Free-energy calculation of structure-H hydrates

A molecular-dynamics (MD) simulation of structure-H hydrates was performed under constant pressure and temperature with 6120 TIP4P water molecules, 900 OPLS-UA methane molecules, and 180 large molecular guest substance (LMGS) molecules. The LMGS molecules were represented in the form of a one-site Lennard-Jones (LJ) model using the LJ parameters sigma and epsilon. In order to clarify the thermodynamic stability of structure-H hydrates, we calculated the free-energy difference, changing on the sigma and epsilon only of the LMGS molecules. In this simulation, stable crystals of structure-H hydrates and a minimum value of DeltaG were obtained at sigma approximately 6.2 A and large values of epsilon. All simulations were performed using the special-purpose computer hardware MDGRAPE-2.     

Electronic angular momentum polarizations of photofragments: a case study of ICN photodissociation from a perpendicular transition

A quantum treatment on ICN photodissociation from an initial perpendicular transition (Omega'=+/-1<--Omega"=0) to the asymptote CN(|Sigma+,J'M'N'1/2>)+I(2P3/2) is presented. Density matrices of both photofragments are derived and explicit expressions of the state multipoles in terms of the angular momentum coupling coefficients and the rotation-bending factors have been obtained. To perceive the physical origin of electronic angular momentum polarizations of the iodine photofragments, a correlation scheme which considers the magnetic dipolar and the electrostatic dipole-quadrupole interactions between I and CN cofragments is proposed. For ICN precursors in the vibrational ground state or in the equally populated l-type split levels, the alignment parameters of the iodine photofragments in the molecular frame can be calculated according to this long-range interaction model. For the perpendicular transition |1Pi1><--|1Sigma0+>, its alignment parameters of I(2P3/2) from the incoherent and coherent transitions to the |Omega'=1> and |Omega'=-1> components are rho(0)2(1Pi1)=0.756 and rho2(2)(1Pi1)=-0.656, respectively. For the perpendicular transition to |3Pi1>, rho(0)2(3Pi1)=-0.878 and rho2(2)(3Pi1)=0.328 are from the incoherent transition, whereas rho(0)2(3Pi1)=0.122 and rho2(2)(3Pi1)=0.328 are from the coherent transition. To analyze the photoion images of iodine photofragments, angular distributions of I+ from the 2+1 resonance-enhanced multiphoton ionization detection scheme are derived.     

Photophysics of aromatic molecules with low-lying pi sigma* states: fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S(1)) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S(1) state of pi pi(*) character for fluorinated benzenes with four or less F atoms, and S(1) state of pi sigma(*) character for PFB and HFB. The pi sigma(*) character of the S(1) state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the (1)pi pi(*) (L(b))<--S(0) transition, which can be identified with the predicted low-energy (1)pi sigma(*)<--S(0) absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the (1)pi sigma(*) state and efficient S(1) (pi sigma(*))-->S(0) internal conversion between two electronic states of very different geometries.     

Thermal isomerizations of ketenimines to nitriles: evaluations of sigma-Dot (sigma(*)) constants for spin-delocalizations

Rate constants (k(Y)) of the isomerizations of 11 diphenyl N-(substituted benzyl) ketenimines were measured at 40, 50, 60, and 70 degrees C. Activation parameters DeltaH()(Y) and DeltaS()(Y) were obtained using the Eyring equation. The relative rates (k(Y)/k(H)) were fitted into Hammett single correlations (log k(Y)/k(H) = rhosigma and log k(Y)/k(H) = rho(*)sigma(*)). The single correlations have been compared with Hammett dual correlations (log k(Y)/k(H) = rhosigma + rho(*)sigma(*) ). Separate treatments of para and meta substituents yielded even better correlations. Para substituents control the rates through spin-delocalizations and inductive effects. The former outweighs the latter when the latter exerts a modest but distinct influence on the rates. On the other hand, inductive effects are the "major" or the sole interactions triggered by meta substituents.     

