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My dissertation work uses numerical models to examine geophysical problems and processes related to small and large impact craters on three different planetary bodies. Two projects examine the post-impact modification and potential removal of small craters through topographic relaxation based on the subsurface structures, material rheology, and thermal environments on Mars and the Moon. For Mars I investigate the potential subsurface ice extent and abundance around the Phoenix landing site through its capability to relax small craters to resemble observed shallow palimpsest craters in the region. For the Moon I demonstrate that small craters emplaced on freshly solidified melt sheets of mare or highlands material are unable to relax due to the stiff rheologies despite significantly elevated subsurface thermal profiles. In my third project I use hydrocode modeling to simulate the formation of large-scale impact basins on Europa’s seafloor, exploring various scenarios of its interior layering.

Prior to European settlement, swamps, marshes, lakes, and streams were the major landscape features of the Chicago region. Much of this has been altered or lost in the past one-hundred-and-fifty years. We examined the changes in aquatic environments and fauna in Cook County, Illinois, using 1890–1910 and 1997–2017 as our focus intervals. The extents of aquatic features from historical topographic maps were imported into a GIS database and compared with the modern USGS National Hydrography Dataset. Historical and modern spatial data on aquatic animals were collected from museum collection and survey reports. Overall, the total area of wetlands and water bodies has decreased by about a third, while swamps and marshes have been drained or converted into lakes and ponds. Fifty-four of 80 historic molluscan species are not recently recorded, and 26 current species are not recorded in the historical data, including 6 considered to be invasive. Of 54 fish species in the historic data, 23 are no longer present and about 50% of the remaining species have undergone range reductions. Three out of 10 species of reptiles have disappeared. All 25 aquatic birds reported from the 1890–1910 period are still extant and an additional 13 have been reported in recent times, possibly due to increased collection effort. The Chicago region provides a case study of the impact of anthropogenic landscape change on biota. Large scale extirpations of native species have corresponded to a shift to a highly fragmented landscape of small ponds and reduced swamps and marshes.

The variations in near-surface properties and regolith structure of asteroids are currently not well constrained by remote sensing techniques. Radar is a useful tool for such determinations of Near-Earth Asteroids (NEAs) as the power of the reflected signal from the surface is dependent on the bulk density, ρbd, and dielectric permittivity. In this study, high precision complex permittivity measurements of powdered aluminum oxide and dunite samples are used to characterize the change in the real part of the permittivity with the bulk density of the sample. In this work, we use silica aerogel for the first time to increase the void space in the samples (and decrease the bulk density) without significantly altering the electrical properties. We fit various mixing equations to the experimental results. The Looyenga–Landau--Lifshitz mixing formula has the best fit and the Lichtenecker mixing formula, which is typically used to approximate planetary regolith, does not model the results well. We find that the Looyenga--Landau–Lifshitz formula adequately matches Lunar regolith permittivity measurements, and we incorporate it into an existing model for obtaining asteroid regolith bulk density from radar returns which is then used to estimate the bulk density in the near surface of NEA’s (101955) Bennu and (25143) Itokawa. Constraints on the material properties appropriate for either asteroid give average estimates of ρ b d = 1.27 ± 0.33 g/cm 3 for Bennu and ρ b d = 1.68 ± 0.53 g/cm 3 for Itokawa. We conclude that our data suggest that the Looyenga-Landau-Lifshitz mixing model, in tandem with an appropriate radar scattering model, is the best method for estimating bulk densities of regoliths from radar observations of airless bodies.

The 1640 km diameter Caloris basin is the largest impact basin on Mercury and hosts three distinct suites of tectonic structures in its substantially deformed smooth plains, indicative of the basin’s complex history. These structures, i.e. radial graben comprising Pantheon Fossae, concentric graben/troughs arranged in an irregular pattern, and wrinkle ridges, are found at various regions throughout the basin and occasionally interact. We document the locations, orientations, and cross-cutting relationships of these structures using high-resolution MESSENGER images. We suggest that Caloris shows a history of continuous deformation, resulting from the contemporaneous formation of sets of strain compatible tectonic structures, rather than discrete stages of deformation producing each suite independently in temporal sequence. We propose that radial graben formed continuously in the basin’s early deformational history, following initial wrinkle ridge formation possibly representative of a multi-ring basin structure, overlapping with continued wrinkle ridge formation, and persisting with the formation of concentric graben after contraction ceased.

