ALBERT

Notes: In a series of articles M. Garfinkle has presented an empirical thermodynamic approach to chemical reactions from an initial nonequilibrium state to equilibrium in a closed isothermal system. He claims that (1) (essential points made by M. Garfinkle are numbered for reference later in the text) "a stoichiometric chemical reaction in a closed system traverses a unique natural path from reaction initiation to equilibrium. Along such a natural reaction path the time rate of change of the thermodynamic functions can be analytically described independently of phenomenological or mechanistic consideration'' [M. Garfinkle, J. Phys. Chem. 93, 2158 (1989)]. We show these and other claims not to be correct; this approach has validity limited to: reaction mechanisms with essentially only one velocity (mechanisms with one rate-determining step or mechanisms in a quasi-stationary state); reactions occurring at times close to the initial time; and no products present at the initial time. Garfinkle's method of plotting kinetic data, as a function of t−1, suppress information at later times, which shows the inadequacy of his proposed form of the affinity decay, and leads to the erroneous conclusion that a parameter in this form, tk equal to the most probable time to reach equilibrium, is finite, when in fact it is infinite. The affinity decay rate in general depends on the reaction mechanism, or on the order of the empirical rate equation if determined experimentally.

Notes: The kinetics of some gas-phase alcohol/alkoxide proton-transfer reactions is slower than predicted by simple Rice–Ramsberger–Kassel–Marcus (RRKM) rate theory modeling on the near-barrierless reaction surfaces. Reaction dynamics can be investigated in isolation from nonequilibrium and/or thermodynamic considerations through the study of a generic isoergic ion–molecule system X−+X−H. Monte Carlo quasiclassical trajectory simulations on barrierless reaction surfaces show that the slow experimental kinetics is consistent with both (i) locking of the external rotations of the reactants and (ii) passage over the (orbital angular momentum) centrifugal barrier being the rate-determining steps in bimolecular association, rather than only the latter process. In addition, there may be non-RRKM product selectivity.

Notes: Previous reports of threshold resonances occurring in the photodetachment spectra of molecular anions have provided detailed information about the nature of dipole-supported states and the dynamics of autodetachment from the vibrationless level. In this paper we report the first observation and analysis of rotational band structure in an excited vibrational level of a dipole-supported state. The 1 cm−1 resolution laser photodetachment spectrum of cyanomethyl anion (CH2CN−), the conjugate base of acetonitrile, was recorded in the 12 500–13 700 cm−1 region using ion cyclotron resonance spectrometry. Rotational assignment of the resonances occurring in this region provides evidence for vibrational-to-electronic coupling in the autodetachment process.

Notes: Electron photodetachment spectra of two complex anions, C6H5CH2OHF− and CH3OHOCH−3, have been recorded, using an ion cyclotron resonance spectrometer to generate, store, and detect the ions. The threshold energies determine the potential well depths of the ion–molecule reactions for which the anions are presumable intermediates. The detachment energies for C6H5CH2OHF− and CH3OHOCH−3 are ≤3.05±0.06 and 2.26±0.08 eV, respectively. The ability to measure absolute photodetachment cross sections is demonstrated in the latter case. The electron affinity of C6H5CH2O is 2.14±0.02 eV (49.3±0.5 kcal/mol). One would predict that complex ions of the form ROHF− where ROH is less acidic than HF would have poor Franck–Condon factors for adiabatic threshold photodetachment. This prediction is confirmed; five such ions display cross sections smaller than about 10−20 cm2 at all wavelengths above 365 nm. A useful correlation between ion structure and observation or nonobservation of visible/near-UV photodetachment is therefore demonstrated.

Notes: High resolution photodetachment spectroscopy of acetyl fluoride enolate anion has revealed (approximate)200 narrow resonances near the photodetachment threshold, corresponding to excitation of the anion to a diffuse state in which the electron is weakly bound by the field of the molecular dipole. An analysis of the rotational transitions between the ground valence state and the excited dipole-supported state has been carried out, yielding spectroscopic constants for both states. The binding energy of the dipole-supported state is found to be less than 35 cm−1 . The dependence of autodetachment lifetimes upon rotational quantum numbers of the dipole-supported state has been measured. The selection rules and dynamics of autodetachment from the dipole-supported state are discussed. The results are compared with those obtained previously for acetaldehyde enolate anion.

Notes: Bound excited electronic states are observed in the anions of 7,7,8,8-tetracyanoquinodimethane (TCNQ), 2,5-dimethyl-N,N'-dicyanoquinodiimine (Me2–DCNQI), chloranil, and hexacyano-butadiene using electron photodetachment spectroscopy. Intensity dependence studies and two color experiments are consistent with a two-photon detachment mechanism. A mechanism is proposed to explain the observed photodetachment. The nature of the electronic state responsible for these excited states is discussed and other anions are suggested which should possess bound excited electronic states.

