Your browser doesn't support javascript.
Mostrar: 20 | 50 | 100
Resultados 1 - 14 de 14
Filtrar
Filtros adicionais











Intervalo de ano
1.
Phys Chem Chem Phys ; 21(26): 14042-14052, 2019 Jul 03.
Artigo em Inglês | MEDLINE | ID: mdl-30652179

RESUMO

Ammonia and amines are emitted into the troposphere by various natural and anthropogenic sources, where they have a significant role in aerosol formation. Here, we explore the significance of their removal by reaction with Criegee intermediates, which are produced in the troposphere by ozonolysis of alkenes. Rate coefficients for the reactions of two representative Criegee intermediates, formaldehyde oxide (CH2OO) and acetone oxide ((CH3)2COO) with NH3 and CH3NH2 were measured using cavity ring-down spectroscopy. Temperature-dependent rate coefficients, k(CH2OO + NH3) = (3.1 ± 0.5) × 10-20T2 exp(1011 ± 48/T) cm3 s-1 and k(CH2OO + CH3NH2) = (5 ± 2) × 10-19T2 exp(1384 ± 96/T) cm3 s-1 were obtained in the 240 to 320 K range. Both the reactions of CH2OO were found to be independent of pressure in the 10 to 100 Torr (N2) range, and average rate coefficients k(CH2OO + NH3) = (8.4 ± 1.2) × 10-14 cm3 s-1 and k(CH2OO + CH3NH2) = (5.6 ± 0.4) × 10-12 cm3 s-1 were deduced at 293 K. An upper limit of ≤2.7 × 10-15 cm3 s-1 was estimated for the rate coefficient of the (CH3)2COO + NH3 reaction. Complementary measurements were performed with mass spectrometry using synchrotron radiation photoionization giving k(CH2OO + CH3NH2) = (4.3 ± 0.5) × 10-12 cm3 s-1 at 298 K and 4 Torr (He). Photoionization mass spectra indicated production of NH2CH2OOH and CH3N(H)CH2OOH functionalized organic hydroperoxide adducts from CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. Ab initio calculations performed at the CCSD(T)(F12*)/cc-pVQZ-F12//CCSD(T)(F12*)/cc-pVDZ-F12 level of theory predicted pre-reactive complex formation, consistent with previous studies. Master equation simulations of the experimental data using the ab initio computed structures identified submerged barrier heights of -2.1 ± 0.1 kJ mol-1 and -22.4 ± 0.2 kJ mol-1 for the CH2OO + NH3 and CH2OO + CH3NH2 reactions, respectively. The reactions of NH3 and CH3NH2 with CH2OO are not expected to compete with its removal by reaction with (H2O)2 in the troposphere. Similarly, losses of NH3 and CH3NH2 by reaction with Criegee intermediates will be insignificant compared with reactions with OH radicals.

2.
Nat Commun ; 9(1): 4343, 2018 10 19.
Artigo em Inglês | MEDLINE | ID: mdl-30341291

RESUMO

Methanol is a benchmark for understanding tropospheric oxidation, but is underpredicted by up to 100% in atmospheric models. Recent work has suggested this discrepancy can be reconciled by the rapid reaction of hydroxyl and methylperoxy radicals with a methanol branching fraction of 30%. However, for fractions below 15%, methanol underprediction is exacerbated. Theoretical investigations of this reaction are challenging because of intersystem crossing between singlet and triplet surfaces - ∼45% of reaction products are obtained via intersystem crossing of a pre-product complex - which demands experimental determinations of product branching. Here we report direct measurements of methanol from this reaction. A branching fraction below 15% is established, consequently highlighting a large gap in the understanding of global methanol sources. These results support the recent high-level theoretical work and substantially reduce its uncertainties.

