Investigating the impact of peer supplemental instruction on underprepared and historically underserved students in introductory STEM courses.

Background: Supplemental instruction (SI) is a well-established mode of direct academic support, used in a wide variety of courses. Some reports have indicated that SI and similar peer-led academic support models particularly benefit students identifying with historically underserved racial/ethnic groups in STEM. However, these studies have not explicitly examined the role of prior academic experiences, an important consideration in college success. We report on the impact of a modified SI model, Peer Supplemental Instruction (PSI), on student success in introductory STEM courses at a diverse access institution. This study focuses on PSI's impact on the academic performance of students identifying with historically underserved racial/ethnic groups, while also considering the effects of prior academic experiences. Results: Data were aggregated for nine courses over five semesters to produce a robust data set (n = 1789). PSI attendees were representative of the overall student population in terms of previous academic experiences/performance (as determined by high school GPA) and self-identified racial/ethnic demographics. Frequent PSI attendance was correlated with a significant increase in AB rates (average increase of 29.0 percentage points) and reduction in DFW rates (average decrease of 26.1 percentage points) when comparing students who attended 10 + vs. 1-2 PSI sessions. Overall, students identifying as Black/African American received the largest benefit from PSI. These students experienced a significant increase in their final course GPA when attending as few as 3-5 PSI sessions, and exhibited the largest increase in AB rates (from 28.7 to 60.5%) and decrease in DFW rates (from 47.1 to 14.8%) when comparing students who attended 10 + vs. 1-2 sessions. However, students with similar HS GPAs experienced similar benefits from PSI, regardless of self-identified race/ethnicity. Conclusions: The data presented here suggest that PSI particularly benefitted underprepared students in their introductory STEM courses. Since students identifying with historically underserved racial/ethnic groups have traditionally had inequitable K-12 educational experiences, they enter college less prepared on average, and thus particularly benefit from PSI. PSI, in conjunction with additional strategies, may be a useful tool to help rectify the results of systemic educational inequities for students identifying with historically underserved racial/ethnic groups.

Three-state trajectory surface hopping studies of the photodissociation dynamics of formaldehyde on ab initio potential energy surfaces.

Full-dimensional, three-state, surface hopping calculations of the photodissociation dynamics of formaldehyde are reported on ab initio potential energy surfaces (PESs) for electronic states S(1), T(1), and S(0). This is the first such study initiated on S(1) with ab initio-calculated spin-orbit couplings among the three states. We employ previous PESs for S(0) and T(1), and a new PES for S(1), which we describe here, as well as new spin-orbit couplings. The time-dependent electronic state populations and the branching ratio of radical products produced from S(0) and T(1) states and that of total radical products and molecular products at three total energies are calculated. Details of the surface hopping dynamics are described, and a novel pathway for isomerization on T(1) via S(0) is reported. Final translational energy distributions of H + HCO products from S(0) and T(1) are also reported as well as the translational energy distribution and final rovibrational distributions of H(2) products from the molecular channel. The present results are compared to previous trajectory calculations initiated from the global minimum of S(0). The roaming pathway leading to low rotational distribution of CO and high vibrational population of H(2) is observed in the present calculations.

Flexible, ab initio potential, and dipole moment surfaces for water. I. Tests and applications for clusters up to the 22-mer.

We report full-dimensional, ab initio potential energy and dipole moment surfaces, denoted PES and DMS, respectively, for arbitrary numbers of water monomers. The PES is a sum of 1-, 2-, and 3-body potentials which can also be augmented by semiempirical long-range higher-body interactions. The 1-body potential is a spectroscopically accurate monomer potential, and the 2- and 3-body potentials are permutationally invariant fits to tens of thousands of CCSD(T)/aug-cc-pVTZ and MP2/aug-cc-pVTZ electronic energies, respectively. The DMS is a sum of 1- and 2-body DMS, which are covariant fits to tens of thousands MP2/aug-cc-pVTZ dipole moment data. We present the details of these new 2- and 3-body potentials and then extensive applications and tests of this PES are made to the structures, classical binding energies, and harmonic frequencies of water clusters up to the 22-mer. In addition, we report the dipole moment for these clusters at various minima and compare the results against available and new ab initio calculations.