Correlation between surface tension, density, and sound velocity of liquid metals

The inter-relationship of surface tension sigma, density rho, and sound velocity c has been examined for forty-one liquid metals. Sound velocities correlate with the equation, log c=0.5526log(sigma/rho)+5.4364. Unknown sound velocities may be estimated using surface tension and liquid density values.     

Properties of hydration shells of protein molecules at their pressure- and temperature-induced native-denatured transition.

Properties of water at the surface of biomolecules are important for their conformational stability. The behaviour of hydrating water at protein transition (t) pressures P(t) and temperatures T(t) , with the points (P(t),T(t) ) lying in the Native-Denatured (N-D) transition line, is studied. Hydration shells at the hydrophilic regions of protein molecules with surface charge density sigma are investigated with the help of the equation of state of water in an open system. The local values of sigma rather close to each other (sigma(D) approximately 0.3 C m(-2)) are found for six different experimental lines of the N-D transition found in the literature. The values sigma(D) correspond to the crossings of the total pressure (P(t)+Pi) vs sigma isotherms at different T(t) (Pi-electrostriction pressure). The pressures P(t) and temperatures T(t) appear to be related with some selected sites at the surfaces of the protein molecules.     

Stereoelectronic effects in ring-chain tautomerism of 1,3-diarylnaphth[1,2-e][1,3]oxazines and 3-alkyl-1-arylnaphth[1,2-e][1,3]oxazines

The disubstitution effects of X and Y in 1-(Y-phenyl)-3-(X-phenyl)-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines on the ring-chain tautomerism, the delocalization of the nitrogen lone pair (anomeric effect), and the (13)C NMR chemical shifts were analyzed by using multiple linear regression analysis. Study of the three-component equilibrium B<==>A<==>C revealed that the chain<==>trans (A<==>B) equilibrium constants are significantly influenced by the inductive effect (sigma(F)) of substituent Y on the 1-phenyl ring. In contrast, no significant substituent dependence on Y was observed for the chain<==>cis (A<==>C) equilibrium. There was an analogous dependence for the epimerization (C<==>B) constants of 1-(Y-phenyl)-3-alkyl-2,3-dihydro-1H-naphth[1,2-e][1,3]oxazines. With these model compounds, significant overlapping energies of the nitrogen lone pair was observed by NBO analysis in the trans forms B (to sigma*(C1-C1'), sigma*(C1-C10b), and sigma*(C3-O4)) and in the cis forms C (to sigma*(C1-H), sigma*(C1-C10b), and sigma*(C3-O4)). The effects of disubstitution revealed some characteristic differences between the cis and trans isomers. However, the results do not suggest that the anomeric effect predominates in the preponderance of the trans over the cis isomer. When the (13)C chemical shift changes induced by substituents X and Y (SCS) were subjected to multiple linear regression analysis, negative rho(F)(Y) and rho(F)(X) values were observed at C-1 and C-3 for both the cis and trans isomers. In contrast, the positive rho(R)(Y) values at C-1 and the negative rho(R)(X) values at C-3 observed indicated the contribution of resonance structures f (rho(R) > 0) and g (rho(R) < 0), respectively. The classical double bond-no-bond resonance structures proved useful in explaining the substituent sensitivities of the donation energies and the behavior of the SCS values.     

Solvolysis of para-substituted cumyl chlorides. Brown and Okamoto's electrophilic substituent constants revisited using continuum solvent models

Brown and Okamoto (J. Am. Chem. Soc. 1958, 80, 4979) derived their electrophilic substitutent constants, sigma(p)+, from the relative rates of solvolysis of ring-substituted cumyl chlorides in an acetone/water solvent mixture. Application of the Hammett equation to the rates for the meta-substituted cumyl chlorides, where there could be no resonance interaction with the developing carbocation, gave a slope, rho(+) = -4.54 ( identical with 6.2 kcal/mol free energy). Rates for the para-substituted chlorides were then used to obtain sigma(p)+ values. We have calculated gas-phase C-Cl heterolytic bond dissociation enthalpy differences, Delta BDE(het) (= BDE(het)(4-YC(6)H(4)CMe(2)Cl) - BDE(het)(C(6)H(5)CMe(2)Cl)), for 16 of the 4-Y substituents employed by Brown and Okamoto. The plot of Delta BDE(het) vs sigma(p)+ gave rho(+) (SD) = 16.3 (2.3) kcal/mol, i.e., a rho(+) value roughly 2.5 times greater than experiment. Inclusion of solvation (water) energies, calculated using three continuum solvent models, reduced rho(+) and SD. The computationally least expensive model used, SM5.42R (Li et al. Theor. Chem. Acc. 1999, 103, 9) gave the best agreement with experiment. This model yielded rho(+) (SD) = 7.7 (0.9) kcal/mol, i.e., a rho(+) value that is only 24% larger than experiment.     