3.
Annu Rev Phys Chem ; 67: 41-63, 2016 05 27.
Artigo em Inglês | MEDLINE | ID: mdl-26980312

RESUMO

Attosecond science has paved the way for direct probing of electron dynamics in gases and solids. This review provides an overview of recent attosecond measurements, focusing on the wealth of knowledge obtained by the application of isolated attosecond pulses in studying dynamics in gases and solid-state systems. Attosecond photoelectron and photoion measurements in atoms reveal strong-field tunneling ionization and a delay in the photoemission from different electronic states. These measurements applied to molecules have shed light on ultrafast intramolecular charge migration. Similar approaches are used to understand photoemission processes from core and delocalized electronic states in metal surfaces. Attosecond transient absorption spectroscopy is used to follow the real-time motion of valence electrons and to measure the lifetimes of autoionizing channels in atoms. In solids, it provides the first measurements of bulk electron dynamics, revealing important phenomena such as the timescales governing the switching from an insulator to a metallic state and carrier-carrier interactions.

4.
Science ; 350(6256): 78-82, 2015 Oct 02.
Artigo em Inglês | MEDLINE | ID: mdl-26430117

RESUMO

Despite decades of study, the structures adopted to accommodate an excess proton in water and the mechanism by which they interconvert remain elusive. We used ultrafast two-dimensional infrared (2D IR) spectroscopy to investigate protons in aqueous hydrochloric acid solutions. By exciting O-H stretching vibrations and detecting the spectral response throughout the mid-IR region, we observed the interaction between the stretching and bending vibrations characteristic of the flanking waters of the Zundel complex, [H(H2O)2](+), at 3200 and 1760 cm(-1), respectively. From time-dependent shifts of the stretch-bend cross peak, we determined a lower limit on the lifetime of this complex of 480 femtoseconds. These results suggest a key role for the Zundel complex in aqueous proton transfer.

5.
Science ; 346(6215): 1348-52, 2014 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-25504716

RESUMO

Electron transfer from valence to conduction band states in semiconductors is the basis of modern electronics. Here, attosecond extreme ultraviolet (XUV) spectroscopy is used to resolve this process in silicon in real time. Electrons injected into the conduction band by few-cycle laser pulses alter the silicon XUV absorption spectrum in sharp steps synchronized with the laser electric field oscillations. The observed ~450-attosecond step rise time provides an upper limit for the carrier-induced band-gap reduction and the electron-electron scattering time in the conduction band. This electronic response is separated from the subsequent band-gap modifications due to lattice motion, which occurs on a time scale of 60 ± 10 femtoseconds, characteristic of the fastest optical phonon. Quantum dynamical simulations interpret the carrier injection step as light-field-induced electron tunneling.

6.
J Chem Phys ; 140(20): 204508, 2014 May 28.
Artigo em Inglês | MEDLINE | ID: mdl-24880302

RESUMO

The infrared spectra of aqueous solutions of NaOH and other strong bases exhibit a broad continuum absorption for frequencies between 800 and 3500 cm(-1), which is attributed to the strong interactions of the OH(-) ion with its solvating water molecules. To provide molecular insight into the origin of the broad continuum absorption feature, we have performed ultrafast transient absorption and 2DIR experiments on aqueous NaOH by exciting the O-H stretch vibrations and probing the response from 1350 to 3800 cm(-1) using a newly developed sub-70 fs broadband mid-infrared source. These experiments, in conjunction with harmonic vibrational analysis of OH(-)(H2O)n (n = 17) clusters, reveal that O-H stretch vibrations of aqueous hydroxides arise from coupled vibrations of multiple water molecules solvating the ion. We classify the vibrations of the hydroxide complex by symmetry defined by the relative phase of vibrations of the O-H bonds hydrogen bonded to the ion. Although broad and overlapping spectral features are observed for 3- and 4-coordinate ion complexes, we find a resolvable splitting between asymmetric and symmetric stretch vibrations, and assign the 2850 cm(-1) peak infrared spectra of aqueous hydroxides to asymmetric stretch vibrations.