Roaming radicals.

Roaming is a recently verified unusual pathway to molecular products from unimolecular dissociation of an energized molecule. Here we present the evidence for this pathway for H(2)CO and CH(3)CHO. Theoretical analysis shows that this path visits the plateau region of the potential energy surface near dissociation to radical products. It is not clear whether roaming is a distinct isolated pathway, in addition to the conventional one via the well-known molecular saddle-point transition state. Evidence is presented to suggest that the two pathways may originate from a single, but highly complicated, dividing surface. Other examples of unusual reaction dynamics are also reviewed.

Full-dimensional, ab initio potential energy and dipole moment surfaces for water.

We report full-dimensional, ab initio potential energy (PES) and dipole moment surfaces (DMS) for water. The PES is a sum of one-, two- and three-body terms. The three-body potential is a fit, reported here, to roughly 30,000 intrinsic three-body energies obtained with second-order Møller-Plesset perturbation theory (MP2) and using the aug-cc-pVTZ basis set (avtz). The one- and two-body potentials are from an ab initio water dimer potential [Shank et al., J. Chem. Phys. 130, 144314 (2009)]. The predictive accuracy of the PES is demonstrated for the water trimer, tetramer, and hexamer by comparing the energies and harmonic frequencies obtained from the PES and new high level ab initio calculations at the respective global minima. The DMS is constructed from one- and two-body dipole moments, based on fits to MP2/avtz dipole moments. It is shown to be very accurate for the hexamer by comparison with direct calculations of the hexamer dipole. To illustrate the anharmonic character of the PES one-mode calculations of the 18 monomer fundamentals of the hexamer are reported in normal coordinates.

Quasiclassical trajectory calculations of the HO2 + NO reaction on a global potential energy surface.

We report quasiclassical trajectory calculations of the HO(2) + NO reaction using a new full dimensional, singlet potential energy surface (PES) which is a fit to more than 67 000 energies obtained with density functional theory-B3LYP/6-311G(d,p)-calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), saddle points connecting them and the OH + NO(2), HO(2) + NO channels. Quasiclassical trajectory calculations of cross-sections for the HO(2) + NO to form HOONO, HONO(2) and OH + NO(2) are done using this PES, for reactants in the ground vibrational state and rotational states sampled from a 300 K Boltzmann distribution. Trajectory calculations illustrate the pathway that HO(2) + NO takes to the energized HOONO complex, which dissociates to products OH + NO(2), reactants HO(2) + NO, or isomerizes to HONO(2). The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) in the high-pressure limit, and formation of products OH + NO(2) in the low-pressure limit.

Full-dimensional ab initio potential energy surface and vibrational configuration interaction calculations for vinyl.

The potential energy landscape and two permutationally invariant, full-dimensional ab initio-based potential energy surfaces (PESs) for the doublet vinyl radical, C(2)H(3), are described. The first of the two surfaces, denoted as PES/S, describes the equivalent CH(2)CH global minimum and the saddle point separating them, planar and nonplanar H-atom migration saddle points, a methylcarbyne local minimum that is due to a Jahn-Teller conical intersection, and the saddle point connecting it with the global minimum. The second PES, denoted PES/D, contains all stationary points of PES/S and in addition describes dissociation to C(2)H(2)+H fragments, including the saddle point to dissociation along a least-energy path. The surfaces are least-squares fits to electronic energies obtained with use of the spin-restricted coupled cluster singles and doubles with perturbative treatment of triples method and augmented correlation consistent polarized valence triple zeta basis sets, using permutationally invariant polynomials in "Morse variables" and a many-body expansion. PES/S is a fit to roughly 34,000 and PES/D to roughly 50,000 electronic structure energies. PES/S is used in full-dimensional, vibrational configuration interaction calculations of the vinyl zero-point energy and fundamental vibrational energies, which are compared to recent experiments.

Accurate ab initio potential energy surface, dynamics, and thermochemistry of the F+CH4-->HF+CH3 reaction.

