High-resolution infrared spectroscopy in the 1,200-1,300 cm(-1) region and accurate theoretical estimates for the structure and ring-puckering barrier of perfluorocyclobutane

We present experimental infrared spectra and theoretical electronic structure results for the geometry, anharmonic vibrational frequencies, and accurate estimates of the magnitude and the origin of the ring-puckering barrier in C4F8. High-resolution (0.0015 cm-1) spectra of the nu12 and nu13 parallel bands of perfluorocyclobutane (c-C4F8) were recorded for the first time by expanding a 10% c-C4F8 in helium mixture in a supersonic jet. Both bands are observed to be rotationally resolved in a jet with a rotational temperature of 15 K. The nu12 mode has b2 symmetry under D2d that correlates to a2u symmetry under D4h and consequently has +/- <-- +/- ring-puckering selection rules. A rigid rotor fit of the nu12 band yields the origin at 1292.56031(2) cm-1 with B' = 0.0354137(3) cm-1 and B' ' = 0.0354363(3) cm-1. The nu13 mode is of b2 symmetry under D2d that correlates to b2g under D4h, and in this case, the ring-puckering selection rules are +/- <-- -/+ . Rotational transitions from the ground and first excited torsional states will be separated by the torsional splitting in the ground and excited vibrational states, and indeed, we observe a splitting of each transition into strong and weak intensity components with a separation of approximately 0.0018 cm-1. The strong and weak sets of transitions were fit separately again using a rigid rotor model to give nu13(strong) = 1240.34858(4) cm-1, B' = 0.0354192(7) cm-1, and B' ' = 0.0354355(7) cm-1 and nu13(weak) = 1240.34674(5) cm-1, B' = 0.0354188(9) cm-1, and B' ' = 0.0354360(7) cm-1. High-level electronic structure calculations at the MP2 and CCSD(T) levels of theory with the family of correlation consistent basis sets of quadruple-zeta quality, developed by Dunning and co-workers, yield best estimates for the vibrationally averaged structural parameters r(C-C) = 1.568 A, r(C-F)alpha = 1.340 A, r(C-F)beta = 1.329 A, alpha(F-C-F) = 110.3 degrees , thetaz(C-C-C) = 89.1 degrees , and delta(C-C-C-C) = 14.6 degrees and rotational constants of A = B = 0.03543 cm-1 and C = 0.02898 cm-1, the latter within 0.00002 cm-1 from the experimentally determined values. Anharmonic vibrational frequencies computed using higher energy derivatives at the MP2 level of theory are all within <27 cm-1 (in most cases <5 cm-1) from the experimentally measured fundamentals. Our best estimate for the ring-puckering barrier at the CCSD(T)/CBS (complete basis set) limit is 132 cm-1. Analysis of the C4F8 electron density suggests that the puckering barrier arises principally from the sigmaCC-->sigmaCF hyperconjugative interactions that are more strongly stabilizing in the puckered than in the planar form. These interactions are, however, somewhat weaker in C4F8 than in C4H8, a fact that is consistent with the smaller barrier in the former (132 cm-1) with respect to the latter (498 cm-1).     

Effect of interaction shape on the condensed toroid of the semiflexible chain

We investigate how different microscopic interactions between semiflexible chain segments can qualitatively alter the physical properties of the condensed toroid. We propose a general form of the Hamiltonian of the toroid and discuss its analytic properties. For different interactions, the theory predicts different scaling behaviors of the mean toroidal and cross sectional radii, rc and rcross, as functions of the contour length L: (rc,rcross) approximately Lnu(Nc), with nu=(1/5,2/5) for the van der Waals-type, nu=(-1/3,2/3) for the Coulomb-type, and nu=(-1,1) for the delta-function-type attractions in the asymptotic limit. For the toroids with finite winding number Nc=100-400, we find nu approximately 0 for the Yukawa interaction with screening parameter kappa=0.5-1.0 and nu=0.1-0.13 for the van der Waals-type interactions. These findings could provide a possible explanation for the experimentally well known observation nu approximately 0 of the condensed DNA toroids. Conformational transitions are also discussed.     