Notes: Diverse aspects of the potential surface for the proton-transfer reaction CH3OH+F−→CH3O−+HF have been investigated by means of high-level ab initio electronic structure methods based on single-reference wave functions, namely, Møller–Plesset perturbation theory from second through fourth order (MP2–MP4), the configuration interaction and coupled-cluster singles and doubles methods (CISD and CCSD), and CCSD theory augmented by a perturbative correction for connected triple excitations [CCSD(T)]. The one-particle Gaussian basis sets for (C,O,F;H) ranged in quality from [4s2p1d;2s1p] to [14s9p6d4f;9s6p4d], including as many as 482 atomic orbitals for the CH3OHF− system. The ion–molecule complex on the proton-transfer surface is a tight, hydrogen-bonded structure of CH3OH⋅F− character, exhibiting a nearly linear -OHF− framework, an elongated O–H distance of 1.07(1) A(ring), and a small interfragment separation, r(H–F)=1.32(1) A(ring). Improved structural data for F−⋅H2O are obtained for calibration purposes. A large fluoride affinity is found for the CH3OHF− adduct, D0=30.4±1 kcal mol−1, and a bonding analysis via the Morokuma decomposition scheme reveals considerable covalent character. The harmonic stretching frequencies within the -OHF− moiety are predicted to be 421 and 2006 cm−1, the latter protonic vibration being downshifted 1857 cm−1 relative to ω1(O–H) of free methanol.A systematic thermochemical analysis of the reactants and products on the CH3OHF− surface yields a proton-transfer energy of 10.6 kcal mol−1, a gas-phase acidity for methanol of 381.7±1 kcal mol−1, and D0(CH3O–H)=104.1±1 kcal mol−1, facilitating the resolution of previous inconsistencies in associated thermochemical cycles. A minimum-energy path in geometric configuration space is mapped out and parametrized on the basis of constrained structural optimizations for fixed values of an aptly chosen reaction variable. The evaluation of numerous energy points along this path establishes the nonexistence of either a proton-transfer barrier, an inflection region, or a secondary minimum of CH3O−⋅HF type. The mathematical considerations for a classical multipole analysis of reaction path asymptotes are outlined for ion–dipole systems and applied to the CH3OHF− surface with due concern for bifurcations in the exit channel for the proton-transfer process. A global analytic surface for vibrational stretching motion in the -OHF− moiety of the CH3OHF− system is constructed, and a suitable dynamical model is tested which involves an effective, triatomic hydrogen pseudobihalide anion, [-OHF]−. Converged variational eigenstates of [-OHF]− to one-half its dissociation limit are determined using vibrational configuration interaction expansions in terms of self-consistent-field modals. The fundamental stretching frequencies of the CH3OHF− complex predicted by the [-OHF]− model are 504 (+84) and 1456 (−549) cm−1, the corresponding anharmonicities appearing in parentheses.

Notes: The design and operational characteristics of a capacitance bridge detector and accompanying signal lock system are presented. The signal lock is especially well suited for the measurement of small changes in ion concentration during photochemical experiments, thereby alleviating problems caused by resonant frequency shifts.

Notes: Two-pulse techniques using high power lasers have been developed for the study of collisional deactivation of vibrationally excited molecular ions in the gas phase. Using these techniques, absolute rate constants have been determined for quenching of excited bromo-3-(trifluoromethyl)-benzene cation in a variety of bath gases. These rates are compared to the predictions of two models for collisional energy transfer in ion–molecule collisions. These studies also show that both IR photodissociation after internal conversion of a visible photon and visible photodissociation after IR multiple photon absorption can be operative mechanisms for the "IR-enhanced'' visible photodissociation of molecular ions.

Notes: We report the 1 cm−1 resolution electron photodetachment spectra of cyanomethyl anion and its deuterated analog, trapped and detected in an ion cyclotron resonance spectrometer. Many sharp resonances were observed in the threshold region corresponding to rovibrational transitions from the ground electronic state to a dipole-supported state of the anion which subsequently undergo electron autodetachment. An assignment of the rotational transitions has been carried out, yielding rotational constants. Using spectral band intensities, we have estimated the electron binding energy of the dipole-supported state as 0.020 (±0.006) eV. The adiabatic electron affinities of the neutral radicals ⋅CH2CN and ⋅CD2CN are found to be 1.560±0.006 and 1.549±0.006 eV, respectively. Features of the dipole-supported state in this system have been compared to those in acetaldehyde enolate.