Method to derive the infrared complex refractive indices n(λ) and k(λ) for organic solids from KBr pellet absorption measurements.

Obtaining the complex refractive index vectors n(ν~) and k(ν~) allows calculation of the (infrared) reflectance spectrum that is obtained from a solid in any of its many morphological forms. We report an adaptation to the KBr pellet technique using two gravimetric dilutions to derive quantitative n(ν~)/k(ν~) for dozens of powders with greater repeatability. The optical constants of bisphenol A and sucrose are compared to those derived by other methods, particularly for powdered materials. The variability of the k values for bisphenol A was examined by 10 individual measurements, showing an average coefficient of variation for k peak heights of 5.6%. Though no established standards exist, the pellet-derived k peak values of bisphenol A differ by 11% and 31% from their single-angle- and ellipsometry-derived values, respectively. These values provide an initial estimate of the precision and accuracy of complex refractive indices that can be derived using this method. Limitations and advantages of the method are discussed, the salient advantage being a more rapid method to derive n/k for those species that do not readily form crystals or specular pellets.

Photofragment imaging differentiates between one- and two-photon dissociation pathways in MgI.

The bond strength and photodissociation dynamics of MgI+ are determined by a combination of theory, photodissociation spectroscopy, and photofragment velocity map imaging. From 17 000 to 21 500 cm-1, the photodissociation spectrum of MgI+ is broad and unstructured; photofragment images in this region show perpendicular anisotropy, which is consistent with absorption to the repulsive wall of the (1) Ω = 1 or (2) Ω = 1 states followed by direct dissociation to ground state products Mg+ (2S) + I (2P3/2). Analysis of photofragment images taken at photon energies near the threshold gives a bond dissociation energy D0(Mg+-I) = 203.0 ± 1.8 kJ/mol (2.10 ± 0.02 eV; 17 000 ± 150 cm-1). At photon energies of 33 000-41 000 cm-1, exclusively I+ fragments are formed. Over most of this region, the formation of I+ is not energetically allowed via one-photon absorption from the ground state of MgI+. Images show the observed product is due to resonance enhanced two-photon dissociation. The photodissociation spectrum from 33 000 to 38 500 cm-1 shows vibrational structure, giving an average excited state vibrational spacing of 227 cm-1. This is consistent with absorption to the (3) Ω = 0+ state from ν = 0, 1 of the (1) Ω = 0+ ground state; from the (3) Ω = 0+ state, absorption of a second photon results in dissociation to Mg* (3P° J) + I+ (3PJ). From 38 500 to 41 000 cm-1, the spectrum is broad and unstructured. We attribute this region of the spectrum to one-photon dissociation of vibrationally hot MgI+ at low energy and ground state MgI+ at higher energy to form Mg (1S) + I+ (3PJ) products.

Bonding, Thermodynamics, and Dissociation Dynamics of NiO+ and NiS+ Determined by Photofragment Imaging and Theory.

We use photofragment ion imaging and ab initio calculations to determine the bond strength and photodissociation dynamics of the nickel oxide (NiO+) and nickel sulfide (NiS+) cations. NiO+ photodissociates broadly from 20350 to 32000 cm-1, forming ground state products Ni+(2D) + O(3P) below ∼29000 cm-1. Above this energy, Ni+(4F) + O(3P) products become accessible and dominate over the ground state channel. In certain images, product spin-orbit levels are resolved, and spin-orbit propensities are determined. Image anisotropy and the results of MRCI calculations suggest NiO+ photodissociates via a 3 4Σ- ← X 4Σ- transition above the Ni+(4F) threshold and via 3 4Σ-, 2 4Σ-, and/or 2 4Π and 3 4Π excited states below the 4F threshold. The photodissociation spectrum of NiS+ from 19900 to 23200 cm-1 is highly structured, with ∼12 distinct vibronic peaks, each containing underlying substructure. Above 21600 cm-1, the Ni+(2D5/2) + S(3P) and Ni+(2D3/2) + S(3P) product spin-orbit channels compete, with a branching ratio of ∼2:1. At lower energy, Ni+(2D5/2) is formed exclusively, and S(3P2) and S(3P1) spin-orbit channels are resolved. MRCI calculations predict the ground state of NiS+ to be one of two nearly degenerate states, the 1 4Σ- and 1 4Δ. Based on images and spectra, the ground state of NiS+ is assigned as 4Δ7/2, with the 1 4Σ3/2- and 1 4Σ1/2- states 81 ± 30 and 166 ± 50 cm-1 higher in energy, respectively. The majority of the photodissociation spectrum is assigned to transitions from the 1 4Δ state to two overlapping, predissociative excited 4Δ states. Our D0 measurements for NiO+ (D0 = 244.6 ± 2.4 kJ/mol) and NiS+ (D0 = 240.3 ± 1.4 kJ/mol) are more precise and closer to each other than previously reported values. Finally, using a recent measurement of D0(NiS), we derive a more precise value for IE (NiS): 8.80 ± 0.02 eV (849 ± 1.7 kJ/mol).