Scaling and the Smoluchowski equations

The Smoluchowski equations, which describe coalescence growth, take into account combination reactions between a j-mer and a k-mer to form a (j+k)-mer, but not breakup of larger clusters to smaller ones. All combination reactions are assumed to be second order, with rate constants K(jk). The K(jk) are said to scale if K(lambda j,gamma k) = lambda(mu)gamma(nu)K(jk) for j < or = k. It can then be shown that, for large k, the number density or population of k-mers is given by Ak(a)e(-bk), where A is a normalization constant (a function of a, b, and time), a = -(mu+nu), and b(mu+nu-1) depends linearly on time. We prove this in a simple, transparent manner. We also discuss the origin of odd-even population oscillations for small k. A common scaling arises from the ballistic model, which assumes that the velocity of a k-mer is proportional to 1/square root of m(k) (Maxwell distribution), i.e., thermal equilibrium. This does not hold for the nascent distribution of clusters produced from monomers by reactive collisions. By direct calculation, invoking conservation of momentum in collisions, we show that, for this distribution, velocities are proportional to m(k)(-0.577). This leads to mu+nu = 0.090, intermediate between the ballistic (0.167) and diffusive (0.000) results. These results are discussed in light of the existence of systems in the experimental literature which apparently correspond to very negative values of mu+nu.     

Further insights into the spectroscopic properties, electronic structure, and kinetics of formation of the heme-peroxo-copper complex [(F8TPP)FeIII-(O2(2-)-CuII(TMPA)]+

In the further development and understanding of heme-copper O2-reduction chemistry inspired by the active-site chemistry in cytochrome c oxidase, we describe a dioxygen adduct, [(F8TPP)FeIII-(O22-)-CuII(TMPA)](ClO4) (3), formed by addition of O2 to a 1:1 mixture of the porphyrinate-iron(II) complex (F8TPP)FeII (1a) {F8TPP = tetrakis(2,6-difluorophenyl)porphyrinate dianion} and the copper(I) complex [(TMPA)CuI(MeCN)](ClO4) (1b) {TMPA = tris(2-pyridylmethyl)amine}. Complex 3 forms in preference to heme-only or copper-only binuclear products, is remarkably stable {t1/2 (RT; MeCN) approximately 20 min; lambda max = 412 (Soret), 558 nm; EPR silent}, and is formulated as a peroxo complex on the basis of manometry {1a/1b/O2 = 1:1:1}, MALDI-TOF mass spectrometry {16O2, m/z 1239 [(3 + MeCN)+]; 18O2, m/z 1243}, and resonance Raman spectroscopy {nu(O-O) = 808 cm-1; Delta16O2/18O2 = 46 cm-1; Delta16O2/16/18O2 = 23 cm-1}. Consistent with a mu-eta2:eta1 bridging peroxide ligand, two metal-O stretching frequencies are observed {nu(Fe-O) = 533 cm-1, nu(Fe-O-Cu) = 511 cm-1}, and supporting normal coordinate analysis is presented. 2H and 19F NMR spectroscopies reveal that 3 is high-spin {also muB = 5.1 +/- 0.2, Evans method} with downfield-shifted pyrrole and upfield-shifted TMPA resonances, similar to the pattern observed for the structurally characterized mu-oxo complex [(F8TPP)FeIII-O-CuII(TMPA)]+ (4) (known S = 2 system, antiferromagnetically coupled high-spin FeIII and CuII). M?ssbauer spectroscopy exhibits a sharp quadrupole doublet (zero field; delta = 0.57 mm/s, |DeltaEQ| = 1.14 mm/s) for 3, with isomer shift and magnetic field dependence data indicative of a peroxide ligand and S = 2 formulation. Both UV-visible-monitored stopped-flow kinetics and M?ssbauer spectroscopic studies reveal the formation of heme-only superoxide complex (S)(F8TPP)FeIII-(O2-) (2a) (S = solvent molecule) prior to 3. Thermal decomposition of mu-peroxo complex 3 yields mu-oxo complex 4 with concomitant release of approximately 0.5 mol O2 per mol 3. Characterization of the reaction 1a/1b + O2 --> 2 --> 3 --> 4, presented here, advances our understanding and provides new insights to heme/Cu dioxygen-binding and reduction.     