Gas-phase methyl cation affinities (MCAs) for rare gases Ne, Kr, and Xe were measured with a pulsed electron-beam high-pressure mass spectrometer. The MCAs for Ne and Kr were determined to be 1.2 ± 0.3 and 19.8 ± 2.0 kcal/mol, respectively, by the observation of the clustering reaction, CH3+ + Rg = CH3+(Rg) (Rg = Ne and Kr). The MCA of Xe was measured to be 2.0 ± 0.6 kcal/mol larger than that of N2 by the observation of the substitution reaction CH3+(N2) + Xe = CH3+(Xe) + N2. Based on the MCA of N2 of 44.1 kcal/mol proposed by McMahon et al., the MCA of Xe is determined to be 46.1 ± 0.6 kcal/mol. Molecular orbital calculations at six different levels consistently gave almost identical MCA values for each of the rare gases. At QCISD(T)(full)/6-311++G(2df,p)//B3LYP/6-311++G(d,p), the calculated values (all in kcal/mol) are as follows: He, 0.6; Ne, 2.2; Ar, 15.9; Kr, 24.1; and N2, 43.2. For Xe at B3LYP/DZVP//B3LYP/DZVP, the calculated MCA is 39.0 kcal/mol. The ethyl cation affinities of Ar, Kr, and Xe were ...

Mass spectrometric reaction rate measurements are reported that show the remarkable occurrence of the breaking of four C−F bonds and the formation of four new Nb+−F bonds in gas-phase collisions of Nb+ with hexafluoro- and tetrafluorobenzene. A mechanism involving sequential intramolecular fluorine pick-up is proposed.

Direct bonding has been observed to occur between SiF3 + and the rare gases xenon, krypton, and argon in the gas phase at low collision energies and, in the case of xenon and krypton, also at high collision energies. The kinetics of low-energy addition have been monitored at room temperature in He buffer gas at 0.35 Torr using the selected-ion flow tube (SIFT) tandem mass-spectrometer technique. An investigation of the energetics of dissociation of the adduct ions in multi-collision induced dissociation experiments revealed the formation of adduct ions at high collision energies. Experimental and theoretical results are presented for the formation and stability of two possible low-energy isomers of the gaseous trifluorosilylxenon cation, F 3SiXe + , one formed by the direct bonding of Xe to Si and the other by Xe being attached to one F atom. Two possible high-energy isomers are proposed that contain a Si-Xe-F linkage formed by the insertion of Xe into SiF3 + or aF -Xe-F linkage formed by bond redisposition. The carbon analogue CF3 + did not form adduct ions with Xe under the same experimental conditions. Analogous experimental and theoretical studies are reported for the reactions of SiF3 + with Kr and Ar. Experimental results also are reported for the reaction of SiF 3 + with N2 and O2.

Proton migration in protonated glycylglycylglycine (GGG) has been investigated by using density functional theory at the B3LYP/6-31++G(d,p) level of theory. On the protonated GGG energy hypersurface 19 critical points have been characterized, 11 as minima and 8 as first-order saddle points. Transition state structures for interconversion between eight of these minima are reported, starting from a structure in which there is protonation at the amino nitrogen of the N-terminal glycyl residue following the migration of the proton until there is fragmentation into protonated 2-aminomethyl-5-oxazolone (the b(2) ion) and glycine. Individual free energy barriers are small, ranging from 4.3 to 18.1 kcal mol(-)(1). The most favorable site of protonation on GGG is the carbonyl oxygen of the N-terminal residue. This isomer is stabilized by a hydrogen bond of the type O-H.N with the N-terminal nitrogen atom, resulting in a compact five-membered ring. Another oxygen-protonated isomer with hydrogen bonding of the type O-H.O, resulting in a seven-membered ring, is only 0.1 kcal mol(-)(1) higher in free energy. Protonation on the N-terminal nitrogen atom produces an isomer that is about 1 kcal mol(-)(1) higher in free energy than isomers resulting from protonation on the carbonyl oxygen of the N-terminal residue. The calculated energy barrier to generate the b(2) ion from protonated GGG is 32.5 kcal mol(-)(1) via TS(6--7). The calculated basicity and proton affinity of GGG from our results are 216.3 and 223.8 kcal mol(-)(1), respectively. These values are 3-4 kcal mol(-)(1) lower than those from previous calculations and are in excellent agreement with recently revised experimental values.

