Using theory to guide the choice of pulse shape, we have synthesized frequency-chirped laser pulses and used them to control the evolution of vibrational wave packets on the 8 excited state of iodine.A negatively chirped pulse produces a wave packet at the The spread of the target in position and momentum is given by the formulas Ax = Qh/2m' and Ap = /mesh/2, where m is the reduced mass of the iodine molecule and the parameter co (units of frequency) was chosen to correspond to 250 cm '. The target is centered about a chosen position (0.372 nm) and a chosen momentum (corresponding to a kinetic energy of 50 meV).In this case, the momentum is negative, meaning that the iodine atoms are moving toward each other. The target is achieved by using a tailored, ultrashort light field to create an initial wave packet that evolves under the influ- state at a later time. The wave packet evolution is monitored by exciting the wave packet to the higher-lying e state with a second ultrashort pulse which opens up a "window" in internuclear distance (shown schematically by the striped box).The experimental signal is the total laser-induced fluorescence (LIF) from the e state as a function of delay between the pump and probe pulses, allowing detection of the wave packet density in a region of internuclear distance selected by the probe wavelength.ence of the molecular Hamiltonian to yield a wave packet that has maximum overlap with the target distribution at a later time. To reach the target in a reasonable amount of time, the electric field is allowed to be nonzero only during a temporal interval of our choice (550 fs, in the present case). Thus, the task of theory is to find the electric field F(t) that best steers the system to our chosen target at the chosen time. We note that to achieve this target, the optimal field must overcome the wave packet spreading which ordinarily occurs in anharmonic potentials. 33600031-9007/95/74(17)/3360(4)$06. 00

Molecular dynamics computer simulations of CH3CI, modeled as a vibrating diatomic, in water solvent reveal a rapid vibrational energy relaxation for the polar solute, over a wide range of initial vibrational excitations. A Landau-Teller formula is found to describe well the computed relaxation times.

The vibrational energy relaxation of a model methyl chloride molecule in water is studied through equilibrium and nonequilibrium molecular dynamics simulations. Previous work [Whitnell, Wilson, and Hynes, J. Phys. Chem. 94, 8625 (1990)] has demonstrated the validity of a Landau–Teller formula for this system in which the relaxation rate is equal to the frequency-dependent friction that the solvent exerts on the bond. In the present work, an analysis of this friction is used to test the isolated binary interaction (IBI) approximation for vibrational energy relaxation. In this system, where long-range electrostatic Coulomb forces dominate the interaction between the water solvent and the CH3Cl molecule, we show that the binary approximation to the friction only partially accounts for the rapid relaxation of the vibrational energy. We attribute the importance of cross correlations between different solvent molecules to the overlap of the CH3Cl vibrational frequency with the librational band of the water solvent. The dominance of the long-range Coulomb forces is further explored in nonequilibrium simulations. The vibrational energy relaxation is effected by a hysteresis in the Coulomb forces that the solvent exerts on the solute such that the force as the CH3Cl bond compresses is different from that as it expands. The non-Coulomb forces do not show this hysteresis to any significant extent. This hysteresis is reflected in the spatial distributions for the average dipole moment of the water solvent molecules. These spatial distributions also show that a large number of solvent molecules participate in the energy flow out of the CH3Cl molecule and that most of these important molecules are at positions perpendicular to the CH3Cl bond. The overall picture we develop here is of a process that is more complex than a simple binary interaction description can accurately portray.

We consider the control of molecular dynamics using tailored light fields, based on a phase space theory of control Ty. J. Yan ef al., J. Phys. Chem. 97, 2320 (1993)]. This theory enables us to calculate, in the weak field (one-photon) limit, the globally optimal light field that produces the best overlap for a given phase space target. We present as an illustrative example the use of quantum control to overcome the natural tendency of quantum wave packets to delocalize on excited state potential energy curves. Three cases are studied: (i) a "molecular cannon" in which we focus an outgoing continuum wave packet of I2 in both position and momentum, (ii) a "reflectron" in which we focus an incoming bound wave packet of I,, and (iii) the focusing of a bound wave packet of Na, at a turning point on the excited state potential using multiple light pulses to create a localized wave packet with zero momentum. For each case, we compute the globally optimal light field and also how well the wave packet produced by this light field achieves the desired target. These globally optimal fields are quite simple and robust. While our theory provides the globally optimal light field in the linear, weak field regime, experiment can in reality only provide a restricted universe of possible light fields. We therefore also consider the control of molecular quantum dynamics using light fields restricted to a parametrized functional form which spans a set of fields that can be experimentally realized. We fit the globally optimal electric field with a functional form consisting of a superposition of subpulses with variable parameters of amplitude, center time, center frequency, temporal width, relative phase, and linear and quadratic chirp. The best fit light fields produce excellent quantum control and are within the range of experimental possibility. We discuss relevant experiments such as ultrafast spectroscopy and ultrafast electron and x-ray diffraction which can in principle detect these focused wave packets.