Rheological and structural studies of linear polyethylene melts under planar elongational flow using nonequilibrium molecular dynamics simulations

We present various rheological and structural properties of three polyethylene liquids, C50H102, C78H158, and C128H258, using nonequilibrium molecular dynamics simulations of planar elongational flow. All three melts display tension-thinning behavior of both elongational viscosities, eta1 and eta2. This tension thinning appears to follow the power law with respect to the elongation rate, i.e., eta approximately epsilon(b), where the exponent b is shown to be approximately -0.4 for eta1 and eta2. More specifically, b of eta1 is shown to be slightly larger than that of eta2 and to increase in magnitude with the chain length, while b of eta2 appeared to be independent of the chain length. We also investigated separately the contribution of each mode to the two elongational viscosities. For all three liquids, the intermolecular Lennard-Jones (LJ), intramolecular LJ, and bond-stretching modes make positive contributions to both eta1 and eta2, while the bond-torsional and bond-bending modes make negative contributions to both eta1 and eta2. The contribution of each of the five modes decreases in magnitude with increasing elongation rate. The hydrostatic pressure shows a clear minimum at a certain elongation rate for each liquid, and the elongation rate at which the minimum occurs appears to increase with the chain length. The behavior of the hydrostatic pressure with respect to the elongation rate is shown to correlate with the intermolecular LJ energy from a microscopic viewpoint. On the other hand, R(ete)2 and R(g)2 appear to be correlated with the intramolecular LJ energy. The study of the effect of the elongational field on the conformation tensor c shows that the degree of increase of tr(c)-3 with the elongation rate becomes stronger as the chain length increases. Also, the well-known linear reaction between sigma and c does not seem to be satisfactory. It seems that a simple relation between sigma and c would not be valid, in general, for arbitrary flows.     

Dimethylselenide as a probe for reactions of halogenated alkoxyl radicals in aqueous solution. Degradation of dichloro- and dibromomethane

Using pulse radiolysis and steady-state gamma-radiolysis techniques, it has been established that, in air-saturated aqueous solutions, peroxyl radicals CH 2HalOO (*) (Hal = halogen) derived from CH 2Cl 2 and CH 2Br 2 react with dimethyl selenide (Me 2Se), with k on the order of 7 x 10 (7) M (-1) s (-1), to form HCO 2H, CH 2O, CO 2, and CO as final products. An overall two-electron oxidation process leads directly to dimethyl selenoxide (Me 2SeO), along with oxyl radical CH 2HalO (*). The latter subsequently oxidizes another Me 2Se molecule by a much faster one-electron transfer mechanism, leading to the formation of equal yields of CH 2O and the dimer radical cation (Me 2Se) 2 (*+). In absolute terms, these yields amount to 18% and 28% of the CH 2ClO (*) and CH 2BrO (*) yields, respectively, at 1 mM Me 2Se. In competition, CH 2HalO (*) rearranges into (*)CH(OH)Hal. These C-centered radicals react further via two pathways: (a) Addition of an oxygen molecule leads to the corresponding peroxyl radicals, that is, species prone to decomposition into H (+)/O 2 (*-) and formylhalide, HC(O)Hal, which further degrades mostly to H (+)/Hal (-) and CO. (b) Elimination of HHal yields the formyl radical H-C(*)=O with a rate constant of about 6 x 10 (5) s (-1) for Hal = Cl. In an air-saturated solution, the predominant reaction pathway of the H-C(*)=O radical is addition of oxygen. The formylperoxyl radical HC(O)OO (*) thus formed reacts with Me 2Se via an overall two-electron transfer mechanism, giving additional Me 2SeO and formyloxyl radicals HC(O)O(*). The latter rearrange via a 1,2 H-atom shift into (*)C(O)OH, which reacts with O2 to give CO2 and O2(*)(-). The minor fraction of H-C(*)=O undergoes hydration, with an estimated rate constant of k approximately 2 x 10(5) s(-1). The resulting HC(*)(OH)2 radical, upon reaction with O2, yields HCO 2H and H (+)/O2(*-). Some of the conclusions about the reactions of halogenated alkoxyl radicals are supported by quantum chemical calculations [B3LYP/6-31G(d,p)] taking into account the influence of water as a dielectric continuum [by the self-consistent reaction field polarized continuum model (SCRF=PCM) technique]. Based on detailed product studies, mechanisms are proposed for the free-radical degradation of CH 2Cl 2 and CH 2Br 2 in the presence of oxygen and an electron donor (namely, Me 2Se in this study), and properties of the reactive intermediates are discussed.     

