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.

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.

A combination of photodissociation spectroscopy, ion imaging, and high-level theory is employed to refine the bond strength of the aluminum dimer cation (Al) and elucidate the electronic structure and photodissociation dynamics between 38 500 and 42 000 cm. Above 40 400 cm, structured photodissociation is observed from an extremely anharmonic excited state, which calculations identify as the double minimum G Σ state. The photodissociation spectrum of the G Σ ← X Σ transition in Al gives an average vibrational spacing of 170 cm for the G Σ state and ν = 172 cm for the ground state. Photofragment images of G Σ ← X Σ transitions indicate that once the Al (P) + Al (S) 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 Σ state. The photofragment images also yield D (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 D (Al-Al) with IE (Al) and IE (Al) gives an improved neutral D (Al-Al) = 136.9 ± 1.8 kJ/mol.

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 D 0 measurements for NiO+ (D 0 = 244.6 ± 2.4 kJ/mol) and NiS+ (D 0 = 240.3 ± 1.4 kJ/mol) are more precise and closer to each other than previously reported values. Finally, using a recent measurement of D 0(NiS), we derive a more precise value for IE (NiS): 8.80 ± 0.02 eV (849 ± 1.7 kJ/mol).

Transition metal oxide and sulfide cations activate C-H bonds in the gas phase; several oxide cations activate methane and convert it to methanol at room temperature. However, a lack of experimental data on the energetics and dynamics of these species makes it difficult to model their reactions with hydrocarbons. We perform photofragment ion imaging experiments 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 25000 to 32000 cm −1 via an excited 4 Σ − state, exclusively forming Ni + ( 4 F) + O ( 3 P) products at photolysis energies above 29000 cm −1 . Photofragment images of NiO + show resolved Ni + ( 4 F 9/2 ) + O ( 3 P) and Ni + ( 4 F 7/2 ) + O ( 3 P) product spin-orbit channels; the lower energy (J = 9/2) channel dominates at photolysis energies of 29000 to 32000 cm −1 . The photofragment spectrum of NiS + from 19800 to 23200 cm −1 is highly structured, with 12 distinct vibronic peaks each containing underlying spin-orbit structure. Photofragment images of NiS + collected over this region show slight parallel anisotropy, suggesting the highly structured photodissociation spectrum of NiS + in this region predominantly arises from a ∆Λ = 0 transition. Above 21600 cm −1 , the Ni + ( 2 D 5/2 ) + S ( 3 P) and Ni + ( 2 D 3/2 ) + S ( 3 P) product spin-orbit channels compete, with a branching ratio of 0.5. Temperature-dependent spectra suggest peaks below 20150 cm −1 result from hot bands. Images taken slightly above the dissociation threshold (20600 cm −1 ) show resolved S ( 3 P 2 ) and S ( 3 P 1 ) spin-orbit channels. Our D 0 measurements for NiO + (D 0 = 244.6 +/-2.4 kJ/mol) and NiS + (D 0 = 240.3 +/-1.4 kJ/mol) are more precise and closer to each other than previously reported values.