David J. Nesbitt

Definition of the hydrogen bond (IUPAC Recommendations 2011)

A novel definition for the hydrogen bond is recommended here. It takes into account the theoretical and experimental knowledge acquired over the past century. This definition insists on some evidence. Six criteria are listed that could be used as evidence for the presence of a hydrogen bond.

Defining the hydrogen bond: An account (IUPAC Technical Report)

The term “hydrogen bond” has been used in the literature for nearly a century now. While its importance has been realized by physicists, chemists, biologists, and material scientists, there has been a continual debate about what this term means. This debate has intensified following some important experimental results, especially in the last decade, which questioned the basis of the traditional view on hydrogen bonding. Most important among them are the direct experimental evidence for a partial covalent nature and the observation of a blue-shift in stretching frequency following X–H···Y hydrogen bond formation (XH being the hydrogen bond donor and Y being the hydrogen bond acceptor). Considering the recent experimental and theoretical advances, we have proposed a new definition of the hydrogen bond, which emphasizes the need for evidence. A list of criteria has been provided, and these can be used as evidence for the hydrogen bond formation. This list is followed by some characteristics that are observed in typical hydrogen-bonding environments.

Nonexponential “blinking” kinetics of single CdSe quantum dots: A universal power law behavior

Single molecule confocal microscopy is used to study fluorescence intermittency of individual ZnS overcoated CdSe quantum dots (QDs) excited at 488 nm. The confocal apparatus permits the distribution of “on” and “off” times (i.e., periods of sustained fluorescence emission and darkness) to be measured over an unprecedentedly large dynamic range (109) of probability densities, with nonexponential behavior in τoff over a 105 range in time scales. In dramatic contrast, these same τoff distributions in all QDs are described with remarkable simplicity over this 109-fold dynamic range by a simple inverse power law, i.e., P(τoff)∝1/τoff1+α. Such inverse power law behavior is a clear signature of distributed kinetics, such as predicted for (i) an exponential distribution of trap depths or (ii) a distribution of tunneling distances between QD core/interface states. This has important statistical implications for all previous studies of fluorescence intermittency in semiconductor QDs and may have broader implicatio...

“On”/“off” fluorescence intermittency of single semiconductor quantum dots

Single molecule confocal microscopy is used to investigate the detailed kinetics of fluorescence intermittency in colloidal II–VI (CdSe) semiconductor quantum dots. Two distinct modes of behavior are observed corresponding to (i) sustained “on” episodes (τon) of rapid laser absorption/fluorescence cycling, followed by (ii) sustained “off” episodes (τoff) where essentially no light is emitted despite continuous laser excitation. Both on-time and off-time probability densities follow an inverse power law, P(τon/off)∝1/τon/offm, over more than seven decades in probability density and five decades in time. Such inverse power law behavior is an unambiguous signature of highly distributed kinetics with rates varying over 105-fold, in contrast with models for switching between “on” and “off” configurations of the system via single rate constant processes. The unprecedented dynamic range of the current data permits several kinetic models of fluorescence intermittency to be evaluated at the single molecule level a...

Slow vibrational relaxation in picosecond iodine recombination in liquids

A characteristic time scale of order 100 ps has been observed by several workers in picosecond spectroscopy experiments on iodine recombination in liquid CCl4 and other solvents. This time scale has been thought to be associated with the diffusive relative motion, prior to actual recombination, of photodissociated I atoms. In this paper, we point out that extensive vibrational deactivation of nascent iodine molecules formed in the recombination is required for spectroscopic detection of I2. We propose that slow vibrational relaxation of nascent I2 may play an important role in such recombination studies. We examine the relaxation of highly vibrationally excited I2 in liquid CCl4. Both vibrational‐translational (VT) and vibrational–vibrational (VV) energy transfer mechanisms are considered. We estimate that the characteristic time scale for the vibrational relaxation of I2 in CCl4 is of order 100 ps. VV energy transfer is found to play a critical role in the relaxation. Possible experimental avenues for te...