Isotopic variation of experimental lifetimes for the lowest 1 Pi u states of N2

Lifetimes of several (1)Pi(u) states of the three natural isotopomers of molecular nitrogen, (14)N(2), (14)N(15)N, and (15)N(2), are determined via linewidth measurements in the frequency domain. Extreme ultraviolet (XUV)+UV two-photon ionization spectra of the b (1)Pi(u)(v=0-1,5-7) and c(3) (1)Pi(u)(v=0) states of (14)N(2), b (1)Pi(u)(v=0-1,5-6) and c(3) (1)Pi(u)(v=0) states of (14)N(15)N, and b (1)Pi(u)(v=0-7), c(3) (1)Pi(u)(v=0), and o (1)Pi(u)(v=0) states of (15)N(2) are recorded at ultrahigh resolution, using a narrow band tunable XUV-laser source. Lifetimes are derived from the linewidths of single rotationally resolved spectral lines after deconvolution of the instrument function. The observed lifetimes depend on the vibrational quantum number and are found to be strongly isotope dependent.     

Nanoscale colloids in a freely adsorbing polymer solution: a Monte Carlo simulation study

A key issue in nanoscale materials and chemical processing is the need for thermodynamic and kinetic models covering colloid-polymer systems over the mesoscopic length scale (approximately 1-100 nm). We have applied Monte Carlo simulations to attractive nanoscale colloid-polymer mixtures toward developing a molecular basis for models of these complex systems. The expanded ensemble Monte Carlo simulation method is applied to calculate colloid chemical potentials (micro(c)) and polymer adsorption (gamma) in the presence of freely adsorbing Lennard-Jones (LJ) homopolymers (surface modifiers). gamma and micro(c) are studied as a function of nanoparticle diameter (sigma(c)), modifier chain length (n) and concentration, and colloid-polymer attractive strength over 0.3 < Rg/sigma(c) < 6 (Rg is the polymer radius of gyration). In the attractive regime, nanocolloid chemical potential decreases and adsorbed amount increases as sigma(c), or n is increased. The scaling of gamma with n from the simulations agrees with the theory of Aubouy and Raphael (Macromolecules 1998, 31, 4357) in the extreme limits of Rg/sigma(c). When Rg/sigma(c) is large, the "colloid" approaches a molecular size and interacts only locally with a few polymer segments and gamma approximately n. When Rg/sigma(c) is small, the system approaches the conventional colloid-polymer size regime where multiple chains interact with a single particle, and gamma approximately sigma(c)2, independent of n. In contrast, adsorption in the mesoscopic range of Rg/sigma(c) investigated here is represented well by a power law gamma approximately n(p), with 0 < p < 1 depending on concentration and LJ attractive strength. Likewise, the chemical potential from our results is fitted well with micro(c) approximately n(q)sigma(c)3, where the cubic term results from the sigma(c) dependence of particle surface area (approximately sigma(c)2) and LJ attractive magnitude (approximately sigma(c)). The q-exponent for micro(c) (micro(c) approximately n(q)) varies with composition and LJ attractive strength but is always very close to the power exponent for gamma (gamma approximately n(p)). This result leads to the conclusion that in attractive systems, polymer adsorption (and thus polymer-colloid attraction) dominates the micro(c) dependence on n, providing a molecular interpretation of the effect of adsorbed organic layers on nanoparticle stability and self-assembly.     