Reversal phenomenon of reaction velocity in a mixed reaction system: silylations between simple alcohols and alpha- or beta-hydroxyketones using anilinosilanes and catalytic TBAF agent

We encountered a unique phenomenon of reaction velocity reversal during the silylation between mixed simple alcohols (A; AOH) and alpha- or beta-hydroxyketones (B; BOH). The present reaction system using anilinosilanes 1a [PhNH(TMS)] (TMS-A) and 1b [PhNH(TES)] (TES-A) with the catalytic TBAF (0.02 equiv) reagent showed that the order of silylation-velocity between the independent system and the mixed system was distinctively reversed [nu(A) > nu(B) approximately nu(B(mix)) > nu(A(mix))]. A total of seven examples indicate its high generality: AOH = linalool, 3-octanol; BOH = valeroin, benzoin, 5-hydroxy-4-methyl-3-undecanone. Time-dependent IR spectroscopy analysis rationally supported this speculation.     

Nickel dithiolenes revisited: structures and electron distribution from density functional theory for the three-member electron-transfer series [Ni(S2C2Me2)2]0,1-,2-

The complexes [Ni(S2C2Me2)2](z) (z = 0, 1-, 2-) have been isolated for the purpose of investigating their electronic structures in a reversible three-member electron-transfer series. Members are interrelated by reversible redox reactions with E(1/2)(0/1-) = -0.15 V and E(1/2)(1-/2-) = -1.05 V versus SCE in acetonitrile. The three complexes have nearly planar structures of idealized D(2)(h) symmetry. As the series is traversed in the reducing direction, Ni-S and C-S bond lengths increase; the chelate ring C-C bond length decreases from the neutral complex to the monoanion and does not change significantly in the dianion. Structural trends are compared with previous results for [Ni(S2C2R2)2)](1-,2-). Following the geometrical changes, values of nu(Ni)(-)(S) and nu(C)(-)(S) decrease, while the value of nu(C)(-)(C) increases with increased reduction. Geometry optimizations at the density functional theory (DFT) level were performed for all members of the series. Geometrical parameters obtained from the calculations are in good agreement with the experimental findings. The 5b(2g) orbital was identified as the LUMO in [Ni(S2C2Me2)2], the SOMO in [Ni(S2C2Me2)2](1-), and the HOMO in [Ni(S2C2Me2)2]2-. Unlike in the situation in the [M(CO)2-(S2C2Me2)2]z series (M = Mo, W; z = 0, 1-, 2-), the apparent contribution from the metal d orbital in the electroactive orbital is not constant. In the present series, the d(xz) contribution increases from 13 to 20 to 39% upon passing from the neutral to the monoanionic to the dianionic complex. Accurate calculation of EPR g-values of [Ni(S2C2Me2)2]1- by DFT serves as a test for the reliability of the electronic structure calculations.     

IR/UV double resonant spectroscopy of the methyl radical: determination of nu3 in the 3p(z) Rydberg state

IR+UV double resonant ion-dip and ion-enhancement spectroscopies are employed to study the nu3 asymmetric CH stretch vibration fundamental of CH3 in the ground and 3p(z) Rydberg electronic states. CH3 radical is synthesized in the supersonic jet expansion by flash pyrolysis of azomethane (CH3NNCH3) prior to the expansion. The Q band of the 3(1) (1) 3p(z)<--X transition of CH3, not detected by conventional UV resonantly enhanced multiphoton ionization (REMPI) spectroscopy, is determined to lie at 59,898 cm(-1) using IR+UV REMPI spectroscopy. Energy of the asymmetric CH stretch of CH3 in the 3p(z) Rydberg state, nu3(3p(z)), is 3087 cm(-1), redshifted by approximately 74 cm(-1) with respect to ground state nu3(X).     