7.
Nat Chem ; 5(11): 935-40, 2013 Nov.
Artigo em Inglês | MEDLINE | ID: mdl-24153371

RESUMO

The ability of liquid water to dissipate energy efficiently through ultrafast vibrational relaxation plays a key role in the stabilization of reactive intermediates and the outcome of aqueous chemical reactions. The vibrational couplings that govern energy relaxation in H2O remain difficult to characterize because of the limitations of current methods to visualize inter- and intramolecular motions simultaneously. Using a new sub-70 fs broadband mid-infrared source, we performed two-dimensional infrared, transient absorption and polarization anisotropy spectroscopy of H2O by exciting the OH stretching transition and characterizing the response from 1,350 cm(-1) to 4,000 cm(-1). These spectra reveal vibrational transitions at all frequencies simultaneous to the excitation, including pronounced cross-peaks to the bend vibration and a continuum of induced absorptions to combination bands that are not present in linear spectra. These observations provide evidence for strong mixing of inter- and intramolecular vibrations in liquid H2O, and illustrate the shortcomings of traditional relaxation models.


Assuntos
Água/química , Ligações de Hidrogênio , Modelos Moleculares , Termodinâmica , Vibração
8.
J Phys Chem B ; 117(49): 15319-27, 2013 Dec 12.
Artigo em Inglês | MEDLINE | ID: mdl-23638966

RESUMO

The vibrational dynamics of liquid water, which result from a complex interplay between internal molecular vibrations and the fluctuating hydrogen bond network, are fundamental to many physicochemical and biological processes. Using a new ultrafast broadband mid-infrared light source with over 2000 cm(-1) of bandwidth, we performed ultrafast time-resolved infrared spectroscopy to study the vibrational couplings and relaxation dynamics of the stretching and bending vibrations of the mixed isotopologue, HOD, in D2O. Analysis of cross-peaks and induced absorptions in the two-dimensional infrared spectrum and transient absorption spectrum shows that the hydroxyl stretch of HOD is coupled to the HOD bending mode via Fermi resonance, with a 70° angle between their transition dipole moments. We see that HOD is also anharmonically coupled to the D2O solvent modes. From transient absorption spectra, we conclude that vibrational relaxation occurs through a number of paths. The strongly hydrogen-bonded OH oscillators have the highest propensity to relax through the bending mode, while the weakly hydrogen bonded oscillators relax through other modes.


Assuntos
Espectrofotometria Infravermelho , Água/química , Óxido de Deutério/química , Ligações de Hidrogênio , Fatores de Tempo
9.
J Chem Phys ; 136(13): 134507, 2012 Apr 07.
Artigo em Inglês | MEDLINE | ID: mdl-22482572

RESUMO

We present an approach for calculating nonlinear spectroscopic observables, which overcomes the approximations inherent to current phenomenological models without requiring the computational cost of performing molecular dynamics simulations. The trajectory mapping method uses the semi-classical approximation to linear and nonlinear response functions, and calculates spectra from trajectories of the system's transition frequencies and transition dipole moments. It rests on identifying dynamical variables important to the problem, treating the dynamics of these variables stochastically, and then generating correlated trajectories of spectroscopic quantities by mapping from the dynamical variables. This approach allows one to describe non-Gaussian dynamics, correlated dynamics between variables of the system, and nonlinear relationships between spectroscopic variables of the system and the bath such as non-Condon effects. We illustrate the approach by applying it to three examples that are often not adequately treated by existing analytical models--the non-Condon effect in the nonlinear infrared spectra of water, non-Gaussian dynamics inherent to strongly hydrogen bonded systems, and chemical exchange processes in barrier crossing reactions. The methods described are generally applicable to nonlinear spectroscopy throughout the optical, infrared and terahertz regions.