Infrared vibrational spectroscopy of cis-dichloroethene in Rydberg states

We have measured the infrared (IR) vibrational spectrum for cis-dichloroethene (cis-ClCH[Double Bond]CHCl) in excited Rydberg states with the effective principal quantum numbers n(*)=9, 13, 17, 21, 28, and 55 using the vacuum ultraviolet-IR-photoinduced Rydberg ionization (VUV-IR-PIRI) scheme. Although the IR frequencies observed for the vibrational bands nu(11) (*) (asymmetric C-H stretch) and nu(12) (*) (symmetric C-H stretch) are essentially unchanged for different n(*) states, suggesting that the IR absorption predominantly involves the ion core and that the Rydberg electron behaves as a spectator; the intensity ratio for the nu(11) (*) and nu(12) (*) bands [R(nu(11) (*)nu(12) (*))] is found to decrease smoothly as n(*) is increased. This trend is consistent with the results of a model ab initio quantum calculation of R(nu(11) (*)nu(12) (*)) for excited cis-ClCH[Double Bond]CHCl in n(*)=3-18 states and the MP26-311++G(2df,p) calculations of R(nu(11)nu(12)) and R(nu(11) (+)nu(12) (+)), where R(nu(11)nu(12))[R(nu(11) (+)nu(12) (+))] represents the intensity ratio of the nu(11)(nu(11) (+)) asymmetric C-H stretching to the nu(12)(nu(12) (+)) symmetric C-H stretching vibrational bands for cis-ClCH[Double Bond]CHCl (cis-ClCH[Double Bond]CHCl(+)). We have also measured the IR-VUV-photoion (IR-VUV-PI) and IR-VUV-pulsed field ionization-photoelectron depletion (IR-VUV-PFI-PED) spectra for cis-ClCH[Double Bond]CHCl. These spectra are consistent with ab initio calculations, indicating that the IR absorption cross section for the nu(12) band is negligibly small compared to that for the nu(11) band. While the VUV-IR-PIRI measurements have allowed the determination of nu(11) (+)=3067+/-2 cm(-1), nu(12) (+)=3090+/-2 cm(-1), and R(nu(11) (+)nu(12) (+)) approximately 1.3 for cis-ClCH=CHCl(+), the IR-VUV-PI and IR-VUV-PFI-PED measurements have provided the value nu(11)=3088.5+/-0.2 cm(-1) for cis-ClCH=CHCl.     

An explanation of the unusual strength of the hydrogen bond in small water clusters

We seek to explain why the hydrogen bond possesses unusual strength in small water clusters that account for many of the complex behaviors of water. We have investigated and visualized the donation of covalent character from covalent (sigma) to hydrogen bonds by calculating the eigenvector coupling properties of quantum theory of atoms in molecules (QTAIM), stress tensor σ ( r ), and Ehrenfest Force F ( r ) on the F ( r ) molecular graph. The next-generation three-dimensional (3-D) bond-path framework sets are presented, and only the F ( r ) bond-path framework sets reproduce the earlier finding on the coupling between covalent (sigma) and hydrogen bonds that possess a degree of covalent character. Exploration of the bond-path between the covalent (sigma) and hydrogen bond's critical points provides an explanation for the previously obtained coupling results. The directional character of the covalent (sigma) and hydrogen bonds' 3-D bond-path framework sets for the F ( r ) explains differences found in the earlier results from QTAIM and the stress tensor σ ( r ).     