An accurate full-dimensional global potential energy surface (PES) for the F+CH(4)-->HF+CH(3) reaction has been developed based on 19 384 UCCSD(T)/aug-cc-pVTZ quality ab initio energy points obtained by an efficient composite method employing explicit UCCSD(T)/aug-cc-pVDZ and UMP2/aug-cc-pVXZ [X=D,T] computations. The PES contains a first-order saddle point, (CH(4)- -F)(SP), separating reactants from products, and also minima describing the van der Waals complexes, (CH(4)- - -F)(vdW) and (CH(3)- - -HF)(vdW), in the entrance and exit channels, respectively. The structures of these stationary points, as well as those of the reactants and products have been computed and the corresponding energies have been determined using basis set extrapolation techniques considering (a) electron correlation beyond the CCSD(T) level, (b) effects of the scalar relativity and the spin-orbit couplings, (c) diagonal Born-Oppenheimer corrections (DBOC), and (d) zero-point vibrational energies and thermal correction to the enthalpy at 298 K. The resulting saddle point barrier and ground state vibrationally adiabatic barrier heights (V(SP) and V(VAGS)), dissociation energy of (CH(3)- - -HF)(vdW) (D(e) and D(0)), and the reaction enthalpy (DeltaH(e) ( degrees ), DeltaH(0) ( degrees ), and DeltaH(298) ( degrees )) are (240+/-40 and 245+/-200 cm(-1)), (1070+/-10 and 460+/-50 cm(-1)), and (-10000+/-50, -11200+/-80, and -11000+/-80 cm(-1)), respectively. Variational vibrational calculations have been carried out for (CH(3)- - -HF)(vdW) in full (12) dimensions. Quasiclassical trajectory calculations of the reaction using the new PES are reported. The computed HF vibrational and rotational distributions are in excellent agreement with experiment.

Photodissociation dynamics of formaldehyde initiated at the T1/S0 minimum energy crossing configurations.

The photodissociation dynamics of H(2)CO is known to involve electronic states S(1), T(1) and S(0). Recent quasiclassical trajectory (QCT) calculations, in conjunction with experiment, have identified a "roaming" H-atom pathway to the molecular products, H(2)+CO [Townsend; et al. Science 2004, 306, 1158.]. These calculations were initiated at the global minimum (GM) of S(0), which is where the initial wave function is located. The "roaming" mechanism is not seen if trajectories are initiated from the molecular transition state saddle point (SP). In this Letter we identify the minimum energy-crossing configurations and energy of the T(1)/S(0) potentials as a step toward studying the multisurface nature of the photodissociation. QCT calculations are initiated at these configurations on a revised potential energy surface and the results are compared to those initiated, as previously, from the S(0) GM as well as the S(0) SP. The product state distributions of H(2) + CO from trajectories initiated at the T(1)/S(0) crossing are in excellent agreement with those initiated at the S(0) GM.

Roaming is the dominant mechanism for molecular products in acetaldehyde photodissociation.

Reaction pathways that bypass the conventional saddle-point transition state (TS) are of considerable interest and importance. An example of such a pathway, termed "roaming," has been described in the photodissociation of H(2)CO. In a combined experimental and theoretical study, we show that roaming pathways are important in the 308-nm photodissociation of CH(3)CHO to CH(4) + CO. The CH(4) product is found to have extreme vibrational excitation, with the vibrational distribution peaked at approximately 95% of the total available energy. Quasiclassical trajectory calculations on full-dimensional potential energy surfaces reproduce these results and are used to infer that the major route to CH(4) + CO products is via a roaming pathway where a CH(3) fragment abstracts an H from HCO. The conventional saddle-point TS pathway to CH(4) + CO formation plays only a minor role. H-atom roaming is also observed, but this is also a minor pathway. The dominance of the CH(3) roaming mechanism is attributed to the fact that the CH(3) + HCO radical asymptote and the TS saddle-point barrier to CH(4) + CO are nearly isoenergetic. Roaming dynamics are therefore not restricted to small molecules such as H(2)CO, nor are they limited to H atoms being the roaming fragment. The observed dominance of the roaming mechanism over the conventional TS mechanism presents a significant challenge to current reaction rate theory.