Polymers confined between two parallel plane walls

Single three-dimensional polymers confined to a slab, i.e., to the region between two parallel plane walls, are studied by Monte Carlo simulations. They are described by N-step walks on a simple cubic lattice confined to the region 1< or = z < or = D. The simulations cover both regions DRF (where RF approximately Nnu is the Flory radius, with nu approximately 0.587), as well as the cross-over region in between. Chain lengths are up to N=80 000, slab widths up to D=120. In order to test the analysis program and to check for finite size corrections, we actually studied three different models: (a) ordinary random walks (mimicking Theta polymers); (b) self-avoiding walks; and (c) Domb-Joyce walks with the self-repulsion tuned to the point where finite size corrections for free (unrestricted) chains are minimal. For the simulations we employ the pruned-enriched-Rosenbluth method with Markovian anticipation. In addition to the partition sum (which gives us a direct estimate of the forces exerted onto the walls), we measure the density profiles of monomers and of end points transverse to the slab, and the radial extent of the chain parallel to the walls. All scaling laws and some of the universal amplitude ratios are compared to theoretical predictions.     

Thermochemistry of N-N containing ions: a threshold photoelectron photoion coincidence spectroscopy study of ionized methyl- and tetramethylhydrazine

The unimolecular dissociation reactions of the methylhydrazine (MH) and tetramethylhydrazine (TMH) radical cations have been investigated using tandem mass spectrometry and threshold photoelectron photoion coincidence spectroscopy in the photon energy ranges 9.60-31.95 eV (for the MH ion) and 7.74-29.94 eV (for the TMH ion). Methylhydrazine ions (CH3NHNH2(+*)) have three low-energy dissociation channels: hydrogen atom loss to form CH2NHNH2(+) (m/z 45), loss of a methyl radical to form NHNH2(+) (m/z 31), and loss of methane to form the fragment ion m/z 30, N2H2(+*). Tetramethylhydrazine ions only exhibit two dissociation reactions near threshold: that of methyl radical loss to form (CH3)2NNCH3(+) (m/z 73) and of methane loss to form the fragment ion m/z 72 with the empirical formula C3H8N2(+*). The experimental breakdown curves were modeled with Rice-Ramsperger-Kassel-Marcus theory, and it was found that, particularly for methyl radical loss, variational transition state theory was needed to obtain satisfactory fits to the data. The 0 K enthalpies of formation (delta(f)H0) for all fragment ions (m/z 73, m/z 72, m/z 45, m/z 31, and m/z 30) have been determined from the 0 K activation energies (E0) obtained from the fitting procedure: delta(f)H0[(CH3)2NNCH3(+)] = 833 +/- 5 kJ mol(-1), delta(f)H0 [C3H8N2(+*)] = 1064 +/- 5 kJ mol(-1), delta(f)H0[CH2NHNH2(+)] = 862 +/- 5 kJ mol(-1), delta(f)H0[NHNH2(+)] = 959 +/- 5 kJ mol(-1), and delta(f)H0[N2H2(+*)] = 1155 +/- 5 kJ mol(-1). The breakdown curves have been measured from threshold up to h nu approximately 32 eV for both hydrazine ions. As the photon energy increases, other dissociation products are observed and their appearance energies are reported.     