The presence of an interacting water or methanol molecule has been shown to catalyze the 1,3-proton shift in a peptide linkage between the tautomers of protonated formamide and glycylglycylglycine. Density functional theory calculations at the B3LYP/6-31++G(d,p) level of theory show that, for glycylglycylglycine, the forward barrier of this shift decreases from a free energy at 298 K of 39.6 kcal/mol in the absence of solvent to 26.7 kcal/mol in the presence of water and to 22.0 kcal/mol in the presence of methanol. Protonation at the amide nitrogen of the second residue results in a large increase in the C -N bond distance from 1.336 to 1.519 A, whereas protonation at the carbonyl oxygen leads to a decrease in the C-N bond distance from 1.336 to 1.321 A. Solvent-catalyzed tautomerism may play an important role in the fragmentation of electrosprayed, protonated peptides in the gas phase.

Molecular orbital calculations are reported for N2H+ and N2CH3+ and for the transition structures for the rearrangement of these ions by 1,2-shifts of H and CH3. All reaction profiles were also calculated with one atom of noble gas, M, present (M = Ne, Ar, Kr). Structure optimizations were performed at B3LYP/6-311++G(d,p) and, in the case of N2H+···M, single point calculations were also performed at QCISD(T)(full)/6-311++G(2df,p). For N2H+, inclusion of one noble gas atom reduces the barrier to rearrangement from 46.6 kcal mol-1 for the uncatalysed reaction to 42.6 kcal mol-1 (by Ne), to 21.4 kcal mol-1 (by Ar), and to 11.0 kcal mol-1 (by Kr). For N2CH3+, the barrier of 36.4 kcal mol-1 is reduced to 35.1 kcal mol-1 by Ne, to 27.4 kcal mol-1 by Ar, and to 18.4 kcal mol-1 by Kr.Key words: catalysis, molecular orbital theory, argon, krypton.

Molecular orbital calculations are reported for the monoxides, XO, of group 14 elements ( X = C , Si , Ge and Sn ) and for both isomers, XOH + and HXO + , of the protonated monoxides. Structure optimisation has been carried out using the Density Functional Theory employing the B3LYP procedure and at both Hartree-Fock and MP2 (full) levels, all with a variety of medium-sized Gaussian basis sets. In all XO molecules the oxygen atom is the preferred site for protonation, except when X = C where HCO + is the lower energy isomer. Barriers to interconversion between the two isomers XOH + and HXO + are overestimated by the Hartree-Fock calculations, but with wave functions that include electron correlations they generally fall into the range 27–44 kcal mol −1 . Proton affinities increase as the atomic number of X increases, and values calculated by averaging over all wave functions that include electron correlation, give the following proton affinities: for CO, 141.5; for SiO, 189.3; for GeO, 196.1; and for SnO, 215.6 (all in kcal mol −1 ).

Molecular orbital calculations using density functional theory at the B3LYP/6-311++G(d,p) level have been used to optimize structures for ions COR+···M and M···RCO+ and also for the transition structures COR+···M(ts) for their interconversion (R = H, CH3 and M = Ar and N2). For the unsolvated ions and for ions COH+···M, M···HCO+, and COH+···M(ts) the optimized structures were used for single-point calculations at QCISD(T)(full)/6-311++G(2df,p). Critical points on the COH+ and ArCOH+ potential energy surfaces were also optimized at MP2(full)/6-311++G(3df,3pd). For the uncomplexed ions COR+, the barriers to 1,2-migration of R+ at B3LYP/6-311++G(d,p) are 35.4 kcal mol-1 for R = H and 14.2 kcal mol-1 for R = CH3. Inclusion of a dinitrogen molecule removes this barrier by permitting COR+ to deposit R+ on N2 followed by CO retrieving the R+ to produce the lower energy isomer, RCO+. Argon has a lower R+ affinity than the oxygen atom of CO and does not remove R+ from COR+. Preferential stabilization by argon of t...

The potential energy surface for C4H5+ as calculated by ab initio molecular orbital theory is reported at two levels of theory, HF/6-31G(d,p) and MP2(full)/6-311G(d,p). Fourteen minima have been lo...

Abinitio: molecular orbital calculations at MP2(FULL)/6-311G(d,p) or MP2(FULL)/6-31G(d,p) are reported for carbocations RR′CCHO+, RR′CCHS+, RR′CCONH2+, and RR′CCSNH2+where R and R′ are H, CH3, C-C3H5, and C6H5. Primary (R=R′=H), secondary (R=H, R′=alkyl or phenyl), and tertiary (R′=R′=CH3) ions prefer the cyclic oxiranyl or thiiranyl structure 9, with the open structure 8 being a transition structure for ring opening. Tertiary carbocations with R=R′=phenyl or cyclopropyl and the 9-thioformamidyl-9-fluorenyl cation have the open structure. Isodesmic reactions show CONH2 to be weakly stabilizing in the methyl cation, and CSNH2 has a larger stabilizing effect, roughly equivalent to that of a methyl group. An α-thioamide substituent is less stabilizing in the ethyl cation and even less stabilizing in the isopropyl cation. In C6H5CHCSNH2+ the CSNH2 group is slightly destabilizing and, by extrapolation, is more destabilizing in Ar2CCSNH2+. The rearrangement of the α-thioformamidyl-benzyl cation to benzothiophene is calculated to have a low barrier, 15.4 kcal/mol at HF/6-31G(d,p). Keywords: molecular orbitals, destabilized carbocations, rearrangement.