H atom transfer along an ammonia chain: tunneling and mode selectivity in 7-hydroxyquinoline.(NH3)3

Excitation of the 7-hydroxyquinoline(NH(3))(3) [7HQ(NH(3))(3)] cluster to the S(1) (1)pi pi(*) state results in an O-H-->NH(3) hydrogen atom transfer (HAT) reaction. In order to investigate the entrance channel, the vibronic S(1)<-->S(0) spectra of the 7HQ.(NH(3))(3) and the d(2)-7DQ.(ND(3))(3) clusters have been studied by resonant two-photon ionization, UV-UV depletion and fluorescence techniques, and by ab initio calculations for the ground and excited states. For both isotopomers, the low-frequency part of the S(1)<--S(0) spectra is dominated by ammonia-wire deformation and stretching vibrations. Excitation of overtones or combinations of these modes above a threshold of 200-250 cm(-1) for 7HQ.(NH(3))(3) accelerates the HAT reaction by an order of magnitude or more. The d(2)-7DQ.(ND(3))(3) cluster exhibits a more gradual threshold from 300 to 650 cm(-1). For both isotopomers, intermolecular vibrational states above the threshold exhibit faster HAT rates than the intramolecular vibrations. The reactivity, isotope effects, and mode selectivity are interpreted in terms of H atom tunneling through a barrier along the O-H-->NH(3) coordinate. The barrier results from a conical intersection of the optically excited (1)pi pi(*) state with an optically dark (1)pi sigma(*) state. Excitation of the ammonia-wire stretching modes decreases both the quinoline-O-H...NH(3) distance and the energetic separation between the (1)pi pi(*) and (1)pi sigma(*) states, thereby increasing the H atom tunneling rate. The intramolecular vibrations change the H bond distance and modulate the (1)pi pi(*)<-->(1)pi sigma(*) interaction to a much smaller extent.     

Continuum and discrete pulsed cavity ring down laser absorption spectra of Br2 vapor

The absorption cross-sections at room temperature are reported for the first time, of Br2 vapor in overlapping bound-free and bound-bound transition of A(3)pi1u <-- Xsigma(g)+, X(1)pi1u <-- X(1)sigma(g)+ and B(3)pi0u <-- X(1)sigma(g)+, using cavity ring down spectroscopy (CRDS) technique. We reported here, the A(3)pi1u <-- X(1)sigma(g)+, transition is included along with the two stronger X(1)pi1u <-- X(1)sigma(g)+ and B(3)pi0u <-- X(1)sigma(g) transitions of Br2. We obtained discrete absorption cross-section in the rotational structure, the continuum absorption cross-sections, and were also able to measure the absorption cross-section in separate contribution of A(3)pi1u <-- X(1)sigma(g)+, (1)pi1u <-- X(1)sigma(g)+, and B(3)pi0u <-- X(1)sigma(g)+ transitions using CRDS method to use quantum yield of Br*((2)P(1/2)). We obtained absorption cross-section order 10(-19) cm2 and detection 10(13) molecule cm(-3) (1 mTorr) of Br2. The absorption cross-sections are increasing with increasing excitation energy in the wavelength region 510-535 nm.     

Topological analysis of the electron density distribution in perturbed systems. I. Effect of charge on the bond properties of hydrogen fluoride

Within the framework of the molecular orbital (MO) theory, the addition of one electron to the 4sigma antibonding orbital of the neutral (F...H) system or the removal of one electron from its pi nonbonding orbitals, leading to (F...H)- and to (F...H)+, has permitted the investigation of these charge perturbations on the bond properties of the hydrogen fluoride molecule by using the topological analysis of rho(r). For (F...H), (F...H)-, and (F...H)+, the topological and energetic properties calculated at the F...H bond critical point (BCP) have been related to the 3sigma bonding molecular orbital (BMO) distribution, as this orbital is the main contributor to rho(r) at the interatomic surface. The analysis has been carried out at several F...H internuclear distances, ranging from 0.8 to 3.0 A. As far as the BMO distribution results from its interaction with the average Coulomb and exchange potential generated by the charge filling the other MOs, and in particular by the pi and 4sigma electrons, the comparison between the BCP properties calculated for the charged systems and those corresponding to the neutral one permits the interpretation of the differences in terms of the charge perturbation on BMO. Along with the BCP properties of (F...H), (F...H)-, and (F...H)+, the interaction energy magnitudes of these systems have been also calculated within the same range of internuclear distances, indicating that the applied perturbations do not break the F-H bond but soften it, giving rise to the stable species (F-H)- and (F-H)+. Comparing the three systems at their equilibrium geometries, the most stable configuration, which corresponds to the unperturbed (F...H) system, shows the highest quantity and the most locally concentrated charge density distribution, along with the largest total electron energy density magnitude, at the interatomic surface as a consequence of the BMO contraction toward the fluorine nucleus in (F...H)+ and of the BMO expansion toward both nuclei in (F...H)-. On the other hand, if the comparison is carried out at the equilibrium distance of (F...H) (d(eq)0), this one exhibits both the smallest total energy density magnitude and the largest quantity of bonding charge at the interatomic surface. Hence, being the signature of the most stable configuration, the characteristic magnitudes of the neutral system rho(d(eq)0), inverted triangle2 rho(d(eq)0), and H(d(eq)0) appear as boundary conditions at the interatomic surface of its unperturbed and relaxed electron distribution.     