Charge-sensitive vibrations in p-chloranil: the strange case of the C=C antisymmetric stretching

We have combined DFT calculations with single-crystal polarized infrared spectra to reinvestigate the assignment of the C=C antisymmetric stretching mode b(2u)nu(18) of p-chloranil (CA). The frequency of this mode indeed seems to display a nonlinear dependence on the average charge on the CA molecule (rho), at variance with the behavior of the antisymmetric C=O stretching frequency. The DFT calculations show that the origin of the problem is a drastic, 2 orders of magnitude decrease of the infrared intensity of the C=C antisymmetric stretching upon electron addition. Therefore, no infrared band can be easily associated to this mode in charge-transfer (CT) solids with rho approximately > 0.5. On the other hand, a linear relationship between rho and the b(2u)nu(18) frequency is found in quasi-neutral CT complexes of CA.     

Cyclam kappa4 to kappa3 ligand denticity change upon mono-n-substitution with a carboxypropyl pendant arm in a ruthenium nitrosyl complex

The complex fac-[Ru(NO)Cl2(kappa(3)N(4),N(8),N(11)(1-carboxypropyl)cyclam)]Cl.H2O (1-carboxypropyl)cyclam=3-(1,4,8,11-tetraazacyclotetradecan-1-yl)propionic acid) was prepared in a one pot reaction by mixing equimolar amounts of RuNOCl 3 and (1-carboxypropyl)cyclam and was characterized by X-ray crystallography, electrospray ionization tandem mass spectrometry (ESI-MS/MS), elemental analysis, NMR, and electronic and vibrational (IR) spectroscopies. fac-[Ru(NO)Cl 2(kappa(3)N(4),N(8),N(11)(1-carboxypropyl)cyclam)]Cl.H2O crystallizes in the triclinic, space group P1, No. 2, with unit cell parameters of a=8.501(1) A, b=9.157(1) A, c=14.200(1) A, alpha=72.564(5) degrees , beta=82.512(5) degrees , gamma=80.308(5) degrees , and Z=2. The Ru-N interatomic distance and bond angle in the [Ru-NO] unit are 1.739(2) A and 167.7(2) degrees , respectively. ESI-MS/MS shows characteristic dissociation chemistry that initiates by HCl or NO loss. The IR spectrum displays a nu(NO) at 1881 cm(-1) indicating a nitrosonium character. The electronic spectrum shows absorptions bands at 264 nm (log epsilon=3.27), 404 nm (log epsilon=2.53), and 532 nm (log epsilon=1.88). (1)H and (13)C NMR are in agreement with the proposed molecular structure, which shows a very singular architecture where the cyclam ring N (with the carboxypropyl pendant arm) is not coordinated to the ruthenium resulting in a kappa(3) instead of the expected kappa(4) denticity.     

Inelastic neutron scattering study of electron reduction in Mn12 derivatives

We report inelastic neutron scattering (INS) studies on a series of Mn(12) derivatives, [Mn(12)O(12)(O2CC6F5)16(H2O)4]z, in which the number of unpaired electrons in the cluster is varied. We investigated three oxidation levels: z = 0 for the neutral complex, z = -1 for the one-electron reduced species and z = -2 for the two-electron reduced complex. For z = 0, the ground state is S = 10 as in the prototypical Mn12-acetate. For z = -1, we have S = 19/2, and for z = - 2, an S = 10 ground state is retrieved. INS studies show that the axial zero-field splitting parameter D is strongly suppressed upon successive electron reduction: D = -0.45 cm(-1) (z = 0), D = -0.35 cm(-1) (z = -1), and D approximately -0.26 cm(-1) (z = -2). Each electron reduction step is directly correlated to the conversion of one anisotropic (Jahn-Teller distorted) Mn3+ (S = 2) to one nearly isotropic Mn2+ (S = 5/2).     