10.
J Chem Phys ; 135(5): 054509, 2011 Aug 07.
Artigo em Inglês | MEDLINE | ID: mdl-21823714

RESUMO

Rearrangements of the hydrogen bond network of liquid water are believed to involve rapid and concerted hydrogen bond switching events, during which a hydrogen bond donor molecule undergoes large angle molecular reorientation as it exchanges hydrogen bonding partners. To test this picture of hydrogen bond dynamics, we have performed ultrafast 2D IR spectral anisotropy measurements on the OH stretching vibration of HOD in D(2)O to directly track the reorientation of water molecules as they change hydrogen bonding environments. Interpretation of the experimental data is assisted by modeling drawn from molecular dynamics simulations, and we quantify the degree of molecular rotation on changing local hydrogen bonding environment using restricted rotation models. From the inertial 2D anisotropy decay, we find that water molecules initiating from a strained configuration and relaxing to a stable configuration are characterized by a distribution of angles, with an average reorientation half-angle of 10°, implying an average reorientation for a full switch of ≥20°. These results provide evidence that water hydrogen bond network connectivity switches through concerted motions involving large angle molecular reorientation.


Assuntos
Espectrofotometria Infravermelho , Água/química , Anisotropia , Ligações de Hidrogênio , Simulação de Dinâmica Molecular , Espectrofotometria Infravermelho/métodos
11.
J Phys Chem A ; 115(16): 3957-72, 2011 Apr 28.
Artigo em Inglês | MEDLINE | ID: mdl-21314148

RESUMO

While it is generally recognized that the hydroxide ion can rapidly diffuse through aqueous solution due to its ability to accept a proton from a neighboring water molecule, a description of the OH(-) solvation structure and mechanism of proton transfer to the ion remains controversial. In this report, we present the results of femtosecond infrared spectroscopy measurements of the O-H stretching transition of dilute HOD dissolved in NaOD/D(2)O. Pump-probe, photon echo peak shift, and two-dimensional infrared spectroscopy experiments performed as a function of deuteroxide concentration are used to assign spectral signatures that arise from the OH(-) ion and its solvation shell. A spectral feature that decays on a ∼110 fs time scale is assigned to the relaxation of transiently formed configurations wherein a proton is equally shared between a HOD molecule and an OD(-) ion. Over picosecond waiting times, features appear in 2D IR spectra that are indicative of the exchange of population between OH(-) ions and HOD molecules due to deuteron transfer. The construction of a spectral model that includes spectral relaxation, chemical exchange, and thermalization processes, and self-consistently treats all of our data, allows us to qualitatively explain the results of our experiments and gives a lower bound of 3 ps for the deuteron transfer kinetics.


Assuntos
Hidróxidos/química , Prótons , Espectrofotometria Infravermelho , Fatores de Tempo , Água/química
12.
Proc Natl Acad Sci U S A ; 106(36): 15154-9, 2009 Sep 08.
Artigo em Inglês | MEDLINE | ID: mdl-19666493

RESUMO

It is generally accepted that the anomalous diffusion of the aqueous hydroxide ion results from its ability to accept a proton from a neighboring water molecule; yet, many questions exist concerning the mechanism for this process. What is the solvation structure of the hydroxide ion? In what way do water hydrogen bond dynamics influence the transfer of a proton to the ion? We present the results of femtosecond pump-probe and 2D infrared experiments that probe the O-H stretching vibration of a solution of dilute HOD dissolved in NaOD/D(2)O. Upon the addition of NaOD, measured pump-probe transients and 2D IR spectra show a new feature that decays with a 110-fs time scale. The calculation of 2D IR spectra from an empirical valence bond molecular dynamics simulation of a single NaOH molecule in a bath of H(2)O indicates that this fast feature is due to an overtone transition of Zundel-like H(3)O(2)(-) states, wherein a proton is significantly shared between a water molecule and the hydroxide ion. Given the frequency of vibration of shared protons, the observations indicate the shared proton state persists for 2-3 vibrational periods before the proton localizes on a hydroxide. Calculations based on the EVB-MD model argue that the collective electric field in the proton transfer direction is the appropriate coordinate to describe the creation and relaxation of these Zundel-like transition states.