Subcutaneous infusion (hypodermoclysis) is a simple, safe, and effective technique used to administer fluids into the subcutaneous tissues. This technique is indicated for fluid replacement when there is a need for repetitive venipuncture, veins are inaccessible, or there is recurring infiltration at intravenous sites. It can be started and maintained for prolonged periods by health care professionals with minimal training. The main risk, infection at the infusion site, can be avoided by frequently alternating the site of the infusion.

Background: Repeat cognitive testing is an essential diagnostic strategy to measure changes in cognition over time when following people with memory problems. Alternate forms may avert practice effects that can mimic improvements in cognition. We evaluated alternate forms of verbal fluency and logical memory (paragraph recall) tasks to evaluate their equivalence for clinical use. Methods: Participants with mild cognitive impairment (MCI) and dementia were recruited from five outpatient memory clinics and one nursing home. Participants with normal cognition (NC) were recruited from family members or friends. Verbal fluency categories of animals, cities & towns, fruits & vegetables and first names were used. Scores were recorded for 0–30 seconds, 31–60 seconds and errors. For the logical memory task, participants were read one of three different paragraphs and then were asked to recall the story. Immediate recall and delayed recall scores were recorded. The Standardized Mini-mental State Examination, the AB Cognitive Screen and the 15-point Geriatric Depression Scale were administered as part of the assessment. Analyses were performed using means, frequency distributions, t -tests, receiver-operating characteristic curves and effect sizes. Results: There were 46 NC participants, 45 with MCI and 55 with dementia. For verbal fluency, the mean number of animals, cities & towns, names or fruits & vegetables named in 60 seconds did not differ significantly within each cognitive group. First names was an easier category than the others: NC named 16.9–22.3 items, MCI named 11.6–14.4 items and dementia named 8.1–11.4 items. The mean number of items immediately recalled in logical memory was not significantly different for the three paragraphs. The verbal fluency task (in 60 seconds) and logical memory immediate recall were highly sensitive and specific to differences between NC and MCI (areas under the curves 0.87 and 0.76, respectively). Conclusions: Alternate forms allow serial testing without learning bias. Verbal fluency and logical memory tasks are sensitive to early cognitive changes.

Background: Depression is common in elderly people but physicians may not screen for it because of the length of time required by current screening instruments. We have developed a short screening instrument for depression for use in elderly people with normal cognition, mild cognitive impairment or early dementia. Methods: Participants were aged 55 years or more, had scored 20 or more on the standardized Mini-mental State Examination (SMMSE) and had been referred to a specialist geriatric outpatient memory clinic. Scores on the 30-item Geriatric Depression Scale (GDS) were analyzed. A composite GDS score, consisting of the top five individual question scores that correlated to depression (GDS ≥ 14), were analyzed using a receiver operating curve analysis. Results: There were 810 patients with SMMSE scores of 20 or greater, of whom 202 (24.9%) scored 14 or more on the GDS, indicating depression. GDS question 16, "Do you often feel downhearted and blue?," had the highest correlation with the overall scores of 14 or more on the 30-point instrument (r = 0.64, p < 0.001). The next four questions with the highest correlates were Q10, "Do you often feel helpless?" (r = 0.56, p < 0.001), Q3, "Do you feel that your life is empty?" (r = 0.54, p < 0.001), Q9, "Do you feel happy most of the time?" (r = 0.52, p < 0.001), and Q1, "Are you basically satisfied with your life?" (r = 0.50, p < 0.001). The negative predictive value of "Do you often feel downhearted and blue?" answered negatively for depression was 96%. These five questions were used as a short screening instrument. The positive predictive value of four or five positive responses was 97%. These data were not significantly different whether the patient's SMMSE score was 20-25 or 26-30. Conclusions: The AB Clinician Depression Screen (ABCDS), comprising five questions, can rapidly identify patients with depression or eliminate that diagnosis. In this population, these five questions may be used instead of the longer 30-question GDS scale.