalyzed biotransformation to the pharmacologically active triazine metabolite (cycloguanil), which inhibits plasmodial dihydrofolate reductase. This cyclization is catalyzed by CYP2C19 and many anti‐malarial lead compounds are being designed and synthesized to exploit this pathway. Quantum chemical calculations were performed using the model species (Cpd I for active species of cytochrome and N4‐isopropyl‐N6‐methylbiguanide for proguanil) to elucidate the mechanism of the cyclization pathway. The overall reaction involves the loss of a water molecule, and is exothermic by approximately 55 kcal/mol, and involves a barrier of approximately 17 kcal/mol. The plausible reaction pathway involves the initial H‐radical abstraction from the isopropyl group by Cpd I, followed by two alternative paths‐ (i) oxygen rebound to provide hydroxyl derivative and (ii) loss of additional H‐radical to yield 1,3,5‐triazatriene, which undergoes cyclization. This study helped in understanding the role of the active species of cytochromes in this important cyclization reaction. © 2014 Wiley Periodicals, Inc.     

Quantum dynamics of h(2)(+) in intense laser fields on time-dependent potential energy surfaces

We have exploited the fully time-dependent Born-Oppenheimer approximation to develop time-dependent potential energy surfaces for the lowest two states of H(2)(+) in the presence of intense, time-varying, few-cycle laser fields of 2-8 fs duration. Quantum dynamics are explored on these field-dressed, time-dependent potentials. Our results show that the potential well in the lowest-energy state of H(2)(+) (i) collapses as the laser pulse reaches its peak amplitude and (ii) regains its form on the trailing edge of the pulse, and (iii) the trapped nuclear wavepacket has a higher probability of leaking out from the well in the case of longer laser pulses. The carrier envelope phase is found to have negligible effect on the nuclear dynamics.     

Quantum dynamics driven by continuous laser fields under measurements: towards measurement-assisted quantum dynamics control

We study quantum system dynamics driven by continuous laser fields under the measurement process. In order to take into account the system transition due to the measurement, we define the superoperator which eliminates the coherence relevant to the measured quantum states. We clarify that the dynamics of the measured states is frozen in the frequent measurement limit, while the space spanned by unmeasured states is isolated from the original system. We also derive the effective Liouvillian which governs incoherent population dynamics under the condition, in which measurements are frequently applied. We apply the formulation to two-level and Lambda-type three-level systems and clarify how the quantum measurements hinder the coherent population dynamics driven by the continuous laser fields in practical examples. Analysis on the laser field amplitude dependency of the final distribution in the t-->infinity limit suggests the possibility of the measurement-assisted quantum control.     

Quantum chemical methods and their applications to chemical reactions

This review is concerned with the impact of quantum chemistry on chemical reactions. Starting from the mid-sixties it focusses on those developments which have enabled us to predict essential features of simple chemical reactions. Thus model theories and computational methods are presented which provide the tools for these predictions. Then procedures to characterize potential surfaces and search methods for reaction paths are described. It is also attempted to relate these features to the terminology of the experimentalist. Finally a systematic survey of the main types of reaction (rearrangement, addition, elimination, substitution) is given.     

Large molecules in high-intensity laser fields

New research fields have opened up that are related to the interactions between molecules and high-intensity optical fields where the laser intensity ranges from 10¹²–10¹⁷ W cm⁻². A broad outline of this area will be described from the perspective of products and new techniques for beam generation. Studies of large molecules have begun and some examples are introduced herein. Parent ions with little fragmentation are found to form in the intensity region below 10¹⁶ W cm⁻². The formation of intact ions can be used in femtosecond laser mass spectrometry. In the intensity region above 10¹⁶ W cm⁻², electrons are stripped from the molecules by optical field ionization and the highly charged ions can undergo a Coulomb explosion. Coulomb explosions of benzene and C₆₀ have been demonstrated, and the mechanism can be analyzed by means of molecular dynamics simulations. A high intensity femtosecond laser beam can be converted to radiation sources of coherent VUV light, X-rays etc. and some possibilities for new chemical applications will be discussed.     

Quantum simulations of toroidal electric ring currents and magnetic fields in linear molecules induced by circularly polarized laser pulses

Circularly polarized laser pulses may excite state selective unidirectional toroidal electric ring currents around the axis of oriented linear molecules. These in turn induce state selective magnetic fields. Quantum simulations for AlCl show that these effects are about one or even more than three orders of magnitudes larger than those which may be prepared in oriented planar molecules such as Mg-porphyrin, by means of either circularly polarized laser pulses, or by traditional magnetic fields, respectively.     

Quantum effects in semiclassical treatment of molecular reactions

By the use of the known Coulomb wavefunctions, “quantal” Coulomb streamlines are defined for the eikonal of the system. Comparisons of such streamlines with the classical Coulomb trajectories are made. Use of such “quantum” Coulomb streamlines is proposed for reaction systems containing the Coulomb interaction U. The derived equations of motion are of particular use for cases where the residual potential V₁ (V₁ = V ? U) has either no classical forbidden region or has relatively small effects on trajectories.     