Rovibrationally selected and resolved state-to-state photoionization of ethylene using the infrared-vacuum ultraviolet pulsed field ionization-photoelectron method

By preparing ethylene [C2H4(X1Ag)] in selected rotational levels of the nu11(b1u), nu2+nu12(b1u), or nu9(b2u) vibrational state with infrared (IR) laser photoexcitation prior to vacuum ultraviolet (VUV) laser photoionization, we have recorded rotationally resolved pulsed field ionization-photoelectron (PFI-PE) spectra for C2H4+(X2B3u) in the energy region of 0-3000 cm(-1) above the ionization energy (IE) of C2H4(X1Ag). Here, nu2(ag), nu9(b2u), nu11(b1u), and nu12(b1u) represent the C-C stretching, CH2 stretching, CH2 stretching, and CH2 bending modes of C2H4(X1Ag), respectively. The fully rovibrationally resolved spectra have allowed unambiguous symmetry assignments of the observed vibrational bands, which in turn have provided valuable information on the photoionization dynamics of C2H4. The IR-VUV photoionization of C2H4(X1Ag) via the nu11(b1u) or nu2+nu12(b1u) vibrational states is found to predominantly produce vibrational states of C2H4+(X2B3u) with b1u symmetry, which cannot be observed in single-photon VUV-PFI-PE measurements of C2H4(X1Ag). The analysis of the observed IR-VUV-PFI-PE bands has provided the IE(C2H4) = 84,790.2(2) cm(-1) and accurate vibrational frequencies for the nu4+(au)[84.1(2) cm(-1)], nu12+(b1u)[1411.7(2) cm(-1)], nu4+ +nu12+(b1g)[1482.5(2) cm(-1)], nu2+(ag)[1488.3(2) cm(-1)], nu2+ + nu4+(au)[1559.2(2) cm(-1)], 2nu4+ + nu12 +(b1u)[1848.5(2) cm(-1)], 4nu4+ + nu12 +(b1u)[2558.8(2) cm(-1)], nu2+ + nu12 +(b1u)[2872.7(2) cm(-1)], and nu11+(b1u)[2978.7(2) cm(-1)] vibrational states of C2H4+(X2B3u), where nu4+ is the ion torsional state. The IE(C2H4) and the nu4+(au), nu2+(ag), and nu2+ + nu4+ (au) frequencies are in excellent accord with those obtained in previous single-photon VUV-PFI-PE measurements. The other ion vibrational frequencies represent new experimental determinations. We have also performed high-level ab initio anharmonic vibrational frequency calculations for C2H4(X1Ag) and C2H4+(X2B3u) at the CCSD(T)/aug-cc-pVQZ level for guidance in the assignment of the IR-VUV-PFI-PE spectra. All theoretical vibrational frequencies for the neutral and ion, except the ion torsional frequency, are found to agree with experimental vibrational frequencies to better than 1%.     

On the derivative of the associated Legendre function of the first kind of integer order with respect to its degree (with applications to the construction of the associated Legendre function of the second kind of integer degree and order)

In our recent works (R. Szmytkowski, J. Phys. A 39:15147, 2006; corrigendum: 40:7819, 2007; addendum: 40:14887, 2007), we have investigated the derivative of the Legendre function of the first kind, P _ν(z), with respect to its degree ν. In the present work, we extend these studies and construct several representations of the derivative of the associated Legendre function of the first kind, \({P_{\nu}^{\pm m}(z)}\), with respect to the degree ν, for \({m \in \mathbb{N}}\). At first, we establish several contour-integral representations of \({\partial P_{\nu}^{\pm m}(z)/\partial\nu}\). They are then used to derive Rodrigues-type formulas for \({[\partial P_{\nu}^{\pm m}(z)/\partial\nu]_{\nu=n}}\) with \({n \in \mathbb{N}}\). Next, some closed-form expressions for \({[\partial P_{\nu}^{\pm m}(z)/\partial\nu]_{\nu=n}}\) are obtained. These results are applied to find several representations, both explicit and of the Rodrigues type, for the associated Legendre function of the second kind of integer degree and order, \({Q_{n}^{\pm m}(z)}\); the explicit representations are suitable for use for numerical purposes in various regions of the complex z-plane. Finally, the derivatives \({[\partial^{2}P_{\nu}^{m}(z)/\partial\nu^{2}]_{\nu=n}, [\partial Q_{\nu}^{m}(z)/\partial\nu]_{\nu=n}}\) and \({[\partial Q_{\nu}^{m}(z)/\partial\nu]_{\nu=-n-1}}\), all with m > n, are evaluated in terms of \({[\partial P_{\nu}^{-m}(\pm z)/\partial\nu]_{\nu=n}}\). The present paper is a complementary to a recent one (R. Szmytkowski, J. Math. Chem 46:231, 2009), in which the derivative \({\partial P_{n}^{\mu}(z)/\partial\mu}\) has been investigated.     