"Roaming" dynamics in CH3CHO photodissociation revealed on a global potential energy surface.

We present a quasiclassical trajectory study of the photodissociation of CH3CHO to molecular and radical products, CH4 + CO and CH3 + HCO, respectively, using global ab initio-based potentials energy surfaces. The molecular products have a well-defined potential barrier transition state (TS) but the dynamics exhibit strong deviations from the TS pathway to these products. The radical products are formed via a variational TS. Calculations are reported at total energies corresponding to photolysis wavelengths of 308, 282, 264, 248 and 233 nm. The results at 308 nm focus on a comparison with experiment [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] and the elucidation of the nature and extent of non-TS reaction dynamics to form the molecular products, CH4 + CO. At the other wavelengths the focus is the branching ratio of these products and the radical products, CH3 + HCO.

Determination of the rate constant for sulfur recombination by quasiclassical trajectory calculations.

The sulfur recombination reaction has been thought of as one of the most important chemical reactions in the volcanic activities of the planet. It is also important in determining the propagation of elemental sulfur in the atmosphere. There have been two experimental attempts to determine the reaction rate of the S+S-->S(2) recombination, however their results differ by four orders of magnitude. In this work, we determine the rate constant of S+S-->S(2) from quasiclassical trajectory calculations. The third order rate constant at 298.15 K predicted by the present calculations is 4.19 x 10(-33) cm(6) molecules(-2) s(-1), which is in excellent agreement with the determination of Fair and Thrush [Trans. Faraday Soc. 65, 1208 (1969)]. The temperature dependent rate constant is determined to be 3.94 x 10(-33) exp[205.56(1T-1298.15)], which was determined from the temperature range of 100-500 K.

Hg+Br-->HgBr recombination and collision-induced dissociation dynamics.

A global potential energy surface has been constructed for the system HgBr+Ar-->Hg+Br+Ar to determine temperature dependent rate constants for the collision-induced dissociation (CID) and recombination of Hg and Br atoms. The surface was decomposed using a many-body expansion. Accurate two-body potentials for HgBr, HgAr, and ArBr were calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)], as well as the multireference averaged coupled pair functional method. Correlation consistent basis sets were used to extrapolate to the complete basis set limit and corrections were included to account for scalar and spin-orbit relativistic effects, core-valence correlation, and the Lamb shift. The three-body potential was computed with the CCSD(T) method and triple-zeta quality basis sets. Quasiclassical trajectories using the final analytical potential surface were directly carried out on the CID of HgBr by Ar for a large sampling of initial rotational, vibrational, and collision energies. The recombination rate of Hg and Br atoms is a likely first step in mercury depletion events that have been observed in the Arctic troposphere during polar sunrise. The effective second order rate constant for this process was determined in this work from the calculated CID rate as a function of temperature using the principle of detailed balance, which resulted in k(T) = 1.2 x 10(-12) cm(3) molecule(-1) s(-1) at 260 K and 1 bar pressure.

Quasiclassical trajectory calculations of the OH+NO2 association reaction on a global potential energy surface.

We report a full-dimensional potential energy surface (PES) for the OH+NO(2) reaction based on fitting more than 55,000 energies obtained with density functional theory-B3LYP6-311G(d,p) calculations. The PES is invariant with respect to permutation of like nuclei and describes all isomers of HOONO, HONO(2), and the fragments OH+NO(2) and HO(2)+NO. Detailed comparison of the structures, energies, and harmonic frequencies of various stationary points on the PES are made with previous and present high-level ab initio calculations. Two hydrogen-bond complexes are found on the PES and confirmed by new ab initio CASPT2 calculations. Quasiclassical trajectory calculations of the cross sections for ground rovibrational OH+NO(2) association reactions to form HOONO and HONO(2) are done using this PES. The cross section to form HOONO is larger than the one to form HONO(2) at low collision energies but the reverse is found at higher energies. The enhancement of the HOONO complex at low collision energies is shown to be due, in large part, to the transient formation of a H-bond complex, which decays preferentially to HOONO. The association cross sections are used to obtain rate constants for formation of HOONO and HONO(2) for the ground rovibrational states in the high-pressure limit.