Ligands of structurally diverse natures are able to bind at the CB 1 cannabinoid receptor, suggesting the existence of multiple binding sites on the receptor. Modeling studies have implicated Ser2.60(173) and Ser7.39(383) as possible interaction site(s) for CB 1 agonists. To test the importance of these residues for receptor recognition, recombinant human CB 1 receptors, stably expressed in human embryonic kidney 293 cells, were used to investigate the consequences of mutating Ser2.60 (to S2.60A) or Ser7.39 (to S7.39A) in radioligand binding and guanosine 5Ј-3-O-(thio)triphosphate functional assays. The S7.39A mutant resulted in a total ablation of(1-naphthalenyl)methanone (WIN55,212-2) binding properties at S7.39A were comparable with those of the wild-type (WT) receptor. The binding affinity of (Ϫ)-11␤-hydroxy-3-(1Ј,1Ј-dimethylheptyl)hexahydrocannabinol (AM4056) and (Ϫ)-11-hydroxydimethylheptyl-⌬ 8 -tetrahydrocannabinol (HU210) were drastically reduced (50-to 100-fold) at the S7.39A mutant. Likewise, the EC 50 for HU210 and AM4056-mediated activation of the S7.39A receptor was increased by Ͼ200-fold. In contrast, the binding affinity and potency of WIN55,212-2, CP55,940, HU210, and AM4056 were unaltered at the S2.60A mutant compared with WT human CB 1 receptors. These results clearly suggest that Ser7.39, but not Ser2.60, plays a crucial role in mediating ligand specific interactions for CP55,940, HU210, and AM4056 at the human CB 1 receptor. Our modeling studies predict that Ser7.39 in a gϪ1 conformation may induce a helix bend in TMH7 that provides docking space for CP55,940 binding; the S7.39A mutation may alter this binding space, precluding CP55,940 binding.The CB 1 cannabinoid receptor is a member of the G-protein coupled receptor (GPCR) family 1A, which includes the CB 2 receptor and the prototype rhodopsin (Howlett et al., 2002;Reggio, 2005). The human CB 1 and CB 2 receptors share only 44% amino acid overall homology, with a higher homology (68%) within the seven transmembrane domains (Munro et al., 1993). Both the CB 1 and CB 2 receptors share common signal transduction pathways, such as negative modulation of adenylyl cyclase activity (Felder et al., 1995) and also share certain common structural features with rhodopsin, including an extracellularly oriented N terminus, an intracellular carboxyl terminus, and hydrophobic transmembrane helices (TMHs).Although neither CB 1 nor CB 2 proteins have been crystallized, the crystal structure of rhodopsin (Palczewski et al., 2000) serves as a valuable template to model the putative CB 1 ligand binding domains. Ligands of structural diverse This study was supported by National Institutes of Health grants DA09978 and DA05274 (to M.E.A.), DA00489 and DA039434 (to P.H.R.), and DA09158 (to A.M. and M.E.A.).Article, publication date, and citation information can be found at

We lay some theoretical foundations to deal with the experimental realities faced in controlling molecular dynamics with tailored light fields: the nonideality of the light, the mixed rather than pure quantum-state nature of matter, and environmental and solvent effects. The optimal control of molecular dynamics using light fields is formulated in terms of the density matrix in Liouville space, generalizing existing wave-function-based formulations. This formulation allows the inclusion of mixed states, so that thermal and other nonpure quantum states of matter can be treated, as well as reduced descriptions useful in studying dense gas and condensed phases. In addition, it allows for general constraints and arbitrary coherent and partially coherent radiation fields and provides a unified picture for quantum, semiclassical, and classical molecular dynamics. For weak fields, the calculation simplifies and is given in terms of a molecular response function which itself does not depend upon the field. The solution of an eigenequation then directly gives the globally optimal field and the yield with respect to the target. As a demonstration, we explicitly consider a two electronic surface displaced harmonic oscillator molecular system in a Brownian oscillator solvent at finite temperatures, including nuclear and electronic solvation effects. Numerical illustrations are presented for the quantum control of thermal samples using phase-locked and random-phase light fields in the presence of solvent.

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