Rydberg spectra of trans-1,2-dibromoethylene

(2+1) resonance-enhanced multiphoton ionization spectra of jet-cooled trans-1,2-dibromoethylene are reported for the first time. The two-photon spectral region between 149.7 and 141.2 nm was examined. A 4p(z)<--pi Rydberg transition between 66,800 and 68,000 cm(-1) with A(g) excited state symmetry was analyzed, as well as two 4f<--pi Rydberg transitions with B(g) excited state symmetry and one 4f<--pi Rydberg transition with A(g) excited state symmetry between 68,000 and 70,800 cm(-1). All Rydberg transitions observed in this work belong to series that converge to the first ionization potential of the molecule. The short vibrational progressions observed involve two totally symmetric in-plane normal modes: C=C-H bending (nu(3)) and C=C-Br bending (nu(5)) with average excited state frequencies of 829 and 226 cm(-1), respectively.     

High resolution photofragment translational spectroscopy studies of the near ultraviolet photolysis of phenol

The fragmentation dynamics of gas phase phenol molecules following excitation at many wavelengths in the range 279.145 > or = lambdaphot > or = 206.00 nm have been investigated by H Rydberg atom photofragment translational spectroscopy. Many of the total kinetic energy release (TKER) spectra so derived show structure, the analysis of which confirms the importance of O-H bond fission and reveals that the resulting phenoxyl cofragments are formed in a very limited subset of their available vibrational state density. Spectra recorded at lambdaphot > or = 248 nm show a feature centered at TKER approximately 6500 cm(-1). These H atom fragments, which show no recoil anisotropy, are rationalized in terms of initial S1<--S0 (pi*<--pi) excitation, and subsequent dissociation via two successive radiationless transitions: internal conversion to ground (S0) state levels carrying sufficient O-H stretch vibrational energy to allow efficient transfer towards, and passage around, the conical intersection (CI) between the S0 and S2(1pisigma*) potential energy surfaces (PESs) at larger R(O-H), en route to ground state phenoxyl products. The observed phenoxyl product vibrations indicate that parent modes nu16a and nu11 can both promote nonadiabatic coupling in the vicinity of the S0S2 CI. Spectra recorded at lambdaphot < or = 248 nm reveal a faster, anisotropic distribution of recoiling H atoms, centered at TKER approximately 12,000 cm(-1). These we attribute to H+phenoxyl products formed by direct coupling between the optically excited S1(1pi pi*) and repulsive S2(1pi sigma*) PESs. Parent mode nu16b is identified as the dominant coupling mode at the S1/S2 CI, and the resulting phenoxyl radical cofragments display a long progression in nu18b, the C-O in-plane wagging mode. Analysis of all structured TKER spectra yields D0(H-OC6H5) = 30,015 +/- 40 cm(-1). The present findings serve to emphasize two points of wider relevance in contemporary organic photochemistry: (i) The importance of 1) pi sigma* states in the fragmentation of gas phase heteroaromatic hydride molecules, even in cases where the 1pi sigma* state is optically dark. (ii) The probability of observing strikingly mode-specific product formation, even in "indirect" predissociations, if the fragmentation is driven by ultrafast nonadiabatic couplings via CIs between excited (and ground) state PESs.     