Structure and Spectroscopy of Furan:H2O Complexes.

An analysis of the 1:1 complex of furan and water is presented. In this study, computation and matrix isolation FTIR were used to determine stable complexes of furan:water. Density functional theory and Møller-Plesset second-order, perturbation theory calculations found four, unique geometries for the complex. Two complexes were characterized by C-H···O interactions, one complex was characterized by O-H···O, and the fourth complex was characterized by O-H···π. Optimizations completed using B3LYP, B3LYP-GD3BJ, M05-2X, and MP2 showed the most stable species to be bound by O-H···O interactions. Matrix isolation experiments of mixtures of furan and water held in nitrogen at 15 K showed evidence of stable complexes when probed by FTIR. These signatures grew in intensity when matrices were annealed at 30 K. These vibrational features were predominately associated with perturbation of the water monomer. Additionally, the spectra of complexes containing water isotopologues were recorded. Analysis of spectral features pointed to the presence of a single geometry formed in the matrix, which is best described as a 1:1 complex stabilized by a O-H···O interaction.

Photofragment imaging and electronic spectroscopy of Al2.

A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al2+) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm-1. Above 40 400 cm-1, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G 2Σ+u state. The photodissociation spectrum of the G 2Σ+u ← X 2Σ+g transition in Al2+ gives an average vibrational spacing of 170 cm-1 for the G 2Σ+u state and ν0 = 172 cm-1 for the ground state. Photofragment images of G 2Σ+u ← X 2Σ+g transitions indicate that once the Al (4P) + Al+ (1S) product channel is energetically accessible, it dominates the lower energy, spin-allowed pathways despite being spin-forbidden. This is explained by a proposed competition between radiative and non-radiative decay pathways from the G 2Σ+u state. The photofragment images also yield D0 (Al+-Al) = 136.6 ± 1.8 kJ/mol, the most precise measurement to date, highlighting the improved resolution achieved from imaging at near-threshold energies. Additionally, combining D0 (Al+-Al) with IE (Al) and IE (Al2) gives an improved neutral D0 (Al-Al) = 136.9 ± 1.8 kJ/mol.

Slowed Diffusion and Excluded Volume Both Contribute to the Effects of Macromolecular Crowding on Alcohol Dehydrogenase Steady-State Kinetics.

To understand the consequences of macromolecular crowding, studies have largely employed in vitro experiments with synthetic polymers assumed to be both pure and "inert". These polymers alter enzyme kinetics by excluding volume that would otherwise be available to the enzymes, substrates, and products. Presented here is evidence that other factors, in addition to excluded volume, must be considered in the interpretation of crowding studies with synthetic polymers. Dextran has a weaker effect on the Michaelis-Menten kinetic parameters of yeast alcohol dehydrogenase (YADH) than its small molecule counterpart, glucose. For glucose, the decreased Vmax values directly correlate with slower translational diffusion and the decreased Km values likely result from enhanced substrate binding due to YADH stabilization. Because dextran is unable to stabilize YADH to the same extent as glucose, this polymer's ability to decrease Km is potentially due to the nonideality of the solution, a crowding-induced conformational change, or both. Chronoamperometry reveals that glucose and dextran have surprisingly similar ferricyanide diffusion coefficients. Thus, the reduction in Vmax values for glucose is partially offset by an additional macromolecular crowding effect with dextran. Finally, this is the first report that supplier-dependent impurities in dextran affect the kinetic parameters of YADH. Taken together, our results reveal that caution should be used when interpreting results obtained with inert synthetic polymeric agents, as additional effects from the underlying monomer need to be considered.