Quasiclassical trajectory calculations of acetaldehyde dissociation on a global potential energy surface indicate significant non-transition state dynamics.

A recent experimental study [Houston, P. L.; Kable, S. H. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 16079] of the photodissociation of acetaldehyde (CH(3)CHO) has suggested two distinct mechanisms for the production of the molecular products CH(4) + CO. One corresponds to the traditional transition state mechanism and the other to a transition state-skirting path similar to the roaming channel previously reported in formaldehyde. To investigate this theoretically, a full-dimensional potential energy surface (PES) has been constructed. The PES was fit with permutationally invariant polynomials to 135,000 points calculated using coupled cluster theory with single and double excitations and a perturbative treatment of triple excitations [CCSD(T)] and correlation consistent basis sets of double- and triple-zeta quality. To test the accuracy of the PES additional CCSD(T) and multireference configuration interaction calculations were carried out. Quasiclassical trajectory calculations were run on the PES starting at the acetaldehyde equilibrium geometry and also at the conventional transition state (TS) for the molecular products CH(4) + CO. The former calculations agree well with the experimental results of Houston and Kable; however, those from the TS do not. The implications for a non-transition state, roaming mechanism in this molecule are discussed.

Aqueous microsolvation of mercury halide species.

The effects of aqueous solvation on the thermochemistry of reactions between mercury and small halogen molecules has been investigated by the microsolvation approach using ab initio and density functional theory (DFT) calculations. The structures, vibrational frequencies, and binding energies of 1, 2, and 3 water molecules with mercury-halide (HgBr2, HgBrCl, HgCl2, HgBr, and HgCl) and related mercury and halogen species (Br2, BrCl, Cl2, Cl, Hg, and Br) have been computed with second order Møller-Plesset perturbation theory (MP2) and the B3LYP density functional method. Accurate incremental water binding energies have been obtained at the complete basis set (CBS) limit using sequences of correlation consistent basis sets, including higher order correlation effects estimated from coupled cluster calculations. The resulting energetics were used to calculate the influence of water molecules on the thermochemistry of a number of reactions between mercury and small halogen-containing molecules. In general, the presence of water favors the formation of oxidized mercury halide species.

On the spectroscopic and thermochemical properties of ClO, BrO, IO, and their anions.

A coupled cluster composite approach has been used to accurately determine the spectroscopic constants, bond dissociation energies, and heats of formation for the X1(2)II(3/2) states of the halogen oxides ClO, BrO, and IO, as well as their negative ions ClO-, BrO-, and IO-. After determining the frozen core, complete basis set (CBS) limit CCSD(T) values, corrections were added for core-valence correlation, relativistic effects (scalar and spin-orbit), the pseudopotential approximation (BrO and IO), iterative connected triple excitations (CCSDT), and iterative quadruples (CCSDTQ). The final ab initio equilibrium bond lengths and harmonic frequencies for ClO and BrO differ from their accurate experimental values by an average of just 0.0005 A and 0.8 cm-1, respectively. The bond length of IO is overestimated by 0.0047 A, presumably due to an underestimation of molecular spin-orbit coupling effects. Spectroscopic constants for the spin-orbit excited X2(2)III(1/2) states are also reported for each species. The predicted bond lengths and harmonic frequencies for the closed-shell anions are expected to be accurate to within about 0.001 A and 2 cm-1, respectively. The dissociation energies of the radicals have been determined by both direct calculation and through use of negative ion thermochemical cycles, which made use of a small amount of accurate experimental data. The resulting values of D0, 63.5, 55.8, and 54.2 kcal/mol for ClO, BrO, and IO, respectively, are the most accurate ab initio values to date, and those for ClO and BrO differ from their experimental values by just 0.1 kcal/mol. These dissociation energies lead to heats of formation, DeltaH(f) (298 K), of 24.2 +/- 0.3, 29.6 +/- 0.4, and 29.9 +/- 0.6 kcal/mol for ClO, BrO, and IO, respectively. Also, the final calculated electron affinities are all within 0.2 kcal/mol of their experimental values. Improved pseudopotential parameters for the iodine atom are also reported, together with revised correlation consistent basis sets for this atom.