Structure, dynamics, and phase transitions of tethered membranes: a Monte Carlo simulation study

A coarse-grained model of a self-avoiding tethered membrane with hexagonal coordination, embedded in three-dimensional space, is studied by means of extensive Monte Carlo computer simulations. The simulations are performed at various temperatures for membranes with linear size 5< or =L< or =50. We find that the membrane undergoes several folding transitions from a high-temperature flat phase to multiple-folded structure as the temperature is steadily decreased. Using a suitable order parameter and finite size scaling analysis, these phase transitions are shown to be of first order. The equilibrium shape of the membranes is analyzed by calculating the eigenvalues lambda(max) (2)> or =lambda(med) (2)> or =lambda(min) (2) of the inertia tensor. We present a systematic finite size scaling analysis of the radius of gyration and the eigenvalues of the inertia tensor at different phases of the observed folding transitions. In the high-temperature flat phase, the radius of gyration R(g) grows with the linear size of the membrane L as R(g) proportional to L(nu), where the exponent nu is approximately equal to 1.0. The eigenvalues of the inertia tensor scale as lambda(max) proportional to lambda(med) proportional to L(nu) and lambda(min) proportional to L(nu(min) ), whereby the roughness exponent nu(min) is approximately equal to 0.7. We also find that the time tau(R) of a self-avoiding membrane to diffuse a distance R(g) scales as tau(R) proportional to L(2nu+2), which is in good agreement with the theoretical predictions.     

Examination of the charge-sensitive vibrational modes in bis(ethylenedithio)tetrathiafulvalene

We reinvestigated the two C=C stretching modes of the five-membered rings of ET (ET = bis(ethylenedithio)tetrathiafulvalene), namely, nu(2) (in-phase mode) and nu(27) (out-of-phase mode). The frequency of the nu(27) mode of ET(+) was corrected to be approximately 1400 cm(-1), which was identified from the polarized infrared reflectance spectra of (ET)(ClO(4)), (ET)(AuBr(2)Cl(2)), and the deuterium- or (13)C-substituted compounds of (ET)(AuBr(2)Cl(2)). It was clarified from DFT calculations that the frequency of the nu(27) mode of the flat ET(0) molecule was significantly different from that of the boat-shaped ET(0) molecule. We obtained the linear relationship between the frequency and the charge on the molecule, rho, for the flat ET molecule, which was shown to be nu(27)(rho) = 1398 + 140(1 - rho) cm(-1). The frequency shift due to oxidation is remarkably larger than that reported in previous studies. The fractional charges of several ET salts in a charge-ordered state can be successfully estimated by applying this relationship. Therefore, the nu(27) mode is an efficient probe to detect rho in the charge-transfer salts of ET. Similarly, a linear relationship for the nu(2) mode was obtained as nu(2)(rho) = 1447 + 120(1 - rho). This relationship was successfully applied to the charge-poor molecule of theta-type ET salts in the charge-ordered state but could not be applied to the charge-rich molecule. This discrepancy was semiquantitatively explained by the hybridization between the nu(2) and nu(3) modes.     

Stimulated emission pumping spectroscopy of the [X](1)A' state of CHF

We have recorded stimulated emission pumping (SEP) spectra of the A1A' ' 1A' system of CHF, which reveal rich detail concerning the rovibronic structure of the 1A' state up to approximately 7000 cm-1 above the vibrationless level. Using several intermediate A1A' ' state levels, we obtained rotationally resolved spectra for 16 of the 33 levels observed in our previous single vibronic level (SVL) emission study (Fan et al., J. Chem. Phys. 2005, 123, 014314), in addition to one new level. An anharmonic effective Hamiltonian model poorly reproduces the term energies even with the improved set of data because of the extensive interactions among levels in a given polyad (p) having combinations of nu1, nu2, nu3, which satisfy the relationship p = 2nu1 + nu2 + nu3. However, the precise A rotational constants determined from the SEP data were invaluable in clarifying the assignments for these strongly perturbed levels, and the data are well reproduced using a multiresonance effective Hamiltonian model. The derived vibrational parameters are in good agreement with high level ab initio calculations. The experimental frequencies were combined with those of CDF to derive a harmonic force field and average (rz,r(z)e) structures for the ground state.