Intermolecular motion in strong infrared laser fields

The effect of a strong infrared laser field on the collision between two rare gas atoms is examined as a model of intermolecular motion under such conditions. After examination of the classical collision dynamics, three novel classes of collisions are identified, all of which result from interaction with the angular potential well. The signatures of these “collision mechanisms” in the classical deflection function and differential cross-section are determined. The generality of the results and the feasibility of using a modified crossed molecular beam experiment to observe these laser-induced effects are discussed.     

Multipolymer solution-phase reactions: application to the Mitsunobu reaction

The realization of the first polymer-on-polymer Mitsunobu reaction, in which a polymeric phosphine is used simultaneously with a polymeric azodicarboxylate, is reported. This strategy employs the use of soluble oligomers generated from ring-opening methathesis polymerization. 31P NMR analysis revealed that the two polymers were interacting to generate the Mitsunobu products. Application to several substrates, as well as comparison experiments with other polymeric reagents, is described.     

Quantum limited frequency modulation laser spectroscopy

Frequency modulation spectroscopy (FMS) is an optical heterodyne technique which allows high-resolution, high-sensitivity measurements of absorptions or dispersions associated with narrow spectral features. With new methods, the sensitivity limitations of FMS (caused by residual amplitude modulation) can be overcome, and quantum-limited performance can be readily achieved.     

Quantum nonadiabatic dynamics of hydrogen exchange reactions

In continuation of our earlier effort to understand the nonadiabatic coupling effects in the prototypical H + H₂ exchange reaction [Jayachander Rao et al. Chem. Phys. 333 (2007) 135], we present here further quantum dynamical investigations on its isotopic variants. The present work also corrects a technical scaling error occurred in our previous studies on the H + HD reaction. Initial state-selected total reaction cross sections and Boltzmann averaged thermal rate constants are calculated with the aid of a time-dependent wave packet approach employing the double many body expansion potential energy surfaces of the system. The theoretical results are compared with the experimental and other theoretical data whenever available. The results re-establish our earlier conclusion, on a more general perspective, that the electronic nonadiabatic effects are negligible on the important quantum dynamical observables of these reactive systems reported here.     

Quantum aspects of self-organized periodic chemical reactions

It is a remarkable empirical fact, known for a long time, that in certain self-organized periodic chemical reactions, such as Liesegang's or Belousov-Zhabotinsky's reactions, the product of molecular weight of precipitate, precipitation length period, and speed of precipitation is of the order of universal Planck's quantum of action h. Based on the fact that the classical and quantum diffusions are processes, which are indistinguishable in the configuration space, a quantum criterion in terms of diffusion constants has been established. This criterion enables one to find out conditions under which the quantum behavior of self-organized periodic reactions can be observed.     

Quantum mechanical calculations on phosphate hydrolysis reactions

Multiple biological processes are regulated by kinases and phosphatases. This study aims to provide nonenzymatic models for phosphorylation and dephosphorylation of serine, threonine, and tyrosine phosphate using ab initio guantum mechanical calculations. We reduce the problem to methyl phosphate hydrolysis to model serine/threonine, and the hydrolysis of phenyl phosphate to model the tyrosine. HF, B3LYP, and MP2 calculations with a 6‐31+G(d) basis set were employed. The effect of water as a catalyst was also analyzed. As expected, the activation energy barrier is lowered. © 2000 John Wiley & Sons, Inc. J Comput Chem 21: 43–51, 2000     

几个硫-叶立德反应机理的量子拓扑研究

采用MP2(FC)/6-311++G(d,p)对硫叶立德和类硫叶立德自由基反应机理进行了探讨。优化了中间体、过滤态和产物的几何构型。本文侧重从量子拓扑学的角度,对IRC(内禀反应坐标)反应进程中各点进行电子密度拓扑分析,讨论了反应过程中化学键的断裂、生成和化学键的变化规律。找到了这类反应的能量过渡态和结构过渡态,上述两个反应都是先经历一个没有形成三元环拓扑结构的能量过渡态,再经历一个形成了三元环拓扑结构的结构过渡态,最后到达产物。     

Infrared laser induced chemical reactions

It is shown that a high power infrared laser can enhance a chemical reaction (by lowering the activation energy) even if the reactants are infrared inactive.     

Excitation of two-level atoms in mode-locked laser fields

The excitation of two-level atoms in a laser field comprising many equally spaced coupled laser modes corresponding to a coherent pulse train is examined. The atom-field interaction is analysed via the optical Bloch equations for a rotating wave. In the limit of weak excitation they can be solved analytically and the time-averaged atomic excitation turns out to be a linear superposition of the contributions from the individual laser modes. Thus excitation spectra simply reflect the mode structure of the laser spectrum. Excitation spectra for strong fields are obtained by numerical integration of the optical Bloch equations. They exhibit a saturation behaviour differing significantly from the well known single-mode case. The temporal evolution towards the steady state is calculated for several numerical examples to clarify the origin of this behavior. For achieving maximum excitation, the laser pulse area, as in the single-pulse case, should be an odd multiple of π, and the mode spacing (pulse repetition rate) should exceed the natural linewidth of the atomic transition considerably. Under these conditions the time-averaged excited-state population approaches 1/2 while saturation broadening ensures nearly frequency-independent excitation within an extended fraction of the laser bandwidth.