Chemically accurate thermochemistry of cadmium: An ab initio study of Cd + XY (X = H, O, Cl, Br; Y = Cl, Br).

Using a composite coupled cluster method employing sequences of correlation consistent basis sets for complete basis set (CBS) extrapolations and with explicit treatment of core-valence correlation and scalar and spin-orbit relativistic effects, the 0 K enthalpies of a wide range of cadmium-halide reactions, namely, Cd + (HCl, HBr, ClO, BrO, Cl2, BrCl, Br2) have been determined to an estimated accuracy of +/-1 kcal/mol. In addition, accurate equilibrium geometries, harmonic frequencies, and dissociation energies have been calculated at the same level of theory for all the diatomic (e.g., CdH, CdO, CdCl, CdBr) and triatomic (CdHCl, CdHBr, CdClO, CdBrO, CdCl2, CdBrCl, CdBr2) species involved in these reactions, some for the very first time. Like their mercury analogues, all of the abstraction reactions are predicted to be endothermic, while the insertion reactions are strongly exothermic with the formation of stable linear, Cd-centric complexes. With the exception of CdH and the reactions involving this species, the present results for the remaining Cd-containing systems are believed to be the most accurate to date.

Ab initio thermochemistry involving heavy atoms: an investigation of the reactions Hg + IX (X = I, Br, Cl, O).

Accurate 0 K enthalpies have been calculated for reactions of mercury with a series of small iodine-containing molecules (I2, IBr, ICl, and IO). The calculations have been carried out with the coupled cluster singles and doubles method with a perturbative correction for connected triple excitations [CCSD(T)] using sequences of correlation consistent basis sets and accurate relativistic pseudopotentials. Corrections have been included to account for core-valence correlation, spin-orbit coupling, scalar relativity, and the Lamb shift. In a few cases coupled cluster calculations with iterative triple (CCSDT) and quadruple (CCSDTQ) excitations have been carried out to estimate the effects of higher order electron correlation. The pseudopotential calculations have also been compared to all electron calculations using second- and third-order Douglas-Kroll-Hess Hamiltonians. In addition to the reaction enthalpies, heats of formation, bond lengths, and harmonic vibrational frequencies have been calculated for the stable triatomic products HgI2, HgIBr, HgICl, and HgIO. Accurate dissociation energies, equilibrium bond lengths, and harmonic vibrational frequencies have also been calculated for each of the diatomic molecules involved in this study (HgI, HgBr, HgCl, HgO, I2, IBr, ICl, and IO). The reported enthalpies are expected to have accuracies of 1 kcal/mol or better.

Accurate global potential energy surface and reaction dynamics for the ground state of HgBr2.

A global potential energy surface (PES) for the (1)A' ground state of HgBr(2) has been constructed in order to determine the rate constants for atmospherically important reactions involving mercury and bromine. The total energy of HgBr(2) was calculated by the multireference configuration interaction level of theory with series of correlation consistent basis sets up to quadruple-zeta quality with subsequent extrapolation to the complete basis set limit. An additive correction for spin-orbit coupling was also included. The global PES was represented piecewise by interpolating three separate parts of the surface with the reproducing kernel Hilbert space method and connecting them smoothly by switch functions. Quasiclassical trajectory calculations carried out on the surface yielded 298 K thermal rate constants of 3.89 x 10(-11) cm(3)/(mol.s) for the abstraction reaction HgBr + Br --> Hg + Br(2), 2.98 x 10(-11) cm(3)/(mol.s) for the recombination reaction Br + HgBr --> HgBr(2), and 3.97 x 10(-11) cm(3)/(mol.s) for the exchange reaction Br + HgBr --> BrHg + Br. The insertion reaction Hg + Br(2) --> HgBr(2) was found to have a high barrier of 27.2 kcal/mol and a very small rate constant of just 2.74 x 10(-31) cm(3)/(mol.s) determined by the microcanonical variational transition state theory method. The implications of the obtained results to the description of the mechanism of recently observed polar tropospheric mercury depletion events are briefly discussed.