O+HCl→OH+Cl反应的立体动力学的准经典轨线研究

运用准经典轨线方法, 基于Peterson从头计算势能面对O+HCl→OH+Cl反应的立体动力学性质进行了研究. 讨论了在31.77和51.04 kJ/mol两种碰撞能情况下极化依赖的微分反应截面(2π/σ)(dσ₀₀/dω_t), (2π/σ)(dσ₂₀/dω_t), (2π/σ)(dσ₂₂₊/dω_t)和(2π/σ)(dσ_21-/dω_t)以及描述k-j′两矢量相关和k-k′-j′三矢量相关的分布函数P(θ_r)和P(φ_r). 计算得到的P(θ_r)分布表明, 产物分子的转动角动量j′具有强烈的取向分布, 并且产物转动角动量的取向效应对散射能的变化比较敏感. 而P(φ_r)的分布表明, 产物分子虽然具有沿着y轴的取向效应, 但是没有明显的定向效应.     

Resonant two-photon ionization spectroscopy of BNB

Triatomic BNB has been produced by laser ablation of a boron nitride rod in a supersonic expansion of helium carrier gas and has been investigated using resonant two-photon ionization spectroscopy in the visible region. The B 2Pi(g)-X 2Sigma(u)+ band system has been recorded near 514 nm and is dominated by a strong origin band, which has been rotationally resolved and analyzed. Both the (11)B(14)N(11)B (64% natural abundance) and the (10)B(14)N(11)B (32% natural abundance) isotopic modifications have been analyzed, leading to the spectroscopic constants (and their 1sigma error limits) of B0"(X 2Sigma(u)+)=0.466 147(70), B0'(B 2Pi(g))=0.467 255(75), and A0'(B 2Pi(g))=6.1563(38) cm(-1) for (10)B(14)N(11)B, corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 47(10) A and r(B-N)'(B 2Pi(g))=1.310 92(11) A. Very similar values are obtained for the more abundant isotopomer, (11)B(14)N(11)B: B0"(X 2Sigma(u)+)=0.444 493(69), B0'(B 2Pi(g))=0.445 606(70), A0'(B 2Pi(g))=6.1455(38) cm(-1), corresponding to r(B-N)"(X 2Sigma(u)+)=1.312 41(10) A and r(B-N)'(B 2Pi(g))=1.310 77(10) A. These results are discussed as they relate to Walsh's rules and are compared to results for related molecules.     

The role of pi sigma* state in intramolecular electron-transfer dynamics of 4-dimethylaminobenzonitrile and related molecules

Evidence is presented which indicates that the photoinduced intramolecular charge transfer (ICT) in 4-dimethylaminobenzonitrile proceeds by a new mechanism in which pi sigma(C triple bond N) (*) state is the intermediate of a consecutive process that takes the initially excited pi pi(*) state to the fully charge-separated ICT state. The absence of the ICT-state formation in 4-aminobenzonitrile is attributed to the smaller electron-donor strength of the amino group relative to the dimethylamino group, which hinders the pi sigma(*)-->ICT charge-shift reaction.