Dynamically weighted multiconfiguration self-consistent field: Multistate calculations for F+H2O→HF+OH reaction paths

A novel method of dynamically adjusted weighting factors in state-averaged multiconfiguration self-consistent-field calculations (SA-MCSCF) is described that is applicable to systems of arbitrary dimensionality. The proposed dynamically weighted approach automatically weights the relevant electronic states in each region of the potential energy surface, smoothly adjusting between these regions with an energy dependent functional. This method is tested on the F(2P)+H2O-->HF+OH(2Pi) reaction, which otherwise proves challenging to describe with traditional SA-MCSCF methods due to (i) different asymptotic degeneracies of reactant (threefold) and product (twofold) channels, and (ii) presence of low-lying charge transfer configurations near the transition state region. The smoothly varying wave functions obtained by dynamically weighted multiconfigurational self-consistent field represent excellent reference states for high-level multireference configuration interaction calculations and offer an ideal starting point for construction of multiple state potential energy surfaces.

Visible absorption and magnetic‐rotation spectroscopy of 1CH2: The analysis of the b̃ 1B1 state

The b1B1←a 1A1 spectrum of CH2 radical has been recorded with Doppler‐limited resolution in the 15 600–18 650 cm−1 wavelength region by laser flash‐kinetic absorption spectroscopy. Singlet methylene is produced by photolysis of ketene at 308 nm. Assignments for 477 transitions originating from 1A1(0,0,0) and (0,1,0) levels with J≤8 and Ka≤5 are reported from some 10 000 lines observed. Term values are given for these 1A1 levels and 1B1(0,v2,0) levels with 13≤v2≤17. Magnetic‐rotation signals were observed for 60% of the 424 lines studied. This anomalous Zeeman effect is explained by singlet–triplet mixing in both a and b states. Extensive singlet–triplet mixing occurs for 1B1 levels with Ka≠0; this mixing is allowed in second order by Renner–Teller coupling of b 1B1 with a 1A1 and by spin‐orbit coupling via a 1A1 to X 3B1. The consequent shifts prevent determination of accurate rovibrational structural parameters for the 1B1 state.

Vibrational mode mixing in terminal acetylenes: High‐resolution infrared laser study of isolated J states

Mode–mode vibrational coupling in the acetylinic CH stretch at 3330 cm−1 of 1‐butyne and 1‐pentyne is studied via high‐resolution, direct absorption infrared spectroscopy. As in our previous study of propyne, mixing of the CH stretch vibration carrying oscillator strength (the bright state) with the bath of multiquantum combination states (the dark, or background, states) manifests itself in the spectrum via fragmentation of the isolated bright state transitions into clusters of closely spaced spectral lines in a ∼0.01 cm−1 window about the zeroth order acetylinic CH stretch position. In the 1‐butyne spectrum, we find an experimental density of mixed states of 114±30 states/cm−1 compared to a direct state count prediction of 90 total states/cm−1, and thus quantitatively determine that all possible states appear in the spectrum. The 1‐butyne line spacing distribution suggests the Wigner distribution expected for a quantum mechanically ergodic system. Analysis of the mode mixing as a function of J’ shows th...

High sensitivity, high‐resolution IR laser spectroscopy in slit supersonic jets: Application to N2HF ν1 and ν5+ν1−ν5

A difference frequency IR spectrometer is combined with a slit supersonic expansion for high‐resolution (≤50 MHz FWHM) direct absorption investigations of jet‐cooled species. The 1.25 cm long nozzle provides a long path length and high densities suitable for synthesis and observation of van der Waals clusters, with a gradual spatial temperature gradient that permits experimental control of low frequency vibrational populations. Due to collisional quenching of velocity distributions, absorption linewidths are reduced and peak absorbance increased five‐ to sevenfold compared to pinhole expansions. Minimum detectable concentrations of HF containing complexes are 2×109 molecules/cm3/quantum state in a 2.5 cm path length. The combination of high sensitivity, sub‐Doppler resolution, long path lengths, and temperature control make direct absorption in slit nozzle expansions a powerful and general technique for high‐resolution study of jet‐cooled species. The spectometer is used to obtain the near‐infrared spectr...

Slit pulsed valve for generation of long-path-length supersonic expansions

We describe a valve for production of jet‐cooled species in a pulsed, long‐path‐length (1.2‐cm) supersonic expansion. The valve produces 150–600‐μs‐duration pulses at repetition rates up to 60 Hz from a nozzle with variable slit width, and is suitable for use with corrosive gases and vapors.