Assuntos
Hidróxidos/química , Prótons , Água/química , Simulação por Computador , Ligações de Hidrogênio , Modelos Químicos , Espectrofotometria Infravermelho , Vibração
13.
Acc Chem Res ; 42(9): 1239-49, 2009 Sep 15.
Artigo em Inglês | MEDLINE | ID: mdl-19585982

RESUMO

Compared with other molecular liquids, water is highly structured because of its ability to form up to four hydrogen bonds, resulting in a tetrahedral network of molecules. However, this underlying intermolecular structure is constantly in motion, exhibiting large fluctuations and reorganizations on time scales from femtoseconds to picoseconds. These motions allow water to play a key role in a number of chemical and biological processes. By exploiting the fact that the OH stretching frequency of dilute HOD in liquid D(2)O is highly dependent upon the configuration of the neighbor nearest to the proton, researchers have been able to track water's time-dependent structure using two-dimensional infrared (2D IR) spectroscopy, which tags molecules at an initial frequency and then watches as that frequency evolves with respect to time. Recent advances in molecular dynamics simulation techniques allow for the calculation of 2D IR spectra, providing an atomistic interpretation tool of 2D IR spectra in terms of the underlying dynamics of the liquid. In this Account, we review recent ultrafast 2D IR studies at MIT that provide new information on the mechanism of hydrogen-bond rearrangements in liquid water. The 2D IR spectra of the OH stretching vibration of HOD in D(2)O appear highly asymmetric. In the frequency range indicative of hydrogen-bonded molecules (<3300 cm(-1)), the 2D spectra remain fairly compact. By contrast, in the frequency range in which molecules having weak or broken hydrogen bonds absorb (>3500 cm(-1)), the 2D spectra broaden over a time scale of approximately 60 fs, consistent with librations (hindered rotations) of water molecules. This broadening indicates that molecules forming weak or broken hydrogen bonds are unstable and reorient rapidly to return to a hydrogen-bonded configuration. These conclusions are supported by the results of molecular dynamics simulations, which suggest that water molecules undergo a large-angle reorientation during the course of hydrogen-bond exchange. The transition state for hydrogen-bond rearrangements is found to resemble a bifurcated hydrogen bond. Roughly half of the hydrogen-bond exchange events in the simulation are found to involve the insertion of a water molecule across a hydrogen bond, suggesting that hydrogen-bond exchange in water involves the correlated motion of water molecules as far away as the second solvation shell. The combination of ultrafast 2D IR spectroscopy with simulation-based modeling is leading to self-consistent descriptions of the underlying dynamics in liquid water. Moreover, these results also demonstrate a more general, unique characteristic of the spectroscopy: if a spectral signature of the transition state exists, then 2D IR can effectively serve as a transition-state spectroscopy.


Assuntos
Água/química , Ligações de Hidrogênio , Hidróxidos/química , Modelos Moleculares , Conformação Molecular , Movimento (Física) , Probabilidade , Espectrofotometria Infravermelho , Termodinâmica , Fatores de Tempo , Vibração
14.
J Phys Chem B ; 112(42): 13167-71, 2008 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-18821789

RESUMO

Hydrogen-bonded interfaces are essential structural elements in biology. Furthermore, they can mediate electron transport by coupling the electron to proton transfer within the interface. The specific hydrogen-bonding configuration and strength have a large impact on the proton transfer, which exchanges the hydrogen-bonded donor and acceptor species (i.e., NH...O --> N...HO). Modulations of the hydrogen-bonding environment, such as the hydrogen-bond stretch and twist modes, affect the proton-transfer dynamics. Here, we present transient grating and echo peak shift measurements of the NH stretch vibrations of four doubly hydrogen-bonded cyclic dimers in their electronic ground state. The equilibrium vibrational dynamics exhibit strong coherent modulations that we attribute to coupling of the high-frequency NH vibration to the low-frequency interdimer stretch and twist modes and not to interference between multiple Fermi resonances that dominate the substructure of the linear spectra.


Assuntos
Dimerização , Hidrogênio/química , Nitrogênio/química , Vibração , Ligações de Hidrogênio , Espectroscopia de Infravermelho com Transformada de Fourier , Fatores de Tempo
SELEÇÃO DE REFERÊNCIAS
DETALHE DA PESQUISA